| 1. |
- Aikio, A T, et al.
(författare)
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On the origin of the high-altitude electric field fluctuations in the auroral zone
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A12, s. 27157-27170
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Tidskriftsartikel (refereegranskat)abstract
- Intense fluctuations in the electric field at high altitudes in the auroral zone are frequently measured by the Viking satellite. We have made an analysis of the origin of electric and magnetic fluctuations in the frequency range of 0.1 - 1 Hz by assuming four different sources for the signals: (I) spatial structures, (2) spatial structures with a parallel potential drop below the satellite, (3) traveling; shear Alfven waves, and (4) interfering shear Alfven waves. We will shaw that these different sources of the signals may produce similar amplitude ratios and phase differences between the perpendicular electric and magnetic fields. Since the different sources have different frequency dependencies, this can be used as an additional test if the signals are broadband. In other cases, additional information is needed, for example, satellite particle measurements or ground; magnetic measurements. The ideas presented in the theory were tested for one Viking eveningside pass over Scandinavia, where ground-based magnetometer and EISCAT radar measurements were available. The magnetic conditions were active during this pass and several interfering shear Alfven waves were found. Also, a spatial structure with a parallel potential drop below the satellite was identified. The magnitude of the 10-km-wide potential drop was at least 2 kV and the upward field-aligned current 26 mu A m(-2) (value mapped to the ionospheric level). The held-aligned conductance was estimated as 1.3 - 2.2x10(-8) S m(-2).
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| 2. |
- Cumnock, Judy, et al.
(författare)
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High-Latitude Ionospheric Convection Pattern During Steady Northward Interplanetary Magnetic Field
- 1995
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 100, s. 14537-14555
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Tidskriftsartikel (refereegranskat)abstract
- The DMSP F8 satellite’s coverage of Earth’s polar regions provides horizontal ion drift velocities along the dawn-dusk meridian at approximately 835 km altitude in each hemisphere during the similar to 100 min orbital period. We examine the ionospheric convection signatures observed by this spacecraft in the summer and winter hemispheres during periods when the interplanetary magnetic field (IMF) is directed northward for at least 45 min prior to the satellite entering the polar region and remains northward throughout the polar pass. These convection signatures can be readily categorized by the number of sunward and antisunward flow regions and by their potential distributions. Here we describe the most frequently identifiable and reproducible features of the convection pattern that exist during steady northward IMF conditions. In addition to IMF B-z, the influences on the convection pattern of the IMF B-z/\textbackslashB-y\textbackslash ratio, season, latitude, and solar wind velocity are all considered. The ratio B-z/\textbackslashB-y\textbackslash provides a first order organization of the signatures that occur on the dayside of the dawn-dusk meridian. Sunward flow at highest latitudes on the dayside of the dawn-dusk meridian is the dominant feature seen in the large-scale convection signature during steady northward IMF; however, sunward flow at highest latitudes does not imply the existence of a particular number of convection cells.
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| 3. |
- Feldstein, Y I, et al.
(författare)
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Electromagnetic characteristics of the high-latitude ionosphere during the various phases of magnetic substorms
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A9, s. 19921-19936
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Tidskriftsartikel (refereegranskat)abstract
- Model calculations of the electrodynamics of the high-latitude ionosphere are compared to measurements made by the Viking satellite during July-August 1986. The model calculations are based on the IZMEM procedure, where the electric field and currents in the ionosphere are given as functions of the interplanetary magnetic field. The events chosen correspond to the growth, the expansion, and the recovery phases of substorms. During the growth and expansion phases the correlation between the model results and the satellite data is rather good. During recovery phase the correlation is not as good. The correlation between modeled and observed quantities suggest that during growth and expansion phase the magnetosphere is mainly directly driven by the solar wind, whereas during recovery phase it is mainly driven by internal processes, i.e., loading-unloading. Best fit is obtained when averaging the measured quantities over a few minutes, which means adjusting the spatial resolution of the measurements to the resolution of the model. Different time delays between the interplanetary magnetic field observations and those of Viking were examined. Best agreement was obtained, not surprisingly, for time delays corresponding to the estimated information transit time from the solar wind spacecraft to the ionosphere.
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| 4. |
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| 5. |
- Hole, Lars Robert, et al.
(författare)
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Modeling of Sound Propagation in the Atmospheric Boundary Layer : Application of the MIUU Mesoscale Model
- 1999
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 104:D10, s. 11891-11901
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Tidskriftsartikel (refereegranskat)abstract
- A mesoscale atmospheric model, the Meteorological Institute of Uppsala University model, has been used to simulate local wind and temperature profiles in an area with hilly terrain in eastern Norway. Simulated wind and temperature profiles (up to 250 m height above ground) have been used as input to an acoustical model, called Oases. In this way, sound propagation out to 1.4 km distance along ground has been determined for a site situated in a valley bottom in the center of the model domain. Measured ground conditions are also included in the acoustical model. During the investigated period, meteorological measurements were carried out in conjunction with measurements of low-frequency (≲100 Hz) impulse sound propagation out to a range of 1400 m along a flat, uniform, and snow-covered ground. For a case with geostrophic winds of the order of 25 m s-1 the predicted profiles are compared with profiles measured by meteorological towers and a tethered balloon launched at the site. Acoustic predictions based on both measured and simulated atmospheric profiles agree well with measured single-frequency sound levels in most cases. This suggests that forecasts of sound propagation using both a nested mesoscale atmospheric model and an acoustical model should be possible in future.
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| 6. |
- Marklund, Göran, et al.
(författare)
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Observations of the electric field fine structure associated with the westward traveling surge and large-scale auroral spirals
- 1998
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 103:A3, s. 4125-4144
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Tidskriftsartikel (refereegranskat)abstract
- The characteristics of the fine scale electric field associated with the westward traveling surge and large-scale auroral spirals and surges are investigated using high-resolution electric field, magnetic field, particle and UV imager observations from four eveningside auroral oval crossings by the Freja satellite. Three of the crossings were associated with signatures of auroral substorms and one crossing went directly through the head of a surge close in time and space to substorm onset. Three passes were adjacent to auroral spiral formations, one poleward of and one equatorward of such forms and one through the multiple arc region near the front of an extended region of auroral activity. The ambient electric field was found to intensify in the direction toward the spiral head (or the center of the auroral activity region) over a region comparable to the size of the visible auroral forms. These results confirm previous findings that the spiral or surge head is associated with negative space charge and an intense upward field-aligned current. The fourth pass, directly through the surge head reveals a very complicated structure of the surge region. Narrowly structured, intense (up to 700 mV/m) and mostly converging electric fields associated with intense electron precipitation (of both high and medium energies) and balanced field-aligned currents (up to 30 μA/m2) are seen near the edge of the surge head and adjacent to auroral structures in the wake. These narrow regions are embedded within more extended regions of intense high-energy electron precipitation but very weak electric fields and field-aligned currents. According to some existing models of the surge, a pronounced westward electric field component and a southward polarisation electric field is expected within the entire high-conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field-aligned currents from the near surroundings. The Freja electric field is typically seen to intensify at the edges of or in-between bright auroral structures and to decrease within the arcs similar to what is observed in the ionosphere. The surge electric field is, however, much more intense than previously observed or anticipated at these altitudes with characteristics rather similar to those observed in the auroral acceleration region. Since the particle data indicate that most of the acceleration takes place above Freja altitudes, it seems as if Freja traversed the lower part of the auroral acceleration region associated with the surge.
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| 7. |
- Norqvist, P, et al.
(författare)
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Ion cyclotron heating in the dayside magnetosphere
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A6, s. 13179-13193
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Tidskriftsartikel (refereegranskat)abstract
- Observations of waves and particles obtained by the Freja satellite at altitudes around 1700 km in the dayside high-latitude magnetosphere are used to study ion energization. We find that ions, including O+, during several events of intense ion energization can be heated perpendicularly to the geomagnetic field to mean energies df up to about 20 eV via the process of cyclotron resonance by broadband waves around the ion gyrofrequencies. There is a good correlation between such broadband waves and the ion energization. The waves show no spectral features at the O+ gyrofrequency. The observed wave amplitudes are used as an input to a Monte Carlo simulation to obtain the observed ion energies. The waves around the ion gyrofrequencies may be generated either by field-aligned electrons or by nonlinear processes transferring energy from waves with lower frequencies. Not only the mean energy but also the shape of the particle distribution agrees with the cyclotron resonance heating mechanism. Other mechanisms, such as heating by lower hybrid waves or by a slowly varying electric field, are investigated but are found to be less important than cyclotron heating in this region of space.
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| 8. |
- Ohtani, S, et al.
(författare)
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4 Large-scale field-aligned current systems in the dayside high-latitude region
- 1995
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 100, s. 137-153
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Tidskriftsartikel (refereegranskat)abstract
- A system of four current sheets of large-scale field-aligned currents (FACs) was discovered in the data set of simultaneous Viking and DMSP-F7 crossings of the dayside high-latitude region. This paper reports four examples of this system that were observed in the prenoon sector. The flow polarities of FACs are upward, downward, upward, and downward, from equatorward to poleward. The lowest-latitude upward current is flowing mostly in the CPS precipitation region, often overlapping with the BPS at its poleward edge, and is interpreted as a region 2 current. The pair of downward and upward FACs in the middle of the structure are collocated with structured electron precipitation. The precipitation of high-energy (>1 keV) electrons is more intense in the lower-latitude downward current sheet. The highest-latitude downward flowing current sheet is located in a weak, low-energy particle precipitation region, suggesting that this current is flowing on open field lines. Simultaneous observations in the postnoon local time sector reveal the standard three-sheet structure of FACs, sometimes described as region 2, region 1, and mantle (referred to the midday region 0) currents. A high correlation was found between the occurrence of the four FAC sheet structure and negative interplanetary magnetic field (IMF) B-Y, We discuss the FAC structure in terms of three types of convection cells: the merging, viscous, and lobe cells. During strongly negative IMF B-Y two convection reversals exist in the prenoon sector; one is inside the viscous cell, and the other is between the viscous cell and the lobe cell. This structure of convection flow is supported by the Viking electric field and auroral UV image data. Based on the convection pattern, the four FAC sheet structure is interpreted as the latitudinal overlap of midday and morning FAC systems. We suggest that the four-current sheet structure is common in a certain prenoon local time sector during strongly negative IMF B-Y.
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| 9. |
- Ohtani, S, et al.
(författare)
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Altitudinal comparison of dayside field-aligned current signatures by viking and DMSP-F7 : Intermediate-scale field-aligned current systems
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A7, s. 15297-15310
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Tidskriftsartikel (refereegranskat)abstract
- Dayside large-scale and intermediate-scale field-aligned current (FAG) signatures are examined with multi-instrument measurements from Viking and DMSP-F7 at magnetic conjunctions. The present paper reports four such conjunction events, with an emphasis on an event that occurred on October 13, 1986. In these four events both Viking and DMSP-F7 crossed prenoon FAC systems approximately along meridians. The altitude of DMSP-F7 was 835 km, whereas that of Viking ranged from 8500 to 12,000 km. The electric to magnetic field ratio measured by Viking indicates that intermediate-scale FAC systems, as well as large-scale FAC systems, are often quasi-stationary, This is also supported by the comparison between the Viking and DMSP-F7 magnetic measurements. The only obvious exception is the equatorward part of the October 13 event, in which the Viking and DMSP-F7 measurements are better explained in terms of Alfven waves. In two other events the Viking signature projected to the DMSP-F7 altitude is significantly more structured than the DMSP-F7 signature, although the electric to magnetic field ratio observed by Viking suggests that the associated FACs were quasistationary. This apparent discrepancy is possibly ascribed to the fact that Viking stays longer in FAC systems and therefore has more chance to observe temporal changes in FACs. However, such temporal effects must operate longer than the Alfven transit time so that FAC systems become quasi-stationary. Although the generation mechanism(s) of intermediate-scale FAC systems remains an open question, possibilities include a localized shear of plasma convection and a localized merging between the solar wind and magnetospheric field lines.
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| 10. |
- Ohtani, S, et al.
(författare)
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Simultaneous prenoon and postnoon observations of 3 field-aligned current systems from Viking and DMSP F7
- 1995
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 100:A1, s. 119-136
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Tidskriftsartikel (refereegranskat)abstract
- The spatial structure of dayside large-scale field-aligned current (FAG) systems is examined by using Viking and DMSP-F7 data. We focus on four events in which the satellites simultaneously observed postnoon and prenoon three FAC systems: the region 2, the region 1, and the mantle (referred to as midday region 0) systems, from equatorward to poleward. These events provide the most solid evidence to date that the midday region 0 system is a separate and unique FAC system, and is not an extension of the region 1 system from other local times. The events are examined comprehensively by making use of a multi-instrument data set, which includes magnetic field, particle flux, electric field, auroral UV image data from the satellites, and the Sondrestrom convection data. The results are summarized as follows: (1) Region 2 currents flow mostly in the CPS precipitation region, often overlapping with the BPS at their poleward edge. (2) The region 1 system is located in the core part of the auroral oval and is confined in a relatively narrow range in latitude which includes the convection reversal. The low-latitude boundary layer, possibly including the outer part of the plasma sheet, and the external cusp are the major source regions of dayside region 1 currents. (3) Midday region 0 currents flow on open field lines and are collocated with the shear of antisunward convection flows with velocities decreasing poleward. On the basis of these results we support the view that both prenoon and postnoon current systems consist of the three-sheet structure when the distortion of the convection pattern associated with interplanetary magnetic field (IMF) B-y is small and both morningside and eveningside convection cells are crescent-shaped. We also propose that the midday region 0 and a part of the region 1 systems are closely coupled to the same source.
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| 11. |
- Potemra, T A, et al.
(författare)
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A survey of Pc 5 pulsations in the dayside high-latitude regions observed by Viking
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A11, s. 24801-24813
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Tidskriftsartikel (refereegranskat)abstract
- We have examined magnetic and electric field observations acquired with the polar orbiting Viking satellite near its 3 R(E) apogee and at local times from dawn through noon to dusk. Our objective was to determine the statistical properties of Pc 5 (150-600 s period) pulsations at different dayside local times and their relationship to large-scale Birkeland currents. The Viking data examined in detail here were acquired from 41 orbits during the period from March 26, 1986, to July 19, 1986. Pulsations could be readily identified on prenoon orbits as evidenced in a series of 10 consecutive orbits in one case and six consecutive orbits in another. The waveforms in the postnoon hours are more complicated than in the morning, and this made it more difficult to identify pulsations in this region. The simultaneous electric and magnetic field measurements from Viking were used to distinguish the magnetic variations associated with large-scale Birkeland currents from those associated with waves. These observations also made it possible to derive values for Pedersen and wave conductivities and Poynting fluxes for the waves. The average wave period for the 40 cases studied here is 4.8 min (frequency of 3.5 mHz), the average magnetic field amplitude is 30 nT, the average electric field amplitude (for the 11 cases when the electric field observations were available) is 7 mV/m, and the average incident Poynting flux is 85 mu W/m(2). The principal characteristics of the waves include the following: (1) From morning to noon, the magnetic field pulsations are polarized primarily in the east-west direction, and the associated electric field oscillations lag by 60 degrees-80 degrees and are polarized close to the north-south direction. Consequently, these pulsations are interpreted as the fundamental modes of resonant toroidal magnetic field oscillations. (2) In the afternoon hours, the magnetic field pulsations have a strong north-south component and include higher-frequency components (possibly the second harmonic). (3) In both the morning and afternoon hours, the pulsations occur 2 degrees-5 degrees equatorward of the interface between the counterflowing region 1 and region 2 Birkeland currents. (4) The average wave periods increase from similar to 3 min near 0800 magnetic local time (MLT) to similar to 5 min near noon. (5) The average wave amplitudes range from similar to 40 nT near 0900-1000 MLT to similar to 25 nT near 1300-1600 MLT.
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| 12. |
- Rakov, Vladimir A., et al.
(författare)
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Mechanism of the lightning M component
- 1995
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Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 100:D12, s. 25701-25710
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Tidskriftsartikel (refereegranskat)abstract
- Analysis of simultaneous measurements of the channel-base current and the vertical electric field 30 m from triggered lightning reveals that the fields associated with M components, although essentially electrostatic, appear to be proportional to the time derivatives of the associated M currents. Based on this finding, coupled with other observations and modeling, a mechanism for the lightning M component is proposed. According to this mechanism an M component involves a downward progressing incident wave (the analog of a leader) followed by an upward progressing reflected wave (the analog of a return stroke). However, as opposed to a leader-return stroke sequence in which the latter removes the charge deposited by the former, both the upward and the downward processes contribute about equally to the total charge flowing from the bottom of the channel at any instant of time. Such a mode of charge transfer to ground, distinctly different from a leader-return stroke sequence, is possible because of the presence of a path capable of supporting the propagation of a traveling wave (facilitated by a continuing current flowing to ground) and the fact that the ground is essentially a short circuit for the downward incident wave, so that the magnitude of the current reflection coefficient at ground is virtually equal to unity. We show that some observed properties of M components can be explained if the lightning channel traversed by an M-current wave is represented as a linear R-C transmission line. In this view, the preferential attenuation of the higher-frequency components on an R-C line is responsible for the lack of frequencies above several kilohertz in both the M-current pulses measured at the channel base and the M-light pulses observed in the bottom 1 km or so of the channel. Further, the relatively high characteristic impedance of the channel, of the order of tens to hundreds of kilohms for frequencies below some kilohertz, inferred from the linear R-C line approximation, is consistent with the observation that even a relatively poor ground is sensed by an incident M wave as essentially a short circuit. However, on a linear R-C transmission line the higher-frequency components travel faster than lower-frequency components (this velocity dispersion implying that the original pulse would spread while propagating along the line), whereas the shape of the M-light pulses does not change much within the bottom 1 km or so, as if the channel were a distortionless transmission line. We speculate, on physical grounds, that the front of the traveling M-current pulse heats the channel so that the pulse tail encounters a lowered resistance and, as a result, accelerates. By virtue of these two opposing effects, velocity dispersion and channel nonlinearity, an M pulse is formed whose more-or-less symmetrical shape is preserved over a relatively large distance, as in the case of a soliton.
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| 13. |
- Rakov, V. A., et al.
(författare)
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New insights into lightning processes gained from triggered-lightning experiments in Florida and Alabama
- 1998
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Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 103:D12, s. 14117-14130
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Tidskriftsartikel (refereegranskat)abstract
- Analyses of electric and magnetic fields measured at distances from tens to hundreds of meters from the ground strike point of triggered lightning at Camp Blanding, Florida, and at 10 and 20 m at Fort McClellan, Alabama, in conjunction with currents measured at the lightning channel base and with optical observations, allow us to make new inferences on several aspects of the lightning discharge and additionally to verify the recently published “two-wave†mechanism of the lightning M component. At very close ranges (a few tens of meters or less) the time rate of change of the final portion of the dart leader electric field can be comparable to that of the return stroke. The variation of the close dart leader electric field change with distance is somewhat slower than the inverse proportionality predicted by the uniformly charged leader model, perhaps because of a decrease of leader charge density with decreasing height associated with an incomplete development of the corona sheath at the bottom of the channel. There is a positive linear correlation between the leader electric field change at close range and the succeeding return stroke current peak at the channel base. The formation of each step of a dart-stepped leader is associated with a charge of a few millicoulombs and a current of a few kiloamperes. In an altitude-triggered lightning the downward negative leader of the bidirectional leader system and the resulting return stroke serve to provide a relatively low-impedance connection between the upward moving positive leader tip and the ground, the processes that follow likely being similar to those in classical triggered lightning. Lightning appears to be able to reduce, via breakdown processes in the soil and on the ground surface, the grounding impedance which it initially encounters at the strike point, so at the time of channel-base current peak the reduced grounding impedance is always much lower than the equivalent impedance of the channel. At close ranges the measured M-component magnetic fields have waveshapes that are similar to those of the channel-base currents, whereas the measured M-component electric fields have waveforms that appear to be the time derivatives of the channel-base current waveforms, in further confirmation of the “two-wave†M-component mechanism.
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| 14. |
- Rothwell, P.L, et al.
(författare)
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Particle dynamics in a spatially varying electric field
- 1995
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 100, s. 14875-14885
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Tidskriftsartikel (refereegranskat)abstract
- For an MHD description of a plasma a distinct separation between the macroscopic and microscopic spatial and temporal scales is assumed. In this paper we solve the particle dynamics with finite first and second spatial derivatives in the electric field. We find that (1) MHD (ideal and nonideal) becomes invalid for a sufficiently strong constant electric field gradient perpendicular to the magnetic field; (2) a sufficiently large second derivative in the electric field can cause heavy ions to become chaotically untrapped; (3) for an electric field with a constant gradient the ion drift velocity is equal to (ExB)/\textbackslashB\textbackslash(2) as long as the orbit-averaged value off is used. There are no finite currents associated with the ion drift for such an electric field; (4) perturbation technique gives a poor approximation to the ion drift velocity even for values of the second derivative that may well occur in the magnetosphere. Results 1 and 2 provide necessary criteria for the applicability of magnetospheric MHD models of spatially varying electric fields. They also predict an asymmetry in the heavy ion fluxes, a feature that could be useful in inferring magnetospheric electric field structure. We illustrate these results by application to the Harang discontinuity. It is found that if the interplanetary magnetic field swings northward under substorm growth conditions the orbits of the equatorial O+ may dramatically change due to result 2. This effect may contribute to the substorm onset process.
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| 15. |
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| 16. |
- Rubenstein, M., et al.
(författare)
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Characterization of vertical electric fields 500 m and 30 m from triggered lightning
- 1995
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Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 100:D5, s. 8863-8872
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Tidskriftsartikel (refereegranskat)abstract
- Vertical electric field waveforms of leader-return stroke sequences measured 500 m and 30 m from rocket-triggered lightning are presented. The 500-m data were recorded during the summer of 1986, the 30-m data during the summer of 1991, both at the NASA Kennedy Space Center, Florida. The 40 leader-return stroke field waveforms at 500 m and the 8 waveforms at 30 m all appear as asymmetrical V-shaped pulses, the bottom of the V being associated with the transition from the leader to the return stroke. Only two waveforms at 30 m were suitable for quantitative analysis. The widths of the V at half of peak value for these are 1.8 and 5.0 Όs, while for the 500-m data they are 1 to 2 orders of magnitude greater, with a median value of 100 Όs. Applying a widely used and simple leader model to the measured leader electric fields at 500 m, we infer, for the bottom kilometer or so of the leader channel, leader speeds between 2×106 and 2×107 m/s and leader charges per unit length of 0.02×10−3 to 0.08×10−3 C/m. From the two measured leader electric field changes at 30 m we infer, using the same leader model, for the bottom 100 meters or so of the leader channel, speeds of 3×107 and 1×107 m/s (the corresponding measured waveform half widths are 1.8 Όs and 5.0 Όs) and charges per unit length of 0.14×10−3 and 0.02×10−3 C/m (the corresponding measured leader field changes are 81 kV/m and 12 kV/m). The corresponding measured return stroke peak currents for the above two cases are 40 kA and 7 kA, respectively. A positive correlation is observed between the magnitude of the leader field change at 500 m and the ensuing return stroke current peak.
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| 17. |
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| 18. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
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Distribution of charge along the lightning channel: Relation to remote electric and magnetic fields and to return stroke models
- 1997
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 102, s. 6987-7006
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Tidskriftsartikel (refereegranskat)abstract
- We derive exact expressions for remote electric and magnetic fields as a function of the time‐ and height‐varying charge density on the lightning channel for both leader and return‐stroke processes. Further, we determine the charge density distributions for six return‐stroke models. The charge density during the return‐stroke process is expressed as the sum of two components, one component being associated with the return‐stroke charge transferred through a given channel section and the other component with the charge deposited by the return stroke on this channel section. After the return‐stroke process has been completed, the total charge density on the channel is equal to the deposited charge density component. The charge density distribution along the channel corresponding to the original transmission line (TL) model has only a transferred charge density component so that the charge density is everywhere zero after the wave has traversed the channel. For the Bruce‐Golde (BG) model there is no transferred, only a deposited, charge density component. The total charge density distribution for the version of the modified transmission line model that is characterized by an exponential current decay with height (MTLE) is unrealistically skewed toward the bottom of the channel, as evidenced by field calculations using this distribution that yield (1) a large electric field ramp at ranges of the order of some tens of meters not observed in the measured electric fields from triggered‐lightning return strokes and (2) a ratio of leader‐to‐return‐stroke electric field at far distances that is about 3 times larger than typically observed. The BG model, the traveling current source (TCS) model, the version of the modified transmission line model that is characterized by a linear current decay with height (MTLL), and the Diendorfer‐Uman (DU) model appear to be consistent with the available experimental data on very close electric fields from triggered‐lightning return strokes and predict a distant leader‐to‐return‐stroke electric field ratio not far from unity, in keeping with the observations. In the TCS and DU models the distribution of total charge density along the channel during the return‐stroke process is influenced by the inherent assumption that the current reflection coefficient at ground is equal to zero, the latter condition being invalid for the case of a lightning strike to a well‐grounded object where an appreciable reflection is expected from ground.
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| 19. |
- Thottappillil, Rajeev, 1958-, et al.
(författare)
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Properties of M components from currents measured at triggered lightning channel base
- 1995
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Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 100:D12, s. 25711-25720
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Tidskriftsartikel (refereegranskat)abstract
- Channel base currents from triggered lightning were measured at the NASA Kennedy Space Center, Florida, during summer 1990 and at Fort McClellan, Alabama, during summer 1991. An analysis of the return stroke data and overall continuing current data has been published by Fisher et al. [1993]. Here an analysis is given of the impulsive processes, called M components, that occur during the continuing current following return strokes. The 14 flashes analyzed contain 37 leader-return stroke sequences and 158 M components, both processes lowering negative charge from cloud to ground. Statistics are presented for the following M current pulse parameters: magnitude, rise time, duration, half-peak width, preceding continuing current level, M interval, elapsed time since the return stroke, and charge transferred by the M current pulse. A typical M component in triggered lightning is characterized by a more or less symmetrical current pulse having an amplitude of 100–200 A (2 orders of magnitude lower than that for a typical return stroke [Fisher et al., 1993]), a 10–90% rise time of 300–500 Όs (3 orders of magnitude larger than that for a typical return stroke [Fisher et al., 1993]), and a charge transfer to ground of the order of 0.1 to 0.2 C (1 order of magnitude smaller than that for a typical subsequent return stroke pulse [Berger et al., 1975]). About one third of M components transferred charge greater than the minimum charge reported by Berger et al. [1975] for subsequent leader-return stroke sequences. No correlation was found between either the M charge or the magnitude of the M component current (the two are moderately correlated) and any other parameter considered. M current pulses occurring soon after the return stroke tend to have shorter rise times, shorter durations, and shorter M intervals than those which occur later. M current pulses were observed to be superimposed on continuing currents greater than 30 A or so, with one exception out of 140 cases, wherein the continuing current level was measured to be about 20 A. The first M component virtually always (one exception out of 34 cases) occurred within 4 ms of the return stroke. This relatively short separation time between return stroke and the first M component, coupled with the observation of Fisher et al. [1993] that continuing currents lasting longer than 10 ms never occur without M current pulses, implies that the M component is a necessary feature of the continuing current mode of charge transfer to ground.
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| 20. |
- Tjernström, Michael, et al.
(författare)
-
Thermal mesoscale circulations on the Baltic coast 1. Numerical case study
- 1996
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 101, s. 18979-18997
-
Tidskriftsartikel (refereegranskat)abstract
- A three-dimensional mesoscale numerical model is utilized to investigate a well-documented seabreeze event taken from experimental data. The simulations are not set up as a model validation. Instead, the model is driven by generalized and simplified initial and boundary conditions extracted from the field experiment data. The aim is to simulate the main characteristics, if not all the details, of the observed case. This generates a comprehensive set of fully three-dimensional and time dependent data that can be used to obtain a more complete description of the case characteristics. Analyzing these data facilitates understanding of the flow mechanisms in more detail. Once such a control simulation is done, the case can be resimulated for slightly different forcing and the cause of observed features can be illuminated. Significant features in the simulation are the significant temperature difference between land and sea, moderate coastal terrain, and an almost 90° change in coastline orientation. Among the conclusions from the control simulation presented here is that the seabreeze stage is preceded by a coastal wind jet stage with a significant influence on the flow by the quite moderate topography. This is due to the ratio of the depth of the marine boundary layer to the terrain height; the terrain is acting on the flow in three ways. First, the stability impedes vertical motions and the terrain thus acts as a barrier inhibiting geostrophic balance. The flow is blocked and deflected down the synoptic-scale pressure gradient. Second, it generates a perturbation in the temperature field causing local baroclinicity and a thermal wind. Finally, a slope flow is also generated that acts to enhance the temperature perturbation by advection of cold air to the coastal zone. A seabreeze evolves on both coastlines. The first forms on the offshore flow coast but remains stationary at the shoreline until the second, later, seabreeze on the almost perpendicular coastline, where the coastal jet dominates the flow well into the morning, moves inland and removes the offshore flow balancing the first seabreeze at the shoreline. After this obstruction is removed, the initial seabreeze rapidly propagates inland.
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| 21. |
- Vaivads, A, et al.
(författare)
-
Energy transport during O+ energization by ELF waves observed by the Freja satellite
- 1999
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 104:A2, s. 2563-2572
-
Tidskriftsartikel (refereegranskat)abstract
- We present a study of strong ELF waves correlated with Of ion heating. We investigate the kinetic energy flux of oxygen ions and electrons parallel to the ambient magnetic field and the Poynting flux along the magnetic field of waves in different frequency regions. We show that the Poynting flux of lower hybrid waves and waves with frequencies below the oxygen gyrofrequency is downward but much less than the upgoing oxygen ion energy flux. The Poynting flux of ELF waves with frequencies at about half the local proton gyrofrequency is also downward, and it is comparable to, or larger, than the upward oxygen energy flux. Our results are consistent with a scenario where ELF waves with frequencies around half the proton gyrofrequency are generated above the satellite and transfer energy to oxygen ions in a region below the satellite.
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| 22. |
- Yamauchi, M, et al.
(författare)
-
Dynamic response of the cusp morphology to the solar wind : A case study during passage of the solar wind plasma cloud on February 21, 1994
- 1996
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A11, s. 24675-24687
-
Tidskriftsartikel (refereegranskat)abstract
- On February 21, 1994, both Geotail and LMP 8 satellites detected an interplanetary plasma cloud with intense interplanetary magnetic field (IMF>50 nT) and high dynamic pressure (> 50 nPa). During this interval the Freja satellite detected intense cusp-like plasma injections in four out of six dayside traversals. The first two traversals are carefully studied, During the first traversal the overall morphology of the ion injection is characterized by a ''multiple-injection'' signature over a wide magnetic local time (MLT) range, whereas it is characterized by a ''single-injection'' signature with narrow injection region at 8 MLT in the second traversal, The solar wind conditions were also quite different between these two periods: while both dynamic and magnetic pressures stayed high during entire period, the dynamic beta was much higher during the first Freja traversal than during the second traversal. Between these two traversals, the cusp plasma injection is detected by the Sondre Stromfjord radar. The radar signature of the plasma injection is identified using the satellite particle data when the satellite and the radar were conjugate (the satellite's footprint was in the radar's field of view.) The cusp position and dynamics observed by the Sondre Stromfjord radar again show a very good correlation to the solar wind condition, especially to the dynamic pressure. The result indicates the following. (1) During southward IMF the cusp morphology differs for conditions of high or low solar wind dynamic pressure. High dynamic pressure widens the cusp (with multiple injections), whereas high magnetic pressure narrows it (with single injection), The effect of the IMF on the cusp locations and morphology becomes dominant only when the dynamic pressure is not very high, (2) Such a morphological difference reflects dynamic pressure more than dynamic beta during southward IMF at least during times of high solar wind dynamic pressure. (3) The cusp morphology responds very quickly to the changes in the solar wind conditions.
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| 23. |
- Andre, M, et al.
(författare)
-
Ion energization mechanisms at 1700 km in the auroral region
- 1998
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 103:A3, s. 4199-4222
-
Tidskriftsartikel (refereegranskat)abstract
- Observations obtained by the Freja satellite at altitudes around 1700 km in the high-latitude magnetosphere are used to study ion energization perpendicular to the geomagnetic field. Investigations of ions, electrons, plasma densities, electric and magnetic wave fields, and field-aligned currents are used to study O+ heating mechanisms. Three ion heating events are studied in detail, and 20 events are used in a detailed statistical study. More than 200 events are classified as belonging to one of four major types of ion heating and are ordered as a function of magnetic local time. The most common types of ion heating are associated with broadband low-frequency electric wave fields occurring at all local times. These waves cover frequencies from below one up to several hundred hertz and correspond to the most intense O+ energization. Heating by these waves at frequencies of the order of the O+ gyrofrequency at 25 Hz seems to be the important energization mechanism, causing O+ ion mean energies up to hundreds of eV. The broadband waves are associated with Alfven waves with frequencies up to at least a few hertz and with field-aligned currents. Other types of O+ energization events are less common. During these events the ions are heated by waves near the lower hybrid frequency or near half the proton gyrofrequency. These waves are generated by auroral electrons or in a few cases by precipitating ions.
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| 24. |
- Barthelmie, RJ, et al.
(författare)
-
Observations and simulations of diurnal cycles of near-surface wind speeds over land and sea
- 1996
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:D16, s. 21327-21337
-
Tidskriftsartikel (refereegranskat)abstract
- The role of stability and roughness changes in producing diurnal cycles of wind speed offshore is examined using experimental data and numerical simulations. Overnight, the transition from stable conditions over land to less stable conditions over sea com
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| 25. |
- Eliasson, L, et al.
(författare)
-
Observations of electron conics by the Viking satellite
- 1996
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A6, s. 13225-13238
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Electron angular distributions peaked at oblique angles to the magnetic field, electron conics, are frequently found in the Viking data at all magnetic local times, but with a maximum in the dusk sector. Several types of electron conics have been observed
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| 26. |
- ELPHINSTONE, RD, et al.
(författare)
-
OBSERVATIONS IN THE VICINITY OF SUBSTORM ONSET - IMPLICATIONS FOR THE SUBSTORM PROCESS
- 1995
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 100:A5, s. 7937-7969
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthal
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| 27. |
- Frey, H U, et al.
(författare)
-
Freja and ground-based analysis of inverted-V events
- 1998
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 103:A3, s. 4303-4314
-
Tidskriftsartikel (refereegranskat)abstract
- During two campaigns, ground-based auroral observations were performed in coordination with Freja. The high temporal and spatial resolution of the satellite instruments as well as the real-time recording with a stereoscopic camera system from the ground enabled detailed comparison of small- and large-scale optical phenomena with particle and field data measured by the satellite. Three passes of the satellite over inverted-V auroral arcs and over precipitation regions with strong field-aligned electron spectra are investigated. Brightness modulations within auroral arcs coincide with modulations of primary electron fluxes. The dynamics of small-scale structures within arcs as well as the proper motion of arcs are analyzed and compared with electric fields measured by the satellite and with BARS radar measurements. Energy fluxes independently determined from the ground and from the satellite are used to calculate the field-aligned conductance. The results agree with predictions of the kinetic theory of the mirror force, if we allow for variations of the density and thermal energy of the electrons in the source region of the magnetosphere. Detailed comparison of electron spectra and electric and magnetic field perturbations provide evidence of different acceleration mechanisms for the electrons, electrostatic acceleration inside inverted-V's, and wave acceleration in transient regions.
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| 28. |
- Grisogono, B, et al.
(författare)
-
Thermal mesoscale circulations on the Baltic coast .2. Perturbation of surface parameters
- 1996
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:D14, s. 18999-19012
-
Tidskriftsartikel (refereegranskat)abstract
- A three-dimensional numerical study of an observed seabreeze (SE) is presented in a companion paper, part 1. This paper, part 2, addresses the influence of various forcing parameters emanating from the physical surface and perturbing the control run. The
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| 29. |
- Grün, E., et al.
(författare)
-
Galileo observes electromagnetically coupled dust in the Jovian magnetosphere
- 1998
-
Ingår i: Journal of Geophysical Research. - 2156-2202. ; 103:E9, s. 20011-20022
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of dust coupled to the Jovian magnetosphere have been obtained with the dust detector on board the Galileo spacecraft. We report on data obtained during the first four orbits about Jupiter that had flybys of the Galilean satellites: Ganymede (orbits 1 and 2), Callisto (orbit 3), and Europa (orbit 4). The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by up to 2 orders of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileo's position relative to the Jovian magnetic field. Around 20 RJ (Jupiter radius, RJ=71, 492 km) in bound a dip in the impact rate has been found consistently. At the same times, reversals by 180° in impact direction occurred. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. At times of satellite flybys, enhanced rates of dust impacts have been observed, which suggests that all Galilean satellites are sources of ejecta particles. Inside about 20 RJ impacts of micrometer-sized particles have been recorded that could be particles on bound orbits about Jupiter.
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| 30. |
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| 31. |
- Makela, JS, et al.
(författare)
-
Evolution of mesoscale auroral cavities before substorm onset
- 1999
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 104:A8, s. 17201-17215
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We have used density measurements made by the Freja satellite to analyze the dynamical behavior of mesoscale auroral cavities at an altitude of 1700 km. Freja's orbital period of 109 min enabled a very large data set of cavity occurrences to be collected.
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| 32. |
- Marengo, José A., et al.
(författare)
-
Trends in streamflow and rainfall in tropical South America : Amazonia, eastern Brazil, and northwestern Peru
- 1998
-
Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996. ; 103:D2, s. 1775-1783
-
Tidskriftsartikel (refereegranskat)abstract
- Long hydrological records, from the Amazon Basin, northeastern Brazil, and northwestern Peru spanning most of this century, are examined for trends in rainfall (three wettest months) and runoff (three months of highest flow) or stage, where no rating curves exist. Trends are tested for significance using the Mann-Kendall statistic. In basins where large soil, aquifer, or man-made reservoirs give rise to appreciable over-year storage, flows and water levels may be serially correlated. Where serial correlation exists, the usual statistical tests (linear regression, t-test, and Mann-Kendall) will overestimate the significance of trends, showing significance where none exists. Analysis for trend therefore requires particular care when data are serially correlated, and to avoid misleading results, additional supportive evidence must be sought. For example, rainfall records within the same river basin can be checked for trends; serial correlation in rainfall records, in particular, is less likely to be present, so the validity of any trends in rainfall is less open to question. Strong negative trends were found in flow data from the coast of northern Peru and the São Francisco River, while positive significant trends were detected in the Parnaíba River basin. No significant trends were found in the discharge or stage records from Amazonia, while rainfall in northeastern Brazil shows a slow increase over long periods. In the Parnaíba and in some rivers of northern Peru unusually large discharges at the beginning or end of the records seem to account for the direction and significance of trends.
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| 33. |
- Marklund, Göran, et al.
(författare)
-
On low-altitude particle acceleration and intense electric fields and their relationship to black aurora
- 1997
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 102:A8, s. 17509-17522
-
Tidskriftsartikel (refereegranskat)abstract
- Recent findings by the Freja satellite have shown the existence of extremely intense (1-2 V/m) and small-scale (1 km) diverging electric fields which are interpreted to be associated with east-west aligned dark striations or black auroral curls. Precipitating or transversely energized ions, downward field-aligned currents carried by upward fluxes of ionospheric electrons and dropouts of energetic electron precipitation, are found to be characteristic features of such events. A comparison of these characteristics to those of the aurora point at a symmetry between the aurora and the black aurora, the aurora being associated with negative divergence of the electric field and the black aurora with positive divergence. The diverging field events typically occur during winter conditions within the midnight to early morning sector of the auroral oval. Estimates of the ambient conductivity due to solar EUV radiation for each of these events show a clear anticorrelation with the electric field magnitude. The black auroral structures are likely to be associated with localized ionospheric density depletions below that of the ambient density and caused by the upward flow of ionospheric electrons. The efficiency by which such density holes are created in regions of downward field-aligned current flow have recently been demonstrated in model studies. The electric field magnitudes are found to decrease with the scale size, not inversely as suggested in recent theoretical work but with a power law exponent of 0.6-0.8. At lower altitudes (around 800 km) the maximum intensities for a majority of the events are in the range of values that have been reported from rocket and radar measurements in the ionosphere, i.e., around 150-200 mV/m. However, close to magnetic midnight and during winter conditions small-scale diverging electric fields of 1 V/m are occasionally found to exist down to at least 800 km. We suggest that the diverging electric fields observed by Freja are associated with low-altitude and narrow ( approximate to 1-2 km) potential structures similar to the auroral potential structures at higher altitude but associated with a positive space charge and a downward parallel electric field. This is supported by Freja observations of narrow upward beams of 2 keV electrons in good agreement with a 2 kV positive peak in the electrostatic potential for a black aurora event. The existence of a downward parallel electric field at low altitudes is also supported by low-altitude observations by the S3-3 and Viking satellites. If such low-altitude potential structures do exist as our results suggest, an outstanding problem for future investigation is how they may be formed and maintained.
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| 34. |
- Masson, A, et al.
(författare)
-
Observation of nonlinear interactions in large-scale density enhancements of the high-latitude ionosphere
- 1999
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 104:A10, s. 22499-22510
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Simultaneous measurements of electron density and ELF electric field by the AUREOL-3 satellite have allowed Cerisier et al. [1985] to point out the existence of large-scale density irregularities in the high-latitude topside F region and to establish the
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| 35. |
- Mishin, V M, et al.
(författare)
-
A study of the CDAW 9C substorm of May 3, 1986, using magnetogram inversion technique 2, and a substorm scenario with two active phases
- 1997
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 102:A9, s. 19845-19859
-
Tidskriftsartikel (refereegranskat)abstract
- One of the CDAW 9C substorms is investigated in this paper using the database reported by Hones et al. and supplemented with magnetogram inversion technique (MIT) 2 data. These latter have provided information about the dynamics of the open tail magnetic Aux, current systems in the ionosphere, and the size and dynamics of the current wedge. We have identified the growth: expansion, and recovery phases of this substorm, with characteristics expected from a generally accepted scenario. However, specific signatures were observed in the interval (0919-0935) UTI i.e., between the growth and expansion phases, indicating the concurrent development of the substorm onset and corresponding instabilities in the innermost current sheet, and small-scale cross-tail current disruptions without the open tail reconnection. In addition to signatures of small-scale dipolarization, an increase of the open tail magnetic flux, and a current system of the type close to DP 2 were observed at (0919-0935) UT, which is more likely to suggest predominance of the tail-stretching process than magnetic collapse, This fact was interpreted in tel ms of a relevant simple model as a signature of the growth of the energy input from the solar wind which ensures the observable disturbance power. Hence the disturbance st (0919-0935) UT was more likely a driven one than an unloading one. The aforementioned signatures make it possible to identify the interval (0919-0935) UT as the ''phase of multiple onsets'' or: (equivalently) the ''first active phase,'' which was previously defined by Mishin [1991., and references therein] as one of the four standard phases of a typical substorm (in addition to the expansion phase). Thus the case study supports the substorm scenario with two active phases and, accordingly, with two different kinds of physics, This case study illustrates also the informativity of MIT 2 data and their ability to effectively complement the database traditionally used in substorm studies.
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| 36. |
- Norqvist, P, et al.
(författare)
-
Ion cyclotron heating in the dayside magnetosphere
- 1996
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A6, s. 13179-13193
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Observations of waves and particles obtained by the Freja satellite at altitudes around 1700 km in the dayside high-latitude magnetosphere are used to study ion energization. We find that ions, including O+, during several events of intense ion energizati
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| 37. |
- Norqvist, P, et al.
(författare)
-
Isotropic and perpendicular energization of oxygen ions at energies below 1 eV
- 1998
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 103:A3, s. 4223-4239
-
Tidskriftsartikel (refereegranskat)abstract
- We present observations of low-energy O+ ion distributions obtained by the Freja satellite at altitudes between 1100 and 1600 km in the dayside high-latitude magnetosphere. The particle observations are obtained by the Freja ion mass spectrometer designed to observe ion energies up to several keV. We show that such instruments can also be used to accurately measure temperatures less than 1 eV, by operating the instrument at a fixed energy level of a few eV. When the mass spectrometer is operated at an energy approximately equal to the energy of O+ ions moving with the satellite velocity (several km/s), the satellite motion and the detector viewing directions become important. The idea is that the satellite ram effect together with different instrument viewing directions related to the satellite spin can allow sampling of different regions of ion velocity space. We find that isotropic O+ energization dominates at mean energies below about 0.4 eV, while energization perpendicular to the geomagnetic field dominates at higher energies. The isotropic ion distributions may exhibit a bulk motion up or down along the geomagnetic field, while the perpendicularly heated distributions are moving upward. The perpendicular energization occurs in the dayside auroral region including the cusp/cleft, the source region of the so-called cleft ion fountain. The perpendicular heating to a few eV can be associated with weak broadband low-frequency electric wave fields, similar to the stronger emissions causing O+ energization to hundreds of eV. However, at low energies other mechanisms map also be important.
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| 38. |
- Ohtani, S, et al.
(författare)
-
Response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field
- 1997
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 102:A10, s. 22247-22260
-
Tidskriftsartikel (refereegranskat)abstract
- The response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field (IMF) is examined with multi-instrument data from the Viking and DMSP-F7 satellites as well as ground magnetometer data. The event selected, which occurred on October 5, 1986, was previously identified as a high-latitude dayside form by Murphree and Elphinstone [1988]. IMF B-Y was positive during most of the Viking auroral UV observation, whereas IMF B-Z made a transient southward excursion, followed by a sharp increase to +8 nT. The results are summarized as follows: (1) There were two auroral belts extending westward from the early afternoon sector. The equatorward belt was persistent throughout the event and was embedded in the CPS/BPS precipitation region, whereas the poleward belt was in the open field line region and was associated with northward IMF B-Z. (2) A new auroral spot emerged in the afternoon sector just poleward of the previously active region, delayed 10 min from the sudden increase in IMF B-Z. This time lag is ascribed to the travel time of the new IMF orientation from the subsolar point to a solar wind-magnetosphere interaction site tailward of the dayside cusp, as well as to the response time of the dayside auroral acceleration process. (3) The new auroral activity expanded both eastward and westward during the first few minutes and then expanded primarily westward across the noon meridian, forming the poleward belt. The speed of the westward extension in the second phase was comparable to the speed of the ionospheric convection. (4) The dayside auroral process has a finite decay constant, which is inferred to be at least 10 min. (5) The response time of the midday ionospheric convection to IMF variations is estimated to be a few to several minutes.
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| 39. |
- Olsson, A, et al.
(författare)
-
Particle precipitation in auroral breakups and westward traveling surges
- 1996
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A11, s. 24661-24673
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We have used data from the European Incoherent Scatter (EISCAT) facility to determine the characteristics of particle precipitation before and within auroral breakups and westward traveling surges. The EISCAT data of electron density versus altitude were
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| 40. |
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| 41. |
- SEYLER, CE, et al.
(författare)
-
THEORY AND SIMULATION OF LOW-FREQUENCY PLASMA-WAVES AND COMPARISON TO FREJA SATELLITE-OBSERVATIONS
- 1995
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 100:A11, s. 21453-21472
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- One-dimensional models of obliquely propagating nonlinear plasma waves were formulated and solved both analytically and numerically to interpret recent Freja satellite observations of low-frequency plasma waves detected in the low-altitude auroral magneto
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| 42. |
- Seyler, CE, et al.
(författare)
-
Theory of nearly perpendicular electrostatic plasma waves and comparison to Freja satellite observations
- 1996
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A10, s. 21795-21813
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The linear and nonlinear two-fluid nature of oblique electrostatic plasma waves is explored and clarified. It is found that two distinct wave regimes exist corresponding to propagation angles greater than or less than epsilon = root m(e)/m(i) with respect
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| 43. |
- SHEN, GY, et al.
(författare)
-
MEASUREMENT OF MELTING TEMPERATURES OF SOME MINERALS UNDER LOWER MANTLE PRESSURES
- 1995
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH. - 0148-0227. ; 100:B9, s. 17699-17713
-
Tidskriftsartikel (refereegranskat)abstract
- Melting temperature measurements of six minerals (stishovite (SiO2), corundum(Al2O3), diopside (CaMgSi2O6), and three perovskites (MgSiO3, CaSiO3, Mg3Al2Si3O12)) at high pressures were carried out in a YAG laser-heated diamond anvil cell with rhenium meta
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| 44. |
- Stasiewicz, K, et al.
(författare)
-
Parametric instabilities of Langmuir waves observed by Freja
- 1996
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A10, s. 21515-21525
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Tidskriftsartikel (refereegranskat)abstract
- We have analyzed complete spectrum of waves (0-4 MHz) and electron distributions during events of modulated Langmuir waves observed by Freja in the topside polar ionosphere. Modulated Langmuir waves are observed with amplitudes 1-1000 mV/m in association
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| 45. |
|
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| 46. |
- Tjernstrom, M, et al.
(författare)
-
Thermal mesoscale circulations on the Baltic coast .1. Numerical case study
- 1996
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:D14, s. 18979-18997
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Tidskriftsartikel (refereegranskat)abstract
- A three-dimensional mesoscale numerical model is utilized to investigate a well-documented seabreeze event taken from experimental data. The simulations are not set up as a model validation. Instead, the model is driven by generalized and simplified init
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| 47. |
- Uvo, Cintia, et al.
(författare)
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Regionalization and spatial properties of Ceará State rainfall in northeast Brazil
- 1996
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996. ; 101:D2, s. 4221-4233
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Tidskriftsartikel (refereegranskat)abstract
- The overall daily rainfall pattern of Ceará for 19 years, 1974-1992, was analyzed in terms of physiography and atmospheric circulation. This information was used to suggest a regionalization of the state's rainfall into homogeneous areas. The three large-scale precipitation mechanisms (Intertropical Convergence Zone, cold fronts, and upper air vortices) interact and create a certain rainfall pattern over the state during different months. An example of this interaction is the opposite behavior that the northern and southern parts of the state tend to have when considering the amount of precipitation during the rainy season; That is, in general, large amounts of rain in the southern part of the state tend to be associated with small amounts of rain in the northern part and vice versa, as influenced by the opposite influence of the cold fronts and the upper air vortices. At the same time, the orographical influence enhances the precipitation in the northern part of the state and decreases precipitation in the central part of the state. The state could be divided into six homogeneous precipitation regions: (1) the northwestern part of the state, locally influenced by the mountain massif of Serra da Ibiapaba; (2)-(3) the northern and northeastern Ceará influenced locally by sea breeze and the slopes of the Serra de Baturité; (4)-(5) the central state, influenced by the Chapada do Apodi and the Serra de Uruburetama; and (6) the southern part of the state, influenced mostly by cold fronts and by the slopes of the Serra do Araripe.
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| 48. |
- Volwerk, M, et al.
(författare)
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Solitary kinetic Alfven waves: A study of the Poynting flux
- 1996
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A6, s. 13335-13343
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Using a particular mode of the FREJA wave experiment (three magnetic and one electric simultaneous measurements), we have investigated the electromagnetic structure of the solitary kinetic Alfven waves observed in the topside ionosphere, It is shown that
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