| 1. |
- Alfsen, K. H., et al.
(author)
-
Electric field and plasma observations near the magnetopause and bow shock during a rapid compression.
- 1984
-
In: Achievements of the International Magnetospheric Study (IMS). ; , s. 99-104
-
Journal article (peer-reviewed)abstract
- A fast compressional motion of the magnetopause resulting from the interaction of an interplanetary shock and the Earth's magnetosphere is discussed. The ISEE-1 and 2 satellites were in the frontside magnetosphere before the shock. A magnetosonic wave front, the magnetopause, and the bow shock passed them in a very short time. By a combination of electric and magnetic field data it is possible to determine the magnetosonic and the magnetopause velocity. -from STAR, 23(14), 1985
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| 2. |
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| 3. |
- Alfvén, Hannes, et al.
(author)
-
Voyager saturnian ring measurements and the early history of the solar-system
- 1986
-
In: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 34:2, s. 145-154
-
Journal article (peer-reviewed)abstract
- The mass distribution in the Saturnian ring system is investigated and compared with predictions from the plasma cosmogony. According to this theory, the matter in the rings has once been in the form of a magnetized plasma, in which the gravitation is balanced partly by the centrifugal force and partly by the electromagnetic forces. As the plasma is neutralized, the electromagnetic forces disappear and the matter can be shown to fall in to of the original saturnocentric distance. This causes the so called “cosmogonic shadow effect”, which has been demonstrated earlier for the asteroidal belt and in the large scale structure of the Saturnian ring system.The relevance of the cosmogonic shadow effect is investigated for parts of the fine structures of the Saturnian ring system. It is shown that many structures of the present ring system can be understood as shadows and antishadows of cosmogonic origin. These appear in the form of double rings centered around a position a factor 0.64 (slightly less than) closer to Saturn than the causing feature. Voyager data agree with an accuracy better than 1%.
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| 4. |
- DUBOULOZ, N, et al.
(author)
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DETAILED ANALYSIS OF BROAD-BAND ELECTROSTATIC NOISE IN THE DAYSIDE AURORAL-ZONE
- 1991
-
In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - SWEDISH INST SPACE PHYS,S-75590 UPPSALA,SWEDEN. ROYAL INST TECHNOL,S-10044 STOCKHOLM 70,SWEDEN.. - 0148-0227. ; 96:A3, s. 3565-3579
-
Journal article (peer-reviewed)abstract
- The great number of bursts of broadband electrostatic noise (BEN) recorded during crossings of the dayside auroral zone by the Viking satellite enables their statistical study. The angular distribution of BEN with respect to the Earth's magnetic field is shown to be most likely isotropic, implying that it cannot consist of a unique linear plasma mode. Most of the bursts evidence a power law spectrum from the lower hybrid and ion plasma frequencies up to frequencies sometimes much higher than the electron plasma frequency, suggesting the presence of nonlinear effects. This is confirmed by their high intensity, and by the correlation between their amplitude and their frequency extension. BEN emissions are associated with ion conical distributions and with field-aligned electron beams. Although most of the power is concentrated at very low frequencies and around the lower hybrid and ion plasma frequencies, electron acoustic and beam mode waves may contribute to the high-frequency extension of BEN. The most intense BEN emissions are also correlated with sharp cold plasma density gradients and probably involve drift instabilities. Strong quasi-static perpendicular electric fields, which induce high-speed plasma flows, are also measured, so that the Doppler effect may contribute to the broadening of the BEN spectrum.
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| 5. |
- Ergun, R. E., et al.
(author)
-
The Axial Double Probe and Fields Signal Processing for the MMS Mission
- 2016
-
In: Space Science Reviews. - : Springer Netherlands. - 0038-6308 .- 1572-9672. ; 199:1-4, s. 167-188
-
Research review (peer-reviewed)abstract
- The Axial Double Probe (ADP) instrument measures the DC to similar to 100 kHz electric field along the spin axis of the Magnetospheric Multiscale (MMS) spacecraft (Burch et al., Space Sci. Rev., 2014, this issue), completing the vector electric field when combined with the spin plane double probes (SDP) (Torbert et al., Space Sci. Rev., 2014, this issue, Lindqvist et al., Space Sci. Rev., 2014, this issue). Two cylindrical sensors are separated by over 30 m tip-to-tip, the longest baseline on an axial DC electric field ever attempted in space. The ADP on each of the spacecraft consists of two identical, 12.67 m graphite coilable booms with second, smaller 2.25 m booms mounted on their ends. A significant effort was carried out to assure that the potential field of the MMS spacecraft acts equally on the two sensors and that photo- and secondary electron currents do not vary over the spacecraft spin. The ADP on MMS is expected to measure DC electric field with a precision of similar to 1 mV/m, a resolution of similar to 25 mu V/m, and a range of similar to 1 V/m in most of the plasma environments MMS will encounter. The Digital Signal Processing (DSP) units on the MMS spacecraft are designed to perform analog conditioning, analog-to-digital (A/D) conversion, and digital processing on the ADP, SDP, and search coil magnetometer (SCM) (Le Contel et al., Space Sci. Rev., 2014, this issue) signals. The DSP units include digital filters, spectral processing, a high-speed burst memory, a solitary structure detector, and data compression. The DSP uses precision analog processing with, in most cases, > 100 dB in dynamic range, better that -80 dB common mode rejection in electric field (E) signal processing, and better that -80 dB cross talk between the E and SCM (B) signals. The A/D conversion is at 16 bits with similar to 1/4 LSB accuracy and similar to 1 LSB noise. The digital signal processing is powerful and highly flexible allowing for maximum scientific return under a limited telemetry volume. The ADP and DSP are described in this article.
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| 6. |
- Erickson, P. J., et al.
(author)
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Multipoint MMS observations of fine-scale SAPS structure in the inner magnetosphere
- 2016
-
In: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 43:14, s. 7294-7300
-
Journal article (peer-reviewed)abstract
- We present detailed observations of dynamic, fine-scale inner magnetosphere-ionosphere coupling at approximate to 3.9R(E) in the Region 2 Birkeland field-aligned current (FAC). We find that observed electrodynamic spatial/temporal scales are primarily characteristic of magnetically mapped ionospheric structure. On 15 September 2015, conjugate Magnetospheric Multiscale (MMS) spacecraft and Millstone Hill radar observations show plasmasphere boundary region subauroral polarization stream (SAPS) electric fields at L = 4.0-4.2 near 21 MLT. MMS observations reveal high-altitude approximate to 1mV/m fine-scale radial and azimuthal electric field perturbations over 0.15L with high spatial coherence over 2-3min and show outward motion within a broader FAC of approximate to 0.12A/m(2). Our analysis shows that MMS electric field fluctuations are most likely reflective of SAPS ionospheric structure at scales of approximate to 22km and with ionospheric closure of small-scale filamentary FAC perturbations. The results highlight the ionosphere's importance in regulating fine-scale magnetosphere-ionosphere structure.
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| 7. |
- Eriksson, Elin, et al.
(author)
-
Strong current sheet at a magnetosheath jet : Kinetic structure and electron acceleration
- 2016
-
In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 121:10, s. 9608-9618
-
Journal article (peer-reviewed)abstract
- Localized kinetic-scale regions of strong current are believed to play an important role in plasma thermalization and particle acceleration in turbulent plasmas. We present a detailed study of a strong localized current, 4900 nA m(-2), located at a fast plasma jet observed in the magnetosheath downstream of a quasi-parallel shock. The thickness of the current region is similar to 3 ion inertial lengths and forms at a boundary separating magnetosheath-like and solar wind-like plasmas. On ion scales the current region has the shape of a sheet with a significant average normal magnetic field component but shows strong variations on smaller scales. The dynamic pressure within the magnetosheath jet is over 3 times the solar wind dynamic pressure. We suggest that the current sheet is forming due to high velocity shears associated with the jet. Inside the current sheet we observe local electron acceleration, producing electron beams, along the magnetic field. However, there is no clear sign of ongoing reconnection. At higher energies, above the beam energy, we observe a loss cone consistent with part of the hot magnetosheath-like electrons escaping into the colder solar wind-like plasma. This suggests that the acceleration process within the current sheet is similar to the one that occurs at shocks, where electron beams and loss cones are also observed. Therefore, electron beams observed in the magnetosheath do not have to originate from the bow shock but can also be generated locally inside the magnetosheath.
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| 8. |
- Escoubet, C P, et al.
(author)
-
Density in the magnetosphere inferred from ISEE 1 spacecraft potential
- 1997
-
In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - ROYAL INST TECHNOL,ALFVEN LAB,DIV PLASMA PHYS,S-10044 STOCKHOLM,SWEDEN.. ; 102:A8, s. 17595-17609
-
Journal article (peer-reviewed)abstract
- The potential of ISEE 1 spacecraft has been used to derive the plasma density in the magnetosphere and its environment. First, we show that using the equilibrium of currents flowing in and out of a spacecraft, we could numerically derive a relation between the spacecraft potential and the density of the surrounding plasma, After verifying: that this relation was in good agreement with the measurement of the density made by other instruments during selected periods of time, we apply this relation to the spacecraft potential measured continuously from 1977 to 1984 on ISEE 1, An image of the plasma density in the magnetosphere and its environment is obtained as a result, All principal magnetospheric regions are clearly identified: the solar wind with a density around 5 cm(-3) : the magnetosheath with a density around 50 cm(-3), the magnetosphere with a density around 1 cm(-3) the plasma sheet with a density around 0.5 cm(-3), and finally, the more tenuous tail lobes with a density below 0.1 cm(-3), The plasma density was observed slightly higher on the dawnside than on the duskside of the magnetosphere, In addition, the magnetosheath was closer to the Earth on the dawnside than on the duskside, When the magnetic activity increased (recorded by the AE index), the dayside magnetosphere was compressed/eroded by about 1 to 2 R-E, while the plasmasphere/inner magnetosphere became quite irregular and expanded in the dawn-midnight and in the dusk-noon sectors, In addition, during high magnetic activity, the plasma sheet at Y-GSM = 0 was thicker and slightly denser than at low-activity, On the other hand, the flanks of the plasma sheet were thinner and slightly less dense during high-activity than during low-activity.
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| 9. |
- Escoubet, C. P., et al.
(author)
-
Imaging the magnetosphere using ISEE-1 spacecraft potential
- 1996
-
In: European Space Agency, (Special Publication) ESA SP. - 0379-6566. ; :392, s. 179-187
-
Journal article (peer-reviewed)abstract
- The potential of ISEE 1 spacecraft has been used to derived the plasma density in the magnetosphere and its environment. First we show that using the equilibrium of currents flowing in and out of a spacecraft, we could derive numerically a relation between the spacecraft potential and the density of the surrounding plasma. After verifying that this relation was in good agreement with the measurement of the density made by other instruments during selected periods of time, we apply this relation to the spacecraft potential measured continuously from 1977 to 1984 on ISEE-1. An image of the plasma density in the magnetosphere and its environment is obtained as a result. All principal magnetospheric regions are clearly identified, from the solar wind with a density around 5 cm-3 , the magnetosheath around 50 cm-3, the magnetosphere around 1 cm-3, the plasma sheet around 0.5 cm-3 and finally the more tenuous tail lobes below 0.1 cm-3. The plasma density was observed slightly higher on the dawnside than on the dusk side of the magnetosphere. In addition the magnetosheath was closer to the Earth on the dawnside than on the duskside. When the magnetic activity increased (recorded by the AE index), the dayside magnetosphere was compressed/eroded by about 1 to 2 RE while the plasmasphere/inner magnetosphere became quite irregular and expanded in the dawn-midnight and in the dusk-noon sectors. In addition, during high magnetic activity, the plasmasheet at Ygsm = 0 was thicker and denser than at low activity. On the other hand the flanks of the plasmasheet were thinner and less dense during high activity than during low activity.
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| 10. |
- Farrugia, C. J., et al.
(author)
-
Magnetospheric Multiscale Mission observations and non-force free modeling of a flux transfer event immersed in a super-Alfvenic flow
- 2016
-
In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 43:12, s. 6070-6077
-
Journal article (peer-reviewed)abstract
- We analyze plasma, magnetic field, and electric field data for a flux transfer event (FTE) to highlight improvements in our understanding of these transient reconnection signatures resulting from high-resolution data. The similar to 20 s long, reverse FTE, which occurred south of the geomagnetic equator near dusk, was immersed in super-Alfvenic flow. The field line twist is illustrated by the behavior of flows parallel/perpendicular to the magnetic field. Four-spacecraft timing and energetic particle pitch angle anisotropies indicate a flux rope (FR) connected to the Northern Hemisphere and moving southeast. The flow forces evidently overcame the magnetic tension. The high-speed flows inside the FR were different from those outside. The external flows were perpendicular to the field as expected for draping of the external field around the FR. Modeling the FR analytically, we adopt a non-force free approach since the current perpendicular to the field is nonzero. It reproduces many features of the observations.
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| 11. |
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| 12. |
- Graham, Daniel B., et al.
(author)
-
Electron currents and heating in the ion diffusion region of asymmetric reconnection
- 2016
-
In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 43:10, s. 4691-4700
-
Journal article (peer-reviewed)abstract
- In this letter the structure of the ion diffusion region of magnetic reconnection at Earth's magnetopause is investigated using the Magnetospheric Multiscale (MMS) spacecraft. The ion diffusion region is characterized by a strong DC electric field, approximately equal to the Hall electric field, intense currents, and electron heating parallel to the background magnetic field. Current structures well below ion spatial scales are resolved, and the electron motion associated with lower hybrid drift waves is shown to contribute significantly to the total current density. The electron heating is shown to be consistent with large-scale parallel electric fields trapping and accelerating electrons, rather than wave-particle interactions. These results show that sub-ion scale processes occur in the ion diffusion region and are important for understanding electron heating and acceleration.
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| 13. |
- HULTQVIST, B, et al.
(author)
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ON THE UPWARD ACCELERATION OF ELECTRONS AND IONS BY LOW-FREQUENCY ELECTRIC-FIELD FLUCTUATIONS OBSERVED BY VIKING
- 1991
-
In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : American Geophysical Union (AGU). - 0148-0227. ; 96:A7, s. 11609-11615
-
Journal article (peer-reviewed)abstract
- The dependence of the generation of upward electron beams in the to side ionosphere on the amplitude of the slow electric field fluctuations and the magnitude of the dc potential difference in the acceleration region is found to agree with the model involving the fluctuating electric field component presented in 1988 by Hultqvist. The ratio between the amplitude of the electric field fluctuations, E(f), determined by the dipole electric field experiment, and the dc acceleration voltage V(acc), given by the energy of the upflowing ion beam, is much higher on average for satellite spins showing an upward electron beam than for the spins without. It is also shown that low plasma density favors the generation of electron beams.
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| 14. |
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| 15. |
- KLETZING, CA, et al.
(author)
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Electric-fields derived from electron-drift measurements
- 1994
-
In: GEOPHYSICAL RESEARCH LETTERS. - : American Geophysical Union (AGU). - 0094-8276. ; 21:17, s. 1863-1866
-
Journal article (peer-reviewed)abstract
- The first observations of electric fields derived from electron E x B drift measurements aboard the Freja spacecraft are presented. The instrument injects a weak beam of 3 keV electrons and measures the displacement of the returning electrons after one gyroperiod. After removing effects due to beam-detector geometry and applying an empirical calibration based upon comparison with the computed v x B electric field induced by the spacecraft motion, good agreement is found when the electron drift measurements are compared with the electric field components measured by the double probe experiment. Examples are presented in which moderately large electric fields are observed near the edges of or adjacent to electron precipitation regions with little or no electric field within.
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| 16. |
- Laakso, H., et al.
(author)
-
Multi-point electric field observations in the high-latitude magnetosphere
- 2001
-
In: European Space Agency, (Special Publication) ESA SP. ; , s. 27-34
-
Conference paper (peer-reviewed)abstract
- We use multi-point electric field observations from the four Cluster satellites to study the dynamical behavior of the high-latitude magnetosphere on February 13-14, 2001, 20-02 UT. At 20:00 UT the vehicles enter the cusp where three satellites observe a 500-volt potential drop. It implies that at lower altitudes there likely exist some parallel electric fields that accelerate electrons downward and ions upward. In the following 2-3 hours the satellites move over the southern polar cap where all four satellites pass through a number of stationary, large-scale density enhancements that are associated with 200-volt potential drops. The observed events are possibly ionospheric ion outflows, triggered by geomagnetic activity. At 23:20 UT, the satellites move in the distant plasma sheet, and an hour later they have a brief encounter with the auroral region where a density cavity of a few degrees wide is observed. At the equatorward edge of the cavity, large electric fields of 100 mV/m are observed, which are likely related to an auroral arc. Similar observations are collected from all four satellites within a few minutes, but a detailed comparison reveals plenty of differences, apparently due to small spatial and temporal scale sizes. Near the perigee pass, the vehicles traverse the plasma trough near local midnight, where they all detect a ULF wave event. A preliminary analysis of the event shows that it is a resonant mode of a 120-sec period. Surprisingly the observations from four satellites are not well correlated, which suggests a short spatial and temporal scale for the event. A possible source mechanism for ULF waves at this local time sector is drifting ring current protons.
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| 17. |
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| 18. |
- Lindqvist, Per-Arne, 1954-
(author)
-
POTENTIAL OF ISEE IN DIFFERENT PLASMA ENVIRONMENTS.
- 1983
-
In: European Space Agency, (Special Publication) ESA SP. ; , s. 25-33
-
Conference paper (peer-reviewed)abstract
- The quasistatic electric field experiment on the ISEE-1 satellite uses spherical double probes to measure the electric field. The potential of each probe relative to the plasma potential can be controlled with a bias current, which compensates for the loss of photoelectrons from the probe. Thus, keeping a probe at plasma potential, the satellite potential is measured with reference to the probe. The dependence of the satellite potential on the ambient plasma electron flux is discussed. It is shown theoretically and found experimentally that the satellite potential is proportional to log(n//eT//e** one-half ). Examples are given of how the satellite potential varies in different regions of space penetrated by ISEE.
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| 19. |
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| 20. |
- Lindqvist, Per-Arne, 1954-, et al.
(author)
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The Spin-Plane Double Probe Electric Field Instrument for MMS
- 2016
-
In: Space Science Reviews. - : Springer Science+Business Media B.V.. - 0038-6308 .- 1572-9672. ; 199:1-4, s. 137-165
-
Research review (peer-reviewed)abstract
- The Spin-plane double probe instrument (SDP) is part of the FIELDS instrument suite of the Magnetospheric Multiscale mission (MMS). Together with the Axial double probe instrument (ADP) and the Electron Drift Instrument (EDI), SDP will measure the 3-D electric field with an accuracy of 0.5 mV/m over the frequency range from DC to 100 kHz. SDP consists of 4 biased spherical probes extended on 60 m long wire booms 90(a similar to) apart in the spin plane, giving a 120 m baseline for each of the two spin-plane electric field components. The mechanical and electrical design of SDP is described, together with results from ground tests and calibration of the instrument.
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| 21. |
- MALINGRE, M, et al.
(author)
-
SPORADIC ELECTROMAGNETIC EMISSIONS IN THE AKR FREQUENCY-RANGE ASSOCIATED WITH ELECTROSTATIC PLASMA TURBULENCE
- 1992
-
In: GEOPHYSICAL RESEARCH LETTERS. - : American Geophysical Union (AGU). - 0094-8276. ; 19:13, s. 1339-1342
-
Journal article (peer-reviewed)abstract
- We present heretofore unreported Viking observations of sporadic AKR bursts occurring well above the electron gyrofrequency together with spiky electrostatic emissions recorded just above the lower hybrid frequency, which exhibit a high degree of correlation in time. Both types of waves show simultaneous large amplitude variations of the order of 30-40 dB on time scales of a few hundreds of ms. Possible interpretations which may account for the observed correlation are investigated. Arguments are given which suggest that such events could be the signature of small scale non-linear structures in the plasma.
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| 22. |
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| 23. |
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| 24. |
- Mursula, K, et al.
(author)
-
Nonbouncing Pc 1 wave bursts
- 1997
-
In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : American Geophysical Union (AGU). ; 102:A8, s. 17611-17624
-
Journal article (peer-reviewed)abstract
- On April 11, 1986, at about 0600 UT a long Pc 1 wave event of the hydromagnetic chorus type started on the ground, as registered by the Finnish pulsation magnetometer network. The main pulsation band at about 0.3 Hz was observed for several hours. Soon after start, this band smoothly extended to higher frequencies, forming another separate wave band which finally reached up to 0.5 Hz. During the event the Viking satellite was on its southbound pass over Scandinavia, close to the MLT sector of the ground network. From 0650 until 0657 UT, Viking observed a chain of Pc 1 bursts with increasing frequency. The strongest bursts could be grouped into two separate wave regions whose properties differed slightly. The higher-latitude region had a frequency of 0.3 Hz, well in agreement with the main Pc 1 band on the ground. The lower-latitude region contained the highest frequencies observed on the ground at about 0.5 Hz. The latitudinal extent of both wave regions was about 0.5 degrees. They had slightly different normalized frequencies, Alfven velocities, and repetition periods. Most interestingly, the repetition periods of both wave sources were too short for the bursts to be due to a wave packet bouncing between the two hemispheres. The results give new information about the high-latitude Pc 1 waves, showing that they can consist of separate repetitive but nonbouncing bursts. We suggest that the long-held bouncing wave packet hypothesis is generally incorrect and discuss two alternative models where the burst structure is formed at the equatorial source region of the waves.
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| 25. |
- Oieroset, M., et al.
(author)
-
MMS observations of large guide field symmetric reconnection between colliding reconnection jets at the center of a magnetic flux rope at the magnetopause
- 2016
-
In: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 43:11, s. 5536-5544
-
Journal article (peer-reviewed)abstract
- We report evidence for reconnection between colliding reconnection jets in a compressed current sheet at the center of a magnetic flux rope at Earth's magnetopause. The reconnection involved nearly symmetric inflow boundary conditions with a strong guide field of two. The thin (2.5 ion-skin depth (d(i)) width) current sheet (at similar to 12 d(i) downstream of the X line) was well resolved by MMS, which revealed large asymmetries in plasma and field structures in the exhaust. Ion perpendicular heating, electron parallel heating, and density compression occurred on one side of the exhaust, while ion parallel heating and density depression were shifted to the other side. The normal electric field and double out-of-plane (bifurcated) currents spanned almost the entire exhaust. These observations are in good agreement with a kinetic simulation for similar boundary conditions, demonstrating in new detail that the structure of large guide field symmetric reconnection is distinctly different from antiparallel reconnection.
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| 26. |
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| 27. |
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| 28. |
- Popielawska, B., et al.
(author)
-
An imprint of the quiet plasma sheet structure at the orbit of viking : Magnetosphere without substorms
- 1996
-
In: European Space Agency, (Special Publication) ESA SP. - 0379-6566. ; :389, s. 133-139
-
Journal article (peer-reviewed)abstract
- We present a study of the plasma sheet structure as seen on VIKING during prolonged periods of magnetic quiescence. The ion and electron spectrograms, the electric field data and field-aligned current signatures in the magnetic field data are examined. A special attention is paid to cases when the Low Latitude Boundary Layer-like plasma is seen deep in the nightside at latitudes which apparently map to close geocentric distances as it implies an exotic plasma circulation in the tail. An attempt has been made to infer from VIKING data how the quiet plasma sheet is formed, whether it originates mainly from the mantle source via the interaction with the distant tail current sheet or whether the lateral transport of the Low Latitude Boundary Layer (LLBL) from the tail flanks (with essentially adiabatic particle motion) takes place followed by the sunward convection in the central part of the near-Earth tail. The VIKING plasma and electric field data suggest that the last mechanism is decisive for plasma sheet structuring during prolonged periods of northward IMF.
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| 29. |
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| 30. |
- SANDAHL, I, et al.
(author)
-
Electron populations above the nightside auroral oval during magnetic quiet times
- 1990
-
In: Planetary and Space Science. - : Pergamon Press. - 0032-0633 .- 1873-5088. ; 38:8, s. 1031-1049
-
Journal article (peer-reviewed)abstract
- In several studies of particle morphology above the nightside auroral oval, the electrons have been divided into two separate spatial regions, often called the BPS (from "boundary plasma sheet") and the CPS (from "central plasma sheet") (Winningham et al., 1975, J. geophys. Res. 80, 3148). The names were derived from the source regions suggested by Winningham et al. In many cases this classification has worked well, but there are also many cases in which it has not. In this paper an alternative classification is proposed and explored by investigating the spatial distribution of electrons at altitudes between 2000 and 13,500 km, using particle spectrograms from the Viking satellite. A major difference between the newly proposed and the earlier classification is that spatial regions of populations may overlap in this new scheme. Electrons above the auroral oval could be divided into two populations. The first one is spatially unstructured and has a characteristic energy of a few kiloelectron volts. It is usually trapped in its equatorward part, while it is isotropic in its poleward part. The second one is spatially structured and normally has a characteristic energy of 100 eV or less. It is always present when there are signs of electron acceleration along magnetic field lines. The global distributions of both the structured and the unstructured electrons are ring-shaped. The two regions partially overlap, and the average latitude of the structured electrons is higher than the average latitude of the unstructured electrons. The majority of bright auroras appear in the region of overlap. The average poleward edge of the overlap region seems to coincide with the average poleward edge of region 1 field-aligned currents. We suggest that this boundary maps to the boundary between the central plasma sheet and the plasma sheet boundary layer. We also suggest that the sources for the region where only structured electrons are present are the low-latitude boundary layer and plasma sheet boundary layer. The conclusions concerning source regions are supported by mapping of the particle population regions into the equatorial plane of the magnetosphere using the Tsyganenko (1987, Planet. Space Sci. 35, 1347) magnetic field model. The average boundary between region 1 and region 2 field-aligned currents in the afternoon and evening is approximately at the average equatorward boundary of unstructured electrons. Through the midnight, morning and prenoon sectors it is at the average equatorward boundary of structured electrons.
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| 31. |
- Stasiewicz, K, et al.
(author)
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Cavity resonators and Alfven resonance cones observed on Freja
- 1997
-
In: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 102:A2, s. 2565-2575
-
Journal article (peer-reviewed)abstract
- Multiresolution wavelet analysis of magnetic field, electric field, and plasma density records taken on Freja during strong auroral events shows evidence for cavity Alfven resonators in the topside ionosphere. The cavity (or transverse) resonators consist of standing perpendicular wave modes which are trapped inside plasma cavities of different perpendicular scales. The smallest size cavities have perpendicular widths comparable to the electron skin depth, lambda(s) = 2 pi c/omega(pe), and are presumably associated with the resonance cones of Alfven waves launched by a magnetospheric source. The Alfven resonance cones (ARCs) carry intense field-aligned currents, support strong parallel electric fields and represent discharge and heating channels for auroral particles. We have made a detailed analysis of the electromagnetic properties of two singular auroral structures associated with ARCs. Field-aligned currents at the resonance structures reach intensities of 100-300 rho Am-2 in the upward and downward directions and are carried mainly by cold ionospheric plasma in both directions. The parallel electric field of ARCs is observed at amplitudes up to 100 mV/m, which is 2 orders of magnitude larger than expected for the unbounded Alfven waves. Field-aligned electron beams accelerated inside ARCs are observed to drive Langmuir waves with parallel electric field occasionally exceeding 1 V/m. One of the analyzed ARC structures has electromagnetic and particle properties characteristic of ''black aurora'' with electric field diverging from the center of the cavity; the other has converging electric field.
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| 32. |
- Stawarz, J. E., et al.
(author)
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Observations of turbulence in a Kelvin-Helmholtz event on 8 September 2015 by the Magnetospheric Multiscale mission
- 2016
-
In: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 121:11, s. 11021-11034
-
Journal article (peer-reviewed)abstract
- Spatial and high-time-resolution properties of the velocities, magnetic field, and 3-D electric field within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two-dimensional turbulence in association with the KHI. The behavior of kinetic-scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI.
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| 33. |
- Torbert, R. B., et al.
(author)
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The FIELDS Instrument Suite on MMS : Scientific Objectives, Measurements, and Data Products
- 2016
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In: Space Science Reviews. - : Springer Science+Business Media B.V.. - 0038-6308 .- 1572-9672. ; 199:1-4, s. 105-135
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Research review (peer-reviewed)abstract
- The FIELDS instrumentation suite on the Magnetospheric Multiscale (MMS) mission provides comprehensive measurements of the full vector magnetic and electric fields in the reconnection regions investigated by MMS, including the dayside magnetopause and the night-side magnetotail acceleration regions out to 25 Re. Six sensors on each of the four MMS spacecraft provide overlapping measurements of these fields with sensitive cross-calibrations both before and after launch. The FIELDS magnetic sensors consist of redundant flux-gate magnetometers (AFG and DFG) over the frequency range from DC to 64 Hz, a search coil magnetometer (SCM) providing AC measurements over the full whistler mode spectrum expected to be seen on MMS, and an Electron Drift Instrument (EDI) that calibrates offsets for the magnetometers. The FIELDS three-axis electric field measurements are provided by two sets of biased double-probe sensors (SDP and ADP) operating in a highly symmetric spacecraft environment to reduce significantly electrostatic errors. These sensors are complemented with the EDI electric measurements that are free from all local spacecraft perturbations. Cross-calibrated vector electric field measurements are thus produced from DC to 100 kHz, well beyond the upper hybrid resonance whose frequency provides an accurate determination of the local electron density. Due to its very large geometric factor, EDI also provides very high time resolution (similar to 1 ms) ambient electron flux measurements at a few selected energies near 1 keV. This paper provides an overview of the FIELDS suite, its science objectives and measurement requirements, and its performance as verified in calibration and cross-calibration procedures that result in anticipated errors less than 0.1 nT in B and 0.5 mV/m in E. Summaries of data products that result from FIELDS are also described, as well as algorithms for cross-calibration. Details of the design and performance characteristics of AFG/DFG, SCM, ADP, SDP, and EDI are provided in five companion papers.
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| 34. |
- Torkar, K., et al.
(author)
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Long-Term Study of Active Spacecraft Potential Control
- 2008
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In: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 36:5, s. 2294-2300
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Journal article (peer-reviewed)abstract
- Emitters based on the liquid metal ion source principle have been operating on the Cluster spacecraft between 2000 and 2004, in order to control the spacecraft potential. The resulting reduction of positive spacecraft potential reduces perturbations to the plasma measurements on board. Ion currents up to 40 [LA have been applied, which reduced the energy band in which photoelectrons disturb the plasma electron measurements to values close to the lower detection limit of the instrument. The experience with this method, meanwhile, covers both the maximum and minimum of the present solar cycle and allows one to study the variations of photoemission and other long-term trends and their effects on the measurements. A long-term trend of the controlled spacecraft potential is indeed observed. In addition, it appears that reconstruction of the uncontrolled spacecraft potential from the controlled one is possible if certain conditions are fulfilled. Spacecraft potential control can thereby improve the plasma measurements while still allowing one to estimate the total plasma density from the residual potential.
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| 35. |
- VOGELSANG, H, et al.
(author)
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An ionospheric travelling convection vortex event observed by ground‐based magnetometers and by Viking
- 1993
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In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 20:21, s. 2343-2346
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Journal article (peer-reviewed)abstract
- A transient ionospheric travelling convection vortex (ITCV) event was recorded by the EISCAT magnetometer cross in northern Scandinavia on April 21, 1986 around 8:40 MLT. Simultaneously, the near-conjugate satellite VIKING observed strong magnetic and electric field variations on closed magnetic field lines. The signatures seen by the spacecraft are consistent with a passage through two oppositely directed field-aligned current tubes. The total upward and downward currents turned out to be matched within a region of approximately 1000 km in the ionosphere. Because of the close spatial and temporal proximity of both observations we interpret the field-aligned currents as being associated with the ITCV. This is the first time that in-situ measurements of the currents connecting the ITCV to the source region close to the magnetopause have been published. Both the current density and the dimensions of the current tubes could be estimated here.
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