| 41. |
- Buchert, Stephan
(författare)
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Entangled dynamos and Joule heating in the Earth's ionosphere
- 2020
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 38, s. 1019-1030
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Tidskriftsartikel (refereegranskat)abstract
- The Earth's neutral atmosphere is the driver of the well-known solar quiet (Sq) and other magnetic variations observed for more than 100 years. Yet the understanding of how the neutral wind can accomplish a dynamo effect has been incomplete. A new viable model is presented where a dynamo effect is obtained only in the case of winds perpendicular to the magnetic field B that do not map along B. Winds where uxB is constant have no effect. We identify Sq as being driven by wind differences at magnetically conjugate points and not by a neutral wind per se. The view of two different but entangled dynamos is favoured, with some conceptual analogy to quantum mechanical states. Because of the large preponderance of the neutral gas mass over the ionized component in the Earth's ionosphere, the dominant effect of the plasma adjusting to the winds is Joule heating. The amount of global Joule heating power from Sq is estimated, with uncertainties, to be much lower than Joule heating from ionosphere-magnetosphere coupling at high latitudes in periods of strong geomagnetic activity. However, on average both contributions could be relatively comparable. The global contribution of heating by ionizing solar radiation in the same height range should be 2-3 orders of magnitude larger.
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| 42. |
- Buchert, Stephan, et al.
(författare)
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The Pedersen current carried by electrons : a non-linear response of the ionosphere to magnetospheric forcing
- 2008
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Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 26:9, s. 2837-2844
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Tidskriftsartikel (refereegranskat)abstract
- Observations by the EISCAT Svalbard radar show that electron temperatures T-e in the cusp electrojet reach up to about 4000 K. The heat is tapped and converted from plasma convection in the near Earth space by a Pedersen current that is carried by electrons due to the presence of irregularities and their demagnetising effect. The heat is transfered to the neutral gas by collisions. In order to enhance T-e to such high temperatures the maximally possible dissipation at 50% demagnetisation must nearly be reached. The effective Pedersen conductances are found to be enhanced by up to 60% compared to classical values. Conductivities and conductances respond significantly to variations of the electric field strength E, and "Ohm's law" for the ionosphere becomes non-linear for large E.
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| 43. |
- Collier, Andrew, et al.
(författare)
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Evidence of standing waves during a Pi2 pulsation event observed on Cluster
- 2006
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 24:10, s. 2719-2733
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Tidskriftsartikel (refereegranskat)abstract
- Observations of Pi2 pulsations at middle and low latitudes have been explained in terms of cavity mode resonances, whereas transients associated with field-aligned currents appear to be responsible for the high latitude Pi2 signature. Data from Cluster are used to study a Pi2 event observed at 18:09 UTC on 21 January 2003, when three of the satellites were within the plasmasphere (L=4.7,4.5 and 4.6) while the fourth was on the plasmapause or in the plasmatrough (L=6.6). Simultaneous pulsations at ground observatories and the injection of particles at geosynchronous orbit corroborate the occurrence of a substorm. Evidence of a cavity mode resonance is established by considering the phase relationship between the orthogonal electric and magnetic field components associated with radial and field-aligned standing waves. The relative phase between satellites located on either side of the geomagnetic equator indicates that the field-aligned oscillation is an odd harmonic. Finite azimuthal Poynting flux suggests that the cavity is effectively open ended and the azimuthal wave number is estimated as m similar to 13.5.
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| 44. |
- Collier, Andrew, et al.
(författare)
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Modelling substorm chorus events in terms of dispersive azimuthal drift
- 2004
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 22:12, s. 4311-4327
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Tidskriftsartikel (refereegranskat)abstract
- The Substorm Chorus Event (SCE) is a radio phenomenon observed on the ground after the onset of the substorm expansion phase. It consists of a band of VLF chorus with rising upper and lower cutoff frequencies. These emissions are thought to result from Doppler-shifted cyclotron resonance between whistler mode waves and energetic electrons which drift into a ground station's field of view from an injection site around midnight. The increasing frequency of the emission envelope has been attributed to the combined effects of energy dispersion due to gradient and curvature drifts, and the modification of resonance conditions and variation of the half-gyrofrequency cutoff resulting from the radial component of the E x B drift. A model is presented which accounts for the observed features of the SCE in terms of the growth rate of whistler mode waves due to anisotropy in the electron distribution. This model provides an explanation for the increasing frequency of the SCE lower cutoff, as well as reproducing the general frequency-time signature of the event. In addition, the results place some restrictions on the injected particle source distribution which might lead to a SCE.
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| 45. |
- Collier, Andrew, et al.
(författare)
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Seasonal and diurnal variation of lightning activity over southern Africa and correlation with European whistler observations
- 2006
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 24:2, s. 529-542
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Tidskriftsartikel (refereegranskat)abstract
- Lightning Imaging Sensor (LIS) data have been analysed to ascertain the statistical pattern of lightning occurrence over southern Africa. The diurnal and seasonal variations are mapped in detail. The highest flash rates (107.2 km(-2) y(-1)) occur close to the equator but maxima are also found over Madagascar (32.1 km(-2) y(-1)) and South Africa (26.4 km(-2) y(-1)). A feature of the statistics is a relatively steady contribution from over the ocean off the east coast of South Africa that appears to be associated with the Agulhas current. Lightning statistics are of intrinsic meteorological interest but they also relate to the occurrence of whistlers in the conjugate region. Whistler observations are made at Tihany, Hungary. Statistics reveal that the period of most frequent whistler occurrence does not correspond to the maximum in lightning activity in the conjugate region but is strongly influenced by ionospheric illumination and other factors. The whistler/flash ratio, R, shows remarkable variations during the year and has a peak that is narrowly confined to February and March.
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| 46. |
- Cosgrove, R., et al.
(författare)
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Radar detection of a localized 1.4 Hz pulsation in auroral plasma, simultaneous with pulsating optical emissions, during a substorm
- 2010
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 28:10, s. 1961-1979
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Tidskriftsartikel (refereegranskat)abstract
- Many pulsating phenomena are associated with the auroral substorm. It has been considered that some of these phenomena involve kilometer-scale Alfven waves coupling the magnetosphere and ionosphere. Electric field oscillations at the altitude of the ionosphere are a signature of such wave activity that could distinguish it from other sources of auroral particle precipitation, which may be simply tracers of magnetospheric activity. Therefore, a ground based diagnostic of kilometer-scale oscillating electric fields would be a valuable tool in the study of pulsations and the auroral substorm. In this study we attempt to develop such a tool in the Poker Flat incoherent scatter radar (PFISR). The central result is a statistically significant detection of a 1.4 Hz electric field oscillation associated with a similar oscillating optical emission, during the recovery phase of a substorm. The optical emissions also contain a bright, lower frequency (0.2 Hz) pulsation that does not show up in the radar backscatter. The fact that higher frequency oscillations are detected by the radar, whereas the bright, lower frequency optical pulsation is not detected by the radar, serves to strengthen a theoretical argument that the radar is sensitive to oscillating electric fields, but not to oscillating particle precipitation. Although it is difficult to make conclusions as to the physical mechanism, we do not find evidence for a plane-wave-like Alfven wave; the detected structure is evident in only two of five adjacent beams. We emphasize that this is a new application for ISR, and that corroborating results are needed.
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| 47. |
- Cumnock, Judy A., et al.
(författare)
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Small-scale characteristics of extremely high latitude aurora
- 2009
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 27:9, s. 3335-3347
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Tidskriftsartikel (refereegranskat)abstract
- We examine 14 cases of an interesting type of extremely high latitude aurora as identified in the precipitating particles measured by the DMSP F13 satellite. In particular we investigate structures within large-scale arcs for which the particle signatures are made up of a group of multiple distinct thin arcs. These cases are chosen without regard to IMF orientation and are part of a group of 87 events where DMSP F13 SSJ/4 measures emissions which occur near the noon-midnight meridian and are spatially separated from both the dawnside and duskside auroral ovals by wide regions with precipitating particles typical of the polar cap. For 73 of these events the high-latitude aurora consists of a continuous region of precipitating particles. We focus on the remaining 14 of these events where the particle signatures show multiple distinct thin arcs. These events occur during northward or weakly southward IMF conditions and follow a change in IMF B-y. Correlations are seen between the field-aligned currents and plasma flows associated with the arcs, implying local closure of the FACs. Strong correlations are seen only in the sunlit hemisphere. The convection associated with the multiple thin arcs is localized and has little influence on the large-scale convection. This also implies that the sunward flow along the arcs is unrelated to the overall ionospheric convection.
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| 48. |
- Cumnock, Judy A., et al.
(författare)
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Transpolar arc evolution and associated potential patterns
- 2004
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 22:4, s. 1213-1231
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Tidskriftsartikel (refereegranskat)abstract
- We present two event studies encompassing detailed relationships between plasma convection, field-aligned current, auroral emission, and particle precipitation boundaries. We illustrate the influence of the Interplanetary Magnetic Field B, component on theta aurora development by showing two events during which the theta originates on both the dawn and dusk sides of the aurora] oval. Both theta then move across the entire polar region and become part of the opposite side of the aurora] oval. Electric and magnetic field and precipitating particle data are provided by DMSP, while the Polar UVI instrument provides measurements of auroral emissions. Utilizing satellite data as inputs, the Royal Institute of Technology model provides the high-latitude ionospheric electrostatic potential pattern calculated at different times during the evolution of the theta aurora, resulting from a variety of field-aligned current configurations associated with the changing global aurora.
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| 49. |
- Dahlgren, Hanna, et al.
(författare)
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Energy and flux variations across thin auroral arcs
- 2011
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 29:10, s. 1699-1712
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Tidskriftsartikel (refereegranskat)abstract
- Two discrete auroral arc filaments, with widths of less than 1 km, have been analysed using multi-station, multi-monochromatic optical observations from small and medium field-of-view imagers and the EISCAT radar. The energy and flux of the precipitating electrons, volume emission rates and local electric fields in the ionosphere have been determined at high temporal (up to 30 Hz) and spatial (down to tens of metres) resolution. A new time-dependent inversion model is used to derive energy spectra from EISCAT electron density profiles. The energy and flux are also derived independently from optical emissions combined with ion-chemistry modelling, and a good agreement is found. A robust method to obtain detailed 2-D maps of the average energy and number flux of small scale aurora is presented. The arcs are stretched in the north-south direction, and the lowest energies are found on the western, leading edges of the arcs. The large ionospheric electric fields (250 mV m(-1)) found from tristatic radar measurements are evidence of strong currents associated with the region close to the optical arcs. The different data sets indicate that the arcs appear on the boundaries between regions with different average energy of diffuse precipitation, caused by pitch-angle scattering. The two thin arcs on these boundaries are found to be related to an increase in number flux (and thus increased energy flux) without an increase in energy.
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| 50. |
- Dahlgren, Hanna, 1980-, et al.
(författare)
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Morphology and dynamics of aurora at fine scale : first results for the ASK instrument
- 2008
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 26:5, s. 1041-1048
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Tidskriftsartikel (refereegranskat)abstract
- The ASK instrument (Auroral Structure and Kinetics) is a narrow field auroral imager, providing simultaneous images of aurora in three different spectral bands at multiple frames per second resolution. The three emission species studied are O-2(+) (5620 angstrom), O+ (7319 angstrom) and O (7774 angstrom). ASK was installed and operated for the first time in an observational campaign on Svalbard, from December 2005 to March 2006. The measurements were supported by data from the Spectrographic Imaging Facility (SIF). The relation between the morphology and dynamics of the visible aurora and its spectral characteristics is studied for selected events from this period. In these events it is found that dynamic aurora is coupled to high energy electron precipitation. By studying the O-2(+)/O intensity ratio we find that some auroral filaments are caused by higher energy precipitation within regions of lower energy precipitation, whereas other filaments are the result of a higher particle flux compared to the surroundings.
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