Downward ion acceleration at auroral latitudes: cause of parallel electric field

Hultqvist, B.

doi:10.5194/angeo-20-1117-2002

Cited by 31 publications

(44 citation statements)

References 18 publications

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“…Another indicator of acceleration processes in the downward current region is downflowing ion beams, as has been reported on by Klumpar et al (1999) based on FAST observations and by Hultqvist (2002) based on Freja observations. Freja observed electric fields of 1.8 V/m as far down as 800 km during winter solstice and close to magnetic midnight, that is during extreme dark conditions, when the ambient plasma density and ionospheric conductivity is very low.…”

Section: Freja Results In the Downward Current Regionmentioning

confidence: 70%

Electric Fields and Plasma Processes in the Auroral Downward Current Region, Below, Within, and Above the Acceleration Region

Marklund

2008

Space Sci Rev

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The downward field-aligned current region plays an active role in magnetosphereionosphere coupling processes associated with aurora. A quasi-static electric field structure with a downward parallel electric field forms at altitudes between 800 km and 5000 km, accelerating ionospheric electrons upward, away from the auroral ionosphere. A wealth of related phenomena, including energetic ion conics, electron solitary waves, low-frequency wave activity, and plasma density cavities occur in this region, which also acts as a source region for VLF saucers. Results are presented from sounding rockets and satellites, such as Freja, FAST, Viking, and Cluster, to illustrate the characteristics of the electric fields and related parameters, at altitudes below, within, and above the acceleration region. Special emphasis will be on the high-altitude characteristics and dynamics of quasi-static electric field structures observed by Cluster. These structures, which extend up to altitudes of at least 4-5 Earth radii, appear commonly as monopolar or bipolar electric fields. The former are found to occur at sharp boundaries, such as the polar cap boundary whereas the bipolar fields occur at soft plasma boundaries within the plasma sheet. The temporal evolution of quasi-static electric field structures, as captured by the pearls-on-a-string configuration of the Cluster spacecraft indicates that the formation of the electric field structures and of ionospheric plasma density cavities are closely coupled processes. A related feature of the downward current often seen is a broadening of the current sheet with time, possibly related to the depletion process. Preliminary studies of the coupling of electric fields in the downward current region, show that small-scale structures appear to be decoupled from the ionosphere, similar to what has been found for the upward current region. However, exceptions are also found where small-scale electric fields couple perfectly between the ionosphere and Cluster altitudes. Recent FAST results indicate that the degree of coupling differs between sheet-like and curved structures, and that it is typically partial. The mapping depends on the current-voltage relationship in the downward current region, which is highly non-linear and still unclear, as to its specific form.G.T. Marklund ( ) Space and Plasma Physics,

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Section: Freja Results In the Downward Current Regionmentioning

confidence: 70%

Electric Fields and Plasma Processes in the Auroral Downward Current Region, Below, Within, and Above the Acceleration Region

Marklund

2008

Space Sci Rev

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“…This study is focussed on Cluster observations in the return current region where downward field-aligned electric fields associated with quasi-static U-or S-shaped (or combinations thereof) equipotential structures, accelerate beams of electrons away from Earth and ions towards Earth (undetectable by Cluster but detectable by low-altitude orbiting satellites, such as Freja, Hultqvist, 2002). The quasi-static acceleration is often accompanied by Alfvén wave acceleration , sometimes adjacent to, sometimes colocated with, the quasi-static acceleration.…”

Section: Discussionmentioning

confidence: 99%

Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster

Marklund

Karlsson

Figueiredo

et al. 2004

Nonlin. Processes Geophys.

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Abstract. Temporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 km altitude) peak amplitudes of ≈1 V/m, bipolar or unipolar profiles, perpendicular scale sizes of ≈10 km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of ≈10 µA/m 2 , and upward electron beams with characteristic energies of a few hundred eV to a few keV. Two events illustrate the temporal evolution of bipolar, diverging electric field structures, indicative of positive U-shaped potentials increasing in magnitude from less than 1 kV to a few kV on a few 100 s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-inhand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. The total observation time for these events was typically 10-20 s, too short for monitoring the evolution of the structure, but yet of interest for revealing their short term stability. The locations of the two bipolar events at the poleward boundary of the central plasma sheet and of the two unipoCorrespondence to: G. T. Marklund (marklund@plasma.kth.se) lar events at the polar cap boundary, suggest that the special profile shape depends on whether plasma populations, dense enough to support upward field-aligned currents and closure of the return current, exist on both sides, or on one side only, of the boundary.

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“…This suggests that the magnetic footprint of the ASAID event in the equatorial plane is located somewhere inside the plasmasphere, close to its edge. The existence of a dispersive signature, the spread in energy, the coincidence with electron precipitation suggest that the source of the precipitation is the plasma sheet (Newell et al, 1996;Kauristie et al, 1999;Hultqvist, 2002). Towards the end of the event (starting around 22:20 UT) the boundary of ion precipitation starts to move equatorward, so that when the ASAID ends and turns into a SAID (at 01:40 UT), the ion precipitation is almost completely south of the zero-energy boundary of the electron precipitation.…”

Section: The Case Of 21 September 2003mentioning

confidence: 99%

Eastward sub-auroral ion drifts or ASAID

Voiculescu

Roth

2008

Ann. Geophys.

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Abstract. From satellite data sampling the top ionosphere in the Northern Hemisphere we have identified strong eastward ion drifts, with speeds larger than 1 km/s, widths of 1 • -2 • , occurring at similar temporal and spatial locations as rapid westward ion drifts known as sub-auroral ion drifts (SAID). We have called these events "abnormal sub-auroral ion drifts" (ASAID). Two events observed in the 20:00-22:00 MLT interval are discussed: the first occurring on 21 September 2003 and the other on 12 October 2003. Tomographic reconstructions of the electron density in the Fregion, based on satellite data, provided by the Scandinavian tomography chain, were also available. We have observed that ASAID are accompanied by upward flows with a speed of the same order as that of the zonal ion drift. They coincide with deep, narrow troughs in the total ion density, both at the altitude of the F15 DMSP satellite (850 km) and in the F-region of the ionosphere, but do not seem to be a feature of the convective transport. During the entire duration of ASAID the electron temperature is very high while, contrary to SAID, the ion temperature has no clear variation. Both events described in this paper end up turning into classical SAID. Satellite data indicate that the generator of ASAID could be located inside the plasmasphere close to the plasmapause and we suggest a possible mechanism for their formation.

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Downward ion acceleration at auroral latitudes: cause of parallel electric field

Cited by 31 publications

References 18 publications

Electric Fields and Plasma Processes in the Auroral Downward Current Region, Below, Within, and Above the Acceleration Region

Electric Fields and Plasma Processes in the Auroral Downward Current Region, Below, Within, and Above the Acceleration Region

Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster

Eastward sub-auroral ion drifts or ASAID

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