12 research outputs found
Statistical study on the occurrence of ASAID electric fields
The first statistical results on the occurrence of abnormal subauroral ion drifts (ASAID) are presented based on electric and magnetic field measurements from the low-altitude Astrid-2 satellite. ASAID are narrow regions of rapid eastward ion drifts observed in the subauroral ionosphere. They correspond to equatorward-directed electric fields with peak amplitudes seen to vary between 45 mV/m and 185 mV/m, and with latitudinal extensions between 0.2&deg; and 1.2&deg; Corrected Geomagnetic Latitude (CGLat), reaching in some cases up to 3.0&deg; CGLat. <br><br> Opposite to subauroral ion drifts (SAID) that are known to be substorm-related, ASAID are seen to occur predominantly during extended periods of low substorm activity. Our results show that ASAID are located in the vicinity of the equatorward edge of the auroral oval, mainly in the postmidnight sector between 23:00 and 03:00 magnetic local time. They are associated with a local current system with the same scale-size as the corresponding ASAID, composed by a region of downward field-aligned currents (FACs) flowing in the ASAID poleward side, and a region of upward flowing FACs in the equatorward side. The FACs have densities between 0.5 and 2.0 &mu;A/m<sup>2</sup>. The data suggest that ASAID do not contribute significantly to the reduction of the ionospheric conductivity. ASAID are seen to have life times of at least 3.5 h. <br><br> A discussion on possible mechanisms for the generation of ASAID is presented. We speculate that the proximity of the electron to the ion plasma sheet inner boundaries and of the plasmapause to the ring current outer edge, during extended quiet times, is an important key for the understanding of the generation of ASAID electric fields
Scale sizes of intense auroral electric fields observed by Cluster
The scale sizes of intense (>0.15 V/m, mapped to the
ionosphere), high-altitude (4–7 RE geocentric distance)
auroral electric fields (measured by the Cluster EFW instrument)
have been determined in a statistical study. Monopolar and bipolar
electric fields, and converging and diverging events, are
separated. The relations between the scale size, the intensity and
the potential variation are investigated.
The electric field scale sizes are further compared with the scale
sizes and widths of the associated field-aligned currents (FACs).
The influence of, or relation between, other parameters (proton
gyroradius, plasma density gradients, and geomagnetic activity),
and the electric field scale sizes are considered.
The median scale sizes of these auroral electric field structures
are found to be similar to the median scale sizes of the
associated FACs and the density gradients (all in the range
4.2–4.9 km) but not to the median proton gyroradius or the proton
inertial scale length at these times and locations (22–30 km).
(The scales are mapped to the ionospheric altitude for reference.)
The electric field scale sizes during summer months and high
geomagnetic activity (Kp>3) are typically 2–3 km, smaller than
the typical 4–5 km scale sizes during winter months and low
geomagnetic activity (Kp≤3), indicating a dependence on
ionospheric conductivity
Uncertainty in recent near-surface wind speed trends: a global reanalysis intercomparison
On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries
The profile of intense high-altitude electric fields on auroral
field lines has been studied using Cluster data. A total of 41
events with mapped electric field magnitudes in the range between
0.5–1 V/m were examined, 27 of which were co-located with a plasma
boundary, defined by gradients in particle flux, plasma density
and plasma temperature. Monopolar electric field profiles were
observed in 11 and bipolar electric field profiles in 16 of these
boundary-associated electric field events. Of the monopolar
fields, all but one occurred at the polar cap boundary in the late
evening and midnight sectors, and the electric fields were
typically directed equatorward, whereas the bipolar fields all
occurred at plasma boundaries clearly within the plasma sheet.
These results support the prediction by Marklund et al. (2004), that the
electric field profile depends on whether plasma populations, able
to support intense field-aligned currents and closure by Pedersen
currents, exist on both sides, or one side only, of the boundary
Magnetosphere-ionosphere coupling during periods of extended high auroral activity: a case study
Results are presented from a case study of a plasma boundary
crossing by the Cluster spacecraft during an extended period of
high auroral activity. The boundary between the magnetotail lobe
region of the Southern Hemisphere and the plasma sheet boundary
layer, was characterized by intense electric and magnetic field
variations, structured upward accelerated ion beams, narrow-scale
large field-aligned Poynting fluxes directed upward away from the
ionosphere, and a relatively sharp plasma density gradient.
The observations are shown to be consistent with the concept of a
multi-layered boundary with temporal and/or spatial variations in
the different layers. H+ and O+ ion beams are seen to be
accelerated upwards both by means of a field-aligned electric
field and by magnetic pumping caused by large-amplitude and
low-frequency electric field fluctuations. The peak energy of the
ion beams may here be used as a diagnostic tool for the temporal
evolution of the spatial structures, since the temporal changes
occur on a time-scale shorter than the times-of-flight of the
detected ion species.
The case study also shows the boundary region to be mainly
characterized by a coupling of the detected potential structures
to the low ionosphere during the extended period of high auroral
activity, as indicated by the intense field-aligned Poynting
fluxes directed upward away from the ionosphere