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Ann. Geophys., 14, 1265-1273, 1996
www.ann-geophys.net/14/1265/1996/
© European Geosciences Union 1996
On the current-voltage relationship in auroral breakups and westwards-travelling surges
A. Olsson1, A. I. Eriksson1, and P. Janhunen1,2
1Swedish Institute of Space Physics, S-755 91 Uppsala, Sweden
2Finnish Meteorological Institute, Helsinki, Finland
Abstract. Auroral precipitating electrons pass through
an acceleration region before entering the atmosphere. Regardless of what
produces it, a parallel electric field is assumed to cause the acceleration. It
is well known that from kinetic theory an expression for the corresponding
upward field-aligned current can be calculated, which under certain assumptions
can be linearized to j∥=KV. The K
constant, referred to as the Lyons-Evans-Lundin constant, depends on the source
density and thermal energy of the magnetospheric electrons; it is an important
parameter in magnetosphere-ionosphere coupling models. However, the K
parameter is still rather unknown, and values are found in a wide range of 10–8–10–10
S m–2. In this study, we investigated how the type of auroral
structure affects the K values. We look at onset and westwards-travelling
surge (WTS) events and make comparisons with earlier results from
observations of more stable auroral arcs. A new analysis technique for studying
those magnetospheric parameters using ground-based measurements is introduced.
Electron density measurements are taken with the EISCAT radar, and through an
inversion technique the flux-energy spectra are calculated. Source densities,
thermal energies and potential drops are estimated from fittings of accelerated
Maxwellian distributions. With this radar technique we have the possibility to
study the changes of the mentioned parameters during the development of onsets
and the passage of surges over EISCAT. The study indicates that the
linearization of the full Knight formulation holds even for the very high
potential drops and thermal temperatures found in the dynamic onset and WTS
events. The values of K are found to be very low, around 10–11
S m–2 in onset cases as well as WTS events. The results may establish
a new technique where ionospheric measurements are used for studying the
ionosphere-magnetosphere coupling processes.
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