Auroral Electric Fields From Satellite Observations and Numerical Modelling

Sammanfattning: This thesis is about electrodynamics of thehigh-latitude/auroral region of near-Earth space. The work ismainly based on electric field measurements made by the doubleprobe instrument on the Freja satellite at altitudes ofapproximately 800-1700 km, together with measurements fromother instruments on the same satellite, and ground-basedinstrumentation. A useful tool for interpreting observationaldata is also numerical modelling, and this is the subject ofpart of the work. The electric field measurements address threesubjects. The first one is that of very intense, divergingelectric fields at Freja altitude. Statistics and case studiesshow that these structures have scale-sizes of the order of 1km, and are associated with regions devoid of electronprecipitation (which at times can be identified with theoptical phenomenon of black aurora), with downward currents,ion heating, density depletions, and upward acceleratedelectron beams. A numerical model is used to study theionospheric response to intense small-scale current systems. Itis shown that on time scales of the order of 1 minute, deepdensity cavities and enhancements of the electric fieldresults. The second subject is the electrodynamics of thewestward traveling surge (WTS) and other large-scale auroralspirals. Freja measurements show that the electric field isdirected towards the surge/spiral center, and that the surgehead is associated with extremely intense, small-scale,converging electric fields, and field-aligned currents. Datasuggest that a significant part of the upward current of thesurge head is closed by localized, downward-directed currents,whereas no clear signature of a Cowling channel feeding thecurrent to the surge head from distributed downward currents inthe wake of the surge is observed. The third type ofmeasurements concerns the phenomenon of subauroral ion drifts(SAID), or equivalently subauroral electric fields (SAEF). Acomprehensive statistics shows the distribution in local timeand latitude of the SAID, and their correlation withgeomagnetic activity. It is concluded that the SAID areassociated with the low-conductivity region of the mid-latitudetrough, and that they may be a consequence of closure ofsubstorm-related currents through this region, which may becomeeven more low-conducting by the ionospheric response to theapplied current and electric field. Finally a new formulationof a classification scheme of auroral arc-associated electricfields is given. It is shown that this generalization enables aflexible way of modelling and predicting the arc-associatedfield in various situations.Keywords: Satellite measurements, electric fields,ionosphere, magnetosphere, diverging electric fields, downwardcurrent region, black aurora, westward traveling surge, auroralbulge, subauroral ion drifts (SAID), subauroral electric fields(SAEF), mid-latitude trough, numerical modelling, auroral arcclassification, auroral arc-associated electric field.