On the current-voltage relationship in auroral breakups and westwards-travelling surges

[摘要] Auroral precipitating electrons pass throughan acceleration region before entering the atmosphere. Regardless of whatproduces it, a parallel electric field is assumed to cause the acceleration. Itis well known that from kinetic theory an expression for the correspondingupward field-aligned current can be calculated, which under certain assumptionscan be linearized to j_∥=KV. The Kconstant, referred to as the Lyons-Evans-Lundin constant, depends on the sourcedensity and thermal energy of the magnetospheric electrons; it is an importantparameter in magnetosphere-ionosphere coupling models. However, the Kparameter is still rather unknown, and values are found in a wide range of 10^–8–10^–10S m^–2. In this study, we investigated how the type of auroralstructure affects the K values. We look at onset and westwards-travellingsurge (WTS) events and make comparisons with earlier results fromobservations of more stable auroral arcs. A new analysis technique for studyingthose magnetospheric parameters using ground-based measurements is introduced.Electron density measurements are taken with the EISCAT radar, and through aninversion technique the flux-energy spectra are calculated. Source densities,thermal energies and potential drops are estimated from fittings of acceleratedMaxwellian distributions. With this radar technique we have the possibility tostudy the changes of the mentioned parameters during the development of onsetsand the passage of surges over EISCAT. The study indicates that thelinearization of the full Knight formulation holds even for the very highpotential drops and thermal temperatures found in the dynamic onset and WTSevents. The values of K are found to be very low, around 10^–11S m^–2 in onset cases as well as WTS events. The results may establisha new technique where ionospheric measurements are used for studying theionosphere-magnetosphere coupling processes.

[学科分类] 地球科学(综合)