Polar rain gradients and field‐aligned polar cap potentials

Abstract

[1] ACE SWEPAM measurements of solar wind field-aligned electrons have been compared with simultaneous measurements of polar rain electrons precipitating over the polar cap and detected by DMSP spacecraft. Such comparisons allow investigation of cross-polar-cap gradients in the intensity of otherwise-steady polar rain. The generally good agreement of the distribution functions, f, from the two data sources confirms that direct entry of solar electrons along open field lines is indeed the cause of polar rain. The agreement between the data sets is typically best on the side of the polar cap with most intense polar rain but the DMSP f's in less intense regions can be brought into agreement with ACE measurements by shifting all energies by a fixed amounts that range from tens to several hundred eV. In most cases these shifts are positive which implies that field-aligned potentials of these amounts exist on polar cap field lines which tend to retard the entry of electrons and produce the observed gradients. These retarding potentials undoubtedly appear in order to prevent the entry of low-energy electrons and maintain charge quasi-neutrality that would otherwise be violated since most tailward flowing magnetosheath ions are unable to follow polar rain electrons down to the polar cap. In more limited regions near the boundary of the polar cap there is sometimes evidence for field-aligned potentials of the opposite sign that accelerate polar rain electrons. A solar electron burst is also studied and it is concluded that electrons from such bursts can enter the magnetotail and precipitate in the same manner as polar rain.

Plasma flow channels at the dawn/dusk polar cap boundaries: momentum transfer on old open field lines and the roles of IMF <I>B<sub>y</sub></I> and conductivity gradients

“…Equatorward of the BPS precipitation a zone of magnetospheric energetic electrons (CPS type), using the terminology of Fairfield et al (2008) (see their Fig. 2), is detected.…”

On the field‐aligned electric field in the polar cap

“…Ion and electron outflows, which are absent in the model, may affect the charge quasi-neutrality calculation and can complicate model-data comparisons, as discussed next. In any case, the net potential drop in the polar rain has been observed to range from several tens to hundreds of volts [Fairfield et al, 2008].…”

“…On occasion, however, the shift was in the opposite direction. Examples extracted from Fairfield et al [2008] are shown in Figures 1b-1e for the 31 July 2000 and 24 October 2003 events. Figures 1b and 1d show the original measurements from ACE (heavy red trace) and DMSP (thinner colored lines).…”

“…The existence of this accelerating potential drop at intermediate altitudes is not inconsistent with the simultaneous existence of a retarding potential drop at high altitude near the magnetopause. Thus, the retarding potential drop obtained by Fairfield et al [2008] gives a lower bound estimate of the potential drop needed to maintain charge quasi-neutrality near the magnetopause. Figure 2b, the poleward boundary of the auroral oval is located at MLAT~66.8°(14:35:38 UT).…”

“…In a study of variations in the otherwise uniform precipitation of solar wind electrons over the polar cap, Fairfield et al [2008] plotted the solar wind electron phase space density obtained by ACE along with the corresponding polar rain electrons simultaneously measured at low altitude by DMSP satellites. They found that the DMSP polar rain spectra would usually match ACE solar wind electron spectra if the DMSP electrons were shifted up in energy.…”