Effects of high-speed solar wind on energetic electron activity in the auroral regions during July 1–2, 2005

This study concerns the properties and behaviour of energetic ( > 30 keV ) electrons in the magnetosphere in relation to an enhancement of the solar wind caused by the sub-Earth meridional crossing of a trans-equatorial coronal hole during late June 2005. It covers periods of slow and fast solar wind, each of about 12 h duration, separated by a rapid increase of speed on July 1st. The observations were made at about 850 km altitude by four polar orbiting NOAA spacecraft, covering a full range of local times. We select invariant latitudes from 57° to 77°, the region which includes the auroral zone where electrons of these energies are sporadically precipitated, and we consider the variations of intensity and spectrum, the relation between the precipitating and mirroring fluxes, and the relative spectral hardness of these components. In general, all properties show considerable variability, but also with significant trends. The flux of mirroring electrons was greater during the period of fast solar wind than before it, but the change was relatively gradual and the flux was decreasing again towards the end of the period although the solar wind was still fast. The spectrum was softest during the transition. The ratio of precipitating to mirroring electrons generally increased as the solar wind speeded up, though with a marked dependence on local time. The precipitating spectrum tended to be harder than the mirroring spectrum during the period of slow wind, but with the increased fluxes during the faster wind the precipitating spectrum tended to be softer. The random variability was least in the noon sector where there was a progressive hardening of both the precipitating and mirroring spectra as the flux declined towards the end of the fast wind period. The data suggest that the ratio of precipitating to mirroring flux is proportional to the mirroring flux for both the energy ranges > 30 and > 100 keV , and the characteristic energy of the precipitated spectrum is half that of the mirrored spectrum during this period. Use of a magnetospheric model suggests that a change in the character of the particle fluxes in the afternoon sector around the time of transition between slow and fast wind was due to the field lines being pulled back into the tail at a time when the pressure exerted by the solar wind was at its greatest.