Enhancement of equatorial energetic electron fluxes near L = 4.2 as a result of high speed solar wind streams

Abstract

We examine the relationship of energetic equatorial electron flux enhancements occurring near L = 4.2 and 6.6 associated with 26 well‐defined high‐speed solar wind streams (HSSWS) detected by Wind between December 1994 and September 1996. Events were selected for having high‐energy (>2 MeV) geosynchronous electron daily average fluxes surpassing 103 cm−2 s−1 sr−1 for at least a day as measured by GOES 7 or GOES 9. Los Alamos differential‐energy electron data from SOPA (0.2 – 2.0 MeV) at L = 6.6 and the GPS BDD‐II dosimeters (0.2 – 3.2 MeV) at L = 4.2 illustrate that flux dropouts are typically observed in all energy channels at both equatorial altitudes within the first day of each event. While SOPA consistently records postdropout flux enhancements, GPS dosimeters detect equatorial postdropout enhancements in 1.6–3.2 MeV electron fluxes in only 15 of 26 events and all are either concurrent (1 event) with or follow (14 events) the geosynchronous increases of electrons with similar values of the first adiabatic invariant, μ ∼ 2.1 × 103 MeV G−1. In addition, 10 of 15 GPS growth periods produced electron enhancements above predropout levels. For all 26 events the phase space density for electrons of similar μ is consistently greater at geosynchronous altitude than at GPS equatorial altitude. The critical factor leading to GPS L = 4.2 electron flux enhancements is elevated geomagnetic activity levels of Kp ∼ 3.0 – 3.5 and above for extended periods. A combination of enhanced solar wind ram pressure, electric field (with B‐south), and velocity also appears to be necessary. If outward phase space density gradients are combined with the large electric fields generally accompanying elevated Kp, then sufficient conditions may exist to promote the inward radial diffusive transport of equatorial electrons that ultimately lead to electron flux enhancements at GPS altitudes. Comparison of observed and theoretically estimated electron growth rates is consistent with this picture of inward radial transport for these equatorially mirroring particles with μ ∼ 2.1 × 103 MeV G−1 at L = 4.2.