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 Septelnber 1996. Events were selected fbr 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 oe -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 fittx 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,/t -• 2.1 x 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 3t 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 3t ~ 2.1 x 103 MeV G -1 at L -4.2.

Online NARMAX model for electron fluxes at GEO

“…The review by Friedel et al (2002) includes all these mechanisms for acceleration. The main candidates are based on either local diffusion due to particle interactions with waves (Temerin et al, 1994;Shprits et al, 2008;Reeves et al, 2009;Omura et al, 2007;Horne et al, 2005;Summers and Thorne, 2003;Summers et al, 1998Summers et al, , 2002Summers et al, , 2004Albert, 2003Albert, , 2005 or radial diffusion (Falthammar, 1968;Schulz and Lanzerotti, 1974;Hilmer et al, 2000;Ingraham et al, 2001;McAdams et al, 2001). …”

Identifying the radiation belt source region by data assimilation

“…[3] It has been known since the late 1960s that radial diffusion is a key mechanism influencing radiation belt dynamics, [e.g., Cornwall, 1968;Fälthammar, 1968;Schulz and Lanzerotti, 1974;Brautigam and Albert, 2000;Hilmer et al, 2000]. Recently, new observations and increased monitoring evidenced that other processes play an important role as well [Reeves et al, 1998].…”

Dynamic Radiation Belt Modeling at the Air Force Research Laboratory

“…and Brautigam and Albert [2000] took a = 2, f3 = 2, while Hilmer et al [2000] took (3 = 1.6 and ne glected the magnetic term. Brautigam and Albert [2000] and Hilmer et al [2000] explicitly parameterized the amplitudes by Kp. Figure 8 shows the values of these expressions evaluated at L = 4.2 for different values of p and Kp.…”

“…The radial diffusion coefficients expressing the effects of magnetic and electric field fluctuations, whose power spectra are assumed to depend on frequency as u~a and UJ~P', respectively [Claflin and White, 1974]. and Brautigam and Albert [2000] took a = 2, f3 = 2, while Hilmer et al [2000] took (3 = 1.6 and ne glected the magnetic term. Brautigam and Albert [2000] and Hilmer et al [2000] explicitly parameterized the amplitudes by Kp.…”

“…This is quantified by determining a rise rate A of the GPS levels and an average value of Kp during the rise [Hilmer et a/., 2000]. As shown in Figure 7, the GPS recovery rate A is virtually zero when the averaged Kp is below a threshold of about 3.5, and increases with Kp beyond the threshold.…”