Untangling the solar wind and magnetospheric drivers of the radiation
belt electrons
Abstract
Many solar wind parameters correlate with one another, which complicates
the causal-effect studies of solar wind driving of the magnetosphere.
Conditional mutual information (CMI) is used to untangle and isolate the
effect of individual solar wind and magnetospheric drivers of the
radiation belt electrons. The solar wind density
(nsw) negatively correlates with electron phase
space density (PSD) (average energy ~ 1.6 MeV) with time
lag (t) = 15 hr. This effect of nsw on PSD has
been attributed to magnetopause shadowing losses, but when the effect of
solar wind velocity (Vsw) is removed, t shifts to
7–11 hr, which is a more accurate time scale for this process. The peak
correlation between Vsw and PSD shifts from t =
38 to 46 hr, when the effect of nsw is removed.
This suggests that the time scale for electron acceleration to 1–2 MeV
is about 46 hr following Vsw enhancements. The
effect of nsw is significant only at L* = 4.5–6
(L* > 6 is highly variable) whereas the effect of
Vsw is significant only at L* = 3.5–6.5. The
peak response of PSD to Vsw is the shortest and
most significant at L* = 4.5–5.5. As time progresses, the peak response
broadens and shifts to higher t at higher and lower L*, consistent with
local acceleration at L* = 4.5–5.5 followed by outward and inward
diffusion. The outward radial diffusion time scale at L* = 5–6 is
~40 hr per RE.