On the relationship between relativistic electron flux and solar wind velocity: Paulikas and Blake revisited

Abstract

[1] Thirty years ago Paulikas and Blake (1979) showed a remarkable correlation between geosynchronous relativistic electron fluxes and solar wind speed (Vsw). This seminal result has been a foundation of radiation belt studies, space weather forecasting, and current understanding of solar wind radiation belt coupling. We have repeated their analysis with a considerably longer-running data set from the Los Alamos National Laboratory energetic particle instruments with several surprising results. Rather than the roughly linear correlation between Vsw and log (flux), our results show a triangle-shaped distribution in which fluxes have a distinct velocity-dependent lower limit but a velocity-independent upper limit. The highest-electron fluxes can occur for any value of Vsw with no indication of a Vsw threshold. We also find a distinct solar cycle dependence with the triangle-shaped distribution evident in 2 declining phase years dominated by high-speed streams but essentially no correlation in 2 solar maximum years. For time periods that do show a triangle-shaped distribution we consider whether it can be explained by scatter due to other parameters. We examine the role of time dependence and time lag in producing the observed distribution. We also look at the same statistical relationship but at energies 1 MeV. We conclude that the relationship between radiation belt electron fluxes and solar wind velocity is substantially more complex than suggested by previous statistical studies. We find that there are important ways in which the "conventional wisdom" stating that high-velocity wind drives high-MeV electron fluxes is, in general, either misleading or unsupported.

Coronal mass ejections and their sheath regions in interplanetary space

“…Any response during a storm is a complex combination of depletion associated with rapid reconfiguration and changes in the electron drift Li et al (2017), copyright by the authors paths, acceleration processes associated with magnetotail reconnection events and with induced electric fields during substorms, and acceleration and loss processes associated with wave-particle interactions, including ULF, electromagnetic ion cyclotron (EMIC), VLF chorus and plasmaspheric hiss waves (e.g., Summers et al 2014). Early works by Paulikas and Blake (1979) recognized the role of high-speed solar wind in creating electron flux enhancements of the outer Van Allen electron belt, but the response is much more subtle than being a simple function of speed (e.g., Reeves et al 2011).…”

High‐speed solar wind with southward interplanetary magnetic field causes relativistic electron flux enhancement of the outer radiation belt via enhanced condition of whistler waves

“…[4] However, HSS events do not always lead to large flux enhancement [Kim et al, 2006;Li et al, 2011;Reeves et al, 2011], suggesting the involvement of other important solar wind parameters. Miyoshi and Kataoka [2008] conducted a superposed epoch analysis of MeV electrons at geosynchronous orbit about the 179 stream interface crossing events, which are precursor of the arrival of HSS events during solar cycle 23.…”

The coupling between the solar wind and proton fluxes at GEO

“…The ERR study has shown that for energies < 800 keV, the main control parameter is the solar wind velocity, showing a similar triangular dependance observed with electron fluxes (Reeves et al, 2011). The lag of the velocity is between 2-3 days which indicates that the solar wind velocity supplies the seed population of protons, which is then accelerated over two days to the higher energies.…”

“…Figure 1 shows a scatter plot for the 80-110 keV proton flux and velocity recorded two days prior. This indicates a complex triangular relationship, similar to the electron fluxes (Reeves et al, 2011), where the proton fluxes have a velocity dependant lower limit but are independent in the upper limit. Figure 1 displays a higher and much narrower range of proton fluxes when the solar wind velocity is high compared to when the velocity is low.…”