Exploring the Properties of the Electron Strahl at 1 AU as an Indicator of the Quality of the Magnetic Connection Between the Earth and the Sun

Joseph E Borovsky

doi:10.3389/fspas.2021.646443

Abstract

In this report some properties of the electron strahl at 1 AU are examined to assess the strahl at 272 eV as an indicator of the quality of the magnetic connection of the near-Earth solar wind to the Sun. The absence of a strahl has been taken to represent either a lack of magnetic connection to the corona or the strahl not surviving to 1 AU owing to scattering. Solar-energetic-electron (SEE) events can be used as indicators of good magnetic connection: examination of 216 impulsive SEE events finds that they are all characterized by strong strahls. The strahl intensity at 1 AU is statistically examined for various types of solar-wind plasma: it is found that the strahl is characteristically weak in sector-reversal-region plasma. In sector-reversal-region plasma and other slow wind, temporal changes in the strahl intensity at 1 AU are examined with 64 s resolution measurements and the statistical relationships of strahl changes to simultaneous plasma-property changes are established. The strahl-intensity changes are co-located with current sheets (directional discontinuities) with strong changes in the magnetic-field direction. The strahl-intensity changes at 1 AU are positively correlated with changes in the proton specific entropy, the proton temperature, and the magnetic-field strength; the strahl-intensity changes are anti-correlated with changes in the proton number density, the angle of the magnetic field with respect to the Parker-spiral direction, and the alpha-to-proton number-density ratio. Reductions in the strahl intensity are not consistent with expectations for a simple model of whistler-turbulence scattering. Reductions in the strahl intensity are mildly consistent with expectations for Coulomb scattering, however the strongest-observed plasma-change correlations are unrelated to Coulomb scattering and whistler scattering. The implications of the strahl-intensity-change analysis are that the change in the magnetic-field direction at a strahl change represents a change in the magnetic connection to the corona, resulting in a different strahl intensity and different plasma properties. An outstanding question is: Does an absence of an electron strahl represent a magnetic disconnection from the Sun or a poor strahl source in some region of the corona?

Highlights

Using the 64-sec-resolution ACE plasma SWEPAM and magnetic-field MAG data for the year 2005, 528 strong changes in the intensity of the electron strahl at 1 AU were analyzed: 282 of those 528 strahl changes occurred when the solar-wind plasma was categorized as sector-reversal-region plasma and 246 of the strahl changes occurred in wind with vsw < 460 km/s that was categorized as either ejecta or streamerbelt-origin plasma
Being aware that different mechanisms can act on different energy electrons in the strahl, examining 272 eV strahlintensity changes in sector-reversal-region plasma and other slow solar wind (Table 1) found that increases or decreases of the strahl intensity are positively correlated with Sp, Tp, and Bmag increases or decreases and that strahl-intensity increases or decreases are anti-correlated with np, θPS, and α/p increases or decreases
The results of the 272 eV strahl-change analysis are inconsistent with a constant-amplitude model for whistlerturbulence scattering of the strahl as the reason for the strahlintensity changes; for whistler scattering the changes in the strahl intensity should be correlated with np changes and anti-correlated with Bmag changes and the opposite correlations are observed

Summary

Introduction

Knowing the magnetic connectivity between the Earth and the Sun is desirable for several reasons: e.g., 1) to gain a global understanding of the heliospheric magnetic structure, (e.g. McComas et al, 1989; Crooker et al, 2002; Crooker and Pagel, 2008; Crooker and Owens, 2012; Lockwood, 2013; Viall and Borovsky, 2020), 2) for understanding solar energetic particle (proton and electron) events and particle transport (Mazur et al, 2000; Chollet and Giacalone, 2011; Trenchi et al, 2013), and 3) for assessing the likelihood that Earth will or will not undergo an energetic-particle event.One promising method to assess the magnetic connectivity between the near-Earth solar wind and the Sun is to interpret observations of the electron strahl in the near-Earth solar wind.

Results

Conclusion