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

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?

The Electron Structure of the Solar Wind

“…There is no reason to argue that the differing flux tubes should not act independently. At 1 AU it is seen that the intensity of the electron stralh can vary from flux tube to flux tube (Borovsky, 2021): this strahl variation is an indication that different flux tubes at 1 AU are connected to different features in the corona (Gosling et al, 2004a,b;FIGURE 7 | For 15.6 min of time-series measurements by Parker Solar Probe on January 30, 2020 the relationship between jumps in the coreelectron temperature (blue curve) and current-sheet crossings as indicated by strong temporal changes in the magnetic-field direction (green curve) are examined. Borovsky, 2021).…”

“…If the solar-wind plasma is an admixture of open flux tubes plus other magnetic structures (e.g., Figure 24C of Khabarova et al, 2021) then the exosphere-model picture is more complicated. Further investigation of field and core-electron measurements at 1 AU, guided with measurements of the strahl intensity to judge magnetic connectivity to the Sun (Borovsky, 2021), is called for. In any closed-magnetic structure, the interpretation of T core as an indication of the local electrical potential ϕ (e.g., Feldman et al, 1975;Boldyrev et al, 2020;Moncuquet et al, 2020) might no longer be possible.…”

“…Certainly, it is well known that closed-field plasma blobs can be found in solar-wind plasma originating from coronal streamer stalks (Wang et al, 1999;Sheeley and Rouillard, 2010;Viall et al, 2010;Viall and Vourlidas, 2015;Kepko et al, 2016;Di Matteo et al, 2019). Relatedly, sector-reversalregion plasma (in which the heliospheric current sheet is found) tends to have magnetic fields that are less Parker-spiral oriented and tends to have a weaker electron strahl (Borovsky, 2021), both being indicative of impulsive emission of plasma from the Sun with poor magnetic connections back to the Sun. In the non-ejecta solar wind the magnetic flux tubes meander along the Parker-spiral direction (Borovsky, 2010).…”

“…2) Similarly, current sheets at 1 AU are seen co-located with jumps in the intensity of the electron strahl (Borovsky, 2020a): for these current sheets the strahl jump is an indication that the current sheet is coherent back to the Sun (Gosling et al, 2004a,b;Borovsky, 2020a). The jumps in strahl intensity from tube to tube implies that different flux tubes in the magnetic spaghetti magnetically connect to different spots on the Sun (Gosling et al, 2004a,b;Borovsky, 2021). (Similarly, see Trenchi et al (2013a,b) for coherence indicated by energeticparticle observations).…”

Do Impulsive Solar-Energetic-Electron (SEE) Events Drive High-Voltage Charging Events on the Nightside of the Moon?

“…darkness much of the time that the Moon is on the sunward side of the bow shock in the supersonic solar wind (Properties of the electron strahl and the solar-wind plasma at Earth during these 216 SEE events are further analyzed in Borovsky (2021)).…”