Examining the Evolution of Solar Wind Strahl: What Can We Determine About Potential Scattering Mechanisms?
Abstract
Field-aligned beams of suprathermal electrons called the `strahl' and approximately isotropic suprathermal electrons called the `halo' make up key components of solar wind plasma. However, the exact nature of the solar wind suprathermal electrons' origins and heliospheric evolution is yet to be fully understood. Magnetic field strength decreases with distance from the Sun and suprathermal electrons travelling outwards into the heliosphere will experience strong adiabatic focussing. Hence, in the absence of other effects, by 1 AU the electrons should form a highly collimated beam. This contrary to what is observed at 1 AU, where an approximately isotropic halo and strahl beams with highly variable width are observed. Strahl beam width has also been shown to broaden with heliospheric radial distance and studies have shown that the relative density of the strahl and halo populations vary oppositely with distance, suggesting that the strahl may be scattered into the halo. Hence, strahl electrons must be subject to some form of pitch angle scattering, of which the most likely candidate is wave-particle interactions. We present the results of two studies that examine strahl width with distance and electron energy. In the first, we examine the evolution of strahl beam width with heliospheric radial distance and find that the strahl width continues to broaden out to at least 5.5 AU, most likely being scattered into the diffuse halo population. In the second, we use a novel technique to estimate the path length travelled by the strahl along the interplanetary magnetic field. We find find an energy relation for strahl beam width broadening in both studies that suggests a dominant pitch angle scattering mechanism is affecting the strahl.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMSH32A..05G
- Keywords:
-
- 7829 Kinetic waves and instabilities;
- SPACE PLASMA PHYSICS;
- 7846 Plasma energization;
- SPACE PLASMA PHYSICS;
- 7859 Transport processes;
- SPACE PLASMA PHYSICS;
- 7867 Wave/particle interactions;
- SPACE PLASMA PHYSICS