TY - JOUR

T1 - The evolution of solar wind strahl with heliospheric distance

AU - Graham, G. A.

AU - Rae, I. J.

AU - Owen, C. J.

AU - Walsh, A. P.

AU - Arridge, C. S.

AU - Gilbert, L.

AU - Lewis, G. R.

AU - Jones, G. H.

AU - Forsyth, C.

AU - Coates, A. J.

AU - Waite, J. H.

PY - 2017/4

Y1 - 2017/4

N2 - Field-aligned beams of suprathermal electrons, known as strahl, are a frequently observed constituent of solar wind plasma. However, the formation and interplanetary evolution of the strahl electron populations has yet to be fully understood. As strahl electrons travel away from the Sun, they move into regions of decreasing magnetic field strength and thus are subject to adiabatic focusing. However, the widths of strahl pitch angle distributions observed at 1AU are significantly broader than expected. Previous investigations have found that the average observed strahl pitch angle width actually increases with heliocentric radial distance. This implies that strahl electrons must be subjected to some form of pitch angle scattering process or processes, details of which as of yet remain elusive. In this paper, we use Cassini electron measurements to examine strahl beams across a distance range of approximately 8 AU, from its Earth Flyby in 1999 until its insertion into orbit around Saturn in 2004. We find that, in general, there is a relatively constant rate of broadening of strahl pitch angle distributions with distance between similar to 1 and 5.5 AU. Our results from beyond this distance indicate that the strahl population is likely to be completely scattered, presumably to form part of the halo. We find multiple energy dependences at different radial distances implying that there are multiple strahl scattering mechanisms in operation.

AB - Field-aligned beams of suprathermal electrons, known as strahl, are a frequently observed constituent of solar wind plasma. However, the formation and interplanetary evolution of the strahl electron populations has yet to be fully understood. As strahl electrons travel away from the Sun, they move into regions of decreasing magnetic field strength and thus are subject to adiabatic focusing. However, the widths of strahl pitch angle distributions observed at 1AU are significantly broader than expected. Previous investigations have found that the average observed strahl pitch angle width actually increases with heliocentric radial distance. This implies that strahl electrons must be subjected to some form of pitch angle scattering process or processes, details of which as of yet remain elusive. In this paper, we use Cassini electron measurements to examine strahl beams across a distance range of approximately 8 AU, from its Earth Flyby in 1999 until its insertion into orbit around Saturn in 2004. We find that, in general, there is a relatively constant rate of broadening of strahl pitch angle distributions with distance between similar to 1 and 5.5 AU. Our results from beyond this distance indicate that the strahl population is likely to be completely scattered, presumably to form part of the halo. We find multiple energy dependences at different radial distances implying that there are multiple strahl scattering mechanisms in operation.

KW - ELECTRON HEAT-FLUX

KW - VELOCITY DISTRIBUTION-FUNCTIONS

KW - MAGNETIC-FIELD

KW - ULYSSES OBSERVATIONS

KW - SPECTROMETER

KW - WAVES

KW - SPACECRAFT

KW - EVENTS

KW - EARTH

KW - HALO

U2 - 10.1002/2016JA023656

DO - 10.1002/2016JA023656

M3 - Journal article

VL - 122

SP - 3858

EP - 3874

JO - Journal of Geophysical Research: Space Physics

JF - Journal of Geophysical Research: Space Physics

SN - 2169-9380

IS - 4

ER -