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Title: Electron temperature of the solar wind
Solar wind provides an example of a weakly collisional plasma expanding from a thermal source in the presence of spatially diverging magnetic-field lines. Observations show that in the inner heliosphere, the electron temperature declines with the distance approximately as T e ( r ) ∼ r − 0.3 … r − 0.7 , which is significantly slower than the adiabatic expansion law ∼ r − 4 / 3 . Motivated by such observations, we propose a kinetic theory that addresses the nonadiabatic evolution of a nearly collisionless plasma expanding from a central thermal source. We concentrate on the dynamics of energetic electrons propagating along a radially diverging magnetic-flux tube. Due to conservation of their magnetic moments, the electrons form a beam collimated along the magnetic-field lines. Due to weak energy exchange with the background plasma, the beam population slowly loses its energy and heats the background plasma. We propose that no matter how weak the collisions are, at large enough distances from the source a universal regime of expansion is established where the electron temperature declines as T e ( r ) ∝ r − 2 / 5 . This is close to the observed scaling of the electron temperature in the inner heliosphere. Our first-principle kinetic derivation may thus provide an explanation for the slower-than-adiabatic temperature decline in the solar wind. More broadly, it may be useful for describing magnetized collisionless winds from G-type stars.  more » « less
Award ID(s):
1707272
PAR ID:
10175644
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Proceedings of the National Academy of Sciences
Volume:
117
Issue:
17
ISSN:
0027-8424
Page Range / eLocation ID:
9232 to 9240
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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