Solar wind heating by Fermi acceleration

Details Solar wind heating by Fermi acceleration Solar wind heating by Fermi acceleration

Jun 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aipc..471..453h&link_type=abstract

The solar wind nine conference. AIP Conference Proceedings, Volume 471, pp. 453-456 (1999).

Computer Science

Solar Wind Plasma, Sources Of Solar Wind, Particle Emission, Solar Wind

Scientific paper

Heating by Fermi acceleration offers a unifying explanation of several disparate properties of solar wind ions. Inside 1 AU, undamped Alfvén wave amplitudes, δB, decrease less rapidly than the field magnitude, maintaining |δB|/B>~1 and forming wave fronts about a proton gyroradius thick. Fermi acceleration in the compressible medium heats ions to a thermal speed about equal to the Alfvén wave amplitude, |δVA| the heating mechanism is ion reflection from the wave fronts. Because |δVA|~ρ1/4, the heating results in a proton polytropic index of 3/2 (while the adiabatic index is 5/3 for a fully ionized gas with three degrees of freedom), consistent with observational results and the observed entropy increase of protons in fast wind between 0.3 and 1 AU. Because ions are heated to a thermal speed that is independent of mass, ion temperature is proportional to mass in accord with observations. Helium and heavy ions, with larger gyroradii than protons, are more efficiently reflected by the wave fronts and cross them with difficulty; they are therefore carried by the wave and have differential velocities with respect to protons of about the Alfvén velocity, also in agreement with observations. The Fermi acceleration mechanism is inapplicable to the inner corona where |δB|/B<<1 but offers a plausible explanation of an inference by S. Olbert of a temperature maximum in the outer corona near the Alfvén critical point where the wave amplitude is maximum.

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