On the determination of electron polytrope indices within coronal mass ejections in the solar wind

Details On the determination of electron polytrope indices within coronal mass ejections in the solar wind On the determination of electron polytrope indices within coronal mass ejections in the solar wind

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999jgr...10419851g&link_type=abstract

Journal of Geophysical Research, Volume 104, Issue A9, p. 19851-19858

Physics

13

Interplanetary Physics: Ejecta, Driver Gases, And Magnetic Clouds

Scientific paper

Electron temperature and density often appear to be negatively correlated with one another as a coronal mass ejection, CME, in the solar wind passes over a spacecraft at 1 AU and beyond. It has been suggested that this negative correlation within magnetic clouds, which form a subset of all CMEs, implies that the electron polytropic index for the plasma within the clouds is significantly less than 1.0. We argue that single-point measurements of density and temperature within clouds do not speak to the issue of the coupled evolution of density and temperature that occurs as the clouds expand out into the heliosphere, and thus do not provide a measure of the polytropic index within them. Moreover, we show that observed electron temperatures within magnetic clouds and CMEs close to the Sun, at 1 AU, and at large heliocentric distances do not agree with predictions based upon electron polytropic indices significantly less than 1.0. We suggest that the negative correlation between electron temperature and density often observed within CMEs and magnetic clouds at 1 AU and beyond simply reflects the presence of structure within the CMEs and the plasma's tendency to achieve local pressure balance as it evolves outward from the Sun. Our results indicate that self-similar models that require the electron polytrope index to be less than 1.0 in order for a magnetic cloud to expand are physically unrealistic.

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