1112.3850 (J. F. Lemaire)

Determination of coronal temperatures from electron density profiles    [PDF]

J. F. Lemaire

The most popular method for determining coronal temperatures is the scale-height-method (shm). It is based on electron density profiles inferred from White Light (WL) brightness measurements of the corona during solar eclipses. This method has been applied to several published coronal electron density models. The calculated temperature distributions reach a maximum at r > 1.3 RS, and therefore do not satisfy one of the conditions for applying the shm method. Another method is the hydrostatic equilibrium method (hst), which enables coronal temperature distributions to be determined, providing solutions to the hydrostatic equilibrium equation. The temperature maximas using the hst method are almost equal to those obtained using the shm method, but the temperature peak is always at significantly lower altitude when the hst-method is used than when the shm-method is used. A third and more recently developed method, dyn, can be used for the same published electron density profiles. The temperature distributions obtained using the dyn method are regular solutions of the hydrodynamic equations. They depend on the expansion velocity of the coronal plasma considered as a free input parameter in the calculations. The larger the solar wind expansion velocity at 1AU, the larger the new temperature maximum that develops in the range of altitudes (about 3 RS) where the outward acceleration rate of the coronal plasma is greatest. At the base of the solar corona, where the coronal bulk velocity is small (subsonic), the dyn and hst methods give similar temperature values. More significant differences are found at higher altitudes where the expansion velocity approaches and exceeds the velocity of sound. The effects of (i) super-radial expansion flux tubes; (ii) alpha particle concentration; and (iii) ratios of ion over electron temperature.

View original: http://arxiv.org/abs/1112.3850