This model study shows that heating of the protons in the corona may lead to high proton temperature and large asymptotic flow speed of the solar wind. These solutions can be obtained when the electron density in the inner corona is low.
For spherically symmetric outflow the solar wind flow speed in the inner corona is low, and there is significant thermal coupling between electrons and protons, even for quite low electron densities. Thus, heating of the protons in the inner corona will lead to heat transfer to the electron gas and to a significant heat conductive flux into the chromosphere-corona transition region. Most of this energy flux is lost as radiation from the transition region. The increase of the solar wind energy flux in the transition region is smaller than the radiative energy loss, so a significant inward heat flux is consistent with a quite large transition region pressure, a high electron density in the inner corona, a quite large solar wind proton flux, and a low asymptotic flow speed. By allowing for a more gradual damping of the energy flux from the sun, such that the outer corona is heated, we find a reduced proton flux and an increased asymptotic flow speed.
In rapidly expanding flow geometries the solar wind flow speed in the inner corona is higher than in spherically symmetric flow (for the same solar wind proton flux density at the orbit of Earth), the expansion rate of the plasma is larger, the thermal coupling between electrons and protons is weaker, and a smaller fraction of the energy flux deposited in the corona is lost as inward heat conductive flux. This may result in a more rapid acceleration and higher asymptoic speed of the solar wind than what we find in an flow geometry.
We conclude that in reasonable models of the corona-solar wind system, heating of the inner corona leads to a significant heat flux density into the transition region, a large transition region pressure, a large coronal base electron density, and a large solar wind proton flux. In order to obtain speeds comparable to the high speeds measured by the Ulysses spacecraft, a significant fraction of the energy flux from the sun must be deposited in the proton gas in the outer corona, where the proton heat conductive flux is small and the collisional coupling to the electrons is weak. This may lead to a low transition region pressure, a low coronal electron density, a high proton temperature, and a high asymptotic flow speed. Heating of the electron gas (as well as heating of the transition region and of the inner corona) will tend to increase the solar wind proton flux and reduce the asymptotic flow speed.
© European Southern Observatory (ESO) 1998
Online publication: December 8, 1997