Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...510l..63e&link_type=abstract
The Astrophysical Journal, Volume 510, Issue 1, pp. L63-L67.
Astronomy and Astrophysics
Astronomy
124
Sun: Solar Wind, Sun: Corona, Sun: Uv Radiation
Scientific paper
Recent observations of the Lyalpha lambda1216, Mg X lambda625, and O VI lambda1038 spectral lines carried out with the Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO at distances in the range 1.35-2.1 R_S in the northern coronal hole are used to place limits on the turbulent wave motions of the background plasma and the thermal motions of the protons and Mg^+9 and O^+5 ions. Limits on the turbulent wave motion are estimated from the measured line widths and electron densities derived from white light coronagraph observations, assuming WKB approximation at radial distances covered by the observations. It is shown that the contribution of the turbulent wave motion to the widths of the measured spectral lines is small compared to thermal broadening. The observations show that the proton temperature slowly increases between 1.35 and 2.7 R_S and does not exceed 3x10^6 K in that region. The temperature of the minor ions exceeds the proton temperature at all distances, but the temperatures are neither mass proportional nor mass-to -charge proportional. It is shown, for the first time, that collision times between protons and minor ions are small compared to the solar wind expansion times in the inner corona. At 1.35 R_S the expansion time exceeds the proton Mg^+9 collision time by more than an order of magnitude. Nevertheless, the temperature of the Mg ions is significantly larger than the proton temperature, which indicates that the heating mechanism has to act on timescales faster than minutes. When the expansion time starts to exceed the collision times a rapid increase of the O^+5 ion spectral line width is seen. This indicates that the heavier and hotter ions lose energy to the protons as long as collision frequencies are high, and that the ion spectral line width increases rapidly as soon as this energy loss stops.
Dobrzycka Danuta
Edgar Richard J.
Esser Ruth
Fineschi Silvano
Guhathakurta Madhulika
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