MEASUREMENT OF THE ELECTRON-TEMPERATURE GRADIENT IN A SOLAR CORONAL HOLE

It has long been established that the high speed solar wind streams ob served at 1 A.U. originate from the coronal hole regions of the Sun. T heoretical modelling of the acceleration mechanism depends critically on the value of the maximum of temperature existing close to the Sun. Measurements of the temperature in coronal holes prior to SOHO are unr eliable. The very low luminosity leads to extreme observational diffic ulties, in particular due to light scattering in the instrument telesc opes. Using the two SOHO spectrometers CDS and SUMER, electron tempera tures have now been measured as a function of height above the limb in a polar coronal hole. Temperatures of around 0.8 MK are found close t o the limb, rising to a maximum of less than 1 MK at 1.15 R-.,then fal ling to around 0.4 MK at 1.3 R-.. With these low temperatures, the cla ssical Parker mechanism cannot alone explain the high wind velocities, which must therefore be due to the direct transfer of momentum from M HD waves to the ambient plasma.