Letter to the Editor Measurement of the electron temperature gradient in a solar coronal hole

It has long been established that the high speed solar wind streams observed at 1 A.U. originate from the coronal hole regions of the Sun. Theoretical 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 unreliable. The very low luminosity leads to extreme observational difficulties, in particular due to light scattering in the instrument telescopes. Using the two SOHO spectrometers CDS and SUMER, electron temperatures 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 to the limb, rising to a maximum of less than 1 MK at 1.15 R , then falling to around 0.4 MK at 1.3 R . With these low temperatures, the classical Parker mechanism cannot alone explain the high wind velocities, which must therefore be due to the direct transfer of momentum from MHD waves to the ambient plasma.