Oxygen freeze-in temperatures measured with SOHO/CELIAS/CTOF

Abstract

We use the charge time-of-flight (CTOF) mass and charge spectrometer of the charge, element, and isotope analysis system (CELIAS) on board the Solar and Heliospheric Observatory (SOHO) to determine the solar wind oxygen freeze-in temperature T₇₆ from the O⁷⁺ and O⁶⁺ abundance ratios in the period from days 92 to 229 of 1996 (Carrington Rotations 1908 to 1912). The freeze-in temperature is a conserved property of the solar wind because the charge states do not change after a distance of a few solar radii. Therefore it is an ideal in situ diagnostic for remote sensing of the inner solar corona. We determine the mean freeze-in temperature during the selected period to be 1.6×10⁶K. We use it to map coronal regions of different temperatures and to determine the separation between such regions based on our observation of abrupt transitions of the freeze-in temperature. We find a upper limit for the separation in the inner corona of 1000 km.