Viking electron temperature measurements: Evidence for a magnetic field in the Martian ionosphere

Further analysis of the Viking RPA data has now provided measurements of the thermal electron temperature in the upper Martian ionosphere. It is found that T e is several thousand degrees K, i.e., only of the order of twice the ion temperature. The sum of all the measured partial plasma pressures, including ions and suprathermal electrons, has a minimum value of ∼5×10 −10 dyn cm −2 near 350 km and is found to be insufficient to balance the measured electron pressure in the shocked solar wind near 1000 km altitude, by a factor of the order of 4. Thus there is no doubt that a magnetic field of at least 30 to 40 nT permeates the ionosphere. This conclusion is not inconsistent with previous assessments, but it now has a firm observational basis. These data do not uniquely establish whether the magnetic field is intrinsic or induced, but our assessment is that a significant intrinsic moment is not required.