Comparison of Terrestrial and Martian TEC at Dawn and Dusk during Solstices

Planetary ionospheres undergo many changes at dawn and dusk due to both photochemical and transport processes.  The relative importance of these different processes vary depending on a variety of factors, including the amount of solar radiation, the composition of the thermosphere, and the characteristics of any local magnetic fields.  This study uses the similarities between the ionospheres on Mars and Earth to examine the behaviour of the ionosphere at dawn and dusk.  It has notable implications for comparative aeronomy, as a solid understanding of ionospheric processes on planets with and without magnetic fields is important for characterising the environments of solar and exoplanets, as well as atmospheric evolution over long time scales.

The amount of plasma present in the ionosphere was measuring using the total electron content (TEC), and grouped so that both solstices and different phases of the solar cycle could be examined.  To allow comparisons between the ionospheres of Mars and Earth, which differ greatly in density, the rate of change of TEC as a function of solar zenith angle was used to compare the plasma production and losses in the main layer of each planetary ionosphere.  Examination of the dawn and dusk TEC slopes shows that, to first order, the Martian slopes are symmetric while those at Earth are not.  This symmetry is interpreted as an indicator of photochemical equilibrium, and different reasons for deviations from symmetry were explored.  The presence or absence of a magnetic field played a large role in shaping plasma transport, with photochemical processes in both ionospheres behaving similarly in the absence of a magnetic field.  At Mars, it was found that transport processes were most important at solar maximum, while at Earth transport processes were most important at solar minimum.