Abstract
AbstractSolar cycle variations in solar radiation create density changes in any planetary ionosphere, which are well established in the Earth's case. At Mars, however, the ionospheric response to such changes is not well understood. We show the solar cycle impact on the topside ionosphere of Mars, using data from the Mars Advance Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express. Topside ionospheric variability during the solar cycle is analyzed through neutral scale height behavior. For moderate and high solar activity phases, the topside electron density profile is reproduced with an altitude‐variable scale height. However, for the period of extremely low solar activity in 2008 and 2009, the topside was smaller in density than in the other phases of the solar cycle, and there is evidence that it could be reproduced with either a constant scale height or a height‐variable scale height with lower electron density. Moreover, the ionosphere during this time did not show any apparent dependence on the EUV flux. This singular behavior during low solar activity may respond to the presence of an induced magnetic field which can penetrate to lower ionospheric altitudes than in other phases of the solar cycle due to the reduced thermal pressure. Numerical simulations of possible scenarios for two different solar cycle phases indicate that this hypothesis is consistent with the observations.
Highlights
Solar extreme ultraviolet (EUV) and X-ray photon flux are the main sources of ionization of a planetary atmosphere, especially in the inner solar system [e.g., Witasse et al, 2008; Schunk and Nagy, 2009]
In order to analyze the ionospheric response to solar cycle changes, the Active Ionospheric Sounding (AIS) data set was split into four periods according to notable changes in the solar spectrum, as seen in Figure 1 by considering all the panels together
In order to assess the impact of the planetary orbital distance in the EUV spectrum, we show the topside total electron content (TEC) estimated from the profiles used in this study as a function of the EUV solar irradiance at 30.5 nm scaled to Mars, for each phase of the solar cycle (Figure 2)
Summary
Solar extreme ultraviolet (EUV) and X-ray photon flux are the main sources of ionization of a planetary atmosphere, especially in the inner solar system [e.g., Witasse et al, 2008; Schunk and Nagy, 2009]. At Earth during the recent solar minimum the thermosphere became much colder and lower in density, it is not clear if this was the case at Mars For this reason, we look for evidence of similar or otherwise exceptional behavior as we analyze the response of Mars’ ionosphere during this period of extreme low solar activity. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) [Picardi et al, 2004] on board Mars Express has been probing the topside ionosphere of Mars since June 2005, currently covering 10 years and with the best planetary coverage yet archived When this instrument operates in the Active Ionospheric Sounding (AIS) mode, which occurs for short periods close to periapsis, the topside ionosphere is sounded [Gurnett et al, 2005] following exactly the same physical principles of ionosondes on Earth [Sánchez-Cano et al, 2012; Morgan et al, 2013]. This may seem trivial, but we note that Mars Express periapsis is in the nightside of the planet for large periods of time
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
