Variations in Titan’s Ionospheric Densities over a 10 Year Time Period of the Cassini Mission

Abstract:

Titan hosts the most chemically complex atmosphere in the solar system. The first encounter of the Cassini spacecraft with Titan was in October 2004 with a closest approach altitude of 1147km. Since then the Cassini has made measurements of the ionospheric species over multiple flybys and has provided a unique opportunity to study long term effects on Titan's ionosphere including solar activity cycle. It has been shown that solar extreme ultraviolet radiation is the main source of ion production and production rates are highly dependent on solar zenith angle. We use data from the ion and Neutral Mass Spectrometer (INMS) and the Radio and Plasma Wave Science-Langmuir probe (RPWS-LP) instruments onboard the Cassini spacecraft to investigate changes in the ion composition of the ionosphere. We compare the observations with the results of our ionospheric models. Data from 21 targeted Titan flybys are used, covering the declining phase of solar cycle 23 into the double peak of solar cycle 24. We show that the ion densities have in fact increased during the high solar activity, especially at altitudes near the ionospheric peak, and unlike Earth, the Titan's thermosphere is contracted during high solar activity. Charge neutrality is assumed near the peak of the ionosphere, however difference exists between INMS total ion densities and RPWS-LP electron densities and it becomes more noticeable at high solar activity. This may be for several reasons including enhanced negative ion densities and increased electron recombination rates. The model results agree reasonably well with the in-situ measurements for most ion species at low and high solar activity conditions although overproduction exists for some ions (e.g. HCNH+) and underproduction is seen for some heavier species such as C7H7+.