Solar cycle variations in ion composition in the dayside ionosphere of Titan

Abstract

One Titanian year spans over two complete solar cycles, and the solar irradiance has a significant effect on ionospheric densities. Solar cycle 24 has been one of the quietest cycles on record. In this paper we show data from the Cassini ion and neutral mass spectrometer (INMS) and the radio and plasma wave science Langmuir probe spanning the time period from early 2005, at the declining phase of solar cycle 23, to late 2015 at the declining phase of solar cycle 24. Densities of different ion species measured by the INMS show a consistent enhancement for high solar activity, particularly near the ionospheric peak. The density enhancement is best seen in primary ion species such as CH3+ rather than heavier ion species such as HCNH+. Unlike at Earth, where the ionosphere and atmosphere thermally expand at high solar activity, at Titan the altitude of the ionospheric peak decreases, indicating that the underlying neutral atmosphere was less extensive. Among the major ion species, CH5+ shows the largest decrease in peak altitude, whereas heavy ions such as C3H5+ show very little decrease. We also calculate the ion production rates using a theoretical model and a simple empirical model using INMS data and show that these effectively predict the increased ion production rates at high solar activity.

CME impact on comet 67P/Churyumov-Gerasimenko

“…Hence, we conclude that the plasma environment was significantly compressed during the impact, due to the increased solar wind dynamic pressure. This is in agreement with the background magnetic field strength increasing by a factor of about 2-3, which would then explain the increased suprathermal electron fluxes as an effect of adiabatic compression (Madanian et al 2016).…”

Titan's ionosphere: A survey of solar EUV influences

“…It should also be noted that the nightside EUV trends are clear both with the median normalization and SLT detrending, although the error margins are much greater in the latter case. Interestingly, both peak and max n + and n À correlate with F EUV on the dayside, a factor ≈2 (≈4000 cm À3 ) increases between minimum and maximum solar activity (similar enhancements have also been observed by the Cassini INMS for positive ions <100 amu [Madanian et al, 2016]). At the same time, the charge densities strongly anticorrelate with F EUV on the nightside of Titan, a factor ≈3-4 (≈3000-4000 cm À3 ) Figure 6.…”

“…It should also be noted that the nightside EUV trends are clear both with the median normalization and SLT detrending, although the error margins are much greater in the latter case. Interestingly, both peak and max n + and n − correlate with F EUV on the dayside, a factor ≈2 (≈4000 cm −3 ) increases between minimum and maximum solar activity (similar enhancements have also been observed by the Cassini INMS for positive ions <100 amu [ Madanian et al , ]). At the same time, the charge densities strongly anticorrelate with F EUV on the nightside of Titan, a factor ≈3–4 (≈3000–4000 cm −3 ) decrease (Figures d and e; fit coefficients are summarized in Table ), and despite the fact that there are no measurements of nightside for high F EUV (≳ 40 μW m −2 , see Figures d and e) that cover the altitudes <1200 km, the trends of both n + and n − are consistent (as expected due to the coupled ion‐ion reactions [ Shebanits et al , ]).…”

“…The ionizing solar EUV flux evidently plays a key role in the complex organic chemistry of Titan's dayside ionosphere as the main source of energy. Recent research revealed the influence of the solar cycle on Titan's ionosphere, showing that electron [ Edberg et al , ] and positive ion number densities for lighter species (<100 amu) [ Madanian et al , ] are enhanced during the solar maximum, including the ions not directly produced by the ionizing EUV flux. Moreover, Sagnières et al [] showed that the ion number densities correlate with the local ionization rate, although the correlation is significantly stronger for the short‐lived ions than for the long‐lived ones.…”

Titan's Variable Ionosphere During the T118 and T119 Cassini Flybys

“…The structure and variability of this ionosphere are sensitive to several factors. On the dayside of the moon, the ionosphere is created mainly through photoionization by extreme ultraviolet (EUV) radiation of the atmospheric N 2 and CH 4 (Ågren et al, ; Galand et al, ), which means that Titan's ionospheric properties will vary with the solar rotation and the phase of the solar cycle (Edberg et al, ; Madanian et al, ; Shebanits et al, ). Particle impact ionization also contributes and is the main ionization source on the nightside of the moon (Ågren et al, ; Cravens et al, , ; Vigren et al, ).…”