Ion and aerosol precursor densities in Titan's ionosphere: A multi‐instrument case study

Abstract

The importance of the heavy ions and dust grains for the chemistry and aerosol formation in Titan's ionosphere has been well established in the recent years of the Cassini mission. In this study we combine independent in situ plasma (Radio Plasma and Wave Science Langmuir Probe (RPWS/LP)) and particle (Cassini Plasma Science Electron Spectrometer, Cassini Plasma Science Ion Beam Spectrometer, and Ion and Neutral Mass Spectrometer) measurements of Titan's ionosphere for selected flybys (T16, T29, T40, and T56) to produce altitude profiles of mean ion masses including heavy ions and develop a Titan‐specific method for detailed analysis of the RPWS/LP measurements (applicable to all flybys) to further constrain ion charge densities and produce the first empirical estimate of the average charge of negative ions and/or dust grains. Our results reveal the presence of an ion‐ion (dusty) plasma below ~1100 km altitude, with charge densities exceeding the primary ionization peak densities by a factor ≥2 in the terminator and nightside ionosphere (ne/ni ≤ 0.1). We suggest that ion‐ion (dusty) plasma may also be present in the dayside ionosphere below 900 km (ne/ni < 0.5 at 1000 km altitude). The average charge of the dust grains (≥1000 amu) is estimated to be between −2.5 and −1.5 elementary charges, increasing toward lower altitudes.

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

“…We can also note that the ion density derived from the LP (not shown) is generally also showing the same kind of profiles as the electron density, and the LP measurements of negative ions (Shebanits et al, , ) do not show any distinct increases when the electron density decreases to abnormally low values during T118. This rules out any effect of dust, which could otherwise cause a depletion of free electrons as the charges attach to the dust.…”

Titan's ionosphere: A survey of solar EUV influences

“…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 SZA varies during each flyby and it is not guaranteed that these values reflect the location of density maxima along the vertical direction. The location of the maxima may be close to the maxima of the ion-electron pair production rate (P e ) but can also be shifted downward by an increased fraction of negative ions with decreasing altitude [Vigren et al, 2014;Shebanits et al, 2016]. In any case, we consider that estimates of P e along the dayside Titan flybys are needed for an interpretation of the presented trend of decreasing (increasing) altitude of the positive (negative) ion number density peaks with increasing EUV flux.…”

“…In the deepest ionosphere (880–1000 km altitude), the in situ observations by the Cassini spacecraft (S/C) have revealed negatively charged ions/dust particles (~5 nm) [ Coates et al , ], with abundances comparable to or higher than the free electrons [ Ågren et al , ; Shebanits et al , , ]. In this region, the number density of the positive ions [singly charged, Thissen et al , ] is enhanced due to the (comparatively low) ion production rate being compensated by a slow chemical loss rate [ Lavvas et al , ; Vigren et al , ; Shebanits et al , ].…”

Saturn's Ionosphere: Electron Density Altitude Profiles and D‐Ring Interaction From The Cassini Grand Finale

“…From the voltage—current characteristics of the LP a number of plasma parameters can be estimated and derived (the spacecraft potential U sc , the electron number density n e , the electron temperature T e , the ion density n i , the ion ram speed v i , and the average ion mass m i ). More details regarding the derivation of the parameters can be found in Holmberg et al () and Shebanits et al (, ). In the continuous mode, the electron current ( I e ) is sampled at a constant bias voltage (+11 V for ±32‐V sweep mode and +4 V for ±4‐V sweep mode) with 20 samples per second and is proportional to Using T e and U sc estimates from the voltage sweeps (with lower temporal resolution) and assuming T e and U sc ≈ constant between the sweeps, we can derive n e with a resolution of 20 Hz (Ågren et al, , ).…”