Titan’s atomic hydrogen corona

Abstract

Based on measurements performed by the Hydrogen Deuterium Absorption Cell (HDAC) aboard the Cassini orbiter, Titan's atomic hydrogen exosphere is investigated. Data obtained during the T9 encounter are used to infer the distribution of atomic hydrogen throughout Titan's exosphere, as well as the exospheric temperature.The measurements performed during the flyby are modeled by performing Monte Carlo radiative transfer calculations of solar Lyman-α radiation, which * Corresponding author, Tel. +33 557 776 127 ACCEPTED MANUSCRIPTis resonantly scattered on atomic hydrogen in Titan's exosphere. Two different atomic hydrogen distribution models are applied to determine the best fitting density profile. One model is a static model that uses the Chamberlain formalism to calculate the distribution of atomic hydrogen throughout the exosphere, whereas the second model is a particle model, which can also be applied to non-Maxwellian velocity distributions. The density distributions provided by both models are able to fit the measurements although both models differ at the exobase: Best fitting exobase atomic hydrogen densities of n H = (1.5 ± 0.5) · 10 4 cm −3 and n H = (7 ± 1) · 10 4 cm −3 were found using the density distribution provided by both models, respectively. This is based on the fact that during the encounter, HDAC was sensitive to altitudes above about 3,000 km, hence well above the exobase at about 1,500 km. Above 3,000 km, both models produce densities which are comparable, when taking into account the measurement uncertainty.The inferred exobase density using the Chamberlain profile is a factor of about 2.6 lower than the density obtained from Voyager 1 measurements and much lower than the values inferred from current photochemical models. However, when taking into account the higher solar activity during the Voyager flyby, this is consistent with the Voyager measurements. When using the density profile provided by the particle model, the best fitting exobase density is in perfect agreement with the densities inferred by current photochemical models.Furthermore, a best fitting exospheric temperature of atomic hydrogen in the range of T H = (150 − 175) ± 25 K was obtained when assuming an isothermal exosphere for the calculations. The required exospheric temperature depends on the density distribution chosen. This result is within the temperature range determined by different instruments aboard Cassini. The inferred temperature is close to the critical temperature for atomic hydrogen, above which it can escape hydrodynamically after it diffused through the heavier background gas.

The Structure of Titan’s N2 and CH4 Coronae

“…For illustrative purpose, the best-fit corona models for T83 are given by the red lines in Figure 1(c), with an inferred exobase temperature of 177K for N 2 and 159K for CH 4 , respectively. The same approach has also been used in previous analyses of Titan's H 2 corona (Cui et al , 2011, atomic H corona (Hedelt et al 2010), as well as N 2 and CH 4 coronae (De La Haye et al 2007a).…”

“…De La Haye et al (2007b) later evaluated the role of exothermic ion-neutral chemistry, but found it inadequate to produce the suprathermal N 2 and CH 4 populations. In addition, the atomic H corona on Titan was sampled by the Cassini Hydrogen Deuterium Absorption Cell (HDAC) instrument, and existing studies using the HDAC data did not support the presence of a hot atomic H population (Hedelt et al 2010).…”

Exospheres and Energetic Neutral Atoms of Mars, Venus and Titan

“…Two main instruments on board Cassini have been able to investigate the Titan thermosphere and exosphere: the Ultraviolet Imaging Spectrograph (UVIS) (Esposito et al 2004;Hedelt et al 2010) and the Ion and Neutral Mass Spectrometer (INMS) (Waite et al 2004). Cassini also has the capability of detecting in situ high energy plasma particles using the Magnetospheric Imaging Instrument (MIMI).…”

“…Its Hill sphere radius (Hill 1878), which gives the limit of its gravitational influence and thus a good estimation for the upper limit of its exosphere, is about 50,000 km. As a consequence, the measurements available today (up to 2000 km altitude, for an exobase around 1400-1500 km) are only able to provide information in the lower part of the corona, except for molecular hydrogen with observations up to 7000 km altitude or atomic hydrogen with the UVIS experiment (Hedelt et al 2010).…”

“…5.4, the Titan exosphere extends up to several 10,000 km, but the Cassini instruments cannot provide neutral densities in this upper part of the exosphere (except for atomic hydrogen up to 10,000 km; Hedelt et al 2010). Brandt et al (2009Brandt et al ( , 2011 studied the ENA images of the extended H 2 corona during several moon encounters among 3 years of observations.…”