Unraveling the connection of charge distribution in Titan’s upper and lower ionosphere

In Titan’s atmosphere, haze formation is initiated in the upper atmosphere (~900-1200 km) through photodissociation of N2 and CH4 , producing radicals, ions, and increasing complexity molecules [1, 2]. These chemical processes give rise to an upper ionosphere whose structure and composition have been revealed by the Cassini mission that subsequently performed in situ measurements of this upper ionosphere down to 950 km altitude. Mass spectra and charge distribution measurements revealed complex molecules up to ~100 amu with the Ion and Neutral Mass Spectrometer (INMS), and a plasma of heavy positively and negatively charged molecules up to 350-1000 amu with the Cassini Plasma Spectrometer Ion Beam Spectrometer (CAPS-IBS) [1, 3]. These heavy ions were shown to be directly linked to the photochemical haze formation mechanism at ~1000 km [4]. Magnetospheric ions have also been shown to contribute to a fraction of upper ionosphere ionization [5, 6, 4], espacially during nightside [7]. Aerosols embedded in an ionosphere acquire preferentially negative charges due to the higher mobility of electrons relative to positive ions, thereby depleting the electron population. Below 1000 km, the ionosphere behaves as a dusty plasma [4], and ions become the dominant mobile charge carriers, as the negatively charged dust grains are comparatively heavier. Aerosol growth in the ionosphere occurs primarily through collisions between negatively charged haze particles and positive ions, and their electric charge governs their coagulation efficiency, and coupling to the ionospherere. 

In 2005, the Huygens probe obtained in situ measurements of conductivity during its descent using the Mutual Impedance (MI) and Relaxation Probe (RP) sensors [8, 9], revealing a second ionospheric layer near 65 km [10, 11] where the electron density peaks [12]. Unlike the upper ionosphere, this layer is almost independent of the solar cycle, as ionization is driven by the most energetic galactic cosmic rays (GCRs) particles capable of penetrating to such depths [13, 14, 15, 16]. GCRs ionize neutral constituents, producing positive ions and electrons. MI and RP sensors data indicate peak charge densities of positive ions and electrons between 60 and 70 km [8, 9]. 

The conductivity of the lower atmosphere is a topic of investigation for reproducing Huygens measurements [17]. Recent studies [12] find that the inclusion of aerosol particles reduces charge concentrations, with the reduction being more pronounced for electrons, and deduced that the electron trapping in aerosols leads a charge per radius of ≈ 60 charges/µm. 

Here we use a self consistent model of photochemistry, microphysics and radiative transfer to investigate charge distribution across a full atmopsheric column. The model was originally developed by [18, 19, 20] and previously applied to the study of the upper ionosphere [21, 4]. We adapted its latest version, used by [22] to study the southern polar region during the post-equinox period. In Titan’s atmosphere, haze particles acquire charge through two competing mechanisms : (i) photoelectric emission driven by the solar UV flux, and (ii) collisional capture of ions and electrons from the gas phase. To compute those rates, we follow [23] for the photoelectric yield and electron sticking efficiency, and [24] for the collisional charging rates. Electron and ion density profiles, along with charged particles connect the upper and lower ionosphere. We will discuss the resulting particle charge distributions and compare with available observations through out the atmosphere. Moreover, we will evaluate the feedback between the gas and particulate phases through the involved heterogeneous processes.

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