Conductivities of Titan's Dusty Ionosphere

• Published in: Journal of geophysical research. Space physics Vol. 127; no. 2
• Main Authors: Shebanits, O., Wahlund, J.‐E., Waite, J. H., Dougherty, M. K.
• Format: Journal Article
• Language: English
• Published: 01.02.2022
• Subjects: Cassini; conductivity; dusty plasma; ionosphere; Titan
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2021JA029910

Titan's ionosphere hosts a globally distributed non‐trivial dusty ion‐ion plasma, providing an environment for studies of dusty ionospheres that is in many aspects unique in our solar system. Thanks to the Cassini mission, Titan's ionosphere also features one of the largest dusty plasma data sets from 126 flybys of the moon over 13 years, from 2004 to 2017. Recent studies have shown that negatively charged dust dramatically alters the electric properties of plasmas, in particular planetary ionospheres. Utilizing the full plasma content of the moon's ionosphere (electrons, positive ions, and negative ions/dust grains), we derive the electric conductivities and define the conductive dynamo region. Our results show that using the full plasma content increases the Pedersen conductivities at ∼1,100–1,200 km altitude by up to 35% compared to the estimates using only electron densities. The Hall conductivities are in general not affected but several cases indicate a reverse Hall effect at ∼900 km altitude (closest approach) and below. The dayside conductivities are shown to be factor ∼7–9 larger than on the nightside, owing to higher dayside plasma densities. Plain Language Summary Titan (largest moon of Saturn) is famous for its signature orange haze, formed in the top layer of its atmosphere–ionosphere. The complex organic chemistry initiated mainly by sunlight forms grains of dust that at ∼1,000 km altitude reach a few nanometers in size (comparable to finely ground flour). In the ionosphere, these grains of dust absorb the free electrons (depleting them) and become charged. Below ∼1,000 km altitude there is very little electrons and the plasma consists primarily of ions–called “ion‐ion” or “dusty” plasma. In the absence of light electrons (negative charge), the positively charged ions instead become the dominant mobile charge carrier, as the negatively charged dust is much heavier. Such a reversal of charge mobility has a large impact on the electric properties of an ionosphere, increasing its electric conductivity and changing the direction of its electric currents. We use a Cassini mission data set spanning an entire solar cycle, nearly half a Titan year (≈15 Earth years), to calculate the electric conductivities of Titan's ionosphere and show that dusty plasma typically contributes up to 35%. We also find indications of the charge mobility reversal below 1,000 km although it is not a persistent feature. Key Points Titan's ionospheric conductivities from in‐situ measurements with full plasma content (electrons, pos. ions, neg. ions/dust grains) Indications of reverse Hall effect near ∼900 km altitude Most impact of dusty plasma at ∼1,100–1,200 km altitude—up to 35% increase for Pedersen conductivities