Small scale ionospheric density irregularities at Mars: the first resolved density measurements, made by the MAVEN spacecraft

Abstract:

Remote and in-situ plasma measurements have been made for several decades at Mars and have provided the basis for our understanding of the basic large scale structure and properties of the Martian ionosphere. Observations of smaller scale (~<20 km) structure and dynamics have however been lacking, due to limitations in instrument capabilities and coverage, combined with the difficulties associated in measuring thermal plasma densities with an orbiting spacecraft that travels at supersonic velocity through the ionosphere. We present here the first resolved observations of small scale (1-10 km) density irregularities at Mars, made in a special “high time cadence” ion measurement mode with the NGIMS instrument onboard the MAVEN spacecraft. The density irregularities form in a region analogous to the terrestrial E region (Ω_ion << ν_ion-neutral; Ω_electron >> ν_{electron-neutral}), and are associated with magnetic fluctuations that can occur both in and out of phase with the density fluctuations. Events demonstrate a range of characteristics and form in a variety of background conditions, and it is not yet clear if a variety of instability “types” exist at Mars, as is the case at Earth. The gradient drift and two stream instabilities are possible candidates for driving these instabilities at Mars, but further work is needed to conclusively identify the driving mechanisms. Interestingly, the draping pattern of the solar wind magnetic field about the Martian ionosphere may play a role in the formation of these instabilities, in contrast to Earth where the dipole field dominates the magnetic environment within the ionosphere. The study of small scale ionospheric irregularities at Mars is in its infancy, and comparative studies between Mars and Earth have the potential to yield new insight into how such instabilities form in ionospheres throughout the solar system.