Alfvén Waves at Mars

Abstract

The solar wind upstream of Mars's bow shock can be described in terms of Alfvénic turbulence, with an incompressible energy cascade rate of $10^{-17}$ J m${}^{-3}$ s${}^{-1}$ at magnetohydrodynamics (MHD) scales. The solar wind has more Alfvén waves propagating outwards from the Sun (than inwards) and a median Alfvén ratio of $\sim 0.33$. Newly ionized planetary protons associated with the extended hydrogen corona generate waves at the local proton cyclotron frequency. These 'proton cyclotron waves' (PCW) mostly correspond to fast magnetosonic waves, although the ion cyclotron (Alfvénic) wave mode is possible for large Interplanetary Magnetic Field cone angles. PCW do not show significant effects on the solar wind energy cascade rates at MHD scales but could affect smaller scales. The magnetosheath displays high amplitude wave activity, with high occurrence rate of Alfvén waves. Turbulence appears not fully developed in the magnetosheath, suggesting fluctuations do not have enough time to interact in this small-size region. Some studies suggest PCW affect turbulence in the magnetosheath. Overall, wave activity is reduced inside the magnetic pile-up region and the Martian ionosphere. However, under certain conditions, upstream waves can reach the upper ionosphere. So far, there have not been conclusive observations of Alfvén waves in the ionosphere or along crustal magnetic fields, which could be due to the lack of adequate observations.

Accepted for publication in 'Alfvén Waves across Heliophysics: Progress, Challenges, and Opportunities' (American Geophysical Union). Further reproduction or electronic distribution is not permitted