Alfvén Waves at Mars
Alfvén Waves Across Heliophysics Geophysical Monograph Series(2024)
摘要
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 ∼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.
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