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New experimental results on studies of sounder accelerated particles onboard Mars Express

Presentation #211.06 in the session Planetary Space Physics (Poster)

Published onOct 23, 2023
New experimental results on studies of sounder accelerated particles onboard Mars Express

Acceleration of plasma particle in the vicinity of a powerful sounding radar opens opportunities for the development of a new technique for space plasma diagnostic as well as studies of fundamental wave – particle interactions. Due to the unique combination of instruments Mars Express (MEX) is the only spacecraft, currently in operation, that can be used to study sounder accelerated ions (SAI). MEX carries Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) – a radar that routinely accelerates local ionospheric plasma up energies of 800 eV when operating in the ionospheric sounding mode. Fluxes of accelerated electrons and ions are measured by Electron Spectrometer (ELS) and Ion Mass Analyzer (IMA) of the particle suite ASPERA-3.

When operating in the ionospheric sounding mode (AIS), MARSIS transmits 150 radio pulses within 1.25 second with frequencies from 100 kHz to 5.5 MHz. Such broad frequency range is far from ideal for SAI studies, as most efficient acceleration of electrons and ions in the Martian ionosphere occurs when the pulse frequency is close to the local plasma frequency or its harmonics. In order to improve the measurements of the SAI spectra, new mode of MARSIS operation was introduced when all 150 pulses have the same fixed frequency (FF), that was chosen to be match the plasma frequency at the altitudes of 400 – 500 km. MARSIS tested the fixed frequency ionospheric mode during dozens of orbits in summer of 2021 and in winter 2023. During the tests MARSIS was alternating between AIS and FF modes in 5 minutes intervals.

Introduction of the new mode for MARSIS allowed much more detailed study of the spectra of the SAI. SAI signals have higher intensity and duration when MARSIS operates in FF mode in comparison to operations in AIS mode. Preliminary analysis of the new data revealed existence of two distinct population of the SAI, one with energies close to 100 eV and one with energies above 200 eV. Ions of the first group are typically detected coming from the directions parallel to the MARSIS antenna. It has been also found that the SAI spectra strongly depend on the local ionospheric conditions, explanation of these dependencies requires further theoretical analysis.

In addition, FF mode showed potential for the ionospheric studies. Because the frequency is fixed during an entire MARSIS sweep, the MARSIS ionograms can be used to study variation in the electron density, both local and of the underling ionospheric layer, at the timescales bellow 1.25 seconds. The new mode of operation revealed a very complex structure of the ionosphere with irregularities down to few km’s.

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