Mars Express investigations of the Martian ionosphere using ASPERA-3 and new MARSIS fixed frequency modes

The plasma package Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) onboard Mars Express has observed ions and electrons accelerated by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar when it operates in its active ionospheric sounding mode. To better study the processes involved, new operational modes for MARSIS have been developed. In the first, a fixed frequency mode, the transmitter does not sweep over a range of frequencies, as normal, but instead transmits pulses at a fixed frequency. This frequency has been chosen to be close to the fundamental frequency of the local ionosphere around the spacecraft, which in all cases is < 350 kHz. Also, an alternating mode has been introduced, where observations in the fixed frequency mode are interleaved with observations that sweep over frequencies in order to investigate how ions are accelerated. Here we describe the new operational modes and present the results of several tests performed in the years 2020, 2021, and 2023.