Jacob Parrott

Radio occultation (RO) is a very powerful technique as it offers great opportunities to study planetary atmospheres, providing information about their ionosphere and neutral atmosphere. Standard methods use a radio link at S and/or X bands between a spacecraft orbiting a planet and a ground station on the Earth. At Mars, such measurements have been conducted since the 60s. Three most recent data sets are from Mars Global Surveyor, Mars Express and the Mars Atmosphere and Volatile Evolution satellites. Furthermore, the possibility to obtain information about the Martian atmosphere with mutual RO events, using data from NASA Mars Odyssey and Mars Reconnaissance Orbiters, has been demonstrated by NASA scientists in 2015. Taking advantage of two European spacecraft in orbit around Mars, the ESA is currently performing an experiment that consists of mutual radio occultations between Mars Express (MEX) and ExoMars Trace Gas Orbiter (TGO). In preparation for MEX and TGO data inversion and analysis, a simulation‐based strategy has been adopted and an algorithm, including the associated software, able to retrieve vertical electron density profiles from Doppler shift measurements has been implemented and validated. Subsequently, to test the mentioned algorithm with experimental data, the same three mutual RO events considered by NASA scientists have been re‐processed. In this work, the research activities carried out through the simulation studies and the results obtained by the application of the mentioned inversion algorithm to experimental data are presented.

Solar energetic events can have considerable effects on planetary ionospheres. However, the erratic nature of these solar energetic events make observations difficult. Here we show a mutual radio occultation observation, which serendipitously occurred just 10 minutes after a large solar flare impacted Mars. This resulted in the largest lower ionospheric layer ever recorded, where it was 278% its typical size. We used in-situ soft x-ray irradiance measurements to show a threefold increase in flux. This infers a different relation of soft X-ray to this layer’s density than previously thought, with variations depending on the amount of spectrum ‘hardening’ leading to the increase of ionisation from secondaries.

This study presents a comprehensive analysis of the Martian ionosphere using Mutual Radio Occultation (RO) observations between Mars Express and Trace Gas Orbiter, featuring 71 full vertical profiles out of a total of 124 measurements. Among these, 35 measurements were taken from regions with Solar Zenith Angles lower than 40°. The profiles also represent the largest data set for the lower M1 ionospheric layer during the midday ever measured. This paper has also been submitted with a comprehensive data set, which marks the first time MEX‐TGO RO data has been made available to the community. Additionally, neutral temperature profiles have been extracted from the measurements. We find unexpected features in the lower thermosphere temperature behavior which we conclude is likely due to the effects of local circulation and associated dynamical heating rather than solar‐controlled.

Spacecraft‐to‐spacecraft radio occultations experiments are being conducted at Mars between Mars Express (MEX) and Trace Gas Orbiter (TGO), the first ever extensive inter‐spacecraft occultations at a planet other than Earth. Here we present results from the first 83 such occultations, conducted between 2 Nov 2020 and 5th of July 2023. Of these, 44 observations have to‐date resulted in the extraction of vertical electron density profiles. These observations are the successful results of a major feasibility study conducted by the European Space Agency to use pre‐existing relay communication equipment for radio science purposes. Mutual radio occultations have numerous advantages over traditional spacecraft‐to‐ground station occultations. In this work, we demonstrate how raw data are transformed into electron density values and validated with models and other instruments.

Conventional Radio Occultation measurements take place between a spacecraft in orbit around a planet (or during an occasional fly-by), and a receiving ground station on the Earth. When the straight line between the sender and receiver crosses the ionosphere and the atmosphere of the planet the signal is affected by the electrons in the ionosphere and the molecules in the neutral atmosphere. This signal change is observed as a slight frequency shift in the received radio signal with respect to the transmitted signal. The shift is very small compared to the Doppler shift in the frequency due to the differential relative velocity of the spacecraft and the Earth, but can for a stable system be isolated, and profiles of electron density in the ionosphere and profiles of atmospheric density in the neutral atmosphere can be derived.Here we are attempting to do occultation measurements between two spacecraft at Mars, Mars Express and ExoMars Trace Gas Orbiter, using the UHF link normally used for communicating between the orbiters and the landers on the surface of Mars, rather than using the X- or S-band transmitter used for the communication between a spacecraft and the Earth. This was first exercised between NASA’s Odyssey and MRO at three occasions in 2007 (Ao, et al., Radio Sci., 2015), but has not been repeated since then.Such a Mutual Occultation technique has several significant advantages over the traditional spacecraft to Earth occultation measurements. In particular, it allows a much more even distribution of the samples over the surface of the planet and in time, and the observations are independent of Earth occultation seasons. Due to the short period of the orbit of TGO many occultations take place every day. The distance between the two spacecraft is much smaller than the distance to the Earth and therefore a much higher signal to noise ratio can be achieved, resulting in very low statistical errors. The Mutual Occultation s/c to s/c link also avoids that the signal passes through the Earth’s ionosphere and atmosphere. The compensation for the Earth atmosphere and ionosphere is far from perfect and results in major contributions to the systematic errors. An additional positive effect is that the interplanetary plasma disturbance to the signal is a minimum due to the short distance between the two s/c and the large distance from the Sun.The first experiment was carried out on the 2nd of November 2020 and demonstrated the feasibility of the technique. Several more experiments took place during November-December. These initial test used a lander (Beagle2) hail signal from Mars Express/Melacom, consisting of a series of 20 seconds of modulated and unmodulated transmissions interspersed with 2 seconds without transmission. In March 2021 a software update was applied to Melacom, which allowed it to generate an uninterrupted, unmodulated carrier-only signal, since. This has improved the performance and the quality of the measurements significantly. To date four passes with this improved performance have been executed. These data are presently being analysed. This presentation will summarise recent simulations and show the latest results from these new experimental measurements. Example of Occultation Geometry: The orbits of TGO and MEX are indicated by tan respective red lines and the direct path of the radio signal is indicated by a white line. The part of the propagation through the atmosphere is indicated by a yellow box.

Radio Occultation (RO) is a very powerful technique to probe a planetary atmosphere, in providing vertical density profiles of the neutral atmosphere and ionosphere. The standard method uses a radio link between a spacecraft and an Earth ground station. Nevertheless, the possibility to obtain information about the Martian atmosphere with mutual RO events, using data from NASA Mars Odyssey and Mars Reconnaissance Orbiters (MRO), has been demonstrated by Ao et al. (2015).Taking advantage of two European spacecraft in orbit around Mars, the European Space Agency is currently preparing experiments of mutual RO between Mars Express (MEX) and the ExoMars Trace Gas Orbiter (TGO). In preparation of MEX and TGO data inversion and analysis, a simulation-based strategy has been adopted and an algorithm able to retrieve vertical electron density profiles from Doppler shift measurements has been implemented and validated. Subsequently, in order to test the mentioned algorithm with experimental data, the same three RO events considered in the paper by Ao et al. (2015) have been processed. In particular, for each RO event, having the information about the satellites’ orbit, the (excess) Doppler shift values corresponding to the Mars Odyssey-MRO ray-paths have been converted to bending angles as a function of impact parameter. Then, assuming a spherical symmetry (Fjeldbo et al., 1971) for the ionosphere electron density, the bending angles have been transformed (through Abel integral) to a vertical refractivity profile, which, in turn, has been converted to an ionospheric electron density profile.In this work, the results obtained by the application of the mentioned inversion algorithm to experimental data will be presented, with particular focus on the retrieval of the ionospheric electron density profiles.ReferencesAo, C. O., C. D. Edwards Jr., D. S. Kahan, X. Pi, S. W. Asmar, and A. J. Mannucci (2015), A first demonstration of Mars crosslink occultation measurements, Radio Sci., 50, 997–1007, doi:10.1002/2015RS005750.Fjeldbo, G., A. J. Kliore, and V. R. Eshleman (1971), The neutral atmosphere of Venus as studied with the Mariner V radio occultationexperiments, Astron. J., 76, 123–140.

Spacecraft-to-spacecraft radio occultations experiments are being conducted at Mars between the Mars Express (MEX) and Trace Gas Orbiter (TGO) spacecraft, the first ever periodic inter-spacecraft occultations at a planet other than Earth. Here we present results from the first 83 such occultations, conducted between 2\textsuperscript{nd} Nov 2020 and 5\textsuperscript{th} of July 2023. Of these, 44 observations have to-date resulted in the extraction of vertical electron density profiles. These observations are the successful results of a major feasibility study conducted by the European Space Agency to use pre-existing relay communication equipment for radio science purposes. Mutual radio occultations have numerous advantages over traditional spacecraft-to-ground station occultations. In this work, we demonstrate how raw data are transformed into electron density values and validated with models and other instruments.