Potential Crustal Magnetic Field Shielding Using Mars Radar Observations

Mars has no global intrinsic magnetic field; however, it does have strong remnant crustal magnetic fields concentrated mainly in the southern hemisphere of the planet.  Whether or not these crustal magnetic fields shield Mars from radiation or allow the precipitation of high energy particles is currently an unanswered question.  To help address this outstanding problem, we have analysed radar observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on Mars Express at 0.1 – 5.5 MHz and the SHAllow RADar sounder (SHARAD) on Mars Reconnaissance Orbiter (MRO) at 20 MHz, both across both the dayside and nightside of the planet. 

During solar energetic particle events, high energy particles from the sun create a sporadic ionospheric layer at 60 – 90km in Mars’ ionosphere, a region not typically ionised. This leads to the attenuation of the MARSIS and SHARAD radar signals which propagate through the sporadic layer as shown in Figure 1.  The extent of the radar signal attenuation is inversely proportional to the radar frequency.  Since SHARAD uses a higher frequency, it is in principle more difficult to attenuate and this can help us understand the threshold of solar energetic particle fluxes and energies for shielding effects.  The absence of the sporadic layer over the crustal field regions - when the sporadic layer appears elsewhere - indicates that an extra process has limited the ability of incoming solar particles to penetrate into the atmosphere.  The extent of attenuation is also affected by the energy of the incoming solar particles, whether they are electrons or ions, the Martian season, column of the atmosphere, and the solar cycle. 

In our study, we focus on three major events that occurred at different phases of the solar cycle, with all of them occurring on the day and nightsides.  We show that the lack of attenuation of the MARSIS and SHARAD radars in the southern hemisphere of Mars may be due to localised crustal magnetic field shielding during solar energetic particle events. 

By analysing where the attenuation of these two radars occurs over the planet, we have found indications that potential regional shielding may be in place over the stronger crustal magnetic fields.  We have developed a catalogue of attenuated measurements during radar observations from 2006 to 2024 and found that statistically, there is a 50% greater probability of finding partial radio attenuation outside of the strong crustal field regions in the northern hemisphere, indicating a potential magnetic shielding effect.  We compare three major solar energetic particle events, namely the 10 September 2017 event, the 15 February 2022 event, and the 20 May 2024 event, using analysis of radar attenuation as well as incoming particle fluxes and energies using Mars Atmosphere and Volatile Mission’s (MAVEN) Solar Energetic Particle (SEP) instrument.  We have found evidence of shielding during at least the 15 February 2022 event and the 20 May 2024 event, with greater particle fluxes required to form the sporadic layer over the strong crustal field regions of the planet.   This indicates a potential affect of the solar cycle in local magnetic shielding when comparing the two latest events to the 10 September 2017 event.

Figure 1: A schematic showing possible radar interactions with Mars’ ionosphere and surface.  The spacecraft housing the radar is shown in grey.  The purple shows the radar beam’s reflection from the main ionospheric layer.  The green shows the radar beam’s path in the case of a surface reflection.  The red shows the beam’s path when absorption by the sporadic layer causes attenuation and a non-reflection of the beam.