Origin of the Extended Mars Radar Blackout of September 2017

Abstract

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express, which operates between 0.1 and 5.5 MHz, suffered from a complete blackout for 10 days in September 2017 when observing on the nightside (a rare occurrence). Moreover, the Shallow Radar (SHARAD) onboard the Mars Reconnaissance Orbiter, which operates at 20 MHz, also suffered a blackout for three days when operating on both dayside and nightside. We propose that these blackouts are caused by solar energetic particles of few tens of keV and above associated with an extreme space weather event between 10 and 22 September 2017, as recorded by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Numerical simulations of energetic electron precipitation predict that a lower O 2 + nighttime ionospheric layer of magnitude~10 10 m −3 peaking at~90-km altitude is produced. Consequently, such a layer would absorb radar signals at high frequencies and explain the blackouts. The peak absorption level is found to be at 70-km altitude. Plain Language Summary Several instrument operations, as well as communication systems with rovers at the surface, depend on radio signals that propagate throughout the atmosphere of Mars. This is the case also for two radars that are currently working in Mars' orbit, sounding the ionosphere, surface, and subsurface of the planet. In mid-September 2017, a powerful solar storm hit Mars, producing a large amount of energetic particle precipitation over a 10-day period. We have found that high-energy electrons ionized the atmosphere of Mars, creating a dense layer of ions and electrons at~90 km on the Martian nightside. This layer attenuated radar signals continuously for 10 days, stopping the radars to receive any signal from the planetary surface. In this work, we assess the properties of this layer in order to understand the implications of this kind of phenomenon for radar performance and communications.

Mars Express Observations of Cold Plasma Structures in the Martian Magnetotail

“…In the right half of Figure 3, where MEX descends at lower than 1,200 km altitudes, and for sounding frequencies greater than 2.5 MHz, MARSIS should detect the reflected signal from the surface when there is no absorption in the lower atmosphere caused by a layer of energetic particles. Data from the Solar Energetic Particle (SEP) instrument (Larson et al., 2015) on board MAVEN (not shown here) indicate an increase in the flux of energetic electrons during MEX orbit 16148, which explains the attenuation of the MARSIS signal resulting in the lack of a surface signature (Espley et al., 2007; Němec et al., 2014; Sánchez‐Cano et al., 2019). The absence of the surface echo in orbit 16130 though is rather baffling since the flux of energetic charged particles detected was insignificant at this time.…”

“…Surface echoes are largely affected by low‐altitude ionization. Enhanced plasma densities at collisional altitudes cause efficient absorption of the MARSIS signal and consequently, weaker surface reflections or no reflections at all are observed (Espley et al., 2007; Němec et al., 2014; Sánchez‐Cano et al., 2019). Local physical properties of the surface may also play a role in determining how efficiently the MARSIS signal is reflected.…”

“…The formation process of the nightside ionosphere became somewhat clearer when it was shown that additionally to the ion transport from the dayside, magnetospheric electrons have sufficient energy to act as another source through electron impact ionization, assuming electron fluxes at high altitudes do actually precipitate into the atmosphere (Haider et al, 1992;Verigin et al, 1991). Fox et al (1993) calculated that the maximum electron densities in the nightside ionosphere, including both ion transport from the dayside and electron precipitation, at altitudes 159-179 km should lie within the range of 1.3-1.9 × 10 4 cm −3 , while at an altitude of ∼90 km, ionospheric modeling predicts that electron precipitation after a solar storm can result in ion densities of 10 4 cm −3 (Sánchez-Cano et al, 2019). Němec et al (2010) analyzed thousands of ionograms from the nigthside ionosphere and concluded that the majority of the ionograms with a detected ionospheric echo show peak electron densities lower than ∼2 × 10 4 cm −3 .…”

Effects of the 10 September 2017 Solar Flare on the Density and Composition of the Thermosphere of Mars

“…At the time of the baseline orbits, the Martian upper atmosphere was not contaminated by other significant sources of density and compositional change such as significant dust storms (not present), CMEs (arrived later), or SEPs (primarily affect altitudes lower than those observed by NGIMS) (C. O. Lee, Jakosky, et al., 2018; Mayyasi et al., 2018; Sánchez‐Cano et al., 2019). Other flares of magnitude M and greater from same solar event were also not present at the time of or directly preceding the five baseline orbits (Chamberlin et al., 2018).…”

Comparison of terrestrial and Martian TEC at dawn and dusk during solstices

“…The Martian nightside ionosphere near the terminator is maintained thanks to dayto-night plasma transport (e.g., Chen et al 1978;Cui et al 2015) together with impact ionisation by precipitating electrons (e.g., Cao et al 2019), where densities of the major ions decrease with the solar zenith angle (SZA) across the terminator (e.g., Girazian et al 2017;Withers et al 2012). Far from the terminator, electron precipitation is the dominant source of ionisation, which is more important over regions of strong crustal magnetic fields (as will be described in the Section "Planetary environments") (e.g., Nĕmec et al 2010;Lillis et al 2018), and during solar storms (e.g., Sánchez-Cano et al 2019). Transport also plays a large role maintaining the nighttime ionosphere at Earth, with current systems leading to E×B drifts and neutral winds driving field-aligned motions that may both lift and move plasma to less dense regions where ion loss through photochemical recombination occurs more slowly (e.g., Heelis 2004).…”