D. D. Morgan

Nightside ionosphere of Mars studied with local electron densities: A general overview and electron density depressions

altitude of about 275 km, we measured the maximum average densities as 1000 cm −3 on the nightside. The electron density profiles on the nightside are highly variable. An inverse exponential relationship is observed between the electron density and the altitude. At low altitudes, the median electron density decreases with increasing solar zenith angle (SZA). However, at high altitudes no dependence on SZA is observed. Steep electron density gradients, similar to the ionopause at Venus, are also observed in 15% of the passes in the nightside ionosphere. A commonly encountered structure on the nightside is an ionospheric density depression, which is a deep trough in the electron density. Nightside density depressions are large features, with an average width of 950 km. In some cases, the depressions in MARSIS data are associated with ion flow features in the Analyzer of Space Plasma and Energetic Atoms (ASPERA‐3) data. In other cases, the depressions correspond to density depletion regions. Half of the depressions are aligned with the edge of the dayside‐generated photoelectrons. It is concluded that several different conditions can cause the electron density depressions.

Effects of a strong ICME on the Martian ionosphere as detected by Mars Express and Mars Odyssey

We present evidence of a substantial ionospheric response to a strong interplanetary coronal mass ejection (ICME) detected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board the Mars Express (MEX) spacecraft. A powerful ICME impacted the Martian ionosphere beginning on 5 June 2011, peaking on 6 June, and trailing off over about a week. This event caused a strong response in the charged particle detector of the High-Energy Neutron Detector (HEND) on board the Odyssey spacecraft. The ion mass spectrometer of the Analyzer of Space Plasmas and Energetic Atoms instrument on MEX detected an increase in background counts, simultaneous with the increase seen by HEND, due to the flux of solar energetic particles (SEPs) associated with the ICME. Local densities and magnetic field strengths measured by MARSIS and enhancements of 100 eV electrons denote the passing of an intense space weather event. Local density and magnetosheath electron measurements and remote soundings show compression of ionospheric plasma to lower altitudes due to increased solar wind dynamic pressure. MARSIS topside sounding of the ionosphere indicates that it is extended well beyond the terminator, to about 116° solar zenith angle, in a highly disturbed state. This extension may be due to increased ionization due to SEPs and magnetosheath electrons or to plasma transport across the terminator. The surface reflection from both ionospheric sounding and subsurface modes of the MARSIS radar was attenuated, indicating increased electron content in the Mars ionosphere at low altitudes, where the atmosphere is dense.

Total electron content in the Martian atmosphere: A critical assessment of the Mars Express MARSIS data sets

The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of the Martian ionosphere because it contains information on the total amount of free electrons, the m ...

Control of the topside Martian ionosphere by crustal magnetic fields

We present observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument onboard Mars Express of the thermal electron plasma density of the Martian ionosphere and investigate the extent to which it is influenced by the presence of Marss remnant crustal magnetic fields. We use locally measured electron densities, derived when MARSIS is operating in active ionospheric sounding (AIS) mode, covering an altitude range from ∼300 km to ∼1200 km. We compare these measured densities to an empirical model of the dayside ionospheric plasma density in this diffusive transport-dominated regime. We show that small spatial-scale departures from the averaged values are strongly correlated with the pattern of the crustal fields. Persistently elevated densities are seen in regions of relatively stronger crustal fields across the whole altitude range. Comparing these results with measurements of the (scalar) magnetic field also obtained by MARSIS/AIS, we characterize the dayside strength of the draped magnetic fields in the same regions. Finally, we provide a revised empirical model of the plasma density in the Martian ionosphere, including parameterizations for both the crustal field-dominated and draping-dominated regimes.

Interpreting Mars ionospheric anomalies over crustal magnetic field regions using a 2‐D ionospheric model

The spatially inhomogeneous, small-scale crustal magnetic fields of Mars influence the escape of planetary atmospheric species and the interaction of the solar wind with the ionosphere. Understanding the plasma response to crustal magnetic field regions can therefore provide insight to ionospheric structure and dynamics. To date, several localized spatial structures in ionospheric properties that have been observed over regions of varying magnetic field at Mars have yet to be explained. In this study, a two-dimensional ionospheric model is used to simulate the effects of field-aligned plasma transport in regions of strong crustal magnetic fields. Resulting spatial and diurnal plasma distributions are analyzed and found to agree with observations from several spacecraft and offer compelling interpretations for many of the anomalous ionospheric behaviors observed at or near regions of strong crustal magnetic fields on Mars.

Enhanced ionization of the Martian nightside ionosphere during solar energetic particle events

Electron densities in the Martian nightside ionosphere are more than 90% of time too low to be detected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding radar sounder on board the Mars Express spacecraft. However, the relative number of ionograms with peak electron density high enough to be detected represents a good statistical proxy of the ionospheric density. We focus on solar energetic particle (SEP) events, and we analyze their effects on ionospheric formation. SEP time intervals were identified in situ using the background counts recorded by the ion sensor of the ASPERA-3 instrument on board Mars Express. We show that peak electron densities during the SEP events are large enough to be detected in more than 30% of measurements, and, moreover, the reflections of the sounding signal from the ground almost entirely disappear. Nightside electron densities during SEP events are thus substantially increased as compared to normal nightside conditions.

An ionized layer in the upper atmosphere of Mars caused by dust impacts from comet Siding Spring

We report the detection of a dense ionized layer in the upper atmosphere of Mars caused by the impact of dust from comet Siding Spring. The observations were made by the ionospheric radar sounder on the Mars Express spacecraft during two low-altitude passes approximately 7 h and 14 h after closest approach of the comet to Mars. During these passes an unusual transient layer of ionization was detected at altitudes of about 80 to 100 km with peak electron densities of (1.5 to 2.5) × 105 cm−3, much higher than normally observed in the Martian ionosphere. From comparisons to previously observed ionization produced by meteors at Earth and Mars, we conclude that the layer was produced by dust from the comet impacting and ionizing the upper atmosphere of Mars.

MARSIS observations of the Martian nightside ionosphere dependence on solar wind conditions

Despite the absence of solar radiation on the Martian nightside, a weak, irregular, and variable ionosphere is produced there. The nightside ionosphere is thought to be maintained by two main sources: dayside-nightside plasma transport and electron precipitation. Observations by Mars Express (MEX) and Mars Global Surveyor (MGS) have shown that these plasma sources are either hindered or favored by the presence of strong crustal magnetic fields and that these effects are modulated by external parameters, such as the solar wind dynamic pressure and the orientation of the interplanetary magnetic field (IMF). These external drivers are expected to influence the supply of plasma to the nightside and thus the formation of the irregular nightside ionosphere. We here present a statistical study of the Martian ionosphere at solar zenith angle greater than 107° from November 2005 to May 2006, using remote measurements of ionospheric echoes with the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) radar sounder onboard MEX and using MGS-based proxies for the solar wind dynamic pressure and the IMF clock angle. We find that the peak densities increase with the dynamic pressure and also that cases of very high peak density are almost always associated with Westward IMF orientation. We find that, using MEX/ASPERA-3 electron data, these cases often seem to be linked to accelerated electrons. Plasma transport is known to be important in the near nightside. On the other hand, electron precipitation prevails when the dynamic pressure is high enough to compress the ionosphere and in vertical field regions where the IMF orientation matters.

Solar cycle variations in the ionosphere of Mars as seen by multiple Mars Express datasets

The response of the Martian ionosphere to solar activity is analyzed by taking into account variations in a range of parameters during four phases of the solar cycle throughout 2005–2012. Multiple Mars Express data sets have been used (such as Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) in Active Ionospheric Sounding, MARSIS subsurface, and MaRS Radio Science), which currently cover more than 10 years of solar activity. The topside of the main ionospheric layer behavior is empirically modeled through the neutral scale height parameter, which describes the density distribution in altitude, and can be used as a dynamic monitor of the solar wind-Martian plasma interaction, as well as of the mediums temperature. The main peak, the total electron content, and the relationship between the solar wind dynamic pressure and the maximum thermal pressure of the ionosphere with the solar cycle are assessed. We conclude that the neutral scale height was different in each phase of the solar cycle, having a large variation with solar zenith angle during the moderate-ascending and high phases, while there is almost no variation during the moderate-descending and low phases. Between end-2007 and end-2009, an almost permanent absence of secondary layer resulted because of the low level of solar X-rays. Also, the ionosphere was more likely to be found in a more continuously magnetized state. The induced magnetic field from the solar wind, even if weak, could be strong enough to penetrate more than at other solar cycle phases.

Empirical model of the Martian dayside ionosphere: Effects of crustal magnetic fields and solar ionizing flux at higher altitudes

We use electron density profiles measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on board the Mars Express spacecraft to investigate the effects of possible controlling parameters unconsidered in the empirical model of Nemec et al. (2011, hereafter N11). Specifically, we focus on the effects of crustal magnetic fields and F-10.7 proxy of the solar ionizing flux at higher altitudes. It is shown that while peak electron densities are nearly unaffected by crustal magnetic fields, electron densities at higher altitudes are significantly increased in areas of stronger magnetic fields. The magnetic field inclination appears to have only a marginal effect. Moreover, while the N11 empirical model accounted for the variable solar ionizing flux at low altitudes, the high-altitude diffusive region was parameterized only by the solar zenith angle and the altitude. It is shown that this can lead to considerable inaccuracies. A simple correction of the N11 model, which takes into account both the crustal magnetic field magnitude and the effect of F-10.7 at higher altitudes, is suggested.