David Morgan

Radar Soundings of the Ionosphere of Mars

We report the first radar soundings of the ionosphere of Mars with the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on board the orbiting Mars Express spacecraft. Several types of ionospheric echoes are observed, ranging from vertical echoes caused by specular reflection from the horizontally stratified ionosphere to a wide variety of oblique and diffuse echoes. The oblique echoes are believed to arise mainly from ionospheric structures associated with the complex crustal magnetic fields of Mars. Echoes at the electron plasma frequency and the cyclotron period also provide measurements of the local electron density and magnetic field strength.

Solar control of radar wave absorption by the Martian ionosphere

[1]xa0The MARSIS active sounder aboard the Mars Express spacecraft, under certain conditions in the Martian ionosphere, fails to detect the planetary surface. We have generated a statistical measure of the surface reflection visibility, which we plot as a time series and compare with both in situ particle data taken at Mars and solar x-ray data taken at Earth. We show that loss of the surface signal is closely related to the influx of solar protons at tens of MeV energies. We infer that the influx of high energy solar protons causes impact ionization, increasing the electron density in the Martian ionosphere. At altitudes close to or below 100 km, where the electron-neutral collision frequency is high and the electron density typically has a local maximum, the increased electron density raises the damping coefficient to levels sufficient for complete absorption of the sounding wave over an altitude range of a few tens of kilometers.

A clear view of the multifaceted dayside ionosphere of Mars

[1]xa0By examining electron density profiles from the Mars Express Radio Science Experiment MaRS, we show that the vertical structure of the dayside ionosphere of Mars is more variable and more complex than previously thought. The top of the ionosphere can be below 250xa0km (25% occurrence rate) or above 650xa0km (1%); the topside ionosphere can be well-described by a single scale height (10%) or two/three regions with distinct scale heights (25% or 10%), where those scale heights range between tens and hundreds of kilometers; the main layer of the ionosphere can have a sharply pointed (5%), flat-topped (6%), or wavy (8%) shape, in contrast to its usual Chapman-like shape; a broad increase in electron density is detected at 160–180xa0km (10%); a narrow increase in electron density is sometimes found in strongly-magnetized regions; and an additional layer is present between the M1 and M2 layers (3%).

Absorption of MARSIS radar signals: Solar energetic particles and the daytime ionosphere

[1]xa0We present observations from the subsurface sounding mode of the MARSIS instrument onboard Mars Express that imply radar wave absorption because of increased amounts of ionization in the upper Martian atmosphere during the fall of 2005. On at least two occasions these radar disruptions lasted for several days and we find that these periods are correlated with periods when other instruments indicate elevated levels of solar energetic particles. Another disruption lasted for over a month and we find that it was likely caused by a combination of solar activity and observing through the daytime ionosphere. There is no evidence in the present results for the constant ionospheric layer predicted to be created by the normal infall of cosmic dust, although the effects of enhanced infall during meteor showers remains uncertain. The effects of dust activity also remain uncertain but will be tested during the 2007 dust season.

Oblique reflections in the Mars Express MARSIS data set: Stable density structures in the Martian ionosphere

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the European Space Agencys Mars Express (MEX) spacecraft routinely detects evidence of localized plasma density structures in the Martian dayside ionosphere. Such structures, likely taking the form of spatially extended elevations in the plasma density at a given altitude, give rise to oblique reflections in the Active Ionospheric Sounder data. These structures are likely related to the highly varied Martian crustal magnetic field. In this study we use the polar orbit of MEX to investigate the repeatability of the ionospheric structures producing these anomalous reflections, examining data taken in sequences of multiple orbits which pass over the same regions of the Martian surface under similar solar illuminations, within intervals lasting tens of days. Presenting three such examples, or case studies, we show for the first time that these oblique reflections are often incredibly stable, indicating that the underlying ionospheric structures are reliably reformed in the same locations and with qualitatively similar parameters. The visibility, or lack thereof, of a given oblique reflection on a single orbit can generally be attributed to variations in the crustal field within the ionosphere along the spacecraft trajectory. We show that, within these examples, oblique reflections are generally detected whenever the spacecraft passes over regions of intense near-radial crustal magnetic fields (i.e., with a “cusp-like” configuration). The apparent stability of these structures is an important feature that must be accounted for in models of their origin.

A new semiempirical model of the peak electron density of the Martian ionosphere

[1]xa0Observations of the ionosphere of Mars have now reached a sufficient number to begin discussions on how best to create an empirically based model of its global morphology. Here we use nearly 113,000 values of maximum electron density (Nmax) obtained from 2005 to 2012 by the Mars Advanced Radar for Subsurface and Ionospheric Sounding on board the Mars Express satellite. At the altitude of peak density, photochemical processes dominate over dynamical effects, and thus values of Nmax can be organized using three basic parameters: solar flux, solar zenith angle, and orbital distance. The model can be used retrospectively to provide Nmax values for any date starting in 1965. Forecasts are possible using predicted solar flux values extending to the end of solar cycle 24. Validations using Viking in situ observations and radio occultation measurements from several satellite missions provide encouraging results for a useful semiempirical climatological model.

MARSIS remote sounding of localized density structures in the dayside Martian ionosphere: A study of controlling parameters

Enhanced topside electron densities in the dayside Martian ionosphere have been repetitively observed in areas of near-radial crustal magnetic fields, for periods of tens of days, indicating their long-term spatial and temporal stability despite changing solar wind conditions. We perform a statistical study of these density structures using the ionospheric mode of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express. We estimate the apparent extents of these structures relative to the altitude of the surrounding ionosphere. The apex of the density structures often lies higher than the surrounding ionosphere (median vertical extent of 18u2009km), which indicates upwellings. These structures are much wider than they are high, with latitudinal scales of several degrees. The radar reflector regions are observed above both moderate and strong magnetic anomalies, and their precise locations and latitudinal extents match quite well with the locations and latitudinal extents of magnetic structures of given magnetic polarity (oblique to vertical fields), which happen to be regions where the field lines are open part of the time. The majority of the density structures occur in regions where ionospheric plasma is dominant, indicating closed field regions shielded from shocked solar wind plasma.

Evidence of scale height variations in the Martian ionosphere over the solar cycle

Solar cycle variations in solar radiation create density changes in any planetary ionosphere, which are well established in the Earths case. At Mars, however, the ionospheric response to such changes is not well understood. We show the solar cycle impact on the topside ionosphere of Mars, using data from the Mars Advance Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express. Topside ionospheric variability during the solar cycle is analyzed through neutral scale height behavior. For moderate and high solar activity phases, the topside electron density profile is reproduced with an altitude-variable scale height. However, for the period of extremely low solar activity in 2008 and 2009, the topside was smaller in density than in the other phases of the solar cycle, and there is evidence that it could be reproduced with either a constant scale height or a height-variable scale height with lower electron density. Moreover, the ionosphere during this time did not show any apparent dependence on the EUV flux. This singular behavior during low solar activity may respond to the presence of an induced magnetic field which can penetrate to lower ionospheric altitudes than in other phases of the solar cycle due to the reduced thermal pressure. Numerical simulations of possible scenarios for two different solar cycle phases indicate that this hypothesis is consistent with the observations.

A case study of a density structure over a vertical magnetic field region in the Martian ionosphere

One of the discoveries made by the radar sounder on the Mars Express spacecraft is the existence of magnetically controlled structures in the ionosphere of Mars, which result in bulges in the ionospheric electron density contours. These bulges lead in turn to oblique echoes, which show up as hyperbola-shaped features in the echograms. A hyperbola-shaped feature observed over an isolated region of strong crustal magnetic field is associated with a plasma cavity in the upper ionosphere and a corresponding density enhancement in the lower levels of the ionosphere. We suggest that along open magnetic field lines, the solar wind electrons are accelerated downward and the ionospheric ions are accelerated upward in a manner similar to the field line-driven auroral acceleration at Earth. This heating due to precipitating electrons may cause an increase in the scale height and may drive a loss of ionospheric plasma at high altitudes.

The Transient Topside Layer and Associated Current Sheet in the Ionosphere of Mars

Radar soundings from the MARSIS instrument on board the Mars Express spacecraft have shown that transient layers exist in the dayside upper ionosphere of Mars. The most prominent of these features is a second layer at an altitude near 200u2009km, well above that of the main photo-ionization layer. While the general properties of this layer have been studied previously, the inner workings of this layer, and the mechanisms that drive it, are only now becoming clear. With the addition of solar wind, particle, and magnetic field instruments carried by the MAVEN spacecraft, a more detailed analysis has now been completed. Results show the existence of local current sheets in the upper Martian ionosphere in conjunction with the appearance of the second layer. These currents reveal an important magnetic aspect to the transient layer, and point to a variety of possible explanations for its formation, including the Kelvin-Helmholtz instability, magnetic flux ropes, x-type magnetic reconnection, and solar wind magnetic field rotations.