A. J. Kopf

Transient layers in the topside ionosphere of Mars

[1]xa0Radar soundings from the MARSIS instrument on board the Mars Express spacecraft have shown that distinct layers can occur in the topside ionosphere of Mars, well above the main photo-ionization layer. These layers appear as cusps, or sometimes steps, in plots of the time delay as a function of frequency. Usually only one topside layer is observed, typically at altitudes from 180 to 240 km. However, occasionally an additional layer occurs at even higher altitudes. The layers are transient features and are present about 60% of the time near the subsolar point, decreasing with increasing solar zenith angle to less than 5% at the terminator and the nightside. The transient nature of the layers suggests that they are produced by a dynamical process, most likely involving an interaction with the solar wind in the upper levels of the ionosphere.

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 7u2009h and 14u2009h 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 100u2009km with peak electron densities of (1.5 to 2.5)u2009×u2009105u2009cm−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.

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.

Ionospheric Irregularities at Mars Probed by MARSIS Topside Sounding

The upper ionosphere of Mars contains a variety of perturbations driven by solar wind forcing from above and upward propagating atmospheric waves from below. Here we explore the global distribution and variability of ionospheric irregularities around the exobase at Mars by analyzing topside sounding data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on board Mars Express. As irregular structure gives rise to off-vertical echoes with excess propagation time, the diffuseness of ionospheric echo traces can be used as a diagnostic tool for perturbed reflection surfaces. The observed properties of diffuse echoes above unmagnetized regions suggest that ionospheric irregularities with horizontal wavelengths of tens to hundreds of kilometers are particularly enhanced in the winter hemisphere and at high solar zenith angles. Given the known inverse dependence of neutral gravity wave amplitudes on the background atmospheric temperature, the ionospheric irregularities probed by MARSIS are most likely associated with plasma perturbations driven by atmospheric gravity waves. Though extreme events with unusually diffuse echoes are more frequently observed for high solar wind dynamic pressures during some time intervals, the vast majority of the diffuse echo events are unaffected by varying solar wind conditions, implying limited influence of solar wind forcing on the generation of ionospheric irregularities. Combination of remote and in situ measurements of ionospheric irregularities would offer the opportunity for a better understanding of the ionospheric dynamics at Mars.

Mars Initial Reference Ionosphere (MIRI) Model: Updates and Validations Using MAVEN, MEX, and MRO Data Sets

The Mars Initial Reference Ionosphere (MIRI) model is a semiempirical formulation designed to provide climatological estimates of key parameters of the Martian ionosphere. For the new MIRI-2018 version, an expanded database is used from the Mars Express/Mars Advanced Radar for Subsurface and Ionosphere Sounding/Active Ionospheric Sounding (MEX/MARSIS/AIS) instrument, consisting of 215,818 values of maximum electron density of the M2-layer (NmM2) from the years 2005–2015. These data are organized by photochemical-equilibrium equations to obtain a functional dependence of NmM2 upon solar drivers (flux and solar zenith angle). The resulting peak density is used to calibrate normalized electron density profiles [Ne (h)] derived from theory and an empirical model. The MIRI-2018 thus provides estimates of NmM2, Ne (h), and total electron content (TEC) for any date past or future. Validation using Mars Atmosphere and Volatile EvolutioN (MAVEN)’s new radio occultation science experiment (ROSE) was successful for NmM2 values, but MIRI was found to overestimate TEC values. The validation failure for TEC was traced to overestimates of plasma at low altitudes (M1 layer). A separate module for TEC was derived using 126,055 values from the Mars Reconnaissance Orbiter/SHAllow RADar (MRO/SHARAD) TEC database from 2006 to 2014. Validation of this new TEC module with ROSE data was successful. Future improvements to MIRI-2018 require new ways to characterize the bottomside ionosphere’s contribution to the TEC integral for midday (low solar zenith angle) conditions. This requires new simulation studies of secondary ionization rates by photoelectrons produced via the primary X-ray ionization process for the M1 layer.

Shapes of Magnetically Controlled Electron Density Structures in the Dayside Martian Ionosphere

Nonhorizontal localized electron density structures associated with regions of near‐radial crustal magnetic fields are routinely detected via radar oblique echoes on the dayside of Mars with the ionospheric sounding mode of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) radar onboard Mars Express. Previous studies mostly investigated these structures at a fixed plasma frequency and assumed that the larger apparent altitude of the structures compared to the normal surrounding ionosphere implied that they are bulges. However, the signal is subjected to dispersion when it propagates through the plasma, so interpretations based on the apparent altitude should be treated with caution. We go further by investigating the frequency dependence (i.e., the altitude dependence) of the shape of 48 density structure events, using time series of MARSIS electron density profiles corrected for signal dispersion. Four possible simplest shapes are detected in these time series, which can give oblique echoes: bulges, dips, downhill slopes, and uphill slopes. The altitude differences between the density structures and their edges are, in absolute value, larger at low frequency (high altitude) than at high frequency (low altitude), going from a few tens of kilometers to a few kilometers as frequency increases. Bulges dominate in numbers in most of the frequency range. Finally, the geographical extension of the density structures covers a wide range of crustal magnetic fields orientations, with near‐vertical fields toward their center and near‐horizontal fields toward their edges, as expected. Transport processes are suggested to be a key driver for these density structures.

MARSIS Observations of Field‐Aligned Irregularities and Ducted Radio Propagation in the Martian Ionosphere

Knowledge of Marss ionosphere has been significantly advanced in recent years by observations from Mars Express (MEX) and lately MAVEN. A topic of particular interest are the interactions between the planets ionospheric plasma and its highly structured crustal magnetic fields, and how these lead to the redistribution of plasma and affect the propagation of radio waves in the system. In this paper, we elucidate a possible relationship between two anomalous radar signatures previously reported in observations from the MARSIS instrument on MEX. Relatively uncommon observations of localized, extreme increases in the ionospheric peak density in regions of radial (cusp-like) magnetic fields and spread-echo radar signatures are shown to be coincident with ducting of the same radar pulses at higher altitudes on the same field lines. We suggest that these two observations are both caused by a high electric field (perpendicular to

Ionospheric Electron Densities at Mars: Comparison of Mars Express Ionospheric Sounding and MAVEN Local Measurements

mathbf{B}

Ionospheric Electron Densities at Mars: Comparison of Mars Express Ionospheric Sounding and MAVEN Local Measurements: MARS EXPRESS VS MAVEN ELECTRON DENSITIES

) having distinctly different effects in two altitude regimes. At lower altitudes, where ions are demagnetized and electrons magnetized, and recombination dominantes, a high electric field causes irregularities, plasma turbulence, electron heating, slower recombination and ultimately enhanced plasma densities. However, at higher altitudes, where both ions and electrons are magnetized and atomic oxygen ions cannot recombine directly, the high electric field instead causes frictional heating, a faster production of molecular ions by charge exchange, and so a density decrease. The latter enables ducting of radar pulses on closed field lines, in an analogous fashion to inter-hemispheric ducting in the Earths ionosphere.

The transient topside layer and associated current sheet in the ionosphere of Mars: Transient Martian Ionospheric Layer

We present the first direct comparison of Martian ionospheric electron densities measured by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) topside radar sounder on board the Mars Express spacecraft and by the Langmuir Probe and Waves (LPW) instrument on board the Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft. As low electron densities are not measured by MARSIS due to the low power radiated at low sounding frequencies, MARSIS electron density profiles between the local electron density and the first data point from the ionospheric sounding (≈104 cm−3) rely on an empirical electron density profile shape. We use the LPW electron density measurements to improve this empirical description, and thereby the MARSIS derived electron density profiles. We further analyze four coincident events, where the two instruments were measuring within a five degree solar zenith angle (SZA) interval within one hour. The differences between the electron densities measured by the MARSIS and LPW instruments are found to be within a factor of two in 90% of measurements. Taking into account the measurement precision and different locations and times of the measurements, these differences are within the estimated uncertainties.