Meredith Elrod

MAVEN observations of the response of Mars to an interplanetary coronal mass ejection

Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

Structure and composition of the neutral upper atmosphere of Mars from the MAVEN NGIMS investigation

Abstract The Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) provides sensitive detections of neutral gas and ambient ion composition. NGIMS measurements of nine atomic and molecular neutral species, and their variation with altitude, latitude, and solar zenith angle are reported over several months of operation of the MAVEN mission. Sampling NGIMS signals from multiple neutral species every several seconds reveals persistent and unexpectedly large amplitude density structures. The scale height temperatures are mapped over the course of the first few months of the mission from high down to midlatitudes. NGIMS measurements near the homopause of 40Ar/N2 ratios agree with those reported by the Sample Analysis at Mars investigation and allow the altitude of the homopause for the most abundant gases to be established.

Early MAVEN Deep Dip campaign reveals thermosphere and ionosphere variability

The Mars Atmosphere and Volatile Evolution (MAVEN) mission, during the second of its Deep Dip campaigns, made comprehensive measurements of martian thermosphere and ionosphere composition, structure, and variability at altitudes down to ~130 kilometers in the subsolar region. This altitude range contains the diffusively separated upper atmosphere just above the well-mixed atmosphere, the layer of peak extreme ultraviolet heating and primary reservoir for atmospheric escape. In situ measurements of the upper atmosphere reveal previously unmeasured populations of neutral and charged particles, the homopause altitude at approximately 130 kilometers, and an unexpected level of variability both on an orbit-to-orbit basis and within individual orbits. These observations help constrain volatile escape processes controlled by thermosphere and ionosphere structure and variability.

Simultaneous observations of atmospheric tides from combined in situ and remote observations at Mars from the MAVEN spacecraft

We report the observations of longitudinal variations in the Martian thermosphere associated with nonmigrating tides. Using the Neutral Gas Ion Mass Spectrometer (NGIMS) and the Imaging Ultraviolet Spectrograph (IUVS) on NASAs Mars Atmosphere and Volatile EvolutioN Mission (MAVEN) spacecraft, this study presents the first combined analysis of in situ and remote observations of atmospheric tides at Mars for overlapping volumes, local times, and overlapping date ranges. From the IUVS observations, we determine the altitude and latitudinal variation of the amplitude of the nonmigrating tidal signatures, which is combined with the NGIMS, providing information on the compositional impact of these waves. Both the observations of airglow from IUVS and the CO2 density observations from NGIMS reveal a strong wave number 2 signature in a fixed local time frame. The IUVS observations reveal a strong latitudinal dependence in the amplitude of the wave number 2 signature. Combining this with the accurate CO2 density observations from NGIMS, this would suggest that the CO2 density variation is as high as 27% at 0–10° latitude. The IUVS observations reveal little altitudinal dependence in the amplitude of the wave number 2 signature, varying by only 20% from 160 to 200 km. Observations of five different species with NGIMS show that the amplitude of the wave number 2 signature varies in proportion to the inverse of the species scale height, giving rise to variation in composition as a function of longitude. The analysis and discussion here provide a roadmap for further analysis as additional coincident data from these two instruments become available.

Photochemical escape of oxygen from Mars: First results from MAVEN in situ data

Photochemical escape of atomic oxygen is thought to be one of the dominant channels for Martian atmospheric loss today and played a potentially major role in climate evolution. MAVEN is the first mission capable of measuring, in situ, the relevant quantities necessary to calculate photochemical escape fluxes. We utilize 18 months of data from three MAVEN instruments: LPW, NGIMS and STATIC. From these data we calculate altitude profiles of the production rate of hot oxygen atoms from the dissociative recombination (DR) of O2+ and the probability that such atoms will escape the Mars atmosphere. From this we determine escape fluxes for 815 periapsis passes. Derived average dayside hot O escape rates range from 1.2 to 5.5 x 1025 s-1 depending on season and EUV flux, consistent with several pre-MAVEN predictions and in broad agreement with estimates made with other MAVEN measurements. Hot O escape fluxes do not vary significantly with dayside solar zenith angle or crustal magnetic field strength, but depend on CO2 photoionization frequency with a power law whose exponent is 2.6 ± 0.6, an unexpectedly high value which may be partially due to seasonal and geographic sampling. From this dependence and historical EUV measurements over 70 years, we estimate a modern-era average escape rate of 4.3 x 1025 s-1. Extrapolating this dependence to early solar system EUV conditions gives total losses of 13, 49, 189, and 483 mb of oxygen over 1, 2, 3, and 3.5 Gyr respectively, with uncertainties significantly increasing with time in the past.

Ionopause‐like density gradients in the Martian ionosphere: A first look with MAVEN

For unmagnetized planets, the top of the ionosphere is often marked by a sharp change in electron density and other plasma properties, called an ionopause. Here we present a statistical study of dayside ionopause-like density gradients observed in 54% of ion density profiles from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission spacecraft at Mars. Prior studies of the Martian ionopause have lacked simultaneous comprehensive measurements of plasma and magnetic field properties. Therefore, we use MAVEN observations of the electron density, magnetic field, and ion and electron energy spectra to study the factors that influence properties of the ionopause. On average, profiles with an ionopause are accompanied by a higher energy flux of protons at high altitudes and stronger magnetic field at low altitude than profiles without an ionopause. At altitudes above ~300 km, the O+/O2+ ratio is significantly larger for profiles with an ionopause than those without an ionopause. These findings enhance our understanding of this important plasma boundary at Mars.

Photoelectrons and solar ionizing radiation at Mars: Predictions versus MAVEN observations

Understanding the evolution of the Martian atmosphere requires knowledge of processes transforming solar irradiance into thermal energy well enough to model them accurately. Here we compare Martian photoelectron energy spectra measured at periapsis by Mars Atmosphere and Volatile Evolution MissioN (MAVEN) with calculations made using three photoelectron production codes and three solar irradiance models as well as modeled and measured CO2 densities. We restricted our comparisons to regions where the contribution from solar wind electrons and ions were negligible. The two intervals examined on 19 October 2014 have different observed incident solar irradiance spectra. In spite of the differences in photoionization cross sections and irradiance spectra used, we find the agreement between models to be within the combined uncertainties associated with the observations from the MAVEN neutral density, electron flux, and solar irradiance instruments.

Comparison of model predictions for the composition of the ionosphere of Mars to MAVEN NGIMS data

Prior to the arrival of the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft at Mars, the only available measurements of the composition of the planets ionosphere were those acquired by the two Viking Landers during their atmospheric entries. Many numerical models of the composition of the ionosphere of Mars have been developed, but these have only been validated for species, altitudes, and conditions for which Viking data exist. Here we compare the ionospheric composition and structure predicted by 10 ionospheric models at solar zenith angles of 45–60° against ion density measurements acquired by the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS). The most successful models included three-dimensional plasma transport driven by interactions with the surrounding space environment but had relatively simple ionospheric chemistry.

Global Distribution and Parameter Dependences of Gravity Wave Activity in the Martian Upper Thermosphere Derived from MAVEN NGIMS Observations

Wavelike perturbations in the Martian upper thermosphere observed by the Neutral Gas Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft have been analyzed. The amplitudes of small-scale perturbations with apparent wavelengths between ~100 and ~500 km in the Ar density around the exobase show a clear dependence on temperature (T0) of the upper thermosphere. The average amplitude of the perturbations is ~10% on the dayside and ~20% on the nightside, which is about 2 and 10 times larger than those observed in the Venusian upper thermosphere and in the low-latitude region of Earths upper thermosphere, respectively. The amplitudes are inversely proportional to T0, suggesting saturation due to convective instability in the Martian upper thermosphere. After removing the dependence on T0, dependences of the average amplitude on the geographic latitude and longitude and solar wind parameters are found to be not larger than a few percent. These results suggest that the amplitudes of small-scale perturbations are mainly determined by convective breaking/saturation in the upper thermosphere on Mars, unlike those on Venus and Earth.

Changes in the thermosphere and ionosphere of Mars from Viking to MAVEN

We compare Viking and Mars Atmosphere and Volatile EvolutioN mission (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) observations of the thermosphere and ionosphere of Mars in order to test predictions of large variations in conditions over the solar cycle and with season. Substantial differences exist between the Viking observations at solar minimum and near aphelion and the MAVEN NGIMS observations at moderate solar activity and near perihelion. Differences in the O/CO2 ratio, the O + ionospheric peak, ion densities at high altitude, and neutral and ion scale heights can be attributed to differences in solar activity and season, but the relative importance of solar activity and season for these differences was not established. Current models do not explain the observed differences in the mixing ratios of N, NO, and O2. These results place new constraints on models of how the thermosphere and ionosphere of Mars vary over the solar cycle and with season.