S. W. Bougher

Polar warming in the Mars thermosphere: Seasonal variations owing to changing insolation and dust distributions

Received 13 July 2005; revised 16 December 2005; accepted 27 December 2005; published 27 January 2006. [1] Warming of the martian lower thermosphere (100–130 km) at north polar latitudes near the perihelion/winter solstice (Ls = 270) was recently observed. No analogous warming has been observed within the south polar thermosphere during its aphelion/winter season (Ls 90). Detailed global model simulations are required to investigate the physical processes driving these seasonal variations. New simulations are conducted for conditions approximating the atmosphere during these Mars Global Surveyor (MGS) and Odyssey (ODY) aerobraking periods. Strong northern winter polar warming features are calculated near 120 km, yielding nightside mean temperatures 10–15 K warmer than observed ODY values. No southern winter polar warming trend is simulated; however, nightside mean temperatures are 20– 30 K warmer than observed by MGS. The stronger interhemispheric circulation during northern winter is clearly driven by stronger insolation and dust heating near perihelion, resulting in subsidence and warmer temperatures in the northern polar night. Citation: Bougher, S. W., J. M. Bell, J. R. Murphy, M. A. Lopez-Valverde, and P. G. Withers (2006), Polar warming in the Mars thermosphere: Seasonal variations owing to changing insolation and dust distributions, Geophys. Res. Lett., 33, L02203, doi:10.1029/2005GL024059.

On the origin of aurorae on Mars

] We report observations by Mars Global Surveyor(MGS) of thousands of peaked electron energy spectrasimilar to terrestrial auroral electrons. They are observed onthe Martian nightside, near strong crustal magnetic sources.The spectra have peak energies ranging from 100 eV –2.5 keV, and fluxes near the peak are 10–10000 timeshigher than typical nightside spectra. They occur onmagnetic field lines that connect the shocked solar windto crustal magnetic fields, and on adjacent closed field lines.Their detection is directly controlled by the solar wind,suggesting that magnetic reconnection is required for theirobservation. We calculate that the most energeticdistributions could produce atmospheric emission withintensity comparable to that recently reported from theMars Express (MEX) spacecraft. Half of the most energeticexamples occur during the passage of space weather eventspast Mars, suggesting that a disturbed plasma environmentis favorable for electron acceleration along magnetic fieldlines.

Mars Global Ionosphere‐Thermosphere Model: Solar cycle, seasonal, and diurnal variations of the Mars upper atmosphere

A new Mars Global Ionosphere-Thermosphere Model (M-GITM) is presented that combines the terrestrial GITM framework with Mars fundamental physical parameters, ion-neutral chemistry, and key radiative processes in order to capture the basic observed features of the thermal, compositional, and dynamical structure of the Mars atmosphere from the ground to the exosphere (0–250 km). Lower, middle, and upper atmosphere processes are included, based in part upon formulations used in previous lower and upper atmosphere Mars GCMs. This enables the M-GITM code to be run for various seasonal, solar cycle, and dust conditions. M-GITM validation studies have focused upon simulations for a range of solar and seasonal conditions. Key upper atmosphere measurements are selected for comparison to corresponding M-GITM neutral temperatures and neutral-ion densities. In addition, simulated lower atmosphere temperatures are compared with observations in order to provide a first-order confirmation of a realistic lower atmosphere. M-GITM captures solar cycle and seasonal trends in the upper atmosphere that are consistent with observations, yielding significant periodic changes in the temperature structure, the species density distributions, and the large-scale global wind system. For instance, mid afternoon temperatures near ∼200 km are predicted to vary from ∼210 to 350 K (equinox) and ∼190 to 390 k (aphelion to perihelion) over the solar cycle. These simulations will serve as a benchmark against which to compare episodic variations (e.g., due to solar flares and dust storms) in future M-GITM studies. Additionally, M-GITM will be used to support MAVEN mission activities (2014–2016).

The ancient oxygen exosphere of Mars: Implications for atmosphere evolution

The evolution of the Martian atmosphere, particularly with regard to water, is influenced by (1) “nonthermal” escape of oxygen atoms created by dissociative recombination and (2) by oxygen ion pickup by the solar wind. Both processes depend on the intensity of solar EUV radiation, which affects atmosphere temperatures (scale heights) and photoionization rates, and thereby the exosphere and the fluxes of escaping atoms and ions. This study involves the calculation, by the two-stream model method of Nagy and Cravens (1988), of the exospheric hot oxygen densities for “ancient” atmospheres and ionospheres (e.g., for different EUV fluxes), and the associated neutral escape fluxes. The ion production rates above nominal ionopause altitudes are also computed and are considered to be the upper limits to losses by direct solar wind pickup. Since we do not consider the pickup ion precipitation and additional neutral escape due to the sputtering process described by Luhmann and Kozyra (1991), the results presented here represent conservative estimates of the neutral escape fluxes, but somewhat generous estimates of ion loss rates. We find that when the inferred increased solar EUV fluxes of the past are taken into account, oxygen equivalent to that in several tens of meters of water, planet-wide, should have escaped to space over the last 3 Gyr.

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.

Solar cycle variability of Mars dayside exospheric temperatures: Model evaluation of underlying thermal balances

[1] The response of the Mars dayside exospheric temperatures to short and long term solar flux changes was recently established. Characterization of the relative importance of various thermospheric heating and cooling mechanisms for maintaining these Mars exospheric temperatures requires the systematic application of modern global dynamical models that capture both lower and upper atmosphere thermal and dynamical processes. Coupled Mars General Circulation Model (MGCM) plus Mars Thermospheric General Circulation Model (MTGCM) simulations are utilized for this study, closely matching conditions during Mars Global Surveyor drag measurements. Simulations confirm the major balance of EUV heating and thermal heat conduction at dayside exospheric altitudes. However, the influence of variable Martian global winds is significant and must be carefully considered when investigating the global regulation of Mars exospheric temperatures over the solar cycle and Martian seasons. Finally, the present MGCM-MTGCM heating and cooling processes suggests that an EUV-UV heating efficiency of 19% yields net heating in accord with MGS exospheric temperatures.

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.

Monte Carlo model of electron transport for the calculation of Mars dayglow emissions

[1] A model of the photoelectron collision-induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on a stochastic solution of the Boltzmann equation using the direct simulation Monte Carlo method. The neutral atmosphere is taken from outputs of a global circulation model, and recent inelastic collision cross sections are adopted. The calculated vertical profiles of the CO Cameron bands and CO2 doublet emissions integrated along the line of sight compare well with the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) limb profiles observed with the SPICAM spectrograph on board Mars Express made at Ls = 166 during the summer season at northern midlatitudes. The comparison shows agreement to within the uncertainties of the excitation cross sections. Seasonal changes in the brightness and the altitude of the emission peaks are predicted with intensity variations in the range 15–20%.

New observations of molecular nitrogen in the Martian upper atmosphere by IUVS on MAVEN

We identify molecular nitrogen (N2) emissions in the Martian upper atmosphere using the Imaging Ultraviolet Spectrograph (IUVS) on NASAs Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We report the first observations of the N2 Lyman-Birge-Hopfield (LBH) bands at Mars and confirm the tentative identification of the N2 Vegard-Kaplan (VK) bands. We retrieve N2 density profiles from the VK limb emissions and compare calculated limb radiances between 90 and 210 km against both observations and predictions from a Mars general circulation model (GCM). Contrary to earlier analyses using other satellite data, we find that N2 abundances exceed GCM results by about a factor of 2 at 130 km but are in agreement at 150 km. The analysis and interpretation are enabled by a linear regression method used to extract components of UV spectra from IUVS limb observations.