A document by Krishnaprasad Chirakkil. Click on the document to view its contents.

Mars once had a dense atmosphere enabling liquid water existing on its surface, however, much of that atmosphere has since escaped to space. We examine how incoming solar and solar wind energy fluxes drive escape of atomic and molecular oxygen ions (O+ and O2+) at Mars. We use MAVEN data to evaluate ion escape from February 1, 2016 through May 25, 2022. We find that Martian O+, and O2+ all have increased escape flux with increased solar wind kinetic energy flux. Increased solar wind electromagnetic energy flux also corresponds to increased O+ and O2+ escape flux. Increased solar irradiance (both total and ionizing) does not obviously increase escape of O+ and O2+. Together, these results suggest that the solar wind electromagnetic energy flux should be considered along with the kinetic energy flux, and that other parameters should be considered when evaluating solar irradiance’s impact on O+ and O2+ escape.

Mars once had a dense atmosphere enabling liquid water existing on its surface, however, much of that atmosphere has since escaped to space. We examine how incoming solar and solar wind energy fluxes drive escape of atomic and molecular oxygen ions (O+ and O2+) at Mars. We use MAVEN data to evaluate ion escape from February 1, 2016 through May 25, 2022. We find that Martian O+ and O2+ both have increased escape flux with increased solar wind kinetic energy flux and this relationship is generally logarithmic. Increased solar wind electromagnetic energy flux also corresponds to increased O+ and O2+ escape flux, however, increased solar wind electromagnetic energy flux seems to first dampen ion escape until a threshold level is reached, at which point ion escape increases with increasing electromagnetic energy flux. Increased solar irradiance (both total and ionizing) does not obviously increase escape of O+ and O2+. Our results suggest that the solar wind electromagnetic energy flux should be considered along with the kinetic energy flux as an important driver of ion escape, and that other parameters should be considered when evaluating solar irradiance’s impact on O+ and O2+ escape.

A key element of successful aerobraking operations at Mars is accurate thermospheric density predictions. Evidence suggests that much of the longitude variability in Mars’ aerobraking region is associated with atmospheric tides, and the day-to-day variability is connected with tidal modulation by longer-period global-scale waves. Specifically, ultra-fast Kelvin waves (UFKWs) and their modulation of the tidal spectrum play a key role in coupling Mars’ lower ($<$$\sim$80 km) and middle ($\sim$80-100 km) atmosphere with the aerobraking region above. In this study, over 5 years of Mars Atmosphere and Volatile Evolution (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) CO$_2$ density observations are employed to reveal prominent, frequent, and persistent 2.5- to 4.5-day UFKW packets in the whole Martian middle and upper thermosphere (ca. 150-200 km), and large secondary waves arising from their nonlinear interactions with the tidal spectrum. Detailed analyses focusing on a prominent $\sim$2.5-day UFKW event in late 2015 demonstrate primary and secondary wave amplitudes growing twofold with altitude from $\sim$7-14\% near 150 km to $\sim$12-25\% near 200 km and their combined effects to account for $\sim$60-80\% of the altitude-longitudinal variability of Mars’ thermospheric density. Concurrent temperature measurements from Mars Reconnaissance Orbiter (MRO) Mars Climate Sounder (MCS) reveal consistent wave signatures near 80 km altitude suggesting propagation of both primary and secondary waves from the lower atmosphere. This study demonstrates that UFKWs and secondary waves from UFKW-tide interactions are sources of significant altitude-longitude variability in the Mars’ aerobraking region that should be accounted for when analyzing satellite observations and nonlinear models.

The Flare Irradiance Spectral Model (FISM) is an important tool for estimating solar variability for a myriad of space weather research studies and applications, and FISM Version 2 (FISM2) has recently been been released. FISM2 is an empirical model of the solar ultraviolet irradiance created to fill spectral and temporal gaps in the measurements, where these measurements are infrequent as they need to be made from space due to their absorption in the planetary atmospheres. FISM2 estimates solar ultraviolet irradiance variations due to solar cycle, solar rotation, and solar flare variations. The major improvement provided by FISM2 is that it is based on multiple new, more accurate instruments that have now captured almost a full solar cycle and thousands of flares, drastically improving the accuracy of the modeled FISM2 solar irradiance spectra. FISM2 is also improved to 0.1 nm spectral bins across the same 0-190 nm spectral range, and is already being used in research to estimate space weather changes due to solar irradiance variability in planetary thermospheres and ionospheres.

The MAVEN/EUVM solar soft x-ray (SXR) and Lyman-α measurements are compared with analogous measurements made from Earth to characterize the typical error introduced when phase-shifting solar EUV irradiance measurements made from Earth to other points in the solar system according to the 27.27 day synodic solar rotation period. The phase-shifting error, ε, measured at SXR and Lyman-α are extrapolated to the full EUV spectrum by assuming it is proportional to the variability that occurs over the 27-day timescale of solar rotation. Values for ε as a function of wavelength are reported and used to find the typical error for estimates of photoionization frequencies of some major species found in planetary upper atmospheres derived by phase-shifted EUV irradiance. This study finds that the typical extrapolation error for the CO photoionization frequency is 5.7% of the solar cycle mean value, and 87% of the typical 27-day variability.

Martian sub-solar electron temperatures obtained below 250 km are examined using data obtained by instruments on the Mars Atmosphere Evolution Mission (MAVEN) during the three sub-solar deep dip campaigns and a one-dimensional fluid model. This analysis was done because of the uncertainty in MAVEN low electron temperature observations at low altitudes and the fact that the Level 2 temperatures reported from the MAVEN Langmuir Probe and Waves (LPW) instrument are more than 400 Kelvin above the neutral temperatures at the lowest altitudes sampled (~120 km). These electron temperatures are well above those expected before MAVEN was launched. We find that an empirical normalization parameter, neutral pressure divided by local electron heating rate, organized the electron temperature data and identified a similar altitude (~160 km) and time scale (~2,000 s) for all three deep dips. We show that MAVEN data are not consistent with a plasma characterized by electrons in thermal equilibrium with the neutral population at 100 km. Because of the lack data below 120 km and the uncertainties of the data and the cross sections used in the one dimensional fluid model above 120 km, we cannot use MAVEN observations to prove that the electron temperature converges to the neutral temperature below 100 km. However, the lack of our understanding the electron temperature altitude profile below 120 km does not impact our understanding of the role of electron temperature in determining ion escape rates because ion escape is determined by electron temperatures above 180 km.

A method is presented for retrieving temperature and composition from 150-350 km in Earth’s thermosphere using total number density measurements made via EUV solar occultations by the PROBA2/LYRA instrument. Systematic and random uncertainties are calculated and found to be less than 5% for the temperature measurements and 5-20% for the composition measurements. Regression coefficients relating both temperature and the [O]/[N2] abundance ratio with EUV irradiance at 150, 275 and 350 km are reported. Additionally, it is shown that the altitude where [O] equals [N2] decreases with increasing solar EUV irradiance, an effect attributed to thermal expansion. Temperatures from 2010 to 2017 are compared with estimates from the MSIS empirical model and show good agreement at the dawn terminator but LYRA is markedly cooler at the dusk terminator, with the MSIS-LYRA temperature difference increasing with solar activity. Anthropogenic cooling can explain this discrepancy at periods of lower solar activity, but the divergence of temperature with increasing solar activity remains unexplained. LYRA measurements of the exospheric sensitivity to EUV irradiance are compared with contemporaneous measurements made at Mars, showing that the exospheric temperature at Mars is approximately half as sensitive to EUV variability as that of Earth.

Soft x-ray and EUV radiation from the Sun is absorbed by and ionizes the atmosphere, creating both the ionosphere and thermosphere. Temporal changes in irradiance energy and spectral distribution can have profound impacts on the ionosphere, impacting technologies such as satellite drag and radio communication. Because of this, it is necessary to estimate and predict changes in Solar EUV spectral irradiance. Ideally, this would be done by direct measurement but the high cost of solar EUV spectrographs makes this prohibitively expensive. Instead, scientists must use data driven models to predict the solar spectrum for a given irradiance measurement. In this study, we further develop the Synthetic Reference Spectral Irradiance Model (SynRef). The SynRef model, which uses broadband EUV irradiance data from the MAVEN EUVM at Mars, was created to mirror the SORCE XPS model which uses data from the TIMED SEE instrument and the SORCE XPS instrument at Earth. Both models superpose theoretical Active Region and Quiet Sun spectra generated by CHIANTI to match daily measured irradiance data, and output a modeled solar EUV spectrum for that day. We use the broadband EUVM measurements to estimate Active Region temperature. This will allow us to select from a library of AR reference spectra with different temperatures. We also investigate how the prevalence of solar minimum coronal holes affects our measurements and how to account for them. We present this updated SynRef model to more accurately characterize the Solar EUV and soft x-ray spectra.

A two-dimensional nonlinear numerical model has been developed to study atmospheric coupling due to vertically propagating Gravity Waves (GWs) on different planets. The model is able to simulate both acoustic and gravity waves due to inclusion of compressibility. The model also considers dissipative effects due to viscosity, conduction and radiative damping. The hyperbolic inviscid advection equations are solved using the Lax-Wendroff method. The parabolic diffusion terms are solved implicitly using a linear algebra-based Direct method. The model is validated by comparing numerical solutions against analytical results for linear propagation, critical level absorption and breaking. A case study of tsunami-generated GWs is presented for the 2004 Sumatra earthquake whereby the model is forced through tsunamigenic sea-surface displacement. The properties of simulated GWs closely match those derived from ionospheric sounding observations reported in literature. Another application for Martian ice cloud formation is discussed where GWs from topographic sources are shown to create cold pockets with temperatures below the CO2 condensation threshold. The simulated cold pockets coincide with the cloud echo observations from the Mars Orbiting Laser Altimeter (MOLA) aboard Mars Global Survey (MGS) spacecraft.