The Sun's Influence on the vertical structure of the ionospheres of Venus and Mars

The ionospheres of Venus and Mars are important components of the planet-space boundary that play a major role in atmospheric escape processes. Characterization of these regions reveals the physical processes that control them and provides a foundation for more detailed studies of chemistry, dynamics, and energetics. At both planets the ionospheres contain two layers: the main layer, which is formed by photoionization from extreme ultraviolet radiation (EUV, λ<120 nm), and the lower layer, which is formed by photoionization from soft X-rays (SXRs, λ<10 nm) and subsequent electron impact ionization. In this dissertation I investigate how the solar EUV and SXR irradiance controls these layers at Venus and Mars. First, I develop an empirical model of the ultraviolet (UV, λ<190 nm) solar spectrum as a function of F10.7, which is a commonly used proxy of the UV irradiance. I derive power-law relationships between F10.7 and the ionizing irradiance for five neutral species and show that the relationships are nonlinear. These relationships can be used to estimate the EUV irradiance when no solar spectrum measurements are available. Second, I show that the peak electron densities in the ionospheres of Venus and Mars are proportional to the square-root of the ionizing irradiance, which is in contrast to previous studies that have used F10.7 as their representation of the UV irradiance. This finding ameliorates a discrepancy between theory and observations and is in agreement with the prediction that dissociative recombination is the main ion loss mechanism near the ionospheric peaks at Venus and Mars. Third, using a numerical model and electron density profiles from Venus Express, I examine the behavior of the peak altitude, peak density, and morphology of the lower layer at Venus. I show that the peak altitudes and densities in the lower and main layers vary similarly with solar zenith angle (SZA). This implies that neutral and electron thermal gradients at these altitudes vary little with SZA. I also show that, compared to the main layer, the lower layer morphology and peak density varies more over the solar cycle due to the hardening of the solar spectrum

The Dependence of Peak Electron Density on Solar Irradiance in the Ionosphere of Mars

National Aeronatics and Space Administration (NASA) (NNX08AN56G, NNX08AP96G, NNX12AJ39G

Properties of Mars' Dayside Low-Altitude Induced Magnetic Field and Comparisons with Venus

Our research objective is to characterize Mars' low-altitude (250 km) induced magnetic fields using data from NASA's MAVEN (Mars Atmosphere and Volatile EvolutioN) Mission. We aim to assess how the induced magnetic fields behave under different solar zenith angles and solar wind conditions, and additionally, understand how planet-specific properties (such as Mars crustal magnetism) alter the formation and structure of the magnetic fields. We then use data from the Pioneer Venus Orbiter to compare induced magnetic fields at Venus with those at Mars. At Venus, the vertical structure of the magnetic field tends to exist in one of two states (magnetized or unmagnetized) but we find the induced fields at Mars are more complicated, and we are unable to use this simple classification scheme. We also find the low-altitude induced field strength in the ionospheres of both Venus and Mars vary with as cosine of the angle between solar wind velocity and the magnetic pileup boundary. The low-altitude field strength at Venus tends to be higher than Mars. However, Venus field strengths are lower than theoretical predictions assuming pressure balance and negligible thermal pressure. For Mars, low-altitude field strengths are higher than expected given these assumptions. Induced field strengths exhibit a trend with solar wind dynamic pressure that is consistent with pressure balance expectations at both planets, however there is significant uncertainty in the Venus fit due to lack of upstream solar wind data. Our results highlight major differences between the induced magnetic fields at Venus and Mars, suggesting planet-specific properties such as size and the presence of crustal magnetism affect the induced ionospheric magnetic fields at non magnetized planets.Comment: 20 pages, 7 figures. To be submitted to AGU Journa

Advancing Our Understanding of Martian Proton Aurora through a Coordinated Multi-Model Comparison Campaign

Proton aurora are the most commonly observed yet least studied type of aurora at Mars. In order to better understand the physics and driving processes of Martian proton aurora, we undertake a multi-model comparison campaign. We compare results from four different proton/hydrogen precipitation models with unique abilities to represent Martian proton aurora: Jolitz model (3-D Monte Carlo), Kallio model (3-D Monte Carlo), Bisikalo/Shematovich et al. model (1-D kinetic Monte Carlo), and Gronoff et al. model (1-D kinetic). This campaign is divided into two steps: an inter-model comparison and a data-model comparison. The inter-model comparison entails modeling five different representative cases using similar constraints in order to better understand the capabilities and limitations of each of the models. Through this step we find that the two primary variables affecting proton aurora are the incident solar wind particle flux and velocity. In the data-model comparison, we assess the robustness of each model based on its ability to reproduce a MAVEN/IUVS proton aurora observation. All models are able to effectively simulate the data. Variations in modeled intensity and peak altitude can be attributed to differences in model capabilities/solving techniques and input assumptions (e.g., cross sections, 3-D versus 1-D solvers, and implementation of the relevant physics and processes). The good match between the observations and multiple models gives a measure of confidence that the appropriate physical processes and their associated parameters have been correctly identified and provides insight into the key physics that should be incorporated in future models

Mars’ plasma system. Scientific potential of coordinated multipoint missions: “The next generation”

The objective of this White Paper, submitted to ESA’s Voyage 2050 call, is to get a more holistic knowledge of the dynamics of the Martian plasma system, from its surface up to the undisturbed solar wind outside of the induced magnetosphere. This can only be achieved with coordinated multi-point observations with high temporal resolution as they have the scientific potential to track the whole dynamics of the system (from small to large scales), and they constitute the next generation of the exploration of Mars analogous to what happened at Earth a few decades ago. This White Paper discusses the key science questions that are still open at Mars and how they could be addressed with coordinated multipoint missions. The main science questions are: (i) How does solar wind driving impact the dynamics of the magnetosphere and ionosphere? (ii) What is the structure and nature of the tail of Mars’ magnetosphere at all scales? (iii) How does the lower atmosphere couple to the upper atmosphere? (iv) Why should we have a permanent in-situ Space Weather monitor at Mars? Each science question is devoted to a specific plasma region, and includes several specific scientific objectives to study in the coming decades. In addition, two mission concepts are also proposed based on coordinated multi-point science from a constellation of orbiting and ground-based platforms, which focus on understanding and solving the current science gaps

Mars Thermospheric Water Abundance in Mars Years 32-36

&lt;p&gt;This file contains the derived thermospheric water abundances and mixing ratios along with the associated solar longitude, latitude, solar zenith angle, local time and global dust optical depth.&lt;/p&gt

Characterizing the V1 layer in the Venus ionosphere using VeRa observations from Venus Express

The Venus Radio Science Experiment (VeRa) on the Venus Express spacecraft sounds the Venus atmosphere during Earth occultations to obtain vertical profiles of electron density in the ionosphere. The resultant profiles reveal the vertical structure of the Venus ionosphere from the topside down to below the lower layers (\u3c 115 km). On the dayside, the dominant plasma layer is the V2 layer at ~142 km, which is produced primarily by photoionization of CO2. Embedded on the bottomside of the V2 layer is the less prominent, and much less studied, V1 layer at ~127 km. The V1 layer is also produced by photoionization of CO2, but secondary ionization due to energetic photoelectrons is much more important. Here we investigate properties of the V1 layer using VeRa profiles from 2006 to 2012 during which the Sun went from the deep solar minimum of Solar Cycle 23 to the rising solar activity levels of Solar Cycle 24. We investigate how the peak electron density and peak altitude of the V1 layer depend on solar zenith angle. We also characterize the shapes of the V1 layer and show how they are related to the solar activity level. Solar spectra from the Solar EUV Experiment (SEE) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft are used to characterize the shapes of the V1 layer with solar activity

Characterization of the lower layer in the dayside Venus ionosphere and comparisons with Mars

The influence of solar zenith angle (SZA) and solar irradiance has been well characterized for the V2 layer in the Venus ionosphere, but not the V1 layer, where previous efforts were limited by data scarcity and incomplete SZA coverage. Here we use more than 200 radio occultation profiles from Venus Express with good SZA coverage to characterize how the V1 peak altitude, peak density, and morphology respond to changes in SZA and solar activity. The V1 and V2 peak altitudes vary little with SZA, and both peak electron densities vary with SZA in an approximately Chapman-like manner. These results imply that the thermal structures of the atmosphere and ionosphere between similar to 125 km and similar to 140 km vary little with SZA. As solar activity increases, the ratio of the V1 to V2 peak density increases, and the V1 morphology changes more than the V2 morphology. These results are due to the soft X-ray flux increasing relative to the EUV flux as solar activity increases. We compare the behavior of the V1 layer to the analogous M1 layer at Mars, and find that their peak altitudes respond differently to changes in SZA and solar activity. The V1 peak density also increases more with solar activity than the M1 peak density. These distinct behaviors arise from differences in their underlying neutral atmospheres. (C) 2015 Elsevier Ltd. All rights reserved