Superthermal Electrons at Mars: Photoelectrons, Solar Wind Electrons, and Dust Storm Influences.

Mars is unique in the solar system in terms of its interaction with solar wind because it lacks of a significant intrinsic global magnetic field but possesses localized strong crustal fields. This interaction results in a very complex magnetic topology at Mars so that superthermal electrons, mainly including photoelectrons and solar wind electrons, can be distinctively important for such a complicated planetary space environment. These energetic electrons ($sim 1-1000$ electron volts) can carry and rapidly redistribute energy along the magnetic field lines. They are also a reliable tool to deduce the Martian magnetic topology, which is critical to understand the electromagnetic dynamics of the Martian space environment. The investigation methodology involves both data analysis and modeling. This dissertation mainly investigates three topics of superthermal electrons at Mars. (1) This dissertation confirms that the long-lived influence of Martian low-altitude dust storms on high-altitude photoelectron fluxes is common for a wide range of energy and pitch angles and determines that this effect originates from the thermosphere-ionosphere source region of the photoelectrons, rather than at exospheric altitudes at or above MGS. Through simulations, the results suggest that the global dust storm altered the photoelectron fluxes by causing CO$_2$ to be the dominant species at a much larger altitude range than usual. (2) Because the integral of the production rate above the superthermal electron exobase is about the same for all solar zenith angles, quite counterintuitively, it is found, observationally and numerically/theoretically, that the high-altitude photoelectron fluxes are quite independent of solar zenith angle. (3) Based on the energy spectral (flux against energy) difference between photoelectrons and solar wind electrons, a statistical approach is taken to distinguish the two populations and also allows us to quantify the occurrence rate of solar wind electron precipitation and also these electrons' energy deposition. The broad impact and future work of this dissertation is also briefly discussed, especially with the comprehensive neutral and plasma measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to further our understanding of the Martian space environment.PhDAtmospheric, Oceanic and Space SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116715/1/xussui_1.pd

Misbehaving High‐Energy Electrons: Evidence in Support of Ubiquitous Wave‐Particle Interactions on Dayside Martian Closed Crustal Magnetic Fields

Multiple studies have reported either isotropic or trapped pitch angle distributions of high‐energy (>100 eV) electrons on closed crustal field lines on the dayside of Mars. These pitch angle distributions are not to be expected from collisional scattering and conservation of adiabatic invariants alone. We use 2 years of data from the Mars Atmosphere and Volatile EvolutioN mission to analyze the pitch angle distributions of superthermal electrons on dayside‐closed crustal magnetic fields and compare to results from an electron transport model. Low‐energy electrons (10–60 eV) have pitch angle distributions in agreement with modeling results, while high‐energy electrons (100–500 eV) do not. High‐energy electrons have a flux peak at perpendicular pitch angles which suggests there is a ubiquitous energization process occurring on crustal fields. Wave‐particle interactions seem to be the most likely candidate. Trapping of high‐energy electrons may impact the nightside ionosphere dynamics.Plain Language SummarySuperthermal electrons are electrons with energies between 1 and 1,000 eV and can be produced from ionizing a neutral atmospheric molecule (photoelectron). These electrons are efficient at shifting energy around in space environments due to their high speeds and their ability to interact with the more ubiquitous lower energy (thermal) plasma. Past studies have investigated the distribution of photoelectrons on the crustal magnetic fields of Mars, and they do not always agree with past modeling results and a basic understanding of electron transport. In this study, we use data from the Mars Atmosphere and Volatile EvolutioN mission in order to understand the distribution of these electrons throughout the Mars space environment, previously impossible due to spacecraft orbits. We find that the lower energy electrons (10–60 eV) behave as expected but the higher‐energy electrons (100–500 eV) do not. We find that the type of distribution statistically seen by Mars Atmosphere and Volatile EvolutioN for these high‐energy electrons suggests that a ubiquitous energization process is occurring on the dayside crustal magnetic fields of Mars. We consider multiple physical processes capable of producing such observed distributions and conclude that wave‐particle interactions are the most likely candidate.Key PointsLow‐energy photoelectrons on the Martian dayside have pitch angle distributions consistent with conservation of adiabatic invariantsHigh‐energy electrons on the Martian dayside have a flux peak at perpendicular pitch angles, indicating a ubiquitous energization processWave‐particle interactions are the most likely candidate to produce such distributions for high‐energy electronsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153259/1/grl59709-sup-0001-2019GL084919-Figure_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153259/2/grl59709_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153259/3/grl59709.pd

Mars nightside electrons over strong crustal fields

We investigated 7 years worth of data from the electron reflectometer and magnetometer aboard Mars Global Surveyor to quantify the deposition of photoelectron and solar wind electron populations on the nightside of Mars, over the strong crustal field region located in the southern hemisphere. Just under 600,000 observations, each including energy and pitch angle distributions, were examined. For solar zenith angles (SZA) less than 110°, photoelectrons have the highest occurrence rate; beyond that, plasma voids occur most often. In addition, for SZA >110°, energy deposition of electrons mainly occurs on vertical field lines with median pitch angle averaged energy flux values on the order of 107–108 eV cm−2 s−1. The fraction of downward flux that is deposited at a given location was typically low (16% or smaller), implying that the majority of precipitated electrons are magnetically reflected or scattered back out. The average energy of the deposited electrons is found to be 20–30 eV, comparable to typical energies of photoelectrons and unaccelerated solar wind electrons. Median electron flux values, from near‐vertical magnetic field lines past solar zenith angle of 110°, calculated in this study produced a total electron content of 4.2 × 1014 m−2 and a corresponding peak density of 4.2 × 103 cm−3.Key PointsElectron populations on the nightside of Mars are exploredOccurrence rates and energy deposition values are calculatedMedian precipitating flux yields a peak ne of 4.2 × 103 cm−3Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137525/1/jgra52560.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137525/2/jgra52560_am.pd

Mars photoelectron energy and pitch angle dependence on intense lower atmospheric dust storms

We have conducted a survey of the Mars Global Surveyor (MGS) electron data across all the pitch angles of 12 usable energy bins (11–746 eV) for dayside photoelectron observations over regions of strong crustal fields. Studies have shown that solar EUV flux is the main controlling factor, but dust storms play an important role as well. Our study of different energies and pitch angles has shown that the unusual bimodal solar flux dependence is not a common feature but mainly found in low energies and a few bins of higher‐energy channels. By multiplying time‐history dust opacity with a solar EUV proxy as a new controlling function, the statistically significant increase of the correlation of photoelectron flux against this function indicates that dust storms have a long‐lasting influence on high‐altitude photoelectron fluxes, especially at low energies and the pitch angle source regions of high‐energy channels. The correlation increases experienced by the pitch angle source regions of all examined energy channels suggest that dust storms' influence most likely happens in the thermosphere‐ionosphere source region of the photoelectrons, rather than at exospheric altitudes at or above MGS. Furthermore, by isolating the global‐scale dust storm in Mars year 25 (2001) from the rest, the results suggest that this storm is entirely responsible for the second solar flux‐dependent trend. While not excluding the possibility of this phenomenon being a one‐time event, we hypothesize that there is a threshold of dust opacity at which the low‐altitude dust's influence on high‐altitude photoelectron fluxes begins to be significant. Key Points Dust storms' influence is strongest in the thermosphere‐ionosphere source region Hypothesize a dust opacity threshold for a long‐lived effect on the ionospherePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108280/1/fs01_fism010nm.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/108280/2/fs02_fism050nm.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/108280/3/jgre20282.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/108280/4/fs03_fism50100nm.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/108280/5/README.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/108280/6/fs04.pd

Ionospheric ambipolar electric fields of Mars and Venus: Comparisons between theoretical predictions and direct observations of the electric potential drop.

We test the hypothesis that their dominant driver of a planetary ambipolar electric field is the ionospheric electron pressure gradient (∇P e). The ionospheres of Venus and Mars are mapped using Langmuir probe measurements from NASAs Pioneer Venus Orbiter (PVO) and Mars Atmosphere and Volatile Evolution (MAVEN) missions. We then determine the component of the ionospheric potential drop that can be explained by the electron pressure gradient drop along a simple draped field line. At Mars, this calculation is consistent with the mean potential drops measured statistically by MAVEN. However, at Venus, contrary to our current understanding, the thermal electron pressure gradient alone cannot explain Venus strong ambipolar field. These results strongly motivate a return to Venus with a comprehensive plasmas and fields package, similar to that on MAVEN, to investigate the physics of atmospheric escape at Earths closest analog

Ionospheric control of the dawn‐dusk asymmetry of the Mars magnetotail current sheet

This study investigates the role of solar EUV intensity at controlling the location of the Mars magnetotail current sheet and the structure of the lobes. Four simulation results are examined from a multifluid magnetohydrodynamic model. The solar wind and interplanetary magnetic field (IMF) conditions are held constant, and the Mars crustal field sources are omitted from the simulation configuration. This isolates the influence of solar EUV. It is found that solar maximum conditions, regardless of season, result in a Venus‐like tail configuration with the current sheet shifted to the −Y (dawnside) direction. Solar minimum conditions result in a flipped tail configuration with the current sheet shifted to the +Y (duskside) direction. The lobes follow this pattern, with the current sheet shifting away from the larger lobe with the higher magnetic field magnitude. The physical process responsible for this solar EUV control of the magnetotail is the magnetization of the dayside ionosphere. During solar maximum, the ionosphere is relatively strong and the draped IMF field lines quickly slip past Mars. At solar minimum, the weaker ionosphere allows the draped IMF to move closer to the planet. These lower altitudes of the closest approach of the field line to Mars greatly hinder the day‐to‐night flow of magnetic flux. This results in a buildup of magnetic flux in the dawnside lobe as the S‐shaped topology on that side of the magnetosheath extends farther downtail. The study demonstrates that the Mars dayside ionosphere exerts significant control over the nightside induced magnetosphere of that planet.Plain Language SummaryMars, which does not have a strong magnetic field, has an induced magnetic environment from the draping of the interplanetary magnetic field from the Sun. It folds around Mars, forming two “lobes” of magnetic field behind the planet with a current sheet of electrified gas (plasma) behind it. The current sheet is not directly behind the planet but rather shifted toward the dawn or dusk direction. It is shown here that one factor controlling the location of the current sheet is the dayside ionosphere. At solar maximum, the ionosphere is dense, the magnetic field slips easily by the planet, and the current sheet is shifted toward dawn. At solar minimum, the ionosphere is relatively weak, the magnetic field slippage is slowed down, and the current sheet shifts toward dusk.Key PointsThere is a systematic Y (i.e., dawn‐dusk) asymmetry in the location of the Martian magnetotail current sheet in modified MSE coordinatesThe asymmetry is controlled by ionospheric conditions, shifting to the dawn (‐Y) during solar maximum and to the dusk during solar minimumThe shift found in this study is not a function of crustal fields, which were omitted, or solar wind conditions, which were held constantPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137681/1/jgra53609_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137681/2/jgra53609.pd

Martian low‐altitude magnetic topology deduced from MAVEN/SWEA observations

The Mars Atmosphere and Volatile Evolution mission has obtained comprehensive particle and magnetic field measurements. The Solar Wind Electron Analyzer provides electron energy‐pitch angle distributions along the spacecraft trajectory that can be used to infer magnetic topology. This study presents pitch angle‐resolved electron energy shape parameters that can distinguish photoelectrons from solar wind electrons, which we use to deduce the Martian magnetic topology and connectivity to the dayside ionosphere. Magnetic topology in the Mars environment is mapped in three dimensions for the first time. At low altitudes (<400 km) in sunlight, the northern hemisphere is found to be dominated by closed field lines (both ends intersecting the collisional atmosphere), with more day‐night connections through cross‐terminator closed field lines than in the south. Although draped field lines with ~100 km amplitude vertical fluctuations that intersect the electron exobase (~160–220 km) in two locations could appear to be closed at the spacecraft, a more likely explanation is provided by crustal magnetic fields, which naturally have the required geometry. Around 30% of the time, we observe open field lines from 200 to 400 km, which implies three distinct topological layers over the northern hemisphere: closed field lines below 200 km, open field lines with foot points at lower latitudes that pass over the northern hemisphere from 200 to 400 km, and draped interplanetary magnetic field above 400 km. This study also identifies open field lines with one end attached to the dayside ionosphere and the other end connected with the solar wind, providing a path for ion outflow.Key PointsPitch angle‐resolved electron energy shape parameters are used to deduce magnetic topologyClosed magnetic field lines dominate low altitudes (<400 km) of the northern hemisphere on the daysideThe 3‐D view of the Martian magnetic topology is presented for the first timePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136484/1/jgra53291.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136484/2/jgra53291_am.pd

Closed magnetic topology in the Venusian magnetotail and ion escape at Venus.

Venus, lacking an intrinsic global dipole magnetic field, serves as a textbook example of an induced magnetosphere, formed by interplanetary magnetic fields (IMF) enveloping the planet. Yet, various aspects of its magnetospheric dynamics and planetary ion outflows are complex and not well understood. Here we analyze plasma and magnetic field data acquired during the fourth Venus flyby of the Parker Solar Probe (PSP) mission and show evidence for closed topology in the nightside and downstream portion of the Venus magnetosphere (i.e., the magnetotail). The formation of the closed topology involves magnetic reconnection-a process rarely observed at non-magnetized planets. In addition, our study provides an evidence linking the cold Venusian ion flow in the magnetotail directly to magnetic connectivity to the ionosphere, akin to observations at Mars. These findings not only help the understanding of the complex ion flow patterns at Venus but also suggest that magnetic topology is one piece of key information for resolving ion escape mechanisms and thus the atmospheric evolution across various planetary environments and exoplanets

Observations and Modeling of the Mars Low‐Altitude Ionospheric Response to the 10 September 2017 X‐Class Solar Flare

Solar extreme ultraviolet and X‐ray photons are the main sources of ionization in the Martian ionosphere and can be enhanced significantly during a solar flare. On 10 September 2017, the Mars Atmosphere and Volatile EvolutioN orbiter observed an X8.2 solar flare, the largest it has encountered to date. Here we investigate the ionospheric response before, during, and after this event with the SuperThermal Electron Transport model. We find good agreement between modeled and measured photoelectron spectra. In addition, the high photoelectron fluxes during the flare provide adequate statistics to allow us to clearly and repeatedly identify the carbon Auger peak in the ionospheric photoelectron energy spectra at Mars for the first time. By applying photochemical equilibrium, O2+ and CO2+ densities are obtained and compared with Mars Atmosphere and Volatile EvolutioN observations. The variations in ion densities during this event due to the solar irradiance enhancement and the neutral atmosphere expansion are discussed.Plain Language SummarySolar extreme ultraviolet and X‐ray photons are the main source of ionization in the Martian ionosphere, photoionizing the neutral particles and producing photoelectrons and ions. These short‐wavelength photon fluxes can be enhanced by a factor of a few to orders of magnitudes during a solar flare (the result of the rapid conversion of magnetic energy to kinetic energy in the solar corona). On 10 September 2017, the Mars Atmosphere and Volatile EvolutioN mission encountered the largest solar flare (X8.2) to date. The comprehensive measurements from Mars Atmosphere and Volatile EvolutioN provide us with an opportunity to evaluate the ionospheric response to this flare event in detail with models. In particular, we investigate the photoelectron flux and ion density response to the flare with an electron transport model. The modeled and measured photoelectron fluxes are in a good agreement. Ion density enhancement at a fixed altitude is from tens of percent to 1500% due to a combination of intensified solar photon fluxes and the heated and then expanded neutral atmosphere during this flare event.Key PointsThe modeled and measured photoelectron spectra are in good agreement during an X8.2 solar flare eventThe carbon Auger peak is clearly and repeatedly identified in electron energy spectra of the Martian ionosphere for the first timeThe ion density enhancement due to the flare at a fixed altitude is from tens to 1,500%Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145576/1/grl57692.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145576/2/grl57692_am.pd