The ULF wave foreshock boundary: Cluster observations

The interaction of backstreaming ions with the incoming solar wind in the upstream region of the bow shock gives rise to a number of plasma instabilities from which ultra-low frequency (ULF) waves can grow. Because of their finite growth rate, the ULF waves are spatially localized in the foreshock region. Previous studies have reported observational evidences of the existence of a ULF wave foreshock boundary, which geometrical characteristics are very sensitive to the interplanetary magnetic field (IMF) cone angle. The statistical properties of the ULF wave foreshock boundary is examined in detail using Cluster data. A new identification of the ULF wave foreshock boundary is presented using specific and accurate criterion for a precises determination of boundary crossings. The criterion is based on the degree of IMF rotation as Cluster crosses the boundary. The obtained ULF wave foreshock boundary is compared with previous results reported in the literature as well as with theoretical predictions. Also, we examined the possible connexion between the foreshock boundary properties and the ion emission mechanisms at the bow shock

Theory for planetary exospheres: III. Radiation pressure effect on the Circular Restricted Three Body Problem and its implication on planetary atmospheres

The planetary exospheres are poorly known in their outer parts, since the neutral densities are low compared with the instruments detection capabilities. The exospheric models are thus often the main source of information at such high altitudes. We present a new way to take into account analytically the additional effect of the stellar radiation pressure on planetary exospheres. In a series of papers, we present with an Hamiltonian approach the effect of the radiation pressure on dynamical trajectories, density profiles and escaping thermal flux. Our work is a generalization of the study by Bishop and Chamberlain (1989). In this third paper, we investigate the effect of the stellar radiation pressure on the Circular Restricted Three Body Problem (CR3BP), called also the photogravitational CR3BP, and its implication on the escape and the stability of planetary exospheres, especially for Hot Jupiters. In particular, we describe the transformation of the equipotentials and the location of the Lagrange points, and we provide a modified equation for the Hill sphere radius that includes the influence of the radiation pressure. Finally, an application to the hot Jupiter HD 209458b reveals the existence of a blow-off escape regime induced by the stellar radiation pressure

Theory for planetary exospheres: I. Radiation pressure effect on dynamical trajectories

The planetary exospheres are poorly known in their outer parts, since the neutral densities are low compared with the instruments detection capabilities. The exospheric models are thus often the main source of information at such high altitudes. We present a new way to take into account analytically the additional effect of the radiation pressure on planetary exospheres. In a series of papers, we present with an Hamiltonian approach the effect of the radiation pressure on dynamical trajectories, density profiles and escaping thermal flux. Our work is a generalization of the study by Bishop and Chamberlain (1989). In this first paper, we present the complete exact solutions of particles trajectories, which are not conics, under the influence of the solar radiation pressure. This problem was recently partly solved by Lantoine and Russell (2011) and completely by Biscani and Izzo (2014). We give here the full set of solutions, including solutions not previously derived, as well as simpler formulations for previously known cases and comparisons with recent works. The solutions given may also be applied to the classical Stark problem (Stark,1914): we thus provide here for the first time the complete set of solutions for this well-known effect in term of Jacobi elliptic functions

Making waves: Mirror Mode structures around Mars observed by the MAVEN spacecraft

We present an in-depth analysis of a time interval when quasi-linear mirror mode structures were detected by magnetic field and plasma measurements as observed by the NASA/Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We employ ion and electron spectrometers in tandem to support the magnetic field measurements and confirm that the signatures are indeed mirror modes. Wedged against the magnetic pile-up boundary, the low-frequency signatures last on average $\sim$10 s with corresponding sizes of the order of 15-30 upstream solar wind proton thermal gyroradii, or 10-20 proton gyroradii in the immediate wake of the quasi-perpendicular bow shock. Their peak-to-peak amplitudes are of the order of 30-35 nT with respect to the background field, and appear as a mixture of dips and peaks, suggesting that they may have been at different stages in their evolution. Situated in a marginally stable plasma with $\beta_{||}\sim$1, we hypothesise that these so-called magnetic bottles, containing a relatively higher energy and denser ion population with respect to the background plasma, are formed upstream of the spacecraft behind the quasi-perpendicular shock. These signatures are very reminiscent of magnetic bottles found at other unmagnetised objects such as Venus and comets, also interpreted as mirror modes. Our case study constitutes the first unmistakable identification and characterisation of mirror modes at Mars from the joint points of view of magnetic field, electron and ion measurements. Up until now, the lack of high-temporal resolution plasma measurements has prevented such an in-depth study.Comment: 37 pages, 11 figures, 1 tabl

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

Low Electron Temperatures Observed at Mars by MAVEN on Dayside Crustal Magnetic Field Lines

An edited version of this paper was published by AGU. Copyright 2019 American Geophysical Union.The ionospheric electron temperature is important for determining the neutral/photochemical escape rate from the Martian atmosphere via the dissociative recombination of O2+. The Langmuir Probe and Waves instrument onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) measures electron temperatures in the ionosphere. The current paper studies electron temperatures in the dayside for two regions where (1) crustal magnetic fields are dominant and (2) draped magnetic fields are dominant. Overall, the electron temperature is lower in the crustal‐field regions, namely, the strong magnetic field region, which is due to a transport of cold electrons along magnetic field lines from the lower to upper atmosphere. The electron temperature is also greater for high solar extreme ultraviolet conditions, which is associated with the local extreme ultraviolet energy deposition. The current models underestimate the electron temperature above 250‐km altitude in the crustal‐field region. Electron heat conduction associated with a photoelectron transport in the crustal‐field regions is altered due to kinetic effects, such the magnetic mirror and/or ambipolar electric field because the electron mean free path exceeds the relevant length scale for electron temperature. The mirror force can affect the electron and heat transport between low altitudes, where the neutral density and related electron cooling rates are the greatest, and high altitudes, while the ambipolar electric field decelerates the electron's upward motion. These effects have not been included in current models of the electron energetics, and consideration of such effects on the electron temperature in the crustal‐field region should be considered for future numerical simulations

ION ACCELERATION AT THE QUASI-PARALLEL BOW SHOCK: DECODING THE SIGNATURE OF INJECTION

Collisionless shocks are efficient particle accelerators. At Earth, ions with energies exceeding 100 keV are seen upstream of the bow shock when the magnetic geometry is quasi-parallel, and large-scale supernova remnant shocks can accelerate ions into cosmic rays energies. This energization is attributed to diffusive shock acceleration, however, for this process to become active the ions must first be sufficiently energized. How and where this initial acceleration takes place has been one of the key unresolved issues in shock acceleration theory. Using Cluster spacecraft observations, we study the signatures of ion reflection events in the turbulent transition layer upstream of the terrestrial bow shock, and with the support of a hybrid simulation of the shock, we show that these reflection signatures are characteristic of the first step in the ion injection process. These reflection events develop in particular in the region where the trailing edge of large-amplitude upstream waves intercept the local shock ramp and the upstream magnetic field changes from quasi-perpendicular to quasi-parallel. The dispersed ion velocity signature observed can be attributed to a rapid succession of ion reflections at this wave boundary. After the ions' initial interaction with the shock, they flow upstream along the quasi-parallel magnetic field. Each subsequent wave front in the upstream region will sweep the ions back toward the shock, where they gain energy with each transition between the upstream and the shock wave frames. Within three to five gyroperiods, some ions have gained enough parallel velocity to escape upstream, thus completing the injection process.Comment: 30 pages, 10 figures. Accepted for publication in The Astrophysical Journa

Model insights into energetic photoelectrons measured at Mars by MAVEN

Photoelectrons are important for heating, ionization, and airglow production in planetary atmospheres. Measured electron fluxes provide insight into the sources and sinks of energy in the Martian upper atmosphere. The Solar Wind Electron Analyzer instrument on board the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft measured photoelectrons including Auger electrons with 500 eV energies. A two-stream electron transport code was used to interpret the observations, including Auger electrons associated with K shell ionization of carbon, oxygen, and nitrogen. It explains the processes that control the photoelectron spectrum, such as the solar irradiance at different wavelengths, external electron fluxes from the Martian magnetosheath or tail, and the structure of the upper atmosphere (e.g., the thermal electron density). Our understanding of the complex processes related to the conversion of solar irradiances to thermal energy in the Martian ionosphere will be advanced by model comparisons with measurements of suprathermal electrons by MAVEN

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