Thomas Edward Cravens

Magnetic flux ropes are twisted magnetic field lines that are commonly observed in space and astrophysical plasmas. At Mars, these structures are especially interesting because the planet does not have a global magnetic field like Earth, but instead has remanent magnetic fields preserved in its crust. In this study, we report a close conjunction observation by MAVEN and Tianwen-1 of a magnetic flux rope within the Martian ionosphere. The two spacecraft observed closely related magnetic signatures at near-identical locations and altitudes, allowing us to examine the spatial structure of the event. Our analysis suggests that the observed flux rope may have a loop-like geometry, with magnetic-field lines connected to the Martian dayside ionosphere at both ends. This structure may have formed or evolved through ionospheric instabilities under unusually low solar-wind dynamic pressure or internal reconnection between neighboring crustal magnetic fields. This event shows that Mars may produce a loop-like magnetic flux rope that resembles, in geometry, twisted magnetic loops seen in the solar atmosphere, providing a useful connection between planetary and solar plasma processes.

The total power of the Jovian aurora is about 10 ¹³ –10 ¹⁴ W with ultraviolet emission observed in the Lyman and Werner bands of molecular hydrogen. X‐ray, radio, and infrared emissions have also been observed. The Jovian Auroral Distributions Experiment and the Jovian Energetic Particle Detector Instrument onboard the Juno spacecraft, orbiting Jupiter, measured large downward and upward fluxes of energetic electrons and ions in the polar regions. The main auroral oval is mainly due to energetic electron precipitation, but proton and heavy ion (sulfur and oxygen) precipitation contribute to the aurora poleward of the main oval. The current paper presents the results of a Monte Carlo model of energy deposition by proton precipitation into Jupiter's molecular hydrogen atmosphere for incident energies ranging from tens of keV up to 25 MeV. Cross sections for H ⁺ , H, and H ⁻ collisions with H ₂ were determined using a classical trajectory Monte Carlo method. The model generates ionization, dissociation, and excitation rates of H ₂ for a wide range of incident proton energy and altitude. The contribution of secondary electrons to the overall energy deposition is included. The model results indicate that 50 keV protons deposit their energy near 500 km, where the H ₂ density is about 3 × 10 ¹¹ cm ⁻³ , with 10 MeV protons depositing their energy near 200 km, where the H ₂ density is about 5 × 10 ¹⁵ cm ⁻³ , which is in the stratosphere.

The Cassini mission made the first in situ measurements of the ionosphere of Saturn during its Grand Finale in 2017. The Ion and Neutral Mass Spectrometer (INMS) instrument found molecular hydrogen and helium as well as minor species including water, methane, ammonia, and organics. INMS also made measurements of light ion species (H ⁺ , H ₂ ⁺ , H ₃ ⁺ , He ⁺ ). For a single orbital pass through the ionosphere, INMS measured the energies of ionospheric protons and demonstrated the existence of flow along magnetic field lines at some latitudes and at high altitudes. Flow speeds of a few km/s were found. This work applies a multi‐fluid transport model to the equatorial ionosphere of Saturn in order to explore plasma flow along magnetic field lines. Two field line cases were considered: one for an L‐shell of 1.1 and the other for L = 1.2. For the second case, the field line enters the shadow of the B‐ring on the atmosphere and ionosphere. Simple ion chemistry is adopted for the main ionosphere, but the focus of this work is on ion transport. Diurnal variation of the ionosphere is studied by varying the solar photoionization rate. Model results are generally within a factor of 2–3 of INMS ionospheric data. The role of ring shadowing of the ionosphere is shown to be important for determining plasma flow. Without ring shadowing, proton velocities are on the order of 0.1 km/s and are symmetric across hemispheres. With ring shadowing, proton velocities can exceed 8 km/s flowing southward for northern summer.

Magnetic reconnection in collisional plasmas has been widely studied in solar and laboratory plasma disciplines, but in situ measurements in space plasmas have rarely been utilized to explore this reconnection regime. Here we study collisional effects on magnetic reconnection in the Martian ionosphere by analyzing in situ data obtained by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. A case study and statistical results demonstrate that current sheets are commonly observed in both collisionless and collisional regions in the Martian ionosphere. Meanwhile, reconnection ion jets become hardly detectable in collisional current sheets, suggesting suppression of reconnection outflows by ion‐neutral friction effects. MAVEN observations allow us to access multiple regimes of magnetic reconnection, thereby providing valuable opportunities for cross‐disciplinary studies of collisional reconnection.

Magnetic fields measured in the ionosphere of Mars by the magnetometers onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) and the Mars Global Surveyor (MGS) spacecraft display a wide range of spatial scales, ranging from global to small‐scale. Magnetic structures advect with the plasma flow from the solar wind into the ionosphere via the magnetosheath and the pile‐up regions. Structures evolve as they move deeper into the ionosphere. Helical force‐free rope‐like structures are frequently observed at lower altitudes in the ionospheres of both Venus and Mars. Force‐free means that the magnetic forces on a structure are zero (or are very small). MAVEN data and simple magnetohydrodynamic (MHD) analysis are used to study structure evolution in the dayside ionosphere. The Kelvin‐Helmholtz instability is shown to be possible for altitudes greater than about 300 km and the Rayleigh‐Taylor instability possible for altitudes below about 250 km. The study also explains how structures can become force‐free due to the Hall and ambipolar electric field terms in the generalized Ohm's law. Timescales for structure evolution are estimated. The role of force‐free ropes and associated field‐aligned currents in linking different regions of the ionosphere is discussed.

Mars, being a small planet with a tenuous atmosphere, does not have a sharp boundary between regions dominated by solar wind plasma and planetary plasma. Instead, this transition is typically extended, allowing the interplanetary magnetic field (IMF) to penetrate into the Martian ionosphere. However, the depth of this penetration is not well understood. Using 6 years of MAVEN data, we statistically assess locations where a transition exists between the dominance of magnetic versus cold (1 eV), thermal plasma pressure to better understand the reach of the IMF. We identify the presence or absence of pressure transitions from 200 to 800 km altitude for each MAVEN orbit and find a clear transition in 55% of cases. The pressure transition locations are mapped in different coordinate systems that provide insight into the solar and planetary driving conditions that cause a detected transition region. Transitions are more likely to occur under weak‐to‐nominal solar wind conditions, away from strong crustal magnetic fields, near the terminator, on the dusk side of the planet compared to the dawn side, and in the negative solar wind motional electric field hemisphere. We speculate on possible causes for asymmetries that arise in the mapped locations of these pressure transitions and the effect that penetrated IMF may have on driving plasma dynamics in the Martian ionosphere.

Statistically ion and electron densities are enhanced above strong crustal magnetic field regions according to measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. Plasma created by ionization of neutrals in the lower ionosphere (where chemistry dominates) flows upward and becomes trapped on closed magnetic field loops. Enhanced ion density in the ionosphere (particularly O2+) is associated with enhanced photochemical escape of atomic oxygen. This paper presents a quasi‐1D multi‐fluid time‐dependent model of the Martian ionosphere for nine ion species. Ionospheric temperatures are adopted but ion densities and velocities (along the field lines) are determined using a numerical solution of the continuity and momentum equations. Diurnal effects are explored by varying photoionization rates. Three crustal field cases are considered: a low altitude closed, a high altitude closed, and a high altitude open field line. Additionally, a case with no crustal field is modeled to provide a comparison between regions with and without crustal fields in the upper Martian ionosphere. Model results show higher ion and electron densities in the crustal field cases than in the purely induced field case. Additionally, we find that densities are generally higher on the closed field lines than on the open field lines, and ion velocities are generally up the field lines, away from the Martian surface. We also find that velocities are larger on the open field line case. We compare modeled density results to MAVEN data and find general agreement. Implications for atmospheric escape, particularly photochemical escape of O, are also discussed.

The two main sources of the magnetic field in the Martian ionosphere are the solar wind interaction with the planet, and, mainly in the southern hemisphere, remnant crustal magnetization. The magnetic fields measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) and Mars Global Surveyor spacecraft displayed a wide range of spatial scales, from the global (i.e., L ≈ 10³ km) to mesoscale (L ≈ 10² km) to small‐scale (L < 10 km). Hamil et al. (2022) used MAVEN magnetometer and Langmuir Probe data to study these structures and suggested that they might be advected into the ionosphere from the solar wind and magnetosheath. In the current study, we apply a Fourier analysis to the fields and interpret the resulting power spectral density profiles versus frequency. The power spectral density function found from MAVEN data resembles that of the solar wind magnetic field (or interplanetary magnetic field) (i.e., power law with an index of about −2), but shifted upward in frequency by a factor of about 100. From a comparison of ionospheric power spectra with solar wind power spectra, we deduce that plasma, carrying a magnetic field with it moves from the magnetic pile‐up region downward into the ionosphere with speeds of roughly tens of meters per second. The derived power spectra in the ionosphere, in addition to the basic power law shape, show hints of extra power at a spatial scale of about 10 km, and this might be due to the creation of a magnetic structure within the ionosphere itself.