We present the first large-scale statistical survey of the Jovian main emission (ME) to map auroral properties from their ionospheric locations out into the equatorial plane of the magnetosphere, where they are compared directly to in-situ spacecraft measurements. We use magnetosphere-ionosphere (MI) coupling theory to calculate currents from the auroral brightness as measured with the Hubble Space Telescope and from plasma flow speeds measured in-situ with the Galileo spacecraft. The effective Pedersen conductance of the ionosphere (\(\Sigma_P^*\)) remains a free parameter in this comparison. We first show that the field-aligned currents per radian of azimuth calculated from the auroral observations, found to be \(I_{||}=9.54^{+11.5}_{-6.35}\) MA rad-1 and \(I_{||}=10.64^{+11.1}_{-6.11}\) MA rad-1 in the north and south, respectively, are consistent with previous results. Then, we calculate the Pedersen conductance from the combined datasets, and find it ranges from \(0.02<\Sigma_P^*<2.26\) mho overall with averages of \(0.14^{+0.31}_{-0.08}\) mho in the north and \(0.14^{+0.26}_{-0.09}\) mho in the south. Taking the currents and effective Pedersen conductance together, we find that the average ME intensity and plasma flow speed in the middle magnetosphere (10-30 RJ) RJ) are broadly consistent with one another under MI coupling theory. We find evidence for peaks in the distribution of \(\Sigma_P^*\) near 7, 12, and 14 hours magnetic local time (MLT). This variation in Pedersen conductance with MLT may indicate the importance of conductance in modulating MLT- and local-time-asymmetries in the ME, including the apparent subcorotation of some auroral features within the ME.

Observations of the Lyman-a emissions from Interplanetary Hydrogen (IPH) atoms are made from Mars’ orbit using a high spectral resolution instrument in echelle configuration. The measurements can uniquely be used to resolve IPH from planetary H emissions and to subsequently determine the brightness, velocity, and thermal broadening of the IPH flow along the instrument line of sight. Planned as well as serendipitous observations, both upwind and downwind of the flow, are analyzed to determine these IPH properties and to examine the variability of IPH brightness with solar activity through the declining phase of Solar Cycle 24. A heliospheric interface model was used to simulate and interpret the derived IPH properties. The results show that the IPH brightness trends with solar irradiance, the flow is fainter downwind than upwind, the IPH brightness is variable and non-negligible compared with planetary emissions, and that deriving thermal properties of IPH requires higher spectral resolution than is presently available. These results can improve the theoretical understanding of solar system dynamics by providing empirical constraints to simulations from the inner boundary of the heliosphere and can guide the development of future interplanetary missions.

A NASA sponsored study conducted at John Hopkins University Applied Physics Lab culminated in a community-inspired heliospheric mission concept called the Interstellar Probe (ISP). The ISP’s science goals include understanding our habitable astrosphere by investigating its interactions with the interstellar medium, and determining the structure, composition, and variability of its constituents. A suite of instruments were proposed to achieve these and other science objectives. The instruments include a Lyman-a spectrograph for velocity-resolved measurements of neutral H atoms. The capability to address key components of the ISP’s science objectives by utilizing high spectral resolution Lyman-a measurements are described in this presentation. These findings have been submitted as a community White Paper to the recent Heliophysics decadal survey.

The upper atmosphere of Mars is directly affected by solar activity and the resulting solar irradiance impinging upon it. Variations in solar forcing can affect the rate at which atmospheric species escape from the planetary system. Remotely sensed observations of the upper atmosphere of Mars have been made during solar activity extrema of Solar Cycles 22 and 24. These observations were made of D and H Lyman-a emissions using the Mars Atmosphere and Volatile Evolution (MAVEN) mission and the Hubble Space Telescope (HST) high resolution spectrographs. Data obtained from the two missions are analyzed and used to derive densities and escape rates of D and H from the martian upper atmosphere. The results show that the properties of these two water-spawned atoms vary with solar cycle, and display significant inter-annual variability, mainly due to variations in atmospheric temperature. The findings suggest that cooler atmospheric temperatures due to reduced solar EUV flux may enhance the abundance of H atoms in the upper atmosphere of Mars, yet this does not increase their escape rates.

High-resolution observations made by the Hubble Space Telescope have developed the modern picture of the Jovian main oval emission as the signature of field-aligned corotation-enforcement currents which keep magnetospheric plasma rotating at the same rate as Jupiter's magnetic field. This model explains the slowly changing behavior and bright emissions of the main oval and allows the magnetosphere to be studied by remote observations, as the auroral oval directly reflects processes in the magnetosphere and properties of the plasma therein. Recent results from the Juno mission have called these results into question, as the strong, field-aligned currents required by the corotation-enforcement current system have not been measured. Where is the corotation-enforcement current-- which must exist to move magnetospheric plasma-- a dominant driver of the main oval emission? Where do other processes drive the main oval aurora instead? We present one of the widest surveys of Jupiter's main oval auroral features to date by combining over 180 hours of Hubble Space Telescope observations to address these questions. This comprehensive survey is the first to measure the corotation rate, an important tool for distinguishing auroral drivers, of all individual auroral features. This analysis is made possible due to the development of automated pipelines to reduce observations, produce keograms, identify discrete auroral emission features, and measure the corotation rates of these features, among other properties. We present the measured properties of emission features as a function of location along the main oval, in terms of longitude, local time, auroral local time, and magnetic local time. These results serve as the foundation for comparison to in-situ measurements from both the Galileo and Juno missions, which will ultimately help reveal which magnetospheric conditions are likely responsible for driving corotating emissions and which are responsible for sub-corotating emissions, such as the dawn storms.

The Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high-spectral resolution designed to resolve D and H Lyman-α emissions. The high-spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman-α emissions. This work presents improved characterizations of the high-spectral resolution mode using in-orbit observations that allow for more robust detections of the faint D Lyman-α emission line. Additionally, the instrument was re-calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high-spectral resolution instrument. Comparisons to Lyman-α observations made with the low-resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint-emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data-reduction pipeline with favorable implications for the science utility of D and H Lyman-α emissions.

Hubble Space Telescope images of Jupiter’s UV aurora show that the main emission occasionally contracts or expands, shifting toward or away from the magnetic pole by several degrees in response to changes in the solar wind dynamic pressure and Io’s volcanic activity. When the auroral footprints of the Galilean satellites move with the main emission this indicates a change in the stretched field line configuration that shifts the ionospheric mapping of a given radial distance at the equator. However, in some cases, the main emission shifts independently from the satellite footprints, indicating that the variability stems from some other part of the corotation enforcement current system that produces Jupiter’s main auroral emissions. Here we analyze HST images from the Galileo era (1996-2003) and compare latitudinal shifts of the Ganymede footprint and the main auroral emission. We focus on images with overlapping Galileo measurements because concurrent measurements are available of the current sheet strength, which indicates the amount of field line stretching and can influence both the main emission and satellite footprints. We show that the Ganymede footprint and main auroral emission typically, but do not always, move together. Additionally, we find that the auroral shifts are only weakly linked to changes in the current sheet strength measured by Galileo. We discuss implications of the observed auroral shifts in terms of the magnetospheric mapping. Finally, we establish how the statistical reference main emission contours vary with CML and show that the dependence results from magnetospheric local time asymmetries.