M. O. Fillingim

MAVEN observations of the response of Mars to an interplanetary coronal mass ejection

Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

Determination of the substorm initiation region from a major conjunction interval of THEMIS satellites

[1] We investigate in detail the time history of substorm disturbances in the magnetotail observed during a major tail conjunction of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites on 29 January 2008, 0700―0900 UT. During this interval, all THEMIS satellites were closely aligned along the tail axis near midnight and were bracketed in local time by GOES 11 and 12. The radial distance covered ranges from the geosynchronous altitude to ∼30 R E in the tail. This interval consists of three activations detected by the THEMIS satellites with good ground all-sky-camera observations of auroral activity. The first activation is a small substorm with spatially limited disturbance in the tail. The onset arc was equatorward of an undisturbed arc. The second activation is a moderate size substorm with the onset arc also being equatorward of an undisturbed arc. The third activation is an intensification of the substorm with its onset indicated by the second activation. The active auroral arc for this intensification was near the poleward boundary of the auroral oval. Analysis of these observations indicates that the first activation is a small substorm initiated in the near-Earth plasma sheet and does not involve magnetic reconnection of open magnetic field lines. Magnetic reconnection on closed field lines can be ruled out for this substorm because it cannot generate the observed high-speed plasma flow. The second and third activations are part of a moderate size substorm initiated also in the near-Earth plasma sheet, with a subsequent substorm intensification involving activity initiated tailward of ∼30 R E . Overall, the time history of substorm activity for these two substorms is consistent with the near-Earth initiation model.

Early MAVEN Deep Dip campaign reveals thermosphere and ionosphere variability

The Mars Atmosphere and Volatile Evolution (MAVEN) mission, during the second of its Deep Dip campaigns, made comprehensive measurements of martian thermosphere and ionosphere composition, structure, and variability at altitudes down to ~130 kilometers in the subsolar region. This altitude range contains the diffusively separated upper atmosphere just above the well-mixed atmosphere, the layer of peak extreme ultraviolet heating and primary reservoir for atmospheric escape. In situ measurements of the upper atmosphere reveal previously unmeasured populations of neutral and charged particles, the homopause altitude at approximately 130 kilometers, and an unexpected level of variability both on an orbit-to-orbit basis and within individual orbits. These observations help constrain volatile escape processes controlled by thermosphere and ionosphere structure and variability.

Time development of field‐aligned currents, potential drops, and plasma associated with an auroral poleward boundary intensification

[1] We present a detailed case study of the plasma and fields measured by the Cluster spacecraft fleet at the high-altitude auroral zone (-3.5 R E ) across the plasma sheet boundary layer and into the polar cap. This event, which occurred during quiet geomagnetic conditions (Kp = 1 + , AE = 50 nT), is of particular interest in that Cluster provides measurements at key instances during the time development of a new large-scale auroral arc system. Central to the formation of the arc system is the depletion of ionospheric plasma through a region of small-scale, field-aligned currents having the properties of Alfven waves. This depletion occurred prior to the growth of and ultimately bounded a well-defined equatorward moving, upward and downward current sheet pair. In association with the transverse scales approaching the electron inertial scale, the Alfvenic currents have amplitudes that appear to be attenuated subsequent to the formation of the cavity. Potential structures essentially time invariant over particle transit times (quasi-static) associated with the current pair are identified and observed to drive a poleward boundary intensification (PBI) identified in coincident IMAGE satellite far ultraviolet measurements. The PBI formed in association with a local thickening of the plasma sheet via the injection of new magnetospheric plasma, which may be the result of a bursty, patchy reconnection process. Estimates of the ionospheric equatorward velocity and thickness of the PBI are consistent with their ionospheric mapped cavity counterparts, suggesting that the motion and thickness are controlled by the plasma and electrodynamic features at or above the altitude sampled by Cluster. The magnitude of the upward and downward current region parallel potentials is correlated with the temperature of the newly injected electrons suggesting that the electron temperature is an important controlling factor. These novel observations indicate that quasi-static systems of field-aligned currents do form out of the highly dynamic Alfvenic region at the plasma sheet boundary layer, and perhaps suggest that the Alfvenic region can be the initial stage in the development of quasi-static systems. The observed time sequence of the currents is qualitatively similar to the expectations of transient response models of magnetospheric-ionospheric coupling, however, such models may need to be modified to account for the attenuation of electron inertial scale currents/Alfven waves.

Electrodynamics of the Martian dynamo region near magnetic cusps and loops

Strong and inhomogeneous remanent magnetization on Mars results in a complex pattern of crustal magnetic fields. The geometry and topology of these fields lead to atmospheric electrodynamic structures that are unique among the bodies of the solar system. In the atmospheric dynamo region (∼100−250 km altitude), ions depart from the gyropath due to collisions with neutral particles, while electron motion remains governed by electromagnetic drift. This differential motion of the charge carriers generates electric currents, which induce a perturbation field. The electromagnetic changes ultimately alter the behavior of the local ionosphere beyond the dynamo region. Here we use multifluid modeling to investigate the dynamics around an isolated magnetic cusp and around magnetic loops or arcades representative of the magnetic topology near, for example, Terra Sirenum. Our results show consistent, circular patterns in the electric current around regions with high local field strength, with possible consequences on atmospheric escape of charged particles.

On wind-driven electrojets at magnetic cusps in the nightside ionosphere of Mars

Mars has a complex magnetic topology where crustal magnetic fields can interact with the solar wind magnetic field to form magnetic cusps. On the nightside, solar wind electron precipitation can produce enhanced ionization at cusps while closed field regions adjacent to cusps can be devoid of significant ionization. Using an electron transport model, we calculate the spatial structure of the nightside ionosphere of Mars using Mars Global Surveyor electron measurements as input. We find that localized regions of enhanced ionospheric density can occur at magnetic cusps adjacent to low density regions. Under this configuration, thermospheric winds can drive ionospheric electrojets. Collisional ions move in the direction of the neutral winds while magnetized electrons move perpendicular to the wind direction. This difference in motion drives currents and can lead to charge accumulation at the edges of regions of enhanced ionization. Polarization fields drive secondary currents which can reinforce the primary currents leading to electrojet formation. We estimate the magnitude of these electrojets and show that their magnetic perturbations can be detectable from both orbiting spacecraft and the surface. The magnitude of the electrojets can vary on diurnal and annual time scales as the strength and direction of the winds vary. These electrojets may lead to localized Joule heating, and closure of these currents may require field-aligned currents which may play a role in high altitude acceleration processes.

Spectral observations of FUV auroral arcs and comparison with inverted-V precipitating electrons

[1] This paper presents first simultaneous observations of far ultraviolet (FUV) spectra of discrete and diffuse auroras, together with precipitating electrons measured on the same spacecraft, to emphasize the importance of high-resolution FUV images for accurate estimation of precipitating energy flux in the auroral region. An FUV spectrograph image with ∼2 km × 3 km resolution show small-scale features were embedded in the auroral arcs. Comparison of peak energies of inverted-V events with the corresponding FUV spectra shows that the observed long band N 2 Lyman-Birge-Hopfield (LBH) emission (long LBH band (LBHL): 160.0-171.5 nm) varied more sensitively to the peak energies compared to the short band. Comparison of the inverted-V structures and their energy fluxes with the LBHL irradiance for ∼10 km × 10 km regions show they are well correlated for peak energy >2 keV. When the data are averaged over a larger area (70 km × ∼140 km), on the other hand, the LBHL irradiance becomes less bright for the corresponding electron energy flux due to the contribution from the low-intensity background diffuse aurora produced by secondary electrons. This study demonstrates a reliable relationship between precipitating electron energy flux and LBHL intensity is obtained only if the precipitating region and FUV intensity are locally matched with a scale of less than 10 km corresponding to the size of discrete auroras.

Auroral precipitating energy during long magnetic storms

The power energy input carried by precipitating electrons into the auroral zone is an important parameter for understanding the solar wind-magnetosphere energy transfer processes and magnetic storms triggering. Some magnetic storms present a peculiar long recovery phase, lasting for many days or even weeks, which can be associated with the intense and long-duration auroral activity named HILDCAA (High Intensity Long Duration Continuous AE Activity). The auroral energy input during HILDCAAs has been pointed out as an essential key issue although there have been very few quantitative studies on this topic. In the present work, we have estimated the auroral electron precipitating energy during the events of long (LRP) and short (SRP) storm recovery phase. The energy has been calculated from the images produced by the Ultraviolet Imager (UVI) on board the Polar satellite. In order to obtain accurate energy values, we developed a dayglow estimate method to remove solar contamination from the UVI images, before calculating the energy. We compared the UVI estimate to the Hemispheric Power (HP), to the empirical power obtained from the AE index and to the solar wind input power. Our results showed that the UVI electron precipitating power for the LRP events presented a quasi-periodic fluctuation, which has been confirmed by the other estimates. We found that the LRP events are a consequence of a directly driven system, where there is no long-term energy storage in the magnetosphere, and the auroral electrojets during these events are directly affected by the electron precipitating power.

Field-Aligned Electrostatic Potentials Above the Martian Exobase From MGS Electron Reflectometry: Structure and Variability: Mars Parallel Electrostatic Potentials

3 Robert J. Lillis, J. S. Halekas, M. O. Fillingim, A. R. Poppe, G. Collinson, David A. Brain, D. L. 4 Mitchell 5 6 Space Sciences Laboratory, University of California Berkeley, 7 Gauss Way, Berkeley, CA 94720, 7 USA 8 Department of Physics and Astronomy, University of Iowa, Iowa city, Iowa, USA 9 NASA Goddard Space Flight Ctr., Greenbelt, MD, USA 10 Laboratory for Atmospheric and Space Physics, 3665 Discovery Drive, University of Colorado, 11 Boulder, CO 80309, USA 12