Y. J. Ma

Multifluid Block‐Adaptive‐Tree Solar wind Roe‐type Upwind Scheme: Magnetospheric composition and dynamics during geomagnetic storms—Initial results

[1]xa0The magnetosphere contains a significant amount of ionospheric O+, particularly during geomagnetically active times. The presence of ionospheric plasma in the magnetosphere has a notable impact on magnetospheric composition and processes. We present a new multifluid MHD version of the Block-Adaptive-Tree Solar wind Roe-type Upwind Scheme model of the magnetosphere to track the fate and consequences of ionospheric outflow. The multifluid MHD equations are presented as are the novel techniques for overcoming the formidable challenges associated with solving them. Our new model is then applied to the May 4, 1998 and March 31, 2001 geomagnetic storms. The results are juxtaposed with traditional single-fluid MHD and multispecies MHD simulations from a previous study, thereby allowing us to assess the benefits of using a more complex model with additional physics. We find that our multifluid MHD model (with outflow) gives comparable results to the multispecies MHD model (with outflow), including a more strongly negative Dst, reduced CPCP, and a drastically improved magnetic field at geosynchronous orbit, as compared to single-fluid MHD with no outflow. Significant differences in composition and magnetic field are found between the multispecies and multifluid approach further away from the Earth. We further demonstrate the ability to explore pressure and bulk velocity differences between H+ and O+, which is not possible when utilizing the other techniques considered.

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.

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-dependent global MHD simulations of Cassini T32 flyby: From magnetosphere to magnetosheath

When the Cassini spacecraft flew by Titan on 13 June 2007, at 13.6 Saturn local time, Titan was directly observed to be outside Saturns magnetopause. Cassini observations showed dramatic changes of magnetic field orientation as well as other plasma flow parameters during the inbound and outbound segments. In this paper, we study Titans ionospheric responses to such a sudden change in the upstream plasma conditions using a sophisticated multispecies global MHD model. Simulation results of three different cases (steady state, simple current sheet crossing, and magnetopause crossing) are presented and compared against Cassini Magnetometer, Langmuir Probe, and Cassini Plasma Spectrometer observations. The simulation results provide clear evidence for the existence of a fossil field that was induced in the ionosphere. The main interaction features, as observed by the Cassini spacecraft, are well reproduced by the time-dependent simulation cases. Simulation also reveals how the fossil field was trapped during the interaction and shows the coexistence of two pileup regions with opposite magnetic orientation, as well as the formation of a pair of new Alfven wings and tail disconnection during the magnetopause crossing process.

Dynamical and magnetic field time constants for Titan's ionosphere: Empirical estimates and comparisons with Venus

plasma flow speed relative to the neutral gas speed is approximately 1 m s −1 near an altitude of 1000 km and 200 m s −1 at 1500 km. For comparison, the thermospheric neutral wind speed is about 100 m s −1 . The ionospheric plasma is strongly coupled to the neutrals below an altitude of about 1300 km. Transport, vertical or horizontal, becomes more important than chemistry in controlling ionospheric densities above about 1200–1500 km, depending on the ion species. Empirical estimates are used to demonstrate that the structure of the ionospheric magnetic field is determined by plasma transport (including neutral wind effects) for altitudes above about 1000 km and by magnetic diffusion at lower altitudes. The paper suggests that a velocity shear layer near 1300 km could exist at some locations and could affect the structure of the magnetic field. Both Hall and polarization electric field terms in the magnetic induction equation are shown to be locally important in controlling the structure of Titan’s ionospheric magnetic field. Comparisons are made between the ionospheric dynamics at Titan and at Venus.

Martian ionospheric responses to dynamic pressure enhancements in the solar wind

As a weakly magnetized planet, Mars ionosphere/atmosphere interacts directly with the shocked solar wind plasma flow. Even though many numerical studies have been successful in reproducing numerous features of the interaction process, these earlier studies focused mainly on interaction under steady solar wind conditions. Recent observations suggest that plasma escape fluxes are significantly enhanced in response to solar wind dynamic pressure pulses. In this study, we focus on the response of the ionosphere to pressure enhancements in the solar wind. Through modeling of two idealized events using a magnetohydrodynamics model, we find that the upper ionosphere of Mars responds almost instantaneously to solar wind pressure enhancements, while the collision dominated lower ionosphere (below ~150u2009km) does not have noticeable changes in density. We also find that ionospheric perturbations in density, magnetic field, and velocity can last more than an hour after the solar wind returns to the quiet conditions. The topside ionosphere forms complicated transient shapes in response, which may explain unexpected ionospheric behaviors in recent observations. We also find that ionospheric escape fluxes do not correlate directly with simultaneous solar wind dynamic pressure. Rather, their intensities also depend on the earlier solar wind conditions. It takes a few hours for the ionospheric/atmospheric system to reach a new quasi-equilibrium state.

The influence of production mechanisms on pick‐up ion loss at Mars

[1]xa0This study quantifies the influence of ionization production mechanisms on ion escape and transport through near-Mars space. The Mars Test Particle simulation calculates the detailed ion velocity space distribution through a background magnetic and electric field model at specific locations. The main objective of this work is to extensively probe the sources of O+ ion escape relative to the production mechanisms: photoionization, charge exchange, and electron impact. Seven production methods are explored and compared, resulting in total production and loss rates differing up to two orders of magnitude. Photoionization was compared as a function of solar zenith angle and optical shadow. Charge exchange O+ production was studied with three methods: a constant rate assuming cold ion collisions, a constant rate proportional to the reaction cross-section and upstream solar wind bulk velocity, and finally a novel approach proportional to the cross-section and both the random and bulk velocity. Finally, electron impact ionization was considered as a constant and as a function of electron temperature. Of these methods, a baseline of the most physically relevant ion mechanisms was selected. Additionally, energy distributions at specific spatial locations highlight the individual ion populations in velocity space, revealing asymmetric and nongyrotropic features due to specific ionization methods. Analysis of the O+ flux and loss is in agreement with observations and also indicates a strong polar plume in the northern hemisphere for a given interplanetary magnetic field orientation. We calculate the total production and escape to be 2.5u2009×u20091025 and 6.4u2009×u20091024, respectively.

Time‐varying magnetospheric environment near Enceladus as seen by the Cassini magnetometer

[1]xa0In 2008, the Cassini spacecraft made four close Enceladus flybys along similar trajectories. During these flybys the magnetometer recorded the time-varying magnetic field associated with the plasma interaction with Enceladus and its plume. Close to Enceladus, the Cassini magnetometer observed 4% to 7% enhancement in the magnetic field magnitude, associated with the slowing down of the ambient plasma. Herein we examine these four flybys, estimate the deceleration of the flow, locate the momentum-loading center for each pass, and compare their pass to pass variability. Even though the spacecraft trajectories were similar, two different types of perturbations were observed at distances greater than 5 Enceladus radii downstream, and to the north of the moon. Ion-cyclotron waves were observed during each of the flybys, with pass to pass wave amplitudes varying in a similar manner as the enhancement of the field magnitude.

Interaction of Saturn's magnetosphere and its moons: 2. Shape of the Enceladus plume

[1]xa0The Saturnian moons in the inner magnetosphere are immersed in a plasma disk that rotates much faster than the moons Keplerian speed. The interaction of the rotating plasma with the moons results in a disturbance in the Saturnian magnetospheric plasma that depends on the nature of obstacle that the moon represents. In particular at Enceladus, such perturbations in the magnetic field and flowing plasma enable us to infer the 3-D shape of the Enceladus plume and its outgassing rate. In this paper, we apply our 3-D magnetohydrodynamic model to extensively study the effects of different plume and disk plasma conditions on the interaction. By finding the best agreement with the observations of two diagnostic flybys, one with its point of closest approach on the upstream side and the other on the downstream side, we determine the plume intensity and configuration. We find that mass loading in the plume is less efficient close to the surface of the moon, where the neutral density is the highest. For E2 and E5, the opening angle of the plume is about 20°, and the plume is tilted toward the corotating direction. The upstream density has a significant effect on the mass loading rate, while its effect on the magnitude of the magnetic perturbation is less significant. An upstream velocity component in the Saturn direction helps to explain the observed magnetic perturbation in the By component and signals the need to consider Enceladuss effect on the global plasma circulation in addition to the local effect. Quantitative comparisons of the simulated and observed interaction are provided.

Test particle comparison of heavy atomic and molecular ion distributions at Mars

This study uses the Mars Test Particle simulation to create virtual detections of O+, O2+, and CO2+ in an orbital configuration in the Mars space environment. These atomic and molecular planetary pickup ions are formed when the solar wind directly interacts with the neutral atmosphere, causing the ions to be accelerated by the background convective electric field. The subsequent ion escape is the subject of great interest, specifically with respect to which species dominates ion loss from Mars. O+ is found to be the dominant escaping ion because of the large sources of transported ions in the low-energy ( 1 keV) range. O2+ and CO2+ are observed at these energy ranges but with much lower fluxes and are generally only found in the tail between 10 eV and 1 keV. Using individual particle traces, we reveal the origin and trajectories of the low-energy downtail O+ populations and high-energy polar O+ populations that contribute to the total escape. Comparing them against O2+ and CO2+ reveals that the extended hot oxygen corona contributes to source regions of high- and low-energy escaping ions. Additionally, we present results for solar minimum and maximum conditions with respect to ion fluxes and energies in order to robustly describe the physical processes controlling planetary ion distributions and atmospheric escape.