James P. McFadden

Tail Reconnection Triggering Substorm Onset

Magnetospheric substorms explosively release solar wind energy previously stored in Earths magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at ∼10 RE (RE: Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at ∼20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 RE, at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

MHD model results of solar wind interaction with Mars and comparison with MAVEN plasma observations

The Mars Atmosphere and Volatile EvolutioN mission (MAVEN), launched on 18 November 2013, is now in its primary science phase, orbiting Mars with a 4.5 h period. In this study, we use a time-dependent MHD model to interpret plasma observations made by MAVEN particle and field instruments. Detailed comparisons between the model and the relevant plasma observations from MAVEN are presented for an entire Mars rotation under relatively quiet solar wind conditions. Through comparison along MAVEN orbits, we find that the time-dependent multispecies single-fluid MHD model is able to reproduce the main features of the plasma environment around Mars. Using the model results, we find that photoionization beyond the terminator is the dominant ion source as compared with day-night transport in maintaining the nightside ionosphere. Model results also show that both the time-varying solar wind conditions and the continuously rotating crustal field work together to control the ion escape variation with time.

Magnetotail dynamics at Mars: Initial MAVEN observations

We report on the complex nature of the induced Martian magnetotail using simultaneous magnetic field and plasma measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Two case studies are analyzed from which we identify (1) repetitive loading and unloading of tail magnetic flux as the field magnitude changes dramatically, exhibiting signatures similar to substorm activity within intrinsic magnetospheres; (2) multiple current sheet crossings indicative of plasma sheet flapping; (3) tailward flowing high-energy planetary ions (O+ and O2+), confined exclusively to the cross-tail current sheet, contributing to atmospheric escape; and (4) signatures of magnetic flux ropes, suggesting the occurrence of tail reconnection. These events illustrate the complexity of the Martian magnetotail as MAVEN provides key observations relevant to the unanswered questions of induced magnetosphere dynamics.

Response of Mars O+ pickup ions to the 8 March 2015 ICME: Inferences from MAVEN data‐based models

We simulate and compare three phases of the Mars-solar wind interaction with the 8 March interplanetary coronal mass ejection (ICME) event using Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observations in order to derive heavy ion precipitation and escape rates. The MAVEN observations provide the initial conditions for three steady state MHD model cases, which reproduce the observed features in the solar wind density, velocity, and magnetic field seen along the MAVEN orbit. Applying the MHD results to a kinetic test particle model, we simulate global precipitation and escape maps of O+ during the (1) pre-ICME phase, (2) sheath phase, and (3) ejecta phase. We find that the Case 1 had the lowest precipitation and escape rates of 9.5 × 1025 and 4.1 × 1025 s−1, Case 2 had the highest rates of 9.5 × 1025 and 4.1 × 1025 s−1, and Case 3 had rates of 3.2 × 1025 and 1.3 × 1025 s−1, respectively. Additionally, Case 2 produced a high-energy escaping plume >10 keV, which mirrored corresponding STATIC observations.

Estimation of the spatial structure of a detached magnetic flux rope at Mars based on simultaneous MAVEN plasma and magnetic field observations

Simultaneous Mars Atmosphere and Volatile EvolutioN mission (MAVEN) plasma and magnetic field observations reveal a detached magnetic flux rope in the Martian induced magnetosphere. The flux rope was identified by an increase in the magnetic field amplitude accompanied by smooth vector rotations. In addition, MAVEN observed a pronounced ion composition change across the structure, with solar wind ions dominating outside and planetary ions dominating within. Grad-Shafranov reconstruction is applied to determine the two-dimensional spatial structure of the flux rope. The event occurred near the dusk terminator, downstream from strong crustal magnetic fields. One possibility is that the flux rope was created by magnetic reconnection associated with interplanetary and/or crustal magnetic fields. A weak interplanetary coronal mass ejection (ICME) arrived at Mars a few hours before the event. A pressure pulse and turbulent magnetic fields due to the ICME might be responsible for driving magnetic reconnection to detach the flux rope from the crustal source.

Photochemical escape of oxygen from Mars: First results from MAVEN in situ data

Photochemical escape of atomic oxygen is thought to be one of the dominant channels for Martian atmospheric loss today and played a potentially major role in climate evolution. MAVEN is the first mission capable of measuring, in situ, the relevant quantities necessary to calculate photochemical escape fluxes. We utilize 18 months of data from three MAVEN instruments: LPW, NGIMS and STATIC. From these data we calculate altitude profiles of the production rate of hot oxygen atoms from the dissociative recombination (DR) of O2+ and the probability that such atoms will escape the Mars atmosphere. From this we determine escape fluxes for 815 periapsis passes. Derived average dayside hot O escape rates range from 1.2 to 5.5 x 1025 s-1 depending on season and EUV flux, consistent with several pre-MAVEN predictions and in broad agreement with estimates made with other MAVEN measurements. Hot O escape fluxes do not vary significantly with dayside solar zenith angle or crustal magnetic field strength, but depend on CO2 photoionization frequency with a power law whose exponent is 2.6 ± 0.6, an unexpectedly high value which may be partially due to seasonal and geographic sampling. From this dependence and historical EUV measurements over 70 years, we estimate a modern-era average escape rate of 4.3 x 1025 s-1. Extrapolating this dependence to early solar system EUV conditions gives total losses of 13, 49, 189, and 483 mb of oxygen over 1, 2, 3, and 3.5 Gyr respectively, with uncertainties significantly increasing with time in the past.

Proton cyclotron waves occurrence rate upstream from Mars observed by MAVEN: associated variability of the Martian upper atmosphere

Measurements provided by the Magnetometer and the Extreme Ultraviolet Monitor (EUVM) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft together with atomic H exospheric densities derived from numerical simulations are studied for the time interval from October 2014 up to March 2016. We determine the proton cyclotron waves (PCWs) occurrence rate observed upstream from Mars at different times. We also study the relationship with temporal variabilities of the high altitude Martian hydrogen exosphere and the solar EUV flux reaching the Martian environment. We find that the abundance of PCWs is higher when Mars is close to perihelion, and decreases to lower and approximately constant values after the Martian Northern Spring Equinox. We also conclude that these variabilities cannot be associated with biases in MAVENs spatial coverage or changes in the background magnetic field orientation. Higher H exospheric densities on the Martian day side are also found when Mars is closer to perihelion, as a result of changes in the thermospheric response to variability in the ultraviolet flux reaching Mars at different orbital distances. A consistent behavior is also observed in the analyzed daily irradiances measured by the MAVEN EUVM. The latter trends point towards an increase in the planetary proton densities upstream from the Martian bow shock near perihelion. These results then suggest a method to indirectly monitor the variability of the H exosphere up to very high altitudes during large time intervals (compared to direct measurements of neutral particles), based on the observed abundance of PCWs.

The Impact and Solar Wind Proxy of the 2017 September ICME Event at Mars

We study a large ICME event impacting Mars in mid-September 2017 numerically. During this time period, MAVEN remained inside the Martian bow shock, and therefore could not measure the solar wind directly. We first simulate the event using three steady-state cases with estimated solar wind conditions, and find that these cases were able to reproduce the general features observed by MAVEN. However, these time-stationary runs cannot capture the response of the system to large variations in the solar wind associated with the event. To address this question, we derive a solar wind proxy based on MAVEN observations in the sheath region and their comparison with steady-state MHD model results. The derived solar wind proxy is then used to drive a time-dependent MHD model, and we find that the datamodel comparison is greatly improved, especially in the magnetosheath. We are able to reproduce some detailed structures observed by MAVEN during the period, despite the lack of a direct measurement of the solar wind, indicating the derived solar wind conditions are reliable. Finally, we examine in detail the impact of the event on the Martian system: including variations of the three typical plasma boundaries and the ion loss rates. Our results show that these plasma boundary locations varied drastically during the event, and the total ion loss rate was enhanced by more than an order of magnitude.

MAVEN observations on a hemispheric asymmetry of precipitating ions toward the Martian upper atmosphere according to the upstream solar wind electric field

MAVEN observations show that the global spatial distribution of ions precipitating toward the Martian upper atmosphere has a highly asymmetric pattern relative to the upstream solar wind electric field. MAVEN observations indicate that precipitating planetary heavy ion fluxes measured in the downward solar wind electric field (−E) hemisphere are generally larger than those measured in the upward electric field (+E) hemisphere, as expected from modeling. The −E(+E) hemispheres are defined by the direction of solar wind electric field pointing towards (or away from) the planet. On the other hand, such an asymmetric precipitating pattern relative to the solar wind electric field is less clear around the terminator. Strong precipitating fluxes are sometimes found even in the +E field hemisphere under either strong upstream solar wind dynamic pressure or strong interplanetary magnetic field periods. The results imply that those intense precipitating ion fluxes are observed when the gyro radii of pickup ions are estimated to be relatively small compared with the planetary scale. Therefore, the upstream solar wind parameters are important factors in controlling the global spatial pattern and flux of ions precipitating into the Martian upper atmosphere.

Statistical Study of Relations Between the Induced Magnetosphere, Ion Composition, and Pressure Balance Boundaries around Mars Based on MAVEN Observations†

Direct interaction between the solar wind (SW) and the Martian upper atmosphere forms a characteristic region, called the induced magnetosphere between the magnetosheath and the ionosphere. Since the SW deceleration due to increasing mass loading by heavy ions plays an important role in the induced magnetosphere formation, the ion composition is also expected to change around the induced magnetosphere boundary (IMB). Here we report on relations of the IMB, the ion composition boundary (ICB), and the pressure balance boundary based on a statistical analysis of about 8-months of simultaneous ion, electron, and magnetic field observations by Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We chose the period when MAVEN observed the SW directly near its apoapsis to investigate their dependence on SW parameters. Results show that IMBs almost coincide with ICBs on the dayside and locations of all three boundaries are affected by the SW dynamic pressure. A remarkable feature is that all boundaries tend to locate at higher altitudes in the southern hemisphere than in the northern hemisphere on the nightside. This clear geographical asymmetry is permanently seen regardless of locations of the strong crustal B fields in the southern hemisphere, while the boundary locations become higher when the crustal B fields locate on the dayside. On the nightside, IMBs usually locate at higher altitude than ICBs. However, ICBs are likely to be located above IMBs in the nightside, southern, and downward ESW hemisphere when the strong crustal B fields locate on the dayside.