Stephen A. Ledvina

A Comparison between Global Solar Magnetohydrodynamic and Potential Field Source Surface Model Results

The large-scale, steady-state magnetic field configuration of the solar corona is typically computed using boundary conditions derived from photospheric observations. Two approaches are typically used: (1) potential field source surface (PFSS) models, and (2) the magnetohydrodynamic (MHD) models. The former have the advantage that they are simple to develop and implement, require relatively modest computer resources, and can resolve structure on scales beyond those that can be handled by current MHD models. However, they have been criticized because their basic assumptions are seldom met. Moreover, PFSS models cannot directly incorporate time-dependent phenomena, such as magnetic reconnection, and do not include plasma or its effects. In this study, we assess how well PFSS models can reproduce the large-scale magnetic structure of the corona by making detailed comparisons with MHD solutions at different phases in the solar activity cycle. In particular, we (1) compute the shape of the source surface as inferred from the MHD solutions to assess deviations from sphericity, (2) compare the coronal hole boundaries as determined from the two models, and (3) estimate the effects of nonpotentiality. Our results demonstrate that PFSS solutions often closely match MHD results for configurations based on untwisted coronal fields (i.e., when driven by line-of-sight magnetograms). It remains an open question whether MHD solutions will differ more substantially from PFSS solutions when vector magnetograms are used as boundary conditions. This will be addressed in the near future when vector data from SOLIS, the Solar Dynamics Observatory, and Solar-B become incorporated into the MHD models.

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.

The role of the Martian crustal magnetic fields in controlling ionospheric loss

A hybrid particle code has been used to examine the interaction of the solar wind with Mars. It is found that the presence of the crustal magnetic fields modifies the heavy ion (O+ and O2+) loss rates. In the case of the solar minimum situation the modification was found to be significant and reported in Brecht and Ledvina (2012). In this paper both solar minimum and solar maximum results are reported and compared with data. The crustal magnetic fields reduce the ionospheric loss rate; and when the energy limits imposed on the data fits are considered, the results of the simulations are in reasonable agreement with data. The agreement with the data provides a strong argument for the physical control demonstrated by the simulations being realistic.

Control of ion loss from Mars during solar minimum

A hybrid particle code has been used to examine the interaction of the solar wind with Mars during solar minimum. The results were surprising as they produced ion loss rates from Mars far in excess of what is estimated from MEX. The results are analyzed and found to be consistent with the competition between photochemical rates and advection of the ionosphere. The simulations showed significant erosion of the ionosphere at altitudes between 200 km and 250 km altitudes. Addition of the crustal magnetic fields reduced the erosion and reduced the ion loss rates to a level consistent with the data.

The orientation of Titan’s dayside ionosphere and its effects on Titan’s plasma interaction

A hybrid particle code has been used to examine how Titan’s interaction with Saturn’s magnetosphere is effected by the orientation of the dayside ionosphere with respect to the incident magnetospheric flow. The hybrid code self-consistently includes a version of Titan’s ionosphere represented by 7 generic ion species, over 40 ionneutral chemical reactions, ion-neutral collisions and Hall and Pederson conductivities. Emphasis is placed on what effects the orientation angle has on the ion loss rates, ion densities, and the electric and magnetic fields. The results are analyzed and regardless of the orientation angle the ionosphere is found to be within photochemical equilibrium below 1200 km altitude. The ion loss rates and field structures also show little dependence on the orientation of the dayside ionosphere. It is found to first order illumination angle does not have a significant effect on these features of the Titan interaction.

Pickup ion measurements by MAVEN: A diagnostic of photochemical oxygen escape from Mars

A key process populating the oxygen exosphere at Mars is the dissociative recombination of ionospheric O2+, which produces fast oxygen atoms, some of which have speeds exceeding the escape speed and thus contribute to atmospheric loss. Theoretical studies of this escape process have been carried out and predictions made of the loss rate; however, directly measuring the escaping neutral oxygen is difficult but essential. This paper describes how energetic pickup ion measurements to be made near Mars by the SEP (Solar Energetic Particle) instrument on board the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft can be used to constrain models of photochemical oxygen escape. In certain solar wind conditions, neutral oxygen atoms in the distant Martian exosphere that are ionized and picked up by the solar wind can reach energies high enough to be detected near Mars by SEP.

Ionospheric loss from Mars as predicted by hybrid particle simulations

This paper reports the results of hybrid simulations of the Mars-solar wind interaction. The focus is on the loss rates of O+ and O2+ from the Martian ionosphere while examining the importance of including 3-D dimensional neutral atmospheres with their associated neutral winds. The simulation loss rates are found to be in excellent agreement with loss rates reported by Lundin et al. [2013]. Further, the results follow the functional form of the loss rates vs. solar EUV as determined by Lundin et al. [2013]. This paper will discuss numerical issues as well as the implementation of 3-D neutral atmospheres, including neutral winds and co-rotation flows.

Chemical interactions between Saturn’s atmosphere and its rings

Cassinis final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planets upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planets aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planets upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturns atmosphere. Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382 INTRODUCTION Past remote observations of Saturn by Pioneer 11, Voyager 1 and 2, Earth-based observatories, and the Cassini prime and solstice missions suggested an inflow of water from the rings to the atmosphere. This would modify the chemistry of Saturn’s upper atmosphere and ionosphere. In situ observations during the Cassini Grand Finale provided an opportunity to study this chemical interaction. RATIONALE The Cassini Grand Finale consisted of 22 orbital revolutions (revs), with the closest approach to Saturn between the inner D ring and the equatorial atmosphere. The Cassini Ion Neutral Mass Spectrometer (INMS) measured the composition of Saturn’s upper atmosphere and its chemical interactions with material originating in the rings. RESULTS Molecular hydrogen was the most abundant constituent at all altitudes sampled. Analysis of the atmospheric structure of H2 indicates a scale height with a temperature of 340 ± 20 K below 4000 km, at the altitudes and near-equatorial latitudes sampled by INMS. Water infall from the rings was observed, along with substantial amounts of methane, ammonia, molecular nitrogen, carbon monoxide, carbon dioxide, and impact fragments of organic nanoparticles. The infalling mass flux was calculated to be between 4800 and 45,000 kg s−1 in a latitude band of 8° near the equator. The interpretation of this spectrum is complicated by the Cassini spacecraft’s high velocity of 31 km s−1 relative to Saturn’s atmosphere. At this speed, molecules and particles have 5 eV per nucleon of energy and could have fragmented upon impact within the INMS antechamber of the closed ion source. As a result, the many organic compounds detected by INMS are very likely fragments of larger nanoparticles. Evidence from INMS indicates the presence of molecular volatiles and organic fragments in the infalling material. Methane, carbon monoxide, and nitrogen make up the volatile inflow, whereas ammonia, water, carbon dioxide, and organic compound fragments are attributed to fragmentation inside the instrument’s antechamber of icy, organic-rich grains. The observations also show evidence for orbit-to-orbit variations in the mixing ratios of infalling material; this suggests that the source region of the material is temporally and/or longitudinally variable, possibly corresponding to localized source regions in the D ring. CONCLUSION The large mass of infalling material has implications for ring evolution, likely requiring transfer of material from the C ring to the D ring in a repeatable manner. The infalling material can affect the atmospheric chemistry and the carbon content of Saturn’s ionosphere and atmosphere. INMS mass spectra from the Grand Finale. The graphic depicts the Cassini spacecraft as it passes from north to south between Saturn and its rings. The inset spectrum shows the mass deconvolution of compounds measured by INMS on rev 290. The x axis is in units of mass per charge (u) and extends over the full mass range of INMS (1 to 99 u). The y axis is in counts per measurement cycle integrated over the closest-approach data. The mass influx rate for rev 290, derived from mass deconvolution of the rev-integrated spectrum, is shown as embedded text in the spectrum. The side panel gives the average of the mass deconvolution of revs 290, 291, and 292 in mass density units (g cm–3). The composition of the ring-derived compounds in terms of percentage mass density is also shown. IMAGE COURTESY OF NASA/JPL-CALTECH/SWRI The Pioneer and Voyager spacecraft made close-up measurements of Saturn’s ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn’s atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft’s Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H2 ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.

Ion Escape Rates from Mars: Results from Hybrid Simulations Compared to MAVEN Observations

Daily averaged heavy ion escape rates from HALFSHEL hybrid simulations of the solar wind interaction with the Martian ionosphere are compared to the ion escape rates reported by Brain et al., [2015]. The simulation rates are found to be in agreement with the rates measured by MAVEN. When the simulation rates are adjusted for known variability in the Martian system, the ion escape rates are within 40% of the MAVEN results. The ion escape rate is found to vary linearly with the solar wind speed. Using the simulation results to scale the MAVEN ion escape rate to include ions of all kinetic energies, we predict a total heavy ion escape rate of 1.2 x 1025 ions/s. The assumptions used to derive the total ion escape by Brain et al., [2015] are tested against the simulation results and are found to be excellent.

Estimates of Ionospheric Transport and Ion Loss at Mars

Ion loss from the topside ionosphere of Mars associated with the solar wind interaction makes an important contribution to the loss of volatiles from this planet. Data from NASAs Mars Atmosphere and Volatile Evolution (MAVEN) mission combined with theoretical modeling are now helping us to understand the processes involved in the ion loss process. Given the complexity of the solar wind interaction, motivation exists for considering a simple approach to this problem and for understanding how the loss rates might scale with solar wind conditions and solar extreme ultraviolet irradiance. This paper reviews the processes involved in the ionospheric dynamics. Simple analytical and semi-empirical expressions for ion flow speeds and ion loss are derived. In agreement with more sophisticated models and with purely empirical studies, it is found that the oxygen loss rate from ion transport is about 5% (i.e., global O ion loss rate of Qion ≈ 4 x 1024 s-1) of the total oxygen loss rate. The ion loss is found to approximately scale as the square root of the solar ionizing photon flux and also as the square root of the solar wind dynamic pressure. Typical ion flow speeds are found to be about 1 km/s in the topside ionosphere near an altitude of 300 km on the dayside. Not surprisingly, the plasma flow speed is found to increase with altitude due to the decreasing ion-neutral collision frequency.