J. G. Luhmann

Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure

The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.

Dayside pickup oxygen ion precipitation at Venus and Mars: Spatial distributions, energy deposition and consequences

The fluxes and energy spectra of picked-up planetary O+ ions incident on the dayside atmospheres of Venus and Mars are calculated using the neutral exosphere models of Nagy and Cravens (1988) and the Spreiter and Stahara (1980) gasdynamic model of the magnetosheath electric and magnetic field. Cold (∼10 eV) O+ ions are launched from hemispherical grids of starting points covering the daysides of the planets and their trajectories are followed until they either impact the dayside “obstacle” or cross the terminator plane. The impacting, or precipitating, ion fluxes are weighted according to the altitude of the hemispherical starting point grid in a manner consistent with the exosphere density models and the local photoion production rate. Maps of precipitating ion number flux and energy flux show the asymmetrical distribution of dayside energy deposition expected from this source which is unique to the weakly magnetized planets. Although the associated heating of the atmosphere and ionosphere is found to be negligible compared to that from the usual sources, backscattered or sputtered neutral oxygen atoms are produced at energies exceeding that needed for escape from the gravitational fields of both planets. These neutral “winds,” driven by pickup ion precipitation, represent a possibly significant loss of atmospheric constituents over the age of the solar system.

The solar wind interaction with Venus

This paper assesses our current understanding of the solar wind interaction with Venus in light of developments since the last major reviews were published in 1983. Suggestions for making further progress in the area of solar wind interactions with planetary atmospheres and ionospheres are offered based on the available observations and techniques, and from the viewpoint of forthcoming missions to Mars.

Relationships between coronal mass ejection speeds from coronagraph images and interplanetary characteristics of associated interplanetary coronal mass ejections

With an eye toward space weather forecasting and the planned Solar Terrestrial Relations Observatory mission, a combination of Solwind and SMM coronagraph data and Helios-1 and Pioneer Venus Orbiter interplanetary field and plasma data are used to study statistical relationships between the speeds of coronal mass ejections (CMEs) observed near the Sun and key characteristics of the associated interplanetary disturbances (interplanetary coronal mass ejections (ICMEs)) detected near the ecliptic at ≤ 1 AU. When confident associations can be made between the coronagraph observations and interplanetary observations, a predictable relationship is found between observed coronagraph CME speeds and subsequently observed ICME bulk plasma speeds. Consistent with earlier work, the CMEs, regardless of their speed, produce ICMEs moving at least as fast as the minimum solar wind speed. As a rule, the CMEs observed at speeds below the average solar wind speed produce ICMEs that travel faster than the associated CME, implying acceleration, while CMEs with coronagraph speeds above the average solar wind speed produce ICMEs that travel slower than the associated CME, implying deceleration of initially fast low-heliolatitude ejecta. A formula is provided for estimating ICME speed from CME speed. As also previously found, faster CMEs tend to produce ICMEs with larger internal magnetic field magnitudes. While the size and occurrence of southward Bz in an ICME are not generally related to the observed CME speed, Bz in the sheath region preceding the ICME shows some positive correlation. These observations confirm that while the occurrence of large interplanetary magnetic field magnitudes and high bulk plasma speeds associated with ICME passage may be predictable from coronagraph-derived CME speeds, other important ICME features like large-magnitude southward Bz require other diagnostics and tools for forecasts.

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.

Solar cycle evolution of the structure of magnetic clouds in the inner heliosphere

Nearly ten years of continuous magnetic field observations by the Pioneer Venus spacecraft allows us to study the correlation between the structure of magnetic clouds in the inner heliosphere and the phase of the solar cycle. Fifty-six magnetic clouds have been identified in the PVO data at .7AU during 1979–1988. As this period spans nearly two solar maxima and one solar minimum we can study the evolution of the structure of these magnetic clouds through varying solar activity and under various orientations of the coronal streamer belt. Until shortly after the 1979 solar maximum the majority of clouds had an initially southward magnetic field which turned northward as the cloud was traversed, while in the period leading up to the 1988 solar maximum the majority had a northward field that turned southward. In the declining phase of solar activity magnetic clouds continued to occur, but only a minority can be classified as having south-to-north and north-to-south rotations. The majority of these clouds occurred with the field remaining entirely north or south relative to the solar equator. These results confirm observations using Helios and ISEE data indicating that the structure of magnetic clouds varies in response to changes in the magnetic structure of the source region. By interpreting these observations to imply that the leading magnetic field in magnetic clouds is controlled by the polarity of the suns global field and that the inclination of the coronal streamer belt controls the axis of symmetry of the clouds, we can predict preferred magnetic cloud structure and orientation during varying phases of the solar cycle. The helicity of the observations does not seem to be ordered by the solar cycle.

The magnetic barrier at Venus

The magnetic barrier at Venus is a region within which the magnetic pressure dominates all other pressure contributions. The barrier is formed in the inner region of the dayside magnetosheath to transfer solar wind momentum flux to the ionosphere. Passes through the dayside magnetosheath and ionopause with Pioneer Venus have allowed us to probe the magnetic barrier directly. These passes have been used to construct altitude profiles of the barrier. Here we define the ionopause as the lower boundary of the barrier. The upper boundary is defined as the altitude where the magnetosheath magnetic pressure is equal to half of the upstream solar wind dynamic pressure corrected by the boundary normal angle. The magnetic barrier is strongest at the subsolar point and weakens as expected with increasing solar zenith angle. The existence of a north-south asymmetry in the barrier strength is also demonstrated. The magnetic barrier is about 200 km thick at the subsolar point and 800 km thick at the terminator, which is comparable with the so-called “mantle.” We find that the magnetic barrier transfers most of the solar wind dynamic pressure to the ionosphere via the enhanced magnetic pressure. The convected field gasdynamic model is found to predict the correct bow shock location if the magnetic barrier is treated as the obstacle.

The loss of ions from Venus through the plasma wake

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth’s also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Topological Evolution of a Fast Magnetic Breakout CME in Three Dimensions

We present the extension of the magnetic breakout model for CME initiation to a fully three-dimensional, spherical geometry. Given the increased complexity of the dynamic magnetic field interactions in three dimensions, we first present a summary of the well known axisymmetric breakout scenario in terms of the topological evolution associated with the various phases of the eruptive process. In this context, we discuss the analogous topological evolution during the magnetic breakout CME initiation process in the simplest three-dimensional multipolar system. We show that an extended bipolar active region embedded in an oppositely directed background dipole field has all the necessary topological features required for magnetic breakout, i.e., a fan separatrix surface between the two distinct flux systems, a pair of spine field lines, and a true three-dimensional coronal null point at their intersection. We then present the results of a numerical MHD simulation of this three-dimensional system where boundary shearing flows introduce free magnetic energy, eventually leading to a fast magnetic breakout CME. The eruptive flare reconnection facilitates the rapid conversion of this stored free magnetic energy into kinetic energy and the associated acceleration causes the erupting field and plasma structure to reach an asymptotic eruption velocity of 1100 km s−1 over an ~15 minute time period. The simulation results are discussed using the topological insight developed to interpret the various phases of the eruption and the complex, dynamic, and interacting magnetic field structures.

Geomagnetic response to magnetic clouds of different polarity

The polarity of a magnetic cloud refers to its changing magnetic field direction. It is classified as S-N polarity when the magnetic field rotates from southward to northward and N-S polarity when the field is initially northward and rotates southward. A study of 29 magnetic cloud events has found that 40–45% of magnetic clouds, independent of polarity, are followed by a fast solar wind stream which compresses the tail end of the cloud. The compression results in an increase in the solar wind plasma density and in 64% of the cases an increase in the magnetic field strength towards the latter part of the cloud. Such tail end compression can have a significant effect upon geomagnetic storm intensity if the magnetic cloud is of N-S polarity. This is because only in the N-S polarity case does the compression coincide with the southward IMF portion of the cloud. To test the “geoeffectiveness” of N-S versus S-N magnetic clouds three selected magnetic cloud events, two of S-N polarity and one of N-S polarity, are investigated in terms of their geomagnetic response through measured and estimated Dst values. It is found that there is an increased geoeffectiveness of N-S polarity clouds due to both an increased solar wind dynamic pressure and a compressed southward field associated with a following fast solar wind stream.