Herbert I. M. Lichtenegger

Phenomenology of the Lense-Thirring effect in the solar system

Recent years have seen increasing efforts to directly measure some aspects of the general relativistic gravitomagnetic interaction in several astronomical scenarios in the solar system. After briefly overviewing the concept of gravitomagnetism from a theoretical point of view, we review the performed or proposed attempts to detect the Lense-Thirring effect affecting the orbital motions of natural and artificial bodies in the gravitational fields of the Sun, Earth, Mars and Jupiter. In particular, we will focus on the evaluation of the impact of several sources of systematic uncertainties of dynamical origin to realistically elucidate the present and future perspectives in directly measuring such an elusive relativistic effect.

Determining the mass loss limit for close-in exoplanets: what can we learn from transit observations?

Aims. We study the possible atmospheric mass loss from 57 known transiting exoplanets around F, G, K, and M-type stars over evolutionary timescales. For stellar wind induced mass loss studies, we estimate the position of the pressure balance boundary between Coronal Mass Ejection (CME) and stellar wind ram pressures and the planetary ionosphere pressure for non- or weakly magnetized gas giants at close orbits. Methods. The thermal mass loss of atomic hydrogen is calculated by a mass loss equation where we consider a realistic heating efficiency, a radius-scaling law and a mass loss enhancement factor due to stellar tidal forces. The model takes into account the temporal evolution of the stellar EUV flux by applying power laws for F, G, K, and M-type stars. The planetary ionopause obstacle, which is an important factor for ion pick-up escape from non- or weakly magnetized gas giants is estimated by applying empirical power-laws. Results. By assuming a realistic heating efficiency of about 10–25% we found that WASP-12b may have lost about 6–12% of its mass during its lifetime. A few transiting low density gas giants at similar orbital location, like WASP-13b, WASP-15b, CoRoT-1b or CoRoT-5b may have lost up to 1–4% of their initial mass. All other transiting exoplanets in our sample experience negligible thermal loss (≤1%) during their lifetime. We found that the ionospheric pressure can balance the impinging dense stellar wind and average CME plasma flows at distances which are above the visual radius of “Hot Jupiters”, resulting in mass losses <2% over evolutionary timescales. The ram pressure of fast CMEs cannot be balanced by the ionospheric plasma pressure for orbital distances between 0.02–0.1 AU. Therefore, collisions of fast CMEs with hot gas giants should result in large atmospheric losses which may influence the mass evolution of gas giants with masses <MJup. Depending on the stellar luminosity spectral type, planetary density, heating efficiency, orbital distance, and the related Roche lobe effect, we expect that at distances between 0.015–0.02 AU, Jupiter-class and sub-Jupiter-class exoplanets can lose several percent of their initial mass. At orbital distances ≤0.015 AU, low density hot gas giants in orbits around solar type stars may even evaporate down to their coresize, while low density Neptune-class objects can lose their hydrogen envelopes at orbital distances ≤0.02 AU.

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

Atmosphere and water loss from early mars under extreme solar wind and extreme ultraviolet conditions

The upper limits of the ion pickup and cold ion outflow loss rates from the early martian atmosphere shortly after the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) were investigated. We applied a comprehensive 3-D multi-species magnetohydrodynamic (MHD) model to an early martian CO(2)-rich atmosphere, which was assumed to have been exposed to a solar XUV [X-ray and extreme ultraviolet (EUV)] flux that was 100 times higher than today and a solar wind that was about 300 times denser. We also assumed the late onset of a planetary magnetic dynamo, so that Mars had no strong intrinsic magnetic field at that early period. We found that, due to such extreme solar wind-atmosphere interaction, a strong magnetic field of about approximately 4000 nT was induced in the entire dayside ionosphere, which could efficiently protect the upper atmosphere from sputtering loss. A planetary obstacle ( approximately ionopause) was formed at an altitude of about 1000 km above the surface due to the drag force and the mass loading by newly created ions in the highly extended upper atmosphere. We obtained an O(+) loss rate by the ion pickup process, which takes place above the ionopause, of about 1.5 x 10(28) ions/s during the first < or =150 million years, which is about 10(4) times greater than today and corresponds to a water loss equivalent to a global martian ocean with a depth of approximately 8 m. Consequently, even if the magnetic protection due to the expected early martian magnetic dynamo is neglected, ion pickup and sputtering were most likely not the dominant loss processes for the planets initial atmosphere and water inventory. However, it appears that the cold ion outflow into the martian tail, due to the transfer of momentum from the solar wind to the ionospheric plasma, could have removed a global ocean with a depth of 10-70 m during the first < or =150 million years after the Sun arrived at the ZAMS.

Hot oxygen and carbon escape from the martian atmosphere

Abstract The escape of hot O and C atoms from the present martian atmosphere during low and high solar activity conditions has been studied with a Monte-Carlo model. The model includes the initial energy distribution of hot atoms, elastic, inelastic, and quenching collisions between the suprathermal atoms and the ambient cooler neutral atmosphere, and applies energy dependent total and differential cross sections for the determination of the collision probability and the scattering angles. The results yield a total loss rate of hot oxygen of 2.3 – 2.9 × 10 25 s − 1 during low and high solar activity conditions and is mainly due to dissociative recombination of O2+ and CO2+. The total loss rates of carbon are found to be 0.8 and 3.2 × 10 24 s − 1 for low and high solar activity, respectively, with photodissociation of CO being the main source. Depending on solar activity, the obtained carbon loss rates are up to ~40 times higher than the CO2+ ion loss rate inferred from Mars Express ASPERA-3 observations. Finally, collisional effects above the exobase reduce the escape rates by about 20–30% with respect to a collionless exophere.

On the gravitomagnetic clock effect

General relativity predicts that two freely counter-revolving test particles in the exterior field of a central rotating mass take different periods of time to complete the same full orbit; this time difference leads to the gravitomagnetic clock effect. The effect has been derived for circular equatorial orbits; moreover, it has been extended via azimuthal closure to spherical orbits around a slowly rotating mass. In this Letter, a general formula is derived for the main gravitomagnetic clock effect in the case of slow motion along an arbitrary elliptical orbit in the exterior field of a slowly rotating mass. Some of the implications of this result are briefly discussed.

Energetic neutral atoms at Mars 3. Flux and energy distributions of planetary energetic H atoms

[1] The energetic neutral atom (ENA) distribution around Mars is characterized by ENAs which have their origin in the solar wind (H ENA SW ) and by a second particle population which comes from the planetary atmosphere (H ENA pl ). Since planetary energetic neutral hydrogen atoms contain information about the Martian water inventory, we calculated their flux and energy distribution. We used a test particle model which involves the motion in the external electric and magnetic field. It is shown that after a planetary neutral hydrogen atom is transformed into an ion via charge exchange with solar wind particles, solar UV, or electron impact, it is accelerated to higher energies by the convective electric field and gradually guided by the solar wind plasma flow around the planetary obstacle. Some of the newly born planetary ions take part in a charge exchange reaction with particles of the upper Martian atmosphere and will thus be transformed into planetary ENAs. The integral flux of planetary hydrogen ENAs is found to be highest in the magnetosheath with a distinct asymmetry between the direction of the solar wind magnetic and the solar wind electric field. For typical solar wind parameters the maximum planetary ENA flux can reach almost 10% of the unperturbed solar wind flux at solar minimum in some beams generated at middle latitudes in the dayside magnetosphere. The energy of planetary ENAs upstream of the bow shock can exceed the solar wind energy, while the downtail magnetosheath region is populated by ENAs with energies below that of ENAs originating from the solar wind. Our results suggest that the ENA detector on board of the Mars Express spacecraft can separate the hydrogen ENA populations from the solar wind ENAs by a careful analysis of the particle energies.

Particle simulation in the Martian magnetotail

A gasdynamic model of the magnetosheath is extended into the tail region by incorporating a cometary tail field produced by mass loading. By means of test particle simulations, a picture of the martian pickup ion wake is described and contrasted with particle measurements of the Phobos 2 automatic space plasma experiment with a rotating analyzer (ASPERA). It is shown that the convection electric field alone is not sufficient to explain the observations. If magnetic shear stresses of the draped field are taken into account, the flux of low-energy oxygen ions close to the central wake of Mars (a persistent observational feature in the particle data) is reproduced. Simulation results suggest that the low-energy ion observations (E ≤ 350 eV) of the ASPERA particle instrument are due to particles picked up in a source region at lower altitude close to the terminator plane at low latitudes, while moderate energy ions are created at high areographic latitudes, depending on the orientation of the transverse interplanetary magnetic field component. High-energy ions (E ≥ 2 keV) usually originate from more distant regions in the magnetosheath or in the solar wind above the dayside of Mars.

Geophysical and Atmospheric Evolution of Habitable Planets

The evolution of Earth-like habitable planets is a complex process that depends on the geodynamical and geophysical environments. In particular, it is necessary that plate tectonics remain active over billions of years. These geophysically active environments are strongly coupled to a planets host star parameters, such as mass, luminosity and activity, orbit location of the habitable zone, and the planets initial water inventory. Depending on the host stars radiation and particle flux evolution, the composition in the thermosphere, and the availability of an active magnetic dynamo, the atmospheres of Earth-like planets within their habitable zones are differently affected due to thermal and nonthermal escape processes. For some planets, strong atmospheric escape could even effect the stability of the atmosphere.

Model calculations of the planetary ion distribution in the Martian tail

Based on a recent model of the Martian atmosphere/exosphere and a model of the magnetic field and solar wind flow around Mars, the distribution of different planetary ion species in the near tail is calculated. Three main regions are identified: 1) “clouds” of pickup ions with distinct mass separation travel along cycloidal trajectories; 2) another group of ions forms a distinct plasma mantle in the magnetosphere; 3) a third population fills up the plasma sheet. Further, the energy of ions in different locations is also analyzed. Finally, comparison of observations made onboard the Phobos-2 spacecraft shows a reasonable agreement with simulation results.