No abstract

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. 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.

A new kind of magnetohydrodynamic instability and waves are analyzed for a current sheet in the presence of a small normal magnetic field component varying along the sheet. These waves and instability are related to the existence of two gradients of the tangential (B_{tau}) and normal (B_{n}) magnetic field components along the normal (nabla_{n}B_{tau}) and tangential (nabla_{tau}B_{n}) directions with respect to the current sheet. The current sheet can be stable or unstable if the multiplication of two magnetic gradients is positive or negative. In the stable region, the kinklike wave mode is interpreted as so-called flapping waves observed in Earth's magnetotail current sheet. The kink wave group velocity estimated for the Earth's current sheet is of the order of a few tens of kilometers per second. This is in good agreement with the observations of the flapping motions of the magnetotail current sheet.

A new kind of magnetohydrodynamic waves are analyzed for a current sheet in a presence of a small normal magnetic field component varying along the sheet. As a background, two simplified models of a current sheet are considered with a uniform and nonuniform current distributions in the current sheet. On a basis of these two models, the flapping‐type waves are obtained which are related to a coexistence of two gradients of the tangential and normal magnetic field components along the normal and tangential directions with respect to the current sheet. A stable situation for the current sheet is associated with a positive result of the multiplication of the two magnetic gradients, and unstable (wave growth) condition corresponds to a negative result of the product. In the stable region, the “kink”‐like wave mode is interpreted as so called flapping waves observed in the Earth's magnetotail current sheet.

A new model of the electron pressure anisotropy in the electron diffusion region in collisionless magnetic reconnection is presented for the case of antiparallel configuration of magnetic fields. The plasma anisotropy is investigated as source of collisionless dissipation. By separating electrons in the vicinity of the neutral line into two broad classes of inflowing and accelerating populations, it is possible to derive a simple closure for the off-diagonal electron pressure component. The appearance of these two electron populations near the neutral line is responsible for the anisotropy and collisionless dissipation in the magnetic reconnection. Particle-in-cell simulations verify the proposed model, confirming first the presence of two particle populations and second the analytical results for the off-diagonal electron pressure component. Furthermore, test-particle calculations are performed to compare our approach with the model of electron pressure anisotropy in the inner electron diffusion region by Fujimoto and Sydora [Phys. Plasmas 16, 112309 (2009)].

[1] We present a method to determine the location of the reconnection site and the amount of reconnected magnetic flux out of an analytical time-dependent reconnection model and apply this method to disturbances observed on 2 February 2008 at about 0200 and 0815 UT by THEMIS B (P1). During these events, P1 detected two tailward propagating traveling compression regions, associated with typical variations in B z and B x . We find the reconnection site to be located at about À16 R E for the event at 0200 UT and À17.5 R E for the event at 0815 UT. These locations are consistent with simple timing considerations with respect to disturbances detected by the inner THEMIS spacecraft. The amount of reconnected flux in our 2-D model can be found to be in the order of 10 8 nT m for both events. The calculations for the reconnection site's location are done by using two approaches, i.e., by using the B z and the B x signals, yielding consistent results. The reconnected flux can be determined using B z and v z . Also, these results are in good agreement. A comparison between the disturbances detected by P1 and the modeled variations shows that our model describes disturbances in the magnetic field and the background plasma very well.

No abstract

The discovery of high concentrations of water-ice just below the Martian surface polar areas made by Mars Odyssey has strengthened the debate about the search for life on Mars. Generally it is believed that life on Earth emerged in liquid water from the processing of organic molecules. Thus, the possible origin of life on early Mars should have been related to the evolution of the planetary water inventory, consequently it is important to know the amount of water-ice stored below the planetary surface. The search and mapping of the present subsurface water and ice reservoirs will be carried out experimentally by Mars Express with its Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) ground-penetrating radar in the near future. We estimate the present and past water-ice reservoirs, which are and were in exchange with the atmosphere by using the observed D/H ratio in the atmospheric water vapour, measured D/H ratios in Martian SNC meteorites and D/H isotope ratios based on a study by Lunine et al. (2003) regarding asteroid and cometary water delivery to early Mars. Using the results of this study with initial D/H ratios of about 1.2-1.6 times the terrestrial sea water (TSW) ratio and the assumption that these ratios were not fractionated by XUV-driven hydrodynamic escape due to a more active young Sun prior to 3.5 Ga, one finds a present water-ice reservoir, which can exchange with the Martian atmosphere, equivalent to a global ocean layer with a thickness of about 3.3-15 m. By assuming that hydrodynamic escape fractionated the D/H ratio to a value that is stored in the old Martian SNC meteorites with a measured average enrichment of about 2.3 times the TSW ratio we estimate a present water-ice reservoir equivalent to a global layer with a thickness of about 11-27 m. From the obtained range of the estimated present water-ice deposit, we estimate a water-ice reservoir exchangeable with the atmosphere on Mars 3.5 Ga equivalent to a global ocean with a thickness of between 17 and 61 m. All the estimated reservoirs depend on the escape of water from Mars since 3.5 Ga equivalent to a global ocean with a thickness of about 14 m (minimum) to 34 m (maximum). The main uncertainties in the estimate of the minimal and maximal water-ice reservoir is related to the present uncertainties in the efficiency of atmospheric escape rates triggered by plasma instabilities and momentum transfer effects between the solar wind and the ionosphere. However, these uncertainties will be reduced in the near future, since both loss processes will be studied in detail by the Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA-3) on-board Mars Express. The obtained results combined with the discovery of the present water-ice subsurface reservoirs by the MARSIS radar and isotope studies as presented in this work, will also give us an idea of how enriched the atmosphere was in D compared with H after the heavy bombardment corresponding to a better understanding of the efficiency of the hydrodynamic escape process ...