SM34A-01
Solar wind interaction with Venus at solar minimum: Venus Express magnetic field observations
The launch of Venus Express provides a new opportunity to study the solar wind interaction with Venus. Although a wealth of knowledge about the interaction of Venus and the solar wind has been obtained from earlier missions, notably the long lasting PVO mission, Venus Express will greatly improve our view of the Venus plasma environment due to many unique characteristics of the mission such as the improved capability of the onboard instrumentation, the unique orbital trajectory and the solar minimum observations at a low altitude compared with PVO. It is the purpose of this paper to illustrate some of the new science obtained by the Venus Express. We find that the bow shock stands off from the planet a smaller distance than at solar maximum. On the dayside, the magnetic field piles up to form a magnetic barrier in the inner magnetosheath. This magnetic barrier, an induced magnetosphere on the dayside, acts as an obstacle to the solar wind in analog to the Earth's magnetosphere. The magnetic barrier is bounded by the ionopause at its lower boundary and a "magnetopause" at its upper boundary. Both ionopause and magnetopause extend to nightside. The magnetopause on the nightside separates the magnetosheath and magnetotail which is formed by the anchored, draped magnetic fields. In contrast to its unmagnetized state at solar maximum, the ionosphere appears to be completely magnetized and its upper boundary, the ionopause, significantly lowered. The magnetic draping configuration on the dayside becomes reverse draping on the night, forming a near toroidal magnetic field at low altitude.
SM34A-02
Upstream Cyclotron Waves from Venus Express Observations
The escape of particles from planetary atmospheres, especially hydrogen, is an important key towards understanding the atmospheric composition and evolution over the lifetime of the solar system. For an unmagnetized planet such as Venus or Mars when the neutral exosphere extends into the flowing solar wind plasma, loss of pick-up ions can play a significant role. Cyclotron waves from pick-up hydrogen in the solar wind have been previously observed at Mars but not at Venus. Here we report proton-cyclotron waves seen well upstream from the planet both outside and inside the planetary foreshock region, giving direct evidence that the solar wind is removing hydrogen from the planetary exosphere. Properties of the wave-occurrences are extensively studied and statistics are shown, i.e. specific aspects of the spectra, analysis in the magnetic field principal axes system, direction of propagation. The proton cyclotron waves appear to be intermittent; moreover, from PVO observations there were not reported. This raises the question why the situation at Venus is so different from Mars.
SM34A-03
Numerical Simulations of the Magnetic Topology Near Mars Auroral Observations
Magnetohydrodynamic (MHD) simulation results are presented of the magnetic topology near the location where auroral emissions were observed. MHD simulations were conducted for this scenario with high spatial resolution centered on the region where the Mars Express SPICAM instrument observed intense aurora-like emissions (as published in Nature). This is also the region of strong magnetic field source in the Mars crust. A dense array of field lines were then extracted from the MHD simulation results in order to determine the most likely source population for these emissions. Specifically, two sources are being evaluated: solar wind electrons precipitating from the magnetosheath along open field lines, and atmospheric photoelectrons crossing the terminator on closed field loops. The ASPERA-3 ELS instrument on Mars Express observed photoelectrons near the time of the SPICAM measurements of auroral emissions, raising the possibility of photoelectrons as the auroral energy source. It is shown that closed field loops can indeed cross the terminator, thus depositing atmospheric photoelectrons into the nightside ionosphere, and possibly causing auroral/airglow emissions. However, for the specific location of the SPICAM auroral observations, the MHD results show only open field lines connected to the solar wind. The conclusion is therefore that the ELS in situ observations of atmospheric photoelectrons are most likely unrelated to the SPICAM limb scan optical measurements of auroral emissions.
SM34A-04 INVITED
What new Plasma and Aeronomy Measurements are Required to Constrain the Climate Evolution of Mars and Venus?
Solar forcing is the main driver for the climate and atmospheric evolution of Mars and Venus. Climate evolution is a complex issue that strongly couples to the evolution of the atmosphere and ionosphere. On basis of new data from Mars Express (MEX) and Venus Express (VEX) we have reached a somewhat better understanding of the consequences of solar wind forcing for the upper atmosphere and ionosphere of Venus and Mars. For instance, we observe a strong dependence of the ionospheric mass escape on solar wind forcing for Mars. Some new results from MEX and VEX relevant for solar forcing will be presented. Besides solar wind forcing we have the solar X-ray, EUV and UV radiation interacting with the planetary ionosphere and upper atmosphere. The combined solar forcing may vary substantially with time, on short-terms ranging from hours (e.g. CMEs) to decades (solar cycle). The long-term variability/trend is even more pronounced and important for the evolution of a planetary atmosphere, considering that the forcing terms may have been up to a factor of 1000 times higher in the early period of the solar system. However, knowing the impact of the short- term variability (up to a factor of 10) better, we should be able to improve our understanding of the long-term evolution as well. To understand the short-term solar forcing effects on planets such as Mars and Venus will require special tailored missions and adequate instrumentation. The cause - solar forcing (solar wind, X-ray, EUV and UV), and the effect - (e.g. atmospheric expansion, ionospheric ion- and sputtering outflow) should be measured simultaneously. However, with missions carried out properly the prospects are intriguing. We may be able to scale, on basis of the present solar forcing variability, back to the decisive time periods that made the Earth-like planets evolve so differently.
SM34A-05 INVITED
Simulations of solar wind interactions with the terrestrial planets and asteroids: Recent achievements
The interaction of the solar wind with magnetized planets and asteroids is examined using global hybrid simulations that treat ions kinetically through particle-in-cell methods and the electrons form a massless fluid. These simulations allow us to investigate the nature of the resulting magnetospheres whose sizes range from proton kinetic scales such as gyroradius and skin depth to much larger systems. For a given solar wind condition, the global properties of dipolar magnetospheres are found to increase in complexity with increasing dipole strength. In its simplest form, the magnetosphere consists of a whistler wake while magnetospheres larger than about 1/30 that of the Earth have terrestrial characteristics. In this talk, we review these results and their implications for magnetospheres of asteroids and the inner planets. We also examine the scaling properties of magnetospheric regions and processes such as the bow shock, magnetosheath, magnetopause and magnetic reconnection. The results indicate that while different regions and processes have differing scaling properties, for magnetospheres larger than 1/30 of the Earth the same basic processes are operative.
SM34A-06 INVITED
Global vs. Mini-Magnetospheres: Differences and Similarities
Remanent crustal magnetic fields such as those found on the Moon, Mars, and possibly asteroids provide us with a unique space physics laboratory. Lunar crustal sources in particular allow us to explore direct solar wind interaction with magnetic obstacles of varying scale sizes. This interaction shares many features with a global magnetosphere, with various types of waves generated by charged particles interacting with magnetic field obstacles. For small electron-scale obstacles, a "whistler wake" can form, with waves similar to the "One Hz" waves commonly observed upstream from a global magnetosphere. Larger obstacles can generate ion-scale (magnetosonic) waves which can steepen to form shocks upstream from the magnetic obstacle, analogous to the bow shock of a global magnetosphere. Presumably, for a large enough crustal magnetic source, a full magnetosphere could be produced, with bow shock, magnetopause, etc. However, from a fundamental space physics perspective, perhaps the most interesting regime is that transitional between electron and ion scales, where electrons and ions likely decouple and kinetic behavior may prove most important. This transitional regime (which may be analogous in some ways to the reconnection diffusion region) is not well understood, and will require careful observations with a complete suite of particle and fields instrumentation - in concert with modern simulation techniques - to fully unravel.
SM34A-07 INVITED
The Solar Wind Interactions With Mercury, Moon, and Asteroids: Effect on Evolution of Planetary Surfaces
When the solar wind flows toward the Moon, there is no forewarning for the streaming plasma that it is about to encounter an obstacle. Thus, solar wind particles strike the lunar surface and interact directly with the lunar regolith. A solar wind ion implants itself in the grain of regolith that it comes into contact with. As the ion enters the grain, it breaks chemical bonds in the grain lattice until it loses enough energy to come to rest. Therefore, the solar wind is retained in trace amounts in the lunar regolith. As the deposited energy recoils through the lattice, particles may be ejected from the surface of the grain into the thin lunar exosphere (sputtering). Thus, solar wind sputtering has a major influence on the lunar atmosphere. We review how the solar wind interaction with the Moon affects the evolution of the lunar surface, specifically the physical, chemical, and spectral modifications of the surface from exposure to the solar wind as found in analysis of Apollo samples, laboratory experiments, and remote sensing. Finally, we compare the solar wind interaction with the Moon to similar bodies, e.g. Mercury and asteroids, to examine how magnetic fields affect the solar wind interaction.
SM34A-08
Interplanetary Magnetic Flux Enhancements in the Inner Solar System: Possible Remote Observation of comet McNaught
The asteroid, 2201 Oljato, that crosses the orbit of Venus, was found to be associated with a series of sharply peaked magnetic disturbances both ahead and behind its three inferior conjunction with Venus during the Pioneer Venus mission. Ulysses saw similar disturbances one of which was identified with comet 122P/De Vico (Jones et al 2003). These events have been interpreted as signifying the interaction of the solar wind with charged dust particles produced along their orbits by asteroids and comets. During the passage of an ICME on December 15, 2006 STEREO A and B, Wind and ACE detected a magnetic disturbance that was very unusual, lasting a length of time similar to previously observed IFEs, but different in that it lacked a compressional signature. At the time of observation STEREO A and B, Wind and ACE were crossing the orbit plane of comet McNaught that was approaching perihelion well within IAU. Comet McNaught was not aligned with the spacecraft. This disturbance would have to be caused an interaction with the dust trail and not the nucleus or coma itself.