SA33C-01 INVITED
Comparative interactions of the ionospheres of Mars and Venus with the solar wind
When dealing with the general problem of the interaction of a fast-flowing magnetized plasma with a neutral gas environment, the comparison of the interaction of the solar wind with the atmospheres of Mars and Venus is obviously of high relevance, in particular for the understanding of the loss of volatiles. Their environment consists of characteristic boundaries separating regions with very different plasma conditions and dynamics (upstream region, collisionless shock, magnetosheath, "induced" magnetosphere, ionosphere, tail, plasma sheet). The most important features are the formation of a magnetic barrier in front of the highly conducting ionosphere and an induced magnetic tail. The outer edge of the magnetic barrier on the dayside appears as a sharp boundary, reported from the magnetometer of Mars Global Surveyor and Venus Express. Recent results from the IMA mass spectrometer onboard Mars Express and Venus Express also offers us the unique opportunity to compare the plasma regimes at both planets with the same ion instrument. Recent achievements on this topic are presented.
SA33C-02
Solar Forcing and the Variability of the Planetary Ion Escape from Mars
An orbit-by-orbit study with ASPERA-3 on Mars Express indicates a great variability of the ionospheric ion outflow from Mars. The study comprises 46 orbits covering a range of solar wind parameters. Solar wind conditions were determined during inbound and outbound, assuming stable solar wind conditions throughout the traversal of the Mars induced magnetosphere. A comparision between planetary ion outflow and the solar wind/sheath flux was made. The study shows evidence for a strong coupling between ion escape and solar wind momentum and energy flux. A fairly high correlation coefficient (R=0.85) between the solar wind dynamic pressure and the ion escape mass flux is obtained. The inferred ion escape mass flux varied up to two orders of magnitude. The good correlation agrees with the hypothesis of a direct energy- and momentum transfer between the solar wind and the ionospheric plasma at Mars. By normalizing the Mars obstacle response/size with the solar wind/sheath flux we obtain and increased correlation coefficient (R=0.95). The improved correlation can be interpreted as an EUV/UV control of the obstacle size, since the obstacle size (ionospheric boundary) without any changes of EUV/UV should be entirely controlled by solar wind parameters. We argue that the strong variability of the escape flux from Mars is the consequence of the variability in solar forcing, the solar forcing originating from the solar wind energy- and momentum flux, and solar EUV/UV. We note that the solar EUV/UV and solar wind forcing are not necessarily coincident, considering the difference in travel time to the planet.
SA33C-03
Ion Pickup at Mars
With the continual presence of spacecraft in the near space region of Mars, one of the most intriguing questions has become: What happened to the water on Mars? This question has a direct connection to the issues of Mars loss of atmosphere/ionosphere due to solar wind intereaction with this planet. In recent years simulations have been employed to estimate the loss rate under different solar wind conditions. A variety of scientists in Asia, Europe, and the U.S. have performed such simulations. The results have produced mix results. In this paper our simulation efforts will be discussed. Specific emphasis will be placed on the electromagnetic environment surrounding Mars as various models are changed. We will discuss where the sensitivities seem to reside. We will discuss the sensitivities we have seen based on model choices.
SA33C-04
On the Escape of Ions from the Venus Ionosphere
Using high and low solar activity models of the Venus ionosphere, We compute the upper limits to the upward fluxes of ions. In a 1-D model, these fluxes probably represent the divergence of the horizontal ion fluxes, except near the subsolar point. The maximum transterminator fluxes are determined by the production rate of of ions above the photochemical equilibrium boundary. The transterminator ion fluxes at low altitudes may converge on the nightside and flow downward, producing the nightside ionosphere. Some of the ion fluxes may escape from the gravitational field of the planet. We compare our upper limits to the transterminator ion fluxes measured by the RPA on Pioneer Venus, and estimates of the fluxes necessary to produce the observed nightside ionosphere. If data are available, we will compare our maximum fluxes to those measured by the Venus Express spacecraft. We also consider the effect of the upward fluxes on the measured and model ion density profiles. We find, that unlike Mars at low solar activity, fluxes close to the maxima are not necessary to reproduce the measured ion density profiles.
SA33C-05
Magnus Force in the Dawnward Displacement of the Venus Transterminator Ionospheric Flow
An empirical value of the Magnus force coefficient suitable to conditions applicable to the Venus ionosphere is derived by comparing the dawnward displacement of the Venus trans-terminator ionospheric flow expected from the Magnus force with that implied from the PVO measurements. The Magnus force is produced by a pressure difference that is set across that planet by the different speed that results when the rotation motion of the ionosphere and the trans-terminator flow are in the same sense (dusk side terminator) and that where both motions are opposite to each other (dawn side terminator). The Magnus force coefficient is inferred by using measured values of the parameters involved in the Magnus force equation (density and rotation frequency of the Venus upper ionosphere together with the speed and the dawnward deflection of the trans-terminator flow downstream from that planet) and leads to values of the Reynolds number that would be representative in comparable fluid dynamic problems.
SA33C-06
Observations of Solar Wind Interactions with Earth, Mars and Venus with Cluster, MEX and VEX
Clearly one of the most important controlling factors influencing the interaction of the solar wind with planetary ionospheres is the presence of a planetary magnetic field. We will present results using data from the ESA Cluster, Mars Express, and Venus Express missions. These missions provide data from Earth with its fully developed dynamo driven magnetic field, from Mars with its remnant field, and from Venus with no field. We will examine data from the electron sensors on these missions. Just internal to the dusk boundary between the shocked, sheath electron populations at each planet one observes heated electrons. For Mars and Venus the transits from the solar wind and into the planetary environment are strikingly similar. For all three planets the heated electron spectra just inside the planetary boundary are very similar. We will examine the possible mechanisms for this heating process. Clearly it does not have to involve any form of merging, reconnection, etc., since it occurs for unmagnetized Venus.
SA33C-07
The Martian bow shock and magnetic pileup boundary: two plasma boundaries induced by the interaction of the solar wind with the planetary ionosphere
The Phobos 2 and Mars Global Surveyor (MGS) planetary missions have confirmed and/or established that the interaction of the solar wind with the space environment of Mars was complex and could be unique in the solar system. The MGS observations have, in particular, definitely proved that no dipole-like planetary magnetic field exists at Mars and therefore, on the global scale, the interaction should be between the heavy ions of the ionosphere/exosphere and the solar wind protons. This interaction leads to the creation of two outer permanent plasma boundaries, the bow shock and the magnetic pile-up boundary, MPB. A great many Martian bow shock and magnetic pile-up boundary crossings have been identified in the Phobos 2 and Mars Global Surveyor, MGS, data. From these observations the positions and shapes of the bow shock and magnetic pile-up boundary, MPB, have been derived and modelled, using curve-fitting techniques. The purpose of the paper is to show the results obtained from the mixing of the Phobos-2 and MGS data bases and to compare the derived bow shock and MPB models with the ones obtained previously. The underlying objective was to see whether it was possible to determine improved bow shock and MPB models or not. The answer is definitely yes, and particularly for the MPB, thanks to the complementary nature of the observations. The boundaries crossed close to the subsolar direction or mostly far downstream by Phobos 2 indeed allow a better coverage of the Martian space environment to be considered.
SA33C-08
Angular Electron Distributions at Mars and Venus
Suprathermal electron measurements have been recently made at both Venus and Mars by several spacecraft. Directional information in the form of pitch angle distributions are particularly useful because they allow us to probe magnetic field topology near planets, as well as the physical processes affecting plasma populating magnetic flux tubes. We will present angular electron distributions measured at both Mars and Venus. We analyze measurements made by the magnetometer and electron reflectometer (MAG/ER) on Mars Global Surveyor over a period of more than six years to identify the locations of open and closed magnetic field lines near Mars, and variability with respect to solar wind pressure and the direction of the interplanetary magnetic field. At Venus we discuss the utility of electron angular distributions for determining when draped solar wind magnetic field lines intersect the collisional atmosphere, and present recent progress made toward construction of pitch angle distributions from the Venus Express magnetometer and electron sensor (MAG and ASPERA-4 ELS).