P13B-1305
Upstream Proton Cyclotron Waves at Venus and Consequences for the Venus Exosphere
Magnetometer data from two Venus-years of the Venus Express mission are investigated for the occurrence of proton cyclotron waves. Recently, proton cyclotron waves (PCW) were detected in the upstream region, in and upstream of the foreshock region and over a large volume of space; they are a direct indication of pick up of planetary protons from the exosphere of Venus and loss of hydrogen to interplanetary space. A statistical survey from long term observations from start of the nominal orbit (May 2006) over two Venus- years gives insight into the waves' spatial occurrence and specific properties. The waves are found up to large distances from the planet (~ 9 Rv); the positions of observation in Solar Wind Magnetic coordinates (with Z along the convective electric field) show no clear organization with the electric field direction. No known mechanisms exist to propagate PCWs across the magnetic fieldlines nor ions into directions of negative electric field. Furthermore, generation of the waves closer to the planet and subsequent propagation in the solar wind outward to the observed position can be excluded for several reasons. Therefore, the PCWs have to be generated locally from available neutral hydrogen, even at large distances. Pick-up from different sources of neutral hydrogen is discussed, leading to a non-satisfying result for non- planetary particles. The PCW observations from VEXMAG are interpreted as indication for the existence of an extended neutral hydrogen exosphere at Venus. The nature of this extended exosphere is still an open issue and under investigation.
P13B-1306
Solar Wind Control of the Venus Bow Shock
The characteristics of the Venusian bow shock depend on both the prevailing solar wind conditions and shock geometry. This paper uses magnetometer data from the ESA Venus Express mission to investigate the shock properties depending on solar wind conditions.
P13B-1307
VISCOUS BOUNDARY LAYER IN THE VENUS IONOSHEATH: EVIDENCE FROM THE VENUS EXPRESS PLASMA DATA
Measurements conducted with the ASPERA-4 instrument in the Venus Express spacecraft further support the presence of a plasma transition located at the flanks of the Venus ionosheath downstream from the bow shock and that had previously been inferred in the data obtained in earlier missions at Venus. Across this transition there are sudden changes in the plasma properties including lower speed and density values as well as higher temperatures of the shocked solar wind in its downstream side. In addition there is evidence that the planetary ion component becomes enhanced in the downstream side of that transition with fluxes that lead to significantly larger densities than those measured in the upstream side. That plasma transition has been interpreted as representing the outer extent of a viscous boundary layer formed by the transport of solar wind momentum to the Venus upper ionosphere, and the ASPERA-4 data provide for the first time information on the kinetic properties of the planetary ion population that is seen to stream mostly in the solar wind direction but with velocities that remain smaller than those of the solar wind. From the analysis of a collection of orbits with evidence of that transition it has been possible to derive that its position varies significantly with the downstream distance from the planet. Furthermore it has also been found that the momentum flux of the dominant component of planetary ions measured downstream from the plasma transition can be accounted for from the momentum flux of the freestream solar wind protons. In most cases the momentum flux of the planetary ions represents 80 to 90% of the incident momentum flux of the solar wind and implies that there is an approximate balance in the momentum between both populations as would result from the transport of solar wind momentum to the Venus ionospheric plasma.
P13B-1308
Venus Express observations of magnetic field fluctuations in the magnetosheath
Magnetic field fluctuations within a planetary magnetosheath play an important role in the solar wind interaction with the planet, since they can reconfigure the plasma flow and the magnetic field and transfer energy from the bow shock to the lower boundary. Many studies have been presented on the fluctuations in the terrestrial magnetosheath; however, hardly any studies have so far been carried out for Venusian magnetosheath fluctuations, except for Luhmann et al. [1983] and Vörös et al. [2008] who performed some case studies on the magnetosheath fluctuations at Venus. It was shown that the fluctuations are probably convected from the vicinity of the quasi-parallel bow shock along the streamlines. Based on the Venus Express observations in 2006 and 2007, we investigate the spatial distributions of magnetic field fluctuations in the Venus magnetosheath statistically.
P13B-1309
Venus Express observations of solar wind magnetic field draping around Venus
Although Venus has no intrinsic magnetic moment, the piled-up interplanetary magnetic field in dayside inner magnetosheath forms a magnetic barrier to the solar wind. This magnetic barrier, an induce magnetosphere on the dayside, acts as an obstacle to the solar wind in analog to the Earth's magnetosphere. On the nightside, a magnetotail is formed by the anchored, draped magnetic fields. Previous studies show that at low altitudes, the magnetic draping configuration on the dayside might become reverse draping on the night, forming a near toroidal magnetic field. In this study, we use Venus Express magnetometer measurements to map the magnetic field environment and examine the solar wind magnetic field draping around Venus.
P13B-1310
Current Sheets and Substroms in Venus' Magnetotail
We investigate 2 years of Venus Express FGM data when the spacecraft is crossing Venus' magnetotail. Using only data when the IMF is within ± 20° of the nominal Parker spiral ~ 37° and little variation of the field before and after the tail crossing, we obtain a quiet-time magnetic field profile along the orbit of the spacecraft. There is an Earth-like tail field in the region less then 3 RV downstream from Venus and the current sheet is in the XYVSO-plane, with an estimated current density of 3 nA/m2. We also investigate two cases, one for a quiet tail and one for an acitive tail, using both FGM and ASPERA data. Increased but constant pressure of the solar wind is shown to increase the esitmated cross- tail current. Most importantly, strongly varying solar wind magnetic field is shown to lead to observations that can be interpreted as reconnection in Venus' magnetotail.
P13B-1311
The Effect Of Fast And Slow Solar Wind On The Venusian Upper Atmosphere.
Using the ASPERA-4 instrument onboard Venus Express the change in the atmospheric boundary layers are investigated. Using the instrument when it is in the apoapsis period of the polar orbit allows pure Solar Wind measurements to be taken. Composition ratios are determined thus identifying periods of fast and slow solar wind. This is then applied to ion maps of the Venusian atmosphere to determine the response after noise calibration methods. The Species reactions to the differing Solar Wind are also used to investigate the inner boundary movements.
P13B-1312
Photoelectrons at Venus in the Ionosphere and the Tail
Pioneer Venus Orbiter has shown that there is no intrinsic magnetic field at Venus. Instead, there is an induced magnetosphere caused by the solar wind interaction with the upper atmosphere of the planet. This interaction causes the solar wind's magnetic field lines to drape around the planet, dragging through the ionosphere where they continue on to form a magnetotail. Ionisation of the ionosphere by the solar HeII 30.4nm line leads to the production of ionospheric photoelectrons, recognisable by their characteristic spectral shape in the electron energy spectrum. These photoelectrons have been seen at Venus by the electron spectrometer, part of the ASPERA instrument, onboard the Venus Express spacecraft due to its energy resolution of ~7%. In addition, these ionospheric photoelectrons have now been seen in the induced magnetotail for the first time at Venus. Tailward photoelectrons have previously been seen at Earth, Mars and Titan. At Titan, they have been linked to ambipolar diffusion.
P13B-1313
Numerical Simulation of the Formation of Venus Ionospheric Channels due to Viscous Dragging by the Solar Wind
We present results of the numerical simulation of the process of formation of ionospheric channels in Venus as a consequence of the viscous interaction of the solar wind with ionospheric gas. A 2D, hydrodynamical, two species, computational code has been developed to solve the Navier-Stokes, continuity and energy equations allowing us to study simultaneously the dynamics of the solar wind flow and material in the upper layers of the ionosphere. The results of our simulations depend mainly on the value of the 3 adimensional parameters of the problem: the Mach number of the incident solar wind, the effective Reynolds number of the shocked solar wind and ionospheric material and the ratio of the timescale for interspecies coupling to the crossing time for the shocked solar wind. On the basis of our results we estimate that the timescale for channel formation is typically of the order of tens of minutes. Complementing our 2D results with analytical calculations we can also estimate the amount of mass removed from the ionosphere of the planet as a function of time and the shape of the resulting ionospheric channels, although these features are more strongly dependent on the parameters of the simulation. Finally, we compare our results to the known properties of ionospheric channels as derived from the in situ measurements in the ionosphere of Venus.
P13B-1314
A hybrid modelling case study of the Venus Express particle and magnetic observations
We present a case study of the Venus-solar wind interaction as observed by the Venus Express spacecraft. In the study we compare the particle observations from the ASPERA-4 instrument and the magnetic field observations from the MAG magnetometer to a global hybrid simulation. The HYB hybrid code is used to model a single orbit case with a reasonably steady interplanetary magnetic field and the solution is used to study the local in-situ spacecraft observations in a global planetary context. Also, we present test particle simulations for the case using the global electric and magnetic fields from the simulation.
P13B-1315
Ionospheric Plasma Energization and Escape from Mars: Composition and Flow Properties
Recent results from Mars Express (Lundin et al., GRL 2008) display a comet-like behaviour of the Martian ionospheric plasma escape. Low-energy (cold) ionospheric plasma is swept from the dayside, expanding into the nightside/tail, eventually picking up speed in the central and deep tail. The cause of the plasma escape, i.e. the processes that bring ionospheric plasma to just above escape velocity (5 - 10 km/s), is of particular interest. In analogy with the polar wind of the Earth, ionospheric plasma is expected to become energized by waves and electric fields generated by solar wind energy and momentum transfer processes. The comet-like flow of low-energy ionospheric plasma, streaming along the external sheath flow, suggests a 'viscous-like' coupling between the sheath plasma and the expanding ionospheric plasma. Moreover, the tailward outflow is structured, frequently modulated in the same manner as the ULF wave activity in the Martian magnetosheath. This implies that wave activity is involved in the energization and escape of ionospheric ions. Another interesting feature in the ionospheric plasma escape from Mars is a large abundance of molecular ions, predominantly molecular heavy ions (e.g. O2+ and CO2+). However, we also find a significant fraction of low-energy/cold m/q=2 ionospheric ions; most likely molecular hydrogen (H2+) because the content is perhaps too high to qualify as Deuterium (D+). Whether H2+ or D+, the finding provides important information on the long-term, and short term, weathering and dissociation of water at Mars.
P13B-1316
Evidence of a Transient Ionopause at Mars Identified from the Excitation of Local Electron Plasma Oscillations
One way to measure the ionospheric electron density with the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft is from the excitation of local plasma oscillations. With this method it is possible to obtain local electron densities at spacecraft altitudes typically between 250 and 1300 km. A study of the electron density versus time for more than 500 orbits revealed that in some orbits it is possible to identify a very sharp step in the electron density that we interpret as an ionopause, similar to the ionopause boundary that is commonly observed at Venus. Measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) Electron Spectrometer (ELS) instrument on MEX verify that these sharp decreases in the electron density occur somewhere between the end of ionospheric photoelectrons and the magnetosheath. This study shows that the average altitude of the ionopause is almost constant, around 500 km, from subsolar region up to solar zenith angles of 60o, after which the ionopause altitude shows a slight increase. Investigation of the effect of crustal magnetic fields on ionopause altitude shows that the ionopause boundary is raised at the locations where strong crustal magnetic fields are located.
P13B-1317
Hydrogen and Oxygen Hot Ion Precipitation in the Martian Ionosphere
High energy H/H+ ion precipitation into Earth's upper atmosphere has previously been modeled (Fang et al., 2004; 2005). Recently, we have extended this work for the Martian ionosphere using different cross sections for relevant Martian "background" species. Additionally, we have included O+/O precipitation with the view to keeping track of secondary hot ions/neutrals in future work, which important for sputtering in the Martian upper atmosphere. Atmospheric effects of these precipitating hot ions in the Martian atmosphere are studied and reported on.
P13B-1318
Modeling variability in the night side ionosphere of Mars
The night side ionosphere of Mars is known to be highly variable: essentially nonexistent in certain geographic locations, while occasionally nearly as strong as the photoionization-produced dayside ionosphere in other places. Using a coupled kinetic Monte Carlo/multi-fluid approach, we model the dynamics of precipitating solar wind electrons on the Martian night side and calculate resulting ionospheric density profiles. We investigate the broad observed range of precipitating electron properties by varying the electron pitch angle distributions (i.e. field-aligned versus isotropic) and energy spectra (i.e. typical versus low-energy flux enhancement versus accelerated). We further investigate the coupled geographic and seasonal variations in nightside ionospheric profiles by varying the strength of the crustal magnetic field, the angle between the crustal field and external field and the atmospheric neutral density profile. We compare these modeling results to precipitating electron spectra, observed total electron content (TEC) measurements and nightside density profiles measured by the ASPERA-3 and MARSIS radar sounding experiments on Mars Express. These comparisons provide insight into the processes, both internal (seasonal variation in thermospheric neutral densities, plasma transport by neutral winds) and external (precipitating plasma properties), that control the night side ionosphere.
P13B-1319
Ionopause Features of Mars as Observed by the Radio Science Experiment MaRS on Mars Express
The ionopause of a planet is defined as the boundary between its ionosphere and the solar wind regime. It was first described for Venus when a sharp decrease in electron density towards very small values was found at certain altitudes. So far, the ionopause at Mars has not been well observed. One reason is that the noise of the Viking profiles was relatively high and did not drop below 500 el/cc. The MGS data base is inconclusive concerning the ionopause. The highly elliptical orbit of Mars Express allows us to investigate the electron density of the Martian ionosphere up to an altitude of about 1500 km. Based on these data we want to define the ionopause feature at Mars as an electron density gradient starting well above the topside ionospheric main peak, tending to decrease the electron density towards noisy values around zero. The Radio Science Experiment MaRS on Mars Express sounded the Martian atmosphere and ionosphere, starting from April 2004. So far, more than 400 vertical profiles of the ionospheric electron density could be derived, covering all planetary latitudes and almost all local times on the northern hemisphere. This presentation shows the high variability of the ionopause structures of Mars, based on the MaRS radio occultation data from the years 2005 and 2006.
P13B-1320
Observation of Large Plasmoids Removing Atmosphere From Mars
Atmospheric escape is known to occur at Mars today, and may have been the dominant loss process for the atmosphere since the end of the late heavy bombardment. Planetary ions escape to space through a number of different processes, including ion pickup, ion outflow, and bulk escape. Simulations and theoretical predictions suggest that bulk escape - or removal of coherent portions of ionospheric plasma - can contribute significantly to the total atmospheric escape rate, but there have been relatively few observational studies of this process. We present observations from Mars Global Surveyor (MGS) magnetometer and electron data of very strong (>100 nT) magnetic field signatures at 400 km altitudes near the Martian terminator, similar to plasmoids (flux ropes) observed in Earth's magnetotail. To our knowledge these are the strongest flux ropes reported at Venus or Mars, and may be the strongest sampled anywhere in the solar system. We will present the results of a statistical study that suggests that the plasmoids are observed downstream from regions of strong crustal magnetic field, at times of moderate or high solar wind pressure. We will present a case study that shows that the strongest plasmoid was observed shortly after a change in direction of the draped interplanetary magnetic field (IMF), and is filled with ionospheric plasma. These features clearly formed through interaction of the draped IMF and crustal magnetic anomalies, either through magnetic reconnection or a Kelvin-Helmholtz type instability. Either mechanism will result in removal of coherent portions of ionospheric plasma, indicating that we are observing bulk escape processes as they occur. We will present results of model fits of the total magnetic and plasma flux contained in the ropes, and the resulting implications for the role of bulk escape in Martian atmospheric evolution.
P13B-1321
Escape of Mars Photoionized Electrons from Carbon Dioxide and Atomic Oxygen in 2004
Photoelectron peaks in the atmosphere caused by the ionization of carbon dioxide and atomic oxygen by Solar 30.4 nm photons have been observed by the Electron Spectrometer (ELS), a component of the Mars Express (MEx) Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) Experiment. Most of the ionizations occur near the Mars exobase, with a significant fraction of the photoelectrons following the local magnetic field line. A fraction of these are able to escape the planetary ionosphere by flowing down the tail and (presumably) out of the Martian environment. In this paper we use properties of the measured photoelectron spectrum to estimate the loss rate of electrons produced by the above ionization processes. Specifically, we use ELS data taken between 5 January 2004 and 25 January 2005 to collect statistics of occurrence of the characteristic photoelectron spectral peaks; using example spectra, we demonstrate the techniques of identifying and selecting the spectral peaks; and we obtain an estimate of the flux of electrons in the peaks flowing away from the planet. Our results yield an average rate of escape from Mars of 2.47x105 electrons /(cm2 s) over the year 2004. At a distance of 1.5 RMars tailward of the planetary center, an estimate of the outflow area, 1.16x1018 cm2, allows estimation of the electron escape rate of 2.85x1023 electrons/s. This gives about 9x1030 electrons or 15 Mmole which escaped Mars in 2004 due to the ionization of carbon dioxide and atomic oxygen by the He 30.4 nm line.
P13B-1322
Comparing ASPERA 3/4 Pick-up Ion Signatures at Mars and Venus
The solar wind interacts directly with the atmospheres of planets that do not have intrinsic magnetic fields, leading to direct atmospheric escape. The Mars Express and Venus Express spacecraft measure this interaction with the ASPERA 3 and 4 Ion Mass Analyzer instruments. Energy-time spectrograms of high mass ions show dispersed energy signatures that can be associated with the ion pick-up process which depends on the conditions in the solar wind. The subject of this study is a comparison between the pick-up ion signatures seen at Mars and Venus. A survey of the data reveals that high energy pick-up signatures are more common at Venus than Mars. The features gain energy with altitude at both planets, but achieve higher peak energies at Venus. The differences in what is observed may be due to ion gyroradius effects and/or orbital sampling bias. Modeling is necessary to understand these issues.
P13B-1323
Unique properties of solar-wind-Mars interaction in terms of ion escape and mass-loading processes
Because of low gravity in the Mars environment, Mars has an extended oxygen and hydrogen corona. The solar-wind-Mars interaction is profoundly influenced by both the mass loading and ion escape mechanisms operating around Mars. Although several attempts have been made in numerical simulations to account for the observationally obtained unique features of Mars, e. g. existence of the magnetic pile-up boundary and the time and spatially varying ion loss rates, it cannot be said we have obtained any definite conclusion about the physical mechanisms leading to these features. Here we discuss these points in light of a newly developed MHD multi-fluid code focusing upon the relative significance of the bulk ionopspheric outflow vs. ion pick up loss mechanisms associated with oxygen/hydrogen corona. Although not all the observed features are accounted for by numerical simulations, several things have been made clear: (1) Cooling of magnetosheath plasma due to chemical processes occurring just above the dayside ionosphere has a significant effect to determine the plasma flow around Mars. In extreme cases it leads to a formation of planetward/vortex motions behind the planet. (2) Magnetic tension whose effect is strongest in the plane perpendicular to the IMF makes the escaping ions to converge to the center portion of the tail. Thus the tail center ray is produced by magnetic tension (actually by electric field associated with magnetic tension). On the other hand, ion escape from the top of the ionosphere leads to the boundary ion ray flux in the tail. (3) It is presently difficult to reproduce the magnetic pile-up boundary via chemical processes with the present knowledge of the distribution of oxygen/hydrogen corona. The crucial points, which would be applicable to both MHD and hybrid codes, are: (A) Oxygen and hydrogen coronae have too large scale heights to reproduce chemically a very sharp boundary as observed at MPB. (B)In the dayside low-altitude magnetosheath, the plasma beta value is already low due to the flow divergence along field lines so that the thermal effects produced by chemical processes cannot change the magnetic field magnitude/configurations appreciably. The only exception will be the case where the oxygen corona is much denser than predicted.
P13B-1324
Martian Ionospheric and Atmospheric Interaction Under Extreme Solar Wind Conditions
As a weakly magnetized planet, Mars interacts directly with the solar wind. During periods of enhanced solar activities, solar radiation, as well as the interplanetary magnetic field (IMF), solar wind plasma density and flow speed can be greatly enhanced. The planetary particle escape rate is estimated to be more than an order of magnitude larger under extreme solar wind conditions than in normal situations. To understand the responses and its long-term consequences of the Martian ionosphere and atmosphere to extreme space weather events, we study Martian ionospheric and atmospheric interaction with extreme solar wind conditions using two sophisticated 3D models. We use a multi-species global MHD model to simulate cases with enhanced solar wind density, flow speed, magnetic field and intensity of the solar radiation, respectively to quantify the effect of each parameter and to identify the most significant one that cause the enhancement of the particle escape. A newly developed 3D test particle model, which traces the motion of pick-up ions is also used to examine the same problem based on the corresponding MHD electro-magnetic field results. The calculated ion escape fluxes for different ion species will be compared with MHD model results.
P13B-1325
A two-spacecraft study of the Martian magnetic pileup boundary: Mars Express and Mars Global Surveyor observations
We present initial results from a study of the Martian magnetic pileup boundary (MPB) using simultaneous measurements from Mars Global Surveyor (MGS) and Mars Express (MEX). MGS and MEX had almost three years of overlapping data coverage at Mars, from 2004 until late 2006 with very complementary instrumentation. MGS measured vector magnetic field and suprathermal electrons from a ~400 km circular orbit fixed in local time whereas the MEX ASPERA-3 instrument measures suprathermal electrons and ions from a precessing elliptical orbit. We use these highly complementary datasets to study asymmetries and variability in the shape of the MPB. The suprathermal electron measurements can be used to identify times when each spacecraft is in the magnetosheath " above the MPB. Simultaneous measurements will help to constrain the shape. Correlation of the boundary shape with upstream solar wind and IMF conditions measured directly by MEX and indirectly by MGS allow us to quantify the variability in the shape. We will present both case studies and a statistical analysis using these simultaneous observations.
P13B-1326
Hybrid code simulations of the solar wind interaction with Pluto
Pluto's low gravity implies that the atmosphere is only weakly bound and significant hydrodynamic outflow can exist. Though surface spectroscopy of Pluto has revealed methane frost the dominant escaping neutral gas is thought to be N2. These escaping neutrals are photoionized and the heavy ions (N2+) move away from Pluto in the direction perpendicular to the solar wind flow (i.e. nearly unmagnetized relative to the length scales of the plasma interaction region). The turning distance of the solar wind protons at the magnetic pileup boundary is large compared to the interaction region. As a result, large ion gyroradius affects determine Pluto's highly asymmetric interaction with the solar wind. We use a three-dimensional hybrid code (fluid electrons, kinetic ions) to investigate the geometry of the interaction region for a variety of of possible atmospheric escape rates in anticipation of the New Horizons encounter with Pluto. We find considerable structuring in the wake region due to bi-ion waves and Kelvin-Helmholtz waves. The shock structures vary from a simple Mach cone for low escape rates (~ 2 × 1026 s-1) to a full detached bow shock for large escape rates (~ 2 × 1028 s-1).