SA31B-01
An upper limit for the mass los rate of HD 209458b
HD 209458b is a Jovian like planet orbiting around a star with similar characteristic to the Sun. It was discovered by its repeated transit across the stellar disk. The orbital period is 3.5 days, with a mean distance to the star of 0.045 AU (closer than Mercury to the Sun in our system). The peculiar characteristic of this system is that the planet shows an apparent larger size when observed in Lyman (1214-1217 A) than in optical wavelength (Na I). This discrepancy is thought to be due to the existence of an extended upper atmosphere or wake (comet like wake). In this work we present 3D numerical simulations of this system (done with the adaptive grid Yguazú-a code), and study the absorption produced by this extended atmosphere. The atmosphere is modelled as a wind outgoing from the surface of the planet at velocities of the order of the escape velocity (60 km s-1). Four runs where carried out considering different mass loss rates for the planet in the range [ ]MSUNyr-1. By comparing the model with the observations, we found an upper limit for the mass loss rate of HD 209458b (previous works only found a lower limit. We compare HD 209458b characteristics with solar system planets.
SA31B-02
2D Numerical Study of Superthermal Particles in Martian Exosphere
Hot atomic oxygen two-dimensional velocity distributions, atmospheric loss rates and profiles of density and temperature have been calculated for Mars exosphere by using our new Direct Simulation Monte Carlo (DSMC) flow solver on an unstructured 2D-axisymmetric mesh. As a test of the 2D version of our code, the result of the 1D distribution function obtained by a two-stream transport model by Nagy et al. [J. Geophys. Res.,106, 21565, 2001.] was used as boundary condition. For the full 2D Solar Zenith Angle (SZA) dependent run however, a more complete approach was adopted and the 2D average of the well-established 3D thermospheric model of Bougher et al. [Geophys. Res. Lett., 33, doi:10.1029/2005GL024059, 2006.] has been used to calculate the production of hot O and its collisional thermalization with the dominant background species (O and CO2). Dissociative recombination (DR) of O2+ ions was assumed to be the main production of hot O and calculations were performed for equinox conditions and for both low and high solar activity cases. Due to the methodology of the DSMC method, the velocity distribution at the exobase describes the gas system of the Martian exosphere and allows us to provide a complete set of its macroscopic parameters.
SA31B-03
Mars Global Surveyor Nightside Electron Observations
We present observations of electrons above the Martian night side, using data from the Mars Global Surveyor Electron Reflectometer. These electron observations allow us to investigate fundamental space physics at Mars. Electron angular distributions allow us to understand the constantly evolving Martian magnetic field topology. Energized electron distributions observed in remanent magnetic field cusps and near crossings of the induced magnetotail current sheet inform us about Martian physical processes which may be analogous to those observed in the terrestrial auroral zone, magnetospheric cusp, and plasma sheet. Anisotropic electron distributions commonly observed near the current sheet (but also in other regions) allow us to trace magnetic connectivity to the solar wind by tracking the anisotropic suprathermal electron population from the solar wind to the Martian night side (as previously demonstrated using Phobos data). Detailed investigation of electron angular and energy distributions may also allow us to investigate other fundamental space physics processes, including wave particle interactions.
SA31B-04
Localized Ionization Patches on the Nightside of Mars and Their Dependence Upon Atmospheric Variations
Using an electron transport model, we investigate the effect of ionization due to electron precipitation on the
electron density and total electron content in the nightside ionosphere of Mars. As input we use typical tail
electron spectra and recently reported auroral-like peaked electron spectra that appear to have undergone an
acceleration process. The accelerated electron spectra increase the maximum number density and total electron
content by nearly a factor of 3 over that produced by typical tail spectra. The regions of enhanced ionization are
localized in space and correspond to magnetic cusps formed by the interaction of the Martian crustal sources with
the interplanetary magnetic field. We find that our modeled peak ion production rates are several times less than
previous calculations and that our calculated maximum ionospheric electron densities are less than recent
observations. The main source of the discrepancy between our results and previous work appears to arise from
differences in the neutral atmosphere profile. The upper atmosphere of Mars changes significantly with season
and solar cycle. Likewise, peak ionization rates and maximum ionospheric electron number densities can also
change significantly, with these quantities being minimum for conditions of large atmospheric scale heights;
however, the thickness of the ionospheric layer is greatest under these conditions leading to a small net change
in total electron content.
http:sprg.ssl.berkeley.edu/matt/AGUS2007/mars/
SA31B-05
Simulations of the Mars Ionosphere during a Solar Flare
Increased fluxes of X-rays during solar flares have been observed to enhance electron densities in the lower ionosphere of Mars. The photochemical timescale at these altitudes is on the order of minutes, so these electron density enhancements do not persist for substantially longer than the flare duration. We present the results of photochemical model simulations of the Mars ionosphere driven by temporally-varying solar fluxes, concentrating on 15 and 26 April, 2001. The Mars Global Surveyor (MGS) Radio Science (RS) instrument observed flare- enhanced electron densities on these dates. Solar fluxes are derived from the Flare Irradiance Spectral Model, which outputs the solar spectrum at 1 nm intervals from 0.5 to 195.5 nm every 1 minute. This empirical model is based on TIMED SEE, UARS SOLSTICE, GOES, and other observations. For a short period at the peak of a large solar flare, X-ray photoionization rates exceed EUV photoionization rates and the electron density peak altitude decreases by tens of kilometres. Simulations will be compared to MGS RS measurements of electron density profiles.
SA31B-06
Low frequency plasma waves at Mars: New results from the magnetosheath
The interaction between the solar wind and the Martian ionosphere produces copious amounts of plasma waves and instabilities. We first review observations of low frequency (< 1 Hz) waves throughout the Martian system. We then focus specifically on new results regarding the high amplitude waves found in the magnetosheath region. The dayside magnetosheath waves are nonlinear (deltaB/|B| > 0.1) and are dominated by power near the local proton cyclotron frequency (~ 0.1 Hz). However, we also find intervals marked by strong spectral power at the local helium and oxygen gyrofrequencies. Some intervals also show strong bursts of wave activity bracketed by periods of relative calm; we speculate that these bursts could be related to plasma clouds detached from the Martian ionosphere. The nightside magnetosheath is comparatively quiet but intervals of ion cyclotron wave activity do exist.
SA31B-07
3d Hybrid Simulation of the Martian Plasma Environment and Comparison With Mars Express and Rosetta Flyby Data
The interaction of the solar wind with the Martian magnetosphere is simulated by a 3d hybrid code model. This approach is applied as the gyroradii of the solar wind protons are in the range of several hundred kilometers and therefore they are comparable to the characteristic scale of the subsolar ionospheric interaction region. Characteristic boundaries as bow shock and ion composition boundary are identified and they are compared with Mars-Express data and the Mars flyby of the Rosetta spacecraft.
SA31B-08
Low Altitude Characteristics of Ion-Neutral Coupling in the Venus Nightside Ionosphere; an Examination Using Pioneer Venus Orbiter Data
Archived data, available from Pioneer Venus Orbiter (PVO) covering over a full solar cycle from before the peak of cycle 21 (December 1978) past the peak of cycle 22 (October 1992), are used to study the uncoupling of the ionosphere from the neutral atmosphere. The early measurements provide solar maximum observations when there was characteristically a substantial nightside ionosphere. The late mission measurements were made under quiet solar conditions, more comparable to the state of the ionosphere that Venus Express has encountered. Evidence of ionospheric flow is sought by a comparison of the deduced ion densities from the Orbital Ion Mass Spectrometer (which assumed collection at the spacecraft velocity) with the measured electron density from the Orbital Electron Density Probe (which was independent of the ion velocity) and by the limited availability of Orbital Retarding Potential Analyzer Ion Drift Measurements. The evidence for flow at high altitudes is very evident during solar maximum in the comparison of the ion-electron measurements and perhaps at lower altitudes during solar minimum. The influence on the measurements of superthermal ions and impact ionization at low altitudes is discussed. A search was made for the presence of ion flows at low altitudes to determine where the ion dynamics uncoupled from the neutral atmosphere. In the region of strong-ion neutral coupling the neutral composition measurements (from the Orbital Neutral Mass Spectrometer) can show departures from nominal ambient conditions when the ion velocities and densities are sufficient to drag the neutrals. Analysis of the datasets of neutral, ion, and electron densities along with the magnetic field exhibits the correlation between the ions and neutrals on a variety of scales from large-scale behavior to small-scale fluctuations.
SA31B-09
Fine-scale Structure of Magnetized Ionospheres
SA09 Interaction of the Solar Wind with Planetary Ionospheres Index terms: 6295, 6225 Fine-scale Structure of Magnetized Ionospheres The ionospheres of Titan, Venus and Mars are magnetized by their interactions with flowing magnetized plasma corotating with Saturn in the former case and convected by the supersonic solar wind in the latter two cases. At solar maximum, the Venus ionosphere was found to be generally field-free by the PVO eccentric orbiter, albeit threaded with twisted flux ropes. Venus Express is now orbiting Venus at solar minimum and detects a largely magnetized ionosphere. Similarly, the repeated low altitude passes of Cassini through the Titan atmosphere reveal a "strongly" magnetized ionosphere. Nevertheless there too we detect signs of twisted magnetic fields. In this paper we examine the fine-scale structures revealed by these missions at Titan and Venus.