SM23B-01
Search for Na+ Pickup Ion Generated Waves at Mercury
Telescopic observations by Potter et al. [2002] have discovered that Mercury's Sodium exosphere has a tail extending 10's of Mercury radii. Theory predicts that the shape of and the amount of Sodium [Smyth, 1986, 1995; Ip 1986, 1990] in this exospheric tail is highly dependent upon the true anomaly of Mercury. The exospheric Na that is not reabsorbed on Mercury's surface will be photo-ionized. Computations by Ip [1986] indicated that ionized exospheric Na could significantly mass load the plasma population in Mercury's magnetosphere. These freshly created ions will be rapidly energized by the convection electric field in Mercury's magnetosphere and sheath and should be highly unstable to the generation of plasma waves. These waves could play an important role in the thermalization and retention of the Na+. Because the gyro radii of Na+ can be comparable to the scale sizes in Mercury's geospace there is an open question whether Mercury's geospace can sustain such waves. After a brief review of what was observed in the Mariner 10 magnetometer data, we will present analytic calculations of the expected pickup ion distributions, the expected unstable waves, their frequencies, wavelengths and Doppler shifts, their variation with location in Mercury's geospace and Mercury's true anomaly for both high and low solar wind convection electric fields. We will assess if and when such waves can be generated and sustained.
SM23B-02
Solar Wind Influences on Waves in the Magnetic Field Measured on the Lunar Dayside
We analyze the high-resolution magnetic field data from Lunar Prospector to study waves in the 0.5-4.5 Hz frequency range in the vicinity of the Moon. We study the position of these waves in selenographic and solar wind coordinates. The wave power decreases with solar zenith angle and while the Moon is in the Earth's magnetotail, indicating that the solar wind drives them. We further discuss the influence of solar wind conditions on the waves through comparison with upstream solar wind parameters.
SM23B-03
Dependence of Lunar Surface Charging on Ambient Plasma Conditions and Solar Irradiation
The surface of the Moon is electrically charged by solar ultraviolet radiation incident on its dayside and the highly variable plasma environment that surrounds it. Lunar surface charging and the associated transport of charged dust could present hazards to future explorers, so developing a predictive capability for this environment will be a high priority. The main electric current sources come from the photoemission of electrons, plasma electrons, plasma ions, and the secondary emission of electrons. All four current sources can be very dynamic, which in turn results in a highly variable electrostatic potential and electric field at the lunar surface, both temporally and spatially. We present predictions for lunar surface potentials and electric fields for a variety of steady-state solar wind conditions. In addition, we also consider what happens when the Moon enters the hotter and more tenuous lobe and plasma sheet regions in the Earth's magnetotail. The main assumptions in deriving these predictions are that all the charged particle populations have a Maxwellian velocity distribution, and that as far as these populations are concerned the Moon's surface is an infinite plane. Since we focus mainly on the solar wind-lunar interaction, we initially neglect the effects of secondary electron emission, since this is often not a significant current source. The intention of this work is to develop a basic theoretical approach to making lunar surface charging predictions, which can be augmented by improvements in (1) our understanding of the current sources, (2) observational constraints, and (3) laboratory measurements. These initial predictions establish a "baseline" against which future theoretical and observational results may be compared, not just for the lunar case, but for all airless bodies such as Mercury and asteroids.
SM23B-04
Surface Charging on Airless Bodies
Although the Moon and asteroids are often thought of as having relatively dormant environments, in fact the Moon at least is very electrically active. The surfaces of airless bodies are directly exposed to solar UV and X-rays, as well as solar wind plasma and energetic particles. This bombardment creates a complex electric field and plasma environment, with the surface typically charging positive in sunlight and negative in shadow, and surface potentials varying over orders of magnitude in response to changing solar illumination and plasma conditions. We present the first efforts to derive the exact magnitude of the nightside lunar surface electric potential from orbit (which involves correcting for spacecraft charging effects), rather than the lower limits which have been derived before. We then compare these measurements to simple theoretical models and other predictions for lunar surface charging in shadow during quiet times. In addition, we present a complete survey of lunar surface charging (utilizing data from Apollo surface observations and Lunar Prospector orbital observations, in concert with theory and modeling) for all lunar locations and solar and plasma conditions, in order to demonstrate the wide range of charging conditions that can occur on airless bodies. By validating surface charging models for the Moon, we can gain confidence in the application of these models to other airless bodies such as asteroids, moons, and Mercury. It is important to have confidence in these theoretical tools, so we can apply them to problems such as dust levitation and transport - which may be of fundamental importance both at the Moon and on asteroids.
SM23B-05
Episodic Atmospheric Heating at Venus and Mars by Solar Storms
The structure, dynamics, chemistry, and evolution of planetary atmospheres are in large part determined by the available sources of energy. One potentially important energy source is solar energetic particle (SEP) events consisting of large fluxes of charged particles accelerated near the Sun during and following fast coronal mass ejections. While other mechanisms provide more constant sources of energy, SEP events can significantly affect an atmosphere for short periods, possibly enhancing atmospheric loss and driving chemical reactions. At unmagnetized planets, in particular, SEPs of all energies have direct access to the atmosphere and so provide a more substantial energy source than at planets having protective global magnetic fields. Therefore quantification of the atmospheric energy input from SEP events is an important component of our understanding of the processes that control their state and evolution of planetary atmospheres. Here we present the results calculations of the energy input by a single large SEP event in the CO2 atmospheres of Venus and Mars. Using simplifying assumptions we calculate the penetration depth and energy deposition profile of energetic protons having different incident energies, and weight the profiles by the event-integrated energy spectrum of a SEP event to estimate the total energy deposition. We also use a more sophisticated radiation code (TRIM/HZETRN) to include the effects of secondary particles on the energy deposition profiles.
SM23B-06
Small-scale Physical Processes in the Plasma Environment of Mars
The global plasma interaction at Mars is well-known to resemble that at Venus and at comets, where the solar wind is slowed and deflected around the conducting planetary ionosphere, interplanetary magnetic field lines drape around the ionospheric obstacle and form a two-lobed induced magnetotail, and newly-formed planetary ions are added to the external plasma flow. Particle and field measurements reveal all of these features at Mars, and also contain evidence for a number of smaller scale physical processes that are commonly observed in the magnetospheres of Earth and other planets. We present evidence of four non-global plasma features evident in Mars Global Surveyor magnetometer and electron reflectometer (MAG/ER) measurements. Magnetic flux ropes with very large core fields (>150 nT) are observed in association with large crustal magnetic fields. Electron butterfly distributions (or conics) are also observed on the planetary night side. Magnetic field perturbations on short timescales, consistent with localized currents are frequently seen. And a variety of localized plasma waves are seen in current sheets and near crustal magnetic fields. In all four cases we draw comparison with similar features measured in Earth's global magnetic field and elsewhere in the inner solar system.
SM23B-07
Observations of Photoelectron Energy Peaks Below 400 km in the Dayside Ionosphere of Mars
The Mars Express Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) experiment determines the electron, ion, and neutral particle components of plasma using four instruments: Electron Spectrometer (ELS), Ion Mass Analyzer (IMA), Neutral Particle Imager (NPI), and Neutral Particle Detector (NPD). The ELS instrument determines the electron energy spectrum by collecting 128 logarithmically spaced samples of the electron spectrum between 1 eV and 20 keV every four seconds. When the ASPERA-3 makes measurements within the dayside Martian ionosphere, it detects electrons from the 30.4 nm photoionization peaks of carbon dioxide and atomic oxygen. These photoelectron peaks are typically observed in all ELS directional sectors when they are detected. Thus, spectrograms in the ionosphere have been examined for one ELS sector in a pilot study to determine if and where the electron photoelectron peaks are observed with respect to the planet surface. The locations of these peaks are compared to the map of the radial component of the crustal magnetic fields to determine if the crustal magnetic fields influence the locations of photoelectrons. The comparison is restricted to the region below an altitude of 400 km and bounded at low altitudes by the spacecraft periapsis (250-300 km).