SH12A-0849
Directional ENA Flow in the Earth's Magnetosheath: Estimation of the Magnetopause Subsolar Distance
Recent studies with the low-energy neutral atom imager (LENA) on the IMAGE spacecraft have shown that directional ENAs are emitted in the Earth's magnetosheath flow through charge exchange with the Earth's hydrogen exosphere. In order to obtain the subsolar distance of the magnetopause from the LENA observation we have modeled ENA flux distributions using the gasdynamic theory for magnetosheath ions and the hydrogen exospheric density model, and compared with the observations. The line of sight integration of the distribution of the ENA flux starting at a relatively high-latitude point inside the magnetosphere with arbitrary dimension shows that the distribution has a clear peak in the direction looking into the low-latitude magnetosheath, which is consistent with the LENA observation in the near-noon sector. By adapting the direction of the peak emission in the model to the one in the LENA observation, we have calculated the subsolar distance. The test for several cases shows that the magnetopause inward motion triggered by the sharp increase of the solar wind dynamic pressure can be taking place during several minutes. Simultaneous observations from in-situ GOES 8 are consistent with this result. This approach thus opens a new way of the empirical modeling of the response of the subsolar distance for the sharp solar wind variation, which was estimated from previous empirical models based on in-situ spacecraft observations to be instantaneous.
SH12A-0850
Conjugate observations of ENA signals in the high-altitude cusp and proton auroral spot in the low-altitude cusp with IMAGE spacecraft
Dayside proton auroral spots in the ionosphere are frequently observed by the Far Ultraviolet Instrument (FUV) onboard the IMAGE spacecraft. On 28 April 2001 (0500-0630 UT), when the solar wind density was about 10 cm- 3 and IMF was northward except for a few short periods, FUV observed proton auroral spots in the cusp ionosphere. The intensification of the proton auroral spot was simultaneous with ENA signals observed by the Low Energy Neutral Atom (LENA) imager on IMAGE also from the direction of the high-altitude cusp. If the primary source of the LENA cusp signal were the proton injection caused by reconnection, then the LENA signal in the high-altitude cusp and the FUV spot in the low-altitude cusp should have a common source. The magnetic field line tracing with the Tsyganenko-96 model shows that the possible source location of the LENA cusp signal maps on the proton auroral spot, and the temporal variations of both intensities also show a correlation. During the interval of interest a brief period has been identified in which the proton aurora moves toward the pre-noon side and the direction of the LENA cusp signal moves poleward. Considering the solar wind variations for this event, we have interpreted these motions as being due to the detached reconnected flux tubes created by a brief southward tilting of the IMF, and not the shift of the reconnection point which is often invoked for the motion of the FUV spot in the cusp. Detailed characteristics about the correspondence between the LENA signal and the FUV spot are presented, and the field line geometry for the detached reconnected flux tube will be discussed.
SH12A-0851
Neutral Solar Wind Generated by Lunar Exospheric Dust at the Terminator
We calculate the flux of neutral solar wind observed on the lunar surface at the terminator due to solar wind protons penetrating exospheric dust with: (1) grains larger that 0.1μm and (2) grains larger than 0.01μm. For grains larger than 0.1μm, the ratio of the neutral solar wind to solar wind flux is estimated to be ~10-4-10-3 at solar wind speeds in excess of 800 km/s, but much lower (<10-5) at average to low solar wind speeds. However, when the smaller grain sizes are considered, the ratio of the neutral solar wind flux to solar wind flux is estimated to be ≥10-5 at all speeds and at speeds in excess of 700 km/s reaches 10-3-10-2. These neutral solar wind fluxes are easily measurable with current low energy neutral atom instrumentation. Observations of neutral solar wind from the surface of the Moon could provide a very sensitive determination of the distribution of very small dust grains in the lunar exosphere and would provide data complementary to optical measurements at ultraviolet and visible wavelengths. Furthermore, neutral solar wind, unlike its ionized counterpart, is not held-off by magnetic anomalies, and may contribute to greater space weathering than expected in certain lunar locations.
SH12A-0852
Quantifying the Exospheric Component of Soft X-ray Emission
High charge state heavy ions in the solar wind exchange charge with ambient neutral gas. This process creates a product ion in an excited state. During the radiative cascade process, EUV and X-ray photons are emitted with energies in the range of about 100 eV to 1 keV. Because the terrestrial exospheric density at the nominal magnetopause location is relatively high, ~10 cm-3, solar wind charge exchange, or SWCX, can be observed by Earth-orbiting soft X-ray instruments such as the ROSAT Position Sensitive Proportional Counters (PSPC). In this presentation, we will compare simulated and observed soft X-ray emission during an event on August 18-19, 1991 and discuss the role of exospheric SWCX emission for this and other events.
SH12A-0853
A New Instrument Design for Imaging Low Energy Neutral Atoms
The MidSTAR-2 satellite, to be built at the US Naval Academy as a follow-on to the successful MidSTAR-1 satellite, will launch in 2011 and carry three Goddard Space Flight Center (GSFC) experiments developed under Goddard's Internal Research and Development (IRAD) program. One of these GSFC instruments, the Miniature Imager for Neutral Ionospheric atoms and Magnetospheric Electrons (MINI-ME) builds on the heritage of the Goddard-developed Low-Energy Neutral Atom (LENA) imager launched on the IMAGE spacecraft in 2000. MINI- ME features a Venetian-blind conversion surface assembly that improves both light rejection and conversion efficiency in a smaller and lighter package than LENA making this an highly effective instrument for viewing solar wind charge exchange with terrestrial and planetary exospheres. We will describe the MINI-ME prototyping effort and its science targets, also including time variability of ENA fluxes and charge-exchange interactions in the upper atmosphere from the terrestrial ring current source. http://web.ew.usna.edu/~midstar/
SH12A-0854
Application of ENA data assimilation to global modeling of space plasma environments
Data assimilation is a technique which combines information of observations and our knowledge about physical laws described by a simulation model, and it offers a general framework for providing a best estimate of the system state based on both of observations and physical consistency. Data assimilation techniques have been originally developed for applications in meteorology, and those framework are applicable to various dynamical systems in geophysics. We are developing a data assimilation scheme which incorporates ENA observations from IMAGE/HENA into a kinetic ring current model in order to obtain a realistic model of the ring current evolution. We outline this data assimilation scheme, and we will also discuss how ENA data assimilation is effective for modeling studies concerning space plasma.
SH12A-0855
Seeking to improve low energy neutral atom detection in space
The detection of energetic neutral atoms allows for the remote examination of the interactions between plasmas and neutral populations in space. Before these neutral atoms can be measured, they must first be converted to ions. For the low energy end of this spectrum, interaction with a conversion surface is often the most efficient method to convert neutrals into ions. It is generally thought that the most efficient surfaces are low work functions materials. However, by their very nature, these surfaces are highly reactive and unstable, and therefore are not suitable for space missions where conditions cannot be controlled as they are in a laboratory. We therefore are looking to optimize a stable surface for conversion efficiency. Conversion efficiency can be increased either by changing the incident angle of the neutral particles to be grazing incidence and using stable surfaces with high conversion efficiencies. We have examined how to increase the angle of incidence from ~80 degrees to ~89 degrees, while maintaining or improving the total active conversion surface area without increasing the overall volume of the instrument. We are developing a method to micro-machine silicon, which will reduce the volume to surface area ratio by a factor of 60. We have also examined the material properties that affect the conversion efficiency of the surface for stable surfaces. Some of the parameters we have examined are work function, smoothness, and bond structure. We find that for stable surfaces, the most important property is the smoothness of the surface.
SH12A-0856
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., 2006, 2004). 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, which are important for sputtering in the Martian upper atmosphere. Atmospheric effects of these precipitating hot ions in the Martian atmosphere are studied and reported on.
SH12A-0857
Energetic Neutral Atoms Around the Extrasolar Planet HD 209458b
Observed atomic hydrogen absorption in the stellar Lyman-alpha line by the planet HD 209458b shows hydrogen with high velocity at great distances from the planet. This has been interpreted as hydrogen atoms undergoing hydrodynamic escape. We estimate the production of energetic neutral atoms from the interaction of the stellar wind with the extrasolar planet HD 209458b, and the implications for observations of atomic hydrogen. The estimated production of hydrogen-energetic neutral atoms is compared with observations.
SH12A-0858
ENA Emission From the Cusp of the Earth's Magnetosphere
The Earth's magnetosheath ions are injected into the cusp along newly reconnected field lines. ENAs produced by charge exchange of these ions with the Earth's hydrogen exosphere have been detected by the Low-Energy Neutral Atom imager (LENA) on the IMAGE spacecraft in the magnetosphere. The evidence is from a case study of simultaneous observations from IMAGE/LENA and the Thermal Ions Dynamic Experiment onboard the in-situ Polar spacecraft. Theoretical considerations also show that LENA can detect the ENAs from the cusp when looking into the cusp roughly along the magnetic field lines. Hence, LENA can monitor the ion injection region continuously for a few hours. Detailed analyses of the LENA cusp emission data have revealed several characteristics of the ion injection region. We present the results for various IMF conditions, in particular the IMF By dependence of the ion injection region, the poleward/equatorward drift of the injection region during fluctuating IMF Bz, and the gradual motion of the injection region for northward IMF.
SH12A-0859
Response of the lunar alkali exosphere to plasma
The response of the lunar exosphere to plasma when the Moon traverses the terrestrial magnetosphere is investigated. A published series of ground-based observations of the lunar sodium emission were obtained while Lunar Prospector was in orbit around the Moon (Potter et al., 2000), and the data of column density and temperature are correlated to LP in situ plasma measurements. Results from this analysis indicate that the variability of the observed column density correlates to the total electron flux measured by Lunar Prospector, which is a proxy for the ion sputtering flux. However, direct ion sputtering is found to be relatively inefficient and cannot explain the evident regulation of the total column by ions. We conclude that the efficiency of the photon- stimulated source is augmented by ion surface impact. Ion-enhanced PSD becomes the leading source of the exosphere in the magnetosheath and solar wind, while shielding of ions by the Earth's magnetosphere emphasizes release of sodium by impact vaporization
SH12A-0860
Lunar Surface Charging During Solar Energetic Particle Events
We have previously shown, using Lunar Prospector data, that the nightside lunar surface can reach potentials as large as several kilovolts negative during SEP events. These large surface charging events may have significant effects on the lunar surface and exospheric environment (including dust), and therefore have intriguing implications from both a pure science and an exploration perspective. We now build on this initial result to further investigate the response of the lunar surface to SEP events. We present the results of investigations into two aspects of this response: 1. How does the response of the lunar surface to different SEP events vary, and what features of specific SEP events make them more efficient at charging the surface to large potentials? 2. How does the lunar surface response vary as a function of solar zenith angle, and can we explain this response in terms of the expected charged particle access to the surface? By answering these questions, we can make significant steps towards understanding the complex coupled system formed by the interactions of the lunar surface, lunar exosphere, and solar plasma.
SH12A-0861
Supercharging of the Lunar Surface by Solar Wind Halo Electrons
Lunar surface potentials can reach several kilovolts negative during Solar Energetic Particle (SEPs) events, as indicated by recent analysis of data from the Lunar Prospector Electron Reflectometer (LP/ER). The lunar surface- plasma interactions that result in such extreme surface potentials are poorly characterized and understood. Extreme lunar surface charging, and the associated electrostatic discharges and transport of charged dust, will likely present significant hazards to future human explorers. This is of particular concern near the terminator and polar regions, such as the South Pole/Aiken Basin site planned for NASA's manned outpost. It is the flux of electrons from the ambient plasma that charges the surface of the Moon to negative potentials. In the solar wind, the electron temperature is typically ~10 eV which tends to charge the lunar surface to ~100 V negative in shadow. However, during space weather events the solar wind electrons are often better described by the sum of two Maxwellian distributions, referred to as the "core" and "halo" components. The core electrons are relatively cool and dense (e.g., ~10 eV and ~10/cc), whereas the halo electrons are hot and tenuous (e.g., ~100 eV and ~0.1/cc). Despite, the tenuous nature of the halo electrons, our surface charging model - using core and halo electron data derived from the Solar Wind Experiment (SWE) aboard the Wind spacrcraft - predicts that they are capable of "supercharging" the lunar surface to kilovolt potentials during space weather events, which could explain the LP/ER observations.
SH12A-0862
Global 3D Kinetic Simulations of the Solar Wind Interaction with the Moon and the Formation of the Lunar Wake-Tail
When the solar wind interacts with the Moon, the lunar surface acts as a diamagnetic obstacle removing plasma from the solar wind and a density depletion region is formed on the nightside. To examine the refilling of the density depletion region and the formation of the Moon's wake-tail, a high resolution 3D global hybrid simulation is used. The hybrid simulation (particle ions, fluid electrons) uses realistic spatial scales for the lunar obstacle and the system size. The Moon's tail refilling process can be described approximately by a plasma expansion into a vacuum driven by the thermal motion of particles along the interplanetary magnetic field lines. Parallel electric fields created due to the pressure gradient at the edge of the density cavity further accelerate ions into the density depletion region and highly anisotropic distribution functions are formed. These anisotropic ion distribution functions lead to the generation of electromagnetic waves near the ion cyclotron frequency along the length of the wake-tail further downstream. A density increase at the center of the plasma depletion region also tends to form downstream where the ions beams from either side of the cavity meet and interact. Results will be compared with data from Wind when the satellite made flybys through the Moon's wake tail.