Geomagnetic Activity and Directional Dependences of Low Altitude Energetic Neutral Atom Intensities
Energetic neutral atom (ENA) stereo images are routinely obtained by the dual-spacecraft Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission. During the quiet to moderately disturbed geomagnetic conditions that have prevailed during 2008-2009, the ENA images have contained emissions from low altitudes, i.e., close to the limb of the Earth. Stereo imaging (observation from two widely-separated vantage points) is used to analyze the directional dependence of these low-altitude ENA emissions. With this directional dependence characterized, we determine the dependences of the occurrence and intensity of these emissions versus several key indicators of geomagnetic activity, including available solar wind data from the Advanced Composition Explorer (ACE) and various geomagnetic indices (Kp, Dst, AE, etc.).
Computation of Emergent Intensities of ENAs and Ions From Low Altitude Emission
The energetic neutral atom (ENA) cameras on the TWINS spacecraft have been observing low-altitude emission (LAE) from the auroral regions during recent weak geomagnetic activitiy [ Bazell et al.; McComas et al., this Special Session]. When the observing viewpoint is favorable, LAE can be the brightest ENA emission in the TWINS images, even during large geomagnetic storms. The production of LAE is a "thick- target" process involving many collisions at altitudes <500km with monatomic oxygen (O): charge exchange of H+, stripping of neutral H, as well as energy losses due to ionization and excitation of the O when the energetic hydrogen is in either its charged (H+) or neutral (H) state. The theory of LAE therefore requires two coupled transport equations for the ion and ENA intensities. These have been developed in the extreme forward-scattering approximation. Pitch-angle changes in the Earth's magnetic field are also included, because they profoundly influence the emerging intensities of both ENAs and ions. Analytic solutions for these intensities have been obtained [ Roelof, this Special Session] that are expressed in two "eigen-functions" that are linear combinations of the ENA and ion intensities. They are functions of a "helical column density" that is accumulated by a gyrating particle during its downward path to its mirror point and then upward until its emergence as either an ENA or an ion. The solution also includes the history of the particle's energy, which constantly decreases during its trajectory due to the energy loss in every kind of atomic collision. The emergent spectra are highly directional (both with respect to the zenith and the local magnetic field vector), and they depend upon the shape of the precipitating ion spectrum. Numerical examples from the analytic solutions are presented for these functions and the emergent intensities of both ENA emission and energetic ion albedo.
Determining the ENA Composition From TWINS Flight Data: A Data-Analysis Technique
We present a data-analysis technique that is able to estimate the mass composition of the TWINS (Two Wide-
angle Imaging Neutral-atom Spectrometers) measured energetic neutral atom (ENA) data using the
amplitudes of electron distributions derived from the micro-channel plates (MCP). These distributions are
generated through the registration of secondary electrons emitted from a thin carbon foil as the TWINS
instruments measure the time-of-flight of incoming ENAs. For a given particle time-of-flight (or ENA velocity),
the secondary electron yield depends on ENA energy or mass. Utilizing the theory of multiple emissions of
secondary electrons, we can fit the distributions from the flight data using a superposition of a standard
number of peaks, each represented by a three-parameter Cauchy-Lorentzian function. Through this we
estimate an effective secondary electron yield value through comparison to TWINS calibration data. From the
number of valid events for a given set of data and its effective SE-yield value, we can extract the composition of
the flight data for a selected velocity range based on the anticipated most abundant two-species of
magnetospheric ENAs, i.e., hydrogen (H) and oxygen (O). We present the results of applying this technique to
selected periods of TWINS flight data over the past year.
The Role of Plasma Sheet Conditions in Ring Current Formation and Energetic Neutral Atom Emissions: TWINS Results and CRCM Comparison
The dynamics of the ring current is sensitive to plasma sheet density and temperature. The situation is further complicated by ionospheric feedback and the existence of electric shielding at low latitudes. Most of the ring current pressure is carried by ions with energies of ~5-50 keV. In this energy range, H-H+ charge exchange cross section falls sharply with increasing energy. As a result, the intensity of energetic neutral atoms (ENA) emitted from the ring current is very sensitive to the ion energy distribution, which, in turn, is controlled by the plasma sheet temperature. Using the Comprehensive Ring Current Model (CRCM) with different plasma sheet models, we calculate ENA emissions during several moderate storms in years 2008 and 2009. We compare the simulated images with those from the TWINS imagers and study the effects of plasma sheet conditions on the ring current and the associated ENA emissions.
Comparison of ENA Ion Temperature Calculation Technique to Model Predictions
Because ion heating has been correlated with several magnetospheric phenomena that occur on varying spatial and temporal scales, including magnetic reconnection, instabilities, and convection of plasma through different regions of the magnetosphere, it is inherently difficult to establish a reliable model of ion temperatures in the magnetosphere. Using a technique to calculate effective ion temperatures based on the charge- exchange cross section-corrected energetic neutral atom (ENA) flux versus energy spectrum, we have recently shown that, during certain storm intervals, there is a significant difference between our calculated plasma sheet ion temperatures and those predicted using a statistical correlation with the solar wind velocity. We will present our ion temperature calculations in comparison with those predicted by two such correlation models given by Borovsky et al.  and Tsyganenko and Mukai . The Comprehensive Ring Current Model (CRCM) is often using in conjunction with ENA data to study the physical mechanisms responsible for features seen in the data. We will present a comparison of our ion temperature calculation technique applied ENA flux maps generated by CRCM with those calculated directly within the model.
First Ion Temperature Images From TWINS Data
Ion heating has been correlated with several magnetospheric phenomena, including magnetic reconnection, instabilities, and convection of plasma through different regions of the magnetosphere. Thus, it important to be able to measure ion temperatures throughout the magnetosphere to better understand the physics of these phenomena. Effective ion temperatures based on the charge-exchange cross section-corrected energetic neutral atom (ENA) flux versus energy spectrum can be calculated from TWINS data. Effective ion temperatures calculated from the Medium Energy Neutral Atom (MENA) imager on the IMAGE spacecraft using this technique were shown to have excellent (within ~30%) agreement with in-situ measurements from MPA instruments on LANL geosynchronous spacecraft and GEOTAIL. In order to achieve adequate statistics for reliable ion temperature calculations, we can use either data with significant ENA flux rates, such as during geomagnetic storms, or superpositions of multiple data sets. We present ion temperature images from the few small storms that have occurred so far in the TWINS mission. We will present both 'skymap' images as well as 2-D maps created using an algorithm to project the data along each pixel's line-of-sight to the equatorial plane of the Geocentric Solar Magnetospheric (GSM) coordinate system. During intervals for which data is available from both TWINS I and II, we will compare the locations of interesting features as seen by the two satellites.
Acceleration of Magnetospheric Relativistic Electrons by Ultra-Low Frequency Waves: A Comparison Study
In a statistical study O'Brien et al.  showed that magnetospheric relativistic electrons (MRE) can be accelerated significantly during the recovery phase of a magnetic storm when there exists sustained solar wind speeds of > 450 km s-1 and long duration of Pc5 ULF activity. While this statistical result supports the general tendency for MRE's to depend on solar wind speeds and ULF activity, individual events can behave quite differently. For example, during the storm recovery phases on September 25, 2001 and November 25, 2001, when the solar wind speeds were > 600 kms-1, the MRE data measured by LANL and GOES spacecraft indicate a 4-fold and negligible increases of 1.1-1.5 MeV electrons during ~3-hour strong ULF wave activity periods as observed by Cluster spacecraft at noon and dusk, respectively. In this paper, we present detailed comparisons between these two events. Our results show that the main difference between the two events is a strong day-night asymmetry of ULF wave distributions in the September event in contrast with the uniform ULF wave distribution in the November event. O'Brien, T. P., et al., 2001, Which magnetic storms produce relativistic electrons at geosynchronous orbit? J. Geophys. Res., 106, 15,533.
Investigation of Energy and Pitch Angle Cross Diffusion Effects on the Outer Radiation Belts Through a Kinetic Radiation Belt Environment Model
Understanding the dynamics of the Earth's outer radiation belt is important for better modeling and forecasting the intensities of energetic electrons in space. It is established that wave diffusion processes are responsible for loss and acceleration of electrons in the radiation belts. Several recent studies indicate pitch angle and energy cross-diffusion cannot be ignored when considering the total diffusive effects. In this study, we incorporate the electron energy and pitch angle cross diffusion into the current Radiation Belt Environment model and investigate its effects on the dynamics of radiation belt electrons.