Transport Equations for Low Altitude Emission of Energetic Neutral Atoms Caused by Precipitating Magnetospheric Ions
Low altitude energetic neutral atom emission (LAE) is produced when precipitating magnetospheric ions (mainly H+) undergo charge-exchange collisions with exospheric monatomic oxygen atoms (O) at altitudes below 500 km. LAE has been previously observed by ENA cameras on several spacecraft (Astrid/PIPPI, IMAGE/MENA, and IMAGE/HENA). Recent comparisons of observations of LAE in ENA images from TWINS with simultaneous in situ DMSP measurements of precipitating ions are presented in related papers in this Special Session [ Bazell et al.; McComas et al.]. LAE is strongly directional, and so is observable by the TWINS spacecraft when either spacecraft is at moderate to high geomagnetic latitudes and precipitation is occurring at MLT about 12h around from the location of the TWINS spacecraft. LAE is a "thick-target" process involving many collisions with 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. Two coupled transport equations for the ion and ENA intensities are developed in the extreme forward-scattering approximation (which does, however, include an energy loss in every kind of collision). 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, and they are applied to obtain a simplified relation between the precipitating ion intensity and the emergent ENA and ion intensities. Details of the computations are presented in a related poster in this Special Session [ Nair and Roelof].
Comparison of TWINS Images of Low-Altitude ENA Emissions With DMSP Precipitating Ion Fluxes
Despite the low levels of geomagnetic activity over the past year, ENA cameras on both TWINS spacecraft have been detecting strong low-altitude emission (LAE) of energetic neutral atoms (ENAs) in the range 1-30 keV/nuc. The emission appears to come from auroral latitudes. The TWINS image pixels (4°× 4°) cannot resolve the auroral zone in latitude (MLAT), but do allow a resolution ~ 3h in local time (MLT) under favorable viewing conditions. We have been comparing the ENA spectra in the LAE to the precipitating ion spectra 0.3-40 keV/Q measured in situ by DMSP at the same UT and as near as possible to the MLT of the ENA emission. We present comparisons of TWINS and DMSP spectra during the modest geomagnetic activity during the 24-hour period of 11 October 2008. Assuming the source to be precipitating protons, we estimate the proton spectrum implied by the TWINS ENA spectrum using the "thick target" calculations for multiple charge-exchange transport in the monatomic oxygen exosphere at altitudes below 500km [Roelof; Nair and Roelof; this Special Session]. The spectra often show good agreement in terms of spectral shape, though the TWINS spectra can be up to a factor of ten lower in intensity than the DMSP spectra. This is consistent with the large (4°× 4°) TWINS pixel size compared to the narrow latitudinal extent of the auroral oval.
Composition of the ENA low altitude emissions as viewed by TWINS
The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) instruments consistently view bright ENA emission coming from low altitude (<500 km altitude). These low altitude emissions are typically the most intense ENA signal seen in the TWINS ENA energy range (~1keV to 100keV), especially during quiet times. The relative composition of the incident ENA's are determined by measuring the relative yield of secondary electrons liberated from the ultra-thin carbon foil as the ENA passes through the TWINS sensors. While this technique cannot determine the exact species on a particle per particle basis, it can statistically determine the ratio of heavy to light ENA's (O and H). We present the composition of the low altitude emission as a function of energy and local time for several events.
ENA Emissions from low Altitude as Seen from IMAGE
The Medium (MENA) and High (HENA) Energy Neutral Atom imagers on IMAGE routinely observed ENA emissions from low altitude. These emissions are far more profuse during the main phase of geomagnetic storms and seem to emanate from limited regions of invariant latitude, in limited pitch angle bands. The HENA observations of ENAs at higher energies see both Oxygen and Hydrogen emissions. In some cases both detailed observations of emissions of ENAs from low altitude and in situ observations of precipitating ions are available. We will show comparison of fluxes and invariant latitude variations with magnetic local time in these cases.
EMIC wave generation and associated loss of energetic particles during magnetospheric compressions.
Electromagnetic ion cyclotron (EMIC) waves are believed to play an important role in the dynamics of energetic particles in the inner magnetosphere, causing them to precipitate into the ionosphere via resonant interaction. In order to incorporate the EMIC-related loss processes into global magnetospheric models one needs to know solar wind and magnetospheric conditions favorable for EMIC wave excitation as well as localization of the waves in the magnetosphere. In this work, we investigate the role of magnetospheric compression in the generation of EMIC waves in the inner magnetosphere. Theoretical studies have shown that in the inner magnetosphere resonant interaction with EMIC waves may be important for MeV electron loss from the radiation belts, especially in regions of high plasma density and low magnetic field, e.g., outer regions of quiet- time plasmasphere. We will present multiple examples of EMIC wave observation - both ground-based and in- situ and simultaneous registration of energetic proton precipitation into the ionosphere. Such precipitations were previously found to be co-located with those of relativistic electrons. Based on our observations, we will discuss the following possibility for the inner magnetosphere loss processes: enhancements in solar wind density -> magnetospheric compression -> generation of EMIC waves in regions of high plasma density - > consequent loss of energetic particles.
Visualization of EMIC waves-particle interaction region through isolated proton auroras at subauroral latitudes
We investigated details of an isolated proton auroral aurora observed at Athabasca, Canada (MLAT: 62, L: 4.6) at 2100-2240 MLT on 5 September 2005, using a multi-data set obtained by simultaneous ground and satellite observations. The event clearly demonstrates that a localized enhancement of 30-80 keV ions, which was observed by the NOAA 17 satellite, precipitated into an isolated proton aurora equatorward of the auroral oval. The appearance interval of the isolated proton aurora was coincident with a burst of Pc 1 geomagnetic pulsations in the frequency range of the electromagnetic ion cyclotron (EMIC) wave. The DMSP F13 satellite observed an ionospheric plasma trough, which is a signature of the footprint of the plasmapause, at the conjugate point of the isolated proton aurora in the southern hemisphere. These observations support the following scenario: EMIC waves are generated near the plasmapause in the inner magnetosphere and resonantly scatter energetic ring current ions into loss cone. The precipitating ions excite proton aurora in the ionosphere. The EMIC waves propagated into the ionosphere are detected on the ground as Pc 1 geomagnetic pulsations. Based upon multi-event analysis of one-year auroral imaging and magnetic field data at Athabasca in September 2005 - September 2006, we found that the isolated auroras were always observed simultaneously with Pc 1 geomagnetic pulsations in the frequency range of He+-band EMIC waves at the conjugate magnetic equator for all 13 events. The isolated auroras had narrow latitudinal widths (less than ~230 km) and limited longitudinal lengths (250-800 km) at an altitude of 120 km. The isolated auroras moved equatorward as associated Pc 1 frequencies increased, since the ion gyrofrequency becomes higher at smaller L values. These facts indicate that the isolated proton auroras are connected rigidly with the source region of EMIC waves by a magnetic flux tube and that the wave-particle interaction occurs in a very small region in the inner magnetosphere as indicated by the localization of the isolated auroras.
Modeling the Radiation Belts During a Geomagnetic Storm
We utilize the Radiation Belt Environment (RBE) model to simulate the radiation belt electrons during a geomagnetic storm. Particularly, we focus on the relative contribution of whistler mode wave-particle interactions and radial diffusion associated with rapid changes in the magnetospheric magnetic field. In our study, the RBE model obtains a realistic magnetic field from the BATS-R-US magnetosphere model at a regular, but adjustable, cadence. We simulate the storm with and without wave particle interactions, and with different frequencies for updating the magnetic field. The impacts of the wave-particle interactions, and the rapid variations in the magnetospheric magnetic field, can then be studied. Simulation results are also extracted along various satellite trajectories for direct comparison where appropriate.