Low-Altitude Emission of Energetic Neutral Atoms from Precipitating Magnetospheric Ions: IMAGE/HENA Mission-Long Survey

Remote sensing of ring current losses should be of considerable utility to the RBSP mission. Low-altitude emission (LAE) of energetic neutral atoms (ENAs) yields a sensitive measurement of the precipitation of magnetospheric energetic ions into the dense monatomic oxygen (O) exosphere at altitudes ∼350 km. LAE is the brightest source of ENAs with energies 1<E<200 keV in the magnetosphere. It is much brighter than ring-current emission (RCE) of ENAs produced by the same energetic ion population directly by charge exchange with the neutral hydrogen atom (H) geocorona. LAE has been detected by many spacecraft (including the NASA TWINS spacecraft now in orbit), but the HENA imager on the IMAGE spacecraft (operating during 2000-2005) had the highest angular resolution (6^o×6^opixels) and sensitivity (G=1.6 cm² sr) over the energy range of the bulk of the ring current (20<E<200 keV). HENA forms an all-sky image every spin period (2 minutes). During each perigee pass (every ∼14 hours, altitude ∼2000 km), some half-dozen consecutive 2-min images were obtained. As the initial step in a study of LAE*, the mission-long archive of 10-min (5-spin) IMAGE/HENA images has been scanned by eye and cataloged according to maximum (average) intensity in the images for the three channels: 27-60 keV H; 60-120 keV H; and 52-180 keV O. When the LAE intensities were compared to 1-hour magnetic indices D_st and AE, two correlations emerged. Order-of-magnitude increases in 52-180 keV ENA O occurred during only the largest geomagnetic storms, while abrupt order-of-magnitude decreases in the ENA H (some lasting only a day or two) showed a nearly 1:1 correlation with isolated days of near-zero AE< 20nT. The latter association of a lack of auroral electro-jet activity with much-reduced LAE intensity would seem straight-forward if the energetic ions were precipitating on auroral-zone field lines. However, some case studies comparing LAE imaged by the TWINS imagers with DMSP in situ measurements of precipitating 2-30 keV ions [Bazell et al., JGR, 2010] revealed that relatively quiet-time precipitation (D_st>-40 nT) was restricted to ring current invariant latitudes 60-65 deg, well below the auroral zone. The next stage of our HENA mission-long study* will examine the latitude range of in situ ion precipitation measured on DMSP passes at ∼800 km altitude, above the reduced-intensity HENA LAE during days of near-zero AE. *Supported by NASA Grant NNX12AJ59G.