POES satellite observations of EMIC-wave driven relativistic electron precipitation during 1998-2010

[1] Using six Polar Orbiting Environmental Satellites (POES) satellites that have carried the Space Environment Module-2 instrument package, a total of 436,422 individual half-orbits between 1998 and 2010 were inspected by an automatic detection algorithm searching for electromagnetic ion cyclotron (EMIC) driven relativistic electron precipitation (REP). The algorithm searched for one of the key characteristics of EMIC-driven REP, identified as the simultaneity between spikes in the P1 (52 keV differential proton flux channel) and P6 (>800 keV electron channel). In all, 2331 proton precipitation associated REP (PPAREP) events were identified. The majority of events were observed at L-values within the outer radiation belt (3 < L < 7) and were more common in the dusk and night sectors as determined by magnetic local time. The majority of events occurred outside the plasmasphere, at L-values ~1 Re greater than the plasmapause location determined from two different statistical models. The events make up a subset of EMIC-driven proton spikes investigated by Sandanger et al. (2009), and potentially reflect different overall characteristics compared with proton spikes, particularly when comparing their location to that of the plasmapause, i.e., EMIC-driven proton precipitation inside the plasmapause, and potentially EMIC-driven REP outside the plasmapause. There was no clear relationship between the location of plasmaspheric plumes and the locations of the PPAREP events detected. Analysis of the PPAREP event occurrence indicates that high solar wind speed and high geomagnetic activity levels increase the likelihood of an event being detected. The peak PPAREP event occurrence was during the declining phase of solar cycle 23, consistent with the 2003 maximum in the geomagnetic activity index, Ap

Precipitating radiation belt electrons and enhancements of mesospheric hydroxyl during 2004–2009

Energetic particle precipitation leads to enhancement of odd hydrogen (HOx) below 80 km altitude through water cluster ion chemistry. Using measurements from the Microwave Limb Sounder (MLS/Aura) and Medium Energy Proton and Electron Detector (MEPED/POES) between 2004–2009, we study variations of nighttime OH caused by radiation belt electrons at geomagnetic latitudes 55–65°. For those months with daily mean 100–300 keV electron count rate exceeding 150 counts/s in the outer radiation belt, we find a strong correlation (r ≥ 0.6) between OH mixing ratios at 70–78 km (0.046–0.015 hPa) and precipitating electrons. Correlations r ≥ 0.35, corresponding to random chance probability p ≤ 5%, are observed down to52 km (0.681 hPa), while no clear correlation is observed at altitudes below. This suggests that the fluxes of ≥3 MeV electrons were not high enough to cause observable changes in OH mixing ratios. At 75 km, in about 34% of the 65 months analyzed we find a correlation r ≥ 0.35. Although similar results are obtained for both hemispheres in general, in some cases the differences in atmospheric conditions make the OH response more difficult to detect in the South. Considering the latitude extent of electron forcing, we find clear effects on OH at magnetic latitudes 55–72°, while the lower latitudes are influenced much less. Because the time period 2004–2009 analyzed here coincided with an extended solar minimum, and the year 2009 was anomalously quiet, it is reasonable to assume that our results provide a lower-limit estimation of the importance of energetic electron precipitation at the latitudes considered

Contrasting the efficiency of radiation belt losses caused by ducted and nonducted whistler-mode waves from ground-based transmitters

It has long been recognized that whistler-mode waves can be trapped in plasmaspheric whistler ducts which guide the waves. For nonguided cases these waves are said to be "nonducted", which is dominant for L < 1.6. Wave-particle interactions are affected by the wave being ducted or nonducted. In the field-aligned ducted case, first-order cyclotron resonance is dominant, whereas nonducted interactions open up a much wider range of energies through equatorial and off-equatorial resonance. There is conflicting information as to whether the most significant particle loss processes are driven by ducted or nonducted waves. In this study we use loss cone observations from the DEMETER and POES low-altitude satellites to focus on electron losses driven by powerful VLF communications transmitters. Both satellites confirm that there are well-defined enhancements in the flux of electrons in the drift loss cone due to ducted transmissions from the powerful transmitter with call sign NWC. Typically, similar to 80% of DEMETER nighttime orbits to the east of NWC show electron flux enhancements in the drift loss cone, spanning a L range consistent with first-order cyclotron theory, and inconsistent with nonducted resonances. In contrast, similar to 1% or less of nonducted transmissions originate from NPM-generated electron flux enhancements. While the waves originating from these two transmitters have been predicted to lead to similar levels of pitch angle scattering, we find that the enhancements from NPM are at least 50 times smaller than those from NWC. This suggests that lower-latitude, nonducted VLF waves are much less effective in driving radiation belt pitch angle scattering