Physics of auroral phenomena : proceedings of the 37th Annual seminar, Apatity, 25 - 28 February, 2014 / [ed. board: A. G. Yahnin, N. V. Semenova]. - Апатиты : Изд-во Кольского научного центра РАН, 2014. - 125 с. : ил., табл.

“Physics o fAuroral Phenomena”, Proc. XXXVII A nnual Seminar, Apatity, pp. 46 50, 2 0 1 4 © Kola Science Centre, Russian Academy of Science, 2014 Polar Geophysical Institute RELATIVISTIC ELECTRON PRECIPITATION AS SEEN BY NOAA POES A.G. Yahnin, T.A. Yahnina, N.V. Semenova, B.B. Gvozdevsky (Polar G eophysical Institute, Apatity) Abstract. We performed a survey of relativistic electron precipitation (REP) events, which had been seen with MEPED P6 telescope onboard NOAA POES during a 38-days interval. Combining P6 data with simultaneous energetic (>30 keV) electron and proton observations we divided all REP events into three groups. One group consists of REP enhancements forming the isotropy zone at the poleward edge of relativistic electron fluxes. These REP events are observed on the night side, and are, evidently, produced by isotropization process related to non- adiabatic motion of particles in the stretched night side magnetic field. Second group consists of REP events related to simultaneous enhancements of energetic >30-300 keV electrons. These events have a wider ML Г range of occurrence with maximum in the pre-midnight sector. These REP events can be related to interaction of electrons with waves in a wide range of frequencies. Some REP events correlate with burst-like precipitation of >30-keV protons. Such proton bursts indicate the location of the EMIC wave source. Thus, these REP events could be related to scattering of the relativistic electrons by EMIC waves. However, even in such cases the relativistic electrons are always associated with precipitations of energetic (>30 keV) electrons. This fact poses a question: If the relativistic electrons are indeed precipitated by EMIC waves or some other waves precipitating electrons in a wide range of energies are involved? 1. Introduction Investigation of the relativistic electron precipitation (REP) morphology is important because of the REP atmospheric impact and because it may help in solving the problem of mechanisms of radiation belt losses. Imhof et al. (1991) using the data from three-axis-stabilized S81-1 spacecraft concluded that REP at the outer limit of electron trapping is due to the particle scattering in that region of the magnetosphere where the radius of the field line curvature is comparable with the gyroradius of the electron (e.g. Sergeev and Tsyganenko, 1982). Another study by Imhof at al. (1986) based on the data from several low-orbiting satellites dealt with narrow (with duration of equal or less than 10 s) spikes of relativistic electrons well inside the trapping boundary. Some of these electron spikes were associated with energetic (tens of keV) protons flux enhancements. The authors concluded that the precipitation was due to cyclotron interaction of radiation belt particles with low-frequency waves. Some of the REP spikes were observed near noon, and most of the events including those associated with energetic proton precipitation were found in the late evening sector. Nakamura et al. (2000) presented the morphology o f the precipitation o f > 1 MeV electrons based on SAMPEX observations. These authors divided REP events into three categories relatively to their duration (<1 s, <10 s, and <30 s). The first category (microbursts) was observed mainly in the morning sector during magnetic storms. MLT distributions of REP bursts with the duration <10 s and <30 s were found to be similar. Their occurrence maximum was in pre-midnight hours for non-storm time, and they were observed at all MLTs during storms. NOAA Polar-orbiting Operational Environmental Satellites (POES) data have been already used in some statistical studies of REP. Thus, global distribution and variations of the intensity of the ~1 MeV electron precipitation during geomagnetic storms were presented by Horne et al. (2009). They found the enhanced precipitation in the vicinity of the South Atlantic Magnetic Anomaly (SAMA) and nearly uniform distribution of the precipitating flux intensity around the pole. Recently, Carson et al. (2012) and Wang et al. (2014) constructed maps of occurrence of those REP events, which associate with the localized precipitation of energetic (30-80 keV) protons. The localized precipitation of energetic protons equatorward of the isotropy boundary is suggested to be a signature of the interaction of the ring current/plasma sheet protons with EMIC waves (e.g., Yahnin and Yahnina, 2007). Due to huge amount of data (12 years of observations of several satellites), Carson et al. applied an automated algorithm for the event selection. Surprisingly, the revealed map of occurrence of “EMIC driven” REP events showed a maximal occurrence on the night side. This seems to be inconsistent with both theoretical predictions and observational statistics of the location of EMIC waves, which could be responsible for scattering of the relativistic electrons ( Thorne and Kennel, 1971; Jordanova et al., 2008; Chen et al 2009 Meredith et al 2003). Wang et al. (2014) revisited the Carson’s et al. study using more rigorous algorithm of the event selection and revealed the maximal occurrence of the “EMIC driven” REP events in the evening sector In this paper advantages of the multi-spacecraft MLT coverage by NOAA POES will be used to study the REP morphology and to compare it with results of some previous studies. To distinguish between REP events of different nature we will use not only data from detectors measuring relativistic electrons (P6), but also measurements of energetic (>30 keV) protons and electrons. Bellow (section 2) we present the used data and selection criteria. In section 3 three kinds of REP observed with NOAA POES as well as their morphological features are described. A discussion is presented in section 4. 46