A Statistical Study of Quasi-trapped Electron Energy Spectrum: DEMETER Measurements

Quasi-trapped electrons (pitch angle in DLC) in Earth's inner radiation belt are an important particle population whose behaviors help reveal the scattering effects of trapped electrons and quantify the intensity of electron precipitation. However, a detailed systematic characterization of quasi-trapped electron energy spectra in inner belt has not yet been investigated. Here we present a statistical study of quasi-trapped electron energy spectra based on 8-year DEMETER measurements. The electron energy spectral are classified into three categories: CRAND-produced, ROH (Raise-On-Head), and exponential-type. More than 95% of the energy spectra can be categorized as one of these three energy spectrum types, while 10.12% are CRAND energy spectra, 75.78% are ROH energy spectra, and 9.19% are exponential energy spectra. Through event analysis and statistical analysis of distributions of spectral types and characteristic parameters, we analyze the spatiotemporal evolution of quasi-trapped electrons in the inner radiation belt and investigate the source mechanisms of quasi-trapped electrons. The CRAND energy spectrum is predominantly distributed at L<=1.2, which is very stable and remains unchanged during active geomagnetic activities. The ROH energy spectrum is the most dominant type of quasi-trapped electron in the inner radiation belt. The exponential energy spectrum occurs at L~3 during intense magnetic storms, with a tendency moving to lower L. Notably, exponential energy spectra are also observable at L ~ 2.5 with the longitude =240°-300° during geomagnetically quiet periods, which may be due to electron drift accumulation. These results provide new insights of inner belt electron dynamics.