In situ electron density profiles obtained from Arase in the night magnetic local time (MLT) sector and from RBSP-B covering all MLTs are used to study the small-scale density irregularities present in the plasmasphere and near the plasmapause. Electron density perturbations with amplitudes >10% from background density and with time-scales less than 30-min are investigated here as the small-scale density irregularities. The statistical survey of the density irregularities is carried out using nearly 2 years of density data obtained from RBSP-B and 4 months of data from Arase satellites. The results show that density irregularities are present globally at all MLT sectors and L-shells both inside and outside the plasmapause, with a higher occurrence at L > 4. The occurrence of density irregularities is found to be higher during disturbed geomagnetic and interplanetary conditions. The case studies presented here revealed: (1) The plasmaspheric density irregularities observed during both quiet and disturbed conditions are found to coexist with the hot plasma sheet population. (2) During quiet periods, the plasma waves in the whistler-mode frequency range are found to be modulated by the small-scale density irregularities, with density depletions coinciding well with the decrease in whistler intensity. Our observations suggest that different source mechanisms are responsible for the generation of density structures at different MLTs and geomagnetic conditions.
Bibliographical noteFunding Information:
The authors would like to thank NASA’s Van Allen Probe team for the use of EFW LEVEL 3 data product obtained from ( http://www.space.umn.edu/rbspefw-data/ ), EMFISIS data from ( https://emfisis.physics.uiowa.edu/data/index ), the plasma data of HOPE and MagEIS instruments from ( https://rbsp-ect.newmexicoconsortium.org/science/DataDirectories.php ) and the plasma data of RBSPICE instrument from ( http://rbspice.ftecs.com/Data.html ). The geomagnetic and solar wind indices used in the study are downloaded from NASA’s OMNI data base in Coordinated Data Analysis web ( https://cdaweb.gsfc.nasa.gov/index.html/ ). This work is supported by JSPS KAKENHI (15H05815, 16H06286, 15H05747, 17H00728, and 20H01959).
Science data of the ERG (Arase) satellite were obtained from the ERG Science Center operated by ISAS/JAXA and ISEE/Nagoya University (http://ergsc.isee.nagoya-u.ac.jp/, Miyoshi et?al.,?2018a). The Arase satellite data is publicly available via ERG Science Center. The present study analyzed Level-2 omni-directional flux data of LEP-e v01.01, LEP-i v03.00, MEP-e v01.02, MEP-i v01.03, and orbit Level-2 v02 and PWE/HFA Level-3 v01.02 data. Arase satellite data sets for this research are available in these in-text data citation references: Wang et?al.?(2018), Asamura et?al.?(2018b), S. Kasahara et?al.?(2018a), Yokota et?al.?(2018), and Miyoshi et?al.?(2018b).
© 2021. The Authors.
- electron density
- inner magnetosphere
- small-scale density irregularities
- Van Allen Probes