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Occurrence of EMIC waves and plasmaspheric plasmas derived from THEMIS observations in the outer magnetosphere: Revisit
Author(s) -
Kim GiJeong,
Kim KhanHyuk,
Lee DongHun,
Kwon HyuckJin,
Park JongSun
Publication year - 2016
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2016ja023108
Subject(s) - noon , physics , plasmasphere , magnetosphere , local time , morning , astrophysics , geophysics , ionosphere , atmospheric sciences , atomic physics , plasma , astronomy , nuclear physics , statistics , mathematics
We have statistically studied the relationship between electromagnetic ion cyclotron (EMIC) waves and cold plasmaspheric plasma ( N sp ) in the L range of 6–12 using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) data for 2008–2011. The important observational results are as follows: (1) Under quiet geomagnetic conditions ( K p ≤ 1), the maximum occurrence rate of the hydrogen (H) band EMIC waves appears in the early morning sector (0600–0900 MLT) at the outermost region ( L = 10–12). (2) Under moderate and disturbed conditions ( K p ≥ 2), the H‐band occurrence rate is higher in the morning‐to‐early‐afternoon sector for L > 10. (3) The high‐occurrence region of helium (He) band waves for K p ≤ 1 varies from L = 7 to 12 in radial distances along the local time (i.e., at L ∼ 7 near noon and at L = 8–12 near late afternoon). (4) The He‐band waves for K p ≥ 2 are mainly localized between 1200 and 1800 MLT with a peak around 1500–1600 MLT at L = 8–10. (5) N sp is much higher for the He‐band intervals than for the H‐band intervals by a factor of 10 or more. The He‐band high occurrence appears at a steep N sp gradient region. (6) The morning‐afternoon asymmetry of the normalized frequency seen both in H‐band and He‐band is similar to the asymmetric distribution of N sp along the local time. These observations indicate that the cold plasma density plays a significant role in determining the spectral properties of EMIC waves. We discuss whether a morning‐afternoon asymmetry of the EMIC wave properties can be explained by the spatial distribution of cold plasmaspheric plasma.