A New Type of Polar Cap Arc Observed in the ~1500 MLT Sector: 1. Northern Hemisphere Observations

Abstract

AbstractA new type of polar cap aurora observed in ~1500 MLT sector, abbreviated as 15MLT‐PCA, is identified on examining the long‐term spectrographic imager observations on the DMSP satellite over the Northern Hemisphere. Apparent dependencies of 15MLT‐PCA on the solar cycle, season, universal time, and interplanetary magnetic field (IMF) are presented, which indicate that the 15MLT‐PCA occurs in the northern summer under conditions of large dipole tilt angle, long‐lasting positive IMF By, and predominant negative Bx. The IMF Bz and other solar wind conditions show less control over the occurrence of 15MLT‐PCA. The particle precipitations of 15MLT‐PCA show magnetosheath‐like or mantle‐like properties with the ions showing clear energy dispersion property. We suggest that the 15MLT‐PCA is caused by reconnection between a twisted lobe field line and an antisunward IMF field line, which differs from the mechanism for High‐latitude dayside aurora (HiLDA) that share some common observational features with the 15MLT‐PCA.