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

Abstract

A 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 B y , and predominant negative B x. The IMF B z 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.

DMSP Observations of High‐Latitude Dayside Aurora (HiLDA)

“…In addition, the case shown in Figure 4 by Han et al. (2020) is very similar to the case we shown in Figure 5 panels (d) and (j). In our two events, the magnetic local times of the aurora do not remain around 15 MLT during their long life time.…”

“…The occurrences of both aurora are depend on very similar solar wind and IMF conditions and typically occur in the summer hemisphere, as shown in their both statistical studies. In addition, the case shown in Figure 4 by Han et al (2020) is very similar to the case we shown in Figure 5 panels d and j. In our two events, the magnetic local times of the aurora do not remain around 15 MLT during their long life time.…”

“…A recently study by Han et al. (2020) shows the DMSP observations of the arc‐like structures at around 15 MLT in the polar cap. Although Han et al.…”

“…A statistical study based on the IMAGE FUV data showed that the occurrence of HiLDA in the northern hemisphere is typically connected with strong positive IMF B y , positive B z , negative B x , and low solar wind density (Frey et al, 2004). Han et al (2020) found a significant UT dependence in the northern hemisphere. Those statistical studies also showed that HiLDA is most frequently observed in the summer, and almost absent in the winter.…”

“…Han et al. (2020) found a significant UT dependence in the northern hemisphere. Those statistical studies also showed that HiLDA is most frequently observed in the summer, and almost absent in the winter.…”