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.

Using auroral photographs from the Defense Meteorological Satellite Program (DMSP), Snyder and Akasofu (1976) noticed that the midday sector of the auroral oval is a "permanent" gap, which has been called the "auroral midday gap." The auroral midday gap was suggested to be the cusp's optical signature (Meng, 1981), and several studies on the midday gap have been carried out (Dandekar, 1979;Dandekar & Pike, 1978).Later, it was noticed that the midday gap is not permanent because the ground-based instruments (Eather et al., 1981) and satellite imagers (Meng & Lundin, 1986;Zhang et al., 2005) frequently observed "non-gap" events, that is, full of the aurora, in this sector. The gap is always observed in the midday sector, indicating that the auroral intensity in this region is generally weak. When the auroral intensity cannot reach the sensitivity threshold of the instruments, a "gap" is observed.Despite the average intensity being early noticed to be weak, the auroras in the cusp region have been extensively studied. Based on the ground-based optical observations, the auroras observed in the cusp region

The polar cap is defined as the area surrounded by the poleward boundary of auroral oval. The auroral arcs observed in the polar cap, called polar cap arcs (PCAs) for short, have been extensively studied. With the help of the global auroral imagers, some larger-scale PCAs, also referred to as transpolar arcs (TPAs), were noticed (e.g., Frank et al., 1982;Zhang et al., 2020;Zhu et al., 1997). According to different forms of evolution, the PCAs have been classified into different types, such as Sun-aligned arc (Berkey et al., 1976), theta aurora (Frank et al., 1986, and oval-aligned arc (Murphree & Cogger, 1981). Special attention is needed to distinguish the hook-shaped TPA and bending arcs. The former is Sun-aligned with a hook-shaped tailward arc (Ismail & Meng, 1982;Murphree et al., 1982). In contrast, the latter is a faint PCA splitting from the dayside auroral oval with anti-sunward moving (Kullen et al., 2015). These PCAs tend to occur during northward interplanetary magnetic field (IMF) with relatively quiet magnetic conditions (

<p>Throat auroras frequently observed near local noon have been confirmed to correspond to magnetopause indentations, but the generation mechanisms for these indentations and the detailed properties of throat aurora are both not fully understood. Using all‐sky camera and magnetometer observations, we reported some new observational features of throat aurora as follows. (1) Throat auroras can occur under stable solar wind conditions and cause clear geomagnetic responses. (2) These geomagnetic responses can be simultaneously observed at conjugate geomagnetic meridian chains in the Northern and Southern Hemispheres. (3) The initial geomagnetic responses of throat aurora show concurrent onsets that were observed at all stations along the meridians. (4) Immediately after the concurrent onsets, poleward moving signatures and micropositive bays were observed in the <em>X </em>components at higher‐ and lower‐latitude stations, respectively. We argue that these observations provide evidence for throat aurora being generated by low‐latitude magnetopause reconnection. We suggest that the concurrent onsets reflect the instantaneous responses of the reconnection signal arriving at the ionosphere, the followed poleward moving signatures reflect the antisunward dragging of the footprint of newly opened field lines, and the micropositive bays may result from a pair of field‐aligned currents generated during the reconnection. This study may shed new light on the geomagnetic transients observed at cusp latitude near magnetic local noon.</p>

The cusp is a region in which the solar wind plasma can directly penetrate into the ionosphere. Most previous studies mainly focused on the low‐ and high‐latitude boundaries of the cusps, the identification of the dawnside and duskside boundaries of them appears to be questionable. The auroras are the ionospheric manifestations of solar‐terrestrial energy coupling. This paper presents a new approach to diagnose the duskside boundary of the cusp with the observation of a particular polar cap arc (PCA) located near the ∼1500 magnetic local time sector (15MLT‐PCA). Using measurements from Defense Meteorological Satellite Program satellite, we selected several 15MLT‐PCA examples where the satellite flew over different regions and analyze the particle characteristics. We identified the precipitating particles in dawnside and duskside regions next to 15MLT‐PCA are from the low latitude boundary layer/cusp and boundary plasma sheet/central plasma sheet respectively. The statistical result further confirms our findings. Therefore, we proposed that 15MLT‐PCA can be used as an identifiable optical signature of the duskside boundary of the cusp. This paper provides a valuable approach to identify the cusp boundaries, and the results are critical for understanding the generation mechanism of 15MLT‐PCA.