A Study on the Potential Mechanisms Underlying the Seasonal Dependence of 15MLT-PCA

Abstract

Specific polar cap auroras, such as 15MLT-PCA, linked to lobe reconnection due to the influence of the interplanetary magnetic field (IMF) B_y component, were only observed in the summer. Although the variance in ionospheric conductivity between winter and summer has been proposed as a potential explanation for this seasonal dependency, it has also been argued that the differences in lobe reconnection between the winter and summer hemispheres could be the cause. To address this debate, we examined two data periods with similar IMF conditions when the northern hemisphere was in summer and winter, respectively. Using DMSP/SSUSI and AMPERE observations, we detected clear 15MLT-PCA and associated field-aligned currents in the summer, but not in the winter. These observations were compared with global MHD simulations from OpenGGCM. Lobe reconnection signatures were identified for both winter and summer in the simulation results. However, a detailed analysis showed that the pattern of lobe reconnection in the winter hemisphere was different from that in the summer. Based on the combined observation and simulation results, we suggest that particular lobe reconnection in summer is critical for generating 15MLT-PCA, while the winter's reconnection may lead to transient or small-scale auroral responses that were not easily identified by DMSP/SSUSI observations as a 15MLT-PCA event.