HiLDAs, cusp aurora and their connection to transpolar arcs: Classification, Conjugacy and Origins

Tranpolar arcs (TPAs) are large-scale auroral structures that span the polar cap from the nightside to the dayside during predominantly northward IMF conditions. TPAs, especially when several of them appear simultaneously, typically converge towards the auroral signature of the cusp. In a recent study, we have shown that, at times, multiple TPAs can be all connected to one isolated cusp spot, while in other cases, each arc can be connected to a different cusp spot. Additionally, we presented a case where TPAs were attached to a High Latitude Dayside Aurora (HiLDA), while the cusp appeared between HiLDA and oval. The reason why TPAs may merge with the HiLDA region instead of the auroral cusp is not yet understood. This is especially intriguing as HiLDAs typically appear in the summer hemisphere, while TPAs often occur simultaneously in both hemispheres. 

We, thus, focus in the present study on the interhemispheric conjugacy of multiple TPAs and their connection to different dayside auroral phenomena.  In this work, we examine global auroral images from DMSP SSUSI from 2015 and 2016. Through this dataset, we identify the dayside connection points of TPAs and classify them into three categories: isolated cusp spots, multiple cusp spots covering an extended region, and HiLDAs. In order to study the interhemispheric conjugacy of those aurora forms, we have only selected the events where dayside auroral oval data were available for both hemispheres at the same time (maximal 15-min difference). We present a statistical study of their interhemispheric behaviour and examine the solar wind and interplanetary magnetic field conditions associated with them.