IMF BX and Dipole Tilt Dependence on Transpolar Arcs

Transpolar arcs (TPAs) are predicted by many models to appear in both hemispheres, as so-called conjugate TPAs. However, some observations have suggested that this is not always the case, and that there is an IMF B_x dependence on whether TPAs appear on both hemispheres or not. Specifically, it has been suggested that TPAs only appear on the northern hemisphere for negative IMF B_X and vice versa for positive IMF B_X. Furthermore, a positive Earth dipole tilt is predicted to have a similar effect on TPA occurrences as a negative IMF B_X and vice versa. It is also known that TPAs appear on different locations on the auroral oval, i.e., dawn-, dusk- or both sides of the oval, depending on IMF B_Y. However, the role of IMF B_X and IMF B_Z for the TPA location remains unclear, with some previous observations suggesting a correlation with IMF B_X.

In this study, we investigate the influence of IMF B_X and dipole tilt on TPAs by statistically analyzing observational data. We analyze TPA datasets from four previous studies, as well as our own TPA dataset, created from DMSP satellite measurements. At first glance, the data suggests that there is a strong correlation between both IMF B_X and dipole tilt, and TPA observations in a specific hemisphere. However, when normalizing the data to the solar wind distribution and when taking observational bias into account, this correlation disappears. We therefore conclude that there is no clear correlation between neither IMF B_X nor dipole tilt and in which hemisphere a TPA appears. We further analyze four of the five datasets with respect to dawn-dusk appearances of TPAs and its correlation to IMF B_X, B_Y and B_Z. Here, the results for the datasets mostly agree with previous observations. Finally, we discuss the potential causes for the few non-conjugate TPAs, by studying our own TPA dataset in further detail.