Quantifying the Lobe Reconnection Rate During Dominant IMF B_y Periods and Different Dipole Tilt Orientations

Abstract

Lobe reconnection is usually thought to play an important role in geospace dynamics only when the Interplanetary Magnetic Field (IMF) is mainly northward. This is because the most common and unambiguous signature of lobe reconnection is the strong sunward convection in the polar cap ionosphere observed during these conditions. During more typical conditions, when the IMF is mainly oriented in a dawn‐dusk direction, plasma flows initiated by dayside and lobe reconnection both map to high‐latitude ionospheric locations in close proximity to each other on the dayside. This makes the distinction of the source of the observed dayside polar cap convection ambiguous, as the flow magnitude and direction are similar from the two topologically different source regions. We here overcome this challenge by normalizing the ionospheric convection observed by the Super Dual Aurora Radar Network (SuperDARN) to the polar cap boundary, inferred from simultaneous observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). This new method enable us to separate and quantify the relative contribution of both lobe reconnection and dayside/nightside (Dungey cycle) reconnection during periods of dominating IMF By. Our main findings are twofold. First, the lobe reconnection rate can typically account for 20% of the Dungey cycle flux transport during local summer when IMF By is dominating and IMF Bz ≥ 0. Second, the dayside convection relative to the open/closed boundary is vastly different in local summer versus local winter, as defined by the dipole tilt angle.

Explicit IMF By‐Dependence of Energetic Protons and the Ring Current

“…Evidence toward the former hypothesis was provided by Reistad et al. ( 2021 ) who showed that there is an explicit B y ‐dependence in the cross‐polar cap potential which is consistent with a similar B y ‐dependence of the substorm occurrence frequency (Liou et al., 2020 ; Ohma et al., 2021 ).…”

“…They suggested that IMF B y either modulates the dayside reconnection rate or the magnetotail response to solar wind driving. Evidence toward the former hypothesis was provided by Reistad et al (2021) who showed that there is an explicit B y -dependence in the cross-polar cap potential which is consistent with a similar B y -dependence of the substorm occurrence frequency (Liou et al, 2020;Ohma et al, 2021).…”

Local Mapping of Polar Ionospheric Electrodynamics

“…The HiLDA spot/space hurricane is clearly visible in the WIC image displayed in Figure 9 . It is a signature of lobe reconnection during times when the interplanetary magnetic field has a strong positive B y component (or negative, if observed in the Southern hemisphere; Reistad et al., 2021 ).…”

“…The HiLDA spot/space hurricane is clearly visible in the WIC image displayed in Figure 9. It is a signature of lobe reconnection during times when the interplanetary magnetic field has a strong positive B y component (or negative, if observed in the Southern hemisphere; Reistad et al, 2021). The magnetic field components measured by CHAMP (with the International Geomagnetic Reference Field (IGRF) main magnetic field subtracted), as solid lines.…”

The relationship between interhemispheric asymmetries in polar ionospheric convection and the magnetic field line footpoint displacement field

“…A large region surrounding the magnetic poles do not connect to the opposite hemisphere but instead connect out in the solar wind. In these regions of open magnetic flux (the polar caps), we expect to see mismatches in the potentials (Crooker and Rich, 1993;Reistad et al, 2019Reistad et al, , 2021 that are unrelated to magnetic field line displacements. In regions that map to reconnection, we also expect there to be magnetic fieldaligned potential differences between hemispheres (Siscoe et al, 2001).…”