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

• Published in: Journal of geophysical research. Space physics Vol. 126; no. 11
• Main Authors: Reistad, J. P., Laundal, K. M., Østgaard, N., Ohma, A., Burrell, A. G., Hatch, S. M., Haaland, S., Thomas, E. G.
• Format: Journal Article
• Language: English
• Published: 01-11-2021
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2021JA029742

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
B
y
. 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
B
y
is dominating and IMF
B
z
 ≥ 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.
Plain Language Summary
Reconnection of magnetic field lines at high latitudes, tailward of the magnetic poles, is most often thought to play a big role in near‐Earth space dynamics only when the magnetic field carried by the solar wind, the Interplanetary Magnetic Field or IMF, has a large northward component. This is because such conditions lead to the distinct strong sunward movement of plasma at polar latitudes in the ionosphere. During more typical conditions (when the IMF is dawn/dusk directed), identifying and quantifying the effect of such high‐latitude reconnection becomes much more challenging. This paper presents a new technique that enables this separation even when the dawn‐dusk component of the IMF is dominating, by normalizing the convection to the boundary between open and closed magnetic field lines. We make two main findings. First, the summer and winter dayside plasma flows near the boundary between open and closed magnetic field lines are vastly different. Second, plasma circulation at polar latitudes (interpreted as lobe reconnection rate) can on average account for
of the total plasma transport during local summer when IMF is mostly directed in the dawn‐dusk direction and IMF B
z
is northward.
Key Points
We demonstrate a novel technique to quantify the polar cap plasma circulation statistically
During local summer and By dominated Interplanetary Magnetic Field (IMF), lobe reconnection is estimated to amount to ∼20% of the dayside reconnection rate
In the local summer hemisphere, dayside polar cap convection is more vortical compared to the local winter hemisphere during IMF
B
y
periods