Evolution of the magnetic field structure outside the magnetopause under radial IMF conditions

Abstract

We use the Time History of Events and Macroscale Interactions during Substorms data to investigate the magnetic field structure just outside the magnetopause and its time evolution for radial interplanetary magnetic field (IMF) events. When the magnetic field drapes around the magnetopause in the magnetosheath region, an asymmetric magnetic field orientation in different hemispheres is expected. Our two‐case study reveals some conflicts with the predicted draped field configuration in the Southern Hemisphere. The magnetosheath Bz component had a different sign depending on the upstream IMF Bx component's polarity at the beginning of the radial IMF intervals. With time, the observed Bz became northward in both cases with increasing positive values through the events. The increasing value of the Bz component may be explained by two possible mechanisms: by a change of the upstream IMF and by a reconnection between magnetosheath and magnetospheric field lines. Our study shows that both mechanisms contributed to the observed changes. Thus, there was a correlation between the change of the upstream IMF conditions and an increase in the magnetosheath northward magnetic field component. The observed formation of the boundary layer near the magnetopause proves that the reconnection process was ongoing at least for a part of the time. We suggest two possible reconnection scenarios: one near subsolar point and another tailward of the one cusp due to lobe reconnection. The asymmetry of reconnection locations causes rearrangement of the magnetic field structure near the magnetopause and turns the observed magnetosheath Bz component even further into positive values.

Formation of the Dayside Magnetopause and Its Boundary Layers Under the Radial IMF

“…During earthward IMF, reconnection occurs equatorward of the cusp in the Southern Hemisphere and tailward of the cusp in the Northern Hemisphere. This simulation result supports the illustration in Pi et al (; see Figure 6 in that paper) that was based on considerations only. Moreover, the simulation allows us to follow the process of the gradual reconstruction of the magnetic field structures near the dayside magnetopause.…”

“…The magnetic field direction in the magnetosheath adjacent to the magnetopause turned to the northward orientation in a region from Z = 0 to Z = −5 R E because dayside reconnection shifted the plane separating different B z polarities southward. This time evolution was also mentioned by Pi et al (). Dayside reconnection in the Southern Hemisphere leads to deformation of the open‐closed boundary in Figure b (around Z = −10 R E ) and suggests that reconnection proceeds even further to the south than that in the illustration suggested by Pi et al ().…”

“…The BATS‐R‐US model provides a global image of the magnetosphere but with a limited spatial resolution (around 0.25–1 R E ); thus, the detailed structure of boundary layers cannot be resolved in simulations. The global structure of the dayside magnetosphere and its evolution will be discussed using the scenario suggested by Pi et al ().…”

“…Reconnection in both hemispheres forms open field lines that pile up at the northern dayside magnetopause. It should form the boundary layer in the Northern Hemisphere, and this boundary layer would extend to the Southern Hemisphere in the course of time (Pi et al, ).…”

“…Tang et al () studied the magnetospheric structures for the radial IMF by the magnetohydrodynamic (MHD) simulation, and the asymmetric reconnection locations can be seen also in their results. Two‐case study based on the Time History of Events and Macroscale Interactions during Substorm (THEMIS) data by Pi et al () revealed that a region of the northward magnetic field in the magnetosheath adjacent to the magnetopause is enlarging over time and the authors explained this phenomenon by lobe reconnection in one hemisphere supplemented with dayside reconnection in the other hemisphere. Simultaneous reconnection piles up the open field lines at the dayside magnetopause and enlarges the region of the magnetopause influenced by the northward magnetic field.…”