On the role of pressure and flow perturbations around dipolarizing flux bundles

Abstract

As a dipolarizing flux bundle (DFB) moves earthward, it creates pressure and flow perturbations. These perturbations may play a significant role in controlling DFB motion and generating field-aligned currents (FACs) which render the DFB a "wedgelet", a traveling building block of the substorm current wedge. To investigate this hypothesis, we use DFB observations from the Time History of Events and Macroscale Interactions during Substorms mission to reconstruct the spatial profiles of the thermal and total (thermal plus magnetic) pressures and of the plasma flow near the DFB. The total pressure reaches maximum inside the dipolarization front (DF, the leading edge of the DFB). The resultant pressure gradient force pushes ambient plasma in the direction normal to the front and exerts a gradient force density of ~0.15 nPa/RE against the DFB motion. The thermal pressure in the equatorial plane is strongest immediately ahead of the DFB's leading point; it decreases with distance from that peak: toward the ambient plasma, toward the DFB interior, and toward the DFB flanks. Combining our estimate of the flux tube volume distortion with the measured equatorial thermal pressure distribution, we obtain a region-1-sense FAC inside the DF layer and region-2-sense FAC in the ~1 RE thick region immediately ahead of it. This system of FACs is indeed consistent with a wedgelet. Key Points Pressure at the dipolarization front requires field-aligned currents (FACs) The pressure-related FACs are consistent with the "wedgelet" configuration A wedgelet (element of substorm current wedge) can support its FACs by itself ©2013. American Geophysical Union. All Rights Reserved.