Storm-time Ring Current S Ohtani A Y Ukhorskiy

Storm-time Ring Current S. Ohtani, A. Y. Ukhorskiy, P. C. Brandt, and D. G. Mitchell (The Johns Hopkins University Applied Physics Laboratory) Fig. 1. (top) The Asy-H index along with the 60 -120 ke. V hydrogen ENA intensity measured by the IMAGE/HENA instrument for the two-day interval centered at 12: 00 UT on August 12, 2000. (bottom) The Sym-H (red) and Asy-H (blue) indices for the same interval. The Sym-H index is decomposed into the trend and AR (auto-regression) components. Fig. 2. ENA images of (top) 27 -60 ke. V hydrogen, (middle) 60 -120 ke. V hydrogen, and (bottom) < 160 ke. V oxygen measured by the IMAGE/HENA instrument (left) before and (right) after a substorm onset at ~1130 UT on August 12, 2000. . The same color code is used for all panels. The circle at the center represents the Earth, and also shown at L=4 and 8 dipole field lines at four local times.

Storm-time Ring Current S. Ohtani, A. Y. Ukhorskiy, P. C. Brandt, and D. G. Mitchell (The Johns Hopkins University Applied Physics Laboratory) Goal: To estimate the variation of the storm-time ring current intensity from ground magnetic indices. Problem: The Dst/Sym-H index is widely used as a measure of the ring current intensity. However, the substorm-related change of Dst/Sym-H is actually anticorrelated to that of the ring current intensity, and is therefore misleading. Observational Background: Substorm-related variations of Sym-H, Asy-H, and the ENA intensity are all related (Fig. 1): • The ring current is intensified due to particle injection during the substorm expansion phase (Fig. 2), but the expected decrease in the ground H component is overcompensated by the effect of the tail current reduction (Fig. 1 shows the sharp recovery of Sym-H at ~11: 30 UT for the interval of Fig. 2. ) • The ground H component increases inside the substorm current wedge but it decreases outside, and consequently Asy-H increases. • Substorm injects fresh particles into the ring current, which enhances the ENA intensity (Fig. 1 -top). Approach: 1) Evaluate substorm-related variations from ground magnetic indices (Sym-H & Asy-H). 2) Calculate the associated change of the ENA intensity 3) Seek for an empirical formula to estimate the change of the ENA intensity from Sym-H and Asy-H. 4) Modify Sym-H using the obtained formula so that the new index better reflects the change of the ring current intensity.