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Storm-time Ring Current S. Ohtani, A. Y. Ukhorskiy, P. C. Brandt, and D. G. Mitchell

Storm-time Ring Current S. Ohtani, A. Y. Ukhorskiy, P. C. Brandt, and D. G. Mitchell (The Johns Hopkins University Applied Physics Laboratory).

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Storm-time Ring Current S. Ohtani, A. Y. Ukhorskiy, P. C. Brandt, and D. G. Mitchell

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  1. 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 keV 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 keV hydrogen, (middle) 60-120 keV hydrogen, and (bottom) < 160 keV 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.

  2. 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.

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