Using Auroral Asymmetries to Test MHD Models

Abstract:

From 16:40 to 19:00 UT on August 17th, 2001, the IMAGE satellite viewed the Northern aurora, while the POLAR satellite simultaneously observed the Southern aurora. Unlike typical cases where the aurora is nearly conjugate, the Y-component of Interplanetary Magnetic Field (IMF By) during this storm ranged from +20 to +32 nT, causing the polar cap to shift towards the dusk in the Southern hemisphere and towards the dawn in the Northern. Using satellite images in the 130 nm to 160 nm wavelength range, we have been able to identify the polar cap boundary in both hemispheres throughout the event, and calculated the Dawn-Dusk Offset, ∆L, which ranged from 0° to 15° latitude. We then found correlations of 0.90 in the Northern Hemisphere and 0.83 in the Southern Hemisphere between ∆L and IMF By. ∆L also correlated well against IMF Clock Angle (ϴ_C) and the Epsilon parameter (ϵ = vB²sin[ϴ_C/2]) when using specific time averages of these parameters. The same methods are then applied to compute the polar cap boundaries in the BATSRUS, OpenGGCM, and LFM-MIX models that were run to simulate the event. We find that none of the models accurately describe the observed open-closed field line boundary during this event, with BATSRUS producing boundaries that are too ideal and symmetric, whereas OpenGGCM and LFM-MIX produced boundaries that are highly distorted and random. The LFM-MIX model gave the best average offset but did not match the observed variation with solar wind parameters.