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Asymmetric Index To mathematically quantify the “inter-hemispheric asymmetry” in the magnetosphere-ionosphere-thermosphere (M-I-T) system, we defined the “asymmetry index”. Specifically, for the I-T system, we use the averaged or integrated values of the I-T quantities between the two hemispheres to quantify the asymmetry:

I =2(YNH-YSH)/(YNH+YSH) × 100%

Here, YNH and YSH are the spatially averaged values of the I-T quantities (e.g., E-region altitude electron density, F-region altitude neutral mass density and horizontal neutral wind speed), or the hemispheric-integrated values (e.g., Joule heating) in northern hemisphere (NH) and southern hemisphere (SH), respectively. The asymmetry index I is positive if the NH/SH ratio exceeds 1 and negative if it is less than 1. For example, a NH/SH ratio of 1.5 corresponds approximately to an asymmetry index I equal 40%.

The following figure shows the distributions of the electron density, neutral mass density, horizontal neutral wind and Joule heating in NH and SH along with their differences at 12 UT on the northern summer solstice from GITM idealized simulations (IMF Bz =-5nT, By=0nT and F10.7=150sfu). Positive values in the bottom row represent higher quantities in the NH. Clearly, the asymmetric solar irradiance at summer solstice leads to significant asymmetries in all the quantities. To better examine their inter-hemispheric asymmetries in the high- and mid-latitudes (|geographic latitude| > 45o), based on our definition, the asymmetry index I for these quantities are roughly 95.60%, 15.07%, 32.19% and 38.25%, respectively. 