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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.

For more information, refer to this AGU poster –

Figure 1. Distributions of electron density at ~120 km altitude (1st column), neutral mass density at ~300 km (2nd column), horizontal neutral wind at ~300 km (3rd column), and height-integrated Joule heating (4th column) for NH (top row), SH (middle row), and difference between them, NH – SH (bottom row) from northern summer solstice at 12UT. The geographic latitude (GLAT) and solar local time (SLT) are used in each plot. The geographic latitude of the perimeter latitude in each plot is 40o. In the third column, the color contour represents the horizontal neutral wind speed and the arrows represent the horizontal neutral wind vectors with the scale of 500 m/s.

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