Hemispheric Asymmetries in the Ionospheric Total Electron Content during 1999-2019

This paper studies the hemispheric asymmetry of the total electron content (TEC) by means of the Global Ionosphere Map (GIM), which has been routinely publishing every 2 hours by the Center for Orbit Determination in Europe (CODE) during 1999-2019.  The study period including the solar cycles 23 and 24 allows us globally uniformly examine the ionospheric asymmetry phenomenon of the GIM TEC in the different solar activities.  At each time point, summations of the GIM TEC at low-, mid-, and high-latitude in the northern and southern hemispheres are computed.  Results reveal the maximum summations of the GIM TEC occurring in April or November and the minimum summations of the GIM TEC occurring in July, which indicates the extreme TEC values in each year are not at the equinox or solstice.  We compare the sums of the two hemispheres in various local times, months, and solar activities.  By computing the annual magnitude of the asymmetry phenomenon, it is found that the asymmetric phenomenon is more prominent during the low solar activity, although its fluctuation is proportional to the F10.7 index.  We further study the asymmetry phenomenon in various latitudes and find the latitudes with prominent signatures.