Polar Cap Patches and the Tongue of Ionization on 16 July 2017

The UT and seasonal variation of polar cap F-region density patches and/or the tongue of ionization (TOI) has been shown in both observational and theoretical work to have an empty period during winter in each hemisphere, around 0300-1200 UT in the Northern hemisphere, and 1200-2300 in the Southern hemisphere (Sojka et al 1994, David et al 2016). This period is generally devoid of TOIs owing to the relative positions of the solar terminator and the magnetic pole, such that it is not possible under ordinary conditions that ExB transport will move dayside high-density plasma through the cusp and anti-sunward into the polar cap. This is the "hole" seen in the figure below, which shows the observed UT/seasonal pattern of patches/TOIs for the Southern hemisphere in 2017, based on the GPS Total Electron Content (TEC) maps from the Madrigal data server.

Under unusual conditions, a tongue of ionization or a series of polar cap density patches may form during this normally empty period. An outstanding example of this occurs in the middle of the month of July 2017 (note that July is in the dead of winter in the southern hemisphere). On 16 July in the figure we see a prominent TOI or patch event around 1200-1600 UT, right in the heart of the winter "hole" where normally no such event would be able to occur. We analyze this event and find that, rather than being an anomaly, it is "the exception that proves the rule", as a large tongue of ionization is formed and persists for several hours, owing to the expansion of the polar cap convection electric field during a geomagnetic storm. Of particular interest is the fact that this storm was of only moderate intensity, but notably was preceded by several days of unbroken very quiet geomagnetic conditions.

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References:

David, M., Sojka, J. J., Schunk, R. W., & Coster, A. J. ( 2016). Polar cap patches and the tongue of ionization: A survey of GPS TEC maps from 2009 to 2015. Geophysical Research Letters, 43, 2422- 2428. https://doi.org/10.1002/2016GL068136

Sojka, J. J., M. D. Bowline, and R. W. Schunk (1994), Patches in the polar ionosphere: UT and seasonal dependence, J. Geophys. Res., 99(A8), 14,959-14,970, doi:10.1029/93JA03327.