Winter Anomaly in the Critical Frequency of the Nighttime Polar Ionosphere’s E Layer
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Analysis of the properties of the winter anomaly in E layer critical frequency foE in the nighttime (22.00–02.00 LT) polar ionosphere is performed based on data of the Tromso digital ionospheric station during 1995–1998. It is found that for these conditions, the winter anomaly in foE, i.e., the excess of the winter values of foE over summer values is typical not only for the median, but for the values of foE averaged over a month as well. The amplitude of the winter anomaly in foE is minimal for quiet geomagnetic conditions and reaches a maximum under moderate and enhanced geomagnetic activity (Kp = 3–4), mainly due to stronger increase in foE with geomagnetic activity increase in winter. This property of the winter anomaly is qualitatively and even quantitatively similar to the property of winter–summer asymmetry in fluxes of accelerated electrons to which discrete aurorae are related. That is why foE data from ionospheric stations could serve as an indicator of such fluxes in the polar ionosphere.
The data on critical frequencies foE of the Tromso digital ionospheric station and on indices of solar and geomagnetic activity were taken from the sites of Space Physics Interactive Data Resource (SPIDR, http://spidr.ionosonde.net/spidr), the World Data Center for Solar-Terrestrial Physics, Chilton (http://www.ukssdc.ac.uk/wdcc1/), and World Data Center for Geomagnetism, Kyoto (http://wdc.kugi. kyoto-u.ac.jp/). Calculations by the ISRIM were performed at the site (http://madrigal.haystack.mit. edu/models). Calculations by the Storm-E model were performed with the help of the subprogram presented in the IRI model at the site (http://irimodel. org/). The work was partly supported by the Russian Foundation for Basic Research (project no. 17-05-00427) and by Program 28 of the Presidium of the Russian Academy of Sciences.
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