Characteristic Features of the Magnetic and Ionospheric Storms on December 21–24, 2016

Solar storms accompanied by solar flares, coronal mass ejections, and high-speed flows result in considerable disturbances in the Sun–interplanetary medium–magnetosphere–ionosphere–atmosphere–Earth (internal geospheres) system. As a result, geospace storms with synergistically interacting magnetic, ionospheric, atmospheric, and electrical storms arise in our planet. Magnetic and ionospheric storms have been studied for a long time, but atmospheric storms and electrical storms have been studied considerably to a less extent. Geospace storms and their components exhibit significant variability. It may be asserted that no identical two storms exist. Therefore, a comprehensive study of each new geospace storm and its manifestations and features is an urgent scientific issue. This will contribute to a process of their adequate simulation and, in the long term, forecasting. The purpose of this article is to describe the observed features of the ionospheric and magnetic storms accompanying the geospace storm on December 21–24, 2016. The state of the geomagnetic field has been observed via the fluxgate magnetometer located at the Magnetometer Observatory of the Karazin Kharkiv National University (49°38′ N, 36°56′ E). The dynamics of the ionospheric plasma has been monitored by a vertical incidence Doppler radar and a digisonde located at the Radio Physics Observatory of the Karazin Kharkiv National University (49°38′ N, 36°20′ E). The Doppler radar operate at 3.2 and 4.2 MHz; however, only measurements performed at 3.2 MHz are given below, since a frequency of 4.2 MHz turned out to be inefficient at nighttime when F2 layer critical frequency median f_{0 F2} ≈ 2 MHz, which prevented signal reflection from the ionosphere even at 3.2 MHz. Prior to the beginning of the magnetic storm on December 20, 2016, the level of the H and D components rarely exceeded 0.2–0.7 nT. The sudden commencement of a storm between 06:00 and 10:00 UTC virtually did not affect this level. During the second half of the day on December 21, 2016, the level of exhibited sporadic fluctuations increased from approximately 1 to 3–4 nT. During the next few days, up to December 25, 2016, their level showed variations mostly from approximately 1 nT to approximately 2 nT. Increases in the level were predominantly observed in the period from 05:00 to 15:00 UTC for the H component and from 10:00 to 20:00 UTC for the D component. The weak (power 20 GJ/s and energy approximately 0.45 PJ) geospace storm in the period of December 21–24, 2016, was accompanied by a moderate positive ionospheric storm, as well as by three negative ionospheric storms, one of which was very strong, and the other two were strong and moderate. The geospace storm was accompanied by a moderate magnetic storm with an energy of approximately 2 PJ and a power of approximately 56 GW. The positive ionospheric storm barely affects the level of the signal reflected from the ionosphere, whereas the reflected signal may be very weak or totally absent during the negative ionospheric storms. The positive ionospheric storm has a substantial effect on the Doppler shift when the wave activity enhanced in the ionosphere. The relative amplitude of disturbances in the electron density increases from a few percent to approximately 50%, and their period increases from 6–12 to 40 min. It is impossible to follow wave activity during the negative ionospheric storms. In the course of a long magnetic storm, the level of D and H components in a subrange of 200–1000 s of the period increased from 0.2–0.3 and 0.3–0.5 nT to 1.0–2.0 and 1.0–1.8 nT, respectively. In a subrange of 50–200 s of the period, the corresponding levels increased from 0.3–0.5 and 0.3–0.5 nT to 0.5–1.0 and 1.5–2.0 nT, respectively. Within a subrange of 10–50 s the period, the corresponding levels increased from 0.05–0.06 and 0.10–0.15 nT to 0.20–0.30 and 0.5–1.0 nT, respectively. Comparative studies of two geospace storms that occurred on December 21–24, 2016, and March 21–23, 2017, show that their ionospheric and magnetic effects are comparable, even if the storms are different.