Variations in the parameters of the propagation of decameter waves on two auroral radio paths, Dikson–Pevek (D = 2800 km) and Lovozero–Pevek (D = 4200 km), were studied via oblique ionospheric sounding for two days, one weakly disturbed and one strongly disturbed. The following parameters were analyzed: F2 MOF and Es MOF (the maximum observed frequencies when the signal is reflected from the F2 layer and the Es layer), F2Spread (diffuse reflection), and lateral reflections. The following new results have been obtained: the F2 MOF level for the strongly disturbed day is higher than that for the weakly disturbed day on both paths; however, the F2 MOF values during substorms are either small or absent due to increased absorption. The Es MOF values on the Dikson–Pevek path increase during the period of disturbances. As for the Lovozero–Pevek path, the probability of Es reflections decreases during the disturbed day. The diffusion in the F2 layer on the Dikson–Pevek path is mostly typical for a strongly disturbed period, while that on the Lovozero–Pevek path, conversely, is mostly typical for weakly disturbed conditions. Lateral reflections on the Dikson–Pevek path occur rather rarely. On the Lovozero–Pevek path, there are very few lateral reflections on the weakly disturbed day and no lateral reflections on the strongly disturbed day.

We analyze ionospheric oblique sounding data on three high-latitude and one high-latitude–midlatitude HF radio paths for February 15 and 16, 2014, when two substorms and one magnetic storm occurred. We investigate cases of anomalous propagation of signals: their reflection from sporadic layer Es, lateral reflections, type “M” or “N” modes, the presence of traveling ionospheric disturbances, and the diffusivity of signals and triplets. The most significant results are the following. In geomagnetically undisturbed times, sporadic Es-layers with reduced maximum observed frequencies (MOFEs) on three high-latitude paths were observed in both days. The values of MOFEs during disturbances are large, which leads to the screening of other oblique sounding signals reflected from the ionosphere. On all four paths, the most frequently traveling ionospheric disturbances due to the terminator were observed in quiet hours from 03:00 to 15:00 UT on the first day and from 06:00 to 13:00 UT on the second day of the experiment. In addition, both the sunset terminator and the magnetic storm on the high-latitude–mid-latitude path were found to generate traveling ionospheric disturbances jointly. No such phenomenon was found on high-latitude paths.

Studies were performed on three oblique ionospheric sounding (OIS) paths: Gor’kovskaya-Lovozero (St. Petersburg) with a length of 890 km, Sodankyla-Gor’kovskaya (800 km), and Sodankyla-Lovozero (360 km). The data for March 17 and April 14, 2012, the days during the recovery phase of the corresponding magnetic storms, have been analyzed. According to the observations performed at Sodankyla, riometer absorption in the morning-daytime hours was high against a background of very weak magnetic disturbances registered with a magnetometer; a high absorption level was also typical of the second day but during a substantial magnetic disturbance. The signal propagation mode structure and intensity on different paths were compared for the indicated days. The main results achieved are as follows. The OIS signal mode structure at weak (for April 14, 2012) and strong (for March 17, 2012) absorption substantially differed when magnetic disturbances were weak at the same instant. Diffuse reflections from the F2 layer were observed on the first two paths during a magnetic disturbance at night of April 14, 2012. At the same time, diffusivity was absent on the same paths under quiet magnetic conditions in the morning and daytime hours on March 17, 2012. A short-term abrupt increase in the maximum observed frequency of the Es layer (MOF Es) by 30–80% was registered half an hour before a substantial absorption burst on the first path. Signal reflections from the sporadic Es layer were observed only on the first path on March 17, 2012, and April 14, 2012, during the absorption maximum (A = 6 dB) according to the Sodankyla data, and the signal characteristics differed on those days.

Проанализированы проявления так называемого главного эффекта в ионосфере во время геомагнитных бурь/суббурь в характере распространения декаметровых радиоволн. На КВ-радиотрассах главный эффект обнаруживает себя в виде изменений амплитуды сигнала и диапазона рабочих частот МНЧ–ННЧ аналогично критической частоте слоя F2 ионосферы. А именно, происходит рост указанных параметров перед активной фазой возмущения, их уменьшение во время активной фазы и снова рост после активной фазы. На трассах во время бурь/суббурь также рассмотрено распространение вне дуги большого круга, смена механизмов распространения, поведение КВ-радиошумов. Приводятся соображения о возможности прогнозирования начала развития бурь/суббрь.

The manifestations of the so-called main ionospheric effect during geomagnetic storms (sub-storms) in the character of decameter-wave propagation are analyzed. On HF radio paths, the main effect is observed as variations in the signal amplitude and the MOF-LOF working frequency band similarly to the critical frequency of the ionospheric F2 layer. Specifically, these parameters increase before the disturbance active phase, decrease during the active phase, and increase again after this phase. The propagation outside the great circle arc, the change in the propagation processes, and the HF radio noise behavior were also considered on these paths during storms (substorms). It is assumed that the storm (substorm) development onset can be predicted.

Оценено изменение радиуса корреляции ионосферы во время магнитосферной суббури 14.02.2011 г., который на средних широтах принято считать равным 500 км. Для анализа использованы данные вертикального зондирования (ВЗ) обсерваторий Санкт-Петербург и Соданкюля, Финляндия, а также данные наклонного зондирования (НЗ) на радиотрассе Соданкюля–Санкт-Петербург длиной 790 км. Особенность эксперимента состояла в том, что в приемном пункте трассы НЗ Санкт-Петербурге синхронно принимались сигналы передатчика ВЗ обс. Соданкюля. Точка отражения трассы НЗ расположена на расстоянии 400 км от точки отражения ВЗ. Сопоставлены ионограммы, характерные для точек отражения сигналов ВЗ и НЗ в ионосфере, отстоящих друг от друга на расстояние несколько меньшее, чем радиус корреляции ионосферы 500 км, и данные ионозондов ВЗ Соданкюля и Санкт-Петербург. Показано, что горизонтальный радиус корреляции в 400 км во время магнитного возмущения может рассматриваться как приемлемый только на трех этапах возмущения. Это – начальная фаза, когда еще не началась перестройка ионосферы, взрывная фаза (максимум возмущения), когда отражающим слоем в ионосфере является только спорадический слой Es, и фаза восстановления, когда возмущение уже заканчивается, и ионосфера возвращается к исходному невозмущенному состоянию. На других этапах возмущения радиус корреляции, если он существует, значительно меньше, чем 400 км.

Исследован результат воздействия магнитосферных бурь и суббурь на ионосферу – так называемый главный эффект. Он проявляется в том, что критическая частота и высота F-области во время возмущения изменяются следующим специфическим образом. Сначала происходит рост критической частоты перед активной фазой бури/суббури, затем ее спад во время активной фазы и снова рост после активной фазы. Высота F-области, наоборот, возрастает во время активной фазы и уменьшается после ее окончания. Предложен подход для краткосрочного прогнозирования (2–3 ч) развития бурь/суббурь как главных элементов возмущенной космической погоды.

A change in the correlation radius of the ionosphere during the magnetospheric substorm of February 14, 2011, which is considered to be 500 km at midlatitudes, has been estimated. The vertical sounding (VS) data from the St. Petersburg and Sodankyla (Finland) observatories, as well as the data of oblique incidence sounding (OIS) at the Sodankyla-St. Petersburg path with a length of 790 km, have been analyzed. A specific feature of the experiment consisted in that the signals of a VS transmitter from Sodankyla were synchronously received at the receiving point on the OIS path in St. Petersburg. The OIS path reflection point is located at a distance of ∼400 km from the VS reflection point. Ionograms typical of the VS and OIS signal reflection points in the ionosphere, the distance between which was slightly smaller than the correlation radius of the ionosphere (500 km), and the data of the Sodankyla and St. Petersburg ionosondes have been compared. It has been indicated that a horizontal correlation radius of 400 km can only be considered acceptable during three disturbance phases: the initial phase before the reconfiguration of the ionosphere; the explosion phase (the disturbance maximum), when only the sporadic Es layer is the reflecting ionospheric layer; and the recovery phase, when a disturbance already ceases and the ionosphere returns to its initial undisturbed state. During other disturbance phases, the correlation radius (if it exists) is much smaller than 400 km.

The result of the effect of magnetospheric storms and substorms on the ionosphere (the so-called main effect) has been studied. The effect consists in that the critical frequency and altitude of the F region vary specifically during a disturbance. The critical frequency first increases before the storm (substorm) active phase, then decreases during the active phase, and increases again after this phase. On the contrary, the F region altitude increases during the active phase and decreases after this phase. An approach to the short-term (2–3 h) prediction of the development of storms (substorms) as the main disturbed space weather elements has been proposed.

The ionospheric effects during the main phase of the extremely intense magnetic storm of November 20, 2003 (Dst = -472 nT), caused by the M-class solar flare of November 18, 2003, have been considered. During the storm, HF Doppler measurements in the auroral zone were simultaneously performed at several frequencies and in several directions using the method of bi-static backscatter observations of diagnostic HF radiowaves scattered by small-scale ionospheric field-aligned irregularities. The observations by the method of oblique-incidence ionospheric sounding at a sweeping frequency were also performed on two high-latitude radio paths. The state of the ionosphere in the region of scattering of diagnostic HF sionals was controlled based on data of the Tromso dynasonde, which registered the F2 layer critical frequencies (foF2 = 4-7 MHz), anomalously high for the main phase of the magnetic storm on the nightside. It has been found out that short-period wave processes in the ionosphere with predominant periods of 1.5-2.0 and 4-8 min can be caused by the generation of Pi2 and Pc5-6 geomagnetic pulsations, respectively. It has been established that decametric radio-waves were absent on auroral radio paths for several hours.

The effects of the May 15-16, 1997, magnetic storm in the midlatitude and low-latitude ionosphere of the Northern Hemisphere have been studied. The observations at three chains of stations extended approximately along the 20degrees, 140degrees, and 280degrees E meridians in the 13degrees-65degrees N range of geomagnetic latitudes have been used. Zonal electric fields have been estimated. An analysis provided the following results. Durable intense negative disturbances during the growth and recovery phases of the storm were observed at all stations. Before the storm, positive disturbances were observed at all stations of the European and American chains regardless of local time. The positive peak in DeltafoF2 during the storm growth phase ("the dusk effect") was seen only at the Asiatic chain of stations in evening hours. The strongest response to the storm in the F region was registered at the Asiatic chain where the difference between the geographical and magnetic poles is maximum. The obtained differences in the DeltafoF2 variations could be caused by the local time of the sudden storm commencement and by the magnetic dip.