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In this paper is analyzed the reaction of the ionosphere over Bulgaria during the G3 (Strong) geomagnetic storm on 3-4 November 2021 on the basis of data from constantly acting monitoring of the state of the ionosphere at the National Institute of Geophysics, Geodesy and Geography – Bulgarian Academy of Sciences. The global geomagnetic indices (Kp and Dst), which characterize the time development of the storm and the data for the solar wind, are presented. The model values (MAK model, working on the NIGGG website) of the global index of geomagnetic activity, which is calculated in real time from solar wind data show good agreement with definitive values of this index from global data centers. The values of Total Electron Content (TEC) and the forecast values of the critical frequencies of the ionosphere (foF2 and MUF3000) for Bulgaria during the storm are presented. Due to the lack of data from the ionospheric station Sofia, the values foF2 and MUF3000 are calculated on the basis of TEC and are used to prepare daily forecasts for the propagation of radio waves on the territory of our country. A very good coincidence between the model values and the measured ones was reported. As a result, the reaction of the ionosphere was monitored, which in the considered interval 3-5 November and in the conditions of a geomagnetic storm is positive. An explanation of the observed behaviour of the critical frequencies of the ionosphere is also purposed. All parameters discussed in this work are subject to monitoring activities under the Project “National Geoinformation Center” (NGIC) financed by the National Roadmap for Scientific Infrastructure 2017-2023., Bulgarian Geophysical Journal, vol. 44, p. 43-52

The study of ultra-low frequency (ULF) waves and geomagnetic pulsations plays an important role in better understanding the mechanisms of their generation and spread in the magnetosphere and on the ground. The magnetospheric ULF waves, which provide useful information about the conditions in the solar wind and in the magnetosphere, can be detected on the ground by different types of magnetometers and recorded as geomagnetic pulsations � continuous and irregular. This paper aims to study the characteristics of Pc5 geomagnetic continuous pulsations recorded at mid latitudes during the strong geomagnetic storm (Kp =7) that occurred on May 12, 2021. The sets of time series of data at sampling period 1s, recorded along the three geomagnetic directions (X, Y and Z), are shown and analyzed. A spectral analysis, based on the Morlet Wavelet transform, is applied. It shows powerful geomagnetic disturbances in the Pc5 band (1.7-6.7 mHz) in two-time intervals: 00-02 UTC- before the beginning of the storm and 10-15 UTC- during the storm. Furthermore, the Fast Fourier Transform (FFT) band pass filter is applied to the data series, and Pc5 pulsations are shown. It was concluded that their emergence was correlated with the dynamics of changes in the interplanetary magnetic field (IMF) Bz component, solar wind plasma speed, and flow dynamic pressure.

The spectral characteristics of the ULF waves (1mHz -1Hz) provide useful information about the conditions in the solar wind and in the magnetosphere. A series of geomagnetic field variations data are collected by the tri-axial search-coil instrument and acquisition system located at Panagjuriste, Geomagnetic Observatory (PAG), Bulgaria and other series of data are collected from two different instruments located at Conrad Observatory (WIC), Austria and Ebro Observatory (EBR), Spain, which operate within INTERMAGNET - the global network of geomagnetic observatories. We analyze the time series corresponding to the period 00 - 01 UT (02-03 LT) on 29 December 2020. To determine the spectral characteristics of observed geomagnetic pulsations, we apply the autocorrelation analysis to obtain the autocorrelation function and the power spectra, as well as Morlet wavelet analysis of the considered time series. Pulsations with an amplitude modulation period of about 15 min. (~900 s.) and main periods at 84, 75 and 68 seconds are determined. The specific spectral characteristics of these pulsations are obtained on the basis of the Morlet dynamic spectra. They show continuous ULF pulsations represented in four wave “packets” of sinusoidal oscillations with varying periods from 30 seconds till 115 seconds and main periods at 84 s., 75 s. and 68 seconds. We conclude that the recorded pulsations at Geomagnetic Observatory Panagjuriste, Bulgaria, located at mid latitudes, on 29 December 2020 during the time interval 00-01 UT are continuous ULF pulsations within the range Pc4 (45-150 seconds). Similar ULF continuous pulsations in the Pc4 range, with similar power spectra during the same time are observed both at Ebro and Conrad Observatories.

The study of the dynamic behavior of the Earth's magnetic field and its interaction with the Solar wind is an important for better understanding of the processes in the magnetosphere and on the surface of the Earth's ground. This dynamic system is created due to solar-terrestrial interactions. The effects on the variations of the magnetic field induced by geomagnetic storms are strongly dependent on the local time and the latitude. Our goal is to study of the spectral characteristics of the ULF geomagnetic field variations with moderate geomagnetic activity conditions. The data sets for the ULF geomagnetic field variations, recorded at Geomagnetic Observatory Panagjuriste, Bulgaria, for the Bz component of the IMF (Interplanetary Magnetic Field) and for the Kp index value are analyzed. Applying Morlet wavelet analysis over time intervals with duration of four hours, the dynamic spectra of the geomagnetic field variations were obtained for three days: 16 July 2017, 31 August 2017 and 20 April 2018. Original results of the geomagnetic field variations over different time scales are obtained. These are disturbed by moderate geomagnetic activity over all time scales. The disturbances are compared by the time of their occurrence and periods of manifestation. It is found that all disturbances behave in a similar way along the three days. The geomagnetic variations over time scales 150-700 seconds (in Pc5 range) are strongly affected by the geomagnetic storms. They are related with energy transfer from the solar wind to the magnetosphere and many factors connected with magnetosphere-ionosphere system interactions. For time scales smaller than 150 seconds (in Pc2-Pc4 range) these type of disturbances in the ULF geomagnetic field variations behave differently on 31 August 2017 for the time intervals 00-03UT, 12-15UT and 20-23UT, where in contrast to the same intervals of the other days, they appear predominantly with time scales smaller than 150 seconds: 10-45 seconds, 45-150 seconds and 25-75 seconds.