Initial interpolated geomagnetic field data (after the removal of interference and noise spikes) for 2025, Panagjurishte Magnetic Observatory (Bulgaria).

The pattern of time-frequency spectra in the 2–48 hour range is almost entirely determined by two factors: the ionospheric dynamo (regular currents) and magnetosphere-ionosphere interaction (magnetic storms) (Figure 2). The data analysis consisted of three stages: cleaning the magnetograms of interference and noise spikes; interpolation and filtering (using a Butterworth bandpass filter, which yields minimal edge effects and almost no "false ringing" artifacts); and constructing time-frequency wavelet spectra based on the Morlet wavelet function.

Main resonance (24 hours): The brightest and thickest red band across all components. It is caused by the global current system in the ionospheric E-layer (at an altitude of 100–120 km). Solar radiation ionizes the gas on the dayside of the Earth, while high-altitude winds move this plasma across the magnetic field, generating giant current vortices. Bulgaria is located relatively close to the focus of the northern Sq-vortex, which is why diurnal fluctuations are highly pronounced here.

Harmonics (12 and 8 hours): The clearly visible bands at 12 hours and, slightly weaker, at 8 hours correspond to semidiurnal and 8-hour thermal atmospheric tides. The atmosphere is heated unevenly, resulting in "tidal waves" that modulate the ionosphere's conductivity and give rise to these harmonics.

Wavelet spectrum of the filtered geomagnetic field data in the 2–48 hour period band, designed to isolate the harmonics of solar quiet daily variations for 2025, for the geomagnetic field components X (northward), Y (eastward), Z (vertical), and F (total geomagnetic field induction). The period scale is in hours. The lines on the spectra indicate the periods of the spectral power maxima and their transitions. 

In this example, the primary energy of the magnetic variations is concentrated in the 24-hour, 12-hour, and 8-hour harmonics. It is evident how geomagnetic disturbances cause distortions in the periods of the solar quiet daily variations, specifically through disruptions to the structure of ionospheric electric currents. During periods of a quiet Sun, the line of spectral maxima remains steady at 24 hours. Upon the impact of a magnetic storm, the spectrum becomes "cluttered" with intense short-period noise, the dominant energy shifts, and the line of maxima abruptly drops (to 12, 8, or even 4 hours). The greater the number of such fractured segments in the spectral skeleton, the more disturbed the geomagnetic environment.

X-component (North-South): The line of maxima here "breaks" much more frequently, and the background noise between the harmonics is higher. The X-component is directed poleward and is highly sensitive to the equatorial ring current. Any magnetic storm (even a minor one) introduces a significant contribution to the X-axis, overpowering local ionospheric effects.

The Baltic region is a highly sensitive zone for ionospheric disturbances. Seasonal and latitudinal effects are pronounced here, making local observations of the geomagnetic field essential. The methodologies derived from the Bulgarian geomagnetic monitoring will be utilized to analyze data from the Latvian AlphaLab magnetometer in Irbene at the VIRAC radio astronomy observatory, serving as the first steps toward establishing a Latvian space weather monitoring system.

The above-mentioned activities were carried out within the framework of the research application “Studies of space weather conditions during the 25th solar cycle observed along the Struve Geodetic Arc”, No. 1.1.1.9/LZP/1/24/048. The application is implemented within Project No. 1.1.1.9/1/24/I/001, co-financed by the European Regional Development Fund.