Modern geophysical science is currently experiencing a renewed interest in the long-term dynamics of the Sq-components (solar quiet daily geomagnetic variation) of the geomagnetic field, and this is no coincidence. The scientific community has recognized that Sq-variations are much more than mere "background daily ripples" in magnetometer data. They have proven to be the key to understanding processes previously considered disparate: from solar-terrestrial relations to climatic trends and the dynamic evolution of the ionosphere. 1. Sq-variations as a unique long-term indicator of the state of the ionosphere and thermosphere Sq-components are generated by currents in the ionospheric E-region, which means they directly reflect: Plasma conductivity. The dynamics of thermospheric winds (the upper layer of the Earth's atmosphere, approximately 80 to 600 km, where temperatures rise to extremely high values due to the absorption of solar ultraviolet and X-ray radiation). The level of solar ultraviolet radiation. The structure of atmospheric tides in the Earth's upper atmosphere. This makes Sq-variations a natural archive of the upper atmosphere's state, which can be studied over multi-decadal scales using data from magnetic observatories. Today, as climatic and atmospheric changes accelerate, the Sq-curve has become a crucial tool for tracking long-term trends. 2. Growing interest in solar-terrestrial relations and space weather as a global "geospace": Modern infrastructure—satellites, navigation systems, power grids – is highly sensitive to severe geomagnetic disturbances. Sq-variations: Serve as a baseline for the quiet state of the magnetic field. Enable the detection of hidden changes in the solar wind. Help assess the frequency and intensity of disruptions to the normal rhythm of ionospheric currents. The long-term dynamics of Sq-components reveal how the interaction between the Earth and the Sun evolves over decadal scales, encompassing solar activity cycles. 3. Access to century-long magnetic data: The digitization of observatory archives (Greenwich, Potsdam, Irkutsk, Mikhnevo, etc.) has opened the possibility to: Analyze Sq-variations over a period of more than 100 years . Compare epochs of different solar cycles . Study the impact of major geophysical events (e.g., superstorms). This has created an entirely new stratum of research: historical geomagnetic climatology. The present example demonstrates the long-term evolution of the solar quiet daily (Sq) variations of the geomagnetic field based on data from the Bulgarian geomagnetic station Panagjurishte (Geophysical Institute of BAS, Sofia - Bulgaria). The data (Figure 1) represent time series of geomagnetic field variations for the year 2025, in form of individual geomagnetic field components (X, Y, Z) as well as the total geomagnetic field induction (utilizing modern high-precision magnetometers manufactured in Denmark and Canada).

The decametric radio emission of Jupiter (DAM) is one of the most powerful and structurally complex radio sources in the Solar System. Jupiter’s signals (an example is shown in Figure 1) and their fine spectral structure constitute a unique natural laboratory for plasma astrophysics. The Io–Jupiter electrodynamic circuit: the Jovian satellite Io, moving through the planet’s powerful magnetic field, acts as a giant unipolar inductor. This generates an enormous potential difference and drives currents of millions of amperes flowing along the magnetic field lines (within the so-called Io flux tube).