**2. Spatial-temporal variability of geomagnetic field at different time scales**

The Earth's magnetic field interacts with all planetary shells – the core, mantle, and crust of the solid Earth, as well as with the atmosphere, hydrosphere, and biosphere. It comprises information about both the state of near-earth space and the internal structure of our planet. The Earth's magnetic field is continuously changing in space and time. The sources of its variations are located inside and outside of the planet. The amplitude and periodicity of geomagnetic variations are very different, which affects the methods used for data acquisition.

## **2.1 Long-term variations related to the heterogeneity at the core-mantle boundary**

The longest periods of reoccurrence have geomagnetic *reversals*, followed by geomagnetic *excursions*. Geomagnetic *reversals* define the exchange of positions of the North and South magnetic poles. For the last million years, the geomagnetic field has changed its polarity four times. The last one happened about 780,000 years ago. During the inversion of geomagnetic polarity, the magnetic field's strength drops dramatically, leading to a severe weakening of the planetary magnetic shielding, which protects living organisms from harmful cosmic radiation. This is the argument of some scientists to suggest that episodes of mass extinctions of terrestrial biota could be attributed to geomagnetic reversal [6, 7]. No systematic pattern was found in the occurrence of inversions and they are treated as a random process.

The palaeomagnetic records reveals also the existence of shorter periods (with a duration of several thousand years) when the field has departed from its near-axial configuration. Such short-term events are called geomagnetic *excursions*. The *excursions* are usually defined as a deviation of the virtual geomagnetic pole equatorward of 45° latitude, or as a short-term change in the direction of a geomagnetic field, whose amplitude is at least three times greater than the *secular variations* for a given period of time. *Excursions* are short-term impulse fluctuations, which are mostly replaced by smoother secular variations in geomagnetic field intensity. The nature of geomagnetic polarity reversals and excursions is not fully understood. Their general characteristics suggest that they could be considered manifestations of various processes within the Earth's liquid core.

*Secular variations* are another long-term variability of geomagnetic field ranging from decades to several thousands of years. It is generally accepted that the geomagnetic secular variations are associated with changes at the core-mantle boundary. They are studied using all available methods – paleomagnetic, archeomagnetic, and direct observations. Over the period of instrumental observations (approximately 120 years), secular variations are grouped in 4 intervals: 60–70, 30–40, 18–25, 10–11 years. Variations with a period of about 60 years have the greatest amplitude.

In the first approximation, the magnetic field is interpolated as the field of a magnetic dipole. However, the empirical models (incorporating all available measurements of field intensity), as well as satellite measurements, reveal the existence of a non-dipolar component in the real geomagnetic field. The irregularities in the spatial distribution of geomagnetic field intensity are well visible in **Figure 1**, based on the 13th generation of the International Geomagnetic References Field model (IGRF) [8]. The two-wave distribution of field intensity in the Northern Hemisphere and a singlewave in the Southern Hemisphere are well visible in **Figure 1**.

Moreover, the temporal evolution of geomagnetic field also differs in different regions over the world. The greatest amplitude of changes is observed in the Western Hemisphere, in the regions of the Canadian (**Figure 2a**) and South Atlantic (**Figure 2c**) world anomalies. In the Eastern Hemisphere amplitudes of these changes are smaller (**Figure 2b** and **d**). The spatial structure of these irregularities is well visible in the maps of geomagnetic secular variations (**Figure 3**), which are calculated by the formula: *Fsv* ¼ *Ft*<sup>2</sup> � *Ft*<sup>1</sup> ð Þ*=*ð Þ *t*<sup>2</sup> � *t*<sup>1</sup> , where *Ft*<sup>1</sup> and *Ft*<sup>2</sup> are field intensity in two moments in time, and ð Þ *t*<sup>2</sup> � *t*<sup>1</sup> is the length of the period in years.

**Figure 3** illustrates fairly well that focuses of the strongest secular variations evolve with time, in their strength and position over the globe. All the features of the spatio-temporal structure of the geomagnetic field, the problems of its observations and modeling, are described in great details in [9].
