**2.2 Heterogeneous interplanetary environment and its imprint on the geomagnetic field's short-term variability**

Short-term changes in geomagnetic field (from seconds to days) are caused by the external sources – i.e. the current systems in the magnetosphere and ionosphere. In the absence of solar-terrestrial disturbances, the Earth's magnetic field shows regular daily variations with small amplitude (�tens of nT), which are primarily composed of 24, 12, 8, and 6-hour spectral components [10–12]. These variations are known as

#### **Figure 1.**

*Spatial structure of the modulus of the total vector of the geomagnetic field intensity, calculated for 2021 by the IGRF-13 model. (https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#igrfgrid).*

**Figure 2.**

*Geomagnetic field changes in the regions of world geomagnetic anomalies: (a) Canadian, (b) Siberian, (c) South Atlantic and (d) Geomagnetic pole in the southern hemisphere.*

solar quiet (Sq) variations. Today it is well understood that Sq variations are induced by the electric currents existing in the ionospheric dynamo region (between 90 and 150 km), where the neutral wind drives an electromotive force – through the ionospheric wind dynamo mechanism [13, 14]. The Sq variations are sensitive to the sunspot numbers [10]. For example, the midlatitude Sq currents' intensity is approximately twice higher in solar maximum than in solar minimum conditions [15, 16].

In addition, the geomagnetic perturbations at the planetary surface also have *lunar* spectral components. The stronger one is the semidiurnal lunar variation with a period of 12 hours in lunar time or 12.42 hours in solar time. The typical amplitude of *lunar* variation is much smaller – approximately one-tenth of the Sq variation [17]. Geomagnetic lunar variability is a consequence of atmospheric lunar tides, inducing ionospheric currents in the ionospheric dynamo region, which are furthermore projected on the ground [18, 19].

The maximum amplitude of quiet Sq and lunar variations has a maximum during the daytime hours, and when the moon is in opposition. These are smooth periodic variations with intensities reaching 200nT, increasing from the equator to the poles [20].

The quiet conditions, however, are frequently disturbed by active processes on the Sun (e.g. solar flares, coronal mass ejection, coronal holes, etc.). The ejected solar mass and magnetic fields propagate in the interplanetary magnetic field (IMF) as a shock wave, which distorts significantly geomagnetic field when it splashes on the Earth. Only 1% of energy carried by the solar wind is transferred to the Earth's magnetic field because the reconnection between interplanetary and geomagnetic fields depends on their directions. It is well established that the southward direction of the interplanetary magnetic field favors its reconnection with the Earth's magnetic field.

*Coupling between Geomagnetic Field and Earth's Climate System DOI: http://dx.doi.org/10.5772/intechopen.103695*

**Figure 3.**

*Secular variations of the first two decades of twenty-first century, based on the IGRF-13 model. (https://www.ngdc. noaa.gov/geomag/calculators/magcalc.shtml#igrfgrid).*

The energy transferred to the magnetosphere in such periods abrupt dramatically by one-two orders of magnitude, reaching power of ≥1011 W [21]. These periods are known as *geomagnetic storms* or *substorms.*

The dominant interplanetary phenomena causing *intense* magnetic storms (with an equatorial Dst index lower than 100 nT) depends on the solar cycle. Around the solar maximum, the interplanetary medium is dominated by fast coronal mass ejections (CMEs). Two interplanetary structures are important for the development of storms, involving intense southward IMFs: the *sheath* region just behind the forward shock, and the CME ejecta itself. Whereas the *initial phase* of a storm (manifesting itself as a sudden impulse in geomagnetic field) is caused by the increase of plasma pressure at, and behind the shock, the storm's *main phase* is due to southward IMFs. The storm recovery begins when the IMF turns less southward, with delays of ≈ 1–2 hours, and has typically a decay time of 10 hours [22].

Magnetic clouds are large-scale interplanetary formations, caused by coronal mass ejection on the Sun, in which the magnetic field strength, propagating speed, and plasma concentration are higher than in the surrounding flows [23]. The vertical Bz component of IMF slowly changes from negative to positive sign in SN clouds, and vice versa in NS clouds. The interaction of the Earth's magnetosphere with magnetic clouds, as a rule, is accompanied by intense geomagnetic disturbances [24, 25].

According to some estimates, the geoeffectiveness of magnetic clouds to disturb Earth's magnetic field is 77% [25, 26].

During solar minimum, high-speed streams from coronal holes dominate the interplanetary medium activity. The high-density, low-speed streams (associated with the heliospheric current sheet plasma) impinging upon the Earth's magnetosphere cause positive Dst values in the initial phase of the storm. In the absence of shocks, sudden impulses are infrequent in periods of low solar activity. The interaction between fast stream (emanated from coronal holes) and the slow heliospheric current sheet plasma leads to the formation of a compression region with a high magnetic gradient, called *Corotating Interaction Region* (CIR)*.* The main phase of magnetic storms generated by the CIR is typically weaker and highly irregular. The recovery of geomagnetic storms that happened in periods of inactive Sun is also quite different – lasting from many days to weeks. The southward magnetic field component of Alfvén waves, existing in the high-speed plasma stream, causes intermittent reconnection and substorm activity, as well as sporadic injections of plasma sheet energy into the outer portion of the ring current, prolonging its final decay to quiet day values [22].

For certain classes of magnetic storms, the interaction of CIR with the Earth's magnetosphere is more efficient than CME [27]. On the other hand, comparisons of the geoeffectiveness of various interplanetary structures, such as shock waves, magnetic clouds, IMF sectors boundaries, and CIR, showed that 33% of CIR are accompanied by moderate or intense storms. This means that every third phenomenon of the observed CIR at the Earth is geoeffective [28].

It is statistically confirmed that geoeffective disturbances can be caused by a whole spectrum of various phenomena on the Sun: *flares* (especially with the release of high-energy protons); the *sudden disappearance of filaments*, followed by a transition to a *coronal mass ejection*; *high-speed streams* of solar wind; Earth passage through the IMF sectors' boundaries, etc. However, the features of the magnetic storms are primarily determined by changes in IMF and solar wind parameters [29–32].

The influence of geomagnetic storms on the lower atmospheric variables is studied by many authors. The storm imprint on the near-surface pressure and temperature has been reported by [33–34], on circulation by [35–40], on total ozone density by [41], etc. The latter authors have compared geomagnetic storm manifestation in upper, middle, and lower atmosphere, emphasizing on differences in the atmospheric response to geomagnetic storms. Their main conclusions are summarized as follow: (i) unlike the prevailing latitudinal dependence of storm impact on the upper-middle atmosphere, the tropospheric effects manifest itself with a well pronounced *regionality*; (ii) the weak seasonal dependence of the storm effect in the upper middle atmosphere is altered by a *strong seasonal dependence* of detected tropospheric response; and (iii) the geomagnetic effect in the upper middle atmosphere are caused primarily by energetic particles, while the *origin of tropospheric effect* is still not well understood [41].

All these effects are due to the short-term geomagnetic disturbances, initiated by the external influence – i.e. solar variability and inhomogeneity of interplanetary medium. Although important, these fluctuations of Earth's magnetic field are shortlasting and their impact on the climate system is negligible. Oppositely, this publication is focused on the long-term variations of geomagnetic field on interdecadal and multidecadal time scales (initiated at the core-mantle boundary) and their relation to climate variability with its regional specifics.
