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Geomagnetic Field Effects on Living Systems

Even during the Maunder Minimum, changes in the number of sunspots in the 11-year periodic cycle

can be read from changes in the number of sunspots (Schröder, 1992). However, even if the activity of

sunspots decreased, it is considered that there was almost no change in the amount of sunlight, so there

was no signifcant efect on the temperature of the Earth. Te relationship has not been clarifed between

the activity of sunspots and the climate change of the Earth.

Meanwhile, solar activity induces an explosive phenomenon called the “solar fare” that occurs

abruptly in the solar active region. At this time, the solar wind plasma and the coronal MF may be

blown into outer space together. Tis plasma ejection is termed “coronal mass ejection (CME).” Tis

CME, unlike the solar fare which is light, causes a large mass to reach the Earth, similar to the solar

wind. CMEs travel through interplanetary space with occasional serious impacts on the solar-terrestrial

environment (Gutierrez et al., 2021). Terefore, CMEs are thought to have a greater efect on the GMF

than solar fares (Gosling, 1991).

Most large-scale magnetic storms occur when the solar fare causes CMEs, which blow into the geo­

magnetosphere with a strong southward MF. Magnetic storms are more likely to occur when the solar

activity with many sunspots is high because the solar fare is associated with sunspot activity. In the case

of magnetic storms, a severe aurora storm ofen occurs, and in that case, a severe change in the GMF

intensity is also observed, especially in high latitude areas.

In addition to the 11-year periodic cycle due to the solar activity, there are other periods of variation

in the geomagnetic activity: 1 day cycle, due to the Earth’s rotation; a 6–7-day cycle, corresponding to

one sector of the solar wind; a 13–14-day cycle, corresponding to the passage of two solar wind sectors; a

27-day cycle, corresponding to the period of the Sun’s rotation around its own axis (Volpe, 2003). Other

rhythms are also correlated with these cycles: a 29.5-day cycle, corresponding to the synodic period of

the Moon; a 1-year cycle, corresponding to the period of the Earth’s revolution; some cycles of the solar

activity that are not so prominent when compared to the main 11-year cycle (2-, 3-, 8-, 22-, and 35-year

cycles) (Volpe, 2003).

A magnetic storm is a worldwide geomagnetic disturbance, distinct from regular diurnal variations.

Geomagnetic activity is natural variations in the GMF, which is classifed into quiet, unsettled, active,

and magnetic storm levels (Poole, 2002; Alabdulgader et al., 2018; Table 6.1). Geomagnetic activity indi­

ces, e.g., K, Kp, A, and Ap indices, were designed to describe variation in the GMF caused by the irregular

current systems (Poole, 2002; Alabdulgader et al., 2018; Table 6.1).

K index is a quasi-logarithmic local index of the 3-hourly range in magnetic activity relative to an

assumed quiet-day curve for a single geomagnetic observatory site. First introduced by a German geo­

physicist, Julius Bartels in 1949, it consists of a single-digit 0 through 9 for each 3-hour interval of the

universal time day (Bartels, 1949).

Kp index is the planetary 3-hour-range index, which is the mean standardized K index from 13 geo­

magnetic observatories between 44° and 60° Northern or southern geomagnetic latitudes. Te scale

is zero to nine expressed in thirds of a unit; e.g., 5− is 4^2/3, 5 is 5, and 5+ is 5^1/3. Tis planetary index is

designed to measure solar particle emissions by their magnetic efects.

TABLE 6.1 Te General Relationship between Ap and K Values

Category

Ap Index Range

K Index Range

Quiet

<8

<2

Unsettled

8–16

2–3

Active

16–30

3–5

Minor storm

30–50

5–6

Major storm

50–100

6–7

Severe storm

>100

>7

Source: Compiled from Poole (2002) and Alabdulgader et al. (2018).