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Bioelectromagnetism

No matter how magnifcent the aurora is and the MFs around the Earth change, it is not called a

magnetic storm by itself. Whether or not a magnetic storm is becoming larger in size is determined by

changes in the MF at mid and low latitudes, not by the aurora. A westward current called “ring current”

fows around the equatorial plane of the Earth. Tis current generates an MF in the opposite direction

to the normal GMF, and thus by the interference efects, leads to weakening the MF at mid-low latitudes.

If such a state lasts for 2–3 days, this is the magnetic storm. Of course, at that time, active aurora occurs

continuously at high latitudes. Te solar wind has the greatest efect on these magnetic storms.

Te solar wind from the Sun reaches the Earth ~150 million kilometers away. Te solar wind that

reaches the Earth’s orbit collides with the geomagnetosphere (Borovsky and Valdivia, 2018). Te geo­

magnetosphere is also called a “magnetic comet” (Gonzales, 1969) because its shape with a long mag­

netic tail blown by the solar wind is just like the tail of a comet. Te range of the geomagnetosphere

extends tens of thousands of kilometers on the Sun side and like a tail on the other side, reaching more

than hundreds of thousands of kilometers.

As shown in Figure 6.13, the solar wind and the geomagnetosphere are presented by Borovsky and

Valdivia (2018).

Bow shock indicates the shock front along which the solar wind encounters the GMF (Borovsky and

Valdivia, 2018) (Figure 6.13). A magnetic storm is a temporary disturbance of the geomagnetosphere

caused by a solar wind shock wave and/or cloud of the MF that interacts with the GMF. Te reason why

the magnetosphere becomes asymmetrical in this way is that the GMF is afected by the solar wind and

is blown away to the opposite side of the Sun. On the other hand, most of the solar wind that collides

with the magnetosphere is bounced of and fows into outer space, but the large current generated at

that time is transmitted to the sky near the N and S poles of the Earth. Tus, the GMF acts as a shield to

defect potentially harmful charged particles mainly coming from the solar wind.

Te most well-known solar activity is the activity of sunspots, which repeats in a cycle of ~11 years.

Te dark spots are because the temperature is ~4,000°C, which is lower than the normal photosphere

temperature of ~6,000°C on the Sun’s surface. Te activity of sunspots is considered to be caused by the

periodic changes of the MF of the Sun. Te greater the number of sunspots, the more active the solar

activity, and such a state is called “the maximum period of solar activity.” In contrast, the state where

the number of sunspots is small is “the minimum period of solar activity.” However, during the 70 years

from 1645 to 1715, there was a period when the number of sunspots decreased drastically. Tis period is

called the “Maunder Minimum”(1645–1715) (Nymmik, 2011; Riley, 2012). Tis name comes from Edward

Walter Maunder, a solar astronomy researcher who studied the past records of the disappearance of the

sunspots. During the 30 years during the Maunder Minimum, only about 50 sunspots were observed.

Normally, except for during the Maunder Minimum, about 40,000–50,000 sunspots are observable.

FIGURE 6.13 Te solar wind and the geomagnetosphere (Borovsky and Valdivia, 2018). (Reproduced with per­

mission from Borovsky and Valdivia (2018), Copyright 2018, Elsevier. It is licensed under the Creative Commons

Attribution 4.0 International.)