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Geomagnetic Field Effects on Living Systems

Te current chron, called the “Brunhes normal polarity chron,” began about 774,000 years ago (774

ka) (ka is an abbreviation for kilo annum, 1 ka = 10³years) (Valet et al., 2019; Simon et al., 2019). Te

immediate prior chron from 2.581 to 0.774 million years (Myr) ago (2.581–0.774 Ma) (Ma is an abbrevia­

tion for Mega annum, 1 Ma = 10⁶years) is the “Matuyama reversed polarity chron.” It was named afer

Motonori Matuyama, but at the time when he proposed “the theory of geomagnetic reversal” in 1929

(Matuyama, 1929), like the theory of ground motion and continental movement, it was not accepted at

frst. Subsequently, the chron from 3.58 to 2.581 Ma is the “Gauss normal” (Clague et al., 2006), and the

chron from 5.894 to 3.58 Ma is the “Gilbert reverse” (Hill et al., 2006).

Te typical periods of GMF reversals are as follows: the Gilbert–Gauss reversal, 3.58 Ma (Hill et al.,

2006); the Gauss–Matuyama reversal, 2.581 Ma (Clague et al., 2006); the Matuyama–Brunhes (M–B)

reversal, 774 ka (Valet et al., 2019; Simon et al., 2019). As described above, the rate of reversals in the

GMF has varied widely over time. It is not yet known when and how the GMF reversal will occur, but

the mechanism by which the Earth becomes a magnet has been elucidated by the “dynamo theory (geo­

dynamo theory)” (Elsasser, 1950; Bullard and Gellman, 1954). Tis dynamo theory was frst proposed

by a German-born American physicist, Walter M. Elsasser in 1946 (Elsasser, 1950), and is the frst math­

ematical model to show that the GMF is generated by the current (~3 × 10⁹ A) induced by the convection

in the Earth’s outer core (Elsasser, 1950). Te dynamo theory was proposed not only by Elsasser but also

by a British geophysicist, Edward Bullard during the mid-1900s (Bullard and Gellman, 1954). Bullard

showed that the movement of fuid in the outer core can generate the GMF (Bullard and Gellman, 1954;

Massey, 1980).

Tere is a dynamo (generator) in the Earth’s outer core (Elsasser, 1950), from which an MF is gener­

ated, creating dipoles that cover the entire Earth. In the 1990s, research on the Earth’s dynamo (geody­

namo), which had been developed only in theory for a long time, became possible on a large scale due

to advances in computers, and research began to clarify more specifc mechanisms of geomagnetic gen­

eration. Te key area was located on the core-mantle boundary (CMB) (Glatzmaier and Olson, 2005).

Moreover, observations by artifcial satellites have revealed that there are magnetic fux patches on

this CMB that are opposite to the normal orientation. Tese are called “reverse magnetic fux patches”

(Glatzmaier and Olson, 2005). Te largest reverse magnetic fux patches extend from the southern tip

of Africa to below the southern tip of South America. As a result of comparison with past observations,

it was found that reverse magnetic fux patches were formed one afer another on the CMB (Glatzmaier

and Olson, 2005). It seems plausible that the reverse magnetic fux patches are formed when the MF lines

are afected by the Coriolis force of the Earth’s rotation, and/or by the MF in the east-west direction

(Glatzmaier and Olson, 2005).

In 1995, the team of Satoshi Kageyama and Tetsuya Sato of National Institute for Fusion Science, Japan

(Kageyama et al., 1995), and the team of Gary A. Glatzmaier of the Los Alamos National Laboratory and

Paul H. Roberts of the University of California, Los Angeles (Glatzmaier and Roberts, 1995), announced

that both teams succeeded in simulating the geodynamo independently. Each team has developed a

program that can simultaneously calculate the temperature, pressure, density, fuid movement, and

even the generated MF in the center of the Earth, and using state-of-the-art supercomputers, they have

succeeded in simulating the geodynamo, which has a dominant dipole MF closer to the real Earth

(Suganuma, 2020). Tese were the frst simulations of the geodynamo, albeit with some simple cal­

culations (Suganuma, 2020). Te three-dimensional MF structure simulated by supercomputers was

completely diferent from the GMF structure that had been imagined from the time of Gilbert to the

present day (Suganuma, 2020). As a result of calculations to reproduce hundreds of thousands of years, a

dipole MF was generated, reverse magnetic fux patches were formed on the CMB, and then the MF was

reversed (Glatzmaier and Roberts, 1995; Glatzmaier and Olson, 2005). However, no one can be sure that

these simulations are occurring in the core of the Earth, because all calculations use approximate val­

ues (Glatzmaier and Olson, 2005). Te heat convection in the core should be complex and there should

be many small turbulences (Glatzmaier and Olson, 2005). It is impossible to handle this turbulence

three-dimensionally with current supercomputers (Glatzmaier and Olson, 2005). While research on