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Bioelectromagnetism

 

forms the inner edge of the magnetosphere. It has practical importance because, among other functions,

it infuences radio propagation to distant places on the Earth (Rawer, 1993). Te coupling currents or

feld-aligned currents fow along magnetic feld (MF) lines between the magnetosphere and ionosphere.

One of the manifestations of the coupling currents is the auroral oval (e.g., Nagata and Kokubun, 1962).

We can see the schematic of the Earth’s inner core and outer core motion and the GMF generation

from the following source image: USGS FAQs (2013). Tis diagram shows the relationship between the

motion of conducting fuid organized into rolls by the Coriolis force, and the MF that the motion gener­

ates (see USGS FAQs, 2013). Te current understanding of the origin of the GMF is that in the outer core

of the Earth, a liquid metal containing iron and nickel as the main component generates electric cur­

rents (eddy currents) by heat convection while receiving the efect of rotation, and these electric currents

generated the GMF (Hale, 1987; Selkin et al., 2000; Smirnov et al., 2003).

Te inner core is primarily a solid ball with a radius of about 1,220 km, which is about 20% of Earth’s

radius (Monnereau et al., 2010). Te temperature of the inner core can be estimated from the melting

temperature of impure iron (Fe) at the pressure which Fe is under at the boundary of the inner core about

330 GPa (Giga-Pascals). Anzellini et al. (2013) obtained experimentally a substantially higher tempera­

ture for the melting point of Fe, 6,230 ± 500 K. Fe can be solid at such high temperatures only because

its melting temperature increases dramatically at pressures of that magnitude (Aitta, 2006, 2008). Static

compression experiments showed that the hexagonal close-packed structure of Fe is stable up to 377 GPa

and 5,700 K, corresponding to inner core conditions (Tateno et al., 2010). Te observed weak tempera­

ture dependence of the c/a axial ratio suggests that hcp-Fe is elastically anisotropic at core temperatures

(Tateno et al., 2010). Te solid inner core has a faster rotation rate toward the east relative to the mantle

(Alboussière et al., 2010; Dumberry and Mound, 2010). Tis should have generated a stronger and pos­

sibly more stable “dipolar” MF counteracting a decreasing solar ionizing fux (Doglioni et al., 2016). Te

heavy elements started to sink, and the inner core initiated to solidify at about 1 (1.5–0.5) Ga due to the

Earth’s cooling. Since the solid inner core rotates faster than the external core, this possibly would have

generated a stronger dipolar MF and a thicker atmospheric shield. While the intensity of the magnetic

dipole may have slightly increased, the high-energy solar fux hitting the Earth was decreasing. X- and

UV-rays were 100–1,000 times higher in the early stages with respect to the present solar radiation.

Doglioni et al. (2016) speculated that the development of life on Earth was signifcantly afected by

the growth of the solid inner core and the natural evolution of the Earth. Te fast diversifcation of

basal eukaryotes may have been triggered by a more stable atmosphere as a consequence of the stronger

magnetic shield exerted by the newly developed rotating solid inner core and weaker X-, γ-, and UV-rays

(Doglioni et al., 2016). Te GMF strength should increase in the core itself, possibly being related also to

the tidally sheared liquid outer core (Bufett, 2010).

It was announced that a jet-stream of rapidly moving liquid iron is moving at around 50 km per

year (Amos, 2016; Livermore et al., 2017). Te liquid outer core must be convective in order to maintain

the MF against “Ohmic dissipation” of an electric current (Bufett, 2010; Jackson and Livermore, 2009;

Jackson et al., 2011). Bufett (2010) determined that the average MF in the liquid outer core is about

2.5 mT, which is about 40 times the maximum strength at the surface. He started from the known fact

that the Moon and Sun cause tides in the liquid outer core, just as they do on the oceans on the surface

(Bufett, 2010). He observed that motion of the liquid through the local MF creates electric currents,

which dissipate energy as heat according to Ohm’s law (Bufett, 2010). Tis dissipation, in turn, damps

the tidal motions and explains previously detected anomalies in Earth’s nutation, which is, in this case,

the variation over time of the orientation of the axis of rotation of the Earth. From the magnitude of the

latter efect, he could calculate the MF (Bufett, 2010). Te feld inside the inner core presumably has a

similar strength.

Te MF generated by the fow in the direction of rotation is called the “toroidal MF,” and the impor­

tant cause of the convection is in the outer core. As convection, both “thermal convection” and “com­

positional convection” are estimated to occur (Jones, 2015). Te mantle determines how much heat is

released from the core. A spirally rotating fuid fow is also essential in order to generate the GMF. Te