was a Japanese physicist specializing in the near-Earth space environment. He served as Secretary General of the International Association of Geomagnetism and Aeronomy

International Association of Geomagnetism and Aeronomy

International Association of Geomagnetism and Aeronomy is the premier international scientific association promoting the study of terrestrial and planetary magnetism and space physics....

 from 1975 to 1983. In 1990 he was awarded the AGU

American Geophysical Union

The American Geophysical Union is a nonprofit organization of geophysicists, consisting of over 50,000 members from over 135 countries. AGU's activities are focused on the organization and dissemination of scientific information in the interdisciplinary and international field of geophysics...

's Waldo E. Smith Medal

Waldo E. Smith Medal

The Waldo E. Smith Medal is given out by the American Geophysical Union to recognize "individuals who have played unique leadership roles in such diverse areas as scientific associations, education, legislation, research, public understanding of science, management, and philanthropy, and whose...

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He is best known for proving that, under certain conditions, the magnetic field from a field-aligned current

Birkeland current

A Birkeland current is a set of currents which flow along geomagnetic field line connecting the Earth’s magnetosphere to the Earth's high latitude ionosphere. They are a specific class of magnetic field-aligned currents. Lately, the term Birkeland currents has been expanded by some authors to...

 and the magnetic field from the associated Pederson current in the ionosphere

Ionosphere

The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

 would exactly cancel at the surface of the Earth. The magnetic equivalence of field-aligned currents with Pederson currents is referred to colloquially as Fukushima's Theorem

Fukushima's Theorem

In physics, Fukushima's Theorem holds that for all points beneath the ionosphere the magnetic fields from field-aligned currents and their corresponding Pederson currents exactly cancel...

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