  The Earth is a planet with a dipolar magnetic field which is agitated by a
magnetized plasma wind streaming from the Sun. The magnetic field shields the
Earth's surface from penetrating high energy solar wind particles, as well as
interstellar cosmic rays. The magnetic dipole has reversed sign some hundreds
of times over the last 400 million years. These polarity reversals correspond
to drastic breakdowns of the dynamo action. The question arises what the
consequences for the Earth's atmosphere, climate, and, in particular, biosphere
are. It is shown by kinematic estimates and three-dimensional plasma-neutral
gas simulations that the solar wind can induce very fast a magnetic field in
the previously completely unmagnetized Earth's ionosphere that is strong enough
to protect Earth from cosmic radiations comparable to the case of an intact
magnetic dynamo.
