The Polar Aurora
A woodcut of the aurora, by explorer Fridtjof Nansen.
Many people view the polar aurora ("northern lights") as a rare phenomenon.
It is indeed rare at most centers of population, but in Alaska, for instance, or
in much of Canada, the aurora is quite common. Its greenish arcs, often
consisting of many parallel rays (picture above), may stretch quietly across the
sky, with the rays constantly fading as new ones appear. At other times the
arcs may be agitated, move rapidly, expand or fade, as if manipulated by an
What is the aurora? Where does its light come from, and what causes it? In
the 1800s it was already evident that the Earth's magnetic field was involved:
auroral rays seemed to follow the Earth's magnetic field lines, and the
frequency with which aurora was observed depended on the distance from the
magnetic pole, not from the geographic one. The typical height of the aurora
turned out to be about 100 km or 60 miles, placing it in the upper fringes of
In the late 1800s scientists experimented with electric phenomena in glass
containers from which most of the air had been pumped out, and produced there
beams of what seemed to be negatively charged particles, later named electrons.
When electrons hit an obstacle, they can produce light (television screens and
computer monitors operate that way), leading to the idea that perhaps the aurora
was produced like that, too, when beams of electrons from outer space entered
The idea gained support when Kristian Birkeland in Norway, around 1895, aimed
an electron beam at a spherical magnet and found it was guided towards the
magnetic poles of the sphere. Instrumented rockets and satellites later
confirmed the existence of such electron beams in the aurora, measured their
energies and even photographed from above the world-wide distribution of aurora.
But we are still learning about how this happens and where the aurora's energy
More about the aurora
Matter is made up of atoms, each consisting of electrically charged
parts: a central nucleus, charged positively, surrounded by one or more negative electrons. The nucleus contains most of the mass, whereas the electrons are
lightweight, nimble and relatively easy to separate from the rest of the atom. A
glowing wire, for instance, emits electrons and can serve as an electron source
for the beam used in TV tubes and computer monitors.
Electrons are also most useful in encoding and processing information
electrically, a field known as electronics. Nowadays this usually involves
transistors, where electrons are loosely held inside a semiconducting material;
but at one time all electronic devices--radios, TVs, even early
computers--relied on vacuum tubes, in which a hot wire emitted electrons and an
arrangement of electrified grids and coils controlled their motion.
Short electromagnetic waves carry enough energy to eject electrons from matter, in
particular ultra-violet light and x-rays. A near-vacuum is necessary for any
such procedure to be effective, because in ordinary air free electrons collide
with molecules, lose their energy and are recaptured. In most of space however
matter is so rarefied and encounters are so few that free electrons persist
for a long time.
As we climb upwards through the atmosphere, space conditions begin at about 70 km or 45
miles, where electrons liberated by sunlight last long enough to allow
air to conduct electricity to a significant degree. That is the beginning of
the ionosphere, a layer with enough free electrons (and ions) to play an
important role in radio communications. At sunset the electrons of the lowest
part of the ionosphere are quickly recaptured and that layer disappears.
However, at about 200 km (120 miles), where the density of free electrons is the
greatest (up to a million in each cubic centimeter), collisions are so few that the ionosphere persists day
More about electrons
When one or more electons are torn off an atom, the remaining atom becomes
positively charged and is known as a positive ion. Positive ions carry most of
the energy and electrical current in the magnetosphere, and are the main
component of both the inner and the outer radiation belts. Fast ions are also
produced by the Sun as a continuous outflow in all directions, known as the
solar wind, which initiates and powers magnetic storms and similar phenomena.
The simplest atom is the one of hydrogen, with just one electron. Tearing
off that electron gives the simplest ion, the proton. The proton has a close relative, the neutron--nearly the same mass, but no electric charge--and
together these two form the basic building blocks from which the nuclei of all
atoms are constructed.
Most of the fast ions in the magnetosphere and in the solar wind are protons.
In the ionosphere one would expect to see ions of oxygen or nitrogen, the main
atmospheric gases, and in fact most ions there are O+, oxygen atoms which have
lost one electron (out of eight). Some O+ ions end up in the radiation belt,
greatly energized by magnetic storms.
More about positive ions
Ions and electrons in space are usually intimately mixed, in a "soup" containing
equal amounts of positive and negative charges. Such a mixture is known as a
plasma (the same term has a different meaning in medicine; see the history of plasma). In
many respects it behaves like a gas, but when electric and magnetic forces are
present, additional properties come to light, quite unlike those of ordinary
The ionosphere above our heads is a plasma. Unlike air, it conducts
electricity, and in fact, the ionosphere in the polar regions carries large
electric currents, as is discussed in a later section. The electric
conductivity of the ionosphere, unlike that of metals or seawater, is very much
influenced by the Earth's magnetic field. This is a rather special plasma,
because the ionosphere also contains a fairly high number of neutral atmospheric
molecules, with which the ions and electrons constantly collide.
In contrast, collisions are extremely rare in the solar wind. If this were an
ordinary gas, or if the Earth lacked a magnetic field, the solar wind would have
penetrated all the way to the top of the atmosphere and would then have flowed
around the Earth, the way water flows around a rock in a stream. Something like
that in fact happens at the planet Venus, which seems to have no magnetic field
of its own. At Earth, however, a strong magnetic field confronts the solar
wind, forming a much bigger obstacle than the Earth itself. Because the solar
wind is a plasma, it is forced to detour around the Earth's field, creating a
large shielded cavity around the Earth--the magnetosphere.
The explanation of space phenomena thus requires a good understanding of
plasma physics. Unfortunately, no laboratory can duplicate the large dimensions
and the very low particle collision rates found in space plasmas. The behavior
of such plasmas can be sometimes simulated by computers, but ultimately, to
figure what actually happens, one needs to send instruments into space and study
More about plasmas