The Sun emits more than visible light. It also gives off ultraviolet, x-ray, gamma-ray and radio emissions that are much more variable than its visible emissions.
Above the solar surface or photosphere, there is a solar atmosphere
whose temperature rises rapidly with height. The upper atmosphere of
the Sun, the corona, is
highly ionized because of its high temperature.
The larger scale fields extend into the corona where they produce the structure seen in the pictures above.
The solar magnetic field undergoes
an eleven year cycle during which the number of sunspots waxes and
wanes as shown in the introduction, and the solar dipole contribution
to the field reverses polarity. The solar magnetic cycle is behind
essentially all of space weather.
The upper atmosphere of the Sun in the high corona is not static but flows away from the Sun at supersonic speeds (on average about 400 kilometers per second). The coronal magnetic fields behave as if they are frozen into this ionized, outflowing 'solar wind.' and are carried with it into interplanetary space. The rotation of the Sun makes the magnetic field in the plane of the Earth's orbit around the Sun appear to wind up like the spray around a garden hose.
The solar wind at the Earth's distance from the Sun (1 astronomical unit)
has a density of about 8 particles per cubic centimeter and is composed
of mostly hydrogen ions (or protons) and electrons in equal numbers. The
interplanetary magnetic field strength at the Earth is about 5 nanoteslas
or roughly 1/10,000 times that of the Earth's own field at the equator.
It typically points along the direction indicated by the garden hose
pattern
described above, but its 'polarity' (toward the Sun or away from it)
depends
on the coronal field polarity at its roots.
The solar wind perceives the dipolar field of the Earth as an obstacle. As a result, the geomagnetic field makes a cavity in it that is compressed on the dayside and extends into a long 'magnetotail' on the nightside. This cavity forms the boundary of the 'magnetosphere'.
The Earth's magnetic field connects with the interplanetary magnetic
field in the stretched out regions near the polar caps. The auroral oval is
located around the base of the interconnected field region.
Interconnection between the interplanetary and geomagnetic fields
has been found to be greatest when the former points southward, or
opposite to the Earth's field.
Charged particles find their way into the magnetosphere from both the
solar wind on the outside and the upper atmosphere and ionosphere on the
inside. Once within the magnetosphere, they can be energized and
transported
around by a variety of processes that are still under study.
Several concentrations of different
particle populations with different origins, densities, and energies are
found in the magnetosphere. The Van Allen
radiation belts are only one such population.
At the base of the magnetosphere lies the upper reaches of the Earth's atmosphere , and within it the ionosphere. The upper atmsophere becomes increasingly thin with altitude. At the typical altitude of the shuttle orbit (about 300 to 400 km), the atmospheric density is the order of 10**8 particles per cubic centimeter (CHECK) or about 1/10,000,000,000 times the density of air at the surface. At typical weather satellite orbit altitudes (about 850 kilometers) it is down to 1/10 to 1/100 times the density at the shuttle orbit.
The region where the atmosphere is significantly ionized by solar ultraviolet and extreme ultraviolet radiation is located above about 50 km, below which almost all of this ionizing radiation has been absorbed. The ions and electrons that are produced from the local air atoms and molecules can persist for long times because the densities are so low at these heights. The numbers of ions relative to unionized or 'neutral' particles varies with altitude. The contribution of the ionized portion is highest at the highest altitudes, but because the atmosphere gets thinner with height, the largest ion and electron densities are found at about 300 km. Even here there are many more neutral atoms than ions and electrons.
Under the aurora more ions are produced in about the same height
range by the energetic particles that cause the atmospheric gases
to emit light.
In addition to the space environment described above, the Earth is immersed in an extremely tenuous bath of very high energy charged particles from the galaxy called 'cosmic rays'. The Earth's magnetic field acts like a shield, deflecting many of these particles. However, cosmic rays can gain access to low altitudes in the polar regions where the Earth's field has been greatly stretched and interconnects with the interplanetary field. Some of the most energetic cosmic rays reach cloud levels (around 10 km altitude) where they can affect cloud electrification.
