Broadly speaking, terrestrial space weather events are usually due to
Space weather can be summarized in terms of the following headings.
Before proceeding to describe several well-defined periods of space weather activity, we next attempt to integrate the above lists in a more physical way.
Increases in solar X-rays, UV and other radiation during solar flares constitute space weather events for at least two reasons. First, the increased radiation levels can be dangerous to astronauts (e.g., by damaging biological tissue) and can damage (``radiation damage'') spacecraft materials and sensors. Second, the ionosphere expands and heats due to absorption of the increased radiation fluxes, leading to increased drag for spacecraft, increased transport of ionospheric plasma into the magnetosphere, and increased conductivity in the ionosphere (particularly the auroral ionosphere).
Space weather associated with energetic particles includes the following. First, damage by energetic particles (``penetrating radiation'') to spacecraft and humans, both near Earth and in the solar wind (Figure 15.1). Second, energetic particles can implant themselves in non-conducting spacecraft materials, causing dielectric charging, arcing, and eventually destruction of spacecraft electronics and components once the potential becomes large enough.
Figure 15.1: Degradation of the solar cell array on a spacecraft with time due to damage by
energetic particles [Goldhammer et al., 1976]. The step function decrease was due to the
protons from one large solar flare.
Third, particles accelerated by solar flares or by
CME and CIR shocks can cause these problems in the solar wind and, due to transport through the cusps and
enhanced magnetic reconnection at the magnetopause, in the magnetosphere. Fourth, energetic particles
are injected near geosynchronous orbit ( 
) and midnight during magnetic substorms.
These particles enter the ring current and radiation belts, with associated changes in magnetic
field near Earth and enhanced precipitation into the ionosphere.
Compression of the magnetosphere and entry of plasma particles from flares and the solar wind leads to a number of space weather phenomena. First, the decreased distance to the magnetopause current layer leads to a small increase in the magnetic field observed at Earth's surface, the so-called sudden impulse or sudden storm commencement (lecture 14). Second, increased auroral displays and particle precipitation due to plasma funneling down the cusp field lines to the auroral regions. Third, enhanced plasma densities and energies and magnetic field energy densities stored in the plasmasheet and the magnetotail, which are subsequently released in magnetic substorms. Fourth, radiation damage and dielectric charging etc. as described above.
Changes in magnetic field and associated currents cause many effects in space and on the ground. First, the induction of electromotive forces and currents in long, transoceanic and transcontinental, cables (Figure 15.2).
Figure 15.2: Currents induced in transformer windings of power stations in the northern USA
(top), a magnetogram trace from Canada for the same period (middle), and time variations
in the index 
for the ring current associated with a substorm [Williams, 1979].
Second, the induced currents can overload the transformers of electric utilities, leading to widespread power blackouts on Earth. The March 1989 event, for instance, involved over 6 million people being without power for over 9 hours in Canada and Sweden. Third, the fluctuating magnetic fields can lead to difficulties in operating prospecting and communications equipment and high-tech manufacturing equipment. Fourth, these currents can cause increased corrosion in long pipelines.
Finally, changes in the ionosphere due to space weather effects are important in at least the following ways. First, difficulties in radio communications, navigation, and use of GPS systems due to increased radio scintillations (caused by increased density turbulence) and the creation of plasma density holes and enhancements in the ionosphere (which affect the propagation of signals). Second, difficulties in maintaining satellite orbits and orientations due to increased drag from the expanded and denser ionosphere. Third, increased auroral displays and particle precipitation affect the ionospheric density. Fourth, the auroral region and associated displays can move to dramatically different locations rather rapidly. Fifth, the enhanced current and flow of the auroral electrojets leads to large, rapidly changing magnetic fields and associated induced currents on Earth.