Technological systems in space and on the Earth's surface are subject to adverse effects from solar driven space weather effects. The increasing deployment of radiation-, current-, and field-sensitive technological systems over the last few decades, the increasing complexity of interlocking components such as those represented by the national electric power grid, and the increasing presence of systems in space combine to make society more vulnerable to solar-terrestrial disturbances [ Allen and Wilkinson, 1993]. This has been emphasized by the large number of problems associated with the severe magnetic storms between 1989 and 1991 as the 11-year solar activity cycle peaked. This solar cycle (#22) maximum was greater than the past two, but not nearly as great as the cycle #19 maximum in 1958, when a coordinated effort was made to investigate the geophysical environment under the auspices of the International Geophysical Year (IGY). It was more comparable to the pre IGY cycle #18 which peaked in 1947. We now may anticipate a few years of lower activity as we approach solar minimum. However, of great concern are predictions of even greater activity than we have just experienced for the next solar maximum , which will be a typically mightier odd-numbered cycle, [i.e. Kopecký, 1991].
The cause of these disturbances is episodic energy and mass releases from the sun. Soft X-rays from a solar flare arrive within 10 minutes, creating ionization enhancements in the ionosphere. Energetic protons can arrive anywhere from a few minutes after the soft X-rays to hours later. Their arrival increases ionization in the polar ionosphere, increases the frequency of upsets in satellite electronics and begins solar panel degradation effects from the enhanced high energy fluxes. Depending on the speed of the solar wind, magnetic storms begin 1 to 2 days later when the shock wave impacts the earth's magnetosphere [see Cliver et al., 1990].
However, not all solar flares result in magnetic storms, and, even more significantly, not all storms can be associated with solar flares. While solar flares capture ones attention, coronal mass ejections (CMEs), sometimes associated with flares and sometimes not, now appear to be a primary cause of geomagnetic activity [ Kahler, 1992; Gosling, 1993]. CMEs are not easily detectable unless they occur on the limbs of the sun. CMEs that are ejected toward the Earth produce geomagnetic activity when the associated shock front arrives. The severity of the storm is related to the polarity of the north-south component of the interplanetary magnetic field (IMF) as well as the velocity of the solar wind. The distribution of effects is further influenced by the dawn-dusk component of the IMF. Prediction of geomagnetic storms based on observation of events on the Sun only will have a high degree of error, since we presently can not accurately model the direction of the interplanetary magnetic field and the speed of the disturbance.
Figure 1 shows the sequence of events leading up to the major magnetic storm of March 24, 1991 [ Shea and Smart, 1993]. The enhancement of the X-rays is followed by the solar proton enhancement and eventually the magnetic storm. The timing of some of the space weather related disruptions and events are shown at the bottom. On this storm a third radiation belt was injected between the inner and outer belts. Communication disruptions, power surges in ground transmission lines, satellite single event upsets and other effects are scattered throughout the time period. Aurora was seen as far south as Georgia. Admittedly, this was a major storm. But to a lesser degree, smaller storms exhibit these effects and present similar hazards.
A second factor in predicting geomagnetic activity and associate hazards is that within a major storm the magnetosphere releases energy to the high latitude ionosphere in episodic events called substorms. The triggering mechanism for substorms is not understood physically. Thus peaks in activity can occur at various times within the main storm. Substorms also occur on an aperiodic routine basis during periods of moderate activity. These factors are reflected in the constantly changing AE (auroral electrojet) magnetic activity index, and are manifested in a highly variable ionosphere that impacts communications, navigation systems (LORAN and GPS) and the operation and tracking of low Earth-orbiting satellites.
Large magnetic storms can occur throughout the 11 year solar cycle, although they are more prevalent near solar maximum. In the declining phase of the solar cycle, recurrent moderate storms occur when high speed streams from particular regions on the sun rotate by the Earth with the 27 day solar rotation. These are accompanied by several days of very enhanced fluxes of energetic electrons in the outer radiation belt as observed at geosynchronous orbit. It was during one of these events in January, 1994, that control of the Canadian communications satellite Anik was lost. Thus, space weather hazards are always present but are, on average, more frequent and more intense near solar maximum.