the radiation belt for weeks or months, adding to other transient populations that follow the passage of the shock front through the magnetosphere. Moreover, by virtue of their disturbance of the entire magnetosphere, it is the CMEs that cause widespread effects including auroral activity, ionospheric disturbances, and induced ground currents.
Further discoveries have resulted from the observational capabilities that became available during the present solar minimum period. These illustrated the special role of high-speed solar wind streams emanating from coronal holes during solar inactive periods. These high-speed wind streams somehow control the intensity of energetic (>1-MeV) electrons in the radiation belts with a fair degree of predictability.
We are now witnessing a transition from the high-speed solar wind stream activity characteristic of a solar minimum to the less predictable eruptive events that characterize a solar maximum. This change in behavior manifested itself most spectacularly with the event of January 6-11, 1997, when a CME produced an enormous magnetic cloud that expanded toward Earth. The CME produced an interplanetary shock that hit the magnetosphere on January 10, shortly after midnight Universal Time. The shock was followed roughly 24 hours later by an interval of unusually high-density solar wind (150 cm−3, roughly 15 times the average solar wind density) that compressed the magnetosphere on the dayside within the geosynchronous orbit. This strong pressure pulse was followed by a stream of higher than average speed (~600 km/s). A rapid buildup within the inner magnetosphere of energetic (>1 MeV) electrons by 3 to 4 orders of magnitude was under way before arrival of the high-density solar wind impulse. Figure 1 illustrates the connections that can now be made between solar, interplanetary, and geospace observations during such events.
What was particularly significant about this January 6-11, 1997, event was the first-ever opportunity to observe the effects of a CME from cradle to grave: Observations from the Solar and Heliospheric Observatory (SOHO) and Yohkoh showed the changes in the photosphere and corona that preceded the eruption. The coronagraph on SOHO then imaged the dense plasma accompanying the erupting fields, the CME, as it moved away from the Sun. Radio waves emitted from the approaching shock front were detected on board the Wind spacecraft (as was the passage of the leading interplanetary shock and the trailing density pulse), while the Polar spacecraft and other magnetospheric satellites and ground-based observatories measured the shock's effects on geospace. Theory teams sponsored by the International Solar-Terrestrial Physics (ISTP) program produced a global numerical simulation of the event, the longest to date, to capture the entire period of magnetic cloud passage and to analyze the physical causes of the energetic particle increases and atmospheric effects. Researchers have never been so well prepared to study the complete life cycle of this type of complex and important natural event.
The past 15 years have also produced explosive growth in our knowledge of our Sun's outputs and the responses of Earth's atmosphere to them. Accumulating records of the total solar radiative output1 are finally yielding accurate measures of the degree to which the “solar constant” actually varies (by ~0.1%) as the contemporary Sun goes through the extremes of its ~11-year activity cycle. In addition, images of the Sun in wavelengths over a broad range of the
Board on Global Change, National Research Council, Solar Influences on Global Change, National Academy Press, Washington, D.C., 1994.