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Readiness for the Upcoming Solar Maximum
wavelength bands show that the fields extend far above the sunspots, producing sometimes complex patterns of loops and arcades. These images also indicate the presence of weaker magnetic fields between the sunspots that underlie larger-scale systems of loops and arcades visible in the corona during solar eclipses.
Most frequently at solar maximum, the interactions between the magnetic fields and the moving, ionized solar atmosphere can twist strong fields above sunspot groups into unstable configurations storing substantial amounts of energy. This energy can be suddenly released in solar flares, which temporarily enhance the solar radiant output at radio and x-ray wavelengths by several orders of magnitude. The larger-scale magnetic structures also undergo frequent eruptions during solar maximum. The ejecta from these coronal mass ejections (CMEs) can travel outward through the background solar wind at speeds of up to 2,000 km/s. These fast CMEs cause interplanetary shocks as they move, thereby greatly enhancing the density and fields in the solar wind. The large-scale character of solar magnetic fields over the cycle also determines the large-scale pattern of coronal fields carried by the solar wind.
Although the solar magnetic field is considered ultimately responsible for all solar variability, the physics underlying its character is not yet understood. Models of the solar dynamo are able to simulate a cyclical behavior, but the changes they predict are inconsistent with observations. This problem seems to stem in large part from an an inaccurate description of the dynamics of the solar interior. Helioseismology techniques now allow researchers to probe the interior structure and dynamics of a star. In just the last few years it has been learned that the differential solar rotation observed on the surface persists inside the visible Sun to about two-thirds of its radius from the center, where there is a strong, thin shear layer. Inside that radius, the Sun rotates almost as a solid body. The implications of this new knowledge are just beginning to be explored but raise even more important questions: Does the situation change in any way as the solar activity cycle progresses? Are the origins of sunspots and CMEs apparent beneath the surface? Do the magnetic fields ever become dynamically important in the solar interior? Is this a critical factor in explaining the Sun's activity cycle? The intensity of the solar maximum varied dramatically over time. What aspect of the solar dynamo determines what the solar maximum intensity will be? What are the consequences for Earth?
EARTH'S SPACE ENVIRONMENT RESPONSE
Only since the last solar maximum have researchers begun to understand the fundamental geospace responses to the maximum and minimum of solar activity. Even the limited spacecraft capabilities that were available for solar and space environment observations during the last solar maximum resulted in a revolution of scientific understanding of the causes and consequences of geomagnetic activity. It is now clear that conditions in interplanetary space and geospace can become extreme and highly dynamic.
In particular, the Van Allen radiation belts, previously regarded as slowly varying and predictable features, were found to be much more dynamic and transient than any statistical model would imply. The bulk of the major geospace disturbances were a consequence of the CMEs, which on occasion compress the magnetosphere to altitudes well inside the geosynchronous orbit (at 6.6 Earth radius [RE equatorial). Those CMEs that travel fast enough with respect to the low-speed (~400 km/s) ecliptic solar wind, moreover, produce leading shock waves that were a major cause of solar energetic proton events. These events can contribute to