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mechanisms, and to the troposphere through the global electric circuit, cannot be ignored. Understanding how the upper atmosphere varies naturally, and how it may be affected by human activities, is necessary from a societal and economic perspective because of the critical role played by the upper atmosphere in communications, navigation, national defense, and a wide assortment of space related endeavors, including the presence of humans in space. Furthermore, current modeling studies indicate that the upper atmosphere may itself be sensitive to global change caused by human activities.

Solar EUV and UV Radiation

The Sun's ultraviolet radiation at wavelengths less than about 180 nm varies considerably more than does the UV radiation that is absorbed in the middle atmosphere and the visible radiation that penetrates to the Earth's surface (see Figure 1.1). Solar cycle changes of 100 percent are typical in solar radiation at wavelengths from 10 to 100 nm; the soft X-rays (1 to 10 nm) that penetrate to the lowest layers of the upper atmosphere vary by an order of magnitude. This highly variable energy from the Sun is deposited entirely in the terrestrial upper atmosphere via absorption of the primary constituents, O2, N2, and O. Without heating from the absorption of solar extreme ultraviolet (EUV) and UV radiation, the thermosphere and the ionosphere would not exist at all. This heating, which varies with solar activity, is responsible for the increase of temperature with height above about 100 km (see Figure 1.2) and for driving most of the bulk motions of the gases within the entire region. Large variability in the basic properties of both the thermosphere and ionosphere is the direct result of the variability in the solar EUV and UV input (as illustrated in Figure 1.2 by the change in the temperature profile from minimum to maximum solar activity).

Measurements of Solar EUV Spectral Irradiance

Current knowledge of the magnitude and variability of the solar EUV energy deposited in the upper atmosphere is based almost entirely on a brief four-year period of measurements made by the Atmosphere Explorer