The aurorae are the result of collisions with the atmosphere of energetic particles that have their origin in the solar wind, and reach the atmosphere after having undergone varying degrees of acceleration and redistribution within the Earth's magnetosphere. The global scale phenomenon represented by the aurorae therefore contains considerable information concerning the solar-terrestrial connection. For example, by correctly measuring specific auroral emissions, and with the aid of comprehensive models of the region, we can infer the total energy flux entering the atmosphere and the average energy of the particles causing these emissions. Furthermore, from these auroral emissions we can determine the ionospheric conductances that are part of the closing of the magnetospheric currents through the ionosphere, and from these we can in turn obtain the electric potentials and convective patterns that are an essential element to our understanding of the global magnetosphereionosphere-thermosphere-mesosphere. Simultaneously acquired images of the auroral oval and polar cap not only yield the temporal and spatial morphology from which we can infer activity indices, but in conjunction with simultaneous measurements made on spacecraft at other locations within the magnetosphere, allow us to map the various parts of the oval back to their source regions in the magnetosphere. This paper describes the Ultraviolet Imager for the Global Geospace Sciences portion of the International Solar-Terrestrial Physics program. The instrument operates in the far ultraviolet (FUV) and is capable of imaging the auroral oval regardless of whether it is sunlit or in darkness. The instrument has an 8 ~ circular field of view and is located on a despun platform which

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Increases in the solar ultraviolet flux (λ < 1250Å) over the past five years of rising solar activity have been larger than anticipated. This increase in UV flux dramatically affects the production of ionization of the various constituents in the thermosphere. In this paper we use measurements of the solar UV flux by the Atmosphere Explorer satellites to determine ionization frequencies for the major thermospheric species for various dates exhibiting notably different levels of solar activity. For the convenience of users of such dataa reduced set of cross section and flux data is presented for the wavelength range below 1027Åcomprising 37 wavelength intervals.

Long-lived or metastable excited states of various atoms and molecules in the thermosphere provide reservoirs for the temporary storage of a considerable portion of the solar EUV photon energy deposited in the thermosphere. These species permit the redistribution of energy via collision processes yielding kinetic or vibrational heating, ion formation, the formation of other metastable species and the nonlocal deposition of the energy, as opposed to spontaneous radiative decay. While thermospheric species such as O(•S) have been studied for decades, and indeed provided the first evidence for forbidden transitions, others have a relatively short history. The body of information concerning metastable constituents of relevance to the thermosphere has grown considerably over the past decade, to the point where it is timely to consider the status and future needs of research in this area. In this paper we review the developments leading to the current photochemical picture of O(•D), O(1S), O+(2D), O+(2P), N(2D), N(2p), N+(•S), N+(•D), N2(A3Zu+), NO+(a3Z), O2+(a4•'u), and the vibrationally excited states of N2, 02, O2 +, and N2 +. Because a primary significance of these constituents is their role in the thermospheric energy budget, we quantify the major' metastable channels by which solar EUV radiation is redistributed in the thermosphere. A few of the major highlights are as follows: A major fraction of the kinetic heating of the thermosphere takes place via the single constituent O(•D); N2(A3•u +) and vibrationally excited N2 also are important heating channels. O +(2D) is a primary factor in determining the thermospheric ionic composition. Species such as O + (2p) and N+(•S) provide valuable ways in which to determine optically the concentrations of certain major species; measurement of emissions from the former yield the atomic oxygen concentration as well as the ion drift speed, and measurement of emissions from the latter provide the concentration of N2; N(2D) remains a major source of NO; and a recent finding is that vibrationally excited N2 + plays a vital role in converting N2 + into O +.

Detailed spectra of the extreme ultraviolet solar flux at the earth were provided by instruments on the Atmosphere Explorer satellites. These data have been used for aeronomical purposes in a large number of studies. An important parameter for such studies is the rate of production of various ions through the photoionization process. This parameter, known as an ionization frequency, is the integral over wavelength of the product of the solar flux and the cross section for the ionization of the particular constituent. Thus the determination of the ionization rate is dependent on a good knowledge of the solar EUV intensities for the solar period in question. Over the past few years the EUV solar spectra that have been developed for use by aeronomers as reference spectra for such photochemical and ionospheric studies have been improved. In this paper we report the results of a redetermination of the most important ionization frequencies using the revised solar fluxes. The impact is found to be more significant at solar maximum, amounting to a reduction of 12–21% in the ionization frequencies of the major terrestrial thermospheric constituents for solar minimum conditions and 21–33% for solar maximum conditions. The corrections are apart from the ongoing debate concerning the absolute intensity of EUV solar flux measurements for the solar cycle 21 maximum period.