The role of metastable species in the thermosphere

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(¹S) 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(¹D), O(¹S), O+(²D), O+(²P), N(²D), N(²P), N+(¹S), N+(¹D), N2(A³Σu+), NO+(a³Σ), O2+(a4πu), and the vibrationally excited states of N2, O2, 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(¹D); N2(A³Σu+) 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+(¹S) 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+.