1.1. INTRODUCTION
Joule heating, which is the irreversible conversion of electromagnetic energy into heat through ohmic currents, is a significant source of energy for the highâlatitude thermosphere (Cole, 1962; Thayer, 2000; Lu et al., 2016). Unlike heating by solar ultraviolet and extreme ultraviolet radiation, Joule heating occurs over only a small fraction of the Earth, and can drive large vertical velocities that alter the thermospheric circulation, leading to local and global temperature increases and changes in the structure of thermospheric composition, temperature, and density (e.g., Taeusch et al., 1971; Mayr & Volland, 1972, 1973; Mayr et al., 1978; Volland, 1979; Roble et al., 1983; Rees & FullerâRowell, 1989; Rees, 1995; Liu & LĂŒhr, 2005; Sutton et al., 2005; Lei et al., 2010; Liu et al., 2010; Fedrizzi et al., 2012; FullerâRowell, 2013). Thermospheric responses to Joule heating during magnetic storms can be dramatic (e.g., Prölss, 1980, 1995; Rishbeth, 1991; FullerâRowell et al., 1994, 1997; Rees, 1995; Lu et al., 2016; Deng et al., 2018). In addition to temperature increases, which produce large density increases in the upper thermosphere, the upwelling in the highâlatitude region of heating induces a global circulation within several hours (Volland & Mayr, 1971; Mayr & Volland, 1973), accompanied by downwelling at lower latitudes. The circulation dampens the upperâthermosphere density response at high latitudes and spreads this response globally. The upwelling decreases the O/N2 ratio at high latitudes (Taeusch et al., 1971; Mayr & Volland, 1972; Lu et al., 2016). Rapid variations of the heating generate thermospheric gravity waves in the lower thermosphere that propagate globally into the upper thermosphere, causing oscillations of wind, temperature, composition, and density as well as largeâscale traveling ionospheric disturbances (e.g., Wright, 1960; Lu et al., 2016). The effects of Joule heating depend not only on its highly variable intensity and its distribution over the polar regions, but also on the altitude distribution of the heating. Effects observed in the upper thermosphere have a complex relation to the heating distribution, such that thermospheric density increases usually do not coincide with regions of maximum heat input, due not only to the presence of gravity waves, but also to the fact that circulation changes rapidly redistribute density (Johnson, 1960). Furthermore, temperature changes are coupled to composition changes, such that the temperature and the thermospheric O/N2 ratio tend to be inversely correlated in space. This is due to the tendency of the circulation to smooth out horizontal variations of the pressure scale height (Hays et al., 1973). This effect contributes to the fact that horizontal variations of density and composition during magnetic storms can be very different (e.g., Lei et al., 2010).
1.2. PHYSICS OF JOULE HEATING
The physics of thermospheric Joule heating involves collisional interactions among electrons, positive ions, and neutral molecules. These species have differential bulk motions owing to the presence of electric and magnetic fields, so that collisions result in frictional momentum exchange and heating (e.g., Brekke & Kamide, 1996; Thayer & Semeter, 2004; Zhu et al., 2005; Vasyliunas & Song, 2005; Strangeway, 2012). The sum of frictional heating of all species gives the total Joule heating. The frictional heating causes the species to have different temperatures, with the electron and ion temperatures exceeding the neutral temperature (e.g., St. Maurice & Hanson, 1982; Heelis & Coley, 1988; St. Maurice et al., 1999), and additional collisions transfer heat from hotter to cooler species. On timescales longer than...
