Auroral hydrogen emissions: a historic survey
Auroral spectroscopy provided the first tool for remotely sensing the compositions and dynamics of the high-latitude ionosphere. In 1885, Balmer discovered that the visible hydrogen spectrum consists of a series of discrete lines whose wavelengths follow a simple mathematical pattern, which ranks among the first steps toward developing this tool. On 18 October 1939 Lars Vegard discovered the Hα (656.3 nm) and Hβ (486.1 nm) spectral lines of Balmer series emissions, emanating from a diffuse structure, located equatorward of the auroral zone. Intense, first positive bands of N2+ nearly covered the Hα emissions. With more advanced instrumentation after World War II, auroral spectroscopists Vegard, Gartlein and Meinel investigated other characteristics of the auroral hydrogen emissions. The first three lines of the Balmer series, including Hγ at 410 nm, were identified in ground-based measurements prior to the space age. Based on satellite observations, the Balmer lines Hδ and Hε at 410.13 and 396.97 nm, respectively, as well as extreme ultraviolet (EUV) Lyman α (121.6 nm) hydrogen emissions, were also detected. Doppler blue shifts in hydrogen emissions, established in the 1940s, indicated that emitting particles had energies well into the kiloelectron volt range, corresponding to velocities >1000 km s−1. Systematic spatial separations between the locations of electron- and proton-generated aurorae were also established. These observations in turn, suggested that protons, ultimately of solar origin, precipitate into the topside ionosphere, where they undergo charge-exchange events with atmospheric neutrals. Newly generated hydrogen atoms were left in excited states and emitted the observed Balmer radiation. Sounding rocket data showed that most of the hydrogen radiation came from altitudes between 105 and 120 km. Space-age data from satellite-borne sensors made two significant contributions: (1) energetic particle detectors demonstrated the existence of regions in the magnetosphere, conjugate to nightside proton aurora, where conditions for breaking the first adiabatic invariants of kiloelectron volt protons prevail, allowing them to precipitate through filled loss cones. (2) EUV imagers showed that dayside hydrogen emissions appear in response to changes in solar wind dynamic pressure or the polarity of the north–south component of the interplanetary magnetic field.