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Auroral hydrogen emissions: a historic survey

Abstract. 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.
