AURORA on Earth

Photo Credit: Jan Curtis, UAF, GI
Proton Precipitation
into the Atmosphere


Introduction to proton precipitation

AURORA on Earth

Photo Credit: Jan Curtis, UAF, GI

Energetic protons, precipitating into the atmosphere, interact with the ambient neutrals, leading to excitation, ionization, elastic scattering, and, for MeV protons, dissociation. In addition, a proton can capture an electron, producing an energetic H atom, which has enough energy to interact, in turn, with the ambient neutrals. This H atom can also get stripped of its electron, becoming a proton again. Because of these charge-changing reactions, the incident proton beam penetrating the atmosphere becomes a mixture of protons and H atoms.  In addition, electrons are produced inside the proton beam through ionization and stripping reactions. These electrons, also called proto-electrons, can be energetic enough to excite and ionize atmospheric gases.  All these interactions lead to electron and ion production, heating, and excitation, and to the spectacular auroral emissions. Excitation can be produced directly by the energetic particles or indirectly via chemistry. Protons in the keV energy range deposit most of their energy in the E region (100-160 km), whereas MeV proton energy deposition will occur at lower altitudes, typically in the D region and below.

H emissions resulting from excited H atoms inside the proton beam are a unique signature of proton precipitation.  In the region where precipitating particles deposit their energy, the ambient H atom density is too low to produce a significant amount of auroral emissions from excitation by energetic particle precipitation. Since the hydrogen atoms retain the energy of the protons on charge exchange, the emissions of excited H atoms are Doppler-broadened and -shifted. Observed from ground along the magnetic zenith, the H emission profile is blue-shifted, because most of the energetic H atoms are moving downward.  Unlike electron aurora, proton aurora is diffuse owing to the contribution of H atoms whose path, independent of the magnetic field configuration, produces a spreading of the incident proton beam [see, e.g., Eather, Phys. Rev., 5, p.207-285, 1967].

Proton precipitation is observed at different locations around the Earth.  At high latitudes, keV protons contribute to the auroral ovals, precipitating from the plasma sheet, the magnetosheath, the low-latitude boundary layer, and through the cusp [see, e.g., Hardy et al., J. Geophys. Res., 94, p.370-392, 1989]. MeV protons from solar particle events [Gosling, J. Geophys. Res., 98, p.18,937-18,949, 1993] induce the polar cap absorption (PCA) events. The additional E and D region (20-120km) ionization caused by proton precipitation results in the enhancement of absorption of the background high-frequency cosmic radio waves across the entire polar cap [see, e.g., Bailey, Planet. Space Sci., 12, p.495-541, 1964]. Finally, energetic neutral atoms (ENA) and protons originating in the ring current produce low-latitude and
midlatitude aurorae [see, e.g., Rassoul et al., J. Geophys. Res., 98, p.7695- 7709, 1993].


Chamberlain, J. W., Physics of the Aurora and Airglow,
Classics in Geophysics, American Geophysical Union, 1995.

Eather, R. H., Auroral proton precipitation and hydrogen emissions,
Phys. Rev., 5, p.207-285, 1967.

Galand M., Introduction to the special section: proton precipitation into the atmosphere, J. Geophys. Res., December 2000. 

Rees, M. H., Physics and Chemistry of the Upper Atmosphere,
Cambridge Atmos. Space Sci. Ser., Cambridge Univ. Press, New York, 1989.

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