Fast electrons in the night sky

Atoms and molecules in Earth’s upper atmosphere are ionized by UV photons from the Sun. The layer of plasma that results—the ionosphere—has a temperature of about 2000 K, equivalent to a thermal energy kT of around 0.2 eV. By contrast, the photoelectrons that are kicked out during ionization have energies of around 10 eV. When those fast electrons whiz through ionospheric plasma, they excite waves. Thanks to the waves’ motion, a radar beam bounced off the ionosphere acquires frequency-shifted side lines on its return. Such “plasma lines” are strongest when the Sun is shining. Indeed, it was a surprise when in 1982 Herbert Carlson of Utah State University detected weak plasma lines at night in the skies above Arecibo Observatory. Now Carlson has returned to Arecibo with the aim not just of detecting and characterizing nighttime plasma lines but also of determining their origin. Because Earth’s magnetic field lines funnel fast electrons, Carlson and his collaborators had to ensure that they made their observations when the other, Southern Hemisphere end of the field line that arcs over Arecibo was also in darkness. From their observations, conducted in February of this year, the researchers discovered that nighttime plasma lines are frequent and variable. Although the source of the fast electrons that engender the lines remains unclear, one strong possibility is leakage from Earth’s radiation belts. (H. C. Carlson et al., Geophys. Res. Lett., in press.)