Optical evidence for mid-latitude charged particle precipitation

Measurements of the N 2 + λ427.8 nm emission from the atmosphere at a mid-latitude station show intensities correlated with Kp, typical values being about 1R at Kp = 3 to 4. It is concluded that charged particle precipitation is a major cause of the emission. Assuming the particles to be electrons of typical auroral energies they make a major contribution to the maintanance of the night-time E-region ionization. On one night the rotational temperature inferred from the N 2 + band shape was approximately 170 K, implying particle penetration to 80–90 km. Attempts to record H β emission from the atmosphere put an upper limit of 0.005 R on the intensity. It is concluded that proton precipitation is not a significant contributor to the E-region ionization.