Right‐hand polarized 4fce auroral roar emissions: 1. Observations

A receiving system installed at Sondrestrom, Greenland, was used to monitor all detectable auroral radio emissions at the fourth harmonic of the electron cyclotron frequency (called 4fce roar emissions) between May 2015 and March 2016. Of 88 events detected, 86 occurred during daylit conditions and were left‐hand polarized. Two occurred during darkness conditions and were right‐hand polarized. The left‐hand and right‐hand polarized events had entirely different frequency distributions. One of the right‐hand polarized 4fce emissions occurred at the same time as and at exactly twice the frequency of a second harmonic emission (2fce roar). The occurrence rate of 4fce emissions during premidnight hours under daylit conditions at Sondrestrom is 5%, comparable to previously reported occurrence rates of 2fce roar in darkness conditions at optimum latitudes of occurrence, but the occurrence rate of 4fce emissions during dark conditions is much lower, suggesting that if the right‐hand polarized events arise from coalescence of 2fce waves, only for a small fraction of nighttime 2fce roar emissions does such a process yield 4fce emissions detectable at ground level. Key Points Systematic study of 4fce emissions shows daytime events are distinct from nighttime events Daytime 4fce emissions have higher occurrence rate, left polarization, and higher frequency: consistent with linear conversion of upper hybrids Nighttime 4fce emissions are rarer, right‐polarized, and have lower frequency: consistent with nonlinear coalescence of 2fce upper hybrids