Empirical Model of the Lowest Cutoff Altitude of Global Hiss Near Magnetic Equator

Van Allen Probe observations indicate that whistler-mode hiss waves below 1 kHz are absorbed at low altitudes near magnetic equator. The lowest cutoff frequency of equatorial hiss is close to the gyrofrequency of hydrogen ions. The lowest cutoff altitude of global hiss is extracted when its occurrence rate is equal to 0.005 on the plane of altitude (L in R_E) and magnetic local time (MLT). By fitting the lowest cutoff altitude of global hiss, we constructed the empirical model of the lowest cutoff altitude of equatorial hiss under geomagnetically quiet (AE < 200 nT) and active (AE ≥ 200 nT) conditions. The enhanced substorm activities reduce the lowest cutoff altitude of hiss waves on the dawnside (MLT ∼ 1–5 hr), whereas the lowest cutoff altitude of the dayside hiss is nearly fixed at ∼1.1 R_E (MLT ∼ 6–20 hr). From the dayside to the nightside (MLT ∼ 0–6 hr and 20–24 hr), the lowest cutoff altitude of equatorial hiss raises gradually from 1.1 R_E to 1.4 R_E.