Statistical Study of the Properties of Magnetosheath Lion Roars

Lion roars are narrowband whistler wave emissions that have been observed in several environments, such as planetary magnetosheaths, the Earth’s magnetosphere, the solar wind, downstream of interplanetary shocks, and the cusp region. We present measurements of more than 30,000 such emissions observed by the Magnetospheric Multiscale spacecraft with high-cadence (8,192 samples/s) search coil magnetometer data. A semiautomatic algorithm was used to identify the emissions, and an adaptive interval algorithm in conjunction with minimum variance analysis was used to determine their wave vector. The properties of the waves are determined in both the spacecraft and plasma rest frame. The mean wave normal angle, with respect to the background magnetic field (B(0)), plasma bulk flow velocity (V(b)), and the coplanarity plane (V(b) × B(0)) are 23°, 56°, and 0°, respectively. The average peak frequencies were ~31% of the electron gyrofrequency (ω(ce)) observed in the spacecraft frame and ~18% of ω(ce) in the plasma rest frame. In the spacecraft frame, ~99% of the emissions had a frequency < ω(ce), while 98% had a peak frequency < 0.72 ω(ce) in the plasma rest frame. None of the waves had frequencies lower than the lower hybrid frequency, ω. From the probability density function of the electron plasma β(e), the ratio between the electron thermal and magnetic pressure, ~99.6% of the waves were observed with β(e) < 4 with a large narrow peak at 0.07 and two smaller, but wider, peaks at 1.26 and 2.28, while the average value was ~1.25.