Characteristics of hiss‐like and discrete whistler‐mode emissions

Abstract

The characteristics of hiss‐like and discrete (rising and falling tones) whistler‐mode waves in the lower‐band wave frequency range (0.1–0.5 of equatorial electron gyrofrequency) are investigated using waveform data from near‐equatorially orbiting multiple THEMIS spacecraft outside the plasmasphere. Statistical results show that wave polarization properties of hiss‐like emissions are similar to rising tones, but are significantly different from falling tones. The magnetic wave amplitudes of hiss‐like bands and rising tones are generally larger than those of falling tones. Wave normal angles of broadband hiss‐like emissions and rising tones tend to be quasi field‐aligned, whereas falling tones are very oblique. Importantly, discrete emissions including rising and falling tones are predominantly observed in the region of low f pe / f ce (the ratio of plasma frequency to electron gyrofrequency), whereas hiss‐like bands alone preferentially occur in the region of high f pe / f ce . These important features of hiss‐like and discrete whistler‐mode emissions should be considered when evaluating their interactions with energetic electrons.