Ground-based observations of hiss-like emissions changed from chorus emissions and related pulsating aurora

Chorus and hiss-like emissions in the VLF band (several hundred Hz to 10 kHz) are the most common and intense whistler mode waves in the inner magnetosphere. Though frequency spectra of chorus waves having rising tone frequencies are different from those of hiss-like emissions, their wave amplitude and propagation characteristics are similar [1]. A recent simulation study shows that geomagnetic field gradient along a field line near the equatorial plane plays an important role in the generation of hiss-like emissions and chorus emissions [2]. When the geomagnetic field gradient varies to more gradual one, a threshold amplitude for triggering chorus emissions becomes smaller in the nonlinear wave growth theory. Then, multiple chorus emissions are frequently generated, and hiss-like emissions are produced by merging these chorus emissions in frequency spectra. We observed a high correlation between luminosity of a pulsating aurora (PA) and intensities of grouped chorus and hiss-like emissions on 7 February 2013 at Athabasca (L=4.3) in Canada. In this event, the periodic PA luminosity disappeared when the chorus emissions changed to hiss-like emissions. For this event, variations of the geomagnetic field gradient near the equator were estimated by using the Tsyganenko model [3]. The estimated geomagnetic field gradient became more gradual when the hiss-like emissions were observed. It is consistent with the nonlinear wave growth theory of the generation process of hiss-like emissions. Also, these observation results suggest a close relationship between periods of PA and hiss-like emissions. In this presentation, we will discuss the importance of the effect of geomagnetic field gradient on periods of PA and hiss-like emissions.