Pulsating proton aurora caused by rising tone Pc1 waves

Abstract

We found rising tone emissions with a dispersion of ∼1 Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over ∼30 min from 0250 UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7°N, 246.7°E, magnetic latitude 61.7°N). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all‐sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, ∼10% variations in intensity, and fine structures of 3° in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one‐to‐one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of ∼1 Hz in Pc1 rising tones. We suggest that EMIC‐triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time.

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes

“…The spatial distribution of the high correlation (over 0.75) is concentrated around a proton spot, as shown in Figures Figure 3b also shows the amplitude envelope of the Pc1 pulsations (red curve) and the BG3 intensity (blue curve) at the proton spot, which have a time resolution of 1/8 s. The fine structures of the Pc1 wave packets are seen in accordance with successive discrete elements. The time resolution of 1/8 s for a PPA is the best to our knowledge and 24 times faster than the previous study by Nomura et al [2016]. Thus, the observed amplitude modulation of the Pc1 wave packet would be equivalent to subpacket structures of EMIC waves.…”

“…The EMIC waves may play a crucial role in the loss of relativistic (MeV energy) electrons from the radiation belts via wave-particle interactions [e.g., Miyoshi et al, 2008;Rodger et al, 2008;Usanova et al, 2014]. Isolated proton aurora and related Pc1 pulsations are simultaneously observed on the ground at subauroral latitudes [Sakaguchi et al, 2007Miyoshi et al, 2008;Nomura et al, 2012Nomura et al, , 2016. A portion of the scattered energetic ions falls into the upper atmospheric loss cone, and the ion precipitation can then be observed as a proton aurora [Frey et al, 2004;Yahnin et al, 2007].…”

“…Also, EMIC waves are capable of scattering magnetospheric energetic ions (keV to 100 keV) [Erlandson and Ukhorskiy, 2001;Yahnina et al, 2003;Usanova et al, 2010]. Nomura et al [2016] showed that the luminosity of pulsating proton aurora (PPA) oscillates with the same period between successive elements as the Pc1 pulsations, i.e., several tens of seconds. Isolated proton aurora and related Pc1 pulsations are simultaneously observed on the ground at subauroral latitudes [Sakaguchi et al, 2007Miyoshi et al, 2008;Nomura et al, 2012Nomura et al, , 2016.…”

Ground-based instruments of the PWING project to investigate dynamics of the inner magnetosphere at subauroral latitudes as a part of the ERG-ground coordinated observation network

“…As shown by Jun et al (2014Jun et al ( , 2016, comparison of Pc1 pearl structures (amplitude modulation) at different stations may help understanding the generation mechanisms of the pearl structure through beating of different waves during duct propagation. Comparison with all-sky airglow/aurora imager data gives us an interesting opportunity to monitor interaction between EMIC waves and ring-current protons/relativistic electrons (e.g., Sakaguchi et al 2007Sakaguchi et al , 2008Sakaguchi et al , 2012Miyoshi et al 2008;Nomura et al 2011Nomura et al , 2012Nomura et al , 2016Ozaki et al 2016). …”

The ARASE (ERG) magnetic field investigation

“…Nomura et al (2012Nomura et al ( , 2016 reported proton aurora events that were supposedly caused by EMIC wave scattering. Sakaguchi et al (2013) showed an EMIC rising tone event observed by Akebono in the deep inner magnetosphere and suggested the precipitation of relativistic electrons via EMIC waves.…”