[P-EM15] Dynamics of Magnetosphere and Ionosphere

[PEM15-10] Energy of pulsating auroral electrons dependent on intensities of the internal modulation

*Kiyoka Murase^1,2, Takanori Nishiyama¹, Mizuki Fukizawa¹, Ryoichi Fujii¹, Masaki Tsutsumi¹, Kaoru Sato³, Ryuho Kataoka¹ (1.National Institute of Polar Research, 2.Kitami Institute of Technology, 3.The University of Tokyo)

Keywords:Pulsating aurora, Internal modulation

Pulsating aurora is a major type of diffuse aurora, characterized by quasi-periodic pulsations in brightness ranging from a few seconds to several tens of seconds. They sometimes coexist with shorter-period variations of a few Hz, called internal modulations. The internal modulation is caused by repetitive electron precipitation, which is attributed to interactions with discrete elements of chorus waves generated at the magnetic equator. As the waves propagate along the field line to higher latitudes, they interact and scatter electrons with a wide energy range from keV to 100s keV. The elements of auroral intensity modulation are expected to be more clearly separated by the precipitation of higher energy (>100s keV) electrons since energetic electrons are scattered only at high latitudes, making the time dispersion of precipitation shorter compared to lower energy electrons. We then proposed a hypothesis that the modulations are intensified when the energy spectrum of precipitating electrons becomes harder.
This study investigates how the intensity of internal modulation depends on the energy spectra of precipitating electrons. We found 86 events of pulsating aurora with clear internal modulations from the 3-year dataset of 10-Hz sampling images obtained by the High-speed Auroral Imaging system at Syowa Station. We started with an event on May 28, 2017, where the modulation intensity was the most distinct, with enhanced atmospheric ionization caused by electron precipitation as observed by the PANSY radar and imaging riometer. In that event, the modulation intensity showed a decreasing trend with spectral softening inferred from the rising ionization altitude and decreasing CNA intensity. This is consistent with the hypothesis that the energy spectrum of precipitating electrons controls the intensity of the internal modulation. In this presentation, we will discuss, with other examples, the possibility that the intensity of the internal modulation can be a proxy for the time variation of the electron energy.