[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-P01] Fine structures of 4f_ce auroral roar emissions

*佐藤 由佳^1,2、門倉 昭³、小川 泰信³、熊本 篤志²、加藤 雄人²、小原 隆博² (1.日本工業大学、2.東北大学、3.国立極地研究所)

キーワード：オーロラ現象、高分解能観測、電波放射

The terrestrial auroral ionosphere is a natural source of electromagnetic waves in the MF/HF ranges (up to 6 MHz) as well as well-known intense VLF/LF emissions (auroral kilometric radiation and auroral hiss). In the MF/HF ranges, three types have been identified at ground level: auroral hiss, medium frequency burst (MFB), and auroral roar. Auroral roar is a relatively narrow band emission (δf/f < 0.1 ) near ionospheric cyclotron harmonics (nf_ce; n = 2, 3, 4, and 5). Previous studies revealed that 2f_ce auroral roar consists of various types of fine structures: multiple narrow band features drifting up and down in frequency in complicated patters (LaBelle et al., 1995; Shepherd et al., 1998) and ~ 10 Hz pulsations in the amplitude called “flickering auroral roar” (Hughes and LaBelle, 2001). However, no observational work has investigated fine structures of higher harmonics of auroral roar. In this study, we report on the first high-resolution measurement of 4f_ce auroral roar, using a ground-based passive waveform receiver installed in March 2015 at Kjell Henriksen Observatory (KHO) in Svalbard (latitude 78.15° N, longitude 16.04° E, 75.2° magnetic latitude). This receiver recorded waveform data digitalized at a rate of 12.5 M samples/s (Nyquist frequency 6.25 MHz) for a selected 40-min to 2-hour interval per day. In 2015 and 2016, the high-resolution measurement detected 22 events of 4f_ce auroral roar. All these events were detected in sunlit conditions, which are attributed to conversion of upper hybrid waves to O-mode electromagnetic waves, excited by auroral electrons under the matching condition of f_UH ~ 4f_ce. While 4f_ce auroral roar typically appeared weak and steady, ~ 3 Hz pulsations in amplitude with a short duration (< tens of seconds) were observed in 12 events. These pulsations might be attributed to ~3 Hz modulations in the distribution function of the auroral electrons at the 4f_ce auroral roar source region (~ 240 km altitude).