Journal Article Quasi-periodic rapid motion of pulsating auroras

10 ( 3 )  , pp.183 - 191 , 2016-09
We report rapid motion of pulsating auroras associated with so called ​3 ± 1 Hz modulations embedded in the main pulsations. During the pulsation ON phase, repetitive expansions are often observed around the edges of pulsating patches. Some events show a few detached expansions traveling away from the main deformed pulsating patch. Approximately 80% of all expansion speeds were found to be less than 70 km s−1 at ionospheric altitudes, which is less than the projected Alfvén speed from the magnetospheric equator to the ionosphere. The rapid motions with speeds of tens of km s−1 are unlikely to be explained by obliquely propagating chorus elements, which are known to cause the 3 ± 1 Hz modulation, because the perpendicular speed of the oblique chorus waves is higher than the Alfvén speed. We discuss the slow-mode Alfvén wave as a candidate modulation source to generate the rapid motions. A few non-repetitive expansion events with a speed of more than 150 km s−1 also appear at the onset of the ON phase. These non-repetitive expanding motions are characterized by a long displacement compared to the repetitive expanding motions. The differences in the expansion speeds indicate different formation mechanisms of the patch motions.
秒単位で発光強度が変化する脈動オーロラには、数Hzの強度変化が伴うことが知られている。高速カメラを用いることで、その数Hzの強度変化に対応したオーロラ形状の高速変化を多数確認することができた。本研究では、その空間変化のスピードは電離圏高度で70 km s−1以下となる例が大半であり、典型的な磁気赤道面でのアルベンスピードよりも遅いという事を明らかにした。より高速な空間変化が期待されるコーラス波動との対応では説明できないため、スローモードのアルベン波が原因である可能性を新たに提唱した。また、電離圏高度で150 km s−1以上の高速変化も発見されたが、単発的で比較的長距離移動する特徴を持っており、数10 km s−1の変化とは異なる形成メカニズムを持つと考えられる。

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