Physics – Geophysics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm51a1320t&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SM51A-1320
Physics
Geophysics
[1914] Informatics / Data Mining, [2704] Magnetospheric Physics / Auroral Phenomena, [2790] Magnetospheric Physics / Substorms, [3270] Mathematical Geophysics / Time Series Analysis
Scientific paper
Until now, several substorm studies have discussed that about substorm precursors, which are observed in the auroral region and preceded to onset of substorm expansion phase by 1-3 minutes. Kepko et al., [2004] indicated that high-latitude magnetogram shows a very small negative deflection in the H-component before auroral arc brightening. Morioka et al. [2008] reported the dH/dt component from a search-coil magnetometer at ground shows that a few minutes prior to high-altitude AKR breakup, the quasi-DC component begins a negative exclusion that is nearly synchronized with the start of the gradual enhancement of the low-altitude AKR. These observations suggest that the precursor signature on the H-comp. of geomagnetic filed observed in the auroral region reflects the gradual enhancement of parallel electric filed before expansion onset and it plays an essential role to drive a subsequent enhancement of the upward FAC. Therefore, it is important to elucidate spatio-temporal evolutions of the substorm precursors to understand the onset mechanism associated with the rapid development of the M-I coupling via FAC. In most cases, it is difficult to determine the accurate timing because substorm precursor has a gradual onset. In this paper, we will present a new algorithm based on Singular Spectrum Analysis (SSA), described in the next paragraph, for determining the onset times of substorm precursors and expansion phase. SSA has been developed and applied in the field of geophysics. Ide and Inoue [2005] demonstrated the effectiveness of a SSA based change-point detection method, which is called Singular Spectrum Transformation (SST), to detect a structure change of various types of time series. Since this approach is completely data adaptive, it is probably a powerful method for our aim: to detect a small and irregular perturbation such as substorm precursors on the geomagnetic data observed in the auroral region. We have applied SST for a substorm event occurred on January 8, 1997 around 14:35UT. We used ground-magnetometer data from the Circum-Pan Pacific Magnetometer Network (CPMN) [Yumoto et al., 2001]. As a result, two clear change-points were detected from H-component of KTN data around 14:35:30UT and 14:37:30UT. These two change-points remain their essential structure over a wide range of window lengths, which should be pre-adjusted. By comparison with auroral global image provided by Polar/UVI and AKR wave spectrograms provided by Polar/PWI electric field observations, it is confirmed that the first change-point appeared almost synchronized with an initial brightening of aurora and an enhancement of low-altitude AKR. In addition, it is confirmed that the second change-point appeared almost synchronized with the auroral breakup and an enhancement of high-altitude AKR.
Higuchi Takeo
Morioka Akira
Nakamura Kazuo
Tokunaga Takashi
Uozumi Teiji
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