Fig. 8
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The squared difference between variations observed by three NMs. (SOPO-SYO_NM)2, (SOPB-SYO_NM)2 and (SOPO-SOPB)2 are shown by black open circles, red open triangles and blue open squares, respectively. Differences became smaller after 2019, except in several short periods. Minor tick marks on the horizontal axis are indicated every 27 days.
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