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I just exploring the sequential analysis with ARIMA (2-month data, period = 15 minutes, lags=360)
My suggestion: As ACF is a measure of the correlation between the timeseries with a lagged version of itself, so it seems like each observation is correlated to its adjacent observations. Also I can see the downwards trend but that indicated the correlation decreasing after certain period.
Any help and thoughts are appriciated Data available here