Removing Diurnal Signals and Longer Term Trends From Electron Flux and ULF Correlations: A Comparison of Spectral Subtraction, Simple Differencing, and ARIMAX Models

Simultaneously cycling space weather parameters may show high correlations even if there is no immediate relationship between them. We successfully remove diurnal cycles using spectral subtraction, and both longer (e.g., the 27 days solar cycle) with a difference transformation. Other methods of cycle removal (daily averaging, moving averages [MAs], simpler subtraction regression) are less successful at removing cycles. apply (a finite impulse response equiripple bandstop filter) to hourly electron flux (Los Alamos National Laboratory satellite data) ground-based ULF index 24 hr noise signal. This results in smoother time series appropriate for short-term (approximately < 1 week) correlation observational studies. However, not such as 11 yr A differencing transformation (yt – yt−24) removes only signal but also cycle, autocorrelation, trends. low over long periods (maximum 0.1). Correlations wind velocity (differenced yt yt−1) lower than previously reported (≤0.1). An autoregressive, MA transfer function model (ARIMAX) shows that significant cumulative effects on activity periods, waves seen shorter spans more homogeneous geomagnetic levels.