Spectral and Causality Analysis of Fluctuations in a 25 Year Record of Schumann Resonances.

Abstract:

We report here on the spectral and causality analysis of the 25 year record of Schumann resonance (SR) signals measured at West Greenwich Rhode Island, U.S.A. The SR signals are the values of the fit-parameters of the first six modes of an eight-mode, three-parameters per mode, Lorentzian-line-shape model spanning the frequency range 3 Hz - 56 Hz. The three fit-parameters for each mode of the model are the (1) peak-center frequency, (2) peak-quality factor, and (3) peak intensity.

Using the LIS/OTD derived HRAC total lightning data set, the uniform cavity model, and the normal mode equations, approximately 50% of the variance in the Rhode Island Schumann data can be accounted for. The sunspot-cycle accounts for approximately 30% of the Of the remaining 50% variance.

We then examined factors that may contribute to the residual variations in the SR signals. To do this, we chose 60 data sets that ranged from direct measures of solar activity to terrestrial temperature anomalies. Two different tests were performed to assess the magnitude of the various putative influences upon the residual SR time series. The first test is the Granger causality test, and the second is a spectral comparison test.

The results indicate that direct measures of solar activity (e.g. sunspot number and area) most strongly Granger-cause peak-center frequency and peak-quality factor (median R² = 0.50) and less so the peak intensity (median R² = 0.02). Terrestrial temperature signals (e.g. Ocean temperature anomalies) influence peak intensity (median R² = 0.15) but not peak-center frequency nor peak-quality factor (median R² = 0.01). The variable most highly correlated is the long wave 10 cm radio emissions from the Sun; it is most highly correlated with (peak-center frequency) variations and not correlated with the Intensity variations.

We examined the spectral characteristics of the SR signals and the CDS. Both the ``Frequency'' and ``Quality-factor" parameters display strong peaks at 180 day, 365 day, and 11 years and a small peak at 27 days. The peak at 27 days could be due to the Space weather variables and/or the Direct Solar variables as both show peaks in the region of 27 days.

From these results, we can direct our modeling of the Schumann resonance signals to those variables showing the most influence upon the SR signals.