Intensity and Variability of Geomagnetic Time Derivatives

Abstract:

Time derivatives of the geomagnetic field are studied for more than a decade of observations at more than a dozen sites in northern Canada. In the auroral zone the derivative magnitude observed by 5-second fluxgate magnetometers often has a lognormal distribution. Parameter estimates corresponding to intensity (log-mean) and variability (log-variance) are nearly independent and have very different statistical properties. Variability is essentially a random variable, while intensity autocorrelation times are on the order of tens of minutes. Observed intensities are highly correlated with AE, and increase with solar wind speed and the magnitude of $B_z<0$. Both variability and intensity have local-time maxima before and after midnight, but with different patterns that combine to produce a larger post-midnight peak. Post-midnight variability is almost completely determined by latitude, with largest values at subauroral sites and smallest values in the polar cap. Intensity depends on latitude, but also has a site-specific element which may be due to local conductivity.