Complexity signatures in the geomagnetic <i>H</i> component recorded by the Tromsø magnetometer (70° N, 19° E) over the last quarter of a century

Abstract

Abstract. Solar disturbances, depending on the orientation of the interplanetary magnetic field, typically result in perturbations of the geomagnetic field as observed by magnetometers on the ground. Here, the geomagnetic field's horizontal component, as measured by the ground-based observatorystandard magnetometer at Tromsø (70 • N, 19 • E), is examined for signatures of complexity. Twenty-five year-long 10 s resolution data sets are analysed for fluctuations with timescales of less than 1 day. Quantile-quantile plots are employed first, revealing that the fluctuations are better represented by Cauchy rather than Gaussian distributions. Thereafter, both spectral density and detrended fluctuation analysis methods are used to estimate values of the generalized Hurst exponent, α. The results are then compared with independent findings. Inspection and comparison of the spectral and detrended fluctuation analyses reveal that timescales between 1 h and 1 day are characterized by fractional Brownian motion with a generalized Hurst exponent of ∼ 1.4, whereas including timescales as short as 1 min suggests fractional Brownian motion with a generalized Hurst exponent of ∼ 1.6.

Spectral characteristics of high latitude raw 40 MHz cosmic noise signals

“…By using a different parameterization of the distribution width than half-maximum full-width, viz. 1/e of maximum, a wider Cauchy distribution is modelled that fits the data better (suggested by Hall, 2014b). Qualitatively the Cauchy model describes the distribution 10 considerably better than the Gaussian, but all the same, a heavy "shoulder" is evident for reduced values of cosmic noise.…”

“…One can envisage an auroral arc moving across the sky; although this will be in one position for a short time interval, it will be in the riometer antenna beam for much longer. The response of a ground-based magnetometer is similar as described by Hall (2014b), 20 although current systems causing perturbations in the geomagnetic field typically occur at higher altitude than ionisation modulating cosmic noise at 40 MHz. It can therefore be hypothesized that the scaling exponent for periods > 1 h is associated with absorption events, and, at that, caused by high-energy precipitation, since the riometer is within the auroral oval and receives at 40 MHz.…”

“…This is then subtracted from the original and the residual deemed largely stochastic. A corresponding method was employed by Hall (2014b). As will be seen, spectral and therefore frequencies in the spectrum, these spikes impose insignificant influence when determining the spectral slope.…”

“…One example is an analysis of Disturbance storm time (Dst) index by Balasis et al (2006), reporting scaling in the range (5 days-2 h) with α between ∼ 1.4 25 and ∼ 1.6. For the geomagnetic field in the auroral oval, Hall (2014b) of the order of minutes to hours, exhibiting 1/f scaling, an intra-day timescale with a spectral slope of ∼ 1.4, and finally a longer timescale regime also exhibiting 1/f scaling. For the first of these it is difficult to identify a specific underlying physical process; for the second, similarity with intra day variability in the geomagnetic field and with solar disturbances suggests solar origins and therefore cosmic noise absorption: 5 the last has similarities with slow fluctuations in independent determinations of cosmic ray intensity therefore confirming origins in cosmic ray sources.…”

Characterization of the South Atlantic Anomaly

“…The next step is to determine the power spectral density and its scaling with respect to frequency (Hall, 2014). The spectral analysis of a time series allows one to infer something regarding the characteristic time scales of the phenomena which give rise to the observed variations (Santis et al, 1997).…”