Intermittency of storms and substorms: is it related to the critical behaviour?

Abstract

Abstract. Intermittency is one of the possible means of quantifying dynamics of fractal processes. In this paper, the analysis of the intermittency of magnetospheric storms and substorms is presented. The analysis allows for a classification of the processes in terms of the power-law scaling of the magnitude of deviations of the index values from the values at quiet times (normal state), and the relative timings of occurrences of such deviations. These are expressed in terms of the co-dimension and the Fano factor. The relationship between the two is related to the nature of the processes behind the observed storm and substorm dynamics. The results suggest that there is a similarity between the two, and therefore it is possible that there are common dynamical processes behind the storms and substorms. In particular, it appears that both of them behave consistently with what would be expected for critical systems, which is consistent with the conclusions of several previous works.

Intermittency and multifractional Brownian character of geomagnetic time series

“…Figures 10 and 11 show the results of the local Hurst exponent H loc (t), obtained using the DMA technique on a moving window of 801 points, in the case of the two selected time intervals. We notice that the average value H loc (t) of local Hurst exponents is always higher than 0. , suggesting that the mean character of geomagnetic time series is well in agreement with that of a persistent mBm, as already observed in geomagnetic index studies (e.g., Dobias and Wanliss, 2009), and with what is found in the case of a near-criticality dynamics (Woodard et al, 2007). However, we emphasize that the structure of the fluctuation field is sometimes not in agreement with a simple mBm being the value of the local Hurst exponent higher than 1.…”

“…The origin of this intermittent and multifractal character has been interpreted as a direct consequence of dynamical changes and turbulent dynamics. In this framework, Dobias and Wanliss (2009) recently noticed that the intermittent character observed during magnetic substorms and storms displays similar features when it is investigated in terms of fractal processes. This observation suggests that during magnetic storms and substorms there are common dynamical processes at the basis of the critical behavior of the Earth's magnetospheric dynamics in response to solar wind changes (Dobias and Wanliss, 2009).…”

“…It has been found that the fractal character of several geomagnetic time series, for example those relative to the geomagnetic indices AE, Dst and SYM-H, changes during magnetic storms and substorms (Uritsky and Pudovkin, 1998;Wanliss, 2005;Balasis et al, 2006;Wanliss and Dobias, 2007;Dobias and Wanliss, 2009). In particular, it has been noticed that the self-similarity properties of these time series change when an intense magnetospheric/geomagnetic activity event approaches.…”

“…Its complex dynamics, which results from the superposition of the internal dynamics and the external driving due to the changes of solar wind conditions, has been widely investigated during the past two decades. Several studies have been devoted to examine the occurrence of chaos, turbulence and criticality during geomagnetic storms and substorms, which are the most important manifestations of the magnetospheric activity (e.g., Klimas et al, 1996Klimas et al, , 2000Consolini and Chang, 2001;Dobias and Wanliss, 2009). These studies have clearly evidenced that the magnetospheric evolution is characterized by a near-criticality dynamics, which manifests both in the scale-invariant nature of magnetotail relaxation events and fractal properties of geomagnetic time series.…”

“…We notice that, the average value H loc (t) of local Hurst exponents is always higher than 0. indices studies (e.g. Dobias and Wanliss, 2009), and with what is found in the case of a nearcriticality dynamics (Woodard et al, 2007). However, we emphasize that the structure of the fluctuation field is sometimes not in agreement with a simple mBm being the value of the local Hurst As the third step in the analysis of the mBm character of geomagnetic time series, we com- Clearly, this point requires a different and more accurate analysis, which is outside the aim of this work.…”