Comparison of chaotic aspects of magnetosphere under various physical conditions using AE index time series

K. Unnikrishnan

doi:10.5194/angeo-26-941-2008

Abstract

Abstract. The deterministic chaotic behaviour of magnetosphere was analyzed, using AE index time series. The significant chaotic quantifiers like, Lyapunov exponent, spatio-temporal entropy and nonlinear prediction error for AE index time series under various physical conditions were estimated and compared. During high solar activity (1991), the values of Lyapunov exponent for AE index time series representing quiet conditions (yearly mean = 0.5±0.1 min−1) have no significant difference from those values for corresponding storm conditions (yearly mean = 0.5±0.17 min−1). This implies that, for the cases considered here, geomagnetic storms may not be an additional source to increase or decrease the deterministic chaotic aspects of magnetosphere, especially during high solar activity. During solar minimum period (1994), the seasonal mean value of Lyapunov exponent for AE index time series belong to quiet periods in winter (0.7±0.11 min−1) is higher compared to corresponding value of storm periods in winter (0.36±0.09 min−1). This may be due to the fact that, stochastic part, which is Dst dependent could be more prominent during storms, thereby increasing fluctuations/stochasticity and reducing determinism in AE index time series during storms. It is observed that, during low solar active period (1994), the seasonal mean value of entropy for time series representing storm periods of equinox is greater than that for quiet periods. However, significant difference is not observed between storm and quiet time values of entropy during high solar activity (1991), which is also true for nonlinear prediction error for both low and high solar activities. In the case of both high and low solar activities, the higher standard deviations of yearly mean Lyapunov exponent values for AE index time series for storm periods compared to those for quiet periods might be due to the strong interplay between stochasticity and determinism during storms. It is inferred that, the external driving forces, mainly due to solar wind, make the solar-magnetosphere-ionosphere coupling more complex, which generates many active degrees of freedom with various levels of coupling among them, under various physical conditions. Hence, the superposition of a large number of active degrees of freedom can modify the stability/instability conditions of magnetosphere.

Highlights

The Earth’s magnetosphere is mainly affected by solar wind and interplanetary magnetic field, and it responds to external drivers in a highly organized and complex way (Klimas et al, 1996)
Studies have appeared on the role that chaos, turbulence and near-criticality dynamics might play in the magnetospheric dynamics (Baker et al, 1990; Roberts et al, 1991; Vassiliadis et al, 1990)
A time series, Sn is the sequence of scalar measurements of some quantity, which depends on the current state of a system, taken at multiples of a fixed sampling time ( t), and Fig. 1a represents the typical time series of auroral electrojet index (AE) index

Summary

Introduction

The Earth’s magnetosphere is mainly affected by solar wind and interplanetary magnetic field, and it responds to external drivers in a highly organized and complex way (Klimas et al, 1996). This complex behaviour is due to a nonlinear dynamics related to the energy storage, transport and release in the geomagnetic tail regions. A common feature of systems displaying complexity is that all these systems are generally made of a huge number of interconnected and cross-coupled parts The investigation of such systems allowed the introduction of several

Results

Discussion

Conclusion