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

摘要

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