|
|
 |
|
|
Ann. Geophys., 26, 941-953, 2008
www.ann-geophys.net/26/941/2008/
© European Geosciences Union 2008
Comparison of chaotic aspects of magnetosphere under various physical conditions using AE index time series
K. Unnikrishnan
School of Pure and Applied Physics, Mahatma Gandhi University, Priyadarshini Hills, Kottayam – 686 560, Kerala, India
also at: Department of Physics, N. S. S. Hindu College, Changanacherry – 686 102, Kerala, India
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.
Full Article in PDF (916 KB) |
|
|
