An essential starting point in understanding the dynamics of the magnetotail is the nature of the particle trajectories. It is this motion that ultimately determines the electric currents and subsequent magnetic fields. For years, this motion was calculated using approximate analytical techniques even though it was known that often times the approximations failed. Just over a decade ago, a more complete understanding of the "nonlinear dynamical" nature of particle motion was initiated by numerical experiments. Among the more significant results were numerical existence proofs of chaotic behavior and the discovery that the particle phase space is partitioned into three dynamically distinct regions: transient, stochastic, and regular. Although many investigators have suggested applications of this newly recognized behavior, the underlying "cause" of the chaos remains hotly debated.
Using a computer simulation of charged particle dynamics in the modified Harris magnetic field,
(a standard model to the magnetotail magnetic field), we have begun
an investigation into the nature and underlying causes of the chaos. In particular, we calculate the Lyapunov exponent, a Benettin and Strelcyn, in which the divergence of two numerical algorithms. First, we use the method of dimensional phase space. We then calculate the Lyapunov exponet by using the equations of deviation of the system:
where is the
deviation vector in the phase space and is Jacobian Matrix
of the equations of motion. Both calculations of the Lyapuvov exponent give nearly
identical results and behaviors. One should be careful in the interpretation of the results, since the Lyapunov exponent is defined as a time asymptotic quantity and we are dealing with a chaotic scattering system where the particles have a finite residence time. It is important to note, however, that we are able to see distinctly different characteristics of the Lyapunov exponent for each of our orbit types (transient, stochastic, and
regular.)
Introduction
Earth's Magnetosphere
Equations of Motion
reversal same as reversal
Poincare Surface of Section
The Three Disjoint Classes of Orbits
SOS as a Function of Energy
Fig.4.1. A sketch of trajectories in a three-dimensional state
space. Notice how two nearby trajectories, starting near the origin, can
continue to behave quite differently from each other yet remain boundec by
weaving in and out and over and under each other.
Hilborn: Chaos and Nolinear Dynamics
The parameter
is called the Lyapunov exponent.
Benettin And Strelcyn
Numerical experiments on the free motion of a point mass moving in
a plane convex region: Stochastic transition and entropy
Method of calculating the Lyapunov exponent utilizing the
divergence of two near by orbits.
Orbit
Lyapunov Exponent vs Time
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bn = 0.1
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Stochastic Orbits bn = 0.1
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Transient Orbits
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bn = 0.05
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bn = 0.3
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Stochastic Orbits
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- Tthe Lyapunov exponent is reasonable measure of "chaos" of the system.
- Different classes of orbits exhibit different Lyapunov exponent behaviors.
- Resonnace surfaces have higher average stochastic Lyapunov
exponents, as is expected because stochastic particles are trapped in
the system longer and so have more interations with the current
sheet.
- As the decreases the average
Lyapunov exponent also decreases, as is expected because when the system becomes completely integrable
(i.e. no chaos)
Further Work:
- Use double precision on the Bulirsch-Stoer integrator.
- Use a differnt integrator.
- Use other measures of chaos (i.e. Kolomagorov Entropy,
Topological pressure).
- Attempt to understand the origin of the chaos (i.e. stretching,
folding. separatrix crossings).
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