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| Titel |
The self-similar, non-linear evolution of rotating magnetic flux ropes |
| VerfasserIn |
C. J. Farrugia, V. A. Osherovich, L. F. Burlaga |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 13, no. 8 ; Nr. 13, no. 8, S.815-827 |
| Datensatznummer |
250011955
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| Publikation (Nr.) |
 copernicus.org/angeo-13-815-1995.pdf |
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| Zusammenfassung |
| We study, in the ideal MHD approximation, the
non-linear evolution of cylindrical magnetic flux tubes differentially rotating
about their symmetry axis. Our force balance consists of inertial terms, which
include the centrifugal force, the gradient of the axial magnetic pressure, the
magnetic pinch force and the gradient of the gas pressure. We employ the
"separable" class of self-similar magnetic fields, defined recently.
Taking the gas to be a polytrope, we reduce the problem to a single, ordinary
differential equation for the evolution function. In general, two regimes of
evolution are possible; expansion and oscillation. We investigate the specific
effect rotation has on these two modes of evolution. We focus on critical values
of the flux rope parameters and show that rotation can suppress the oscillatory
mode. We estimate the critical value of the angular velocity Ωcrit,
above which the magnetic flux rope always expands, regardless of the value of
the initial energy. Studying small-amplitude oscillations of the rope, we find
that torsional oscillations are superimposed on the rotation and that they have
a frequency equal to that of the radial oscillations. By setting the axial
component of the magnetic field to zero, we study small-amplitude oscillations
of a rigidly rotating pinch. We find that the frequency of oscillation
ω is inversely proportional to the angular velocity of rotation
Ω; the product ωΩbeing proportional to the
inverse square of the Alfvén time. The period of large-amplitude oscillations
of a rotating flux rope of low beta increases exponentially with the energy of
the equivalent 1D oscillator. With respect to large-amplitude oscillations of a
non-rotating flux rope, the only change brought about by rotation is to
introduce a multiplicative factor greater than unity, which further increases
the period. This multiplicative factor depends on the ratio of the azimuthal
speed to the Alfvén speed. Finally, considering interplanetary magnetic clouds
as cylindrical flux ropes, we inquire whether they rotate. We find that at 1 AU
only a minority do. We discuss data on two magnetic clouds where we interpret
the presence in each of vortical plasma motion about the symmetry axis as a sign
of rotation. Our estimates for the angular velocities suggest that the
parameters of the two magnetic clouds are below critical values. The two clouds
differ in many respects (such as age, bulk flow speed, size, handedness of the
magnetic field, etc.), and we find that their rotational parameters reflect some
of these differences, particularly the difference in age. In both clouds, a
rough estimate of the radial electric field in the rigidly rotating core,
calculated in a non-rotating frame, yields values of the order mV m–1. |
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