299 research outputs found
Neural network prediction of geomagnetic activity: a method using local H\"{o}lder exponents
Local scaling and singularity properties of solar wind and geomagnetic time
series were analysed using H\"{o}lder exponents . It was shown that in
analysed cases due to multifractality of fluctuations changes from
point to point. We argued there exists a peculiar interplay between regularity
/ irregularity and amplitude characteristics of fluctuations which could be
exploited for improvement of predictions of geomagnetic activity. To this end
layered backpropagation artificial neural network model with feedback
connection was used for the study of the solar wind - magnetosphere coupling
and prediction of geomagnetic index. The solar wind input was taken
from principal component analysis of interplanetary magnetic field, proton
density and bulk velocity. Superior network performance was achieved in cases
when the information on local H\"{o}lder exponents was added to the input
layer.Comment: 17 pages, 7 figure
Scaling and singularity characteristics of solar wind and magnetospheric fluctuations
Preliminary results are presented which suggest that scaling and singularity
characteristics of solar wind and ground based magnetic fluctuations appear to
be a significant component in the solar wind - magnetosphere interaction
processes. Of key importance is the intermittence of the "magnetic turbulence"
as seen in ground based and solar wind magnetic data. The methods used in this
paper (estimation of flatness and multifractal spectra) are commonly used in
the studies of fluid or MHD turbulence. The results show that single
observatory characteristics of magnetic fluctuations are different from those
of the multi-observatory AE-index. In both data sets, however, the influence of
the solar wind fluctuations is recognizable. The correlation between the
scaling/singularity features of solar wind magnetic fluctuations and the
corresponding geomagnetic response is demonstrated in a number of cases. The
results are also discussed in terms of patchy reconnection processes in
magnetopause and forced or/and self-organized criticality (F/SOC) of internal
magnetosphere dynamics.Comment: 28 pages, 12 figure
Magnetic reconnection associated fluctuations in the deep magnetotail: ARTEMIS results
On the basis of ARTEMIS two-probe mission magnetic reconnection (MR) outflow associated magnetic fluctuations and turbulence are analyzed on 19 February 2011. In the deep-tail, at distances between <i>X</i> = 45 &ndash; 51 <i>R</i><sub>E</sub>, evidence for reconnection associated plasma sheet thinning was found, accompanied by heating of the plasma sheet. Correlated flow and field reversals and the large-scale Hall-effect signatures indicated the presence of the reconnection <i>X</i>-line. Within fast reconnection plasma outflows, magnetic fluctuations exhibit the same spectral scaling features and kinked spectra as magnetic fluctuations in the solar wind or in various parts of geospace. It was shown that the proton scale magnetic fluctuations are constrained by oblique firehose, proton cyclotron and mirror instability thresholds. For parallel plasma &beta;<sub>||</sub> > 1, where the thresholds converge, perpendicular magnetic fluctuations are enhanced. Magnetic compressibility decreases with the distance to the neutral sheet, however, near the instability thresholds it is comparable to the values obtained in the solar wind
Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind
Solar wind plasma is supposed to be structured in magnetic flux tubes carried
from the solar surface. Tangential velocity discontinuity near the boundaries
of individual tubes may result in Kelvin-Helmholtz instability, which may
contribute into the solar wind turbulence. While the axial magnetic field may
stabilize the instability, a small twist in the magnetic field may allow to
sub-Alfvenic motions to be unstable. We aim to study the Kelvin-Helmholtz
instability of twisted magnetic flux tube in the solar wind with different
configurations of external magnetic field. We use magnetohydrodynamic equations
in the cylindrical geometry and derive the dispersion equations governing the
dynamics of twisted magnetic flux tube moving along its axis in the cases of
untwisted and twisted external fields. Then we solve the dispersion equations
analytically and numerically and found thresholds for Kelvin-Helmholtz
instability in both cases of external field. Both analytical and numerical
solutions show that the Kelvin-Helmholtz instability is suppressed in the
twisted tube by external axial magnetic field for sub-Alfvenic motions.
However, even small twist in the external magnetic field allows the
Kelvin-Helmholtz instability to be developed for any sub-Alfvenic motions. The
unstable harmonics correspond to vortices with high azimuthal mode numbers,
which are carried by the flow. Twisted magnetic flux tubes can be unstable to
Kelvin-Helmholtz instability when they move with small speed relative to main
solar wind stream, then the Kelvin-Helmholtz vortices may significantly
contribute into the solar wind turbulence.Comment: 8 pages, 3 figures, accepted in A&
Magnetic Fluctuations and Turbulence in the Venus Magnetosheath and Wake
Recent research has shown that distinct physical regions in the Venusian
induced magnetosphere are recognizable from the variations of strength and of
wave/fluctuation activity of the magnetic field. In this paper the statistical
properties of magnetic fluctuations are investigated in the Venusian
magnetosheath, terminator, and wake regions. The latter two regions were not
visited by previous missions. We found 1/f fluctuations in the magnetosheath,
large-scale structures near the terminator and more developed turbulence
further downstream in the wake. Location independent short-tailed non-Gaussian
statistics was observed.Comment: 16 pages, 4 figure
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