265 research outputs found
Wavelet analysis of magnetic turbulence in the Earth's plasma sheet
Recent studies provide evidence for the multi-scale nature of magnetic
turbulence in the plasma sheet. Wavelet methods represent modern time series
analysis techniques suitable for the description of statistical characteristics
of multi-scale turbulence. Cluster FGM (fluxgate magnetometer) magnetic field
high-resolution (~67 Hz) measurements are studied during an interval in which
the spacecraft are in the plasma sheet. As Cluster passes through different
plasma regions, physical processes exhibit non-steady properties on
magnetohydrodynamic (MHD) and small, possibly kinetic scales. As a consequence,
the implementation of wavelet-based techniques becomes complicated due to the
statistically transitory properties of magnetic fluctuations and finite size
effects. Using a supervised multi-scale technique which allows existence test
of moments, the robustness of higher-order statistics is investigated. On this
basis the properties of magnetic turbulence are investigated for changing
thickness of the plasma sheet.Comment: 17 pages, 5 figure
Thin current sheets in the magnetotail at lunar distances: statistics of ARTEMIS observations
The magnetotail current sheet's spatial configuration and stability control
the onset of magnetic reconnection - the driving process for magnetospheric
substorms. The near-Earth current sheet has been thoroughly investigated by
numerous missions, whereas the midtail current sheet has not been adequately
explored. This is especially the case for the long-term variation of its
configuration in response to the solar wind. We present a statistical analysis
of 1261 magnetotail current sheet crossings by the Acceleration, Reconnection,
Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS)
mission orbiting the moon (X~-60 RE), collected during the entirety of Solar
Cycle 24. We demonstrate that the magnetotail current sheet typically remains
extremely thin, with a characteristic thickness comparable to the thermal ion
gyroradius, even at such large distances from Earth's dipole. We also find that
a substantial fraction (~one quarter) of the observed current sheets have a
partially force-free magnetic field configuration, with a negligible
contribution of the thermal pressure and a significant contribution of the
magnetic field shear component to the pressure balance. Further, we quantify
the impact of the changing solar wind driving conditions on the properties of
the midtail around the lunar orbit. During active solar wind driving
conditions, we observe an increase in the occurrence rate of thin current
sheets, whereas quiet solar wind driving conditions seem to favor the formation
of partially force-free current sheets
Global and local disturbances in the magnetotail during reconnection
We examine Cluster observations of a reconnection event at <I>x</I><sub>GSM</sub>=&minus;15.7 <I>R<sub>E</sub></I> in the magnetotail on 11 October 2001, when Cluster recorded the current sheet for an extended period including the entire duration of the reconnection event. The onset of reconnection is associated with a sudden orientation change of the ambient magnetic field, which is also observed simultaneously by Goes-8 at geostationary orbit. Current sheet oscillations are observed both before reconnection and during it. The speed of the flapping motions is found to increase when the current sheet undergoes the transition from quiet to active state, as suggested by an earlier statistical result and now confirmed within one single event. Within the diffusion region both the tailward and earthward parts of the quadrupolar magnetic Hall structure are recorded as an x-line passes Cluster. We report the first observations of the Hall structure conforming to the kinks in the current sheet. This results in relatively strong fluctuations in <I>B<sub>z</sub></I>, which are shown to be the Hall signature tilted in the <I>yz</I> plane with the current sheet
Multi-scale magnetic field intermittence in the plasma sheet
This paper demonstrates that intermittent magnetic field fluctuations in the
plasma sheet exhibit transitory, localized, and multi-scale features. We
propose a multifractal based algorithm, which quantifies intermittence on the
basis of the statistical distribution of the 'strength of burstiness',
estimated within a sliding window. Interesting multi-scale phenomena observed
by the Cluster spacecraft include large scale motion of the current sheet and
bursty bulk flow associated turbulence, interpreted as a cross-scale coupling
(CSC) process.Comment: 18 pages, 7 figure
Alfvén waves in the near-PSBL lobe: Cluster observations
Electromagnetic low-frequency waves in the magnetotail lobe close to the PSBL (Plasma Sheet Boundary Layer) are studied using the Cluster spacecraft. The lobe waves show Alfvénic properties and transport their wave energy (Poynting flux) on average toward the Earth along magnetic field lines. Most of the wave events are rich with oxygen (O+) ion plasma. The rich O+ plasma can serve to enhance the magnetic field fluctuations, resulting in a greater likelihood of observation, but it does not appear to be necessary for the generation of the waves. Taking into account the fact that all events are associated with auroral electrojet enhancements, the source of the lobe waves might be a substorm-associated instability, i.e. some instability near the reconnection site, or an ion beam-related instability in the PSBL
Pressure and intermittency in passive vector turbulence
We investigate the scaling properties a model of passive vector turbulence
with pressure and in the presence of a large-scale anisotropy. The leading
scaling exponents of the structure functions are proven to be anomalous. The
anisotropic exponents are organized in hierarchical families growing without
bound with the degree of anisotropy. Nonlocality produces poles in the
inertial-range dynamics corresponding to the dimensional scaling solution. The
increase with the P\'{e}clet number of hyperskewness and higher odd-dimensional
ratios signals the persistence of anisotropy effects also in the inertial
range.Comment: 4 pages, 1 figur
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