4,492 research outputs found
Solar filament eruptions and their physical role in triggering Coronal Mass Ejections
Solar filament eruptions play a crucial role in triggering coronal mass
ejections (CMEs). More than 80 % of eruptions lead to a CME. This correlation
has been studied extensively during the past solar cycles and the last long
solar minimum. The statistics made on events occurring during the rising phase
of the new solar cycle 24 is in agreement with this finding. Both filaments and
CMEs have been related to twisted magnetic fields. Therefore, nearly all the
MHD CME models include a twisted flux tube, called a flux rope. Either the flux
rope is present long before the eruption, or it is built up by reconnection of
a sheared arcade from the beginning of the eruption. In order to initiate
eruptions, different mechanisms have been proposed: new emergence of flux,
and/or dispersion of the external magnetic field, and/or reconnection of field
lines below or above the flux rope. These mechanisms reduce the downward
magnetic tension and favor the rise of the flux rope. Another mechanism is the
kink instability when the configuration is twisted too much. In this paper we
open a forum of discussions revisiting observational and theoretical papers to
understand which mechanisms trigger the eruption. We conclude that all the
above quoted mechanisms could bring the flux rope to an unstable state.
However, the most efficient mechanism for CMEs is the loss-of-equilibrium or
torus instability, when the flux rope has reached an unstable threshold
determined by a decay index of the external magnetic field.Comment: 23 pages, 13 figures, revie
Tzitzeica solitons versus relativistic CalogeroâMoser three-body clusters
We establish a connection between the hyperbolic relativistic CalogeroâMoser systems and a class of soliton solutions to the Tzitzeica equation (also called the DoddâBulloughâZhiberâShabatâMikhailov equation). In the 6N-dimensional phase space Omega of the relativistic systems with 2N particles and N antiparticles, there exists a 2N-dimensional PoincarĂ©-invariant submanifold OmegaP corresponding to N free particles and N bound particle-antiparticle pairs in their ground state. The Tzitzeica N-soliton tau functions under consideration are real valued and obtained via the dual Lax matrix evaluated in points of OmegaP. This correspondence leads to a picture of the soliton as a cluster of two particles and one antiparticle in their lowest internal energy state
Does the spacecraft trajectory strongly affect the detection of magnetic clouds?
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections
(ICMEs) where a magnetic flux rope is detected. Is the difference between MCs
and ICMEs without detected flux rope intrinsic or rather due to an
observational bias? As the spacecraft has no relationship with the MC
trajectory, the frequency distribution of MCs versus the spacecraft distance to
the MCs axis is expected to be approximately flat. However, Lepping and Wu
(2010) confirmed that it is a strongly decreasing function of the estimated
impact parameter. Is a flux rope more frequently undetected for larger impact
parameter? In order to answer the questions above, we explore the parameter
space of flux rope models, especially the aspect ratio, boundary shape, and
current distribution. The proposed models are analyzed as MCs by fitting a
circular linear force-free field to the magnetic field computed along simulated
crossings.
We find that the distribution of the twist within the flux rope, the
non-detection due to too low field rotation angle or magnitude are only weakly
affecting the expected frequency distribution of MCs versus impact parameter.
However, the estimated impact parameter is increasingly biased to lower values
as the flux-rope cross section is more elongated orthogonally to the crossing
trajectory. The observed distribution of MCs is a natural consequence of a
flux-rope cross section flattened in average by a factor 2 to 3 depending on
the magnetic twist profile. However, the faster MCs at 1 AU, with V>550 km/s,
present an almost uniform distribution of MCs vs. impact parameter, which is
consistent with round shaped flux ropes, in contrast with the slower ones. We
conclude that either most of the non-MC ICMEs are encountered outside their
flux rope or near the leg region, or they do not contain any
Global axis shape of magnetic clouds deduced from the distribution of their local axis orientation
Coronal mass ejections (CMEs) are routinely tracked with imagers in the
interplanetary space while magnetic clouds (MCs) properties are measured
locally by spacecraft. However, both imager and insitu data do not provide
direct estimation on the global flux rope properties. The main aim of this
study is to constrain the global shape of the flux rope axis from local
measurements, and to compare the results from in-situ data with imager
observations. We perform a statistical analysis of the set of MCs observed by
WIND spacecraft over 15 years in the vicinity of Earth. With the hypothesis of
having a sample of MCs with a uniform distribution of spacecraft crossing along
their axis, we show that a mean axis shape can be derived from the distribution
of the axis orientation. In complement, while heliospheric imagers do not
typically observe MCs but only their sheath region, we analyze one event where
the flux-rope axis can be estimated from the STEREO imagers. From the analysis
of a set of theoretical models, we show that the distribution of the local axis
orientation is strongly affected by the global axis shape. Next, we derive the
mean axis shape from the integration of the observed orientation distribution.
This shape is robust as it is mostly determined from the global shape of the
distribution. Moreover, we find no dependence on the flux-rope inclination on
the ecliptic. Finally, the derived shape is fully consistent with the one
derived from heliospheric imager observations of the June 2008 event. We have
derived a mean shape of MC axis which only depends on one free parameter, the
angular separation of the legs (as viewed from the Sun). This mean shape can be
used in various contexts such as the study of high energy particles or space
weather forecast.Comment: 13 pages, 12 figure
Radiated Immunity Testing of a Device with an External Wire: Repeatibility of Reverberation Chamber Results and Correlation with Anechoic Chamber Results
We present the experimental radiated immunity results of an electronic device with an external wire obtained in reverberation and anechoic chambers. Repeatability and reproducibility of reverberation chamber measurements are investigated by repeating the test in three reverberation chambers with different characteristics. We show how the current state of the art allows a statistical control of RC measurement repeatability within an industrial installation, and that a statistical correlation with AC results frequency by frequency is possible in particular cases relevant to automotive application
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