38 research outputs found
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
Thermal stability analysis of the fine structure of solar prominences
The linear thermal stability of a 2D periodic structure (alternatively hot and cold) in a uniform magnetic field is analyzed. The energy equation includes wave heating (assumed proportional to density), radiative cooling and both conduction parallel and orthogonal to magnetic lines. The equilibrium is perturbed at constant gas pressure. With parallel conduction only, it is found to be unstable when the length scale 1// is greater than 45 Mn. In that case, orthogonal conduction becomes important and stabilizes the structure when the length scale is smaller than 5 km. On the other hand, when the length scale is greater than 5 km, the thermal equilibrium is unstable, and the corresponding time scale is about 10,000 s: this result may be compared to observations showing that the lifetime of the fine structure of solar prominences is about one hour; consequently, our computations suggest that the size of the unresolved threads could be of the order of 10 km only
Global and local expansion of magnetic clouds in the inner heliosphere
Observations of magnetic clouds (MCs) are consistent with the presence of
flux ropes detected in the solar wind (SW) a few days after their expulsion
from the Sun as coronal mass ejections (CMEs). Both the \textit{in situ}
observations of plasma velocity profiles and the increase of their size with
solar distance show that MCs are typically expanding structures. The aim of
this work is to derive the expansion properties of MCs in the inner heliosphere
from 0.3 to 1 AU.We analyze MCs observed by the two Helios spacecraft using
\textit{in situ} magnetic field and velocity measurements. We split the sample
in two subsets: those MCs with a velocity profile that is significantly
perturbed from the expected linear profile and those that are not. From the
slope of the \textit{in situ} measured bulk velocity along the Sun-Earth
direction, we compute an expansion speed with respect to the cloud center for
each of the analyzed MCs. We analyze how the expansion speed depends on the MC
size, the translation velocity, and the heliocentric distance, finding that all
MCs in the subset of non-perturbed MCs expand with almost the same
non-dimensional expansion rate (). We find departures from this general
rule for only for perturbed MCs, and we interpret the departures as the
consequence of a local and strong SW perturbation by SW fast streams, affecting
the MC even inside its interior, in addition to the direct interaction region
between the SW and the MC. We also compute the dependence of the mean total SW
pressure on the solar distance and we confirm that the decrease of the total SW
pressure with distance is the main origin of the observed MC expansion rate. We
found that was for non-perturbed MCs while was
for perturbed MCs, the larger spread in the last ones being due
to the influence of the environment conditions on the expansion
Understanding CME and associated shock in the solar corona by merging multi wavelengths observation
Using multi-wavelength imaging observations, in EUV, white light and radio,
and radio spectral data over a large frequency range, we analyzed the
triggering and development of a complex eruptive event. This one includes two
components, an eruptive jet and a CME which interact during more than 30 min,
and can be considered as physically linked. This was an unusual event. The jet
is generated above a typical complex magnetic configuration which has been
investigated in many former studies related to the build-up of eruptive jets;
this configuration includes fan-field lines originating from a corona null
point above a parasitic polarity, which is embedded in one polarity region of
large Active Region (AR). The initiation and development of the CME, observed
first in EUV, does not show usual signatures. In this case, the eruptive jet is
the main actor of this event. The CME appears first as a simple loop system
which becomes destabilized by magnetic reconnection between the outer part of
the jet and the ambient medium. The progression of the CME is closely
associated with the occurrence of two successive types II bursts from distinct
origin. An important part of this study is the first radio type II burst for
which the joint spectral and imaging observations allowed: i) to follow, step
by step, the evolution of the spectrum and of the trajectory of the radio
burst, in relationship with the CME evolution; ii) to obtain, without
introducing an electronic density model, the B-field and the Alfven speed.Comment: 17 pages, 13 figure
Eruption of a Kink-Unstable Filament in Active Region NOAA 10696
We present rapid-cadence Transition Region And Coronal Explorer (TRACE)
observations which show evidence of a filament eruption from active region NOAA
10696, accompanied by an X2.5 flare, on 2004 November 10. The eruptive
filament, which manifests as a fast coronal mass ejection some minutes later,
rises as a kinking structure with an apparently exponential growth of height
within TRACE's field of view. We compare the characteristics of this filament
eruption with MHD numerical simulations of a kink-unstable magnetic flux rope,
finding excellent qualitative agreement. We suggest that, while tether
weakening by breakout-like quadrupolar reconnection may be the release
mechanism for the previously confined flux rope, the driver of the expansion is
most likely the MHD helical kink instability.Comment: Accepted by ApJ Letters. 4 figures (Fig. 3 in two parts). For MPEG
files associated with Figure 1, see:
http://www.mssl.ucl.ac.uk/~drw/papers/kink/ktrace.mpg
http://www.mssl.ucl.ac.uk/~drw/papers/kink/kmdi.mpg
http://www.mssl.ucl.ac.uk/~drw/papers/kink/ksimu.mp
Plasma Upflows Induced by Magnetic Reconnection Above an Eruptive Flux Rope
One of the major discoveries of Hinode's Extreme-ultraviolet Imaging Spectrometer (EIS) is the presence of upflows at the edges of active regions. As active regions are magnetically connected to the large-scale field of the corona, these upflows are a likely contributor to the global mass cycle in the corona. Here we examine the driving mechanism(s) of the very strong upflows with velocities in excess of 70 km s(-1), known as blue-wing asymmetries, observed during the eruption of a flux rope in AR 10977 (eruptive flare SOL2007-12-07T04:50). We use Hinode/EIS spectroscopic observations combined with magnetic-field modeling to investigate the possible link between the magnetic topology of the active region and the strong upflows. A Potential Field Source Surface (PFSS) extrapolation of the large-scale field shows a quadrupolar configuration with a separator lying above the flux rope. Field lines formed by induced reconnection along the separator before and during the flux-rope eruption are spatially linked to the strongest blue-wing asymmetries in the upflow regions. The flows are driven by the pressure gradient created when the dense and hot arcade loops of the active region reconnect with the extended and tenuous loops overlying it. In view of the fact that separator reconnection is a specific form of the more general quasi-separatrix (QSL) reconnection, we conclude that the mechanism driving the strongest upflows is, in fact, the same as the one driving the persistent upflows of approximate to 10 - 20 km s(-1) observed in all active regions
Structure et stabilite thermique et magnetique des protuberances solaires
SIGLEINIST T 72608 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc