251 research outputs found
Coronal magnetic reconnection driven by CME expansion -- the 2011 June 7 event
Coronal mass ejections (CMEs) erupt and expand in a magnetically structured
solar corona. Various indirect observational pieces of evidence have shown that
the magnetic field of CMEs reconnects with surrounding magnetic fields,
forming, e.g., dimming regions distant from the CME source regions. Analyzing
Solar Dynamics Observatory (SDO) observations of the eruption from AR 11226 on
2011 June 7, we present the first direct evidence of coronal magnetic
reconnection between the fields of two adjacent ARs during a CME. The
observations are presented jointly with a data-constrained numerical
simulation, demonstrating the formation/intensification of current sheets along
a hyperbolic flux tube (HFT) at the interface between the CME and the
neighbouring AR 11227. Reconnection resulted in the formation of new magnetic
connections between the erupting magnetic structure from AR 11226 and the
neighboring active region AR 11227 about 200 Mm from the eruption site. The
onset of reconnection first becomes apparent in the SDO/AIA images when
filament plasma, originally contained within the erupting flux rope, is
re-directed towards remote areas in AR 11227, tracing the change of large-scale
magnetic connectivity. The location of the coronal reconnection region becomes
bright and directly observable at SDO/AIA wavelengths, owing to the presence of
down-flowing cool, dense (10^{10} cm^{-3}) filament plasma in its vicinity. The
high-density plasma around the reconnection region is heated to coronal
temperatures, presumably by slow-mode shocks and Coulomb collisions. These
results provide the first direct observational evidence that CMEs reconnect
with surrounding magnetic structures, leading to a large-scale re-configuration
of the coronal magnetic field.Comment: 12 pages, 12 figure
Observation of An Evolving Magnetic Flux Rope Prior To and During A Solar Eruption
Explosive energy release is a common phenomenon occurring in magnetized
plasma systems ranging from laboratories, Earth's magnetosphere, the solar
corona and astrophysical environments. Its physical explanation is usually
attributed to magnetic reconnection in a thin current sheet. Here we report the
important role of magnetic flux rope structure, a volumetric current channel,
in producing explosive events. The flux rope is observed as a hot channel prior
to and during a solar eruption from the Atmospheric Imaging Assembly (AIA)
telescope on board the Solar Dynamic Observatory (SDO). It initially appears as
a twisted and writhed sigmoidal structure with a temperature as high as 10 MK
and then transforms toward a semi-circular shape during a slow rise phase,
which is followed by fast acceleration and onset of a flare. The observations
suggest that the instability of the magnetic flux rope trigger the eruption,
thus making a major addition to the traditional magnetic-reconnection paradigm.Comment: 13 pages, 3 figure
Temporal Correlation of Hard X-rays and Meter/Decimeter Radio Structures in Solar Flares
We investigate the relative timing between hard X-ray (HXR) peaks and
structures in metric and decimetric radio emissions of solar flares using data
from the RHESSI and Phoenix-2 instruments. The radio events under consideration
are predominantly classified as type III bursts, decimetric pulsations and
patches. The RHESSI data are demodulated using special techniques appropriate
for a Phoenix-2 temporal resolution of 0.1s. The absolute timing accuracy of
the two instruments is found to be about 170 ms, and much better on the
average. It is found that type III radio groups often coincide with enhanced
HXR emission, but only a relatively small fraction ( 20%) of the groups
show close correlation on time scales 1s. If structures correlate, the HXRs
precede the type III emissions in a majority of cases, and by 0.690.19 s
on the average. Reversed drift type III bursts are also delayed, but
high-frequency and harmonic emission is retarded less. The decimetric
pulsations and patches (DCIM) have a larger scatter of delays, but do not have
a statistically significant sign or an average different from zero. The time
delay does not show a center-to-limb variation excluding simple propagation
effects. The delay by scattering near the source region is suggested to be the
most efficient process on the average for delaying type III radio emission
Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections
A clear understanding of the nature of the pre-eruptive magnetic field
configurations of Coronal Mass Ejections (CMEs) is required for understanding
and eventually predicting solar eruptions. Only two, but seemingly disparate,
magnetic configurations are considered viable; namely, sheared magnetic arcades
(SMA) and magnetic flux ropes (MFR). They can form via three physical
mechanisms (flux emergence, flux cancellation, helicity condensation) . Whether
the CME culprit is an SMA or an MFR, however, has been strongly debated for
thirty years. We formed an International Space Science Institute (ISSI) team to
address and resolve this issue and report the outcome here. We review the
status of the field across modeling and observations, identify the open and
closed issues, compile lists of SMA and MFR observables to be tested against
observations and outline research activities to close the gaps in our current
understanding. We propose that the combination of multi-viewpoint multi-thermal
coronal observations and multi-height vector magnetic field measurements is the
optimal approach for resolving the issue conclusively. We demonstrate the
approach using MHD simulations and synthetic coronal images.
Our key conclusion is that the differentiation of pre-eruptive configurations
in terms of SMAs and MFRs seems artificial. Both observations and modeling can
be made consistent if the pre-eruptive configuration exists in a hybrid state
that is continuously evolving from an SMA to an MFR. Thus, the 'dominant'
nature of a given configuration will largely depend on its evolutionary stage
(SMA-like early-on, MFR-like near the eruption).Comment: Space Science Reviews, accepted for publicatio
Review article: MHD wave propagation near coronal null points of magnetic fields
We present a comprehensive review of MHD wave behaviour in the neighbourhood
of coronal null points: locations where the magnetic field, and hence the local
Alfven speed, is zero. The behaviour of all three MHD wave modes, i.e. the
Alfven wave and the fast and slow magnetoacoustic waves, has been investigated
in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null
points, for a variety of assumptions, configurations and geometries. In
general, it is found that the fast magnetoacoustic wave behaviour is dictated
by the Alfven-speed profile. In a plasma, the fast wave is focused
towards the null point by a refraction effect and all the wave energy, and thus
current density, accumulates close to the null point. Thus, null points will be
locations for preferential heating by fast waves. Independently, the Alfven
wave is found to propagate along magnetic fieldlines and is confined to the
fieldlines it is generated on. As the wave approaches the null point, it
spreads out due to the diverging fieldlines. Eventually, the Alfven wave
accumulates along the separatrices (in 2D) or along the spine or fan-plane (in
3D). Hence, Alfven wave energy will be preferentially dissipated at these
locations. It is clear that the magnetic field plays a fundamental role in the
propagation and properties of MHD waves in the neighbourhood of coronal null
points. This topic is a fundamental plasma process and results so far have also
lead to critical insights into reconnection, mode-coupling, quasi-periodic
pulsations and phase-mixing.Comment: 34 pages, 5 figures, invited review in Space Science Reviews => Note
this is a 2011 paper, not a 2010 pape
Could Public Restrooms Be an Environment for Bacterial Resistomes?
PMCID: PMC3547874This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
The evolution of writhe in kink-unstable flux ropes and erupting filaments
Copyright © 2014 IOP Publishing Ltd. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence (http://creativecommons.org/licenses/by/3.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.The helical kink instability of a twisted magnetic flux tube has been suggested as a trigger mechanism for solar filament eruptions and coronal mass ejections (CMEs). In order to investigate if estimations of the pre-emptive twist can be obtained from observations of writhe in such events, we quantitatively analyze the conversion of twist into writhe in the course of the instability, using numerical simulations. We consider the line tied, cylindrically symmetric GoldâHoyle flux rope model and measure the writhe using the formulae by Berger and Prior which express the quantity as a single integral in space. We find that the amount of twist converted into writhe does not simply scale with the initial flux rope twist, but depends mainly on the growth rates of the instability eigenmodes of higher longitudinal order than the basic mode. The saturation levels of the writhe, as well as the shapes of the kinked flux ropes, are very similar for considerable ranges of initial flux rope twists, which essentially precludes estimations of pre-eruptive twist from measurements of writhe. However, our simulations suggest an upper twist limit of ~6Ï for the majority of filaments prior to their eruption.European Commissionâs Seventh Framework ProgrammeScience & Technology Facilities Council (STFC)Hungarian Researc
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