222 research outputs found
Granular-Scale Elementary Flux Emergence Episodes in a Solar Active Region
We analyze data from Hinode spacecraft taken over two 54-minute periods
during the emergence of AR 11024. We focus on small-scale portions within the
observed solar active region and discover the appearance of very distinctive
small-scale and short-lived dark features in Ca II H chromospheric filtergrams
and Stokes I images. The features appear in regions with close-to-zero
longitudinal magnetic field, and are observed to increase in length before they
eventually disappear. Energy release in the low chromospheric line is detected
while the dark features are fading. In time series of magnetograms a diverging
bipolar configuration is observed accompanying the appearance of the dark
features and the brightenings. The observed phenomena are explained as
evidencing elementary flux emergence in the solar atmosphere, i.e small-scale
arch filament systems rising up from the photosphere to the lower chromosphere
with a length scale of a few solar granules. Brightenings are explained as
being the signatures of chromospheric heating triggered by reconnection of the
rising loops (once they reached chromospheric heights) with pre-existing
magnetic fields as well as to reconnection/cancellation events in U-loop
segments of emerging serpentine fields. We study the temporal evolution and
dynamics of the events and compare them with the emergence of magnetic loops
detected in quiet sun regions and serpentine flux emergence signatures in
active regions. Incorporating the novel features of granular-scale flux
emergence presented in this study we advance the scenario for serpentine flux
emergence.Comment: 24 pages, 9 figures. Accepted for publication in Solar Physic
Investigating the Dynamics and Density Evolution of Returning Plasma Blobs from the 2011 June 7 Eruption
This work examines infalling matter following an enormous Coronal Mass
Ejection (CME) on 2011 June 7. The material formed discrete concentrations, or
blobs, in the corona and fell back to the surface, appearing as dark clouds
against the bright corona. In this work we examined the density and dynamic
evolution of these blobs in order to formally assess the intriguing morphology
displayed throughout their descent. The blobs were studied in five wavelengths
(94, 131, 171, 193 and 211 \AA) using the Solar Dynamics Observatory
Atmospheric Imaging Assembly (SDO/AIA), comparing background emission to
attenuated emission as a function of wavelength to calculate column densities
across the descent of four separate blobs. We found the material to have a
column density of hydrogen of approximately 2 10 cm,
which is comparable with typical pre-eruption filament column densities.
Repeated splitting of the returning material is seen in a manner consistent
with the Rayleigh-Taylor instability. Furthermore, the observed distribution of
density and its evolution are also a signature of this instability. By
approximating the three-dimensional geometry (with data from STEREO-A),
volumetric densities were found to be approximately 2 10 g
cm, and this, along with observed dominant length-scales of the
instability, was used to infer a magnetic field of the order 1 G associated
with the descending blobs.Comment: 9 pages, 13 figures, accepted for publication in Ap
Flux cancellation and the evolution of the eruptive filament of 2011 June 7
We investigate whether flux cancellation is responsible for the formation of
a very massive filament resulting in the spectacular 2011 June 7 eruption. We
analyse and quantify the amount of flux cancellation that occurs in NOAA AR
11226 and its two neighbouring ARs (11227 & 11233) using line-of-sight
magnetograms from the Heliospheric Magnetic Imager. During a 3.6-day period
building up to the filament eruption, 1.7 x 10^21 Mx, 21% of AR 11226's maximum
magnetic flux, was cancelled along the polarity inversion line (PIL) where the
filament formed. If the flux cancellation continued at the same rate up until
the eruption then up to 2.8 x 10^21 Mx (34% of the AR flux) may have been built
into the magnetic configuration that contains the filament plasma. The large
flux cancellation rate is due to an unusual motion of the positive polarity
sunspot, which splits, with the largest section moving rapidly towards the PIL.
This motion compresses the negative polarity and leads to the formation of an
orphan penumbra where one end of the filament is rooted. Dense plasma threads
above the orphan penumbra build into the filament, extending its length, and
presumably injecting material into it. We conclude that the exceptionally
strong flux cancellation in AR 11226 played a significant role in the formation
of its unusually massive filament. In addition, the presence and coherent
evolution of bald patches in the vector magnetic field along the PIL suggests
that the magnetic field configuration supporting the filament material is that
of a flux rope.Comment: 18 pages, 7 figures. Submitted to ApJ in December 2015, accepted in
June 201
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
Emergence of small-scale magnetic loops through the quiet solar atmosphere
We investigate the emergence of magnetic flux in the quiet Sun at very small
spatial scales, focusing on the magnetic connection between the photosphere and
chromosphere. The observational data consist of spectropolarimetric
measurements and filtergrams taken with the Hinode satellite and the Dutch Open
Telescope. We find that a significant fraction of the magnetic flux present in
internetwork regions appears in the form of Omega-shaped loops. The emergence
rate is 0.02 loops per hour and arcsec^{-2}, which brings 1.1 x 10^12 Mx s^{-1}
arcsec^{-2} of new flux to the solar surface. Initially, the loops are observed
as small patches of linear polarization above a granular cell. Shortly
afterwards, two footpoints of opposite polarity become visible in circular
polarization within or at the edges of the granule and start to move toward the
adjacent intergranular space. The orientation of the footpoints does not seem
to obey Hale's polarity rules. The loops are continuously buffeted by
convective motions, but they always retain a high degree of coherence.
Interestingly, 23% of the loops that emerge in the photosphere reach the
chromosphere (16 cases out of 69). They are first detected in Fe I 630 nm
magnetograms and 5 minutes later in Mg I b 517.3 nm magnetograms. After about 8
minutes, some of them are also observed in Ca II H line-core images, where the
footpoints produce small brightness enhancements.Comment: Accepted for publication in Ap
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
Why are CMEs large-scale coronal events: nature or nurture?
The apparent contradiction between small-scale source regions of, and large-scale coronal response to, coronal mass ejections (CMEs) has been a long-standing puzzle. For some, CMEs are considered to be inherently large-scale events – eruptions in which a number of flux systems participate in an unspecified manner, while others consider magnetic reconnection in special global topologies to be responsible for the large-scale response of the lower corona to CME events. Some of these ideas may indeed be correct in specific cases. However, what is the key element which makes CMEs large-scale? Observations show that the extent of the coronal disturbance matches the angular width of the CME – an important clue, which does not feature strongly in any of the above suggestions. We review observational evidence for the large-scale nature of CME source regions and find them lacking. Then we compare different ideas regarding how CMEs evolve to become large-scale. The large-scale magnetic topology plays an important role in this process. There is amounting evidence, however, that the key process is magnetic reconnection between the CME and other magnetic structures. We outline a CME evolution model, which is able to account for all the key observational signatures of large-scale CMEs and presents a clear picture how large portions of the Sun become constituents of the CME. In this model reconnection is driven by the expansion of the CME core resulting from an over-pressure relative to the pressure in the CME's surroundings. This implies that the extent of the lower coronal signatures match the final angular width of the CME.Fil: van Driel Gesztelyi, Lidia. Centre National de la Recherche Scientifique. Observatoire de Paris; FranciaFil: Attrill, G. D. R.. University College London; Estados UnidosFil: Démoulin, Pascal. Centre National de la Recherche Scientifique. Observatoire de Paris; FranciaFil: Mandrini, Cristina Hemilse. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Harra, L. K.. University College London; Estados Unido
Can we detect local helioseismic parameter shifts in coronal holes?
Changes in helioseismic mode parameters in active regions and across the solar disk are well documented, but local magnetic activity and geometric effects may not account for all of the scatter seen in the results. We use results from theHelioseismic and Magnetic Imagerring-diagram pipeline for Carrington rotation 2113 to look for differences in mode amplitude and frequency between coronal holes and other quiet-Sun regions. While we do not find a systematic difference, the results do suggest that the correlation between magnetic activity index and mode parameters shows less scatter in coronal hole regions than in general quiet Sun
Evolution and Flare Activity of Delta-Sunspots in Cycle 23
The emergence and magnetic evolution of solar active regions (ARs) of
beta-gamma-delta type, which are known to be highly flare-productive, were
studied with the SOHO/MDI data in Cycle 23. We selected 31 ARs that can be
observed from their birth phase, as unbiased samples for our study. From the
analysis of the magnetic topology (twist and writhe), we obtained the following
results. i) Emerging beta-gamma-delta ARs can be classified into three
topological types as "quasi-beta", "writhed" and "top-to-top". ii) Among them,
the "writhed" and "top-to-top" types tend to show high flare activity. iii) As
the signs of twist and writhe agree with each other in most cases of the
"writhed" type (12 cases out of 13), we propose a magnetic model in which the
emerging flux regions in a beta-gamma-delta AR are not separated but united as
a single structure below the solar surface. iv) Almost all the "writhed"-type
ARs have downward knotted structures in the mid portion of the magnetic flux
tube. This, we believe, is the essential property of beta-gamma-delta ARs. v)
The flare activity of beta-gamma-delta ARs is highly correlated not only with
the sunspot area but also with the magnetic complexity. vi) We suggest that
there is a possible scaling-law between the flare index and the maximum umbral
area
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