548 research outputs found
Magnetic shuffling of coronal downdrafts
Channelled fragmented downflows are ubiquitous in magnetized atmospheres, and
have been recently addressed from an observation after a solar eruption. We
study the possible back-effect of the magnetic field on the propagation of
confined flows. We compare two 3D MHD simulations of dense supersonic plasma
blobs downfalling along a coronal magnetic flux tube. In one, the blobs move
strictly along the field lines; in the other, the initial velocity of the blobs
is not perfectly aligned to the magnetic field and the field is weaker. The
aligned blobs remain compact while flowing along the tube, with the generated
shocks. The misaligned blobs are disrupted and merged by the chaotic shuffling
of the field lines, and structured into thinner filaments; Alfven wave fronts
are generated together with shocks ahead of the dense moving front. Downflowing
plasma fragments can be chaotically and efficiently mixed if their motion is
misaligned to field lines, with broad implications, e.g., disk accretion in
protostars, coronal eruptions and rain.Comment: 9 pages, 4 figures, proposed for acceptance, movies available upon
request to the first autho
Guided flows in coronal magnetic flux tubes
There is evidence for coronal plasma flows to break down into fragments and
to be laminar. We investigate this effect by modeling flows confined along
magnetic channels. We consider a full MHD model of a solar atmosphere box with
a dipole magnetic field. We compare the propagation of a cylindrical flow
perfectly aligned to the field to that of another one with a slight
misalignment. We assume a flow speed of 200 km/s, and an ambient magnetic field
of 30 G. We find that while the aligned flow maintains its cylindrical symmetry
while it travels along the magnetic tube, the misaligned one is rapidly
squashed on one side, becoming laminar and eventually fragmented because of the
interaction and backreaction of the magnetic field. This model could explain an
observation of erupted fragments that fall back as thin and elongated strands
and end up onto the solar surface in a hedge-like configuration, made by the
Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The
initial alignment of plasma flow plays an important role in determining the
possible laminar structure and fragmentation of flows while they travel along
magnetic channels.Comment: 11 pages, 8 figures, accepted for publication, movies available upon
request to the first autho
Bright hot impacts by erupted fragments falling back on the Sun: magnetic channelling
Dense plasma fragments were observed to fall back on the solar surface by the
Solar Dynamics Observatory after an eruption on 7 June 2011, producing strong
EUV brightenings. Previous studies investigated impacts in regions of weak
magnetic field. Here we model the km/s impact of fragments
channelled by the magnetic field close to active regions. In the observations,
the magnetic channel brightens before the fragment impact. We use a 3D-MHD
model of spherical blobs downfalling in a magnetized atmosphere. The blob
parameters are constrained from the observation. We run numerical simulations
with different ambient density and magnetic field intensity. We compare the
model emission in the 171\AA~ channel of the Atmospheric Imaging Assembly with
the observed one. We find that a model of downfall channelled in a MK
coronal loop confined by a magnetic field of G, best explains
qualitatively and quantitatively the observed evolution. The blobs are highly
deformed, further fragmented, when the ram pressure becomes comparable to the
local magnetic pressure and they are deviated to be channelled by the field,
because of the differential stress applied by the perturbed magnetic field.
Ahead of them, in the relatively dense coronal medium, shock fronts propagate,
heat and brighten the channel between the cold falling plasma and the solar
surface. This study shows a new mechanism which brightens downflows channelled
by the magnetic field, such as in accreting young stars, and also works as a
probe of the ambient atmosphere, providing information about the local plasma
density and magnetic field.Comment: 17 pages, 14 figure
Thermal structure of hot non-flaring corona from Hinode/EIS
In previous studies a very hot plasma component has been diagnosed in solar
active regions through the images in three different narrow-band channels of
SDO/AIA. This diagnostic from EUV imaging data has also been supported by the
matching morphology of the emission in the hot Ca XVII line, as observed with
Hinode/EIS. This evidence is debated because of unknown distribution of the
emission measure along the line of sight. Here we investigate in detail the
thermal distribution of one of such regions using EUV spectroscopic data. In an
active region observed with SDO/AIA, Hinode/EIS and XRT, we select a subregion
with a very hot plasma component and another cooler one for comparison. The
average spectrum is extracted for both, and 14 intense lines are selected for
analysis, that probe the 5.5 < log T < 7 temperature range uniformly. From
these lines the emission measure distributions are reconstructed with the MCMC
method. Results are cross-checked with comparison of the two subregions, with a
different inversion method, with the morphology of the images, and with the
addition of fluxes measured with from narrow and broad-band imagers. We find
that, whereas the cool region has a flat and featureless distribution that
drops at temperature log T >= 6.3, the distribution of the hot region shows a
well-defined peak at log T = 6.6 and gradually decreasing trends on both sides,
thus supporting the very hot nature of the hot component diagnosed with
imagers. The other cross-checks are consistent with this result. This study
provides a completion of the analysis of active region components, and the
resulting scenario supports the presence of a minor very hot plasma component
in the core, with temperatures log T > 6.6.Comment: 12 pages, 8 figures, accepted for publicatio
X-Raying the Dark Side of Venus - Scatter from Venus Magnetotail?
This work analyzes the X-ray, EUV and UV emission apparently coming from the
Earth-facing (dark) side of Venus as observed with Hinode/XRT and SDO/AIA
during a transit across the solar disk occurred in 2012. We have measured
significant X-Ray, EUV and UV flux from Venus dark side. As a check we have
also analyzed a Mercury transit across the solar disk, observed with Hinode/XRT
in 2006. We have used the latest version of the Hinode/XRT Point Spread
Function (PSF) to deconvolve Venus and Mercury X-ray images, in order to remove
possible instrumental scattering. Even after deconvolution, the flux from Venus
shadow remains significant while in the case of Mercury it becomes negligible.
Since stray-light contamination affects the XRT Ti-poly filter data from the
Venus transit in 2012, we performed the same analysis with XRT Al-mesh filter
data, which is not affected by the light leak. Even the Al-mesh filter data
show residual flux. We have also found significant EUV (304 A, 193 A, 335 A)
and UV (1700 A) flux in Venus shadow, as measured with SDO/AIA. The EUV
emission from Venus dark side is reduced when appropriate deconvolution methods
are applied; the emission remains significant, however. The light curves of the
average flux of the shadow in the X-ray, EUV, and UV bands appear different as
Venus crosses the solar disk, but in any of them the flux is, at any time,
approximately proportional to the average flux in a ring surrounding Venus, and
therefore proportional to the average flux of the solar regions around Venus
obscuring disk line of sight. The proportionality factor depends on the band.
This phenomenon has no clear origin; we suggest it may be due to scatter
occurring in the very long magnetotail of Venus.Comment: This paper has been accepted in The Astrophysical Journa
Asymmetric Twisting of Coronal Loops
The bright solar corona entirely consists of closed magnetic loops rooted in the photosphere. Photospheric motions are important drivers of magnetic stressing, which eventually leads to energy release into heat. These motions are chaotic and obviously different from one footpoint to the other, and in fact, there is strong evidence that loops are finely stranded. One may also expect strong transient variations along the field lines, but at a glance, coronal loops ever appear more or less uniformly bright from one footpoint to the other. We aim to understand how much coronal loops can preserve their own symmetry against asymmetric boundary motions that are expected to occur at loop footpoints. We investigate this issue by time-dependent 2.5D MHD modelling of a coronal loop, including its rooting and beta-variation in the photosphere. We assume that the magnetic flux tube is stressed by footpoint rotation but also that the rotation has a different pattern from one footpoint to the other. In this way, we force strong asymmetries because we expect independent evolution along different magnetic strands. We found that until the Alfven crossing-travel time relative to the entire loop length is much lower than the twisting period, the loop's evolution depends only on the relative velocity between the boundaries, and the symmetry is efficiently preserved. We conclude that the very high Alfven velocities that characterise the coronal environment can explain why coronal loops can maintain a very high degree of symmetry even when they are subjected to asymmetric photospheric motions for a long time
MHD Modelling of Coronal Loops: Injection of High-Speed Chromospheric Flows
Context. Observations reveal a correspondence between chromospheric type II spicules and bright upward-moving fronts in the corona observed in the extreme-ultraviolet (EUV) band. However, theoretical considerations suggest that these flows are probably not the main source of heating in coronal magnetic loops. Aims. We investigate the propagation of high-speed chromospheric flows into coronal magnetic flux tubes and the possible production of emission in the EUV band. Methods. We simulated the propagation of a dense 104 K chromospheric jet upward along a coronal loop by means of a 2D cylindrical MHD model that includes gravity, radiative losses, thermal conduction, and magnetic induction. The jet propagates in a complete atmosphere including the chromosphere and a tenuous cool (approximately 0.8 MK) corona, linked through a steep transition region. In our reference model, the jet initial speed is 70 km per second, its initial density is 10(exp 11) per cubic centimeter, and the ambient uniform magnetic field is 10 G. We also explored other values of jet speed and density in 1D and different magnetic field values in 2D, as well as the jet propagation in a hotter (approximately 1.5 MK) background loop. Results. While the initial speed of the jet does not allow it to reach the loop apex, a hot shock-front develops ahead of it and travels to the other extreme of the loop. The shock front compresses the coronal plasma and heats it to about 10(exp 6) K. As a result, a bright moving front becomes visible in the 171 Angstrom channel of the SDO/AIA mission. This result generally applies to all the other explored cases, except for the propagation in the hotter loop. Conclusions. For a cool, low-density initial coronal loop, the post-shock plasma ahead of upward chromospheric flows might explain at least part of the observed correspondence between type II spicules and EUV emission excess
Whole-body magnetic resonance imaging (WB-MRI) for cancer screening in asymptomatic subjects of the general population: review and recommendations.
BACKGROUND:The number of studies describing the use of whole-body magnetic resonance imaging (WB-MRI) for screening of malignant tumours in asymptomatic subjects is increasing. Our aim is to review the methodologies used and the results of the published studies on per patient and per lesion analysis, and to provide recommendations on the use of WB-MRI for cancer screening. MAIN BODY:We identified 12 studies, encompassing 6214 WB-MRI examinations, which provided the rates of abnormal findings and findings suspicious for cancer in asymptomatic subjects, from the general population. Eleven of 12 studies provided imaging protocols that included T1- and T2-weighted sequences, while only five included diffusion weighted imaging (DWI) of the whole body. Different categorical systems were used for the classification and the management of abnormal findings. Of 17,961 abnormal findings reported, 91% were benign, while 9% were oncologically relevant, requiring further investigations, and 0.5% of lesions were suspicious for cancer. A per-subject analysis showed that just 5% of subjects had no abnormal findings, while 95% had abnormal findings. Findings requiring further investigation were reported in 30% of all subjects, though in only 1.8% cancer was suspected. The overall rate of histologically confirmed cancer was 1.1%. CONCLUSION:WB-MRI studies of cancer screening in the asymptomatic general population are too heterogeneous to draw impactful conclusions regarding efficacy. A 5-point lesion scale based on the oncological relevance of findings appears the most appropriate for risk-based management stratification. WB-MRI examinations should be reported by experienced oncological radiologists versed on WB-MRI reading abnormalities and on onward referral pathways
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