788 research outputs found
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
Principal Component Analysis to correct data systematics. Case study: K2 light curves
Instrumental data are affected by systematic effects that dominate the errors
and can be relevant when searching for small signals. This is the case of the
K2 mission, a follow up of the Kepler mission, that, after a failure on two
reaction wheels, has lost its stability properties rising strongly the
systematics in the light curves and reducing its photometric precision. In this
work, we have developed a general method to remove time related systematics
from a set of light curves, that has been applied to K2 data. The method uses
the Principal Component Analysis to retrieve the correlation between the light
curves due to the systematics and to remove its effect without knowing any
information other than the data itself. We have applied the method to all the
K2 campaigns available at the Mikulski Archive for Space Telescopes, and we
have tested the effectiveness of the procedure and its capability in preserving
the astrophysical signal on a few transits and on eclipsing binaries. One
product of this work is the identification of stable sources along the ecliptic
plane that can be used as photometric calibrators for the upcoming Atmospheric
Remote-sensing Exoplanet Large-survey mission.Comment: 20 pages, 7 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
SASE FEL Storage Ring
We explore the possibility of operating a SASE FEL with a Storage Ring. We
use a semi-analytical model to obtain the evolution inside the undulator by
taking into account the interplay on the laser dynamics due to the induced
energy spread and to the radiation damping. We obtain the Renieri's limit for
the stationary output power and discuss the possibility of including in our
model the effect of the beam instabilities.Comment: 5 page
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