143 research outputs found
Velocity vectors of a quiescent prominence observed by Hinode/SOT and the MSDP (Meudon)
The dynamics of prominence fine structures is a challenge to understand the
formation of cool plasma prominence embedded in the hot corona. Recent
observations from the high resolution Hinode/SOT telescope allow us to compute
velocities perpendicularly to the line-of-sight or transverse velocities.
Combining simultaneous observations obtained in H-alpha with Hinode/SOT and the
MSDP spectrograph operating in the Meudon solar tower we derive the velocity
vectors of a quiescent prominence. The velocities perpendicular to the
line-of-sight are measured by time slice technique, the Dopplershifts by the
bisector method. The Dopplershifts of bright threads derived from the MSDP
reach 15 km/s at the edges of the prominence and are between +/- 5 km/s in the
center of the prominence. Even though they are minimum values due to seeing
effect, they are of the same order as the transverse velocities. These
measurements are very important because they suggest that the
verticalstructures shown in SOT may not be real vertical magnetic structures in
the sky plane. The vertical structures could be a pile up of dips in more or
less horizontal magnetic field lines in a 3D perspective, as it was proposed by
many MHD modelers. In our analysis we also calibrate the Hinode H-alpha data
using MSDP observations obtained simultaneously.Comment: 7 pages, 7 figures, submitted to A &
A Search for High-Frequency Coronal Brightness Variations in the 21 August 2017 Total Solar Eclipse
We report on a search for short-period intensity variations in the green-line
FeXIV 530.3 nm emission from the solar corona during the 21 August 2017 total
eclipse viewed from Idaho in the United States. Our experiment was performed
with a much more sensitive detection system, and with better spatial
resolution, than on previous occasions (1999 and 2001 eclipses), allowing fine
details of quiet coronal loops and an active-region loop system to be seen. A
guided 200-mm-aperture Schmidt-Cassegrain telescope was used with a
state-of-the-art CCD camera having 16-bit intensity discrimination and a
field-of-view 0.43 degree x 0.43 degree that encompassed approximately one
third of the visible corona. The camera pixel size was 1.55 arcseconds, while
the seeing during the eclipse enabled features of approx. 2 arcseconds (1450 km
on the Sun) to be resolved. A total of 429 images were recorded during a 122.9
second portion of the totality at a frame rate of 3.49 images per second. In
the analysis, we searched particularly for short-period intensity oscillations
and travelling waves, since theory predicts fast-mode magneto-hydrodynamic
(MHD) waves with short periods may be important in quiet coronal and
active-region heating. Allowing first for various instrumental and photometric
effects, we used a wavelet technique to search for periodicities in some 404
000 pixels in the frequency range 0.5-1.6 Hz (periods: 2 second to 0.6 second).
We also searched for travelling waves along some 65 coronal structures.
However, we found no statistically significant evidence in either. This
negative result considerably refines the limit that we obtained from our
previous analyses, and it indicates that future searches for short-period
coronal waves may be better directed towards Doppler shifts as well as
intensity oscillations
The Influence of Solar Flares on the Lower Solar Atmosphere: Evidence from the Na D Absorption Line Measured by GOLF/SOHO
Solar flares presumably have an impact on the deepest layers of the solar
atmosphere and yet the observational evidence for such an impact is scarce.
Using ten years of measurements of the Na D and Na D Fraunhofer
lines, measured by GOLF onboard SOHO, we show that this photospheric line is
indeed affected by flares. The effect of individual flares is hidden by solar
oscillations, but a statistical analysis based on conditional averaging reveals
a clear signature. Although GOLF can only probe one single wavelength at a
time, we show that both wings of the Na line can nevertheless be compared. The
varying line asymmetry can be interpreted as an upward plasma motion from the
lower solar atmosphere during the peak of the flare, followed by a downward
motion.Comment: 13 pages, 7 figure
Magnetic Field Structures in a Facular Region Observed by THEMIS and Hinode
The main objective of this paper is to build and compare vector magnetic maps
obtained by two spectral polarimeters, i.e. THEMIS/MTR and Hinode SOT/SP, using
two inversion codes (UNNOFIT and MELANIE) based on the Milne-Eddington solar
atmosphere model. To this end, we used observations of a facular region within
active region NOAA 10996 on 23 May 2008, and found consistent results
concerning the field strength, azimuth and inclination distributions. Because
SOT/SP is free from the seeing effect and has better spatial resolution, we
were able to resolve small magnetic polarities with sizes of 1" to 2", and we
could detect strong horizontal magnetic fields, which converge or diverge in
negative or positive facular polarities. These findings support models which
suggest the existence of small vertical flux tube bundles in faculae. A new
method is proposed to get the relative formation heights of the multi-lines
observed by MTR assuming the validity of a flux tube model for the faculae. We
found that the Fe 1 6302.5 \AA line forms at a greater atmospheric height than
the Fe 1 5250.2 \AA line.Comment: 20 pages, 9 figures, 3 tables, accepted for publication in Solar
Physic
Svestka's Research: Then and Now
Zdenek Svestka's research work influenced many fields of solar physics,
especially in the area of flare research. In this article I take five of the
areas that particularly interested him and assess them in a "then and now"
style. His insights in each case were quite sound, although of course in the
modern era we have learned things that he could not readily have envisioned.
His own views about his research life have been published recently in this
journal, to which he contributed so much, and his memoir contains much
additional scientific and personal information (Svestka, 2010).Comment: Invited review for "Solar and Stellar Flares," a conference in honour
of Prof. Zden\v{e}k \v{S}vestka, Prague, June 23-27, 2014. This is a
contribution to a Topical Issue in Solar Physics, based on the presentations
at this meeting (Editors Lyndsay Fletcher and Petr Heinzel
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
First light observations of the solar wind in the outer corona with the Metis coronagraph
In this work, we present an investigation of the wind in the solar corona that has been initiated by observations of the resonantly scattered ultraviolet emission of the coronal plasma obtained with UVCS-SOHO, designed to measure the wind outflow speed by applying Doppler dimming diagnostics. Metis on Solar Orbiter complements the UVCS spectroscopic observations that were performed during solar activity cycle 23 by simultaneously imaging the polarized visible light and the H?» I Lyman-α corona in order to obtain high spatial and temporal resolution maps of the outward velocity of the continuously expanding solar atmosphere. The Metis observations, taken on May 15, 2020, provide the first HI Lyman-α images of the extended corona and the first instantaneous map of the speed of the coronal plasma outflows during the minimum of solar activity and allow us to identify the layer where the slow wind flow is observed. The polarized visible light (580-640 nm) and the ultraviolet HI Lyα (121.6 nm) coronal emissions, obtained with the two Metis channels, were combined in order to measure the dimming of the UV emission relative to a static corona. This effect is caused by the outward motion of the coronal plasma along the direction of incidence of the chromospheric photons on the coronal neutral hydrogen. The plasma outflow velocity was then derived as a function of the measured Doppler dimming. The static corona UV emission was simulated on the basis of the plasma electron density inferred from the polarized visible light. This study leads to the identification, in the velocity maps of the solar corona, of the high-density layer about ±10° wide, centered on the extension of a quiet equatorial streamer present at the east limb - the coronal origin of the heliospheric current sheet - where the slowest wind flows at about 160 ± 18 km s-1 from 4 R⊙ to 6 R⊙. Beyond the boundaries of the high-density layer, the wind velocity rapidly increases, marking the transition between slow and fast wind in the corona
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