35 research outputs found
EIS/Hinode observations of Doppler flow seen through the 40 arcsec wide slit
The Extreme ultraviolet Imaging Spectrometer (EIS) on board Hinode is the
first solar telescope to obtain wide slit spectral images that can be used for
detecting Doppler flows in transition region and coronal lines on the Sun and
to relate them to their surrounding small scale dynamics. We select EIS lines
covering the temperature range 6x10^4 K to 2x10^6 K that give spectrally pure
images of the Sun with the 40 arcsec slit. In these images Doppler shifts are
seen as horizontal brightenings. Inside the image it is difficult to
distinguish shifts from horizontal structures but emission beyond the image
edge can be unambiguously identified as a line shift in several lines separated
from others on their blue or red side by more than the width of the
spectrometer slit (40 pixels). In the blue wing of He II, we find a large
number of events with properties (size and lifetime) similar to the
well-studied explosive events seen in the ultraviolet spectral range.
Comparison with X-Ray Telescope (XRT) images shows many Doppler shift events at
the footpoints of small X-ray loops. The most spectacular event observed showed
a strong blue shift in transition region and lower corona lines from a small
X-ray spot that lasted less than 7 min. The emission appears to be near a cool
coronal loop connecting an X-ray bright point to an adjacent region of quiet
Sun. The width of the emission implies a line-of-sight velocity of 220 km/s. In
addition, we show an example of an Fe XV shift with a velocity about 120 km/s,
coming from what looks like a narrow loop leg connecting a small X-ray
brightening to a larger region of X-ray emission.Comment: 12 pages, 8 figures, to be published in Solar Physic
Off-limb (spicule) DEM distribution from SoHO/SUMER observations
In the present work we derive a Differential Emission Measure (DEM) dis-
tribution from a region dominated by spicules. We use spectral data from the
Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer
on-board the Solar Heliospheric Observatory (SoHO) covering the entire SUMER
wavelength range taken off-limb in the Northern polar coronal hole to construct
this DEM distribution using the CHIANTI atomic database. This distribution is
then used to study the thermal properties of the emission contributing to the
171 {\AA} channel in the Atmospheric Imaging Assembly (AIA) on-board the Solar
Dynamics Observatory (SDO). From our off-limb DEM we found that the radiance in
the AIA 171 {\AA} channel is dominated by emission from the Fe ix 171.07 {\AA}
line and has sparingly little contribution from other lines. The product of the
Fe ix 171.07 {\AA} line contribution function with the off-limb DEM was found
to have a maximum at logTmax (K) = 5.8 indicating that during spicule
observations the emission in this line comes from plasma at transition region
temperatures rather than coronal. For comparison, the same product with a quiet
Sun and prominence DEM were found to have a maximum at logT max (K) = 5.9 and
logTmax (K) = 5.7, respectively. We point out that the interpretation of data
obtained from the AIA 171 {\AA} filter should be done with foreknowledge of the
thermal nature of the observed phenomenon. For example, with an off-limb DEM we
find that only 3.6% of the plasma is above a million degrees, whereas using a
quiet Sun DEM, this contribution rises to 15%.Comment: 12 pages, 6 figures accepted by Solar Physic
Investigation of quasi-periodic varaiations in hard X-rays of solar flares
The aim of the present paper is to use quasi-periodic oscillations in hard
X-rays (HXRs) of solar flares as a diagnostic tool for investigation of
impulsive electron acceleration. We have selected a number of flares which
showed quasi-periodic oscillations in hard X-rays and their loop-top sources
could be easily recognized in HXR images. We have considered MHD standing waves
to explain the observed HXR oscillations. We interpret these HXR oscillations
as being due to oscillations of magnetic traps within cusp-like magnetic
structures. This is confirmed by a good correlation between periods of the
oscillations and the sizes of the loop-top sources. We argue that a model of
oscillating magnetic traps is adequate to explain the observations. During the
compressions of a trap particles are accelerated, but during its expansions
plasma, coming from chromospheric evaporation, fills the trap, which explains
the large number of electrons being accelerated during a sequence of strong
impulses. The advantage of our model of oscillating magnetic traps is that it
can explain both the impulses of electron acceleration and quasi-periodicity of
their distribution in time.Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physic
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
Extreme Ultra-Violet Spectroscopy of the Lower Solar Atmosphere During Solar Flares
The extreme ultraviolet portion of the solar spectrum contains a wealth of
diagnostic tools for probing the lower solar atmosphere in response to an
injection of energy, particularly during the impulsive phase of solar flares.
These include temperature and density sensitive line ratios, Doppler shifted
emission lines and nonthermal broadening, abundance measurements, differential
emission measure profiles, and continuum temperatures and energetics, among
others. In this paper I shall review some of the advances made in recent years
using these techniques, focusing primarily on studies that have utilized data
from Hinode/EIS and SDO/EVE, while also providing some historical background
and a summary of future spectroscopic instrumentation.Comment: 34 pages, 8 figures. Submitted to Solar Physics as part of the
Topical Issue on Solar and Stellar Flare
Spectroscopic Coronal Observations during the Total Solar Eclipse of 11 July 2010
The flash spectrum of the solar chromosphere and corona was measured with a
slitless spectrograph before, after, and during the totality of the solar
eclipse, of 11 July 2010, at Easter Island, Chile. This eclipse took place at
the beginning of the Solar Cycle 24, after an extended minimum of solar
activity. The spectra taken during the eclipse show a different intensity ratio
of the red and green coronal lines compared with those taken during the total
solar eclipse of 1 August 2008, which took place towards the end of the Solar
Cycle 23. The characteristic coronal forbidden emission line of forbidden Fe
XIV (5303 {\AA}) was observed on the east and west solar limbs in four areas
relatively symmetrically located with respect to the solar rotation axis.
Subtraction of the continuum flash-spectrum background led to the
identification of several extremely weak emission lines, including forbidden Ca
XV (5694 {\AA}), which is normally detected only in regions of very high
excitation, e.g., during flares or above large sunspots. The height of the
chromosphere was measured spectrophotometrically, using spectral lines from
light elements and compared with the equivalent height of the lower
chromosphere measured using spectral lines from heavy elements.Comment: 14 pages, 8 figures, 1 table; Solar Physics, 2012, Februar
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Propagating Disturbances in Coronal Loops: A Detailed Analysis of Propagation Speeds
Quasi-periodic disturbances have been observed in the outer solar atmosphere
for many years now. Although first interpreted as upflows (Schrijver et al.
(1999)), they have been widely regarded as slow magnetoacoustic waves, due to
observed velocities and periods. However, recent observations have questioned
this interpretation, as periodic disturbances in Doppler velocity, line width
and profile asymmetry were found to be in phase with the intensity oscillations
(De Pontieu et al. (2010),Tian1 et al. (2011))}, suggesting the disturbances
could be quasi-periodic upflows. Here we conduct a detailed analysis of the
velocities of these disturbances across several wavelengths using the
Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory
(SDO). We analysed 41 examples, including both sunspot and non sunspot regions
of the Sun. We found that the velocities of propagating disturbances (PDs)
located at sunspots are more likely to be temperature dependent, whereas the
velocities of PDs at non sunspot locations do not show a clear temperature
dependence. We also considered on what scale the underlying driver is affecting
the properties of the PDs. Finally, we found that removing the contribution due
to the cooler ions in the 193 A wavelength suggests that a substantial part of
the 193 emission of sunspot PDs can be contributed to the cool component of
193\AA.Comment: 26 Papges, 15 Figure
An Observational Overview of Solar Flares
We present an overview of solar flares and associated phenomena, drawing upon
a wide range of observational data primarily from the RHESSI era. Following an
introductory discussion and overview of the status of observational
capabilities, the article is split into topical sections which deal with
different areas of flare phenomena (footpoints and ribbons, coronal sources,
relationship to coronal mass ejections) and their interconnections. We also
discuss flare soft X-ray spectroscopy and the energetics of the process. The
emphasis is to describe the observations from multiple points of view, while
bearing in mind the models that link them to each other and to theory. The
present theoretical and observational understanding of solar flares is far from
complete, so we conclude with a brief discussion of models, and a list of
missing but important observations.Comment: This is an article for a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011