54 research outputs found
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
Outflows at the Edges of an Active Region in a Coronal Hole: A Signature of Active Region Expansion?
Outflows of plasma at the edges of active regions surrounded by quiet Sun are
now a common observation with the Hinode satellite. While there is
observational evidence to suggest that the outflows are originating in the
magnetic field surrounding the active regions, there is no conclusive evidence
that reveals how they are driven. Motivated by observations of outflows at the
periphery of a mature active region embedded in a coronal hole, we have used a
three-dimensional simulation to emulate the active region's development in
order to investigate the origin and driver of these outflows. We find outflows
are accelerated from a site in the coronal hole magnetic field immediately
surrounding the active region and are channelled along the coronal hole field
as they rise through the atmosphere. The plasma is accelerated simply as a
result of the active region expanding horizontally as it develops. Many of the
characteristics of the outflows generated in the simulation are consistent with
those of observed outflows: velocities up to 45 km per sec, properties akin to
the coronal hole, proximity to the active region's draining loops, expansion
with height, and projection over monopolar photospheric magnetic
concentrations. Although the horizontal expansion occurs as a consequence of
the active region's development in the simulation, expansion is also a general
feature of established active regions. Hence, it is entirely possible and
plausible that the expansion acceleration mechanism displayed in the simulation
is occurring in active regions on the Sun and, in addition to reconnection, is
driving the outflows observed at their edges.Comment: 19 pages, 9 figure
EUV Analysis of a Quasi-Static Coronal Loop Structure
Decaying active region 10942 is investigated from 4:00-16:00 UT on February
24, 2007 using a suite of EUV observing instruments. Results from Hinode/EIS,
STEREO and TRACE show that although the active region has decayed and no
sunspot is present, the physical mechanisms that produce distinguishable loop
structures, spectral line broadening, and plasma flows still occur. A coronal
loop that appears as a blue-shifted structure in Doppler maps is apparent in
intensity images of log(T) = 6.0-6.3 ions. The loop structure is found to be
anti-correlated with spectral line broadening generally attributed to
nonthermal velocities. This coronal loop structure is investigated physically
(temperature, density, geometry) and temporally. Lightcurves created from
imaging instruments show brightening and dimming of the loop structure on two
different time scales; short pulses of 10-20 min and long duration dimming of
2-4 hours until its disappearance. The coronal loop structure, formed from
relatively blue-shifted material that is anti-correlated with spectral line
broadening, shows a density of 10^10 to 10^9.3 cm-3 and is visible for longer
than characteristic cooling times. The maximum nonthermal spectral line
broadenings are found to be adjacent to the footpoint of the coronal loop
structure.Comment: 26 pages, 13 figures; Solar Physics 201
EUV Spectra of the Full Solar Disk: Analysis and Results of the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS)
We analyze EUV spectra of the full solar disk from the Cosmic Hot
Interstellar Plasma Spectrometer (CHIPS) spanning a period of two years. The
observations were obtained via a fortuitous off-axis light path in the 140 --
270 Angstrom passband. The general appearance of the spectra remained
relatively stable over the two-year time period, but did show significant
variations of up to 25% between two sets of Fe lines that show peak emission at
1 MK and 2 MK. The variations occur at a measured period of 27.2 days and are
caused by regions of hotter and cooler plasma rotating into, and out of, the
field of view. The CHIANTI spectral code is employed to determine plasma
temperatures, densities, and emission measures. A set of five isothermal
plasmas fit the full disk spectra well. A 1 -- 2 MK plasma of Fe contributes
85% of the total emission in the CHIPS passband. The standard Differential
Emission Measures (DEMs) supplied with the CHIANTI package do not fit the CHIPS
spectra well as they over-predict emission at temperatures below log(T) = 6.0
and above log(T) = 6.3. The results are important for cross-calibrating TIMED,
SORCE, SOHO/EIT, and CDS/GIS, as well as the recently launched Solar Dynamics
Observatory.Comment: 27 Pages, 13 Figure
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
- …