1,522 research outputs found
The coronal source of extreme-ultraviolet line profile asymmetries in solar active region outflows
High resolution spectra from the Hinode EUV Imaging Spectrometer (EIS) have
revealed that coronal spectral line profiles are sometimes asymmetric, with a
faint enhancement in the blue wing on the order of 100 km/s. These asymmetries
could be important since they may be subtle, yet diagnostically useful
signatures of coronal heating or solar wind acceleration processes. It has also
been suggested that they are signatures of chromospheric jets supplying mass
and energy to the corona. Until now, however, there have been no studies of the
physical properties of the plasma producing the asymmetries. Here we identify
regions of asymmetric profiles in the outflows of AR 10978 using an asymmetric
Gaussian function and extract the intensities of the faint component using
multiple Gaussian fits. We then derive the temperature structure and chemical
composition of the plasma producing the asymmetries. We find that the
asymmetries are dependent on temperature, and are clearer and stronger in
coronal lines. The temperature distribution peaks around 1.4-1.8 MK with an
emission measure at least an order of magnitude larger than that at 0.6 MK. The
first ionization potential bias is found to be 3-5, implying that the high
speed component of the outflows may also contribute to the slow speed wind.
Observations and models indicate that it takes time for plasma to evolve to a
coronal composition, suggesting that the material is trapped on closed loops
before escaping, perhaps by interchange reconnection. The results, therefore,
identify the plasma producing the asymmetries as having a coronal origin.Comment: ApJ Letters (in press
Determining heating time scales in solar active region cores from AIA/SDO Fe XVIII images
We present a study of the frequency of transient brightenings in the core of
solar active regions as observed in the Fe XVIII line component of AIA/SDO 94 A
filter images. The Fe XVIII emission is isolated using an empirical correction
to remove the contribution of "warm" emission to this channel. Comparing with
simultaneous observations from EIS/Hinode, we find that the variability
observed in Fe XVIII is strongly correlated with the emission from lines formed
at similar temperatures. We examine the evolution of loops in the cores of
active regions at various stages of evolution. Using a newly developed event
detection algorithm we characterize the distribution of event frequency,
duration, and magnitude in these active regions. These distributions are
similar for regions of similar age and show a consistent pattern as the regions
age. This suggests that these characteristics are important constraints for
models of solar active regions. We find that the typical frequency of the
intensity fluctuations is about 1400s for any given line-of-sight, i.e. about
2-3 events per hour. Using the EBTEL 0D hydrodynamic model, however, we show
that this only sets a lower limit on the heating frequency along that
line-of-sight.Comment: Submitted to Ap
Temporal Variability of Active Region Outflows
Recent observations from the Extreme-ultraviolet Imaging Spectrometer (EIS)
on board Hinode have shown that low density areas on the periphery of active
regions are characterized by strong blue-shifts at 1 MK. These Doppler shifts
have been associated with outward propagating disturbances observed with
Extreme-ultraviolet and soft X-ray imagers. Since these instruments can have
broad temperature responses we investigate these intensity fluctuations using
the monochromatic imaging capabilities of EIS and confirm their 1 MK nature. We
also find that the Fe XII 195.119 A blue shifted spectral profiles at their
footpoints exhibit transient blue wing enhancements on timescales as short as
the 5 minute cadence. We have also looked at the fan peripheral loops observed
at 0.6 MK in Si VII 275.368 A in those regions and find no sign of the
recurrent outward propagating disturbances with velocities of 40 - 130 km/s
seen in Fe XII. We do observe downward trends (15 - 20 km/s) consistent with
the characteristic red-shifts measured at their footpoints. We, therefore, find
no evidence that the structures at these two temperatures and the intensity
fluctuations they exhibit are related to one another.Comment: Movies are available at
http://tcrb.nrl.navy.mil/~iuu/out/papers/2010_flows/. To be submitted to Ap
Measurements of Non-Thermal Line Widths in Solar Active Regions
Spectral line widths are often observed to be larger than can be accounted
for by thermal and instrumental broadening alone. This excess broadening is a
key observational constraint for both nanoflare and wave dissipation models of
coronal heating. Here we present a survey of non-thermal velocities measured in
the high temperature loops (1--5MK) often found in the cores of solar active
regions. This survey of Extreme Ultraviolet Imaging
Spectrometer (EIS) observations covers 15 non-flaring active regions that span
a wide range of solar conditions. We find relatively small non-thermal
velocities, with a mean value of 17km s, and no significant trend with
temperature or active region magnetic flux. These measurements appear to be
inconsistent with those expected from reconnection jets in the corona,
chromospheric evaporation induced by coronal nanoflares, and Alfv\'en wave
turbulence models. Furthermore, because the observed non-thermal widths are
generally small their measurements are difficult and susceptible to systematic
effects.Comment: This is the revised version to be published in Ap
The Multi-Instrument (EVE-RHESSI) DEM for Solar Flares, and Implications for Non-Thermal Emission
Solar flare X-ray spectra are typically dominated by thermal bremsstrahlung
emission in the soft X-ray (10 keV) energy range; for hard X-ray
energies (30 keV), emission is typically non-thermal from beams of
electrons. The low-energy extent of non-thermal emission has only been loosely
quantified. It has been difficult to obtain a lower limit for a possible
non-thermal cutoff energy due to the significantly dominant thermal emission.
Here we use solar flare data from the EUV Variability Experiment (EVE)
on-board the Solar Dynamics Observatory (SDO) and X-ray data from the Reuven
Ramaty High Energy Spectroscopic Imager (RHESSI) to calculate the Differential
Emission Measure (DEM). This improvement over the isothermal approximation and
any single-instrument DEM helps to resolve ambiguities in the range where
thermal and non-thermal emission overlap, and to provide constraints on the
low-energy cutoff.
In the model, thermal emission is from a DEM that is parametrized as multiple
gaussians in . Non-thermal emission results from a photon spectrum
obtained using a thick-target emission model. Spectra for both instruments are
fit simultaneously in a self-consistent manner.
Our results have been obtained using a sample of 52 large (GOES X- and
M-class) solar flares observed between February 2011 and February 2013. It
turns out that it is often possible to determine low-energy cutoffs early (in
the first two minutes) during large flares. Cutoff energies are typically low,
less than 10 keV, with most values of the lower limits in the 5--7 keV range.Comment: 10 pages, 12 figures, 2 tables; accepted for publication in
Astrophysical Journal; then withdrawn; now 19 pages, 24 figure
Solar Coronal Loops Resolved by Hinode and SDO
Despite decades of studying the Sun, the coronal heating problem remains
unsolved. One fundamental issue is that we do not know the spatial scale of the
coronal heating mechanism. At a spatial resolution of 1000 km or more it is
likely that most observations represent superpositions of multiple unresolved
structures. In this letter, we use a combination of spectroscopic data from the
Hinode EUV Imaging Spectrometer (EIS) and high resolution images from the
Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory to
determine the spatial scales of coronal loops. We use density measurements to
construct multi-thread models of the observed loops and confirm these models
using the higher spatial resolution imaging data. The results allow us to set
constraints on the number of threads needed to reproduce a particular loop
structure. We demonstrate that in several cases million degree loops are
revealed to be single monolithic structures that are fully spatially resolved
by current instruments. The majority of loops, however, must be composed of a
number of finer, unresolved threads; but the models suggest that even for these
loops the number of threads could be small, implying that they are also close
to being resolved. These results challenge heating models of loops based on the
reconnection of braided magnetic fields in the corona.Comment: Revised version accepted by ApJ Letter
Observations of Thermal Flare Plasma with the EUV Variability Experiment
One of the defining characteristics of a solar flare is the impulsive
formation of very high temperature plasma. The properties of the thermal
emission are not well understood, however, and the analysis of solar flare
observations is often predicated on the assumption that the flare plasma is
isothermal. The EUV Variability Experiment (EVE) on the Solar Dynamics
Observatory (SDO) provides spectrally resolved observations of emission lines
that span a wide range of temperatures (e.g., Fe XV-Fe XXIV) and allow for
thermal flare plasma to be studied in detail. In this paper we describe a
method for computing the differential emission measure distribution in a flare
using EVE observations and apply it to several representative events. We find
that in all phases of the flare the differential emission measure distribution
is broad. Comparisons of EVE spectra with calculations based on parameters
derived from the GOES soft X-ray fluxes indicate that the isothermal
approximation is generally a poor representation of the thermal structure of a
flare.Comment: Paper has not been submitted. Comments are welcome
A Systematic Survey of High Temperature Emission in Solar Active Regions
The recent analysis of observations taken with the EIS instrument on Hinode
suggests that well constrained measurements of the temperature distribution in
solar active regions can finally be made. Such measurements are critical for
constraining theories of coronal heating. Past analysis, however, has suffered
from limited sample sizes and large uncertainties at temperatures between 5 and
10 MK. Here we present a systematic study of the differential emission cores.
We focus on measurements in the "inter-moss" region, that is, the region
between the loop footpoints, where the observations are easier to interpret. To
reduce the uncertainties at the highest temperatures we present a new method
for isolating the Fe XVIII emission in the AIA/SDO 94 channel. The resulting
differential emission measure distributions confirm our previous analysis
showing that the temperature distribution in an active region core is often
strongly peaked near 4 MK. We characterize the properties of the emission
distribution as a function of the total unsigned magnetic flux. We find that
the amount of high temperature emission in the active region core is correlated
with the total unsigned magnetic flux, while the emission at lower
temperatures, in contrast, is inversely related. These results provide
compelling evidence that high temperature active region emission is often close
to equilibrium, although weaker active regions may be dominated by evolving
million degree loops in the core.Comment: Figures are degraded to reduce file sizes. Email the first author for
a higher resolution version of the pdf. Submitted to ApJ. Minor revisions
have been made to the manuscript. The AIA data were reprocessed and the
emission measures have changed slightl
Chromospheric Evaporation in an M1.8 Flare Observed by the Extreme-ultraviolet Imaging Spectrometer on Hinode
We discuss observations of chromospheric evaporation for a complex flare that
occurred on 9 March 2012 near 03:30 UT obtained from the Extreme-ultraviolet
Imaging Spectrometer (EIS) on the Hinode spacecraft. This was a multiple event
with a strong energy input that reached the M1.8 class when observed by EIS.
EIS obtained a full-CCD spectrum of the flare. Chromospheric evaporation
characterized by 150-200 km/s upflows was observed in multiple locations in
multi-million degree spectral lines of flare ions such as Fe XXII, Fe XXIII,
and Fe XXIV, with simultaneous 20-60 km/s upflows in million degree coronal
lines from ions such as Fe XII - Fe XVI. The behavior of cooler, transition
region ions such as O VI, Fe VIII, He II, and Fe X is more complex, but upflows
were also observed in Fe VIII and Fe X lines. At a point close to strong energy
input in space and time, the flare ions Fe XXII, Fe XXIII, and Fe XXIV reveal
an isothermal source with a temperature close to 14 MK and no strong
blueshifted components. At this location there is a strong downflow in cooler
active region lines from ions such as Fe XIII and Fe XIV. We speculate that
this downflow may be evidence of the downward shock produced by reconnection in
the current sheet seen in MHD simulations. A sunquake also occurred near this
location.Comment: 14 pages, 1 table, 17 figure
Properties and Modeling of Unresolved Fine Structure Loops Observed in the Solar Transition Region by IRIS
Recent observations from the Interface Region Imaging Spectrograph (IRIS)
have discovered a new class of numerous low-lying dynamic loop structures, and
it has been argued that they are the long-postulated unresolved fine structures
(UFS) that dominate the emission of the solar transition region. In this
letter, we combine IRIS measurements of the properties of a sample of 108 UFS
(intensities, lengths, widths, lifetimes) with 1-D non-equilibrium ionization
simulations using the HYDRAD hydrodynamic model to examine whether the UFS are
now truly spatially resolved in the sense of being individual structures rather
than composed of multiple magnetic threads. We find that a simulation of an
impulsively heated single strand can reproduce most of the observed properties
suggesting that the UFS may be resolved, and the distribution of UFS widths
implies that they are structured on a spatial scale of 133km on average.
Spatial scales of a few hundred km appear to be typical for a range of
chromospheric and coronal structures, and we conjecture that this could be an
important clue to the coronal heating process.Comment: This is the version to be publishe
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