442 research outputs found
Spectroscopic Observations of Fe XVIII in Solar Active Regions
The large uncertainties associated with measuring the amount of high
temperature emission in solar active regions represents a significant
impediment to making progress on the coronal heating problem. Most current
observations at temperatures of 3 MK and above are taken with broad band soft
X-ray instruments. Such measurements have proven difficult to interpret
unambiguously. Here we present the first spectroscopic observations of the Fe
XVIII 974.86 AA emission line in an on-disk active region taken with then SUMER
instrument on SOHO. Fe XVIII has a peak formation temperature of 7.1 MK and
provides important constraints on the amount of impulsive heating in the
corona. Detailed evaluation of the spectra and comparison of the SUMER data
with soft X-ray images from the XRT on Hinode confirm that this line is
unblended. We also compare the spectroscopic data with observations from the
AIA 94 AA channel on SDO. The AIA 94 AA channel also contains Fe XVIII, but is
blended with emission formed at lower temperatures. We find that is possible to
remove the contaminating blends and form relatively pure Fe XVIII images that
are consistent with the spectroscopic observations from SUMER. The observed
spectra also contain the Ca XIV 943.63 AA line that, although a factor 2 to 6
weaker than the Fe XVIII 974.86 AA line, allows us to probe the plasma around
3.5 MK. The observed ratio between the two lines indicates (isothermal
approximation) that most of the plasma in the brighter Fe XVIII active region
loops is at temperatures between 3.5 and 4 MK.Comment: 12 pages, 5 figures. Submitted as letter to Ap
Constraints on the Heating of High Temperature Active Region Loops: Observations from Hinode and SDO
We present observations of high temperature emission in the core of a solar
active region using instruments on Hinode and SDO. These multi-instrument
observations allow us to determine the distribution of plasma temperatures and
follow the evolution of emission at different temperatures. We find that at the
apex of the high temperature loops the emission measure distribution is
strongly peaked near 4 MK and falls off sharply at both higher and lower
temperatures. Perhaps most significantly, the emission measure at 0.5 MK is
reduced by more than two orders of magnitude from the peak at 4 MK. We also
find that the temporal evolution in broad-band soft X-ray images is relatively
constant over about 6 hours of observing. Observations in the cooler SDO/AIA
bandpasses generally do not show cooling loops in the core of the active
region, consistent with the steady emission observed at high temperatures.
These observations suggest that the high temperature loops observed in the core
of an active region are close to equilibrium. We find that it is possible to
reproduce the relative intensities of high temperature emission lines with a
simple, high-frequency heating scenario where heating events occur on time
scales much less than a cooling time. In contrast, low-frequency heating
scenarios, which are commonly invoked to describe nanoflare models of coronal
heating, do not reproduce the relative intensities of high temperature emission
lines and predict low-temperature emission that is approximately an order of
magnitude too large. We also present an initial look at images from the SDO/AIA
94 A channel, which is sensitive to Fe XVIII.Comment: Movies are available at
http://tcrb.nrl.navy.mil/~hwarren/temp/papers/active_region_core/ Paper has
been refereed and revise
Evidence for Steady Heating: Observations of an Active Region Core with Hinode and TRACE
Previous observations have not been able to exclude the possibility that high
temperature active region loops are actually composed of many small scale
threads that are in various stages of heating and cooling and only appear to be
in equilibrium. With new observations from the EUV Imaging Spectrometer (EIS)
and X-ray Telescope (XRT) on \textit{Hinode} we have the ability to investigate
the properties of high temperature coronal plasma in extraordinary detail. We
examine the emission in the core of an active region and find three independent
lines of evidence for steady heating. We find that the emission observed in XRT
is generally steady for hours, with a fluctuation level of approximately 15% in
an individual pixel. Short-lived impulsive heating events are observed, but
they appear to be unrelated to the steady emission that dominates the active
region. Furthermore, we find no evidence for warm emission that is spatially
correlated with the hot emission, as would be expected if the high temperature
loops are the result of impulsive heating. Finally, we also find that
intensities in the "moss", the footpoints of high temperature loops, are
consistent with steady heating models provided that we account for the local
expansion of the loop from the base of the transition region to the corona. In
combination, these results provide strong evidence that the heating in the core
of an active region is effectively steady, that is, the time between heating
events is short relative to the relevant radiative and conductive cooling
times.Comment: Minor changes based on the final report from the referee; Movies are
available from the first autho
The Heating of the Solar Atmosphere: from the Bottom Up?
The heating of the solar atmosphere remains a mystery. Over the past several decades, scientists have examined the observational properties of structures in the solar atmosphere, notably their temperature, density, lifetime, and geometry, to determine the location, frequency, and duration of heating. In this talk, I will review these observational results, focusing on the wealth of information stored in the light curve of structures in different spectral lines or channels available in the Solar Dynamic Observatory's Atmospheric Imaging Assembly, Hinode's X-ray Telescope and Extreme-ultraviolet Imaging Spectrometer, and the Interface Region Imaging Spectrograph. I will discuss some recent results from combined data sets that support the heating of the solar atmosphere may be dominated by low, near-constant heating events
Institutions, Developmental Alliances, and Economic Development in Korea and Brazil (1950-1985)
This paper compares the development of Korea and Brazil, 1950-85. These newly industrialized countries developed at above-average rates among less developed countries. Korea developed more rapidly than Brazil. The paper contends that institutions, interest groups (especially firms) and the state, enter into developmental alliances. Alliances affect policies. Policies, then, affect development.
Findings reveal interesting trends in the 1950s\u27 democracies of the cases. Both countries had semi-autonomous states, equivocally committed to industrialization. Industry was the growth point in each. Korea used local firms to industrialize; Brazil used foreign firms. In both cases, the state allied itself with firms. Policy mostly favored industrialization. Both countries experienced comparable rates of industrialization.
Findings of the 1960s-1980s\u27 bureaucratic-authoritarianisms (B-A) of the cases also reveal interesting trends. While the state became more autonomous with B-A in both cases, the Korean state became more autonomous. Korea developed an unequivocal commitment to industrialization; Brazil retained its lukewarm industrial commitment. Brazil formed an alliance with public firms to modernize in the 1970s; Korea turned to large local capital. Korea\u27s policy was myopically pro-industrial; Brazil\u27s policy was not as narrowly pro-industrial. Korea displayed phenomenally higher rates of development during B-A than it exhibited in democracy. Brazil continued at a pace comparable to the democratic period.
The paper finds the following conclusions. Autonomous states with high commitment to economic development maximize development. Firm types that optimize economic development are local. Primary developmental alliances of state with fins augment development. Pervasive, pro-industrial policies expand development. To the extent that both cases possessed these characteristics, they developed rapidly. Since Korea possessed these qualities to a greater extent than Brazil, Korea developed more rapidly
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