336 research outputs found
Hinode EUV Imaging Spectrometer Observations of Solar Active Region Dynamics
The EUV Imaging Spectrometer (EIS) on the Hinode satellite is capable of
measuring emission line center positions for Gaussian line profiles to a
fraction of a spectral pixel, resulting in relative solar Doppler-shift
measurements with an accuracy of less than a km/s for strong lines. We show an
example of the application of that capability to an active region sit-and-stare
observation in which the EIS slit is placed at one location on the Sun and many
exposures are taken while the spacecraft tracking keeps the same solar location
within the slit. For the active region examined (NOAA 10930), we find that
significant intensity and Doppler-shift fluctuations as a function of time are
present at a number of locations. These fluctuations appear to be similar to
those observed in high-temperature emission lines with other space-borne
spectroscopic instruments. With its increased sensitivity over earlier
spectrometers and its ability to image many emission lines simultaneously, EIS
should provide significant new constraints on Doppler-shift oscillations in the
corona.Comment: 7 Pages, 7 figure
Observations of Transient Active Region Heating with Hinode
We present observations of transient active region heating events observed
with the Extreme Ultraviolet Imaging Spectrometer (EIS) and X-ray Telescope
(XRT) on Hinode. This initial investigation focuses on NOAA active region 10940
as observed by Hinode on February 1, 2007 between 12 and 19 UT. In these
observations we find numerous examples of transient heating events within the
active region. The high spatial resolution and broad temperature coverage of
these instruments allows us to track the evolution of coronal plasma. The
evolution of the emission observed with XRT and EIS during these events is
generally consistent with loops that have been heated and are cooling. We have
analyzed the most energetic heating event observed during this period, a small
GOES B-class flare, in some detail and present some of the spectral signatures
of the event, such as relative Doppler shifts at one of the loop footpoints and
enhanced line widths during the rise phase of the event. While the analysis of
these transient events has the potential to yield insights into the coronal
heating mechanism, these observations do not rule out the possibility that
there is a strong steady heating level in the active region. Detailed
statistical analysis will be required to address this question definitively
Network oscillations at the boundary of an equatorial coronal hole
We investigate intensity oscillations observed simultaneously in the quiet
chromosphere and in the corona, above an enhanced network area at the boundary
of an equatorial coronal hole. A Fourier analysis is applied to a sequence of
images observed in the 171 A and 1600 A passbands of TRACE. Four interesting
features above the magnetic network are further investigated by using a wavelet
analysis. Our results reveal that, in both the 171 A and 1600 A passbands,
oscillations above the magnetic network show a lack of power at high
frequencies (5.0-8.3 mHz), and a significant power at low (1.3-2.0 mHz) and
intermediate frequencies (2.6-4.0 mHz). The global 5-min oscillation is clearly
present in the 4 analyzed features when seen in the 1600 A passband, and is
also found with enhanced power in feature 1 (leg of a large coronal loop) and
feature 2 (legs of a coronal bright point loop) when seen in the 171 A
passband. Two features above an enhanced network element (feature 3 and feature
4) show repeated propagating behaviors with a dominant period of 10 min and 5
min, respectively. We suggest these oscillations are likely to be slow
magneto-acoustic waves propagating along inclined magnetic field lines, from
the lower solar atmosphere into the corona. The energy flux carried by these
waves is estimated of the order of 40 erg cm\^{-2} s\^{-1} for the 171 A
passband and is far lower than the energy required to heat the quiet corona.
For the 1600 A passband, the energy flux is about 1.4*10^6 erg cm\^{-2}
s\^{-1}, which is about one third of the required energy budget for the
chromosphere.Comment: 7 pages, 8 figure
Observations of Active Region Loops with the EUV Imaging Spectrometer on Hinode
Previous solar observations have shown that coronal loops near 1 MK are
difficult to reconcile with simple heating models. These loops have lifetimes
that are long relative to a radiative cooling time, suggesting quasi-steady
heating. The electron densities in these loops, however, are too high to be
consistent with thermodynamic equilibrium. Models proposed to explain these
properties generally rely on the existence of smaller scale filaments within
the loop that are in various stages of heating and cooling. Such a framework
implies that there should be a distribution of temperatures within a coronal
loop. In this paper we analyze new observations from the EUV Imaging
Spectrometer (EIS) on \textit{Hinode}. EIS is capable of observing active
regions over a wide range of temperatures (\ion{Fe}{8}--\ion{Fe}{17}) at
relatively high spatial resolution (1\arcsec). We find that most isolated
coronal loops that are bright in \ion{Fe}{12} generally have very narrow
temperature distributions ( K), but are not
isothermal. We also derive volumetric filling factors in these loops of
approximately 10%. Both results lend support to the filament models.Comment: Submitted to ApJ
Using a Differential Emission Measure and Density Measurements in an Active Region Core to Test a Steady Heating Model
The frequency of heating events in the corona is an important constraint on
the coronal heating mechanisms. Observations indicate that the intensities and
velocities measured in active region cores are effectively steady, suggesting
that heating events occur rapidly enough to keep high temperature active region
loops close to equilibrium. In this paper, we couple observations of Active
Region 10955 made with XRT and EIS on \textit{Hinode} to test a simple steady
heating model. First we calculate the differential emission measure of the apex
region of the loops in the active region core. We find the DEM to be broad and
peaked around 3\,MK. We then determine the densities in the corresponding
footpoint regions. Using potential field extrapolations to approximate the loop
lengths and the density-sensitive line ratios to infer the magnitude of the
heating, we build a steady heating model for the active region core and find
that we can match the general properties of the observed DEM for the
temperature range of 6.3 Log T 6.7. This model, for the first time,
accounts for the base pressure, loop length, and distribution of apex
temperatures of the core loops. We find that the density-sensitive spectral
line intensities and the bulk of the hot emission in the active region core are
consistent with steady heating. We also find, however, that the steady heating
model cannot address the emission observed at lower temperatures. This emission
may be due to foreground or background structures, or may indicate that the
heating in the core is more complicated. Different heating scenarios must be
tested to determine if they have the same level of agreement.Comment: 16 pages, 9 figures, accepted to Ap
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