1,952 research outputs found
Absolute radiometric calibration of the EUNIS-06 170-205 A channel and calibration update for CDS/NIS
The Extreme-Ultraviolet Normal-Incidence Spectrograph sounding-rocket payload
was flown on 2006 April 12 (EUNIS-06), carrying two independent imaging
spectrographs covering wave bands of 300-370 A in first order and 170-205 A in
second order, respectively. The absolute radiometric response of the EUNIS-06
long-wavelength (LW) channel was directly measured in the same facility used to
calibrate CDS prior to the SOHO launch. Because the absolute calibration of the
short-wavelength (SW) channel could not be obtained from the same lab
configuration, we here present a technique to derive it using a combination of
solar LW spectra and density- and temperature-insensitive line intensity
ratios. The first step in this procedure is to use the coordinated, cospatial
EUNIS and SOHO/CDS spectra to carry out an intensity calibration update for the
CDS NIS-1 waveband, which shows that its efficiency has decreased by a factor
about 1.7 compared to that of the previously implemented calibration. Then,
theoretical insensitive line ratios obtained from CHIANTI allow us to determine
absolute intensities of emission lines within the EUNIS SW bandpass from those
of cospatial CDS/NIS-1 spectra after the EUNIS LW calibration correction. A
total of 12 ratios derived from intensities of 5 CDS and 12 SW emission lines
from Fe Fe X - Fe XIII yield an instrumental response curve for the EUNIS-06 SW
channel that matches well to a relative calibration which relied on combining
measurements of individual optical components. Taking into account all
potential sources of error, we estimate that the EUNIS-06 SW absolute
calibration is accurate to about 20%.Comment: 11 pages, 10 figures, 4 tables. 2010, ApJ Suppl. In pres
Underflight calibration of SOHO/CDS and Hinode/EIS with EUNIS-07
Flights of Goddard Space Flight Center's Extreme-Ultraviolet Normal-Incidence
Spectrograph (EUNIS) sounding rocket in 2006 and 2007 provided updated
radiometric calibrations for SOHO/CDS and Hinode/EIS. EUNIS carried two
independent imaging spectrographs covering wavebands of 300-370 A in first
order and 170-205 A in second order. After each flight, end-to-end radiometric
calibrations of the rocket payload were carried out in the same facility used
for pre-launch calibrations of CDS and EIS. During the 2007 flight, EUNIS, SOHO
CDS and Hinode EIS observed the same solar locations, allowing the EUNIS
calibrations to be directly applied to both CDS and EIS. The measured CDS NIS 1
line intensities calibrated with the standard (version 4) responsivities with
the standard long-term corrections are found to be too low by a factor of 1.5
due to the decrease in responsivity. The EIS calibration update is performed in
two ways. One is using the direct calibration transfer of the calibrated
EUNIS-07 short wavelength (SW) channel. The other is using the insensitive line
pairs, in which one member was observed by EUNIS-07 long wavelength (LW)
channel and the other by EIS in either LW or SW waveband. Measurements from
both methods are in good agreement, and confirm (within the measurement
uncertainties) the EIS responsivity measured directly before the instrument's
launch. The measurements also suggest that the EIS responsivity decreased by a
factor of about 1.2 after the first year of operation. The shape of the EIS SW
response curve obtained by EUNIS-07 is consistent with the one measured in
laboratory prior to launch. The absolute value of the quiet-Sun He II 304 A
intensity measured by EUNIS-07 is consistent with the radiance measured by CDS
NIS in quiet regions near the disk center and the solar minimum irradiance
obtained by CDS NIS and SDO/EVE recently.Comment: 16 pages, 14 figures, 5 tables, accepted by ApJ Supplement (Sep.
2011
Emission lines of Fe XI in the 257--407 A wavelength region observed in solar spectra from EIS/Hinode and SERTS
Theoretical emission-line ratios involving Fe XI transitions in the 257-407 A
wavelength range are derived using fully relativistic calculations of radiative
rates and electron impact excitation cross sections. These are subsequently
compared with both long wavelength channel Extreme-Ultraviolet Imaging
Spectrometer (EIS) spectra from the Hinode satellite (covering 245-291 A), and
first-order observations (235-449 A) obtained by the Solar Extreme-ultraviolet
Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and 276.36 A
lines of Fe XI are detected in two EIS spectra, confirming earlier
identifications of these features, and 276.36 A is found to provide an electron
density diagnostic when ratioed against the 257.55 A transition. Agreement
between theory and observation is found to be generally good for the SERTS data
sets, with discrepancies normally being due to known line blends, while the
257.55 A feature is detected for the first time in SERTS spectra. The most
useful Fe XI electron density diagnostic is found to be the 308.54/352.67
intensity ratio, which varies by a factor of 8.4 between N_e = 10^8 and 10^11
cm^-3, while showing little temperature sensitivity. However, the 349.04/352.67
ratio potentially provides a superior diagnostic, as it involves lines which
are closer in wavelength, and varies by a factor of 14.7 between N_e = 10^8 and
10^11 cm^-3. Unfortunately, the 349.04 A line is relatively weak, and also
blended with the second-order Fe X 174.52 A feature, unless the first-order
instrument response is enhanced.Comment: 9 pages, 5 figures, 13 tables; MNRAS in pres
Analysis and Modeling of Two Flare Loops Observed by AIA and EIS
We analyze and model an M1.0 flare observed by SDO/AIA and Hinode/EIS to
investigate how flare loops are heated and evolve subsequently. The flare is
composed of two distinctive loop systems observed in EUV images. The UV 1600
\AA emission at the feet of these loops exhibits a rapid rise, followed by
enhanced emission in different EUV channels observed by AIA and EIS. Such
behavior is indicative of impulsive energy deposit and the subsequent response
in overlying coronal loops that evolve through different temperatures. Using
the method we recently developed, we infer empirical heating functions from the
rapid rise of the UV light curves for the two loop systems, respectively,
treating them as two big loops of cross-sectional area 5\arcsec by 5\arcsec,
and compute the plasma evolution in the loops using the EBTEL model (Klimchuk
et al. 2008). We compute the synthetic EUV light curves, which, with the
limitation of the model, reasonably agree with observed light curves obtained
in multiple AIA channels and EIS lines: they show the same evolution trend and
their magnitudes are comparable by within a factor of two. Furthermore, we also
compare the computed mean enthalpy flow velocity with the Doppler shift
measurements by EIS during the decay phase of the two loops. Our results
suggest that the two different loops with different heating functions as
inferred from their footpoint UV emission, combined with their different
lengths as measured from imaging observations, give rise to different coronal
plasma evolution patterns captured both in the model and observations.Comment: Accepted for publication in Ap
Emission lines of Fe X in active region spectra obtained with the Solar Extreme-ultraviolet Research Telescope and Spectrograph
Fully relativistic calculations of radiative rates and electron impact
excitation cross sections for Fe X are used to derive theoretical emission-line
ratios involving transitions in the 174-366 A wavelength range. A comparison of
these with solar active region observations obtained during the 1989 and 1995
flights of the Solar Extreme-ultraviolet Research Telescope and Spectrograph
(SERTS) reveals generally very good agreement between theory and experiment.
Several Fe X emission features are detected for the first time in SERTS
spectra, while the transition at 195.32 A is identified for the first time (to
our knowledge) in an astronomical source. The most useful Fe X electron density
diagnostic line ratios are assessed to be 175.27/174.53 and 175.27/177.24,
which both involve lines close in wavelength and free from blends, vary by
factors of 13 between Ne = 1E8 and 1E13 cm-3, and yet show little temperature
sensitivity. Should these lines not be available, then the 257.25/345.74 ratio
may be employed to determine Ne, although this requires an accurate evaluation
of the instrument intensity calibration over a relatively large wavelength
range. However, if the weak 324.73 A line of Fe X is reliably detected, the use
of 324.73/345.74 or 257.25/324.73 is recommended over 257.25/345.74.Comment: 11 pages, 10 figures, MNRAS in pres
The Absolute Abundance of Iron in the Solar Corona
We present a measurement of the abundance of Fe relative to H in the solar
corona using a technique which differs from previous spectroscopic and solar
wind measurements. Our method combines EUV line data from the CDS spectrometer
on SOHO with thermal bremsstrahlung radio data from the VLA. The coronal Fe
abundance is derived by equating the thermal bremsstrahlung radio emission
calculated from the EUV Fe line data to that observed with the VLA, treating
the Fe/H abundance as the sole unknown. We apply this technique to a compact
cool active region and find Fe/H
= 1.56 x 10^{-4}, or about 4 times its value in the solar photosphere.
Uncertainties in the CDS radiometric calibration, the VLA intensity
measurements, the atomic parameters, and the assumptions made in the spectral
analysis yield net uncertainties of order 20%. This result implies that low
first ionization potential elements such as Fe are enhanced in the solar corona
relative to photospheric values.Comment: Astrophysical Journal Letters, in pres
The Structure and Properties of Solar Active Regions and Quiet-Sun Areas Observed in Soft X-Rays with Yohkoh/SXT and in the Extreme-Ultraviolet with SERTS
We observed two solar active regions (NOAA regions 7563 and 7565), quiet-Sun areas, and a coronal hole region simultaneously with Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS) and with the Yohkoh Soft X-ray Telescope (SXT) on 1993 August 17. SERTS provided spatially resolved active region and quiet-Sun slit spectra in the 280 to 420 A wavelength range, and images in the lines of He II λ303.8, Mg IX λ368.1, Fe XV λ284.1, and Fe XVI λλ335.4 and 360.8 SXT provided images through multiple broadband filters in both the full-frame imaging mode and the partial-frame imaging mode. The SERTS images in Fe XV (log Tmax = 6.33, where Tmax is the temperature which maximizes the fractional ion abundance in the available ionization equilibrium calculations, i.e., the formation temperature) and Fe XVI (log Tmax = 6.43) exhibit remarkable morphological similarity to the SXT images. Whereas the Fe XV and XVI images outline the loop structures seen with SXT, the cooler He II (log Tmax = 4.67) and Mg IX (log Tmax = 5.98) images outline loop footpoints. In addition, the Mg IX emission outlines other structures not necessarily associated with the hot loops; these may be cool (T 1 × 106 K) loops. From the spatially resolved slit spectra, we obtained emission-line profiles for lines of He II λ303.8, Mg IX λ368.1, Fe XIII λ348.2, Si XI λ303.3, Fe XIV λ334.2, Fe XV λ284.1, and Fe XVI λ335.4 for each spatial position. Based upon the spatial variations of the line intensities, active region 7563 systematically narrows when viewed with successively hotter lines, and appears narrowest in the broadband soft X-ray emission. The active region width (full width at half-maximum intensity) diminishes linearly with log Tmax; the linear fit yields an extrapolated effective log Tmax of 6.51 ± 0.01 for the X-ray emission. The most intense, central core straddles the magnetic neutral line. Active region and quiet-Sun one-dimensional temperature scans were derived from intensity ratios of spatially resolved SERTS slit spectral lines, and from coregistered SXT filter ratios. The highest plasma temperatures were measured in the most intense, central core of region 7563. The temperatures derived from Fe XVI λ335.4/Fe XV λ284.1 and Fe XVI λ335.4/Fe XIV λ334.2 vary significantly (based upon the measurement uncertainties) but not greatly (factors of less than 1.5) across the slit. The average log T values derived from the above two ratios for region 7563 are 6.39 ± 0.04 and 6.32 ± 0.02, respectively. Somewhat larger systematic variations were obtained from all available SXT filter ratios. The average active region log T values derived from the SXT AlMgMn/thin Al, thick Al/thin Al, and thick Al/AlMgMn filter ratios are 6.33 ± 0.03, 6.45 ± 0.02, and 6.49 ± 0.03, respectively. Active region and quiet-Sun one-dimensional density scans were derived from intensity ratios of spatially resolved SERTS slit spectral lines of Fe XIII and Fe XIV. The derived densities show neither systematic nor significant variations along the slit in either the active region or the quiet-Sun, despite the fact that the intensities themselves vary substantially. This indicates that the product of the volume filling factor and the path length (fΔl) must be greater by factors of 3-5 in the active region core than in the outskirts. Furthermore, the derived active region densities are ~2 times the quiet-Sun densities. This density difference is adequate to explain the factor of ~4 intensity difference in Fe XII and Fe XIII between the active and quiet areas, but it is not adequate to explain the factor of ~8 intensity difference in Fe XIV between the active and quiet areas. We attribute the latter to a greater fΔl in the active regions. Statistically significant Doppler shifts are not detected in region 7563 or in the quiet-Sun with any of the EUV lines
Remote Sensing and the Earth
A text book on remote sensing, as part of the earth resources Skylab programs, is presented. The fundamentals of remote sensing and its application to agriculture, land use, geology, water and marine resources, and environmental monitoring are summarized
Flows in the solar atmosphere due to the eruptions on the 15th July, 2002
<p>Which kind of flows are present during flares? Are they compatible with the present understanding of energy release and which model best describes the observations? We analyze successive flare events in order to answer these questions. The flares were observed in the magnetically complex NOAA active region (AR) 10030 on 15 July 2002. One of them is of GOES X-class. The description of these flares and how they relate to the break-out model is presented in Gary & Moore (2004). The Coronal Diagnostic Spectrometer on board SOHO observed this active region for around 14 h. The observed emission lines provided data from the transition region to the corona with a field of view covering more than half of the active region. In this paper we analyse the spatially resolved flows seen in the atmosphere from the preflare to the flare stages. We find evidence for evaporation occurring before the impulsive phase. During the main phase, the ongoing magnetic reconnection is demonstrated by upflows located at the edges of the flare loops (while downflows are found in the flare loops themselves). We also report the impact of a filament eruption on the atmosphere, with flows up to 300 km s<sup>-1</sup> observed at transition-region temperatures in regions well away from the location of the pre-eruptive filament. Our results are consistent with the predictions of the break out model before the impulsive phase of the flare; while, as the flare progresses, the directions of the flows are consistent with flare models invoking evaporation followed by cooling and downward plasma motions in the flare loops.</p>
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