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Deeper by the Dozen: Understanding the Cross-Field Temperature Distributions of Coronal Loops  

Joan Schmelz   Submitted: 2013-01-25 09:18

Spectroscopic analysis of coronal loops has revealed a variety of cross-field temperature distributions. Some loops appear to be isothermal while others require multithermal plasma. The EUV Imaging Spectrometer on Hinode has the spatial resolution and temperature coverage required for differential emission measure (DEM) analysis of coronal loops. Our results also use data from the X-Ray Telescope on Hinode as a high-temperature constraint. Of our 12 loops, two were post-flare loops with broad temperature distributions, two were narrow but not quite isothermal, and the remaining eight were in the mid range. We consider our DEM methods to be a significant advance over previous work, and it is also reassuring to learn that our findings are consistent with results available in the literature. For the quiescent loops analyzed here, 10 MK plasma, a signature of nanoflares, appears to be absent at a level of approximately two orders of magnitude down from the DEM peak. We find some evidence that warmer loops require broader DEMs. The cross-field temperatures obtained here cannot be modeled as single flux tubes. Rather, the observed loop must be composed of several or many unresolved strands. The plasma contained in each of these strands could be cooling at different rates, contributing to the multithermal nature of the observed loop pixels. An important implication of our DEM results involves observations from future instruments. Once solar telescopes can truly resolve X-ray and EUV coronal structures, these images would have to reveal the loop substructure implied by our multithermal results.

Authors: Schmelz, J.T., Pathak, S. Jenkins, B.S., Worley, B.T.
Projects: Hinode/EIS

Publication Status: 2013, ApJ, 764, 53
Last Modified: 2013-01-28 09:28
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Atmospheric Imaging Assembly Response Functions: Solving the Fe VIII Problems with Hinode EIS Bright Point Data  

Joan Schmelz   Submitted: 2013-01-09 10:23

The Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory is a state-of-the-art imager with the potential to do unprecedented time-dependent multi-thermal analysis at every pixel on scales short compared to the radiative and conductive cooling times. Recent results, however, have identified missing spectral lines in the CHIANTI atomic physics data base, which is used to construct the instrument response functions. This is not surprising since the wavelength range from 90 ? to 140 ? has rarely been observed with solar spectrometers, and atomic data for many of these ions are simply not available in the literature. We have done differential emission measure analysis using simultaneous AIA and Hinode/EIS observations of six X-ray bright points. Our results not only support the conclusion that CHIANTI is incomplete near 131 Å, but more importantly, suggest that the peak temperature of the Fe VIII emissivity/response is likely to be closer to log T = 5.8 than to the current value of log T = 5.7. Using a revised emissivity/response calculation for Fe VIII, we find that the observed AIA 131-? flux can be underestimated by ~1.25, which is smaller than previous comparisons. Making these adjustments brings not only the AIA 131-? data but also the EIS Fe VIII lines into better agreement with the remainder of the bright point data. In addition, we find that CHIANTI is reasonably complete in the AIA 171- and 193-? bands. For the AIA 211-, 335-, and 94-? channels, we recommend that more work be done with AIA?EIS DEM comparisons using observations of active region cores, coronal structures with more warm emission measure than our bright points. Then direct comparisons can be made with a variety of EIS iron lines.

Authors: J.T. Schmelz, B.S. Jenkins, J.A. Kimble
Projects: SDO-AIA

Publication Status: Solar Physics, 2013, DOI 10.1007/s11207-012-0208-1
Last Modified: 2013-01-09 10:40
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Composition of the Solar Corona, Solar Wind, and Solar Energetic Particles  

Joan Schmelz   Submitted: 2012-06-08 09:26

Along with temperature and density, the elemental abundance is a basic parameter required by astronomers to understand and model any physical system. The abundances of the solar corona are known to differ from those of the solar photosphere via a mechanism related to the first ionization potential of the element, but the normalization of these values with respect to hydrogen is challenging. Here we show that the values used by solar physicists for over a decade and currently referred to as the ``coronal abundances'' do not agree with the data themselves. As a result, recent analysis and interpretation of solar data involving coronal abundances may need to be revised. We use observations from coronal spectroscopy, the solar wind, and solar energetic particles as well as the latest abundances of the solar photosphere to establish a new set of abundances that reflect our current understanding of the coronal plasma.

Authors: J.T. Schmelz, D.V. Reames, R. von Steiger, S. Basu
Projects: None

Publication Status: ApJ in press
Last Modified: 2012-06-08 14:46
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Warm and Fuzzy: Temperature and Density Analysis of an Fe XV EIS Loop  

Joan Schmelz   Submitted: 2011-07-11 09:40

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: J.T. Schmelz, L.A. Rightmire, S.H. Saar, J.A. Kimble, B.T. Worley, S. Pathak
Projects: Hinode/EIS

Publication Status: ApJ (in press)
Last Modified: 2011-07-11 10:02
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Isothermal and Multithermal Analysis of Coronal Loops Observed with AIA: II. 211 Å Selected Loops  

Joan Schmelz   Submitted: 2011-07-08 10:41

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: J.T. Schmelz, B.T. Worley, D.J. Anderson , S. Pathak, J.A. Kimble, B.S. Jenkins, S.H. Saar
Projects: SDO-AIA

Publication Status: ApJ (in press)
Last Modified: 2011-07-08 11:23
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Isothermal and Multithermal Analysis of Coronal Loops Observed with AIA  

Joan Schmelz   Submitted: 2011-02-14 09:48

The coronal filters in the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory peak at different temperatures; the series covers the entire active region temperature range, making AIA ideal for multi-thermal analysis. Here we analyze coronal loops from several active regions that have been observed by AIA. We have specifically targeted cool loops (or at least loops with a cool component) that were chosen in the 171Å channel of AIA, which has a peak response temperature of Log T = 5.8. We wanted to determine if the loops could be described as isothermal or multithermal. We find that several of our 12 loops have narrow temperature distributions, which may be consistent with isothermal plasma; these can be modeled with a single flux tube. Other loops have intermediate-width temperature distributions, appear well-constrained, and should be multi-stranded. The remaining loops, however, have unrealistically broad DEMs. We find that this problem is the result of missing low-temperature lines in the AIA 131Å channel. If we repeat the analysis without the 131Å data, these loops also appear to be well-constrained and multi-stranded.

Authors: Schmelz, Jenkins, Worley, Anderson, Pathak, Kimble
Projects: SDO-AIA

Publication Status: ApJ accepted for publication
Last Modified: 2011-02-14 11:04
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AIA Multithermal Loop Analysis: First Results  

Joan Schmelz   Submitted: 2011-02-14 09:44

The Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory has state-of-the-art spatial resolution and shows the most detailed images of coronal loops ever observed. The series of coronal filters peak at different temperatures, which span the range of active regions. These features represent a significant improvement over earlier coronal imagers and make AIA ideal for multi-thermal analysis. Here we targeted a 171Å coronal loop in AR 11092 observed by AIA on 2010 August 3. Isothermal analysis using the 171-to-193 ratio gave a temperature of Log T ∼ 6.1, similar to the results of EIT and TRACE. Differential Emission Measure analysis, however, showed that the plasma was multithermal, not isothermal, with the bulk of the emission measure at Log T > 6.1. The result from the isothermal analysis, which is the average of the true plasma distribution weighted by the instrument response functions, appears to be deceptively low. These results have potentially serious implications: EIT and TRACE results, which use the same isothermal method, show substantially smaller temperature gradients than predicted by standard models for loops in hydrodynamic equilibrium and have been used as strong evidence in support of footpoint heating models. These implications may have to be re-examined in the wake of new results from AIA.

Authors: Schmelz, Kimble, Jenkins, Worley, Anderson, Pathak, Saar
Projects: SDO-AIA

Publication Status: ApJ Letters, 725, L34, 2010
Last Modified: 2011-02-14 11:04
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Hinode X-Ray Telescope Detection of Hot Emission from Quiescent Active Regions: A Nanoflare Signature?  

Joan Schmelz   Submitted: 2009-02-13 14:52

The X-Ray Telescope (XRT) on the Japanese/USA/UK {it Hinode (Solar-B)} spacecraft has detected emission from a quiescent active region core that is consistent with nanoflare heating. The fluxes from 10 broadband X-ray filters and filter combinations were used to constructed Differential Emission Measure (DEM) curves. In addition to the expected active region peak at Log T = 6.3-6.5, we find a high-temperature component with significant emission measure at Log T > 7.0. This emission measure is weak compared to the main peak - the DEM is down by almost three orders of magnitude - which accounts of the fact that it has not been observed with earlier instruments. It is also consistent with spectra of quiescent active regions: no Fe XIX lines are observed in a CHIANTI synthetic spectrum generated using the XRT DEM distribution. The DEM result is successfully reproduced with a simple two-component nanoflare model.

Authors: Schmelz, J.T., Saar, S.H., DeLuca, E.E., Golub, L., Kashyap, V.L., Weber, M.A. Klimchuk, J.A.
Projects: Hinode/XRT

Publication Status: Accepted for publication in ApJ Letters
Last Modified: 2009-02-14 10:40
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Are Coronal Loops Isothermal or Multithermal? Yes!  

Joan Schmelz   Submitted: 2009-01-20 13:03

Surprisingly few solar coronal loops have been observed simultaneously with TRACE and SOHO/CDS, and even fewer analyses of these loops have been conducted and published. The SOHO Joint Observing Program 146 was designed in part to provide the simultaneous observations required for in-depth temperature analysis of active region loops and determine whether these loops are isothermal or multithermal. The data analyzed in this paper were taken on 2003 January 17 of AR 10250. We used TRACE filter ratios, emission measure loci, and two methods of differential emission measure analysis to examine the temperature structure of three different loops. TRACE and CDS observations agree that Loop 1 is isothermal with Log T = 5.85, both along the line of sight as well as along the length of the loop leg that is visible in the CDS field of view. Loop 2 is hotter than Loop 1. It is multithermal along the line of sight, with significant emission between 6.2 < Log T < 6.4, but the loop apex region is out of the CDS field of view so it is not possible to determine the temperature distribution as a function of loop height. Loop 3 also appears to be multithermal, but a blended loop that is just barely resolved with CDS may be adding cool emission to the Loop 3 intensities and complicating our results. So, are coronal loops isothermal or multithermal? The answer appears to be yes!

Authors: J.T. Schmelz, K. Nasraoui, L.A. Rightmire, J.A. Kimble, G. Del Zanna, J.W. Cirtain, E.E. DeLuca, H.E. Mason
Projects: SoHO-CDS

Publication Status: The Astrophysical Journal, 691:503?515, 2009 January 20
Last Modified: 2009-01-21 08:50
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All Coronal Loops are the Same: Evidence to the Contrary  

Joan Schmelz   Submitted: 2005-06-02 15:48

The 1998 April 20 spectral line data from CDS on SOHO shows a coronal loop on the solar limb. Our original analysis of these data showed that the plasma was multi-thermal, both along the length of the loop and along the line of sight. However, more recent results by other authors indicate that background subtraction might change these conclusions, so we consider the effect of background subtraction on our analysis. We show Emission Measure (EM) Loci plots of three representative pixels: loop apex, upper leg, and lower leg. Comparisons of the original and background-subtracted intensities show that the EM Loci are more tightly clustered after background subtraction, but that the plasma is still not well represented by an isothermal model. Our results taken together with those of other authors indicate that a variety of temperature structures may be present within loops.

Authors: J.T. Schmelz, K. Nasraoui, V.L. Richardson, P.J. Hubbard, C.R. Nevels, J.E. Beene (University of Memphis)
Projects: Soho-CDS

Publication Status: ApJ Letters -- accepted for publication
Last Modified: 2005-06-02 15:48
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Abstracts by Author
Deeper by the Dozen: Understanding the Cross-Field Temperature Distributions of Coronal Loops
Atmospheric Imaging Assembly Response Functions: Solving the Fe VIII Problems with Hinode EIS Bright Point Data
Composition of the Solar Corona, Solar Wind, and Solar Energetic Particles
Warm and Fuzzy: Temperature and Density Analysis of an Fe XV EIS Loop
Isothermal and Multithermal Analysis of Coronal Loops Observed with AIA: II. 211 A Selected Loops
Isothermal and Multithermal Analysis of Coronal Loops Observed with AIA
AIA Multithermal Loop Analysis: First Results
Hinode X-Ray Telescope Detection of Hot Emission from Quiescent Active Regions: A Nanoflare Signature?
Are Coronal Loops Isothermal or Multithermal? Yes!
All Coronal Loops are the Same: Evidence to the Contrary

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