J. T. Schmelz

OBSERVATIONAL CONSTRAINTS ON CORONAL HEATING MODELS USING CORONAL DIAGNOSTICS SPECTROMETER AND SOFT X-RAY TELESCOPE DATA

We have constructed a multithermal diUerential emission measure distribution for several pixels along a relatively isolated coronal loop on the southwest limb of the solar disk using spectral line data from the Solar and Heliospheric Observatory coronal diagnostics spectrometer (CDS) and broadband data from the Yohkoh soft X-ray telescope. The temperature distributions are clearly inconsistent with isothermal plasma along either the line of sight or the length of the loop. These conclusions disagree with some recent results that used an isothermal approximation derived from narrowband —lter ratios to calculate loop temperature pro—les. The diUerences between their results and ours could be attributed to pixel

COMPOSITION OF THE SOLAR CORONA, SOLAR WIND, AND SOLAR ENERGETIC PARTICLES

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.

HINODE X-RAY TELESCOPE DETECTION OF HOT EMISSION FROM QUIESCENT ACTIVE REGIONS: A NANOFLARE SIGNATURE?

The X-Ray Telescope (XRT) on the Japanese/USA/UK 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 construct 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.

Neon lights up a controversy : The solar Ne/O abundance

The standard solar model was so reliable that it could predict the existence of the massive neutrino. Helioseismology measurements were so precise that they could determine the depth of the convection zone. This agreement between theory and observation was the envy of all astrophysics—until recently, when sophisticated three-dimensional hydrodynamic calculations of the solar atmosphere reduced the metal content by a factor of almost 2. Antia & Basu suggested that a higher value of the solar neon abundance, ANe/AO = 0.52, would resolve this controversy. Drake & Testa presented evidence in favor of this idea from a sample of 21 Chandra stars with enhanced values of the neon abundance, ANe/AO = 0.41. In this Letter, we have analyzed solar active region spectra from the archive of the Flat Crystal Spectrometer on the Solar Maximum Mission, a NASA mission from the 1980s, as well as full-Sun spectra from the pioneering days of X-ray astronomy in the 1960s. These data are consistent with the standard neon-to-oxygen abundance value, ANe/AO = 0.15 (Grevesse & Sauval). We conclude, therefore, that the enhanced-neon hypothesis will not resolve the current controversy.

The Inadequacy of Temperature Measurements in the Solar Corona through Narrowband Filter and Line Ratios

We analyze the determination of coronal line-of-sight temperatures with the technique of narrowband filter ratios that is currently employed for data obtained with the Transition Region and Coronal Explorer and the EUV Imaging Telescope on board the Solar and Heliospheric Observatory. We demonstrate that the simple fact that the observed differential emission measure curves in coronal loops have a broad plateau everywhere along the length of the loop leads to the finding of isothermal loops with different temperatures for each pair of filters. We show that none of the temperatures thus obtained correctly describe the state of the loop plasma, which instead must be characterized by the full differential emission measure per pixel. We conclude that the recent discovery of a new class of isothermal loops is probably a mere artifact of the narrowband filter ratio method and show that the shift in the location of the plateau in the differential emission measure along the loop indicates significant heating near the loop tops.

Fe XVII Soft X-Ray Lines: Theory and Data Comparisons

Fe XVII soft X-ray spectral emission lines are examined using data from the Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission satellite. Results are compared with theoretical calculations and with other recent observational results. Disparate findings from different studies on the inferred opacity of the bright resonance line at 15.01 A and on its center-to-limb behavior are reviewed. Present limitations on the use of resonance scattering to infer coronal plasma densities and absolute elemental abundances are discussed. An analysis is made of the temperature-insensitive ratio of the 15.01 A line of Fe XVII to the optically thin 16.78 A line. This analysis shows that approximately half of the photons expected in the 15.01 A line are missing from the bright emission cores of quiescent active regions on the solar disk; the missing fraction increases at most by 50% near the solar limb. If the missing flux has been resonantly scattered out of the line of sight, then the equivalent optical depth at line center of the 15.01 A line is τ0~2 on the disk, based on a simple escape probability treatment for a slab geometry. This suggests that the effects of resonance scattering for other FCS lines, with the possible exception of the O VIII doublet at 18.97 A, should be negligible for quiescent active region conditions. This is consistent with the lack of systematic center-to-limb dependence found previously for FCS lines other than Fe XVII at 15.01 A. Yohkoh Soft X-ray Telescope measurements of the expected lines of sight for active regions as a function of location on the solar disk, and resonance scattering results from other soft X-ray active region data sets all support a trend of increased opacity at the limb compared to disk center.

ARE CORONAL LOOPS ISOTHERMAL OR MULTITHERMAL

Surprisingly few solar coronal loops have been observed simultaneously with TRACE and SOHO/Coronal Diagnostics Spectrometer (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 the 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.

Multithermal Analysis of a SOHO/CDS Coronal Loop

The observations from 1998 April 20 taken with the Coronal Diagnostics Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) of a coronal loop on the limb have shown that the plasma was multi-thermal along each line of sight investigated, both before and after background subtraction. The latter result relied on Emission Measure (EM) Loci plots, but in this Letter, we used a forward folding technique to produce Differential Emission Measure (DEM) curves. We also calculate DEM-weighted temperatures for the chosen pixels and find a gradient in temperature along the loop as a function of height that is not compatible with the flat profiles reported by numerous authors for loops observed with the EUV Imaging Telescope (EIT) on SOHO and the Transition Region and Coronal Explorer (TRACE). We also find discrepancies in excess of the mathematical expectation between some of the observed and predicted CDS line intensities. We demonstrate that these differences result from well-known limitations in our knowledge of the atomic data and are to be expected. We further show that the precision of the DEM is limited by the intrinsic width of the ion emissivity functions that are used to calculate the DEM, which for the EUV lines considered is of the order dlog(T) 0.2 - 0.3. Hence we conclude that peaks and valleys in the DEM, while in principle not impossible, cannot be confirmed from the data.

Coronal magnetic structures observing campaign. I - Simultaneous microwave and soft X-ray observations of active regions at the solar limb

Using simultaneous microwave and soft X-ray measurements made with the VLA at 6 and 20 cm and the X-ray Polychromator (XRP) aboard the Solar Maximum Mission, two active regions near the solar limb have been studied. The images in soft X-rays and at 20 cm wavelength are similar : both show peaks above the active regions and extended bridge of emission 200000 km long connecting the two regions. The brightness temperature of the 20 cm emission is lower than that predicted from the X-ray emitting material, however; it can be attributed to free-free emission in cooler plasma not visible to XRP, with an optical depth ∼ 1.

Isothermal bias of the filter ratio method for observations of multithermal plasma

An early result from the Transition Region and Coronal Explorer (TRACE) was that the EUV filter ratios for many narrow coronal loops (widths of a few arcseconds) were found to cluster within the small range 0.50-1.70, as functions of position along loop length. The most common interpretation is that the temperature along the loop is in fact nearly constant with a value between 1.1 and 1.3 MK. This interpretation has resulted in a class of TRACE loop models with heating close to the footpoints. We analyze the filter ratio method to show that the constant TRACE 195 A/173 A ratios can be reproduced by multithermal differential emission measures (DEMs) along the line of sight over a wide range of peak temperatures, so long as the distribution is relatively flat and spans the temperature response of both channels. Furthermore, in the limit of flat (i.e., very multithermal) DEMs, the filter ratio method is biased toward the ratio of the integrals of the temperature response functions. This result is general to any measurement of intensity ratios that are formed over a nonzero temperature range (e.g., narrow passbands and ion emission lines).