Differential Emission Measure Curves Research Articles

Temperature and density structure of a recurring active region jet

We present a study of a recurring jet observed on October 31, 2011 by SDO/AIA, Hinode/XRT and Hinode/EIS. We discuss the physical parameters of the jet such as density, differential emission measure, peak temperature, velocity and filling factor obtained using imaging and spectroscopic observations. A differential emission measure (DEM) analysis was performed at the region of the jet-spire and the footpoint using EIS observations and also by combining AIA and XRT observations. The DEM curves were used to create synthetic spectra with the CHIANTI atomic database. The plasma along the line-of-sight in the jet-spire and jet-footpoint was found to be peak at 2.0 MK. We calculated electron densities using the Fe XII ($\lambda$186/$\lambda$195) line ratio in the region of the spire (Ne = 7.6x$10^{10}$ $cm^{-3}$) and the footpoint (1.1x$10^{11}$ $cm^{-3}$). The plane-of-sky velocity of the jet is found to be 524 km/s. The resulting EIS DEM values are in good agreement with those obtained from AIA-XRT. There is no indication of high temperatures, such as emission from Fe XVII ($\lambda$254.87) (log T [K] = 6.75) seen in the jet-spire. In case of the jet-footpoint, synthetic spectra predict weak contributions from Ca XVII ($\lambda$192.85) and Fe XVII ($\lambda$254.87). With further investigation, we confirmed emission from the Fe XVIII ($\lambda$93.932) line in the AIA 94 ${\AA}$ channel in the region of the footpoint. We also found good agreement between the estimated and predicted Fe XVIII count rates. A study of the temporal evolution of the jet-footpoint and the presence of high-temperature emission from the Fe XVIII (log T [K] = 6.85) line leads us to conclude that the hot component in the jet-footpoint was present initially that the jet had cooled down by the time EIS observed it.

MULTIMODAL DIFFERENTIAL EMISSION MEASURE IN THE SOLAR CORONA

The Atmospheric Imaging Assembly (AIA) telescope on board the Solar Dynamics Observatory (SDO) provides coronal EUV imaging over a broader temperature sensitivity range than the previous generations of instruments (EUVI, EIT, and TRACE). Differential emission measure tomography (DEMT) of the solar corona based on AIA data is presented here for the first time. The main product of DEMT is the three-dimensional (3D) distribution of the local differential emission measure (LDEM). While in previous studies, based on EIT or EUVI data, there were 3 available EUV bands, with a sensitivity range $\sim 0.60 - 2.70$ MK, the present study is based on the 4 cooler AIA bands (aimed at studying the quiet sun), sensitive to the range $\sim 0.55 - 3.75$ MK. The AIA filters allow exploration of new parametric LDEM models. Since DEMT is better suited for lower activity periods, we use data from Carrington Rotation 2099, when the Sun was in its most quiescent state during the AIA mission. Also, we validate the parametric LDEM inversion technique by applying it to standard bi-dimensional (2D) differential emission measure (DEM) analysis on sets of simultaneous AIA images, and comparing the results with DEM curves obtained using other methods. Our study reveals a ubiquitous bimodal LDEM distribution in the quiet diffuse corona, which is stronger for denser regions. We argue that the nanoflare heating scenario is less likely to explain these results, and that alternative mechanisms, such as wave dissipation appear better supported by our results.

Synthetic differential emission measure curves of prominence fine structures

Synthetic differential emission measure curves of prominence fine structures

Synthetic differential emission measure curves of prominence fine structures

Aims. This study is the first attempt to combine the prominence observations in Lyman, UV, and EUV lines with the determination of the prominence differential emission measure derived using two different techniques, one based on the inversion of the observed UV and EUV lines and the other employing 2D non-LTE prominence fine-structure modeling of the Lyman spectra. Methods. We use a trial-and-error method to derive the 2D multi-thread prominence fine-structure model producing synthetic Lyman spectra in good agreement with the observations. We then employ a numerical method to perform the forward determination of the DEM from 2D multi-thread models and compare the synthetic DEM curves with those derived from observations using inversion techniques. Results. A set of available observations of the June 8, 2004 prominence allows us to determine the range of input parameters, which contains models producing synthetic Lyman spectra in good agreement with the observations. We select three models, which represent this parametric-space area well and compute the synthetic DEM curves for multi-thread realizations of these models. The synthetic DEM curves of selected models are in good agreement with the DEM curves derived from the observations. Conclusions. We show that the evaluation of the prominence fine-structure DEM complements the analysis of the prominence hydrogen Lyman spectra and that its combination with the detailed radiative-transfer modeling of prominence fine structures provides a useful tool for investigating the prominence temperature structure from the cool core to the prominence-corona transition region.

BRIGHT POINTS AND JETS IN POLAR CORONAL HOLES OBSERVED BY THE EXTREME-ULTRAVIOLET IMAGING SPECTROMETER ONHINODE

We present observations of polar coronal hole bright points (BPs) made with the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft. The data consist of raster images of BPs in multiple spectral lines from mostly coronal ions, e.g., Fe X-Fe XV. The BPs are observed for short intervals and thus the data are snapshots of the BPs obtained during their evolution. The images reveal a complex unresolved temperature structure (EIS resolution is about 2''), with the highest temperature being about 2 × 106 K. Some BPs appear as small loops with temperatures that are highest near the top. But others are more point-like with surrounding structures. However, the thermal time evolution of the BPs is an important factor in their appearance. A BP may appear quite different at different times. We discuss one BP with an associated jet that is bright enough to allow statistically meaningful measurements. The jet Doppler speed along the line of sight is about 15-20 km s–1. Electron densities of the BPs and the jet are typically near 109 cm–3, which implies path lengths along the line of sight on the order of a few arcsec. We also construct differential emission measure curves for two of the best observed BPs. High spatial resolution (significantly better than 1'') is required to fully resolve the BP structures.

SOME LIKE IT HOT: CORONAL HEATING OBSERVATIONS FROMHINODEX-RAY TELESCOPE ANDRHESSI

We have used Hinode X-Ray Telescope observations and RHESSI upper limits together to characterize the differential emission measure (DEM) from a quiescent active region. We find a relatively smooth DEM curve with the expected active region peak at log T = 6.4. We also find a high-temperature component with significant emission measure at log T approx> 7. This curve is consistent with previous observations of quiescent active regions in that it does not produce observable Fe XIX lines. It is different from that generated with X-Ray Telescope (XRT) data alone-RHESSI rules out the possibility of a separate high-temperature component with a peak of approximately log T = 7.4. The strength and position of the high-temperature peak in this XRT-only analysis was, however, poorly determined; adding RHESSI flux upper limits in the 4-13 keV energy range provide a strong high-temperature constraint which greatly improves the multi-thermal findings. The results of the present work as well as those from a growing number of papers on this subject imply that our previous understanding of the temperature distribution in active regions has been limited. Hot plasma (log T approx 7) appears to be prevalent, although in relatively small quantities as predicted by nanoflare models.more » Other models may need to be adjusted or updated to account for these new results.« less

QUANTITATIVE, THREE-DIMENSIONAL ANALYSIS OF THE GLOBAL CORONA WITH MULTI-SPACECRAFT DIFFERENTIAL EMISSION MEASURE TOMOGRAPHY

A previous paper (Frazin et al. 2005b) introduced the concept of differential emission measure tomography (DEMT), which is a three-dimensional (3D) extension of the classical differential emission measure technique for determining the distribution of temperatures in a volume of plasma. The information for the reconstruction in the three spatial dimensions is provided by solar rotation and/or multi-spacecraft views. This paper describes, quantitatively, the procedure for implementing DEMT with data from NASA's STEREO/EUVI instrument, including the radiometry, line-of-sight geometry, and image preparation steps. An example of a quantitative, multiband, 3D reconstruction and local differential emission measure curves are given, and it is demonstrated that, when applicable, DEMT is a simple 3D analysis tool that obviates the need for structure-specific modeling.

Prominence modelling: from observed emission measures to temperature profiles

Aims. We outline the construction of prominence - corona transition region models based upon the observations of one particular prominence. Methods. The differential emission measure curves from observations were approximated by simple analytical functions. On this basis we constructed the temperature curve and calculated the radiative losses, the gains by thermal conduction, and some estimates for the wave heating. Results. The temperature curve was calculated in the range between 23 000 K and 450 000 K. The resulting transition region can be divided into an inner region where the temperature is low and the radiative losses are very large, a part with a very steep temperature rise from 40 000 K to around 250 000 K over a width of only 500 km, and an extended high temperature region. Both the conductive heating and our estimates for a possible wave cooling/heating peak very sharply in the region with the very large temperature gradient. The consequences for the energy balance are discussed.

Coronal Diagnostic Spectrometer Observations of Isothermal and Multithermal Coronal Loops

A data set obtained on 2003 January 17 with the Coronal Diagnostic Spectrometer (CDS) shows two loops sitting side by side on the solar disk. These loops are oriented along the CDS slit, so all pixels in each loop were observed simultaneously. So, although the instrument has a relatively slow time cadence, changes as a function of time that may occur during the CDS raster buildup will not affect the loop temperature results. Differential emission measure (DEM) analysis using a forward-folding technique shows different results for the two loops. For the first loop, the intensities of the lines that remain after background subtraction are well fit with a DEM curve that collapses to a single spike. In other words, the loop plasma at this location is isothermal. This analysis is confirmed with an emission measure loci method and agrees with the results obtained recently by other authors that show that the moderate spatial resolution of CDS can detect isothermal structures. For the second loop, the background-subtracted line intensities require a broad DEM, not consistent with isothermal plasma. This conclusion is confirmed with an automatic-inversion DEM method. In this Letter, we specifically address some of the concerns raised about CDS temperature analysis: the slow CDS temporal resolution, the moderate CDS spatial resolution, the inherent smoothing associated with DEM inversion, and line-of-sight effects on the DEM distribution.

An AB Initio Approach to Solar Coronal Loops

Data from recent numerical simulations of the solar corona and transition region are analysed and the magnetic field connection between the low corona and the photosphere is found to be close to that of a potential field. The fieldline to fieldline displacements follow a power law distribution with typical displacements of just a few Mm. Three loops visible in emulated Transition Region And Coronal Explorer (TRACE) filters are analysed in detail and found to have significantly different heating rates and distributions thereof, one of them showing a small scale heating event. The dynamical structure is complicated even though all the loops are visible in a single filter along most of their lengths. None of the loops are static, but are in the process of evolving into loops with very different characteristics. Differential Emission Measure (DEM) curves along one of the loops illustrate that DEM curves have to be treated carefully if physical characteristics are to be extracted.

Analysis of a coronal loop on the limb

The ability to accurately measure the temperature and density of the solar atmosphere is essential to understanding the physical nature of Coronal loops. After data has been corrected to account for cosmic ray incidents on the CCD and instrumental effects, there are additional sources of error that must also be addressed. We applied a correction to the intensities for spectral lines to account for line of sight effects. Having performed these procedures, the corrected intensities from the Coronal Diagnostic Spectrometer are then used to produce differential emission measure curves with much improved error estimates

Coronal loops: Isothermal OR multithermal?

Are coronal loops isothermal? A controversy over this question has arisen recently because different investigators using different techniques have obtained very different answers. Analysis of data using narrowband filter ratios to obtain temperature maps has produced several key publications that suggest that coronal loops may be isothermal. We have constructed a multi-thermal distribution for several pixels along a relatively isolated coronal loop on the southwest limb of the solar disk using spectral line data from the Coronal Diagnostics Spectrometer (CDS), on SOHO taken on 1998 April 20. These distributions are clearly inconsistent with isothermal plasma along either the line of sight or the length of the loop, and suggested rather that the temperature increases from the footpoints to the loop top. We convolved these Differential Emission Measure curves with two of the Soft X-ray Telescope (SXT) response functions. This gives us the intensity (in Data Numbers/sec) of what the instrument would “see” in these filters if it were observing the same loop. We then took a ratio of these values, and used the regular Yohkoh software to calculate a temperature at each pixel. The instrument “sees” a loop that is marginally consistent with an almost uniform temperature, but a linear or quadratic model is a much better fit to the data. These results are different from those of a similar analysis with narrow-passband instruments that produced data consistent with a uniform temperature loop, even though the actual temperature input was multi-thermal both along the line of sight and along the length of the loop. We suspect that these apparent uniform-temperature loops may be an unfortunate byproduct of the simplistic filter-ratio method that is used for temperature analysis. Our results are consistent with earlier analysis of Yohkoh data, where there were strong indications that the SXT temperatures were a kind of Differential Emission Measure-weighted temperature. There is a problem, however, when we compare these temperatures with those calculated with the actual SXT data, which are much higher than even the hottest plasma observed by CDS.

Solving the Discrepancy between the Extreme‐Ultraviolet and Microwave Observations of the Quiet Sun

The aim of the present work is to understand the origin of the long-standing discrepancy between the EUV/UV-based predictions of the quiet-Sun microwave spectrum and the observed one. We compare accurate measurements of the quiet-Sun microwave brightness temperature (Tb) with theoretical calculations obtained by using the differential emission measure (DEM) of the plasma derived from UV and EUV spectral line intensities observed by the SUMER and CDS instruments on board the Solar and Heliospheric Observatory (SOHO). No agreement can be found between the observed Tb and calculations carried out using the standard DEM curves obtained from the EUV/UV observations. In order to obtain agreement, it is necessary (1) to modify the temperature range in which the DEM is usually defined in order to take into account the presence of an isothermal corona, (2) to separate the contribution of the cell and the network structures in the transition region, and (3) to substitute the EUV/UV-based DEM values at very low temperature (log T ≤ 4.3) with values based on the Vernazza, Avrett, & Loeser model. In the present work we are able to solve a long-standing discrepancy between microwave and EUV/UV results, and we demonstrate the great potential of the simultaneous use of observations in these two spectral ranges.

Are Coronal Loops Isothermal?

A controversy over the temperature distribution of coronal loops has arisen recently because different investigators using different techniques have obtained very different answers. Analysis of Solar and Heliospheric Observatory (SOHO) Extreme ultraviolet Imaging Telescope (EIT) and Transition Region and Coronal Explorer (TRACE) data using narrowband ratios to obtain temperature maps has produced several key publications that suggest that coronal loops may be isothermal. On the other hand, our analysis of the SOHO Coronal Diagnostics Spectrometer (CDS) spectral line data taken on 1998 April 20 for several pixels along a relatively isolated coronal loop on the southwest limb of the solar disk is clearly inconsistent with isothermal plasma along either the line of sight or the length of the loop. We have constructed a differential emission measure (DEM) distribution for each pixel; these distributions, taken together, suggested that the DEM-weighted temperature increases from the footpoints to the loop top. We convolved these DEM curves with the three different EIT coronal response functions. This gives us the intensity (in units of DN s-1) of what EIT would see in the 171, 195, and 284 A passbands if it were observing the CDS loop. We take a ratio of these values (171 A/195 A and 195 A/284 A), and use the regular EIT software to calculate a temperature at each pixel for each ratio. EIT sees a loop with an almost uniform temperature, but the derived temperatures are different for the different ratios. These uniform-temperature loops arise even though the actual temperature input is multithermal both along the line of sight and along the length of the loop. We suspect that these apparent uniform-temperature loops may be an unfortunate by-product of the simplistic filter-ratio method that is used for both EIT and TRACE temperature analysis. Our results indicate that narrow-passband EUV observations must be used in conjunction with other diagnostics to draw quantitative conclusions about the properties of coronal plasma.

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.

Determining coronal heating of plasma loops through differential emission measure analysis

Insights into the nature of the heating mechanism in coronal plasmas can be gained through the analysis of differential emission measure curves localized along coronal loops. This technique is especially desirable since it does not require the isothermal approximation for potentially dynamic loop plasmas. Of particular interest are the parameters describing the heating rate per unit volume in the corona as a function of radial height and/or arc length along loop structures. Using simultaneous observations taken on 20 April 98 with the Solar and Heliospheric Observatory Coronal Diagnostics Spectrometer and the Yohkoh Soft X-ray Telescope, plasma temperature distributions were constructed in the relevant temperature domain (Log T = 5.5–7.5) along an isolated coronal loop on the west limb, ranging from one footpoint to the loop top. Subsequent analysis of the differential emission measure curves using abundance values from Fludra and Schmelz (1999) at each pixel combined with knowledge of the loop geometry helped to pin down the coronal heating mechanism using the method described by Priest et al. (1998).

DEM Study of Selected Quiet Sun Regions

In the present work EUV spectra of quiet Sun regions, observed with the Coronal Diagnostic Spectrometer (CDS) on SOHO, are analysed in order to determine the Differential Emission Measure (DEM) of selected areas of the field of view. The purpose of the present work is to study the differences between the DEM curves of the quiet Sun cell centers areas, intermediate areas and network boundaries.

Analysis of a Solar Active Region Extreme‐Ultraviolet Spectrum from SERTS‐97

Goddard Space Flight Center's Solar EUV Research Telescope and Spectrograph was flown on 1997 November 18, carrying an intensified CCD detector and a multilayer-coated toroidal diffraction grating with enhanced sensitivity over that of a standard gold-coated grating throughout the instrument's 299-353 A spectral bandpass. Spectra and spectroheliograms of NOAA Active Region 8108 (N21°, E18°) were obtained with a spectral resolution (instrumental FWHM) of 115 mA. Nearly 100 emission lines were observed in the spatially averaged active region spectrum. Spectra and spectroheliograms of quiet areas south of the region were also obtained. An end-to-end radiometric calibration of the rocket instrument was carried out at the Rutherford-Appleton Laboratory in the same facility that was used to calibrate the Coronal Diagnostic Spectrometer experiment on SOHO and using the same EUV light source. The accuracy of this calibration is confirmed by the excellent agreement between the measured and theoretical values of density- and temperature-insensitive line intensity ratios. Nine emission lines of Fe XV are identified in our spectrum; however, large differences between wavelengths in the CHIANTI database and some of the measured solar wavelengths, as well as inconsistencies of various theoretical intensity ratios, suggest a need for improvement in the Fe XV atomic physics parameters and/or the presence of unidentified blending lines. Density-sensitive line intensity ratios of Fe XI λλ308.55/352.67, Fe XII λλ338.27/352.11, Fe XIII λλ320.80/312.17, and Fe XV λλ321.78/327.03 yield logarithmic electron densities (in cm-3) of 9.92 ± 0.28, 9.74 ± 0.28, 9.52 ± 0.30, and 9.62 ± 0.26, respectively. Using the strongest emission line observed for each ionization stage of Fe from X through XVI and Ni XVIII, we find that all of the measured nonthermal line widths yield velocities consistent with 35 km s-1. The differential emission measure curve derived from the observed line intensities exhibits a relative minimum at log T ~ 5.7, a broad maximum centered around log T ~ 6.3, and a rapid decline for temperatures above log T ~ 6.6.

Coronal Magnetography of a Solar Active Region Using Coordinated SERTS and VLA Observations

We observed NOAA region 7563 simultaneously with Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrogaph (SERTS) and with the Very Large Array (VLA) on 1993 August 17. SERTS obtained spectra in the 280-420 A wavelength range, and images in the lines of Mg IX λ368.1, Fe XV λ284.1, and Fe XVI λ335.4. The VLA obtained microwave images at 20 and 6 cm wavelengths. The microwave emission depends upon the coronal temperature, density, column emission measure, and magnetic field; therefore, the coronal magnetic field can be derived when all of these other quantities are measured. Here we demonstrate this approach by using the SERTS data to derive all the relevant plasma parameters and then fitting the radio observations to a magnetic field model in order to determine the magnetic field structure. We used the method of Monsignori-Fossi & Landini and the coronal elemental abundances of Feldman et al. to derive the differential emission measure (DEM) curve for region 7563 from numerous EUV emission lines in spatially averaged SERTS spectra. A similar curve was estimated for each point (i.e., each pixel or each spatial location) in the two-dimensional region by scaling the average DEM curve with corresponding pixel intensities in the Mg IX, Fe XV, and Fe XVI images. We integrated each such DEM over narrow temperature ranges to obtain the column emission measure (CEM) as a function of temperature, CEM(T). We also obtained electron density measurements from EUV line intensity ratios in the spatially averaged spectrum for several ionization stages of iron. These were used to derive a functional relation between density and temperature, ne(T). We derived the temperature dependence of the coronal magnetic field [B(T)] at each point in the two-dimensional region by incorporating CEM(T) and ne(T) into expressions for the thermal bremsstrahlung and the gyroresonance opacities, and varying B(T) so as to minimize the difference between the calculated and the observed microwave intensities. The resulting calculated 20 and 6 cm microwave intensity images reproduce the observed images very well. We found that thermal bremsstrahlung alone is not sufficient to produce the observed microwave intensities: gyroemission is required. Further, contrary to several earlier studies, we found no evidence for cool, absorbing plasma in the solar corona above the active region. The coronal magnetic fields derived with our method typically exceed the coronal fields extrapolated with a simple potential model, suggesting the presence of coronal electric currents. However, in the diminutive sunspot which dominates the 6 cm emission this difference is relatively small, suggesting that the sunspot magnetic field itself is nearly potential. Although we cannot firmly establish the uniqueness of our solution in this particular case, the method is quite powerful and should be repeated with other similar data sets. Variations in the coronal elemental abundances could affect the determination of the microwave emission mechanism(s), introduce evidence for the presence of cool coronal plasma, and alter the strengths of the derived coronal magnetic fields.

Measuring Active and Quiet-Sun Coronal Plasma Properties with Extreme-Ultraviolet Spectra from SERTS

view Abstract Citations (166) References (65) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Measuring Active and Quiet-Sun Coronal Plasma Properties with Extreme-Ultraviolet Spectra from SERTS Brosius, Jeffrey W. ; Davila, Joseph M. ; Thomas, Roger J. ; Monsignori-Fossi, Brunella C. Abstract We obtained high-resolution extreme-ultraviolet (EUV) spectra of solar active regions, quiet-Sun areas, and off-limb areas during 1991 May 7 and 1993 August 17 flights of NASA/Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS). The 1991 flight was the first time a multilayer coated diffraction grating was ever used in space. Emission lines from the eight ionization stages of iron between Fe+9 (Fe x) and Fe+16 (Fe XVII) were observed. Values of numerous density- and temperature-insensitive line intensity ratios agree with their corresponding theoretical values. Intensity ratios among various lines originating in a common stage of ionization provide measurements of coronal electron density. Numerous density-sensitive ratios are available for Fe xiii, and they yield active region density (cm-3) logarithms of 9.66±0.49 and 9.60±0.54 for the 1993 and 1991 flights, respectively, and a quiet-Sun density of 9.03±0.28 for the 1993 flight. Filling factors, calculated from the derived densities assuming a path length of 1 × 109 cm, range from several thousandths to nearly unity. Intensity ratios among lines originating in different ionization stages of iron yield measurements of coronal electron temperature in the isothermal approximation. The line ratios yield temperatures ranging from 1.1 × 106 to 5.2 × 106 K for the active regions, and 1.0 × 106 to 2.1 × 106 K for the quiet Sun, depending upon the ionization stages used. The derived temperature diminishes with decreasing ionization stages. Fe XVII emission, detected in the active regions but not in the quiet areas, accounts for the higher maximum active region temperature. Derived active region temperatures are greater than their quiet-Sun counterparts for ratios that include lines from Fe xiv through Fe XVI; however, the derived active region and quiet-Sun temperatures are not statistically significantly different for line intensity ratios that involve only Fe x through Fe xiii. The latter similarity in derived temperatures suggests the presence of similar thermal structures in all the areas observed, although the active regions also harbor hotter material. Differential emission measure (DEM) distributions were constructed for the active region and quiet- Sun observations obtained during both flights. The two quiet-Sun DEM curves and the 1993 active region DEM curve all show peaks between log T = 6.1 and 6.2. The 1993 active region DEM has a second peak between log T = 6.6 and 6.7, and the 1991 active region DEM has only one peak, between log T = 6.5 and 6.6. Thus, the 1993 active region DEM curve appears, in some sense, to be a composite of the quiet-Sun DEM curve and the 1991 active region DEM curve. The 1991 active region exhibited flaring activity, yielded higher line ratio temperatures, and contained greater photo spheric magnetic fields than the 1993 active region. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1996 DOI: 10.1086/192332 Bibcode: 1996ApJS..106..143B Keywords: SUN: ACTIVITY; SUN: CORONA; SUN: UV RADIATION full text sources ADS |