FLOWS AND DYNAMICS IN THE CORONA OBSERVED WITH THE CORONAL DIAGNOSTIC SPECTROMETER (CDS)

EUV spectra obtained with the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) show significant flows of plasma in active region loops, both at coronal and transition region temperatures. Wavelength shifts in the coronal lines Mgix 368 Å and Mgx 624 Å corresponding to upflows in the plasma reaching velocities of 50 km s^-1 have been observed in an active region. Smaller velocities are detected in the coronal lines Fexvi 360 Å and Sixii 520 Å. Flows reaching 100 km s^-1 are observed in spectral lines formed at transition region temperatures, i.e., Ov 629 Å and Oiii 599 Å, demonstrating that both the transition region and the corona are clearly dynamic in nature. Some high velocity events show even higher velocities with line profiles corresponding to a velocity dispersion of 300–400 km s^-1. Even in the quiet Sun there are velocity fluctuations of 20 km s^-1 in transition region lines. Velocities of the magnitude presented in this paper have never previously been observed in coronal lines except in explosive events and flares. Thus, the preliminary results from the CDS spectrometer promise to put constraints on existing models of the flows and energy balance in the solar atmosphere. The present results are compared to previous attempts to observe flows in the corona.     

FIRST RESULTS OF THE SUMER TELESCOPE AND SPECTROMETER ON SOHO – I. Spectra and Spectroradiometry

SUMER – the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) – observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 Å (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 Å. The second-order spectra of detectors A and B cover 330 to 805 Å and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 × 10⁶ K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mÅ is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona.     

Analysis of Two Coronal Loops with Combined TRACE and SOHO/CDS Data

We use an innovative research technique to analyze combined images from the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE). We produce a high spatial and temporal resolution simulated CDS raster or “composite” map from TRACE data and use this composite map to jointly analyze data from both instruments. We show some of the advantages of using the “composite” map method for coronal loop studies. We investigate two postflare loop structures. We find cool material (250 000 K) concentrated at the tips or apex of the loops. This material is found to be above its scale height and therefore not in hydrostatic equilibrium. The exposure times of the composite map and TRACE images are used to give an estimate of another loop’s cooling time. The contribution to the emission in the TRACE images for the spectral lines present in its narrow passband is estimated by using the CDS spectral data and CHIANTI to derive synthetic spectra. We obtain cospatial and cotemporal data collected by both instruments in SOHO Joint Observations Program (JOP) 146 and show how the combination of these data can be utilized to obtain more accurate measurements of coronal plasmas than if analyzed individually. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Magnetic Stereoscopy of Coronal Loops in NOAA 8891

The Solar TErrestrial RElations Observatory (STEREO) requires powerful tools for the three-dimensional (3D) reconstruction of the solar corona. Here we test such a program with data from SOHO and TRACE. By taking advantage of solar rotation, a newly developed stereoscopy tool for the reconstruction of coronal loops is applied to the solar active region NOAA 8891 observed from 1 March to 2 March 2000. The stereoscopic reconstruction is composed of three steps. First, we identify loop structures in two TRACE images observed from two vantage viewpoints approximately 17 degrees apart, which corresponds to observations made about 30 hours apart. In the second step, we extrapolate the magnetic field in the corona with the linear force-free field model from the photospheric line-of-sight SOHO/MDI data. Finally, combining the extrapolated field lines and one-dimensional loop curves from two different viewpoints, we obtain the 3D loop structures with the magnetic stereoscopy tool. We demonstrate that by including the magnetic modeling this tool is more powerful than pure geometrical stereoscopy, especially in resolving the ambiguities generated by classical stereoscopy. This work will be applied to the STEREO mission in the near future.     

Multiwavelength Analysis of an Active Region

We study active region NOAA 8541, observed with instruments on board SOHO, as well as with TRACE. The data set mainly covers the transition region and the low corona. In selected loops studied with SUMER on SOHO, the VIII 770 Å line is systematically redshifted. In order to estimate the plasma velocity, we combine the Doppler shifts with proper motions (TRACE) along these loops. In the case of an ejection, apparently caused by the emergence of a parasitic polarity, proper motions and Doppler shifts give consistent results for the velocity. A cooler loop, observed in the same active region with CDS, shows a unidirectional motion reminiscent of a siphon flow. The derived electron temperature and density along a large steady loop confirm that it cannot be described by hydrostatic models.     

Correlation Height of the Source Region of Si II Emission Lines in Coronal Hole Regions

The height of the source region of Si II emission lines characterizes the height of the bottom layer of solar atmosphere's transition region. The correlation analysis of the intensities of ultraviolet spectral lines and the threedimensional structure of magnetic field yielded by force-free extrapolation is a new method for determining the height of ultraviolet emission lines’ source region. It has been found that the height thus obtained is larger than that given by traditional viewpoint. Because the existing numerical analyses with this method are scarce, this result has to be further verified with more observational materials. In this work, this method is applied to the Si II emission lines observed by SOHO/SUMER for the solar surface region beneath the solar coronal hole at southern pole and to the magnetic fields measured by National Solar Observatory/Kitt Peak (NSO/KP) in U.S.A. The correlation height of the source region of Si II emission lines in coronal hole region is approximately 5.0 Mm. This result supports the conclusion that the height of the bottom layer of transition region in coronal hole region is larger than that in quiet regions. Moreover, some new phenomena have been discovered and their causes are probed.     

SUMER - Solar Ultraviolet Measurements of Emitted Radiation

The instrument SUMER - Solar Ultraviolet Measurements of Emitted Radiation is designed to investigate structures and associated dynamical processes occurring in the solar atmosphere, from the chromosphere through the transition region to the inner corona, over a temperature range from 10⁴ to 2 × 10⁶ K and above. These observations will permit detailed spectroscopic diagnostics of plasma densities and temperatures in many solar features, and will support penetrating studies of underlying physical processes, including plasma flows, turbulence and wave motions, diffusion transport processes, events associated with solar magnetic activity, atmospheric heating, and solar wind acceleration in the inner corona. Specifically, SUMER will measure profiles and intensities of EUV lines; determine Doppler shifts and line broadenings with high accuracy; provide stigmatic images of the Sun in the EUV with high spatial, spectral, and temporal resolution; and obtain monochromatic maps of the full Sun and the inner corona or selected areas thereof. SUMER will be flown on the Solar and Heliospheric Observatory (SOHO), scheduled for launch in November, 1995. This paper has been written to familiarize solar physicists with SUMER and to demonstrate some command procedures for achieving certain scientific observations.     

The heating of the solar corona

The heating of the solar corona has been a fundamental astrophysical issue for over sixty years. Over the last decade in particular, space-based solar observatories (Yohkoh, SOHO and TRACE) have revealed the complex and often subtle magnetic-field and plasma interactions throughout the solar atmosphere in unprecedented detail. It is now established that any energy release mechanism is magnetic in origin - the challenge posed is to determine what specific heat input is dominating in a given coronal feature throughout the solar cycle. This review outlines a range of possible magnetohydrodynamic (MHD) coronal heating theories, including MHD wave dissipation and MHD reconnection as well as the accumulating observational evidence for quasi-periodic oscillations and small-scale energy bursts occurring in the corona. Also, we describe current attempts to interpret plasma temperature, density and velocity diagnostics in the light of specific localised energy release. The progress in these investigations expected from future solar missions (Solar-B, STEREO, SDO and Solar Orbiter) is also assessed.Received: 6 February 2003, Published online: 14 November 2003 Correspondence to: R. W. Walsh     

What Is Hatching in the Egg?

We present observations of several large two-ribbon flares observed with both the Transition Region and Coronal Explorer (TRACE) and the soft X-ray telescope on Yohkoh. The high spatial resolution TRACE observations show that solar flare plasma is generally not confined to a single loop or even a few isolated loops but to a multitude of fine coronal structures. These observations also suggest that the high-temperature flare plasma generally appears diffuse while the cooler ( less, similar2 MK) postflare plasma is looplike. We conjecture that the diffuse appearance of the high-temperature flare emission seen with TRACE is due to a combination of the emission measure structure of these flares and the instrumental temperature response and does not reflect fundamental differences in plasma morphology at the different temperatures.     

Oscillations Above Sunspots

The 3-min oscillations in the sunspot atmosphere are discussed, based on joint observing with the Transition Region and Coronal Explorer – TRACE and the Solar and Heliospheric Observatory – SOHO. We find that the oscillation amplitude above the umbra increases with increasing temperature, reaches a maximum for emission lines formed close to 1–2× 10⁵ K, and decreases for higher temperatures. Oscillations observed with a high signal-to-noise ratio show deviations from pure linear oscillations. The results do not support the sunspot filter theory, based on the idea of a chromospheric resonator. Whereas the filter theory predicts several resonant peaks in the power spectra, equally spaced 1 mHz in frequency, the observed power spectra show one dominating peak, close to 6 mHz. Spectral observations show that the transition region lines contribute less than 13 percent to the TRACE 171 Å channel intensity above the umbra. The 3-min oscillations fill the sunspot umbra in the transition region. In the corona the oscillations are concentrated to smaller regions that appear to coincide with the endpoints of sunspot coronal loops, suggesting that wave propagation along the magnetic field makes it possible for the oscillations to reach the corona.     

Different spatial structures between network regions and active regions indicated by TRACE 171 Å observation

By comparing TRACE 171 Å observations with photospheric magnetograms, we find that the root of TRACE 171 Å emission is centered in magnetic elements in simultaneous Huairou photospheric magnetogram and the luminosity of TRACE 171 Å emission is not always in proportion to the strength of the corresponding photospheric magnetic field. While TRACE emission from an active region shows an obvious upward extension as a whole, fibril-like emissions from network elements show little extension along the structure from the root of each emission to 40′′ higher up in the solar atmosphere. Together with previous studies by Zhang and Zhang (1999, 2000), it is suggested that the magnetic fields in active regions and quiet-Sun regions present different spatial structures from the solar photosphere to the chromosphere and maybe even in the corona.     

Multi-Wavelength Analysis of the 29 September 2002 M2.6/2N Flare

Using TRACE EUV 171 Å line, Hα line, Zürich radio, RHESSI, and HXRS observations the 29 September 2002 flare (M2.6), which occurred in AR NOAA 0134, was analyzed. Flaring structures were compared with a potential magnetic field model (field lines and quasi-separatrix layers) made from SOHO/MDI full-disk magnetogram. Series of high-resolution SOHO/MDI magnetograms and TRACE white-light images were used to find changes in the active region at the photosphere during the flare. The flare began with a rising of a small dark loop followed by the flare brightening observed in 171 Å with TRACE and Hα lines. In radio wavelengths, first type III bursts were observed 5 min prior to the start of hard X-ray emission, indicating a pre-flare coronal activity. The main hard X-ray emission peak (at 06:36 UT) was associated with the second type III burst activity and several slowly negatively drifting features, all starting from one point on the radio spectrum (probably a shock propagating through structures with different plasma parameters). After this time a huge loop formed and three minutes later it became visible in absorption both in Hα and 171 Å EUV lines. The phase of huge dark loop formation was characterized by long-lasting, slowly negatively drifting pulsations and drifting continuum. Finally, considering this huge loop as a surge an evolution of the event under study is discussed.     

The Magnetic Structure and Generation of EUV Flare Ribbons

The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE, and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2′′, and thus to investigate the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations. There are several respects in which the observations differ from what would be expected in the commonly-adopted models for flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them, apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate. Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly, the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection. We describe the interpretation of our observations within the standard model, and the implications for the distribution of magnetic fields in this active region.     

Coronal response of Bi-directional Jets

EUV bi-directional jets are a prominent class of phenomena characterizing the solar transition region. Using simultaneously obtained SUMER observations in the chromospheric Si ii 1251.16 Å and C i 1251.17 Å, transition region N v 1238.8 Å and coronal Mg x 625 Å lines we show an example of a bi-directional jet observed in the chromospheric and the transition region lines but not showing any detectable signature in the coronal line. The phenomenon, however, was also clearly detected by the TRACE imager with the 171 Å filter. This discrepancy is explained here with a non-Maxwellian electron distribution which makes a significant fraction of the plasma in the TRACE 171 Å pass-band to be derived from temperatures around ≈ 300 000 K, as opposed to ≈ 800 000 K. This could have implications for other phenomena observed in the TRACE pass-bands, including the transition region ‘moss’ and the 3- and 5-min oscillations.     

Intensity Distribution and Contrast of the Solar EUV Network

Intensity distributions of the EUV network and the cell interior in the solar atmosphere have been obtained in fourteen emission lines from Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) observations. The formation temperature of the observed lines is in the range log T=4.90 – 6.06 (T in Kelvin), and hence they represent increasing heights in the solar atmosphere from the upper chromosphere and the transition region to the low corona. Intensity distributions of the cell interior have been found to be different in the quiet Sun and the coronal hole even at the lower transition region, which is at variance with some earlier results. The intensity contrast of the network with respect to the cell interior has been obtained for each line, and differences in the quiet Sun and the coronal hole have been examined. The network contrast, in general, is lower for the coronal hole as compared to the quiet Sun, but becomes equal to it in the upper transition region. The maximum contrast for both the regions is at about log T=5.3. Also obtained are the relative contributions of the network and the cell interior to the total intensity. The implications of the results for models of the transition region are briefly mentioned.     

Physics of the sun’s hot atmosphere

This article briefly overviews the physics of the Sun’s hot atmosphere, using observations from recent solar spacecrafts: Yohkoh, SOHO, TRACE and RHESSI.     

White-Light Flares: A TRACE/RHESSI Overview

The Transition Region and Coronal Explorer (TRACE) instrument includes a “white light” imaging capability with novel characteristics. Many flares with such white-light emission have been detected, and this paper provides an introductory overview of these data. These observations have 0.5″ pixel size and use the full broad-band response of the CCD sensor; the images are not compromised by ground-based seeing and have excellent pointing stability as well as high time resolution. The spectral response of the TRACE white-light passband extends into the UV, so these data capture, for the first time in images, the main radiative energy of a flare. This initial survey is based on a sample of flares observed at high time resolution for which the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) had complete data coverage, a total of 11 events up to the end of 2004. We characterize these events in terms of source morphology and contrast against the photosphere. We confirm the strong association of the TRACE white-light emissions - which include UV as well as visual wavelengths – with hard X-ray sources observed by RHESSI. The images show fine structure at the TRACE resolution limit, and often show this fine structure to be extended over large areas rather than just in simple footpoint sources. The white-light emission shows strong intermittency both in space and in time and commonly contains features unresolved at the TRACE resolution. We detect white-light continuum emission in flares as weak as GOES C1.6. limited by photon statistics and background solar fluctuations, and support the conclusion of Neidig (1989) that white-light continuum occurs in essentially all flares.     

Relative Velocities and Linewidths in a Coronal Hole and Outside

Relative Doppler velocities and spectral linewidths in a coronal hole and in the quiet Sun region outside have been obtained from Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) observations. Five strong emission lines in the CDS wavelength range (namely, O? iii 599 Å, O?v 630 Å, Ne?vi 562.8 Å, He?ii 304 Å, and Mg?ix 368 Å), whose formation temperatures represent different heights in the solar atmosphere from the lower transition region to the inner corona, have been used in the study. As reported earlier, relative velocities in the coronal hole are generally blueshifted with respect to the quiet Sun, and the magnitude of the blueshifts increases with height. It has been found that the polar coronal hole has larger relative velocities than the equatorial extension in the inner corona. Several localized velocity contours have been found mainly on network brightenings and in the vicinity of the coronal hole boundary. The presence of velocity contours on the network may represent network outflows whereas the latter could be due to localized jets probably arising from magnetic reconnection at the boundary. All spectral lines have larger widths in the coronal hole than in the quiet Sun. In O?v 630 Å an extended low-linewidth region is seen in the coronal hole?–?quiet Sun boundary, which may indicate fresh mass transfer across the boundary. Also polar coronal holes have larger linewidths in comparison with the equatorial extension. Together with larger relative velocities, this suggests that the solar wind emanating from polar hole regions is faster than that from equatorial hole regions.     

HIGH-RESOLUTION OBSERVATIONS OF THE EXTREME ULTRAVIOLET SUN

This paper presents first results of the Coronal Diagnostic Spectrometer (CDS) recently launched aboard the Solar and Heliospheric Observatory (SOHO). CDS is a twin spectrometer, operating in the extreme ultraviolet range 151–785 Å. Thus, it can detect emission lines from trace elements in the corona and transition region which will be used to provide diagnostic information on the solar atmosphere. In this paper, we present early spectra and images, to illustrate the performance of the instrument and to pave the way for future studies.