Time Variability of Active Region Loops Observed with the Coronal Diagnostic Spectrometer (Cds) on Soho

Monochromatic images from the Coronal Diagnostic Spectrometer (CDS) of loops above active regions show clear evidence of rapid time variability. The rapidly changing conditions that we observe give a new conception of loop systems that has never before been seriously considered. Loop systems, particularly in emission lines formed at temperatures in the 1–5 × 10⁵ K range, traditionally thought of as transition region temperatures, are seen to change significantly over a period of 1 hour. Loops may appear or disappear in certain emission lines, may show rapid variations in the distribution of the emission along their lengths, or may change shape or expand outward, all on time scales of 10–20 min. At other temperatures below 1.5 MK the variability appears less striking, but is still pronounced. At high temperatures, i.e., T ≥ 1.5 MK, conditions are normally much more stable. Examples exist, however, of loop systems showing violent changes in images at all temperatures up to Fe xvi formed at 2.7 MK. The structural variability is accompanied by high Doppler shifts, especially in the O v line. Corresponding velocities typically amount to 50–100 km s^-1, but values as high as 300 km s^-1 have been recorded. Animations with illustrative examples of loop variability have been prepared and are found on the enclosed CD-ROM. In addition we briefly discuss other structural and dynamical properties of active region loops, particularly those with temperatures below 1.5 MK. Theoretical models of loops cannot explain the present observations, but models that combine extreme fine structure, episodic heating and magneto-acoustic wave disturbances propagating in the loop legs seem promissing. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005031711233     

The XUV structure of solar active regions

XUV spectroheliograms of 2 active regions are studied. The images are due to lines emitted at temperatures between 8 x 10⁴ K and 2 x 10⁶ K and thus are indicative of transition region and coronal structures. The hot coronal lines are formed solely in loop structures which connect regions of opposite photospheric magnetic polarity but are not observed over sunspots. Transition region lines are emitted in plages overlying regions of intense photospheric magnetic field and in loops or loop-segments connecting such regions. The hot coronal loops are supported hydrostatically while only some of the transition zone loops are. The coronal and transition zone loops are distinctly separated and are not coaxial. A comparison of direct measurements of electron densities using density sensitive line ratios with indirect measurements using emission measures and path lengths shows the existence of fine structures of less than a second of arc in transition region loops. From a similar analysis, hot coronal loops do not have any fine structure below about 2 seconds of arc.     

Counter-streaming Mass Flow and Transient Brightening in Active Region Loops

An active region loop system was observed in a decaying active region for three hours by TRACE and BBSO in a joint campaign on September 27, 1998. Continuous mass motion was seen in Hα offband filtergrams throughout the three hours, and some UV loops were exhibited transient brightenings. We find that: (1) cool material was flowing along the loops at a speed of at least 20 km s^?1. Further, in Hα red and blue wings, we see mass motion along different loops in opposite directions. This is the first report of a counter-streaming pattern of mass motion in an Hα loop system. (2) Transient brightenings of different UV loops at different times were observed at C?iv 1550 Å. These brightened UV loops were located in the same region and at the same altitudes as the Hα loops. The observations show a clear correlation between the transient brightenings of UV loops and mass motion in Hα loops. (3) Both footpoints of the loop system were located in regions of mixed magnetic polarities. Frequent micro-flares at one footpoint of the loops with small-scale brightenings spreading along the loop leg were observed before the brightening and rising of one C?iv loop. Similar to the case of a filament, the continuous mass motion along the loops seems important for maintaining the cool Hα loop system at coronal height. There may be an indication that the mass motion in cool Hα loops and the correlated transient brightening of the active region loops were due to the small-scale chromospheric magnetic reconnection at the footpoint regions of the loop system.

     

The temperature structure and pressure balance of magnetic loops in active regions

EUV observations show many active region loops in lines formed at temperatures between 10⁴K and 2×l0⁶K. The brightest loops are associated with flux tubes leading to the umbrae of sunspots. It is shown that the high visibility of certain loops in transition region lines is due principallly to a sharp radial decrease of temperature to chromospheric values toward the loop axis. The plasma density of these cool loops is not significantly greater than in the hot gas immediately surrounding it. Consequently, the internal gas pressure of the cool material is clearly lower. The hot material immediately surrounding the cool loops is generally denser than the external corona by a factor 3–4. When the active region is examined in coronal lines, this hot high pressure plasma shows up as loops that are generally parallel to the cool loops but significantly displaced laterally. In general the loop phenomenon in an active region is the result of temperature variations by two orders of magnitude and density variations of around a factor five between adjacent flux tubes in the corona.     

Type I noise storms and the structure of the extreme ultraviolet corona

The spatial fine structure of the solar corona as observed in the EUV line Fexv is compared with the occurrence of major type I metric noise storms. In all cases, strong changes in the loop structure of the corona are observed. On the disk, these coronal changes are correlated to the emergence of new magnetic flux in the vicinity of existing large active regions. The reverse is demonstrated: during noise storm free periods no coronal changes can be observed. Noise storms at the limb seem to originate in open field configurations over active regions. In all cases, reconnection of coronal magnetic fields over large distances are the cause of noise storms rather than changes of magnetic fields within an active region. Noise storms disappear or are weak at the limb because of foreground absorption in chains of active regions. The observed intensities of active region loops at the limb show that a density of 1.3 × 10⁹ cm^?3 which corresponds to a plasma frequency of 100 MHz can occur over a wide variety of altitudes because active region loops are not in hydrostatic equilibrium.     

Dynamical Features and Evolutional Characteristics of Brightening Coronal Loops

We present a detailed study of coronal loop brightenings observed in an active region on the solar limb. These brightening loops show expanding and shrinking motions in EUV coronal line images and also show downflow along the loops in Lα and Hα images. By means of time-slice analysis of the images, we found that both the expanding and shrinking motions of the loops are not real motions of plasma but apparent motions like post-flare loops, where the loops at the different height are successively heated and cooled. From a temperature analysis, the time delay between the brightenings of hot 195 Å and cool Lα loops is found to be nearly equal to the time-scale of the conduction cooling. We conclude that these loop brightenings are sources of so called Hα coronal rains.     

Active Regions Observed in Extreme Ultraviolet Light by the Coronal Diagnostic Spectrometer on Soho

We present observations of five active regions made by the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO). CDS observes the Sun in the extreme ultraviolet range 150–780 Å. Examples of active region loops seen in spectral lines emitted at various temperatures are shown. Several classes of loops are identified: those that are seen in all temperatures up to 2 x 10⁶ K; loops seen at 10⁶ K but not reaching 1.6 x 10⁶ K; those at temperatures 2– 4 x 10^-5 K and occasionally at 6 x 10^-5 K but not reaching 10⁶ K. An increasing loop size with temperature and the relationship between the cool and hot structures is discussed. CDS observations reveal the existence of loops and other unresolved structures in active regions, at temperatures between 1.5– 4 x 10^-5 K, which do not have counterparts in lines emitted above 8 x 10^-5 K. Bright compact sources only seen in the transition region lines are investigated. These sources can have lifetimes of up to several days and are located in the vicinity of sunspots. We study the variability of active region sources on time scales from 30 sec to several days. We find oscillatory behaviour of Hei and Ov line intensities in an active region on time scales of 5–10 min.     

Spatial structure and temporal development of a solar X-ray flare observed from Skylab on June 15, 1973

A solar flare on June 15, 1973 has been observed with high spatial and temporal resolution by the S-054 grazing-incidence X-ray telescope on Skylab. Both morphological and quantitative analyses are presented. Some of the main results are: (a) the overall configuration of the flare is that of a compact region with a characteristic size of the order of 30 at the intensity peak, (b) this region appears highly structured inside with complex systems of loops which change during the event, (c) a brightening over an extended portion of the active region precedes the flare onset, (d) the impulsive phase indicated by the non-thermal radio emission is a period during which a rapid brightening occurs in loop structures, (e) the X-ray emission is centered over the neutral line of longitudinal magnetic field, and the brightest structures at the flare onset bridge the neutral line, (f) loop systems at successively increasing heights form during the decay phase, finally leading to the large loops observed in the postflare phase, (g) different parts of the flare show distinctly different light curves, and the temporal development given by full disk detectors is the result of integrating the different intensity vs time profiles.The implications of these observations for mechanisms of solar flares are discussed. In particular, the flux profiles of different regions of the flare give strong evidence for continued heating during the decay phase, and a multiplicity of flare volumes appears to be present, in all cases consisting of loops of varying lengths.On leave from Arcetri Astrophysical Observatory, Florence, Italy.     

Opening of active region’s large-scale overlying magnetic loops and kinking motion of a magnetic loop triggered by a vicinal small eruption

In this paper, we present a clear case of a small eruption near active region NOAA 10990 leading to opening of active region’s large-scale magnetic loops and kinking motion of a magnetic loop in this active region on April 16, 2008. From a sequence of TRACE 171 ? images, we find that the upper large-scale overlying magnetic loops of the active region expanded upwards from the arcaded configuration to the vertical one accompanying the vicinal small eruption. At the same time, the topology of a magnetic loop inside the active region changed from the inverted “U” shape to the almost vertical one with its two legs intertwining each other. From the temporal analysis of the observations, we find that the opening of the upper large-scale overlying magnetic loops resulted in the instability of the magnetic loop inside the active region. By analyzing the surrounding coronal environment, we suggest that the existence of the lower overlying magnetic loops prevented the magnetic loop from eruption. Generally, the kinking motion of the flux rope is taken as occurrence of kink instability. In this event, the top of the magnetic loop first began the kinking motion and then the two legs intertwined each other. According to our knowledge, the latter process has not been reported. Because there was only one active region on the solar disk on April 16, 2008, it turned out to be a very good candidate for us to investigate the impact of the vicinal eruption on the active region. These findings provide evidence that the active region is very sensitive to the vicinal eruptions.     

EUV Analysis of a Quasi-static Coronal Loop Structure

Decaying active region 10942 is investigated from 4:00?–?16:00 UT on 24 February 2007 using a suite of EUV observing instruments. Results from Hinode/EIS, STEREO and TRACE show that, although the active region has decayed and no sunspot is present, the physical mechanisms that produce distinguishable loop structures, spectral line broadening, and plasma flows still occur. A coronal loop that appears as a blue-shifted structure in Doppler maps is apparent in intensity images of log(T)=6.0?–?6.3 ions. The loop structure is found to be anti-correlated with spectral line broadening generally attributed to non-thermal velocities. This coronal loop structure is investigated physically (temperature, density, geometry) and temporally. Light curves created from imaging instruments show brightening and dimming of the loop structure on two different time scales; short pulses of 10?–?20?min and long duration dimming of two?–?four hours until its disappearance. The coronal loop structure, formed from relatively blue-shifted material that is anti-correlated with spectral line broadening, shows a density of 10¹⁰ to 10^9.3?cm^?3 and is visible for longer than characteristic cooling times. The maximum non-thermal spectral line broadenings are found to be adjacent to the footpoint of the coronal loop structure.     

Erratum to: The Maunder Minimum and the Sun as the Possible Source of Particles Creating Increased Abundance of the <Superscript>14</Superscript>C Carbon Isotope

Solar flares take place in regions of strong magnetic fields and are generally accepted to be the result of a resistive instability leading to magnetic reconnection. When new flux emerges into a pre-existing active region it can act as a flare and coronal mass ejection trigger. In this study we observed active region 10955 after the emergence of small-scale additional flux at the magnetic inversion line. We found that flaring began when additional positive flux levels exceeded 1.38×10²⁰ Mx (maxwell), approximately 7 h after the initial flux emergence. We focussed on the pre-flare activity of one B-class flare that occurred on the following day. The earliest indication of activity was a rise in the non-thermal velocity one hour before the flare. 40 min before flaring began, brightenings and pre-flare flows were observed along two loop systems in the corona, involving the new flux and the pre-existing active region loops. We discuss the possibility that reconnection between the new flux and pre-existing loops before the flare drives the flows by either generating slow mode magnetoacoustic waves or a pressure gradient between the newly reconnected loops. The subsequent B-class flare originated from fast reconnection of the same loop systems as the pre-flare flows.     

Transverse oscillations in coronal loops observed with TRACE – I. An Overview of Events,Movies, and a Discussion of Common Properties and Required Conditions

We study transverse loop oscillations triggered by 17flares and filament destabilizations; only 2 such cases have been reported in the literature until now. Oscillation periods are estimated to range over a factor of ∼15, with most values between 2 and 7 min. The oscillations are excited by filament destabilizations or flares (in 6% of the 255 flares inspected, ranging from about C3 to X2). There is no clear dependence of oscillation amplitude on flare magnitude. Oscillations occur in loops that close within an active region, or in loops that connect an active region to a neighboring region or to a patch of strong flux in the quiet Sun. Some magnetic configurations are particularly prone to exhibit oscillations: two active regions showed two, and one region even three, distinct intervals with loop oscillations. The loop oscillations are not a resonance that builds up: oscillations in loops that are excited along their entire length are likely to be near the fundamental resonance mode because of that excitation profile, but asymmetrically excited oscillations clearly show propagating waves that are damped too quickly to build up a resonance, and some cases show multiple frequencies. We discuss evidence that all oscillating loops lie near magnetic separatrices that outline the large-scale topology of the field. All magnetic configurations are more complicated than a simple bipolar region, involving mixed-polarities in the interior or vicinity of the region; this may reflect that the exciting eruptions occur only in such environments, but this polarity mixing likely also introduces the large-scale separatrices that are involved. Often the oscillations occur in conjunction with gradual adjustments in loop positions in response to the triggering event. We discuss the observations in the context of two models: (a) transverse waves in coronal loops that act as wave guides and (b) strong sensitivity to changes in the field sources for field lines near separatrices. Properties that favor model b are (1) the involvement of loops at or near separatrices that outline the large-scale topology of the field, (2) the combined occurrence of oscillations and loop translations, (3) the small period spread and similar decay time scale in a set of oscillating loops in one well-observed event, and (4) the existence of loops oscillating in antiphase with footpoints close together in two cases. All other properties are compatible with either model, except the fact that almost all of the oscillations start away from the triggering event, suggestive of an outward-pushing exciting wave more in line with model a. The spread in periods from event to event suggests that the oscillations may reflect the properties of some driver mechanism that is related to the flare or mass ejection. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014957715396     

Dynamical interpretation of the very hot region appearing at the top of the loop

In order to explain the appearance of a hard X-ray source at the top of a loop, we present a model in which the dynamical effects of the dark filament mass infallng along the loop in association with the “disparition brusque” plays an important role. The crash of the infalling mass produces high temperature regions in the low corona above the two footpoints of the loop, and the up-going shocks, created in the crash and strengthened in propagating upwards along the steep density gradient in the tail of the infalling mass, produce a very high temperature (10⁸ K) region upon colliding with each other near the top of the loop. Successive occurrence of this process in successively higher loops in magnetic arcade may account for the sources of gradual hard X-ray bursts appearing at the top of the loop-like structure.     

Interactive flare sites within an active region complex

The problem of physical relationships between different active regions has been dealt with only rarely and mainly in connection with flares. How sympathetic activity can be triggered between distant regions is therefore, so far, largely unknown. Soft X-ray images of large-scale coronal structures connecting different active regions were obtained by Skylab nearly 20 years ago, while SMM, more recently, did not provide any clear evidence for this kind of loop. As a consequence, we do not know how common these features are nor how they form nor whether they represent the only means by which distant active regions may be linked. In the latter case, however, interconnecting loops should be detected by analyzing the interaction between different active regions. We examine here a set of images of an active region complex, acquired on June 24-25, 1980, by the Hard X-ray Imaging Spectrometer on SMM, with the purpose of establishing whether there was any interplay between the frequent activity observed at different sites in the activity center and, in such a case, how the interaction was established. By analyzing both quiet and active orbits we show that, as a rule, activity originating in one region triggers the other region's activity. However, we find little unambiguous evidence for the presence of large-scale interconnecting loops. A comparison of X-ray images with magnetic field observations suggested that we interpret the active region behavior in terms of the interaction between different loop systems, in a scenario quite analogous to the interacting bipole representation of individual flares. We conclude that active region interplay provides an easily observable case to study the time-dependent topology and the mechanisms for the spreading of activity in transient events over all energy scales.     

Magnetic interactions during sympathetic solar eruptions

We present the first evidence for occurrences of magnetic interactions between a jet, a filament and coronal loops during a complex event, in which two flares sequen-tially occurred at different positions of the same active region and were closely associated with two successive coronal mass ejections (CMEs), respectively. The coronal loops were located outside but nearby the filament channel before the flares. The jet, originating from the first flare during its rise phase, not only hit the filament body but also met one of the ends of the loops. The filament then underwent an inclined eruption followed by the second flare and met the same loop end once more. Both the jet and the filament erup- tion were accompanied by the development of loop disturbances and the appearances of brightenings around the meeting site. In particular, the erupting filament showed clear manifestations of interactions with the loops. After a short holdup, only its portion passed through this site, while the other portion remained at the same place. Following the fila-ment eruption and the loop disappearance, four dimmings were formed and located near their four ends. This is a situation that we define as "quadrupolar dimmings." It appears that the two flares consisted of a sympathetic pair physically linked by the interaction between the jet and the filament, and their sympathy indicated that of the two CMEs.Moreover, it is very likely that the two sympathetic CMEs were simultaneously associ-ated with the disappearing loops and the quadrupole dimmings.     

Reversal of the polarization of cyclotron radiation in a hot coronal loop

Spectropolarimetric features of thermal cyclotron radiation of solar coronal loops and the possibility of interpretation of the observed reversal of the sense of polarization of centimeter and decimeter waves are discussed. To this end, thermal cyclotron radiation is computed in terms of the simplest model of a three-dimensional hot loop (a half-torus). Such a loop is shown to be capable of changing appreciably the properties of the radiation of a solar active region at centimeter and decimeter wavelengths. A detailed analysis is performed to determine the conditions under which the radiation spectrum of an active region containing a coronal loop may have a complex pattern with several maxima or relatively narrow-band cyclotron lines, and the sense of polarization may change several times in the wavelength interval considered. These conditions are modelled by such parameters as the structure of the magnetic field, electron density, and size of the loop. The results of the computations of two-dimensional brightness temperature distributions at different wavelengths for ordinary and extraordinary waves at fixed points of the loop and the integrated parameters of the flux and polarization of radiation in terms of the model discussed are reported. Cases are considered where the line of sight is crossed by one or two loops. The expected distribution of polarization across the source in the model considered is compared to the results of RATAN-600 observations of the solar active region AR 7962 made on May 12–14, 1996.     

Spectroscopic far ultraviolet observations of transition zone instabilities and their possible role in a pre-flare energy build-up

(1) Highly flare-productive new emerging active regions are characterized by numerous small low-lying loops which frequently show a chaotic pattern. (2) Flare activity in such a region subsides as the chaotic loop structures relax and expand into a bipolar configuration. (3) The transition zone in such an active region is highly unstable as shown by broadened and shifted non-thermal line profiles of medium ionized elements like Si iii, Si iv, C iv, etc. (4) These transition zone instabilities which occur as isolated events in active regions of low flare productivity are often observed prior to flares. (5) Transition zone instabilities can be traced to the footpoints of active loops, and seem to be accompanied by heating of the loop. (6) The loops vary in size and show differing degrees of activity, with the brightest and most compact ones seemingly being in a pre-flare state which results in the catastrophic energy release along the loop during a flare.     

Catastrophic cooling and high-speed downflow in quiescent solar coronal loops observed with TRACE

Observations with the Transition Region and Coronal Explorer, TRACE, show frequent catastrophic cooling and evacuation of quiescent solar coronal loops over active regions. We analyze this process using image sequences taken in passbands showing plasma from a few million degrees down to less than 100 000 K, taken at a cadence of 90 s. The loop evacuation often occurs after plasma high in the corona has cooled to transition-region or even chromospheric temperatures. The cooling loops frequently show Lyman-α and C iv emission developing initially near the loop tops; later, that cool plasma usually slides down on both sides of the loop. The relatively cool material often forms clumps that move at speeds of up to 100 km s⁻¹. The downward acceleration is no more than 80 m s⁻², less than of the surface gravity. Cooling appears to progress with delays of the order of up to 10³ s between thin, neighboring strands within flux bundles with cross-sections of at least 1–2 Mm, so that hot and cool loops are transiently outlined at essentially the same location. The falling material at temperatures of ≲ 0.1 MK shows no evidence of loop braiding on scales above the resolution of ∼1 Mm; loop cross-sections appear independent of height. Existing numerical models suggest that the observed catastrophic loop-top cooling in non-flaring conditions can occur if the loop heating precipitously drops by 1.5 orders of magnitude or more, first and most strongly high in the corona. Using order-of-magnitude geometrical arguments, we estimate that loop bundles in the interior of an active region undergo catastrophic cooling on average once every 2 days, while in a decayed bipolar region that time interval is approximately a week. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005211925515     

The Effect of Mass Motions Inside the Coronal Loops on Emission Line Profiles

The observed green coronal emission line profiles have been often found to have multi-components. Further examinations reveal that the occurrence of multi-components in line profiles is related to the solar cycle variations as well as the activity of the coronal region. The spatial correspondence between the intense loops in active regions and strong multi-components in line profiles suggests that the presence of loops affects the line shapes. The emission line profiles have been found to be fitted well with single or multi-Gaussians with line-of-sight velocities up to 70 km s–1. A simple radiative transfer model of coronal emission line profiles is developed which shows that coronal loops with mass motions inside may give rise to multi-components in line profiles. The effects of loop parameters such as electron density, flow velocity and kinetic temperature and the line-of-sight variations are studied. It is found that line profiles strongly reflect the physical conditions inside the loop.     

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.