Coronal loops and active region structure

We intercompared synoptic H, Ca K, magnetograph and Skylab soft X-ray and EUV data for the purpose of identifying the basic coronal magnetic structure of loops in a typical active region and studying its evolution. We focussed on a complex of activity in July 1973, especially McMath 12417. Our principal results are: (1) Most of the brightest loops connected the bright f plage to either the sunspot penumbra or to p satellite spots; no non-flaring X-ray loops end in umbrae; (2) short, bright loops had one or both ends in regions of emergent flux, strong fields or high field gradients; (3) stable, strongly sheared loop arcades formed over filaments; (4) EFRs were always associated with compact X-ray arcades; and (5) loops connecting to other active regions had their bases in outlying plage of weak field strength in McM 417 where H fibrils marked the direction of the loops. We conclude that a typical loop brightens in response to magnetic field activity at its feet, which heats the plasma. This suggests that the loop acts as a trap for gas convected from its base.     

Evidence for opposed currents in active region loops

EUV and X-ray images of the Sun are used as tracers of the magnetic field structure in the Solar atmosphere in order to study the existence of currents in active regions. Criteria are suggested for comparing the data with theoretical extrapolations of magnetic field lines above the level of magnetograph observations. Analysis of the data presented in conjunction with force-free field calculations suggests the existence of currents flowing in opposite directions in nearby sets of loops in the active regions shown. Some simple qualitative implications of opposed current structures are suggested.     

On heating and cooling in some active region loops

It is shown that the neglect of radiative losses by Antiochos and Sturrock (1976) in investigating conduction cooling is not justified. It is further shown that the anomalous current dissipation leads to substantial amount of heating contrary to remarks made by Ionson (1982).     

The heating and cooling in certain active region loops

Narain and Kumar (1985) have mixed up various situations which are quite different from each other. In view of that, the conclusions drawn by them may not be reliable.     

Propagating disturbances along fan-like coronal loops in an active region

Propagating disturbances are often observed in active region fan-like coronal loops. They were thought to be due to slow mode magnetohydrodynamic waves based on some of the observed properties. However, recent studies involving spectroscopy indicate that they could be due to high speed quasi-periodic upflows which are difficult to distinguish from upward propagating slow waves. In this context, we have studied a fan loop structure in the active region AR 11465 using simultaneous spectroscopic and imaging observations from the Extreme Ultraviolet Imaging Spectrometer onboard Hinode and Atmospheric Imaging Assembly onboard Solar Dynamics Observatory. Analysis of the data shows significant oscillations at different locations. We explore the variations in different line parameters to determine whether the waves or flows could cause these oscillations to improve the current understanding of the nature of these disturbances.     

What causes solar active region loops to exist at transition region temperatures?

High-lying, dynamic loops have been observed at transition region temperatures since Skylab observations. The nature of these loops has been debated for many years with several explanations having been put forward. These include that the loops are merely cooling from hotter coronal loops, that they are produced from siphon flows, or that they are loops heated only to transition region temperatures. In this paper we will make use of combined SOHO-MDI (Michelson-Doppler Imager), SOHO-CDS (Coronal Diagnostic Spectrometer) and Yohkoh SXT (Soft X-ray Telescope) datasets in order to determine whether the appearance of transition region loops is related to small-scale flaring in the corona, and to estimate the magnetic configuration of the loops. The latter allows us to determine the direction of plasma flows in the transition region loops. We find that the appearance of the transition region loops is often related to small-scale flaring in the corona and in this case the transition region loops appear to be cooling with material draining down from the loop top.     

What causes solar active region loops to exist at transition region temperatures?

High-lying, dynamic loops have been observed at transition region temperatures since Skylab observations. The nature of these loops has been debated for many years with several explanations having been put forward. These include that the loops are merely cooling from hotter coronal loops, that they are produced from siphon flows, or that they are loops heated only to transition region temperatures. In this paper we will make use of combined SOHO-MDI (Michelson-Doppler Imager), SOHO-CDS (Coronal Diagnostic Spectrometer) and Yohkoh SXT (Soft X-ray Telescope) datasets in order to determine whether the appearance of transition region loops is related to small-scale flaring in the corona, and to estimate the magnetic configuration of the loops. The latter allows us to determine the direction of plasma flows in the transition region loops. We find that the appearance of the transition region loops is often related to small-scale flaring in the corona and in this case the transition region loops appear to be cooling with material draining down from the loop top.     

Observations of the Mg i and ii resonance lines in an active region

Spectra from 2678-2931 Å were obtained of an active region during the 19 June 1974, flight of the University of Hawaii rocket-borne echelle spectrograph. We report behavior of the Mg i and ii resonance line cores in quiet Sun, plage, sunspot, and filament structures. Among the interesting variations in these lines we discern a strong suppression of the red Mg ii emission peaks and possible rapid changes in the Mg i core in the spatially partially resolved sunspot.     

Asymmetric flux loops in active regions,I

We investigate asymmetries of bipolar sunspot groups. We find that the magnetic field distribution of simple bipolar sunspot groups is significantly asymmetrical: the polarity inversion line is usually nearer to the main following polarity spot than to the main preceding one. This asymmetry grows with the age of the sunspot group. We suggest that this asymmetry has a causal link with two long-established asymmetries- the one in the proper motions of young sunspots, the other in the relative stability of p and f spots.In our view, these asymmetries together indicate that emerging flux loops, making sunspot groups, are not symmetrical but tilted eastward. The tilt is presumably caused by drag forces due to radial differential rotation in subphotospheric layers. In this paper we present observational indications supporting this hypothesis.     

Asymmetric flux loops in active regions,II

We propose that magnetic flux loops in the subphotospheric layers of the Sun are seriously asymmetrical as a consequence of the drag force exerted on them because of the different rotational rate of the surrounding plasma. In numerical models of stationary slender flux loops in the plane parallel approximation we show that a serious tilt is both possible and probable. Observational facts (see van Driel-Gesztelyi and Petrovay, 1989; Paper I) strongly support the case for high asymmetry. The different stability of p and f spots may also be related to such an asymmetry.The tilts are very sensitive to the rotational profile and to the magnetic field structure. Nevertheless the characteristic maximal tilts can be tentatively estimated to be 20° for thin flux tubes and 5° for thick tubes.For two of the five observational consequences of such a tilt (described in detail in Paper I) order-of-magnitude estimates of the effects are given. The estimates are in reasonable accord with observations.We also explore the possibilities of an inverse treatment of the problem whereby subphotospheric rotation and/or flux tube shapes can be inferred from observations of velocities of photospheric spot motions. In particular we demonstrate how analytic inverse solutions can be obtained in special cases.     

Fourier parameters and moments of polarization profiles of magnetically active lines. Fourier vector magnetograph

A new method is proposed to determine all components of the solar magnetic fields using the cumulants of the profile of a magnetic sensitive line. The method is based on polarization measurements in a number of points of the line profile and subsequent calculation of the amplitudes and phases of its two first Fourier-harmonics.     

Evolution of active galactic nuclei broad-line region clouds: low- and high-ionization lines

The formation of quasar broad-line region (BLR) clouds via thermal instability in the presence of Alfvén heating has been discussed by Gonçalves, Jatenco-Pereira & Opher. In particular, these studies showed the relevance of Alfvén heating in establishing the stability of BLR clouds in the intercloud medium. The present paper shows the results of time-dependent calculations (we use a time-dependent hydrodynamic code) following the evolution of BLR clouds, since their formation from the 10⁷-K intercloud medium. We also calculate the UV and optical line emission associated with the clouds in order to compare with observations. Our results are compared with those of UV and optical monitoring of well-studied AGN, which suggest that the BLR is most probably composed of at least two different regions, each one giving rise to a kind of line variability, since low- and high-ionization lines present different patterns of variability. We discuss the alternative scenario in which lines of different ionization could be formed at the same place but heated/excited by distinct mechanisms, considering the Alfvén heating as the non-radiative mechanism.     

Spatial and temporal variations of solar coronal loops

Skylab EUV observations of an active region near the solar limb were analyzed. Both cool (T < 10⁶ K) and hot (T > 10⁶ K) loops were observed in this region. For the hot loops the observed intensity variations were small, typically a few percent over a period of 30 min. The cool loops exhibited stronger variations, sometimes appearing and disappearing in 5 to 10 min. Most of the cool material observed in the loops appeared to be caused by the downward flow of coronal rain and by the upward ejection of chromospheric material in surges. The frequent EUV brightenings observed near the loop footpoints appear to have been produced by both in situ transient energy releases (e.g. subflares) and the infall/impact of coronal rain. The physical conditions in the loops (temperatures, densities, radiative and conducting cooling rates, cooling times) were determined. The mean energy required to balance the radiative and conductive cooling of the hot loops is approximately 3 × 10^–3 erg cm^–3 s^–1. One coronal heating mechanism that can account for the observed behavior of the EUV emission from McMath region 12634 is heating by the dissipation of fast mode MHD waves.     

Correction of observational profiles of spectral lines

The profiles obtained from radial-velocity observations of the Rosette Nebula with a Fabry-Pérot (F.P.) interferometer (Meaburn andSmith, 1968) had to be corrected, for the spreading action of the instrumental function, and for motion of the observer. Two current methods are compared and a method devised for fitting a sum of Gaussians to a noisy output profile.     

Non-LTE profiles of the Ali autoionization lines

A non-LTE formulation is given for the transfer of radiation in the autoionizing lines of neutral aluminum at λ1932 and λ1936 through both the Bilderberg and Harvard-Smithsonian model atmospheres. Numerical solutions for the common source function of these lines and their theoretical line profiles are calculated and compared with the corresponding LTE profiles. Our results show that the non-LTE profiles provide a better match with the observations. They also indicate that the continuous opacity of the standard solar models should be increased in this wavelength region if the center-limb variations of observed and theoretical profiles of these lines are to be in reasonable agreement.

     

Fe xiii emission lines in active region spectra obtained with the Solar Extreme-Ultraviolet Research Telescope and Spectrograph

Recent fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe  xiii are used to generate emission-line ratios involving 3s²3p²–3s3p³ and 3s²3p²–3s²3p3d transitions in the 170–225 and 235–450 Å wavelength ranges covered by the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS). A comparison of these line ratios with SERTS active region observations from rocket flights in 1989 and 1995 reveals generally very good agreement between theory and experiment. Several new Fe  xiii emission features are identified, at wavelengths of 203.79, 259.94, 288.56 and 290.81 Å. However, major discrepancies between theory and observation remain for several Fe  xiii transitions, as previously found by Landi and others, which cannot be explained by blending. Errors in the adopted atomic data appear to be the most likely explanation, in particular for transitions which have 3s²3p3d ¹D₂ as their upper level. The most useful Fe  xiii electron-density diagnostics in the SERTS spectral regions are assessed, in terms of the line pairs involved being (i) apparently free of atomic physics problems and blends, (ii) close in wavelength to reduce the effects of possible errors in the instrumental intensity calibration, and (iii) very sensitive to changes in N _e over the range  10⁸–10¹¹ cm⁻³  . It is concluded that the ratios which best satisfy these conditions are 200.03/202.04 and 203.17/202.04 for the 170–225 Å wavelength region, and 348.18/320.80, 348.18/368.16, 359.64/348.18 and 359.83/368.16 for 235–450 Å.     

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.     

On the location of energy release and temperature profiles along coronal loops

Several mechanisms have been suggested to contribute to the heating of the solar corona, each of which deposits energy along coronal loops in a characteristic way. To compare the theoretical models with observations one has to derive observable quantities from the models. One such parameter is the temperature profile along a loop. Here numerical experiments of flux braiding are used to provide the spatial distribution of energy deposition along a loop. It is found that braiding produces a heat distribution along the loop which has slight peaks near the footpoints and summit and whose magnitude depends on the driving time. Using different examples of the heat deposition, the temperature profiles along the loop are determined assuming a steady state. Along with this, different methods for providing average temperature profiles from the time-series have been investigated. These give summit temperatures within approximately 10% of each other. The distribution of the heating has a significant impact on both the summit temperature and the temperature distribution along the loop. In each case the ratio between the heat deposited and radiation provides a scaling for the summit temperature.