Connection of coronal holes with active regions

We have determined two qualitative parameters for 643 coronal holes (CHs). Parameter A characterizes the magnetic-field variation in CHs depending on the height. Parameter B characterizes the connection between the magnetic field in a CH and the polar field at the photosphere level. A comparison of these parameters corresponding to CH with and without active regions (ARs) has led to the following conclusions: the formation of AR in CH is a frequent phenomenon, since the former exist in every other CH for at least 1 day; unlike the case of a CH without an AR, the configuration of the magnetic field above the CH with an AR often changes with the height: two out of three CHs have the opposite sign of the magnetic field in photosphere, as compared to the sign of the polar magnetic field; the areas of ARs in CHs do not differ from those for many other ARs outside of CHs.     

The analysis and interpretation of solar X-ray photographs

This paper discusses the methods used to analyse and interpret X-ray filtergrams obtained by solar soft X-ray telescopes such as the S-056 Skylab instrument. First, an appropriate definition of the line-of-sight emission measure _L(T) is developed, and it is shown how the X-ray data may be analysed to obtain an approximation to _L(T). The accuracy of this approximation is severely limited by the mathematical ill-conditioning of the problem, and additional constraints on the solutions must be imposed through the use of a specific model of the coronal region under study. Such a model is also required for the proper interpretation of the results in terms of coronal plasma processes. Examples of such models are provided and the forms of _L(T) derived from them compared with other, semi-empirical forms.The filter ratio method (a simplified form of analysis in which the region under study is assumed isothermal) is discussed. It is shown that in the presence of line-of-sight temperature gradients, the values of effective temperature and emission measure yielded by this method cannot be directly related to the physical state of the plasma and so are of little utility in the study of coronal processes.Now at: Institute for Plasma Research, Stanford University, Via Crespi, Stanford, Calif. 94305, U.S.A.     

A comparison of the temperature and emission measure of X-ray active regions with coronal magnetic fields

The distribution of temperature and of emission measure in X-ray active regions relative to the coronal magnetic fields has been investigated. The position of maximum temperature and the position of maximum emission measure were found to lie along the magnetic neutral line, with the maximum temperature tending to lie above the position of an abrupt change in direction of the neutral line. Several simple structural models of these regions are compared to the emission measure. The total magnetic energy and the total emission measure appear to be related by a power law in the regions studied by us.     

The coronal structure of active regions

A four-parameter model which assumes a Gaussian dependence of both temperature and pressure on distance from center is used to fit the compact part of coronal active regions as observed in X-ray photographs from a rocket experiment. The four parameters are the maximum temperature T _M, the maximum pressure P _M= 2N_MkT_M, the width of the pressure distribution σ _P, and the width of the temperature distribution σ _T = α^1/2σ_P. The maximum temperature T _M ranges from 2.2 to 2.8 × 10⁶K, and the maximum density N _M from 2 to 9 × 10⁹cm^?3. The range of σ _P is from 2 to 4 × 10⁹ cm and that of α from 2 to 7.     

A comparison of coronal X-ray structures of active regions with magnetic fields computed from photospheric observations

The appearances of several X-ray active regions observed on March 7, 1970 and June 15, 1973 are compared with the corresponding coronal magnetic field topology. Coronal fields have been computed from measurements of the longitudinal component of the underlying magnetic fields, under the current-free hypothesis. An overall correspondence between X-ray structures and calculated field lines is established, and the magnetic counterpart of different X-ray features is also examined. A correspondence between enhanced X-ray emission and the location of compact closed field lines is suggested by this study. Representative magnetic field values calculated under the assumption of current-free fields are given for heights up to 200″.     

The interpretation of simultaneous soft X-ray spectroscopic and imaging observations of an active region

Simultaneous soft X-ray spectroscopic and broad band imaging observations of an active region have been analyzed together to determine the parameters which describe the coronal plasma. From the spectroscopic data, models of temperature-emission measure-elemental abundance have been constructed which provide acceptable statistical fits. By folding these possible models through the imaging analysis, models which are not self-consistent can be rejected. In this way, only the oxygen, neon and iron abundances of Pottasch (1967), combined with either an isothermal or exponential temperature-emission measure model are consistent with both sets of data. Contour maps of electron temperature and density for the active region have been constructed from the imaging data. The implications of the analysis to the determination of coronal abundances and to future satellite experiments are discussed.     

Further remarks on the analysis and interpretation of solar X-ray photographs

In the light of a recent discussion by Gerassimenko and Nolte (1978) we re-examine the use of the filter ratio method for determining the thermodynamic parameters of coronal plasmas. We conclude that the method gives reliable results only when the observed plasma is nearly isothermal and that presently available data do not demonstrate that active region plasmas generally meet this condition.     

Properties of the magnetic fields of coronal holes with active regions

We determine the structure of the magnetic fields of coronal holes (CHs) and investigate its change in connection with the emergence of active regions (ARs) in CHs. Based on our observations in the HeI 1083 nm line performed with the CrAO TST-2 telescope, we have selected CHs of two types: without (15 CHs) and with (28 CHs) ARs. Magnetograms obtained at the Kitt Peak National Solar Observatory have been used to calculate the magnetic fields of the same objects.     

Distribution of lifetimes for coronal soft X-ray bright points

We have measured the lifetimes of all compact emission features visible on three sets of high time resolution soft X-ray images. The spectrum of lifetimes is found to be heavily weighted toward short lifetimes. The number of features present on the disk which live 2–48 hours is at least ten times as great as the number living more than 48 hours. The distribution of lifetimes can be fit in all three cases by a four-parameter function N(t) = N _s exp(-t/ _s ) + N _L exp(-t/ _L ), with _s = 8.7±0.2, _L = 35±4 and N _s 10N _L . Features living two days or less have a very broad latitude distribution (Golub et al., 1974, 1975) whereas nearly all longer-lived features are found within 30° of the equator. The growth rates of long-lived vs short-lived points are the same to within 20%, the major difference being that long-lived points continue to grow and generally reach larger sizes.Harvard College Observatory/Smithsonian Astrophysical Observatory.     

An investigation of the structure of coronal active regions

The temperature and density structure of a typical coronal active region is deduced from X-ray observations of several active regions. Observations of the limb transits of three regions from OSO-5 indicate that the X-ray emission originates between 2 × 10⁴ km and 1.5 × 10⁵ km. An emission measure-temperature distribution is deduced from high resolution X-ray spectra obtained with a rocket observation of two similar regions. These observations are combined to give a model of a typical active region, the temperature varying from 2 to 6 × 10⁶ K with corresponding densities between 2 × 10⁹ and 10¹⁰ cms^–3.     

A comparison of three solar active regions based on their soft X-ray line spectra

A rocket-borne, collimated spectrometer has obtained the soft X-ray (1.0–2.2 nm) spectra of three solar active regions. The principal features of the spectra are described and are then used to determine the conditions in the active regions. An isothermal (single temperature) model is not able to describe the observed spectra so that a continuous distribution of emission measure with temperature is introduced.This distribution, based on that proposed by Chambe, is then used to investigate the structure of the active regions. Several simple models are considered. It is shown that each active region has a hot, dense core surrounded by a large outer volume through which the temperature and density fall until normal coronal conditions are reached.Two of the regions exhibited similar characteristics with the cores having electron densities 10¹⁰ cm^–3 and temperatures of at least 4 × 10⁶K. Even the third region, which was much less impressive and quite compact in H, appears to have had a small amount of this dense plasma in its central core.     

Soft X-ray observations of large-scale coronal active region brightenings

One-hundred fifty-six large-scale enhancements of X-ray emission from solar active regions were studied on full-disk filterheliograms to determine characteristic morphology and expansion rates for heated coronal plasma. The X-ray photographs were compared with H observations of flares, sudden filament disappearances, sprays and loop prominence systems (LPS). Eighty-one percent of the X-ray events were correlated with H filament activity, but only forty-four percent were correlated with reported H flares. The X-ray enhancements took the form of loops or arcades of loops ranging in length from 60 000 km to 520 000 km and averaging 15 000 km in width. Lifetimes ranged from 3 hr to >24 hr. Event frequency was 1.4 per day. X-ray loop arcades evolved from sharp-edged clouds in cavities vacated by rising H filaments. Expansion velocities of the loops were 50 km s^-1 immediately after excitation and 1–10 km s^-1 several hours later. These long-lived loop arcades are identified with LPS, and it is suggested that the loops outlined magnetic fields which were reconnecting after filament eruptions. Another class of X-ray enhanced loops stretched outside active regions and accompanied sprays or lateral filament ejections. H brightenings occurred where these loops intersected the chromosphere. Inferred excitation velocities along the loops ranged between 300 and 1200 km s^-1. It is suggested that these loops outlined closed magnetic fields guiding slow mode shocks from flares and filament eruptions.     

Fresnel zone plate telescopes for X-ray imaging I: experiments with a quasi-parallel beam

Combination of Fresnel Zone Plates (FZP) can make an excellent telescope for imaging in X-rays. We present here the results of our experiments with several pairs of tungsten made Fresnel Zone plates in presence of an X-ray source kept at a distance of about 45 ft. The quasi-parallel beam allowed us to study sources placed on the axis as well as off the axis of the telescope. We present theoretical study of the fringe patterns produced by the zone plates in presence of a quasi-parallel source. We compare the patterns obtained from experiments with those obtained by our Monte-Carlo simulations. The images are also reconstructed by deconvolution from both the patterns. We compare the performance of such a telescope with other X-ray imaging devices used in space-astronomy.     

Further X-ray spectra of solar active regions

Measurements of the solar X-ray spectrum between 3 Å and 15 Å are reported. They were made with two slitless Bragg crystal spectrometers flown on a Sun-pointed Skylark rocket on 8 August, 1967.The use of a beryl crystal has provided higher spectral resolution than hitherto in the spectral range 12 Å and 15 Å leading, in particular, to a revised identification of the strong line at 13.71 Å. Separate components of the stronger emission lines are clearly seen from each of three coronal active regions which may be identified on radio and X-ray spectroheliograms.The absolute line fluxes are used to determine a model for each active region in terms of the differential emission measure as a function of electron temperature. Emission lines due to the transition 1s ^{2 1} S ₀ – 1s2s ³ S ₁ in several helium-like ions are identified and values of the local electron density derived from measurement of the line flux in these ions.     

Modeling coronal magnetic field using spherical geometry: cases with several active regions

The magnetic fields in the solar atmosphere structure the plasma, store free magnetic energy and produce a wide variety of active solar phenomena, like flare and coronal mass ejections (CMEs). The distribution and strength of magnetic fields are routinely measured in the solar surface (photosphere). Therefore, there is considerable interest in accurately modeling the 3D structure of the coronal magnetic field using photospheric vector magnetograms. Knowledge of the 3D structure of magnetic field lines also help us to interpret other coronal observations, e.g., EUV images of the radiating coronal plasma. Nonlinear force-free field (NLFFF) models are thought to be viable tools for those task. Usually those models use Cartesian geometry. However, the spherical nature of the solar surface cannot be neglected when the field of view is large. In this work, we model the coronal magnetic field above multiple active regions using NLFFF extrapolation code using vector magnetograph data from the Synoptic Optical Long-term Investigations of the Sun survey (SOLIS)/Vector Spectromagnetograph (VSM) as a boundary conditions. We compare projections of the resulting magnetic field lines solutions with their respective coronal EUV-images from the Atmospheric Imaging Assembly (SDO/AIA) observed on October 15, 2011 and November 13, 2012. This study has found that the NLFFF model in spherical geometry reconstructs the magnetic configurations for several active regions which agrees to some extent with observations. During October 15, 2011 observation, there are substantial number of trans-equatorial loops carrying electric current.     

Pre- and post-flare X-ray variations in active regions

Extremely low background noise of the HXIS experiment aboard the SMM made it possible to detect > 3.5 keV X-ray emissions from non-flaring active regions which are 10³–10⁴ times weaker than the X-ray flux from flares. Short-lived X-ray bursts and long-lived X-ray enhancements of various intensities seem to characterize active regions in different phases of their development. After major two-ribbon flares, giant X-ray arches are seen in the corona, slowly decaying for many hours after the flare end. Associated with these arches appear to be quasi-periodic flare-like variations of purely coronal nature.     

Fresnel zone plate telescopes for X-ray imaging II: numerical simulations with parallel and diverging beams

We present the results of simulations of shadows cast by a zone plate telescope which may have one to four pairs of zone plates. From the shadows we reconstruct the images under various circumstances. We discuss physical basis of the resolution of the telescope and demonstrate this by our simulations. We allow the source to be at a finite distance (diverging beam) as well as at an infinite distance (parallel beam) and show that the resolution is worsened when the source is nearby. By reconstructing the zone plates in a way that both the zone plates subtend the same solid angles at the source, we obtain back high resolution even for sources at a finite distance. We present simulated results for the observation of the galactic center and show that the sources of varying intensities may be reconstructed with accuracy. Results of these simulations would be of immense use in interpreting the X-ray images from recently launched CORONAS-PHOTON satellite.     

The birthplaces of active regions and X-ray bright points

A comparison of soft X-ray pictures of the Sun (S-054 experiment of Skylab) with K-line spectroheliograms (Mount Wilson) shows that the X-ray bright points tend to emerge randomly throughout the Ca network pattern. However, all those bright points that developed into active regions emerged at the boundaries of network cells. This suggests that the magnetic flux of active regions comes from greater depths in the convection zone than the shallow flux that gives rise to the random emergence of bright points.Also Physics Department, C-011, University of California at San Diego, La Jolla, Calif., U.S.A.     

X-ray observations of recurrence of eruptive flares and active regions

In a study of soft X-ray coronal images obtained with the Yohkoh spacecraft, two eruptive flares with remarkably similar X-ray structures were noted – most remarkably because the flares occurred at the same solar location (approximately 10 deg north latitude on the east limb) yet separated in time by three solar rotations. Between the times of the eruptions, the active region responsible for the first flare disappeared from Yohkoh images. An extremely similar X-ray active region replaced it by the third solar rotation. The recurring X-ray active region appearance and recurring flare activity after 86 days suggest that persistent subsurface flux emergence patterns might be responsible, and support previous arguments that active longitudes exist.     

Preflare characteristics of active regions observed in soft X-rays

X-ray images from the AS&E telescope on Skylab are used to investigate coronal conditions in solar active regions during the 20-min periods preceding the X-ray onsets of small flares. The preflare or precursor phase is defined as a phase with a characteristic length or time scale significantly different from that of the rise phase. We show that there is no observational evidence of a requirement for a coronal preflare heating phase with a time scale longer than 2 min for small flares characterized by one or two loops. In 18 out of 25 cases the flaring X-ray structure was not the brightest feature in the preflare active region. The electron densities are estimated for preflare loops.