Coronal changes associated with a disappearing filament

This paper describes Skylab/ATM observations of the events associated with a disappearing filament near the center of the solar disk on January 18, 1974. As the filament disappeared, the nearby coronal plasma was heated to a temperature in excess of 6 × 10⁶K. A change in the pattern of coronal emission occurred during the 11/3 hr period that the soft X-ray flux was increasing. This change seemed to consist of the formation and apparent expansion of a loop-like coronal structure which remained visible until its passage around the west limb several days later. The time history of the X-ray and microwave radio flux displayed the well-known gradual-rise-and-fall (GRF) signature, suggesting that this January 18 event may have properties characteristic of a wide class of X-ray and radio events.In pursuit of this idea, we examined other spatially-resolved Skylab/ATM observations of long-duration X-ray events to see what characteristics they may have in common. Nineteen similar long-lived SOLRAD X-ray events having either the GRF or post-burst radio classification occurred during the nine-month Skylab mission. Sixteen of these occurred during HAO/ATM coronagraph observations, and 7 of these 16 events occurred during observations with both the NRL/ATM slitless spectrograph and the MSFC-A/ATM X-ray telescope. The tabulation of these events suggests that all long-lived SOLRAD X-ray bursts involve transients in the outer corona and that at least two-thirds of the bursts involve either the eruption or major activation of a prominence. Also, these observations indicate that long-lived SOLARD events are characterized by the appearance of new loops of emission in the lower corona during the declining phase of the X-ray emission. However, sometimes these loops disappear after the X-ray event (like the post-flare loops associated with a sporadic coronal condensation), and sometimes the loops remain indefinitely (like the emission from a permanent coronal condensation).Visiting Scientist, Kitt Peak National Observatory, Tucson, Ariz. 85726, U.S.A. operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Presently located at NASA/MSFC, Space Sciences Laboratory, Marshall Space Flight Center, Ala. 35812, U. S.A.     

Vector magnetic field evolution,energy storage,and associated photospheric velocity shear within a flare-productive active region

The evolution of vector photospheric magnetic fields has been studied in concert with photospheric spot motions for a flare-productive active region. Over a three-day period (5–7 April, 1980), sheared photospheric velocity fields inferred from spot motions are compared both with changes in the orientation of transverse magnetic fields and with the flare history of the region. Rapid spot motions and high inferred velocity shear coincide with increased field alignment along the B _L= 0 line and with increased flare activity; a later decrease in velocity shear precedes a more relaxed magnetic configuration and decrease in flare activity. Crude energy estimates show that magnetic reconfiguration produced by the relative velocities of the spots could cause storage of 10³² erg day^–1, while the flares occurring during this time expended 10³¹ erg day^–1.Maps of vertical current density suggest that parallel (as contrasted with antiparallel) currents flow along the stressed magnetic loops. For the active region, a constant-, force-free magnetic field (J = B) at the photosphere is ruled out by the observations.Presently located at NASA/MSFC, Huntsville, Ala. 35812, U.S.A.     

A multiwavelength study of a double impulsive flare

Extensive data from the Solar Maximum Mission (SMM) and ground-based observatories are presented for two flares; the first occurred at 12:48 UT on 31 August, 1980 and the second just 3 min later. They were both compact events located in the same part of the active region. The first flare appeared as a typical X-ray flare: the Caxix X-ray lines were broadened ( 190±40 km s^-1) and blue shifted ( 60±20 km s^-1) during the impulsive phase, and there was a delay of about 30 s between the hard and soft X-ray maxima. The relative brightness of the two flares was different depending on the spectral region being used to observe them, the first being the brighter at microwave and hard X-ray wavelengths but fainter in soft X-rays. The second flare showed no significant mass motions, and the impulsive and gradual phases were almost simultaneous. The physical characteristics of the two flares are derived and compared. The main difference between them was in the pre-flare state of the coronal plasma at the flare site: before the first flare it was relatively cool (3 × 10⁶ K) and tenuous (4 × 10⁹ cm^-3), but owing to the residual effects of the first flare the coronal plasma was hotter (5 × 10⁶ K) and more dense (3 × 10¹¹ cm^-3) at the onset of the second flare. We are led to believe from these data that the plasma filling the flaring loops absorbed most of the energy released during the impulsive phase of the second flare, so that only a fraction of the energy could reach the chromosphere to produce mass motions and turbulence.A simple study of the brightest flares observed by the SMM shows that at least 43% of them are multiple. Thus, the situation studied here may be quite common, and the difference in initial plasma conditions could explain at least some of the large variations in observed flare parameters. We draw a number of conclusions from this study. First, the evolution of the second flare is substantially affected by the presence of the first flare. Secondly, the primary energy release in the second event is in the corona. Thirdly, the flares occur in a decaying magnetic region, probably as a result of the interaction of existing sheared loops; there is no evidence of emerging magnetic flux. Also, magnetic structures of greatly varying size participate in the flare processes. Lastly, there is some indication that the loops are not symmetrical or stable throughout the flares, i.e. the magnetic field does not act as a uniform passive bottle for the plasma, as is often assumed in flare models.NOAA/Space Environment Laboratory, currently at NASA/MSFC, Ala., U.S.A.Now at Sacramento Peak Observatory, Tucson, Ariz., U.S.A.     

Calculation of 5250.216 Å line profiles in sunspots

The assumptions of pure absorption and local thermodynamic equilibrium are sometimes used to calculate approximate spectral line profiles in cases where a rigorous treatment is impractical or impossible. In certain conditions, the profile is not completely defined under these assumptions.This work was done under NASA contract No. NAS8-26376 for the Space Sciences Laboratory, George C. Marshall Space Flight Center, Huntsville, Ala.     

Analysis of X-ray observations of the 15 June 1973 flare in active region NOAA 131

Observations and analyses of the 1B/M3 flare of 15 June, 1973 in active region NOAA 131 (McMath 12379) are presented. The X-ray observations, consisting of broadband photographs and proportional counter data from the Skylab/ATM NASA-MSFC/Aerospace S-056 experiment, are used to infer temperatures, emission measures, and densities for the flaring plasma. The peak temperature from the spatially resolved photographs is 25 × 10⁶ K, while the temperature from the full-disk proportional counter data is 15 × 10⁶ K. The density is 3 × 10¹⁰cm^–3. The X-ray flare emission appears to come primarily from two low-lying curvilinear features lying perpendicular to and centered on the line where the photospheric longitudinal magnetic field is zero. Similarities in the preflare and postflare X-ray emission patterns indicate that no large-scale relaxation of the coronal magnetic configuration was observed. Also discussed are H and magnetic field observations of the flare and the active region. Finally, results of numerical calculations, including thermal conduction, radiative loss and chromospheric evaporation, are in qualitative agreement with the decay phase observations.Presently at NASA/Marshall Space Flight Center.     

Spatial distribution of XUV emission and density in a loop prominence

We have studied the spatial distribution of XUV emission in the 14 August, 1973 loop prominence observed with the NRL spectroheliograph on Skylab. The loop prominence consists of two large loops and is observed in lines from ions with temperatures ranging from 5 × 10⁴ K to 3 × 10⁶ K. The loops seen in low temperature (10⁶K) lines such as from He ii, Ne vii, Mg vii, Mg viii, and Si viii are systematically displaced from loops seen in higher temperature lines such as from Si xii, Fe xv, and Fe xvi. The cross section of the loop, particularly in cooler lines is nearly constant along the loop. For hotter loops in Si xii, Fe xv, and Fe xvi, however, emission at the top of the loop is more intense and extended than that near the footpoints, which makes the loops appear wider at the top.There is no evidence that the 14 August loop prominence consists of a cooler core surrounded by a hot sheath as in some active region and sunspot loops reported by Foukal (1975, 1976). Rather, the observed spatial displacement between cooler and hotter loops suggest that the 14 August loop prominence is composed of many magnetic flux tubes, each with its own temperature.Ball Corporation. Now with NASA/Marshall Space Flight Center.     

High resolution absorption cross section measurements of SO2 at 213 K in the wavelength region 172–240 nm

Laboratory measurements at high resolution of the absorption cross section of SO₂ at the temperature 213 K have been performed in the wavelength region 172–240 nm with a 6.65 m scanning spectrometer/spectrograph operated at an instrumental width of 0.002 nm. The measured cross sections are presented graphically in representative wavelength regions and are available throughout the region 172–240 nm at wavenumber intervals of 0.4–0.1 cm^?1 as a numerical tabulation stored on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. The measured cross sections, which are relevant to the photochemistry of planetary atmospheres, possess significantly more spectroscopic structure, and are more accurate, than previous measurements made at lower resolution.     

Active region magnetic fields inferred from simultaneous VLA microwave maps,X-ray spectroheliograms,and magnetograms

A series of VLA maps at 6 cm wavelength have been generated from observations of a solar active region (NOAA 2363) on 29 and 30 March, 1980. During the same period, X-ray spectroheliograms were acquired for this region in the lines of O viii, Ne ix, Mg xi, Si xiii, S xv, and Fe xxv, with X-rayn Polychromator (XRP) aboard the Solar Maximum Mission (SMM). Intervals of relative quiescence (i.e., when X-ray flares and centimeter wave bursts were not evident) were selected for microwave mapping. The resulting VLA maps have spatial resolution of 4 × 4, and generally show two or more sources whose slowly evolving substructures have spatial scales of 10–30. These maps were co-registered with H photographs (courtesy of AF/AWS SOON, Holloman and Ramey AFB) to an accuracy of ± 8. Similarly, the X-ray spectroheliograms have been co-registered with white light photographs to about the same accuracy. Magnetograms from KPNO and MSFC have also been co-aligned, and the magnetic X-ray, and microwave features compared. In general we have found that (a) the peaks of X-ray and 6 cm emission do not coincide, although (b) the sources in the two wavelength domains tend to overlap. These facts in themselves are evidence for the existence of opacity mechanisms other than thermal bremsstrahlung. In order to quantify this assertion, we have computed differential emission measures to derive densities and temperatures. Using these and calculated force-free magnetic fields from Kitt Peak magnetograms, we present an assessment of the mechanism of gyroresonance absorption at low harmonics of the electron gyrofrequency as the source of opacity responsible for the microwave features. We conclude that large-scale currents must be present in the active region loops to account for the bright 6 cm sources far from sunspots.Lockheed Missiles and Space Company, Palo Alto: currently at GSFC.Currently at NASA/MSFC.     

The continuous emission of low energy cosmic rays during solar flares

It has been recently suggested by several investigators that the accelerated charged particles provide the energy of the optical flare by the ionization loss process. We have examined this mechanism assuming different forms of the spectrum of the accelerated protons at lower chromosphere. The flux and the energy spectrum of protons of energy 0.1–100 MeV have been calculated at successive heights, from 10³ to 40 × 10³ km from the solar surface taking into account the ionization loss, pitch angle distribution and density distribution of the neutral and ionized hydrogen in the chromosphere and lower corona. Hence the energy spectrum of the protons escaping from the Sun and the amount of energy dissipated in the solar chromosphere are computed. Comparing the calculated results with the observational data on the solar event of September 28, 1961 it is found that the ionization loss of the accelerated protons and heavier nuclei in the solar atmosphere may supply a significant part of the energy of the optical flare assuming that the fraction, f, of magnetic tubes of force extending out of the solar atmosphere is about 1 %. The accelerated proton spectrum in the form of power law in kinetic energy seems to be the most appropriate form. In the event of September 28, 1961 best estimates are made on this basis of the total number and the energy spectrum of protons at injection, the flux and energy spectrum of escaping protons and the energy dissipated in the solar atmosphere by the accelerated ions. It is found that the possible range of variation of the height of injection level hardly affects the total energy dissipated. The high variability of the intensity of protons released by the Sun is interpreted in terms of the variations of the parameter, f, determined by the configurations of the magnetic field lines.Preliminary results were presented at the International Symposium on Solar-Terrestrial Physics, Leningrad, May, 1970.Presently at NASA/Goddard Space Flight Center, Greenbelt, Maryland, U.S.A., on leave from T.I.F.R., Bombay.     

Study of the post-flare loops on 29 July 1973

We present revised values of temperature and density for the flare loops of 29 July 1973 and compare the revised parameters with those obtained aboard the SMM for the two-ribbon flare of 21 May 1980. The 21 May flare occurred in a developed sunspot group; the 29 July event was a spotless two-ribbon flare. We find that the loops in the spotless flare extended higher (by a factor of 1.4–2.2), were less dense (by a factor of 5 or more in the first hour of development), were generally hotter, and the whole loop system decayed much slower than in the spotted flare (i.e. staying at higher temperature for a longer time). We also align the hot X-ray loops of the 29 July flare with the bright H ribbons and show that the H emission is brightest at the places where the spatial density of the hot elementary loops is enhanced.presently at Big Bear Solar Observatory, Caltech, California, U.S.A.presently at The Marshall Space Flight Center, Huntsville, Alabama, U.S.A.     

The Solar-A mission

The Institute for Space and Astronautical Sciences (ISAS) is developing a satellite dedicated to high-energy observations of solar flares. The Solar-A will be launched in August–September, 1991, from the Kagoshima Space Center on board a M3S-II vehicle. The instrument complement emphasizes hard X-ray and soft X-ray imaging, and contains instruments supplied in part by U.S. and U.K. experimenters. This paper describes the instrumentation and the tentative observing program.

     

Solar flare injection and propagation of low-energy protons and electrons in the event of 7–9 July, 1966

Simultaneous observations of the 7–9 July 1966 solar particle event by energetic particle detectors on three satellites, IMP-III, OGO-III and Explorer 33 are utilized to show that large spatial gradients are present in the fluxes of 0.5–20 meV protons and 45 keV electrons. The event is divided into three parts: the ordinary diffusive component, the halo, and the core. The core corotates with the interplanetary field, and therefore it and the surrounding halo are interpreted as spatial features which are connected by the interplanetary magnetic field lines to the vicinity of the flare region. Upper limits to the interplanetary transverse diffusion coefficient for 4–20 meV protons at 1 AU are derived from the width of the halo. These are at least two orders of magnitude less than the parallel diffusion coefficient for the same energy particles.It is argued that the observed flux variations cannot be explained by an impulsive point source injection for any physically reasonable diffusion model. Instead, since the interplanetary transverse-diffusion coefficient is small for these low-energy particles, the observed spatial features are interpreted as the projection to 1 AU by the interplanetary field lines of an extensive injection profile at the sun. The geometry of the injection mechanism is discussed and it is suggested that some temporary storage of the flare particles occurs near the sun.Now at NASA, Goddard Space Flight Center, Greenbelt, Md., U.S.A.     

Interpretation of time characteristics of solar X-ray bursts referring to associated microwave bursts

It has been controversial whether the flare-associated hard X-ray bursts are thermal emission or non-thermal emission. Another controversial point is whether or not the associated microwave impulsive burst originates from the common electrons emitting the hard X-ray burst.It is shown in this paper that both the thermal and non-thermal bremsstrahlung should be taken into account in the quantitative explanation of the time characteristics of the hard X-ray bursts observed so far in the photon energy range of 10–150 keV. It is emphasized that the non-thermal electrons emitting the hard X-rays and those emitting the microwave impulsive burst are not common. The model is as follows, which is also consistent with the radio observations.At the explosive phase of the flare a hot coronal condensation is made, its temperature is generally 10⁷ to 10⁸K, the number density is about 10¹⁰ cm^–3 and the total volume is of the order of 10²⁹ cm³. A small fraction, 10^–3–10^–4, of the thermal electrons is accelerated to have power law distribution. Both the non-thermal and thermal electrons in the sporadic condensation contribute to the X-ray bursts above 10 keV as the bremsstrahlung. Fast decay of the harder X-rays (say, above 20 keV) for a few minutes is attributed to the decay of non-thermal electrons due to collisions with thermal electrons in the hot condensation. Slower decay of the softer X-rays including around 10 keV is attributed to the contribution of thermal component.The summary of this paper was presented at the Symposium on Solar Flares and Space Research, COSPAR, Tokyo, May, 1968.     

Statistical properties of MHD fluctuations associated with high-speed streams from Helios-2 observations

A variance analysis of Helios-2 magnetic data has been used to derive several statistical properties of MHD fluctuations associated with the trailing edge of a given stream observed in different solar rotations. Such properties are derived both as a function of distance from the Sun and as a function of the frequency range included in the sample. The most noticeable result is that the radial gradients of various parameters, such as anisotropy and normalized power of the fluctuations, depend from frequency range. In particular the variation with distance of the normalized power does not correspond, for periods 1 hr, to what is expected from WKB propagation effects.Istituto di Fisica, Università di Roma, Rome, Italy.Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Md 20771, U.S.A.     

The flares of August 1972

We present the analysis of observations of the August flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms and spectra. In each flare the observations fit a model of particle acceleration in the chromosphere with emission produced by impact and by heating by the energetic electrons and protons. The region showed inverted polarity and high gradients from birth, and flares appear due to strong magnetic shears and gradients across the neutral line produced by sunspot motions. Post flare loops show a strong change from sheared, force-free fields parallel to potential-field-like loops, perpendicular to the neutral line above the surface.We detected fast (5 s duration) small (1') flashes in 3835 at the footpoints of flux loops in the August 2 impulsive flare at 1838 UT, which may be explained by dumping of > 50 keV electrons accelerated in individual flux loops. The flashes show excellent time and intensity agreement with > 45 keV X-rays. In the less impulsive 2000 UT flare a less impulsive wave of emission in 3835 moved with the separating footpoints. The thick target model of X-ray production gives a consistent model for X-ray, 3835 and microwave emission in the 18:38 UT event.Spectra of the August 7 flare show emission 12 Å FWHM in flare kernels, but only 1 to 2 Å wide in the rest of the flare. The kernels thus produce most of the H emission. The total emission in H in the August 4 and August 7 flares was about 2 × 10³⁰ erg. We belive this dependable value more accurate than previous larger estimates for great flares. The time dependence of total H emission agrees with radio and X-ray data much better than area measurements which depend on the weaker halo.Absorption line spectra show a large (6 km/s^-1) photospheric velocity discontinuity across the neutral line, corresponding to sheared flow across that line.This work has been supported by NASA under NGR 05 002 034, NSF Atmospheric Sciences program under GA 24015, and AFCRL under FI9628-73-C-0085.     

Martens-Kuin models of normal and inverse polarity filament eruptions and coronal mass ejections

An analysis is made of the Martens-Kuin filament eruption model in relation to observations of coronal mass ejections (CMEs). The field lines of this model are plotted in the vacuum or infinite resistivity approximation with two background fields. The first is the dipole background field of the model and the second is the potential streamer model of Low. The assumption is made that magnetic field evolution dominates compression or other effects which is appropriate for a low- coronal plasma. The Martens-Kuin model predicts that, as the filament erupts, the overlying coronal magnetic field lines rise in a manner inconsistent with observations of CMEs associated with eruptive filaments. Initially, the bright arc of a CME broadens in time much more slowly than the dark cavity between it and the filament, whereas in the model they broaden at the same rate or the bright arc broadens more rapidly than the dark cavity, depending on the background field. Thus, this model and, by generalization the whole class of so-called Kuperus-Raadu configurations in which a neutral point occurs below the filament, are of questionable utility for CME modeling. An alternate case is considered in which the directions of currents in the Martens-Kuin model are reversed resulting in a so-called normal polarity configuration of the filament magnetic field. In this case, a neutral line occurs above the current-carrying filament. The background field lines now distort to support the filament and help eject it. While the vacuum field results make this configuration appear very promising, a full two- or more-dimensional MHD simulation is required to properly analyze the dynamics resulting from this configuration.Presently NRC Senior Research Associate at NOAA, Space Environment Laboratory, Boulder, Colorado, U.S.A.At the NASA National Space Data Center.     

High resolution absorption cross section measurements and band oscillator strengths of the (1, 0)−(12, 0) Schumann-Runge bands of O2

Cross sections of O₂ at 300 K have been obtained from photoabsorption measurements at various pressures throughout the wavelength region 179.3–201.5 nm with a 6.65 m photoelectric scanning spectrometer equipped with a 2400 lines mm^?1 grating and having an instrumental width (FWHM) of 0.0013 nm. The measured absorption cross sections of the Schumann-Runge bands (12, 0) through (1, 0) in this wavelength region are absolute, i.e., independent of the instrumental width, a result not achieved previously. The measured cross sections are presented graphically and are available at wavenumber intervals of > sim; 0.1 cm^?1 as numerical complications stored on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. Band oscillator strengths of the (12, 0) through (1, 0) bands have been determined by direct numerical integration of the measured cross sections.     

Prominence mass ejections and their effects on the corona

Skylab soft X-ray observations of two lower coronal limb events and corresponding H observations (Skylab and ground-based) are analyzed. We discuss the morphology and evolution of an eruptive prominence occurring on 21 August 1973, beginning (in H) at about 1300 UT and of a surge on 4 December 1973, beginning at about 1758 UT. For the eruptive prominence, measured X-ray flux is used in the determination of line-of-sight temperatures, emission measures, and electron densities. A peak temperature of 8.5 × l0⁶ K and densities to 3.5 × l0⁹ cm^-3 are derived. A time-dependent, two-dimensional, single-fluid magnetohydrodynamic computer code has been used to simulate the coronal response to these prominences. We find that the coronal response to the observed eruptive prominence may be simulated with a density-dominated pressure pulse at the base of the corona ( 30000 km above photosphere), while a temperature pulse of short duration will simulate the coronal response to the surge. Approximately 10³¹ ergs and 10⁴⁰ particles (or 10¹⁶ g) were deposited into the corona during the eruptive prominence event, while about 10²⁹ ergs and 10³⁸ particles (or 10¹⁴ g) were injected during the surge event. A shock wave formed ahead of the ejected material at about 70000 km above the photosphere in the eruptive prominence event and had a velocity of 275 km s^-1 at 1.5 r above the limb.Presently at NASA / Marshall Space Flight Center.     

Hα surges and associated Soft X-ray loops

A recurrent H surge was observed on 7 October, 1991 on the western solar limb with the Meudon MSDP spectrograph. The GOES satellite recorded X-ray subflares coincident with all three events. During two of the surges high-resolutionYohkoh Soft X-ray Telescope (SXT) images have been taken. Low X-ray loops overlying the active region where the surges occurred were continuously restructuring. A flare loop appeared at the onset of each surge event and somewhat separated from the footpoint of the surge. The loops are interpreted as causally related to the surges. It is suggested that surges are due to magnetic reconnection between a twisted cool loop and open field lines. Cold plasma bubbles or jets squeezed among untwisting magnetic field lines could correspond to the surge material. No detection was made of either X-ray emission along the path of the surges or X-ray jets, possibly because of the finite detection threshold of theYohkoh SXT.     

High resolution absorption cross-section measurements of N2O at 295–299 K in the wavelength region 170–222 nm

High resolution absorption cross-section measurements of N₂O at 295–299 K have been performed in the wavelength region 170–222 nm with a 6.65 m scanning spectrometer/spectrograph of sufficient resolution to yield cross-sections that are independent of the instrumental function. The measured cross-sections are presented graphically and are available throughout the region 44925–58955 cm^?1 at intervals of 0.1–0.2 cm^?1 as a numerical tabulation stored on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. Previously unresolved details of the banded structure which is superposed on the continuous absorption in the region 174–190 nm are observed.