Large-scale active coronal phenomena in Yohkoh SXT images

We discuss Yohkoh SXT observations of stationary giant post-flare arches which occurred on 3–6 May, 1992 and study in detail the last arch, associated with the flare at 19:02 UT on 5 May, which extended above the west limb. The arch was similar to the first giant arch discovered on board the SMM, on 21–22 May, 1980. We demonstrate that the long lifetimes of these structures necessarily imply additional energy input from the underlying active region: otherwise, conduction would cool these arches in less than one hour and even with the unlikely assumption of conduction inhibited, pure radiative cooling would not produce the temperature decrease observed. All arch tops, although varying in brightness, stayed for several days at a fairly constant altitude of 100 000 km, and the arch studied, on 5–6 May, was just a new brightening of the pre-existing decaying structure. The brightening was apparently due to inflow of hot plasma from the flare region. Yohkoh data confirm that these stationary arches are rare phenomena when compared with the rising arches studied in Paper I and with Uchida et al.'s expanding active regions.     

Large-scale active coronal phenomena in Yohkoh SXT images

We have found several occurrences of slowly rising giant arches inYohkoh images. These are similar to the giant post-flare arches previously discovered by SMM instruments in the 80s. However, we see them now with 3–5 times better spatial resolution and can recognize well their loop-like structure. As a rule, these arches followeruptive flares with gradual soft X-ray bursts, and rise with speeds of 1.1–2.4 km s^–1 which keep constant for >5 to 24 hours, reaching altitudes up to 250 000 km above the solar limb. These arches differ from post-flare loop systems by their (much higher) altitudes, (much longer) lifetimes, and (constant) speed of growth. One event appears to be a rise of a transequatorial interconnecting loop.In the event of 21–22 February 1992 one can see both the loop system, rising with a gradually decreasing speed to an altitude of 120 000 km, and the arch, emerging from behind the loops and continuing to rise with a constant speed for many more hours up to 240 000 km above the solar limb. In the event of 2–3 November 1991 three subsequent rising large-scale coronal systems can be recognized: first a fast one with speed increasing with altitude and ceasing to be visible at about 300 000 km. This most probably shows the X-ray signature of a coronal mass ejection (CME). A second one, with gradually decreasing speed, might represent very high rising flare loops. A third one continues to rise slowly with a constant speed up to 230 000 km (and up to 285 000 km after the speed begins to decay), and this is the giant arch. This event, including an arch revival on November 4–5, is very similar to rising giant arches observed by the SMM on 6–7 November 1980. Other events of this kind were observed on 27–28 April 1992, 15 March 1993, and 4–6 November 1993, all seen above the solar limb, where it is much easier to identify them.The temperature in the brightest part of the arch of 2–3 November 1991 was increasing with its altitude, from 2 to 4 × 10⁶ K, which seems to be an effect of slower cooling at lower densities. Under an assumption of line-of-sight thickness of 50 000 km, the emission measure indicates densities from 1.1 × 10¹⁰ cm^–3 at an altitude of 150 000 km to 1.0 × 10⁹ cm^–3 at 245 000 km 11.5 hours later. It appears that the arch is composed of plasma of widely different temperatures, and that hot plasma rises faster than the cool component. Thus the whole arch expands upward, and its density gradient increases with time, which explains whyYohkoh images show only the lowest and coolest parts of the expanding structure. The whole arch may represent an energy in excess of 10³¹ erg, and more if conduction contributes to the arch cooling.We suggest that the rise of the arch is initiated by a CME which removes the magnetic field and plasma in the upper corona, and the coronal structures remaining below this cavity begin to expand into the vacuum left behind the CME. However, we are unable to explain why the speed of rise stays constant for so many hours.     

A semi-analytical approach to time-dependent coronal expansion

In this paper we point out the existence of a special class of solutions to the nonlinear hydrodynamic equations describing the time-dependent solar wind, namely that for which the velocity profile is time-invariant but the density at each point of the corona changes exponentially with time. Theoretical velocity curves are calculated for the case of isothermal expansion and compared with the Parker model for steady-state expansion. These solutions can be used to obtain quantitative estimates for the degree of departure from the latter of a real corona undergoing evolution on a finite time scale.On leave from Los Alamos Scientific Laboratory, Los Alamos, N.M., U.S.A.     

Flare-associated high-speed solar plasma streams

Characteristics of flare-associated high-speed solar plasma streams are investigated using measurements from space probes and Earth-orbiting spacecraft for the period 1964–1982. The maximum observed velocity (V _m) of these streams range from 400 to 850 km s^–1} with peak probability for 600 km s^–1}. These remain for the period of 1–10 days with the peak occurrence 3 days. The difference between the pre-stream velocity (V ₀) and the maximum velocity (V _m) of any high-speed stream serves as the measure of its intensity. For about 60% of the flare associated streams, (V _m-V ₀) is well in excess of 200 km s^–1} and in some cases becomes as large as 450 km s^–1}. The yearly percentage occurrence, total duration and the product of mean (V _m - V ₀) with total duration of the high-speed streams during the year correlates well with solar activity, e.g., maximum during high solar activity period and minimum during low solar activity. The study suggests that presence of sunspots plays a significant role in the generation of flare associated high-speed solar streams.     

Flare-associated surge prominence on 1980 october 30

The Hα observations of a flare-associated surge prominence on 1980 October 30 have been described. Morphology and dynamics of the surge prominence have been presented. From our observations and analysis we have estimated the magnetic field associated with surge material to be about 35 gauss which is in good agreement with the earlier result of Tandberg-Hanssen & Malville (1974). It has been determined that coronal pressure is not acting as a resistive force on the outward expansion of the surge into the corona. The kinetic energy of the surge was about 10²⁸ erg, which is 2 orders less than required for the mass to escape the chromosphere. It appears that the flare-associated surge prominence was perhaps a result of kink instability in the flaring region.     

Effects of diverging coronal fields on the solar wind expansion

The expansion of the solar wind in divergent flux tubes is calculated by taking into account a magnetic acceleration of the particles, analogous to the magnetic mirror effect.The resulting force term included in the magnetohydrodynamical equations describes a conversion of thermal into kinetic energy. This causes an additional acceleration of the solar wind plasma which has never been taken into account before. The force is directed opposite to the magnetic field gradient. Consequently, in this case the solar wind velocity increases faster to its asymptotic value than it does for corresponding nonmagnetic solutions. Therefore inside and close to the solar corona markedly higher velocities are found. Compared to strictly hydrodynamical models, the critical point is shifted towards the Sun, and the radial decrease of the ratio of thermal to kinetic energy is faster.The necessary prerequisites for these calculations are (a) that the gyroperoid _g of the plasma particles is much shorter than the Coulomb collision time _c , and (b) that the collision time _c is shorter than the characteristic time _d in which an appreciable amount of thermal anisotropy is built up. Thus it is (a) insured that the particles have established magnetic moments and follow the guiding center approximation, and (b) an almost isotropic velocity distribution function is maintained which, in this first approximation of a purely radial expansion, justifies the use of isotropic pressures and temperatures.Both (a) and (b) are shown to be fulfilled in a region around the Sun out to about 20R , and thermal anisotropies developing outside of this region could explain the observed magnetically aligned anisotropies at 1 AU.     

Flare-associated shock waves observed by interplanetary scintillation

Daily observations of a grid of scintillating sources during the period January–August 1971 indicate that enhancements in scintillation index which cannot be related to corotating structure, are related to interplanetary shock waves associated with solar flares. Only 3 enhancements in scintillation index associated with shock waves were observed during the eight months period of observations.     

Flare-associated magnetic changes in an active region

We report an instance of localized chromospheric polarity reversal in a rapidly-formed sunspot which appears to be part of new emerging flux. The chromospheric polarity reversal is preceded by extraordinarily fast growth of the transverse magnetic field and an increase in the line-of-sight magnetic flux of the newly formed sunspot in the photosphere. The strength of this reversal is more than 350 G at maximum, in contrast to approximately - 1300 G for the line-of-sight field and 400 G for the transverse field in the photosphere. Continued flare activity takes place around the site of the reversal with progressively increasing flare size and extent. It is suggested that a kinked or knotted flux loop, or a self-closed flux system developed above the fast-forming sunspot. So far, this phenomenon has been revealed in several active regions.     

The effects of diverging coronal fields on the solar wind expansion

The calculations made by Fahret al. (1976) have been subjected to a re-examination, the results of which are described in this letter.     

High-frequency coronal oscillations and coronal heating

At the 1980 total solar eclipse, we searched for high-frequency (0.1–2 Hz) oscillations in the intensity of the 5303-Å coronal green line, as a test of predictions of theories of coronal heating via magnetohydrodynamic waves. Portions of the image 2.5- or 5-arc sec across were fed to cooled photomultipliers using fiber-optic probes. We detected excess power in Fourier transforms of the data for the region between 0.5 and 2 Hz at the level of 1% or 2% of the incident power. Such oscillations could be associated with Alfvén waves that are trapped on loops a few thousand kilometers long or with fast waves that are trapped on loops a few thousand kilometers in diameter. Additional observations at the 1983 eclipse are planned to resolve atmospheric and instrumental contributions.     

Possible Detection of a Flare-associated Coronal Mass Ejection on an M-dwarf

We here report a probable detection of a stellar coronal mass ejection(CME) in active M dwarf KIC 8093473 by performing an analysis on its time resolved X-ray spectra observed by the XMM-Newton satellite. Compared to the value at the quiescent state and the interstellar one, our spectral modeling returns a marginal(and probably evolving) excess of hydrogen column density in the flare state at a significance level of 1σ, which can be understood by an additional absorption due to a flare-associate...     

Evidence linking coronal transients to the evolution of coronal holes

The positions of X-ray coronal transients outside of active regions observed during Skylab were superposed on H synoptic charts and coronal hole boundaries for seven solar rotations. We confirmed a detailed spatial association between the transients and neutral lines. We found that most of the transients were related to large-scale changes in coronal hole area and tended to occur on the borders of evolving equatorial holes.Skylab Solar Workshop Post-Doctoral Appointee, 1975–1977.     

Polar coronal plumes

Polar coronal plumes are modeled using concentrations of magnetic flux at 1.01R , and assuming the field is current-free, or a potential field. Identifying the density enhancement of plumes with magnetic flux concentration produces good agreement between 1.01R and 1.10R , for model conditions of a large background magnetic field and a plume separation of 50 000 to 70 000 km at the base. Beyond 1.10R , both plumes and the potential field diverge very nearly as r ².Also Department of Astrogeophysics, University of Colorado, Boulder, Colo. 80309, U.S.A. Presently visiting Stanford University Institute for Plasma Research, Via Crespi, Stanford, Calif. 94303, U.S.A.     

Solar coronal loop

The conclusions drawn by Narain (1983) appear to be in error.     

Solar coronal streamers

A unique combination of photographic and K-coronameter data were used to study the structure and evolution of two known coronal streamers. In addition, two other K-coronameter enhancements were studied as representing ideal second examples of the known streamers. As a general rule the observations indicate that these features were direct coronal manifestations of photospheric bipolar magnetic regions (BMR) and were of two basic types:active region, by which is meant a coronal streamer which develops radially over a low-latitude active region; andhelmet which denotes a streamer whose structure and development appear to be a consequence of a long-lived complex of activity, composed of both trailing magnetic fields and a parent center of disk activity.The similarity of growth rates during the first solar rotation of life led to derivation of a total streamer density of 4–5 × 10⁸ cm^–3 atr = 1.125R . This density may represent a characteristic maximum density at the base of streamers. The intensity gradient of the inner (r1.5R ) corona was used to establish a qualitative evolutionary model of streamers which synthesizes the observations. Briefly, streamers initially develop over active regions; the streamer growth rate may be as rapid as the disk activity, or at worst lags flare activity by solar rotation. The streamer can be the cause of interplanetary and geomagnetic effects at 1 AU within a solar rotation after birth. Thereafter the streamer follows an evolution dictated by the underlying solar magnetic fields. In any case the lowest level of the coronal enhancement has a lifetime not exceeding that of the solar disk activity.     

Solar coronal streamers

The solar disk locations of 13 coronal streamers were determined from a combination of eclipse, K-coronameter (1 1/8r1 1/2 R ), and balloon-borne coronagraph (2<r<6 R ) observations taken during 1964 and 1965. Of this sample, three were observed twice on photographs taken over intervals of four and 28 days. Most of these streamers could be structurally associated with K-coronameter enhancements to establish their disk locations.Those features having known disk locations all lay above some stage of chromospheric disk activity in the form of active regions and prominences. The average lifetime of three K-coronameter streamer-enhancements, for which all or nearly all of their lifetimes were known, was about 4 solar rotations. Rotation rates for the lower latitude streamer-enhancements (30°) were essentially identical to the underlying surface. One high latitude feature ( 50°) which overlay a quiescent prominence had a rate equivalent to the surface rate at 30° latitude. In general those K-coronameter enhancements associated with streamers came into existence over time periods of 14 days and disappeared by gradually blending into the background coronal pattern. All the observed structures are explained by a model consisting of localized, high density features (streamers) which overlie disk activity and are imbedded in a uniform but weaker azimuthally-symmetric quiet corona.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Threadlike coronal streamers

Threadlike streamers have small cross sections that are almost constant over their whole length. A typical value of their diameter is 30000 km. Threadlike streamers may reach a length of several hundred thousand kilometers or even more than a solar radius. A photometric analysis of a threadlike streamer, observed at the eclipse of 1963 July 20, yields an electron density 8 times larger than that of the undisturbed corona. This ratio undergoes only small variations with the distance from the Sun.     

Progress in coronal physics

This paper reviews research highlights of the past five years. Considerable progress has been made in observing and interpreting coronal mass ejections. The stability of coronal loops is much better understood and new observations of the onset of wind streams in coronal holes have been made. Observations from the Solar Maximum Mission should helpt to clarify the physics of the active corona.The mechanisms that heat the corona and accelerate the high-speed wind streams remain to be identified, however.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Post-flare coronal loop interaction

High-resolution images of post-flare loop systems in Fexiv (5303 ) and Fex (6374 ) display occasional transient enhancements at the projected intersection of some loops. The brightness of a green-line enhancement gradually increases to a marked maximum and then fades with a lifetime of the order of thirty minutes. The red-line image at the same location, although fainter, shows the same overall characteristics, its maximum following that of the green-line on average by 8.6 min. H then becomes more evident and reaches a maximum in extent on average 9.3 min after the red-line maximum. The phenomenon is interpreted as a process of localized loop coalescence involving partial magnetic reconnection. Estimates of the electron density are derived from the cooling time following the initial heating of the plasma in the immediate vicinity of the X-point of interaction. Similar estimates for the energy dissipated, equivalent to a very small flare, are derived by two independent methods.Operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.     

Properties of coronal arches

The properties of coronal arches located on the peripheries of active regions, observed during a sounding rocket flight on March 8, 1973, are discussed. The arches are found to overlie filament channels and their footpoints are traced to locations on the perimeters of supergranulation cells. The arches have a wide range of lengths although their widths are well approximated by the value 2.2 × 10⁹ cm. Comparison of the size of the chromospheric footprint with the arch width indicates that arches do not always expand as they ascend into the corona. The electron temperatures and densities of the plasma contained in the arches were measured and the pressure calculated; typical values are 2 × 10⁶ K, 1 × 10⁹ cm^–3, and 2 × 10^–1 dyne cm^–2, respectively. The variation of these parameters with position along the length of the arch indicates that the arches are not in hydrostatic equilibrium.