Transequatorial loops interconnecting McMath regions 12472 and 12474

We discuss the life-story of a transequatorial loop system which interconnected the newly born active region McMath 12474 with the old region 12472. The loop system was probably born through reconnection accomplished 1.5 to 5 days after the birth of 12474 and the loops were observed in soft X-rays for at least 1.5 days. Transient sharpenings of the interconnection and a striking brightening of the whole loop system for about 6 hr appear to be caused by magnetic field variations in the region 12474. A flare might have been related to the brightening, but only in an indirect way: the same emerging flux could have triggered the flare and at the same time strengthened the magnetic field at the foot-points of the loops. Electron temperature in the loop system, equal to 2.1 × 10⁶ K in its quiet phase, increased to 3.1 × 10⁶ K during the brightening. Electron density in the loop system was 1.3 × 10⁹ cm^–3 and it could be estimated to 7 × 10⁸ cm^–3 prior to the brightening. During the brightening the loops became twisted. There was no obvious effect whatsoever of the activity in 12474 upon the in erconnected old region. The final decay of the loop system reflected the decay of magnetic field in the region 12474.     

Evolution of the high-temperature plasma in the 15 June 1973 flare

The analysis of the high temperature plasma in Fe xxiii–xxiv in the 15 June 1973 flare is presented. The observations were obtained with the NRLXUV spectroheliograph on Skylab. The results are: (1) There was preheating of the active region in which the flare occurred. In particular, a large loop in the vicinity of the flaring region showed enhanced brightness for many hours before the flare. The loop disappeared when the flare occurred, and returned in the postflare phase, as if the energy flux which had been heating the large loop was blocked during the flare and restored after the flare was gone. The large magnetic fields did not change significantly. (2) The flare occurred in low-lying loop or loops. The spatial distribution of flare emission shows that there was a temperature gradient along the loop. (3) The high temperature plasma emitting Fe xxiii and xxiv had an initial upward motion with a velocity of about 80 km s^–1. (4) There was large turbulent mass motion in the high temperature plasma with a random velocity of 100 to 160 km s^–1. (5) The peak temperature of the hot plasma, determined from the Fe xxiii and xxiv intensity ratio, was 14 × 10⁶ K. It decreased slightly and then, for a period of 4 min, remained at 12.6 × 10⁶ K before dropping sharply to below 10 × 10⁶ K. The density of the central core of the hot plasma, determined from absolute intensity of Fe xxiv 255 Å line, was of the order of 10¹¹ cm^–3.The persistence of the high level of turbulence and of the high temperature plateau in the decaying phase of the flare indicates the presence of secondary energy release. From the energy balance equation the required energy source is calculated to be about 3 to 7 ergs cm^–3 s^–1.Ball Brothers Research Corporation.     

Multiple loop activations and continuous energy release in the solar flare of June 15, 1973

The spatial and temporal evolution of the high temperature plasma in the flare of 1973 June 15 has been studied using the flare images photographed by the NRL XUV spectroheliograph on Skylab.The overall event involves the successive activations of a number of different loops and arches bridging the magnetic neutral line. The spatial shifts and brightenings observed in the Fe xxiii–xxiv lines are interpreted as the activation of new structures. These continued for four or five minutes after the end of the microwave burst phase, implying additional energy-release unrelated to the nonthermal phase of the flare. A shear component observed in the coronal magnetic field may be a factor in the storage and release of the flare energy.The observed Fe xxiii–xxiv intensities define a post-burst heating phase during which the temperature remained approximately constant at 13 × 10⁶ K while the Fe xxiv intensity and 0–3 Å flux rose to peak values. This phase coincided with the activation of the densest structure (N _e = 2 × 10¹¹ cm^–3). Heating of higher loops continued into the decay phase, even as the overall temperature and flux declined with the fading of the lower Fe xxiv arches.The observed morphology of individual flaring arches is consistent with the idea of energy release at altitude in the arch (coincident with a bright, energetic core in the Fe xxiv image) and energy flow downward into the ribbons. The Doppler velocity of the Fe xxi 1354 Å line is less than 5 km s^–1, indicating that the hot plasma region is stationary.The relation of this flare to the larger class of flares associated with filament eruptions and emerging magnetic flux is discussed.     

A model of hot loops associated with solar flares

A dynamical model is proposed for the formation of soft X-ray emitting hot loops in solar flares. It is examined by numerical simulations how a solar model atmosphere in a magnetic loop changes its state and forms a hot loop when the flare energy is released in the form of heat liberation either at the top part or around the transition region in the loop.When the heat liberation takes place at the top part of the loop which arches in the corona, the plasma temperature around the loop apex rises rapidly and, as the result, the downward thermal conductive flux is increased along the magnetic tube of force. Soon after the thermal conduction front rushes into the upper chromosphere, a local peak of pressure is produced near the conduction front and the chromospheric material begins to expand into the corona to form a high-temperature (10⁷ K-3 × 10⁷ K at the loop apex) and high-density (10¹⁰ cm^–3-10¹¹ cm^–3 at the loop apex) loop. The velocity of the expanding material can reach a few hundred kilometres per second in the coronal part. The thermal conduction front also plays a role of piston pushing the chromospheric material downward and gives birth to a shock wave which propagates through the minimum temperature region into the photosphere. If, on the other hand, the heat source is placed around the transition region in the loop, the expansion of the material into the corona occurs from the beginning of the flare and the formation process of the hot loop differs somewhat from the case with the heat source at the top part of the loop.Thermal components of radiations emitted from flare regions, ranging from soft X-rays to radio wavelengths, are interpreted in a unified way by using physical quantities obtained as functions of time and position in our flare loop model as will be discussed in detail in a following paper.     

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.     

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.

     

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.     

Revivals of a coronal arch

The giant post-flare arch of 6 November 1980 revived 11 hr and 25 hr after its formation. Both these revivals were caused by two-ribbon flares with growing systems of loops. The first two brightenings of the arch were homologous events with brightness maxima moving upwards through the corona with rather constant speed; during all three brightenings the arch showed a velocity pattern with two components: a slow one (8–12 km^?1), related to the moving maxima of brightness, and a fast one (～ 35 km s^?1), the source of which is unknown. During the first revival, at an altitude of 100000 km, temperature in the arch peaked ～ 1 hr, brightness ～ 2 hr, and emission measure ～ 3.5 hr after the onset of the brightening. Thus the arch looks like a magnified flare, with the scales both in size and time increased by an order of magnitude. At ～ 100000 km altitude the maximum temperature was ?14 × 10⁶K, max.n _e? 2.5 × 10⁹cm^?3, and max. energy density ? 11.2 erg cm^?3. The volume of the whole arch can be estimated to 1.1 × 10³⁰ cm³, total energy ?1.2 × 10³¹ erg, and total mass ?4.4 × 10¹⁵g. The density decreased with the increasing altitude and remained below 7 × 10⁹ cm^?3 anywhere in the arch. The arch cooled very slowly through radiation whereas conductive cooling was inhibited. Since its onset the revived arch was subject to energy input within the whole extent of the preexisting arch while a thermal disturbance (a new arch?) propagated slowly from below. We suggest that the first heating of the revived arch was due to reconnection of some of the distended flare loops with the magnetic field of the old preexisting arch. The formation of the ‘post’-flare loop system was delayed and started only some 30–40 min later. Since that time a new arch began to be formed above the loops and the velocities we found reflect this formation.     

Photographs of the Sun in the XUV-region

X-ray photographs obtained with a zone plate camera on October 3, 1967 in the wavelength band 49.5–52.5 Å have been investigated photometrically.The most intense X-ray emission corresponds with active regions in H and Ca ii. About one quarter of the total solar flux is emitted by the three brightest X-ray sources (A, E and J). X-ray emission from quiet regions is also observed. Limb brightening is found, also at the poles, which indicates a higher electron density at the poles than during solar minimum.The brightest X-ray regions have a very small core of the order of 20. No relation to magnetic field strengths of sunspots has been found. However, a correlation with active prominences cannot be ruled out. X-ray source A is related either to prominence activity or to flare activity. One X-ray region (J) is probably related to flare activity.Assuming an electron temperature of 3 × 10⁶K to 5 × 10⁶K for coronal active regions an emission measure of a few times 10⁴⁹ cm^–3 is derived, which yields an electron density of a few times 10¹⁰ cm^–3.     

Flare loops heated by thermal conduction

A flare observed with the Hard X-Ray Imaging Spectrometer (HXIS) was studied during its rise to maximum temperature and X-ray emission rate. Two proximate flare loops, of lengths 2.8 × 10⁹ cm and 1.1 × 10¹⁰ cm, rose to temperatures of 21.5 × 10⁶ K and 30 × 10⁶ K, respectively, in 30 s. Assuming equal heat flux F into each loop from a thermal source at the point where they met, we derive a simple relationship between temperature T and loop length , which gives a loop temperture ratio of 0.68, in close agreement with the observed ratio of 0.72. The observations imply that heating in each loop was maintained by a thermal flux of 5 × 10⁹ ergs cm^-2 s^-1. It is suggested that conductive heating adequately describes the rise and maximum phase emissions in the loops and that long flare loops reach higher temperatures than short loops during the impulsive phase because of an equipartition of energy between them at their point of interaction.     

The Hα development of an intense limb flare and associated flaring arches

The H analysis of the development of the strong impulsive and faint gradual phase of the June 26, 1983 flare indicates the following: (1) The flare originated from two microprominences on the southeast border of NOAA 4227. Several similar events are summarized in Table II. (2) The main flare structure was a flare cone, which consisted of a bright surge-like stream, elevated above two flare ribbons (located in the cone's base). The flare cone had a height of about 40 × 10³ km and lasted 4 min in H. The upper part of the cone was terminated by a very fine loop, which was bent to the west, where later a chromospheric brightening occurred at the footpoint of a flaring arch. A 300 keV burst and radio spikes were observed during the maximum flare phase. (3) The flaring arch system, with its apex at a height of about 48 × 10³ km, formed the skeleton for the coronal helmet structure (Figure 7(c)). The velocity of the plasma moving along the flaring arch was between 3500 km s^–1} and 6900 km s^–1} during the first brightening (14:07 UT).     

Slow-mode oscillations of large-scale coronal loops

On several occasions, repetitive X-ray brightenings, sometimes accompanied by mass injections into adjacent loops, appeared quasi-periodically with mean periods close to 20 minutes. In all cases when X-ray images were available, the sites of these brightenings were in active regions which were associated with large-scale coronat loops of length (2 – 3) × 10⁵ km. Therefore, the primary source of these long-periodic pulsations might be slow-mode oscillations in these large-scale loops. Free MHD oscillations, proposed earlier by Roberts, Edwin, and Benz (1984), may fit the observed data.     

Structure and dynamics of cool flare loops

MSDP observations of the 16 May, 1981 two-ribbon flare are used to study the physical structure and the dynamical behaviour of cool flare loops. The loops have been detected in the H line just after the flare maximum and they appeared in absorption against the disk. Using the first-order differential cloud model (DCM1) technique, we derive empirically some basic plasma parameters at 15 points along one loop leg. The flow velocities and the true heights have been reconstructed with respect to a geometrical projection. Subsequently, detailed non-LTE models of cool loops have been constructed in order to fit H source function values previously derived from DCM1 analysis. It is demonstrated that this source function is rather sensitive to the radial component of the flow velocity (the so-called Doppler brightening) and to enhanced irradiation of the loops from the underlying flare ribbons. In this way, we have been able to estimate quantitatively all plasma parameters which determine the physical structure of cool loops (i.e., the temperature, pressure, density), as well as the momentum-balance condition within the loops. For these dark loops we have arrived at relatively low gas pressures of the order of 0.1–0.5 dyne cm^-2 with corresponding electron densities around 10¹¹ cm^-3. Pressure-gradient forces have been found to be of small importance in the momentum-balance equation, and thus they cannot explain departures from a free-fall motion found in our MSDP data analysis. We propose three possible solutions to this problem.     

Transition-zone observations of rapid flare events as observed by OSO-8

The rapid dissipation of flare energy has been observed in the transition-zone line of C iv at 1548.2 Å using the University of Colorado spectrometer aboard OSO-8. Impulsive brightenings have been resolved with characteristic risetimes as low as 3.5 s. One event is analyzed in detail, in which it is inferred that the electron density is greater than 2 × 10¹¹ cm^–3 at T = 60 000 K, and that the flare energy is deposited at a rate of 2 ergs cm^–3 s^–1 or greater. The temporal behavior of the intensity at the center of the C iv line is consistent with a non-equilibrium ionization of C iii through C v. If this event is a result of the multiple tearing mode instability as the primary energy release mechanism, then the observations indicate a pre-flare magnetic field of about 175 G.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Coronal X-ray activity preceding solar flares

A comprehensive survey of Skylab S-054 soft X-ray images was performed to investigate the characteristics of coronal enhancements preceding solar flares. A search interval of 30 min before flare onset was used. A control sample was developed and tests of the statistical results performed. X-ray images with preflare enhancements were compared with high resolution H images and photospheric magnetograms.The results are as follows: preflare X-ray enhancements were found in a statistically significant number of the preflare intervals, and consisted of one to three loops, kernels or sinuous features per interval. Typically, the preflare feature was not at the flare site and did not reach flare brightness. There was no systematically observed time within the preflare interval for the preflare events to appear and no correlation of preflare event characteristics with the subsequent flare energy. Gas pressures of several preflare features were calculated to be on the order of several dyne cm^–2, typical of active region loops, not flares. These results suggest that observations with both high spatial resolution and low coronal temperature sensitivity are required to detect these small, low pressure enhancements that preceded the smaller flares typical of the Skylab epoch. H brightenings were associated with nearly all of the preflare X-ray enhancements. Changing H absorption features in the form of surges or filament activations were observed in about half of the cases. These results do not provide observational support for models which involve preheating of the flare loop, but they are consistent with some current sheet models which invoke the brightening of structures displaced from the flare site tens of min before onset.     

A single loop of 21 January 1974 flare

A single loop associated with a flare of 21 January 1974 was studied with NRL spectroheliograms in order to understand the phenomenon of evaporation. The loop seen in the emission lines of Fe xv reached its maximum brightness 15 min after the onset. The loop is different from a flare loop because of the time sequence in which it appeared and is different from a post-flare loop prominence system because of its morphology. The electron density in the loop increases gradually to 4 × 10¹⁰ cm^–3. The material of the loop is thought to be supplied from the lower atmosphere of the chromosphere or the photosphere. The loop is an associated phenomenon of the main flare event distinguished by a longer rise time (15 min) and a lower peak temperature (2 × 10⁶ K).     

The quantitative properties of three soft X-ray flare kernels observed with the AS&E X-ray telescope on Skylab

The physical parameters for the kernels of three solar X-ray flare events have been deduced using photographic data from the S-054 X-ray telescope on Skylab as the primary data source and 1–8 and 8–20 Å fluxes from Solrad 9 as the secondary data source. The kernels had diameters of 5–7 and in two cases electron densities at least as high as 3 × 10¹¹ cm^–3. The lifetimes of the kernels were 5–10 min. The presence of thermal conduction during the decay phases is used to argue: (1) that kernels are entire, not small portions of, coronal loop structures, and (2) that flare heating must continue during the decay phase.We suggest a simple geometric model to explain the role of kernels in flares in which kernels are identified with emerging flux regions. The flare is triggered at the neutral sheet between the EFR and a larger loop structure. We associate the X-ray kernels with H kernels, which previously associated (incorrectly, we believe) with the nonthermal impulsive phases of flares.Most of this work was completed while the author was a Visiting Scientist at the Center for Astrophysics, Cambridge, Massachusetts 02138.     

Estimation of an upper limit for the solar neutron emission during large flares

Solar neutron emission during large flares is investigated by using neutron monitor data from the mountain stations Chacaltaya (Bolivia), Mina Aguilar (Argentine), Pic-du-Midi (France) and Jungfraujoch (Switzerland). Registrations from such days on which large flares appeared around the local noon time of the monitor station are superimposed with the time of the optical flare as reference point.No positive evidence for a solar neutron emission was found with this method, However, by using an extrapolation of the neutron transport functions given by Alsmiller and Boughner a rough estimation of mean upper limits for the solar neutron flux is possible. The flux limits are compared with Lingenfelter's model calculations.From the Chacaltaya measurements it follows: N ₀2.8 × 10^{–3 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P0 = 125 MV N ₀1.4 × 10^{–2 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 60 MV and from Pic-du-Midi measurements: N ₀6.7 × 10^{–3 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 125 MV N ₀4 × 10^{–2 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 60 MV P ₀ = characteristic rigidity of the producing proton spectrum on the Sun.The flux limits estimated for some special proton flares are consistent with Lingenfelter's predictions for the acceleration phase but are too small for the slowing down phase. Therefore it is believed that Lingenfelter's assumption of isotropic proton emission from the flare region is not fulfilled.     

OBSERVATIONS OF FAINT,OUTLYING LOOP SYSTEMS IN LARGE FLARES

Faintly visible, darkened regions in H lying outside but adjacentto bright flare emissionwere found to occur in 10 of 31 major flares investigated. Without exception, the darkenings occur over magnetically neutral areas, and these are usually bordered by ridges ofoppositely-poled field, where one border is shared in common with a flare ribbon. Thedarkenings probably result from the formation of faint, outlying loop systems, similar topost-flare loops seen in absorption, but which are connected to magnetic features outsidethe flare and are unresolved or only marginally resolved in patrol images. Simple modelsfor post-flare loops incorporating the results of statistical equilibrium calculations readilydemonstrate that darkenings of several percent (consistent with our photometric measurements) can be produced by loop structures of cross-sectional diameter 10² km (unresolved by patrol instruments) and containing gas at densities 5 × 10¹⁰–5 × 10¹¹ cm^-3 andtemperatures 8000–15000 K. Outlying loop systems might be formed by magnetic fieldreconnection, analogous to the mechanism ascribed to eruptive two-ribbon flares, butassociated with field structures adjacent to the flare. Alternatively, these outlying loopsystems may not erupt but become visible as a result of heating and chromospheric evaporation at the footpoints shared with the flare ribbon. In either case, the observations presented here have interesting implications for both the spatial scale and the topology of thecoronal magnetic fields in which eruptions occur.