Coronal Arch Research Articles

A solar radio moving type IV burst of expanding arches type involving multi-sources

A complex solar radio moving type IV burst was observed on 23 September 1998 with the broadband (1.0-2.0 GHz and 2.6-3.8 GHz) spectrometers with high temporal and spectral resolutions at National Astronomical Observatories of China (NAOC). Comparing to the high spatial resolution data of Siberian Solar Radio Telescope (SSRT), we find that this burst is a rare type of moving type IV burst which is caused by the expanding arches, and the spatial structure oscillations of the radio sources are related with the time structure pulsations of the radio emission. Furthermore, the burst is associated with the multiple quasi-periodic long-term pulsations, and this suggests the existence of multi-scale magnetic structures in a large expanding coronal arch. We think the moving type IV burst is due to the synchrotron emission of the energetic electrons trapped in the expanding arch, and the multiple quasi-periodic pulsations are due to the second harmonic plasma emission.

Patterning of human dental arch wire blanks using a vector quantization algorithm.

We objectively and automatically classified arch wire blank forms represented by a series of facial surface points on tooth crowns in human adults with normal occlusions using a vector quantization algorithm on the basis of the generalized Lloyd algorithm. We investigated also the descriptive dental arch form parameters that were most effective in distinguishing the classified groups of dental arch wire blanks and examined if they were associated with specific anatomical traits. Dental casts, taken from 79 adults with complete dentitions, were laser scanned with a computer-assisted stereotaxic device. Coordinates of the tooth crown points (FA points) were measured for each dental arch expressed as a vector having positional elements in a series of 14 FA points and categorized into several codes (ie, patterns) according to the similarity of the quantized vector patterns. We found that classifying the dental arches into four patterns maximized the difference between arch wire blank patterns. The classified patterns were differentiated by a gradual broadening of the interarch widths posterior to the lateral incisor. The code with the narrowest arch had the longest coronal arch, whereas the code with the widest arch had the shortest coronal arch (P < .01). The interpremolar and intermolar basal arch widths determined for the code showing the widest arch were significantly greater than those for the code with the narrowest width (P < .01).

Common features in the development of powerful long-duration solar X-ray flares

We have begun an investigation of the possible origins of considerable of powerful solar flares. This effect is manifest, first and foremost, in the existence of high-temperature plasma in flare loops over many hours. Analysis of the soft X-ray emission in two energy bands detected by the GOES satellites for about 20 powerful solar flares reveals long time intervals during the decay phase when the source temperature decreases, in general, exponentially. The characteristic time ti for a decrease in the temperature by a factor of ten is 3–10 hours for most powerful events. In addition, another interval of very slow decrease with a characteristic time ti of tens of hours can be identified in some cases. We found a gradual change in the dependence of the temperature on the square root of the emission measure for the source as a whole, which characterizes the transition from purely coronal processes to powerful flares with a prolonged inflow of plasma from the chromosphere. Modeling the energy balance in a loop can yield the requirements for the source of plasma heating in a long-lived arch system. A necessary condition for the development of prolonged flares seems to be a powerful coronal mass ejection, which initiates the formation of a source of plasma heating at coronal heights. Our analysis shows that a considerable fraction of the energy is often released in the region of the cusp, and that systems of giant coronal arches rising to heights of about 100 000 km above the limb are formed in most prolonged events (called dynamical flares in the terminology of Svestka).

Modelling long duration X-ray flares on the UX Ari late-type subgiant

Abstract Modelling of the gas-dynamic process in giant coronal arches for interpretation of the large prolonged X-ray flare on subgiant UX Ari with less than solar gravity is carried out. The case where plasma is heated near the top of the giant loop is considered taking into account the thermal conduction and radiative losses. The observational duration of the soft X-ray flare radiation and peculiar features of the temporal behaviour of the temperature can be explained if this process is developed under prolonged heating in giant loops with densities more than 1011 cm−3. The model of the X-ray flare source, agreeing with the observed value of the emission measure, includes around 8 loops lifting up to heights around 1011 cm, i.e. more than 0.2 R *. It is proposed than the energy of such non-stationary phenomena is supplied from the large-scale (global) magnetic fields, and these flares are not a result of evolution of local magnetic fields as happens during impulsive stellar flares.

Peculiarities of the plasma mechanism of radio emission from late-type stars

The plasma mechanism of radio emission in the coronas of late-type stars is shown to be considerably more efficient than that in the solar corona because of the high plasma temperature in their magnetic arches. This is attributable to an increase in the length of plasma-wave conversion into electromagnetic waves and a decrease in the optical depth of collisional wave absorption. Magnetic-arch filamentation results in a decrease in the intensity of the fundamental-tone radio emission and in the relative dominance of the second-harmonic radio emission. The efficiency of the fundamental-tone radio emission increases with plasma density in a coronal arch. The plasma mechanism accounts for the high brightness temperature of the flare radio emission from stars (≥1014 K).

New Insights into the Large 1992 July 15–17 Flare on AU Microscopii: The First Detection of Posteruptive Energy Release on a Red Dwarf Star

Extreme Ultraviolet Explorer (EUVE) observations of the long-duration 1992 July 15-17 flaring event on the red dwarf star AU Mic are discussed. The flare was detected in the EUVE Deep Survey (DS) Lexan/B band (65-190 Å) and was made up of a large impulsive event corresponding to an increase in the extreme ultraviolet (EUV) emission by a factor of 15 relative to quiescence, followed by a slow exponential decay interspersed with another smaller impulsive event and further decay. As observed in the EUV, the flare lasted for more than half a day. A similar exponential decay of the flux in the Fe XVIII λ93.9 line was detected in the EUVE short wavelength (SW) spectrometer following the first powerful impulsive phase. The flare decay in the 65-190 Å band and in the Fe XVIII line during this prolonged event lasted a factor of 10 or more longer than would have been the case had the plasma cooling been due only to radiative losses at typical flare plasma densities. An analysis of the simultaneous DS Lexan/B data and the Fe XVIII line flux has allowed us to determine the approximate temporal behavior of the flaring plasma temperature (d log T/dt ≈ -0.5 day-1) and of the emission measure of the EUV source. The total energy emitted in the 1-2000 Å range during the first 12 hours or so of the flaring event was 3 × 1035 ergs. Solar analogs of processes providing similar long-duration soft X-ray radiation are discussed: (1) a coronal mass ejection (CME); (2) a system of giant postflare arches, which can form in powerful solar flares after a CME; (3) typical soft X-ray flaring loops. A general expression describing the energy balance in the EUV source is derived for both CMEs and giant arch systems. As a result, we conclude that the long-duration EUV emission in the AU Mic flare most resembles the solar scenario of emission from a system of giant coronal arches forming after a CME. Such a system on AU Mic would have a size of 1-2 stellar radii and would be energized by reconnection in a vertical current sheet. In this picture, a substantial part of the radiative loss is compensated for by additional postflare energy input, in a similar way as is thought to take place in the posteruptive phases of the most powerful solar flares. Apparently, we are faced here with a new kind of surface activity on late-type stars, which is intermediate between impulsive flares on red dwarf stars and the long-duration, powerful events seen on subgiant components of the RS CVn binaries.

Solar Corona: from Shklovsky to our Days

Fundamental advances in the physics of the solar corona from the 1960's to the present time are briefly reviewed. Progress in coronal investigations has mainly been associated with major space missions, which have discovered such new phenomena as coronal holes and coronal mass ejections, and have enabled detailed study of coronal arch systems. The role of magnetic fields on various scales in quasi-steady-state phenomena is discussed for various phases of the solar activity cycle.

The roots of coronal structure in the Sun's surface

We have compared the structures seen on X-ray images obtained by a flight of the NIXT sounding rocket payload on July 11, 1991 with near-simultaneous photospheric and chromospheric structures and magnetic fields observed at Big Bear. The X-ray images reflect emission of both Mgx and Fexvi, formed at 1 × 106 K and 3 × 106 K, respectively. The brightest Hα sources correspond to a dying sub-flare and other active region components, all of which reveal coronal enhancements situated spatially well above the Hα emission. The largest set of X-ray arches connected plages of opposite polarity in a large bipolar active region. The arches appear to lie in a small range of angle in the meridian plane connecting their footpoints. Sunspots are dark on the surface and in the corona. For the first time we see an emerging flux region in X-rays and find the emission extends twice as high as the Hα arches. Many features which we believe to correspond to ‘X-ray bright points’ (XBPs) were observed. Whether by resolution or spectral band, the number detected greatly exceeds that from previous work. All of the brighter XBPs correspond to bipolar Hα features, while unipolar Hα bright points are the base of more diffuse comet-like coronal arches, generally vertical. These diverge from individual features by less than 30°, and give a good measure of what the ‘canopies’ must do. The Hα data shows that all the Hα features were present the entire day, so they are not clearly disappearing or reappearing. We find a new class of XBPs which we call ‘satellite points’, elements of opposite polarity linked to nearby umbrae by invisible field lines. The satellite points change rapidly in X-ray brightness during the flight. An M1.9 flare occurred four hours after the flight; examination of the pre-flare structures reveals nothing unusual.

Loop models of low coronal structures observed by the Normal Incidence X-Ray Telescope (NIXT)

The X-ray pictures obtained with the Normal Incidence X-Ray Telescope (NIXT), apart from the ubiquitous coronal loops well known from previous X-ray observations, show a new and peculiar morphology: in many active regions there are wide and apparently low-lying areas of intense emission which resemble H alpha plages. By means of hydrostatic models of coronal arches, we analyze the distribution of temperature, density, emission measure, and plasma emissivity in the spectral band to which NIXT is sensitive, and we show that the above morphology can be explained by the characteristics of high pressure loops having a thin region of high surface brightness at the base. We therefore propose that this finding might help to identify high-pressure X-ray emitting coronal regions in NIXT images, and it is in principle applicable to any imaging instrument which has high sensitivity to 10(exp 4) - 10(exp 6) K plasma within a narrow coronal-temperature passband. As a more general result of this study, we propose that the comparison of NIXT observations with models of stationary loops might provide a new diagnostic: the determination of the loop plasma pressure from measurements of brightness distribution along the loop.

An analysis of Skylab X-ray pictures of a giant coronal arch

The limb event of 13/14 August, 1973, imaged by Skylab in soft X-rays, proved to be a giant arch, quite similar to those observed in 1980–1986 on SMM. High spatial resolution (by a factor of 4–5 better than in SMM data) made it possible to see the internal structure of the arch. Its brightest part consisted of loops very similar to, but higher than, post-flare loops, surrounded by a rich system of weak loop structures extending up to altitudes of 260 000 km. While the main brightest structure of the arch was newly formed, the weak very large loops had existed above the active region before and were only enhanced during the event.

Coronal Magnetic Fields from the Effect of the Double Inversion of Circular Polarization of Radio Emission

AbstractWe present preliminary results of radio observations of the Sun with the Panoramic Analyzer of the Spectrum (21 channels in 1.8 – 8.2 cm wavelength range) of the radio telescope RATAN-600. Observations on 1992 January 9 – 10 have shown double inversion of the sign of a local source through the microwave wavelength range with fine spectral structure. This phenomenon can be treated as a result of mode coupling when the electromagnetic wave crosses two regions of quasi-transverse propagation (QT-region). Using this treatment we have found the strength of the coronal magnetic field at a number of points in the corona above an active region. The results also imply that trans-equatorial coronal arches affect the structure of magnetic fields at heights of about 3 . 1010 cm with B ~ 5 – 10 G.

X-ray observations of limb flare loops and post-flare coronal arch

We present observations of another post-flare arch following an eruptive flare, detected in X-ray lines above the western solar limb on 2 May 1985.

A giant coronal arch observed by Skylab

The limb event of 13 August 1973, observed by Skylab in soft X-rays, has all the typical characteristics of the giant post-flare arches observed by HXIS and FCS on board SMM in the 1980s. Skylab images provide us with 4–5 times better angular resolution than the SMM experiments and thus, for the first time, make it possible to distinguish the real fine structure of a giant post-flare arch. The image processing of the 13/14 August 1973 event is now in progress.

A slingshot model for solar flares

Recent observations of intense, impulsive gamma-ray and X-ray-emitting solar flares underline the suddenness of these events. The simultaneous emission of X-rays greater than 40 keV from electron bremsstrahlung and gamma-rays requiring several MeV protons shows that all particles must be accelerated in less than 5 s. This paper proposes a simple model to explain such events, using the energy stored in the stretched field lines of a coronal arch. When reconnection occurs at the top of the arch, field lines retract like stretched rubber bands, sweeping up plasma and acting like a piston or slingshot. When the slug of plasma caught in the magnetic fields strikes the photosphere, it deposits its considerable kinetic energy, heating and compressing the intruding slug. Ten slugs of 100 km radius striking the photosphere may account for the 10 to the 29th ergs radiation from loop flares.

Real-time simulation of a potential magnetic field in a post-flare arch

We analyze the X-ray observations on 7–8 November of the last revival of a giant coronal arch first observed in the morning hours of 6 November, 1980. We compare these data with observations of earlier revivals of this arch, in order to determine whether the successive brightenings repeat without any apparent modification, or whether they show a varying behavior as they keep occurring. We also examine the magnetic structure of the revived arches in order to ascertain whether the subsequent phenomena involve the same magnetic configuration. The information about the magnetic structure of the last revival is the best we have, because the revival started close to the time of the Kitt Peak magnetogram which has been used for the magnetic modelling of the whole series of these giant arches.

The footpoints of giant arches

We have detected chromospheric footpoints of the giant post-flare coronal arches discovered by HXIS a few years ago. Hα photographs obtained at Big Bear and Udaipur Solar Observatories show chromospheric signatures associated with 5 sequential giant arch events observed in the interval from 6 to 10 November, 1980. The set of footpoints at one end of the arches consists of enhancements within a plage at the northeast periphery of the active region and the set of footpoints at the other end of the arch consists of brightenings of the chromosphere south of the active region. Both sets of footpoints show very slow brightness variations correlated in time with the brightness variations of the X-ray arches. Current-free modelling of the coronal magnetic field by Kopp and Poletto (1989), based on a Kitt Peak magnetogram, confirms the identification of the two sets of footpoints by showing magnetic field lines connecting them.

Solar coronal loop heating by cross-field wave transport

Solar coronal arches heated by turbulent ion-cyclotron waves may suffer significant cross-field transport by these waves. Nonlinear processes fix the wave-propagation speed at about a tenth of the ion thermal velocity, which seems sufficient to spread heat from a central core into a large cool surrounding cocoon. Waves heat cocoon ions both through classical ion-electron collisions and by turbulent stochastic ion motions. Plausible cocoon sizes set by wave damping are in roughly kilometers, although the wave-emitting core may be only 100 m wide. Detailed study of nonlinear stabilization and energy-deposition rates predicts that nearby regions can heat to values intermediate between the roughly electron volt foot-point temperatures and the about 100 eV core, which is heated by anomalous Ohmic losses. A volume of 100 times the core volume may be affected. This qualitative result may solve a persistent problem with current-driven coronal heating; that it affects only small volumes and provides no way to produce the extended warm structures perceptible to existing instruments.

Shearing dynamics of magnetic arches and formation of prominences

We study the active role played by shear motion of magnetic field, suggested by observations of prominences, in the formation of the prominences. First, we calculate what physical phenomena will result when the foot-points of a two-dimensional dipole potential field undergo shearing. Three cases were calculated. The results show that, as a general rule, a mushroom-shaped flow will be generated, and, under certain conditions, there will be depression of the arch top. To clarify the cause of the mushroom flow, we explored the linear asymptotic solution for the early stage of evolution, and we found the flow amplitude growing exponentially with time. A qualitative explanation for the arch top depression is given: it is due to the interaction of the magnetic buoyance − J x B z and field deformation damping force J z B x . This interaction also tends to open out the magnetic field in the shear region. Depression of the arch top is an important condition for the formation of prominences. The slight diminution of heating of the coronal arch generates thermal instability, and matter from chromospheric evaporation at the foot-points rises along the magnetic tubes and enters the depression on the top, aggravating the thermal instability: matter becomes denser still, and temperature falls further, leading to the eventual formation of a prominence.

Giant solar arches and coronal mass ejections in November 1980

Using data from the SOLWIND coronagraph and photometers aboard HELIOS-A we examine coronal mass ejections from an active region which produced a series of giant post-flare coronal arches. HXIS X-ray observations reveal that in several cases underlying flares did not disrupt these arch structures, but simply revived them, enhancing their temperature, density and brightness. Thus we are curious to know how these quasi-stationary X-ray structures could survive in the corona in spite of recurrent appearances of powerful dynamic flares below them. We have found reliable evidence that two dynamic flares which clearly revived the preexisting giant arch were not associated with any mass ejection. After two other flares, which were associated with mass ejections, the arch might have been newly formed when the ejection was over. In one of these cases, however, the arch had typical characteristics of a revived structure so that it is likely that it survived a powerful mass ejection nearby. In a magnetic configuration of the arch which results from potential-field modelling (Figure 1(b)) such a survival seems possible.

Slow-shock heating in post-flare arches

The heating of a coronal arch, following the occurrence of a dynamic (two-ribbon) flare, is discussed. We investigate whether slow-shock heating, occurring during the reconnection process in the dynamic flare and responsible for the heating of the post-flare loops, is also a workable proposition for the heating of a coronal arch. Contrary to the flare loops, the shock structure in the arch is generally not modified greatly by thermal conduction effects. As a result slow-shock heating may be investigated in terms of the familiar MHD shock jump relations. The observed enhanced arch density with respect to the surrounding corona is explained as a direct consequence of the reconnection process. For a combination of high arch temperatures and low values of coronal magnetic field and density thermal conduction may become important and will lead to an extra density enhancement in the arch. Our interpretation of the arch of 21–22 May, 1980 suggests that the formation of the arch took approximately one hour, and that observed temperature, density and maximum energy content can be consistently explained by the slow-shock heating mechanism.