Showing papers in "Solar Physics in 1982"

Impulsive phase of flares in soft X-ray emission

Abstract: Observations using the Bent Crystal Spectrometer instrument on the Solar Maximum Mission show that turbulence and blue-shifted motions are characteristic of the soft X-ray plasma during the impulsive phase of flares, and are coincident with the hard X-ray bursts observed by the Hard X-ray Burst Spectrometer. A method for analysing the Ca xix and Fe xxv spectra characteristic of the impulsive phase is presented. Non-thermal widths and blue-shifted components in the spectral lines of Ca xix and Fe xxv indicate the presence of turbulent velocities exceeding 100 km s-1 and upward motions of 300–400 km s-1.

Propagation Speeds and Acoustic Damping of Waves in Magnetic Flux Tubes

Abstract: Propagation speeds are derived for the wave modes of a thin magnetic tube in an otherwise homogeneous magnetized or unmagnetized fluid. These results generalize results obtained by previous authors. There are three types of wave, a (torsional) Alfven wave and two waves which are specific for the thin tube. These are named the longitudinal and transversal tube waves, according to their polarization properties. They can be camped by radiating an MHD or acoustic wave into the surroundings of the tube. Conditions for occurrence of this acoustic damping, and the damping rates, are derived. The behavior of the waves in the solar convection zone and corona is discussed.

Wave propagation in a magnetically structured atmosphere: III: The slab in a magnetic environment

Abstract: The propagation of waves in a magnetic slab embedded in a magnetic environment is investigated. The possible modes of propagation are examined from the general dispersion relation, both analytically and numerically, for disturbances which are evanescent in the environment. Approximate dispersion relations governing propagation in a slender slab of field are derived both from the general dispersion relation and from an application of the slender flux tube approximation.Several different situations, representative of both photospheric and coronal conditions, are considered. In general, the structures are found to support both fast and slow, body and surface, waves. Under coronal conditions, for two dimensional propagation, disturbances propagate as fast and slow body waves. The fast body waves are analogous to the ducted shear waves of seismology (Love waves).

Alfven waves in the solar atmosphere. III - Nonlinear waves on open flux tubes

Abstract: Consideration is given the nonlinear propagation of Alfven waves on solar magnetic flux tubes, where the tubes are taken to be vertical, axisymmetric and initially untwisted and the Alfven waves are time-dependent axisymmetric twists. The propagation of the waves into the chromosphere and corona is investigated through the numerical solution of a set of nonlinear, time-dependent equations coupling the Alfven waves into motions that are parallel to the initial magnetic field. It is concluded that Alfven waves can steepen into fast shocks in the chromosphere, pass through the transition region to produce high-velocity pulses, and then enter the corona, which they heat. The transition region pulses have amplitudes of about 60 km/sec, and durations of a few tens of seconds. In addition, the Alfven waves exhibit a tendency to drive upward flows, with many of the properties of spicules.

The Hanle effect and the diagnostics of turbulent magnetic fields in the solar atmosphere

Abstract: The theory of the Hanle effect is used to interpret the linear polarization measured in a number of spectral lines on the solar disk near the heliographic north and south poles, in search for a turbulent magnetic field in the solar atmosphere. The Hanle depolarization is separated from a number of other effects, including collisional depolarization and scattering geometry. Although the main aim of the paper is to elucidate the physics of the Hanle effect as applied to the Sun, our results indicate the existence of hidden or turbulent magnetic flux near the temperature minimum of the solar atmosphere, with a field strength between 10 and 100 G. This field is hidden in the sense that it is not seen in measurements of the longitudinal Zeeman effect (solar magnetograms). It carries more total magnetic flux than the kG network fields.

Green's Function Methods for Potential Magnetic Fields

Abstract: The Green's function method to calculate potential magnetic field on the Sun, which was first established by Schmidt (1964) in the case that the field component normal to a flat boundary plane is specified, is extended to the following three cases: (a) The field component along the line of sight, which is not generally normal to the flat boundary plane, is specified; (b) the line of sight component on a spherical boundary surface is specified; (c) the normal component on a spherical surface is specified, together with the condition that the field becomes approximately radial on an outer spherical surface (the so-called source surface). Properties of these Green's functions are examined, and the applicability of these methods to solar magnetic data is discussed.

Delta spots and great flares

Abstract: The development of delta spots and the great flares they produce are reviewed based on 18 years of observations Delta groups are found to develop in three ways: (1) by the eruption of a single complex active region formed below the surface; (2) by the eruption of large satellite spots near a large older spot; and (3) by the collision of spots of opposite polarity from different dipoles It is shown that the present sample of 21 delta spots never separate once they lock together, and that the driving force for the shear is spot motion Indicators for the prediction of the occurrence of great flares are identified

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

Abstract: 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.

X-ray imaging of three flares during the impulsive phase

Abstract: The impulsive phases of three flares that occurred on April 10, May 21, and November 5, 1980 are discussed. Observations were obtained with the Hard X-ray Imaging Spectrometer (HXIS) and other instruments aboard SMM, and have been supplemented with Hα data and magnetograms. The flares show hard X-ray brightenings (16–30 keV) at widely separated locations that spatially coincide with bright Hα patches. The bulk of the soft X-ray emission (3.5–5.5 keV) originates from in between the hard X-ray brightenings. The latter are located at different sides of the neutral line and start to brighten simultaneously to within the time resolution of HXIS. Concluded is that: It is found that the total flare energy is always larger than the total energy residing in the beam electrons. However, it is also estimated that at the time of the peak of the impulsive hard X-ray emission a large fraction (at least 20%) of the dissipated flare power has to go into electron acceleration. The explanation of such a high acceleration efficiency remains a major theoretical problem.

Torsional waves on the sun and the activity cycle

Abstract: The detailed relation of the sun's torsional oscillation velocity feature to magnetic activity indicates (1) that these motions represent a fundamental oscillation within the sun which is responsible for the solar activity cycle, and (2) that they are not a natural consequence of an alpha-omega dynamo. It is demonstrated that a solar torsional oscillation with wave number 1/hemisphere exists.

Magnetic clouds: Voyager observations between 2 and 4 AU

Abstract: Magnetic clouds were observed in the solar wind between 2–4 AU by Voyagers 1 and 2, indicating that they are stable enough to persist without major changes out to such distances. The average size in radial extent of the clouds observed at these distances was ∼ 0.47 AU, compared to 0.25 for clouds observed at 1 AU. Assuming that these numbers are representative, we estimate that the clouds were expanding at a speed of the order of 45 km s-1. This is consistent with the expansion speed derived from the difference of the speeds of the front and rear boundaries of the clouds, ∼ 33 km s-1. The average Alfven speed at the front and rear boundaries was 104 km s-1, so our estimated expansion speed is nearly half of the Alfven speed, consistent with an earlier estimate of the expansion speed of clouds between the Sun and 1 AU. The magnetic field configuration cannot be determined uniquely, but it is highly ordered and consistent with the passage of some kind of loop. The simple model of a magnetic tongue with magnetic field lines in planes, e.g., meridian planes, is not consistent with the data.

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

Abstract: 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-α, force-free magnetic field (J = αB) at the photosphere is ruled out by the observations

The effects of sunspots on solar irradiance

Abstract: It is pointed out that the darkness of a sunspot on the visible hemisphere of the sun will reduce the solar irradiance on the earth. Approaches are discussed for obtaining a crude estimate of the irradiance deficit produced by sunspots and of the total luminosity reduction for the whole global population of sunspots. Attention is given to a photometric sunspot index, a global measure of spot flux deficit, and models for the compensating flux excess. A model is shown for extrapolating visible-hemisphere spot areas to the invisible hemisphere. As an illustration, this extrapolation is used to calculate a very simple model for the reradiation necessary to balance the flux deficit.

Extreme ultraviolet spectra of solar active regions and their analysis

Abstract: Extreme ultraviolet spectra of several active regions are presented and analyzed. Spectral intensities of 3 active regions observed with the NRL Skylab XUV spectroheliograph (170–630 A) are derived. From this data density sensitive line ratios of Mg viii, Si x, S xii, Fe ix, Fe x, Fe xi, Fe xii, Fe xiii, Fe xiv, and Fe xv are examined and typically yield, to within a factor of 2, electron pressures of 1 dyne cm−2 (n e T = 6 × 1015 cm−3 K). The differential emission measure of the brightest 35″ × 35″ portion of an active region is obtained between 1.4 × 104 K and 5 × 106 K from HCO OSO-VI XUV (280–1370 A) spectra published by Dupree et al. (1973). Stigmatic EUV spectra (1170–1710 A) obtained by the NRL High Resolution Telescope and Spectrograph (HRTS) are also presented. Doppler velocities as a function of position along the slit are derived in an active region plage and sunspot. The velocities are based on an absolute wavelength scale derived from neutral chromospheric lines and are accurate to ±2 km s−1. Downflows at 105 K are found throughout the plage with typical velocities of 10 km s−1. In the sunspot, downflows are typically 5 to 20 km s−1 over the umbra and zero over the penumbra. In addition localized 90 and 150 km s−1 downflows are found in the umbra in the same 1″ × 1″ resolution elements which contain the lower velocity downflows. Spectral intensities and velocities in a typical plage 1″ resolution element are derived. The velocities are greatest (∼ 10 km s−1) at 105 K with lower velocities at higher and lower temperatures. The differential emission measure between 1.3 × 104 K and 2 × 106 K is derived and is found to be comparable to that derived from the OSO-VI data. An electron pressure of 1.4 dynes cm−2 (n e T = 1.0 × 1016 cm−3 K) is determined from pressure sensitive line ratios of Si iii, O iv, and N iv. From the data presented it is shown that convection plays a major role in determining the structure and dynamics of the active region transition zone and corona.

Magneto-optical effects and the determination of vector magnetic fields from stokes profiles

Abstract: The analysis procedure proposed by Auer et al. (1977) for deducing magnetic field vectors from Stokes profiles has been tested to investigate the influence of magneto-optical effects on the deduced field parameters. The quality of the fit between synthetic profiles generated with the inclusion of magneto-optical effects and the profiles returned by the inversion routine is also investigated. The results show that magneto-optical effects should be included in the inversion routine especially to increase the accuracy of the deduced, azimuth of the magnetic field.

Numerical hydrodynamics of the jet phenomena in the solar atmosphere: I. Spicules

Abstract: We present a spicule model whose eruption occurs as a result of the sudden pressure enhancement at the bright point located at the root of the spicule. To show this, one dimensional (constant cross sectional) and time dependent hydrodynamic equations are solved numerically in the realistic solar atmosphere extending from the photosphere to the corona. Adiabatic motion is assumed. The pressure enhancement by a bright point at the base of the model atmosphere generates a shock wave. The shock gets stronger as it passes upward through the chromosphere and eventually collides with the chromosphere-corona interface which is a kind of a contact discontinuity. As the result, the interface begins to move upward. We identify the matter following behind this interface as the solar spicule. The model explains many observed features, such as the height and the density of the spicules, although such features have been hitherto considered not to be explained easily by shock theories.

The MSFC vector magnetograph

Abstract: The NASA/Marshall Space Flight Center's solar vector magnetograph system is described; this sytem allows measurements of all components of the Sun's photospheric magnetic field over a 5 × 5 or 2.0 × 2.0 arc min square field-of-view with an optimum time resolution of ∼ 100 s and an optimum signal-to-noise of ∼ 1600. The basic system components are described, including the optics, detector, digital system and associated electronics. Automatic sequencing and control functions are outlined as well as manual selections of system parameters which afford unique system flexibility. Results of system calibration and performance are presented, including linearity, dynamic range, uniformity, spatial and spectral resolutions, signal-to-noise, electro-optical retardation and polarization calibration. Scientific investigations which utilize the unique characteristics of the instrument are described and typical results are presented.

Solar rotation measurements at Mount Wilson. III - Meridional flow and limbshift

Abstract: It is shown that the use of a two-parameter limbshift and a meridional flow velocity fits solar velocity data better than the standard analysis defined by Howard et al. (1980). The data used are the coarse residual velocity arrays, with 34 equal intervals in both sine latitude and sine longitude. There are a total of 2899 full-disk observations between January 1, 1967, and December 12, 1980. The original velocity fields are reconstructed by adding into the residual arrays the large-scale patterns that were measured and removed on a daily basis by a standard reduction. Tests of this reconstituded data set show that no significant errors are introduced in the analysis of large-scale velocity fields. The results of the analysis presented here imply that the study of solar velocity pattern at the level of a few m/s requires that magnetic regions be treated separately from nonmagnetic regions.

Observations of a post-flare radio burst in X-rays

Abstract: More than six hours after the two-ribbon flare of May 21, 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which was found to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The bent crystal spectrometer aboard the SMM confirms that the arch emission was basically thermal. Variations in brightness and energy spectrum at one of the supposed footpoints of the arch are seen as correlation in time with radio brightness, suggesting that suprathermal particles from the radio noise regions dumped in variable quantities onto the low corona and transition layer.

Magnetic reconnection and coronal transients

Abstract: Every two-ribbon flare observed during the Skylab period produced an observable coronal transient, provided the flare occurred close enough to the limb. The model presented here treats these two events as a combined process. Transients that occur without flares are believed to involve magnetic fields that are too weak to produce significant chromospheric emission. Adopting the hypothesis that the rising flare loop systems observed during two-ribbon flares are exhibiting magnetic reconnection, a model of a coronal transient is proposed which incorporates this reconnection process as the driving force. When two oppositely directed field lines reconnect a lower loop is created rooted to the solar surface (the flare loop) and an upper disconnected loop is produced which is free to rise. The magnetic flux of these upper loops is proposed as the driver for the transient. The force is produced by the increase in magnetic pressure under the filament and transient. A quantitative model is developed which treats the transient configuration in terms of four distinct parts- the transient itself with its magnetic field and material, the region just below the transient but above the filament, the filament with its magnetic field, and the reconnected flux beneath the filament. Two cases are considered - one in which all the prominence material rises with the transient and one in which the material is allowed to fall out of the transient. The rate of rise of the neutral line during the reconnection process is taken from the observations of the rising X-ray flare loop system during the 29 July, 1973 flare. The MHD equations for the system are reduced to four non-linear ordinary coupled differential equations which are solved using parameters believed to be realistic for solar conditions. The calculated velocity profiles, widths, etc., agree quite well with the observed properties of coronal transients as seen in white light. Since major flares are usually associated with a filament eruption about 10–15 min before the flare and since this model associates the transient with the filament eruption, we suspect that the transient is actually initiated some time before the actual flare itself.

The determination of vector magnetic fields in prominences from the observations of the Stokes profiles in the D3 line of helium

Abstract: A method is presented to measure the magnetic field vector in prominences by means of the polarimetric observations in the D3 line of He obtained with the High Altitude Observatory Stokes polarimeter. The characteristics of the observed Stokes profiles are discussed. The theory of the Hanle effect is reformulated in the representation of the irreducible tensors of the density matrix, and is generalized to derive the circular polarization profiles across the spectral line in terms of the intensity and direction of the prominence magnetic field. The circular polarization profile so deduced can be employed to obtain useful information which adds to that carried by the linear polarization observations. A non-linear least-squares algorithm is proposed to derive the measurement of the magnetic field from the observations, and a consistency check is suggested to test the adequacy of the theoretical model to describe the physics of the He I atomic excitation in prominences.

The measurement of magnetic fields in the solar atmosphere above sunspots using gyroresonance emission

Abstract: An analysis of the local sources (LS) structure of the S-component of solar radio emission confirms the presence of a core component which is characterized by strong circular polarization and a steep growing spectrum at shorter centimeter wavelengths. These details coincide in position with the sunspots' umbra and their height above the photosphere does not generally exceed about 2000 km. Gyroresonance emission of thermal electrons of the corona is generally accepted as being responsible for this type of emission. The spectral and polarization observations of LS made with RATAN-600 using high resolution in the wavelength range 2.0–4.0 cm, allow us to measure the maximum magnetic fields of the corresponding sunspots at the height of the chromosphere-corona transition region (CCTR). This method is based on determining the short wavelength limit of gyroresonance emission of the LS and relating it to the third harmonic of gyrofrequency. An analysis of a large number of sunspots and their LS (core component) has shown a good correlation between radio magnetic fields near the CCTR and optical photospheric ones. The magnetic field in CCTR above a sunspot is found only 10 to 20% lower than in the photosphere. The resulting gradient of the field strength is not less than 0.25 G km−1. This result seems to contradict the lower values of magnetic fields generally found above sunspots using the chromospheric Hα line. Some possible ways of overcoming this difficulty are proposed.

Numerical hydrodynamics of the jet phenomena in the solar atmosphere

Abstract: One-dimensional hydrodynamic simulations of surges are performed in order to make clear their origin and structure. Surges are regarded as the jets resulting from a sudden pressure increase at the base of the model atmosphere. The height of the explosion (h0), which is measured from the level of τ5000 = 1, is regarded as a free parameter. Another free parameter is the strength of the sudden pressure increase (p/p0) at h0. Simulations are performed for values in the ranges of 540 km ≤ h0 ≤ 1920 km and 3 ≤ p/p0 ≤ 30. It was found that for a fixed p/p0 there exists a critical height (hc) in h0, which separates the jet (surge) models into two types. For h0 > hc, jets are produced directly by the pressure gradient force near h0, and made of the matter ejected from the explosion itself. The essential hydrodynamic structure of this type is the same as that in a shock tube (this type is called ‘shock tube’ type). For h0 < hc, jets are not the direct results of the pressure enhancement, but are produced by the shock wave which are generated by the pressure enhancement and which has propagated through the chromosphere (this type is called the ‘crest shock’ type). It is shown that the critical height (hc) ranges from 1000 km to 1500 km for 3 ≤ p/p0 ≤ 30. General properties of both types are investigated in detail. The results are compared with observations and it is concluded that small surges associated with Ellerman bombsbelong to the ‘crest shock’ type, i.e. they are produced by the shock wave.

Type II solar radio events observed in the interplanetary medium. I - General characteristics

Abstract: Fifteen type II solar radio events have been identified in the 2 MHz to 30 kHz frequency range by the radio astronomy experiment on the ISEE-3 satellite over the period from September 1978 to December 1979. These data provide the most comprehensive sample of type II radio bursts hitherto observed at kilometer wavelengths. Dynamic spectra of a number of events are presented. Where possible, the 15 events have been associated with an initiating flare, ground-based radio data, the passage of a shock at the spacecraft and the sudden commencement of a geomagnetic storm. The general characteristics of kilometric type II bursts are discussed.

Evidence for a poleward meridional flow on the sun

Abstract: We define for observational study two subsets of all polar zone filaments, which we call polemost filaments and polar filament bands. The behavior of the mean latitude of both the polemost filaments and the polar filament bands is examined and compared with the evolution of the polar magnetic field over an activity cycle as recently distilled by Howard and LaBonte (1981) from the past 13 years of Mt. Wilson full-disk magnetograms. The magnetic data reveal that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions that originate in active region latitudes and subsequently drift to the poles. After leaving the active-region latitudes, these unipolar f-polarity regions do not spread equatorward even though there is less net flux equatorward; this indicates that the f-polarity regions are carried poleward by a meridional flow, rather than by diffusion. The polar zone filaments are an independent tracer which confirms both the episodic polar field formation and the meridional flow. We find: (1) The mean latitude of the polemost filaments tracks the boundary of the polar field cap and undergoes an equatorward dip during each arrival of additional polar field. (2) Polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m s-1. (3) The Mt. Wilson magnetic data, combined with a simple model calculation, show that the filament drift expected from diffusion alone would be slower than observed, and in some cases would be equatorward rather than poleward. (4) The observation that filaments drift poleward along with the magnetic regions shows that fields of both polarities are carried by the meridional flow, as would be expected, rather than only the f-polarity flux which dominates the strength. This leads to the prediction that in the mid-latitudes during intervals between the passage of f-polarity regions, both polarities are present in nearly equal amounts. This prediction is confirmed by the magnetic data.

Numerical study of line-tied magnetic reconnection

Abstract: A two-dimensional configuration, analogous to that at the start of the main phase in two-ribbon flares, is modelled numerically by self-consistently solving the time-dependent MHD equations. The initial state consists of a vertical current sheet with an external plasma beta value of 0.1 and a magnetic Reynolds number of 10−3. Although the model does not yet include gravity or a full energy equation, many of the principal dynamical features of the main phase in a flare are present. In particular, the numerical results confirm the earlier prediction of the kinematic Kopp-Pneuman (1976) model that a neutral line forms at the base of the corona and rises upwards as open, extended field lines close back down to form loops (i.e., ‘post’-flare loops). By the end of the computation a state of nonlinear reconnection containing slow shocks has developed, and the velocity of the plasma flowing into the neutral line region is approximately 0.06 times the corresponding inflow Alfven velocity - a value consistent with the steady-state nonlinear reconnection theory of Soward and Priest (1977). The speed at which the neutral line rises in the numerical simulation varies from an initial value of ≲ 0.02 to a final value of ∼- 0.12 times the inflow Alfven speed.

The three-dimensional structure of atmospheric magnetic fields in two active regions

Abstract: The magnetic field above two unrelated active regions on September 11-12, 1974 has been studied by means of magnetograms obtained in C I 9111, Fe I 8688, Ca II 8542, and H-alpha. In C I, originating low in the photosphere, the fields are strong and sharply defined, by contrast with Ca II and H-alpha, where they are very diffuse. The similar diffuseness of Fe I is due to the spreading of the field with height to form nearly horizontal magnetic canopies over regions free of field at lower levels. Within a region between two small sunspots some 140 Mm apart, the canopy height is typically 300-400 km, and it is about 150-250 km within a small superpenumbra. It is hypothesized that the chromospheric fibrils taken to delineate the field configuration are not due primarily to lateral variations in field, but rather to differences in density or excitation of gas across the lines of force.

Recurrence of solar activity: Evidence for active longitudes

Abstract: The autocorrelation coefficients of the daily Wolf sunspot numbers over a period of 128 years reveal a number of interesting features of the variability of solar activity. In addition to establishing periodicities for the solar rotation, the solar activity cycle, and perhaps the ‘Gleissberg Cycle’, they suggest that active longitudes do exist, but with much greater strength and persistence in some solar cycles than in others. There is evidence for a variation in the solar rotation period, as measured by sunspot number, of as much as two days between different solar cycles.

Unusual coronal activity following the flare of 6 November 1980

Abstract: For almost 30 hr after the major (gamma-ray) two-ribbon flare on 6 November 1980, 03:30 UT, the Hard X-Ray Imaging Spectrometer (HXIS) aboard the SMM satellite imaged in > 3.5 keV X-rays a gigantic arch extending above the active region over the limb. Like a similar configuration on 22 May 1980, this arch formed the lowest part of a stationary post-flare radio noise storm recorded at metric wavelengths at Nancay and Culgoora. 6.5 hr after the flare a coronal region below the arch started quasi-periodic pulsations in X-ray brightness, observed by several SMM instruments. These brightness variations had no response in the chromosphere (Hα), very little in the transition layer (O v), but they clearly correlated with similar variations in brightness at 169 MHz. There were 13 pulses of this kind, with apparent periodicity of about 20 min, until another flare occurred in the active region at ∼ 15:00 UT. All the brightenings appeared within a localized area of about 30000 km2 in the northern part of the active region, but they definitely did not occur all at the same place. The top of the X-ray arch, at an altitude of ∼ 155 000 km, was continuously and smoothly decaying, taking no part in the striking variations below it. Therefore, the area variable in brightness does not seem to be the footpoint of the arch, as we supposed for similar variations on 22 May. More likely, it is a separate region connected directly with the source of the radio storm; particles accelerated in the storm may be dumped into the low corona and cause the X-ray enhancements. The X-ray arch was enhanced by two orders of magnitude in 3.5–5.5 keV X-ray counts and the temperature increased from ≲ 7.3 × 106 to 9 × 106 K when the new two-ribbon flare occurred at 15:00 UT. Thus, it is possible that energy is brought into the arch via the upper parts of the reconnecting flare loops - a process that can continue for hours.

Two distinct morphological types of magnetic shear development and their relation to flares

Abstract: From a morphological study of the evolution of six active regions, we found two types of processes for the development of magnetic shear configurations between sunspots: (A) collision of two sunspots of opposite magnetic polarities, and (B) successive emergence of twisted magnetic flux ropes. We conclude that the process (B) might be essential for the production of major flares.