On the outburst of flare activity of 26 November, 1973

We draw attention of flare build-up observers to a strong 30 hour-long outburst of homologous flare activity and unusual growth and brightening of coronal loops, seen on Skylab. We suggest that these events might have been closely associated with newly emerging magnetic flux, in spite of the fact that the flux effects in H and EUV were first seen only late after the activity had started, and the flux emerged at the opposite end of the coronal loops from where the flares occurred.     

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

More than six hours after the two-ribbon flare of 21 May 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which proved to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The X-ray arch extended over 3 or more arc minutes to a projected distance of 95 000 km, and its real altitude was most probably between 110 000 and 180 000 km. The mean electron density in the cloud was close to 10⁹ cm^–3 and its temperature stayed for many hours at a fairly constant value of about 6.5 × 10⁶ K. 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 seem to correlate in time with radio brightness suggesting that suprathermal particles from the radio noise regions dumped in variable quantities into the low corona and transition layer; these particles may have contributed to the population of the arch, after being trapped and thermalized. The arch extended along the H = 0 line thus apparently hindering any upward movement of the upper loops reconnected in the flare process. There is evidence from Culgoora that this obstacle may have been present above the flare since 15–30 min after its onset.     

The inclination changes in the problem of two triaxial rigid spheroids

The first-order perturbations of a system of two triaxial rigid spheroids under Hori-Lie transformation are investigated. The time dependence of the configuration of the three angular momentum vectors, two rotational and one orbital, is studied. The problem is simplified by the introduction of a new time parameter , such thatt is the hyperelliptic function of . The projectionsH ₁ andH ₂ of the rotational momentum vectors into the direction of the total angular momentum vector of the system are then harmonic or exponential functions of . The trajectory in theH ₁,H ₂ plane is a part of an ellipse or hyperbola respectively. If this conical section intersects a certain critical contourC, the system is bounced back along the original trajectory. The motion of the relative configuration of the angular momentum vectors is periodical except in a special aperiodic case. The expressions for the periods are given.     

Elastic Parameters of West Bohemian Granites under Hydrostatic Pressure

—The West Bohemian seismoactive region is situated near the contact of the Moldanu bian, Bohemian and Saxothuringian units in which a large volume is occupied by granitoid massifs. The spatial distribution of P-wave velocities and the rock fabric of five representative samples from these massifs were studied. The P-wave velocities were measured on spherical samples in 132 independent directions under hydrostatic pressure up to 400 MPa, using the pulse-transmission method. The pressure of 400 MPa corresponds to a depth of about 15 km in the area under study. The changes of P-wave velocity were correlated with the preferred orientations of the main rock fabric elements, i.e., rock forming minerals and microcracks. The values of the P-wave velocity from laboratory measurements on granite samples fit the velocity model used by seismologists in the West Bohemian seismoactive region.     

The existence of a chaotic region due to the overlap of secular resonances ν5 and ν6

A nonlinear theory of secular resonances is developed. Both terms corresponding to secular resonances ₅ and ₆ are taken into account in the Hamiltonian. The simple overlap criterion is applied and the condition for the overlap of these resonances is found. It is shown that in given approximation the value p = (1 - e²)^1/2(1 - cosI) is an integral of motion, where the mean eccentricity e and mean inclination I are obtained by eliminating short-period perturbations as well as the nonresonant terms from the planets. The overlap criterion yields a critical value of parameter p depending on the semi-major axis a of the asteroid. For p greater than the critical value, resonance overlap occurs and chaotic motion has to be expected. A mapping is presented for fast calculation of the trajectories. The results are illustrated by level curves in surfaces of section method.     

Determination of the geopotential scale factor from satellite altimetry

Summary The geopotential scale factor R ₀ =GM/W ₀ has been determined on the basis of satellite altimetry as R _{0=(6 363 672·5±0·3) m} and/or the geopotential value on the geoid W ₀ =(62 636 256·5±3) m ² s ^–2 . It has been stated that R ₀ and/or W ₀ is independent of the tidal distortion of surface W=W ₀ due to the zero frequency tide.

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¶rt;a nmu amumuu u ama amnmuaa R ₀ =GM/W ₀ =(6 363 672,5±0,3) m u/uu aunmuaa a nmuu¶rt;a W ₀ =(62 636 256,5±3) m² s^–2. m, m R ₀ u/uu W ₀ auum m nm amu a a nuu ¶rt;au nmu W=W ₀ .

     

X-ray bright surges

We present evidence of X-ray emission from surges that are bright in H. These surges have many features common to flaring arches of Martin and vestka (1988); the basic difference between the two is that in flaring arches cold and hot plasma are injected into clearly defined closed magnetic loops, while in the surges the injection goes into large-scale magnetic field structures of which the second footpoint is usually unknown. Because of the steep density gradient in such large-scale structures, the X-ray visibility of bright surges is limited to a few tens of seconds only. A series of repetitive surges, some of them bright and emitting X-rays, occurred on 8 July, 1980 from footpoints of two large-scale coronal structures, which might have been the legs of an enormous arch at least 600 Mm long.     

On the occurrence of sympathetic flares

We have tried to determine whether statistical evidence on the occurrence of sympathetic flares, which is negative for whole-disk data, can be found for particular, physically connected, pairs of active regions. Recently, Simnett (1974) and Gergely and Erickson (1975) claimed to have found such evidence, but their results were based on incorrect computations of the random incidence of flares. If the correct formula is applied, the supposed evidence disappears. The results are negative also for pairs of active regions interconnected with magnetic loops visible in soft X-rays during the Skylab mission. The only positive result (with statistical confidence of 3.4 ) is found for pairs of active regions, which are closer than 30° to each other, without specifying any kind of physical relationship. For such pairs of regions the occurrence of short-time (< 20 min) intervals between flares is increased, but the time interval pattern does not correspond to any mode of propagation of a triggering agent in the solar atmosphere. Therefore, if the increase has real physical significance, it would be indicative of some kind of subphotospheric synchronization of activity in nearby sympathetic active regions.     

The stationary post-flare arch of May 21/22, 1980

On May 21/22, 1980 the Hard X-Ray Imaging Spectrometer aboard the SMM imaged an extensive coronal structure after the occurrence of a two-ribbon flare on May 21, 20:50 UT. The structure was observed from 22:20 UT on May 21 until its disappearence at 09:00 UT on May 22.At 22:20 UT the brightest pixel in the arch was located at a projected altitude of 95 000 km above the zero line of the longitudinal magnetic field. At 23:02 UT the maximum of brightness shifted to a neighbouring pixel with approximately the same projected altitude. This sudden shift indicates that the X-ray structure consisted of (at least) two separate arches at approximately the same altitude, one of which succeeded the other as the brightest arch in the structure at 23:02 UT.From 23:02 UT onwards the maximum of brightness did not change its position in the HXIS coarse field of view. With a spatial resolution of 32 this places an upper limit of 1.1 km s^-1 on the rise velocity of the arch. Thus, contrary to a similar arch observed on November 6/7, where rise velocities of the order of 10 km s^-1 were measured in the same phase of development, the May 22 arch was a stationary structure at an altitude of 145000 km.The following values were estimated for the physically relevant quantities of the May 21/22 arch at the time of its maximum brightness (23:00 UT): temperature T 6.3 × 10⁶ K, electron density n _e 1.1 × 10⁹ cm^-3, total emitting volume V 5 × 10²⁹ cm³, energy density 2.9 erg cm^–3, total energy contents E 1.4 × 10³⁰ erg, total mass M 9 × 10¹⁴ g.The top of the arch was observed at 145 000 km altitude within 1.5 hr after the flare occurrence. Since it seems unlikely that the structure already existed prior to the flare at 20:50 UT, the arch must have risen to its stationary position with an average velocity exceeding 17 km s^–1 (possibly much faster). We speculate that the arch was formed very fast at the flare onset, when (part of) the active region loop system was elevated within minutes to the observed altitude.