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Svestka, Z.

Publications and source records attributed to Svestka, Z..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Post-flare loops embedded in a hot coronal fan-like structure

Limb events were demonstrated on the sun in which rising post-flare loops were embedded in hot structures looking in soft X-rays like fans of rays, formed during the flare and extending high into the corona. One of these structures is analyzed and it is suggested that these fans of rays represent temporary ministreamers, along which mass flows into interplanetary space. This suggestion is supported by maps of solar wind density constructed from scintillation measurements.

Svestka, Z.↗
NASA NTRS1986

X-ray imaging of flare loops and coronal arches

HXIS (Hard X-ray Imaging Spectrometer) results related to preflare, flare-decay, and post-flare phases in the solar corona are summarized. Particularly discussed are miniflares, long-lived X-ray enhancements along filament channels, flare precursors (some signifying the flare position, others possibly related to the onset of mass ejections), growth of post-flare loops, images of sites of field-line reconnections late in the flare development, giant post-flare coronal arches with energy contents comparable to coronal transients, and hot loops interconnecting active regions. The extremely low HXIS background, in particular, made it possible to discover many new, previously unknown features in the solar corona. Also emphasized are some problems that have appeared during the evaluation of HXIS data, for the benefit of those who plan future hard X-ray imaging experiments.

Svestka, Z.↗
NASA NTRS1986

A dynamic flare with anomalously dense flare loops

The dynamic flare of November 6, 1980 developed a rich system of growing loops which could be followed in H-alpha for 1.5 hours. Throughout the flare, these loops, near the limb, were seen in emission against the disk. Theoretical computations of b-values for a hydrogen atom reveal that this requires electron densities in the loops to be close to 10 to the 12th per cu cm. From measured widths of higher Balmer lines the density at the tops of the loops was found to be 4 x 10 to the 12th per cu cm if no nonthermal motions were present. It is now general knowledge that flare loops are initially observed in X-rays and become visible in H-alpha only after cooling. For such a high density a loop would cool through radiation from 10 to the 7th K to 10 to the 4th K within a few minutes so that the dense H-alpha loops should have heights very close to the heights of the X-ray loops. This, however, contradicts the observations obtained by the HXIS and FCS instruments on board SMM which show the X-ray loops at much higher altitudes than the loops in H-alpha. Therefore, the density must have been significantly smaller when the loops were formed and the flare loops were apparently both shrinking and becoming denser while cooling.

Svestka, Z.↗
NASA NTRS1984

H-alpha and hard X-ray development in two-ribbon flares

Morphological features of two-ribbon flares have been studied, using simultaneous ISEE-3 hard X-ray records and high-resolution Big Bear H-alpha movies for more than 20 events. Long-lasting and complex hard X-ray bursts are almost invariably found associated with flares of the two-ribbon type. At least three events are found, namely March 31, 1979, April 10, 1980, and July 1, 1980, where the occurrence of individual spikes in hard X-ray radiation coincides with suddenly enhanced H-alpha emission covering the sunspot penumbra. There definitely exist important (greater than or equal to 1 B) two-ribbon flares without significant hard X-ray emission.

Dwivedi, B. N.↗
NASA NTRS1983

Transient brightenings of interconnecting loops. III Interpretation

Two alternative interpretations of the sudden X-ray brightenings observed in loops that interconnect active regions are presented. A fast tearing mode may be excited in those newly formed interconnecting loops within which sufficient magnetic free energy is stored to drive the mode. Alternatively, anomalous Joule heating driven by an inductive electric field parallel to the magnetic field, varying on a time scale of order of a minute, may cause the brightenings. It is suggested that it is plausible that the fast tearing mode may be the cause of brightenings in the young newly formed interconnecting loops, whereas the anomalous Joule heating might occur in old loop connections when an exteral disturbance propagates through them.

Spicer, D. S.↗
NASA NTRS1983

Purely coronal flare-like variations

Quasiperiodic X-ray, UV, microwave, and metric-wave variations after a solar flare on November 6, 1980 are reported and analyzed, based on observational data from SMM (HXRBS, UVSP, HXIS), GOES-2, the 100-m radiotelescope at Bonn, and the Nancay radioheliograph. The maxima of the 13 brightenings observed are listed and characterized; a comparison is made with a 'normal' flare at 17:26 UT on the same day. The HXIS and UVSP data are discussed in terms of the physical properties, X-ray flux, O V flux, and H-alpha flux. The variations are found to be mainly thermal and purely coronal, with no chromospheric (H-alpha) participation (in contrast to the 17:26 flare). Since strong X-ray emissions were observed which should have involved the chromosphere through magnetic-field-line heat conduction, it is proposed that the variations wre produced ina coronal plasmoid magnetically separate from the chromosphere. A mechanism for the evolution of such a plasmoid from the upper loops of a giant X-ray arch is discussed. An iterative HXIS-image-deconvolution process is presented in an appendix.

Svestka, Z.↗
NASA NTRS1983

The Queen's flare - Its structure and development; precursors, pre-flare brightenings, and aftermaths

A limb flare, which started at about 20:20 UT on April 30, 1980, was observed by several of the instruments on the Solar Maximum Mission (SMM) spacecraft. This flare has been the subject of a joint analysis of the SMM instruments. The present investigation represents a continuation of research reported in part by Woodgate et al. (1981) and Gabriel et al. (1981). Several questions are explored regarding the preflare activity, the evolution of the flare, and its decay. It is concluded that the X-ray brightenings observed before the flare were indicative only of the generally high level of activity from this region. They were not connected with the build-up of energy before the flare since similar brightenings were observed in the region after the flare. At least one brightening occurred at the site of the kernel before the flare. There is also some evidence of a tongue.

De Jager, C.↗
NASA NTRS1982

Unusual coronal activity following the flare of 6 November 1980

The major two-ribbon flare that occurred on November 6, 1980 is discussed, using data from the hard X-ray imaging spectrometer aboard the SMM satellite. The post-flare X-ray arch and loops are analyzed, showing the flare characteristics, the coronal arch, the time variations of X-rays after the flare, and the time variation of the maximum intensity at the top of the coronal arch in the 3.5-5.5 keV range. A comparison is made with an earlier arch. The post-flare conronal brightness variations are discussed, including a correlation with a 169 MHz noise stomr, the absence of chromospheric excitation, the brightness and temperature of the X-ray arch, the locations of the brightenings, and the details of the first and fifth brightenings. Problems posed by the observations are discussed.

Svestka, Z.↗
NASA NTRS1982

Study of the post-flare loops on 29 July 1973. IV - Revision of T and n sub e values and comparison with the flare of 21 May 1980

Revised temperature and density estimates are provided in a synthesis of previous studies of the two-ribbon solar flare of July 29, 1973 and results are compared with observations of a similar event by the Solar Maximum Mission on May 21, 1980. Photographs taken through X ray filters permitted determination of the spatial distribution of temperature and density in the loop system and variations over time. The entire growth process of the flare is detailed, including the magnetic field configurations and the enlargement of the Halpha flare ribbon separation distance. The temperature is revised to 8.8 million K during maximum soft X ray emission and 6.8 million K at the appearance of the last Halpha loops. The 1973 flare is noted to have occurred in an old, decaying spotless region, producing higher loops at lower density than the 1980 flare, which displayed ribbons imbedded in a sunspot group.

Svestka, Z.↗
NASA NTRS1982

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

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.

Svestka, Z.↗
NASA NTRS1981

Transient brightenings of interconnecting loops. II - Dynamics of the brightened loops

Three different kinds of dynamic events related to interconnecting loops observed in soft X-rays aboard Skylab are discussed: (1) a newly born transequatorial loop that was either emerging from subphotospheric layers or gradually filled in with hot plasma; (2) large-scale twists of interconnecting loops which never relax, and often only form after the loop brightenings, and (3) three events where the loop that later interconnected two active regions had been visible long before one of the interconnecting regions was born. Several impacts this observation might have upon the understanding of the process of flux emergence are suggested.

Svestka, Z.↗
NASA NTRS1981

On the outburst of flare activity of 26 November, 1973

The possible source of a strong 30 hour-long outburst of homologous flare activity, and an unusual growth and brightening of coronal loops in the active region McMath 12628, on the eastern solar hemisphere, on Nov. 1973, as seen from Skylab are analyzed. Prior to that date, not many flares and subflares in the region were associated with radio bursts. However, almost all flares after that date were accompanied by microwaves and type III bursts until the last flare of this kind appeared at 03:52 on the 27th. All the type III bursts were accompanied by type U-bursts, giving evidence for strong closed fields rooted in the site of the radio bursts. It is recorded that U-bursts stopped after 07:18 on Nov. 26, and a change noted is attributed to the occurrence of two two-ribbon flares at the site of the recurrent activity which could have destroyed the U-type favorable situation. A two-ribbon flare, according to Kopp and Pneuman (1976), opens the magnetic field-configuration, and it is therefore suggested that a newly emerging magnetic flux was the source of the complex solar situation. It is suggested that this kind of activity should be studied during the FBS-ALERT periods in 1980-81.

Howard, R.↗
NASA NTRS1980

Activated solar filaments and flares

Skylab observations have shown that activations and disruptions of dark H-alpha filaments are one of the basic and most important mechanisms of solar activity. The Kopp-Pneuman theory of post-flare loops shows that the process which disrupts a filament opens the magnetic field and causes a greatly enhanced mass flow along the field lines. The open field lines subsequently reconnect, starting from the bottom of the corona and proceeding upwards. This process can last for many hours. Hot loops are first seen in X-rays, then in extreme UV lines, and, after an appropriate cooling time, in H-alpha as the loop prominence systems.

Svestka, Z.↗
NASA NTRS1980

The thermal X-ray flare plasma

Following a review of current observational and theoretical knowledge of the approximately 10 to the 7th K plasma emitting the thermal soft X-ray bursts accompanying every H alpha solar flare, the fundamental physical problem of the plasma, namely the formation and evolution of the observed X-ray arches, is examined. Extensive Skylab observations of the thermal X-ray plasmas in two large flares, a large subflare and several compact subflares are analyzed to determine plasma physical properties, deduce the dominant physical processes governing the plasma and compare large and small flare characteristics. Results indicate the density of the thermal X-ray plasma to be higher than previously thought (from 10 to the 10th to 10 to the 12th/cu cm for large to small flares), cooling to occur radiatively as much as conductively, heating to continue into the decay phase of large flares, and the mass of the thermal X-ray plasma to be supplied primarily through chromospheric evaporation. Implications of the results for the basic flare mechanism are indicated.

Moore, R.↗
NASA NTRS1979

Slowly moving disturbances in the X-ray corona

Sequences of soft X-ray pictures, taken aboard Skylab between May and November, 1973, have made it possible to detect slowly moving disturbances originating in disrupted filaments and causing subsequent brightenings of distant coronal structures. With speeds decreasing from approximately 400 km/sec shortly after the filament disruption to approximately 10 km/sec four or five hours later, these disturbances appear to be identical with slow waves earlier inferred by Bruzel (1952, 1969), Oehman (1953), and Yajima (1971) from chromospheric observations.

Rust, D. M.↗
NASA NTRS1979

Transient brightenings of interconnecting loops - Morphology of the sudden brightenings

Sudden brightenings of coronal loops that interconnect active regions are studied. Such brightenings often occur within one or two days after the birth of a new interconnecting loop, as well as in some old interconnections. The brightenings of young loops are obviously associated with the emergence of new magnetic flux near their footpoints, whereas some enhancements of old loops may be triggered by slowly moving disturbances propagating from other centers of activity. A few loop brightenings are associated with flares, but the loop does not brighten in consequence of the energy supply from the flare. Both the flare and the loop brightening are independent consequences of one common agent, presumably newly emerging flux. Temperatures in brightened loops are between 3 and 4 x 10 to the 6th K and densities are less than 2 x 10 to the 9th/cu cm, probably less than 5 x 10 to the 8th/cu cm in some old loops. The top part of a loop is the site of the most intense brightening in the initial phase of a loop enhancement. The most frequent lifetime of these brightenings is 6 to 7 hr.

Svestka, Z.↗