A. K. Bhatnagar

Travelling interplanetary disturbances detected using interplanetary scintillation at 327 MHz

Based on the advance predictions of two flare-generated shock fronts, obtained from the Space Environment Centre (SEC, NOAA, Boulder), observations of interplanetary scintillation (IPS) were carried out with the Ooty Radio Telescope (ORT) on a grid of appropriately located sources during the period 31 October to 5 November, 1992. Solar wind velocities were derived by fitting model spectra to the observed spectra and two travelling interplanetary disturbances were detected. Both disturbances were traced back to an active region on the Sun which was located close to a large coronal hole. The roles of flares and coronal holes in producing such disturbances are examined and it is shown that in the present case both the coronal hole and the active region probably played key roles in generating the two IPS disturbances.

Evolution of helically twisted prominence structures of March 11, 1979

Helical structures are generally associated with many eruptive solar prominences. Thus, study of their evolution in the solar atmosphere assumes importance. We present a study of a flare-associated erupting prominence of March 11, 1979, with conspicuous helically twisted structure, observed in Hα line center. We have attempted to understand the role played by twisted force-free magnetic fields in this event. In the analysis, we have assumed that the helical structures visible in Hα outline the field lines in which prominence tubes are embedded. Untwisting of observed prominence tubes and later, formation of open prominence structures provide evidence of restructuring of the magnetic field configuration over the active region during the course of prominence eruption. Temporal evolution of the force-free parameter α is obtained for two main prominence tubes observed to be intertwined in a rope-like structure. Axial electric currents associated with the prominence tubes are estimated to be of the order of 1011 A which decreased with time. Correspondingly, it is estimated that the rate of energy release was ≈ 1028 erg s−1 during the prominence eruption.

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.The brightenings appear as a succession of point-like enhancements whose individual lifetimes are of the time-scale of minutes but which continue to occur for periods of several hours. This behaviour allows us to infer a fine structure in the coronal arches, undetectable in the X-ray images. The discovery of these brightenings and their location at the periphery of the active region also alters our conception of the relationship of the giant arches to the flares that begin concurrently with them. The giant arch phenomenon appears now to be either: (1) a long-lived, semi-permanent, coronal structure which is revived and fed with plasma and energy by underlying dynamic flares, or alternatively (2) a system of high-altitude loops which open at the onset of every such flare and subsequently reconnect over intervals of many hours.

Sunspot proper motions in active region NOAA 2372 and its flare activity during SMY period of 1980 April 4–13

Solar Active Region NOAA 2372 was observed extensively by the Solar Maximum Mission (SMM) satellite and several ground-based observatories during 1980 April 4–13 in the Solar Maximum Year. After its birth around April 4, it underwent a rapid growth and produced a reported 84 flares in the course of its disc passage. In this paper, we have studied photospheric and chromospheric observations of this active region together with Marshall Space Flight Center magnetograms and X-ray data from HXIS aboard the SMM satellite. In particular, we discuss the relationship of the flare-productivity with sunspot proper motions and emergence of new regions of magnetic flux in the active region from its birth to its disappearance at the W-limb.

Eruptive prominences of 1980 april 27 observed during STIP interval — X

Observations and analyses of two similar eruptive prominences on the north-east limb observed on 1980 April 27 at 0231 and 0517 UT, which are associated with the Boulder active region No. 2416 are presented. Both the eruptive prominences gave rise to white-light coronal transients as observed by C/P experiment of High Altitude Observatory on the Solar Maximum Mission. Type II and moving type IV radio bursts are reported in association with the first Hα eruptive prominence at 0231 UT.Both the Hα eruptive prominences showed pulse activity with a quasi-periodicity of about 2–4 min. We estimate a magnetic field in the eruptive prominence of about 100 G and a build-up rate ∼ 1026 ergs-1. The high build-up rate indicates that the shearing of the photospheric magnetic field, which fed the energy into the filament, was rapid. It is proposed that fast-moving Hα features must have initiated the observed coronal transients. From Hα, type II and coronal-transient observations, we estimate a magnetic field of 2.8 G at 1.9R⊙ from the disc centre, which agrees well with the earlier results.

Filament eruptions, flaring arches and eruptive flares

Several cases of erupting filaments showing distinctly their “feet” have been studied. Role of the feet and their anchorage with the photosphere in maintaining filament stability is established; apart from the footpoint separation and height criteria. Further, a homologous series of more energetic events, namely, the flaring arches and eruptive flares of March 5–7, 1991, suggest a repetitive restoration of magnetic field conditions and energy build-up within a day. High resolution H-alpha observations of these events indicate that large amount of ejected material was “siphoned out” from the chromosphere through, the top. of a low-lying compact emission loop within the active region.

Flare Associated Eruptive Prominence Activity of February 1, 1979

Observations and analysis of solar flare activated ascending “Fountain type” prominence of 1 February 1979 are presented. This “Fountain” prominence rose to 180,000 km above the solar surface and gave rise to a number of ascending loops and helical structure. These “helicals” are clear manifestation of magnetic field configuration. From these observations it is shown that, as the “Fountain” prominence rises, it carries along with it the complex magnetic field which unfolds as the prominence material expands into distinct magnetic field lines. Several type III radio bursts were also seen associated with this event. No type II or IV radio emission was reported.