C. Debi Prasad

Imaging polarimetry of Comet P/Halley

Abstract Comet P/Halley was imaged polarimetrically in the coma and tail region on 5th January 1986. In the inner coma a small region has been found which has very low ( 105 km) two separate regions of enhanced polarizations (>8%) are found. These two regions are probably connected with jet ativities in the comet. The low polarization region in the inner coma can also be connected with a fresh dust jet ejecta, where the low polarization is resulting from multiple scattering in a region of high dust concentration, in the fresh ejecta. In general the polarization vectors have been found to be perpendicular to the scattering plane.

Variability of circumsolar dust ring

A study of existing observations and theoretical concepts about the circumsolar emission in infrared radiation and its relation to the dust ring have been carried out. It is shown that the dust ring is temporally variable and depends on the solar cycle - being easily observable during minimum phase of the solar activity. Its constituent must be fine particles, instead of 10 km sized boulders at 4R⊙ from the Sun. It is also shown that the measurement along the coronal features such as streamers give rise to higher flux estimates.A study of existing observations and theoretical concepts about the circumsolar emission in infrared radiation and its relation to the dust ring have been carried out. It is shown that the dust ring is temporally variable and depends on the solar cycle - being easily observable during minimum phase of the solar activity. Its constituent must be fine particles, instead of 10 km sized boulders at 4R⊙ from the Sun. It is also shown that the measurement along the coronal features such as streamers give rise to higher flux estimates.

SOLAR PHOTOSPHERIC AND CHROMOSPHERIC OBSERVATIONS USING A LITHIUM NIOBATE FABRY-PEROT ETALON

We have made a narrow band tunable filter for solar observations using a Lithium Niobate Fabry-Perot etalon. The 60 mm aperture etalon with a free spectral range of 4.22 Å and finesse of 26 at λ = 6122 Å has been procured from CSIRO, Australia. The wavelength tuning is achieved by applying high voltage to the etalon substrate at the rate of 0.45 Å per 1000 Volts. The filter is being used for imaging the sun in Hα line and obtaining Doppler- and Magnetogram in CaI 6122 Å line. In this paper, we present some initial observations carried out with this filter at Udaipur Solar Observatory.

Instrumental broadening caused by the misalignment function in a Fabry-Perot etalon assembly

This Technical Note discusses the contribution to broadening from lack of parallelism of the etalon plates.

Chromospheric evolution and the flare activity of super-active region NOAA 6555

Super-active region NOAA 6555 was highly flare productive during the period March 21st–27th, 1991 of its disk passage. We have st udied its chromospheric activity using high spatial resolution Hα filtergrams taken at Udaipur along with MSFC vector magnetograms. A possible relationship of flare productivity and the variation in shear has been explored. Flares were generally seen in those subareas of the active region which possessed closed magnetic field configuration, whereas only minor flares and/or surges occurred in subareas showing open magnetic field configuration. Physical mechanisms responsible for the observed surges are also discussed.

Optical Interferometric Observations of a Transient Event of 1986 March 13 in the Coma of Comet Halley

During the recent apparition of Comet Halley in 1985-86 a transient ionic event in the form of a blob of H2 O+ emission was recorded in thecoma at ∼ 0h UT on 1986 March 13. Observations were carried out using a special IHW filter for H2 O+ emission at 7000 å/175 å, a 35 cm telescope, a Fabry-Perot interferometer and an image intensifier camera from Gurushikhar, Mt Abu. (24‡39’N,72‡ 47’E). A Fabry-Perot inter-ferogram in Ha taken a few minutes later at the same location reveals strong hydrogen emission (Hα) associated with the blob. The velocity field in the blob is structured with relative velocities upto ∼ 35km s−1. The event is interpreted as arising due to the sector boundary crossing of the interplanetary magnetic field by the comet

A kinematic study of Orion Nebula in the emission line [S II] 6731 Å

Fabry-Perot Interferometric observations were made on Orion Nebula in the forbidden emission line [S II] 6731 Å line. The observed field is about 6′ × 6′ on the face of the nebula and it covers the Orion Trapezium stars and the nearby regions. The relative line of sight velocities which denote the average deviations from the systematic velocities were derived at a large number of points in the nebula. Velocities were found to show a gradient towards the north of Trapezium stars. Large blue shifted velocities were found at the nebular boundaries, probably associated with the ionization fronts. The observed velocities, in general, show agreement with the flow model of Balick, Gull and Smith (1980).

The plasma condensation region in the coma of Halley's Comet

The cometary images taken on 1986 January 8.590 and 8.638 UT (R-0.9 AU, δ ~ 1.29 AU) at Gurushikhar, Mt. Abu, India (24 °39′ N, 72 °43′ E alt: 1700 m) show distinct condensation region in the tail direction. The size of the condensation region is 4 × 103 km. The condensation region showed up strongly in the blue emission, implying the abundance of CO+. It was inferred to be moving with a velocity of 37 ± 3 km/s relative to the comet at a distance of 2.3 × 105 km from the nucleus in the tailward direction.The analysis show that the condensation was a result of rapid ionization mechanism, with a time scale of \~103 to 104 sec. The most probable mechanism for producing the ionization region was found to be the discharge of cross tail electric current passing through the neutral sheet in the near nucleus region followed by an outburst observed in IR wavelengths at 8.1 UT. It was accelerated by J × B drift at a rate of ~24 cm/sec2 to the position observed by us.This feature, most probably is the precursor of the first dramatic Disconnection Event (DE) observed in Halleys Comet at Jan.10.375 UT. This supports the conjecture that the tail features originate in the coma with a velocity of ~20–40 km/s.

Imaging Fabry-Prot observations of [OI] 6300 emission in the coma of Halley's comet 1982i

The observation of [OI] 6300 » emission in the coma of Halleys comet 1982i, using the imaging Fabry-Pérot spectrometer, was carried out from Gurushikhar (24‡39′N, 72‡43′E, 1700m altitude), Mt. Abu, India on March 15, 1986 (R = 0.90 AU, δ = 0.96 AU). The analysis of the interferogram show the absence of the differential velocity of neutral oxygen above 5 km s−1.

Observations on near nucleus activity in comet Halley during January 1986

Abstract A large data base of near nucleus images of Comet Halley in white light, Kodak wratten filters and IHW filters was built up using an image intensifier camera and a 35 cm aperture f/11 telescope at Gurushikhar, Mt.Abu., India (24°39′N, 72°43′E, Alt.1700 m) during 1985–1986. The images taken on January 8.590 and 8.632 UT (R∼0.9 AU, Δ ∼ 1.29 AU) show distinct condensation region in the tail direction. From the relative position of the condensation region in two frames, its velocity was inferred as ∼ 37±3 km/s in the anti-sunward direction. The wide field photographic study shows this feature to be the precursor of the disconnection event observed on 10th Jan 1986. Based on the Near Nucleus photographs obtained by us and wide field photographs from other sources, it is shown that there is a strong correlation of nuclear activity with the tail disturbances observed on 31st Dec 1985, 8th and 10th Jan 1986.