Solar spectroscopy

In the early sixties, the solar group at the institute was interested primarily in studies related to molecules in photosphere, sunspots and faculae on the sun. Dissociation equilibrium calculations and line profile studies of diatomic and tri-atomic molecules in different model atmospheres were carried out. The main aim being to pick up new molecular species and transitions for identifications in the photosphere, sunspot or facular spectra. It also helped in testing photo spheric, sunspot and facular model atmospheres. Additionally molecular and atomic parameters and elemental abundances were derived.

Intensities and line positions of vibration-rotation bands of isotopic lines of molecules with large isotopic abundances have also been predicted. Investigations regarding the behaviour of some unionized molecular and atomic lines in various faculae models have been made. Studies of Brackett series of hydrogen, together with some ionized diatomic molecules were also taken up. The behaviour of hydrogen atomic lines in the visible and in the infrared region of facular spectra have been studied. Also investigations regarding the temperature sensitive lines of atomic oxygen have been made. A suggestion has been made that a simultaneous observation for the C2, Tio and the MGH lines in the sunspot and photosphere should help refine model atmospheres. Diffusion of molecules has been found several orders of magnitudes lower compared to the molecules formation and destruction rates. Also diffusion velocities have been found much smaller than the turbulent velocities in the solar atmosphere.

The various representative photospheric, sunspot and faculae models have been tested with the help of rotational temperature calculations using molecular lines. Amongst the various photospheric model proposed , the one by Holweger and Muller in 1974 was found by us to be the most acceptable one for the center of the disc calculations. Parameters such as the oscillator strengths of MGH Green Bands, C2 Phillips bands, CN Red System, 3993 Å System of SIH+ and the Alpha system of BO molecules have been obtained utilizing the solar spectra. Log gf values of about three hundred lines due to Al, C, Ca, Cr, Fe, K, Mn, Mg, Na, Ni, P, S, Sc, Si, Ti and V giving spectra in the 6209 - 6273 Å and in the J and H bands were obtained by utilizing the high quality FTS solar spectrum.

Amongst the ones predicted by the solar scientists, the presence of SH, SiO and additional Swan band lines of C2 have been confirmed in observations. About 85 research publications and 5 Ph.D. theses have resulted from these studies.

A Bibliography of Papers Published in Solar Spectroscopy at ARIES

Energy transport through solar atmosphere

K- line spectroheliograms obtained from the Kitt Peak National Observatory and the Kodaikanal Observatory were utilized to study the characteristic scale lengths and the frequencies of the energy transport in the Sun. Power spectra is not homogeneous and the different sections on the line profile emphasize different frequencies differently. The directly identifiable features of the K-line profiles are not related even in their scale length variations. A life cycle of a typical feature that emits only in one wing has been studied. Excepting for large features, a collection of small feature profiles exhibits a wide diversity showing the same order of magnitude velocities as observed in spicules which appear all over the solar disc and show no particular affinity to bright network feature. About five research papers and one Ph.D. Thesis have been published on the topic.

Studies of solar flares

It is concluded from the Hα observations of a two-ribbon solar flare of 13 May 1981 that apart from the oscillating behaviour of the strands, twisting/shearing in them is also seen which could be due to the rotation of their foot points. Using the Hα data of 19 October 1981 flare, the magnetic field strength and the kinetic energy of the flare were estimated. The important feature to be noted in the dynamic spectra of the radio event associated with the flare of 14 April 1982 is the appearance of radio pulsation in the metric band, type IV continuum, type III burst storms and SXR before the flare onset in Hα . The measurements of the observed brightness of the 9 and 25 November 1990 Hα flares reveal that the flares were triggered by filament eruption and produced remote chromospheric brightening. The morphological changes of post flare loops of 27 November 1996 in active region NOAA 7997 showed that this loop system was associated with long duration B9.0 class X-ray flare. The study of a dynamic flare on 24 November 2000 from super-active region NOAA 9236 illustrates that the flare showed impulsive nature in different wavelengths and associated with violent full halo CME and proton event. The very impulsive flare on 10 March 2001 was associated with type II radio bursts and CME which is inconsistent with the existing theories. The Hα CCD observations of three small to medium size two ribbon flares observed from giant AR 9433 on 24 April 2001 were compared with observation at other associated wavelengths. The flares are also studied in EUV and UV emissions using TRACE data. A comparison of post flare loop structure of 2 May 2001 with potential field indicates the relaxation of sheared magnetic field into almost potential field.

Studies of prominences and filaments

Morphological-cum temporal behaviour of a large arch type prominence, its eruption and subsequent interaction with a neighbouring prominence observed on 22 November 1980 revealed that the prominence configuration had a complicated magnetic field geometry and transverse velocities of some distinct knot had a range from 20 to 80 km s~1. The eruptive screw-type prominence of 7 October 1981 seems to have originated from the photosphere as a huge twisted rope like structure. It reached a height of about 100,000 km later on split. The analysis of an eruptive prominences on 25 January 1991 associated with the X10.0 class limb flare, showed that the event could be explained with the circuit model of prominence. The evolution of a quiescent prominence seen on the NW solar limb from 11 to 13 June 1991 showed that the prominence migrated pole ward and transformed from normal (N) to inverse (I) magnetic configuration. The helical eruptive prominences of 15 December, 1992 and 14 January, 1993 have also been studied.

The evolution of the sigmoid filament observed from 14-20 February 1994 which produced two ribbon flares on 20 February giving strong radio bursts, MW bursts, CMEs etc.

Studies of solar surges

Morphological, magnetic field and energies associated with two solar surges observed on 19 November 1981 and 25 December 1981 indicated that the maximum mechanical energy associated with non-flare surges can be as much as 1029 ergs. Eight solar surges during 26-28 November 1990 showed homologous nature and the height, mass, radial velocity,mechanical energies and magnetic fields associated with them were estimated. The surge of 28 November 1990 was very dynamic and had twisted screw like structure which also got de-twisted during evolution. A study of solar surge association with microwave (for he period 1980-1983) and HXR (for the period 1981-1982) bursts showed that 20% of solar surges produce microwave bursts, whereas no solar surge produces HXR bursts. It is found that surges are most prolific at 80°, 110°, 260° and 290° of longitudes on the Sun, thus indicating four active longitude on the Sun. These active longitudes are concentrated in two zones and the zone separation is 150° in longitude.

Studies on effects of solar flares on Earth's ionosphere

The effects of the flare X-rays on the ionospheric D-region were studied at the 16kHz VLF radio signal as sudden phase anomalies (SPA). The observed diurnal phase variations of these signals have been found to have 98.5% correlation with those calculated from the mode theory of VLF propagation. The average diurnal change in the height of reflection over the propagation path of the ionospheric D-layer was estimated to be 13.2 km. The SPAs observed at the institute in the received phase of GBR, 16 khz VLF signals during the year 1977 to 1983 have been analysed in the light of their association with the solar X-ray flux in the 0.5- 4Å and 1- 8Å bands. The threshold X-ray flux needed to produce a detectable SPA event has been estimated to be 1.6x107 Wm-2 and 1.8x10-6 wm-2 in the 0.5- 4Å and 1- 8Å X-ray bands respectively.

Expeditions to total solar eclipses

20th June 1955: Total solar eclipse (TSE), Hingurakgoda, SriLanka
16th Feb. 1980: Total solar eclipse (TSE), Palem, Mehboob Nagar, Andhra Pradesh, India
24th Oct. 1995: Total solar eclipse (TSE), Meja Khas, Allahabad, Uttar Pradesh, India
11th Aug. 1999: Total solar eclipse (TSE), Bhuj, Gujarat, India
23rd Nov. 2003: Total solar eclipse (TSE), Antarctica
29th Mar. 2006: Total solar eclipse (TSE), Turkey, Antalya