Indian Journal of Radio & Space Physic\

Vol. 17, December 1988, pp. 296-304

Comet Halley - India~ ObservationsT CHANDRASEKHAR

Physical Research Laboratory, Ahmedabad 380 009

The recent apparition of Comet Halley in 1985-86 witnessed a well planned coordinated study by the spaceprobes as well as by an extensive observational network on the ground which has resulted in a sea change in ourunderstanding of the cometary environment. The Indian astronomers actively participated in the coordinated ob­servational programme under the aegis of the International Halley Watch (IHW) and Indian Halley ObservationProgramme (IHOP) and have obtained several interesting results on the spatial distribution of coma emissions andon some transient events. The observations and some of the results are discussed.

1 Introduction

The apparition of Halley's comet in the winterof 1985-86 was a watershed in the history ofcometary astronomy. Never before had a cometbeen observed so intensively and with such so­phisticated instrumentation. The earlier visitationschronicled since 240 Be or even earlier had been

little more than event markers in history. Duringthe last apparition of Halley's comet in 1910, as­tronomical instrumentation was in its infancy.Apart from photographing the extent of the cometand recording a few spectrographs, which re­vealed emissions in the coma, little else could bedone. For the present apparition however, themost recent advances in technology were put touse to maximize the scientific output. For a periodof about eight months from October 1985 to May1986 the comet v.;as under close observation by avery large number of telescopes around the world.Observations were also carried out from aircraft,balloons, rockets and satellites to overcome theabsorbing effects of the terrestrial atmosphere.

In March 1986 just a month after perihelionwhen the cometary activity was near its peak, anarmada of five spacecraft converged on the com­et. In one eventful week the spacecraft encountersaltered forever our perception of the cometaryenvironment as detailed below:

(i) The cometary nucleus was imaged for thefirst time. A potato or avocado shaped dark nuc­leus (albedo < 4%), 16 km X 8 km x 8 km wasseen with highly asymmetric and localized centresof activity!.

(ii) In situ measurements also detected extreme­ly small grains (- 10 - 16 g) rich in low Z ele­ments - the so called CHON particles. These par­ticles which cannot be detected from the earthsolve the problem of missing carbon in the come-

296

tary atmosphere and underscore the importanceof organic matter in comets.

(iii) A magnetic field free cavity surrounds thenucleus at a distance of 4500 km. This contact

surface separates the outflowing pure cometaryplasma from the solar wind contaminated come­tary plasma.

Notwithstanding the significant achievements ofspacecraft encounters, some very important adv­ances in our understanding of comets were alsomade from earth-based observations. For the first

time, from near infrared spectroscopic observ­ations near 2.65 flm taken from an aircraft, it wasunambiguously established that water is the domi­nant constituent of the cometary nucleus2•

A comet, during its perihelion passage, is usual­ly a higWy time variable object. Apart from itsdramatic evolution from nearly a point source toa spectacular body displaying a structured comaand a long tail, a comet also exhibits a number ofshort term events like dust jets, brightenings, out­bursts, structures in the ion tail, disconnectionevents, etc. In order to continuously monitor thecomet and to coordinate the observations the In­ternational Halley Watch (IHW) was set up withseveral subdisciplines: (a) Astrometry, (b) Infraredspectroscopy, (c) Large scale phenomena, (d)Near nucleus studies, (e) Photometry, (f) Radiostudies, (g) Spectroscopy and (h) Meteor studies.Over a thousand professional astronomers from40 countries were involved in keeping a closewatch on Halley with a variety of techniques.

The Indian astronomical community participat­ed actively in the coordinated Halley observationsunder the aegis of IHw. An Indian Halley Obser­vation Programme (IHOP) comprising astronom­ers from the leading Indian institutions wasformed to coordinate the Indian observations. Af-

I I "I I;;' III 11'11· I

CHANDRASEKHAR :INDIAN OBSERVATIONS ON COMET HALLEY

nucleus, (b) aperture displaced antisunward and(c) aperture displaced normal to the sun-cometaxis. The displacements totalled about i' from thenucleus.

ISRO telemetry,AFU-75 camera (f/3.5, Wide fieldtracking and 210 mm f.1.) photographycommand net- 10° x 14°work (Kavalur) astrometry*The stat mark indicates that these are collaborative experi­ments between PRL and IIA, i.e. PRL instruments used with IIA

telescope.

IHWfilter

imagery

eN rotationaltemperatureMeteor streamstudiesPhotoelectric

spectrometryrange: 3200­7000 AWidefield

photography4°.5 x 6°.5

Schmidt

MeinelspectrographMeteor radar

Spectrometer

10000 m2 103 MHz inter­

planetary scintillation (IPS)

45cm

50 MHz

35 cm Celestron + Imageintensifier camera (Mode 1)

continuumIondensityinhomogeneit­ies

array in the tailTata Institute 327 MHz Ooty radio telescope londensityof Fundamental inhomogeneit-Research ies

(TIFR), Bom,- in the tailbay

35 cm Celestron + Fabry-Perot Imagingoptics + Image intensifier spectroscopycamera (Mode 2) in C2,H,[O]-

interferograms*1 m (Kavalur) + Piezo-electric High resolution

Fabry-Perot spectrometer interferometricspectroscopy in[0I]6300AandNH2(0-8-0)

*1 m (Kavalur) + Photopolari- Polarization inmeter with IHW filters the emission

bands and

Table I - Observational Facilitiesfor Comet HalleyInstitution Telescope Instrument Remarks

Indian Institute 1 m Photoelectric b A =40 A; 20 Aof Astrophysics scanner tange: 3900-(IIA) 6200 A

40 cm Photometer IlIW filter

~otometry4f x 5° photo­graphs

Astrograph AstrometryCentre for 20 cm

Advanced Studyin Astronomy 1.2 m(CASA),Osmania

UniversityUttar Pradesh 1 mState Observa-

tory (UPSO),NainitalPositional 35 cm Celestron with 20 cm

Astronomy Schmidt cameraCentre (PAC),Calcutta

PhysicalResearch

Laboratory(PRL),Ahmedabad

ter the campaign of observations ended and someanalysis had been done, a National Symposium onComet Halley was held in October 1987 in Ban­galore wherein the main results were presentedand summarized. Some of the specific questionswhich the Indian astronomers addressed them­selves to were the following:

(i) What are the values of column densities,production rates and abundances of neutral andionic species in the coma and tail of Comet Hal­ley?

(ii) Are the above values time variable and alsospace variable? What are the scale sizes involved?Can we infer the underlying causes of the varia­bility?

(iii) Are there detectable ion density inhomog­eneities in the tail of the comet? Can we detect

radio scintillatiorts due to these inhomogeneities?(iv) Is there polarization in the emission bands?(v) Are there transient events occurring in the

coma in the time scale of :5 1 hr? Are these trig-gered by the sector boundary crossings by the in­terplanetary magnetic field or by high speed solarwind streams?

Table 1 summarizes the observational facilities

available to the Indian astronomers during theHalley campaign. In addition to the professionalastronomers, groups of enthusiastic amateur as­tronomers also actively participated in the Halleyobservations employing a variety of instrumentsranging from the naked eye to sophisticated 12.5cm aperture Schmidt cameras. A large number ofwide field photographs and sketches have beenobtained by them. Table 2 lists the details of theIHW filters used by many Indian observers.

2 Indian Observations

2.1 Indian Institute of Astrophysics (IIA), Bangalore

An extensive programme of observations con­sisting of spectrophotometry, imaging photometryand astrometry was carried out at the telescopesof the Vainu Bappu Observatory, Kavalur. Resultsof spectrophotometric observations - the fluxes inthe emissions bands and in the continuum as wellas the distribution of fluxes as a function of radialdistance from the nucleus - have been reported3•

The observations consisted of spectral scanstaken with an automated spectral scanner using athermo-electrically cooled photomultiplier tube inthe pulse counting mode at the Cassegrain focusof the 102 em retlector at Kavalur. Entrance ap­erture to the scanner was 25".9. Typically thespectral range covered was 3900 to 6200 A. Thespectral resolution was normally - 40 A.. Observ­ations were taken with (a) aperture centred on the

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INDIAN J RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988

Table 2 - IHW Filters

Species

WavelengthBandwidthRemarksA

(FWHM),A

Continuum

3650100CN

387050(0,0) VioletsystemC,

406070CO+

426065(2,0) ProminentdoubletstructureContinuum

484565

C2

514090(0,0) bandContinuum

684090

H2O+

7000175

Using observations of standard stars, from thespectral energy curves the flux (F)J in the emissionbands of C3, CN, CH, Swan bands, NHz andHzO+ over and above the smoothed level of thecontinuum have been evaluated. The total number

of molecules in the line-of-sight through the com­et for a resonance fluorescence emission is der­ived from the relation

FA 4rr~zN=--­g

where

FA Band flux in the 26" diaphragmg Fluorescence efficiency factor for the emissionconcerned~ Comet-earth distance

Fig. 1 shows the variation of spectral energycurves with the displacement of aperture awayfrom the nucleus. The rapid decrease in the bandemissions away from the imcleus is clearly seen.In Table 3 the total number of molecules (N) ofeach species contained in a cylinder of diameter25".9 in the line-of-sight extending through thenucleus is given.

A comparison of the observed and theoreticalband sequence flux ratios for the Swan bands ofthe Cz molecule has been made. According toSivaraman et af.3 the Swan band sequence flux ra­tio F(~v=+l)/F(~v=O) and F(~v=-l)/F(~ v= 0) obtained from the observations do notmatch the ratio for Cz emission predicted by eventhe sophisticated model of Krishnaswamy andO'DeIl4• The Cz problem, i.e. the discrepancy be­tween observations and theory based on reson­ance fluorescence mechanisms continues to be en­igmatic.

From the observations, emission profiles, i.e. logFA versus log p where p is the distance of centre

298

1.4

1.01

0.6

0.6

o.s

IVI

NIE 0.5u\IIt7l•..~u..-< 1.3

X::J...Ju.. o.s

3.0

2.2

1.4

4000 4400

Fig. 1 - Spectrophotometric observations made on 1986March 31.9 [Plot of flux Fl. versus wavelength correspondingto 6 locations (deisgnated N, 2, 3, 6) of the 26" diaph-ragm within the coma, N being position on the nucleus. The 5successive positions of the diaphragm (- 28" from one an­other) in the anti-sunward direction are shown in the inset.Emission in the bands decreases rapidly as the diaphragm is

displaced away from the nucleus(r= 1.18, Ll = 0.53 AU)]

of aperture from the nucleus, have also been con­structed for C3, Swan bands of Cz and continuumat 4860 A. Emission profiles along the radiusvector, i.e. the sun-comet line show that on thesame night 3 bands of Cz exhibit the same gra­dient while profiles of C3 and the continuum aresteeper. Normal to the radius vector the gradientsare steeper for Cz while the reverse is true for C3

and the continuum. The gradients on either sideof the radius vector are not the same. The pro­files are clearly asymm~tric for all the species.

The ratio of the fluxes in the continuum over

the range 4000-6000 A to the sum of the fluxesin the Swan bands of Cz and C3 emission hasbeen taken as a measure of the dust-to-gas ratio.The ratio thus defined decreased from Nov. 1985

r-4.:

I I II 'I !!'1" '" 'I

------ --------~------------. ~

CHANDRASEKHAR :INDIAN OBSERVATIONS ON COMET HALLEY

Table 3 - Column Densities Integrated over the Aperture (n:R2)

Date

AUR LogNUT

104 !an1985-86

C3Cz (1,0)Cz (0,0)Cz(O,l)NHzHzO+

(4040 A)(4700 A)(5165 A)(5500A)(0,7,0)

Nov. 16.904

1.710.67-293429.3129.2830.0728.54Nov 17.748

1.690.66-29.2929.3229.2530.3528.43Nov 18.762

1.680.65-29.2729.2729.2330.2728.36Dee 8.813

1.380.65-29:2029.1128.13Dee 19.576

1.210.8428.9429.8329.9830.12Dee 20.590

1.200.8628.8229.7830.0530.14Mar 21.918

1.000.7429.0229.7729.9530.12Mar 23.950

1.040.6928.8329.6329.8930.04Mar 28.883

1.110.5829.2529:7829.9230.08Mar 29.896

1.130.5529.16·30.0130.1830.19Mar 30.903

1.140.5328.8229.4929.7029.72Mar31.904

1.160.5129.2329.7730.0230.00

April 5.848

1.240.4329.0729.7929.7729.75

April 12.850

1.340.3929.1729.5629.5429.76

April 17.694

1.420.4429.0029.4729.4229.5930.45

April 30.638

1.620.7228.8329.6529.6429.87

May 7.662

1.720.9329.3929.8029.6629.77

16.904* (r= 1.71 AU) to Dec. 20.590 (r= 1.20AU). After perihelion the ratio increased fromMarch 1986 21.918 (r= 1.0 AU) to May 7622(r= 1.72 AU). The behaviour is consistent withthe breaking up of. the grain into fine particles byUV and solar wind and the general increase ofgaseous component during the preperihelion peri­od considered. In the post perihelion period, theincrease is attributed to grain growth.

In addition to spectrophotometric observationslarge-field photographs of the coma and tail weretaken regularly with the Schmidt telescope. About150 good quality photographs have been obtainedwhich show several time varying features in theplasma tail.

2.2 Centre for Advanced Study in Astronomy (CASA),Osmania University

Photoelectric observations of Comet Halley inthe IHW filters for 10 nights with the 1.2 m tele­scope have been reported. Fluxes, column densit­ies and production rates for the various emissionspecies have been determined.

The CN (0,0) band near A. 3880 A. was re­corded on five spectrograms with the Meinelspectrograph. The spectrograms were analyzed toobtain rotational temperatures in the range 540­630 K in the preperihelion phase. From the ob­servations in the post perihelion period a rota-

* These numbers denote the exact time of observation in UT

as fraction of a day. For example, Nov. 1985 16.904 meansthat the date is 16th Nov. 1985 and the time of the day is0.904, i.e. the fraction of the day eqivalent to is 21.6 hrs.

tional temperature variation with heliocentric dis­tance (r) of 430/,J-A was derived.

Astrometric observations with the 20 cm astro­

graph gave positions of Comet Halley in agree­ment with Yeoman's orbit No. 33 within 1 arc s.

2.3 Uttar Pradesh State Observatory (UPSO), Nainital

Photoelectric spectrophotometry of the coma inthe wavelengtli region 3200-7000 A was carriedout in the post-perihelion period (r-1.4-1.6 AU).Emission features due to· CN, CH, Cz and C3

were clearly seen. Abundances of Cz and CNhave been estimated and production rates derivedin the usual way using band luminosities.

Photographic pho.tometry of the cometary pic­ture taken on 15 Mar. 1986 at the 1 m telescopethrough a Schott GG 385 filter on Kodak 103 a-ohas been carried out. Average B magnitudes permm of the film at various distances from the nuc­leus are determined5• It is found that the integrat­ed total magnitude within a radius of 2 arc mincentred on the nucleus is 5.5.

2.4 Positional Astronomy Centre (PAC), Calcutta

Using a 20 cm aperture Celestron Schmidtwide field photographs 4°.5 x 6°.5 of the cometfield have been obtained during the period Nov.1985 to April 1986. Analysis of a large jet likefeature seen on 17 Mar. 1986 spiralling outwardfrom the nucleus shows a prograde nuclear rota­tion of 53 ± 5 hours with an initial velocity ofejection of 0.65 kmls.2.5 Physical Research Laboratory (PRL), Ahmedabad

The Physical Research Laboratory carried out

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INDIAN J RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988

Table 4 - Gurushikhar Observations: Statistics of Data

Mode 1 (filler imagery)Emission No. of frames

C2 46CO+ 30

H20+ 25CN 2

White light 1I1Comet + laser 27

R·!:J.R = oA/A = vie

where R is th"e radius of the fringe, !:J.R the fringedistortion; V is the line-of-sight (Doppler shift) ve­locity. The analysis shows that all the fringes arecircular within the limits of measurement. Henceit is inferred that Cz outflow velocity is symmetricin the inner coma and the expansion 'velocity is< 5 km/s.

coma have been obtained. Six interferograms havebeen recorded. The statistics of the data areshown in Table 4. During every session of obser­vation a step wedge calibration was invariably im­pressed on one portion of the film, so that thephotographic density distribution could be reli­ably. transformed into relative intensity distribu­tion through the characteristic curve.

Most of the imagery data and the interfero­grams have been digitized. The digitization workwith a 20 !-tmpixel size was carried out at photo­microdensitometric system (PDS) at IIA, Banga­lore. In collaboration with Space ApplicationCentre, Ahmedabad, image processing of someframes involving frame matching, intensity con­version, continuum subtraction and enhancementof faint features has been initiated.

3 Cz InterferogramFig. 2 shows the interferogram recorded on

1986 March 22.985 in (0-0) Cz emission. A preli­minary analysis of the interferogram in the formof careful measurements of fringe diameters hasbeen carried out. If the Cz flow velocity is asym­metric then the interferogram would depart fromcircularity given by

Fig. 2 - Interferogram of Comet Halley in the C2 Swan band(10 m exposure) at f... 5165 A taken on 1986 March 22.985

UT (r= 1.04 AU; L\ = 0.73 AU)

0.92 15

Heliocentric Exposuredistance in time

AU min

0.84 101.04 101.63 300.89 II1.04 10

1986 Mar 15.994

1986 Jan 12.6041986 Mar 22.985

1986 May 1.9651986 Mar 13.0001986 Mar 22.999

H,,(6563 A)Na(5893/

5890 A)[01](6300 A)

Note: Comet + laser frames were taken

to determine the position of the centreof the fringe pattern in the interfero­

grams.

Mode 2 (Inleiferogrlll1ls)Emission Date

UT

wide ranging and sustained observations of Com­et Halley with a variety of sophisticated instru­mentation listed in Table 1.

Imaging spectroscopy of the cometary emis­sions was carried out over a period of eightmonths from 16 Oct. 1985 till 7 May 1986 fromGurushikhar, Mt. Abu. Observations were per­formed with an interferometric image intensifiersystem coupled to a 35 cm .fl11 Celestron tele­scope. It was possible to operate the instrumenta­tion in two modes: Mode 1 - Direct imagingmode without the Fabry-Perot etalon; and Mode2 - Herein the Fabry-Perot and associated opticswere attached to the intensifier camera. The scale

sizes and fields-of-view were, respectively, 83"/mm and 28' for mode 1 and 6T/mm and 21' formode 2. Data were collected on Kodak 2415 35mm film. The interferometric intensifier system iscompact, light weight ( - 7 kg) and has proved tobe very effective in doing quality work with relat­ively small telescope apertures. The exposuretimes range from a few seconds for a white lightcomet image, 1-3 min for IHW direct imagery(mode 1) and 5-30 min for the interferometry(mode 2). Details of the instrumentation are re­ported elsewhere6. A special electronic drive cor­rector was also developed to improve the tele­scope tracking 7.

Over 200 frames af good quality images includ­ing 100 frames of white light images of the inner

300

CHANDRASEKHAR :INDIAN OBSERVATIONS ON COMET HALLEY

Ib.-....-_

Fig. 4-The interferogram in H" (6563 'A.) emission taken on13 Mar. 1986 (Exposure time 11 min centred on 0 hrs 24min 27 s UT. Three fringes in the form of arcs of concentric

circles can be seen.)

ferogram. From careful measurements of thefringe peaks and structures in the Ha interfero­gram it is possible to determine the velocity fieldin the region. The details of the analysis and theevent have been reported9.

Care has been taken to ensure that the blob is

really associated with the comet. A careful searchin the star catalogues indicates no star brighterthan magnitude 11 in the vicinity of the comet atthat time. The blob is definitely brighter than thislimit and is definitely not a compact source. Asearch in the NGC catalogues of non-stellarsources fails to reveal any extended source nearthe comet down to the magnitude limit of 16.

Detailed analysis of the H20 + blob and the Haimerferogram has lead to the following conclu­SIOns:

(a) The blob has a recession radial velocity of30 ± 10 km/s relative to the comet (as seen fromearth).

(b) The Gaussian analysis of the interferogramyields Doppler line width velocities from 5 to 22km/s.

(c) The relative velocity distribution derivedfrom the interferogram (Fig. 5) shows a verystructured distribution. There is clustering of highand low velocity regions. Fringe splitting is ob­served at many places on the sunward side.

(d) An estimate of the mass of the blob assum­ing a symmetric blob and 1/ r2 dependence of theH20+ number density gives a value of 1010 g.

Fig. 3 - The event of 1986 March 13.0 [The image of thecomet and the blob in the light of HzO+ at 7000 'A.. Expo­sure time 5 min centred'on 0 hr 12 min 47 s. The distance tothe blob from the nucleus is 2 x IOj km and its linear extent

3 x 104 km (r= 0.89 AU; ~ = 0.99 AU)]

A more detailed analysis of the interferogram isin progress. Attempts are being made to see if thespatial structures seen in the form of modulationsof fringe intensity distribution can be related tothe structure of dust spirals from the nucleus assome C2 emission can be associated with dustjets. It is also proposed to compare the three C2

interferograms taken at varying heliocentric dis­tances.

4 The Transient Ionic Event of 13 Mar. 1986A dynamic ionic event8 in the inner coma of

Comet Halley was recorded on 13 Mar. 1986 at- 0 hrs UT. The event manifested itself in the

form of a blob of strong H20 + emission quitedetached from the main body of the comet. Aninte"rferogram taken a few minutes later showsstrong fringes due to Ha in the same region.

Fig. 3 shows the blob clearly in H20+ (7000 AI175 A). The position angle of the blob is 1600(measured north through east) with respect to thenucleus. The blob is at an angular distance of 5arc min from the nucleus corresponding to a line­ar distance of 2 x 105 km. The extent of the blobis 3 x 104 km.

Fig. 4 shows the interferogram in Ha 6563 Ataken with 5 A bandwidth filter soon after theH20+ image. At least 3 fringes are distinctly seen.Careful microdensitometry on the interferogramand the H20 + image shows that the Ha emissionis associated with the blob of H20 + emission.The instrumental parameters of the Fabry-Perotetalon were determined by the He-Ne laser inter-

301

INDIAN J RADIO & SPACE PHYS, VOL. 17, DECEMBER 1988

EVENT OF 13 MAR.1986

DIFFERENTIAL VELOCITY MAP DF THE BLOB REGION IN THE PLANE OF T HE SKY IN km Is

..-././ ...-._._ ._._._._3_ .....",,' ..•.•.•.

"

""

+H20 CONTOUR LEVELS _

I 32 % 'mo. (Carnell2 16 %'3 8 %

Sky 2 %

INTERPLANETARY MAGNETICFIELD

HII t H1 0 SECTOR------- BOUNDARY

HIIJ Hl 0H1 0

- "" tan 42HII

MAX ERRORS

o ± 2 km/s

o ± 8 km/s

IoS: SPLIT FRINGE ZONE~o+ ISOPHOTES

o LOW VELOCITY ZONE

~ HIGH VELOCITY ZONE

Fig. 5 - The velocity field in the region of the blob denved from the interferogram of Fig. 4 [A structured velocity distributionwith many pockets of large differential movement (split fringe positions) can be seen]

(e) The dispersal time of the blob for a sym­metric expansion rate of 30 kmls is half an hour.The event is therefore a transient one.

An explanation of the event can be attemptedfrom the state of the interplanetary magnetic fieldgeometry in the vicinity of the comet. The eventoccurred at the time of the sector boundarycrossing of interplanetary magnetic field by thecomet. Disconnection events are known to occur

particularly at these times 10 though other explan­ations for DEs in terms of high speed solar windstreams also exist 11. The event witnessed on 13Mar. 1986 is not a typical disconnection eventwherein a portion of plasma tail gets detachedfrom the comet and accelerates away anti-sunw­ard. In the present case the event occurs in thecoma and its motion appears to be perpendicularto the sun-comet line. A detailed model of the so­

lar wind comet interaction with the reversingmagnetic field of the interplanetary sector boun­dary serving as a trigger for instability processesis needed for a proper explanation of the event.

5 Piezo-electric Fabry-Perot Observations of[01] and NH2In collaboration with scientists from IIA, PRL

made high resolution observations (A/b). - 6 x 104)of the comet in the emissions of [01], NHz (0-8-0)and Ha. The experiment was aimed at studyingthe spatial distribution of hydrogen and oxygenatoms and then velocity fields through high spec­tral and spatial resolution observations of [01]6300 A.. and 6563 A.. emission of Ha' A sophisti­cated piezo-electrically controlled and scannedFabry-Perot interferometer was used for this pur­pose at the 1 m telescope at Kavalur in April1986 (r-1.3-1.4 AU; ~=0.42 AU). Details ofthe instrumentation have been reported1z.

Under low resolution, study of cometary [01]6300 A.. emission is always limited by the blenc­ing with the lines of the NHz (0-8-0) band. In th.sexperiment the high resolution (b). - 0.1 A..) e'}­sured clear separation of cometary [01] from allthe lines of the NHz (0-8-0) band and also fromthe terrestrial airglow emission (Fig. 6). Line pro-

302

lJ< >1'11II'·1 t!l11f 'jl , I I fl'"

CHANDRASEKHAR :INDIAN OBSERVATIONS ON COMETHALLEY

I' I

"I

lSO.0

; e<-EARTHfTERRES1RIAl ~ ·t..l l AIRGLOWI ~ j. l j.w- ~ COMET1 2 3 4 5 ERROR BAR

w>t= -10.0<{-'wex:

-50.00.0 1.0 2.0 3.0 ".0 - 5.0

WAVELENGTHCJ()

Fig. 6 - The high resolution spectrum in the neighbourhoodof the 6300 A obtained with the piezo-electric Fabry-Perotspectrometer [2 indicates position of terrestrial 6300 airglowemission. 1 and 6 refer to the cometary 6300.304 A [01] line.3 and 4 are identified with cometary NH2 (0-8-0) transitions313-303 at 6299.004 A and 212-202 at 6298.6 A, respectively.The line profiles derived from the spectrum are symmetricwith Doppler widths (FWHM) of - 3 km/s. Feature 5 is un-

identified.]

file analysis shows that both [01] and NHz rota­tionallines were symmetric with a Doppler veloc­ity (FWHM) of - 3 km/s. The ratio NHz (212-20z,

0-8-0)/[01] is 0.5 averaged over the 50 arc s ap­erture used. The cometary [01] flux is estimatedto be - 150 ± 40 Rayleighs consistent with mea­surements made earlier. No H emission could be

detected; upper limits of its intensity based on in­strumental noise lead to a value of - 30 R.

6 Event of 8 Jan. 1986The white light images taken on 1986 Jan.

8.590 and Jan. 8.632 UT (r- 0.9 AU; ~ - 1.29AU) show a distinct condensation region in thetail direction at a distance of (2.4-3.0) x 105 kInfrom the nucleus. From the movement of the con­densation relative to the comet in the two framesit is inferred that the average velocity of the fea­ture is -28 ± 3 km/ s in a direction inclined at 56°northward of the ion tail in the plane of the sky.The feature appears to be mainly due to CO + asan image taken with 5900 A high pass filter be­tween the two white light images fails to reveal it.The condensation might be telated to some nuc­lear activity observed in far IR region at 8.1 UTand to the tail structures observed in white fieldr\hotographs taken later at 8.719 UT on.the sameday in Czechoslovakia.

7 PhotopolarimetryPhotopolarimetric observations of Comet Hal-

ley in the IHW filters were carried out with thePRL photopolarlmetric system in March 1986with the 1 m telescope at Kavalur. These observ­ations were also part of the collaborative pro­gramme with IIA on Comet Halley.

It was found that the linear polarization in theneutral emission bands Cz, C3, CN vary from 5 to7% while for the ionic species CO+ and HzO+ alarge polarization - 15% was observed.

Polarization measurements were also made

through narrow band interference filters centredat 3420, 4420, 5270, 5750 and 6420 A. Except5750 A filter which includes NH emission therest are free from cometary emission. The contin­uum linear polarization is found to slightly in­crease with wavelength (17% in UV to 20% in thered).

8 IPS Observations

Using the 10000 mZ 103 MHz IPS array atThaltej, Ahmedabad, observations of the occulta­tion of several scintillating radio sources by theion tail of Comet Halley in Dec. 1985 have beenmade 13. Additional scintillations due to ion dens­

ity inhomogeneities in the cometary ion tail havebeen reported. A scale size for .the inhomogeneit­ies of - 100 kIn and < N; > + -10 el./cm3 havebeen derived.

Similar scintillations at 408 MHz dUe to thecometary ions have been reported by Slee et al.for 29 Mar. 1986 (Ref. 14). However, the observ­ations made at 327 MHz by the Ooty radio tele­scope have not yielded any additional scintill­ations which could be attributed to the cometl5•

Clearly, a coordinated programme of these ob­servations is needed on a future bright comet.

9 ISRO (Kavalur)-AstrometryThe aim of the investigation is to study the

non-gravitational effects (due to jet action) on theactual orbit of Comet Halley. Using an AFO-75Camera (f/3.5, 210 mm focal length) at Kavalur,about 150 photographs of the comet field(10° x 14°) were qbtained during the entire appari­tion. The precise topocentric equatorial coordi­nates of Comet Halley obtained from the photo­graphs were used for orbit determination. Bestconvergence is obtained only when one day inter­vals of observations are used.

Apart from the observations of Comets Halleyand Giacobini Zinner a number of theoretical in­

vestigations pertaining to the comets were alsotriggered by the Halley apparition. Some of thethemes on which theoretical work proceeded aregiven below:

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INDIAN J RADIO & SPACE PHYS, VOL. 17. DECEMBER 1988

I 'I' q,. I

III,Iii 'I'

2 Weaver H A, Mumma M J, Larson H P & Davis D S,

Proceedings of the twentieth ESLAB symposium on theexploration of Halley's comet, Heidelberg, 27-31 Oct.1986, ESA SP-250 (ESA Publications Division, Nether­lands). Vol. J, 1986,329.

3 Sivararnan K R, Babu G S D, Shylaja B S & Raj MohanR, Astron &Astrophys (Germany). 187 (1987) 543.

4 Krishnaswarny K S & O'Dell C R, Astrophys J (USA),216 (1977) 158.

5 Joshi S C & Sanwai B B, Earth, Moon & Planets (Nether­lands), 39 (1987) 203.

6 Chandrasekhar T, Ashok N M, Debi Prasad C & Desai J

N. Opt Eng( USA), 27 (1988) 67.7 Ash<)k N M, Chandrasekhar T & Manian K S B, J Phys

E (GB), 20 (1987) 469.

(! Chandrasekhar T, Debi Prasad C, Desai J N &. Ashok NM, Proceedings of the symposium on diversity and similar­ity of comets, Brussels, 6-10 April 1987, ESA SP (ESAPublications Division, Netherlands), 1987,278.

9 Debi Prasad C, Chandrasekhar T, Desai J N & Ashok NM, J Astrophys &Astron (India), 1988 in press.

10 Niedner M B & Brandt J C, Astrophys J (USA), 223(1978)655 ..

11 Ip W H & Mendis D A, Astrophys J (USA), 223 (1978)671.

12 Chandrasekhar T, Debi Prasad C, Desai J N & Ashok N

M, Opt Eng( USA), 27 (1988) 1088.13 Alurkar S K, Bhonsle R V & Sharma A K, Nature (GB),

322(1986)493.14 Slee 0 B, Mc Connel D, Lirn J & Bobra A D, Nature

(GB), 325 (1987) 699.15 Ananthakrishnan S, Manoharan P K & Venugopal V R,

Nature (GB), 329 (1987) 698.

" n'I""'1 ,,'IIIIIl' I

(i) Helical wave structures in the ;:;omet plasmataiL

(ii) Hydromagnetic turbulence III cometaryplasma.

(iii) Speculations on the Oort cloud and its pos­sible links with interstellar matter.

(iv) Scattering properties of cometary grains.

10 Conclusion

The Indian astronomical community has partici­pated vigorously in the observations of the brigh­test and most well predicted periodic comet ofour times. A number of important results havebeen achieved. More will emerge when the analy­sis of the huge pile of photographic data is com­plete. Many new techniques and instruments weredeveloped for the Halley apparition which willprove to be of great use as and when a bright newcomet appears unheralded suddenly in theheavens.

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AcknowledgementFinancial assistance for many of the Halley ob­

servational programmes came from Departmentof Science and Technology and the Departmentof Space, which is gratefully acknowledged.

4 References1 Keller H U et al., Astron & Astrophys (Germany), 187

(1987) 807 .

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