On the Fabry-Perot investigations of the solar corona: Eclipse observations of large-scale dynamics

Adv. Space Res. Vol. 14, No. 4, pp. (4)45-(4)48, 1994

0273-1177/94 $6.00 + 0.00 Copyright© 1993 COSPAR

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ON THE FABRY-PEROT INVESTIGATIONS OF THE SOLAR CORONA: ECLIPSE OBSERVATIONS OF LARGE-SCALE DYNAMICS I r a i d a S. K i m

Sternberg State Astronomical Institute, Moscow State University Moscow, Universitetsla~i prospect 13, 1 I9899, Russia

ABSTRACT Eclipse data on the large-scale velocity field of the green coronal line obtained by a Fabry-Perot interferometer combined with a blocking interference filter are presented. Eclipse interferograms of the solar corona of 1972 and 1981 indicate that coronal conditions are relatively quiet ,but not homogeneous. Doppler shifts of 95 • profiles 4do not exceed 20 km s-'. Large scale motions with velocities of 10 km s-" are found for coronal regions. Effect of coronal rotation is evidently noted. Mean values of half width are are different for different coronal structures . In terms of "turbulent" velocities Vt equals 19.5, 22.5 and 27.5 in streamers, helmets and coronal holes respectively. A tendency for the decreasing of half width with distance in one helmet, possible increasing in holes and no evident dependence are found.

INTRODUCTION The structure of the inner corona is more evident in the light of the emission lines than in continuum. ~nission lines profiles obtained during eclipses give an unique opportunity to study coronal regions located above the EUV and X-ray observations and lower than white light space-born ones. The first successful eclipse coronal line observations by a Fabry-Perot etalon (FPE) /I-3/ gave an impetus to a n extensive emplo~nent in coronal researches of a FPE combined with an interference filter (IF). The E-corona interferograms status today

i n F e XIV 5 3 0 3 A a n d F e X 6374 A h a v e a c q u i r e d /~ - 1 4 / ~ The so-called "image-epectr~n" of

a trad~itio6A1 the E-corona

(the interferogram) corresponding to a multislit spectrum gives an instant picture of intensity, half width and velocity fields. However the interpreta, tion o f interferograms given by different authors does not result in a definite answer to physical parameters, especially to doppler shifts. This controversy may be explained both by different coronal regions under study and by the YPE demerits, which though known to everyone, are not always taken into account /15/. Instrumentation, observations, problems of calibrations and photometry, preliminary data on the green line intensity vs distance are discussed in /I I, 15-16/. Here data on {he large-scale velocity field of the solar corona of July 10, 1972 and July 31, 1981 are presented. LARGE-SCALE MOTIONS While interpreting interfero~'~ms, care h a v e b e e n t a k e n to select and remove ghost images, defects of films, white corona features, fixed pattern noise of image intensifier used in 1981. For the ~ ~ ~analysis only profiles with peak intensities more than 0.5 er~ cm-~s-'sr -" have been chosen. This level corresponds to the signal-to-graln noise ratio of 3. Correction for the instrumental width were applied to all FWH~ measured. Each photometric scan (Figure 1 ) represents a sum of a smooth background, narrow instrumental profile of the FPE representing by the reference Hg-He or Fel line and a broad gaussian coronal line of Fe XIV. The coronal line profile provided b y a FPE corresponds to emission of coronal features located within the proxiAe. Hence distortions would appear while steep ohsnglng of intensity from fringe to frir~e. Overlapping of the profile wings effects on the derived background level. Detailed comparison with the high resolution white and monochromatic corona pictures /17-18/ was made to select the white corona structures.

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(4)46

I.S.Kiln 0--

2-3--

Fixednois _

_ ~ 1G~m iiageh O s t

7

~

[ \

patternof imageintensifier / ~

~

Ghostimage

or

Fig. I. An exampAe pno~ome~rxo scan of the Fabry-Perot interferogram of the solar corona of duly 31, I981. The microdensitometry slit size corresponds to (20x38) arc sec.

D(str~but(on on the ucZue ol ~oppZe~ ue~oc(t~es. Sinoe the suooessful observations of JazTett and Kluber /2-3/ extremely diverse results on doppler velooities have been obtained by the Fabry-Perot teohnique /2-14/. Aooording to the most part of observers doppler shifts of the emission line were found to be small /3, 7, 10, 11/. The others .found about 10 % ooronal features with doppler shifts reaohing I00 km s'I/4/. A rstrospeotive analysis of the 10 July 1972 interferogram and oomparison with the results of 31 July 1981 were made. The set of fringes extending up to 2.1 Ro permit a oomparison of the derived and measured fringe radius. Departures from the OozTesponding order frir~ge position are oaloulated and represented as doppler velooity shifts after applying the appropriate value of dispersion: V = o 6×Ix = o r 6rlf 2, (I) where x = 5302.81 A aooording to Livingston and Harvey /19/ and 6k is the differenoe between h measured and x, r - the oomputed radius for k, 6r - the differenoe between the oomputed and measured radii, f - the fooal length of the oamera lens. The aoo,~uTaoy of doppler velooity determination depends mainly on aoouraoy of measurements of departures and assooiated oertainly with the value of dispersion and intensity variations aoross the profile. Random error arising from film grain and fixed pattern noise of image tube may be signifioant at low ooronal brightness~1 For o u r , e o l i p s e i n s t r u m e n t a t i o n the aoouraoy varies from 5 Ion s to 8 Ion s " depending on the fringe number. F" e 2 shows the B e e n emission distribution on the value of doppler shrifts. The oorona is relatively quiet. Doppler shifts are found to be small, More than 95 % profiles examined have doppler shifts not exoeeding 20 km s-i Botct(on.

The velooity distribution Figure 3 represents the moderate velooities of 10 looal large-soale motions

on position amgle for the whole oorona given in ooronal rotation and large-soale d.vnamios with a k~/s. The rotation effeot may be partly masked by with velooities of 6-15 km/s existing in the inner

oorona.

HaZl W~dth ol Green L~me (FWNM) D~str(btA~orA On h ~

Interpretation of Fe XIV emission line widths ~as o T = 3.843 x IO v (FWHM) ~, (2) where FWHM is in A, and T in kelvins. All the broadening is assumed to be oaused by thermal motion. The line shape in this oase will be Gaussian in the optioally thin oorona. (FWHM)

in terms of

w~Gth.

temperature follows

EclipseObservationsof Large-ScaleDynamics

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The value of half width of the green line obtained by different authors using the Fabr~-Perot teehnique varies from 0.6 to 2.2 A /4, 7-9/. The aoouraoy of line width determination is limited mainly by the grain noise of the film used and equals 10 % for 1972 and 5 % for I981 observations. The d/stribution for the whole oorona has been obtained with the mean value of YWHM of 0 . 9 2 A and 0.88 A (Figure 4) for the eolipsee 1972 and 1981 z~epeotively. There are 2.7 % profiles with ~ of 1.2+1.4 A. It should be noted that the level of prof~lelPe_~ for these profiles was oompaz~able with the grain noise (0.3 erg ore- o- st- ). The distribution for the whole •Or•ha seems to be the sum of distributions of different ••tonal Stl~Aotu~es.

n

i

i

I

+30L

120

'

'

'

'

'

'

I

'

I

I

I

I

'

I

I e

I

N=323

+20

ioo

Tm +I0

80

So•

60

"r"-"

g-IO,

40

•

•

•

el

•

-20 -30

I

IO

,

20 V, km s-I

Fig. 2. Green emission distribution on the value of doppler velocity (July 31,1981) -----7

T

T

l

,

!

|

90

|

I

f

I

)

i

i

270

180

p•

Fig. 3. Velocity distribution on position angle for the solar corona of July 31, 1981.

~--7

T

7--

I

l

I

I

I

I

I

n

60

N ~ 3 5 0

HELMET

( 1 9 8 1 ) ~ -

P;225 °

96 (1972)--50

@

@ •

40 30 20

•@o I

I0 i

J

20

I

STREAMERS

20 I

I0

HOLE

@

}~vIETS, N=52 P=300-350~

STREAMERS P=45-68 c

~ @

HOLES, N=31 ~=7 °, I55-180 °

0.8 1.0 FWHM, A Fig. 4. Distribution of the green coronal emission on the value of half width. ---- July IO, I972; - July 31, 1981.

I

p=70

I0

20

I

"I

N=70-

65_1P=275_290~ i0 ~

I0

,

[

rn_ -~-J

i

@

• •

I. I

I. 3

@ o•

a

@ •

•

@•@

I. 5

R/Re

Fig. 5. Dependence of half width on radial distance for different coronal structures.

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I S K[m

De[~ende~e on aod(c~ D£st~mce8. Information on maorosoopio motions in the solar oorona may be obtained from the half widths of ooronal lines. Existenoe of an additional meohanism of line broadening is suggested (maorosoopio ohaotio matter motions traditionally but not oorreotly oalled "turbulent" velooities) to explain ~different temperatures derived from the ionization theoz~ (Ti = I .3 x 10 ~ K) and observed values of half widths. A oonvenient expression for Vt is Vt = 0.13

• ~/(T

- Ti)/~

,

(3)

where ~ is the ion mass. Aooording .to the distributions ooronal~temperatures deduoed from lm~HM equal 3 x 10 ~ K in 1972 and 3.2 x 10 ~ K in 1981. In terms of "turbulent" velooities that means Vt = 22.5 km/s and 24 km/s in 1972 and 1981 respeotively. Aoeoz-dln~ to different authors ~ inoreases /4, 11, /, peaks at 1.2 Re /14/ or deoreases /21/ with distanoe. An attempt to find a dependenoe of half width on radial distanoe was made for different struotures (Pi~re 5 ). A tendenoy for the deorsasin~ in one helmet, possible inoreasing in holes and no evident dependenoe due to high data soatterin~ are found. 0~O~Sl0~

Doppler shifts are found to be relatively small. 95 % profiles have shifts oorrespondin~ to doppler velooities less than 20 km/s. Coronal rotation is evidently found at radial distanoes exeeedln~ I .5 Re. Lar~e-seale motions with moderate veloeities of 5 15 km/s always existing in the Lnner oorona san mask this rotation effect. In terms of "turbulent" velooities lamge-seale_1 ooronal struotures may be characterized by the value of Vt of 19-28 ~n s The above-mentioned analysis results us in oonolusion that for reliable interpretation of the ~abry-Pe~ot interfer~grams the high quality preliminary monoohromatizatlon should be used to rsduoe the white oorona oontrlbution. Detailed oomparison with the high resolution white and monoohmomatio oorona piotur~s helps to seleot and remove all instrumental effeots. Soattering light should also be reduoed. The referenoe speotra should be exposed during the all time of exposures.

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