Initial results of geocoronal Balmer Alpha observations

Planet. Spaec Sci. 1967. Vol. 15. pp. 1757 to 1775. Perganmn Press Ltd. Printed in Northern Ireland

INITIAL RESULTS OF GEOCORONAL OBSERVATIONS

BALMER ALPHA

B. A. TINSLSY Southwest Center

for Advanced StudiesDallas, Texas, U.S.A.

(Received in final form 29 June 1967) Abstract-Observations of Balmer Alpha in the night sky from May to November; 1965,have been reduced. After elimination of emissions due to astronomical sources and the effects of the Zodiacal Light the data were analysed in several co-ordinate systems. The smmner maxhnum in Bahner Alpha intensity can be seen to be due to the geometry of solar ilhunination, in fact the nighttime abundances are a little greater in the fall if the effective solar Lyman /J flux did not increase between summer and fall of 1965. There was no large change in abundance at sunset in the fall, and probably none in the summer. The abundances just before dawn were greater than in the evenings, by up to 30 per cent in the summer. Available radiative transfer models allow only a limited amount of comparison with theory, in which a number of approximations have been made. About 1Olp atoms/cm2 of hydrogen above 120 km are called for if a 1962 value for the effective solar Lyman p flux is used. This abundance agrees with models for lower escape rates in the cooler thermosphere at solar minimum. OBSERVATIONS

May, 1965, observations have been made of the Bahner Alpha (Ha) emission at 6563 A, to obtain the intensity distribution over the night sky and its time variations. The emission has been measured with a grille spectrometer located at 3240 m (10,600 ft) altitude near the Langmuir Laboratory (latitude 33’59’N, longitude 107”ll’W) and operated on clear nights when the Moon is below the horizon, from about an hour after sunset to about an hour before sunrise. A normal sky scan consists of readings every 30” or 60” in azimuth, at SO”, 60” and 40” zenith distances, and 120” azimuth intervals at 20” zenith distance, and a single zenith reading. The beamwidth was a square of side 5.5”. The principle and optical design of the grille spectrometer are discussed in Tinsley’s (1966) paper. For the Balmer observations the limiting system noise at about 1 c/s most of the time was about the value for photon shot noise. This arose from the continuum sky background passed through the interference’ pre-filter, of width 150 A at half intensity. However, noticeable low frequency fluctuations, with periods 10 set or longer, presumably arising from atmospheric transparency fluctuations (Rozenberg, 1966, p. 100) have been a serious source of noise on many occasions. The resolution was 2-5 A, and the instrument was set to scan a wavelength range of 9 A centred on the wavelength of Ha. The OH lines at 6553.8 A and 6569.3 A were thus avoided, but could be clearly seen when occasionally the scan was widened. The 9 A wavelength interval is scanned over six or eight times at each azimuth and zenith distance, and the periscope is held stationary while this is done. A complete sky scan takes about an hour. Since

CALIBRATION

After each sky scan an “efficiency” check and internal calibration are made. The efficiency check is necessary because the imaging of the spectrometer optics is not perfect, 1151

1758

B. A. TINSLEY

and because the unstable thermal environment of the site results in differential expansions in the spectrometer, which cause small relative movements of the image of the entrance grille on the exit grille. The ratio of the instrumental response when scanning across a narrow emission line with imperfect imaging, to that with perfect imaging is the efficiency. The change in light transmitted when scanning across a line with perfect imaging is exactly equal to the total light in the emission line transmitted when the spectrometer is set at a wavelength away from that of the line. Thus the efficiency can be measured quite simply by measuring the ratio of the deflection when scanning across the line, to the transient deflection obtained when turning off the source afterwards. An Atomic Laboratories Balmer Lamp is used, with scattering of the light on a distant object. The efficiency normally varies between 50 and 70 per cent. Following the efficiency check the grille is adjusted for maximum efficiency and a further efficiency measurement made. The internal calibration consists of a measurement of the instrument response to a fraction of the light from a small 1-5-V incandescent lamp run at constant current. The lamp provides continuum radiation, of which only that passing through the interference filter is detected. The lamp is turned on and off several times and the deflection noted. This internal calibration lamp is in turn calibrated once per night against an external low brightness calibration source. This external source is a box with an opal glass 6 in2 at one end, illuminated by a 115 V 50-W tungsten filament lamp with an opal bulb, behind a l-3 mm diaphragm at the other end of the box, about 1 m away. The lamp is run at a constant 105 V, and the surface brightness in Rayleighs/A of the opal screen has been measured at a number of wavelengths through the visible by the National Bureau of Standards. The transmission function of the interference filter was also measured by NBS. For data reduction it is necessary to take into account a geometrical factor (0.39) since the low brightness source did not fill the whole aperture of the field lens. This factor was measured by observing a distant broad source with and without a mask, with a hole the size of the opal screen, over the objective lens. DATA REIDUCTION

Figure 1 shows a typical recorder output of three azimuth-zenith distance settings. At the AZ. 180” Z.D. 20” setting four back and forth scans have been made over the Her wavelength. The zero level was drawn in by hand, as an estimate of the mean of the level on either side of the I&x position. This position was established from scans of the emission from the Hydrogen lamp, and the deflection (D,) from the mean level was read at this wavelength. From the above data the apparent brightness (RT) of the Balmer Alpha emission in Rayleighs is calculated, according to Rr = 0.39 x Q x D,

x D,/(D,,

x DI, x XT)

where Q = external source Q value (Gadsden, 1966) D, = deflection for internal source calibration at time of external source calibration DE0 = deflection for external source calibration D,, = internal source deflection interpolated to time of observation X, = efficiency interpolated to time of observation. The observations were all assigned a reliability rating, ranging from 1 for observations taken in very clear weather through 2, 3, and 4 for varying degrees of haze and thin or

INITIAL

RESULTS

OF GEOCORONAL

BALMER

ALPHA

1759

OBSERVATIONS

scattered clouds in the sky, to 5 for a cloud near or in the direction of observation. (Cloud positions were recorded in a log every hour during the observations.) Reliability ratings were changed to 5 during a later calculation if the solar depression angle at the time of observation came out to less than 13”. The reliability code makes it possible to assign weights to individual observations to allow a more sophisticated analysis than has been carried out in the present paper, All that has been done so far is to eliminate observations of reliability 5 from the data.

FIG. 1. TYI'iCALRECORDEROUTPUTFORTHREE AZIMUTH-ZE~DISTANCESE~GS, ~0 3:38 a.m. JuNg 29, 1965. ZODIACAL

3:32

a.m.

LIGHT

The apparent HE intensities were further corrected for the effects of the zodiacal light. The instrumental resolution was insufficient to separate the geocoronal emission line from the Fraunhofer absorption feature in the zodiacal light continuum. Further, the grille Consequently, spectrometer is not set up at present to measure the level of the continue. The differential brightness of the continuum in a correction was applied as follows. Ray~eighs~~ at 6563 A was derived from Table 4 of Smith, Roach and Owen (1965), using the solar spectral distribution (Johnson, 1965) and definition of S,, (vis) of Roach and Smith (1964) to convert the photometric units. A figure of 3.18 A for the equivalent width of the Fraunhofer IIcr feature in integrated sunlight was derived from the data of Evans (1940). The product of differential brightness times equivalent width was considered to be a suf-hciently good approximation to the necessary correction. While the width of the Fraunhofer feature recorded by the Grille Spectrometer in twilight was a little greater than the instrumental width as inferred from measurements on the Hydrogen lamp, the systematic and random errors jn the present data and in Smith, Roach and Owens data do not justify The data in their Table 4 were linearly ~nte~olated in ecliptic a higher appro~mation. co-ordinates to correspond to the ecliptic co-ordinates of each observation. The correction obtained was added to the intensity of the emission line after it had been corrected for extinction.

1760

B. A. TINSLEY EXTIN’CTION

Extinction measurements in the field were not made because they are time consuming, would have required additional instrumentation, and are not directly applicable to extended irregular sources of emission. To provide an approximate extinction correction Chamberlain’s plane parallel model with conservative scattering and finite ground albedo was computed (Chamberlain, 1961, p. 55) using data from Chandrasekhar and Elbert (1952). An extinction coefficient of O-09 per air mass was used, based on data for water vapour, ozone and dust extinction from Allen (1963). Taking values of ground albedo 1, of O-0,0-3 and l-0, Table 1 gives the multiplication factor needed to give the intensity corrected for extinction. The factors less than 1 for smaller zenith distances arise from the contribution of TABLE 1

a, = 0.0 & = 0.3 1, = 1.0

Zenith

Z.D. = 20”

Z.D. = 40”

Z.D. = 60”

Z.D. = 80”

o-95 O-92 0.87

0.95 0.93 0.88

o-97 0.97 0.90

1.03 1*Ol 0.96

l-46 1.42 1.33

scattered light from the source at large zenith distances. The effect of scattering from astronomical sources would be to reduce these factors in a very irregular way, being a function of the non-uniformity and movement of the sources in sky co-ordinates. The extinction correction applied to the data is that given in Table 1 for A,,= O-3. It was not considered worthwhile to attempt a more accurate extinction correction, in view of the complexity of the calculation and the magnitude of the improvement probably being smaller than other systematic and random errors. ASTRONOMICAL

SOURCE CORRECTIONS

It was known from previous work that there were astronomical objects such as gaseous nebulae with emission in Ha up to hundreds of Rayleighs, extending up to tens of degrees across the sky. The brighter of these were easily recognizable in the data as very high readings. A programme to remove these sources from the data was developed with three tests as follows. Test 1 A grid with spacing of 3’ in declination and 0.2 hr in right ascension was considered to cover the whole celestial sphere, and at each point of intersection of grid lines it was noted whether or not there was an astronomical Ha source within a 3” x 0.2-hr rectangle centered on that point. The maps of astronomical Ha sources given by Montbriand, Tinsley and Valiance Jones (1965) were used as reference. The regions within the heavy dashed lines were also considered as sources for the purpose of this test, as were the regions within 6” of the recorded edges of the bright sources in Orion, Cygnus and Ophiuchus. The regions out to 15” from these three sources were included in a second list. The right ascension and declination of the direction in the sky corresponding to the azimuth, zenith distance, and time of each observation were claculated. Then a test was made to see if any of the grid points associated with astronomical sources fell within 6” of the declination, and within 0*4/cos 6 hr (corresponding to 6” of arc, where B is the declination) of the right ascension. If for any observation, a grid point did fall within these limits,

INITIAL RESULTS OF GEOCORONAL

BALMER ALPHA OBSERVATIONS

1761

the observation was removed from the data. Observations were also removed that drew a point on the second listing, if the reliability was 2, 3 or 4. Ha sources fainter than about 60 R and possible unrecorded Ha sources far from the Milky Way had to be guarded against, and further internal tests of the data were made. Test 2 To check that some of the brighter observations were not due to astronomical sources, all those brighter than the arbitrary value of 9 R (before correction for extinction and zodiacal light) were compared with all other observations for several months earlier and later. If there was any case where there was an observation brighter than 9 R, and there were no other observations within 3” of arc in right ascension and declination fainter than 9 R, then these were singled out for further examination as possible astronomical sources. Test 3 Another test was to compare each observation greater than 9 R with all other observations made within 90 min in time and and within 40” in zenith distance and azimuth angle. If the observation was more than double the brightness of the next brightest within that range, then the observation was singled out for further examination as a possible astronomical source. A number of observations singled out be Tests 2 and 3 were examined, but in the end none were rejected, since the observations were found consistent with the distributions based on the remaining data. Evidently, any new sources of the order of 5” in extent and of brightness 9-60 R were sufficiently close to the recorded sources near the galactic plane to be eliminated by Test 1, or else happened to lie on declinations other than the ones corresponding to the azimuth-zenith distance settings used. Later examination of better data from the winters of 1965-6 and 1966-7 showed, however, the existence of a low brightness source of 4-8 R between declinations + 12” and -lo”, and between right ascensions 3~2~and 5h. This is near the emitting regions associated with the Great Nebula in Orion, and may be an outer extension of the complex. The observations made looking in this area were deleted from the data. They were mainly morning fall observations at zenith distances 40” and 60”, in the southeast. We cannot suppose that all effects of astronomical sources have been eliminated from the data, especially where scattering on very thin clouds or aerosols has been a factor. The data are certainly valid as an upper limit on geocoronal emission, and although some of the random fluctuation in the distributions are undoubtedly due to the emission from astronomical sources, the spatial and time variations show clearly the remaining effects of such sources are minor. ECLIPTIC

CO-ORDINATE ANALYSIS

The data were Crst averaged in ecliptic co-ordinates since the earlier results of Shcheglov (1964) had been presented that way. However it was suspected that like sodium and other emissions in twilight, the scattering of solar radiation would be associated with a strong dependence on solar depression angle. Figure 2 is a comparison of plots of intensity vs. ecliptic co-ordinates for two ranges of depression angles. The histograms standing on marked co-ordinate points compare the intensities for depression angles greater than 30” (hatched) both before and after midnight, with morning depression angles less than 30” (unhatched). The data are for the period Aug. IO-Nov. 4, 1965.

1762

B. A. TINSLEY

u&l

fa

Ll

d

a

B fi

r4 I Pa

ed -50

-78

I 0

I4i

30’

60.

90.

120’

ECLIPTIC

150

LONGITUDE

160’ MINUS

210’

240.

LONGITUDE

270. OF

300’

330’

3 ;0*

SUN

FIG. 2. &hWARLWN OF INTENSITY VS. ECLIpnC COORDINATES FOR DEPRJSXON ANGLES GREATER THAN 30” (HATCHED),AVERAGED BEFORE AND AITER MIDNIGHT, WITH DEPRESSION ANGLES LESS THAN 30” AFTERMU)NIG~ (UNHATCHED). Data for Aug. lO-Nov. 4, 1965. The part of the intensity up to the horizontal line is the amount added to correct for the zodiacal light Fraunhofer line. The data has been corrected for extinction and zodiacal light as previously described. The zodiacal light correction is a function of ecliptic co-ordinates only and so does not alter the differences between the two distributions. As can be seen, the intensities are greater for the smaller depression angles in thirty-three out of thirty-nine cases, showing the dependence on terrestrial rather than ecliptic parameters. The amount of the zodiacal light correction is indicated by the horizontal line across the hatched and unhatched histograms. The whole amount of the histograms represent the corrected intensities, and the amount up to the horizontal line represents the correction added. The correction is important only in a narrow zone along the ecliptic equator, close to the longitude of the sun. AZIMUTH-ZENITH

DISTANCE-DEPRESSION

ANGLE

ANALYSES

The data have been plotted as functions of azimuth and zenith distance, for various solar depression angles through the night. A schematic of the depression angle variation with season is shown in Fig. 3. 240

300

270 \

\

\\

330

DEPRESSION=IJ*

0

60

30 0 _$I

$

60

100

A

FIG. 3. VARIATION OF SOLAR DEPRESSION ANGLE AND SOLAR ORSERVATION SITE.

Dots mark off 20”-hr angle intervals.

/

AZIMUTH

//

WITH

SEASON

FOR

INITIAL RESULTS OF GEOCORONAL

1763

BALMER ALPHA OBSERVATIONS

Because of the effects of the Fraunhofer feature in the morning twilight, data for solar depression angles less than 13”, as mentioned earlier, were eliminated from the analysis. Effects of twilight may still be present for depression angles 13-15”, and the intensities for these angles should be regarded as lower limits. For depression angles greater than 15”, however, it is clear from Rosenberg (1966, p. 24) that the brightness level is essentially that of night sky emissions.

t

I

18W

2100

*

2w

2700

I

300-

,

330°

f

O-

I

JO0

1

6cP

8

900

1

120"

I

IV

t

I800

F1ci.4. DLWMLWTION FOR WLU

OF BAWAR ALPHAASAFIJN~TIONOFAZ~.~~~ANDZ~DISTAN~JI, DEPRESSION ANGLE 2.5” TO 30” (B.L.M.), MAY 20 TO AUG. 6, 1965.

Data for depression angles before local midnight (B.L.M.) have been separated from data for the same depression angles after local midnight (A.L.M.). The averaging intervals were over j, 15” in azimuth and f 10” in zenith distance (although nearly all observations were at the tied zenith distances of 0”, 20”, 40”, 60’ and SOO). In depression angle the averages were over f2-S” and f5”. Summer data for 20 May-6 Aug. are averaged separately from fall data 10 Aug.-4 Nov. 1965. Even with this averaging the quantity of output is large. Figures 4-9 show sample results, for evening, middle night, and morning data for summer and fall. The complete set of the same results are tabulated in Tables 2 and 3, which also give in parenthesis with each average, the number of observations averaged for that interva1. The convention used for azimuth angles is north = 0”, through east = 90”, south = 180” and west = 270’.

FIG.%

As FOR FIG. 4, BUT DEPRESSION ANGLE 30” TO 40” (B.L.M.).

B. A. TINSLEY

18cP

210*

2409

2709

3ov

330°

0“

30'

fiG_ 6.AS FOR fiC3.4,BUT D8PRE88ION ANGL8

, 18cP

2w

I

24W

I

2709

I

3000

I

330*

I

09

FtG. 7.As FOR F%G.~, BUT DEPRRSION ANGUS

I

JO0

600

SO"

1200

I500

180.

30'TO 25" (A.L.M.).

I

6W

I

90"

I

1200

I

1500

\

‘18W

20"TO 30" (B.L.M.) AUG. 10 TO Nov. 4.

8CP 18cP

2100

24W

27W

3OCP

330*

Q

foe

600

90"

12cP

Fro. 8.AS POR Fro. 4, BUT QEPRiSSION ANcXI? 60" To 70' TO 60" (B.L.M. AND

Aucs.10TO Nov.4.

l6Q

180

A.LM.),

INITIAL

FIG.

9.

hi

RESULTS

OF GEOCORONAL

BALMER

ALPHA

FOR FIO. 4, BUT DEPR@SSION ANGLE 30” TO 20” (A.L.M.)

Auci. 10 TO Nov.

TABLE2. DISTRIBUTION OF BALMAR ALPHA EMLWON IN AZIMUTH AND ZENITH DISTANCE FOR vicious SOLAR DEPRESSION ANGLES a, FOR 20 MAY TO 6 AUG., 1965 Azimuth

210°

240°

210°

300° a =

330”

o”

13”-15” B.L.M.

30°

60°

1765

OBSERVATIONS

90°

4.

COORDINATES,

120°

15o”

180’

May ‘LO-Aug. 6, 1965

Z.D. O0

2o” 4o” o fro a = W-20’

Z.D.

2::

7*9(l) 11*3(2) 12*9(2) 18*7(3)

40° 60’ 80’

B.L.M. May 20-Aug. 6, 1965

9*8(l)

10*1(2) 12*2(3) 13*3(3)

:8:$; 26*0(2)

a = 20”-25O B.L.M. Z.D. O0 2o” 4o” 60” 80’

12*7(l)

8*2(5) 13*7(2) 15.3(2) 19*8(l)

14*8(l)

7-6(4) 11.8(4) 14*9(3)

16*4(l)

8*6(2) 7*4(4) 8*2(5) 14*3(3)

8*4(2) 7*9(l) 9*4(2)

10*4(2) 7*3(3)

i3+1{2j 8.7(2) 13*5(2)

1?9(2)

7*4(2) 1l-O(6) 12*6(7) 23*8(7)

5.4(8) 5*7(10) 8*1(g) 1l-6(8)

10*3(l)

11*0(l)

8*8(2) 7*3(4)

11.9(3) 7*6(3) 14.5(1

j

May SO-Aug. 6, 1965

7*2(5) lWl(3) 8*5(5) 12*8(6) 25.6(3)

a = 30“-40” B.L.M. 5.2(8) 7*1(8) 6*9(6) 10*5(5)

9*3(2)

May 20-Aug. 6. 1965

8*3(4) 10*2(2) 11*5(4) ;;;i;;

a = 25’-30’ B.L.M.

Z.D. O0 2o” 4o” 60°

14*2(l)

15*2(l)

3*4(l) 9.8(l) 8*5(2) 14*7(5)

2*4(l)

5*9(3)

9*3(4) 9*3(5)

9*0(l)

5.1(2) :‘W;

5.4(4) 6.7(l)

7.9(51

9.30)

May 20-Aug. 6, 1965

5*1(8) 7.3(8) 9*0(10) 11*4(S) 14-l(4)

5.5(4) 10.4(l) 9*6(4)

7*4(2) 8*0(l)

1766

B. A. TINSL.EY TABLE2 (c~ti.)

Azimuth

210°

240’

270°

3oo”

330°

o”

3o”

60”‘

90”

120°

150”

180’

CL = 40”-30” A.L.M. May ZO-Aug. &I965 7*1(S) 7.2(9) 7.4(2) 10*2(2)

6.40)

5*8(g) ;f;;;) 16*7(4)

7-S(6) 7*1(S) 9*7(9) 14*0(5) 24*4(6)

9*7(4) 10*1(6) 11+3(4) 18*7(4)

8*2(S) 6*7(10) 7*7(6) 9*6(4)

7*0(5) 7*5(6) 7*1(S) 10*3(3)

cc= 30*-25’ A.L.M. May 20-Aug. 6,196s 5*9(2) 9-S(6) 10*9(4) 11*9(3)

7*5(2) 8*2(2) 11*0(6) lFS(6)

i:?$] 11*0(7) I&5(6) 24-O(5)

8*1(l) 6*3(2) 12&3(3) 23+(3)

6*3(3) 10*6(g) 9*8(S) 19*6(3)

5*6(3) 7.4(S) 7.1(5} 12*3(4)

9-O(5) 13*0(i) 13*7(3) l&6(3)

S-7(5) 12*2(l) 10*4(4)

i1*5(4) 13*6(7) 16-S(3)

:q:j 10*3(4)

CL = 2.5”--20’A.L.M. May 20-Aug. 6, 1965 9-i(3) 1@6(3) 11.3(l) 10*6(3)

‘% 10.8(2) 12*9(S)

:::;I:; 13*6(S) 23.7(S)

a = 20°-15’ A.L.M.

1%

12.7(l) 1I*2(2) 128(l) 25*1(l)

9*8(2) 1S+(2) 31*9(2)

May PO-Aug. 6.1965

:z$:f 18*3(3) 36*0(l)

123(4) 18*6(4) 18*6(2)

a = 15°-130 A.L.M. May 20-Aug. 6,1965 9-S(3) :i::::j 15*4(l)

10*3(2) 15*3(l) 26*6(l)

14*8(l) 15*3(l)

I 2&4(2)

11*7(3) 11*9(l) 15*3(2)

The total number of observations for the period May 20-Aug. 7 was 1272, of which 362 were rejected by Test 1, and a further 194 were rejected because of reliabihty rating of 5. For the period Aug. IO-Nov. 4 the corresponding numbers were 1812,539 and 87. D~T~C~D~~ON ANGL3X ANALY!iBS SOLAR ~-~ Analysis has been made as a function of solar depression angle (a) of the brightuess near the vertical plane through the Sun, and near the vertical plane perpendicular to this. Graphs are presented of intensity as a function of depression angle for various zenith distances in the azimuth of the Sun, and for azimuths separated by +90”, 3-180” and -90” from the solar azimuth. The summer data are contained in Figs. 10-13, and the fall data in Figs. 1417. The averaging was over &30“ in arc (i.e. ~30°~sin(~~th distance) in azimuth). Smoot~g has been carried out in the fall data by a least squares fittiug of a fifth order polynomial to the data for each zenith distance, for a > 15”. R.M.S. deviations from the polyuomials of data averages in f25” intervals were 2.4, 1.2, 0.9, 0.8 and 0.8 R for the curves in the azimuth of the Sun, for zenith distances 80”, 60”, 40”, 20” and 0”. The curves

INITIAL RESULTS OF GEOCORONAL

1767

BALMER ALPHA OBSERVATIONS

TABLE 3. DJSTRWTION OF BALMAR ALPHA EMISSIONIN AZIMJTH ANLI ZENITH DISTANCECO-ORDINATES, FOR VARIOUSSOLARDEPRESSION ANGLES a, FOR 10 AUG. TO 4 Nov., 1965 Azimuth

210°

240”

270’

3oo” a =

Z.D. O0 2o” 4o” 60’ 80’

19*4(l) 31.1(l)

330”

15’-20’

0’

B.L.M.

18*4(2)

a = 20°-30’

B.L.M.

10*6(3) 12*9(3) 17*5(l) 20*8(l)

13*8(l) 17-S(3) 27*7(2)

11*0(3) 12*3(4) 16*2(6) 24*2(3)

9.2(2)

11-l(3) 10*1(l)

10*8(l) 11*9(l)

7*3(3) 11*3(S) 11-l(7) 16*1(4) a = 40°-50’

Z.D. O0 0 6.7(3)

4*8(l) 5*7(l) 1@7(2)

16.2(l)

S-4(11) 9*4(10)

a = 50”-60’ Z.D. O0 0 6*6(2) 4.8(l)

5*8(5) 5*6(4) 5.9(3) 12*2(l)

7*0(2) 10*6(3) a = 60”-70”-60’

IO-Nov.

HO0

180°

8*6(2) 176(l)

9*4(3) S-4(5) 15*3(2)

4, 1965

13.1(2)

4, 1965

11.7(6) 14.5(5) 17*5(2)

B.L.M.

7*6(2) 18-S(5) 15.5(l)

Aug. IO-Nov.

10*3(7) 10*7(g) 16*5(4)

8*2(l)

B.L.M.

7*0(l) 9*2(2)

IO-Nov.

8*1(12) 1;:;;;

B.L.M.

Aug.

7*7(l)

IO-Nov.

9*6(5) 9.9(5) 8.6(3)

4, 1965

7*4(4) 10*1(7)

Aug.

6*6(2) 9.5(l)

8.5(7) 7*3(11)

9-l(2) 8*3(S)

6*0(14) 6*2(11) 7*9(11)

5*4(3)

6*8(11) 6*0(11) 7.1(10)

4.3(2) 5*4(l)

2*7(2) 46(3) 3.9(3) 7.0(5)

5.3(2) 5*5(l)

4, 1965

FO(5) S-9(8) 10*2(6)

6*7(3)

4, 1965

8.0(2)

5.6(4) &3(3) 9*8(5)

B. & A.L.M.

8*0(l) 8*0(3) 8.9(2) 7.9(l) Aug.

5.0(3) 8.8(l)

IO-Nov.

4*3(4) 3*7(4) 5*3(3) 8.8(5)

4, 1965

4*0(l)

3.7(l)

3.4(3) 4.3(2) 5.6(2) 6*2(4)

3*3(3) 4.6(2) 4.2(l)

6.9(2) 10.4(2) a = 60”-50’

Z.D. O0 0 :x0 60’ 80’

Aug.

120°

4.7(3)

Z.D. O0 0 % 60’ SO0

90’

12*1(2) 6*7(l)

iiG3j

10.6(l) 16-O(4) 22*6(3)

a = 30°-400

% 60” 80

60’

11*2(l)

2:: 40° 60’ 80’

ii0 60” 80’

Aug. lO-Nov.

16-l(2) 26*4(l)

Z.D.

Z.D. O0 20” 4o” 60’ 80’

30°

A.L.M.

6*2(2) 9*5(3)

6*9(l)

3.9(2) 3.4(2) 6*5(3)

5.5(4) 6*5(3) 7*3(5) 11*3(l)

6*8(3) 6*3(l)

3.8(2) 3.8(3) 4*2(7) 4.4(5)

6*9(16) 75(15) 10.2(S) 9*9(2)

5*2(l) 9*7(l)

6*8(11) S-3(6)

4*2(2)

4.8(l) 76(3)

Aug. lO-Nov.

4, 1965

3.7(2)

4*2(6)

4.0(4) 4*2(5) 4.4(6) 3.9(5)

3.3(4)

3=9(l) 8*1(5)

a = 50°dOo Z.D. O0 0

7-O(3)

A.L.M.

6*3(4) 5.2(6) 9*4(6)

5*6(5) 16*1(l)

Aug. IO-Nov.

6*3(l) 6*1(l) 5*4(5) 12-S(7)

8*1(4)

4, 1965

6.4(9)

% 5.4(4) 9*1(l)

6*4(17) 5*1(16) 5*3(S) 7*2(3)

4*0(2) 7*9(l)

5.1(2) 6*2(6) 7*6(7) 8*5(l)

8*0(16) 9*4(10) 12*7(3)

S-4(3)

S-5(14) 7*5(12) 11*2(4) 1F2(2)

12*5(2)

1768

B. A. TINSLEY TABLE3 (cont.)

Azimuth Z.D. O0 2o” 40° 60’ 80’

210’

240°

210°

3oo”

330°

o”

30”

60°

9o”

120°

l5O0

180’

a = 40’~30’ A.L.M. Aug. lo-Nov. 4, 1965 7*7(9)

5*9(l)

6*4(8) 6*9(8)

5*8(l)

8.7(l) 10*7(9) 12*4(7) 8.3(l) 15*6(2) 25*3(l)

9*5(a) 6.8(l)

S-5(6) 6*7(2) 17-O(2) 11*1(l) 22*2(2)

9*6(7) 10-O(6) l;i;;]

7*4(l)

8*7(7) 6*9(7) 7-l(8) 16*2(l)

0: = 30°-20’ A.L.M. Aug. IO-Nov. 4, 1965 Z.D. O0 0 % 60’ 80°

7*2(2) 10*4(l)

8.6(8) 8*9(9) S-8(7) 8*2(4)

11*2(8)

11*2(7) 4*7(2) 13*7(l)

‘Q:‘,:{

12*1(l)

*,

11*7(3) 11*3(7) 10.6(7) 13-l(2) 19*1(l) 12*3(l) 16*1(7) 10.0(8) ;:;(S; 11*6(3) 19*3(S) 19*9(3) 12-O(2) 1Ow) 24*1(l) 341(l) 18*2(l) 11*2(2)

a = 20°-15” A.L.M. Aug. lO-Nov. 4, 1965 Z.D. O0 20° 40° 0 %

12*2(3) 9*0(2) 10*9(4) 9*1(2)

13*1(2) 6*8(l) 23*6(5) 18*3(8) 13*3(l) 15*4(2) 31*3(l)

:::;I# 1l-7(3)

8*9(l) 23*4(6) 23*4(l) 31*4(3) 53.8(l)

lGO(2) 11*7(l) 27*5(l) 23*2(2)

14*2(5) 18*8(2)

a = 15’=-13”A.L.M. Aug. IO-Nov. 4,1965 Z.D. O0 0 % 60’

12*5(6) 122(2)

16*7(2)

16*7(4) 169(l)

10*2(l)

1l-9(3)

22*1(l) 10*0(l)

19.6(2)

11*1(l) 16-l(5) 19*0(l)

164(l)

28 26 -

ZENITH

242220 18 g

16-

(I Ilf 145 K IZto 664eBEFORE LOCAL II I Izr IS@ 2v

O-

SOLAR

MIDNIGHT I 3w DEPRESSION

INTHEsoLARAzlMum, FIG. 10. BRI~~TNFSS ZENITH

DISTANCES

VS.

AFTER I 3w

I 40.

LOCAL MIDNIGHT I I,’ 20. 16. 12.0

ANGLE

SOLAR

DEPRFSSION

MAY 20 TO AUG. 6, 1965.

ANGLE

FOR

VARIOUS

~~L~SUL~

OF GEOCORONAL

0

II I3 13

Fm. If.AS

FOR

Fich

BALMERALPHA

1

6

I

30

40

x,

1 20

10,

BUT

M)R

OBSERVATIONS

I J 20

SOLAR

AZIMUTH

-@o”.

40

30

20

15 13

28 25

ir

13

Fm.12. As

FOR

I3

I

I

20

30

Fm.10, BUT mzt SOLAR

AZIMUTH

f 10 13

3-1803

I769

1770

B. A. TINSJJZY 30 26 26 24 22 20y

IS-

II y

16-

t Gc 1412 IO66-

0

II 13 IS FIG.

13. AS

FOR

I 20

nG.10,

I 30

I

I

,

I,

40

30

20

IS I3

BUT FOR SOLAR AZIMUTH

--9o".

A 38

g 0 y r 2

-

36

-

34

-

32

-

30

-

26

-

26

-

24

-

22

-

20

-

IB16 14 12 IO 664-

BEFORE IS

2v

LOCAL 3v

MIDNIGHT 40=

AFTER XT SOLAR

FIG. 14. As

FOR

I I 60’69

I 60.

SO=

DEPR266lON

ANGLE

LOCAL 40.

MIDNIGHT 30’

2w

FIG. 10, SOLARAZIMUTH;BUTAUG. 10 TO Nov. 4.

I I IY 13.

INITIAL RESULTS OF GEOCORONAL BALMJZRALPHA OBSERVATIONS

*

32 30 28 26 24 22 20jj

l8-

9 ‘65 141210864-

:: Fro,

is* w w w 500 w65*60* f&r 40. 30. 2o*ww 15. As FOR FIG. 10, BUT FOR .%&ARAZIMUTH+!W, AUG. 10 TO Nov. 4.

28 26 24

0

I 19

I

I

I

I

200

3v

4v

XT

FIG.

I

I

I

I

I

60 65. ar

w

40

w

m

I

I

16. As FOR FIG.10,BUT FOR SOLARmm-m

+lSO*,

II IY 13.

AUG. 10 TO Nov.4.

1771

1772

B. A. TINSLEiY

36 34 32 30 26 26 24 22

14 12 IO 6 6 4

FIG. 17. As FORFIG. 10, BUT FOR SOLAR m

-!W,

AUG. 10 TO Nov. 4.

for the other three azimuths had nearly the same values. The polynomial regression technique proved unsuitable for the summer data and they have been left unsmoothed. DISCUSSlON The present results may be compared with past measurements of Ha in the night sky, e.g. Prokudina (1959), Kvifte (1959), Gaynullina and Karyagina (1960), Dufay and Dufay (1960), Fishkova and Markova (1960), Dufay, Dufay and Nguyen (1961), Fishkova (1963), Ingham (1962), Shcheglov (1964). All these measurement were very much more limited in their coverage of time and spatial variations during each night than the present observations, and a number of signiscant points now emerge. First, the variations with solar depression angle and with direction relative to the Sun leave no excitation mechanism open other than fluorescence of the geocoronal hydrogen in solar Lyman t!I. Secondly, the seasonal variations discussed by Fishkova (1963) and by Krassovsky et al. (1965) can be seen to be a result of the changes in the geometry of the solar illumination. The summer maximum of intensity occurs because at Abastumani (Lat. 41”44’N. Long. 42’51’E) the solar depression angle averages a much smaller value (-20”) during the summer nights than in the winter (MW), (as is the situation with about 10” greater depression angles at the Langmuir Lab.). In fact, by comparing Figs. 10 and 14, the intensities at corresponding depression angles can be seen in the present data to average a little higher

INITIAL RESULTS OF GEOCORONAL

BALMER ALPHA OBSERVATIONS

1773

in the fall than in the summer. If there has been no significant increase in Lyman /I between summer and fall of 1965, the overall nighttime abundances must have been greater in the fall. Thirdly, the present results give information on the way in which the hydrogen is distributed around the Earth. It can be seen that the morning intensities are up to 30 per cent greater than the evening intensities at the same depression angles. If there is no large change in the hydrogen abundance at sunset, as appears to be the case for the fall data at least, since the morning curves have the same shape as the evening curves, one may conclude the amount of hydrogen in a vertical column was greater just before dawn than in the evening. This is consistent with Shcheglov’s (1964) results, although his measurements are not referenced to solar depression angle, and are along the ecliptic which is inclined at different angles to the horizon in the evening and the morning; thus his morning and evening results were not necessarily taken under equivalent conditions of solar illumination. The curves in a plane through the zenith perpendicular to the plane through the Sun should show departures from a spherical distribution of hydrogen around the Earth. There are indications (Figs. 11, 13, 15 and 17) that as well as the greater easterly (morning) intensities, those in southerly (equatorial) directions are greater than in northerly regions, but the effect is marginal. The strong enhancement of intensity towards the plane of the ecliptic, reported by Shcheglov (1964) is not present in these results. Donahue (1964) suggested that this enhancement might be an artifact of the observation program. Fourthly, the present results are suitable for comparison with detailed radiative transfer calculations of model hydrogen distributions, with adjustments made to the model until a good fit to the observations is obtained. The analysis of Donahue (1964) may be used as a starting point, although some caution should be observed when referring to this paper. It can be shown that the method of scaling column emission rates discussed by Donahue on pp. 152-3, using the ratio gs,/gZ,, and two higher order scaling factors to account for the non-absorption of Hcc and for non-conservative scattering in Lyman /?, leads to an error by a factor of approximately 6 in the final emission rate. This is because g,, and g,, refer to scatterings per atom, and the scaling is being made for the same optical thickness in La as in L/3, rather than for a given number of atoms. The worked example on p. 152 gives a Ha column emission rate too low by a factor of 6. According to Donahue (private communication) however, the results in his Figs. 2 through 6 were not in fact obtained by this method, but from a set of new computations of source functions using programs developed by Thomas (1963). The source functions were integrated along the line of sight (omitting the term for self absorption, but introducing estimated correction factors for non-conservative scattering in Lyman p) to give the final column emission rates. This method should give reasonably accurate results were it not for the fact that the method of calculating source functions (Thomas, 1963) was developed to allow interpretation of rocket observations, above 120 km, in Lyman a. It treats the region with large hydrogen density below 120 km somewhat like a mirror reflecting 40 per cent of the incident radiation and gives good results when looking upward from the 120&m level, but should not be applied without moditication to ground based Ha observation because it neglects the Ha emission from the region below 120 km. A further caution concerning using Donahue’s (1964) curves in interpreting the present results in that they are derived for a hydrogen distribution corresponding to a temperature of 1250°K all the way down to 120 km. This is an approximation to a real distribution, and

1774

B. A. TINSLEY

also the temperature is considerably higher than what one might expect near solar cycle minimum (Jaccia, 1963). The Lyman /? flux at the center of the solar line probably varies with the solar cycle, as discussed by Donahue, who used g,, = 3.4 x lo-‘, based on 1961 and 1962 measurements. Whether we can use this flux for 1965 is questionable. A greater effective flux in 1965 would mean that Donahue’s curves should be increased for comparison with the 1965 results, a smaller effective flux would reduce them. Donahue’s Fig. 4 refers to zenith distances of 80”. Solar elongations of 25” to 80” refer to depression angles of 15” to 70”, before local midnight, with observations in the azimuth of the Sun. Elongations from 120” to 190” refer to depression angles from 70” to 0”, after local midnight, with observations of solar azimuth +180”. Figure 10, 12, 14 and 16, for zenith distance 80” in the summer and fall of 1965 show corresponding observations. (Unfortunately these are the least reliable of the observations, owing to a large extinction correction and fewer observations than at other zenith distances owing to horizon cloud. The summer curves for 80” zenith distance do not show the generally smaller emission rates than the fall data at corresponding depression angles.) A comparison between the observations and Donahue’s Fig. 4 shows that there is approximate agreement with the shape of the curves for 7,, in Lyman /? = 0.8 or 1.3, for the case of no abundance change at sunset, but the intensities are higher by a factor of 10 than given for TV= 0.8, or a factor of 6 higher than the TV= 1.3 curves (48 x lols atom/cm* above 120 km). One might allow a factor perhaps 2 for the contribution to the Ha emission from regions below 120 km and obtain l-2 x 10r4 atoms/cm2 column above 120 km as a very rough estimate of the hydrogen abundance in the latter half of 1965. This abundance would not be inconsistent with the hydrogen abundance given by Johnson (1965, p. 10) for sunspot cycle minimum, nor with the revised theory of escape (Liwshitz, 1967), nor with the suggestion of 5 x lo5 hydrogen atoms/ems at 1000 km in 1964-65 (Brace et al., 1967). However in view of the uncertainties that have been discussed, this conclusion must be regarded as very preliminary. Very recently Armstrong (1967) has published results of observations made in 1965 Australia. The measurements agree with the present results to the extent of the large scale variation with solar depression angle. His equipment was unfortunately rather susceptible to the effects of extraneous sources of light. The shape of his curves for westerly intensities in morning differs from the present results (Fig. 16), and in the present results the westerly morning variation is very similar to the easterly evening variation (Fig. 16), so that such a large asymmetry as in his Fig. 9 is not supported. His arguments for his Fig. 9 were based partly on comparison with the curves of Donahue (1964) and the cautions mentioned above should be considered in this context also. A set of new detailed radiative transfer calculations are called for, with curves for a number of zenith distances, azimuths, and abundances, and for models with a range of temperatures down to about 600°K. Such calculations should be of considerable help in resolving the present uncertainties, and with or without contemporary solar Lyman B data, we should be able to use the observations to interpret further the nighttime abundance and distribution of hydrogen. considerable help and encouragement has been given by Dr. F. S. Johnson in setting up the observation programme and during the course of the analysisof the results. Most of the observing has been done by Mr. H. T. Goh and A. A. Clarke, and most of the data reduction by R. A. Bohlander

Acknowledgements-Very

INITIAL RESULTS OF GEOCORONAL BALMER ALPHA OBSERVATIONS

1775

and W. C. Penrod. Discussions with Dr. Beatrice M. Tinsley have been helpful. This research has been supported by the Atmospheric Sciences Section, National Science Foundation, N.S.F. Grant GP 3950. REFERENCES ALLEN, C. W. (1963). Astrophysical Quantities. (2nd ea.). Athlone Press, London. hMSTBON0, E. B. (1967). Planet. Space Sci. 15,407. BRACE,L. H., RBDDY, B. M. and MAYR, H. G. (1967). .I. geophys. Res. 72,265. CHAMBERLAIN, J. W. (1961). Physics ofthe Aurora and Airglow. Academic Press, New York. CIWNDRASEKHAR, S. and ELBERT,D. D. (1952) Astrophys. J. 115,269-278. DONAHUE,T. M. (1964). Planet. Space Sci. 12,149-159. DUFAY, M. and DUFAY, J. 1960. C. r. hebd. S&znc. Acud. Sci., Paris 250,41914193. DUFAY, J., DUFAY, M. and NGUYEN, HUU-DOAN (1961). C.r. hebd. S~unc. Acud. Sci., Paris 253,974-977. EVANS, D. S. (1940). Mon. Not. R. ostr. Sot. 100, 156-179. FISHKOVA,L. M. and MARKOVA,G. V. (1960) Astr. Circ. USSR 208,14-17.

FISHKOVA, L. M. (1963). Aurora and Airglow 10, Results of IGY, USSR Acad. Sci. 35-39. GAYNULLINA, R. H. and KARYAGINA,Z. V. (1960) Izv. Astrophys. Inst. Kazakh. SSR Acad. Sci. 10,52-63. INGW, M. F. (1962) Mon. Not. R. astr. Sot. K&523-532. JACCIA,L. G. (1963). Space Res. IZZ, pp. 3-18. North-Holland, Amsterdam. JOHNSON, F. S. 1965. SatelIite Environment Handbook (2nd ed.). Stanford University Press. KWWWSKY, V. I., SHBFOV, N. N. and VAISBERG, 0. L. (1965). paper delivered at Smithsonian Aeronomy Symp., Boston, U.S.A. Kvrrnr, G. (1959). J. atmos. terr. Phys. 16,252-258. Kvrpre, G. (1959). Geofy. Publr. 20, No. 12, 1-19. Lrwsnr~z, M. (1967). J. geophys. Res. 72,285. MONTBRIAND, L. E. J., RNSLEY,B. A. and VALLANCEJONES,A. (1965) Can. J. Phys. 43,782. PROKUDINA, V. S. 1959. Spectral. Electrophotometrical and Radar Researches of Aurora and Airglow 1, 43-44. ROACH,F. E. and Shrrrn, L. L. (1964). N.B.S. Technical Note No. 214. ROZENBERC~, G. V. (1966). Twilight (English Trans.) Plenum Press, New York. SHCHEGLOV, P. V. (1964) (English Trans.) Sov. Astronomy 8,289. SMITH,L. L., ROACH,F. E. and OWEN,R. W. 1965. Planet. Space Sci. 13,207-217. THOMAS, G. 1963, J. geophys. Res. 68,2639-2660. TINSLBY,B. A. (1966). Appl. Uptics 5, 1139-1145. TOUSEY,R. 1963. Space Sci. Rev. 2,3-69. PesIoMe-npHBegeHa pe3ynbTaTar Ha6nweanti JIEIHBYIH, B CBegeHEE HOPHO~O He6a c MaH no HoH6pb 1966 rona. Hocne ncrunoqennfr nanyqenntt OT acTopoHoniwecKUx IlCTO~HElKOBE3~~eKTOB30~EIaKaJIbHOr0CBeTa~aHHhIe~pOaHa3I~3EpOBaH~BHeCKO~bKHX CECTeMaX KOOpAHHaT. aeTHEti MaKCUMyM HHTWUX¶BHOCTli JIEiHElEl Her HBJIHeTCJi pe3yJIbTaTOM reOMeTpKH COJEfeqHOrO OCBeIIIeHEH; AeZtCTBETeJlbHO,COAepHcaHHe BOAOpo~~bnc aTOMOB HeMHOrO 6onbme OCeHbIO, eCJIH @j@eKTEBHbI$i IIOTOK COJIHeqHOrO H3JIyqeHnH C JIIlHIlll &?j p HeHOBbIIJJaJICHMemAyneTOM R: OCeHbPJ 1965r. B OCeHHHe CyMepKH COAep%aHEe aTOMOB He HCIIbITMBaHO 6onbmHx E3MeHeHEti; IlaMeHeHEe He Barn0 BepoRTHo II JIeTOM. COAepEWiFie aTOMOB nepeA BOCXOAOM Ban0 6onbme, UeM Be¶epOM (A0 30% JIeTOM). u3BeCTEhle MOAeJIH pa,IWfaTHBHOrO IlepeHOCa UO3BOJlHIOT UpOBeCTE JIHUIb OrpaHEqeHHOe COJlOCTaBJleHEeCTeOpHe&B KOTOpOi%EClIOJlb3yeTCJIpHA npw6nnmenai. ECJIH IlCIIOJIbaOBaTbIIaHHbIe 1962 rOAa 0 BeJIE14HHe a$.$eKT&iBFIOrOHOTOKa B JIHHHH &j/!?, TO COAepxaHEe BOAOpOAHbIX aTOMOB BbIIIle120 KM COCTaBHT OKOJIO 1Q14 aTOMOB/CM'. TaKaHOlJCHKa COrJIaCyeTCR C MOAeJIHME HU3KOti CKO~OCTE y6eraHEH EITOMOB EI3 6onee XoJIO~OtTepMcC+epH B rOAbI MElHHMYMe COJlHe=IHOMPKTHBEOCTH.