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The zodiacal light
The zodiacal light
998
:[-~AMID, S . . . . . . . . . . . . ]::[OFFMEISTER, C . . . . . . . . . . JACCHIA, L. G., KOPAL, Z. and MILLMAN, P . M . KRAMER, J. N . . . . . . . . . . KRESAK, L. a n d VozXRovX, M . . . . . LEVIN, B. J u . . . . . . . . . .
1951 1948 1950 1951 1953 1953a
Astron. J., 56, 126. Meteorstr6me, B a r t h , Leipzig. Ap. J., 111, 104. Astron. Circ. U.S.S.R., No. 117, 10. Bull. Astron. Inst. Czechoslovakia, 4, 139. Doklady akademii nauk S S S R (Documents of the
Academy of Sciences U.S.S.R., Moscow), 90,
LINDBLAD, B. A .
.
.
.
.
OLIVER, CH. ]3. . . . . PLAVEC, M . . . . . .
PORTER, J. G .
.
.
.
RADZIJEVSKIJ, V. V . WATSO~¢, F. G . . .
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WKIPPLE, F. L. a n d WRIGHT,F. W. WRmrIT, F. W . . . . . . . . . WRmHT, F. W. and WHIPPLE, F. L. WYATT, S. P. and WHIPPLE, F. L.
.
WHIPPLE, F. L .
.
. .
.
.
.
513. 1953b Doklady akademii nauk S S S R (Documents of the Academy of Sciences U.S.S.R., Moscow), 90, 737. 1952 A Radar Investigation of the Delta Aq~arid ZVleteor Shower of 1950, GOteborg. 1925 Meteors, Williams and Wilkins, Baltimore. 1950 C.R. Acad. Sci. (Paris), 231, 434. 1954a Bull. Astron. Inst. Czechoslovakia, 5, 38. 1954b Bull. Astron. Inst. Czechoslovakia, 5, 15. 1954c Bull. Astron. Inst. Czechoslovakia, 6, 20. 1955 Bull. Astron. Inst. Czechoslovakia, 6, 33 (see also 6, pp. 70 76, for summaries). 1952 Comets and Meteor Streams, C h a p m a n and Hall, London. 1952 Astron. J. U.S.S.R., 29, 162. 1934 Harvard Coll. Obs. Bull., 895, 9. 1941 Between the Planets. The Blakiston (~ompany, Philadelphia. 1950 Ap. J., 111, 375. 1951 Ap. J., 118, 464. 1950 Harvard Technical Report No. 6. I951 Harvard Technical Report No. 7. 1953 Astron. J., 58, 49. 1950 A p . . I . , 111, 134.
The Zodiacal Light H~. C. VAN DE H U L S T
Sterrewacht Leiden, Netherlands
ONE of my favourite slides for a popular lecture on astronomy is a drawing. It shows a tropical beach with a palm tree; far out at sea is a schooner with full sails, and over the horizon, far above the ship, stands the majestic cone of the zodiacal light. It seems almost within reach of the hands. The zodiacal light is romantic, because it is vague and near; it is an attractive field of study, since it has not been drawn completely out of the hands of the amateurs. In addition to this poetic attraction, the confidence in technical advances has given many an amateur and professional astronomer the feeling that pretty soon the ships will sail beyond the earthly horizon into those interplanetary seas. This vista makes it somewhat hard to find the selfrestraint for a critical analysis of our present knowledge. In fact, the zodiacal light is a difficult subject to tie down by precise observations and theories. The following remarks are an attempt to sketch some of the present needs. They summarize the main conclusions of a critical study of the literature, that will form a chapter of Vol. IV of The Solar System, edited by G. P. KHPER (University of Chicago Press), 1955; full references may be found there. The photometry of the brightest parts of the zodiacal light, known as the morning and evening cone and extending from 30 ° to 70 ° from the Sun, presents no major problems. The dozen sets of observations made during the past 30 years are in fair
]~. C. VAN DE ~'IULST
999
agreement, but not sufficiently exact, however, to make a reliable estimate of year-toyear and seasonal variations. In this respect HURAttATA'S photoelectric measurements made in the same month during successive years and suggesting a correlation with the position of Comet Encke in its orbit set an example for further work. The short-period fluctuations reported from time to time are in all probability due to changes in the superposed airglow or to faint aurorae. As soon as we wish to go down to fainter parts of the zodiacal light, at large ecliptic latitudes or at longitudes far away from the Sun, the big problem is to eliminate the other sources of sky light correctly. A major advance was made when the night sky emission (now usually called airglow) was recognized as a separate and important component. The standard method now is to separate the zodiacal light from the airglow by means of the nightly variation; and from the galactic light, including star light and diffuse light, by means of the seasonal change. But the set of equations is not sufficient for a complete solution. This can be most easily seen from the fact that a constant brightness over the entire sky, e.g. equal to the faintest light ever observed, might be ascribed to any of these three components. In this way an amount of the order of fifty tenth-magnitude stars per square degree has been pushed back and forth between the various components. The crucial question may also be formulated as follows ; how strong is the zodiacal light near the pole of the ecliptic ? It is clear that the interplanetary dust cloud will seem thicker or flatter, accordingly, as this estimate is made bigger or smaller. Many of the differences among published solutions go back to this uncertainty. It is also clear that photometric measurements alone cannot solve the question, but that (simple) tlheoretical arguments about the expected distribution of zodiacal and/or galactic light have to be invoked. My present guess is that the zodiacal light near the pole of the ecliptic may be at most twenty tenth-magnitude stars per square degree, but a frontal attack on this whole problem is most needed. Another uncertainty, that has become clear recently, refers to the interpretation of the photometric data. The increase of brightness of the zodiacal light towards the Sun was formerly held to arise from an increase of the number of particles with decreasing distance from the Sun. On extending the observational data all the way into the region of the corona this explanation is found to be absurd. Another effect, the strongly forward directed phase function of small particles due to Fraunhofer diffraction, is the only likely explanation here. It thus remains to be found which is the stronger effect for intermediate angles, say for elongations 10°-40 ° . Probably photometric arguments (including colour and polarization) are insufficient and a theoretical discussion of particle sizes and densities m a y be needed to solve this question. A definite advance of the past decade is the recognition of a component in the zodiacal light due to scattering by free electrons in interplanetary space. Several sets of polarimetric observations are in good agreement. When care is taken to eliminate the airglow and to refer the polarization to the zodiacal light only, its degree of polarization is about 19 per cent at 60 ° from the Sun. This is about twice as high as any expected polarization of the dust component. Thus one half or more of the zodiacal light at this elongation may be attributed to electron scattering. It is
1000
The zodiacal light
not necessary to perform complicated integrations in order to derive the electron density. A reliable estimate m a y be made from scratch as follows. At elongation 60 ° the observed surface brightness is 370 visual t e n t h - m a g n i t u d e stars per square degree, and the degree of polarization is 0.19. At 120 ° the surface brightness is 60 and the degree of polarization certainly smaller. The difference 310 comes from scattering b y material along a chord, 1 astronomical unit long, inside the orbit of the E a r t h . About 90 per cent of the polarized light must come from this region. Its surface brightness is 0.9 × 0-19 × 370 ---- 63 t e n t h - m a g n i t u d e stars per square degree or 206,000 t e n t h - m a g n i t u d e stars per steradian. A t t r i b u t i n g all of this to electrons, we m a y use the formula H~ ---- J
. % . N . R,
where
H , ---- polarized c o m p o n e n t of observed surface brightness ---- 206,000 ; J z i n t e n s i t y of sunlight at 1 astronomical unit ~ 0.49 x l015 visual t e n t h m a g n i t u d e stars ; J ( R / r ) 2 = the same at distance r; average r / R for this region: r / R ~ 0.9;
s v ---- scattering cross-section (for polarized component) of one electron for scattering into a steradian. This is (a/47r) . ~ sin 2 0, where ~ 0.66 × l0 -24 and 0 ~ 75 + on the average for this path, so t h a t s~ z 3.7 x 10 -26 cm2; R ~- length of p a t h ~ 1 astronomical unit ~ 1.50 × l013 cm ; ~¥ ---- average n u m b e r of electrons per cubic centimetre. The numerical result is N z 610 electrons per cm a somewhat inside the E a r t h ' s orbit. I t is fully confirmed b y more elaborate computations in which numerical or analytical integrations are made. The u n c e r t a i n t y in the observational d a t a is perhaps 20 per cent; b u t t h a t due to the assumption t h a t light scattered b y dust is unpolarized, is greater. The actual value might be between 0.5 and 1.5 times the estimated one. A t t e m p t s to find the change of electron density with the distance from the Sun must be viewed with great caution. Solutions of integral equations have a t e n d e n c y to give spurious results a n y w a y and the big u n c e r t a i n t y in the d a t a enhances this greatly. A safer guess m a y be made b y simply interpolating between the electron density in the solar corona and near the E a r t h ' s orbit. This gives an electron density proportional to r -r5 or r -rT. BIERMANN has pointed out t h a t b o t h the e x p o n e n t and the absolute density are compatible with the hypothesis t h a t these electrons are simply the electrons ejected from active regions of the Sun. The dust-scattering need not be t r e a t e d in great detail. W e have already m e n t i o n e d above some of the problems in the i n t e r p r e t a t i o n of its dependence on longitude and latitude. A p a r t from the diffraction effect, no direct size-estimates are possible. B u t the dynamical relations, in particular the Poynting-l~obertson effect, are quite different for particles of different sizes. I t thus seems t h a t the time has come when it is clear t h a t we cannot hope to add a discussion of the space distribution and the dynamics as a crown to the p h o t o m e t r i c work. Instead, we m a y h a v e to start from a d y n a m i c a l l y sound working model, fill in the k n o w n d a t a and raise f u r t h e r specific problems t h a t m a y be decided on the basis of observations.
H . C. VAN DE I-IuLsT
1001
One challenge of this kind is posed by the various conflicting theories of the "gegenschein". The gegenschein, a local brightening of the zodiacal light near the anti-solar point, is the most strongly disputed subject in the entire field. Three explanations have been suggested: (1) The brightening is due to an increase of scattering efficiency of one particle near perfect backscatter. (2) The brightening is due to an agglomeration of particles near the libration point in the Sun-Earth system. This libration point is at a distance of 0.01 astronomical units, four times farther than the Moon. (3) The gegenschein is a quite different phenomenon, namely the emission by a gas tail given off by the Earth, that may be compared with the tails of comets. The curious situation now is that the third explanation, in spite of the fact that its observational support is fairly weak, seems the most promising, as there are powerful arguments against the earlier theories. In the first theory the interplanetary particles are required to exhibit to a much stronger degree an effect that is, for example, known from the corrugated surface of the Moon. This seems unlikely for small pieces of rock. Very small particles may exhibit similar effects for quite different reasons. Perhaps this possibility ought to be explored but it would still leave the westerly deviation of one or two degrees unexplained. The theoretical basis of explanation (2) is doubtful, and is made even more doubtful if we consider that the particle density should be increased by a factor of about 100 over the average particle density seen at 1 astronomical unit in that direction. The big question is therefore : is it possible to obtain definite proof or disproof of the gas-tail hypothesis from observations ? Evidently, the spectrum should be studied carefully; this is not easy, as the gegenschein does not exceed the usual airglow in brightness. FESSENKOFFhas linked this hypothesis with the assumption of a terrestrial component in the entire zodiacal light. This assumption had even weaker support, and has been disproved for the main cones by the best spectral observations and by filter photometry. But the gegenschein problem is still open and requires another factor 10 in observational precision for its solution. So, again, we have come to the point where present observations fail, but new stimulus can be obtained from a closer theoretical examination of various working models.
998
:[-~AMID, S . . . . . . . . . . . . ]::[OFFMEISTER, C . . . . . . . . . . JACCHIA, L. G., KOPAL, Z. and MILLMAN, P . M . KRAMER, J. N . . . . . . . . . . KRESAK, L. a n d VozXRovX, M . . . . . LEVIN, B. J u . . . . . . . . . .
1951 1948 1950 1951 1953 1953a
Astron. J., 56, 126. Meteorstr6me, B a r t h , Leipzig. Ap. J., 111, 104. Astron. Circ. U.S.S.R., No. 117, 10. Bull. Astron. Inst. Czechoslovakia, 4, 139. Doklady akademii nauk S S S R (Documents of the
Academy of Sciences U.S.S.R., Moscow), 90,
LINDBLAD, B. A .
.
.
.
.
OLIVER, CH. ]3. . . . . PLAVEC, M . . . . . .
PORTER, J. G .
.
.
.
RADZIJEVSKIJ, V. V . WATSO~¢, F. G . . .
.
. .
.
. . .
.
. . .
.
. . .
.
. . .
.
.
. .
. .
. .
. .
. .
. .
.
.
.
.
.
.
WKIPPLE, F. L. a n d WRIGHT,F. W. WRmrIT, F. W . . . . . . . . . WRmHT, F. W. and WHIPPLE, F. L. WYATT, S. P. and WHIPPLE, F. L.
.
WHIPPLE, F. L .
.
. .
.
.
.
513. 1953b Doklady akademii nauk S S S R (Documents of the Academy of Sciences U.S.S.R., Moscow), 90, 737. 1952 A Radar Investigation of the Delta Aq~arid ZVleteor Shower of 1950, GOteborg. 1925 Meteors, Williams and Wilkins, Baltimore. 1950 C.R. Acad. Sci. (Paris), 231, 434. 1954a Bull. Astron. Inst. Czechoslovakia, 5, 38. 1954b Bull. Astron. Inst. Czechoslovakia, 5, 15. 1954c Bull. Astron. Inst. Czechoslovakia, 6, 20. 1955 Bull. Astron. Inst. Czechoslovakia, 6, 33 (see also 6, pp. 70 76, for summaries). 1952 Comets and Meteor Streams, C h a p m a n and Hall, London. 1952 Astron. J. U.S.S.R., 29, 162. 1934 Harvard Coll. Obs. Bull., 895, 9. 1941 Between the Planets. The Blakiston (~ompany, Philadelphia. 1950 Ap. J., 111, 375. 1951 Ap. J., 118, 464. 1950 Harvard Technical Report No. 6. I951 Harvard Technical Report No. 7. 1953 Astron. J., 58, 49. 1950 A p . . I . , 111, 134.
The Zodiacal Light H~. C. VAN DE H U L S T
Sterrewacht Leiden, Netherlands
ONE of my favourite slides for a popular lecture on astronomy is a drawing. It shows a tropical beach with a palm tree; far out at sea is a schooner with full sails, and over the horizon, far above the ship, stands the majestic cone of the zodiacal light. It seems almost within reach of the hands. The zodiacal light is romantic, because it is vague and near; it is an attractive field of study, since it has not been drawn completely out of the hands of the amateurs. In addition to this poetic attraction, the confidence in technical advances has given many an amateur and professional astronomer the feeling that pretty soon the ships will sail beyond the earthly horizon into those interplanetary seas. This vista makes it somewhat hard to find the selfrestraint for a critical analysis of our present knowledge. In fact, the zodiacal light is a difficult subject to tie down by precise observations and theories. The following remarks are an attempt to sketch some of the present needs. They summarize the main conclusions of a critical study of the literature, that will form a chapter of Vol. IV of The Solar System, edited by G. P. KHPER (University of Chicago Press), 1955; full references may be found there. The photometry of the brightest parts of the zodiacal light, known as the morning and evening cone and extending from 30 ° to 70 ° from the Sun, presents no major problems. The dozen sets of observations made during the past 30 years are in fair
]~. C. VAN DE ~'IULST
999
agreement, but not sufficiently exact, however, to make a reliable estimate of year-toyear and seasonal variations. In this respect HURAttATA'S photoelectric measurements made in the same month during successive years and suggesting a correlation with the position of Comet Encke in its orbit set an example for further work. The short-period fluctuations reported from time to time are in all probability due to changes in the superposed airglow or to faint aurorae. As soon as we wish to go down to fainter parts of the zodiacal light, at large ecliptic latitudes or at longitudes far away from the Sun, the big problem is to eliminate the other sources of sky light correctly. A major advance was made when the night sky emission (now usually called airglow) was recognized as a separate and important component. The standard method now is to separate the zodiacal light from the airglow by means of the nightly variation; and from the galactic light, including star light and diffuse light, by means of the seasonal change. But the set of equations is not sufficient for a complete solution. This can be most easily seen from the fact that a constant brightness over the entire sky, e.g. equal to the faintest light ever observed, might be ascribed to any of these three components. In this way an amount of the order of fifty tenth-magnitude stars per square degree has been pushed back and forth between the various components. The crucial question may also be formulated as follows ; how strong is the zodiacal light near the pole of the ecliptic ? It is clear that the interplanetary dust cloud will seem thicker or flatter, accordingly, as this estimate is made bigger or smaller. Many of the differences among published solutions go back to this uncertainty. It is also clear that photometric measurements alone cannot solve the question, but that (simple) tlheoretical arguments about the expected distribution of zodiacal and/or galactic light have to be invoked. My present guess is that the zodiacal light near the pole of the ecliptic may be at most twenty tenth-magnitude stars per square degree, but a frontal attack on this whole problem is most needed. Another uncertainty, that has become clear recently, refers to the interpretation of the photometric data. The increase of brightness of the zodiacal light towards the Sun was formerly held to arise from an increase of the number of particles with decreasing distance from the Sun. On extending the observational data all the way into the region of the corona this explanation is found to be absurd. Another effect, the strongly forward directed phase function of small particles due to Fraunhofer diffraction, is the only likely explanation here. It thus remains to be found which is the stronger effect for intermediate angles, say for elongations 10°-40 ° . Probably photometric arguments (including colour and polarization) are insufficient and a theoretical discussion of particle sizes and densities m a y be needed to solve this question. A definite advance of the past decade is the recognition of a component in the zodiacal light due to scattering by free electrons in interplanetary space. Several sets of polarimetric observations are in good agreement. When care is taken to eliminate the airglow and to refer the polarization to the zodiacal light only, its degree of polarization is about 19 per cent at 60 ° from the Sun. This is about twice as high as any expected polarization of the dust component. Thus one half or more of the zodiacal light at this elongation may be attributed to electron scattering. It is
1000
The zodiacal light
not necessary to perform complicated integrations in order to derive the electron density. A reliable estimate m a y be made from scratch as follows. At elongation 60 ° the observed surface brightness is 370 visual t e n t h - m a g n i t u d e stars per square degree, and the degree of polarization is 0.19. At 120 ° the surface brightness is 60 and the degree of polarization certainly smaller. The difference 310 comes from scattering b y material along a chord, 1 astronomical unit long, inside the orbit of the E a r t h . About 90 per cent of the polarized light must come from this region. Its surface brightness is 0.9 × 0-19 × 370 ---- 63 t e n t h - m a g n i t u d e stars per square degree or 206,000 t e n t h - m a g n i t u d e stars per steradian. A t t r i b u t i n g all of this to electrons, we m a y use the formula H~ ---- J
. % . N . R,
where
H , ---- polarized c o m p o n e n t of observed surface brightness ---- 206,000 ; J z i n t e n s i t y of sunlight at 1 astronomical unit ~ 0.49 x l015 visual t e n t h m a g n i t u d e stars ; J ( R / r ) 2 = the same at distance r; average r / R for this region: r / R ~ 0.9;
s v ---- scattering cross-section (for polarized component) of one electron for scattering into a steradian. This is (a/47r) . ~ sin 2 0, where ~ 0.66 × l0 -24 and 0 ~ 75 + on the average for this path, so t h a t s~ z 3.7 x 10 -26 cm2; R ~- length of p a t h ~ 1 astronomical unit ~ 1.50 × l013 cm ; ~¥ ---- average n u m b e r of electrons per cubic centimetre. The numerical result is N z 610 electrons per cm a somewhat inside the E a r t h ' s orbit. I t is fully confirmed b y more elaborate computations in which numerical or analytical integrations are made. The u n c e r t a i n t y in the observational d a t a is perhaps 20 per cent; b u t t h a t due to the assumption t h a t light scattered b y dust is unpolarized, is greater. The actual value might be between 0.5 and 1.5 times the estimated one. A t t e m p t s to find the change of electron density with the distance from the Sun must be viewed with great caution. Solutions of integral equations have a t e n d e n c y to give spurious results a n y w a y and the big u n c e r t a i n t y in the d a t a enhances this greatly. A safer guess m a y be made b y simply interpolating between the electron density in the solar corona and near the E a r t h ' s orbit. This gives an electron density proportional to r -r5 or r -rT. BIERMANN has pointed out t h a t b o t h the e x p o n e n t and the absolute density are compatible with the hypothesis t h a t these electrons are simply the electrons ejected from active regions of the Sun. The dust-scattering need not be t r e a t e d in great detail. W e have already m e n t i o n e d above some of the problems in the i n t e r p r e t a t i o n of its dependence on longitude and latitude. A p a r t from the diffraction effect, no direct size-estimates are possible. B u t the dynamical relations, in particular the Poynting-l~obertson effect, are quite different for particles of different sizes. I t thus seems t h a t the time has come when it is clear t h a t we cannot hope to add a discussion of the space distribution and the dynamics as a crown to the p h o t o m e t r i c work. Instead, we m a y h a v e to start from a d y n a m i c a l l y sound working model, fill in the k n o w n d a t a and raise f u r t h e r specific problems t h a t m a y be decided on the basis of observations.
H . C. VAN DE I-IuLsT
1001
One challenge of this kind is posed by the various conflicting theories of the "gegenschein". The gegenschein, a local brightening of the zodiacal light near the anti-solar point, is the most strongly disputed subject in the entire field. Three explanations have been suggested: (1) The brightening is due to an increase of scattering efficiency of one particle near perfect backscatter. (2) The brightening is due to an agglomeration of particles near the libration point in the Sun-Earth system. This libration point is at a distance of 0.01 astronomical units, four times farther than the Moon. (3) The gegenschein is a quite different phenomenon, namely the emission by a gas tail given off by the Earth, that may be compared with the tails of comets. The curious situation now is that the third explanation, in spite of the fact that its observational support is fairly weak, seems the most promising, as there are powerful arguments against the earlier theories. In the first theory the interplanetary particles are required to exhibit to a much stronger degree an effect that is, for example, known from the corrugated surface of the Moon. This seems unlikely for small pieces of rock. Very small particles may exhibit similar effects for quite different reasons. Perhaps this possibility ought to be explored but it would still leave the westerly deviation of one or two degrees unexplained. The theoretical basis of explanation (2) is doubtful, and is made even more doubtful if we consider that the particle density should be increased by a factor of about 100 over the average particle density seen at 1 astronomical unit in that direction. The big question is therefore : is it possible to obtain definite proof or disproof of the gas-tail hypothesis from observations ? Evidently, the spectrum should be studied carefully; this is not easy, as the gegenschein does not exceed the usual airglow in brightness. FESSENKOFFhas linked this hypothesis with the assumption of a terrestrial component in the entire zodiacal light. This assumption had even weaker support, and has been disproved for the main cones by the best spectral observations and by filter photometry. But the gegenschein problem is still open and requires another factor 10 in observational precision for its solution. So, again, we have come to the point where present observations fail, but new stimulus can be obtained from a closer theoretical examination of various working models.