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Comments on lithium in the upper atmosphere
COMMENTS
ON LITHIUM
lJ3 TIXE UPPER ATMOSPHERE
s. n&sILb%mlAN Geophysics Research IXnctomte Air Force Cambridge Research Laboratorim, Bedford, Massachusem (&ce&ed 10 October1961) pub~~tio~ on the ocxmrence of lithium in the upper atmosphere are critically reviewed, It is concluded that insufEcientdata arecurrentlyavailableto decidebetween
AIwaet-R-t
tbe two hypotiem of natural or art&S origin of upperatmosphericlithium. RESULTS GESUMMARY OF OBSERVATIONAL. In September of 1958two observations of a new line at 6707 A in twilight spectra taken as part of the IGY antarctic program were reported. The fist report, that of Dehumoy and
WeilP was based on plates taken in October of 1957. The second report, that of Gadsden and Salmon(*),was based on spectra taken during August and September of 1958. Subsequently, the presence of this line in twilight spectra for August, 1958was con6rmed by other stations in the antarcti~.~~~The new line has been attributed to resonance radiation from the 2%2sp transition in the neutral lithium atom at 6707.8A. These reports stimulated work in other laboratories and three additional pub~~tions sub~quen~y appeared in AC&are.These are listed below together with the place and dates of obviation. 1. A. Khvostikov and T. G. Megrelishvilif*),Abastumani Astrophysical Observatory, Georgia, USSR, during the year beginning December, 1957. Fig. 1 of this paper shows two spectra. On one, taken August 22, 1958,Z = 97”33’-101”46’,the 6707 line is not present, though this may be due to the quality of the reproduction. On the other, taken November 11,1958, Z = 980-101030’,the 6707lines is clearly present. The 6707 line is stated as occurring on more than 10 of the 67 spectrograms taken. The line is believed by the authors to be a blend of a Ns line (1st positive system, 6704.8A) and the Li line (6707.86A). 2. A. V. Jones@)observed the lithium emission at Saskatoon, Saskatchewan, Canada, January 10-21, 1959. The spectrograph was guided according to an exposure program which kept it pointing at the point in the sun-zenith plane where the geometrical shadow height was 80 km. 3. Kvi&@, Aas, Norway, January 18-February 20, 1959. A line was observed at 6707.7 f @5 A. ‘I%elineappeared when the solar depression was less than 145” and 13” with the collimator of the spectrograph elevated at 45” and g-5”,respectively. Kvifte states that these are the same depression angle limits as for sodium, and therefore infers that both lithium and sodium originate in the same atmospheric layers. He observed no decrease in the height of the lithium layer. The observational evidence may then be summarized as follows : Lithium has been observed in the twilight spectrum (either morning or evening) in the antarctic in October of 1957 and August and September, 1958; in Georgia, USSR on November II,1958 but not on August 22,1958; and in Saskatchewan, Canada and Aas, Norway, during January and February, 1959. It should be noted that, with the exception of the Soviet observations, a lack of observation during other periods is not sign&ant iriasmuch as the lithium line was not being looked for. 89
S. M. SILVERMAN
90 -ES
ON THJ3 ORIGIN OF LITHIUM
IN THE UPPER ATMOSPHERE
Both a natural and artificial origin for the lithium in the upper atmosphere have been postulated. The natural sources assumed are either evaporated material from stony meteorites or material caught up from ocean waters. The artificial source assumed is debris from H-bombs. The assumptions involved will be outlined below. The crucial test for the assumption of a natural origin is taken as the comparison of the observed Li/Na abundance in the atmosphere with that in the natural source. Barbier, et u1.t’) calculate a Li/Na ratio in the atmosphere of O-006, based on the spectra of reference (1). Gadsden and Salmon@) obtain a ratio of 0.05 from their spectra. Both calculations are preliminary and order of magnitude. In a later paper, Delannoy w has calculated the Li/Na abundance ratio from twilight spectra taken in the antarctic in August-September 1957 and August-September 1958. He obtains order of magnitude ratios of 7 x 1W for 1957 and 8 x lo-8 for 1958. In a recent paper Gadsden and Salmon w have reported that no lithium emission was found in the 1959 spectra from two stations in the antarctic. In addition, they re-examined the 1958 Hallett spectrograms microphotometrically. These results show no lithium emission before 5 August and after 11 September. During the period from 5 August to 11 September the lithium/sodium intensity ratio rises to a maximum around 16 August and then declines. They point out that Delannoy’s results (plotted on the same graph) are systematically higher by a ratio of about l-9 and suggest that this may be due to a genuine station variation. For comparison with natural sources, Barbier, et al. quote ratios of O-002 for meteorites and 2.3 x lad for ocean water. Donahue, however, has suggested that the lack of linearity between sodium brightness and abundance would mean that the Li/Na intensity ratio would give an incorrect abundance ratio, though this criticism does not apply to the work of Delannoy, where this factor is taken into account. Jones@), using the Saskatoon data, has estimated the Li/Na abundance ratio to be about 45 x lad, intermediate between the meteor and seawater values. The attribution of atmospheric Li to natural sources is thus seen to be indeterminate at the present time for the following reasons: (1) The observations have been sporadic. A continuous program of observation is needed to determine more precisely the conditions of occurrence. It is possible that the reduction of the IGY data, now in progress, will be helpful in this respect. (2) The conversion of observed intensity ratios to abundance ratios is in doubt because of possible non-linearity of the intensity vs. abundance curve. (3) The physical mechanism for atmospheric contamination by either meteors or seawater is not clear. Consider, for example, the meteoric action. Atmospheric contamination presumably takes place by ablation from the surface. The important parameter to be considered is then the residence time of the atoms in the atmosphere. This will depend on their atomic weights, diffusion coefficients and atmospheric mixing. The Saskatchewan group have reported increases in the calcium intensity during the period of the meteoric showers. The residence times of sodium and lithium in the atmosphere, because of their differing atomic weights, would be expected to be different. Consequently, the Li/Na intensity ratio might give an abundance ratio ditferent (and higher) from that in meteors. This factor, as far as I am aware, has not previously been considered. For the seawater hypothesis, the ratio would depend on separation occurring in the atmospheric mixing process, and this is also not known. The second hypothesis about the origin of the lithium lines is that of an artificial origin, specifically, a thermonuclear burst. This was postulated by Gadsden and SahnotP) in their
COMMENTS ON LlTHlUM IN THE UPPER ATMOSPHERE
91
occurmnce. llti8 pot&r&e is ban& on the following evidence: initialmportofthelinea (1) the occurmnce of the lines five days after the thermonuclear burst; (2) a rise and then fall of the intedty, iadicativc of the arrival of a quantity of material and subsequent dispersion; (3) an observed differential change in the emission height of thelithium and sodium. Barber(n) has used (3) as the criterion for rejecting the meteorite hypothesis. He has calculated, on the basis of a descending lithium oxide cloud, that the lithium twilight emission might be expected to be present for about 90 days after a nuclear burst. The hypothesis may be criticized on the basis that no mechanism is provided for transporting a local&d cloud from north of the equator to the antarctic regions. Furthermore, the observed decrease in Li emission height relative to the sodium emission height is based on a prehminary calculation and would need to be checked. Some months after the burst Kvitte’s) has reported no decrease in the height of the lithium layer. In their 1961 paper, Gadsden and Salmon’s), state, on the basis of the later microphotometric measurements, that the relative height measurements may be confused by patchiness of the emissions in the sky and the measures are therefore not given any weight. In connection with the bomb debris hypothesis, they also point out that no intensity maximum followed the 12 August high altitude burst, and suggest that this may be due to the lower altitude of the second burst. If this is indeed the case, then we are faced with the necessity of finding an alternative explanation for the occurrence of lithium in the 1957 twilight spectra, a period when there were no known high altitude bursts at all. We must consider also the mechanism by which lithium may be transported from one part of the atmosphere to another. There are two possible mechanisms for transporting the lithium. One would be by the motion of ions along the lines of force of the earth’s magnetic field, with subsequent neutralization. At least two ionizations and neutralizations would be required. A second mechanism would be by transport by high winds in the ionosphere. Some rocket measurements have been reporteP) which indicate high winds in the ionosphere, but insufficient data are available on the morphology of these winds to allow a decision to be made. Additional wind data might be obtainable from other sources, such as the movement of sporadic E, but I have made no attempt to collect this data. The artificial origin hypothesis thus suffers from the same limitations as the natural origin hypothesis, that is, a lack of observational data and of theoretical calculation, and a final conclusion must, therefore, be postponed for the time being. One final criticism applicable to either the natural or artificial origin hypothesis should be quoted. Blamont, et uZ.(~) have studied the time constant for the disappearance of sodium and lithium emissions from a chemical release at 190 km from a rocket. They find a short time constant and conclude from this that any sudden injection of material into the atmosphere is rapidly absorbed by photochemical processes and that no trace of the influx remains. Consequently, they fael that the diurnal or seasonal variations of the sodium and lithium emissions bear no relationship to the origin of these elements. It should be pointed out, however, that if the fraction of sodium or lithium in the atomic form is relatively independent of the total amount in all forms, then, on a longer time scale, of the order of days or weeks, a net increase would be noted if the residence time in the upper atmosphere were sufficiently long. Here again, however, there is insufficient information available for any definite conclusion to be reached. SUGGESTIONS FOR FUTURE WORK On the basis of the evidence and arguments summari& above, it seems clear that no definite conclusion as to the origin of the lithium observed in the spectra of the night sky
92
S. M. SILVERMAN
can be reached. The following experiments appear to be essential for an understanding of the problem. (1) A continuous monitoring of the lithium line in the night sky spectrum over an extended period of time; (2) A more complete description of upper atmosphere wind morphology on a global basis and of the diurnal and seasonal changes of these winds; (3) Measurements of the residence times of elements at heights of greater than 70 km; (4) More detite information of the photochemical equilibria of sodium and lithium in the upper atmosphere and on the fraction of these elements present in the upper atmosphere in the atomic form. The current (September-October 1961) series of atmospheric nuclear tests by the USSR may provide some of this information. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
REFERENCES J. DBLANNoY and G. WEILL, C.R. Acad. Sci. (Paris), 247,806 (1958). M. Gmmm and D. SUON, Nature, Load., 182,1598 (1958). N. J. OLIVER,private communication. A. Kwcmncov and T. G. MBORBLISHVILI, Nature, Lmd., 183,811 (1959). A. V. Jam, Nature,Lmi., 183, 1315 (1959). G. Km, Nature, Lo&., 183,1384 (1959). D. Bmmaa, J. -or and G. WEILL, C.R. AC&. Sci. (Paris), 247,886 (1958). J; DELANNOY, Am. Gtophys. 19,236 (1960). GADSDEN and K. SUMON,J. Amos. Terr..Phys., 22,75 (1961). T. M. D~NAHUB, Nature, Lmd., 183, 1480 (1959). D. R. BARBBR,Nature, Lord, 183,384 (1959). W. G. Smom, W. R. BANDKBN and W. NORDBWO, IGY Rocket Report Series, No. 1, July 1958, p. 58. J. Bmom, M. L. URY and G. Couam, Ann. GJophys., 16,435 (1960).
ON LITHIUM
lJ3 TIXE UPPER ATMOSPHERE
s. n&sILb%mlAN Geophysics Research IXnctomte Air Force Cambridge Research Laboratorim, Bedford, Massachusem (&ce&ed 10 October1961) pub~~tio~ on the ocxmrence of lithium in the upper atmosphere are critically reviewed, It is concluded that insufEcientdata arecurrentlyavailableto decidebetween
AIwaet-R-t
tbe two hypotiem of natural or art&S origin of upperatmosphericlithium. RESULTS GESUMMARY OF OBSERVATIONAL. In September of 1958two observations of a new line at 6707 A in twilight spectra taken as part of the IGY antarctic program were reported. The fist report, that of Dehumoy and
WeilP was based on plates taken in October of 1957. The second report, that of Gadsden and Salmon(*),was based on spectra taken during August and September of 1958. Subsequently, the presence of this line in twilight spectra for August, 1958was con6rmed by other stations in the antarcti~.~~~The new line has been attributed to resonance radiation from the 2%2sp transition in the neutral lithium atom at 6707.8A. These reports stimulated work in other laboratories and three additional pub~~tions sub~quen~y appeared in AC&are.These are listed below together with the place and dates of obviation. 1. A. Khvostikov and T. G. Megrelishvilif*),Abastumani Astrophysical Observatory, Georgia, USSR, during the year beginning December, 1957. Fig. 1 of this paper shows two spectra. On one, taken August 22, 1958,Z = 97”33’-101”46’,the 6707 line is not present, though this may be due to the quality of the reproduction. On the other, taken November 11,1958, Z = 980-101030’,the 6707lines is clearly present. The 6707 line is stated as occurring on more than 10 of the 67 spectrograms taken. The line is believed by the authors to be a blend of a Ns line (1st positive system, 6704.8A) and the Li line (6707.86A). 2. A. V. Jones@)observed the lithium emission at Saskatoon, Saskatchewan, Canada, January 10-21, 1959. The spectrograph was guided according to an exposure program which kept it pointing at the point in the sun-zenith plane where the geometrical shadow height was 80 km. 3. Kvi&@, Aas, Norway, January 18-February 20, 1959. A line was observed at 6707.7 f @5 A. ‘I%elineappeared when the solar depression was less than 145” and 13” with the collimator of the spectrograph elevated at 45” and g-5”,respectively. Kvifte states that these are the same depression angle limits as for sodium, and therefore infers that both lithium and sodium originate in the same atmospheric layers. He observed no decrease in the height of the lithium layer. The observational evidence may then be summarized as follows : Lithium has been observed in the twilight spectrum (either morning or evening) in the antarctic in October of 1957 and August and September, 1958; in Georgia, USSR on November II,1958 but not on August 22,1958; and in Saskatchewan, Canada and Aas, Norway, during January and February, 1959. It should be noted that, with the exception of the Soviet observations, a lack of observation during other periods is not sign&ant iriasmuch as the lithium line was not being looked for. 89
S. M. SILVERMAN
90 -ES
ON THJ3 ORIGIN OF LITHIUM
IN THE UPPER ATMOSPHERE
Both a natural and artificial origin for the lithium in the upper atmosphere have been postulated. The natural sources assumed are either evaporated material from stony meteorites or material caught up from ocean waters. The artificial source assumed is debris from H-bombs. The assumptions involved will be outlined below. The crucial test for the assumption of a natural origin is taken as the comparison of the observed Li/Na abundance in the atmosphere with that in the natural source. Barbier, et u1.t’) calculate a Li/Na ratio in the atmosphere of O-006, based on the spectra of reference (1). Gadsden and Salmon@) obtain a ratio of 0.05 from their spectra. Both calculations are preliminary and order of magnitude. In a later paper, Delannoy w has calculated the Li/Na abundance ratio from twilight spectra taken in the antarctic in August-September 1957 and August-September 1958. He obtains order of magnitude ratios of 7 x 1W for 1957 and 8 x lo-8 for 1958. In a recent paper Gadsden and Salmon w have reported that no lithium emission was found in the 1959 spectra from two stations in the antarctic. In addition, they re-examined the 1958 Hallett spectrograms microphotometrically. These results show no lithium emission before 5 August and after 11 September. During the period from 5 August to 11 September the lithium/sodium intensity ratio rises to a maximum around 16 August and then declines. They point out that Delannoy’s results (plotted on the same graph) are systematically higher by a ratio of about l-9 and suggest that this may be due to a genuine station variation. For comparison with natural sources, Barbier, et al. quote ratios of O-002 for meteorites and 2.3 x lad for ocean water. Donahue, however, has suggested that the lack of linearity between sodium brightness and abundance would mean that the Li/Na intensity ratio would give an incorrect abundance ratio, though this criticism does not apply to the work of Delannoy, where this factor is taken into account. Jones@), using the Saskatoon data, has estimated the Li/Na abundance ratio to be about 45 x lad, intermediate between the meteor and seawater values. The attribution of atmospheric Li to natural sources is thus seen to be indeterminate at the present time for the following reasons: (1) The observations have been sporadic. A continuous program of observation is needed to determine more precisely the conditions of occurrence. It is possible that the reduction of the IGY data, now in progress, will be helpful in this respect. (2) The conversion of observed intensity ratios to abundance ratios is in doubt because of possible non-linearity of the intensity vs. abundance curve. (3) The physical mechanism for atmospheric contamination by either meteors or seawater is not clear. Consider, for example, the meteoric action. Atmospheric contamination presumably takes place by ablation from the surface. The important parameter to be considered is then the residence time of the atoms in the atmosphere. This will depend on their atomic weights, diffusion coefficients and atmospheric mixing. The Saskatchewan group have reported increases in the calcium intensity during the period of the meteoric showers. The residence times of sodium and lithium in the atmosphere, because of their differing atomic weights, would be expected to be different. Consequently, the Li/Na intensity ratio might give an abundance ratio ditferent (and higher) from that in meteors. This factor, as far as I am aware, has not previously been considered. For the seawater hypothesis, the ratio would depend on separation occurring in the atmospheric mixing process, and this is also not known. The second hypothesis about the origin of the lithium lines is that of an artificial origin, specifically, a thermonuclear burst. This was postulated by Gadsden and SahnotP) in their
COMMENTS ON LlTHlUM IN THE UPPER ATMOSPHERE
91
occurmnce. llti8 pot&r&e is ban& on the following evidence: initialmportofthelinea (1) the occurmnce of the lines five days after the thermonuclear burst; (2) a rise and then fall of the intedty, iadicativc of the arrival of a quantity of material and subsequent dispersion; (3) an observed differential change in the emission height of thelithium and sodium. Barber(n) has used (3) as the criterion for rejecting the meteorite hypothesis. He has calculated, on the basis of a descending lithium oxide cloud, that the lithium twilight emission might be expected to be present for about 90 days after a nuclear burst. The hypothesis may be criticized on the basis that no mechanism is provided for transporting a local&d cloud from north of the equator to the antarctic regions. Furthermore, the observed decrease in Li emission height relative to the sodium emission height is based on a prehminary calculation and would need to be checked. Some months after the burst Kvitte’s) has reported no decrease in the height of the lithium layer. In their 1961 paper, Gadsden and Salmon’s), state, on the basis of the later microphotometric measurements, that the relative height measurements may be confused by patchiness of the emissions in the sky and the measures are therefore not given any weight. In connection with the bomb debris hypothesis, they also point out that no intensity maximum followed the 12 August high altitude burst, and suggest that this may be due to the lower altitude of the second burst. If this is indeed the case, then we are faced with the necessity of finding an alternative explanation for the occurrence of lithium in the 1957 twilight spectra, a period when there were no known high altitude bursts at all. We must consider also the mechanism by which lithium may be transported from one part of the atmosphere to another. There are two possible mechanisms for transporting the lithium. One would be by the motion of ions along the lines of force of the earth’s magnetic field, with subsequent neutralization. At least two ionizations and neutralizations would be required. A second mechanism would be by transport by high winds in the ionosphere. Some rocket measurements have been reporteP) which indicate high winds in the ionosphere, but insufficient data are available on the morphology of these winds to allow a decision to be made. Additional wind data might be obtainable from other sources, such as the movement of sporadic E, but I have made no attempt to collect this data. The artificial origin hypothesis thus suffers from the same limitations as the natural origin hypothesis, that is, a lack of observational data and of theoretical calculation, and a final conclusion must, therefore, be postponed for the time being. One final criticism applicable to either the natural or artificial origin hypothesis should be quoted. Blamont, et uZ.(~) have studied the time constant for the disappearance of sodium and lithium emissions from a chemical release at 190 km from a rocket. They find a short time constant and conclude from this that any sudden injection of material into the atmosphere is rapidly absorbed by photochemical processes and that no trace of the influx remains. Consequently, they fael that the diurnal or seasonal variations of the sodium and lithium emissions bear no relationship to the origin of these elements. It should be pointed out, however, that if the fraction of sodium or lithium in the atomic form is relatively independent of the total amount in all forms, then, on a longer time scale, of the order of days or weeks, a net increase would be noted if the residence time in the upper atmosphere were sufficiently long. Here again, however, there is insufficient information available for any definite conclusion to be reached. SUGGESTIONS FOR FUTURE WORK On the basis of the evidence and arguments summari& above, it seems clear that no definite conclusion as to the origin of the lithium observed in the spectra of the night sky
92
S. M. SILVERMAN
can be reached. The following experiments appear to be essential for an understanding of the problem. (1) A continuous monitoring of the lithium line in the night sky spectrum over an extended period of time; (2) A more complete description of upper atmosphere wind morphology on a global basis and of the diurnal and seasonal changes of these winds; (3) Measurements of the residence times of elements at heights of greater than 70 km; (4) More detite information of the photochemical equilibria of sodium and lithium in the upper atmosphere and on the fraction of these elements present in the upper atmosphere in the atomic form. The current (September-October 1961) series of atmospheric nuclear tests by the USSR may provide some of this information. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
REFERENCES J. DBLANNoY and G. WEILL, C.R. Acad. Sci. (Paris), 247,806 (1958). M. Gmmm and D. SUON, Nature, Load., 182,1598 (1958). N. J. OLIVER,private communication. A. Kwcmncov and T. G. MBORBLISHVILI, Nature, Lmd., 183,811 (1959). A. V. Jam, Nature,Lmi., 183, 1315 (1959). G. Km, Nature, Lo&., 183,1384 (1959). D. Bmmaa, J. -or and G. WEILL, C.R. AC&. Sci. (Paris), 247,886 (1958). J; DELANNOY, Am. Gtophys. 19,236 (1960). GADSDEN and K. SUMON,J. Amos. Terr..Phys., 22,75 (1961). T. M. D~NAHUB, Nature, Lmd., 183, 1480 (1959). D. R. BARBBR,Nature, Lord, 183,384 (1959). W. G. Smom, W. R. BANDKBN and W. NORDBWO, IGY Rocket Report Series, No. 1, July 1958, p. 58. J. Bmom, M. L. URY and G. Couam, Ann. GJophys., 16,435 (1960).