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Considerations on helium emission λ10830Å in aurorae
RBSBARCWNCYTES
286
fhat gives a nssponaethrough one slit and, a short time later, through the other. The two resulting curves am very similar, as would be enpeued, but since individual active rays are in themselves tmnsient phenomena, someminordiEerences occur. Aremrdisahowninthefigure. Thespeedcanbeestimatedbymeasuring ~thnadelaybstweanthepealrsinthatwocumsandthe~~of~tionofthephotometer,asswninga reesonable height for the auroral display, and con&& t&e .geoi&y. A few prelii measuremen ts were made during 10 days’ stay at the Auroral Observatory in Tromse. only21 mlcces& meanvcments were obtained and all were early in the evening. The measured velocities averaged 24 km/set; the highest was 30 km/set, and the lowest 13 l&xc. The height of the aurora was then assumed to be 110 km. The uncertainty in the individual measurements may be as high as 30 per cent. Oftheraysstudied14movedeastwardsand7westwa& The distribution is not symmetric around the average, but is peaked at a somewhat higher velocity. This however is hardly signiticant since the measurements are few. In some of the casec the correlation coe8icient p between the two intensity curves Z&) and Z,(l), was compu@d as a function of “compensating time delay”, P(T) = P(ZV),Zs(t + 7% i.e.acurveZ,wascomlatedwithtbecorPcepondingcurvaZ,atalster~rforsmralv~oftho~ di8emnce. ~~~,~,,of~atwhichphssitsmgximum,shouldrclprassntfhstimcthaauroratakssto travelfiomoneabttotheother. Themaximumvalueofpgivesameasure of how well the aurora keeps its reiativeshapeduringthistime. Theaccordbetweenq,andthetimedelay measumdbetween thepeakswas within 19 per cent. The maximum values of P(T) varred from 065 to Q95. We shall abstain from 8 tInther discusdon of the results now, but it is felt that they are of suilicient im~rtance to justify further work along this lii. Thii will be done with a new instrument which is now bemg built. With this instrument records may be made through 2 or 3 slits, variable in distance and direction. The mtensity through each slit is recorded by its own photomultiplier, with no loss of light. If desired, the intensities may be recorded on magnetic tape and P(T)may be computed by an electronic analogue computer which is being built for a similar analysis of acoustical data at the Physics Department of the University of Oslo. A&towZe&rmen&-The rowarch reported in this paper has been supported in part b the Norwegian Re search Council for Science and the Humanities and by the U.S. Air Force Caunbrr.dy ge Research Center, Geophysics Diirate, under contract AR 61(052)-252 through the European O&e. Tire Norwqian Institute of Cosmic Physics and The Institute of Theoreticul Astrophy&. Blindern-Oslo, Norway
A. OMHOLT
RFSERENCES 1. J. W. m, Physics of the Aurora and Aiglow. Academic Press. New York (1961). 2. J. A. RIM and B. W. Cuaara, Curred.J. Phys. 38,1366 (1960). 3. w. sToFraEoaN, J. Amos. Terr. Phys. 21,257 (1961).
CONSIDERATIONS
ON HELIUM
EMISSION
11083O%LIN AURORAE
(Z?eccloed19 February, 1962)
to Lympute N:
HHlO’ciQ
*~locm-‘,
u - 10-l’ au’,
n*r~1~cm~,
T-101sec
and
Y-3 x 10’cm/sec v-lOsec-l.
Tl?isyMsavaIueZv-3 x ~o’~-‘wbieh~tohim~withobsavationsduringthegrrwt~ auroraeefFebrmuy 18-11,1958,inZ~&igorod.
RBSFJARCH NOTES
287
ThslifetimedfHerSatorns~bedetaminedbythe~p~tbatisbycollisiollll(withN, ando,moleculmandoatoms)inwhichtheneutralcollislon rsionixedandthemetastableatomda ortheproccwwithHe2’Syield: Argon6.6, p”“” excited. Remtt tncmwwm~oftitc3oo“K~~ 10.3,Xe13.9,N,6.4,andH,6.0,alltimea10-1~cms. Tbe~thereforeseemstobeabout sinmanyca#9andwebalievethisisptobablyq~~~trueandtbeoreticallynaclonableas dlmsmed elsewha+. The panning reaction cross-s&ions am essuttially kinetic theory momentum transfer crom+e&ons and hence &&mhted by
where a is the constant in the van der Waals interaction potential expression, #(r) = --or-@. a is determined primarily then by the polarlxabiity and ionization potential of the Penning gas. This dependence is not very sensitive because of the cube root involved. The collision inteeral has been evaluated as 0.434. We therefore expect the 0s cross-section to be essentially &&me as for Ns, and the 0 atom cross-section to be about equal to that for Kr, based on polar&ability and ionization potential considerations. P is M empirical probability factor which is characteristically in the range 0.2-0.3 for the gases so far studied. In the * t of this new data on Penning cross-sections, we believe Shefov’s estimate of 7, the He 2?s lifetime, is atT east two orders of magnitude too large. The reciprocal lifetime or destruction frequency I/T = Y = (Q.v) n, where (u. v) is the average velocity times Penning cross-section and n is the concentration of Penning gas. Appendix IV of Chamberlain f6)based on data from Nicolet is used to determine the upper atmosphere composition. It appears that the metastable lifetime will be determined by reaction with 0 atoms due to their greater abundance, the reaction being He(2%) + 0 -+ He(l%) + Of + e. Taking the altitude as 300 km and n(O) = 10’ cm+, YN lo-“. 10’. 10BN 10-l or T N 10 set as compared with Shefov’s estimated l(r sec. At 200 km with n(O) = 7.8 x 10’. 7 would be only ~1 sec. Even if the 0 atoms cross-section should be much smaller than we believe, the lifetime at 300 km due to Ns alone would still be only 100 set and at lower altitudes correspondingly less. It is indeed conceivable that electron sunerlastic collisions with He 2?9 metastables would reduce the lifetime below 10 set at 300 km. Taking th< noon vahre of xe = l(r cm-’ at 300 km as appropriate for the sunlit aurorae (the midn’ t value is given as 106) from ref. 5 Appendix V and using the superlastic collision cross-se&ion 8 x 15h-I7 cm* from detailed balancing as applied to the excitation cross-section data of Schulz and Fox“) one computes: Y = 11~w 8 x lo-“.lO” x 2.6 x 10’ or 7 w 500 set where an electron velocity corrcspondhrg to a kinetic temperature of 1445°K is usedt6). It is of interest to note that this superlastic collision cross-section should be independent of temperature up to several thousand degrees as a consequence of the linearity of the excitation cross-section over a comparable energy range (O&V). This follows mnnediately from considerations of detailed balancing. It is consequently not clear that resonant light scattering of solar photons can be su!Bcient to agree with the observed intensrtv of 110830A in the sunlight aurorae. Chamberlai@ points out that this is a very important point, sin& a resonant light scatter&g explanation carries the implication that aurorae 11083b emission does not indicate bombardment of helium ions as aurora1 primaries in the same way that & indicates proton bombardment. Further investigations of the 210830 emission intensity and its analysis would appeartobeinorder. A&wwk&emetrts-This work was made possible by the Propulsion Sciences Division, U.S. Air Force GfBceofScientitlcReseamh. Deportment of Physics Wversity Austin
o Tcxar
E. E. ~C3USON
12, 4 exas
1. E. E. Bm, E. E. ~cwSoN, F. A. MA-and W. W. Ronxx-raoNPhys. Rev., submitted for publication. 2. N. I. l?anonov~, Pkanet. Space Sci. $75 (1961). 3. N. N. SHBPOV, Pkmet. Space Sci. 5, 75 (1961). 4. E. E. Fxaouso~, Phys. Rev., submitted for publication. 5. J. W. CruMasRum, Physics of tk Aurora and Airglow, Academic Press, New York, 1961. 5. G. J. Scwu~~ and R. E. Fox, Phys Reu. 106,1179(1957).
286
fhat gives a nssponaethrough one slit and, a short time later, through the other. The two resulting curves am very similar, as would be enpeued, but since individual active rays are in themselves tmnsient phenomena, someminordiEerences occur. Aremrdisahowninthefigure. Thespeedcanbeestimatedbymeasuring ~thnadelaybstweanthepealrsinthatwocumsandthe~~of~tionofthephotometer,asswninga reesonable height for the auroral display, and con&& t&e .geoi&y. A few prelii measuremen ts were made during 10 days’ stay at the Auroral Observatory in Tromse. only21 mlcces& meanvcments were obtained and all were early in the evening. The measured velocities averaged 24 km/set; the highest was 30 km/set, and the lowest 13 l&xc. The height of the aurora was then assumed to be 110 km. The uncertainty in the individual measurements may be as high as 30 per cent. Oftheraysstudied14movedeastwardsand7westwa& The distribution is not symmetric around the average, but is peaked at a somewhat higher velocity. This however is hardly signiticant since the measurements are few. In some of the casec the correlation coe8icient p between the two intensity curves Z&) and Z,(l), was compu@d as a function of “compensating time delay”, P(T) = P(ZV),Zs(t + 7% i.e.acurveZ,wascomlatedwithtbecorPcepondingcurvaZ,atalster~rforsmralv~oftho~ di8emnce. ~~~,~,,of~atwhichphssitsmgximum,shouldrclprassntfhstimcthaauroratakssto travelfiomoneabttotheother. Themaximumvalueofpgivesameasure of how well the aurora keeps its reiativeshapeduringthistime. Theaccordbetweenq,andthetimedelay measumdbetween thepeakswas within 19 per cent. The maximum values of P(T) varred from 065 to Q95. We shall abstain from 8 tInther discusdon of the results now, but it is felt that they are of suilicient im~rtance to justify further work along this lii. Thii will be done with a new instrument which is now bemg built. With this instrument records may be made through 2 or 3 slits, variable in distance and direction. The mtensity through each slit is recorded by its own photomultiplier, with no loss of light. If desired, the intensities may be recorded on magnetic tape and P(T)may be computed by an electronic analogue computer which is being built for a similar analysis of acoustical data at the Physics Department of the University of Oslo. A&towZe&rmen&-The rowarch reported in this paper has been supported in part b the Norwegian Re search Council for Science and the Humanities and by the U.S. Air Force Caunbrr.dy ge Research Center, Geophysics Diirate, under contract AR 61(052)-252 through the European O&e. Tire Norwqian Institute of Cosmic Physics and The Institute of Theoreticul Astrophy&. Blindern-Oslo, Norway
A. OMHOLT
RFSERENCES 1. J. W. m, Physics of the Aurora and Aiglow. Academic Press. New York (1961). 2. J. A. RIM and B. W. Cuaara, Curred.J. Phys. 38,1366 (1960). 3. w. sToFraEoaN, J. Amos. Terr. Phys. 21,257 (1961).
CONSIDERATIONS
ON HELIUM
EMISSION
11083O%LIN AURORAE
(Z?eccloed19 February, 1962)
to Lympute N:
HHlO’ciQ
*~locm-‘,
u - 10-l’ au’,
n*r~1~cm~,
T-101sec
and
Y-3 x 10’cm/sec v-lOsec-l.
Tl?isyMsavaIueZv-3 x ~o’~-‘wbieh~tohim~withobsavationsduringthegrrwt~ auroraeefFebrmuy 18-11,1958,inZ~&igorod.
RBSFJARCH NOTES
287
ThslifetimedfHerSatorns~bedetaminedbythe~p~tbatisbycollisiollll(withN, ando,moleculmandoatoms)inwhichtheneutralcollislon rsionixedandthemetastableatomda ortheproccwwithHe2’Syield: Argon6.6, p”“” excited. Remtt tncmwwm~oftitc3oo“K~~ 10.3,Xe13.9,N,6.4,andH,6.0,alltimea10-1~cms. Tbe~thereforeseemstobeabout sinmanyca#9andwebalievethisisptobablyq~~~trueandtbeoreticallynaclonableas dlmsmed elsewha+. The panning reaction cross-s&ions am essuttially kinetic theory momentum transfer crom+e&ons and hence &&mhted by
where a is the constant in the van der Waals interaction potential expression, #(r) = --or-@. a is determined primarily then by the polarlxabiity and ionization potential of the Penning gas. This dependence is not very sensitive because of the cube root involved. The collision inteeral has been evaluated as 0.434. We therefore expect the 0s cross-section to be essentially &&me as for Ns, and the 0 atom cross-section to be about equal to that for Kr, based on polar&ability and ionization potential considerations. P is M empirical probability factor which is characteristically in the range 0.2-0.3 for the gases so far studied. In the * t of this new data on Penning cross-sections, we believe Shefov’s estimate of 7, the He 2?s lifetime, is atT east two orders of magnitude too large. The reciprocal lifetime or destruction frequency I/T = Y = (Q.v) n, where (u. v) is the average velocity times Penning cross-section and n is the concentration of Penning gas. Appendix IV of Chamberlain f6)based on data from Nicolet is used to determine the upper atmosphere composition. It appears that the metastable lifetime will be determined by reaction with 0 atoms due to their greater abundance, the reaction being He(2%) + 0 -+ He(l%) + Of + e. Taking the altitude as 300 km and n(O) = 10’ cm+, YN lo-“. 10’. 10BN 10-l or T N 10 set as compared with Shefov’s estimated l(r sec. At 200 km with n(O) = 7.8 x 10’. 7 would be only ~1 sec. Even if the 0 atoms cross-section should be much smaller than we believe, the lifetime at 300 km due to Ns alone would still be only 100 set and at lower altitudes correspondingly less. It is indeed conceivable that electron sunerlastic collisions with He 2?9 metastables would reduce the lifetime below 10 set at 300 km. Taking th< noon vahre of xe = l(r cm-’ at 300 km as appropriate for the sunlit aurorae (the midn’ t value is given as 106) from ref. 5 Appendix V and using the superlastic collision cross-se&ion 8 x 15h-I7 cm* from detailed balancing as applied to the excitation cross-section data of Schulz and Fox“) one computes: Y = 11~w 8 x lo-“.lO” x 2.6 x 10’ or 7 w 500 set where an electron velocity corrcspondhrg to a kinetic temperature of 1445°K is usedt6). It is of interest to note that this superlastic collision cross-section should be independent of temperature up to several thousand degrees as a consequence of the linearity of the excitation cross-section over a comparable energy range (O&V). This follows mnnediately from considerations of detailed balancing. It is consequently not clear that resonant light scattering of solar photons can be su!Bcient to agree with the observed intensrtv of 110830A in the sunlight aurorae. Chamberlai@ points out that this is a very important point, sin& a resonant light scatter&g explanation carries the implication that aurorae 11083b emission does not indicate bombardment of helium ions as aurora1 primaries in the same way that & indicates proton bombardment. Further investigations of the 210830 emission intensity and its analysis would appeartobeinorder. A&wwk&emetrts-This work was made possible by the Propulsion Sciences Division, U.S. Air Force GfBceofScientitlcReseamh. Deportment of Physics Wversity Austin
o Tcxar
E. E. ~C3USON
12, 4 exas
1. E. E. Bm, E. E. ~cwSoN, F. A. MA-and W. W. Ronxx-raoNPhys. Rev., submitted for publication. 2. N. I. l?anonov~, Pkanet. Space Sci. $75 (1961). 3. N. N. SHBPOV, Pkmet. Space Sci. 5, 75 (1961). 4. E. E. Fxaouso~, Phys. Rev., submitted for publication. 5. J. W. CruMasRum, Physics of tk Aurora and Airglow, Academic Press, New York, 1961. 5. G. J. Scwu~~ and R. E. Fox, Phys Reu. 106,1179(1957).