- Email: [email protected]
Discrete, continuous and diffuse auroras
RESEARCH
NOTES
1723
The importance of this result for cosmic ray modulation studies is apparent. Modulation is ineffective beyond w 10 A.U. where all radially propagating Alfvenic fluctuations have become longitudinal relative to the field and consequently quickly damped out (Quenby and Sear, 1971). In the absence of an effective unwinding mechanism there is no possibility that the modulation boundary and heliopause are coincident. Taking an even conservatively small value of 5-10 per cent for the particle gradient at 1 GV, the particle density at 10 A.U. is double that at Earth and it is necessary that no further modulation occur between 10 A.U. and the heliopause if the interstellar cosmic ray energy is not to excede the magnetic field energy. Acknowledgements-The author wishes to acknowledge stimulating discussions with Drs. Richard Buckley, John Quenby and Peter Hedgecock. S. WEBB* Department of Physics, Imperial ColIege, London, EngIand REFERENCES BLUM, P. W. and FAHR, H. J. (1969). Nature, Lond. 223, 936. BLUM, P. W. and FAHR, H. J. (1970a). Astron. Astrophys. 4, 280. BLUM, P. W. and FAHR, H. J. (1970b). Astrophys. Lett. 6, 127. COLEMAN,P. J. and WINTER, E. M. (1971). Asilomar Co@ on Solar Wind, March, 1971. CLEMMOW,P. C. and DOUGHERTY,J. P. (1969). Electrodynamics of Particles and Plasmas. Addison-Wesley, New York. Frrs, W. L., SMITH, A. C. H. and STEBBINGS,R. F. (1962). Proc. Roy. Sot. London A, 268,527. HOLZER,T. E. (1972). J. geophys. Res. 77,5407. QUENBY,J. J. and SEAR, 3. F. (1971). 12th Znt. Co@ Cosm. Rays, Hobart, Vol. 2, p. 771. SMITH, E. (1973). Pioneer 10 Symposium, 13th Znt. Con& Cosm. Rays, Denver, August, 1971. STECHER,F. W. (1969). Nature, Lond. 222,865. THOMAS,G. E. (1971). Asilomar Co@ on Solar Wind, March, 1971 WINTER, E. M. and COLEMAN,P. J. (1971a). Paper MOD 9. 12th Znt. Co@ Cosm. Rays, Hobart. WINTER,E. M. and COLEMAN,P. J. (1971b). Pub. 950, Institute of Geophysics & Planetary Physics. * Present address: Division of Clinical Physics Royal Cancer Hospital and Institute of Cancer Research Downs Rd., Sutton, Surrey.
Planet. Space Sci. 1974, Vol. 22, pp. 1723 to 1726. Persamon Press. Printed in Northern Ireland
DISCRETE,
CONTINUOUS
AND DIFFUSE
AURORAS
(Received 11 Jane 1974) Abstract-It is pointed out that there are a number of important differences between discrete auroras and diffuse auroras in the midnight sector. The differences are listed in a tabular form. The identification of the corresponding regions in the magnetotail is an urgent future task. INTRODUCTION Recent airborne and satellite observations of auroras have indicated that there are two ditfuse and broad bands of luminosity, in addition to the oval band of discrete auroras. One of these is a quasi-circular belt approximately along the line of L=6, and the other is called the continuous aurora and is an oval belt which is partially overlapped with and located slightly equatorward of the oval of discrete auroras (Whalen et al., 1971; Buchau et al., 1972). The quasi-circular belt of diffuse auroras and the oval belts of discrete and diffuse aurora lie closly together in the midnight sector, but are separated by a dark gap in the noon sector. Undoubtedly, the mantle aurora (Sandford, 1964, 1967), the so-called ‘hydrogen aurora’ (cf. Eather, 1967; Reid and Rees, 1961) constitute parts of these diffuse bands of luminosity. However, because of their faint, structureless and broad features, it has been difficult to observe them by an all-sky camera. For this reason,
1724
RESEARCH
NOTES
their large-scale morphological features have not been studied in detail, particularly in connection with the aurora1 substorm; note that the substorm features of the diffuse aurora were discussed in terms of the proton aurora substorm by Akasofu (1968, p. 137; Fig. 96). Since satellite data show clearly that these diffuse luminous bands are an integral part of the aurora1 luminosity, it is of great interest to examine whether or not there is any distinction between the discrete auroras and diffuse luminous bands. Here, we contine our attention to their characteristics in the midnight sector where the continuous aurora is partially a background luminosity in the oval of discrete auroras. Thus, we shall be mainly concerned with distinctions between discrete auroras and the quasi-circular diffuse auroras. The purpose of this Note is to show, on the basis of recent publications, that there are indeed important differences (Table 1). The identification of magnetospheric regions corresponding to the regions of aurora1 luminosity is urgently needed. In particular, the first sign of the aurora1 substorm, a sudden brightening of a discrete aurora, occurs along the boundary of the quasi-circular diffuse and discrete aurora1 regions, namely, near the poleward boundary of the diffuse aurora and the equatorward boundary of the belt of discrete auroras. It is of great interest to find where the field lines from the boundary of the two aurora1 regions lie in the magnetotail. TABLE 1
Discrete Aurora Morphological features
Diffuse Aurora
A discrete aurora is a curtain-like structure and appears as a single arc or is separated from other arcs by a dark space of width of order a few tens of kilometers.“J
A diffuse aurora is either a broad band of luminosity of width of at least several tens of kilometers or a group of discrete auroras which are closely packed along a rather narrow east-west belt. Patchy auroras develop in the diffuse aurora. The mantle aurora and the hydrogen aurora may belong to the diffuse aurora.oJ)
Predominantly
Consisting of an oval’sm4)and circular bands which meet together in the midnight sector.(6l
an evening feature.“’
In the morning sector, the diffuse aurora often develops into several discrete auroras; some of the latter spread over the morning half of the polar cap and become the polar cap aurora.(*) It is difficult to distinguish both types of auroras from all-sky camera photographs. The equatorward edge of the diffuse aurora appears like a homogeneous arc.(6,6) Particle precipitation
Varying degree of conjugacy.‘7’
Appears to be conjugate.“)
Inverted V precipitation.‘8J’)
A broad and relatively uniform precipitation.(8~8)
Energy spectrum is considerably variable.(B*O’
Energy spectrum is relatively constant and becomes soft near the equatorward edge.(8J’)
The ‘trapping boundary’ is located between the two aurora1 systems.f8*10) Electric field
Field-aligned currents Radio wave response
The convection electric field appears to change the direction near the poleward boundary of the discrete aurora1 region.ul*laJ Upward currents in the evening sector.us)
Downward currents in the evening sector.03)
Discrete echoes occur in the proximity of discrete auroras. Diffuse echoes occur in a relatively dark region between discrete diffuse aurora1 systems.u4j
RESEARCH
NOTE
1725
TABLE1 (Continued)
Substorm features (auroras)
Discrete Aurora
Diffuse Aurora
Poleward expansion, westward traveling surges, etc.usj
The equatorward boundary is relatively stable. The poleward boundary develops a torch-like structure.“~16r
The first sign of substorm is seen along the boundary of the discrete and diffuse aurora systems as a brightening of a pm-existing arc or as the formation of a new arc. (Magnetic disturbances)
Negative bays.“‘L)
Positive bays in the evening sector. Negative bays in the morning sector.“‘)
Energetic solar electrons
Penetrate into the discrete aurora1 region.“‘)
Cannot penetrate into the diffuse auroral region.“‘)
Relation with the plasma sheet
Possible connection to the upper (N. hemisphere) and lower (S. hemisphere) layers of the plasma sheet.“*~ls)
Connected to the central part of the plasma sheet.‘lB’
Shifts both auroral systems poleward when it is directed northward and southward when directed southward.“or
Interplanetary
magneticfield Magnetic structure
Open region (1)
Closed region ( ?)
1. SNYDER,A. L., Jr. and AKAsomr, S.-I. Major aurora1 substorm features in the dark sector observed by a USAF DMSP satellite. Planet. Space Sci. 22,1511,1974. 2. Lur, A. T. Y. and ANGER, C. D. A uniform belt of diffuse aurora1 emission seen by the ISIS-2 scanning photometer. Planet. Space Sci. 21,799, 1973. 3. WHALEN, J. A., BUCHAU, J. and WAGNER,R. A. Airborne ionospheric and optical measurements of noontime aurora. J. atmos. terr. Phys. 33,661, 1971. 4. BUCHAU,J., G-MANN, G. J., PIKE, C. P., WAGNER,R. A. and WHALEN,J. A. Precipitation patterns in the arctic ionosphere determined from airborne observations. Ann. Geophys. 28,443,1972. 5. Lur, A. T. Y., ANGER, C. D. and AKASOPU,S.-I. The continuity of the aurora1 oval as seen by the ISIS-2 scanner aurora1 photometer. J. geophys. Res. (in press) 1974. 6. Lur, A. T. Y., PERREAULT,P., AKASOFU,S.-I. and ANGER, C. D. The diffuse aurora. Planet. Space Sci. 21,867,1973. 7. STENBAEK-NIELSEN, H. C., Wzscorr. E. M. and DAVIS,T. N. Differences in aurora1 intensity at conjugate points. J. geophys. Res. 78,659, 1973. 8. FRANK, L. A. and ACKERSON,K. Observations of changed particle precipitation into the aurora1 zone. J. PeoDhvs. Res. 70,3612,1971. 9. W&N%HAM, J. D:, YAS.UHARA,F., AKA~OFU,S.-I. and HEMKILA, W. The precipitation pattern of electrons (50-10 keV) during quiet and substorm times in the 22-03 MLT sector. J.geophys. Res. (submitted) 1974. 10. BURROWS,J. R. The plasma sheet in the evening sector. Earth’s Particle and Fields. Advanced Study Institute, Sheffield, England, August, 1973. 11. GURNEY, D. A. and FRANK, L. A. Observed relationships between electric fields and auroral particle precipitation. J.geophys. Z&s. 78,145, 1973. 12. WESCOTT,E. M. Electric field measurements, p. 73. Solar-Terrestrial Relations, Proc. Conf. Univ. Calgary, August 28-Sept. 1,1972. 13. ARMSTRONG,J. C., AKASOFU,S.-I. and ROSTOKER,G. A comparison of satellite observations of Birkeland currents with ground observations of visible aurora and ionospheric currents. J. geophys. Res. (in press) 1974. 14. ROMICK,G. J., ECKLUND, W. L., GREENWALDK. A. and BALSLEY,B. B. The interrelationship between the > 130 kev electron trapping boundary, VHF radar backscatter and the visual aurora. J. geophys. Res. (in press) 1974. 15. AKASOFU,S-I. Polar and Magnetospheric Substorms. Reidel, Dordrecht, Holland, 1968. 16. SNYDER,A. L., AKASOFU,S.-I. and DAX%, T. N. Aurora1 substorms observed from above the north polar region by a satellite, J. geophys. Res. 79, 1393, 1974.
1726
RESEARCH
NOTES
17. BURROWS,J. R. and MCDIARMID, I. B. Trapped particle boundary regions, p. 83, Critical Problems of Magnetospheric Physics. Proc. Joint COSPAR/IAGA/URSI Symposium. (Ed. E. F. DYER), Madrid, Spain. 1972. 18. Fwi, L. A. Plasma in the polar magnetosphere. J. geophys. Res 76,5205, 1971. 19. AKASOFU, S.-I., HONES, E. W., Jr., BAME, S. J., ASBRIDGE,J. R. and Lur, A. T. Y. Magnetotail and boundary layers plasmas at a geocentric distance of 18R,: Vela 5 and 6 observations. J. geophys. Res. 78,725l. 1973. 20. AKASOFU, S.-I., PERREAULT,P. D., YASUHARAF. and MENG, C.-I. Aurora1 substorms and the interplanetary magnetic field. J. geophys. Res. 78,7490,1973. It is quite likely that the diffuse aurora1 region is connected to the main part of the plasma sheet. However, it is not certain if the discrete aurora1 region is connected to the upper (in the northern hemisphere) and lower (in the southern hemisphere) parts of the plasma sheet (that is, the region of ‘closed’ field lines) or if discrete auroras occur along ‘open’ field lines. Some phenomena (i.e. the presence of particles with large (- 90’) pitch angles; the electric field reversal near the poleward boundary) suggest that discrete auroras occur along ‘closed’ field lines, while some other phenomena (i.e. the penetration of solar electrons and the varying degree of conjugacy) are thought to suggest that they occur along ‘open’ field lines. Thus, it is quite dangerous to conclude from a single phenomenon whether those particular field lines are ‘open’ or ‘closed’. Acknowledgements-I would like to thank my colleagues at the Geophysical Institute, University of Alaska and Mr. J. Whalen, AFCRL, for their helpful discussions during the preparation of this Note. The work reported here was supported by a National Science Foundation Grant 9A-36873X. S.-I. AKASOFU Geophysical Institute, University of Alaska, Fairbanks, Alaska, U.S.A. REFERENCES AKA~~FU, S.-I. (1968). Polar and Magnetospheric Substorms. Reidel, Dordrecht, Holland. BUCHAU,J., GASSMANN,G. J., PIKE, C. P., WAGNER,R. A. and WHALEN,J. A. see (4) in Reference for Table. EATHW, R. H. (1967). Aurora1 proton precipitation and hydrogen emissions. Rev. Geophys. 5, 207. REID, G. C. and REES, M. H. (1961). The systematic behavior of hydrogen emission in the aurora-II. Planet. Space Sci. 5, 99. SANDFORD,B. P. (1964). Aurora and airglow intensity variations with time and magnetic activity at southern high latitudes. J. atmos terr. Phys. 26, 749. SANDFORD,B. P. (1967). High latitude night-sky emissions. In AurorasandAirgZow (Ed. B. M. MCCORMAC), pp. 443452. Reinhold, New York. SNYDER,A. L., Jr., AKASOFU,S-1. and DAVIS, T. N. See (16) in Reference for Table. WHALEN,J. A., BUCIUN, J. and WAONER,R. A. See (3) in Reference for Table.
NOTES
1723
The importance of this result for cosmic ray modulation studies is apparent. Modulation is ineffective beyond w 10 A.U. where all radially propagating Alfvenic fluctuations have become longitudinal relative to the field and consequently quickly damped out (Quenby and Sear, 1971). In the absence of an effective unwinding mechanism there is no possibility that the modulation boundary and heliopause are coincident. Taking an even conservatively small value of 5-10 per cent for the particle gradient at 1 GV, the particle density at 10 A.U. is double that at Earth and it is necessary that no further modulation occur between 10 A.U. and the heliopause if the interstellar cosmic ray energy is not to excede the magnetic field energy. Acknowledgements-The author wishes to acknowledge stimulating discussions with Drs. Richard Buckley, John Quenby and Peter Hedgecock. S. WEBB* Department of Physics, Imperial ColIege, London, EngIand REFERENCES BLUM, P. W. and FAHR, H. J. (1969). Nature, Lond. 223, 936. BLUM, P. W. and FAHR, H. J. (1970a). Astron. Astrophys. 4, 280. BLUM, P. W. and FAHR, H. J. (1970b). Astrophys. Lett. 6, 127. COLEMAN,P. J. and WINTER, E. M. (1971). Asilomar Co@ on Solar Wind, March, 1971. CLEMMOW,P. C. and DOUGHERTY,J. P. (1969). Electrodynamics of Particles and Plasmas. Addison-Wesley, New York. Frrs, W. L., SMITH, A. C. H. and STEBBINGS,R. F. (1962). Proc. Roy. Sot. London A, 268,527. HOLZER,T. E. (1972). J. geophys. Res. 77,5407. QUENBY,J. J. and SEAR, 3. F. (1971). 12th Znt. Co@ Cosm. Rays, Hobart, Vol. 2, p. 771. SMITH, E. (1973). Pioneer 10 Symposium, 13th Znt. Con& Cosm. Rays, Denver, August, 1971. STECHER,F. W. (1969). Nature, Lond. 222,865. THOMAS,G. E. (1971). Asilomar Co@ on Solar Wind, March, 1971 WINTER, E. M. and COLEMAN,P. J. (1971a). Paper MOD 9. 12th Znt. Co@ Cosm. Rays, Hobart. WINTER,E. M. and COLEMAN,P. J. (1971b). Pub. 950, Institute of Geophysics & Planetary Physics. * Present address: Division of Clinical Physics Royal Cancer Hospital and Institute of Cancer Research Downs Rd., Sutton, Surrey.
Planet. Space Sci. 1974, Vol. 22, pp. 1723 to 1726. Persamon Press. Printed in Northern Ireland
DISCRETE,
CONTINUOUS
AND DIFFUSE
AURORAS
(Received 11 Jane 1974) Abstract-It is pointed out that there are a number of important differences between discrete auroras and diffuse auroras in the midnight sector. The differences are listed in a tabular form. The identification of the corresponding regions in the magnetotail is an urgent future task. INTRODUCTION Recent airborne and satellite observations of auroras have indicated that there are two ditfuse and broad bands of luminosity, in addition to the oval band of discrete auroras. One of these is a quasi-circular belt approximately along the line of L=6, and the other is called the continuous aurora and is an oval belt which is partially overlapped with and located slightly equatorward of the oval of discrete auroras (Whalen et al., 1971; Buchau et al., 1972). The quasi-circular belt of diffuse auroras and the oval belts of discrete and diffuse aurora lie closly together in the midnight sector, but are separated by a dark gap in the noon sector. Undoubtedly, the mantle aurora (Sandford, 1964, 1967), the so-called ‘hydrogen aurora’ (cf. Eather, 1967; Reid and Rees, 1961) constitute parts of these diffuse bands of luminosity. However, because of their faint, structureless and broad features, it has been difficult to observe them by an all-sky camera. For this reason,
1724
RESEARCH
NOTES
their large-scale morphological features have not been studied in detail, particularly in connection with the aurora1 substorm; note that the substorm features of the diffuse aurora were discussed in terms of the proton aurora substorm by Akasofu (1968, p. 137; Fig. 96). Since satellite data show clearly that these diffuse luminous bands are an integral part of the aurora1 luminosity, it is of great interest to examine whether or not there is any distinction between the discrete auroras and diffuse luminous bands. Here, we contine our attention to their characteristics in the midnight sector where the continuous aurora is partially a background luminosity in the oval of discrete auroras. Thus, we shall be mainly concerned with distinctions between discrete auroras and the quasi-circular diffuse auroras. The purpose of this Note is to show, on the basis of recent publications, that there are indeed important differences (Table 1). The identification of magnetospheric regions corresponding to the regions of aurora1 luminosity is urgently needed. In particular, the first sign of the aurora1 substorm, a sudden brightening of a discrete aurora, occurs along the boundary of the quasi-circular diffuse and discrete aurora1 regions, namely, near the poleward boundary of the diffuse aurora and the equatorward boundary of the belt of discrete auroras. It is of great interest to find where the field lines from the boundary of the two aurora1 regions lie in the magnetotail. TABLE 1
Discrete Aurora Morphological features
Diffuse Aurora
A discrete aurora is a curtain-like structure and appears as a single arc or is separated from other arcs by a dark space of width of order a few tens of kilometers.“J
A diffuse aurora is either a broad band of luminosity of width of at least several tens of kilometers or a group of discrete auroras which are closely packed along a rather narrow east-west belt. Patchy auroras develop in the diffuse aurora. The mantle aurora and the hydrogen aurora may belong to the diffuse aurora.oJ)
Predominantly
Consisting of an oval’sm4)and circular bands which meet together in the midnight sector.(6l
an evening feature.“’
In the morning sector, the diffuse aurora often develops into several discrete auroras; some of the latter spread over the morning half of the polar cap and become the polar cap aurora.(*) It is difficult to distinguish both types of auroras from all-sky camera photographs. The equatorward edge of the diffuse aurora appears like a homogeneous arc.(6,6) Particle precipitation
Varying degree of conjugacy.‘7’
Appears to be conjugate.“)
Inverted V precipitation.‘8J’)
A broad and relatively uniform precipitation.(8~8)
Energy spectrum is considerably variable.(B*O’
Energy spectrum is relatively constant and becomes soft near the equatorward edge.(8J’)
The ‘trapping boundary’ is located between the two aurora1 systems.f8*10) Electric field
Field-aligned currents Radio wave response
The convection electric field appears to change the direction near the poleward boundary of the discrete aurora1 region.ul*laJ Upward currents in the evening sector.us)
Downward currents in the evening sector.03)
Discrete echoes occur in the proximity of discrete auroras. Diffuse echoes occur in a relatively dark region between discrete diffuse aurora1 systems.u4j
RESEARCH
NOTE
1725
TABLE1 (Continued)
Substorm features (auroras)
Discrete Aurora
Diffuse Aurora
Poleward expansion, westward traveling surges, etc.usj
The equatorward boundary is relatively stable. The poleward boundary develops a torch-like structure.“~16r
The first sign of substorm is seen along the boundary of the discrete and diffuse aurora systems as a brightening of a pm-existing arc or as the formation of a new arc. (Magnetic disturbances)
Negative bays.“‘L)
Positive bays in the evening sector. Negative bays in the morning sector.“‘)
Energetic solar electrons
Penetrate into the discrete aurora1 region.“‘)
Cannot penetrate into the diffuse auroral region.“‘)
Relation with the plasma sheet
Possible connection to the upper (N. hemisphere) and lower (S. hemisphere) layers of the plasma sheet.“*~ls)
Connected to the central part of the plasma sheet.‘lB’
Shifts both auroral systems poleward when it is directed northward and southward when directed southward.“or
Interplanetary
magneticfield Magnetic structure
Open region (1)
Closed region ( ?)
1. SNYDER,A. L., Jr. and AKAsomr, S.-I. Major aurora1 substorm features in the dark sector observed by a USAF DMSP satellite. Planet. Space Sci. 22,1511,1974. 2. Lur, A. T. Y. and ANGER, C. D. A uniform belt of diffuse aurora1 emission seen by the ISIS-2 scanning photometer. Planet. Space Sci. 21,799, 1973. 3. WHALEN, J. A., BUCHAU, J. and WAGNER,R. A. Airborne ionospheric and optical measurements of noontime aurora. J. atmos. terr. Phys. 33,661, 1971. 4. BUCHAU,J., G-MANN, G. J., PIKE, C. P., WAGNER,R. A. and WHALEN,J. A. Precipitation patterns in the arctic ionosphere determined from airborne observations. Ann. Geophys. 28,443,1972. 5. Lur, A. T. Y., ANGER, C. D. and AKASOPU,S.-I. The continuity of the aurora1 oval as seen by the ISIS-2 scanner aurora1 photometer. J. geophys. Res. (in press) 1974. 6. Lur, A. T. Y., PERREAULT,P., AKASOFU,S.-I. and ANGER, C. D. The diffuse aurora. Planet. Space Sci. 21,867,1973. 7. STENBAEK-NIELSEN, H. C., Wzscorr. E. M. and DAVIS,T. N. Differences in aurora1 intensity at conjugate points. J. geophys. Res. 78,659, 1973. 8. FRANK, L. A. and ACKERSON,K. Observations of changed particle precipitation into the aurora1 zone. J. PeoDhvs. Res. 70,3612,1971. 9. W&N%HAM, J. D:, YAS.UHARA,F., AKA~OFU,S.-I. and HEMKILA, W. The precipitation pattern of electrons (50-10 keV) during quiet and substorm times in the 22-03 MLT sector. J.geophys. Res. (submitted) 1974. 10. BURROWS,J. R. The plasma sheet in the evening sector. Earth’s Particle and Fields. Advanced Study Institute, Sheffield, England, August, 1973. 11. GURNEY, D. A. and FRANK, L. A. Observed relationships between electric fields and auroral particle precipitation. J.geophys. Z&s. 78,145, 1973. 12. WESCOTT,E. M. Electric field measurements, p. 73. Solar-Terrestrial Relations, Proc. Conf. Univ. Calgary, August 28-Sept. 1,1972. 13. ARMSTRONG,J. C., AKASOFU,S.-I. and ROSTOKER,G. A comparison of satellite observations of Birkeland currents with ground observations of visible aurora and ionospheric currents. J. geophys. Res. (in press) 1974. 14. ROMICK,G. J., ECKLUND, W. L., GREENWALDK. A. and BALSLEY,B. B. The interrelationship between the > 130 kev electron trapping boundary, VHF radar backscatter and the visual aurora. J. geophys. Res. (in press) 1974. 15. AKASOFU,S-I. Polar and Magnetospheric Substorms. Reidel, Dordrecht, Holland, 1968. 16. SNYDER,A. L., AKASOFU,S.-I. and DAX%, T. N. Aurora1 substorms observed from above the north polar region by a satellite, J. geophys. Res. 79, 1393, 1974.
1726
RESEARCH
NOTES
17. BURROWS,J. R. and MCDIARMID, I. B. Trapped particle boundary regions, p. 83, Critical Problems of Magnetospheric Physics. Proc. Joint COSPAR/IAGA/URSI Symposium. (Ed. E. F. DYER), Madrid, Spain. 1972. 18. Fwi, L. A. Plasma in the polar magnetosphere. J. geophys. Res 76,5205, 1971. 19. AKASOFU, S.-I., HONES, E. W., Jr., BAME, S. J., ASBRIDGE,J. R. and Lur, A. T. Y. Magnetotail and boundary layers plasmas at a geocentric distance of 18R,: Vela 5 and 6 observations. J. geophys. Res. 78,725l. 1973. 20. AKASOFU, S.-I., PERREAULT,P. D., YASUHARAF. and MENG, C.-I. Aurora1 substorms and the interplanetary magnetic field. J. geophys. Res. 78,7490,1973. It is quite likely that the diffuse aurora1 region is connected to the main part of the plasma sheet. However, it is not certain if the discrete aurora1 region is connected to the upper (in the northern hemisphere) and lower (in the southern hemisphere) parts of the plasma sheet (that is, the region of ‘closed’ field lines) or if discrete auroras occur along ‘open’ field lines. Some phenomena (i.e. the presence of particles with large (- 90’) pitch angles; the electric field reversal near the poleward boundary) suggest that discrete auroras occur along ‘closed’ field lines, while some other phenomena (i.e. the penetration of solar electrons and the varying degree of conjugacy) are thought to suggest that they occur along ‘open’ field lines. Thus, it is quite dangerous to conclude from a single phenomenon whether those particular field lines are ‘open’ or ‘closed’. Acknowledgements-I would like to thank my colleagues at the Geophysical Institute, University of Alaska and Mr. J. Whalen, AFCRL, for their helpful discussions during the preparation of this Note. The work reported here was supported by a National Science Foundation Grant 9A-36873X. S.-I. AKASOFU Geophysical Institute, University of Alaska, Fairbanks, Alaska, U.S.A. REFERENCES AKA~~FU, S.-I. (1968). Polar and Magnetospheric Substorms. Reidel, Dordrecht, Holland. BUCHAU,J., GASSMANN,G. J., PIKE, C. P., WAGNER,R. A. and WHALEN,J. A. see (4) in Reference for Table. EATHW, R. H. (1967). Aurora1 proton precipitation and hydrogen emissions. Rev. Geophys. 5, 207. REID, G. C. and REES, M. H. (1961). The systematic behavior of hydrogen emission in the aurora-II. Planet. Space Sci. 5, 99. SANDFORD,B. P. (1964). Aurora and airglow intensity variations with time and magnetic activity at southern high latitudes. J. atmos terr. Phys. 26, 749. SANDFORD,B. P. (1967). High latitude night-sky emissions. In AurorasandAirgZow (Ed. B. M. MCCORMAC), pp. 443452. Reinhold, New York. SNYDER,A. L., Jr., AKASOFU,S-1. and DAVIS, T. N. See (16) in Reference for Table. WHALEN,J. A., BUCIUN, J. and WAONER,R. A. See (3) in Reference for Table.