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Reply to professor Rostoker's comments
Planet. Printed
SpaceSci., in Great
Vol. 37, No. 9, pp. 1153-I 154, 1989 Britain.
REPLY
0032-0633/89 03.OOf0.00 Pergamon Press plc
TO PROFESSOR
ROSTOKER’S
J. R. DUDENEY
COMMENTS
and A. S. RODGER
NERC, British Antarctic Survey, Madingley Road, Cambridge, CB3 OET, U.K. (Received in$nal form 24 April 1989)
Recently Dudeney and Rodger (1988) published a case study of the poleward edge of the trough which indicated thinning of the plasma sheet during the growth phase of a substorm (not with the expansive phase as suggested by Professor Rostoker). This paper has been criticised by Professor Rostoker. Here we address his criticisms.
ASSOCIATION
OF
Pi2
WITH
SUBSTORM
EXPANSIVE
PHASE ACTIVITY
Professor Rostoker contends that every Pi2 is followed by an expansive phase of a substorm. We do not believe that this 100% coupling has been established. In the case we reported, we did not rely upon the geomagnetic data from the Southern Hemisphere, but looked at both the aurora1 indices (which are derived from Northern Hemisphere data) and individual traces from selected Northern Hemisphere magnetometer stations (see p. 1287 of our paper). It should also be noted that our analysis period was very close to magnetic midnight, a time for which Rostoker’s model suggests the initial geomagnetic disturbance in the expansive phase occurs. We do not believe that the 10 nT disturbance evident in the Z-component of the Halley magnetometer trace at around 00:55 U.T. is evidence of a substorm, particularly since the South Pole Magnetometer, in the same magnetic meridian about 12” poleward of Halley, shows no discernible disturbance. Further, we have an alternative interpretation of the Halley disturbance, given on p. 1290 of our paper. In summary, our view is that, though there clearly is a very good association between the occurrence of Pi2s and substorms, it is not established that this is exclusive, and our example does not appear to follow the current fashion.
THE USE OF GROUND-BASED
DATA TO EVALUATE
SUB-
STORM MODELS
We owe no particular allegiance to any substorm model or group proposing a model. Only recently
(Rostoker and Eastman, 1987), has there been a publication in the open literature which described the Plasma Sheet Boundary Layer (PSBL) model in any detail. We have based our judgement on many lively discussions between the protagonists of the nearEarth neutral line (NENL) and PSBL models at international meetings, the Hones et al. (1986) discussion of the contrasts of the two models, and Rostoker and Eastman (1987). There has clearly been an evolution in everybody’s thinking over the past few years, from an initial position of extreme polarity, in which one side of the argument was unable to entertain any of the other side’s views. It is not many years since the reality of plasma sheet thinning prior to the expansive phase of a substorm was denied by many members of the community. Though the topic is “alluded” to in Rostoker and Eastman (1987), it is not clear. The topic is not covered in Eastman et al. (1988). We accept that Professor Rostoker has now developed his description to include plasma sheet thinning explicitly, though we presume he does not hold with the limit case of thinning to a near-earth neutral line. We thank him for his clarification of this point. However, it is our impression that the field has now moved on, and the real developments are coming from the attempts to bring together the various competing descriptions of substorm processes [e.g. the NENL, the PSBL, the Magnetosphere-Ionosphere model (Kan et al., 1988)]. From this synergy, we can expect a more comprehensive description which will contain the best elements of all the theories (e.g. Lyons and Nishida, 1988).
INTERPRETATION
OF THE TERM “DIPOLARIZATION”
Our belief is that dipolarization must occur as field-lines close to the earth in the plasma sheet shorten in response to thinning in the growth phase of a substorm. We refer Professor Rostoker to Fig. 2 of Eastman et al. (1988), where field-lines 1 and 2 clearly must shorten, and thus become more dipolar through time period l-5 (the period of interest in our paper). This does not mean that dipolarization is taking place
1153
J. R. DUDENEY and A. S. RODGER
1154
throughout the night-side of the magnetosphere, and we had no intention of suggesting such a possibility. Indeed both NENL and now the PSBL models include stretching of plasma sheet field-lines adjacent to the boundary layer during the growth phase of the substorm. REFERENCES Dudeney, J. R. and Rodger, A. S. (1987) Ionospheric signature of nlasma sheet thinning prior to a substorm. Planet Space SC; 36, 1285. -_ Eastman, T. E., Rostoker, G., Frank, L. A., Huang, C. Y. and Mitchell, D. G. (1988) Boundary layer dynamics in
the description of magnetospheric substorms. J. geophys. Res. 93, 14,411. Hones, E. W., Jr., Fritz, T. A., Birn, J., Cooney, J. and Bane, S. J. (1986) Detailed observations of the plasma sheet during a substorm on April 24, 1979. J. geophys. Res. 91, 6845. Kan, J., Zhu, L. and Akasofu, S.-I. (1988) A theory of substorms; onset and subsidence. J. geophys. Res. 93, 5624. Lyons, L. R. and Nishida, A. (1988) Description of substorms in the tail incorporating boundary layer and neutral line effects. Geophys. Res. Lett. 15, 1337. Rostoker, G. and-Eastman, T. (1987) The boundary layer model for magnetospheric substorms. J. geophys. Res. 92, 12,187.
SpaceSci., in Great
Vol. 37, No. 9, pp. 1153-I 154, 1989 Britain.
REPLY
0032-0633/89 03.OOf0.00 Pergamon Press plc
TO PROFESSOR
ROSTOKER’S
J. R. DUDENEY
COMMENTS
and A. S. RODGER
NERC, British Antarctic Survey, Madingley Road, Cambridge, CB3 OET, U.K. (Received in$nal form 24 April 1989)
Recently Dudeney and Rodger (1988) published a case study of the poleward edge of the trough which indicated thinning of the plasma sheet during the growth phase of a substorm (not with the expansive phase as suggested by Professor Rostoker). This paper has been criticised by Professor Rostoker. Here we address his criticisms.
ASSOCIATION
OF
Pi2
WITH
SUBSTORM
EXPANSIVE
PHASE ACTIVITY
Professor Rostoker contends that every Pi2 is followed by an expansive phase of a substorm. We do not believe that this 100% coupling has been established. In the case we reported, we did not rely upon the geomagnetic data from the Southern Hemisphere, but looked at both the aurora1 indices (which are derived from Northern Hemisphere data) and individual traces from selected Northern Hemisphere magnetometer stations (see p. 1287 of our paper). It should also be noted that our analysis period was very close to magnetic midnight, a time for which Rostoker’s model suggests the initial geomagnetic disturbance in the expansive phase occurs. We do not believe that the 10 nT disturbance evident in the Z-component of the Halley magnetometer trace at around 00:55 U.T. is evidence of a substorm, particularly since the South Pole Magnetometer, in the same magnetic meridian about 12” poleward of Halley, shows no discernible disturbance. Further, we have an alternative interpretation of the Halley disturbance, given on p. 1290 of our paper. In summary, our view is that, though there clearly is a very good association between the occurrence of Pi2s and substorms, it is not established that this is exclusive, and our example does not appear to follow the current fashion.
THE USE OF GROUND-BASED
DATA TO EVALUATE
SUB-
STORM MODELS
We owe no particular allegiance to any substorm model or group proposing a model. Only recently
(Rostoker and Eastman, 1987), has there been a publication in the open literature which described the Plasma Sheet Boundary Layer (PSBL) model in any detail. We have based our judgement on many lively discussions between the protagonists of the nearEarth neutral line (NENL) and PSBL models at international meetings, the Hones et al. (1986) discussion of the contrasts of the two models, and Rostoker and Eastman (1987). There has clearly been an evolution in everybody’s thinking over the past few years, from an initial position of extreme polarity, in which one side of the argument was unable to entertain any of the other side’s views. It is not many years since the reality of plasma sheet thinning prior to the expansive phase of a substorm was denied by many members of the community. Though the topic is “alluded” to in Rostoker and Eastman (1987), it is not clear. The topic is not covered in Eastman et al. (1988). We accept that Professor Rostoker has now developed his description to include plasma sheet thinning explicitly, though we presume he does not hold with the limit case of thinning to a near-earth neutral line. We thank him for his clarification of this point. However, it is our impression that the field has now moved on, and the real developments are coming from the attempts to bring together the various competing descriptions of substorm processes [e.g. the NENL, the PSBL, the Magnetosphere-Ionosphere model (Kan et al., 1988)]. From this synergy, we can expect a more comprehensive description which will contain the best elements of all the theories (e.g. Lyons and Nishida, 1988).
INTERPRETATION
OF THE TERM “DIPOLARIZATION”
Our belief is that dipolarization must occur as field-lines close to the earth in the plasma sheet shorten in response to thinning in the growth phase of a substorm. We refer Professor Rostoker to Fig. 2 of Eastman et al. (1988), where field-lines 1 and 2 clearly must shorten, and thus become more dipolar through time period l-5 (the period of interest in our paper). This does not mean that dipolarization is taking place
1153
J. R. DUDENEY and A. S. RODGER
1154
throughout the night-side of the magnetosphere, and we had no intention of suggesting such a possibility. Indeed both NENL and now the PSBL models include stretching of plasma sheet field-lines adjacent to the boundary layer during the growth phase of the substorm. REFERENCES Dudeney, J. R. and Rodger, A. S. (1987) Ionospheric signature of nlasma sheet thinning prior to a substorm. Planet Space SC; 36, 1285. -_ Eastman, T. E., Rostoker, G., Frank, L. A., Huang, C. Y. and Mitchell, D. G. (1988) Boundary layer dynamics in
the description of magnetospheric substorms. J. geophys. Res. 93, 14,411. Hones, E. W., Jr., Fritz, T. A., Birn, J., Cooney, J. and Bane, S. J. (1986) Detailed observations of the plasma sheet during a substorm on April 24, 1979. J. geophys. Res. 91, 6845. Kan, J., Zhu, L. and Akasofu, S.-I. (1988) A theory of substorms; onset and subsidence. J. geophys. Res. 93, 5624. Lyons, L. R. and Nishida, A. (1988) Description of substorms in the tail incorporating boundary layer and neutral line effects. Geophys. Res. Lett. 15, 1337. Rostoker, G. and-Eastman, T. (1987) The boundary layer model for magnetospheric substorms. J. geophys. Res. 92, 12,187.