Explosive Reconnection: Red Herring or Paradigm Lost?

C. T. Russell

  Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, U.S.A.

Originally published in:
Eos, Vol. 66, No. 32, pp. 581, 1985.


  The recent article by Akasofu [1985] is not an accurate statement of the present status of substorm research. Neither is the magnetospheric substorm "almost exclusively discussed in terms of hypothetical explosive reconnection," nor is it "only recently that the input-output relationship of the magnetosphere has been examined." The question posed by Akasofu, "Is explosive magnetic reconnection. . .the primary energy supply process for substorms or. . .an unworkable hypothesis," attacks a straw man of Akasofu's own de- sign. In the process of attacking this straw man, Akasofu confuses the energization of the magnetosphere as a whole with the energization of magnetospheric substorms and ignores the fact that his present model is essentially in complete accord with the original near-tail reconnection model for substorms [cf. McPherron et al., 1973; Russell and McPherron, 1973]. Since Akasofu claims that misdirection in substorm research has been occurring for 2 decades, we briefly review the history of the reconnection model for magnetospheric energization.

The reconnecting magnetosphere was proposed by Dungey [1961] as a steady state mechanism for the energization of the magnetospheric plasma. This model received much observational support in the 1960's in the form of correlations between the direction of the interplanetary magnetic field and geomagnetic activity [cf. Fairfield and Cahill, 1966; Hirshberg and Colburn, 1969]. In the early 1970's, this work became more quantitative. Arnoldy [1971] proposed the half-wave rectifier model for the interaction of the interplanetary magnetic field with the magnetosphere, in which reconnection occurred only for southward directed interplanetary magnetic fields. A phenomenological model for substorms was developed on the basis of the observed reaction of various regions of the magnetosphere to southward turnings of the interplanetary magnetic field [McPherron et al., 1973; Russell and McPherron, 1973]. The key to explaining the observed phenomena was that there were different time delays in the response of different portions of the magnetosphere to a southward turning of the interplanetary magnetic field (IMF). The dayside magnetosphere and polar cap were almost immediately responsive to the IMF. Magnetic flux and plasma were convected across the polar cap and added to the tail, beginning with the southward turning, but remained in the tail and accumulated there until reconnection in the center plane of the tail could either begin or increase in rate to remove the added flux. The critical observation here was that magnetic flux accumulated in the tail until a substorm expansion phase began.

There are many current systems flowing in the magnetosphere. Only a few of these are specifically associated with substorms. So, too, there are many mechanisms by which energy is deposited in the magnetosphere, only a few of which are associated with substorms. For example, during a geomagnetic storm, of the order of 10 ergs of particle energy are stored in the ring current in the magnetospheric equator. When this ring current is being energized, it is the dominant sink of energy from the solar wind. The control of this energization was studied in the early 1970's, and a quantitative model developed that predicted both the energization and decay of the ring current [Burton et al. 1975]. This model was based on the half-wave rectifier model of reconnection and it was a quasi steady state model. The later model of Perrault and Akasofu [1978] essentially duplicated this model but made an ad hoc change in the dependence on the direction of the IMF (to a modified half-wave rectifier) and in the dependence on the magnetic field strength (from linear to quadratic). They also added a couple of additional magnetospheric energy sinks, but in every case these sinks were a factor of 3 to 10 smaller than the ring current storage. There has been much discussion of the pros and cons of these modifications, but these discussions obscure the fact that the present model advocated by Akasofu [1985] is for all intents and purposes the original growth phase model of substorms [McPherron, 1970; Russell and McPherron, 1973]. If the concurrence of these models can be taken to indicate their accuracy, then magnetospheric research has not been misdirected for the last 20 years.

A legitimate question to ask is how fast reconnection in the tail occurs. When there are steady southward IMF conditions, the magnetosphere appears to enter a steady state disturbed condition, implying that dayside and nightside reconnection are occurring at equal rates as averaged over moderately short time scales (tens of minutes). This steady state disturbance is seen not only in the ring current but also in auroral zone currents [Pytte et al., 1978], in the tail [Caan et al., 1973], and in plasma sheet flows [Hones et al., 1976]. In non-steady state situations, as in the case of a substorm triggered by a northward turning of the IMF after a period of southward field, reconnection may remove flux rapidly from the tail, but such removal has a time scale of the order of half an hour or more, as illustrated by the decay time of the enhanced tail lobe field strength at substorm onset [Russell and McPherron, 1973]. Thus our present understanding of tail reconnection does not demand that it be explosive even though the initiation of reconnection in the tail may be rapid. From a global view- point, there appears to be no need to invoke explosive reconnection to energize either the magnetosphere as a whole or substorms in particular. On the other hand, explosive reconnection is not inconsistent with magnetotail observations [cf. Coroniti, 1985]. In conclusion, it is important to realize that a consensus has developed on the essential details of response of the magnetosphere to solar wind input. The present arguments in this area of research revolve around refinements of that basic model. There are real questions to be answered, and there are rates to be measured. However, to claim that the field has gone off i' the wrong direction is, at a minimum, a gross exaggeration. Explosive reconnection may occur, but existing models do not require it. In fact, in discussions of magnetospheric energization, explosive reconnection is more like a red herring then it is a sacred cow.



Akasofu, S.-l., Explosive magnetic recon- nection: Puzzle to be solved as the ener gy supply process for magnetospheric substorms?, Eos Trans. ACU, 66, 9, 1985.

Arnoldy, R. L., Signature in the interplanetary medium for substorms, J. Geophys. Res., 76, 5189, 1971.

Burton, R. K., R. L. McPherron, and C. T. Russell, An empirical relationship be tween interplanetary conditions and Dst J. Geophys. Res., 80, 4204, 1975.

Caan, M. N., R. L. McPherron, and C. T. Russell, Solar wind and substorm-related changes in the lobes of the geomagnetic tail, J. Geophys. Res., 78, 8087, 1973.

Coroniti, F. V., Explosive tail reconnection: The growth and expansion phase of magnetospheric substorms, J. Geophys. Res., 90, 7427, 1985.

Dungey, J. W., Interplanetary magnetic field and the auroral zones, Phys. Rev., Lett., 6, 47, 1961.

Fairfield, D. H., and L. J. Cahill, Jr., Transition region magnetic field and polar magnetic disturbances, J. Geophys Res., 71, 155, 1966.

Hirshberg, J., and D. S. Colburn, Interplanetary field and geomagnetic variations: A unified view, Planet. Space Sci., 17, 1183, 1969.

Hones, E. W., Jr., S. J. Bame, and J. R. Asbridge, Proton flow measurements in the magnetotail plasma sheet made with Imp 6, J. Geophys. Res., 81, 227, 1976.

McPherron, R. L., Growth phase of magnetospheric substorms, J. Geophys. Res., 75, 5592, 1970.

McPherron R. L. C. T. Russell, and M. P. Aubry Satellite studies of magnetospheric substorms on August 15, 1968, 9, Phenomenological model for sub- storms, J. Geophys. Res., 78, 3131, 1973.

Perreault, P., and S.-l. Akasofu, A study of geomagnetic storms, Geophys. J. R. Astron. Soc., 54, 547, 1978.

Pytte, T., R. L. McPherron, E. W. Hones Jr., and H. I. West, Jr., Multiple-satellite studies of magnetospheric substorms: Distinction between polar magnetic substorms and convection-driven negative bays, J. Geophys. Res., 83, 663, 1978.

Russell, C. T., and R. L. McPherron, The magnetotail and substorms, Space Sci. Rev., 15, 205, 1973.

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