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A Suggestion on How to Get Substorm Research Moving Again

C. T. Russell

Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics University of California, Los Angeles July 1993

in Strategies for the Tail and Substorm Campaign, edited by W.J. Hughes, 24-31, Boston University Center for Space Physics, 1994.

The realization that the interplanetary magnetic field (IMF) joins with the dayside terrestrial magnetic field when the IMF is southward is over 3 decades old [Dungey, 1961], as is the complementary idea of connection between the IMF and magnetotail magnetic field when the IMF is northward [Dungey, 1963]. In these so-called reconnecting magnetospheres, the rate at which magnetic flux becomes linked governs the rate of circulation of the plasma inside the magnetosphere, i.e. the magnetospheric electric field. The ratio of the potential drop across the magnetosphere due to reconnection to the potential drop across and equivalent distance in the solar wind flow might be considered to be the efficiency of reconnection. The flow in this model is steady state. There is no net transfer of magnetic flux or plasma.

It was not long before time dependence was added to the Dungey model to generate a model for magnetospheric substorms. This model has been called the near-earth neutral point model of substorm [McPherron et al., 1973; Russell and McPherron, 1973]. This model is illustrated in Figure 1. Initially the IMF is northward and the magnetosphere is in a quiescent equilibrium state. Then the IMF turns southward, reconnects with the dayside closed magnetic field at a rate, M, and transfers magnetic flux to the magnetotail. This decreases the magnetic flux on the dayside D, and increases the magnetic flux in the lobes of the magnetotail, T. If the reconnection rate in the tail, R, immediately rose to meet that at the nose, the electric field in the magnetosphere would rise and fall, but the magnetic flux in the various regions of the magnetosphere would remain fixed. However, the reconnection rate in the tail is not immediately responsive to the nose reconnection rate and magnetic flux in the tail builds up until such (explosive?) reconnection begins. At that point the rate of connection of magnetic flux from the night plasma sheet to the dayside, C, increases depleting the initial buildup of closed flux in the plasma sheet, PS. If reconnection begins in the tail on initially closed magnetic field lines, a plasmoid or magnetic bubble is formed that is ejected tailward [Russell, 1974].

If there has been any progress in this area in the intervening two decades, it is that reconnection has now become generally accepted as an important magnetospheric process since it was quantitatively verified at the dayside magnetopause [Paschmann et al., 1979]. Thus, essentially all popular substorm models invoke a cycle of reconnection, and the debate over the difference between substorm models has evolved to a discussion of subtle differences in the timing of the sequence of events occurring during the substorm [Fairfield 1991; Kennel, 1993]. However, there is a real remaining substorm problem: what process determines the moment of auroral breakup that by definition [Rostoker et al., 1980] defines the onset of the expansion phase of a substorm. This process could be a global instability, a local instability with global consequences or an external trigger. It is not that ideas are lacking. In fact, several authors have demonstrated that the number of possible models exceeds the number of researchers [ e.g., Baker et al., 1982; Baker and McPherron, 1990]. The problem, as I see it, is that development of the tools for solving the problem has been ignored. It is the purpose of this note to suggest an approach to resolving this impasse.

Not the Road to Progress

Before discussing the positive steps that need to be followed to bring the substorm onset problem to closure, we will mention a few paths that seem to be blind alleys. The first blind alley exists because substorms have a bad name. This bad name has nothing to do with the quality of research done in this field. Rather, the name is bad because substorms are not small geomagnetic storms or the basic building blocks of geomagnetic storms. The intensity of geomagnetic storms seems to be quite independent of the strength of "substorm" activity in the auroral oval [Russell et al., 1974; Feldstein 1992]. The conditions in the solar wind that lead to storms are prolonged strong southward IMF and high solar wind velocities, while the conditions that lead to substorms are short ( ~1 hour) southward IMF periods with small (~ 5 nt) southward fields. The former conditions lead to a buildup of the ring current, but the latter usually do not. Substorms are usually best studied when they are isolated from other activity. Storms can only be studied when activity is high for a very extended period ( ~ days).

Progress has been made in the study of substorms through Coordinated Data Analysis Workshops (CDAWs). While this progress is very welcome, the progress has come at a large price for both the researchers involved and the community at large, as the workshops have become very large and unwieldy. Many researchers have the tools to analyze these data, and they do not have to be studied in large community efforts. These data need to be released to the community at large for individual analysis. Community efforts usually lead to lowest common denominator science and slow the publication of any results.

A regular series of meetings maintains the appearance of progress, but it does not always signal the existence of real progress. It is commendable that the substorm community wishes to converse as often as they do to work on the substorm problem, but a cynic might also view this as evidence that the community is more interested in traveling than on working, especially since the same ideas about substorm processes are being debated today as were being debated twenty years ago. One cynic has even suggested a conspiracy theory of substorms, that the principals have agreed not to solve the problem in order to keep the field alive.

A Quantitative Approach

Most researchers today would say that it is clear that substorms come in a variety of sizes. In fact, the hotly debated topic of pseudo-breakup [Koskenin et al., 1993] may be simply a question of size. Perhaps events below a critical size do not proceed through the full cycle of substorm associated changes. But what is that size? This acceptance of the existence of varying substorm sizes was not always so. The 1979 Victoria conference on the definition of substorms refused to consider size as an important issue in substorms. In fact, the author distinctly remembers one senior scientists stating flatly that substorms did not have different sizes. This statement appears to be true in a practical sense because there is no accepted measure of the size of a substorm despite over 30 years of research on substorms. Atmospheric storms and hurricanes have a measure of size. Earthquakes have measures of size depending on either the destruction caused or the energy released. Solar flares have several measures of size depending on the wavelength viewed. Substorms have no measure of strength. Plots of AE are generally now shown, but the AE index is not a good measure when the substorm is small and is poleward of the usual longitudinal AE chain of stations or if it is large and equatorward of the AE chain. Also, the AE chain is very uneven in its longitudinal coverage. Thus, researchers seem to have shied away from quantifying substorms by their peak AE response. Researchers do not talk about a 500 nT substorm, a 1000 nT substorm, etc.

If we are to make progress in studying the magnetosphere, to test our understanding and our models, we need to understand quantitatively how the current systems respond to changing solar wind conditions. These currents include the Chapman-Ferraro current, Birkeland currents, tail currents and substorm currents. It is clear that these currents do vary with solar wind conditions. For example, the position of the magnetopause moves inward when the IMF is southward, an amount proportional to the southward component of the IMF [Petrinec and Russell, 1992]. This effect is presumably caused by the increase in the dayside Birkeland current system [Russell, 1975]. The response of the surface magnetic field to sudden changes in the solar wind dynamic pressure is less when the IMF is southward than when it is northward {Russell and Ginskey, 1993]. Again this could be due to an increase in the region 1 currents but also tail current increases could contribute. The most clear evidence that the tail currents increase when the IMF is southward is that the strength of the field in the tail lobes increases due to increased flaring of the tail boundary in proportion to the southward component of the IMF [Petrinec and Russell, 1993]. These studies begin to quantify the dependence of the magnetospheric currents on substorms but much more needs to be done, especially for the currents associated with substorms.

Two studies that have addressed the effects of substorms on the current systems (but not the effect of IMF on the substorm) have been carried out by Pulkkinen et al. [1993] and Chun and Russell [1992]. The former study examined how the tail current moved and was enhanced during a substorm. The latter study examined how region 1 or 2 currents varied on average during a substorm. Again, these studies are only a beginning. Much more needs to be done on both topics.

In addition to quantitative studies of the control of the currents by the solar wind and thus control of the currents by the substorm, and important quantitative study that is crying to be done is what are the solar wind conditions that lead to a substorm onset. We know from Kokubun et al. [1977] that a sudden impulse can trigger a substorm if the IMF has been southward before the sudden pressure change reaches the magnetosphere but in the absence of a sudden impulse what conditions are needed? Does the IMF have to be 5 nT southward for an hour? If so, would 10 nT southward for a half hour be just as effective? Figure 4 shows examples of the IMF signature prior to four substorms [Caan et al., 1977]. This work suggests that s northward turning after a southward period can lead to a substorm onset, but no attempt was made to relate the strength of the subsequent goemagnetic activity to the interplanetary or geomagnetic conditions. We need to pursue such cause and effect studies.

The work of Pulkkinen et al. [1992], Chun and Russell [1992] and Petrinec and Russell [1993a,b] among other demonstrates that we have the tools to undertake quantitative studies of the changes in the magnetosphere that take place at substorm times. If we are to go beyond this work we need to have a quantitative measure of the size of the associated substorm effects. There are many possibilities. By default the peak AE index is being used now, at least on occasion. It would seem more appropriate to calculate the intergrated current flowing through the auroral oval as another measure, and the time integrated current over the course of the substorm. Another possible measure would be the size of the mid-latitude positive bay. Again one could look at the maximum disturbance during the substorm, examine the spatial extent of the disturbance and/or look at the time integrated disturbance, the area of the bay. It is not clear a priori which parameter would be best. Perhaps all measures might reveal different information about the substorm. All should therefore be examined.

Recommendations

How can a program like GEM help to get substorm research moving again? First, the working groups should concentrate on determining what are the true outstanding questions. It matters little to the progress of the field who proposed what model when. It does matter if there is a process that once started inevitably leads to the sequence of processes seen in a substorm. Since there is a limited amount of funding, lists of prioritized studies could be prepared. Individual researchers could always argue (in their proposals) with these lists but they would have to make cogent arguments if they wanted to pursue different directions. In particular I would expect that such rational lists would emphasize quantitative studies and hypothesis testing and would be a very positive force for improvement.

Numerical modeling can play a very salutary role in these studies because by its very nature it is quantitative. Efforts that take the observed time sequence of interplanetary conditions and predict magnetospheric response such as the MHD modeling at NRL under Joel Fedder are proving very profitable.

Finally, we need to have an approximate measure of the size of substorms. It is clear that substorms can have a variety of strengths. Until we learn to distinguish strong substorms from weak ones we may be pursuing minor processes in the magnetosphere as much as the major processes and comparing apples and oranges. For if we do not make progress soon in substorm research, there will be more converts who believe the conspiracy theory of substorms.

Acknowledgements. This polemic was supported by the National Science Foundation under research grant ATM 92-13379.

References

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Chun, F. K., and C. T. Russell, The evolution of field-aligned currents as a function of substorm phase, J. Geophys. Res., 96, 15,801, 1991.

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Petrinec. S. M., and C. T. Russell, External and internal influences on the size of the dayside terrestrial magnetosphere, Geophys. Res. Lett., 20, 339, 1993a.

Petrinec, S. M., and C. T. Russell, An empirical model of the size and shape of the near-Earth magnetotail, Geophys. Res. Lett., submitted, 1993b.

Pulkkinen, T. I., D. N. Baker, R. J. Pellinen, J. Buchner, H. E. J. Koskinen, R. E. Lopez, R. L. Dyson, and L. A. Frank, particle scattering and current sheet stability in the geomagnetic tail during the substorm growth phase, J. Geophys. Res., 97, 19,283, 1992.

Rostoker, G., S.-I. Akasofu, J. Foster, R. A. Greenwald, Y. Kamide, K. Kawasaki, A.T.Y. Lui, R. L. McPherron, and C. T. Russell, Magnetospheric substorms-definition and signatures, J. Geophys. Res., 85, 1341, 1980.

Russell, C. T., The solar wind and magnetospheric dynamics, in Correlated Interplanetary and Magnetospheric Observations, edited by D. E. Page, p. 3, D. Reidel Publ. Co., Dordrecht, Holland, 1974.

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

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Russell, C. T., R. L. McPherron, and R. K. Burton, On the cause of geomagnetic storms, J. Geophys. Res., 79, 1105, 1974b.




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