J. Geomag. Geoelectr., 44,109-117,1992

Is Substorm Occurrence a Necessary Condition for a Magnetic Storm?*

Y. KAMIDE

Solar-Terrestrial EnvironmentLaboratory, Nagoya University,Toyokawa 442, Japan

(ReceivedNovember 28,1991)

It is shown that "having substorms" is not a necessary condition for a magnetic storm. The main phase of magnetic storms develops because of sustained, southward interplanetary magnetic field (IMF), not because of frequent occurrence of intense substorms.

1. Introduction

A geomagnetic storm is identified in general by the existence of a main phase during which the magnetic field on the earth's surface is depressed. This depression is caused by the ring current flowing westward in the magnetosphere, and can be monitored by the Dst index. It is commonly assumed that the intensity of magnetic storms can be defined by the minimum Dst value, or the maximum depressed Dst at the main phase. RUSSELL and MCPHERRON(1973) statistically showed the number of storms (per decade) as a function of Dst. Their diagram indicates that we can have more than 100 storms per year if we count the number of storms whose intensity is larger than 20nT in Dst. The statistics tell us that once a solar cycle a great magnetic storm of 600-700nT in Dst can occur, and every 100 years or so a storm having 2000nT in Dst should occur. More seriously, if we are allowed to extend the straight fitted line to the smaller Dst direction, we can see that this line intercepts at N=650 for Dst=-10nT. KAMIDE (1982) counted the total number of substorms (larger than 500nT in the AL magnitude) occurring in one year and found that there were 641 in 1978. We also know that Dst is approximately -10 nT for a typical substorm. This means that it is quite difficult to differentiate between small magnetic storms and large substorms. There are a number of different types of observations which can be confusing, making it difficult to understand the storm-substorm relationship consistently. Most of us tend to believe that the main phase of a magnetic storm is the interval in which many intense substorms must take place successively. Is this really the case? Is this a necessary condition to have a magnetic storm? How many is many? How intense is intense? How successive is successive?

*Portions of this paper were presented as a review talk at the topical symposium entitled "Physics and Predictions of Magnetic Storms and Disturbances" at the XXth IUGG General Assembly, held in Vienna, August, 1991.

109 110 Y,KAMIDE

2. Problems

The following are direct quotations from CHAPMAN(1962) who first defined polar substorms in terms of magnetic storms: A magnetic storm consists of sporadic and intermittent polar disturbances, lifetime being usually one or two hours. These I call polar substorms. Although polar substorms occur most often during magnetic storms, they appear also during rather quiet periods when no significant storm is in progress. This implies that most polar substorms cannot occur without the occurrence of magnetic storms, a condition which differs greatly from our present knowledge. On the other hand, after examining a number of IGY all-sky camera photographs, AKASOFU(1964), in defining the auroral substorm stated: The sequence of auroral events over the entire polar region during the passage from auroral quiet through the various active phases of subsequent calm is called an auroral substorm: it coincides with a magnetic substorm. Each auroral substorm has a lifetime of order 1-3hr. This definition led AKASOFU(1968) to the concept of the magnetospheric substorm, whose manifestations are the auroral substorm, the polar magnetic substorm, etc. The following excerpt is taken from one of the modem definitions by ROSTOKERet al. (1980): A magnetospheric substorm is a transient process initiated on the night side of the earth in which a significant amount of energy derived from the solar wind- magnetosphere interaction is deposited in the auroral ionosphere and magnetosphere. The substorm can thus be independent of the magnetic storm, at least, in their definition. The purpose of this paper is to attempt, like a traffic cop directs traffic, to avoid further confusion regarding the storm-substorm relationship by making clear: 1) What has been known for a long time, 2) What has been assumed or modeled, 3) What has been observed (under what conditions?), 4) What is interpreted (based on what?), S) What is inferred (from what?), 6) What is extrapolated or interpolated (using what data?). Specifically, this paper addresses the following two major questions: (1) What is the physical difference between storm-time substorms and non-storm-time substorms, if any? (2) Is the main phase of magnetic storms a result of (a) the impact of southward interplanetary magnetic field (IMF) which also relates to substorm activity, or (b) the successive occurrence of substorms which also have a direct relationship with southward IMF? Toward the end, it is shown that (1) there is no intrinsic difference between storm-time substorms and usual substorms, and (2) having sustained and large IMF, not the substorm occurrence, is important in generating the main phase of magnetic storms.

3. Storm-Substorm Relationship

Although not all substorms occur during magnetic storms or, more precisely, most substorms occur without being associated with magnetic storms, we know that substorms do occur during the main phase of magnetic storms. Can the main phase start without these Is Substorm Occurrence a Necessary Condition for a Magnetic Storm? 111 substorms? Can we reproduce Dst only from a knowledge of the AE indices which are a measure of substorm activity? The simplest assumption we can make is to suppose schematically that

STORM=ƒ°(SUBSTORM)i.

In the energy-balance equation;

dEldT=Q-E/T

where E is the ring current energy, Q is the rate of energy supply into the ring current, and T is the decay time. To test whether Dst, which is proportional to E (SCKOPKE, 1966), can be reproduced by substorms, it is assumed that Q is simply proportional to substorm activity.

In other words, we assume that the storm main phase is described by a linear superposition of substorm activity. The logic or physics behind this assumption is not, of course, that the auroral electrojets directly in the ionosphere generate the ring current in the magnetosphere, but that there is an energy reservoir in the magnetosphere from which some energy goes to the ring current and some energy goes into the polar ionosphere, generating polar substorms. When there is no energy input to the ring current, the ring current intensity decays with a relaxation time T as

This seems consistent with observations (e.g., DAVISand PARTHASARATHY,1967;FELDSTEIN et al., 1984; PISARSKIJet al., 1989). Many have noticed, however, that this simple assumption does not work very well. As is evident from Fig. 1, for the same intensity of the auroral electrojet, the ring current grows more efficiently during the main phase of magnetic storms (the second half of the time period) than during non-main-phase periods (the first half), indicating that the energy injection rate into the ring current is not simply proportional to substorm activity. The ring current can grow more easily during the main phase than at any other times for the same amount of substorm intensity. The above storm-substorm relationship can then be revised in such a way that

where a is the efficiency, which is largest in the early main phase of a magnetic storm. More practically,

where parameter ƒ¿(0

indicating that although the development of the ring current and the auroral electrojet is 112 Y. KAIVIIDE

Fig. 1. Interplanetaryfield and plasmadata for a two-dayinterval, in whichan intensemagnetic storm with a peakDst valueof -215nT occurred.The letter S in the B panel indicatesthe passageof an interplanetaryshock. After GoNZALEZ et al. (1989).

closely related, the partition of energy injected into the ring current and the polar ionosphere is not always in constant proportion. What determines this partition rate as a function of time? We should not be fully satisfied until we are able to have the physical implications of a. In the mathematical treatment, a is the efficiency of energy going into the ring current. What is controlling this efficiency? Is Substorm Occurrence a Necessary Condition for a Magnetic Storm? 113 114 Y. KAMIDE

Meanwhile, IMF observations come into play. Many authors have shown that there is one-to-one correspondence between southward IMF and substorm occurrence. RoSTOKER and FALTHAMMAR(1967), KOKUBUN (1972), RUSSELLet al. (1974), and BURTONet al. (1975), showed that the main phase of magnetic storms is associated with sustained, large southward IMF. In particular, RUSSELL et al. (1974) found that the southward Bz has to exceed a threshold level in order to trigger a storm main phase. They also showed that weak southward BZ does not necessarily lead to an increase in the ring current, even though such southward fields persist. A new definition of our question, even on the definition of the stormisubstorm relationship, arises: "What are the IMF signatures for magnetic storms?" SISCOE and CROOKER (1974) have developed a linear relation between the time rate of change in Dst, representing the energy transfer to the inner magnetosphere, and the "merging" electric field, by introducing a proportion factor as an explicit function of magnetospheric parameters. BURTON et al. (1975) presented an algorithm for predicting the storm Dst signature solely from knowledge of the velocity and the density of the solar wind and the southward component of the IMF. In referring to the energy budget equation, and assuming the injection rate linearly proportional to the dawn-to-dusk component of the interplanetary electric field, the algorithm pinpoints the causes of various types of storm Dst behavior. In Fig. 3, the predicted and observed Dst are shown to be well agreed, indicating that

Fig. 3. The square root of the solar wind dynamic pressure, the dawn-dusk interplanetary electric field, and the predicted (dashed line) and observed (solid line) Dst. After BURTON et al. (1975). Is SubstormOccurrence a NecessaryCondition for a MagneticStorm? 115 such assumptions are quite essential. The key element of the model is that the magnetosphere is treated as a rectifier for the southward IMF.

4. Requirements

From the discussion given in the previous sections, it is conceivable that the efficiency a(t) is a function of the IMF or the dawn-dusk electric field in the interplanetary medium. The rate of the injection of energetic particles into the inner magnetosphere is regulated by the IMF. It has also been shown that T (the decay time of the ring current) is not constant throughout a magnetic storm. Completely different from CHAPMAN'sdefinition (1962), it is possible that storms and substorms occur as rather independent processes. It is very important, however, to note that in constructing a new model of the storm/substorm relationship, we must not forget the following three points: 1) No magnetic storms have been observed during which intense substorms did not occur. 2) When geomagnetic activity is very high (like Kp = 9), it is always during a magnetic storm. 3) There is no intrinsic difference between storm-time substorms and usual substorms. Another complication in understanding magnetic storm processes and substorm processes in the magnetosphere lies in the existence of the quasi-steady state which is called Convection Bays (PYTTEet al., 1978) or the SMC (Steady Magnetospheric Convection) episode (SERGEEVand LENNARTSSON,1988). This has been shown to relate to continuous southward IMF. In addition, TSURUTANIand GONZALEZ(1987) have shown the existence of HILDCAAs, which stands for high intensity, long-duration, continuous, AE activity.

5. Conclusions

In conclusion, "having substorms" is not a necessary condition for a magnetic storm. Sometimes, even an anticorrelation exists between the substorm intensity during a magnetic storm and the intensity of that storm (AKASOFU,1981). The essence of this new view is that the IMF condition for magnetospheric substorms is included in the IMF condition for magnetic storms. According to KAMIDE et al. (1977), substorms occur with 100% probability whenever the 1-hr average IMF Bz value is less than -3 nT. According to GONZALEZand TSURUTANI(1987), to cause major (Dst<-100nT) magnetic storms, the IMF must be large (>10nT) and sustained (>3hrs). Figure 4 shows a schematic diagram differentiating magnetic storms, substorms, and convection bays. The main phase of magnetic storms starts when the high-speed solar wind along with large IMF hits the magnetosphere. At least, in a working model, it is convenient to assume that substorms occur as a byproduct of enhanced magnetospheric convection, that is generated by the large interplanetary electric field. This view seems to be consistent with the prediction by the Ring Coupling Model by SISCOE(1982), who suggest that the ring current in Dst is a directly driven quantity and the substorm electrojet in AE is an indirectly driven quantity. 116 Y. KAMIDE

Fig. 4. Flow chart outlining the growth of magnetic storms and substorms. Although, whenever the IMF has a large southward component leading to a magnetic storm, substorms do occur with a high probability, those two processes are independent.

This paper is based on an invited talk "Relationship between storms and substorms" presented at the XXth IUGG Assembly held in Vienna in August, 1991. I would like to thank the organizers, C.T. Russell and J.A. Joselyn, for providing an unique opportunity for me to make a critical review on the storm-substorm relationship which is one of the long-standing themes in the field. This paper has also been polished during the 4-day workshop in San Jose de Campos, Brazil, in November 1991. I am grateful to W. Gonzalez, H.W. Kroehl, G. Rostoker, B. T. Tsurutani, V. Vasyliunas, and D J. Williams for their stimulating and constructive discussion.

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