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

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Is Substorm Occurrence a Necessary Condition for a Magnetic Storm?* 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<a<1) expresses the efficiency of the ring current growth relative to the corresponding substorm intensity. The result is shown in Fig. 2 (KAMIDE and FUKUSHIMA, 1971), in which the efficiency is assumed to be an exponential function of time. Both for great and weak storms, observed Dst variations are quite nicely reproduced by a superposition of AL times the efficiency, 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 BURTON et 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.
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