Substorms, Storms, and the Near-Earth Tail. W. BAUMJOHANN* Y. KAMIDE, and R.. NAKAMURA

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1 J. Geomag. Geoelectr., 48, , 1996 Substorms, Storms, and the Near-Earth Tail W. BAUMJOHANN* Y. KAMIDE, and R.. NAKAMURA Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 442, Japan (Received April 1, 1995; Revised October 11, 1995; Accepted October 16, 1995) More than 5, measurements made with the AMPTE IRM satellite in the magnetotail at radial distances between 1 and 19 RE during 4 substorms have been used in a superposed epoch analysis, separating storm-time substorms from those occurring during intervals of low D,t levels. It is found that most of the tail signatures reported previously as typical for substorms are strongly influenced by magnetic storm activity. The magnetic field dipolarization and the Earthward convection in the near-earth tail are much more pronounced for substorms which occur during the main phase of magnetic storms than for non-storm substorms. Moreover, only for storm-time substorms the lobe magnetic pressure decreases during the expansion phase. These findings indicate that not all substorms are alike and that the near-earth neutral line scenario may apply only to the storm-time substorms. 1. Introduction While no magnetic storms are observed in the absence of intense substorms, a substorm can occur independently of a magnetic storm (Gonzalez et at, 1994, and references therein). In fact, the D,t variations are well reproduced through a linear superposition of the AL index, taking into account the efficiency of the energy input into the ring current (Kamide and Fukushima, 1971). It is not clear, however, whether successive occurrence of intense substorms is a necessary condition for a magnetic storm. Nonetheless, a better knowledge of possible differences in the substormassociated changes in the magnetotail for high and low D,t values is crucial to understand the magnetotail behavior associated with the solar wind-magnetosphere-ionosphere coupling during a magnetic storm as well as during a substorm itself. In the present paper we focus on the near-earth plasma sheet between about 1 and 2 RE, which is very dynamic during the substorm expansion phase. Using the major substorm onsets of Baumjohann et al. (1991), we perform a superposed epoch study, to see whether there are any differences between the response of the near-earth tail for substorms that occur during the main phase of magnetic storms and for substorms without any accompanying storm activity. 2. Instrumentation and Data The tail survey data set from the IRM satellite has been described earlier (e.g., Baumjohann et al., 1989; Baumjohann and Paschmann, 1989), and information on the superposed epoch data set can be found in Baumjohann et at (1991). The plasma data were obtained with the plasma instrument on AMPTE IRM (Paschmann et al., 1985). Three-dimensional ion and electron distributions with 128 angle channels, covering the full 4 7r sr solid angle, and 3 energy channels in the range from 2eV/e to 4 kev/e and 15 ev to 3 kev, respectively, are obtained every satellite rotation period of about 4.5 s. From each distribution, microcomputers within the instrument deduce moments of the distribution functions. The spin-averaged magnetic field data originate *On leave from Max-Planck -Institut fiir extraterrestrische Physik, 8574 Garching, Germany. 177

2 178 W. BAUMJOHANN et al. Table 1. Substorm onset times and corresponding D,e values. Substorm.Onset D.9t Value Substornc Onset D. Value Substorm Onset D e Value :4 UT -16 nt :45 UT -6 nt :15 UT -8 nt :32 UT -27 nt :12 UT OnT :58 UT 3nT :26 UT 32 nt :31JT -9 nt :3 UT -1 nt :46 UT 7nT :8 UT -49 nt :2 UT -5 nt :481JT 2nT :5R UT 2nT :28 UT -1 nt :22 UT 3nT :4 UT -21 nt :38 UT -8 nt :71JT -18 nt :2 UT -19 nt :5 UT -3nT :48 UT -1 nt : UT -15nT :49 UT -71nT 8, :2 UT -17 nt :14 UT -16 nt :5 UT -18 nt :2 UT -3 nt :42 UT -RnT :19 UT -8 nt :58 UT 3R nt :36 UT -23 nt :15 UT -6nT :3 UT -12 nt :16 1JT -21 nt :24 UT -19 nt :2 UT -19 nt :44 UT -26 nt :4 UT -6 nt :2 UT 11 nt from the IRM flux gate magnetometer, which has been described by Liihr et al. (1985). This instrument achieves a resolution of.1 nt. Spacecraft charging effects are used to distinguish between the plasma sheet boundary layer and the tail lobe on the one side and the central plasma sheet on the other side. harging effects are noticeable as excess electron densities, caused by inclusion of photoelectrons in the electron moment computations. Whenever NQ > NP''as we concluded that the satellite was outside the central plasma sheet, i.e., in the plasma sheet boundary layer or the lobe (cf. Fig. 5 of Baumjohann et al., 1988). The plasma sheet boundary layer can be distinguished from the lobe by the absence of measurable fluxes of kev electron and ion in the latter. For the eight months covered by the IRM tail survey, we checked data from geosynchronous and near-geosynchronous satellites and the Kakioka ground magnetic station for substorm onset signatures. We took only those onsets where the signatures were well defined and also the AL index showed a clear substorm development. For multiple substorm onsets, the major onset was taken. This way we neglected smaller, localized onsets and obtained a list of 4 substorm onsets which can be classified as major global onsets. Their onset times and corresponding D,t values axe given in Table 1. We then used the D,t index to distinguish between substorms that occurred during times of magnetic storm activity, here somewhat arbitrarily defined as D,t <-25 nt, and non-storm substorms, which occurred when D,t was at levels above -25 nt. We found 33 non-storm substorms and 7 substorms that were accompanied by magnetic storm activity. All of the latter occurred during the storm main phase. 3. Superposed Epoch Analysis Figure 1 shows the average behavior of the AE index, separately for those onsets which occurred during the main phase of a magnetic storm and the non-storm substorms during which the D,t index stayed above -25 nt. On average, the 45 min following the onsets can be characterized as the substorm expansion phase. During the last 45 min, AE either starts to decrease or levels out, and this interval can thus be characterized as the first part of the substorm recovery phase. The AE level of about 2-3 nt before substorm onset may indicate that not all our samples are isolated substorms.

3 Substorms, Storms, and the Near-Earth Tail I 6 F V v 4 A--A 2 ar - 'Al A a Minutes around Substorm Onset 9 Fig. 1. Superposed AE index traces, constructed by averaging 4 AE index traces in 15-min bins with respect to substorm onset. The traces were superposed separately for substorms whose onsets occurred at D,t <-25 nt (solid line, 7 substorms) and those at Det>_ -25 nt (dashed line,. 33 substorms). The vertical error bars show the errors of the mean values. The AE traces look different for the two sets of events, with the storm-time substorms reaching an average maximum level of about 7 nt, while the non-storm substorms peak, on average, at about 4 nt. However, as we will explain later in the discussion section, this difference is not crucial for the different behavior of the near-earth tail plasma and magnetic field seen in most of the remaining figures. Figure 2 shows the average ion temperature in the central plasma sheet, separately for stormtime and non-storm substorms. The traces in Fig. 2, and Figs. 3 and 4 as well, are constructed by binning measurements taken by the IRM satellite in the central plasma sheet with respect to the particular onset and then averaging over all the samples in a particular 15-min bin. Figure 2 clearly illustrates that the heating of the ion population in the central plasma sheet occurs during the substorm expansion phase. The temperature increase from substorm onset to the beginning of the recovery phase is about the same for both types of substorms, of the order of 2.5-1'K, or roughly 2keV. The difference, however, between storm-time and nonstorm substorms lies in the average levels of the ion temperature before the onset, and thus also in the typical energy of the ion populations in the central plasma sheet during the expansion and recovery phases. In addition, the heating seems to occur more rapidly during storm-time expansion phases, resulting in an average ion energy of S kev only 15-3 min after the onset of a storm-time substorm, while the typical central plasma sheet ion has only 3-4 kev during the expansion phase of non-storm substorms. Figure 3 shows a more pronounced difference between the two types of substorms. In both cases, the ions in the central plasma sheet are transported Earthward during the expansion phase, but during the main phase of magnetic storms this transport occurs at a higher velocity. In addition, during magnetic storms the Earthward motion sets in much earlier, soon after substorm onset, while for non-storm substorms the Earthward bulk velocity increases gradually during the expansion phase and reaches its peak value only at the end of the expansion phase.

4 m 18 W. BAUMJOHANN et at. 12 1o Y O _ a 4 L L W a E 6 v F 4 2 L -45 Minutes around Substorm 45 Onset 9 Fig. 2. Average variation of the ion temperature in the central plasma sheet for substorms occurring during a storm main phase (Dat <-25nT; solid line) and for non-storm substorms (D5, >_-25 nt; dashed line). The vertical error bars give the errors of the mean values. The dashed vertical ] inns mark substorm onset and the average start of the recovery phase E Y 1 V W a ul 8 Y a m 6 O L V 4. b 4 2 Ir."4 I I -45 Minutes around Substorm 45 Onset 9 Fig. 3. line) Average variation of the Earthward and non-storm substorms (dashed ion bulk velocity in the central plasma sheet during storm-time (solid line). Otherwise same as Fig. 2.

5 Substorms, Storms, and the Near-Earth Tail 181 Be 5 4 q u 3 N i V a T V 2 N G m S LQ a,, b Minutes around Substorm Onset 9 Fig. 4. Average variation of the magnetic field elevation in the central plasma sheet during storm-time (solid line) and non-storm substorms (dashed dine). In this case, the data points are based on geometric mean values of the ratio between vertical and horizontal magnetic field components. Otherwise same as Fig. 2. It is important to note that the velocities shown in Fig. 3 represent only average values. As first shown by Baumjohann et at (199), the actual plasma transport occurs in a bursty, intermittent fashion, with the maximum bulk speed during the high-speed flow bursts close to the local Alfven speed. In addition, our data indicate that the Earthward transport in the central plasma sheet occurs predominantly perpendicular to the ambient magnetic field and, hence, can justifiably be called Earthward convection. Figure 4 shows the temporal development of the magnetic field elevation angle in the, central plasma sheet, again separately for storm-time and non-storm substorms. The difference between the two types of substorms becomes very clear in the elevation angle. During substorms that are not accompanied by magnetic storm activity, the magnetic field dipolarization appears to be very gradual, reaching its highest elevation angles only during the recovery phase. Moreover, the dipolarization is not very pronounced, with an average maximum elevation angle of only 15. On the other hand, for substorms which occur during the storm main phase, the magnetic field in the central plasma sheet starts to become rather dipolar immediately after substorm onset and the maximum field elevation reaches nearly 5. Figure 5 shows the variation of the lobe magnetic pressure, constructed in the same way as Figs. 2-4, but now using only measurements made while the IRM satellite was located in the tail lobe. Again, the difference between the two traces is most obvious. During non-storm substorms, the lobe magnetic pressure does not change in any systematic way. In fact, one may even say that the lobe is not affected at all by those substorms. On the other hand, the lobe magnetic pressure changes quite drastically during the expansion phase of storm-time substorms. It starts from a somewhat higher level, but even more importantly, it drops to about half of its pre-onset value during the expansion phase. Note that the decrease of the lobe pressure before the storm-time substorm onset is most likely an artifact caused by uneven data coverage.

6 182 W. BAUMJOHANN et a!. 1. m a F.8. a M W L L.4 V m.2.o Minutes around Substorm Onset 9 Fig. 5. Average variation of the magnetic field pressure in the tail lobe during storm-time (solid line) and non-storm substorms (dashed line). Otherwise same as Fig Discussion Before proceeding to the interpretation of our findings, we should note that the different response of the near-earth tail to the two types of substorms is not merely a result of the difference in the average AE levels, i.e., the strength of the substorms. Actually, even the three most intense non-storm substorms, all of which reach maximum AE values of more than 6 nt, did not show any variation in the central plasma sheet or in the lobe significantly different from the average non-storm substorm. For example, the maximum magnetic field elevation observed in the central plasma sheet during these three substorms was a mere 25. Having noted this, we will now proceed to discuss our findings. This will be done in two steps. First, we will argue that the efficiency of energy injection into the ring current is much higher during the storm-time substorms. Subsequently, we will present some arguments that the near-earth tail signatures for the two types of substorms are not only different in size, but that a qualitative difference exists, indicating that there are indeed two kinds of substorms during which different physical processes occur in the near-earth magnetotail. 4.1 Efficiency of ring current injection The D,t variation is the symmetric part of the magnetic disturbance caused by energetic particles encircling the Earth due to the combined effect of the gradient and curvature drift in a near-dipolar field. Hence, what is needed to create a notable D,t variation, is to bring energetic particles from the tail close enough to the Earth so that they experience a gradient and curvature force strong enough to perform complete orbits around the Earth. Since the magnetic drift forces increase with increasing particle energy, more energetic particles will experience a stronger azimuthal drift for the same magnetic gradient and curvature. In addition, they will cause a larger D,t index, since the latter depends on the energy of the ring current particles. Let us now turn to our data, in particular those displayed in Figs. 2 and 4. Without having calculated particle trajectories for the typical plasma and field parameters found in the present

7 Substorms, Storms, and the Near-Earth Tail 183 study, we are unable to prove that the storm-time substorms actually inject particles into the ring current while the non-storm substorms do not. However, the results of our superposed epoch study strongly suggest that this is the case. Apparently, the much stronger dipolarization during the storm-time substorms will bring the heated tail plasma closer to the Earth. Moreover, the plasma brought inward by storm-time substorm activity is more energetic and the more energetic particles will perform closed orbits already at larger radial distances. Finally, once on closed orbits, the more energetic particles will cause a stronger field depression. This does not necessarily mean that no particles are injected into the symmetric ring current during non-storm substorms, but at least the efficiency of energy injection into the ring current is much higher during the storm-time substorms. However, looking at a single substorm may not give the full picture on what is really needed to create a large ring current and thus a large Dat depression. The actual increase in the particle energy during the expansion phase is about the same during both types of substorms. What is clearly different, however, is that the tail plasma is already quite energetic before the onset of the typical storm-time substorm. This must be a result of previous substorm activity, which is much more likely to occur during magnetic storm activity due to the sustained southward interplanetary magnetic field (IMF) and thus enhanced solar wind-magnetosphere coupling typical for the storm main phase (see Gonzalez et al., 1994, and references therein). 4.2 Two different types of substorrns? The persistence of a southward IMF during many hours may not merely result in a more energetic near-earth tail plasma, but it may also be the ultimate cause for generating the systematic difference in the near-earth tail substorm signatures seen during the storm main phase and during non-storm intervals. It may even be the cause of different physical processes dominating the two types of substorms. omparing the results of our superposed epoch study, now taking into account also Figs. 3 and 5, with the signatures to be expected from the near-earth neutral line model (e.g., McPherron et at, 1973; Hones, 1984), only the substorms occurring during the storm main phase show all the typical features. For this type of substorm we can see the Earthward plasma flow, the dipolarization of the tail magnetic field, and the decrease in the strength of the lobe field, all of which are expected to be associated with the formation of a near-earth neutral line tailward of the satellite during substorm onset and subsequent reconnection of closed plasma sheet field lines and open magnetic flux tubes intermediately stored in the tail lobes. During the typical non-storm substorm, we do also see Earthward ion bulk flow and a more dipolar field, but both signatures are weaker and maximize only near the end of the expansion phase, after the AE index has reached its maximum value. Moreover, during this type of substorm, the lobe field strength stays nearly constant. Hence, it is unlikely that open lobe magnetic field lines are reconnected in these cases. Actually, we may speculate that the different behavior of the IMF during storm-time and nonstorm intervals results in quite different types of substorms, in line with arguments presented by owley (1992). During the typical non-storm substorm the enhanced solar wind-magnetosphere coupling due to the southward component of the IMF leads to enhanced convection but the reconnection rate at the distant neutral line may be high enough to allow for the closure of all magnetic flux tubes that have been opened at the dayside magnetopause. Substorm onset can then be the result of an instability developing due to the strongly enhanced current flow associated with the enhanced convection, either of the whole magnetosphere-ionosphere current circuit, as advocated by Kan et al. (1988), or of the enhanced tail current around "RE, as suggested by Lui (1988). The gradual increase of magnetic field elevation and Earthward transport observed between 1 and 19 RE is then the result of the tailward propagating collapse of the tail current that has been initiated much closer to the Earth (e.g., Jacquey et al., 1991; Ohtani et al., 1992). The

8 184 W. BAUMJOHANN et al. collapse may or may not be accompanied by the tearing mode instability or magnetic reconnection, but even if it is accompanied, it occurs closer to the Earth and only closed magnetic field lines are involved in this process. Thus the non-storm substorms lack an important element of the near-earth neutral line scenario, the reconnection of open magnetic flux intermediately stored in the tail lobes. Only when the IMF is due south for a prolonged period of time, like during a magnetic storm, the distant neutral line may be unable to reconnect all the magnetic flux merged at the dayside magnetosphere. In this case there will be a surplus of open magnetic flux that is intermediately stored as magnetic field energy in the tail lobe and then suddenly reconnected at a near-earth neutral line. Since the formation of a new neutral line proceeds via the ion-tearing instability (Schindler, 1974), it may help that the central plasma sheet is already hotter, since more energetic ions will behave less adiabatic in the vicinity of the neutral sheet. Finally, it is interesting to note that the ion temperature in the central plasma sheet increases by nearly the same amount during both types of substorms. Apparently, the heating of the plasma sheet ions is governed by the reconnection of closed plasma sheet field lines, which occurs in both cases, and not by the reconnection of open lobe field lines during the storm-time substorms. 5. onclusions The data presented in this study exhibit a clear difference in the dynamics of the near-earth tail for substorms that occur during the main phase of a magnetic storm and for those that are observed without accompanying magnetic storm activity. This indicates that even though the ionospheric signatures, such as auroral breakup and substorm electrojet, are common for both types of substorms, there may be two qualitatively different types of substorms, which are dominated by two different physical processes in the magnetosphere. If this is really the case, it would resolve some apparently contradictory results from earlier substorm studies. In fact, many of these studies first searched for `explosive' signatures in the data from a limited region of the magnetotail and then verified that these occurred during substorms. This approach often resulted in a typical 8-RE distance substorm, a typical 3-RE-substorm, and so on, which were all quite different. Even if all substorm onsets are included in a study, like done by Baumjohann et al. (1991), the grand average might give a misleading description of the actual response of the magnetosphere-ionosphere system to enhanced solar wind-magnetosphere coupling. One has to consider the whole solar wind-magnetosphere-ionosphere system in order to understand the dynamic behavior of even only one of its components. Our present study shows that at least a first step in this direction might be to look into magnetospheric substorm signatures in different regions of the tail, based on all onsets defined by ionospheric data from auroral latitudes, but at the same time taking into account the state of the solar wind and the IMF. We would like to thank H. Liihr and G. Paschmann for the IRM magnetic field and plasma data and T. Nagai for the original list of substorm onsets. This work was performed while one of the authors (W.B.) stayed at STEL as a Visiting Professor. He would like to thank all those people in this laboratory, and especially the members of the Integrated Studies Division, who made his visit most enjoyable. REFERENES Baumjohann, W. and G. Paschmann, Determination of the polytropic index in the plasma sheet, Geophys. Res. Lett., 16, , Baumjohann, W., G. Paschmann, N. Sckopke,. A. attell, and. W. arlson, Average ion moments in the plasma sheet boundary layer, J. Geophys. Res., 93, 11,57-11,52, 1988.

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