Geophys. Res. Lett., 20, 791-794, 1993
© Copyright 1993 by the American Geophysical Union
Paper Number 93GL00850

Flux Transfer Events: Spontaneous or Driven?

G. Le, C. T. Russell and H. Kuo
Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA

Abstract. Since flux transfer events (FTEs) occur predominantly under conditions of southward interplanetary magnetic field (IMF), one might expect that FTEs could be associated with the southward turning of the IMF which could lead to a transient increase in the merging rate. Lockwood and Wild [1993] recently suggested, in fact, that FTEs are driven by the IMF variations in which the Bz becomes more southward and rather than arising spontaneously during periods of steady IMF. To test this hypothesis, we have surveyed observations of the IMF during the flux transfer events observed in the dayside magnetopause by ISEE spacecraft. The upstream solar wind data from IMP-8 or ISEE 3 are carefully compared with the magnetosheath data from ISEE to determine the time lag between them, which enables us to survey the simultaneous solar wind data during the FTEs. We find no evidence that FTEs are directly correlated with southward turning of the IMF. Rather, the FTEs more frequently occur when the IMF Bz is steady. A survey of 10-minute intervals of simultaneous IMF data preceding each FTE show that the IMF fluctuates between northward and southward for less than 20% events and few of these turnings appear to be consistent with triggering the FTE. For majority of events, the IMF stays southward with fluctuations of clock angle less than 30 over the 10-minute period. Thus we conclude that FTEs do occur spontaneously for steady southward IMF and are not simply directly driven by the observed fluctuations in the IMF Bz component.

Introduction

Flux transfer events are a peculiar signature observed in the magnetic field near the dayside magnetopause [Russell and Elphic, 1978, 1979]. The most noticeable signature of FTEs is the bi-polar perturbation in the component normal to the magnetopause boundary, which can not be explained as the surface waves. They were discovered in the early ISEE data and were interpreted as the evidence of patchy impulsive reconnection by Russell and Elphic [1979]. In their model, the magnetospheric field undergoes a transient reconnection to the upstream magnetic field when the upstream field has a southward component. When the transient reconnection ceases the connected flux tube is carried tailward into the tail lobe by the magnetosheath flow. The magnetosheath field lines not connected to the magnetosphere drape over the connected flux tube, that generates a bi-polar signature at the spacecraft when the connected flux tube passes by. Since then, FTEs have been widely regarded as the signatures of transient reconnection, as strongly supported by a large number of subsequent studies of field data [Berchem and Russell, 1984; Rijnbeek et al., 1984], plasma data [Daly et al., 1981; Scholer et al., 1982; Scudder et al., 1984] or both combined [Paschmann et al., 1982; Elphic and Southwood, 1987; Rijnbeek et al., 1987]. The main evidence is that they occur preferentially during the southward IMF; that their polarity reverses across the magnetic equator [Berchem and Russell, 1984; Rijnbeek et al., 1984]; that a mixture of magnetosheath and magnetospheric plasma is observed within FTEs; and accelerated flows are seen in association with FTEs [Paschmann et al., 1982; Elphic and Southwood, 1987].

One important feature of FTEs is that they occur quasi-periodically near the magnetopause, i.e., they repeat at \sim 8 minutes interval on average [Rijnbeek et al., 1984]. It is not clear what controls this occurrence rate. If FTEs are the results of highly time-dependent reconnection, this quasi-periodic occurrence may suggest that the transient reconnection process has some intrinsic time scale for the energy buildup and release. In this case, the FTEs could arise spontaneously during a period of steady solar wind. Or this occurrence rate may be controlled by the fluctuations in the IMF Bz component, as one might expect that the FTEs could be associated with the southward turning of the IMF, which could lead to a transient increase in the merging rate. In this case, the FTEs are directly triggered by the southward turning of the IMF. Recently, Lockwood and Wild [1993] have suggested that the FTEs are driven by the fluctuations in the IMF Bz component. They found that the time between successive FTEs varies from 1.5 minutes to 18.5 minutes with an average of 8 minutes and the most common value of 3 minutes. They argue that this is not consistent with the 8-minute average value being a natural oscillation time scale of the magnetosphere-ionosphere system, for which they would expect the average value would be the same as the most common value. The wide spectrum of the separation time is similar to the distribution of IMF Bz variability, measured as the time of IMF Bz above a certain threshold which causes FTEs. However, this study is not conclusive because they did not examine simultaneous data of upstream IMF and the magnetopause field. The similarity of the two distributions may be just coincidental.

In this paper, we will test the above two hypotheses. We survey observations of the IMF during the flux transfer events observed in the dayside magnetopause by ISEE 1 and 2 spacecraft. To remove any ambiguity, the upstream solar wind data from IMP-8 or ISEE 3 are carefully compared with the magnetosheath data from ISEE 1 and 2 to determine the time delay between them, which enables us to survey the simultaneous solar wind data during the FTEs.

Observations

We use the ISEE magnetic field data near the magnetopause crossings to identify FTEs in both the magnetosheath and the magnetosphere. The data are at 4-second time resolution, which are obtained by averaging the highest resolution data, either 0.25 or 0.06 second resolution, with a 12-second overlapped window. We survey the data one hour before and after each magnetopause crossing in the years 1977, 1978, 1979 and 1980. The FTEs are identified mainly from the bi-polar signatures in the magnetic component normal to the magnetopause boundary, and the FTE data set only includes bi-polar signatures with at least 10 nT peak-to-peak variation which is distinct from other such neighboring events. The IMF data are from either IMP-8 with 15.3-second time resolution or from ISEE 3 with 64-second time resolution. We found 211 FTEs from 67 magnetopause crossings with simultaneous IMF data available.

Since there is a time lag for the IMF to convect from the upstream solar wind monitor (IMP-8 or ISEE 3) to the spacecraft near the magnetopause (ISEE 1 or 2), one task in this study is to determine this time lag to get the simultaneous IMF data for each magnetopause crossing. This time lag is roughly equal to x/V_{SW} (x is the distance between the two spacecraft along the Sun-Earth line). However, this prediction usually has a very large uncertainty ( \sim 10 minutes), mainly because of the different configurations of magnetic structures in the solar wind and the bow shock slowing factor. Thus, it can not be used in this study since the average occurrence rate of the FTEs is of the same order, i.e., ~ 8 minutes [Rijnbeek et al., 1984]. We also can not compare directly the magnetic field components from the solar wind monitor and the ISEE spacecraft because the solar wind is shocked in the magnetosheath after passing through the bow shock.

One way to overcome this difficulty is to compare the magnetic clock angle, arctan(By/Bz), in the solar wind and in the magnetosheath. According to the Rankine-Hugoniot shock jump condition, the upstream magnetic field, the downstream magnetic field and the shock normal all must be in the same plane, the so-called coplanarity. On the dayside near the subsolar region, the bow shock normal is essentially in the X direction (along the Sun-Earth line). Thus the upstream and downstream magnetic fields should be nearly in the same direction in the YZ plane according to the shock coplanarity. In other words, the clock angle of the magnetic field should stay roughly the same after passing the dayside magnetosheath although the direction of the field relative to flow has been changed. By matching the magnetic field clock angle from the solar wind monitor to that from the ISEE magnetosheath field, we can well define the time lag between the two spacecraft and obtain the simultaneous IMF data for each magnetopause crossing.

Figure 1 shows several examples of simultaneous observations of upstream IMF data and magnetopause crossings when the IMF is extremely steady for an extended time. Figure 1(a) shows a classical occurrence of FTEs (Orbit 289 outbound, September 11, 1979). This magnetopause crossing and its FTEs have been studied by many authors [Berchem and Russell, 1984; Rijnbeek et al., 1982, 1984; Sonnerup et al., 1981; Paschmann et al., 1982]. The top panel shows the IMF data in GSM from ISEE 3, the middle panel shows the magnetic field data in the local boundary normal coordinate system from ISEE 1, and the bottom panel shows the clock angle of the magnetic field from both spacecraft. In this figure, the IMF data have been lagged by the time delay of 60 minutes between the ISEE 3 and ISEE 1. This lag gives the best agreement between the variation of the clock angles of the IMF and the magnetosheath field. There is a very strong FTE centered at 0431:50. There are several smaller FTEs centered at 0408:55, 0418:30 and 0421:15. During the one hour interval displayed in the top panel, the IMF is southward and very steady and there is little variation in the Bz component. The same is true since \sim 0030 UT (ISEE 3), or about two and a half hours prior to the FTEs. In this example, the small fluctuations ( < 1 nT) that are present are not obviously associated with the FTEs detected at the magnetopause.

Figure 1(b) is in the same format as Figure 1(a) and shows another example of FTEs (Orbit 431 outbound, August 15, 1980). The time delay between the IMP-8 and the ISEE 1 is 9 minutes for this crossing. The FTEs are centered near 1350:10 and 1413:30. During this interval, the IMF Bz is very steady prior to the FTEs. This steady southward IMF starts at near 1210 UT (IMP 8), which is more than one hour before the FTEs displayed occur. Thus the southward turning is not the direct trigger of the FTEs. There are some variations in the IMF clock angle, which is mainly caused by the variation of the IMF By component. There is a change in the IMF clock angle near 1400 (IMP 8), about 5 minutes before the FTE at 1413:30 (ISEE 1). However, it makes the IMF less southward. Based on the assumption that FTEs are transient reconnection and more southward turning of the IMF could lead to an increase in the merging rate, this variation toward less southward is unlikely to be the trigger of the FTE at 1413:30. Figure 1(c) is a rare isolated FTE event (Orbit 299 inbound, October 7, 1979) and the time delay between the IMP-8 and the ISEE 1 is 11 minutes. The FTE is centered at 0219:10 (ISEE 1). The IMF turns southward near 0100 (IMP-8) and then stays southward and steady for more than an hour before the FTE. All three examples in Figure 1 demonstrate that the FTEs do occur spontaneously when the IMF is steady over an extended period and the southward turning or fluctuations in IMF Bz are not necessary conditions for their occurrence.

Even for those cases in which the IMF has a southward turning shortly before the FTEs, the data do not support the hypothesis that the IMF variation controls the FTE occurrence rate. Figure 2 shows an example (Orbit 291 inbound, September 17, 1979) in which the IMF turns southward shortly before the FTEs occur. Figure 2 is in the same format as Figure 1. The time delay between the ISEE 3 and ISEE 1 is 63 minutes. The IMF turns from northward to southward about 5 minutes before the first FTE at 2300:20. Then the FTEs repeat about every 6 minutes at 2306:10, 2311:50, 2318:15 and 2325:25. This quasi-periodic nature is spontaneous, and is not associated with any consistent feature in the IMF. As we can see from this example, the FTEs occur following either more southward or more northward variation of the IMF Bz component.

FTEs occur almost exclusively when the IMF is southward [Berchem and Russell, 1984; Rijnbeek et al., 1984], but it is not clear if there is a threshold of negative Bz which causes FTEs. In this study, FTEs occur at various values of southward Bz, thus we test if IMF turning from northward to southward is a direct trigger of FTEs. We have examined all 211 FTEs with simultaneous IMF data and find that for the majority of the events the FTEs are not associated with the southward turning of the IMF. This is demonstrated in Figure 3. We examine if the IMF fluctuates between the northward and southward direction prior to each FTE and determine the time interval for the IMF Bz to stay the same sign preceding each FTE. If the IMF Bz has opposite sign for at least 1 minute (4 consecutive data points for IMP-8 and a single point for ISEE 3) sometime prior to the FTE from the IMF Bz at the center of the FTE, we regard it as changing sign. As shown in Figure 3, for the period of 10 minutes preceding the FTEs, the IMF fluctuates between the southward and northward for only 18.5% events. For the period of 20 minutes preceding the FTEs, only 30.3% events have IMF changing sign. For the period of one hour preceding the FTEs, there are still 46.4% events without IMF changing sign. Considering the FTEs' occurrence rate of 8 minutes on average, it is therefore very unlikely that the northward to southward turning of the IMF triggers the FTEs.

We have also examined the variation of the IMF clock angle during the 10-minute period preceding the FTEs. This is not a test Lockwood and Wild [1993] performed or suggested. The reason we examine the IMF clock angle is that it is one factor which influences the magnetic shear on either side of the magnetopause near the subsolar region, and thus should be directly related to the reconnection rate. The merging rate will increase if the IMF rotates toward more southward direction (increasing magnetic shear). Figure 4 shows the histogram of the maximum change of the IMF clock angle (toward more southward direction) during the 10-minute interval preceding the FTEs. It shows that for the majority of the events, the variation of the IMF clock angle is less than 30 during the 10-minute interval preceding the FTEs. Thus, the fluctuations of greater than 30 in the IMF clock angle can not explain the occurrence of the FTEs, either. If fluctuations in clock angle did act as triggers for most FTEs, they must be smaller than 30. However, we note that such small angular fluctuations become more frequent but FTEs are relatively rare.

Discussion and Conclusions

From the results of data analysis demonstrated in this paper, it is clear that the quasi-periodic nature of the FTE occurrence is not directly caused by the fluctuations between the northward and southward in the IMF Bz component. Figure 3 shows that for a significant fraction of events, the IMF stays southward for over an hour, a time scale much greater than the FTE occurrence rate. Figure 4 also shows that the FTEs are not associated with variations of greater than 30 in the IMF clock angle. The short-time scale variations of the IMF itself are very unlikely to be the trigger of the FTE occurrence.

If the occurrence of the FTEs is not directly driven by the IMF, it must represent some intrinsic time scale of the transient reconnection process, or the time scale for the energy buildup and release at the magnetopause, similar to the substorm process in the magnetotail. This intrinsic time scale may be determined in part by the plasma conditions in the solar wind and the magnetosheath such as Mach number, plasma beta, temperatures, etc, and in part by the magnetosphere itself. Lockwood and Wild [1993] argued against the occurrence rate being an intrinsic time scale of the magnetosphere-ionosphere system. Part of their argument was based on the shape of the distribution of FTE repeat periods. Their distribution is different from that observed previously [Rijnbeek et al., 1984]. It is possible that this difference arises from differences in the operational definition of an FTE. In the cases we have examined we see no evidence for the continuum of separation times presented by Lockwood and Wild [1993].

Although FTEs are widely regarded as highly time-dependent reconnection, it is still possible that continuous reconnection is taking place somewhere on the magnetopause. In fact, both FTEs and continuous reconnection have been observed in the magnetopause crossing only a few minutes away [Rijnbeek et al., 1982]. Moreover steady IMF conditions which one might expect to be associated with continuous reconnections, are seen herein to result in FTE generations. However, we do not yet understand how such continuous reconnection and FTEs may be related.

In conclusion, the origin of the quasi-periodic nature of FTEs is still unknown. However, it is clear that FTEs can occur spontaneously for steady southward IMF. They are not simply driven by the observed short-time scale fluctuations in the IMF itself.

Acknowledgments. We would like to thank M. Lockwood for many valuable comments. The work at UCLA was supported by NSF Grant ATM92-13379.

References

Berchem, J., and C. T. Russell, Flux transfer events on the magnetopause: Spatial distribution and controlling factors, J. Geophys. Res., 89, 6689-6703, 1984.

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Elphic, R. C., and D. J. Southwood, Simultaneous measurements of the magnetopause and flux transfer events at widely separated sites by AMPTE UKS and ISEE 1 and 2,J. Geophys. Res., 92, 13,666, 1987.

Lockwood, M., and M. N. Wild, On the quasi-periodic nature of magnetopause flux transfer events,J. Geophys. Res., in press, 1993.

Paschmann, G., G. Haerendel, I. Papamastorakis, N. Sckopke, S. J. Bame, J. T. Gosling, and C. T. Russell, Plasma and magnetic field characteristics of magnetic flux transfer events, J. Goephys. Res., 87, 2159, 1982.

Rijnbeek, R. P., S. W. H. Cowley, D. J. Southwood, and C. T. Russell, A survey of dayside flux transfer events observed by ISEE 1 and 2 magnetometers, J. Geophys. Res., 89, 786, 1984.

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Russell, C. T., and R. C. Elphic, Initial ISEE magnetometer results: Magnetopause observations,Space Sci. Rev., 22, 681, 1978.

Russell, C. T., and R. C. Elphic, ISEE observation of flux transfer events at the dayside magnetopause,Geophys. Res. Lett., 6, 33, 1979.

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Scudder, J. D., K. W. Ogilvie, and C. T. Russell, The relation of flux transfer events to magnetic reconnection, in Magnetic Reconnection in Space and Laboratory Plasmas, Geophys. Monogr. Ser., Vol. 30, edited by E. W. Hones, Jr., pp. 153-154, AGU, Washington, D.C., 1984.


H. Kuo, G. Le and C. T. Russell, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567.

(Received: November 23, 1992; revised: March 3, 1993; accepted: March 8, 1993.)

Figure Captions

Figure 1. Examples of simultaneous observations of the IMF and the magnetopause crossings with flux transfer events when the IMF is steady over an extended time period. (a) Orbit 289 outbound, September 11, 1979; (b) Orbit 431 outbound, August 15, 1980; (c) Orbit 299 inbound, October7, 1979.

Figure 2. Example of flux transfer events occurring shortly after the IMF turns southward.

Figure 3. Variations of the IMF Bz component prior to each FTE. Shown are the percentages of the events with IMF Bz changing sign and without IMF Bz changing sign for different time intervals preceding the FTEs.

Figure 4. Variations of the IMF clock angle. Shown is the histogram of maximum change in the IMF clock angle during 10-minutes interval preceding each FTE.