Statistical properties of magnetic field fluctuations in the distant plasmasheet

Planet. Space Sci., Vol 35, No. 3, pp. 289-293, 1987 Printed in Great Britain.

00324)633/87 $3.00+0.00 Pergamon Journals Ltd.

STATISTICAL PROPERTIES OF MAGNETIC FIELD FLUCTUATIONS IN THE DISTANT PLASMASHEET BRUCE T. TSURUTANI, MARCIA E. BURTON and EWARD J. SMITH

Space Physics and Astrophysics Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, U.S.A. and D O U G L A S E. J O N E S

Department of Physics, Brigham Young University, Provo, UT 84601, U.S.A. (Received 11 September 1986)

Abstract--Short ( < I min) and long time (> 5 min) variations of the plasmasheet magnetic field have been examined during all intervals when ISEE-3 was at distances x < -200Ro. It is determined that short period magnetic turbulence increases by a factor of ~ 3 with increasing geomagnetic activity, as indicated by AE. In contrast, long period field variations with North-then-South signatures at plasmasheet entry occur ~ 2.5 times more frequently than South-then-North signatures. This result, combined with other previous ISEE-3 results, is in agreement with the interpretation that the Nort~South plasmasheet features are plasmoids propagating tailward. However,a statistical examination of the geomagneticactivity relationship indicates that there does not appear to be any substorm dependence on these North-South events.

INTRODUCTION

Recent I S E E - 3 observations in the Earth's distant magnetotail have detected the presence of magnetic structures in the plasmasheet characterized by large northward-then-southward directional variations, called plasmoids (Hones et al., 1984a,b). These plasmoids are associated with high velocity tailward flows (Hones et al., 1984a,b), isotropic ion and electron fluxes within the plasmoid (Scholer et al., 1984) and precursor streaming ions and electrons prior to the arrival of the plasmoid at I S E E ( Scholer et al., 1984 ; Cowley et al., 1984, Richardson and Cowley, 1985, Tsurutani et al., 1985). Plasmoids are believed to be closed magnetic "bubbles" that are the result of sporadic magnetic reconnection events (as contrasted to steady-state reconnection) taking place inside X = -120R~. Baker et al. (1984) have conducted I S E E - 3 studies at x = - 100 to - 120 Ro, and have detected phenomena with the opposite sense : Souththen-North magnetic signatures, and simultaneously earthward-flowing plasma. Baker et al. (1984) concluded that they had observed reconnection phenomena on the earthward side of the neutral line. The purpose o f this paper is to present the results of a statistical examination of the plasmashect 1-min magnetic field variances and the longer duration ( > 5 m i n ) 0 (North-South) variations at distances beyond 200 Re. Thi~ covers the intervals from day 19 through day 60 and day 141 through day 237, 1983.

We have investigated the change in the North-South field component at the time of entry into the plasmasheet and have attempted to relate the observed changes to the dynamics of the plasmasheet.

RESULTS

From previous studies, I S E E - 3 high resolution magnetic field and plasma data have been examined to identify plasmasheet intervals (Tsurutani et al., 1984a,b). These same intervals are used in this study. Short wavelength structures Figure 1 illustrates the averages of 1-min field variances in the plasmasheet as a function of AE. The AE index was lagged by 30 min to take propagation effects into account. Other delays from 45 to 60min were used and similar results obtained. For this portion of the study, the entire plasmasheet intervals were used. The data were first binned into 100 nT AE intervals. The 1-min variances were then logged and averaged. The one sigma bars represent the standard deviation of the log of the field variances. The figure illustrates that the field component and magnitude variances increase with increasing geomagnetic activity as measured by AE. The logarithm of the field variances asymptotically approach a maximum value with increasing AE. The logs of the variance for By and Bz both increased from ~ - 0 . 7

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PLASMASrmEXMAONETICFmLD y AND Z COMPONENTSAND MAGNITUDE. The data are binned into 200nT AE intervals (AE is lagged by 30min to take propagation effects into account). The error bars represent plus and minus one standard deviation of logcr. The variances of By, Bz and IBI all increase with increasing AE, implying enhanced small scale plasmasheet turbulence during magnetospheric substorms, to - 0 . 3 as AE increased from zero to greater than 400 nT. The log of the variance of |BI increased slightly less over the same AE range, from - 0.8 to -0.5. Because of the large flapping of the distant tail and the short excursions into the plasmasheet, it was impractical to calculate variances for longer invervals. An attempt to do so may create a bias in the data sample, e.g. if greater tail flapping is present during periods ofintensegeomagneticactivity, long duration variances will be biased toward quiet intervals, Lon# wavelength structures For the study of 0 variations in the plasmasheet, high resolution magnetic field data were examined to characterize the magnetic field at entry into the plasmasheet. The plasmoid-like events have the properties that the field rotates to a large northward angle then to a southward angle, and then to zero, or to an in-elliptic orientation. The field can also rotate to large northward angles and then to zero. For brevity, these two cases will be labeled N S and NO events, respectively. Other possible cases are large rotations to southward-then-northward or southward-then-zero 0 orientations, or S N and SO, respectively. Two other

types of plasmasheet magnetic field characteristics have been identified : cases where the plasmasheet field remains in-the-ecliptic and are relatively quiet and another where there are high frequency oscillations in 0, but with a zero average value. Some examples of the different plasmasheet 0 characteristics detected are illustrated in Fig. 2. In each panel, the event is indicated by cross-hatching. A nine station AE index (kindly provided by S.-I. Akasofu of the University of Alaska) is given at the bottom. A delay of 30 min for propagation down the tail is assumed (see Hones et al., 1984a). A broad interval of AE is illustrated in the figure in case the reader chooses to assume other values for propagation delays. Shown in the figure are plasmoids (panels a and b), a turbulent, high-frequency event (panel c), and a South-North and South-Zero event (panels d and e, respectively). All plasmasheet intervals were identified as having one of the above six 0-variation characteristics. The 0-variation characteristics were determined for each plasmasheet entry without prior knowledge of AE or of the plasma velocities. It is believed that the plasmoid expansion and tailward propagation causes the plasmasheet to bulge outward and engulf ISEE-3 (Hones et aL, 1984a). Thus, if this model is correct, plasmoids, if present, should be best identified at the initial entry of ISEE3 into the plasmasheet. In this study we have only used the 0 variation characteristics at initial entry. Besides this scientific justification for data selection, it should be mentioned that it is also difficult to separate North-South from South-North events after initial entry because of the generally turbulent nature of the plasmasheet (Tsurutani et al., 1984a). Two possible models have been considered to aid in interpreting the plasmasheet field variations obtained : a coherent plasmoid-like structure and whole-tail flapping and/or turbulence. These are schematically illustrated in Fig. 3. The figure shows that the magnetic field signature will be very different for the two cases. Plasmoids will have a coherent North-then-South structure while flapping or turbulence will result in more or less random 0 variations. For the latter case, there will be equal numbers of South-North and N o r t h - S o u t h events. For simplicity, we have combined the N S and NO events and the S N and SO events. We call these two types of field variations NS* and SN*, respectively. The results for the above two distributions are nearly identical to that for the N S and S N events (not shown). It is found that beyond 200 Ro there are approximately 2.5 times the n u m b e r of N S events (100) as S N

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F I G . 2. EXAMPLES OF DIFFERENT TYPES OF PLASMASHEET O VARIATIONS FOUND IN THIS STUDY: PANELS (a) AND (b), N O R T H S O U T H ( N S ) VARIATIONS; (c), TURBULENT, HIGH-FREQUENCY STRUCTURES ; (d), SOUTH--NORTH ( S N ) VARIATIONS AND (e), SOUTH-ZERO EVENTS.

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Tail flapping should lead to an equal number of NorthSouth and South-North variations while plasmoid events should consistently give North-South magnetic signatures. events (42), in excellent agreement with the plasmoid picture expected at 200 Re (Hones et al., 1984; Baker et al., 1984). The AE distributions for both the N S * and S N * events are given in Fig. 4 as cross-hatched histograms. Each of the two distributions has been normalized such that the total area is 100%. The number of 5-min averages is given at the top of each bin. For reference the AE distribution for the whole time interval of study (not just when ISEE-3 was in the plasmasheet) is indicated by the bar background. The N S * and S N * normalized distributions appear quite similar to the background AE distribution. No obvious substorm dependence is apparent. If there is any dependence at all, the S N * events occur at slightly higher AE than N S * events. When other delays were used (45 and 60 min), similar results were obtained,

CONCLUSIONS Short period fluctuations present in the plasmasheet show a definite trend towards higher amplitudes during increasing geomagnetic activity. Because coherent waves are typically not found within the plasmasheet proper (Tsurutani et aL, 1985), these enhanced vailances represent a state of increased turbulence, This turbulence is probably not locally generated, but is presumably due to processes occurring closer to the reconnection site. The very high, plasma-sheet convective speeds (400-1000 km s- ]) reported by Zwickl et al. (1984) and the relatively slow Alfvrnic velocities of 30 km s- ] (B ~ 3 nT, n = 0.3 cm- 3) are in agreement with this hypothesis. Clearly the nature and role that this turbulence plays in the reconnection process is an important theoretical problem,

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FIG. 4. NORTn-Sotrm (TOP PANEL) ANn SOUTH-NoRrn (BOTTOMPANEL)PLASMASrm~T0 EWNXS,AS m.rNcrlONSOF AE. Bothhistograms have been normalized to 100%. The number of NS* or SN* events are given at the top of each panel. The AE distribution for the entire interval of study is given by the bar distribution (again normalized to 100%); the numbers of 5 min intervals are given at the bottom. Several featuresare apparent in the figure. The total number of NS* eventsis 2.5 times the number of SN* events, in agreement with most of these features being plasmoids. The AE distributions of the NS* and SN* events are similar, indicating little or no substorm dependence on plasmoid occurrence. The large scale field results indicate the importance of plasmoid-like 0 variations in the distant plasmasheet. The ratio of N S * to S N * events is consistent with plasmoid models of Hones et al. (1984a,b) and others. Tail flapping or general turbulence would lead to equal number of N S * and SN* events. This latter possibility is well out of the realm of statistical fluctuations, based on the large number of samples used in this study. At this time, the authors are not able to suggest an alternative explanation for these results, other than the passage of plasmoids. The AE distribution of the NS* events indicate that plasmoid or plasmoid-like structures occur during all geomagnetic activity levels. There appears to be no substorm dependence on these structures. Scholer et al. (1985) have identified a distant tail ( ~ 140 P~) reconnection event which occurred during quiet geo-

Statistical properties of magnetic field fluctuations magnetic conditions. It is a fundamental question of magnetotail dynamics and evolution to understand

these quiet time plasma phenomena, Acknowledgements--We would like to thank S.-I.Akasofu for providing the AE indices and for his comments on this paper. This work was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. REFERENCES

Baker, D. N., Bame, S. J., Birn, J., Feldman, W. C., Gosling, J. T., Hones, E. W. Jr., Zwickl, R. D., Slavin, J. A., Smith, E. J., Tsurutani, B. T. and Sibeck, D. G. (1984) Direct observations of passages of the distant neutral line (8{) 140/~) following substorm onsets : ISEE-3. Geophys. Res. Lett. l l , 1042. Cowley, S. W. H., Hynds, R. J., Richardson, I. G., Daly, P. W., Sanderson, T. R. and Wenzel, K. P. (1984) Energetic ionre~mesinthedeepgeomagnetictail:ISEE-3. Geophys. Res. Lett. 11,275. Hones, E. W. Jr., Baker, D. N., Bame, S. J., Feldman, W. C., Gosling, J. T., McComas, D. J., Zwickl, R. D., Slavin, J. A., Smith, E. J. and Tsurutani, B. T. (1984a) Structure of the magnetotail at 220 R~ and its response to geomagnetic activity. Geophys. Res. Lett. 11, 5. Hones, E. W. Jr., Birn, J., Baker, D. N., Bame, S . J . , Feldman, W. C., McComas, D. J. and Zwickl, R . D . (1984b) Detailed examination of a plasmoid in the distant

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