GEOPHYSICALRESEARCH LETTERS, VOL. 11, NO. 4, PAGES335-338, APRIL 1984 ISEE-3 WAVE MEASUREMENTS IN THE DISTANT GEOMAGNETIC TAIL AND BOUNDARY LAYER Fß L. Scarfl , F. V. Coroniti1, Cß F. KennelI , Rß Wß Fredricks1 D. A. Gurnett2, andE ß Jß Smith3 1TRWSpace and Technology Group, Redondo Beach, California 90278 2Universityof Iowa,Iowa City, Iowa 52242 3jet PropulsionLaboratory, Pasadena, California 91109 Abstractß The ISEE-3 excursion into the dis- field configuration for the first relatively tant tail region reveals a complex structure with close-in traversal of the tail (r < 94 Re) and several wave, particle and field characteristics for the first deep excursion to r = 225 Re. In that differ significantly from those measured another recent report, Bame et al. (1983) pre- closer to earth. The most striking results are Sented initial results from the ISEE-3 plasma found within the distant boundary layer where in- analyzer, and they showed that all plasma regimes tense electrostatic turbulence levels are de- identified from earlier measurements (plasma tected in association with bi-directional elec- sheet, low latitude boundary levels, mantle, lobe tron distributions. The wave amplitudes appear and magnetosheath) remained recognizable in the to increase with increasing downstream distance distant tail, although the regimes appeared to be and the polarizations are those expected for ion "intermingled" far from earth, and a well-defined acoustic oscillations. Near the boundary of the low density tail lobe was rarely encountered. distant plasmas sheet the turbulence spectra are The distant tail plasma was also characterized by essentially identical to those measured much unusually large flow speeds (generally tailward) closer to earth on IMP-8. We also find that in and by the very common presence of bi-directional the distant tail the continuum radiation spectrum electron distribution functions. has a low frequency cutoff that is much higher Bame et al. selected two twelve-hour intervals than the minimum value for the local plasma fre- on January 24, 27, 1983 to illustrate their quency. general results, and we use these same periods for the initial detailed comparison of wave, par- Introduction ticle and field observations. The bottom panel in Figure 1 shows the magnetic field profile for The decision to move ISEE-3 from its position the first twelve hours on January 24, and the near the upstream libration point to the distant next three panels show the variation in electron geomagnetic tail was based on the expectation temperature, flow speed and plasma density. The that important new plasma physics phenomena would coded bar above the density display indicates the be detected in the region beyond the lunar orbit. various regimes as determined by the plasma From the point of view of plasma processes, the probe; magnetosheath (MS), boundary layer (BL), ISEE-3 wave observations have a special role be- plasma sheet (PS) and lobe (L). In this report, cause no instrument of this type was carried on and in Bame et al., the term boundary layer Explorer 33 or on Explorer 35 to explore even the refers to plasma within the tail lobe directly near-earth tail region out to 80 Re. Pioneer-8 adjacent to the magnetopause and magnetosheath. did have a very rudimentary wave capability that The ISEE-3 plasma wave instrument has a sixteen helped to illuminate the structure of the distant channel spectrum analyzer to measure electric geomagnetic tail and wake at a downstream dis- fields over the frequency range 17 Hz to 100 kHz tance of about 500 Re (Scarf et al., 1970; (two samples per channel per se.cond), and an Siscoe et al., 1970), but these measurements eleven channel analyzer to measure magnetic could not provide information on interaction fields over the range 0.3 Hz to 1 kHz (at lower phenomena. In terms of comprehensive studies of sampling rates). The upper panels in Figure 1 wave-particle interactions in the earth's tail, show peak (line) and average (solid black)wave the most valuable measurements to date have come amplitudes from 19 of the 27 channels (96 second from the earth orbiter, IMP-8 (Gurnett et al., accumulation intervals). For the E-field panels 1976) [the IMP-7 wave instrument (Scarf et al., each amplitude plot is logarithmic with a total 1974; Coroniti e•t al., 1977) also gave limited top-to-bottom range of almost five orders of mag- data in the region out to 35 Re]. ISEE-3 has now nitude; the vertical scale for each B-field chan- provided the first real opportunity to study wave nel is on the order of three orders of magnitude. phenomena in the geomagnetic tail beyond 40 Re, The plasma wave modes are identified by com- the apogee of IMP-8. paring observations with local values of char- acteristic plasma frequencies. In order to fa- Wave, Particle and Field Observations cilitate this comparison, we plot directly on Figure 1 the profiles for the electron cyclotron Slavin et al. (1983) recently described in frequency, fc, and the electron plasma frequency, detail the ISEE-3 trajectory and average magnetic fp. For f < fc, Figure 1 shows wave activity that we generally interpret in terms of detection Copyright 1984 by the American Geophysical Union. of whistler mode turbulence. The magnetosheath is characterized by intense electromagnetic wave Paper number 4L0242. levels, and the lowest frequency spectral compo- 0094-827 6/84/004L-02 42503.00 nents have relatively diffuse amplitude varia- 335 336 Scarf et al.: ISEE-3 Wave Measurements ISEE-3 JANUARY24, 1983 R ß 210Re dent in the 56 kHzpanel. As notedby Gurnett (1974), the measurements of kilometric radiation 100kHz in the near-tail have provided useful indications of auroral and magnetospheric substorm activity 56.2 (see also Gurnett et al•, 1976; Voo•ts et al., 31.1 1977; Green et al., 1979). This correspondence 17.8 persists in the distant tail as well; oh January 24, well-defined substormonsets at 0440 and at 10.0 0820 (D. Baker, private communication, 1983) are 5.62 associated with onsets of 100 and 56 kHz radia- 3.11 tion on ISEE-3. At lower frequencies (31.1, 17.8, 10 and 5.6 kHz) we identify the steady 1.78 signals with high peak to average ratios as elec- 1.00 tron plasma oscillations or upper hybrid reson- ance emissions. 562Hz Figure 1 shows that when ISEE-3 was in the 311 tail lobe, the local plasma frequency was as low 178 as 2 kHz,but in this lowdensity region, no con- tinuum radiation Was detected in the 3.1 kHz 100 channel and, in fact, it was barely detectable at 56.2 5.6 kHz. At Jupiter, the Voyager 1,2 plasma wave investigators (Scarf et al., 1979; Gurnett et 56.2 Hz al., 1979) showed that throughout the encounters the continuum radiation spectrum clearly extended 31.1 down to the local value for fp, and in the region 17.8 out to about 6000 Rj • the lower limit of the con- 8.8 tinuum spectrum was generally used to deduce absolute values for the plasma density (Gurnett et al., 1980; Scarf et al., 1981; Kurth et al., ßMS BL PS L 1982). Thus, 6he result displayed in Figure i suggests that the continuum radiation generation mechanisms at earth and at Jupiter have signifi- cant differences. -1 The most striking new result on January 24 in- ......i......... volves the boundary layer trJversal between 0125 8004 , I '• '1 " and 0430. It can be seen that here the wave in- strument detected intense electric field signals V,km/sec 400• .I •! • with f > fc, while the search coil recorded es- sent'ially no whistler modemagnetic activity, al- though large amplitude ultra-low frequency waves were detected in portions of the boundary layer LOG10T,øK56 ,•,,•• .... -•....• '• ..... .,.... by the DC magnetometer(Tsurutani and Smith, 1983). The absence of low frequency whistler turbulence within the boundary layer clearly dis- tinguishes this region from the magnetosheath and plasma sheet, and we note that the boundary layer 0000 0600 UT 1200 wave phenomena have no real counterpart in any •. •. •[asma wave measurements, ma•e•c •e[d near-earth observations (Rodriguez (1979) did p•o•[e a•d plasma parameters fo• a •we[ve hou• point out tha• E-field wave activity with f > fc period • which •he•e were several c•oss•s •om is a regular component Of magnetosheath turbu- •he ma•e•oshea•h (•S) •o •he bou•da•7 [a7e• lence close to earth, but here the full spectrum (S[), •he plasma shee• (•S) a•d •he •a•[ lobe involves intense whistler mode turbulence as ([), •e•e •p •ep•ese•s •he e[ec•o• plasma •e- well). que•c7 a•d •c •s •he e[ec•o• cyc[o•o• •e- ISEE-3 also had •'four brief traversals of the que•c7. boundary layer on January 29, and in each case it was found that this layer was characterized by the presence of low flow speeds and intense high tions across the magnetopause similar to those frequency electric turbulence along with the ab- detected on IMP-7 near 35 Re (Scarf et al., sence of low frequency electromagnetic wave acti- 1974). Figure 1 also shows electromagnetic noise vity. Detailed comparisonswith the co!o•-coded bursts localized near the boundary of the plasma electron spectrograms of Bame et al. (1983) show sheet, but, as on IMP-8 (Gurnett et al., 1976) that the most intense boundary wave levels are these whistler mode signals are detected in asso- detected in association with electron distribu- ciation with electric field oscillations having a tions that are clearly bi-directional.