GEOPHYSICALRESEARCH LETTERS, VOL. 12, NO. 9, PAGES 613-616, SEPTEMBER1985 ISEE 1 AND 2 OBSERVATION OF THE SPATIAL STRUCTURE OF A COMPRESSIONAL Pc5 WAVE Ka zue Takahashi University of California, Los Alamos National Laboratory ChristopherT. Russell1 Institute of Geophysics and Planetary Physics, University of California Los Angeles Roger R. Anderson Department of Physics and Astronomy, The University of Iowa Abstract. A compressional Pc5 wave was [Russell, 1978]. The da•taare presented in a observed on an ISEE 1 and 2 outbound path on coordinate system where H (9orth) is antiparallel September 28, 1981 at L = 5.6-7.3 near the •to t•he ge•omagneticdipole, D is eastward and magnetic equator at ~10 hr local time during the V = D x H is approximately radially outward. Note recovery phase of a geomagnetic storm. The wave that only deviations from the Olson-Pfizer model propagated westward with a large azimuthal wave field [Olson and Pfttzer, 1977] are plotted. The number of ~30 and exhibited in-phase oscillations total component (not plotted) is essentially the of plasma density and magnetic field magnitude. same as the H component. During this event, component-dependent variations The compressional Pc5 wave was present between in phase and amplitude were observed for the 0400 and 0510 UT. During this interval, the L magnetic field oscillations. The radial and value, local time, and magnetic latitude of the compressional components had a constant phase and satellite changed from (5.6, 0930, 8.2 ø) to (7.3, their amplitude was finite. In contrast, the 1010, 1.9ø). Throughout the wave event the V azimuthal component changed its phase by 180ø and component oscillated without any phase variation its amplitude became zero during the middle of the and had a period of ~400 s. The electric field wave event. We interpret this observation as associated with this magnetic wave did not exhibit indicating the spatial structure of the Pc5 wave. any 400 s oscillation above the measurement noise The polarization reversal is likely to be caused level [Cattell, personal communication, 1984]. by a crossing of a node of a standing wave located In addition to the magnetic field data, data 4ø above the geomagneticequator. from the ISEE 1 and 2 plasma wave experiment [Gurnett et al., 1978] were examined for information on the plasma density. A sharp emission line at the upper hybrid resonance In troduc tion frequency prior to 0410 UT and the plasma frequency cutoff of trapped continuum radiation Compressional Pc5 magnetic waves have been after 0410 UT were used to estimate the plasma observed in the magnetosphere during the recovery density at ISEE 1 and the result is shown at the phase of a geomagnetic storm [Nagano and Araki, bottom of Figure 1. The density decreased from 1983; Takahashi et al., 1985]. Since these ~11/cm3 at L = 4.7 (0330 UT) to ~3/cm3 at observations were made only at the geosynchronous L = 7.1 (0500UT), whichroughly follows the L-4 altitude, the spatial structure of compressional variation characteristic of the outer Pc5 waves is not well understood. In this paper magnetosphere. From an examination of ISEE 9_ we discuss the structure of a compressional Pc5 plasma wave data, the plasmapause was found to be wave based on ISEE 1 and 2 magnetic field located at L ~4.4. observa tions. For a short interval from 0410 to 0426 UT, the cutoff of the continuum radiation was clear enough to be used for determining the phase relation Ob serva tions between oscillations in the plasma density n and the magnetic field magnitudeB T. In Figure 2 we The event of interest wasobserved on September showthe variations in n and BT using a 28, 1981, during the recovery phaseof a magnetic logarithmic scale. Clearly, BT and n oscillated storm. Its overall feature is illustrated in in phase for this interval. Such a phase relation Figure 1, using ISEE 1 magnetic field data was observed by Kivelson et al. [1984] for a compressional Pc5 magnetic pulsation, and they took it as evidence for a global (fast mode) compressional mode. This observation is against 1Alsowith the Departmentof Earthand Space the drift mirror instability [Hasegawa,1975], Sciences, University of California Los Angeles which would produce antiphase oscillations. To discuss the properties of the wave in some Copyright 1985 by the American Geophysical Union. detail we show derrended wave forms for the interval 0400 to 0510 UT in Figure 3. Magnetic Paper number 5L6606. field data from ISEE 2 [Russell, 1978] are also 0094-8276/85/005L-6606503.00. shown. During the wave event ISEE 1 was located 613 614 Takahashiet al.: Structure of a CompressionalPc5 I SEE 1, September 28, 1981 I SEE 1, September 28, 1981 2OO 6.0 0 5.0 bv -lO _ 150 n (nT) BT (cnf• ) -20 4.0 (nT) -3O 5 bD (nT) -5 3.0 f n lOO 2O 0410 0415 0420 0425 0430 Universal Time Fig. 2. Phase relation between oscillations in plasmadensity n and magneticfield magnitudeBT. bH o (nT) -10 Discussion -2O Since compressional Pc5 waves observed during the recovery phase of a geomagnetic storm have a 2ø1 I duration of several hours or longer [Nagano and lO Araki, 1983, Takahashi et al., 1985] and there was no great change in geomagnetic activity during the (cr•-• present wave event, we take the above observations ) •• as showing the spatial structure of the Pc5 wave. There is a marked difference between the 1 I I I I I I I I I observed phase and amplitude variations of the UT(hhmm) 0330 0400 0430 0500 0530 0600 Pc 5 wave and those described by the theory of LT (hhmm) 0900 0930 0950 1006 1018 1028 L 4.7 5.6 6.4 7.1 7.7 field line resonance [Chen and H•segawa, 1974; ), (deg) 13.3 8.2 4.9 2.5 0.9 -0.2 Southwood,1974]. According to the theory, bD Fig. 1. ISEE 1 magnetic field and plasma density should exhibit an amplitude maximum coincident data during a compressional Pc5 wave eventß Local with its phase shiftß In our data, however, the time (LT), L value, and magneticlatitude (l) of amplitudeof bD becameminimum at the point the satellite are shown at the bottomß (~0430 UT) where the phase shift occurredß This discussion against a radial structure may not be valid if the propagation direction is inside and westward of ISEE 2. Apparently, the radial rather than azimuthalß From our wave propagated from ISEE 2 to ISEE 1. If we observations alone we cannot determine the assume that the propagation is strictly azimuthal, propagation directionß However, based on previous this observation means that the wave propagated results [Walker et al., 1982; Takahashi et al., westwardß The average lag of the wave form, 1985], we assume azimuthal propagation. determined with correlation lag analysis, is ~20 s An alternative explanation for the amplitude for the interval of 0400 to 0430 UT and it is variation and polarization reversal could be given ~15 s for 0430 to 0500 UT. These lags correspond in terms of a standing wave structure along the to a constant angular velocity of 0.027ø/s, since ambient magnetic field similar to the one the azimuthal separation AS of the spacecraft originally proposed by Walker et al. [1983]. Our decreased from 0.53 ø at 0415 UT to 0.40 ø at 0445 model, illustrated in Figure 4, differs from the UT. Using this angular velocity and the average Walker et al. model in that the oscillations period of 400 s for the wave, we obtain an betweenb V and bH are out of phaseat the equator. azimuthal wave number of 34. This value, as well Figure 4 illustrates our model at five as the westward propagation, is consistent with different time steps evenly spaced between t = 0 the previous observations at synchronous orbit and T, where t = 0 is defined as the time of [Walker et al., 1982; Takahashi et al., 1985]. bV = 0 at the equator and T is the period of the The amplitude and phase variation is component wave. In the modela fundamentalmode (b D and bH) dependent. The V component has a relatively anda secondharmonic mode (b V) are assumed,but constant amplitude, but the H component has its only the structure near the equator is essential amplitude maximum at ~0430 UT. These components for our discussion. oscillate out of phase. In contrast, the We could interpret our observation in terms of amplitude of the D component became essentially this model as the following. Initially, the zero at ~0430 UT, and at this instance its satellite was at position (A), above the node of oscillation phasechanged by 1800. Before0430 UT bD, whichis indicatedby a horizontalbroken laggedß Thus,bD before by 900 ~0430and after UT, the0430 polarizationUT bV led bDin by bline.m led bHere v byb 900V and . bInH oscillatedpassing acrossout-of-phase the node andas the bv-bD planewas left-handed with respectto t•e satellite movedtoward (B), the phaseof bD the ambientmagnetic field direction, andit was changedby 1800, whereasthat of bV andb H right-handed afterward. remained unchanged. Takahashi et al.: Structure of a Compressional Pc5 615 Detrended Magnetic Field, September 28, 1981 ISEE 1 (thin line) and ISEE2 (thick line) 4 bV (nT) 0 -4 4 J -4 8 0 -8 UT (hhmm) 0400 0410 0420 0430 0440 0450 0500 0510 LT1(hhmm) 0930 0938 0944 0950 0956 1001 1006 1010 L1 5.59 5.88 6.15 6.41 6.66 6.90 7.12 7.32 t, 1 (deg) 8.2 6.9 5.8 4.9 4.0 3.2 2.5 1.9 AR (kin) 711 687 666 646 628 611 595 579 A½ (deg) 0.63 0.56 0.50 0.45 0.41 0.38 0.35 0.32 Fig.