Present observations have revealed a variety of magnetic wave phenomena in the tail, from ULF to ELF frequencies. However, only VLF measurements of electric fields have been made. These measurements reveal that the tail is electrically quiet at VLF frequencies, except in the near Earth plasma sheet during substorm expansion phases. The magnetic waves observed include: waves with periods of about 2 min which cause the plasma sheet boundary position and the neutral sheet location to oscillate; waves from 10-1 to 1 Hz which occur throughout the plasma sheet during plasma sheet expansions; and ELF waves which occur sporadically in the plasma sheet.
A detailed knowledge of the physical processes occurring in the Earth's magnetotail is essential to our understanding of the sub storm process, because the energy released into the night-time magnetosphere is for some period of time stored in the magnetotail. By studying the fluctuations observed in the magnetic and electric fields in the tail, we should be able to identify which instabilities arise there and what forces act on the particles. For example, if we know the wave amplitudes and polarizations we can calculate the effective conductivity due to wave-particle interactions. At the present time we have much information about the magnetic field fluctuations but very little information about the electric field. This is illustrated in Fig. 1.
|FIG. 1. Investigators who have made observations of noise in the tail. The list is ordered by the frequency range of the observations and whether the observations were electric or magnetic.|
This figure shows the frequency range of the various observations of magnetic and electric noise in the tail and the names of some of the researchers who have performed these studies. Turning our attention first to the electric field observations, we see that very little work has been done in this area. In fact, the only electric field measurements actually made in the magnetotail are those of Scarf et al. (1971) on the OGO-5 satellite at VLF frequencies. However, I have included Mozer and Carpenter because their observations of electric fields in the night-time magnetosphere do reflect the variations of the electric field in the tail. F. S. Mozer (1971) has measured these fields with instruments carried on balloons, while D. L. Carpenter (1971) has calculated this field from the motion of whistler ducts.
The magnetic observations listed in the upper panel of this figure are more complete. The work of the researchers in the first list, at periods of several hours, has shown that the overall configuration of the magnetic field in the tail changes in response to substorms. The plasma sheet, in which the field magnitude is reduced due to particle pressure, first thins and then expands during sub storms and the field direction changes to become more dipole-like (Behannon, 1970; Hruska and Hruskova, 1969; 1970; Fairfield and Ness, 1970; Aubry and McPherron, 1971; Russell et al., 1971a; Meng et al., 1971).
Field fluctuations at periods of minutes have been revealed in studies of boundary motions in the tail performed by Mihalov et al. (1970) and Russell et al. (1971b). Waves in the tail with periods of minutes cause the magnetopause, the plasma sheet boundary and the neutral sheet to make periodic crossings of satellites in the tail.
At higher frequencies noise is observed only within the plasma sheet. Noise at frequencies near 1 Hz, that is at ULF frequencies is seen whenever the plasma sheet expands and seldom at other times (Russell et al., 1971b). ELF noise bursts observed by Brody (1970) occur even less frequently than this ULF noise.
In this review, I shall cover only the fluctuations with periods of shorter than several minutes, since the lower frequency observations have already been discussed at this conference in the session on substorms.
Calculations of the eigenmode oscillations of the magnetotail by McClay and Radoski (1967) predict that the tail has natural resonant frequencies with periods of from 5 to 30 min. More recent calculations by Siscoe (1969) and by McKenzie (1970; 1971) predict somewhat shorter periods ranging from about 30 sec to 10 min. Although the amplitudes of these waves are probably quite small, of the order of a gamma or less, they can have dramatic effects in magnetic field data when a satellite is at a boundary across which the magnetic field changes markedly. This is because these waves cause a significant periodic displacement of the boundary. Multiple boundary crossings of the magneto pause, plasma sheet and neutral sheet have been observed. Figure 2 shows multiple neutral sheet crossings observed by Mihalov et al. (1970) with the Explorer 33 Ames Research Center magnetometer. Explorer 33 was 73 R behind the Earth at this time. This figure shows the three vector components of the field expressed in the solar magnetospheric coordinate system for a time period of 1 hr. The neutral sheet crossings are marked by reversals in the polarity of the X-component of the field. The irregular pattern of field variations shown here, of course, cannot be explained by a simple single frequency sinusoidal oscillation of the position of the neutral sheet, but it does show that there are significant oscillations with periods of minutes.
|FIG. 2. The three solar magnetospheric vector components of the magnetic field measured in measured in nanoTeslas during a crossing of the nuetral sheet by explorer 33 spacecraft 73 Re behind the earth (Milhalov et al., 1970).|
Figure 3 shows a histogram of the occurrence of time intervals between successive neutral sheet crossings for this and other neutral sheet crossings studied by Mihalov et al. We see that most oscillations of the neutral sheet have periods of from 1/2 to 10 min with a peak in occurrence about 2 min which is in the range predicted by recent theory.
|FIG. 3. A histogram showing number of occurrances of different seperation intervals between neutral sheet crossings observed by the explorer 33 spacecraft (Milhalov et al., 1970)|
Figure 4 shows the magnetic signature of a multiple crossing of the boundary of the plasma sheet as observed with the University of California, Los Angeles, fluxgate magnetometer carried on board the OGO-5 spacecraft. The satellite was 12.5 R behind the Earth, within 2 R of the midnight meridian and 2 R above the expected position of the neutral sheet at this time. The data are split into two sections in the upper and lower panels each covering a period of 6 min. The total field and the three solar magnetospheric vector components are shown. The entry into the plasma sheet causes a reduction of the strength of the magnetic field. This appears most clearly in the field strength and the X-component which is parallel to the tail axis. I have shaded the region of diamagnetic depression on the X-component to illustrate the crossings more clearly. We interpret these crossings as follows: The plasma sheet is expanding, probably due to a sub storm and the border of the plasma sheet crosses the satellite. Superimposed on this expansion there is an oscillation of the boundary. As the plasma sheet expands, the satellite spends a longer and longer time within the plasma sheet during each oscillation until finally at 0827 the satellite remains in the plasma sheet. The period of this motion is about 2 min in this example, in close accord with the calculations of Siscoe and of McKenzie, and the observations of Mihalov et al.
|FIG. 4. The three solar magntospheric vector components of the magnetic field and the total field measured in nanoTeslas during a crossing of the plasma sheet boundary by the OGO-5 spacecraft. The depressions of the X-component upon entry into the plasma sheet have been shaded for easy identification.|
Examining the magnetic field data in this figure more closely, we can see that, while the field outside the plasma sheet as shown in the upper panel is relatively quiet, the field in the plasma sheet is highly irregular. Before examining the spectral characteristics of this noise, we will first discuss when and where it occurs.
Figure 5 shows 1 min averages of the magnetic field obtained by OGO-5 on a pass inwards towards the Earth near the midnight meridian. During this pass the satellite remained above the neutral sheet and only slowly approached it. The format of these data is quite different from the previous figure. The top panel is the measured field strength minus the dipole field of the Earth. The next panel down is the inclination of the field line, that is the angle between the magnetic field and the local horizontal. If the magnetic field were radial and pointing towards the Earth this angle would be 90o. The declination is the angle of the field projected into the horizontal plane at the satellite; it is an azimuthal angle about the radius vector from the Earth. It is zero for northward pointing fields. The next panel is the solar magnetospheric Z component of the field; that is the component perpendicular to the average neutral sheet. The bottom panel is the root-mean-square amplitude of fluctuations with periods less than 15 sec. Six hr of data are shown.
This figure illustrates the typical long period variations in the magnetic field encountered near the plasma sheet and referred to in the introduction. These variations usually have the form of rapid decreases of the field strength with time scales of about 5 min or less accompanied by rapid rotations of the field. These rapid changes are followed by slow recoveries of the field strength accompanied by slow rotations of the field. Two of these sequences of changes are shown in this figure. The first one begins at 1700 UT and the second about 2010 UT. We interpret these field decreases and increases as expansions and contractions of the plasma sheet across the satellite. These in general are associated with substorms. The increase in the solar magnetospheric Z component during these events shows that the field is becoming more dipolar.
|FIG. 5. One minute averages of the magnetic field obtained by the OGO-5 flux gate magnetometer on a pass inwards in the midnight meridian as the satellite only slowly approached the nuetral sheet. B is the observed field minus the dipole field. B is the solar magnetospheric Z component of the field and is the r.m.s. amplitude of waves with periods <15 sec., all measuerd in nanoTeslas.|
Examining the bottom panel we see that the sudden changes in field magnitude and inclination are accompanied by high frequency turbulence whereas there is little high frequency noise at the time of slow changes in the field magnitude and inclination. Some of the major changes in declination are also accompanied by noise but of a much smaller amplitude.
In order to determine the average amplitude of the noise occurring during these sudden changes in field magnitude and the variation in amplitude with distance down the tail, we have plotted in Fig. 6 the peak amplitude observed at or near these sudden changes. We have restricted our study to regions within 5 Re of the midnight meridian and to changes of the field greater than 3 nT occurring in less than 7 min.
|FIG. 6. The peak amplitude of noise in nanoTeslas at periods <15 sec during plasma sheet expansion as a function of radial distance. The dots indicate the plasma sheet expansions caused a decrease in the field. The crosses indicate the field suddenly increased. The horizontal lines are the medians of the observed amplitudes for 2 R intervals.|
The vertical scale is the amplitude of the noise. The horizontal scale is distance parallel to the Earth-Sun line. Crosses indicate a sudden recovery of the field strength and dots indicate a sudden decrease in the field strength. The horizontal bars represent the median of the data every two earth radii. We see that over the whole range of distances the amplitude of this peak noise during an event varies by an order of magnitude about the median values. On the other hand, the median values do show a decrease with distance down the tail of about a factor of two from 9 to 19 R. However, we should be cautious about the reality of this trend. Not only is there a lot of scatter in these points, but also, the points at large radial distances were obtained on the average further from the neutral sheet.
In order to see if this scatter could be in part due to a variation from one event to another of the distance from the neutral sheet at the time of the observations, we show in Fig. 7 the peak amplitude versus distance from the expected neutral sheet position. In order to remove the effect of the possible variation of amplitude with distance down the tail, we have restricted ourselves to the region from 12 to 16 R behind the Earth. Again, the median values have been plotted for every 1 Re interval. Although it is not possible to predict with any certainty the position of the neutral sheet to within 1/2 R we see some order in this plot. The noise appears to be most intense about 1-2 R from the neutral sheet rather than at the neutral sheet.
|FIG. 7. The peak noise measured in nanoTeslas as a function of distance from the expected nuetral sheet position for the range from 12 to 16 R behond the earth. The same format as FIG. 6.|
We have examined many neutral sheet crossings and have not found any evidence of magnetic turbulence in this frequency range at the neutral sheet. Figure 8 illustrates this. At the top of the figure is the solar magnetospheric X position of the satellite, and the expected distance from the neutral sheet. The top panel is the difference between the measured field and the dipole field, the next panel is the amplitude of the rms deviations of the field and the bottom three panels are the three solar magnetospheric coordinates of the field. There are three neutral sheet crossings in the middle of a broad depression in the field. We note that our prediction of the distance from the neutral sheet is in error by from to 1 R here. There is no noise above 1/10th of a nanoTesla here at any of the crossings.
This brief examination of the location and occurrence of the high frequency turbulence gives us some clues as to what this noise can and cannot do. Its absence in the plasma sheet at quiet times means it plays no role in the maintenance of the quiet time plasma sheet such as providing scattering centers to scatter magnetosheath plasma into the plasma sheet, etc. Its absence during the thinning of the plasma sheet means it is not responsible for loss of particles from the plasma sheet at this time. However, its presence during the plasma sheet expansions means it may play a role in the rapid change in configuration of the tail. We note that although absent near the neutral sheet at quiet times, this noise may be associated with the dissipation process because in the reconnection model the lines of force through the neutral line form the boundary of the plasma sheet. It is precisely just inside this boundary that the noise is most intense. Figure 9 shows the power spectra of the three solar magnetospheric vector components for these three intervals. The spectra cover the frequency range from 0.03 to 3.4 Hz. The vertical scale is the logarithm of the power in gammas squared per hertz. The horizontal scale is the logarithm of the frequency.
|FIG. 8. 1 min averages of the magnetic field measured in nanoTeslas during a passage of OGO-5 completely through the plasma sheet and neutral sheet at quiet times. The distances on the top two scales are the distance along the Earth-Sun-line (X(GSM),) and distance above the expected neutral sheet position. The top panel shows the measured field strength minus the dipole field strength. The panel below it shows the r.m.s amplitude of noise with periods < 15 sec. The bottom three panels show the three vector components of the field in solar magnetospheric coordinates.|
In the first panel, which shows the power 10 min before the entry into the plasma sheet, the power is equal on all three axes within our statistical accuracy as indicated by the error bars. At high frequencies the noise approaches our digital noise level. The second panel shows the spectra of the three components just before the entry into the plasma sheet. Here the two components transverse to the field have increased somewhat but there is little change in the component along the field.
The last panel shows the spectra after the last entry into the plasma sheet. Here all three components have the same power and this power is much greater than before the entry. We note that the spectra are essentially featureless with little or no change at the proton gyro frequency. Also the spectra roughly are proportional to f or f.
Figure 10 shows these same three intervals arranged by component. This illustrates that the turbulence along the X solar magnetospheric direction did not increase until the entry into the plasma sheet although the power in the two transverse directions did begin to increase before the entry.
|FIG. 9. Power spectra of the magnetic field (measured in nt /Hz) in the tail, 10 min. before an entry into the plasma sheet, just before the entry and just after the entry. The X, Y and Z components are in the solar magnetospheric coordinate system. Some of the data from which these spectra were computed are shown in Fig. 4.|
|FIG. 10. The same power spectra as in FIG. 9. Here each panel contains one component at each of the three times.|
To summarize the observations of this high frequency turbulence, it seldom occurs except at times of rapid changes in the field. The amplitude of this noise varies over a wide range but appears to decrease only slightly, if at all, with distance from the Earth. The peak noise is not encountered at the neutral sheet, but rather from about 1 to 2 R from the neutral sheet. No noise is found at neutral sheet crossings. Spectra of the turbulence show that the noise in the plasma sheet is roughly isotropic and the spectra approximately follow a power law of f to f both inside and outside the plasma sheet.
We now turn to observations of ELF and VLF noise in the tail.
Studies of ELF magnetic noise have been carried out by Brody (1970) with the OGO-1 triaxial search coil magnetometer. Figure 11 shows a typical record of some of these data. The twelve traces on this plot are proportional to the square root of the power in frequency bands at 10, 30, 100 and 300 Hz in each of three orthogonal directions. The amplitude scale is in arbitrary units. The direction and frequency corresponding to each trace is indicated by the letter and number to the left of the trace. 2 hr of data are shown. These bursts are quite infrequent, occurring about once per hour when the satellite was in the tail and have relatively low amplitude, about 50 picoTeslas. Figure 12 shows where some of these bursts occurred. The two quantities shown are the amplitude and phase of the field perpendicular to the OGO-1 spin axis. The arrows indicate the occurrence of bursts of ELF noise. All the bursts occur in field depressions, that is in the plasma sheet.
A survey of VLF electric noise in the tail has been carried out by Scarf (personal communication, 1971), using the OGO-5 VLF electric field experiment. This survey revealed no VLF electric field oscillations in the tail with the exception of substorm associated waves in the near tail within about 12 R (Scarf et al., 1971). These signals increase in strength with decreasing radial distances to about 8 R. In this region, signals of up to 100 mV per meter have been observed at 3 kHz.
|FIG. 11 The twelve outputs of the OGO-1 spectrum analyzers at the frequencies 10, 30, 100 and 300 Hz form the three orthogonal coils X, Y and Z showing typical ELF noise bursts in the tail. The vertical scale is in arbitrary units which are a function of frequency (Brody, 1970).|
|FIG. 12. The phase and amplitude of the magnetic field in the spin plane of the OGO-1 satellite during a pass through the near tail. The arrows indicate where ELF noise bursts were seen (Brody, 1970).|
The long term variations of the configuration of the tail in response to substorms indicate a macroscopic instability of the tail. Waves at frequencies with periods less than several seconds are seen in conjunction with these macroscopic changes, which indicate that instabilities on a microscopic level are triggered at these times also. These instabilities have not yet been identified.
The oscillations observed with periods of minutes may be generated by irregularities in the solar wind but alternatively they may be the result of a Kelvin-Helmholtz instability caused by the magnetosheath flow along the tail boundary. In short, present observations have revealed a variety of magnetic wave phenomena in the tail. However, low frequency electric field measurements have not been performed and when they are, these too may reveal other sources of noise.
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Last Modified: May 17, 1996