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First ELF wave measurements with the Equator-S magnetometer
Pergamon www.elsevier.nI/Iocate/asr
0
Adv. Space Res. Vol. 24, NO.1, pp. 11-80,1~9 1999COSPAR. Published by Elsevier Science Ltd. AI1 rights reserved
Printed in Great Britain 0273-l 177/99 $20.00 + 0.00 PII: SO273-1177(99)00428-7
FIRST ELF WAVE MEASUREMENTS WITH THE EQUATOR-S MAGNETOMETER W. Baumjohann’ , G. Haerendel’ , R. A. Treumann’ , T. M. Bauer’ , J. Ruitenbach’ , E. Georgescu’s2, U. Austeg , K. H. Fomacon3, K.-H. GlaBmeies. H. Liihr4, J. BiichneS, B. Nikutowski5, A. Balogh6, and S. W. H. Cowley7
’ Max-Planck-lnstitut fiir extraterrestrische Physik, Garching, Germany ‘Institute of Space Sciences, Bucharest, Romania 3Institut fir Geophysik und Meteorologic, TU Braunschweig, Germany 4GeoForschungsZentrum Potsdam, Potsdam, Germany ‘Max-Planck-Institute fiir Aeronomie, Katlenburg-Lindau, Germany 61mperial College, London, UK ’ Leicester University, Leicester; UK
ABSTRACT The magnetometer onboard the Equator-S satellite is very sensitive and has a high sampling rate of up to 128 Hz. These specifications allow for the first fluxgate magnetometer measurements of ELF waves between the ion cyclotron and the lower hybrid frequencies in the equatorial dayside magnetosheath. The so-called lion roars, typically seen by the Equator-S magnetometer at the bottom of the magnetic troughs of magnetosheath mirror waves, are near-monochromatic packets of electron whistler waves lasting for 0.2 - 1 sec. They are right-hand circularly polarized shear waves with typical amplitudes of 0.5 - 1 nT at frequencies of 15 -40 Hz, i.e., around one tenth of the electron gyrofrequency. 01999 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION More than 20 years ago, Smith and Tsurutani (1976) published the first search-coil magnetometer observations of what they called lion roars inside magnetic troughs in the dayside magnetosheath. They found that the lion roars are narrowbanded right-hand polarized waves, basically electron cyclotron waves, that are relatively short-lived, about 2 seconds, and that they have typical frequencies of about 120 Hz and typical amplitudes of 0.1 nT. Some years later, Tsurutani et al. (1982) studied the same phenomenon with the plasma wave instrument on ISEE. They found these waves at somewhat lower frequencies (50-100 Hz) and also established that lion roars are intimately related to mirror mode waves and that they are caused by the perpendicular electron pressure anisotropy in the magnetic troughs of these structures. However, due to the instrumentation employed in the earlier studies, the actual waveform of the lion roars was unknown until very recently (Zhang et al., 1998). INSTRUMENTATION The Equator-S magnetic field instrument consists of two units with a pair of three-axes fluxgate magnetometers each. The sensors of the primary and the redundant units are mounted on two rigid booms, with the main sensor located at the end of the 1.8-m boom and the other 50 cm further inboard. The inboard magnetometer is only used periodically to determine the amount of interference from the spacecraft. The sampling rate is 128 vectors/s in normal mode, when only the outboard magnetometer is used, and 64 vectors/s for dual mode operation. The amplitude resolution is 16 bit
W Baumjohannetd
0.2
0.0 E
0.4
0.6
0.8 set
0.4
0.6
0.8 set
4ogj
p!
0
LL
*-~I*~-‘~~~‘**-‘~*
0.2
0.0
Fig. 1. Lion roar waveform perpendicular
to the mean field and right-hand polarized wavelet spectrogram.
are selected automatically in steps of 4 between 256 and 64000 nT. The data used in the present study are 128 Hz data sampled with a resolution of about 10 pT in the 256 nT mode.
and ranges
DATA The upper panel of Figure 1 shows a typical lion roar signal wave form, about 10 wavecycles with an amplitude of fl nT. Shown here is one of the components perpendicular to the mean magnetic field. The other perpendicular components looks virtually identical, with the same amplitude, but a 90”-phase shift. The compressional component is very small, less than 0.1 nT. Hence the lion roar wave is a shear wave packet with right-handed circular polarization. Since the lion roar signals are rather short-lived, we use a wavelet analysis to determine spectrograms of the signals. The wavelet used is the so-called Morlet wavelet, which is best suited for this type of analysis (Lui and Najmi, 1987; Torrence and Compo, 1998). By transforming the complex signal (B,l = B, f i By), we calculate the right-hand and left-hand polarization spectrograms. The lower panel of Figure 1 shows the spectrogram of the right-hand polarized signal (which contained all power). The signal is nearly monochromatic with a frequency near 15 Hz. This is less than l/20 of the electron gyrofrequency near 350 Hz.
F .= 0.0
cci
-1.51.
0.2
0.0
0.4
0.6
ti
0.8
set
0.8
set
0
0.0
I.
-
I
0.2
-.
-
’
*
-
‘1.
0.4
Fig. 2. Lion roar waveform perpendicular
“I”
0.6
to the mean field and right-hand polarized wavelet spectrogram.
Magnetosheath
0.6
0.6
i= +,
79
Lion Roars
0.0
0.0
m” -0.6 -0.6 0.0
-iT E.
0.2
0.4
0.6
0.8
1.0
set
0.2
0.0
60
60..
40
40 -
20
20 ;o
. . ’ . .
0.4
1..
0.6
0.8
’ ‘.
. . ’
0.6
0.8
1.0
‘.
set
’ ‘.
._
6 s $ v
0 0.0
0.2
0.4
0.6
0.8
1.0
set
0 . 0.0
..1...1...1...1...1... 0.2 0.4
1.0
set
Fig. 3. Magnetic field magnitude, lion roar shear wave components and right-hand polarized wavelet spectrograms.
Another lion roar signal is shown in Figure 2. Again, the wave train lasts for about one second, but it shows considerable internal structure, with some wave packets lasting only for 3 -4 wavecycles. In addition, one finds that the frequency can vary even during the event, from about 40 Hz to just above 20 Hz, i.e., by a factor of two. In contrast, the electron gyrofrequency drops only slightly, from 360 Hz to 310 Hz, during the event. A similar packet structure can also be seen in Figure 3, which shows two other lion roar wave packet trains and the corresponding magnetic trough. Each wave train lasts about 1.2 set and they occur just 4 set apart on both sides of a stronger core field. While fee= 420 Hz for both events, the lion roar frequency is distinctly different for the two events, 40 and 25 Hz, respectively. SUMMARY The present study has revealed that magnetosheath (1) are near-monochromatic (2) have a wavelet-like
lion roars:
right-hand circularly polarized shear waves;
packet structure with typical durations of 3 - 10 wavecycles (0.1 - 1.O s);
(3) have typical frequencies
between 15 and 40Hz (0.05 - 0.1 fi.<);
(4) have typical amplitudes
of 0.2 - 1.OnT.
DISCUSSION Table 1 shows that the present study complements the earlier observations of lion roars, by extending the frequency range to lower values. While search coil magnetometers and plasma wave instruments are somewhat insensitive to the frequency range below. say, 50 Hz, the Equator-S magnetometer, with a Nyquist frequency of 64 Hz, cannot cover the
80
W. Baumjohannet al.
Table 1. Intercomparison
of lion roar observations
Search Coil Magnetometer OGG-5 (Smith and Tsurutani,
Plasma Wave Instrument 1976)
Right-hand circular polarization
f = 90 - 160Hz f X 0.3 - 0.5 6B = 0.1 nT
fee
with different instruments Fluxgate Magnetometer
ISEE (Tsurutani et al., 1982)
Equator-S (this paper)
-
Right-hand circular polarization
-
Packet-like waveform
f z 50-
100Hz
f=
15-40Hz
f M0.2 - 0.5 &
f X 0.05 - 0.1 fey
-
6B % 0.2 - l.OnT
higher frequency range, but our observations show that lion roar electron whistlers can be observed down to 15 Hz, with the same characteristics as at higher frequencies. The most obvious observation is the narrow-bandedness of the signal. Since we know that it is in the right-hand whistler mode and is generated inside a magnetic trap configuration, whistler linear instability theory for the upper frequency limit suggests a weak electron temperature anisotropy of the order of A, = TJ ql - 1 z 0.1 for all the events. The resonance condition sets a lower limit on the parallel velocity of the resonant electrons which we estimate as umirr z 2.7 u&, roughly 3 times the local electron AlfvCn speed. Similarly the lower frequency cut-off sets a upper limit on the resonant speed ofthe order of u,,,~~ = [A,(1 -cr)+ l].[c~/A,(A,+l)]‘/~ x 1.4 u,,,i,, x 3.8 u,&, where cx = f,nirr/f&x. From the whistler dispersion relation we estimate a parallel wavelength h = UA,/(&‘~~~,) x 5Okm. The changes in the central frequency and frequency cut-offs are most simply interpreted as spatial gradients of the electron temperature anisotropy within the magnetic bottle. Apparently, the anisotropy reaches a maximum in the magnetic field minima and decreases towards the boundaries of the bottle. Glitches in the emission band as in Figure 2, on the other hand, suggest that two different electron populations are encountered inside a mirror bottle on the two opposing sites separated by a local stronger core field. This stronger core field is caused by the magnetic effect of the combined drift and magnetization currents in the magnetic field gradients at the two opposing boundaries of the bottle. The packet form of the emissions suggests that the whistler mode lion roar emissions reach a nonlinear state. In fact, the measured amplitude of 6 B x 1 nT corresponds to a magnetic wave energy density of 4 x lo-l3 Jrnm3 and a whistler wave electric field of 6 E x 1 mVm_’ or an electric wave energy density of 5 x lo-l4 Jm-‘. Compared to an estimated plasma thermal energy density of nkB T M lO-‘O Jmv3 these values correspond to fractions of 4 x 10v3 and 5 x 10p4, respectively. Waves of such a relative intensity will necessarily lead to modulational instability of the plasma and cause selfmodulation of the whistler waves resulting in wave packets as is observed. ACKNOWLEDGMENTS The Equator-S project was made possible through grant 5OOC94024 by the German Space Agency, DLR. Wavelet software was provided by C. Torrence and G. Compo, available at http://paos.colorado.edu/research/wavelets/. REFERENCES Lui, A. T. Y., and A.-H. Najmi, Time-frequency decomposition of signals in a current disruption event, Geophys. Res. Lett., 24,3157 (1997). Smith, E. J., and B. T. Tsurutani, Magnetosheath lion roars, J. Geophys. Rex, 81, 2261 (1976). Torrence, C., and G. P. Compo, A practical guide to wavelet analysis, Bull. Am. Met. Sot., 79,61 (1998). Tsurutani, B. T., E. J. Smith, R. R. Anderson, K. W. Ogilvie, J. D. Scudder, D. N. Baker, and S. J. Bame, Lion roars and nonoscillatory drift mirror waves in the magnetosheath, J. Geophys. Res., 87,606O (1982). Zhang, Y., H. Matsumoto, and H. Kojima, Lion roars in the magnetosheath: The Geotail observations, J. Geophys. Res., 103,4615 (1998).
0
Adv. Space Res. Vol. 24, NO.1, pp. 11-80,1~9 1999COSPAR. Published by Elsevier Science Ltd. AI1 rights reserved
Printed in Great Britain 0273-l 177/99 $20.00 + 0.00 PII: SO273-1177(99)00428-7
FIRST ELF WAVE MEASUREMENTS WITH THE EQUATOR-S MAGNETOMETER W. Baumjohann’ , G. Haerendel’ , R. A. Treumann’ , T. M. Bauer’ , J. Ruitenbach’ , E. Georgescu’s2, U. Austeg , K. H. Fomacon3, K.-H. GlaBmeies. H. Liihr4, J. BiichneS, B. Nikutowski5, A. Balogh6, and S. W. H. Cowley7
’ Max-Planck-lnstitut fiir extraterrestrische Physik, Garching, Germany ‘Institute of Space Sciences, Bucharest, Romania 3Institut fir Geophysik und Meteorologic, TU Braunschweig, Germany 4GeoForschungsZentrum Potsdam, Potsdam, Germany ‘Max-Planck-Institute fiir Aeronomie, Katlenburg-Lindau, Germany 61mperial College, London, UK ’ Leicester University, Leicester; UK
ABSTRACT The magnetometer onboard the Equator-S satellite is very sensitive and has a high sampling rate of up to 128 Hz. These specifications allow for the first fluxgate magnetometer measurements of ELF waves between the ion cyclotron and the lower hybrid frequencies in the equatorial dayside magnetosheath. The so-called lion roars, typically seen by the Equator-S magnetometer at the bottom of the magnetic troughs of magnetosheath mirror waves, are near-monochromatic packets of electron whistler waves lasting for 0.2 - 1 sec. They are right-hand circularly polarized shear waves with typical amplitudes of 0.5 - 1 nT at frequencies of 15 -40 Hz, i.e., around one tenth of the electron gyrofrequency. 01999 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION More than 20 years ago, Smith and Tsurutani (1976) published the first search-coil magnetometer observations of what they called lion roars inside magnetic troughs in the dayside magnetosheath. They found that the lion roars are narrowbanded right-hand polarized waves, basically electron cyclotron waves, that are relatively short-lived, about 2 seconds, and that they have typical frequencies of about 120 Hz and typical amplitudes of 0.1 nT. Some years later, Tsurutani et al. (1982) studied the same phenomenon with the plasma wave instrument on ISEE. They found these waves at somewhat lower frequencies (50-100 Hz) and also established that lion roars are intimately related to mirror mode waves and that they are caused by the perpendicular electron pressure anisotropy in the magnetic troughs of these structures. However, due to the instrumentation employed in the earlier studies, the actual waveform of the lion roars was unknown until very recently (Zhang et al., 1998). INSTRUMENTATION The Equator-S magnetic field instrument consists of two units with a pair of three-axes fluxgate magnetometers each. The sensors of the primary and the redundant units are mounted on two rigid booms, with the main sensor located at the end of the 1.8-m boom and the other 50 cm further inboard. The inboard magnetometer is only used periodically to determine the amount of interference from the spacecraft. The sampling rate is 128 vectors/s in normal mode, when only the outboard magnetometer is used, and 64 vectors/s for dual mode operation. The amplitude resolution is 16 bit
W Baumjohannetd
0.2
0.0 E
0.4
0.6
0.8 set
0.4
0.6
0.8 set
4ogj
p!
0
LL
*-~I*~-‘~~~‘**-‘~*
0.2
0.0
Fig. 1. Lion roar waveform perpendicular
to the mean field and right-hand polarized wavelet spectrogram.
are selected automatically in steps of 4 between 256 and 64000 nT. The data used in the present study are 128 Hz data sampled with a resolution of about 10 pT in the 256 nT mode.
and ranges
DATA The upper panel of Figure 1 shows a typical lion roar signal wave form, about 10 wavecycles with an amplitude of fl nT. Shown here is one of the components perpendicular to the mean magnetic field. The other perpendicular components looks virtually identical, with the same amplitude, but a 90”-phase shift. The compressional component is very small, less than 0.1 nT. Hence the lion roar wave is a shear wave packet with right-handed circular polarization. Since the lion roar signals are rather short-lived, we use a wavelet analysis to determine spectrograms of the signals. The wavelet used is the so-called Morlet wavelet, which is best suited for this type of analysis (Lui and Najmi, 1987; Torrence and Compo, 1998). By transforming the complex signal (B,l = B, f i By), we calculate the right-hand and left-hand polarization spectrograms. The lower panel of Figure 1 shows the spectrogram of the right-hand polarized signal (which contained all power). The signal is nearly monochromatic with a frequency near 15 Hz. This is less than l/20 of the electron gyrofrequency near 350 Hz.
F .= 0.0
cci
-1.51.
0.2
0.0
0.4
0.6
ti
0.8
set
0.8
set
0
0.0
I.
-
I
0.2
-.
-
’
*
-
‘1.
0.4
Fig. 2. Lion roar waveform perpendicular
“I”
0.6
to the mean field and right-hand polarized wavelet spectrogram.
Magnetosheath
0.6
0.6
i= +,
79
Lion Roars
0.0
0.0
m” -0.6 -0.6 0.0
-iT E.
0.2
0.4
0.6
0.8
1.0
set
0.2
0.0
60
60..
40
40 -
20
20 ;o
. . ’ . .
0.4
1..
0.6
0.8
’ ‘.
. . ’
0.6
0.8
1.0
‘.
set
’ ‘.
._
6 s $ v
0 0.0
0.2
0.4
0.6
0.8
1.0
set
0 . 0.0
..1...1...1...1...1... 0.2 0.4
1.0
set
Fig. 3. Magnetic field magnitude, lion roar shear wave components and right-hand polarized wavelet spectrograms.
Another lion roar signal is shown in Figure 2. Again, the wave train lasts for about one second, but it shows considerable internal structure, with some wave packets lasting only for 3 -4 wavecycles. In addition, one finds that the frequency can vary even during the event, from about 40 Hz to just above 20 Hz, i.e., by a factor of two. In contrast, the electron gyrofrequency drops only slightly, from 360 Hz to 310 Hz, during the event. A similar packet structure can also be seen in Figure 3, which shows two other lion roar wave packet trains and the corresponding magnetic trough. Each wave train lasts about 1.2 set and they occur just 4 set apart on both sides of a stronger core field. While fee= 420 Hz for both events, the lion roar frequency is distinctly different for the two events, 40 and 25 Hz, respectively. SUMMARY The present study has revealed that magnetosheath (1) are near-monochromatic (2) have a wavelet-like
lion roars:
right-hand circularly polarized shear waves;
packet structure with typical durations of 3 - 10 wavecycles (0.1 - 1.O s);
(3) have typical frequencies
between 15 and 40Hz (0.05 - 0.1 fi.<);
(4) have typical amplitudes
of 0.2 - 1.OnT.
DISCUSSION Table 1 shows that the present study complements the earlier observations of lion roars, by extending the frequency range to lower values. While search coil magnetometers and plasma wave instruments are somewhat insensitive to the frequency range below. say, 50 Hz, the Equator-S magnetometer, with a Nyquist frequency of 64 Hz, cannot cover the
80
W. Baumjohannet al.
Table 1. Intercomparison
of lion roar observations
Search Coil Magnetometer OGG-5 (Smith and Tsurutani,
Plasma Wave Instrument 1976)
Right-hand circular polarization
f = 90 - 160Hz f X 0.3 - 0.5 6B = 0.1 nT
fee
with different instruments Fluxgate Magnetometer
ISEE (Tsurutani et al., 1982)
Equator-S (this paper)
-
Right-hand circular polarization
-
Packet-like waveform
f z 50-
100Hz
f=
15-40Hz
f M0.2 - 0.5 &
f X 0.05 - 0.1 fey
-
6B % 0.2 - l.OnT
higher frequency range, but our observations show that lion roar electron whistlers can be observed down to 15 Hz, with the same characteristics as at higher frequencies. The most obvious observation is the narrow-bandedness of the signal. Since we know that it is in the right-hand whistler mode and is generated inside a magnetic trap configuration, whistler linear instability theory for the upper frequency limit suggests a weak electron temperature anisotropy of the order of A, = TJ ql - 1 z 0.1 for all the events. The resonance condition sets a lower limit on the parallel velocity of the resonant electrons which we estimate as umirr z 2.7 u&, roughly 3 times the local electron AlfvCn speed. Similarly the lower frequency cut-off sets a upper limit on the resonant speed ofthe order of u,,,~~ = [A,(1 -cr)+ l].[c~/A,(A,+l)]‘/~ x 1.4 u,,,i,, x 3.8 u,&, where cx = f,nirr/f&x. From the whistler dispersion relation we estimate a parallel wavelength h = UA,/(&‘~~~,) x 5Okm. The changes in the central frequency and frequency cut-offs are most simply interpreted as spatial gradients of the electron temperature anisotropy within the magnetic bottle. Apparently, the anisotropy reaches a maximum in the magnetic field minima and decreases towards the boundaries of the bottle. Glitches in the emission band as in Figure 2, on the other hand, suggest that two different electron populations are encountered inside a mirror bottle on the two opposing sites separated by a local stronger core field. This stronger core field is caused by the magnetic effect of the combined drift and magnetization currents in the magnetic field gradients at the two opposing boundaries of the bottle. The packet form of the emissions suggests that the whistler mode lion roar emissions reach a nonlinear state. In fact, the measured amplitude of 6 B x 1 nT corresponds to a magnetic wave energy density of 4 x lo-l3 Jrnm3 and a whistler wave electric field of 6 E x 1 mVm_’ or an electric wave energy density of 5 x lo-l4 Jm-‘. Compared to an estimated plasma thermal energy density of nkB T M lO-‘O Jmv3 these values correspond to fractions of 4 x 10v3 and 5 x 10p4, respectively. Waves of such a relative intensity will necessarily lead to modulational instability of the plasma and cause selfmodulation of the whistler waves resulting in wave packets as is observed. ACKNOWLEDGMENTS The Equator-S project was made possible through grant 5OOC94024 by the German Space Agency, DLR. Wavelet software was provided by C. Torrence and G. Compo, available at http://paos.colorado.edu/research/wavelets/. REFERENCES Lui, A. T. Y., and A.-H. Najmi, Time-frequency decomposition of signals in a current disruption event, Geophys. Res. Lett., 24,3157 (1997). Smith, E. J., and B. T. Tsurutani, Magnetosheath lion roars, J. Geophys. Rex, 81, 2261 (1976). Torrence, C., and G. P. Compo, A practical guide to wavelet analysis, Bull. Am. Met. Sot., 79,61 (1998). Tsurutani, B. T., E. J. Smith, R. R. Anderson, K. W. Ogilvie, J. D. Scudder, D. N. Baker, and S. J. Bame, Lion roars and nonoscillatory drift mirror waves in the magnetosheath, J. Geophys. Res., 87,606O (1982). Zhang, Y., H. Matsumoto, and H. Kojima, Lion roars in the magnetosheath: The Geotail observations, J. Geophys. Res., 103,4615 (1998).