Balloon observations of ultra-low-frequency waves in the electric field ...
transmitted from Siple Station. The large apparent bandwidth of the signal received on the spacecraft results from the presence of electrostatic waves stimulated as the input pulse scattered from magnetic-field-aligned plasma density irregularities located along the propagation path. The top panel of figure 2 shows the measured amplitude of the electrostatic waves shown in figure 1 as a function of the wavelength of these waves. The bottom panel shows the predicted Fourier spectrum of the irregularities which lead to the generation of the electrostatic waves. The dominant predicted wavelengths in the spatial Fourier spectrum of the irregularities lie in the range 27-30 meters. This new diagnostic technique is potentially very powerful since it can be used at altitudes far above that at which groundbased radar is useful and covers a scale range (5-100 meters) that is an order of magnitude better than the resolution of most experiments designed to measure density irregularities directly. Further tests of the technique will be carried out using data acquired at Siple Station in the period 1982-1987 with the ISIS satellite as well as the high altitude ISEE-1 and DE-1 satellites. If the technique can be perfected, it will be of substantial
Balloon observations of ultra-low-frequency waves in the electric field above the South Pole
benefit to the fields of both magnetospheric and ionospheric physics. This research was supported under National Aeronautic and Space Administration grants NGL-05-020-008 and NAS 5-28447.
References Bell, T.F., H.G. James, U.S. man, and J.P. Katsufrakis. 1983. The apparent spectral broadening of VLF transmitter signals during transionospheric propagation. Journal of Geophysical Research, 88(A-6), 4813-4840. Bell, T.F., and H.D. Ngo. 1988. Electrostatic waves stimulated by coherent VLF signals propagating in and near the inner radiation belt. Journal of Geophysical Research, 93(A4), 2599-2618. Bell, T.F., and H.D. Ngo. In preparation. Electrostatic wave stimulation by electromagnetic waves propagating in regions containing magnetic-field-aligned plasma density irregularities. Journal of Geophysical Research.
Fejer, B.C., and M.C. Kelley. 1980. Ionospheric irregularities. Review of Geophysics, 18(2), 401-420.
based fluxgate and induction magnetometers to determine the characteristics of the waves. After float was reached, the electric-field data in figure 1 show large-amplitude, quasi-periodic fluctuations suggesting SOUTH POLE BALLOON CAMPAIGN University of Houston Flight 3
B. LIA0, J.R. BENBROOK, E.A. BERING III, G.J. BYRNE, and J.R. THEALL Physics Department University of Houston Houston, Texas 77204-5504
The physics of ultra-low-frequency waves in the magnetosphere, near the cusp and in the polar cap, is important because this region is one where ultra-low-frequency wave energy from the magnetopause can most easily enter the magnetosphere (Russell, Holzer, and Smith 1969; Russell and Chappel 1971; Lanzerotti, Medford, and Rosenberg 1982). During the 19851986 South Pole balloon campaign (Bering et al. 1986), eight stratospheric balloon payloads were launched from Amundsen-Scott Station, South Geographic Pole, Antarctica, to record data on ultra-low-frequency waves. The payloads were instrumented with three-axis double-probe electric field detectors and X-ray scintillation counters. This paper concentrates on the third flight of this series, which was launched at 2205 universal time on 21 December 1985. Good data were received from the payload until the transmitter failed at 0342 universal time on 22 December. During most of the four hours that the balloon was afloat, an intense ultra-low-frequency wave event was in progress. The electric-field data from this period have been examined in detail and compared with magnetic field data, obtained with ground202
so
:
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60
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40
-420.
-,".',,,,,,, ... 2:
-440
I
so
0000
0100
Universal Time, Da y s 356, 22 December 1985
0300
Figure 1. The three components of the electric field measured by the balloon payload and the three components of the magnetic field measured by ground magnetometers at the South Pole (solid curve) and Iqaluit (dashed curve) are shown for the duration of flight 3. The conventional labels for the three components of the magnetic field are H-poleward, D-eastward, and Z-vertical. (mV/rn denotes millivolts per meter. nT denotes nanoteslas.) ANTARCTIC JOURNAL
the presence of intense ultra-low-frequency wave activity. The electric-field components have an amplitude of 20-30 millivolts per meter and a frequency of about 3 millihertz. The wave amplitude is strongest in the east component for the majority of the time. Ultra-low-frequency wave activity is not visibly present in the unfiltered fluxgate magnetic-field data from either the South Pole or its Northern Hemisphere conjugate point Iqaluit (Lanzerotti, Medford, and Rosenberg 1982; Wolfe et al. 1986). To investigate whether there is a small-amplitude signal present in the magnetic-field data, it is necessary to bandpass filter (Otones and Enochson 1972) the data to enhance the relative strength of any signal that is present at the 3-millihertz frequency of interest (figure 2). The properties of the digital filter are such that a large, abrupt step-like change in input signal will produce a ring of a few cycles. This occurs at a number of places in the fluxgate magnetic-field channels, but there are no periods of extended signal such as are present in the electric-field channel. This figure suggests either that the ultra-low-frequency event is essentially electrostatic or that the wavelength is very short (Boström et al. 1973). A peak near 3 millihertz is readily visible in the electric-field spectrum (figure 3). There is only a hint of a peak at 3 millihertz in the spectrum of the D component of the magnetic field (figure 3, top). The coherence between the two time series is not significant in the frequency range of the ultra-low-frequency waves. A more substantial hint of a peak is evident in the spectrum of the same component measured by the induction (or "search coil") micropulsation magnetometer (figure 3, bottom). A more statistically significant peak can be seen at the Brunt-Väisälä acoustic cut-off frequencies in the spectra of H and Z components of the Iqaluit magnetic-field (not shown). SOUTH POLE BALLOON CAMPAIGN University of Houston Flight 3
IA E-.s
A hodograph analysis indicates that the polarization of the ultra-low-frequency event is mostly linear during the period from 0030 to 0100 universal time. During the next 30 minutes (0100-0130 universal time), the polarization is decidedly elliptic with the amplitude of the signal increasing with time. During the next 30 minutes (0130-0200 universal time), the signal switches from elliptic back to linear polarization, but not in the same direction as in the first interval (0030-0100 universal time), and then back to elliptic polarization. The amplitude of the signal remains high in the third interval (0130-0200 universal time) as in the second interval (0100-0130 universal time). During the next 30-minute period (0200-0230 universal time), the field moves almost to circular polarization and then reverts to linear polarization along the eastward component. During the last 30-minute interval (0230-0300 universal time), the wave changes from linearly polarized along the eastward component to linearly polarized along the poleward component. The changeover takes place in roughly five cycles. Instead of a single ultra-low-frequency wave, an alternate view of these data might describe the event as two separate events, one with a poleward-directed field and one with an eastwarddirected field, with slightly different frequencies. That interpretation would lead to varying polarizations similar to those described, but the amplitude of the two components would remain fixed, rather than shrinking almost to zero as noted. We take this to be good evidence that a single process is responsible for the event. In conclusion, the electric-field signature observed from flight 3 appears to have been essentially an electrostatic event or possibly a short-wavelength hydromagnetic wave with a varying and interesting polarization character. The absence of any signature in any of the other available supporting data sets, such as riometer data, complicates the interpretation of these data. We are continuing the analysis of the data to determine the source of the observed utra-low-frequency waves. We are obliged to Louis J. Lanzerotti and Carol Maclennan of AT&T Bell Laboratories for the use of the Cusp Lab magnetometer data, and also, we thank Roger L. Arnoldy of the University of New Hampshire for the use of the micropulsation data. This data was supported by National Science Foundation grants DPP 84-15203 and DPP 86-14091. References
Os -I 0
0000
0100
0200
0300
UNIVERSAL TIME, DAY 356 DECEMBER 22, 1985
Figure 2. The eastward component of the electric field along with the three components of the magnetic field data from the South Pole after all four components have been subjected to a bandpass filter with a center frequency of 3 millihertz and bandwidth of 0.5 millihertz. (mV/rn denotes millivolts per meter.) 1988 REVIEW
Bering, E.A., III, J.R. Benbrook, D.L. Matthews, and T.J. Rosenberg. 1986. The 1985-1986 South Pole balloon campaign. Antarctic Journal of the U.S., 21(5), 267-269. Boström, R., U. Fahieson, L. Olausson, and G. Hallendal. 1973. Theory of time varying atmospheric electric fields and some applications to fields of ionospheric origin. TRITA-EPP-73-02, Department of Plasma Physics, Royal Institute of Technology, 5-10044 Stockholm 70, Sweden. Lanzerotti, L.J., L.V. Medford, and T.J. Rosenberg. 1982. Magnetic field and particle prescription observations at the South Pole. Antarctic Journal of the U.S., 17(5), 235-236. Otnes, R.K., and L. Enochson. 1972. Digital time series analysis, 108. New York: John Wiley and Sons. Russell, C.T., R.E. Holzer, and E.J. Smith. 1969. Ogo 3 observations of ELF noise in the magnetosphere, 1, spatial extent and frequency of occurrence. Journal of Geophysical Research, 74(3), 755-777. Russell, C.T., and C.R. Chappel. 1971. Ogo 5 observations of the polar cusp on November 1, 1968. Journal of Geophysical Research, 76(28), 6743-6764. Wolfe, A., L.J. Lanzerotti, C.G. Maclennan, and L.V. Medford. 1986. Geomagnetic studies near the magnetospheric cusps. Antarctic Journal of the U.S., 21(5), 277-279. 203
2340 1UT 21 December to 0341 UT 22 December 1985 I 11111111 liii,!!
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I..1 I l'?'r.4._,_r' 01- 1111111 1E-5 1E-4 1E-3 0.01 0.1 Frequency, Hz Figure 3. The power spectra of (top) the eastward electric field and the D component of the South Pole fluxgate magnetic field and (bottom) the eastward electric field and the eastward component (Y) of the induction-magnetometer micropulsation data from the South Pole over the time period that the ultra-low-frequency wave is in progress. The coherence between the two time series is also shown as function of frequency in each plot. The coherence is plotted as a solid line, referring to the linear scale on the left. The power spectra are plotted as dashed lines referring to the logarithmic scale on the right. The electric field spectra are plotted in units of millivolts per meter squared divided by hertz, the fluxgate magnetometer spectrum is in units of nanoteslas squared divided by hertz times 10, and the induction magnetometer spectrum is in units of nanoteslas squared minus hertz times 1.3 x 10'. From left to right, the three vertical lines indicate the Brunt-Väisälä, acoustic cut-off, and balloon neutral buoyancy oscillation frequencies. 204 ANTARCTIC JOURNAL 1 a I I I
I
Balloon observations of ultra-low-frequency waves in the electric field above the South Pole
benefit to the fields of both magnetospheric and ionospheric physics. This research was supported under National Aeronautic and Space Administration grants NGL-05-020-008 and NAS 5-28447.
References Bell, T.F., H.G. James, U.S. man, and J.P. Katsufrakis. 1983. The apparent spectral broadening of VLF transmitter signals during transionospheric propagation. Journal of Geophysical Research, 88(A-6), 4813-4840. Bell, T.F., and H.D. Ngo. 1988. Electrostatic waves stimulated by coherent VLF signals propagating in and near the inner radiation belt. Journal of Geophysical Research, 93(A4), 2599-2618. Bell, T.F., and H.D. Ngo. In preparation. Electrostatic wave stimulation by electromagnetic waves propagating in regions containing magnetic-field-aligned plasma density irregularities. Journal of Geophysical Research.
Fejer, B.C., and M.C. Kelley. 1980. Ionospheric irregularities. Review of Geophysics, 18(2), 401-420.
based fluxgate and induction magnetometers to determine the characteristics of the waves. After float was reached, the electric-field data in figure 1 show large-amplitude, quasi-periodic fluctuations suggesting SOUTH POLE BALLOON CAMPAIGN University of Houston Flight 3
B. LIA0, J.R. BENBROOK, E.A. BERING III, G.J. BYRNE, and J.R. THEALL Physics Department University of Houston Houston, Texas 77204-5504
The physics of ultra-low-frequency waves in the magnetosphere, near the cusp and in the polar cap, is important because this region is one where ultra-low-frequency wave energy from the magnetopause can most easily enter the magnetosphere (Russell, Holzer, and Smith 1969; Russell and Chappel 1971; Lanzerotti, Medford, and Rosenberg 1982). During the 19851986 South Pole balloon campaign (Bering et al. 1986), eight stratospheric balloon payloads were launched from Amundsen-Scott Station, South Geographic Pole, Antarctica, to record data on ultra-low-frequency waves. The payloads were instrumented with three-axis double-probe electric field detectors and X-ray scintillation counters. This paper concentrates on the third flight of this series, which was launched at 2205 universal time on 21 December 1985. Good data were received from the payload until the transmitter failed at 0342 universal time on 22 December. During most of the four hours that the balloon was afloat, an intense ultra-low-frequency wave event was in progress. The electric-field data from this period have been examined in detail and compared with magnetic field data, obtained with ground202
so
:
0 380
60
_400
40
-420.
-,".',,,,,,, ... 2:
-440
I
so
0000
0100
Universal Time, Da y s 356, 22 December 1985
0300
Figure 1. The three components of the electric field measured by the balloon payload and the three components of the magnetic field measured by ground magnetometers at the South Pole (solid curve) and Iqaluit (dashed curve) are shown for the duration of flight 3. The conventional labels for the three components of the magnetic field are H-poleward, D-eastward, and Z-vertical. (mV/rn denotes millivolts per meter. nT denotes nanoteslas.) ANTARCTIC JOURNAL
the presence of intense ultra-low-frequency wave activity. The electric-field components have an amplitude of 20-30 millivolts per meter and a frequency of about 3 millihertz. The wave amplitude is strongest in the east component for the majority of the time. Ultra-low-frequency wave activity is not visibly present in the unfiltered fluxgate magnetic-field data from either the South Pole or its Northern Hemisphere conjugate point Iqaluit (Lanzerotti, Medford, and Rosenberg 1982; Wolfe et al. 1986). To investigate whether there is a small-amplitude signal present in the magnetic-field data, it is necessary to bandpass filter (Otones and Enochson 1972) the data to enhance the relative strength of any signal that is present at the 3-millihertz frequency of interest (figure 2). The properties of the digital filter are such that a large, abrupt step-like change in input signal will produce a ring of a few cycles. This occurs at a number of places in the fluxgate magnetic-field channels, but there are no periods of extended signal such as are present in the electric-field channel. This figure suggests either that the ultra-low-frequency event is essentially electrostatic or that the wavelength is very short (Boström et al. 1973). A peak near 3 millihertz is readily visible in the electric-field spectrum (figure 3). There is only a hint of a peak at 3 millihertz in the spectrum of the D component of the magnetic field (figure 3, top). The coherence between the two time series is not significant in the frequency range of the ultra-low-frequency waves. A more substantial hint of a peak is evident in the spectrum of the same component measured by the induction (or "search coil") micropulsation magnetometer (figure 3, bottom). A more statistically significant peak can be seen at the Brunt-Väisälä acoustic cut-off frequencies in the spectra of H and Z components of the Iqaluit magnetic-field (not shown). SOUTH POLE BALLOON CAMPAIGN University of Houston Flight 3
IA E-.s
A hodograph analysis indicates that the polarization of the ultra-low-frequency event is mostly linear during the period from 0030 to 0100 universal time. During the next 30 minutes (0100-0130 universal time), the polarization is decidedly elliptic with the amplitude of the signal increasing with time. During the next 30 minutes (0130-0200 universal time), the signal switches from elliptic back to linear polarization, but not in the same direction as in the first interval (0030-0100 universal time), and then back to elliptic polarization. The amplitude of the signal remains high in the third interval (0130-0200 universal time) as in the second interval (0100-0130 universal time). During the next 30-minute period (0200-0230 universal time), the field moves almost to circular polarization and then reverts to linear polarization along the eastward component. During the last 30-minute interval (0230-0300 universal time), the wave changes from linearly polarized along the eastward component to linearly polarized along the poleward component. The changeover takes place in roughly five cycles. Instead of a single ultra-low-frequency wave, an alternate view of these data might describe the event as two separate events, one with a poleward-directed field and one with an eastwarddirected field, with slightly different frequencies. That interpretation would lead to varying polarizations similar to those described, but the amplitude of the two components would remain fixed, rather than shrinking almost to zero as noted. We take this to be good evidence that a single process is responsible for the event. In conclusion, the electric-field signature observed from flight 3 appears to have been essentially an electrostatic event or possibly a short-wavelength hydromagnetic wave with a varying and interesting polarization character. The absence of any signature in any of the other available supporting data sets, such as riometer data, complicates the interpretation of these data. We are continuing the analysis of the data to determine the source of the observed utra-low-frequency waves. We are obliged to Louis J. Lanzerotti and Carol Maclennan of AT&T Bell Laboratories for the use of the Cusp Lab magnetometer data, and also, we thank Roger L. Arnoldy of the University of New Hampshire for the use of the micropulsation data. This data was supported by National Science Foundation grants DPP 84-15203 and DPP 86-14091. References
Os -I 0
0000
0100
0200
0300
UNIVERSAL TIME, DAY 356 DECEMBER 22, 1985
Figure 2. The eastward component of the electric field along with the three components of the magnetic field data from the South Pole after all four components have been subjected to a bandpass filter with a center frequency of 3 millihertz and bandwidth of 0.5 millihertz. (mV/rn denotes millivolts per meter.) 1988 REVIEW
Bering, E.A., III, J.R. Benbrook, D.L. Matthews, and T.J. Rosenberg. 1986. The 1985-1986 South Pole balloon campaign. Antarctic Journal of the U.S., 21(5), 267-269. Boström, R., U. Fahieson, L. Olausson, and G. Hallendal. 1973. Theory of time varying atmospheric electric fields and some applications to fields of ionospheric origin. TRITA-EPP-73-02, Department of Plasma Physics, Royal Institute of Technology, 5-10044 Stockholm 70, Sweden. Lanzerotti, L.J., L.V. Medford, and T.J. Rosenberg. 1982. Magnetic field and particle prescription observations at the South Pole. Antarctic Journal of the U.S., 17(5), 235-236. Otnes, R.K., and L. Enochson. 1972. Digital time series analysis, 108. New York: John Wiley and Sons. Russell, C.T., R.E. Holzer, and E.J. Smith. 1969. Ogo 3 observations of ELF noise in the magnetosphere, 1, spatial extent and frequency of occurrence. Journal of Geophysical Research, 74(3), 755-777. Russell, C.T., and C.R. Chappel. 1971. Ogo 5 observations of the polar cusp on November 1, 1968. Journal of Geophysical Research, 76(28), 6743-6764. Wolfe, A., L.J. Lanzerotti, C.G. Maclennan, and L.V. Medford. 1986. Geomagnetic studies near the magnetospheric cusps. Antarctic Journal of the U.S., 21(5), 277-279. 203
2340 1UT 21 December to 0341 UT 22 December 1985 I 11111111 liii,!!
1111111 IIi III III!]
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I..1 I l'?'r.4._,_r' 01- 1111111 1E-5 1E-4 1E-3 0.01 0.1 Frequency, Hz Figure 3. The power spectra of (top) the eastward electric field and the D component of the South Pole fluxgate magnetic field and (bottom) the eastward electric field and the eastward component (Y) of the induction-magnetometer micropulsation data from the South Pole over the time period that the ultra-low-frequency wave is in progress. The coherence between the two time series is also shown as function of frequency in each plot. The coherence is plotted as a solid line, referring to the linear scale on the left. The power spectra are plotted as dashed lines referring to the logarithmic scale on the right. The electric field spectra are plotted in units of millivolts per meter squared divided by hertz, the fluxgate magnetometer spectrum is in units of nanoteslas squared divided by hertz times 10, and the induction magnetometer spectrum is in units of nanoteslas squared minus hertz times 1.3 x 10'. From left to right, the three vertical lines indicate the Brunt-Väisälä, acoustic cut-off, and balloon neutral buoyancy oscillation frequencies. 204 ANTARCTIC JOURNAL 1 a I I I
I
