Asymmetric magnetic pulsations observed at Siple Station
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Figure 3. Magnetic field and riometer variations for an 8-hour interval on 1 June 1980, showing the level of geomagnetic activity during the Impulsive precipitation events. The left panel shows the north-south (H) component variations, the middle panel the east-west (D) component variations, and the right panel the riometer (A) variations. SI = Sipie; nT = nanotesia; dB = decibel; see figure 1 for explanation of other abbreviations.
precipitation and magnetic field variations similar to those in figure 2 occurred are indicated by the horizontal bars. The magnetic field at subauroral latitudes was disturbed at this time, and enhanced particle precipitation, as evidenced by the enhanced absorption of cosmic noise by about 2 decibels, was seen at Siple and over the two most northern conjugate stations. In summary, the Siple magnetic field and nometer data, together with data from the conjugate hemisphere, illustrate new aspects of particle precipitation and magnetic field fluctuation phenomena at subauroral latitudes. Further study of these data and similar data obtained on other days will provide new insights into the nature of magnetosphere particle precipitation phenomena. Logistics support for the Bell Laboratories work at Siple Station was provided by the National Science Foundation, Division
Asymmetric magnetic pulsations observed at Siple Station MARK J. ENGEBRETSON
Department of Physics Augsburg College Minneapolis, Minnesota 55454 LAURENCE J. CAHILL, JR.
School of Astronomy and Physics University of Minnesota Minneapolis, Minnesota 55455 ROGER L. ARNOLD?
Department of Physics University of New Hampshire Durham, New Hampshire 03824
1982 REVIEW
of Polar Programs. The work at the University of Maryland is supported by National Science Foundation grants DPP 80-12901 and ATM 80-06496.
References Arnoldy, R. L., Dragoon, K., Cahill, L. J., Jr., Mende, S. and Rosenberg,
T. J. In preparation. Detailed correlations of magnetic field and riometer observations at L = 4.2 with pulsating aurora.
Rosenberg, T. J., Morris P. B., and Lanzerotti, L. J. 1981. Excitation of magnetospheric hydromagnetic waves by solar-flare-induced charge in ionosphere conductivity. Physical Review Letters, 47, 1343.
As part of a continuing investigation of ultra-low-frequency variations in the Earth's magnetic field, we have investigated a class of irregular magnetic pulsations known as Pi 1 (frequencies in the range .025-1 hertz) using the search coil detector at Siple Station (geographic coordinates, 76°S 84°W). Although Pi 1 pulsations have been studied for decades with magnetometers of the type employed in this study, the Siple search coil magnetometer has the twin advantages of (1) highly accurate 12-bit digital recording and (2) high time resolution of 20 vector samples per second. This instrumentation has allowed us to observe clearly and quite frequently a class of asymmetric pulsations during geomagnetically disturbed periods. Shown in figure 1 are normal- (inset) and high-resolution plots of typical but rather "clean" asymmetric pulsations observed during a sustained Pi 1 event on 10 June 1980. The highamplitude peaks near 80 and 120 seconds in the X (north-south) component appear much more symmetrical in the low-resolution plot than when plotted at full 20 points per second resolution. 243
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Figure 1. Magnetic field (dB/dt) data from a portion of a sustained P11 event on 10 June 1980. The Inset shows a 250-second portion of the event plotted at normal resolution, and the main figure shows full-resolution (20 points per second) data from the digitally sampled search coil magnetometers. nT/s = nanotesla per second.
HR. 4 MIN. 47 SEC. 37 JUNE 12, 1980 HR. 5 MIN. 14 SEC. 17 400 800 1200 1 0
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1 - 0
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800 SECONDS
1200
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1600
Figure 2. Correlated magnetic field and photometer data from a portion of a sustained P11 event on 12 June 1980. The Intensity of the photometer signal (upper trace) increases downward. nT/s = nanotesls per second.
244
Because a search coil measures the derivative of magnetic induction (dB/dt) the rapid rise and slower fall of the observed asymmetric pulse past the zero level represents a transient, one-sided change and subsequent relaxation in B. We wish to emphasize that the observed pulsations are not waves in the usual sense. Although the pulsation near 200 seconds is followed by a train of more sinusoidal variations in both the X and Y (east-west) components, we have found no other examples where asymmetric pulses appeared to stimulate any sinusoidal magnetic variations. It was noted by some of us (Arnoldy et al. in preparation) that there was a close association between the prominent, asymmetric pulses and bursts of auroral light as detected at Siple by a narrowband, 4,278-angstrom, zenith-viewing photometer (Mende et at. 1980). To extend that analysis, we studied the 15 largest amplitude Pi I events noted during 1980, ranging from 26 March to 19 December. Asymmetric pulsations during all 15 events were visually identified. Although asymmetric pulses in dB/dt sometimes occurred without corresponding optical pulsations, nearly every optical pulsation was associated with a magnetic pulsation approximately 1 second later. Arsrrcnc
JOURNAL
changes, optical pulsations, and changes in ionospheric electron density (Arnoldy et al. in preparation), lead us to suggest that this class of pulsations originates at ionospheric altitudes and may be a rather common, but probably not the exclusive, source of auroral-zone irregular magnetic pulsations in this frequency range. This research was supported by National Science Foundation grant DPP 79-23294.
Figure 2 displays simultaneous photometer and search coil magnetometer waveforms for a 1,600-second interval during the 12 June 1980 event. Although the compressed time scale makes examination of the waveform of the magnetic pulsations difficult, it is apparent that the largest and most asymmetric pulsations are like those of figure 1. The background level of the photometer signal is significantly higher near the beginning of the time interval shown and decreases gradually. Individual photometer pulses are no larger at the end of the interval shown than at the beginning, but they appear to be associated with greater magnetic variations. All the major photometer pulses in the second half of the interval shown, and most minor ones as well, clearly are associated with asymmetric pulses in the X and/or Y components of dB/dt. The origin of irregular magnetic pulsations (pi) observed at high latitudes has been a matter of controversy for decades. Observations of these and other asymmetric pulsations, and especially the close relationship between magnetic field
References Arnoldy, R. L., Dragoon, K., Cahill, L. J . , Jr., Mende, S. B., Rosenberg, T. J. , and Lanzerotti, L. J . In preparation. Derailed correlations of magnetic field and riometer observations at L = 4.2 with pulsating aurora.
Mende, S. B., Arnoldy, R. L., Cahill, L. J . , Jr., Doolittle, J . H., Armstrong, W. C., and Fraser-Smith, A. C. 1980. Correlation between X 4278 A optical emissions and a Pc 1 pearl event observed at Siple Station, Antarctica. Journal of Geophysical Research, 85(A3), 1194-1202.
Energy spectrum of precipitating electrons during an artificially triggered wave event
10
190 KM D. L. MATI'HEWS Institute for Physical Science and Technology University of Maryland College Park, Maryland 20742
00 0 000
110 KM oo
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The Nike-Tomahawk rocket payloads launched at Siple Station during the 1980-81 magnetospheric physics campaign included comprehensive energetic electron spectrometers. Results from the solid-state units, which measured electrons of energy greater than 16 kiloelectronvolts, are discussed in a companion article (Siren and Matthews, Antarctic Journal, this issue). Here I discuss the results from the electrostatic analyzers for energies between 0.1 and 16 kiloelectronvolts and give a summary spectrum. Portions of the data in each rocket roll period were contaminated by solar ultraviolet photons. These portions are clearly distinguishable from the good data and are not used. On rocket flight 18.204 (20 December 1980) there is a moderately intense low-energy flux which shows altitude-dependent absorption. The figure shows preliminary differential energy spectra from this flight averaged over several seconds. Three points from the solid-state detector are included; higher energies have been omitted to avoid excessive scale compression. The measurements actually cover 4 decades in energy and 11 in differential flux. Several features of these data merit discussion. There is no obvious correlation between the electron fluxes and simultaneously observed wave events, whether the latter were naturally or artificially triggered. This is not surprising because the waves can spread out hundreds of kilometers horizontally from 1982 REVIEW
Li
f) Jo 6
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ELECTRON FLUXES 20 DEC. SO 1634-1835 LIT
LL_ LL
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102
01 .10 1.00 10.00 100.0 ENERGY KEV Spectrum of precipitating electrons at two altitudes. Solid-state detector points (+) are from the lower altitude. The differential flux values Indicate the number of electrons per unit solid angle (steradian) and unit energy (klloeiectronvolt) passing through 1 square centimeter in 1 second. UT = universal time.
245
03
07
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I200flT ... ..,,.,,,
11 03
07
11
12
16
SI
GV LT PB DU 08
12
16 08
12 UT JUNE 1, 1982
1608
Figure 3. Magnetic field and riometer variations for an 8-hour interval on 1 June 1980, showing the level of geomagnetic activity during the Impulsive precipitation events. The left panel shows the north-south (H) component variations, the middle panel the east-west (D) component variations, and the right panel the riometer (A) variations. SI = Sipie; nT = nanotesia; dB = decibel; see figure 1 for explanation of other abbreviations.
precipitation and magnetic field variations similar to those in figure 2 occurred are indicated by the horizontal bars. The magnetic field at subauroral latitudes was disturbed at this time, and enhanced particle precipitation, as evidenced by the enhanced absorption of cosmic noise by about 2 decibels, was seen at Siple and over the two most northern conjugate stations. In summary, the Siple magnetic field and nometer data, together with data from the conjugate hemisphere, illustrate new aspects of particle precipitation and magnetic field fluctuation phenomena at subauroral latitudes. Further study of these data and similar data obtained on other days will provide new insights into the nature of magnetosphere particle precipitation phenomena. Logistics support for the Bell Laboratories work at Siple Station was provided by the National Science Foundation, Division
Asymmetric magnetic pulsations observed at Siple Station MARK J. ENGEBRETSON
Department of Physics Augsburg College Minneapolis, Minnesota 55454 LAURENCE J. CAHILL, JR.
School of Astronomy and Physics University of Minnesota Minneapolis, Minnesota 55455 ROGER L. ARNOLD?
Department of Physics University of New Hampshire Durham, New Hampshire 03824
1982 REVIEW
of Polar Programs. The work at the University of Maryland is supported by National Science Foundation grants DPP 80-12901 and ATM 80-06496.
References Arnoldy, R. L., Dragoon, K., Cahill, L. J., Jr., Mende, S. and Rosenberg,
T. J. In preparation. Detailed correlations of magnetic field and riometer observations at L = 4.2 with pulsating aurora.
Rosenberg, T. J., Morris P. B., and Lanzerotti, L. J. 1981. Excitation of magnetospheric hydromagnetic waves by solar-flare-induced charge in ionosphere conductivity. Physical Review Letters, 47, 1343.
As part of a continuing investigation of ultra-low-frequency variations in the Earth's magnetic field, we have investigated a class of irregular magnetic pulsations known as Pi 1 (frequencies in the range .025-1 hertz) using the search coil detector at Siple Station (geographic coordinates, 76°S 84°W). Although Pi 1 pulsations have been studied for decades with magnetometers of the type employed in this study, the Siple search coil magnetometer has the twin advantages of (1) highly accurate 12-bit digital recording and (2) high time resolution of 20 vector samples per second. This instrumentation has allowed us to observe clearly and quite frequently a class of asymmetric pulsations during geomagnetically disturbed periods. Shown in figure 1 are normal- (inset) and high-resolution plots of typical but rather "clean" asymmetric pulsations observed during a sustained Pi 1 event on 10 June 1980. The highamplitude peaks near 80 and 120 seconds in the X (north-south) component appear much more symmetrical in the low-resolution plot than when plotted at full 20 points per second resolution. 243
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X-AXIS
"N
Ic2
0 100 200 I I
SECONDS
150
3-
Y-AXIS
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X-AXIS 2 1 -2
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Figure 1. Magnetic field (dB/dt) data from a portion of a sustained P11 event on 10 June 1980. The Inset shows a 250-second portion of the event plotted at normal resolution, and the main figure shows full-resolution (20 points per second) data from the digitally sampled search coil magnetometers. nT/s = nanotesla per second.
HR. 4 MIN. 47 SEC. 37 JUNE 12, 1980 HR. 5 MIN. 14 SEC. 17 400 800 1200 1 0
4M ii I
NNfffT
X-AXIS- 1
1 - 0
:
0
800 SECONDS
1200
0;
1600
Figure 2. Correlated magnetic field and photometer data from a portion of a sustained P11 event on 12 June 1980. The Intensity of the photometer signal (upper trace) increases downward. nT/s = nanotesls per second.
244
Because a search coil measures the derivative of magnetic induction (dB/dt) the rapid rise and slower fall of the observed asymmetric pulse past the zero level represents a transient, one-sided change and subsequent relaxation in B. We wish to emphasize that the observed pulsations are not waves in the usual sense. Although the pulsation near 200 seconds is followed by a train of more sinusoidal variations in both the X and Y (east-west) components, we have found no other examples where asymmetric pulses appeared to stimulate any sinusoidal magnetic variations. It was noted by some of us (Arnoldy et al. in preparation) that there was a close association between the prominent, asymmetric pulses and bursts of auroral light as detected at Siple by a narrowband, 4,278-angstrom, zenith-viewing photometer (Mende et at. 1980). To extend that analysis, we studied the 15 largest amplitude Pi I events noted during 1980, ranging from 26 March to 19 December. Asymmetric pulsations during all 15 events were visually identified. Although asymmetric pulses in dB/dt sometimes occurred without corresponding optical pulsations, nearly every optical pulsation was associated with a magnetic pulsation approximately 1 second later. Arsrrcnc
JOURNAL
changes, optical pulsations, and changes in ionospheric electron density (Arnoldy et al. in preparation), lead us to suggest that this class of pulsations originates at ionospheric altitudes and may be a rather common, but probably not the exclusive, source of auroral-zone irregular magnetic pulsations in this frequency range. This research was supported by National Science Foundation grant DPP 79-23294.
Figure 2 displays simultaneous photometer and search coil magnetometer waveforms for a 1,600-second interval during the 12 June 1980 event. Although the compressed time scale makes examination of the waveform of the magnetic pulsations difficult, it is apparent that the largest and most asymmetric pulsations are like those of figure 1. The background level of the photometer signal is significantly higher near the beginning of the time interval shown and decreases gradually. Individual photometer pulses are no larger at the end of the interval shown than at the beginning, but they appear to be associated with greater magnetic variations. All the major photometer pulses in the second half of the interval shown, and most minor ones as well, clearly are associated with asymmetric pulses in the X and/or Y components of dB/dt. The origin of irregular magnetic pulsations (pi) observed at high latitudes has been a matter of controversy for decades. Observations of these and other asymmetric pulsations, and especially the close relationship between magnetic field
References Arnoldy, R. L., Dragoon, K., Cahill, L. J . , Jr., Mende, S. B., Rosenberg, T. J. , and Lanzerotti, L. J . In preparation. Derailed correlations of magnetic field and riometer observations at L = 4.2 with pulsating aurora.
Mende, S. B., Arnoldy, R. L., Cahill, L. J . , Jr., Doolittle, J . H., Armstrong, W. C., and Fraser-Smith, A. C. 1980. Correlation between X 4278 A optical emissions and a Pc 1 pearl event observed at Siple Station, Antarctica. Journal of Geophysical Research, 85(A3), 1194-1202.
Energy spectrum of precipitating electrons during an artificially triggered wave event
10
190 KM D. L. MATI'HEWS Institute for Physical Science and Technology University of Maryland College Park, Maryland 20742
00 0 000
110 KM oo
T
0
Lu
0000
LU
The Nike-Tomahawk rocket payloads launched at Siple Station during the 1980-81 magnetospheric physics campaign included comprehensive energetic electron spectrometers. Results from the solid-state units, which measured electrons of energy greater than 16 kiloelectronvolts, are discussed in a companion article (Siren and Matthews, Antarctic Journal, this issue). Here I discuss the results from the electrostatic analyzers for energies between 0.1 and 16 kiloelectronvolts and give a summary spectrum. Portions of the data in each rocket roll period were contaminated by solar ultraviolet photons. These portions are clearly distinguishable from the good data and are not used. On rocket flight 18.204 (20 December 1980) there is a moderately intense low-energy flux which shows altitude-dependent absorption. The figure shows preliminary differential energy spectra from this flight averaged over several seconds. Three points from the solid-state detector are included; higher energies have been omitted to avoid excessive scale compression. The measurements actually cover 4 decades in energy and 11 in differential flux. Several features of these data merit discussion. There is no obvious correlation between the electron fluxes and simultaneously observed wave events, whether the latter were naturally or artificially triggered. This is not surprising because the waves can spread out hundreds of kilometers horizontally from 1982 REVIEW
Li
f) Jo 6
N
Li
Li
LU
Li
10 4
0 ELECTROSTATIC fRNALYZEF + 50L[0 55TRTE 5ECT0METS
CE
LU
ROCKET
Ui
ELECTRON FLUXES 20 DEC. SO 1634-1835 LIT
LL_ LL
C]
102
01 .10 1.00 10.00 100.0 ENERGY KEV Spectrum of precipitating electrons at two altitudes. Solid-state detector points (+) are from the lower altitude. The differential flux values Indicate the number of electrons per unit solid angle (steradian) and unit energy (klloeiectronvolt) passing through 1 square centimeter in 1 second. UT = universal time.
245
