Correlated electron and X-ray measurements of quiet
Rosenberg, T.J. 1986. Personal communication. Stiles, CS., F.T. Berkey, and J.R. Doupnik. 1981. Digital ionosonde studies of the ionosphere from Siple Station and Roberval, Quevec. Antarctic Journal of the U.S., 16(5), 213-215. Tiltheridge, J.E. 1985. lonogram analysis with the generalized program POLAN. (World Data Center A for Solar-Terrestrial Physics. Report UAG-93.) Torr, M.R., and D.C. Torr. 1973. The seasonal behaviour of the F2-layer of the ionosphere. Journal of Atmospheric and Terrestrial Physics, 35, 2237-2251.
375 350 Li
Li
Li
D D J0 LJ0
Li
300
0
LiH alley O R ob erval oSiple
U250
200 20
1 4.96
LiHalley ORoberval oSiple
15
o
ZI
0 0 00 0 0 00 0 0 0 0
N
I
2.79 -"
10 LLJ u5
0
Figure 2. This figure presents hmF 2 (upper panel) and N m F2 (lower panel) derived at 15-minute intervals over a 3-hour interval beginfling at 1700 universal time on 13 November 1982. The station identification is the same as that used in figure 1. ("Km" denotes "kilometer:' "mHz" denotes "megahertz." 'rn " denotes "per cubic meter:')
Correlated electron and X-ray measurements of quiet-time electron precipitation near Siple E.E. GAINEs, W.L. IMHOF, W.E. FRANCIS, and M. WALT Lockheed Palo Alto Research Laboratory Palo Alto, California 94304
T.J. ROSENBERG University of Maryland Institute for Physical Science and Technology College Park, Maryland 20742
Several cases of X-ray observations from balloons coordinated with measurements of precipitating electrons were obtained during passes of the low-altitude polar-orbiting satellite P78-1 near Siple, Antarctica. The observations, made during a geomagnetically quiet period from late December 1980 to early January 1981, included high-resolution measurements of the electron energy spectra, pitch angle distributions from two spectrometers on board the satellite (Imhof, Gaines, and Reagan 1981), and very sensitive measurements of bremsstrahlung X-rays from balloon-borne detectors. The data 1986 REVIEW
LLJ 5
0 17
18
- 19
20
0
UT HOUR, Day 317
acquisition limes, satellite-balloon separations, and magnetic indices are summarized in table 1. Cosmic noise absorption measurements were made during this period with a 30-megahertz riometer at Siple Station (Detrick and Rosenberg 1984). Electron energy spectra summed over pitch angles from 65 to 115 degrees are presented in figure 1. These spectra are averaged values from approximately 45 seconds of data for each case during which time the satellite traveled about 300 kilometers. The pitch-angle distributions corresponding to the energy spectra are shown as the upper curves in figure 2 for the nomi nal satellite altitude of 600 kilometers. The lower curve in each panel of figure 2 is the same distribution transformed to 200kilometer altitude for use in bremsstrahlung production calculations. Bremsstrahlung X-rays produced in the atmosphere by the precipitating electrons were computed using the method described in Walt, Newkirk, and Francis (1979). The bremsstrahlung distribution as a function of angle, energy, and altitude thus obtained is then traced through the atmosphere to the balloon altitudes by standard Monte Carlo methods. Electron energy loss to ionization in the atmosphere is obtained from the first part of the bremsstrahlung computation, and this can be used to calculate the expected cosmic noise absorption. The results of these calculations are given in columns three and four of table 2. The measured riometer absorptions are shown in the last column of the table; they show consistently lower values than the calculated ones by 0.2 to 0.4 decibels. The background in the X-ray detectors due to cosmic rays was obtained from a balloon flight on 19 December 1980 at a time when the satellite electron data showed the trapping boundary 259
Table 1. Coordinated electron and X-ray cases Balloon-Satellite Separatlonsa
Date
Time Satellite Minimum (universal longitude Maximum time) at 76° S (in Kilometers) (in Kilometers) 260 250 330 340 200
820 W 810 W 840 W 800 W 800 W
30 Dec 1980 1923 5 Jan 1981 1921 6 Jan 1981 0313 13 Jan 1981 1917 14 Jan 1981 1916
Kp AEC (3 hours)
100 80 130 120 110
3 2 2 0
41 275 63 375 32
a Horizontal distance from balloon to the field line through the satellite at balloon altitude. "p is the 3-hour planetary range index of geomagnetic field variation, an indicator of magnetic storm activity. AE is a magnetic field index which measures global auroral electrojet activity.
- 0313 UT 6 JAN 1981 FE 65 keV
>
a)
-
'r 10 U,
3 E0
E
:
U
a) x10 2 —J LL
zz
310 keV
00,**10\ 0 1
101 0!\ 2 6 0
\0 I - I I
0 0.2 0.6 1.0 0 0,2 0.6 1.
10 4 - 1917 UT 13 JAN 1981
>
a) -
-
U, = U,
\
keV
0
E
U 0
10
E 0 = 28OkeV
2_
- 0 -
00 0 I 1a
10
E= 0 0.2 0.6 1.0 ELECTRON ENERGY (MeV)
Figure 1. Electron energy spectra (65-115 degree pitch angle) measured at 600 kilometers at times of the balloon X-ray measurements. Exponential fits to the data points are illustrated. ("UT" denotes "universal time?' "KeV" denotes "kiloelectronvolts?' "MeV" denotes "megaelectronvolts?" "el/cm 2-s-sr-keV" denotes "electrons per square centimeter per second per steradian per kiloelectronvolt?') 260
ANTARCTIC JOURNAL
1923 UT 30 DEC 1980
1921 UT 5 JAN 1981
0313 UT 6 JAN 1981
X —J U-
10_i -j
0 z
io_2 1917 UT 13 JAN 1981
1916 UT 14 JAN 1981
>< -j U-
LU
ri 10 -
-j
0 z
I I I I • ••I 4• I I I I I 102 •• 40 50 60 70 80 90 40 50 60 70 80 90 PITCH ANGLE (deg) Figure 2. Electron pitch angle distributions at 600 kilometers (upper curves) and transformations to 200 kilometers (lower curves). ("UT" denotes "universal time?' "Km" denotes "kilometer.)"
to be several degrees equatorward from Siple, and no detectable flux of electrons (E is greater than 68 kiloelectronvolts) was observed over Siple. The net measured X-ray fluxes (observed Table 2. Electron energy deposition (greater than 10 kiloelectronvolts) and resulting riometer absorption 30-megahertz Time EDEP ergs per square _ absorption (decibel) (universal centimeter Date time) per second Calculated Measured 30 Dec 1980 1923 9.3 x 10 .20 -.02 5Jan 1981 1921 2.7 x 10-s .63 .33 6Jan 1981 0313 2.1 x 10-s .59 .16 13 Jan 1981 1917 2.2 x 10-s .60 .28 14 Jan 1981 1916 1.7 x 10 .37 .06
1986 REVIEW
minus background) are presented in figure 3 with curves drawn through the data points. Since the statistical error associated with any individual measurement is rather small (typically less than 5 percent in the channel counting rates for E less than 200 kiloelectronvolts), the major uncertainty in the net fluxes arises from the magnitude of the background. An estimate of this error was obtained from the differences in each channel between the lowest fluxes measured during a given flight and the background fluxes described above. This uncertainty is illustrated in the form of vertical dashed bars through the data points in figure 3. The expected bremsstrahlung fluxes at the balloon altitudes are shown as bars calculated for the same energy ranges as the X-ray detector channels with 50 percent uncertainty factors indicated by solid vertical bars. Although no X-ray measurements were made below 25 kiloelectronvolts, the calculated bremsstrahlung in the energy range 10 to 25 kiloelectronvolts is shown to illustrate the expected absorption of lower energy X-rays by the atmosphere. 261
10_i
Ifl
C 0 0 -c a-
10
1923 UT
fit
0313 UT
30 DEC 1980
6 JAN 19811
• BALLOON MEASUREMENT (BACKGROUND SUBTRACTED) I-1 CALCULATED FROM PRECIPITATING ELECTRONS
)< _3 J 10 I-I • i I I I -J Liii I II I U11,1 >II I I II I I TI I I III 1 I I I I iii x II I I 1 I II I. I
0 100 200 300 10_i
I i 1917 UT
0 100 200 300 13 JAN 1981
N
0 100 200 300
1916 UT 14 JAN 1981
tt-^ 102
LL
:1:
0 100 200 300 0 100 200 300 PHOTON ENERGY (keV)
Figure 3. Comparisons between X-ray fluxes observed from balloons with those calculated from precipitating electron measurements with the P78-1 satellite instrumentation. ("UT" denotes "universal time:' "KeV" denotes "kiloelectronvolt.")
In all of the coordinated cases examined here, the electron pitch angle distributions were wider than one taken from a location where the electrons are stably trapped. Thus, even during a fairly extended period of low geomagnetic activity, measurable electron precipitation occurs in the vicinity of Siple, indicating that weak pitch angle diffusion normally occurs at L equals 4.1. Figure 3 illustrates the comparison between the observed Xray flux at balloon altitudes of 28 to 32 kilometers and the expected X-ray flux calculated from the measured precipitating electrons. In this calculation, the detailed nature of the electron flux, energy spectrum, and angular distribution was invoked and few assumptions on the bremsstrahlung production process and photon transport were made. Hence, this comparison clarifies the degree with which one can expect to relate magnetospheric conditions with secondary observations in the atmosphere. Of the five cases considered, three (5, 6, and 13 January) show overall agreement within the overlapping uncertainty estimates over the entire spectral range, the calculated fluxes usually being higher than those measured. This result is consistent with the absorption comparison in table 2. The agreement is as close as could be expected on the basis of uncertainties in the electron angular distribution data. This result also indicates that at the time of these measurements, no unexpected phenomena, 262
such as beam plasma instabilities, altered the electron trajectories between the satellite and the atmosphere. The remaining two cases (December 30 and January 14) have much less satisfactory agreement at low energies; however, the agreement becomes much better above 100 kiloelectronvolts. In both of these cases, the precipitated electron fluxes were much smaller than in the other three cases, making the correction for cosmic ray background much more critical, particularly at low energies. The net X-ray fluxes in these two cases were only 3 percent to 30 percent of the cosmic-ray background, so uncertainties in the background correction may be responsible for the poor agreement on 30 December and 14 January. In summary, it appears that one can expect to relate the precipitating electron flux to the resulting bremsstrahlung X-ray flux in detail over a wide energy range to within a factor of two near the limits of detection providing one has energy spectra extending to high energies and high resolution angular distribution data. The practical threshold to achieve that accuracy in this experiment corresponds to energy deposition of 0.002 ergs per square centimeter per second, and greater accuracy might be expected for higher intensity precipitation. This paper is condensed from Gaines et al. (in press). This research at Lockheed was supported by National Science Foundation grant DPP 82-09967, and the Lockheed Independent Research Program. The University of Maryland balloon proANTARCTIC JOURNAL
grams were supported by National Science Foundation grants DPP 80-12901 and DPP 82-17270, and the riometer data acquisition by grants DPP 79-25014 and DPP 83-04844.
press. Correlated electron and X-ray measurements of quiet time electron precipitation: a comparative study of bremsstrahlung production and transport in the atmosphere, Journal of Geophysical Research.
References
Imhof, W. L., E. E. Gaines, and J. B. Reagan. 1981. High-resolution spectral features observed in the inner radiation belt trapped electron
Detrick, D.L., and T.J. Rosenberg. 1984. Riometry in Antarctica. Antarctic Journal of the U.S., 19 (5), 212. Gaines, E.E., W.L. Imhof, W.E. Francis, M. Walt, andT.J. Rosenberg. In
Walt, M., L.L. Newkirk, and W.E. Francis. 1979. Bremsstrahlung produced by precipitating electrons. Journal of Geophysical Research, 84,
Ducted whistler propagation beyond the plasmapause D.L. CARPENTER STAR Laboratory Stanford University Stanford, California 94305
D. SuLIc Geomagnetic Institute Yugoslavia
Lightning flashes regularly give rise to very-low-frequency (VLF) whistler signals that are observed in the opposite hemisphere after propagation along geomagnetic field aligned paths through the Earth's magnetosphere. Most of these paths are found to have maximum equatorial radii less than about 4 Earth radii and are thus inside the typical location of the region of steep density gradients called the plasmapause, where the plasma density drops sharply by one to two orders of magnitude with increasing distance from the Earth. Whistlers are in fact observed outside the plasmapause; it is the simultaneous propagation on both sides of this boundary that provided the first extensive descriptions of the plasmapause phenomenon (Carpenter 1966). However, the circumstances under which propagation occurs in the outer region are poorly known and are the subject of a recent study. These circumstances are of interest for many reasons, including their relevance to controlled wave experiments and the fact that whistlers in the outer region can act to trigger extended bursts of VLF emissions. These emissions have in turn been observed to induce particle precipitation into the lower ionosphere, such that the phase and amplitude of subionospheric VLF communication signals are perturbed (Dingle and Carpenter 1981). In the study, we have used whistler data published previously, as well as recent data sets from Siple, Antarctica. The figure gives a qualitative indication of results obtained for the Siple longitude during periods of moderate magnetic disturbance. The equatorial regions outside the plasmapause that are regularly penetrated by observed whistlers are shown by shading. Near local dawn there are two regions of activity, one just outside the plasmapause boundary and the other within a band 1986 REVIEW
population. Journal of Geophysical Research, 86, 2341-2347.
967-973.
that is separated from the plasmapause by a "gap" of about 0.5-1.0 Earth radii in extent. In the afternoon sector, the gap disappears and observations occur over a comparatively wide L range and with highest probability. In contrast, ground-toground propagation is essentially absent in the dusk to midnight sector, unless a deep quieting trend is underway. A special study was made of occurrence rates near local dawn, since that time sector is known to be characterized by relatively high levels of natural wave activity and of waveinduced burst precipitation of particles into the ionosphere (e.g., Helliwell et al. 1980). It was found that in the synoptic whistler recordings, which sample activity 1 minute every 5 minutes or 1 every 15 minutes, propagation on one or more paths outside the plasmapause was detected on roughly one half of 45 days surveyed from the austral winters of 1977 and 1982. This agrees well with the results from the limited earlier reports (Carpenter 1968). The equatorial radius of the outermost observed path on a given day was scaled and found to show a steep falloff at about 5.5 Earth radii. This appears to explain recent results from the SCATHA satellite (Koons 1985), in which no whistlers were ob12 :. If. ... . /....HIGHEST. RATES
'N 09 . ...,
\S\\
ro SCATHA L RANGE 181)6MLT PLASMASPHERE
21
03 00
Cross section of the magnetosphere showing equatorial distance as a function of magnetic local time (MLT). Shading indicates those regions beyond the plasmapause that are regularly penetrated by whistlers observed near the meridian of Siple, Antarctica.
263
375 350 Li
Li
Li
D D J0 LJ0
Li
300
0
LiH alley O R ob erval oSiple
U250
200 20
1 4.96
LiHalley ORoberval oSiple
15
o
ZI
0 0 00 0 0 00 0 0 0 0
N
I
2.79 -"
10 LLJ u5
0
Figure 2. This figure presents hmF 2 (upper panel) and N m F2 (lower panel) derived at 15-minute intervals over a 3-hour interval beginfling at 1700 universal time on 13 November 1982. The station identification is the same as that used in figure 1. ("Km" denotes "kilometer:' "mHz" denotes "megahertz." 'rn " denotes "per cubic meter:')
Correlated electron and X-ray measurements of quiet-time electron precipitation near Siple E.E. GAINEs, W.L. IMHOF, W.E. FRANCIS, and M. WALT Lockheed Palo Alto Research Laboratory Palo Alto, California 94304
T.J. ROSENBERG University of Maryland Institute for Physical Science and Technology College Park, Maryland 20742
Several cases of X-ray observations from balloons coordinated with measurements of precipitating electrons were obtained during passes of the low-altitude polar-orbiting satellite P78-1 near Siple, Antarctica. The observations, made during a geomagnetically quiet period from late December 1980 to early January 1981, included high-resolution measurements of the electron energy spectra, pitch angle distributions from two spectrometers on board the satellite (Imhof, Gaines, and Reagan 1981), and very sensitive measurements of bremsstrahlung X-rays from balloon-borne detectors. The data 1986 REVIEW
LLJ 5
0 17
18
- 19
20
0
UT HOUR, Day 317
acquisition limes, satellite-balloon separations, and magnetic indices are summarized in table 1. Cosmic noise absorption measurements were made during this period with a 30-megahertz riometer at Siple Station (Detrick and Rosenberg 1984). Electron energy spectra summed over pitch angles from 65 to 115 degrees are presented in figure 1. These spectra are averaged values from approximately 45 seconds of data for each case during which time the satellite traveled about 300 kilometers. The pitch-angle distributions corresponding to the energy spectra are shown as the upper curves in figure 2 for the nomi nal satellite altitude of 600 kilometers. The lower curve in each panel of figure 2 is the same distribution transformed to 200kilometer altitude for use in bremsstrahlung production calculations. Bremsstrahlung X-rays produced in the atmosphere by the precipitating electrons were computed using the method described in Walt, Newkirk, and Francis (1979). The bremsstrahlung distribution as a function of angle, energy, and altitude thus obtained is then traced through the atmosphere to the balloon altitudes by standard Monte Carlo methods. Electron energy loss to ionization in the atmosphere is obtained from the first part of the bremsstrahlung computation, and this can be used to calculate the expected cosmic noise absorption. The results of these calculations are given in columns three and four of table 2. The measured riometer absorptions are shown in the last column of the table; they show consistently lower values than the calculated ones by 0.2 to 0.4 decibels. The background in the X-ray detectors due to cosmic rays was obtained from a balloon flight on 19 December 1980 at a time when the satellite electron data showed the trapping boundary 259
Table 1. Coordinated electron and X-ray cases Balloon-Satellite Separatlonsa
Date
Time Satellite Minimum (universal longitude Maximum time) at 76° S (in Kilometers) (in Kilometers) 260 250 330 340 200
820 W 810 W 840 W 800 W 800 W
30 Dec 1980 1923 5 Jan 1981 1921 6 Jan 1981 0313 13 Jan 1981 1917 14 Jan 1981 1916
Kp AEC (3 hours)
100 80 130 120 110
3 2 2 0
41 275 63 375 32
a Horizontal distance from balloon to the field line through the satellite at balloon altitude. "p is the 3-hour planetary range index of geomagnetic field variation, an indicator of magnetic storm activity. AE is a magnetic field index which measures global auroral electrojet activity.
- 0313 UT 6 JAN 1981 FE 65 keV
>
a)
-
'r 10 U,
3 E0
E
:
U
a) x10 2 —J LL
zz
310 keV
00,**10\ 0 1
101 0!\ 2 6 0
\0 I - I I
0 0.2 0.6 1.0 0 0,2 0.6 1.
10 4 - 1917 UT 13 JAN 1981
>
a) -
-
U, = U,
\
keV
0
E
U 0
10
E 0 = 28OkeV
2_
- 0 -
00 0 I 1a
10
E= 0 0.2 0.6 1.0 ELECTRON ENERGY (MeV)
Figure 1. Electron energy spectra (65-115 degree pitch angle) measured at 600 kilometers at times of the balloon X-ray measurements. Exponential fits to the data points are illustrated. ("UT" denotes "universal time?' "KeV" denotes "kiloelectronvolts?' "MeV" denotes "megaelectronvolts?" "el/cm 2-s-sr-keV" denotes "electrons per square centimeter per second per steradian per kiloelectronvolt?') 260
ANTARCTIC JOURNAL
1923 UT 30 DEC 1980
1921 UT 5 JAN 1981
0313 UT 6 JAN 1981
X —J U-
10_i -j
0 z
io_2 1917 UT 13 JAN 1981
1916 UT 14 JAN 1981
>< -j U-
LU
ri 10 -
-j
0 z
I I I I • ••I 4• I I I I I 102 •• 40 50 60 70 80 90 40 50 60 70 80 90 PITCH ANGLE (deg) Figure 2. Electron pitch angle distributions at 600 kilometers (upper curves) and transformations to 200 kilometers (lower curves). ("UT" denotes "universal time?' "Km" denotes "kilometer.)"
to be several degrees equatorward from Siple, and no detectable flux of electrons (E is greater than 68 kiloelectronvolts) was observed over Siple. The net measured X-ray fluxes (observed Table 2. Electron energy deposition (greater than 10 kiloelectronvolts) and resulting riometer absorption 30-megahertz Time EDEP ergs per square _ absorption (decibel) (universal centimeter Date time) per second Calculated Measured 30 Dec 1980 1923 9.3 x 10 .20 -.02 5Jan 1981 1921 2.7 x 10-s .63 .33 6Jan 1981 0313 2.1 x 10-s .59 .16 13 Jan 1981 1917 2.2 x 10-s .60 .28 14 Jan 1981 1916 1.7 x 10 .37 .06
1986 REVIEW
minus background) are presented in figure 3 with curves drawn through the data points. Since the statistical error associated with any individual measurement is rather small (typically less than 5 percent in the channel counting rates for E less than 200 kiloelectronvolts), the major uncertainty in the net fluxes arises from the magnitude of the background. An estimate of this error was obtained from the differences in each channel between the lowest fluxes measured during a given flight and the background fluxes described above. This uncertainty is illustrated in the form of vertical dashed bars through the data points in figure 3. The expected bremsstrahlung fluxes at the balloon altitudes are shown as bars calculated for the same energy ranges as the X-ray detector channels with 50 percent uncertainty factors indicated by solid vertical bars. Although no X-ray measurements were made below 25 kiloelectronvolts, the calculated bremsstrahlung in the energy range 10 to 25 kiloelectronvolts is shown to illustrate the expected absorption of lower energy X-rays by the atmosphere. 261
10_i
Ifl
C 0 0 -c a-
10
1923 UT
fit
0313 UT
30 DEC 1980
6 JAN 19811
• BALLOON MEASUREMENT (BACKGROUND SUBTRACTED) I-1 CALCULATED FROM PRECIPITATING ELECTRONS
)< _3 J 10 I-I • i I I I -J Liii I II I U11,1 >II I I II I I TI I I III 1 I I I I iii x II I I 1 I II I. I
0 100 200 300 10_i
I i 1917 UT
0 100 200 300 13 JAN 1981
N
0 100 200 300
1916 UT 14 JAN 1981
tt-^ 102
LL
:1:
0 100 200 300 0 100 200 300 PHOTON ENERGY (keV)
Figure 3. Comparisons between X-ray fluxes observed from balloons with those calculated from precipitating electron measurements with the P78-1 satellite instrumentation. ("UT" denotes "universal time:' "KeV" denotes "kiloelectronvolt.")
In all of the coordinated cases examined here, the electron pitch angle distributions were wider than one taken from a location where the electrons are stably trapped. Thus, even during a fairly extended period of low geomagnetic activity, measurable electron precipitation occurs in the vicinity of Siple, indicating that weak pitch angle diffusion normally occurs at L equals 4.1. Figure 3 illustrates the comparison between the observed Xray flux at balloon altitudes of 28 to 32 kilometers and the expected X-ray flux calculated from the measured precipitating electrons. In this calculation, the detailed nature of the electron flux, energy spectrum, and angular distribution was invoked and few assumptions on the bremsstrahlung production process and photon transport were made. Hence, this comparison clarifies the degree with which one can expect to relate magnetospheric conditions with secondary observations in the atmosphere. Of the five cases considered, three (5, 6, and 13 January) show overall agreement within the overlapping uncertainty estimates over the entire spectral range, the calculated fluxes usually being higher than those measured. This result is consistent with the absorption comparison in table 2. The agreement is as close as could be expected on the basis of uncertainties in the electron angular distribution data. This result also indicates that at the time of these measurements, no unexpected phenomena, 262
such as beam plasma instabilities, altered the electron trajectories between the satellite and the atmosphere. The remaining two cases (December 30 and January 14) have much less satisfactory agreement at low energies; however, the agreement becomes much better above 100 kiloelectronvolts. In both of these cases, the precipitated electron fluxes were much smaller than in the other three cases, making the correction for cosmic ray background much more critical, particularly at low energies. The net X-ray fluxes in these two cases were only 3 percent to 30 percent of the cosmic-ray background, so uncertainties in the background correction may be responsible for the poor agreement on 30 December and 14 January. In summary, it appears that one can expect to relate the precipitating electron flux to the resulting bremsstrahlung X-ray flux in detail over a wide energy range to within a factor of two near the limits of detection providing one has energy spectra extending to high energies and high resolution angular distribution data. The practical threshold to achieve that accuracy in this experiment corresponds to energy deposition of 0.002 ergs per square centimeter per second, and greater accuracy might be expected for higher intensity precipitation. This paper is condensed from Gaines et al. (in press). This research at Lockheed was supported by National Science Foundation grant DPP 82-09967, and the Lockheed Independent Research Program. The University of Maryland balloon proANTARCTIC JOURNAL
grams were supported by National Science Foundation grants DPP 80-12901 and DPP 82-17270, and the riometer data acquisition by grants DPP 79-25014 and DPP 83-04844.
press. Correlated electron and X-ray measurements of quiet time electron precipitation: a comparative study of bremsstrahlung production and transport in the atmosphere, Journal of Geophysical Research.
References
Imhof, W. L., E. E. Gaines, and J. B. Reagan. 1981. High-resolution spectral features observed in the inner radiation belt trapped electron
Detrick, D.L., and T.J. Rosenberg. 1984. Riometry in Antarctica. Antarctic Journal of the U.S., 19 (5), 212. Gaines, E.E., W.L. Imhof, W.E. Francis, M. Walt, andT.J. Rosenberg. In
Walt, M., L.L. Newkirk, and W.E. Francis. 1979. Bremsstrahlung produced by precipitating electrons. Journal of Geophysical Research, 84,
Ducted whistler propagation beyond the plasmapause D.L. CARPENTER STAR Laboratory Stanford University Stanford, California 94305
D. SuLIc Geomagnetic Institute Yugoslavia
Lightning flashes regularly give rise to very-low-frequency (VLF) whistler signals that are observed in the opposite hemisphere after propagation along geomagnetic field aligned paths through the Earth's magnetosphere. Most of these paths are found to have maximum equatorial radii less than about 4 Earth radii and are thus inside the typical location of the region of steep density gradients called the plasmapause, where the plasma density drops sharply by one to two orders of magnitude with increasing distance from the Earth. Whistlers are in fact observed outside the plasmapause; it is the simultaneous propagation on both sides of this boundary that provided the first extensive descriptions of the plasmapause phenomenon (Carpenter 1966). However, the circumstances under which propagation occurs in the outer region are poorly known and are the subject of a recent study. These circumstances are of interest for many reasons, including their relevance to controlled wave experiments and the fact that whistlers in the outer region can act to trigger extended bursts of VLF emissions. These emissions have in turn been observed to induce particle precipitation into the lower ionosphere, such that the phase and amplitude of subionospheric VLF communication signals are perturbed (Dingle and Carpenter 1981). In the study, we have used whistler data published previously, as well as recent data sets from Siple, Antarctica. The figure gives a qualitative indication of results obtained for the Siple longitude during periods of moderate magnetic disturbance. The equatorial regions outside the plasmapause that are regularly penetrated by observed whistlers are shown by shading. Near local dawn there are two regions of activity, one just outside the plasmapause boundary and the other within a band 1986 REVIEW
population. Journal of Geophysical Research, 86, 2341-2347.
967-973.
that is separated from the plasmapause by a "gap" of about 0.5-1.0 Earth radii in extent. In the afternoon sector, the gap disappears and observations occur over a comparatively wide L range and with highest probability. In contrast, ground-toground propagation is essentially absent in the dusk to midnight sector, unless a deep quieting trend is underway. A special study was made of occurrence rates near local dawn, since that time sector is known to be characterized by relatively high levels of natural wave activity and of waveinduced burst precipitation of particles into the ionosphere (e.g., Helliwell et al. 1980). It was found that in the synoptic whistler recordings, which sample activity 1 minute every 5 minutes or 1 every 15 minutes, propagation on one or more paths outside the plasmapause was detected on roughly one half of 45 days surveyed from the austral winters of 1977 and 1982. This agrees well with the results from the limited earlier reports (Carpenter 1968). The equatorial radius of the outermost observed path on a given day was scaled and found to show a steep falloff at about 5.5 Earth radii. This appears to explain recent results from the SCATHA satellite (Koons 1985), in which no whistlers were ob12 :. If. ... . /....HIGHEST. RATES
'N 09 . ...,
\S\\
ro SCATHA L RANGE 181)6MLT PLASMASPHERE
21
03 00
Cross section of the magnetosphere showing equatorial distance as a function of magnetic local time (MLT). Shading indicates those regions beyond the plasmapause that are regularly penetrated by whistlers observed near the meridian of Siple, Antarctica.
263
