Balloon-borne measurements of the global atmospheric-electrical ...
Balloon-borne measurements of the global atmospheric-electrical circuit at the South Pole
'U
8
G.J. BYRNE, J.R. BENBROOK, and E.A. BERING, III DL' pa rtinen t of Ph i,'s ics UnWL'rslti/ of Houston Houston, Texas 77204-5504
In the South Pole balloon campaign during the austral summer of 1985-1986, eight stratospheric balloon payloads were launched from Amundsen-Scott South Pole Station (Bering et al. 1986). The payloads were instrumented to measure the vertical and horizontal components of the atmospheric electric field and the atmospheric conductivity. Over 460 hours of data were acquired from which a variety of scientific questions of the electrical processes in the lower and upper atmosphere can be addressed. In this report, we present some of the observations of the electrical environment of the south polar stratosphere that have bearing on the nature of the Earth's global atmospheric-electrical circuit (Roble and Tzur 1986). The global circuit describes the flow of electrical current through the atmosphere. As an analogy with a simple electrical circuit, the global circuit consists of two capacitive plates (spherical on a global scale), formed by the positively charged ionosphere and the Earth's surface. Current flows from the ionosphere to the surface through the resistive atmosphere, discharging the Earth-ionosphere capacitor. The capacitor is recharged by thunderstorms, which worldwide act collectively as the battery of the global circuit. The atmospheric conduction current J: flowing from the ionosphere to the surface is given by the Ohm's law relation J = or where ads the atmospheric conductivity and E is the vertical component of the electric field. The conductivity of the atmosphere is maintained by cosmic-ray ionization of the atmospheric molecules. The vertical electric field is a function of the ionospheric electrical potential, which is maintained primarily by the global thunderstorm activity. In the polar regions, an additional generator of the ionospheric potential is a magnetospheric dynamo, which is driven by the interaction of the solar wind with the geomagnetic field. Therefore, in the polar regions the global circuit and variations in the circuit are a result of a complex combination of effects of atmospheric (lower and upper) and extraterrestrial origin. The downward air/Earth current that flows to the surface of the antarctic plateau is larger (approximately equal to a factor of 2) than the global average owing to the high elevation of the terrain, which significantly decreases the columnar resistance of the overlying atmosphere (Roble and Tzur 1986). Therefore, because of its orography, Antarctica is strongly coupled to the global atmospheric-electrical circuit. Hence, measurements are needed to characterize the electrical environment of this region to identify and understand the mechanisms of the global circuit. The average profile of the negative ion conductivity measured during the South Pole balloon campaign is shown in figure 1. The positive ion measurements were contaminated by photoelectric emissions (Byrne et al. 1989) and are not shown. The scale height (the e-folding distance) of the South Pole 1989 REVIEW
-c 0
b
IL 10
15 20 25 30 Altitude (km)
Figure 1. A comparison of the profile for the negative ion atmospheric conductivity u - measured at the South Pole with a model profile and profiles measured at Syowa Station and Siple Station. (km denotes kilometer. ohm -1 m 1 denotes per ohm per meter.)
profile is approximately 10 kilometers. The conductivity profiles measured previously at Siple Station, Antarctica, (Byrne et al. 1988) and Syowa Station, Antarctica, (Tanaka, Ogawa, and Kodama 1977) are shown for comparison. The differences in the measured profiles are primarily due to the variation in the cosmic-ray ionization with geomagnetic latitude and solar cycle. Also shown is a model profile (Hays and Roble 1979) which has been used to describe the conductivity in the entire polar cap stratosphere in global models of the atmosphericelectrical circuit. Based on the three measured profiles, the model profile underestimates the conductivity at lower stratospheric altitudes, and the model underestimates the conductivity scale height. The figure illustrates that a single profile as a model for the conductivity of the entire polar cap stratosphere may be an inadequate description of the actual conductivity in this region. Figure 2 shows the electric field and the negative conductivity measurements made during one flight (flight 8) of the South Pole campaign. Also shown is the balloon altitude and the negative ion component of the atmospheric conduction current calculated from Ohm's law. A diurnal variation is evident in the electric field data, witF a maximum near 1800 universal time and a minimum near 0300 universal time. This diurnal variation was not associated with the balloon motion. No significant diurnal variation in the conductivity was observed. During this flight, the Earth's magnetosphere was "quiet" (based upon the geomagnetic activity index K,), which means that the contribution of the magnetospheric dynamo to the total ionospheric potential was relatively small compared to the potential generated by the global thunderstorm activity. Therefore, in this case, the observed diurnal variation in the electric field manifests the diurnal variation of the "battery" of the global circuit, the worldwide thunderstorm activity. For "disturbed" geomagnetic conditions, the contribution to the ionospheric potential of the magnetospheric dynamo generator is comparable to the thunderstorm generator. To estimate the contribution of the magnetospheric dynamo to the electrical parameters, we binned the data set from all 26
-1 0 X
1Ø1vMt
-
0.6
Flight 8 >
0.5
-
'Quiet Magnetosphere' 'Disturbed Magnetosphere'
0.4 I)
0.3 0.2 8
0 x
7 6 I)
0 x
CO C-. I)
4
31 1 I I I 1 I 0 2 4 6 8 10 12 14 16 18 20 22 24 Universal Time
I)
1986 012 1986 013 1986 014 1986 015 1986 016 1986 017
UT, 12 January 1986 to 17 January 1986 Figure 2. The balloon altitude (in kilometers), vertical component of the electric field E1 (x 1O 1 Vm'), and negative conductivity y-(X ohm'm 1 ) measured over a 5-day time span during balloon flight 8 of the South Pole balloon campaign. The atmospheric conduction current J(pA M 2 ) is calculated from E and 'yby Ohm's law. Negative values of E. and J1 denote downward pointing vectors. (UT denotes universal time.) -
5
Figure 3. The measured vertical electric field E and calculated atmospheric conduction current J averaged over the entire data set from the South Pole balloon campaign for the conditions of "quiet" and "disturbed" geomagnetic conditions. Both curves are scaled to the 30-kilometer level, pA r n -2 denotes picoamperes per square meter. V rn' denotes volts per meter.)
10-12
eight flights of the South Pole campaign into 30-minute intervals of universal time and averaged on the basis of "quiet" (K1, 2—) or "disturbed" (K1, 2) geomagnetic conditions. The results are shown in figure 3. The measurements are scaled to the 30-kilometer level. For "quiet" conditions the curves manifest the average diurnal modulation of the global circuit by worldwide thunderstorm activity. For "disturbed" conditions, the amplitudes of the diurnal variations of E and I. are reduced by approximately 20 percent. The suppression of the amplitude of the diurnal universal time variation is the expected effect of the superposition of potentials of the thunderstorm and magnetospheric dynamo generators at high latitudes (Roble and Tzur 1986). This research was supported by the National Science Foun dation grants DPP 84-15203 and DPP 86-14091.
262
References Bering, E.A., J.R. Benbrook, D.L. Mathews, and T.J. Rosenberg. 1986. The 1986-1986 South Pole balloon campaign. Antarctic Journal of the U.S., 21(5), 267-269. Byrne, G.J., J.R. Benbrook, E.A. Bering, D. Oró, CO. Seubert, and W.R. Sheldon. 1988. Observations of the stratospheric conductivity and its variation at three latitudes. Journal of Geophysical Research, 93(D4), 3,879-3,891. Byrne, G.J., E.A. Bering, J.R. Benbrook, and D.M. Oró. In press. Solar radiation (190-230 nm) in the stratosphere: Implications for photoelectric emissions from instrumentation at balloon altitudes. Journal of Geophysical Research.
Hays, PB., and R.G. Roble. 1979. A quasi-static model of global atmospheric electricity: 1. The lower atmosphere. Journal of Geophysical Research, 84(A7), 3,291-3,305. Roble, R.G., and I. Tzur. 1986. The global atmospheric-electrical circuit. In The Earth's electrical environment, Washington, D.C.: National Academy Press. Tanaka, Y., T. Ogawa, and M. Kodama. 1977. Stratospheric electric fields and currents measured at Syowa Station, Antarctica-1. The vertical component. Journal of Atmospheric and Terrestrial Physics, 39, 523-529.
ANTARCTIC JOURNAL
'U
8
G.J. BYRNE, J.R. BENBROOK, and E.A. BERING, III DL' pa rtinen t of Ph i,'s ics UnWL'rslti/ of Houston Houston, Texas 77204-5504
In the South Pole balloon campaign during the austral summer of 1985-1986, eight stratospheric balloon payloads were launched from Amundsen-Scott South Pole Station (Bering et al. 1986). The payloads were instrumented to measure the vertical and horizontal components of the atmospheric electric field and the atmospheric conductivity. Over 460 hours of data were acquired from which a variety of scientific questions of the electrical processes in the lower and upper atmosphere can be addressed. In this report, we present some of the observations of the electrical environment of the south polar stratosphere that have bearing on the nature of the Earth's global atmospheric-electrical circuit (Roble and Tzur 1986). The global circuit describes the flow of electrical current through the atmosphere. As an analogy with a simple electrical circuit, the global circuit consists of two capacitive plates (spherical on a global scale), formed by the positively charged ionosphere and the Earth's surface. Current flows from the ionosphere to the surface through the resistive atmosphere, discharging the Earth-ionosphere capacitor. The capacitor is recharged by thunderstorms, which worldwide act collectively as the battery of the global circuit. The atmospheric conduction current J: flowing from the ionosphere to the surface is given by the Ohm's law relation J = or where ads the atmospheric conductivity and E is the vertical component of the electric field. The conductivity of the atmosphere is maintained by cosmic-ray ionization of the atmospheric molecules. The vertical electric field is a function of the ionospheric electrical potential, which is maintained primarily by the global thunderstorm activity. In the polar regions, an additional generator of the ionospheric potential is a magnetospheric dynamo, which is driven by the interaction of the solar wind with the geomagnetic field. Therefore, in the polar regions the global circuit and variations in the circuit are a result of a complex combination of effects of atmospheric (lower and upper) and extraterrestrial origin. The downward air/Earth current that flows to the surface of the antarctic plateau is larger (approximately equal to a factor of 2) than the global average owing to the high elevation of the terrain, which significantly decreases the columnar resistance of the overlying atmosphere (Roble and Tzur 1986). Therefore, because of its orography, Antarctica is strongly coupled to the global atmospheric-electrical circuit. Hence, measurements are needed to characterize the electrical environment of this region to identify and understand the mechanisms of the global circuit. The average profile of the negative ion conductivity measured during the South Pole balloon campaign is shown in figure 1. The positive ion measurements were contaminated by photoelectric emissions (Byrne et al. 1989) and are not shown. The scale height (the e-folding distance) of the South Pole 1989 REVIEW
-c 0
b
IL 10
15 20 25 30 Altitude (km)
Figure 1. A comparison of the profile for the negative ion atmospheric conductivity u - measured at the South Pole with a model profile and profiles measured at Syowa Station and Siple Station. (km denotes kilometer. ohm -1 m 1 denotes per ohm per meter.)
profile is approximately 10 kilometers. The conductivity profiles measured previously at Siple Station, Antarctica, (Byrne et al. 1988) and Syowa Station, Antarctica, (Tanaka, Ogawa, and Kodama 1977) are shown for comparison. The differences in the measured profiles are primarily due to the variation in the cosmic-ray ionization with geomagnetic latitude and solar cycle. Also shown is a model profile (Hays and Roble 1979) which has been used to describe the conductivity in the entire polar cap stratosphere in global models of the atmosphericelectrical circuit. Based on the three measured profiles, the model profile underestimates the conductivity at lower stratospheric altitudes, and the model underestimates the conductivity scale height. The figure illustrates that a single profile as a model for the conductivity of the entire polar cap stratosphere may be an inadequate description of the actual conductivity in this region. Figure 2 shows the electric field and the negative conductivity measurements made during one flight (flight 8) of the South Pole campaign. Also shown is the balloon altitude and the negative ion component of the atmospheric conduction current calculated from Ohm's law. A diurnal variation is evident in the electric field data, witF a maximum near 1800 universal time and a minimum near 0300 universal time. This diurnal variation was not associated with the balloon motion. No significant diurnal variation in the conductivity was observed. During this flight, the Earth's magnetosphere was "quiet" (based upon the geomagnetic activity index K,), which means that the contribution of the magnetospheric dynamo to the total ionospheric potential was relatively small compared to the potential generated by the global thunderstorm activity. Therefore, in this case, the observed diurnal variation in the electric field manifests the diurnal variation of the "battery" of the global circuit, the worldwide thunderstorm activity. For "disturbed" geomagnetic conditions, the contribution to the ionospheric potential of the magnetospheric dynamo generator is comparable to the thunderstorm generator. To estimate the contribution of the magnetospheric dynamo to the electrical parameters, we binned the data set from all 26
-1 0 X
1Ø1vMt
-
0.6
Flight 8 >
0.5
-
'Quiet Magnetosphere' 'Disturbed Magnetosphere'
0.4 I)
0.3 0.2 8
0 x
7 6 I)
0 x
CO C-. I)
4
31 1 I I I 1 I 0 2 4 6 8 10 12 14 16 18 20 22 24 Universal Time
I)
1986 012 1986 013 1986 014 1986 015 1986 016 1986 017
UT, 12 January 1986 to 17 January 1986 Figure 2. The balloon altitude (in kilometers), vertical component of the electric field E1 (x 1O 1 Vm'), and negative conductivity y-(X ohm'm 1 ) measured over a 5-day time span during balloon flight 8 of the South Pole balloon campaign. The atmospheric conduction current J(pA M 2 ) is calculated from E and 'yby Ohm's law. Negative values of E. and J1 denote downward pointing vectors. (UT denotes universal time.) -
5
Figure 3. The measured vertical electric field E and calculated atmospheric conduction current J averaged over the entire data set from the South Pole balloon campaign for the conditions of "quiet" and "disturbed" geomagnetic conditions. Both curves are scaled to the 30-kilometer level, pA r n -2 denotes picoamperes per square meter. V rn' denotes volts per meter.)
10-12
eight flights of the South Pole campaign into 30-minute intervals of universal time and averaged on the basis of "quiet" (K1, 2—) or "disturbed" (K1, 2) geomagnetic conditions. The results are shown in figure 3. The measurements are scaled to the 30-kilometer level. For "quiet" conditions the curves manifest the average diurnal modulation of the global circuit by worldwide thunderstorm activity. For "disturbed" conditions, the amplitudes of the diurnal variations of E and I. are reduced by approximately 20 percent. The suppression of the amplitude of the diurnal universal time variation is the expected effect of the superposition of potentials of the thunderstorm and magnetospheric dynamo generators at high latitudes (Roble and Tzur 1986). This research was supported by the National Science Foun dation grants DPP 84-15203 and DPP 86-14091.
262
References Bering, E.A., J.R. Benbrook, D.L. Mathews, and T.J. Rosenberg. 1986. The 1986-1986 South Pole balloon campaign. Antarctic Journal of the U.S., 21(5), 267-269. Byrne, G.J., J.R. Benbrook, E.A. Bering, D. Oró, CO. Seubert, and W.R. Sheldon. 1988. Observations of the stratospheric conductivity and its variation at three latitudes. Journal of Geophysical Research, 93(D4), 3,879-3,891. Byrne, G.J., E.A. Bering, J.R. Benbrook, and D.M. Oró. In press. Solar radiation (190-230 nm) in the stratosphere: Implications for photoelectric emissions from instrumentation at balloon altitudes. Journal of Geophysical Research.
Hays, PB., and R.G. Roble. 1979. A quasi-static model of global atmospheric electricity: 1. The lower atmosphere. Journal of Geophysical Research, 84(A7), 3,291-3,305. Roble, R.G., and I. Tzur. 1986. The global atmospheric-electrical circuit. In The Earth's electrical environment, Washington, D.C.: National Academy Press. Tanaka, Y., T. Ogawa, and M. Kodama. 1977. Stratospheric electric fields and currents measured at Syowa Station, Antarctica-1. The vertical component. Journal of Atmospheric and Terrestrial Physics, 39, 523-529.
ANTARCTIC JOURNAL
