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Saturn radio emission and the solar wind: Voyager-2 studies
Adv. Space Res. Vol.5, No.4,
Printed in Great Britain. AU
333—336, 1985 rights reserved. pp.
0273—1177/85 $0.00 + .50 Copyright © COSPAR
SATURN RADIO EMISSION AND THE SOLAR WIND: VOYAGER-2 STUDIES M. D. Desch5 and H. 0. Rucker55 5NASA/Goddard Space Flight Center, Laboratory for Extraterrestrial Physics, Greenbelt, MD 20771, U.S.A. ~Space Research Institute, Austrian Academy of Sciences, Lustbuhel Observatory, A-8042 Graz, Austria
ABSTRACT Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the relationship between parameters of the solar wind/interplanetary medium and the nonthermal Saturn radiation. Solar wind and interplanetary magnetic field properties were combined to form quantities known to be important in controlling terrestrial magnetospheric processes. The Voyager 2 data set used in this investigation consists of 237 days of Saturn preencounter measurements. However, due to the immersion of Saturn and the Voyager 2 spacecraft into the extended Jupiter magnetic tail, substantial periods of the time series were lacking solar wind data. To cope with this problem a superposed epoch method (CHREE analysis) was used. The results indicate the superiority of the quantities containing the solar wind density in stimulating the radio emission of Saturn — a result found earlier using Voyager 1 data — and the minor importance of quantities incorporating the interplanetary magnetic field. INTRODUCTION The observations of both Voyager spacecraft showed that Saturn is an emitter of two different kinds of nonthermal radiation: the Saturn kilometric radiation (SKR) /1/ and the Saturn electrostatic discharges (SED) /2/. The special value of SKR for the present study lies in the fact that this radiation can serve as a remote sensor of Saturnian magneto— spheric activity. Previous studies of the radio wave modulation of SKR on a time scale of days indicate that the radio energy fluctuations are distinctly associated with the solar wind ram pressure and the kinetic energy flux /3,1!!. These findings are consistent with the source location of SKR within the dayside cusp region of Saturn /5/. The present investigation makes use of preencounter Voyager 2 data to study the influence of the solar wind on SKR. SOLAR WIND, INTERPLANETARY MAGNETIC FIELD AND RADIO OBSERVATIONS The present study employs Voyager 2 Planetary Radio Astronomy (PRA), Magnetometer (MAG), and Plasma Science (PLS) data. The analysis interval covers the Saturn preencounter period from 1 January 1981 through 25 August 1981, corresponding to more than 9 solar rotations. During the period from day 1 to day 237, 1981, the spacecraft moved from a distance of 1.5 AU (AU astronomical unit) to 0.1 AU from Saturn. The propagation time for the solar wind plasma to move over the respective distances is 5.5 days and 0.1! days. An overview of the radio and plasma quantities used in this investigation is shown in Figure 1. The top panel shows the isotr~ 8ice~ergy1flux of the Saturn kilometric radiation that exceeded a flux density of 1.3 x 1O~ Wm Hz~ at 1 AU. For comparison the lower panel shows the solar wind kinetic energy flux, PV , which has been ballistically propagated to the position of Saturn. Unlike Voyager 1, the Voyager 2 spacecraft and Saturn were immersed several times in the extended tail of Jupiter. These periods are characterized by low solar wind plasma densities, magnetic field orientations corresponding to the Jovian tail direction, the occurrence of Jovian continuum radiation /6,7/, and the disappearance of SKR /8/. The most significant indicator for Jupiter tail encounter periods are low plasma density events as shown by bars in Figure 1. The frequent interruption of the Voyager 2 solar wind observations made impossible any reliable linear cross correlation analysis as was done in /14/ with Voyager 1 measurements. Therefore the study of the solar wind influence on SKR has been done with the aid of a superposed epoch analysis, or CHREE analysis.
333
334
M.D. Deseh and H.O. Ruoker
6 ____
____________
ERR
In
POWER
1
_
H -
—
i
—
i
.
.—
—
~
0
20
40
60
80
100
120
140
J~ 160
180
.
200
220
240
oo’~’ 1981
Figure 1. Comparison of SKR power and the solar wind kinetic power for the period from day 1 to 237, 1981. The bars In the lower panel indicate Jupiter tail encounter periods.
CHREE ANALYSIS AND DISCUSSION OF RESULTS As explained in detail in /11/ many solar wind parameters have been found important in stimulating terrestrial magnetospheric activity, including auroral kilometric radiation (AKR). However, the aforementioned paper was restricted to Voyager 1 observations, and so the question remains as to how the Voyager 2 data would fit the model developed for the stimulation of SKR as described in /14/. Out of a number of examined solar wind parameters the following 8 quantities were chos~n for Figure 2: solar wind bulk speed ~, density p, momentum pV, solar wind ram pressure PV kinetic solar wind energy flux PV , the Akasofu—parameter E /9/, the interplanetary magnetic field (IMF) magnitude B, and the solar wind electric field magnitude E (~VxB). In Figure 2 no consideration is made for quantities like the IMF—B — component (north/south, aligned to the magnetic dipole axis of Saturn), and the IMF—B — component (toward/away from the sun, associated with the magnetic sector boundary passa~es). These quantities have shown no relationship to SKR. The superposed epoch technique requires identification of the zero epoch in each time series of the examined quantities. The zero epoch is a characteristic point in time within an event and defines a significant onset or change in any time series. In our case it is determined by looking for the time derivative reaching or exceeding the 30 level (0 standard deviation). After having found the solar wind parameter variations before and after the zero epoch the SKR energy fluctuations are aligned in time. Figure 2 shows the relationship between 8 solar wind quantities (dashed line) and the 54CR energy (full line) with respect to time as well as amplitude. The data resolution is one value per Saturn rotation within the period of 10 rotations before and 20 rotations after the zero epoch. SKR energy changes following (lagging) in time the corresponding changes in
Saturn Radio Emission and the Solar Wind from Voyager—2
335
the solar wind quantities indicate a physically realizable relationship. As shown in /14/, lag times greater than 10 hours ( 1 Saturn rotation) cannot be attributed to possible errors due to the propagation of any solar wind feature to Saturn. Obviously, peak—to—peak lead times greater than 1 Saturn rotation are physically irrelevant.
A
:~~ ~
-12.2 83111681
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10.0
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-[0.2
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22.0
-.10.0
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~ -[2.3
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[0.68
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52
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apsi
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58111662 RO1#1168~ 116.60 8~68S[
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;~\;,L~
5811.661 561578368 [10.56 146680]
6.3
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5511568 6018716868 [0.56
80.6858
-12.~
- 2.8
_____
5511685
5815r1368
188.56 10.68S]
Figure 2. CHREE analysis between the S!CR energy (full line) and 8 solar wind quantities (dashed line) for the interval from day 1 to 237, 1981. Solar wind momentum, ram pressure and energy flux are the most significant correlators with the SKR energy.
Although the solar wind speed V was the best predictor of auroral (terrestrial) kilometric radiation /10/, the Saturn kilometric radiation does not respond to speed within physically acceptable lag times. The first peak of the SKR energy occurs 3 Saturn rotations before the velocity peak, and the- second 34CR peak follows 2 rotations after the velocity peak. There is also no physical relationship between the 31CR energy and r. The Akasofu—parameter r describes the solar wind dynamo energy flux over the Saturnian magnetosphere cross section. A remarkable enhancement of r is followed by a rather insignificant 34CR peak, with a lag time of 2 Saturn rotations. This demonstrates the minor importance of solar wind quantities incorporating magnetic field properties in stimulating SKR. This fact is supported by the last plots in Figure 2 showing the relationship between SKR and the IMF magnitude B, and the solar wind electric field E, respectively. The onset of the SKR enhancement leads the corresponding enhancement in B. Finally, there is a peak—to—peak correspondence between E and the first SKR peak, but no E—enhancement for the second SKR peak. Out of all examined parameter combinations the following four related quantities, density, momentum, ram pressure and kinetic solar wind energy flux, exhibit the most significant relationship to the SKR energy fluctuations. For example there is an almost one—to—one correspondence of the ram pressure to SKR within 0 to 7 Saturn rotations of the zero epoch. These four quantities are not independent, but have the solar wind density in common. At Saturn’s distance from the sun, density variations dominate speed variations so it is actually the density that is driving the correlations. When further quantitative comparisons are completed it may be possible to distinguish between each of these quantities.
336
M.D. Desch and H.O. Rucker
Summarizing this Voyager 2 study the present paper confirms the result obtained by the Voyager 1 investigation /14/, which found the best SKR predictors were solar wind quantities involving the density. No evidence is found for any association between SKR and IMF variations or quantities incorporating magnetic field properties. ACKNOWLEDGMENTS We are grateful to the Voyager principal investigators N. F. Ness and H. S. Bridge for making available the magnetometer and plasma data used in this study. REFERENCES 1.
N. L. Kaiser, N. D. Desch, J. W. Warwick, and J. B. Pearce, Voyager detection of nonthermal radio emission from Saturn, Science 209. 1238 (1980)
2.
M. L. Kaiser, J. E. P. Connerney, and M. D. Desch, The source of Saturn electrostatic discharges: Atmospheric storms, Nature 303, 50 (1983)
3.
N. D. Desch, Evidence for solar wind control of the Saturn radio emission, J. Geophys. Res, 87, 145149 (1982)
14•
M. D. Desch and H. 0. Rucker, The relationship between Saturn kilometric radiation and the solar wind, J. Geophys. Res. 88, 8999 (1983)
5.
M. L. Kaiser and M. D. Desch, Saturnian kilometric radiation: Geophys. Res. 87, ‘4555 (1982)
6.
W. S. Kurth, J. D. Sullivan, D. A. Gurnett, F. L. Scarf, H. S. Bridge, and E. C. Sittler, Jr., Observations of Jupiter’s distant magnetotail and wake, J. Geophys. Res. 87, 10373 (1982)
7.
R. P. Lepping, L. F. Burlaga, N. D. Desch, and L. W. Klein, Evidence for a distant (>8700 R~)Jovian magnetotail: Voyager 2 observations, Geophys. Res. Lett. 9, 885 (1982)
8.
N. D. Desch, Radio emission signature of Saturn immersions in Jupiter’s magnetic tail, J. Geophys. Res. 88, 69014 (1983)
9.
S.—I. Akasofu, Energy coupling between the solar wind and the magnetosphere, Space Sci. Rev. 28, 121 (1981) -
Source locations,
,J.
10. D. L. Galagher and N. D’Angelo, Correlations between solar wind parameters and auroral kilometric radiation intensity, Geophys. Res. Lett. 8, 1087 (1981)
Printed in Great Britain. AU
333—336, 1985 rights reserved. pp.
0273—1177/85 $0.00 + .50 Copyright © COSPAR
SATURN RADIO EMISSION AND THE SOLAR WIND: VOYAGER-2 STUDIES M. D. Desch5 and H. 0. Rucker55 5NASA/Goddard Space Flight Center, Laboratory for Extraterrestrial Physics, Greenbelt, MD 20771, U.S.A. ~Space Research Institute, Austrian Academy of Sciences, Lustbuhel Observatory, A-8042 Graz, Austria
ABSTRACT Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the relationship between parameters of the solar wind/interplanetary medium and the nonthermal Saturn radiation. Solar wind and interplanetary magnetic field properties were combined to form quantities known to be important in controlling terrestrial magnetospheric processes. The Voyager 2 data set used in this investigation consists of 237 days of Saturn preencounter measurements. However, due to the immersion of Saturn and the Voyager 2 spacecraft into the extended Jupiter magnetic tail, substantial periods of the time series were lacking solar wind data. To cope with this problem a superposed epoch method (CHREE analysis) was used. The results indicate the superiority of the quantities containing the solar wind density in stimulating the radio emission of Saturn — a result found earlier using Voyager 1 data — and the minor importance of quantities incorporating the interplanetary magnetic field. INTRODUCTION The observations of both Voyager spacecraft showed that Saturn is an emitter of two different kinds of nonthermal radiation: the Saturn kilometric radiation (SKR) /1/ and the Saturn electrostatic discharges (SED) /2/. The special value of SKR for the present study lies in the fact that this radiation can serve as a remote sensor of Saturnian magneto— spheric activity. Previous studies of the radio wave modulation of SKR on a time scale of days indicate that the radio energy fluctuations are distinctly associated with the solar wind ram pressure and the kinetic energy flux /3,1!!. These findings are consistent with the source location of SKR within the dayside cusp region of Saturn /5/. The present investigation makes use of preencounter Voyager 2 data to study the influence of the solar wind on SKR. SOLAR WIND, INTERPLANETARY MAGNETIC FIELD AND RADIO OBSERVATIONS The present study employs Voyager 2 Planetary Radio Astronomy (PRA), Magnetometer (MAG), and Plasma Science (PLS) data. The analysis interval covers the Saturn preencounter period from 1 January 1981 through 25 August 1981, corresponding to more than 9 solar rotations. During the period from day 1 to day 237, 1981, the spacecraft moved from a distance of 1.5 AU (AU astronomical unit) to 0.1 AU from Saturn. The propagation time for the solar wind plasma to move over the respective distances is 5.5 days and 0.1! days. An overview of the radio and plasma quantities used in this investigation is shown in Figure 1. The top panel shows the isotr~ 8ice~ergy1flux of the Saturn kilometric radiation that exceeded a flux density of 1.3 x 1O~ Wm Hz~ at 1 AU. For comparison the lower panel shows the solar wind kinetic energy flux, PV , which has been ballistically propagated to the position of Saturn. Unlike Voyager 1, the Voyager 2 spacecraft and Saturn were immersed several times in the extended tail of Jupiter. These periods are characterized by low solar wind plasma densities, magnetic field orientations corresponding to the Jovian tail direction, the occurrence of Jovian continuum radiation /6,7/, and the disappearance of SKR /8/. The most significant indicator for Jupiter tail encounter periods are low plasma density events as shown by bars in Figure 1. The frequent interruption of the Voyager 2 solar wind observations made impossible any reliable linear cross correlation analysis as was done in /14/ with Voyager 1 measurements. Therefore the study of the solar wind influence on SKR has been done with the aid of a superposed epoch analysis, or CHREE analysis.
333
334
M.D. Deseh and H.O. Ruoker
6 ____
____________
ERR
In
POWER
1
_
H -
—
i
—
i
.
.—
—
~
0
20
40
60
80
100
120
140
J~ 160
180
.
200
220
240
oo’~’ 1981
Figure 1. Comparison of SKR power and the solar wind kinetic power for the period from day 1 to 237, 1981. The bars In the lower panel indicate Jupiter tail encounter periods.
CHREE ANALYSIS AND DISCUSSION OF RESULTS As explained in detail in /11/ many solar wind parameters have been found important in stimulating terrestrial magnetospheric activity, including auroral kilometric radiation (AKR). However, the aforementioned paper was restricted to Voyager 1 observations, and so the question remains as to how the Voyager 2 data would fit the model developed for the stimulation of SKR as described in /14/. Out of a number of examined solar wind parameters the following 8 quantities were chos~n for Figure 2: solar wind bulk speed ~, density p, momentum pV, solar wind ram pressure PV kinetic solar wind energy flux PV , the Akasofu—parameter E /9/, the interplanetary magnetic field (IMF) magnitude B, and the solar wind electric field magnitude E (~VxB). In Figure 2 no consideration is made for quantities like the IMF—B — component (north/south, aligned to the magnetic dipole axis of Saturn), and the IMF—B — component (toward/away from the sun, associated with the magnetic sector boundary passa~es). These quantities have shown no relationship to SKR. The superposed epoch technique requires identification of the zero epoch in each time series of the examined quantities. The zero epoch is a characteristic point in time within an event and defines a significant onset or change in any time series. In our case it is determined by looking for the time derivative reaching or exceeding the 30 level (0 standard deviation). After having found the solar wind parameter variations before and after the zero epoch the SKR energy fluctuations are aligned in time. Figure 2 shows the relationship between 8 solar wind quantities (dashed line) and the 54CR energy (full line) with respect to time as well as amplitude. The data resolution is one value per Saturn rotation within the period of 10 rotations before and 20 rotations after the zero epoch. SKR energy changes following (lagging) in time the corresponding changes in
Saturn Radio Emission and the Solar Wind from Voyager—2
335
the solar wind quantities indicate a physically realizable relationship. As shown in /14/, lag times greater than 10 hours ( 1 Saturn rotation) cannot be attributed to possible errors due to the propagation of any solar wind feature to Saturn. Obviously, peak—to—peak lead times greater than 1 Saturn rotation are physically irrelevant.
A
:~~ ~
-12.2 83111681
-80.0
3.2
RornrlDc
3.2
702.
3.2
2
1O.~[
10.0
0
-[0.2
0.2
[2.6
58111661 RC1flT168~ [10.68
22.0
-.10.0
O.RS[
~ -[2.3
3.3
;__
[0.68
SATl~1R8T2T!68~[10.66
52
0.0
20.3
apsi
0.8
0
58111662 RO1#1168~ 116.60 8~68S[
~ 2.3
[3.2
20.8
11815568 601511368 110.66 [~RS[
;~\;,L~
5811.661 561578368 [10.56 146680]
6.3
aa
5511568 6018716868 [0.56
80.6858
-12.~
- 2.8
_____
5511685
5815r1368
188.56 10.68S]
Figure 2. CHREE analysis between the S!CR energy (full line) and 8 solar wind quantities (dashed line) for the interval from day 1 to 237, 1981. Solar wind momentum, ram pressure and energy flux are the most significant correlators with the SKR energy.
Although the solar wind speed V was the best predictor of auroral (terrestrial) kilometric radiation /10/, the Saturn kilometric radiation does not respond to speed within physically acceptable lag times. The first peak of the SKR energy occurs 3 Saturn rotations before the velocity peak, and the- second 34CR peak follows 2 rotations after the velocity peak. There is also no physical relationship between the 31CR energy and r. The Akasofu—parameter r describes the solar wind dynamo energy flux over the Saturnian magnetosphere cross section. A remarkable enhancement of r is followed by a rather insignificant 34CR peak, with a lag time of 2 Saturn rotations. This demonstrates the minor importance of solar wind quantities incorporating magnetic field properties in stimulating SKR. This fact is supported by the last plots in Figure 2 showing the relationship between SKR and the IMF magnitude B, and the solar wind electric field E, respectively. The onset of the SKR enhancement leads the corresponding enhancement in B. Finally, there is a peak—to—peak correspondence between E and the first SKR peak, but no E—enhancement for the second SKR peak. Out of all examined parameter combinations the following four related quantities, density, momentum, ram pressure and kinetic solar wind energy flux, exhibit the most significant relationship to the SKR energy fluctuations. For example there is an almost one—to—one correspondence of the ram pressure to SKR within 0 to 7 Saturn rotations of the zero epoch. These four quantities are not independent, but have the solar wind density in common. At Saturn’s distance from the sun, density variations dominate speed variations so it is actually the density that is driving the correlations. When further quantitative comparisons are completed it may be possible to distinguish between each of these quantities.
336
M.D. Desch and H.O. Rucker
Summarizing this Voyager 2 study the present paper confirms the result obtained by the Voyager 1 investigation /14/, which found the best SKR predictors were solar wind quantities involving the density. No evidence is found for any association between SKR and IMF variations or quantities incorporating magnetic field properties. ACKNOWLEDGMENTS We are grateful to the Voyager principal investigators N. F. Ness and H. S. Bridge for making available the magnetometer and plasma data used in this study. REFERENCES 1.
N. L. Kaiser, N. D. Desch, J. W. Warwick, and J. B. Pearce, Voyager detection of nonthermal radio emission from Saturn, Science 209. 1238 (1980)
2.
M. L. Kaiser, J. E. P. Connerney, and M. D. Desch, The source of Saturn electrostatic discharges: Atmospheric storms, Nature 303, 50 (1983)
3.
N. D. Desch, Evidence for solar wind control of the Saturn radio emission, J. Geophys. Res, 87, 145149 (1982)
14•
M. D. Desch and H. 0. Rucker, The relationship between Saturn kilometric radiation and the solar wind, J. Geophys. Res. 88, 8999 (1983)
5.
M. L. Kaiser and M. D. Desch, Saturnian kilometric radiation: Geophys. Res. 87, ‘4555 (1982)
6.
W. S. Kurth, J. D. Sullivan, D. A. Gurnett, F. L. Scarf, H. S. Bridge, and E. C. Sittler, Jr., Observations of Jupiter’s distant magnetotail and wake, J. Geophys. Res. 87, 10373 (1982)
7.
R. P. Lepping, L. F. Burlaga, N. D. Desch, and L. W. Klein, Evidence for a distant (>8700 R~)Jovian magnetotail: Voyager 2 observations, Geophys. Res. Lett. 9, 885 (1982)
8.
N. D. Desch, Radio emission signature of Saturn immersions in Jupiter’s magnetic tail, J. Geophys. Res. 88, 69014 (1983)
9.
S.—I. Akasofu, Energy coupling between the solar wind and the magnetosphere, Space Sci. Rev. 28, 121 (1981) -
Source locations,
,J.
10. D. L. Galagher and N. D’Angelo, Correlations between solar wind parameters and auroral kilometric radiation intensity, Geophys. Res. Lett. 8, 1087 (1981)