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The brightness temperature of Saturn at decimeter wavelengths
l&177-180
ICARUS
The
(1973)
Brightness
Temperature
M. J. YERBURY’,
of Saturn J. J. CONDON
Astronomy
National
at Decimeter
and University
Ionosphere Astronomy
Received
July
Wavelengths
AND D. L. JAUNCEY’ Center
and
Cornell-Sydney
Center
17, 1972
Observations of the planet Saturn at wavelengths of 49.5 and 94.3cm are reported. The equivalent disk brightness temperatures were found to be 400 f 65°K and 540 * llO”K, respectively. It is suggested that the enhanced portion of the spectrum of the disk brightness temperature favours the idea that the observed long wavelength radiation comes from the planet’s atmosphere. However, the possibility of a magnetic field associated with Saturn is not rejected by the observations. Part of the excess temperature could be attributed to weak synchrotron emission coming from a region outside the ring system.
I.
INTRODUCTION
During the opposition of 1970, passive radio observations of the planet Saturn were made at the Arecibo Observatory at a wavelength of 49.5cm and again in 1971 at a wavelength of 94.3cm. The purpose of this paper is to describe briefly the techniques used and to discuss the implications of the results. II.
EXPERIMENTAL TECHNIQUE
A linear approximation to the path in right ascension and declination as taken by Saturn over about a g-week period was determined. In each wavelength the observations, which were spread out over this period, required the telescope to track along this line at roughly the sidereal rate. Since the overall motion of the feed system during each track was reduced to a very low level by the choice of tracking velocity and feed start coordinates, the dynamic tracking errors were made negligible. For both experiments, this approach was preferred over the drift-scan procedure because every track comprised an observation of the source together with 1 Cornell 14850. z Arecibo 00612.
University,
Ithaca,
New
York
Observatory,
Arecibo,
Puerto
Rico
Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
the background for earlier and later observations. In fact, as the number of observations becomes large compared with unity, this technique rapidly achieves a 2/2 signal/noise advantage over the driftscan method (Yerbury et al., 1971). The Arecibo telescope is “confusionlimited” at both of the wavelengths used in these observations, the rms confusion being roughly 0.04 f.u.3 at the wavelength of 49.5cm and 0.15f.u. at 94.3cm. The observations at the longer wavelength were made in 1971 when a slightly improved observing procedure had been developed in anticipation of a more difficult experiment. In this case, tracks were grouped under the same antenna start coordinates and the local sidereal time for the start of each group was fixed to within 15 sec. The analysis was carried out in two stages with programs written for the observatory’s CDC 3300 computer. In both experiments, mean sky backgrounds were determined and subtracted from each track, leaving the response due to the planet at the expected time, together with filtered system noise. The respective transit times were then aligned and the data were superimposed to determine the mean response in each case. 3 1 f .u
177
.=
10Tz6
Wm-ZHz-l .
178
M.
J.
YERBURY,
J.
J.
CONDON
AND
TABLE
x
mf.u.
49.5 94.3
28.3 f 8.6 f
III.
4) 5 1.7
1.04 1.12
RESULTS
The calibration source used in both experiments was 3C43 which on the flux density scale of Kellerman, Pauliny-Toth and Williams, 1969 (KPTW) has the value 5.06f.u. at the wavelength of 49.5cm and 7.85f.u. at 94.3cm. There is increasing evidence (Baars and Hartsuijker, 1972; Scott and Shakeshaft, 1971; Wyllie, 1969; Little, 1958) to suggest that the KPTW scale is consistently low for wavelengths longer than about 40cm. Therefore in converting the measured (KPTW) flux densities to equivalent disk temperatures, a correction factor, c(h) was introduced. The equivalent disk brightness temperatures were calculated from the RayleighJeans law T(h) = [x*8(4/21csz,1~@)
+ T,(h),
(1)
where T(X) is the required temperature, X(X) is the measured flux density, c(X) is a
FIG. 1. Plot of Saturn observations
of the at various reported
L.
JAUNCEY
I
In,sterad
SW
cm
D.
T&V
(x10-9)
“K
6.827 6.945
90
15
TO) “K 400 + 65 540 & 110
correction factor for the flux density scale, k is Boltzmann’s constant, Sz, is the mean solid angle subtended by the visible “disk” of the planet during the observing period and TJX) is the sky background temperature. Table I lists the values used in Eq. (1) at the two wavelengths. The value of c (94.3) represents the measured difference between the Wyllie (1969) scale and the KPTW scale (Niell and Jauncey, 1971) and the choice of c (49.5) is discussed by Yerbury et al. (1971). The spectrum of the equivalent blackbody disk temperature is plotted in Fig. 1 where it can be seen that for the wavelength range lmm to about 3.5cm a constant temperature of about 135°K adequately describes the currently available data. For wavelengths beyond about 9cm, however, there is a definite increase in the disk brightness temperature ; the simplest acceptable fit to the data in this enhanced region of the spectrum is a straight line
currently available measurements of the equivalent disk wavelengths. The two open points represent the results in this paper. The simplest acceptable fits to the data
brightness temperature of the 49.5 and 94.3cm are presented.
BRIGHTNESS
with a slope of $0.6, figure. IV.
DISCUSSION
AND
as shown
TEMPERATURE
in the
CONCLUSIONS
Much of the literature dealing with the results of passive radio observations of Saturn has been concerned with attempts to explain the enhancement in the disk brightness temperature at long wavelengths. The ability to provide a convincing interpretation of this phenomenon has been hampered by the sparse and uncertain data in the wavelength range beyond about 10cm. One reason for this paucity of long wavelength observations is the rapid decrease in the intensity of the radiation from Saturn coupled with an increase in the sky background radiation. This fact is exemplified by the 94.3-cm observations reported here in which the final value of the measured flux density is only 6% of the rms confusion for the Arecibo telescope at this wavelength. Turning to the currently available spectral data, a statistical analysis of all the published measurements in the wavelength range 1 mm through 3.5cm gives no reason to reject the idea of a constant temperature of about 136°K in this region. In fact, an exclusive analysis of the six accurate measurements reported by Wrixon and Welch (1970) and surrounding the expected ammonia absorption feature yields a weighted mean temperature of 135 f- 2.7”K. A simple x2 test applied to their results shows that there is about one chance in six of obtaining the observed value of x2 on the hypothesis that the observed temperature is constant. Thus, given even these precise measurements, it appears there is no significant departure from a constant temperature over the region where an ammonia absorption feature might be expected. Perhaps the best evidence for the presence of ammonia in Saturn’s atmosphere lies in the observed enhancement of the brightness temperature at long wavelengths (Berge and Read, 1968). The earlier theoretical calculations by Gulkis et al. (1969) of the run of temperature with wavelength in Saturn’s atmosphere for various ammonia mixing ratios, show a rapid rise at long wave-
OF SATURN
179
lengths. However, later, more accurate calculations (Gulkis and Poynter, 1972) predict a less rapid increase. Berge and Read (1968) have shown that 90% of the observed radiation at the wavelength of 1Ocm comes from the visible “disk” and conclude that the observed enhancement at that wavelength must be associated with the atmosphere. Briggs and Drake (private communication) find that SO-SO% of the emission they observed at the wavelength of 21 cm originates on the visible disk: they obtain a brightness temperature of about 220°K. For the observations reported here the extent of the emitting region is not known. If, as has been assumed, the radiation comes from the visible disk then the temperature at the wavelength of 49.5 cm lends support to the idea of an atmosphere having the cosmic abundance of ammonia. The temperature determined for the wavelength of 94.3 cm fits closely to an extrapolation of the theoretical curve derived by Gulkis and Poynter (1972) for an ammonia-mixing ratio corresponding to the cosmic abundance. However, since the result is outside the range for which their calculations are valid, it will be necessary to await further theoretical work before its relevance to the ammonia absorption theory can be properly assessed. The evidence from the 10 and 21 cm interferometric observations strongly favors atmospheric emission and implies there is no synchrotron radiation. Nevertheless, this does not rule out the possibility that Saturn has a magnetic field. If a strong magnetic field comparable to that on Jupiter exists, most of the field lines would have to intersect the ring system which, because of its high opacity, would preclude the trapping of electrons and the associated synchrotron emission. The possibility of a radiation belt lying outside the ring system remains, however. If the optically tenuous “D” ring possibly detected by Feibelman (1966), is real, then electron screening could be effective out to at least four Saturn radii; synchrotron emission might then be expected to come from a region beyond this limit. The intensity of the radiation from such a region would
M.
180
J.
YERBURY,
J.
J.
CORDON
be required to account only for the observed excess of about 170°K between the measurements at 21 and 49.5cm, and for about 320°K between the measurements at 21 and 94.3cm. In the case of Jupiter, there is some evidence for a broadening of the extent of the emitting region at longer wavelengths (Gulkis, 1970). McAdam (1966) in addition to measuring this broadening, claims to have observed a region at six Jupiter radii which contributes 16-20% of the total flux but this was not supported by the measurements made by Gulkis. The observations reported here help to define more accurately the enhanced region of the spectrum of Saturn’s disk brightness temperature. It is still very desirable, however, to seek more accurate determinations of the emission and its extent, at long wavelengths. V.
ACKNOWLEDGMENTS
The authors are grateful to F. H. Briggs and F. D. Drake for allowing their results to be used in this discussion and to S. Gulkis for valuable discussions. This work was supported under the National Astronomy and Ionosphere Center which is operated by Cornell University under contract to the National Science Foundation. One of the authors (D. L. J.) was partially supported by NSF Grant GP-28942. REFERENCES BAARS, The
J. W. M., AND HARTSUIJKER, A. P. (1972). decrease of flux density of Cassiopeia A
AND
D.
L.
JAUNCEY
and the absolute spectra of Cassiopeia A, Cygnus A and Taurus A. Aatron. Astrophys. 17,172-181. BERQE, G. L., AND READ, R. B. (1968). The microwave emission of Saturn. Aetrophys. J. 152, 755. FEIBEL~, W. A. (1967). Concerning the ‘D’ ring of Saturn. Nature (London) 214,793. G~LEIS, S. (1970). Lunar occultation observations of Jupiter at 74cm and 128cm. Radio sci. 5, 505. GIJLKIS, S., AND POY~ER, R. (1972). Thermal radio emission from Jupiter and Saturn. Phyaiea of the Earth and Planetary Interiors (to be published). GULKIS, S., MCDONOTJGH, T. R., AND CRAFT, H. (1969). The microwave spectrum of Saturn. Icarus 10,421. LITTLE, A. G. (1958). Gain measurements of large aerials used in interferometer and cross-type radio telescopes. Au&. J. Phya. 11, 70. MCADAM, W. B. (1966). The extent of the emission region on Jupiter at 408Mc/s. Planet. Space Sci. 14, 1041. NIELL, A. E., AND JAUNCEY, D. L. (1971). Flux density measurements of Parkes sources at 43OMHz. Bull. A.A.S. 3, 25. Scorr, P. F., AND SHAKESHAFT, J. R. (1971). The flux density scale for radio sources at 81.5MHz. Mon. Not. R. Astr. Sot. 154, 19P. WRIXON, G. T., AND WELCH, W. J. (1970). The millimeter wave spectrum of Saturn. Icarus 13, 163. WYLLIE, D. V. (1969). An absolute flux density scale at 408MHz. Mon. Not. R. A&r. Sot. 142, 229. YERBURY, M. J., CONDON, J. J., AND JAUNCEY, D. L. (1971). Observations of Saturn at a wavelength of 495cm. Icarus 15,459.
ICARUS
The
(1973)
Brightness
Temperature
M. J. YERBURY’,
of Saturn J. J. CONDON
Astronomy
National
at Decimeter
and University
Ionosphere Astronomy
Received
July
Wavelengths
AND D. L. JAUNCEY’ Center
and
Cornell-Sydney
Center
17, 1972
Observations of the planet Saturn at wavelengths of 49.5 and 94.3cm are reported. The equivalent disk brightness temperatures were found to be 400 f 65°K and 540 * llO”K, respectively. It is suggested that the enhanced portion of the spectrum of the disk brightness temperature favours the idea that the observed long wavelength radiation comes from the planet’s atmosphere. However, the possibility of a magnetic field associated with Saturn is not rejected by the observations. Part of the excess temperature could be attributed to weak synchrotron emission coming from a region outside the ring system.
I.
INTRODUCTION
During the opposition of 1970, passive radio observations of the planet Saturn were made at the Arecibo Observatory at a wavelength of 49.5cm and again in 1971 at a wavelength of 94.3cm. The purpose of this paper is to describe briefly the techniques used and to discuss the implications of the results. II.
EXPERIMENTAL TECHNIQUE
A linear approximation to the path in right ascension and declination as taken by Saturn over about a g-week period was determined. In each wavelength the observations, which were spread out over this period, required the telescope to track along this line at roughly the sidereal rate. Since the overall motion of the feed system during each track was reduced to a very low level by the choice of tracking velocity and feed start coordinates, the dynamic tracking errors were made negligible. For both experiments, this approach was preferred over the drift-scan procedure because every track comprised an observation of the source together with 1 Cornell 14850. z Arecibo 00612.
University,
Ithaca,
New
York
Observatory,
Arecibo,
Puerto
Rico
Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
the background for earlier and later observations. In fact, as the number of observations becomes large compared with unity, this technique rapidly achieves a 2/2 signal/noise advantage over the driftscan method (Yerbury et al., 1971). The Arecibo telescope is “confusionlimited” at both of the wavelengths used in these observations, the rms confusion being roughly 0.04 f.u.3 at the wavelength of 49.5cm and 0.15f.u. at 94.3cm. The observations at the longer wavelength were made in 1971 when a slightly improved observing procedure had been developed in anticipation of a more difficult experiment. In this case, tracks were grouped under the same antenna start coordinates and the local sidereal time for the start of each group was fixed to within 15 sec. The analysis was carried out in two stages with programs written for the observatory’s CDC 3300 computer. In both experiments, mean sky backgrounds were determined and subtracted from each track, leaving the response due to the planet at the expected time, together with filtered system noise. The respective transit times were then aligned and the data were superimposed to determine the mean response in each case. 3 1 f .u
177
.=
10Tz6
Wm-ZHz-l .
178
M.
J.
YERBURY,
J.
J.
CONDON
AND
TABLE
x
mf.u.
49.5 94.3
28.3 f 8.6 f
III.
4) 5 1.7
1.04 1.12
RESULTS
The calibration source used in both experiments was 3C43 which on the flux density scale of Kellerman, Pauliny-Toth and Williams, 1969 (KPTW) has the value 5.06f.u. at the wavelength of 49.5cm and 7.85f.u. at 94.3cm. There is increasing evidence (Baars and Hartsuijker, 1972; Scott and Shakeshaft, 1971; Wyllie, 1969; Little, 1958) to suggest that the KPTW scale is consistently low for wavelengths longer than about 40cm. Therefore in converting the measured (KPTW) flux densities to equivalent disk temperatures, a correction factor, c(h) was introduced. The equivalent disk brightness temperatures were calculated from the RayleighJeans law T(h) = [x*8(4/21csz,1~@)
+ T,(h),
(1)
where T(X) is the required temperature, X(X) is the measured flux density, c(X) is a
FIG. 1. Plot of Saturn observations
of the at various reported
L.
JAUNCEY
I
In,sterad
SW
cm
D.
T&V
(x10-9)
“K
6.827 6.945
90
15
TO) “K 400 + 65 540 & 110
correction factor for the flux density scale, k is Boltzmann’s constant, Sz, is the mean solid angle subtended by the visible “disk” of the planet during the observing period and TJX) is the sky background temperature. Table I lists the values used in Eq. (1) at the two wavelengths. The value of c (94.3) represents the measured difference between the Wyllie (1969) scale and the KPTW scale (Niell and Jauncey, 1971) and the choice of c (49.5) is discussed by Yerbury et al. (1971). The spectrum of the equivalent blackbody disk temperature is plotted in Fig. 1 where it can be seen that for the wavelength range lmm to about 3.5cm a constant temperature of about 135°K adequately describes the currently available data. For wavelengths beyond about 9cm, however, there is a definite increase in the disk brightness temperature ; the simplest acceptable fit to the data in this enhanced region of the spectrum is a straight line
currently available measurements of the equivalent disk wavelengths. The two open points represent the results in this paper. The simplest acceptable fits to the data
brightness temperature of the 49.5 and 94.3cm are presented.
BRIGHTNESS
with a slope of $0.6, figure. IV.
DISCUSSION
AND
as shown
TEMPERATURE
in the
CONCLUSIONS
Much of the literature dealing with the results of passive radio observations of Saturn has been concerned with attempts to explain the enhancement in the disk brightness temperature at long wavelengths. The ability to provide a convincing interpretation of this phenomenon has been hampered by the sparse and uncertain data in the wavelength range beyond about 10cm. One reason for this paucity of long wavelength observations is the rapid decrease in the intensity of the radiation from Saturn coupled with an increase in the sky background radiation. This fact is exemplified by the 94.3-cm observations reported here in which the final value of the measured flux density is only 6% of the rms confusion for the Arecibo telescope at this wavelength. Turning to the currently available spectral data, a statistical analysis of all the published measurements in the wavelength range 1 mm through 3.5cm gives no reason to reject the idea of a constant temperature of about 136°K in this region. In fact, an exclusive analysis of the six accurate measurements reported by Wrixon and Welch (1970) and surrounding the expected ammonia absorption feature yields a weighted mean temperature of 135 f- 2.7”K. A simple x2 test applied to their results shows that there is about one chance in six of obtaining the observed value of x2 on the hypothesis that the observed temperature is constant. Thus, given even these precise measurements, it appears there is no significant departure from a constant temperature over the region where an ammonia absorption feature might be expected. Perhaps the best evidence for the presence of ammonia in Saturn’s atmosphere lies in the observed enhancement of the brightness temperature at long wavelengths (Berge and Read, 1968). The earlier theoretical calculations by Gulkis et al. (1969) of the run of temperature with wavelength in Saturn’s atmosphere for various ammonia mixing ratios, show a rapid rise at long wave-
OF SATURN
179
lengths. However, later, more accurate calculations (Gulkis and Poynter, 1972) predict a less rapid increase. Berge and Read (1968) have shown that 90% of the observed radiation at the wavelength of 1Ocm comes from the visible “disk” and conclude that the observed enhancement at that wavelength must be associated with the atmosphere. Briggs and Drake (private communication) find that SO-SO% of the emission they observed at the wavelength of 21 cm originates on the visible disk: they obtain a brightness temperature of about 220°K. For the observations reported here the extent of the emitting region is not known. If, as has been assumed, the radiation comes from the visible disk then the temperature at the wavelength of 49.5 cm lends support to the idea of an atmosphere having the cosmic abundance of ammonia. The temperature determined for the wavelength of 94.3 cm fits closely to an extrapolation of the theoretical curve derived by Gulkis and Poynter (1972) for an ammonia-mixing ratio corresponding to the cosmic abundance. However, since the result is outside the range for which their calculations are valid, it will be necessary to await further theoretical work before its relevance to the ammonia absorption theory can be properly assessed. The evidence from the 10 and 21 cm interferometric observations strongly favors atmospheric emission and implies there is no synchrotron radiation. Nevertheless, this does not rule out the possibility that Saturn has a magnetic field. If a strong magnetic field comparable to that on Jupiter exists, most of the field lines would have to intersect the ring system which, because of its high opacity, would preclude the trapping of electrons and the associated synchrotron emission. The possibility of a radiation belt lying outside the ring system remains, however. If the optically tenuous “D” ring possibly detected by Feibelman (1966), is real, then electron screening could be effective out to at least four Saturn radii; synchrotron emission might then be expected to come from a region beyond this limit. The intensity of the radiation from such a region would
M.
180
J.
YERBURY,
J.
J.
CORDON
be required to account only for the observed excess of about 170°K between the measurements at 21 and 49.5cm, and for about 320°K between the measurements at 21 and 94.3cm. In the case of Jupiter, there is some evidence for a broadening of the extent of the emitting region at longer wavelengths (Gulkis, 1970). McAdam (1966) in addition to measuring this broadening, claims to have observed a region at six Jupiter radii which contributes 16-20% of the total flux but this was not supported by the measurements made by Gulkis. The observations reported here help to define more accurately the enhanced region of the spectrum of Saturn’s disk brightness temperature. It is still very desirable, however, to seek more accurate determinations of the emission and its extent, at long wavelengths. V.
ACKNOWLEDGMENTS
The authors are grateful to F. H. Briggs and F. D. Drake for allowing their results to be used in this discussion and to S. Gulkis for valuable discussions. This work was supported under the National Astronomy and Ionosphere Center which is operated by Cornell University under contract to the National Science Foundation. One of the authors (D. L. J.) was partially supported by NSF Grant GP-28942. REFERENCES BAARS, The
J. W. M., AND HARTSUIJKER, A. P. (1972). decrease of flux density of Cassiopeia A
AND
D.
L.
JAUNCEY
and the absolute spectra of Cassiopeia A, Cygnus A and Taurus A. Aatron. Astrophys. 17,172-181. BERQE, G. L., AND READ, R. B. (1968). The microwave emission of Saturn. Aetrophys. J. 152, 755. FEIBEL~, W. A. (1967). Concerning the ‘D’ ring of Saturn. Nature (London) 214,793. G~LEIS, S. (1970). Lunar occultation observations of Jupiter at 74cm and 128cm. Radio sci. 5, 505. GIJLKIS, S., AND POY~ER, R. (1972). Thermal radio emission from Jupiter and Saturn. Phyaiea of the Earth and Planetary Interiors (to be published). GULKIS, S., MCDONOTJGH, T. R., AND CRAFT, H. (1969). The microwave spectrum of Saturn. Icarus 10,421. LITTLE, A. G. (1958). Gain measurements of large aerials used in interferometer and cross-type radio telescopes. Au&. J. Phya. 11, 70. MCADAM, W. B. (1966). The extent of the emission region on Jupiter at 408Mc/s. Planet. Space Sci. 14, 1041. NIELL, A. E., AND JAUNCEY, D. L. (1971). Flux density measurements of Parkes sources at 43OMHz. Bull. A.A.S. 3, 25. Scorr, P. F., AND SHAKESHAFT, J. R. (1971). The flux density scale for radio sources at 81.5MHz. Mon. Not. R. Astr. Sot. 154, 19P. WRIXON, G. T., AND WELCH, W. J. (1970). The millimeter wave spectrum of Saturn. Icarus 13, 163. WYLLIE, D. V. (1969). An absolute flux density scale at 408MHz. Mon. Not. R. A&r. Sot. 142, 229. YERBURY, M. J., CONDON, J. J., AND JAUNCEY, D. L. (1971). Observations of Saturn at a wavelength of 495cm. Icarus 15,459.