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Mapping of the sodium emission associated with Io and Jupiter
IDARTJ~ 27,425-428
Mapping
(1976)
of the Sodium Emission Associated with lo and Jupiter P. A. WEHINGER,
S. WYCKOFF,
AND
A. FROHLICH
Wise Observatory, Physics-Astronomy Department, Tel-Aviv University, Ramat-Aviv, Iwael Received September 23, 1975 The sodium D-lines are observed in emission in & disklike distribution surrounding 10 and extending outward in the orbital plane of the Galilean setellites to at least 23 R, from Jupiter. A so& length for the sodium emission cloud in the orbital plane and the thickness of the sodium disk perpendicular to the orbital plane are determined. Weak D-line emission is also detected over the poles of Jupiter. Estimates of the apparent emission rates are derived from microdensitometer scans of the spectrograms 8s & function of position in the satellite orbital plane and perpendicular to the orbital plane. No other emission lines were detected down to & limit of ~50R over the spectral range from 3500A to 9000A.
Region C, is an intense disklike cloud ( 20.3 to 5 2 kR) confined to the immediate vicinity of 10 ( 53 to 4 R, from 10) in its orbital plane; Region C, is an extensive envelope of very low intensity ( 20.05 to 0.3kR) observed in the satellite orbital plane outward from Jupiter to at least -23 R, on both sides of Jupiter when IO is at quadrature. The Na D emission intensity is greater on the side of Jupiter nearest 10 by a factor of 5 to 10 times compared to the same position at the opposite elongation at any given time.
After the discovery of the sodium D-lines in emission from 10 (Brown, 1974 ; Brown and Chaffee, 1974), an extended sodium emission cloud was detected at distances of 560arcsec ( 52.6 R,) from 10 (Trafton et al., 1974; Macy and Trafton, 1975; Trafton and Macy, 1975) using a photoelectric echelle coude scanner. Further evidence of the emission cloud was obtained by Miinch and Bergstralh (1975) with a phot,ographic multislit coud& spectrograph. The only other known emission associated with 10 and its environs is a partial Ly-cr torus exOBSERVATIONS tending over 6Odegrees of orbital phase either side of IO in the orbital plane with a Image-tube spectrograms were obtained thickness of less than 2 R, perpendicular to of the environs of 10 and its orbital plane the plane (Carlson and Judge, 1974). using the 40-inch (l-meter) reflector and Calibrated spectroscopic observations of Cassegrain spectrograph at the Wise the D-lines in emission have been secured Observatory. An ITT F4089 image tube which reach detection levels in the range (with an S-25 photocathode) was used with -0.05kR to 2kR in the environs of 10 and the spectrograph at dispersions of 15OA the satellite orbital plane. In order to map -’ covering the spectral region 5OOOA this weak emission, we have employed a E?OdoA with a resolution of 3.5A. Addifast image-tube Cassegrain spectrograph tional data were obtained to search for with a relatively small plate scale. Our obemission from other resonance lines at servations (Wehinger and Wyckoff, 1974; 2408mm-‘, covering the region 35OOA to Wehinger et al,, 1975) constitute the iden85OOA with a resolution of -78. The plate tification of Region C which was defined by scale perpendicular to dispersion was 150 Brown et al. (1975) as an area with D-line arcsecmm-‘. The maximum available slit emission intensities of less than -2kR. length was 12.5 arc min (corresponding to Our observations indicate that Region C 33 R, at the distance of Jupiter) and the can be subdivided into two components: projected slit width was 22microns (3arc Copyright 0 1976 by Academic Press, Inc. 425 All rights of reproduction in any form reserved. Printed in Great Britain
426
WERINCER,
WYCKOFF,
see). The observations, obtained between 1974 August 12 and November 21, were taken with the slit oriented parallel and perpendicular to the satellite orbital plane at various distances from 10 on both sides of Jupiter and also over Jupiter’s polar regions. The spectrograms were calibrated and scanned with a PDS microdensitometer at the Royal Greenwich Observatory. The instrumental sensitivity and atmospheric extinction as a function of wavelength were removed by a point-by-point division of the 10 spectra by an Oke (1965) spectrophotometric standarcl star, observed on the same night at the same zenith angle. Night airglow spectra, taken as a control at the same zenith angle immediately following the 10 observations, showed no trace of the night airglow D lines in 30 minute exposures. In much longer exposures (-OOmin), the night airglow D lines and the 63OOA [01] lines were marginally detectable. The intensity of the D lines associated with 10 and its environs was calibrated relative to the apparent emission rate of the night airglow [01] line at 5577A. The intensity of the 5577A line was assumed to be 250R, as given by Chamberlain (1961, p. 569) and Roach (1963). Diurnal variations of the 5577_.&line are of the order of +20%, according to Brenton and Silverman (1970). Since the 5577A line is formed high in the ionosphere (-lOOkm), to a first approximation, no air mass correction was necessary for observations made at moderate zenith angles ( 530 degrees) (cf. Chamberlain, 1961, p. 487). Intensities of the 5577A line and the D lines were compared from PDS scans of each spectrogram at the same position on the slit. Thus corrections for differences in the instrumental response as a function of wavelength and position along the slit were both accounted for in the reductions. Hence the D-line intensities were scaled to the intensity of the 5577A line taken to be 250 & 50R. SODIUM
EMISSION
REGIONS
Region C,. A set of observations, taken with the slit oriented perpendicular to the
AND FROHLICH
satellite orbital plane, at a series of positions either side of 10, showed evidence of the disklike cloud surrounding 10. This series of exposures was taken with the slit at successive positions from 2 R, to 15 R, from Jupiter surrounding 10 for a given quadrature. The observations show an intense ( 20.3 to 2kR) flattened disk of sodium emission extending from 223 to 5 10 R, from Jupiter (or 53 to 4 R, from 10). The extent of the disk appeared to be slightly asymmetric in shape in the sense that the disk extends outward farther in the orbital plane on the side away from Jupiter. In order to obtain a map of Region C, in a time short compared to the motion of 10 in its orbit, exposures of 6minutes each were taken. The most intense emission mapped in Region C, (at -1 R, from 10) is of approximately the same intensity as obtained photoelectrically by Trafton et al. (1974) at the ame distance from 10. We also found the suggestion of some slight asymmetry in the intensity perpendicular to the orbital plane in some of the spectra, although these effects require further analysis of the plate material. Region C,. In order to cover the full extent of the D-line emission in the Galilean satellite orbital plane (i.e. -30 R, east or west of Jupiter), separate spectra were taken. To intercompare the D-line intensities from one side of the orbit to the other side, additional spectra were taken with the slit centered on Jupiter, which was masked to reduce effects of scattered light. In this way, the emission in the orbital plane was recorded through two slit apertures in the region from 6.6 to 11.2 R, from Jupiter, on both sides of Jupiter. These spectra were taken when 10 was at quadrature, so that the D-line intensity near 10 (through one aperture) could be compared with the intensity at a point equidistant from Jupiter on the opposite side of the orbital plane (through the other aperture). The D-line intensity was -9 times greater on the side near 10 (~1.3 R, from 10) than it was at the opposite elongation -7.3 R, from Jupiter (or -1.3R, from IO’S orbit). For spectra taken with the slit in the orbital plane, the D lines decreased in intensity with increasing distance outward
427
IO SODIUM EMISSION MA.PPINU
from IO’S orbit. The scale length as a function of radial distance outward from 10 in the orbital plane was found to be -2.0 R,, where the scale length is defmed as the distance over which the intensity decreases by a factor e. Other spectrograms, taken with the slit oriented perpendicular to the orbital plane, show the sodium to be confined to a flattened layer with a thickness (perpendicular to the plane) of -4.5R, (FWHM) at -4 R, from 10 at east or west elongation (i.e. -10 R, from Jupiter). Over the range from -7 R, (-1 R, from 10) to 23 R, from Jupiter, the D-line intensity decreased from -2kR to -0.05kR. Beyond about 25 R, the emission level (--5OR) did not decrease significantly with increasing distance and so was probably indistinguishable from the night airglow sodium (-30R). As for other effects, no enhancement in the D-line emission (or in any other possible emission features) was detected near any of the other Galilean satellites. In addition, none of our spectra showed evidence of tilted D lines in contrast to the report by Mekler and Eviatar (1974). Above the polar regions of Jupiter itself weak D line emission was detected with the slit apertures located at 6.6 to 11.2 R, north and south of Jupiter. Intensities, averaged between the same position, north and south of Jupiter, were: 0.2lkR at 7.3R,, 0.2OkR at 8.7R,, and 0.17kR at lO.lR, (measured from Jupiter). It should be noted that special care was taken to mask off the slit aperture from Jupiter’s direct light, so that scattered light in the spectrograph was minimized. Also test spectra obtained in the vicinity of Jupiter indicate that night sky Na-D emission enhanced by preexposure effects due to scattered light from Jupiter on the over-the-pole spectra is probably negligible. SEARCH FOR OTHER EMISSIONLINES Additional spectrograms at lower dispersion (24OAmm-‘) which covered the spectral range 3500A to 8500& and selected long exposures (45min) at 150A mm-’ covering 50008 to 90008, were obtained for the purpose of searching for 1.5
emission from resonance lines of other abundant elements. These spectrograms were taken in the same manner as those described above, and were of those regions where the D-line emission was most intense (i.e. Region C,). However, no evidence was found for emission in CaI 42268 [contrary to the results of Meckler and Eviatar (1974), but in agreement with Brown et al. (1975)] ; nor was evidence found for the presence of KI 7699A and the Al1 3961A lines (to a limit of -5OR). No emission lines other than the sodium D lines were detected in the range from 35008 to 90008. These detection limits thus set further stringent bounds on any hypothetical surface composition for materials which might be sputtered from IO.
ACKNOWLEDGMENTS This research is supported in pert by the Smithsonian Research Foundation through Grant SFC-0-3005. We are most grateful to the Royal Greenwich Observatory for the use of their PDS microdensitometer and for their kind hospitality. Travel support provided by NASA and NSF is acknowledged with special thanks. Discussions with J. T. Bergstralh, R. A. Brown, and D. L. Matson were most helpful and constructive.
REFERENCES BRENTON, J. G., AND SILVERMAN, S. M. (1970). A study of the diurnal variation of the 5577A [01] airglow emission at selected IGY stations. Plartetary Space Sci. 18,641-653. BROW, R. A. (1974). Optical line emission from IO. In Exploration of the Planetary System (A. Woszcyk and C. Iwaniszewska, Eds.), pp. 527-531. Reidel, Dordrecht. BROW, R. A., AND CHAFFEE, F. H. (1974). High-resolution spectra of sodium emission from 10. Astrophya. J. 187, L125-L126. BROWN, R. A., GOODY, R. M., MURCRAY, F. J., AND CEAXFEE, F. H. (1975). Further studies of line emission from IO. Astrophys. J., in press. CARLSON, R. W., AND JUDGE, D. L. (1974). Pioneer 10 uv photometer observetions at Jupiter encounter. J. Geophys. Res. 79, 36233633. CHAMBERLAIN, J. W. (1961). Physics of the Aurora and Airglow, Academic Press, New York.
428
WEHINOER,
WYCKOBF,
MACY, W., AND TRAFTON, L. (1975). A model for IO’S atmosphere and sodium cloud. A&o&s. J. 199, in press. MECKLER, Y., AND EVIATAR, A. (1974). Spectroscopic observations
of 10. Astrophys.
J. 193,
L151-L152. MUNCH, G., Sodium
AND BEROSTR~,
D-line
emission
from
J.
T.
(1976).
IO:
Spatial
brightness distribution from multi&t
spectra.
Bull. Amer. Astron. Sot. 7, 386. OKE, J. B. (1965).
Photoelectric
spectrophoto-
metry of stars suitable for standards. phys. J. 140, 689-693.
Astro-
AND FROHLICH
ROACH, F. E. (1963). The aurora and trapped electrons. J. Gwphys. Res. 68, 1015-1021. TRAFTON, L., PARKINSON, T., AND MACY, W. (1974). The spatial extent of sodium emission around 10. Aatrophya. J. 199, L86-L89. TRAJ?TON, L., AND MACY, W. (1976). The geometry of IO’S sodium cloud. Bull. Amer. Astron. Sot. 7, 386. WEHINGER, P. A., AND WYCKOFF, S. (1974). Jupiter I. IAU Circ. No. 2701 WEHINGER, P. A., WYCKOFF, S., AND FFCOHLICH, A. (1975). The sodium emission cloud associated with 10 and Jupiter. Israel Whys. Sot. 21, 109.
Mapping
(1976)
of the Sodium Emission Associated with lo and Jupiter P. A. WEHINGER,
S. WYCKOFF,
AND
A. FROHLICH
Wise Observatory, Physics-Astronomy Department, Tel-Aviv University, Ramat-Aviv, Iwael Received September 23, 1975 The sodium D-lines are observed in emission in & disklike distribution surrounding 10 and extending outward in the orbital plane of the Galilean setellites to at least 23 R, from Jupiter. A so& length for the sodium emission cloud in the orbital plane and the thickness of the sodium disk perpendicular to the orbital plane are determined. Weak D-line emission is also detected over the poles of Jupiter. Estimates of the apparent emission rates are derived from microdensitometer scans of the spectrograms 8s & function of position in the satellite orbital plane and perpendicular to the orbital plane. No other emission lines were detected down to & limit of ~50R over the spectral range from 3500A to 9000A.
Region C, is an intense disklike cloud ( 20.3 to 5 2 kR) confined to the immediate vicinity of 10 ( 53 to 4 R, from 10) in its orbital plane; Region C, is an extensive envelope of very low intensity ( 20.05 to 0.3kR) observed in the satellite orbital plane outward from Jupiter to at least -23 R, on both sides of Jupiter when IO is at quadrature. The Na D emission intensity is greater on the side of Jupiter nearest 10 by a factor of 5 to 10 times compared to the same position at the opposite elongation at any given time.
After the discovery of the sodium D-lines in emission from 10 (Brown, 1974 ; Brown and Chaffee, 1974), an extended sodium emission cloud was detected at distances of 560arcsec ( 52.6 R,) from 10 (Trafton et al., 1974; Macy and Trafton, 1975; Trafton and Macy, 1975) using a photoelectric echelle coude scanner. Further evidence of the emission cloud was obtained by Miinch and Bergstralh (1975) with a phot,ographic multislit coud& spectrograph. The only other known emission associated with 10 and its environs is a partial Ly-cr torus exOBSERVATIONS tending over 6Odegrees of orbital phase either side of IO in the orbital plane with a Image-tube spectrograms were obtained thickness of less than 2 R, perpendicular to of the environs of 10 and its orbital plane the plane (Carlson and Judge, 1974). using the 40-inch (l-meter) reflector and Calibrated spectroscopic observations of Cassegrain spectrograph at the Wise the D-lines in emission have been secured Observatory. An ITT F4089 image tube which reach detection levels in the range (with an S-25 photocathode) was used with -0.05kR to 2kR in the environs of 10 and the spectrograph at dispersions of 15OA the satellite orbital plane. In order to map -’ covering the spectral region 5OOOA this weak emission, we have employed a E?OdoA with a resolution of 3.5A. Addifast image-tube Cassegrain spectrograph tional data were obtained to search for with a relatively small plate scale. Our obemission from other resonance lines at servations (Wehinger and Wyckoff, 1974; 2408mm-‘, covering the region 35OOA to Wehinger et al,, 1975) constitute the iden85OOA with a resolution of -78. The plate tification of Region C which was defined by scale perpendicular to dispersion was 150 Brown et al. (1975) as an area with D-line arcsecmm-‘. The maximum available slit emission intensities of less than -2kR. length was 12.5 arc min (corresponding to Our observations indicate that Region C 33 R, at the distance of Jupiter) and the can be subdivided into two components: projected slit width was 22microns (3arc Copyright 0 1976 by Academic Press, Inc. 425 All rights of reproduction in any form reserved. Printed in Great Britain
426
WERINCER,
WYCKOFF,
see). The observations, obtained between 1974 August 12 and November 21, were taken with the slit oriented parallel and perpendicular to the satellite orbital plane at various distances from 10 on both sides of Jupiter and also over Jupiter’s polar regions. The spectrograms were calibrated and scanned with a PDS microdensitometer at the Royal Greenwich Observatory. The instrumental sensitivity and atmospheric extinction as a function of wavelength were removed by a point-by-point division of the 10 spectra by an Oke (1965) spectrophotometric standarcl star, observed on the same night at the same zenith angle. Night airglow spectra, taken as a control at the same zenith angle immediately following the 10 observations, showed no trace of the night airglow D lines in 30 minute exposures. In much longer exposures (-OOmin), the night airglow D lines and the 63OOA [01] lines were marginally detectable. The intensity of the D lines associated with 10 and its environs was calibrated relative to the apparent emission rate of the night airglow [01] line at 5577A. The intensity of the 5577A line was assumed to be 250R, as given by Chamberlain (1961, p. 569) and Roach (1963). Diurnal variations of the 5577_.&line are of the order of +20%, according to Brenton and Silverman (1970). Since the 5577A line is formed high in the ionosphere (-lOOkm), to a first approximation, no air mass correction was necessary for observations made at moderate zenith angles ( 530 degrees) (cf. Chamberlain, 1961, p. 487). Intensities of the 5577A line and the D lines were compared from PDS scans of each spectrogram at the same position on the slit. Thus corrections for differences in the instrumental response as a function of wavelength and position along the slit were both accounted for in the reductions. Hence the D-line intensities were scaled to the intensity of the 5577A line taken to be 250 & 50R. SODIUM
EMISSION
REGIONS
Region C,. A set of observations, taken with the slit oriented perpendicular to the
AND FROHLICH
satellite orbital plane, at a series of positions either side of 10, showed evidence of the disklike cloud surrounding 10. This series of exposures was taken with the slit at successive positions from 2 R, to 15 R, from Jupiter surrounding 10 for a given quadrature. The observations show an intense ( 20.3 to 2kR) flattened disk of sodium emission extending from 223 to 5 10 R, from Jupiter (or 53 to 4 R, from 10). The extent of the disk appeared to be slightly asymmetric in shape in the sense that the disk extends outward farther in the orbital plane on the side away from Jupiter. In order to obtain a map of Region C, in a time short compared to the motion of 10 in its orbit, exposures of 6minutes each were taken. The most intense emission mapped in Region C, (at -1 R, from 10) is of approximately the same intensity as obtained photoelectrically by Trafton et al. (1974) at the ame distance from 10. We also found the suggestion of some slight asymmetry in the intensity perpendicular to the orbital plane in some of the spectra, although these effects require further analysis of the plate material. Region C,. In order to cover the full extent of the D-line emission in the Galilean satellite orbital plane (i.e. -30 R, east or west of Jupiter), separate spectra were taken. To intercompare the D-line intensities from one side of the orbit to the other side, additional spectra were taken with the slit centered on Jupiter, which was masked to reduce effects of scattered light. In this way, the emission in the orbital plane was recorded through two slit apertures in the region from 6.6 to 11.2 R, from Jupiter, on both sides of Jupiter. These spectra were taken when 10 was at quadrature, so that the D-line intensity near 10 (through one aperture) could be compared with the intensity at a point equidistant from Jupiter on the opposite side of the orbital plane (through the other aperture). The D-line intensity was -9 times greater on the side near 10 (~1.3 R, from 10) than it was at the opposite elongation -7.3 R, from Jupiter (or -1.3R, from IO’S orbit). For spectra taken with the slit in the orbital plane, the D lines decreased in intensity with increasing distance outward
427
IO SODIUM EMISSION MA.PPINU
from IO’S orbit. The scale length as a function of radial distance outward from 10 in the orbital plane was found to be -2.0 R,, where the scale length is defmed as the distance over which the intensity decreases by a factor e. Other spectrograms, taken with the slit oriented perpendicular to the orbital plane, show the sodium to be confined to a flattened layer with a thickness (perpendicular to the plane) of -4.5R, (FWHM) at -4 R, from 10 at east or west elongation (i.e. -10 R, from Jupiter). Over the range from -7 R, (-1 R, from 10) to 23 R, from Jupiter, the D-line intensity decreased from -2kR to -0.05kR. Beyond about 25 R, the emission level (--5OR) did not decrease significantly with increasing distance and so was probably indistinguishable from the night airglow sodium (-30R). As for other effects, no enhancement in the D-line emission (or in any other possible emission features) was detected near any of the other Galilean satellites. In addition, none of our spectra showed evidence of tilted D lines in contrast to the report by Mekler and Eviatar (1974). Above the polar regions of Jupiter itself weak D line emission was detected with the slit apertures located at 6.6 to 11.2 R, north and south of Jupiter. Intensities, averaged between the same position, north and south of Jupiter, were: 0.2lkR at 7.3R,, 0.2OkR at 8.7R,, and 0.17kR at lO.lR, (measured from Jupiter). It should be noted that special care was taken to mask off the slit aperture from Jupiter’s direct light, so that scattered light in the spectrograph was minimized. Also test spectra obtained in the vicinity of Jupiter indicate that night sky Na-D emission enhanced by preexposure effects due to scattered light from Jupiter on the over-the-pole spectra is probably negligible. SEARCH FOR OTHER EMISSIONLINES Additional spectrograms at lower dispersion (24OAmm-‘) which covered the spectral range 3500A to 8500& and selected long exposures (45min) at 150A mm-’ covering 50008 to 90008, were obtained for the purpose of searching for 1.5
emission from resonance lines of other abundant elements. These spectrograms were taken in the same manner as those described above, and were of those regions where the D-line emission was most intense (i.e. Region C,). However, no evidence was found for emission in CaI 42268 [contrary to the results of Meckler and Eviatar (1974), but in agreement with Brown et al. (1975)] ; nor was evidence found for the presence of KI 7699A and the Al1 3961A lines (to a limit of -5OR). No emission lines other than the sodium D lines were detected in the range from 35008 to 90008. These detection limits thus set further stringent bounds on any hypothetical surface composition for materials which might be sputtered from IO.
ACKNOWLEDGMENTS This research is supported in pert by the Smithsonian Research Foundation through Grant SFC-0-3005. We are most grateful to the Royal Greenwich Observatory for the use of their PDS microdensitometer and for their kind hospitality. Travel support provided by NASA and NSF is acknowledged with special thanks. Discussions with J. T. Bergstralh, R. A. Brown, and D. L. Matson were most helpful and constructive.
REFERENCES BRENTON, J. G., AND SILVERMAN, S. M. (1970). A study of the diurnal variation of the 5577A [01] airglow emission at selected IGY stations. Plartetary Space Sci. 18,641-653. BROW, R. A. (1974). Optical line emission from IO. In Exploration of the Planetary System (A. Woszcyk and C. Iwaniszewska, Eds.), pp. 527-531. Reidel, Dordrecht. BROW, R. A., AND CHAFFEE, F. H. (1974). High-resolution spectra of sodium emission from 10. Astrophya. J. 187, L125-L126. BROWN, R. A., GOODY, R. M., MURCRAY, F. J., AND CEAXFEE, F. H. (1975). Further studies of line emission from IO. Astrophys. J., in press. CARLSON, R. W., AND JUDGE, D. L. (1974). Pioneer 10 uv photometer observetions at Jupiter encounter. J. Geophys. Res. 79, 36233633. CHAMBERLAIN, J. W. (1961). Physics of the Aurora and Airglow, Academic Press, New York.
428
WEHINOER,
WYCKOBF,
MACY, W., AND TRAFTON, L. (1975). A model for IO’S atmosphere and sodium cloud. A&o&s. J. 199, in press. MECKLER, Y., AND EVIATAR, A. (1974). Spectroscopic observations
of 10. Astrophys.
J. 193,
L151-L152. MUNCH, G., Sodium
AND BEROSTR~,
D-line
emission
from
J.
T.
(1976).
IO:
Spatial
brightness distribution from multi&t
spectra.
Bull. Amer. Astron. Sot. 7, 386. OKE, J. B. (1965).
Photoelectric
spectrophoto-
metry of stars suitable for standards. phys. J. 140, 689-693.
Astro-
AND FROHLICH
ROACH, F. E. (1963). The aurora and trapped electrons. J. Gwphys. Res. 68, 1015-1021. TRAFTON, L., PARKINSON, T., AND MACY, W. (1974). The spatial extent of sodium emission around 10. Aatrophya. J. 199, L86-L89. TRAJ?TON, L., AND MACY, W. (1976). The geometry of IO’S sodium cloud. Bull. Amer. Astron. Sot. 7, 386. WEHINGER, P. A., AND WYCKOFF, S. (1974). Jupiter I. IAU Circ. No. 2701 WEHINGER, P. A., WYCKOFF, S., AND FFCOHLICH, A. (1975). The sodium emission cloud associated with 10 and Jupiter. Israel Whys. Sot. 21, 109.