R EPORTS instability in the resistance, which we have 14. Point-contact measurements of Cu/Co multilayers Foundations, the Office of Naval Research (N00014- associated with excitation of the thin-layer mo- have previously shown small GMR values; see (6) and 97-1-0745), and Defense Advanced Research Projects M. V. Tsoi, A. G. M. Jansen, J. Bass, J. Appl. Phys. 81, Agency, and was performed in part at the Cornell ment relative to the thick-layer moment, does 5530 (1997). node of the National Nanofabrication Users Net- have an approximately linear dependence ver- 15. We thank J. Slonczewski, S. Gue«ron, and R. H. Silsbee work, supported by the NSF. E.B.M. was supported by sus H for H greater than the saturation field, for discussions. The work was supported by the NSF a U.S. Department of Education grant. Materials Research Science and Engineering Centers although on a fine scale, there are deviations not program (DMR-9632275), the Sloan and Packard 15 April 1999; accepted 22 June 1999 observed in previous studies (Fig. 3) (7, 9). Subsequent transitions at higher bias, which we have associated with magnetic excitations with- in the Co layers, show more complicated be- Galileo Imaging of Atmospheric havior. The measured zero-field intercepts of Icrit for the first transition are generally a factor Emissions from Io of 2 to 4 lower than those derived from Eq. 2. We consider this satisfactory agreement, P. E. Geissler,1* A. S. McEwen,1 W. Ip,2 M. J. S. Belton,3 since the layers undoubtedly have nonuni- T. V. Johnson,4 W. H. Smyth,5 A. P. Ingersoll6 form thickness. The observed slopes of Icrit versus H also agree with Eq. 2, provided that The Galileo spacecraft has detected diffuse optical emissions from Io in high- we assume a value for the phenomenological resolution images acquired while the satellite was eclipsed by Jupiter. Three ␣ ϭ damping parameter G 0.05 to 0.2, which is distinct components make up Io’s visible emissions. Bright blue glows of more unusually large compared to more macroscopic than 300 kilorayleighs emanate from volcanic plumes, probably due to electron samples (5). However, we are in a new regime, impact on molecular sulfur dioxide. Weaker red emissions, possibly due to and there may be large contributions to damp- atomic oxygen, are seen along the limbs, brighter on the pole closest to the ing near normal/ferromagnetic interfaces (3) plasma torus. A faint green glow appears concentrated on the night side of Io, and from intralayer processes for nanometer- possibly produced by atomic sodium. Io’s disk-averaged emission diminishes scale domains. with time after entering eclipse, whereas the localized blue glows brighten Our results have both good and bad im- instead. plications for applications. The bad news is that the existence of magnetic switching Previous spacecraft and ground-based obser- 10 orbits. Recorded partly to monitor thermal caused by spin transfer places a limit on the vations have yielded several indications of a emission from discrete volcanic centers (11), current (and therefore the signal levels) that tenuous atmosphere on Io. Dominated by these observations provide a detailed look can be used for measuring GMR devices. The SO2 and its dissociation products SO, O, and at visible aurorae on a solar system satellite. good news is that the spin-switching effect S, Io’s atmosphere has been studied at wave- The bulk of the data was acquired with the SSI may enable magnetic random-access memo- lengths ranging from the microwave to the clear filter, which covers wavelengths between ries in which the memory elements are con- ultraviolet (UV) (1, 2). Atomic O, S, Na, and 380 and 1040 nm (12). Two sequences included trolled by local exchange-effect forces rather K have been detected in extended neutral visible color imaging using the SSI violet (380 than by long-range magnetic fields. Slonc- clouds escaping from the satellite (3), and to 445 nm), green (510 to 605 nm), and red zewski predicts that the spin-transfer torques recent Hubble Space Telescope (HST) obser- (615 to 710 nm) filters. Diffuse emissions from should dominate over the effects of the self- vations of Io have imaged intense auroral Io have not been detected in any of the longer magnetic fields from flowing currents for emissions at far-UV wavelengths (4). Visible wavelength infrared SSI filters. Here, we de- devices up to about 100 nm in diameter (2), emissions from Io during eclipse by Jupiter scribe the morphology of the optical emis- so that the point-contact geometry, with its were seen by Voyager 1 (5) and suggested to sions from Io, estimate their brightnesses intrinsically low GMR values, should not be be due to molecular SO2 (6). HST (7) and and radiated powers, and suggest possible necessary to employ the effect. ground-based (8, 9) eclipse observations have interpretations. detected neutral O and Na emissions from Io The most complete set of eclipse images References and Notes at visible wavelengths. was acquired on 31 May 1998 during the first 1. For reviews of the science and applications of GMR The Galileo spacecraft, in orbit around Ju- of two eclipses in orbit E15 (13) (Table 1). materials, see the collection of articles in IBM J. Res. piter since December 1995, can observe optical These pictures were centered near a longitude Dev. 42, ( January 1998). (available electronically at www.research.ibm.com/journal/rd42-1.html) emissions from Io at a higher spatial resolution of 70°W, on the orbital leading hemisphere of 2. J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996). than previously possible and from a variety of Io that is also the location of the plasma wake. 3. L. Berger, Phys. Rev. B 54, 9353 (1996). perspectives unattainable from Earth (10). Dif- One set of color images in the violet, green, red, 4. Ya. B. Bazaliy, B. A. Jones, S.-C. Zhang, ibid. 57, R3213 ␮ (1998). fuse emissions from Io have been seen in 16 and 1- m (935 to 1090 nm) filters was taken 5. J. Slonczewski, J. Magn. Magn. Mater. 195, L261 distinct solid state imager (SSI) observations along with two clear-filter pictures comparing (1999). acquired during 14 eclipses over the course of Io’s appearance 11 min after the start of the 6. R. N. Louie, thesis, Cornell University, Ithaca, NY (1997). eclipse with its appearance 42 min later. The 7. M. Tsoi et al., Phys. Rev. Lett. 80, 4281 (1998); 1Lunar and Planetary Laboratory, University of Arizo- clear-filter images showed diffuse atmospheric erratum, ibid. 81, 493 (1998). na, Tucson, AZ 85711, USA. 2Institute of Astronomy, emissions as well as discrete volcanic hot spots 8. J.-E. Wegrowe et al., Europhys. Lett. 45, 626 (1999). National Central University, Chung-Li, Taiwan 320, 3 on Io’s leading hemisphere, whereas only the 9. J. Z. Sun, J. Magn. Magn. Mater. 202, 157 (1999). Republic of China. National Optical Astronomy Ob- ␮ 10. K. S. Ralls, R. A. Buhrman, R. C. Tiberio, Appl. Phys. servatories, Tucson, AZ 85719, USA. 4Jet Propulsion hot spots were visible in the 1- m filter. Dif- Lett. 55, 2459 (1989). Laboratory, MS 23-201B, 4800 Oak Grove Drive, Pas- fuse emissions with three distinct distributions 11. A. G. M. Jansen, A. P. van Gelder, P. Wyder, J. Phys. C adena, CA 91109, USA. 5Atmospheric and Environ- were seen in the visible color frames (Figs. 1 13, 6073 (1980). mental Research, 840 Memorial Drive, Cambridge, MA and 2A). The brightest emissions were blue 12. Fig. 2, B through E, exhibits samples never exposed to 02139, USA. 6Division of Geology and Planetary Sci- a magnetic field, but there are no qualitative differ- ences, California Institute of Technology, Pasadena, glows close to the equator near the sub- and ences as a function of magnetic history, as long as CA 91125, USA. anti-Jupiter points, extending several hundred H ϭ 0. 13. For a review, see M. A. M. Gijs and G. E. W. Bauer, *To whom correspondence should be addressed. E- kilometers above the limb. They were seen at Adv. Phys. 46, 285 (1997). mail: [email protected] red, green, and violet wavelengths but were 870 6 AUGUST 1999 VOL 285 SCIENCE www.sciencemag.org R EPORTS brightest in the violet bandpass. A second, amount of light scattered by Jupiter’s atmo- orbits G7 and G8 (Fig. 2B) and found that the weaker glow running continuously along the sphere as the satellite moved deeper into the pole with the brightest limb emission alter- limb was seen primarily in the red-filter image planet’s shadow, but the dimming is apparent nated between the north and south. The G7 and was particularly bright along the north po- even on the side of Io facing away from Jupiter. and G8 observations were made while Io was lar limb. The third component was a faint glow More surprisingly, the localized equatorial at System III magnetic longitudes of 165° and against the disk of Io at green-filter wave- plume glows appeared to brighten by 37% over 261°, respectively. At these locations, the lengths. In the E15 observations, this green the same time interval. A similar reduction in center of the plasma torus was to the south of glow was concentrated on the night side of Io. Io’s total radiance with elapsed time in Jupiter’s Io, on the same side as the brighter polar limb Qualitatively similar emission distributions shadow was seen during the second eclipse of glows. Similarly, the limb glow during E15 were seen in the noisier color eclipse data from orbit E15, but quantitative measurements can- (magnetic longitude of 72°) is brighter in the orbit G7, except that the red polar limb glow not be derived from the noisy and badly north, the same side of Io as the plasma torus was brighter in the southern hemisphere than in smeared images.