A Study of the Stability of Jovian Zonal Winds from HST Images: 1995–2000

Icarus 152, 316–330 (2001) doi:10.1006/icar.2001.6646, available online at http://www.idealibrary.com on

A Study of the Stability of Jovian Zonal Winds from HST Images: 1995–2000 E. Garc´ıa-Melendo Observatori Esteve Duran, c/Montseny 46, 08553 Seva, Barcelona, Spain; and Departamento de F´ısica Aplicada, Universitat Polit´ecnica de Catalunya, 08034 Barcelona, Spain

and A. S´anchez-Lavega Departamento F´ısica Aplicada I, E.T.S. Ingenieros, Universidad del Pa´ıs Vasco, Alda. Urquijo s/n, 48013 Bilbao, Spain E-mail: [email protected] Received December 18, 2000; revised April 6, 2001

We present a five-year study of the temporal stability of Jupiter’s zonal wind velocity profile at cloud level based on Hubble Space Telescope images obtained from 1995 to 2000. We used the correlation of east–west albedo scans in pairs of images separated by one jovian rotation to retrieve the zonal winds. The resolution in the HST images ranged from 140 to 190 km pixel−1 . Independent measurements of the motions of individual cloud tracers were used to control the above profiles. Three wavelengths were used in this analysis: 410 nm (violet continuum), 892 nm (methane absorption band) and 953 nm (red continuum). Our study indicates that, globally, Jupiter’s zonal flow did not change during this five-year interval and it does not depend on the observed wavelengths. When comparing our mean profile with that obtained 16 years ago using Voyager data, we find a reasonable agreement. We note, however, a small shift in the latitude of the jets poleward of latitude ±30◦ , probably due to image navigation uncertainties. However, true changes in the intensity of the jets at 24◦ N and 32◦ N are found. Results also indicate that the 7◦ S jet has remained stable since the Voyager era. In addition, we extend in this paper our previous study of the 24◦ N with new data from year 2000. The new data confirm the jet’s stability during the 1995–2000 period. Finally we present the average profile for this period, including error estimation, in a table that extends the latitudinal coverage of Limaye’s (1986, Icarus 65, 335–352) Voyager profile up to latitudes ∼68◦ S and 77◦ N. c 2001 Academic Press °

Key Words: Jupiter; atmosphere; dynamics; meteorology.

1. INTRODUCTION

One of the most important challenges related to the dynamics of jovian atmospheres is to understand the origin of the observed zonal wind flow at the cloud-top level. Any satisfactory theory will undoubtedly shed light on many aspects of Jupiter such as the internal structure of the planet and its meteorology. But any

theory must rely on accurate knowledge of the three-dimensional motions and related temporal changes. The first precise measurements of the zonal winds come with the wealth of information supplied by the Voyager missions in 1979. Voyager 1 and 2 imagery allowed the determination of Jupiter’s zonal wind profile over a four-month interval (e.g., Ingersoll et al. 1979, 1981, Limaye et al. 1982, Limaye 1986, 1989, Magalhaes et al. 1990), the time span that separated the two Voyager fly-bys. These results matched quite well the zonal wind distribution suggested by ground-based observations (Smith and Hunt 1976, Rogers 1995). Most of our knowledge of Jupiter’s global circulation at the cloud deck is based on the Voyager results, which constitute just a snapshot of the possible time-variable phenomena. Because of the importance of this first set of accurate data, much effort has been made to establish a reliable zonal wind profile as well as to monitor possible changes in it. This interest has increased since it became possible to analyze new medium- and high-resolution images supplied by the Hubble Space Telescope (HST) and the Galileo spacecraft. An example of this is the ∼24◦ N jet on the south edge of the North Temperate Belt (NTB), which is the strongest jovian jet at cloud level. Maxworthy (1984) reanalysed Voyager images to establish that this jet had a peak velocity of ∼180 ms−1 . Other results (Magalhaes et al. 1990) pointed in this direction, but there was no general agreement with the results of other authors who obtained slower peak velocities (Ingersoll et al. 1979, 1981, Limaye et al. 1982, Limaye 1986, 1989, Beebe and Hockey 1986). Important morphology changes in the region since the Voyager encounters (Sanchez-Lavega and Quesada 1988, Sanchez-Lavega et al. 1991), monitored recently by the HST, prompted new studies. Simon (1999) reviews Voyager data and obtains a new Voyager zonal wind profile, which she compares to the one measured from the analysis of cloud tracers performed on the October 1995

316 0021-9991/01 $35.00 c 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.

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HST 953-nm images; she inferred that the ∼24◦ N jet receded from ∼180 to ∼140 ms−1 over this period. Garcia-Melendo et al. (2000) concluded that the lower speed has persisted during the entire time HST has been monitoring the planet up to 1998. In fact, the present work shows that the 140 ms−1 speed was present through September 2000, just three months before the Cassini encounter. One of the most complete analyses performed on the stability of the average zonal winds is that of Limaye (1989), who in a systematic fashion measured the zonal wind profile for 111 rotations from Voyager 1 data, and 134 rotations from Voyager 2. He determined variations of over 10 ms−1 in some regions of the planet, although his results show a strong stability for the latitudinal location of the jet stream structure. These wind variations could not unambiguously be related to altitude or temporal changes. From 1995 to 2000, the resolution of the best Wide Field Planetary Camera 2 (WFPC2) HST Jupiter images was ∼150 km pix−1 , which is close to the resolution of ∼160 km pix−1 of the Voyager 1 and 2 images used by Limaye (1989) to measure winds. The similarity of resolutions encouraged the use of correlation techniques to obtain a series of high-resolution zonal wind profiles because comparisons with Voyager-derived winds are likely to be robust. This approach provided a unique opportunity to study the jovian winds during a time span of 5 years, which is about 15 times longer than the 4-month lapse between the two Voyager fly-bys in 1979, 16 years earlier. These data characterize the state of the global circulation a few months before the Cassini encounter at the end of 2000. A total of seven wind profiles from October 5, 1995 to September 2, 2000, in the 410-nm, 889-nm, and 953-nm bands, were obtained from images spanning limited longitude intervals. Based on these data, we present a study of the variability of Jupiter’s zonal jets (location and intensity) during the five-year analysis (1995–2000). One of the most conspicuous results is the strong stability displayed by the zonal flow during the five-year span, although some jet streams showed important alterations in shape and strength, which may be caused by longitudinal or temporal variations in the jet structure. These changes allow us to revisit an open question in the study of zonal flow, namely, its possible dependence on cloud morphology. We also explore the presence of winds at higher latitudes than previously searched (up to 77◦ N and 68◦ S in northern and southern hemispheres, respectively). Our results show the presence of two previously unknown jets over 60◦ N and an additional one to the south of 70◦ S. Wind profiles obtained at various wavelengths were also analyzed, suggesting a negligible influence of a possible color (height) effect. Comparing the HST wind profiles with the Voyager 2 profile obtained by Limaye (1986) in violet light, we note slight latitude discrepancies (maximum 1.5◦ ) for the location of some westerly and easterly jet streams, especially in the planet’s northern hemisphere. We discuss possible causes of this effect. Finally, we supply a reference average zonal wind profile for use in future comparisons with Cassini data.

2. OBSERVATIONS AND ANALYSIS

2.1. Data and Reduction Procedures From 1995 to 1998 we analysed CCD (Charge Coupled Device) frames from the HST imagery archive taken with the WFPC2. Additional observing time was obtained in September 2000. Table I summarizes the log of observations. In all image sets, frames showing the same longitudinal sector were selected in order to obtain longitude albedo scans and perform correlation calculations. This procedure yielded pairs of images separated by one jovian rotation, or about 10 h. Profiles were also calculated from pairs of images taken with F410M (410-nm), FQCH4N (892-nm), and F953N (953-nm) filters. To illustrate Jupiter’s cloud morphology during the 1995–1998 period, Fig. 1 shows four cylindrical projections of Jupiter’s visible cloud deck in the 410-nm band from 1995 through 1998. As a reduction tool we used LAIA software (Cano 1998) which works on a PC platform. Details on how the albedo scans were obtained for the measurement of the mean zonal flow using the correlation approach as well as the navigation procedures for cloud tracking can be found in Garc´ıa-Melendo et al. (2000). In the case of the correlation approach, the latitudinal interval covered by the calculations was determined by the way the planet was framed by the WFPC2, and marginally by the tilt of Jupiter’s rotation axis towards Earth. It also determined the covered longitudinal interval, which ranged between 80 and 120◦ . In some cases, northern and southern hemispheres were both completely imaged, whereas in others only the tropical and northern regions were framed on the CCD chip. Overall, the global latitudinal interval covers the region from ∼70S to ∼80N. As a correlation function we used the expression Rfg ( j) =

N X

f(i)g(i − j),

(1)

i=1

where f(i) and g(i) are two albedo scans performed at the same latitude separated by one Jupiter rotation. Our “best match” criterion was given by the value of j for which Rfg ( j) was maximum. It can be shown (see the Appendix) that Rfg ( j) is TABLE I Observation Log for HST Selected Images

Date

Pixel resolution (km pixel−1 )

Number of individual wind profiles

Filters (nm)

Tilt of Jupiter’s North pole (degrees)

October 5, 1995 May 14, 1996 October 21, 1996 April 4, 1997 June 25, 1997 July 16, 1998 September 2, 2000

190 150 180 180 140 150 160

3 3 2 2 3 5 1

410, 892, 953 410, 953 410, 953 410, 953 410, 953 410, 953 953

−2.63 −1.76 −1.60 −0.32 0.20 2.11 3.11

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FIG. 1. Cylindrical projections of Jupiter’s visible cloud deck in the 410-nm band for the following HST observing opportunities: (1) October 5, 1995, (2) October 21, 1996, (3) November 6, 1997, and (4) July 16, 1998. They illustrate the status of Jupiter’s cloud morphology when mean zonal profiles were computed.

maximum for j = ±1λ (the ± sign depends on the planet’s orientation; the plus sign is for north up), where the zonal translation 1λ = λ2 − λ1 (λ is given in degrees, and the subscripts “1” and “2” refer to the first and second HST frames separated by a complete jovian rotation) is measured in any standard rotation system (I or II), and compensated according to the central meridian displacement 1CM = CM2 − CM1 between the two images in the pair. Finally, the mean zonal velocity hui is computed taking as a reference System III (Davies et al. 1986). As a control of our correlation results, we performed complementary wind measurements by means of cloud tracking of individual elements. Fundamental parameters to be chosen to navigate HST images are the equatorial and polar radii. For this work, the authors chose Re = 71398 km and Rp = 67040 km as given by Davies et al. (1986). These two values allow the transformation between planetocentric (θ ) and planetographic (ϕ) latitudes through the relation µ tan(ϕ) =

Re Rp

¶2 tan(θ).

(2)

In this work, all the latitude values, which appear in the text and figures, are planetographic, except in Fig. 1 and Table III, where planetocentric values are also included. 2.2. Influence of Meteorology on Zonal Wind Measurements When correlation measurements are performed on limited longitude intervals, there could be variability in the zonal wind measurements depending on the extent and intensity of the motions related to the meteorological features. These phenomena can locally change the winds, leading to fluctuations in the reflectivity correlation that could influence the velocity measurement along a latitude circle. This can lead to apparent temporal changes in the profile if comparisons are made for independent single profiles. However, this variability is smoothed out when averages are taken for the various longitude sectors over several years, as in this study. Let us discuss the following kinds of features that represent a problem in the correlation data (see Fig. 2): (a) Convective expanding clouds showing rapid divergent motions. The component of the growth velocity along the latitude circle may mask the true flow motion (Fig. 2, part 1).

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FIG. 1—Continued

(b) Wave-like phenomena where phase velocities can be measured instead of flow motions (Fig. 2, part 2). (c) The large closed anticyclone vortices have two effects on zonal wind measurements. The first is related to their own speed relative to the mean flow (since they do not act as passive tracers of the zonal flow). The second is related to the meanders they induce in their north and south flanks and in their wakes. This occurs in the vicinity of the Great Red Spot and White Ovals. Moreover, the vortex tangential velocity along the southern and northern edges of the vortices could be confused with the true zonal velocity at this latitude (Fig. 2, part 3). (d) Longitudinally long cyclonic cells, in particular in the South Temperate Belt, are another source of errors, since their internal flow superimposes on the surrounding one. These systems have a pattern with many individual “white” cloud tracers aligned in the zonal direction at both the southern and northern edges, so they influence the correlation notoriously. We noted evidence of this in our measurements of winds in the 27◦ S to 32◦ S region during 1997. Conspicuous variability is also found for other southern jets such as the 36◦ S eastward and 46◦ S westward jets where cyclonic systems were prominent. In general

we noted that, during the period analyzed, the south hemisphere presented a higher degree of organization in long cyclonic cells than the north hemisphere (Fig. 2, part 4).

3. RESULTS

3.1. Zonal Wind Profiles at Different Wavelengths Several authors have mentioned the possibility of detecting wind motions at different altitudes using images obtained at various wavelengths that sense different pressure levels. For example, Banfield et al. (1996) and Sanchez-Lavega et al. (1998a) used images in the methane band filters to track the high-altitude motions of the debris left by the SL9 impact. The methaneabsorbing band filter (FQCH4N), operating at an effective wavelength of 892 nm is useful for probing the 300-mb level, whereas the filters F953N (953 nm) and F410M (410 nm) are most sensitive to the cloud tops (Banfield et al. 1998). In fact, the thermal wind relation predicts a vertical shear of the horizontal flow when a horizontal temperature gradient (at constant pressure) is present and geostrophic balance is

FIG. 2. Illustration of the influence of meteorological phenomena on zonal wind measurements. In all idealizations, a given cloud morphology is represented at times t1 and t2 (t2 > t1 ) along with a longitudinal albedo scan, where the peaks on the albedo scans are also depicted for some tracers. (1) Convective expanding clouds showing rapid divergent motions. ±u0 is the spurious growth velocity added to the zonal value hui. (2) Wave-like phenomena where a spurious phase velocity ±c is added to hui. (3) Large closed anticyclonic vortices with their own relative speed ±V. A tracer with relative tangential velocity VT is also represented. (4) Longitudinally long cyclonic cells (C). Some real examples of these features are also shown. Panels 4a and 4b illustrate the effect of large cyclonic cells in zonal wind measurements: panel 4a represents the STB region in the HST image at 410-nm used to compute the June 1997 zonal flow. Horizontal lines represent, from top to bottom, the 32◦ S, 29.1◦ S, 26.4◦ S, and 20◦ S jets. The measured zonal winds are superimposed on the right side. Panel 4b is the STB region in the HST image at 410 nm used to compute the October 1995 zonal flow. White horizontal lines represent, from top to bottom, the 32◦ S, 26.4◦ S, and 20◦ S jets. It can be seen that the presence of cyclonic regions in panel 4a distorts the measured zonal winds. 320

STABILITY OF JOVIAN ZONAL WINDS

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considered (Holton 1992). ∂u R ∂T = , ∂ ln P f ∂y

(3)

where R is the gas constant, f is the Coriolis parameter, T is the temperature, y is the northward distance, u is the zonal wind, and P is the pressure. Based on infrared data supplied by the Voyager IRIS experiment and the use of the thermal wind equation, Gierasch et al. (1986) indicated that the winds decay with height in Jupiter’s upper troposphere. Some differences between the orange and violet zonal wind profiles, calculated by Limaye (1986) and Magalhaes et al. (1990), suggested the possibility of directly detecting this effect by cloud motions. More recently, Simon (1999), using the NICMOS F212N and F273N filters and the FQCH4N filter, was not able to detect significant velocity differences between tracers taken in these various infrared bands. In fact, though not related to measurements dominated by translation rates of cloud tops as is the case in this work, the Doppler wind experiment performed by the Galileo Probe showed that indeed there is a wind decay from the 4-bar level to the tropopause of almost 100 ms−1 ; winds remain constant for pressures exceeding 4 bar (Atkinson et al. 1998). In order to address this effect, we obtained profiles in the 410-nm, 953-nm, and 892-nm filters spanning a latitudinal interval between ∼60◦ S and ∼50◦ N from the HST imagery archive obtained on 5 October 1995. Figure 3 shows the three measured profiles represented separately to allow comparison of their shape. Aside from some differences in the peak velocity of the jet located at 46◦ N, no significant differences are visible beyond the data scatter of ±6 ms−1 . A better comparison of the three profiles is shown in Fig. 4a, which represents their superposition, and Fig. 4b, which represents the wind velocity difference δu between the 410-nm and the 892-nm profile. This difference shows a velocity scatter σ (δu) ∼7.7 m−1 . For random data with Gaussian statistics there is a 0.95 probability for δu ≤ |2σ |. Figure 4b then suggests that there are not statistically significant differences between the 410-nm and 892-nm profiles. It illustrates what can be seen in Fig. 4a, i.e., that there are no clear velocity differences. Most probably we are looking at tracers located at similar cloud levels with these three filters, in agreement with Simon’s (1999) results, or that the vertical variation of the zonal wind between these levels is below our detection limit.

FIG. 3. Three averaged zonal wind profiles at various wavelengths (410 nm, 953 nm, and 892 nm), computed by using the correlation method on HST images taken on October 5, 1995.

3.2. The HST Profiles: 1995–2000 As mentioned in the Introduction, Limaye (1989) performed a study of the zonal flow temporal variability using Voyager 1 and 2 data. His research covered a time span of only 5.5 months, but he was able to analyze over 250 rotations. he claimed that changes over 10 ms−1 were detected. In the present study the situation is exactly the opposite: the zonal flow has been monitored during a time interval of five years, from October 1995

FIG. 4. (top) Superposition of the three average zonal wind profiles measured on HST images taken on October 5, 1995 (they are shown individually in Fig. 3). (bottom) Velocity difference (δu) between the 410-nm and 892-nm profiles. The average overall scatter is ∼7.7 ms−1 . Gray lines mark the ±2σ value around the zero mean (∼15 ms−1 ).

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to September 2000, but only for one rotation, or at most two, in every instance. In this case the goal was to detect changes over a longer time scale (almost half a Jupiter year) and compare the profiles with that of the Voyager era at the same resolution. Starting with 1995, an average zonal profile was computed for each date (except 1999) where images separated by a jovian rotation were obtained by using latitude bins of 0.1 degree, a value close to the maximum HST resolution on Jupiter and equal to the sampling interval of albedo scans. Taking into account that the three different filters did not make any difference in the measured profile, all measurements were given equal weight. We used the expression PN(ϕ) hu(ϕ)i =

ui (ϕ) , N(ϕ)

i=1

(4)

where N(ϕ) is the number of data points in the current bin and ui (ϕ) is the individual velocity of every point within the bin. The total number of points for each profile for a given data ranged between 1500 and 4500, so it was possible to obtain smooth profiles from almost all of them. In the special case of September 2000 only one profile could be computed. The uncertainty for every profile was estimated as (Bevington 1969) s σ (u) =

PN

− hu(ϕi )i)2 , N−1

i=1 (u(ϕi )

(5)

where in this case N is the total number of data points which make up the profile, u(ϕi ) is the individual velocity measurement for latitude ϕi , and hu(ϕi )i is the average value for that latitude. In Table II we show the number of data points for each data set, and the σ scatter. Figure 5 shows individually all the averaged wind profiles from October 1995 to July 1998. They show that globally there is fairly good stability in the latitude location of the eastward and westward jet streams. However, we note that there are some differences in the shape and strength of some of the jet streams. To compare the measured profiles, some differences among them

TABLE II HST Observing Dates and Properties Date

Number of data points

Latitude interval

σ (ms−1 )

October 5, 1995 May 14, 1996 October 21, 1996 April 4, 1997 June 25, 1997 July 16, 1998 September 2, 2000

2290 2302 2395 1481 3452 4476 1124

66S–53N 17S–72N 68S–68N 11S–77N 68S–67N 54S–72N 53S–65N

2.9 3.6 2.3 4.4 3.6 6.0 3.0

Note. This table shows the computed data sets. The number of data points is the total number of correlations computed for every date in all the filters, and zonal wind profiles specified in Table I.

were computed. Figure 6a is the difference between the June 1997 and October 1995 profiles, and Fig. 6b is the difference between July 1998 and October 1995. Figures 6c and 6d are differences between April 1997 and May 1996, and July 1998 and May 1996, respectively. The standard deviation for the southern hemisphere differences are 8.3 m/s and 9.6 m/s (in Figs. 6a and 6b, respectively), whereas the data scatter for the northern hemisphere is 6.0 m/s, 6.5 m/s, 5.2 m/s, and 6.5 m/s (for Figs. 6a, 6b, 6c, and 6d, respectively). In addition, we note changes at ∼37◦ S, ∼10◦ S, and ∼10◦ N, which go beyond the ±2σ scatter and are present in all computed differences. Finally, an interesting variation is the sequence of shape changes of the ∼27◦ S jet and the ∼32◦ S jet strength. In October 1995 and July 1998 the ∼27◦ S jet appears as a single peak feature, whereas in June 1997 it has split into two smaller peaks (see Fig. 5). This feature was unambiguously present in the three measured profiles at 410 nm and 953 nm beyond the data scatter of every individual profile. Most of these phenomena can be interpreted within the framework of longitudinal changes, due to the influence of various meteorological features, as discussed in Section 2.2. In particular, the double peak measured at the ∼27◦ S jet is due to the presence of long cyclonic regions, as explained in Section 2.2. 3.3. The Average 1995–1998 Wind Profile The stability of the zonal flow during the period 1995–1998 justifies the calculation of a mean profile for this period. This new profile can serve as a reference for future study of longterm changes, which is especially timely because of the Cassini encounter with Jupiter at the end of 2000. To obtain a meaningful estimation of data scatter, an average zonal wind profile was computed using the individual profiles listed in Table I, which resulted in a preliminary data set with over 15,000 useful points, which was afterwards split into 0.3◦ bins. This bin size gave a reasonable number of samples at about twice the maximum HST resolution. In every bin, zonal velocities hui and latitudes were averaged (i.e., the “centre of mass” of every bin was computed), and the error given by the scatter of the points was estimated according to (5). This approach yielded (latitude, hui) points which were not separated exactly by 0.3 degree intervals but by a variable latitude interval. The estimated error represents an upper bound to any change, since it includes not only true random data scatter, but also possible small variations in longitude or changes in time. Any important deviations of future zonal wind flow measurements will most probably suggest real changes in the profile. Table III is a list of the individual values of the average zonal flow as a function of planetographic and planetocentric latitudes along with the estimated error. This new profile covers the entire interval between ∼70◦ S and ∼80◦ N, showing two new jet streams at 63◦ N and 68◦ N and another one at 67◦ S which were marginally detected in previous studies (Sanchez-Lavega et al. 1998b, Vincent et al. 2000). Also, and as a consistency check and control, another profile was determined by the individual position measurements on

STABILITY OF JOVIAN ZONAL WINDS

FIG. 5.

Temporal series of average zonal wind profiles. The observational data for each run are presented in Tables I and II.

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TABLE III Averaged Mean Zonal Flow from Hubble Space Telescope Images: 1995–1998

FIG. 6. Temporal differences between Jupiter’s wind profiles. (a) Difference between the June 25, 1997 and October 5, 1995 profiles. (b) Difference profile between those from July 16, 1998 and October 5, 1995. (c) and (d) Differences between the April 4, 1997 and May 14, 1996, and the July 16, 1998 and May 14, 1996 profiles, respectively. Horizontal gray lines mark the data scatter at the ±2σ level.

cloud tracers, using pairs of HST frames taken one or two rotations apart with the F953N filter. The selection and measurement procedures are detailed in Garcia-Melendo et al. (2000). Using the mean profile obtained by correlation methods, we obtain that the estimated error of the cloud tracking profile is 11.6 ms−1 . Figure 7 depicts both profiles. 3.4. Comparison of the HST 1995–1998 Profile to the Voyager Profile When comparing the Voyager and HST profiles it should be noted that any difference greater than that indicated by the σ scatter encompassing each set of mean zonal wind velocities could be attributed to real changes in the profile between the two periods. The reason is that since our HST data are averages of

ϕ (deg)

θ (deg)

−67.8 −67.5 −67.2 −66.9 −66.6 −66.3 −66.0 −65.7 −65.5 −65.1 −64.8 −64.5 −64.2 −63.9 −63.6 −63.3 −63.0 −62.8 −62.4 −62.1 −61.8 −61.6 −61.2 −60.9 −60.6 −60.3 −60.0 −59.8 −59.4 −59.1 −58.8 −58.5 −58.2 −58.0 −57.6 −57.3 −57.0 −56.7 −56.4 −56.1 −55.9 −55.5 −55.2 −54.9 −54.6 −54.3 −54.0 −53.7 −53.4 −53.2 −52.9 −52.5 −52.2 −51.9 −51.6 −51.3 −51.0

−64.9 −64.7 −64.3 −64.0 −63.7 −63.3 −63.0 −62.7 −62.4 −62.0 −61.7 −61.4 −61.1 −60.7 −60.4 −60.1 −59.8 −59.5 −59.1 −58.8 −58.5 −58.2 −57.8 −57.5 −57.2 −56.9 −56.6 −56.3 −55.9 −55.6 −55.3 −55.0 −54.7 −54.4 −54.0 −53.7 −53.4 −53.1 −52.8 −52.5 −52.2 −51.9 −51.5 −51.2 −50.9 −50.6 −50.3 −50.0 −49.7 −49.4 −49.1 −48.7 −48.4 −48.1 −47.8 −47.5 −47.2

hui σ (u) (ms−1 ) (ms−1 ) 27.4 24.1 22.6 30.0 24.5 22.4 25.5 21.4 17.1 16.6 13.5 10.5 7.7 7.5 8.7 9.2 9.8 9.9 12.9 17.3 18.9 24.2 28.6 25.8 19.7 18.2 13.4 10.3 9.0 6.5 5.3 3.8 2.7 1.4 1.2 0.3 0.1 −0.1 −0.6 −0.3 1.0 4.0 5.4 9.9 11.0 12.4 18.0 23.3 30.7 34.5 38.8 44.8 44.7 38.4 33.4 26.4 21.1

15.6 12.3 9.8 10.0 1.8 6.8 8.0 6.9 6.3 3.5 2.4 2.4 2.0 2.7 2.8 2.2 3.6 4.9 7.3 6.7 7.9 10.3 8.4 6.8 3.3 3.1 2.8 5.4 3.2 2.7 2.1 2.7 2.5 2.6 2.7 1.5 2.2 3.4 2.6 1.4 2.8 3.4 4.1 2.7 4.7 4.5 7.0 10.9 13.0 8.3 9.0 6.2 7.5 7.5 7.2 8.0 8.8

N

ϕ θ (deg) (deg)

4 6 8 3 5 9 9 9 7 12 12 12 12 19 15 14 15 15 20 15 15 13 16 12 11 12 17 18 24 18 18 18 18 16 24 18 18 17 18 18 17 19 14 14 16 15 17 19 19 18 16 20 25 28 31 30 30

0.3 0.6 0.9 1.1 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8 5.1 5.4 5.7 5.9 6.3 6.6 6.9 7.2 7.5 7.8 8.1 8.4 8.7 9.0 9.2 9.6 9.9 10.2 10.5 10.8 11.1 11.4 11.7 12.0 12.3 12.6 12.9 13.2 13.4 13.8 14.1 14.3 14.7 15.0 15.2 15.6 15.9 16.1 16.5 16.8 17.0

0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.1 2.3 2.6 2.8 3.1 3.4 3.6 3.9 4.2 4.4 4.7 4.9 5.2 5.5 5.8 6.0 6.3 6.6 6.8 7.1 7.3 7.6 7.8 8.1 8.4 8.6 8.9 9.2 9.4 9.7 10.0 10.3 10.5 10.8 11.0 11.3 11.6 11.8 12.1 12.3 12.6 12.9 13.1 13.4 13.7 13.9 14.2 14.5 14.7 15.0

hui (ms−1 )

σ (u) (ms−1 )

N

77.7 78.4 78.0 79.8 76.8 82.0 81.6 83.5 82.1 85.0 86.8 93.1 93.8 94.4 99.7 98.7 102.5 104.5 101.0 100.3 104.6 104.2 104.4 100.9 105.0 104.1 102.2 100.8 100.0 97.4 92.0 85.8 82.1 79.8 73.2 67.2 63.1 58.3 54.5 47.8 41.5 36.7 33.0 29.6 23.1 17.2 12.9 8.5 3.5 −1.0 −4.1 −8.2 −12.0 −14.5 −15.2 −15.0 −13.9

6.6 4.7 6.6 7.3 6.8 7.5 7.7 4.8 7.8 7.5 8.7 9.8 11.0 10.0 10.1 10.6 10.9 12.3 11.2 13.5 15.7 11.5 10.8 10.0 8.5 12.3 11.2 10.6 9.7 11.1 10.7 11.0 10.8 10.8 11.6 11.3 10.6 11.3 11.0 7.7 8.7 7.7 8.1 8.6 9.1 7.8 7.8 7.9 6.8 6.8 7.5 6.7 7.4 7.3 7.1 6.8 5.9

28 43 30 45 45 31 46 39 28 45 30 45 45 31 42 41 28 38 27 40 38 25 46 45 28 47 48 32 48 32 48 46 30 46 42 29 47 48 32 48 47 32 48 32 48 48 32 48 48 32 48 48 30 48 48 32 48

325

STABILITY OF JOVIAN ZONAL WINDS

TABLE III—Continued ϕ (deg)

θ (deg)

−50.7 −50.4 −50.1 −49.8 −49.5 −49.2 −48.9 −48.6 −48.3 −48.0 −47.8 −47.4 −47.2 −46.9 −46.5 −46.3 −45.9 −45.6 −45.3 −45.0 −44.7 −44.4 −44.1 −43.8 −43.5 −43.2 −42.9 −42.6 −42.3 −42.0 −41.7 −41.4 −41.1 −40.9 −40.6 −40.2 −39.9 −39.6 −39.3 −39.0 −38.7 −38.4 −38.1 −37.9 −37.6 −37.2 −36.9 −36.6 −36.3 −36.0 −35.7 −35.5 −35.1 −34.8 −34.5 −34.2 −33.9 −33.7 −33.3

−46.9 −46.6 −46.3 −46.0 −45.7 −45.4 −45.1 −44.8 −44.5 −44.2 −43.9 −43.6 −43.3 −43.0 −42.7 −42.4 −42.1 −41.8 −41.5 −41.2 −40.9 −40.6 −40.3 −40.0 −39.7 −39.4 −39.1 −38.8 −38.5 −38.2 −37.9 −37.6 −37.3 −37.1 −36.8 −36.5 −36.2 −35.9 −35.6 −35.3 −35.0 −34.7 −34.4 −34.2 −33.9 −33.6 −33.3 −33.0 −32.7 −32.4 −32.1 −31.9 −31.6 −31.3 −31.0 −30.7 −30.4 −30.2 −29.9

hui σ (u) (ms−1 ) (ms−1 ) 16.8 11.2 9.1 5.7 4.4 2.1 −0.3 −1.9 −4.1 −5.1 −5.6 −4.6 −2.7 −0.8 5.2 8.9 14.4 17.3 18.1 20.1 23.6 27.3 30.8 35.8 40.5 43.8 45.6 44.1 40.3 33.0 27.5 22.8 18.3 15.4 9.1 7.9 6.9 3.6 −2.0 −1.4 2.3 8.6 13.8 17.2 24.0 32.3 36.5 39.6 40.2 39.4 38.2 38.2 35.9 32.4 26.3 18.1 14.8 10.0 5.6

10.1 6.6 5.5 5.9 5.6 5.0 5.9 6.6 8.1 8.3 9.7 8.4 9.5 7.7 6.0 9.3 11.7 10.3 9.8 8.5 8.1 6.2 6.8 7.1 6.1 6.9 6.6 7.0 9.7 8.3 8.3 7.7 7.7 8.6 7.2 6.7 6.0 9.0 8.2 8.9 7.3 7.5 8.6 9.1 13.4 11.9 11.1 10.5 8.2 5.2 5.2 6.3 5.5 6.5 9.0 7.0 7.7 6.5 6.2

TABLE III—Continued

N

ϕ θ hui σ (u) (deg) (deg) (ms−1 ) (ms−1 )

33 33 33 33 33 33 33 33 30 33 28 31 28 25 28 31 26 30 31 28 31 33 31 33 33 33 31 33 33 33 33 33 22 33 33 33 33 32 33 33 33 31 22 32 33 33 29 33 32 30 20 32 32 33 33 33 22 32 34

17.4 17.7 18.0 18.3 18.6 18.9 19.2 19.5 19.8 20.1 20.4 20.7 20.9 21.3 21.6 21.9 22.2 22.5 22.8 23.0 23.4 23.7 24.0 24.3 24.6 24.9 25.2 25.5 25.8 26.1 26.3 26.7 27.0 27.3 27.5 27.9 28.2 28.5 28.8 29.1 29.4 29.7 30.0 30.2 30.6 30.9 31.2 31.5 31.7 32.1 32.4 32.7 33.0 33.3 33.5 33.9 34.2 34.5 34.8

15.3 15.6 15.8 16.1 16.4 16.7 16.9 17.2 17.4 17.7 18.0 18.2 18.5 18.8 19.1 19.3 19.6 19.9 20.2 20.4 20.7 21.0 21.3 21.5 21.8 22.1 22.3 22.6 22.9 23.1 23.4 23.7 24.0 24.3 24.5 24.8 25.1 25.4 25.7 25.9 26.2 26.5 26.7 27.0 27.3 27.6 27.9 28.1 28.4 28.7 29.0 29.3 29.6 29.8 30.1 30.4 30.7 31.0 31.2

−11.9 −10.0 −6.1 −3.1 0.1 6.6 13.2 22.9 32.2 44.9 55.8 63.8 71.2 86.6 100.0 106.5 112.7 119.1 125.6 132.2 135.4 134.2 131.0 125.1 114.1 102.8 96.5 88.4 77.3 69.1 59.1 47.3 36.5 29.3 23.6 18.9 15.6 10.3 7.2 2.2 −1.8 −7.4 −10.4 −12.7 −14.7 −14.3 −13.5 −10.6 −7.7 −2.8 2.9 7.2 12.7 18.0 29.3 33.6 37.6 39.7 39.7

5.1 5.5 7.2 6.9 8.8 10.0 9.1 9.4 12.9 13.8 11.1 10.7 11.3 11.5 8.0 8.2 8.0 7.9 7.0 7.8 7.3 7.9 8.0 8.3 10.1 10.4 7.2 8.4 9.3 8.9 7.7 8.0 9.3 7.8 6.7 6.8 7.2 6.0 4.9 7.0 8.1 6.9 6.9 5.7 5.7 6.2 6.5 5.6 6.5 6.3 7.2 5.0 7.1 .2 7.8 6.8 5.4 6.2 6.2

N

ϕ (deg)

θ (deg)

hui σ (u) (ms−1 ) (ms−1 )

N

ϕ θ hui σ (u) (deg) (deg) (ms−1 ) (ms−1 )

N

48 45 30 45 45 30 48 45 30 48 47 29 45 46 46 32 47 45 31 47 45 44 29 48 48 47 30 47 47 32 48 48 48 32 48 45 48 45 30 45 48 48 32 48 48 48 48 32 48 48 47 48 47 29 50 48 48 48 32

−33.0 −32.7 −32.4 −32.2 −31.8 −31.5 −31.2 −30.9 −30.6 −30.3 −30.0 −29.7 −29.4 −29.2 −28.8 −28.5 −28.2 −27.9 −27.6 −27.3 −27.0 −26.7 −26.4 −26.1 −25.8 −25.5 −25.2 −24.9 −24.6 −24.3 −24.0 −23.7 −23.4 −23.2 −22.8 −22.5 −22.2 −21.9 −21.6 −21.3 −21.1 −20.7 −20.4 −20.1 −19.8 −19.5 −19.2 −18.9 −18.6 −18.3 −18.0 −17.7 −17.4 −17.1 −16.8 −16.5 −16.3 −15.9 −15.6

−29.6 −29.3 −29.0 −28.8 −28.5 −28.2 −27.9 −27.6 −27.3 −27.1 −26.8 −26.5 −26.2 −26.0 −25.7 −25.4 −25.1 −24.8 −24.6 −24.3 −24.0 −23.7 −23.5 −23.2 −22.9 −22.6 −22.4 −22.1 −21.8 −21.5 −21.3 −21.0 −20.7 −20.5 −20.2 −19.9 −19.6 −19.4 −19.1 −18.8 −18.6 −18.3 −18.0 −17.7 −17.5 −17.2 −16.9 −16.7 −16.4 −16.1 −15.9 −15.6 −15.3 −15.1 −14.8 −14.5 −14.3 −14.0 −13.7

−0.8 −8.7 −11.6 −11.0 −9.4 −8.4 −4.5 3.0 8.9 14.1 19.1 25.9 29.3 30.9 32.9 30.4 33.5 34.6 38.0 42.9 48.9 54.4 56.8 52.3 39.1 32.9 26.5 24.4 19.4 16.0 11.3 8.7 5.3 1.7 −1.7 −5.9 −12.0 −17.0 −20.0 −25.7 −30.7 −37.2 −43.7 −48.6 −48.1 −48.2 −51.1 −51.3 −46.9 −41.8 −36.3 −34.1 −28.7 −24.1 −17.7 −11.4 −5.4 0.5 5.6

33 33 22 33 33 32 33 32 22 33 31 30 20 30 28 30 33 20 28 24 24 17 27 21 15 10 16 18 18 14 25 27 18 27 27 27 18 27 27 18 27 27 27 18 26 26 18 27 27 18 27 27 18 30 36 24 36 36 24

35.0 35.3 35.7 36.0 36.3 36.6 36.9 37.1 37.4 37.8 38.1 38.4 38.7 39.0 39.3 39.6 39.8 40.1 40.5 40.8 41.1 41.4 41.7 42.0 42.3 42.6 42.9 43.2 43.5 43.8 44.0 44.3 44.6 44.9 45.2 45.5 45.9 46.1 46.5 46.8 47.0 47.4 47.6 47.9 48.2 48.5 48.8 49.1 49.4 49.7 50.1 50.4 50.7 51.0 51.3 51.6 51.9 52.2 52.5

48 48 44 45 43 45 29 44 47 46 42 43 45 46 46 30 48 48 47 48 48 47 47 46 41 35 42 44 40 30 48 47 44 45 48 47 46 48 48 48 47 45 45 42 48 48 49 49 48 51 68 51 46 51 51 46 45 49 45

9.2 9.5 11.1 11.3 9.2 8.3 9.7 7.2 5.8 6.7 8.7 9.8 10.9 12.0 11.2 8.9 6.0 5.9 4.7 4.3 6.6 5.9 8.0 9.1 6.4 5.1 4.4 7.4 4.5 9.8 8.9 6.9 6.5 7.8 6.9 7.4 8.4 8.1 6.8 7.3 8.2 8.5 8.9 7.9 9.2 8.8 7.9 10.1 9.4 11.1 14.2 10.4 8.5 10.1 9.2 8.2 8.5 10.1 7.9

31.5 31.8 32.1 32.4 32.7 33.0 33.2 33.5 33.8 34.1 34.4 34.7 35.0 35.3 35.6 35.9 36.1 36.4 36.7 37.0 37.3 37.6 37.9 38.2 38.5 38.8 39.1 39.4 39.7 40.0 40.2 40.5 40.8 41.1 41.4 41.7 42.0 42.3 42.6 42.9 43.2 43.5 43.8 44.1 44.4 44.7 45.0 45.3 45.6 45.9 46.2 46.6 46.9 47.2 47.5 47.8 48.1 48.4 48.7

38.1 36.6 32.9 28.5 23.4 16.2 11.7 4.0 −0.4 −3.3 −10.8 −15.7 −14.5 −16.0 −14.0 −10.8 −6.5 −1.6 2.8 6.9 10.8 15.8 19.8 25.0 26.4 23.6 16.1 11.7 9.1 6.2 5.7 5.3 6.0 6.4 8.2 10.9 15.5 17.5 18.9 17.8 16.2 13.2 9.9 6.6 5.4 4.0 4.2 4.4 3.0 4.8 4.0 2.7 1.0 0.3 −0.1 −0.3 1.1 0.4 1.5

6.0 7.0 7.7 8.4 11.4 11.6 9.2 10.3 9.9 7.4 9.6 7.9 7.9 8.4 9.5 7.6 7.3 6.8 6.9 7.7 8.4 8.2 7.8 9.7 8.8 7.7 8.0 7.5 7.3 5.4 6.5 6.7 7.0 6.4 6.2 6.7 7.5 6.5 6.3 6.3 6.8 5.9 6.3 6.5 6.2 5.7 5.3 5.4 7.1 7.5 6.7 6.3 6.3 6.1 6.8 4.8 6.0 7.8 7.7

´ GARC´IA-MELENDO AND SANCHEZ-LAVEGA

326 TABLE III—Continued ϕ (deg)

θ (deg)

−15.4 −15.0 −14.7 −14.5 −14.1 −13.8 −13.6 −13.2 −12.9 −12.7 −12.3 −12.0 −11.7 −11.4 −11.1 −10.8 −10.5 −10.2 −9.9 −9.6 −9.3 −9.0 −8.7 −8.5 −8.1 −7.8 −7.5 −7.2 −6.9 −6.6 −6.3 −6.1 −5.7 −5.4 −5.1 −4.8 −4.5 −4.2 −3.9 −3.6 −3.3 −3.0 −2.7 −2.4 −2.1 −1.8 −1.5 −1.3 −0.9 −0.6 −0.3 0.0

−13.5 −13.2 −12.9 −12.7 −12.4 −12.1 −11.9 −11.6 −11.3 −11.1 −10.8 −10.5 −10.3 −10.0 −9.7 −9.5 −9.2 −8.9 −8.7 −8.4 −8.1 −7.9 −7.6 −7.4 −7.1 −6.9 −6.6 −6.3 −6.1 −5.8 −5.5 −5.3 −5.0 −4.7 −4.5 −4.2 −3.9 −3.7 −3.4 −3.1 −2.9 −2.6 −2.4 −2.1 −1.8 −1.6 −1.3 −1.1 −0.8 −0.5 −0.3 0.0

hui σ (u) (ms−1 ) (ms−1 ) 10.3 15.3 18.4 24.2 27.1 31.6 34.4 33.8 34.2 36.4 38.6 40.1 40.8 41.3 41.2 42.8 45.7 48.8 53.3 62.1 73.3 86.8 96.2 107.2 128.3 141.1 148.0 153.6 152.1 150.0 146.2 138.7 132.1 122.7 113.4 107.8 103.1 98.7 95.2 93.7 89.9 87.6 86.1 81.7 81.5 80.0 79.9 77.6 76.1 77.1 76.9 76.7

9.0 9.9 12.5 13.0 12.0 9.6 7.2 10.6 10.8 9.8 9.0 8.2 7.8 6.5 6.9 7.8 6.9 7.5 9.6 12.4 14.1 19.4 16.9 20.1 17.1 9.8 9.3 8.2 9.8 8.6 8.6 9.0 12.7 6.7 12.2 11.2 9.0 8.6 8.5 7.2 7.4 8.5 8.4 6.6 9.3 7.0 8.4 5.5 6.2 4.9 3.4 5.1

N 36 36 24 36 36 24 36 36 24 36 36 24 36 41 30 45 45 32 48 48 32 48 32 48 48 32 46 42 30 42 26 39 39 28 46 47 32 47 47 30 45 30 45 42 28 40 28 39 39 26 39 40

TABLE III—Continued

ϕ θ hui σ (u) (deg) (deg) (ms−1 ) (ms−1 )

N

52.8 53.1 53.4 53.6 53.9 54.3 54.6 54.9 55.2 55.5 55.8 56.1 56.3 56.7 57.0 57.3 57.6 57.9 58.2 58.5 58.8 59.1 59.4 59.7 59.9 60.3 60.6 60.9 61.2 61.4 61.8 62.1 62.4 62.7 62.9 63.3 63.6 63.9 64.2 64.5 64.8 65.1 65.3 65.7 66.0 66.3 66.5 66.9 67.2 67.5 67.7 68.1 68.4 68.7 69.0 69.3 69.6 69.9 70.2

50 49 50 46 36 44 33 36 35 35 35 34 36 46 31 31 31 31 30 42 29 27 28 30 30 37 30 29 27 25 33 26 25 27 26 30 22 18 20 30 22 20 17 26 21 21 21 28 21 18 16 7 6 10 6 7 9 10 6

49.0 49.3 49.6 49.9 50.2 50.5 50.9 51.2 51.5 51.8 52.1 52.4 52.7 53.0 53.4 53.7 54.0 54.3 54.6 54.9 55.3 55.6 55.9 56.2 56.5 56.9 57.2 57.5 57.8 58.1 58.5 58.8 59.1 59.4 59.7 60.1 60.4 60.7 61.0 61.4 61.7 62.0 62.3 62.7 63.0 63.3 63.6 63.9 64.3 64.6 64.9 65.3 65.6 65.9 66.3 66.6 66.9 67.2 67.6

1.8 3.4 4.8 8.7 14.2 18.0 22.3 23.9 23.9 24.1 19.8 15.6 15.0 11.4 6.6 6.3 4.9 3.6 3.3 0.4 0.8 1.0 1.2 1.7 1.8 1.7 3.1 1.9 2.5 2.4 6.7 10.0 11.7 16.9 19.7 21.2 21.6 18.6 15.0 12.0 9.4 7.2 3.9 8.5 11.7 13.9 16.0 18.8 21.8 26.8 33.2 34.7 33.7 26.2 26.8 20.9 16.7 18.1 12.6

8.0 8.4 7.9 7.5 5.4 4.4 6.2 7.5 7.5 7.4 5.8 6.0 7.2 6.7 5.9 7.3 7.1 4.8 6.1 6.2 5.3 5.7 4.8 4.3 5.1 5.1 6.3 6.4 7.1 6.1 7.2 8.1 6.8 9.2 7.9 6.1 6.9 7.4 7.3 5.8 5.0 5.9 5.9 8.2 6.0 7.1 10.2 10.9 9.7 10.4 10.3 3.4 1.8 6.5 1.6 2.6 3.6 4.8 1.5

ϕ (deg)

θ (deg)

hui (ms−1 )

σ (u) (ms−1 )

N

ϕ (deg)

θ (deg)

hui (ms−1 )

σ (u) (ms−1 )

N

70.5 70.8 71.1 71.4 71.7 72.0 72.2 72.6 72.9 73.2 73.5 73.8 74.1 74.4 74.7 74.9 75.3 75.6 75.9 76.2 76.5 76.7

67.9 68.2 68.6 68.9 69.2 69.6 69.9 70.2 70.6 70.9 71.2 71.6 71.9 72.2 72.6 72.9 73.2 73.6 73.9 74.3 74.6 74.9

8.7 7.4 8.7 10.1 8.5 6.0 6.1 9.8 6.6 3.1 3.6 6.2 10.0 9.1 7.1 7.6 8.3 8.2 8.6 12.0 13.4 13.2

2.0 3.3 4.7 6.2 5.0 3.4 2.9 0.8 1.1 0.6 0.5 0.9 0.9 1.0 0.6 0.0 0.1 0.0 0.7 0.8 0.1 0.1

7 12 9 9 12 7 4 4 3 3 4 3 3 4 3 3 4 3 4 3 3 2

different longitudes during a large temporal period, the mistakes in measuring the zonal winds due to local meteorology are now overcome, as discussed in Section 2. This will be our criterion to assign a real change in the jets. In Fig. 8, the averaged HST 1995–1998 and the Voyager 1979 profiles (Limaye 1986) are compared, including the respective error bars. As a reference we also plot the individual profiles for the 23.7◦ N jovian jet by Maxworthy (1984) from Voyager data, and the 7◦ S jet by Vasavada et al. (1998) based on Galileo data. It can be seen that globally there is agreement between the HST and Voyager zonal flow. This result reinforces the idea that Jupiter’s global circulation is very stable in time. Nevertheless there are discrepancies between the two profiles, which can be divided into two parts. (1) There is an apparent systematic difference in the latitudinal positions of the jets above the ±30◦ latitude that in some places reaches ∼ 1◦ . Our jets tend to be located farther poleward than those of Limaye (1986). Probably this difference is not real but originates in navigation errors by both teams. Nevertheless, it must be pointed out that latitude differences are not related to latitude in a simple fashion; for example, whereas the two teams agree on the location of the 24◦ N jet, there are ∼1◦ discrepancies in the locations of the 17◦ N and 30◦ N neighboring westward jets. (2) Despite these problems some jets show significant differences in the magnitude of the peak velocity. These are: the eastward jets at 7◦ S and 24◦ N, the westward jet at 32◦ N, and the jets between 44◦ N and 60◦ N. Let us comment on these changes in more detail.

STABILITY OF JOVIAN ZONAL WINDS

327

FIG. 7. Average zonal wind profile computed using the correlation method (solid line), from the HST images between 1995 and 1998, after averaging the 410-nm and 953-nm profiles. Dots are individual measurements obtained by cloud tracking in the same set of images.

(a) The 24◦ N eastward jet, as already discussed in the Introduction, has attracted the attention of several authors and it is probably the most studied part of the zonal flow. All studies, at the time this paper was written, concluded with an analysis of HST 1998 data. In the work by Simon (1999), and

Garcia-Melendo et al. (2000), there was not agreement about whether the 23.7◦ N jet had recovered its 1979 peak velocity of ∼180 ms−1 . We obtained a new wind profile from the September 2000 data (see Fig. 9), and it is clear that this jet stream has kept a peak velocity of ∼140 ms−1 since 1995, remaining very stable

FIG. 8. Comparison among the HST 1995–1998 average zonal wind profile (this work), the 1979 mean Voyager profile, the 1979 Voyager profile for the 23◦ N jet, and the 1996 Galileo profile for the 7◦ S jet.

328

´ GARC´IA-MELENDO AND SANCHEZ-LAVEGA

FIG. 9. The average 1995–1998 HST profile (solid thin line) compared with the correlation measurements of the September 2000 HST profile in the 953-nm band (square dots).

until these most recent observations. As discussed by GarciaMelendo et al. (2000), the 1979 to 1995 change in the zonal wind is most probably linked to the important local meteorology change that took place during the 1990 NTB disturbance (Sanchez-Lavega et al. 1991). (b) The eastward jet at 7◦ S has not been as thoroughly studied, and it is interesting to review some past results. There seems to be a considerable lack of agreement about the peak velocity of this jet. Ingersoll et al. (1979, 1981) and Limaye (1986, 1989) give maximum velocities of ∼130 ms−1 . Magalhaes et al. (1990) reported important differences between the profiles obtained from the orange and violet Voyager mosaics. In fact, the violet profile does not show a clear peak at 7◦ S as the orange one does. In addition, Beebe et al. (1989) obtained a 150 ms−1 maximum velocity value, and Maxworthy (1985) claimed the peak velocity was 160 ms−1 . To make things a little more confusing, Simon (1999) measured a new profile from Voyager data where the 7◦ S jet peaks at 140 ms−1 . However a study by Vasavada et al. (1998) of new measurements of Voyager 1 orange frames taken with the narrow-angle camera yielded a peak velocity of 150 ms−1 . Vasavada et al. (1998) also measured Galileo images of the same region using the correlation method, which showed very good agreement with their new Voyager results except for a latitude offset of ∼0.5◦ . Previous measurements made by Beebe et al. (1996) on Voyager and HST images using a cloud tracking technique also agree with the results given by Vasavada et al. (1998). All our profiles from 1995 to 2000 show that during this period the peak velocity of the jet has been constant at about 155 ms−1 . Fortunately, 15 days before the Galileo spacecraft took the data analysed by Vasavada et al. (1998), the Hubble Space Telescope imaged the planet, giving us a very good opportunity to check our data consistency. Figure 10 shows a comparison between the Galileo and HST profiles for the 7◦ S jet, along with the

Voyager results by Limaye (1986). The agreement between our HST profile and the Galileo one is very good. If we consider that the actual peak velocity for the Voyager era is 150 ms−1 , then our results suggest that it might have remained stable since then. When compared to the morphology changes, this result is just the opposite of what was stated previously for the NTB. During Voyager’s fly-bys, the southern part of the Equatorial Zone, where the jet resides, was globally a low-albedo region (Beebe et al. 1989), except for the presence of a large White Spot, a long-lived anticyclonic synoptic system ∼10.000 km in length (Sanchez-Lavega and Rodrigo 1985, Maxworthy 1985). This feature moved more slowly than the flow with a velocity of 95 ms−1 . From 1995 to 2000, on the contrary, the region has remained as a high-albedo feature showing a delicate chevron pattern along the 7◦ S jet (Vasavada et al. 1998). But in this case, despite the morphological changes, the jet has remained apparently unchanged as described above. The same appears to be the case during the strong albedo changes suffered by the South Equatorial Belt (SEB) during the development of disturbances at 16◦ S (Sanchez-Lavega and Rodrigo 1985, Sanchez-Lavega and Quesada 1988, Beebe et al. 1989, Sanchez-Lavega et al. 1996). Cloud tracking measurements from ground-based observations during the development of the 1993 SEB disturbance showed the wind velocities to be in good agreement with the zonal flow measured from Voyager data (Sanchez-Lavega et al. 1996). (c) Finally, there appears to be a decrease of ∼15 ms−1 in the intensity of the westward jet at 32◦ N and subtle profile changes from 44◦ N to 60◦ N. The weakening of the 32◦ N jet might be related to the apparition of a broad low-albedo NTB after the 1990 disturbance (Sanchez-Lavega et al. 1991), while possible changes between 44◦ N and 60◦ N are not correlated with any conspicuous morphology changes in the region. Figure 11 shows a comparison between the Voyager (Limaye 1986) and HST profiles for the regions around 32◦ N and between 44◦ N and 60◦ N.

FIG. 10. Comparison among the Galileo profile (November 1996; Vasavada et al. 1998), the HST profile (October 1996; this work), and the Voyager 2 violet filter profile (1979; Limaye 1986) around the 7◦ S eastward jet.

329

STABILITY OF JOVIAN ZONAL WINDS

the zonal winds, especially if in addition we are lucky enough to obtain data during important morphological alterations in the cloud properties. APPENDIX If in Eq. (1) we make g(i) = f(i), then Rff (j) is called the autocorrelation function. It can be shown, using the Cauchy–Schwartz inequality, that Rff (j) ≤ Rff (0). Let A(λ) be an albedo scan at a given latitude. If we assume that after a given time interval 1t we have exactly the same albedo profile but a shift in longitude an amount 1λ due to the zonal wind drift, then the new albedo scan will be A0 (λ) = A(λ ± 1λ). We consequently have RAA0 ( j) = =

X X

A(i)A0 (i − j) A(i)A(i ± 1λ − j) ∼ =

X

A(i)A(i − j0 )

= RAA (j0 ), where ±1λ + j has been replaced by the nearest integer j0 . Obviously A(λ) is a continuous function, 1λ is also a real number, and we are estimating RAA0 (j) through a discrete function at discrete intervals i and j; for this reason the symbol “∼ =” is used. Since RAA (j0 ) ≤ RAA (0), when j0 = 0 then j = ±1λ, and therefore the correlation function is maximum for j = ±1λ. Or in other words, if the correlation function is used as a likelihood criterion and albedo scans are similar, then it is reasonable to assume that the maximum of the function RAA0 (j) will directly indicate the longitude shift due to the zonal wind drift. In ±1λ is included the central meridian displacement 1CM = CM2 − CM1 between the two albedo scans which must also be taken into account.

ACKNOWLEDGMENTS

FIG. 11. Comparison between the Voyager (1979; Limaye 1986) and HST profiles (1995–1998, this work) for the regions around 32◦ N (top), and between 44◦ N and 60◦ N (bottom).

This work has been supported by Gobierno Vasco Research Grant PI 1997-34. We acknowledge the Space Telescope Science Institute and the ESO ST–ECF archive facilities for supplying us with the Hubble Space Telescope images, and the HST director for the discretionary time provided for the observations on 2 September 2000 (DD-8871).

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Our zonal wind measurements from 1995 to 2000 indicate that there have not been important changes during this period in Jupiter’s average zonal flow. In this five-year span, Jupiter’s morphology has remained basically unaltered. These data, also in comparison with Voyager results, support the idea of longterm global stability of Jupiter’s circulation. With the Cassini encounter at the end of December 2000, our opportunities to get new high-resolution data from spacecraft end, at least for the next few years. Nevertheless, this work shows that it is possible to continuously monitor the global flow from space telescopes, in particular the HST, with a resolution similar to that provided by Voyager measurements. In future work, higher precision in the resulting data could probably be achieved if observations are planned to cover total or partial cylindrical projections for at least two rotations. Control of the local meteorology and if possible the altitude of the clouds would be fundamental when retrieving the profile. Close monitoring during the next several years will probably give us much information on the stability of

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