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The impact of comet Shoemaker-Levy 9 on the jovian ionosphere and aurorae
Planet. Space Sci., Vol. 45, No. 10. pp. 1237-1250, 1997 ICY1997 Elsevier Science Ltd
Petgamon
All rights reserved. Printed in Great Britain 0032-0633;97 $17.00 + 0.00 PII: S0032-0633(97)00060-3
The impact of comet Shoemaker-Levy 9 on the jovian ionosphere and aurorae Steven Miller,’ Nicholas Achilleos,’ Hoanh An Lam,’ Bianca Maria Dinelli’ and RenCePrangC,” ‘Department of Physics and Astronomy, University College London, Gower Street, London WClE 6BT. U.K. ‘Istituto di Spettroscopia Molecolare, CNR, Via Gobetti 101, 40129 Bologna, Italy ‘Institut d’Astrophysique Spatiale, CNRS. Bat. 121, Universite de Paris XI. 91405 Orsay Cedex. France Received 7 October 1996; accepted 28 February 1997
n a sensttive tracer of
reserved.
Introduction Logic almost demanded that the jovian ionosphere, as the outermost--and thus first encountered-layer of the atmosphere of Jupiter, should be affected by the impact of Comet Shoemaker--Levy 9 (SL9) in July 1994. Predicted effects varied from a near-complete shutting down of the powerful aurorae (Discussion at the SL9 session of the 1993 AAS Division of Planetary Sciences Meeting) to disruption of the ion-molecule chemistry dominant above the homopause (Cravens, 1994). With this in mind, several of the observing programs prepared for Impact Week set out to measure just what the effect of the cometary fragments might be, whether in terms of the production of novel ionospheric species or significant changes to the power output of the UV and IR aurorae. This review sets out to record what was observed in terms of the final trajectory of a “typical” SL9 fragment, its impact and the aftermath of the collisions. (N.B. Our “typical” fragment, however, is composed of the observations of a number of individual fragments.) Under normal circumstances, far and away the most
powerful ionospheric emission comes from the aurora1 regions. As Jupiter rotates, its magnetic field sweeps up the plasma surrounding the planet. This forms a dense equatorial plasma sheet swirling like a ballerina’s skirt out to over 1 x lo6 km into space. But somewhere between 20 and 30 jovian radii from the centre of Jupiter the plasma speeds required are so great and the field strength is so low, that co-rotation is no longer possible. It is in this region that field aligned currents may accelerate ions and electrons along the magnetic field lines into Jupiter’s upper atmosphere. The brightest aurorae are produced in a tight oval marking the circumpolar footprints of these field lines. At present they are mainly attributed to the precipitation of kiloelectronvolt electrons. But there are also more diffuse aurora1 emissions at lower latitude, possibly also due to other particles scattered by waves generated in the jovian radiation belts, and especially in the outer IO torus (Rezeau et al., 1996) due to the effect of IO’S progress (or. rather, regress) through the magnetic field. (See Thorne, 1983 for a full explanation of these processes.) Since the magnetic poles of Jupiter are displaced from the rotational poles and the field is highly multipolar (Connerney, 1993) the exact location and strength of aurora1 emission is a strong function of the longitude. Jupiter’s aurorae have been studied for many years in the UV (see Livengood et al., 1992), since they were first detected by the Voyager spacecraft and the Earth orbiting FUV observataory IUE. In the UV wavelength region from 1100 to 1700 A emission due to atomic hydrogen Lyman x and the Lyman and Werner bands of Hz is observed. The emitters are collisionally excited by precipitating electrons and then radiate their excess electronic energy away. Typically. Jupiter emits a few x 10” W by this process. But particle impacts can also ionise molecular hydrogen. commencing a chain of reactions which leads to the formation of the H: molecular ion (Atreya and Donahue, 1976). This ion emits strongly in the IR, about 90% of its energy between 3 and 5 pm and the rest mainly at about 2 pm, as
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S. Miller et al.: Impact of SL9 on the jovian ionosphere
vibrationally excited molecules relax back to the ground state. Once more a few x lOI W is a typical IR aurora1 output (Lam et al., 1996). Figure 1 shows images of Jupiter in the UV and IR, which show the aurorae clearly, particularly around the north polar region. The UV image, due to Clarke et al. (1995) shows the bright narrow oval tracing the footprint of the last closed field lines, as well as showing a more diffuse emission region which extends both polewards of the oval and to latitudes which encompass the magnetic footprint of the IO Torus (L = 5.9) on the planet. The IR image, taken at a similar central meridian longitude to the UV as part of the NASA IRTF Galileo Monitoring Program, also shows a similar structure. This indicates that, although they are produced by different species and excited by different mechanisms the UV and IR data are sensitive to similar aurora-producing processes. The intensity of the aurorae is known to vary with time by factors of three or more (Livengood et al., 1992; PrangC et al., 1993 ; Baron et al., 1991). As IO moves through Jupiter’s magnetic field, its dynamo effect causes a current of zz 1O6A to be dumped into the planet’s upper atmosphere. In recent years the aurora1 “spot” produced by this current has been detected in both the IR (Connerney, 1993) and UV (PrangC et al., 1996) clearly separated from the main aurorae. Additionally, it has now been demonstrated that H:emits right across the planet (Ballester et al., 1994), producing an emission pattern which shows both latitudinal and longitudinal variations (Lam et al.. 1996). This is shown in Fig. 2. This brief introduction to the emissions observed from the jovian ionosphere and aurorae indicates that, even under “normal” conditions, they are complicated and spatially and temporally very variable. This is important to remember in attempting to ascertain what changes could be ascribed to the impact of SL9 and what might simply be due to the “normal” variability of the planet. In general, the far UV aurorae appeared to be somewhat fainter in the days prior to, during and after the week from July 16-23, which has become known as Impact Week. With this in mind, we will now try to follow the fate of a “typical” SL9 fragment as it was charted by changes to Jupiter’s ionosphere and aurorae. In what follows, the time of impact is designated to.
t,, - 5 h : “blinking”
aurorae
It is arguable that the first direct effects produced on Jupiter by our composite fragment were on the jovian ionosphere. Images of the south polar region taken between 14 : 10 and 14 :45 UT on July 20 by the Hubble Space Telescope (HST) show the aurora1 oval clearly (Fig. 3). But within the oval, a bright spot-as bright as the aurora1 oval itself-was seen to blink on and off with a period of z20min (Clarke et al., 1995). Owing to the limited amount of FUV images of the Jovian aurorae available at the time, doubts were cast on the actual relationship between these spots and the SL9 event. Observations performed since July 1994 have significantly increased the database of aurora1 measurements. It is now clear that although the Jovian aurorae are quite dynamic,
and transient arcs are observed from time to time, they do not seem to appear inside the south polar cap. and none of them exhibit the same shape as the blinking aurora. Dougherty has estimated that the current producing these bright aurora1 spots was of the order of 10’ to 10’ A (personal communication, 1996). Prangt et al. (1995) showed that the spots were located at the footprint of field lines which connected to the incoming Fragment Q, still 5 h-and 7R,-away from impact (Fig. 3). The lack of conjugate emission on the north polar cap could be explained as field lines the fragment was crossing at the time were open, and there was therefore no direct connection to the northern hemisphere. Further analysis showed that the blinking spot was fixed in local time and did not corotate with the magnetosphere during the 35 min of observations. PrangC et al. (1995) were also able to show that Fragment Q (which exhibited the densest and most extended dust coma) was actually the only one which could be magnetically connected to the bright spots. They suggested that the spots were being produced by energetic particle precipitation triggered by field aligned currents flowing along the magnetic flux tube transiently in excess of the critical current density which gives rise to particle acceleration, followed by some kind of relaxation phase, analogous to the charging and discharging of a capacitor. The currents were due principally to the dynamo effect of the motion of the partially charged fragment dust coma with respect to the ambient corotating magnetospheric plasma.
to - 6 min to impact : ionospheric heating Spectra of the eastern limb of Jupiter at a latitude of S44” taken at the United Kingdom Infrared Telescope (UKIRT) on July 17, shortly before the impact of Fragment C, showed an increase in emission by HTwhich could not be ascribed to “normal” spatial variation (Dinelli rt al., 1996). Around 6min prior to impact, the IR lines around 3.5 pm which were being monitored increased in intensity by 70%. According to Waite et al. (1996), for Fragment K this time coincided with an unusually bright northern X-ray emission. 5 min later, about 1 min prior to impact, the Hllines had brightened by a further 50%. This increase in intensity is unlikely to have been caused by an enhanced column density since incoming cometary material, if anything, would chemically deplete H:(Cravens, 1994 ; see later). The brightening was thus almost certainly due to an increase in temperature, heating being effected by cometary material impacting on the ionosphere prior to the main fragment explosion. The narrow wavelength range monitored made it impossible to determine the Hctemperature. However, a recent study by Lam (1995) has shown that ionospheric temperatures in this region are between 800 K and 900 K. Taking this as a starting temperature, the intensity enhancements correspond to temperature increases of 13&f 50 K at tu - 6 min and 190-280 K at to - 1 min. These temperatures represent averages over pixels which covered nearly 3 x ‘km’ on Jupiter, and may therefore represent only lower limits if the heating were more localised. It is possible to estimate how much energy is represented by
S. Miller et al.: Impact of SL9 on the jovian ionosphere
Fig. 1. (a) HST image of Jupiter taken at 1600 A on July 17. 1994. The planet already shows signs of some of the impact scars (Clarke et a/., 1995). (b) 3.8pm image taken in 1995 (NASA TRTF Galileo Imaging Team). Around the northern polar region. both images show the main amoral oval with additional diffuse emission superimposed on it
1239
240
S. Miller et (11.:Impact of SL9 on the jovian ionosphere
Fig. 2. Global Hlemission from derived from UKIRT spectra 1996). (The intensity scale is in ergs-’ cmP2sr-’ times 10)
of Jupiter
taken in 1993 (Lam et al.,
S. Miller
et al.:
Impact of SL9 on the jovian ionosphere
Fig. 3. HST UV images of Jupiter’s southern polar region showing the “blinking aurora” visible as a spot inside the aurora1 arc, but clearly separated from it. in the top and bottom images, and absent in the middle image (Clarke rt al., 1995)
124
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S. Miller et al.: Impact of SL9 on the jovian ionosphere
Fig. 4. UKIRT echelle spectra of the South 44” line of latitude around the time of the impact of Fragment C (July 17, 1994,07 :12U.T.) (Dinelli et al., 1996). The spectrometer slit is aligned east-west along the line of latitude, with east at the top of the spectral image. The echelle was centred on 3.5337pm. Four Hclines are visible extending across the planet in the first image, about 18 min prior to impact. In the second image, there is a rising continuum on the eastern limb, and the Hzlines are brightened due to heating of the ionosphere. In the third image, the fragment explosion was ejected large quantities of hot methane at speeds up 100 km SK’, and the spectrum of this species swamps all other lines. In the final image, the methane emission has cooled down, to reveal the “normal” ionospheric lines once more
S. Miller ef al.: Impact of SL9 on the jovian ionosphere
Fig. 5. HST UV images of Jupiter 40min and 50min after the impact of Fragment K (Clarke et al., 1995). Bright aurora1 spots are clearly visible in the first image, well separated from the main aurora1 arc and magnetically linked to two fainter spots in the southern hemisphere on either side of the impact site. The northern aurora1 spots are still visible ten minutes later
1243
S. Miller
Fig. 8. HST UV image of Jupiter sites (Clarke et al., 1995)
at the end of Impact
et al.:
Week, showing
Impact of SL9 on the jovian ionosphere
the spreading
of the impact
S. Miller et al.: Impact of SL9 on the jovian ionosphere
Fig. 9. NASA IRTF images of Jupiter at 3.4pm prior to during and after impact week.(Miller et al.. 1995). Note that the southern aurora is almost totally quenched on July 27, compared with a similar view on July 17
I245
S. Miller et ul.: Impact of SL9 on the jovian ionosphere the heating observed. if we assume the Hz temperatures are indicative of the local neutral thermosphere composed mainly of molecular (and atomic) hydrogen. Ionospheric column densities of H: are typically 10” cmw2 and the mixing ratio of this ion is z lop7 in the lo-*-lo-‘bar region where it is important. Taken over the entire pixel, therefore, some 5 x ” mol of Hz would have been heated by between 320 K and 430 K in the 6 min prior to impact. This gives an ionospheric heating, caused by the incoming fragment material, of between 4.5 and 6.0 x “erg.
t, : impact and relative ignorance
From the point of view of studies of the jovian ionosphere, the terminal explosions of the various SL9 fragments were a rather unfortunate occurrence. In all cases, the plumes of hot gas and dust thrown up by the explosions obscured the IR emission from the ionosphere (see Fig. 4). It is clear that large quantities of cometary material and jovian atmosphere were ejected through the ionosphere at velocities anywhere between 20 km s-’ (Hammel et al., 1995) and 100 km sm.’(Dinelli et al., 1996). There is no doubt, therefore, that the ionosphere was enriched by the explosions with chemicals not normally found above the jovian homopause. Between to and to+ lOmin, Ballester et al. (1995) reported that IUE detected H2 emission of an aurora1 nature corresponding to high electron fluxes, presumably due to high temperature ionisation in the rising plume.
to + 20 to 80 min : northern aurora1 counterparts While the south, the aftermath of the impact-the plumes and their subsequent “splashback”-dominated, the effect of the fragments was seen in the northern hemisphere. McGregor and co-workers at the Mount Stromlo observatory in Australia reported a number of bright IR emission spots around the northern latitude of 45”, close to the eastern limb of the planet (McGregor et al., 1996). These emissions were well separated from the normal aurora1 oval and were visible for about half an hour, although the observing program undertaken did not allow them to be monitored continuously. A list of their observations is given in Table 1. Clarke and co-workers using the HST also observed this effect for Fragment K (Fig. 5), and termed it the “northern aurora1 counterpart” (Clarke et al., 1995). Their images showed two well separated northern spots
Table 1. CASPIR
Fragment G K R W
1241
about 45 min after impact, which were no longer visible 30 min later. Hill and Dessler (1995) explained this phenomenon as a result of charged material being ejected into the ionosphere across field lines, creating a Bxv current. This, in turn, accelerated particles along the field lines connecting the affected regions close to the impact sites with their northern magnetic counterparts. Other explanations involve stochastic charged particle acceleration in the turbulent plasma associated with the rising fireball, or increased pitch angle scattering of the equatorial radiation belt by electromagnetic waves generated in the impact site (e.g. Bolton and Thorne, 1995). In this case, the charged particles may have precipitated in the north because the field strength was weaker there than in the south. The times taken for the aurorae to appear were consistent with Alfven wave travel times. t,, + a few hours : ionospheric Hzdepletion
Since the rising plumes placed “metal’‘-rich material (in the astronomical sense) into the jovian ionosphere, unusual chemical reactions could take place. Cravens (1994) demonstrated that the net effect of these reactions would be to deplete the concentration of H,t, through reactions such as : H: +X+XH+
+H,
where X could be molecules such as HzO, CO, NH, or CH, (either dredged up from the lower jovian atmosphere or brought in by the comet) or pyrochemically produced species. Typical rates for such reactions are of the order of lo-’ cm3 s-‘, so that H: depletion would have been rapid. However, the obscuring bright emission from the plumes meant that the reactions could not be monitored via their effect on H: emission intensity at times close to impact. In addition, Hzemission levels are strongly longitudinal dependent, even at latitudes as low as S45’ (see Fig. 6). These factors make interpretation of data
6 I’
1:
Total H3+ Emission
\ !
observations of Northern Emissions First detection a +45 +30 +22 f39
Gone by a +81 f75 +62 +.57
aTime in minutes after to. N.B. First detection does not mean that the phenomenon commenced at this time.
0
‘, 350
I
300
I
I
250
200
I
I
I
I
150
100
50
0
System III Longitude Fig. 6. Hcemission as a function of jovian System III longitude (A,,,) close to the impact latitudes (Lam et al.. 1996)
S. Miller et al.: Impact of SL9 on the jovian ionosphere
1248 !
I
150, x 2 z aJ 100 2
I
I
I
Continuum
I
Subtracted
I
I
I
H,’ Emission
(4 hr after A impact)
1
z ;zi c) 3
I : ._ J F ; 50 fx
1
Profile
aJ .t: rn 4 s
z
: u
; F
I
(2830
,
t-
I
I
1
cm-‘)
“;
I 0’ 180
,,,[,,,; 200
220 Pixels
1
150 -
I
1
Continuum
1
(0.9 arcsec
1
Subtracted
3Q) -#d VI
dh ‘G t 5 100 2 r 1 z 1 ._ 3 t ;
I
1
I
240 / pixel)
I
H,+ Emission
I
1
Profile
I
(2830
,
I
cm-‘)
(10 hr 50 min after E impact)
50 --
X
9 80
1
1
1
200 I
220
240 I
I
Pixels (0.9 arcsec / pixel) H:emission profiles along the S44” line of latitude after Impacts A and E, showing depletion over Site A and possible depletion over Site E (Kim et al., 1996) Fig. 7.
difficult. Kim et al. (1996) were nonetheless able to measure significantly reduced levels of H:emission over the fresh impact sites of Fragments A (clearly) and E (possibly) 4 h 30 min and 10 h 50 min after impact respectively (see Fig. 7). The depletion does not seem to have been long lived, however. Dinelli et al. (1996) report “normal” HTemission levels over the site of Impact G after just 1.2 jovian days.
may have connected to the northern hemisphere in some way. However, it is also possible that the results could be explained by the normal longitudinal variation in H,i emission already remarked upon. Cosmovici et al. (1996) reported detecting radio emission from water high above the jovian homopause a few days after impact. The chemical effect of water would have been, as already noted (Cravens, 1994) to reduce H:concentrations in the ionosphere.
to+ several days : site Hzenhancement and northern aurora1 counterparts
to+ 1 week to ten days : southern aurora1suppression
Schultz et al. (1995) reported enhancement of Hcemission over some of the impact sites and also over their northern magnetic counterparts several days after impact. In particular they found that Hzemission was enhanced over a group of sites clustered around Fragment Q and its northern counterpart also showed this enhancement. It may be that over the impact sites themselves that the ionosphere was warmed slightly by the reflection of solar IR from dust in the stratosphere (Dinelli et al., 1996). This in turn
Images in both UV and IR of the impacts sites 1 week old shows that they were spreading both in longitude and latitude (Fig. 8). Cravens (1994) predicted that material from the impact sites could be carried at high altitudes into the aurora1 regions. IR images taken prior to and during Impact Week showed clear Hzaurorae in both northern and southern spectra (Fig. 9). However, IR spectra and images taken a few days after the week-so between a week and ten days after the impacts them-
1249
S. Miller et ul.: Impact of SL9 on the jovian ionosphere North/south
E(cm1) ratio. data and model
North/south E(cm1) ratio July 25.27 1994
2.5 25
v 3 May 93 x 4 May 93 * 5 May 93
2.0 .s E
1
11
0.5 ,I[ O 350
, 300
,
,
,
,
i
250 200 150 100 System III longitude
I ,g
,
50
0
O
I
,
350
300
Fig. 10. Ratio of H:intensities
in the northern and southern aurorae as a function of A,,, obtained from UKIRT spectra in 1993 (Lam ef al.. 1996)
/
a$
I
250 200 150 100 System III longitude
I
SO
0
North/south E(cml) ratio July 25.27 1994
selves-showed the southern aurora1 region to be much less bright than the north (Miller et al., 1995). Initial comparisons with data obtained a year earlier indicated that there was an enhancement in the northern intensity coupled with a weakening of the southern emission (Miller et al., 1995). However, recalibration of the data showed that the northern enhancement is not more than a factor of two, while the south was depressed by around an order of magnitude. The consequence is that while the ratio of northern to southern emission as a function of longitude is generally between 0.25 and 2.5 (Fig. lo), for data collected on July 25 and 27, 1994, it reached peak values of 20 (Fig. 11). Ruling out other possibilities, it is now considered that chemical depletion of the southern aurora1 Hz along the lines considered by Cravens (1994), probably caused the depression of the emission in that hemisphere (Achilleos et al., work in progress). The transport time of a few days is consistent with that predicted by Cravens (1994) and derivable from the modelling by West et al. (1995) of the behaviour of the particles producing the impact sites. It is also consistent with the detection of high altitude water radio emission by Cosmovici et al. (1996). Depending on the chemical composition of the depleting molecules, concentrations of between lo6 and 107cm-3 would have been required to produce the observed effects (Fig. 12). The southern aurora appeared to re-establish itself over the following week, consistent with estimated settling times (Achilleos et al.. work in progress).
r I O3.50
I
300
I
m
I
250 200 150 IO0 System III longitude
,
so
0
Fig. 11. Ratio of HTintensities
in the northern and southern aurorae as a function of i,,, obtained from UKIRT spectra on July 25 and 27. 1994 (Achilleos rt al., work in progress). Note that the ratio scale is an order of magnitude larger than in Fig. 10, as a result of the quenching of the southern aurora Case A : X = CO Case E : X = H,O
Log ([eI/[eIJ *.......*......
..-a’-OS-
Summary
Following the discussion outlined above, a chronology of Comet SL9’s impact on the jovian ionosphere for a “typical” fragment runs as follows. f= -6to t= -6min
-5h to t,,
“Blinking aurorae” open field lines
Ionospheric heating
as fragment
crosses
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Log [X] (cm-‘) Fig. 12. The effect of chemical depletion by a “metallic” species on the concentration of aurora1 Hz for the case of water and CO. Note that approximately an order of magnitude drop in [Ha is required to account for the observed quenching of the southern aurora on July 25 and 27, 1994
S. Miller et al.: Impact of SL9 on the jovian ionosphere
1250 t=t,to
+40min
t = +20 to 80min r = +a few hours t = + several days
t = +7 to lOdays
Zone of relative ignorance over impact site due to obscuring methane emission. High temperature ionisation in plumes. Northern aurora1 counterpoints Chemical depletion of Hzover impact sites Radio detection of water above homopause Enhancement of Hcemission over impact sites and northern counterparts Quenching of southern IR aurora due to chemical depletion of H:
SL9 has clearly given those interested in the ionosphere and magnetic field structure of Jupiter plenty to think about and work on. Fitting these results into jovian models will provide many useful constraints on parameters otherwise rather too underdetermined for comfort. Acknowledgements. This work was supported by PPPARC (U.K.), CNR (Italy) and CNRS (France), whose financial assistance is gratefully acknowledged.
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The impact of comet Shoemaker-Levy 9 on the jovian ionosphere and aurorae Steven Miller,’ Nicholas Achilleos,’ Hoanh An Lam,’ Bianca Maria Dinelli’ and RenCePrangC,” ‘Department of Physics and Astronomy, University College London, Gower Street, London WClE 6BT. U.K. ‘Istituto di Spettroscopia Molecolare, CNR, Via Gobetti 101, 40129 Bologna, Italy ‘Institut d’Astrophysique Spatiale, CNRS. Bat. 121, Universite de Paris XI. 91405 Orsay Cedex. France Received 7 October 1996; accepted 28 February 1997
n a sensttive tracer of
reserved.
Introduction Logic almost demanded that the jovian ionosphere, as the outermost--and thus first encountered-layer of the atmosphere of Jupiter, should be affected by the impact of Comet Shoemaker--Levy 9 (SL9) in July 1994. Predicted effects varied from a near-complete shutting down of the powerful aurorae (Discussion at the SL9 session of the 1993 AAS Division of Planetary Sciences Meeting) to disruption of the ion-molecule chemistry dominant above the homopause (Cravens, 1994). With this in mind, several of the observing programs prepared for Impact Week set out to measure just what the effect of the cometary fragments might be, whether in terms of the production of novel ionospheric species or significant changes to the power output of the UV and IR aurorae. This review sets out to record what was observed in terms of the final trajectory of a “typical” SL9 fragment, its impact and the aftermath of the collisions. (N.B. Our “typical” fragment, however, is composed of the observations of a number of individual fragments.) Under normal circumstances, far and away the most
powerful ionospheric emission comes from the aurora1 regions. As Jupiter rotates, its magnetic field sweeps up the plasma surrounding the planet. This forms a dense equatorial plasma sheet swirling like a ballerina’s skirt out to over 1 x lo6 km into space. But somewhere between 20 and 30 jovian radii from the centre of Jupiter the plasma speeds required are so great and the field strength is so low, that co-rotation is no longer possible. It is in this region that field aligned currents may accelerate ions and electrons along the magnetic field lines into Jupiter’s upper atmosphere. The brightest aurorae are produced in a tight oval marking the circumpolar footprints of these field lines. At present they are mainly attributed to the precipitation of kiloelectronvolt electrons. But there are also more diffuse aurora1 emissions at lower latitude, possibly also due to other particles scattered by waves generated in the jovian radiation belts, and especially in the outer IO torus (Rezeau et al., 1996) due to the effect of IO’S progress (or. rather, regress) through the magnetic field. (See Thorne, 1983 for a full explanation of these processes.) Since the magnetic poles of Jupiter are displaced from the rotational poles and the field is highly multipolar (Connerney, 1993) the exact location and strength of aurora1 emission is a strong function of the longitude. Jupiter’s aurorae have been studied for many years in the UV (see Livengood et al., 1992), since they were first detected by the Voyager spacecraft and the Earth orbiting FUV observataory IUE. In the UV wavelength region from 1100 to 1700 A emission due to atomic hydrogen Lyman x and the Lyman and Werner bands of Hz is observed. The emitters are collisionally excited by precipitating electrons and then radiate their excess electronic energy away. Typically. Jupiter emits a few x 10” W by this process. But particle impacts can also ionise molecular hydrogen. commencing a chain of reactions which leads to the formation of the H: molecular ion (Atreya and Donahue, 1976). This ion emits strongly in the IR, about 90% of its energy between 3 and 5 pm and the rest mainly at about 2 pm, as
1238
S. Miller et al.: Impact of SL9 on the jovian ionosphere
vibrationally excited molecules relax back to the ground state. Once more a few x lOI W is a typical IR aurora1 output (Lam et al., 1996). Figure 1 shows images of Jupiter in the UV and IR, which show the aurorae clearly, particularly around the north polar region. The UV image, due to Clarke et al. (1995) shows the bright narrow oval tracing the footprint of the last closed field lines, as well as showing a more diffuse emission region which extends both polewards of the oval and to latitudes which encompass the magnetic footprint of the IO Torus (L = 5.9) on the planet. The IR image, taken at a similar central meridian longitude to the UV as part of the NASA IRTF Galileo Monitoring Program, also shows a similar structure. This indicates that, although they are produced by different species and excited by different mechanisms the UV and IR data are sensitive to similar aurora-producing processes. The intensity of the aurorae is known to vary with time by factors of three or more (Livengood et al., 1992; PrangC et al., 1993 ; Baron et al., 1991). As IO moves through Jupiter’s magnetic field, its dynamo effect causes a current of zz 1O6A to be dumped into the planet’s upper atmosphere. In recent years the aurora1 “spot” produced by this current has been detected in both the IR (Connerney, 1993) and UV (PrangC et al., 1996) clearly separated from the main aurorae. Additionally, it has now been demonstrated that H:emits right across the planet (Ballester et al., 1994), producing an emission pattern which shows both latitudinal and longitudinal variations (Lam et al.. 1996). This is shown in Fig. 2. This brief introduction to the emissions observed from the jovian ionosphere and aurorae indicates that, even under “normal” conditions, they are complicated and spatially and temporally very variable. This is important to remember in attempting to ascertain what changes could be ascribed to the impact of SL9 and what might simply be due to the “normal” variability of the planet. In general, the far UV aurorae appeared to be somewhat fainter in the days prior to, during and after the week from July 16-23, which has become known as Impact Week. With this in mind, we will now try to follow the fate of a “typical” SL9 fragment as it was charted by changes to Jupiter’s ionosphere and aurorae. In what follows, the time of impact is designated to.
t,, - 5 h : “blinking”
aurorae
It is arguable that the first direct effects produced on Jupiter by our composite fragment were on the jovian ionosphere. Images of the south polar region taken between 14 : 10 and 14 :45 UT on July 20 by the Hubble Space Telescope (HST) show the aurora1 oval clearly (Fig. 3). But within the oval, a bright spot-as bright as the aurora1 oval itself-was seen to blink on and off with a period of z20min (Clarke et al., 1995). Owing to the limited amount of FUV images of the Jovian aurorae available at the time, doubts were cast on the actual relationship between these spots and the SL9 event. Observations performed since July 1994 have significantly increased the database of aurora1 measurements. It is now clear that although the Jovian aurorae are quite dynamic,
and transient arcs are observed from time to time, they do not seem to appear inside the south polar cap. and none of them exhibit the same shape as the blinking aurora. Dougherty has estimated that the current producing these bright aurora1 spots was of the order of 10’ to 10’ A (personal communication, 1996). Prangt et al. (1995) showed that the spots were located at the footprint of field lines which connected to the incoming Fragment Q, still 5 h-and 7R,-away from impact (Fig. 3). The lack of conjugate emission on the north polar cap could be explained as field lines the fragment was crossing at the time were open, and there was therefore no direct connection to the northern hemisphere. Further analysis showed that the blinking spot was fixed in local time and did not corotate with the magnetosphere during the 35 min of observations. PrangC et al. (1995) were also able to show that Fragment Q (which exhibited the densest and most extended dust coma) was actually the only one which could be magnetically connected to the bright spots. They suggested that the spots were being produced by energetic particle precipitation triggered by field aligned currents flowing along the magnetic flux tube transiently in excess of the critical current density which gives rise to particle acceleration, followed by some kind of relaxation phase, analogous to the charging and discharging of a capacitor. The currents were due principally to the dynamo effect of the motion of the partially charged fragment dust coma with respect to the ambient corotating magnetospheric plasma.
to - 6 min to impact : ionospheric heating Spectra of the eastern limb of Jupiter at a latitude of S44” taken at the United Kingdom Infrared Telescope (UKIRT) on July 17, shortly before the impact of Fragment C, showed an increase in emission by HTwhich could not be ascribed to “normal” spatial variation (Dinelli rt al., 1996). Around 6min prior to impact, the IR lines around 3.5 pm which were being monitored increased in intensity by 70%. According to Waite et al. (1996), for Fragment K this time coincided with an unusually bright northern X-ray emission. 5 min later, about 1 min prior to impact, the Hllines had brightened by a further 50%. This increase in intensity is unlikely to have been caused by an enhanced column density since incoming cometary material, if anything, would chemically deplete H:(Cravens, 1994 ; see later). The brightening was thus almost certainly due to an increase in temperature, heating being effected by cometary material impacting on the ionosphere prior to the main fragment explosion. The narrow wavelength range monitored made it impossible to determine the Hctemperature. However, a recent study by Lam (1995) has shown that ionospheric temperatures in this region are between 800 K and 900 K. Taking this as a starting temperature, the intensity enhancements correspond to temperature increases of 13&f 50 K at tu - 6 min and 190-280 K at to - 1 min. These temperatures represent averages over pixels which covered nearly 3 x ‘km’ on Jupiter, and may therefore represent only lower limits if the heating were more localised. It is possible to estimate how much energy is represented by
S. Miller et al.: Impact of SL9 on the jovian ionosphere
Fig. 1. (a) HST image of Jupiter taken at 1600 A on July 17. 1994. The planet already shows signs of some of the impact scars (Clarke et a/., 1995). (b) 3.8pm image taken in 1995 (NASA TRTF Galileo Imaging Team). Around the northern polar region. both images show the main amoral oval with additional diffuse emission superimposed on it
1239
240
S. Miller et (11.:Impact of SL9 on the jovian ionosphere
Fig. 2. Global Hlemission from derived from UKIRT spectra 1996). (The intensity scale is in ergs-’ cmP2sr-’ times 10)
of Jupiter
taken in 1993 (Lam et al.,
S. Miller
et al.:
Impact of SL9 on the jovian ionosphere
Fig. 3. HST UV images of Jupiter’s southern polar region showing the “blinking aurora” visible as a spot inside the aurora1 arc, but clearly separated from it. in the top and bottom images, and absent in the middle image (Clarke rt al., 1995)
124
242
S. Miller et al.: Impact of SL9 on the jovian ionosphere
Fig. 4. UKIRT echelle spectra of the South 44” line of latitude around the time of the impact of Fragment C (July 17, 1994,07 :12U.T.) (Dinelli et al., 1996). The spectrometer slit is aligned east-west along the line of latitude, with east at the top of the spectral image. The echelle was centred on 3.5337pm. Four Hclines are visible extending across the planet in the first image, about 18 min prior to impact. In the second image, there is a rising continuum on the eastern limb, and the Hzlines are brightened due to heating of the ionosphere. In the third image, the fragment explosion was ejected large quantities of hot methane at speeds up 100 km SK’, and the spectrum of this species swamps all other lines. In the final image, the methane emission has cooled down, to reveal the “normal” ionospheric lines once more
S. Miller ef al.: Impact of SL9 on the jovian ionosphere
Fig. 5. HST UV images of Jupiter 40min and 50min after the impact of Fragment K (Clarke et al., 1995). Bright aurora1 spots are clearly visible in the first image, well separated from the main aurora1 arc and magnetically linked to two fainter spots in the southern hemisphere on either side of the impact site. The northern aurora1 spots are still visible ten minutes later
1243
S. Miller
Fig. 8. HST UV image of Jupiter sites (Clarke et al., 1995)
at the end of Impact
et al.:
Week, showing
Impact of SL9 on the jovian ionosphere
the spreading
of the impact
S. Miller et al.: Impact of SL9 on the jovian ionosphere
Fig. 9. NASA IRTF images of Jupiter at 3.4pm prior to during and after impact week.(Miller et al.. 1995). Note that the southern aurora is almost totally quenched on July 27, compared with a similar view on July 17
I245
S. Miller et ul.: Impact of SL9 on the jovian ionosphere the heating observed. if we assume the Hz temperatures are indicative of the local neutral thermosphere composed mainly of molecular (and atomic) hydrogen. Ionospheric column densities of H: are typically 10” cmw2 and the mixing ratio of this ion is z lop7 in the lo-*-lo-‘bar region where it is important. Taken over the entire pixel, therefore, some 5 x ” mol of Hz would have been heated by between 320 K and 430 K in the 6 min prior to impact. This gives an ionospheric heating, caused by the incoming fragment material, of between 4.5 and 6.0 x “erg.
t, : impact and relative ignorance
From the point of view of studies of the jovian ionosphere, the terminal explosions of the various SL9 fragments were a rather unfortunate occurrence. In all cases, the plumes of hot gas and dust thrown up by the explosions obscured the IR emission from the ionosphere (see Fig. 4). It is clear that large quantities of cometary material and jovian atmosphere were ejected through the ionosphere at velocities anywhere between 20 km s-’ (Hammel et al., 1995) and 100 km sm.’(Dinelli et al., 1996). There is no doubt, therefore, that the ionosphere was enriched by the explosions with chemicals not normally found above the jovian homopause. Between to and to+ lOmin, Ballester et al. (1995) reported that IUE detected H2 emission of an aurora1 nature corresponding to high electron fluxes, presumably due to high temperature ionisation in the rising plume.
to + 20 to 80 min : northern aurora1 counterparts While the south, the aftermath of the impact-the plumes and their subsequent “splashback”-dominated, the effect of the fragments was seen in the northern hemisphere. McGregor and co-workers at the Mount Stromlo observatory in Australia reported a number of bright IR emission spots around the northern latitude of 45”, close to the eastern limb of the planet (McGregor et al., 1996). These emissions were well separated from the normal aurora1 oval and were visible for about half an hour, although the observing program undertaken did not allow them to be monitored continuously. A list of their observations is given in Table 1. Clarke and co-workers using the HST also observed this effect for Fragment K (Fig. 5), and termed it the “northern aurora1 counterpart” (Clarke et al., 1995). Their images showed two well separated northern spots
Table 1. CASPIR
Fragment G K R W
1241
about 45 min after impact, which were no longer visible 30 min later. Hill and Dessler (1995) explained this phenomenon as a result of charged material being ejected into the ionosphere across field lines, creating a Bxv current. This, in turn, accelerated particles along the field lines connecting the affected regions close to the impact sites with their northern magnetic counterparts. Other explanations involve stochastic charged particle acceleration in the turbulent plasma associated with the rising fireball, or increased pitch angle scattering of the equatorial radiation belt by electromagnetic waves generated in the impact site (e.g. Bolton and Thorne, 1995). In this case, the charged particles may have precipitated in the north because the field strength was weaker there than in the south. The times taken for the aurorae to appear were consistent with Alfven wave travel times. t,, + a few hours : ionospheric Hzdepletion
Since the rising plumes placed “metal’‘-rich material (in the astronomical sense) into the jovian ionosphere, unusual chemical reactions could take place. Cravens (1994) demonstrated that the net effect of these reactions would be to deplete the concentration of H,t, through reactions such as : H: +X+XH+
+H,
where X could be molecules such as HzO, CO, NH, or CH, (either dredged up from the lower jovian atmosphere or brought in by the comet) or pyrochemically produced species. Typical rates for such reactions are of the order of lo-’ cm3 s-‘, so that H: depletion would have been rapid. However, the obscuring bright emission from the plumes meant that the reactions could not be monitored via their effect on H: emission intensity at times close to impact. In addition, Hzemission levels are strongly longitudinal dependent, even at latitudes as low as S45’ (see Fig. 6). These factors make interpretation of data
6 I’
1:
Total H3+ Emission
\ !
observations of Northern Emissions First detection a +45 +30 +22 f39
Gone by a +81 f75 +62 +.57
aTime in minutes after to. N.B. First detection does not mean that the phenomenon commenced at this time.
0
‘, 350
I
300
I
I
250
200
I
I
I
I
150
100
50
0
System III Longitude Fig. 6. Hcemission as a function of jovian System III longitude (A,,,) close to the impact latitudes (Lam et al.. 1996)
S. Miller et al.: Impact of SL9 on the jovian ionosphere
1248 !
I
150, x 2 z aJ 100 2
I
I
I
Continuum
I
Subtracted
I
I
I
H,’ Emission
(4 hr after A impact)
1
z ;zi c) 3
I : ._ J F ; 50 fx
1
Profile
aJ .t: rn 4 s
z
: u
; F
I
(2830
,
t-
I
I
1
cm-‘)
“;
I 0’ 180
,,,[,,,; 200
220 Pixels
1
150 -
I
1
Continuum
1
(0.9 arcsec
1
Subtracted
3Q) -#d VI
dh ‘G t 5 100 2 r 1 z 1 ._ 3 t ;
I
1
I
240 / pixel)
I
H,+ Emission
I
1
Profile
I
(2830
,
I
cm-‘)
(10 hr 50 min after E impact)
50 --
X
9 80
1
1
1
200 I
220
240 I
I
Pixels (0.9 arcsec / pixel) H:emission profiles along the S44” line of latitude after Impacts A and E, showing depletion over Site A and possible depletion over Site E (Kim et al., 1996) Fig. 7.
difficult. Kim et al. (1996) were nonetheless able to measure significantly reduced levels of H:emission over the fresh impact sites of Fragments A (clearly) and E (possibly) 4 h 30 min and 10 h 50 min after impact respectively (see Fig. 7). The depletion does not seem to have been long lived, however. Dinelli et al. (1996) report “normal” HTemission levels over the site of Impact G after just 1.2 jovian days.
may have connected to the northern hemisphere in some way. However, it is also possible that the results could be explained by the normal longitudinal variation in H,i emission already remarked upon. Cosmovici et al. (1996) reported detecting radio emission from water high above the jovian homopause a few days after impact. The chemical effect of water would have been, as already noted (Cravens, 1994) to reduce H:concentrations in the ionosphere.
to+ several days : site Hzenhancement and northern aurora1 counterparts
to+ 1 week to ten days : southern aurora1suppression
Schultz et al. (1995) reported enhancement of Hcemission over some of the impact sites and also over their northern magnetic counterparts several days after impact. In particular they found that Hzemission was enhanced over a group of sites clustered around Fragment Q and its northern counterpart also showed this enhancement. It may be that over the impact sites themselves that the ionosphere was warmed slightly by the reflection of solar IR from dust in the stratosphere (Dinelli et al., 1996). This in turn
Images in both UV and IR of the impacts sites 1 week old shows that they were spreading both in longitude and latitude (Fig. 8). Cravens (1994) predicted that material from the impact sites could be carried at high altitudes into the aurora1 regions. IR images taken prior to and during Impact Week showed clear Hzaurorae in both northern and southern spectra (Fig. 9). However, IR spectra and images taken a few days after the week-so between a week and ten days after the impacts them-
1249
S. Miller et ul.: Impact of SL9 on the jovian ionosphere North/south
E(cm1) ratio. data and model
North/south E(cm1) ratio July 25.27 1994
2.5 25
v 3 May 93 x 4 May 93 * 5 May 93
2.0 .s E
1
11
0.5 ,I[ O 350
, 300
,
,
,
,
i
250 200 150 100 System III longitude
I ,g
,
50
0
O
I
,
350
300
Fig. 10. Ratio of H:intensities
in the northern and southern aurorae as a function of A,,, obtained from UKIRT spectra in 1993 (Lam ef al.. 1996)
/
a$
I
250 200 150 100 System III longitude
I
SO
0
North/south E(cml) ratio July 25.27 1994
selves-showed the southern aurora1 region to be much less bright than the north (Miller et al., 1995). Initial comparisons with data obtained a year earlier indicated that there was an enhancement in the northern intensity coupled with a weakening of the southern emission (Miller et al., 1995). However, recalibration of the data showed that the northern enhancement is not more than a factor of two, while the south was depressed by around an order of magnitude. The consequence is that while the ratio of northern to southern emission as a function of longitude is generally between 0.25 and 2.5 (Fig. lo), for data collected on July 25 and 27, 1994, it reached peak values of 20 (Fig. 11). Ruling out other possibilities, it is now considered that chemical depletion of the southern aurora1 Hz along the lines considered by Cravens (1994), probably caused the depression of the emission in that hemisphere (Achilleos et al., work in progress). The transport time of a few days is consistent with that predicted by Cravens (1994) and derivable from the modelling by West et al. (1995) of the behaviour of the particles producing the impact sites. It is also consistent with the detection of high altitude water radio emission by Cosmovici et al. (1996). Depending on the chemical composition of the depleting molecules, concentrations of between lo6 and 107cm-3 would have been required to produce the observed effects (Fig. 12). The southern aurora appeared to re-establish itself over the following week, consistent with estimated settling times (Achilleos et al.. work in progress).
r I O3.50
I
300
I
m
I
250 200 150 IO0 System III longitude
,
so
0
Fig. 11. Ratio of HTintensities
in the northern and southern aurorae as a function of i,,, obtained from UKIRT spectra on July 25 and 27. 1994 (Achilleos rt al., work in progress). Note that the ratio scale is an order of magnitude larger than in Fig. 10, as a result of the quenching of the southern aurora Case A : X = CO Case E : X = H,O
Log ([eI/[eIJ *.......*......
..-a’-OS-
Summary
Following the discussion outlined above, a chronology of Comet SL9’s impact on the jovian ionosphere for a “typical” fragment runs as follows. f= -6to t= -6min
-5h to t,,
“Blinking aurorae” open field lines
Ionospheric heating
as fragment
crosses
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Log [X] (cm-‘) Fig. 12. The effect of chemical depletion by a “metallic” species on the concentration of aurora1 Hz for the case of water and CO. Note that approximately an order of magnitude drop in [Ha is required to account for the observed quenching of the southern aurora on July 25 and 27, 1994
S. Miller et al.: Impact of SL9 on the jovian ionosphere
1250 t=t,to
+40min
t = +20 to 80min r = +a few hours t = + several days
t = +7 to lOdays
Zone of relative ignorance over impact site due to obscuring methane emission. High temperature ionisation in plumes. Northern aurora1 counterpoints Chemical depletion of Hzover impact sites Radio detection of water above homopause Enhancement of Hcemission over impact sites and northern counterparts Quenching of southern IR aurora due to chemical depletion of H:
SL9 has clearly given those interested in the ionosphere and magnetic field structure of Jupiter plenty to think about and work on. Fitting these results into jovian models will provide many useful constraints on parameters otherwise rather too underdetermined for comfort. Acknowledgements. This work was supported by PPPARC (U.K.), CNR (Italy) and CNRS (France), whose financial assistance is gratefully acknowledged.
References Atreya, S. K. and Donahue, T. M. (1976) Model Ionospheres of Jupiter. In Jupiter, ed. T. Gehrels, pp. 304310. University of Arizona Press, Tucson, AZ. Ballester, G. E., Miller, S., Tennyson, J., Geballe, T. R. and Trafton, L. M. (1994) Latitudinal temperature variations of jovian Hz Icarus 107, 189-194. Ballester. G. E. and 21 colleagues (1995) Far ultraviolet emissions during the impact of comet P/Shoemaker-Levy 9 with Jupiter, Geophvs. Res. Lett., 22, 2525-2428. Baron, R., Joseph, R. D., Owen, T., Tennyson, J., Miller, S. and Ballester, G. E. (1991) Imaging Jupiter’s aurorae from H: emissions in the 34 micron band. Nature 353, 539-542. Bolton, S. J. and Thorne, R. M. (1995) Assessment of mechanisms for Jovian synchrotron variability associated with comet SL-9. Geophys. Res. Lett. 22, 1813-1816. Clarke, J. T. et al. (1995) HST far-ultraviolet imaging of Jupiter during the impacts of Comet Shoemaker Levy-g. Science 267, 1302-I 307. Connerney, J. E. P. (1993) Magnetic fields of the outer planets. J. Geophys. Res. 98, 18659-18679. Cosmovici, C. B., Montebugnoli, S., Orfei, A., Pogrebenko. S., and Colom, P. (1996) First evidence of water maser emission indiced by a catastrophic impact. Plan. Space Sci. in press. Cravens, T. E. (1994) Comet SL9 impact with Jupiter : Aeronomical predictions. Geophys. Res. Left. 21, 1 l-14. Dinelli. B. M., Miller, S., Achilleos, N., Lam, H. A., Cahill, M.. Tennyson, J., Jagod, M.-F., Oka. T., Helico, J.-C., and Geballe, T. R. (1996) UKIRT observations of the impact
and consequences of Comet Shoemaker-Levy 9 on Jupiter. Icarus, in press. Hammel, H. B. and 16 colleagues (1995) HST imaging of atmospheric phenomena created by the impact of Comet Shoemaker-Levy 9. Science 267, 1288-1296. Hill, T. W. and Dessler, A. J. (1995) Mid-latitude jovian aurora produced by the impact of Comet Shoemaker-Levy 9. Geophvs. Res. Lett. 22, 1817-1820. Kim, S. J., Orton, G. S., Dumas, C., and Kim, Y. H. (1996) Infrared spectroscopy of Jupiter’s atmosphere after the A and E impacts of Comet Shoemaker-Levy 9. Icarus in press. Lam, H. A. (1995) Monitoring the jovian ionosphere using H: emission as a probe. Ph.D. thesis. University of London. Lam, H. A., Achilleos, N., Miller, S., Tennyson, J., Trafton, L. M., Geballe, T. R., and Ballester, G. E. (1996) A baseline spectroscopic study of the infrared aurorae of Jupiter. Icarus submitted. Livengood, T. A., Moos, H. W., Ballester, G. E. and Prange. R. (1992) Jovian ultraviolet aurora1 activity. 1981-1991. Icarus 97, 2645. Miller, S., Achilleos, N.. Dinelli. B. M., Lam, H. A., Tennyson, J., Jagod, M. F., Geballe, T. R., Trafton, L. M., Joseph, R. D., Ballester, G. E., Baines, K., Brooke. T. Y. and Orton, G. (1995) The effect of the impact of Comet Shoemaker Levy-9 on Jupiter’s aurorae. Geophys. Res. Lett. 12, 162991632. McGregor, P. J.. Nicholson, P. D. and Allen, M. G. (1996) CASPIR observations of the collision of Comet ShoemakerLevy 9 with Jupiter. Icarus 121, 361-388. Prange, R., Zarka, P., Ballester, G. E., Livengood, T. A., Carr, T. A., Reyes, F., Bame, S. and Moos, H. W. (1993) J. Geophys. Res. 98(E9), 18779-l 8792. Prange, R., Engle, I. M., Clarke, J. T., Dunlop, M., Ballester, G. E., Ip, W. H., Maurice, S. and Trauger, J. (1995) Aurora1 signature of Comet Shoemaker Levy-9 in the jovian magnetosphere. Science 267, 13 17-l 320. Prange, R., Rego, D.. Southwood, D., Zarka, P., Miller, S. and Ip, W. (1996) Rapid energy dissipation and variability of the IO-Jupiter electrodynamic circuit. Nature 379, 323-326. Rezeau, L., Cornilleau-Werhlin, N., Belmont, G.. Canu, P., Prange, R.. Balogh, A. and Forsyth, R. J. (1996) Possible role of electromagnetic low frequency waves in the IO torus in the production of Jovian aurorae, submitted to Planet. Space Sci. Schultz, R., Encrenaz, Th., Stuwe, J. and Wiedermann, G. (1995) Monitoring of the near i.r. emission features at the NTT and detection of the northern counterparts. In Proceedings ofthe European SL-g/Jupiter Workshop. Garching, February 1315, 1995, eds. R. M. West and H. Boehnhardt, pp. 363-368. Thorne, R. M. (1983) In Physics of the Jorian Magnetosphere, ed. A.J. Dessler. Cambridge University Press, Cambridge. Waite, J. H., Gladstone, G. R., Lewis. W. S., Fabian A. C. and Brandt. N. (1996) X-ray emission during the SL9 Jupiter impacts. In Abstracts for the Internutionai Conference on the SL9-Jupiter collision, Meudon, 111-6. West, R. A., Karkoschka, E.. Friedson, A. J., Seymour. M., Baines, K. H. and Hammel, H. B. (1995) Impact debris particles in Jupiter’s stratosphere. Science 267, 12961301.