Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 4491c Report Date: 09-May-96:19:38 Version: ********** Check-in Date: ********** 1.Proposal Title: UV SPECTROSCOPY OF THE IO PLASMA TORUS ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID SOLAR SYSTEM GO Sub Category SATELLITES ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Gilda E. Ballester UNIVERSITY OF MICHIGAN USA ------------------------------------------------------------------------------------ 7. Abstract We propose to make the first unambiguous detection of O++ in the hot region of the Io plasma torus near Io's orbit. The abundance of this ion is diagnostic of the ion partioning in this region of the torus where Voyager could not make a definite detection, and upper limits or tentative detections made from ground-based and space -borne instruments give conflicting abundances differing by more than an order of magnitude. The long exposure required for the O++ detection will also make the first measurement of the longitudinal behavior of the strong FUV emission by the S++ torus ions. This needs to be done in the FUV, since longitudinal variations have been observed in the visible and EUV but with different characteristics. Another bonus from this long exposure will be the detection of the neutral S cloud in the torus (far from Io) in its strongest FUV multiplets. These multiplets have more certain atomic physics parameters than for the ones previously detected and will thus provide a better measure of the cloud density relevant to the torus mass loading. We will also make the first high resolution spectroscopic observation of O+, the main torus component near Io. By resolving the emission we will determine the ion's velocity distribution, which is diagnostic of the energy balance and injection of ions into the torus. ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 6.17 par: 0 10. Num targs pri: 1 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: HRS FOS ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 4491c PI: Gilda E. Ballester Proposal Title: UV SPECTROSCOPY OF THE IO PLASMA TORUS ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Pi Gilda E. Ballester 2660 USA Melissa M. Mcgrath 3470 USA Darrell F. Strobel 2380 USA Paul D. Feldman 2380 USA Warren Moos 2380 USA ------------------------------------------------------------------------------------ 2. Scientific Justification. ------------------------------------------------------------------------------------ 3. Description of proposed observations. We propose to make a long FOS G190H exposure with the 4.3" aperture of the hot region of the Io plasma torus near Io's orbit (at a radial distance from Jupiter of 5.7 Jupiter radii). The first goal of this exposure is to make the first unambiguous detection of O++ in its weak 1664 A emission, as the O++ abundance is diagnostic of the ion partioning of this region of the torus. Tentative detections made so far gave conflicting results differing by more than an order of magnitude. This 230 min exposure will span a full Jupiter rotation. The second goal is then to measure the longitudinal behavior of the brighter S++ 1729 A emission, and this will also yield the O++/S++ ion partitioning. The third goal is to measure the abundance of the neutral S cloud of the torus (far from Io), as this cloud may be an important source of torus ions through the pick-up mechanism. We also propose to take a high resolution spectrum of the torus O+ 2470 A emission with GHRS in the Echelle B mode, 2" aperture and 140 min exposure time. This emission was first positively detected with HST (McGrath et al 1993). The sensitivity and effective spectral resolution (0.194 A) are adequate to resolve this emission and determine the O+ velocity distribution in the hot torus. Voyager could not resolve the energy spectrum of each ion and the data was fitted with a common ion temperature of 60-90 eV, while models predict a highly non-Maxwellian velocity distribution for O+ showing the 57km/sec signature of the pick-up process. ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. O++ could not be measured in the hot torus near Io's orbit by the Voyager plasma and UV experiments. Ground-based detection of its 5007 A emission is very difficult, and only upper limits have been obtained. IUE has a camera hot spot overlapping the O++ 1664 A multiplet. HUT made a 2-sigma detection of this multiplet, but there is error in the inferred abundance from the large extension of the aperture and contamination by O+ 834 A emission in 2nd order. HST/FOS is at present the only instrument that can improve the measurement of the O++ abundance. In addition, only HST can measure the longitudinal dependence of the FUV torus emissions, eg, S++ 1729 A: IUE requires more than half of a Jovian rotation. Also, only HST can make a positive detection of the 1814 and 1900 A emission by the neutral S cloud in the torus to provide a reliable density measure since these multiplets have more accurate excitation rates than the 1429 and 1479 A multiplets detected by HUT and the FOT. The 0.194 A spectral resolution of the GRHS Echelle B (extended source in LSA) is adequate to resolve the structure of the O+ 2471 A emission. The ECH-B sensitivity for the brighter S++ 1729 A line is too low. IUE's sensitivity is too low at 2471 A. The velocity distribution is hard to study from the ground: the O+ 3726,3729 A lines observed in low-resolution were weak and their analysis was difficult because they are underlined by a complex solar Fraunhofer background. Page 3 ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. Both observations of the torus west ansa require Io to be far from the field of view, to avoid scattered light contamination and direct signal from this satellite. The same applies to the other Galilean satellites. These are long exposures of a faint target. ------------------------------------------------------------------------------------ 6. Description of special calibration exposures. The long FOS exposure of the torus (SEQ 1.1-1.2) should be made during consecutive HST orbits, in order to assure adequate sampling to measure the longitudinal dependence of S++ emission over a single Jovian rotation (10-hours), as temporal variations may hamper the interpretation of the data. If possible, schedule sequences 2.1-2.2 and 1.1-1.2 close in time for comparison of derived torus parameters, and a more complete set of torus data not affected by temporal variations. ------------------------------------------------------------------------------------ 7. Data reduction and analysis plans. We have extensive experience observing the torus and in reducing HRS and FOS spectra from our cycle 1 Io-torus program as well as other HST programs. The reduction of the new HST spectra should therefore be relatively straightforward. On the analysis, we have a compilation of atomic data for all the ion multiplets involved, and some detailed torus models already exist which can be implemented to address the new aspects of the proposed HST data. One of us has been involved in theoretical modelling of the torus velocity distribution, to be directly compared with the Echelle data. The atomic data for neutral sulfur is limited, but we have already studied the relevant multiplets in detail for our IUE program of SI emission from Io's atmosphere. We have been involved at different levels in observational studies addressing the torus ion-partioning, energetics and mass- loading relevant to the proposed observations. ------------------------------------------------------------------------------------ 8. Additional comments or special requests. None. ------------------------------------------------------------------------------------ 9. Description of previous HST work. Program 2627 was executed on March and May, 1992. The first detection of the Io plasma torus OII 2470 A emission was made, showing significant brightness in most occasions. Based on these results we are now proposing to observe the emission in high dispersion for resolve the ion's velocity distribution. Other results of this program, which mainly concerned Io, include the determination of the spatial extent of the UV emitting region to be within 1 Io radius above the surface, a large improvement over IUE measurements. We also made the first detection of Io's albedo in the 1950--2300 A region. The data shows strong absorption by atmospheric SO2 and detailed modelling of the observed absorption features has yielded SO2 atmospheric abundances much lower than expected from previous upper limit. All Co-Is are involved in other HST projects, but these are not dedicated to studies of the Io torus. ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). Complete computing facilities are in place and available within the Space Physics Research Lab. at the University of Michigan for use on this project. A Sun Sparcstation is available at Michigan and is connected via a local area network, in addition to SPAN and Internet. All Co-I's also have complete computing facilities at their institutions for use on this project, including a Sparcstation dedicated to HST data analysis. Extensive software libraries, including IDL as well as the standard HST data reduction software SDAS, IRAF, etc., are available in these systems. Computer hardware, software, maintenance, and management, will be made available, as well as partial salary support for all Co-Is. Page 4 ------------------------------------------------------------------------------------ 11. Address Information Name: GILDA E. BALLESTER Category: PI Institution: UNIVERSITY OF MICHIGAN Address: AOSS DEPT. City: ANN ARBOR State: MI Zip Code: 481092143 Country: USA Telephone: 313-747-3670 Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST b)Solar System Targets ID = 4491c [ 5] ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: IO-TORUS-W ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: TORUS JUPITER ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = JUPITER | TYPE=TORUS, | LONG=90, | LAT=0, | RAD=4.07E5 | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | SEP OF IO IO-TORUS-W FROM EARTH GT 10", | SEP OF EUROPA IO-TORUS-W FROM EARTH GT | 10", SEP OF GANYMEDE IO-TORUS-W FROM | EARTH GT 10", SEP OF CALLISTO | IO-TORUS-W FROM EARTH GT 10" | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |F-LINE(1729)=2.2 +/- 1 E-14 | WEST ANSA OF IO PLASMA TORUS, AT 2 |W-LINE(1729)=0.2 | RADIAL DISTANCE OF 5.7 JUPITER 3 |F-LINE(1664)=1.1 +/- 0.7 E-15 | RADII, TO BE OBSERVED WITH FOS AND 4 |W-LINE(1664)=0.2 | GHRS. MINIMUM OF ~10" SEPARATION OF 5 |F-LINE(2470)=5.4 +/- 0.5 E-15 | ANSA FROM SATELLITES TO AVOID 6 |W-LINE(2470)=0.4 | CONTAMINATION OF UV SPECTRA. | ------------------------------------------------------------------------------------------------------------------------------------ TARGET LIST b)Solar System Targets ID = 4491c [ 6] ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: IO-TORUS-OFFSET ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = JUPITER | STD = IO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |SURF(V) = 5.0 +/- 0.5 | IMAGES OF IO FOR POINTING ACCURACY 2 |SIZE = 1.0 +/- .2 | INFORMATION OF TORUS OBSERVATIONS. 3 |SURF-CONT(2700) = 2 +/- 1 E-13 | IO IS USED AS AN EXAMPLE - MAY USE | | ANY OTHER GALILEAN SATELLITE IF MORE | | ADEQUATE (EG, IF WEST OF JUPITER). | | CHOSEN SATELLITE MUST BE IN | | SUNLIGHT. THE SURF_CONT(2700) OF | | EUROPA, GANYMEDE AND CALLISTO ARE | | RESPECTIVELY 9.1, 5.6, 2.5 TIMES | | LARGER THAN IO'S (GIVEN IN | | FLUXVAL_3). | ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 4491c [ 7] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1.1 IO-TORUS- FOS/BL IMAGE 4.3 G270H STEP-TIME=1.0 1 4.0 S 10 3 2 GROUP 1.1 - 1.2 OFFSET SUB-STEP=1 NO GAP Y-SIZE=4 CYCLE 3 / 1.1-2.2 Y-SPACE=64 GUIDE TOL 0.5" / COMB=NO 1.1-2.2 Comments: IMAGE OF IO, IF SUNLIT, IN 4.3" FOS APERTURE FOR POINTING ACCURACY INFORMATION. MAY TAKE IMAGE BEFORE OR AFTER TORUS WEST ANSA EXPOSURE 1.2. MAY USE OTHER GALILEAN SATELLITE (IF SUNLIT) WITH SAME EXPOSURE TIME. ------------------------------------------------------------------------------------------------------------------------------------ 1.2 IO-TORUS-W FOS/BL ACCUM 4.3 G190H STEP-PATT=DEF 1 230.0 M 5 1 1 3 5 Comments: G190H SPECTRA OF TORUS WEST ANSA FOR DETECTION OF FAINT O++ EMISSION. TAKE EXPOSURE IN CONSECUTIVE ORBITS TO ATTAIN SECOND SCIENCE GOAL OF MEASURING LONGITUDINAL DEPENDENCE OF S++ EMISSION. ------------------------------------------------------------------------------------------------------------------------------------ 2.1 IO-TORUS- HRS IMAGE 2.0 MIRROR-N2 1 96.8 S 3 2 GROUP 2.1 - 2.2 OFFSET NO GAP Comments: IMAGE OF SUNLIT IO IN HRS LSA FOR POINTING ACCURACY INFO. MAY TAKE IMAGE BEFORE OR AFTER TORUS EXP. 2.2. MAY USE EUROPA, GANYMEDE OR CALLISTO WITH RESPECTIVE EXP. TIMES OF 10.6, 17.4 AND 38.7 S (CORRECT IF NEEDED). ------------------------------------------------------------------------------------------------------------------------------------ 2.2 IO-TORUS-W HRS ACCUM 2.0 ECH-B 2471 STEP-PATT=7 1 140.0 M 5 1 FP-SPLIT=STD COMB=FOUR DOPPLER=ON STEP-TIME=0.2 Comments: ORDER 23, ECHELLE-B SPECTRUM OF TORUS W ANSA OII 2471 A EMISSION, TO MEASURE SPECTRAL PROFILE TO DETERMINE THE VELOCITY DISTRIBUTION OF THESE IONS. IF POSSIBLE, SCHEDULE SEQUENCES 2.1-2.2 AND 1.1-1.2 CLOSE IN TIME. ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 4491c [ 8] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/BL HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters STEP-TIME=1.0 SUB-STEP=1 Y-SIZE=4 Y-SPACE=64 COMB=NO STEP-PATT=DEF STEP-PATT=7 FP-SPLIT=STD COMB=FOUR DOPPLER=ON STEP-TIME=0.2 ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 6.17 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category SATELLITES ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GROUP 1.1 - 1.2 NO GAP; CYCLE 3 / 1.1-2.2; GUIDE TOL 0.5" / 1.1-2.2; GROUP 2.1 - 2.2 NO GAP; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G270H G190H MIRROR-N2 ECH-B ------------------------------------------------------------------------------------------------------------------------------------ Target Names IO-TORUS-W IO-TORUS-OFFSET ------------------------------------------------------------------------------------------------------------------------------------