Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 5466 Report Date: 18-Jul-95:17:27 Version: ********** Check-in Date: ********** 1.Proposal Title: THE INTERACTION BETWEEN IO AND THE PLASMA TORUS: DIRECT MEASUREMENT OF THE MASS LOADING RATE CYCLE4MEDIUM ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID SOLAR SYSTEM GO Sub Category SATELLITES ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Imke De pater 1500 US 510-642-1947 ------------------------------------------------------------------------------------ 7. Abstract We propose to make the first direct measurement of the slow-down of the Io plasma torus in the immediate vicinity of Io. The slow-down is a direct measure of the rate of ionization of neutral material from Io that will provide critical information about the origin and maintenance of the torus, about the details of the interaction between the torus and the satellite, and about the nature and coverage of the Io atmosphere. Current understanding in these areas is extremely tenuous due to the lack of concrete observations. We will unambiguously determine the slow-down at Io by measuring the rotation velocity of the torus from the Doppler shift of OII 2471 A emission observed with Io in the aperture as the torus plane passes through the satellite. This method has been used in our intensive ground-based study to measure the slow-down, and thus the mass loading rate, at almost all points in the torus, however no visible wavelength measurements are possible at the crucial point where the torus passes through Io because the satellite is far too bright at these wavelengths to allow detection of the ion emissions. By contrast, in the UV the Io continuum is negligible compared with the torus emissions, and the measurement of the slow-down is comparatively easy. The results coming from this observation will have a large and immediate impact on our knowledge of the Io atmosphere and of the physical mechanisms involved in the torus-Io interaction. ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 5.05 par: 0 10. Num targs pri: 2 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: HRS ------------------------------------------------------------------------------------ 12. Special sched req: Time Critical obs. ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 5466 PI: Imke De pater Proposal Title: THE INTERACTION BETWEEN IO AND THE PLASMA TORUS: DIRECT MEASUREMENT OF THE MASS LOADING RATE CYCLE4MEDIUM ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Imke De Pater 1500 USA Michael Brown 1500 USA Melissa Mcgrath 3470 USA ------------------------------------------------------------------------------------ 3. Description of proposed observations. We propose high-resolution observations of OII 2471 A emission in the Io plasma torus using the GHRS echelle-B to precisely measure the Doppler shift of the emission line, from which the mass loading rate in the torus can be inferred. The emission line will be observed with Io in the aperture at dusk (western) elongation as the torus plane is passing through Io and also in the same spot but with Io far from the aperture and the torus in the same geometric configuration. A comparison of the velocity shift between these two spectra will yield a direct measure of the slow-down caused by mass loading in the immediate vicinity of the satellite. The OII line was chosen because it will give the most counts of any known torus emission line (the SII emission at 1729 A is brighter, but echelle-B is approximately twenty times less sensitive at that wavelength), and we will observe at western elongation because torus emissions are known to be brightest on that side. Ground-based observations of [SII] 6731 A emission show typical FWHM linewidths of about 25 km/s, so we expect a similar value for the OII emission. Filled-aperture resolution at 2471 A is about 0.17 A, or 20 km/s, so we expect this emission to be marginally resolved. For significant scientific results, we require the ability to distinguish velocity shifts of about 4 km/s, which should be achievable with moderate signal to noise at this resolution. Page 3 ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. The goal of this program is to observe the velocity of the Io plasma torus as it passes through Io. Because Io will be within the aperture of the HRS during the exposures, this observation can only be done in the UV. At visible wavelengths, continuum emission from Io swamps the relatively weak torus emission. This fact has been confirmed by our extensive ground based observations of the rotation velocity of the torus (Brown 1992); we have shown that even with long and repeated exposures and careful continuum subtraction it is impossible to obtain a usable visible wavelength spectrum as the torus plane passes through Io. In contrast, in the UV continuum emission from Io is negligible, so obtaining a good spectrum of the torus passing through Io is comparatively easy. The only other observatory with UV capability is the IUE, but it has neither the required sensitivity (no torus emissions are detectable at high resolution) nor the required spectral resolution to measure a small deviation from corotation velocity (on the order of 4 km/s) in the immediate vicinity of Io. The comparison spectrum, taken without Io in the aperture, could, in principle, be observed elsewhere, but we feel very strongly that because we require the ability to measure a velocity shift as small as 1/5 of a resolution element (filled-aperture resolution at 2471 A is about 20 km/s) precise and reliable scientific results can only be obtained by a direct comparison between two spectra of the same emission line taken with the same instrument under the same observing conditions. The brightness of the OII 2471 A emission has been measured by the GHRS at lower resolution to be about 50 R (McGrath et al. 1993). Using the most recent Echelle -B sensitivity and ripple correction we calculate a count rate of 1.0 X 10^-2 counts/diode-sec. For the S/N calculation we included an average dark count rate of 1.2 X 10^-2 counts/diode-sec, a value of 0.03 for the inter-order background grating scattered light, and 0.11 for the fraction of time spent measuring the inter-order background during the observation. Because these extended emissions will be oversampled by the large aperture, we will bin the data by a factor of two to increase S/N at no expense of resolution. We then find that an exposure time of 160 min is required to maintain a S/N of 6. It is crucial that these observations are made as Io is inside the central part of the torus. Io never stays within this region for more than 120 minutes, so an observation can never be made over more than two orbits. Including time for on-board acquisition during the first orbit, we assume that we will obtain about 40 minutes of on- target exposure during two orbits. Thus the observation with Io in the aperture will require four separate visits to complete a 160 minute exposure. Appropriate torus-Io geometric configurations occur approximately 10 times every 35 days. The exposures without Io in the aperture need to reproduce as closely as possible the torus configuration of the preceding observations, thus we also estimate four separate visits to complete the 160 minute exposure. ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. This program is highly time-critical. The goal is to observe the Io plasma torus as the torus plane passes through Io at western elongation, so the observations must be scheduled carefully. (Correct torus-Io geometric configurations occur about once every 3.5 days.) Because Io only stays within the central part of the torus for about 120 minutes, each observation can last no more than two orbits. We will therefore require four separate visits, each at the appropriate torus-Io geometry, to complete the observation. The spectra will be compared with similar spectra taken with the torus in the same geometric configuration but with Io far from the field of view. These observations will need to be scheduled to reproduce as closely as possible torus configuration of the first observations in order to make a meaningful comparison between the two spectra. (Correct torus geometric configurations with Io far from the aperture occur about once every twenty hours). In addition, all observations should take place when the other Galilean satellites are far removed to avoid contaminating light. Page 4 ------------------------------------------------------------------------------------ 9. Description of previous HST work. 1. GO 2627 - Io's Atmosphere and its Interaction with the Plasma Torus M. McGrath, co-I. 2. GO 2625 - Excitation Processes for the Outer Planet UV Emissions, M. McGrath co-I. 3. GO 3616 - The Upper Atmospheres of Uranus and Neptune, M. McGrath, PI. 4. GO 3617 - The Ultraviolet Emissions of Titan, M. McGrath, PI. 5. GO 3618 - Excitation Processes for the Outer Planet UV Emissions- - Cycle 2 Continuation, M. McGrath, PI. 6. GO 4353 - Hubble Space Telescope Imaging of Io, M. McGrath, PI. 7. GO 4600 - FUV Spectrscopy of Io's SO2 Atmosphere and Surface, M. McGrath, co-I. 1. GO 2627 - cycle 1 program, observations completed successfully in March and May 1992. Observations have resulted in the first detection of [OII]2471 emission from the Io plasma torus; the first UV detection of the Io SO2 atmospheric absorption bands (only the third time the SO2 atmosphere of Io has been detected directly) yielding a measurement of the atmospheric density, pressure, and coverage; and spatial scans of Io neutral S and O atmospheric emissions, resulting in a factor of 3 improvement in the spatial resolution of these constituents compared with previous IUE measurements. 2. GO 4353 - cycle 3 program, not yet completed. Some data obtained on May 10, 1993; remaining observations scheduled for week of May 24, 1993. No results to date. 3. GO 4600 - no data obtained to date. ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). These observation fit into our ongoing ground-based (at Berkeley) and space- based (at STScI) programs to determine the interaction between Io and the plasma torus and the Jovian magnetosphere. The data will be analyzed using the same facilities and techniques that are being used in our current study of torus rotation velocities. We will provide extensive ground-based support from Lick Observatory during the periods of the observations to characterize the state of the torus and the neutral clouds, and we also hope to obtain observations of the Io-torus system from the Keck telescope during these times. ------------------------------------------------------------------------------------ 11. Address Information Name: Imke de Pater Category: PI Institution: 1500 Address: Department of Astronomy University of California City: Berkeley State: CA Zip Code: 94720 Country: USA Telephone: 510-642-1947 Telex (or e-mail): imke@bkyast.berkeley.edu ------------------------------------------------------------------------------------ TARGET LIST b)Solar System Targets ID = 5466 [ 5] ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: TORUS-WITH-IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: TORUS JUPITER ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=JUPITER | STD=IO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | WND1=25-FEB-94 TO 22-JUN-94, | WND2=1-DEC-94 TO 1-JAN-99, | OLG OF IO BETWEEN 246 280, CML OF | JUPITER FROM EARTH BETWEEN 109 235 | SEP OF GANYMEDE IO FROM EARTH GT 5", | SEP OF EUROPA IO FROM EARTH GT 5", | SEP OF CALLISTO IO FROM EARTH GT 5" | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |V=5.02 | EXPOSURE OF THE IO PLASMA TORUS AT 2 |W-LINE(2471)=0.21 +/- 0.05 | THE POSITION OF IO WITH IO AT 2 |SURF-LINE(2471)=13 +/- 4 E-15 | ELONGATION, AND THE TORUS AT A CML 2 |SIZE=20 +/- 5 | OF NEAR 200 IN ORDER FOR IO TO BE IN | | THE DENSEST REGION OF TORUS. CML OF | | JUPITER BETWEEN 289 55 IS | | ACCEPTABLE, AS LONG AS ALL OBS ARE | | MADE IN THIS RANGE. | ------------------------------------------------------------------------------------------------------------------------------------ TARGET LIST b)Solar System Targets ID = 5466 [ 6] ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: TORUS-WITHOUT-IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: TORUS JUPITER ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=JUPITER | TYPE=TORUS, | LONG=90, | LAT=0, | RAD=4.22E05, | POLE_LAT=+90 | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | WND1=25-FEB-94 TO 22-JUN-94, | WND2=1-DEC-94 TO 1-JAN-99, | OLG OF IO BETWEEN 0 180, | CML OF JUPITER FROM EARTH | BETWEEN 109 235 | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |W-LINE(2471)=0.21 +/- 0.05 | EXPOSURE OF THE IO PLASMA TORUS WELL 1 |SURF-LINE(2471)=13 +/- 4 E-15 | AWAY FROM IO, BUT WITH THE SAME CML 1 |SIZE=20 +/- 5 | OF TARGET 1. THE EXPOSURE SHOULD BE | | IN THE ORBITAL PLANE OF IO, AND NOT | | IN THE CENTRIFUGAL PLANE OF THE | | TORUS (HENCE POLE_LATE= +90). CML OF | | JUPITER BETWEEN 289 55 IS | | ACCEPTABLE, AS LONG AS ALL OBS ARE | | MADE IN THIS RANGE. | ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 5466 [ 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 TORUS-WITH- HRS ACQ 2.0 MIRROR-N2 SEARCH-SIZE=5 1 10S 1 1 CYCLE 4 IO BRIGHT=RETURN ONBOARD ACQ FOR 2 GROUP 1-11 WITHIN 120D group 1-3 within 1 d Comments: FROM PREVIOUS IO ACQUISITIONS, WE ESTIMATE A STEP TIME OF 0.4 SEC. BECAUSE OF TEMPORAL CHANGES OBSERVED WITHIN THE TORUS, ALL EXPOSURES SHOULD BE WITHIN ONE JOVIAN APPARITION. ------------------------------------------------------------------------------------------------------------------------------------ 2 TORUS-WITH- HRS ACCUM 2.0 ECH-B 2471 STEP-PATT=DEF 8 5M 6 2 1 CYCLE 4 IO FP-SPLIT=STD 160M CENSOR=YES Comments: GO IMMEDIATELY TO ACCUM MODE. NO PEAKUP IS PERFORMED, BECAUSE TARGET IS TORUS SURROUNDING IO, SO CENTERING OF IO W/O APERTURE IS UNIMPORTANT. ------------------------------------------------------------------------------------------------------------------------------------ 3 TORUS- HRS ACCUM 2.0 ECH-B 2471 STEP-PATT=DEF 11 4.4M 6 1 2 CYCLE 4 WITHOUT-IO FP-SPLIT=STD 160M CENSOR=YES Comments: TARGET IS IO PLASMA TORUS. BLIND POINTING TO WITHIN A FEW ARC-SECS SHOULD BE SUFFICIENT, SO NO ACQUISITION IS PERFORMED. ------------------------------------------------------------------------------------------------------------------------------------ 4 TORUS-WITH- HRS ACQ 2.0 MIRROR-N2 SEARCH-SIZE=5 1 10S 1 1 CYCLE 4 IO BRIGHT=RETURN ONBOARD ACQ FOR 5 group 4-6 within 1 d Comments: FROM PREVIOUS IO ACQUISITIONS, WE ESTIMATE A STEP TIME OF 0.4 SEC. BECAUSE OF TEMPORAL CHANGES OBSERVED WITHIN THE TORUS, ALL EXPOSURES SHOULD BE WITHIN ONE JOVIAN APPARITION. ------------------------------------------------------------------------------------------------------------------------------------ 5 TORUS-WITH- HRS ACCUM 2.0 ECH-B 2471 STEP-PATT=DEF 8 5M 6 2 1 CYCLE 4 IO FP-SPLIT=STD 160M CENSOR=YES Comments: GO IMMEDIATELY TO ACCUM MODE. NO PEAKUP IS PERFORMED, BECAUSE TARGET IS TORUS SURROUNDING IO, SO CENTERING OF IO W/O APERTURE IS UNIMPORTANT. ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 5466 [ 8] ------------------------------------------------------------------------------------------------------------------------------------ 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 ------------------------------------------------------------------------------------------------------------------------------------ 6 TORUS- HRS ACCUM 2.0 ECH-B 2471 STEP-PATT=DEF 11 4.4M 6 1 2 CYCLE 4 WITHOUT-IO FP-SPLIT=STD 160M CENSOR=YES Comments: TARGET IS IO PLASMA TORUS. BLIND POINTING TO WITHIN A FEW ARC-SECS SHOULD BE SUFFICIENT, SO NO ACQUISITION IS PERFORMED. ------------------------------------------------------------------------------------------------------------------------------------ 7 TORUS-WITH- HRS ACQ 2.0 MIRROR-N2 SEARCH-SIZE=5 1 10S 1 1 CYCLE 4 IO BRIGHT=RETURN ONBOARD ACQ FOR 8 group 7-9 within 1 d Comments: FROM PREVIOUS IO ACQUISITIONS, WE ESTIMATE A STEP TIME OF 0.4 SEC. BECAUSE OF TEMPORAL CHANGES OBSERVED WITHIN THE TORUS, ALL EXPOSURES SHOULD BE WITHIN ONE JOVIAN APPARITION. ------------------------------------------------------------------------------------------------------------------------------------ 8 TORUS-WITH- HRS ACCUM 2.0 ECH-B 2471 STEP-PATT=DEF 8 5M 6 2 1 CYCLE 4 IO FP-SPLIT=STD 160M CENSOR=YES Comments: GO IMMEDIATELY TO ACCUM MODE. NO PEAKUP IS PERFORMED, BECAUSE TARGET IS TORUS SURROUNDING IO, SO CENTERING OF IO W/O APERTURE IS UNIMPORTANT. ------------------------------------------------------------------------------------------------------------------------------------ 9 TORUS- HRS ACCUM 2.0 ECH-B 2471 STEP-PATT=DEF 11 4.4M 6 1 2 CYCLE 4 WITHOUT-IO FP-SPLIT=STD 160M CENSOR=YES Comments: TARGET IS IO PLASMA TORUS. BLIND POINTING TO WITHIN A FEW ARC-SECS SHOULD BE SUFFICIENT, SO NO ACQUISITION IS PERFORMED. ------------------------------------------------------------------------------------------------------------------------------------ 10 TORUS-WITH- HRS ACQ 2.0 MIRROR-N2 SEARCH-SIZE=5 1 10S 1 1 CYCLE 4 IO BRIGHT=RETURN ONBOARD ACQ FOR 11 group 10-11 within 1 d Comments: FROM PREVIOUS IO ACQUISITIONS, WE ESTIMATE A STEP TIME OF 0.4 SEC. BECAUSE OF TEMPORAL CHANGES OBSERVED WITHIN THE TORUS, ALL EXPOSURES SHOULD BE WITHIN ONE JOVIAN APPARITION. ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 5466 [ 9] ------------------------------------------------------------------------------------------------------------------------------------ 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 ------------------------------------------------------------------------------------------------------------------------------------ 11 TORUS-WITH- HRS ACCUM 2.0 ECH-B 2471 STEP-PATT=DEF 8 5M 6 2 1 CYCLE 4 IO FP-SPLIT=STD 160M CENSOR=YES Comments: GO IMMEDIATELY TO ACCUM MODE. NO PEAKUP IS PERFORMED, BECAUSE TARGET IS TORUS SURROUNDING IO, SO CENTERING OF IO W/O APERTURE IS UNIMPORTANT. ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 5466 [ 10] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SEARCH-SIZE=5 BRIGHT=RETURN STEP-PATT=DEF FP-SPLIT=STD CENSOR=YES ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 5.05 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category SATELLITES ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CYCLE 4; ONBOARD ACQ FOR 2; GROUP 1-11 WITHIN 120D; group 1-3 within 1 d; CYCLE 4 ONBOARD ACQ FOR 5; group 4-6 within 1 d; ONBOARD ACQ FOR 8; group 7-9 within 1 d; ONBOARD ACQ FOR 11; group 10-11 within 1 d; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N2 ECH-B ------------------------------------------------------------------------------------------------------------------------------------ Target Names TORUS-WITH-IO TORUS-WITHOUT-IO ------------------------------------------------------------------------------------------------------------------------------------