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GAS-instrument
Advances in Space Research 34 (2004) 61–65 www.elsevier.com/locate/asr
Kinetic parameters of interstellar neutral helium: updated results from the ULYSSES/GAS-instrument M. Witte
a,*
, M. Banaszkiewicz
a,1
, H. Rosenbauer a, D. McMullin
b
a
b
Max-Planck-Institut f€ ur Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany Naval Research Laboratory/Praxis, Inc., Space Science Division, Code 7660, 4555 Overlook Ave SW, Washington, DC 20375, USA Received 8 December 2002; received in revised form 20 January 2003; accepted 27 January 2003
Abstract The GAS-instrument onboard the space probe ULYSSES (ULS) is designed to measure the local angular distribution of the flow of interstellar neutral He-atoms within 3 AU distance from the sun; it allows to infer the kinetic parameters (velocity vector, temperature and density) of these particles outside the heliosphere (‘‘at infinity’’). Around the second fast latitude scan of ULYSSES, from 9/2000 to 9/2002 more than 200 new observations were obtained. The average values derived from these observations together with the results of all previous observations, which were recalculated with a refined pointing calibration, are velocity (v1 ¼ 26:3 0:4 km/s), flow direction (ecliptic longitude l1 ¼ 74:7° 0:5°, ecliptic latitude b1 ¼ 5:2° 0:2°) and temperature (T1 ¼ 6300 340 K). From 1990 to 2002, covering a complete solar cycle, no significant temporal variations of these parameters were observed nor variations with solar latitude. In contrast to that, variations in the density n1 values derived from the local observations were obvious and are interpreted to be due to variations in the loss processes, predominantly photo ionization, the particles experience on their way to the observer. While the temporal variations of the ionization rate can be taken into account from instantaneous values of solar EUV-irradiances, which became available from the CELIAS/SEM instrument on SOHO, the residual variations can be explained by latitudinal variations of the solar irradiance. As a result of a simple model a density n1 in the range (1.2–1.6 102 cm3 ) is deduced. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Interstellar medium; Interstellar neutral helium; Kinetic parameters
1. Introduction From the constituents of the local interstellar medium (LISM), besides high energy cosmic rays, only neutral particle such as He-atoms, can penetrate into the inner solar system and are accessible to in situ measurements. The determination of their kinetic parameters (velocity vector, density, and temperature) immediately provides clues about the physical state of the LISM, surrounding our solar system. Therefore, continued effort has been spent over the past decades, to obtain these data using various remote sensing methods like UV-backscatter *
Corresponding author. Tel.: +49-5556-979-0; fax: +49-5556-979240. E-mail address: [email protected] (M. Witte). 1 On leave from Space Research Centre PAS, Bratycka 18A, 00-716 Warsaw, Poland.
observations or measurements of Doppler-shifts of the absorption lines in the emission of stars as well as pickup ions, which originate from the neutral particles through various ionization processes (for a comprehensive summary see e.g. various authors in Von Steiger et al., 1996). A further, very powerful method became available through the in situ detection of low energy Heatoms by means of sputtering secondary charged particles from a surface coated with pure Lithium Fluoride (LiF). Proposed by Rosenbauer and Fahr (1977) this method was realized in space for the first time with the GAS-instrument on board the space probe ULYSSES. In this paper, we shall summarize all results with respect to the flow vector and the temperature obtained since launch of ULYSSES in October 1990. In addition, we present the progress made in the determination of the density n1 , since time-resolved, instantaneous data on the ionization rates became available through the
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.01.037
62
M. Witte et al. / Advances in Space Research 34 (2004) 61–65
measurements of the CELIAS/SEM instrument on board of SOHO.
2. Observations and data evaluation The GAS-instrument observes the local, angular distribution of the flux of incoming interstellar Heatoms. The detector acts like a pinhole-camera, which by means of an instrument-provided stepping platform and the space craft spin can scan the entire celestial sphere or selected subsections (see Witte et al., 1992). From the measured angular distribution, the kinetic parameters of the interstellar He-atoms outside the heliosphere are derived, with an assumed Maxwellian distribution in V1 , n1 , T1 at infinity. Through ‘‘forward modelling’’ by comparing the observed distributions with the modelled ones, best-fit values of the parameters are determined in a multidimensional fitting process (for details, see Banaszkiewicz et al., 1990, 1996). The sputtering process on a clean LiF-surface, used to convert impacting neutral particles into more easily detectable ions, shows a distinct cut-off in the efficiency at energies below 30 eV (Witte et al., 1992, 1999). Therefore, it requires a favourable combination of the space craft velocity vector and the local flow vector of the particles to achieve a relative energy of the particles in the instruments frame of reference, exceeding this detection threshold. This is the case only during parts of the ULYSSES orbit, when the S/C-velocity is high, resulting in three distinct periods of observations: after launch from November 1990 to February 1991 and during the ‘‘fast latitude scans’’ (FLS) around perihelion from September 1994 to June 1996 and from September 2000 to April 2002. Further data would be obtained only around the next FLS after end of 2006, which is beyond the presently scheduled end of the ULS-mission in 2004. Results of the first two observation periods have been presented earlier by Witte et al., 1993, 1996 (see Table 1). In this paper, the about 200 observations obtained during the second FLS are presented together with the earlier results which need to be updated because of a refinement of the pointing calibration.
3. Pointing accuracy The local, apparent arrival direction of the particles is the one and only one independent observable which can be obtained during the observations. The detection method does not provide information about the mass or energy of the primary particles, so that e.g. the velocity of the particles has to be derived from the amount of their gravitational deflection from their direction ‘‘at infinity’’ to their local direction, which in the case of the GAS-observations ranges from about 5°–22°. In a special program an ‘‘end-to-end’’ test of the pointing accuracy was performed: As the instrument has a residual sensitivity to UVphotons, the signal from UV-light emitting star could be clearly registered. Using the instrument housekeeping data about its instantaneous looking direction of the telescopes and the spacecraft attitude data, the position of a star could be calculated in celestial coordinates. Comparing the results of some 70 star observations with the positions well-known from star catalogues, it turned out that there was a small systematic error in the pointing, in the spacecraft coordinate system corresponding to 0.4°–0.8° in cone angle (elevation) which was slightly temperature dependent, and 0.55° in spin angle (azimuth). Compared to the intrinsic fields of view of the two telescopes of 2.4° and 4.7° FWHM, these errors are small and could be resolved only by applying oversampling and statistical methods. After recalibration, the errors were considerably reduced to )0.07° 0.12° in elevation and 0.05° 0.47° in azimuth. Although this correction of about 0.5° seems to be small, it has a sizable impact on the results of the He-velocity vector (see Table 1). As a consequence, next generation instruments should provide knowledge of the pointing accuracy of at least 0.1°.
4. Results of the flow vector and velocity The results of the forward modelling and fitting process, obtained for individual observations are shown
Table 1 Interstellar He–parameter
Velocity V1 (km/s) Ecl. longitude l1 (°) Ecl. latitude b1 (°) Temperature T1 (K) Density n1 (102 cm3 ) At photo ionization bIon (108 s1 ) No. of observations a
Witte et al. (1996)
1994/1996
2000/2002
Best values
25.0 0.6 74.4 0.9 )5.1 0.5 6600 600 1.4–1.7 6–11 20
26.3 0.4 74.7 0.4 )5.2 0.2 6300 340 1.5b (1.2–1.6)b (6–8)a 103
26.3 0.4 74.7 0.5 )5.2 0.2 6466 400 1.4b (1.2–1.6)b (12–21)a 181
26.3 0.4 74.7 0.5 )5.2 0.2 6300 340 tbd (1.2–1.6)b N/A
During data evaluation actual, instantaneous photo ionization rates were derived from SEM/SOHO data for each He-measurement. Depending on the model assumption about the latitudinal variation of the ionization rate, the quoted values are for RðbÞ ¼ 0:5, the ranges for RðbÞ ¼ 0:0 . . . 1:0. b
M. Witte et al. / Advances in Space Research 34 (2004) 61–65
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Fig. 1. Summary of the components of the flow vector (v1 , L1 , b1 ) and the temperature T1 for the three observational periods. Average values are indicated in the panels and are summarized in Table 1 (see Text).
in Fig. 1. Average values of the parameters are indicated in each panel and are summarized in Table 1. During the 12 year mission covering a complete solar cycle there are no significant temporal variations in the parameters. (Increased scatter/variations in the results in 1994 and 2000 are due to disturbing star background in the direction of observation and are not regarded significant). Also, there is no obvious dependence on the solar latitude of the position of ULS, compare Fig. 2. The values of the temperature in the 1990/91 period should be disregarded as they are corrupted by a nutation motion of the space craft spin axis of up to 4°. 2 This effect leads to a significant broadening of the observed angular distribution of the particles which is typically 8° 2
This nutation is a result of a complicated interplay of thermal bending of the spacecrafts semi-flexible axial boom and the gyro-forces of the spinning spacecraft. This boom points away from the earth along the spin axis and as it is heated by the sun on one side a thermal bending away from the sun occures. While the spacecraft rotates (12 rpm) this bending is not stationary and eventually leads to a complicated nutation-like motion of the spacecraft spin axis. This effect disappears at sufficiently large distances from the sun (>2.5 AU), and was absent around the first perihelion for about two month, because the boom was shaded by the spacecraft body. After the initial occurrence in December 1990, the nutation could be kept within a dead band of 0.2° (half cone) by means of e.g. an active attitude control system (CONSCAN) on board the spacecraft and thanks to the continual effort of a dedicated spacecraft control team, optimizing its operation.
FWHM and could not be resolved from the data because of long integration times. As the width of the observed distribution reflects the thermal velocity distribution, the nutation effect immediately leads to too high temperature values. A closer inspection shows that 1° of nutation (half cone) increases the temperature value by about 800 K. This again sets limits on the stability of the S/Cattitude for next-generation instruments. Accordingly, the values of the velocity components are less reliable with larger error ranges. During the perihelion passages, the nutation was kept below 0.2° by active nutation damping using the S/C-attitude control system. While during the period 1995/96 there was longer periods with zero-nutation compared to the 2000/2002 period, the lower temperature value of this period is assumed to be the more appropriate result (see Table 1). For reference, the values published by Witte et al. (1996) are included in Table 1. The discrepancy in the velocity components between those results and the new ones is due to the removal of the small systematic offset in the pointing of the telescopes and the recalibration described in the previous section stressing the importance of a precise knowledge of the pointing. 5. Determination of the density N‘ From the observed local densities the interstellar density n1 of He can be inferred only with sufficient
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M. Witte et al. / Advances in Space Research 34 (2004) 61–65
Fig. 2. Summary of the parameters contributing to the density determination for the second fast latitude scan of ULS in 2000–2002: Upper panel: He densities, obtained with a latitudinal dependence of the ionization rate for ratios (R of 0.0, 0.5, and 1.0). The long-term variations in the density are smallest for ratios around R 0:5 (red curve), indicating that a reasonable result can be obtained under the assumption that 50% of the total ionization rate is latitude dependent. Middle panel: ionization rate for He particles derived from the irradiances observed from CELIAS/SEM (solid line). Effective ionization rate assuming a latitudinal dependence according to the formula for R ¼ 0:5 (red line). The points have been shifted by up to 13 days (half a solar rotation) to compensate for the longitudinal separation of ULS and SEM/SOHO, Lower panel: Latitude b of ULS and the inclination of the particle’s orbital plane relative to the ecliptic plane.
knowledge about the losses these particles experience on their way to the observer. In case of He, the predominant loss process is photo ionization by solar EUV-light (e.g. Rucinski et al., 1996). Sufficiently precise, timeresolved measurements of integral solar irradiances have become available now from the CELIAS/SEM instrument on SOHO (McMullin et al., private communication), which provides full-disk absolute solar flux measurements of the HeII (30.4 nm)-line as well as the absolute integral flux between 17 and 70 nm (Hovestadt et al., 1995). From these data the applicable ionization rates for in-flowing interstellar He can immediately be derived (McMullin et al., 2002). This allowed us for the first time to apply actual ionization rates to the Hedistributions, observed by the GAS-instrument (Fig. 2, middle panel). The resulting densities are shown in Fig. 2, upper panel by (+) symbols (blue curve). In addition to the scatter of the data, there is still a significant residual variation on a time scale of about half a year.
As it is highly unlikely that the interstellar density varies on such a short time scale, and as temporal variation of the ionization rate have been taken into account we shall attempt to explain these variations by a latitudinal dependence of the ionization rate: In principle the bulk of the observed particles moves on hyperbolic orbits in the one orbital plane which includes the position of the Sun as gravitational centre, the position of the observer (ULS) and the vector of the bulk velocity v1 . As the orbital plane of ULS accidentally is almost perpendicular to the flow direction of He, the inclination of the particle’s orbital plane ranges from 0° to 90° relative to the ecliptic plane when ULS performs its latitudinal swing from )80° to +80° solar latitude (Fig. 2, lower panel). At high inclinations, when the particles travel over the solar pole, they are illuminated predominantly by the polar hemisphere. The irradiance of this hemisphere may not be well represented by the irradiances of the equatorial hemisphere observed by the SEM-instrument from 0° latitude (Iðb ¼ 0Þ). In a first attempt to take into account a latitudinal dependence of the irradiance, we have applied a simple cosine-law in latitude to the equatorial irradiance as shown in the formula in Fig. 2. ‘‘R’’ denotes the ratio of the irradiance, (Ib ), modulated with latitude b, to an omni-directional irradiance Io , with the sum of both giving the total irradiance observed by SEM Iðb ¼ 0Þ ¼ Ib þ Io . As one can see, with R ¼ 0:5 (red curve), much of the variation seen in the case R ¼ 1 (blue curve) of a latitude-independent ionization rate is compensated, while for R ¼ 0 (green curve), the variations are clearly overcompensated. The average values obtained for R ¼ 0:5: n1 ¼ 1:4 102 cm3 and R ¼ 1:0: n1 ¼ 1:6 102 cm3 limit the range for the actual density n1 (Table 1). A very similar effect was observed during the first fast latitude scan of ULS in 1995/95. However, at that time, during solar minimum, the total ionization rate was less than half of that during 2000/2002 around solar maximum, hence the latitudinal variations accordingly were much smaller and were masked by the scatter in the data. Applying the same modelling effort as described above results in essentially the same range of densities.
6. Summary and discussion With respect to the flow vector v1 and the temperature T1 of the interstellar Helium we present the final values (Table 1), which have been obtained from observation during the 12 year mission of ULYSSES, covering a complete solar cycle. Further observations can be obtained only, if the ULYSSES mission should be extended to a third fast latitude scan. Due to a special effort, a small systematic error in the pointing determination could be removed, and after recalibration and reprocessing of all data, the values of these parameters
M. Witte et al. / Advances in Space Research 34 (2004) 61–65
have been determined with so far unprecedented accuracy. They are in extremely well agreement with values determined with completely different methods e.g. from Doppler-shift of absorption lines in the emission lines of stars (e.g. Lallement et al., 2004). It should be noted, that the flow vector of He in turn describes the motion of our solar system through the LISM. This is important information, e.g. for astronomers, because much of their knowledge is based on Doppler shifts in the light of stars, which need to be corrected for by the appropriate components of the proper motion of our solar system. With respect to the density n1 of interstellar Helium, significant progress was possible when time-resolved data of the irradiances of the solar HeII became available from the SOHO/CELIAS/SEM observations, which allowed us for the first time to apply actual ionization rates to the observed He-distributions. However, the resulting values of the densities showed a significant variation with solar latitude of the ULYSSES position. By applying a very simple model we demonstrate that these variations can be explained by a latitudinal variation of the solar irradiance. As a result the value of the interstellar density is expected to be in the range of 1.2–1.6 102 cm3 with a most likely value around 1.4 102 cm3 . The latitudinal dependence simply could be a result of the changing aspect geometry. It is known that the solar EUV-emissions are non-uniformly distributed over the solar surface and are enhanced in the activity regions in a belt close to the equator. It is assumed that these regions contribute significantly different to the ionization of a particle over the poles due to their limb proximity than of a particle at the same distance at equatorial latitudes. This definitely needs to be studied in more detail. Presently, a proposal is submitted to NASA to improve this approach by including latitudinal resolved irradiances in the analysis with the help of EIT-data from SOHO, which will enable us to replace the simple model assumptions with more realistic ionization rates.
Acknowledgements The authors are grateful to the two referees and E. Moebius for the helpful suggestions. Two of us (M.W. and D.M.) greatly benefited from stimulating discus-
65
sions during two workshops at the International Science Institute (ISSI) at Bern in 2000 and 2002. This research was funded by the Deutsches Zentrum f€ ur Luft- und Raumfahrt (DLR) under Grant 50 ON 91035. References Banaszkiewicz, M., Rosenbauer, H., Witte, M. An inverse method of determination of the interstellar neutral gas distribution function, in: Grzedzielski, S., Page, D.E. (Eds.), Physics of the Outer Heliosphere, Proceedings of the 1st COSPAR Colloquium on the ‘Physics of the Outer Heliosphere’, Warsaw, Poland, vol. 1. Pergamon Press, PLC, Oxford, pp. 359–362, 1990. Banaszkiewicz, M., Witte, M., Rosenbauer, H. Determination of interstellar helium parameters from the ULYSSES-NEUTRALGAS experiment: method of data analysis. Astron. Astrophys. Suppl. Ser. 120, 587–602, 1996. Hovestadt, D.M. et al. CELIAS – Charge, element and isotope analysis system for SOHO. Sol. Phys. 162, 441–481, 1995. Lallement, R., Raymond, J.R., Vallerga, J. Diagnostics of the Local Interstellar Medium using particles and UV radiation. Adv. Space Res., this issue, 2004, doi:10.1016/j.asr.2003.02.064. McMullin, D.R., Judge, D.L., Phillips, E., M€ obius, E., Bochsler, P., Wurz, P., Hilchenbach, M., Ipavitch, F. Measuring the Ionization Rate of In-flowing Interstellar Helium with the SOHO/CELIAS/ SEM. in: Proceedings of the SOHO 11 Workshop, ESA SP-508, 2002. Rosenbauer, H., Fahr, H.J. Direct measurements of the fluid parameters of the nearby interstellar gas using helium as tracer, Experiment proposal for the out-of-ecliptic mission, Internal Report, Max-Planck-Institut f€ ur Aeronomie, Katlenburg-Lindau, FRG, 1977. Rucinski, D., Cummings, A.C., Gloeckler, G., Lazarus, A.J., Moebius, E., Witte, M. Ionization processes in the heliosphere – rates and methods of their determination. Space Sci. Rev. 78, 73–84, 1996. Von Steiger, R., Lallement, R., Lee, M.A. (Eds.). The heliosphere in the local interstellar medium, in: Proceedings of the First ISSI Workshop, 6–10 November 1995, Space Sci. Rev. vol. 78, pp. 1–2, 1996. Witte, M., Rosenbauer, H., Keppler, E., Fahr, H., Hemmerich, P., Lauche, H., Loidl, A., Zwick, R. The interstellar neutral-gas experiment on ULYSSES. Astron. Astrophys. Suppl. Ser. 92, 333– 348, 1992. Witte, M., Rosenbauer, H., Banaszkiewicz, M., Fahr, H. The ULYSSES neutral gas experiment: determination of the velocity and temperature of the interstellar neutral helium. Adv. Space Res. 13, 121–130, 1993. Witte, M., Banaszkiewicz, M., Rosenbauer, H. Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment. Space Sci. Rev. 78, 289–296, 1996. Witte, M., Bleszynski, S., Banaszkiewicz, M., Rosenbauer, H. Sputtering efficiency of LiF surfaces on impact of low energy neutral helium (20–80 eV): calibration of the interstellar neutral helium instrument on ULYSSES. Rev. Sci. Instrum. 70, 4404–4411, 1999.
Kinetic parameters of interstellar neutral helium: updated results from the ULYSSES/GAS-instrument M. Witte
a,*
, M. Banaszkiewicz
a,1
, H. Rosenbauer a, D. McMullin
b
a
b
Max-Planck-Institut f€ ur Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany Naval Research Laboratory/Praxis, Inc., Space Science Division, Code 7660, 4555 Overlook Ave SW, Washington, DC 20375, USA Received 8 December 2002; received in revised form 20 January 2003; accepted 27 January 2003
Abstract The GAS-instrument onboard the space probe ULYSSES (ULS) is designed to measure the local angular distribution of the flow of interstellar neutral He-atoms within 3 AU distance from the sun; it allows to infer the kinetic parameters (velocity vector, temperature and density) of these particles outside the heliosphere (‘‘at infinity’’). Around the second fast latitude scan of ULYSSES, from 9/2000 to 9/2002 more than 200 new observations were obtained. The average values derived from these observations together with the results of all previous observations, which were recalculated with a refined pointing calibration, are velocity (v1 ¼ 26:3 0:4 km/s), flow direction (ecliptic longitude l1 ¼ 74:7° 0:5°, ecliptic latitude b1 ¼ 5:2° 0:2°) and temperature (T1 ¼ 6300 340 K). From 1990 to 2002, covering a complete solar cycle, no significant temporal variations of these parameters were observed nor variations with solar latitude. In contrast to that, variations in the density n1 values derived from the local observations were obvious and are interpreted to be due to variations in the loss processes, predominantly photo ionization, the particles experience on their way to the observer. While the temporal variations of the ionization rate can be taken into account from instantaneous values of solar EUV-irradiances, which became available from the CELIAS/SEM instrument on SOHO, the residual variations can be explained by latitudinal variations of the solar irradiance. As a result of a simple model a density n1 in the range (1.2–1.6 102 cm3 ) is deduced. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Interstellar medium; Interstellar neutral helium; Kinetic parameters
1. Introduction From the constituents of the local interstellar medium (LISM), besides high energy cosmic rays, only neutral particle such as He-atoms, can penetrate into the inner solar system and are accessible to in situ measurements. The determination of their kinetic parameters (velocity vector, density, and temperature) immediately provides clues about the physical state of the LISM, surrounding our solar system. Therefore, continued effort has been spent over the past decades, to obtain these data using various remote sensing methods like UV-backscatter *
Corresponding author. Tel.: +49-5556-979-0; fax: +49-5556-979240. E-mail address: [email protected] (M. Witte). 1 On leave from Space Research Centre PAS, Bratycka 18A, 00-716 Warsaw, Poland.
observations or measurements of Doppler-shifts of the absorption lines in the emission of stars as well as pickup ions, which originate from the neutral particles through various ionization processes (for a comprehensive summary see e.g. various authors in Von Steiger et al., 1996). A further, very powerful method became available through the in situ detection of low energy Heatoms by means of sputtering secondary charged particles from a surface coated with pure Lithium Fluoride (LiF). Proposed by Rosenbauer and Fahr (1977) this method was realized in space for the first time with the GAS-instrument on board the space probe ULYSSES. In this paper, we shall summarize all results with respect to the flow vector and the temperature obtained since launch of ULYSSES in October 1990. In addition, we present the progress made in the determination of the density n1 , since time-resolved, instantaneous data on the ionization rates became available through the
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.01.037
62
M. Witte et al. / Advances in Space Research 34 (2004) 61–65
measurements of the CELIAS/SEM instrument on board of SOHO.
2. Observations and data evaluation The GAS-instrument observes the local, angular distribution of the flux of incoming interstellar Heatoms. The detector acts like a pinhole-camera, which by means of an instrument-provided stepping platform and the space craft spin can scan the entire celestial sphere or selected subsections (see Witte et al., 1992). From the measured angular distribution, the kinetic parameters of the interstellar He-atoms outside the heliosphere are derived, with an assumed Maxwellian distribution in V1 , n1 , T1 at infinity. Through ‘‘forward modelling’’ by comparing the observed distributions with the modelled ones, best-fit values of the parameters are determined in a multidimensional fitting process (for details, see Banaszkiewicz et al., 1990, 1996). The sputtering process on a clean LiF-surface, used to convert impacting neutral particles into more easily detectable ions, shows a distinct cut-off in the efficiency at energies below 30 eV (Witte et al., 1992, 1999). Therefore, it requires a favourable combination of the space craft velocity vector and the local flow vector of the particles to achieve a relative energy of the particles in the instruments frame of reference, exceeding this detection threshold. This is the case only during parts of the ULYSSES orbit, when the S/C-velocity is high, resulting in three distinct periods of observations: after launch from November 1990 to February 1991 and during the ‘‘fast latitude scans’’ (FLS) around perihelion from September 1994 to June 1996 and from September 2000 to April 2002. Further data would be obtained only around the next FLS after end of 2006, which is beyond the presently scheduled end of the ULS-mission in 2004. Results of the first two observation periods have been presented earlier by Witte et al., 1993, 1996 (see Table 1). In this paper, the about 200 observations obtained during the second FLS are presented together with the earlier results which need to be updated because of a refinement of the pointing calibration.
3. Pointing accuracy The local, apparent arrival direction of the particles is the one and only one independent observable which can be obtained during the observations. The detection method does not provide information about the mass or energy of the primary particles, so that e.g. the velocity of the particles has to be derived from the amount of their gravitational deflection from their direction ‘‘at infinity’’ to their local direction, which in the case of the GAS-observations ranges from about 5°–22°. In a special program an ‘‘end-to-end’’ test of the pointing accuracy was performed: As the instrument has a residual sensitivity to UVphotons, the signal from UV-light emitting star could be clearly registered. Using the instrument housekeeping data about its instantaneous looking direction of the telescopes and the spacecraft attitude data, the position of a star could be calculated in celestial coordinates. Comparing the results of some 70 star observations with the positions well-known from star catalogues, it turned out that there was a small systematic error in the pointing, in the spacecraft coordinate system corresponding to 0.4°–0.8° in cone angle (elevation) which was slightly temperature dependent, and 0.55° in spin angle (azimuth). Compared to the intrinsic fields of view of the two telescopes of 2.4° and 4.7° FWHM, these errors are small and could be resolved only by applying oversampling and statistical methods. After recalibration, the errors were considerably reduced to )0.07° 0.12° in elevation and 0.05° 0.47° in azimuth. Although this correction of about 0.5° seems to be small, it has a sizable impact on the results of the He-velocity vector (see Table 1). As a consequence, next generation instruments should provide knowledge of the pointing accuracy of at least 0.1°.
4. Results of the flow vector and velocity The results of the forward modelling and fitting process, obtained for individual observations are shown
Table 1 Interstellar He–parameter
Velocity V1 (km/s) Ecl. longitude l1 (°) Ecl. latitude b1 (°) Temperature T1 (K) Density n1 (102 cm3 ) At photo ionization bIon (108 s1 ) No. of observations a
Witte et al. (1996)
1994/1996
2000/2002
Best values
25.0 0.6 74.4 0.9 )5.1 0.5 6600 600 1.4–1.7 6–11 20
26.3 0.4 74.7 0.4 )5.2 0.2 6300 340 1.5b (1.2–1.6)b (6–8)a 103
26.3 0.4 74.7 0.5 )5.2 0.2 6466 400 1.4b (1.2–1.6)b (12–21)a 181
26.3 0.4 74.7 0.5 )5.2 0.2 6300 340 tbd (1.2–1.6)b N/A
During data evaluation actual, instantaneous photo ionization rates were derived from SEM/SOHO data for each He-measurement. Depending on the model assumption about the latitudinal variation of the ionization rate, the quoted values are for RðbÞ ¼ 0:5, the ranges for RðbÞ ¼ 0:0 . . . 1:0. b
M. Witte et al. / Advances in Space Research 34 (2004) 61–65
63
Fig. 1. Summary of the components of the flow vector (v1 , L1 , b1 ) and the temperature T1 for the three observational periods. Average values are indicated in the panels and are summarized in Table 1 (see Text).
in Fig. 1. Average values of the parameters are indicated in each panel and are summarized in Table 1. During the 12 year mission covering a complete solar cycle there are no significant temporal variations in the parameters. (Increased scatter/variations in the results in 1994 and 2000 are due to disturbing star background in the direction of observation and are not regarded significant). Also, there is no obvious dependence on the solar latitude of the position of ULS, compare Fig. 2. The values of the temperature in the 1990/91 period should be disregarded as they are corrupted by a nutation motion of the space craft spin axis of up to 4°. 2 This effect leads to a significant broadening of the observed angular distribution of the particles which is typically 8° 2
This nutation is a result of a complicated interplay of thermal bending of the spacecrafts semi-flexible axial boom and the gyro-forces of the spinning spacecraft. This boom points away from the earth along the spin axis and as it is heated by the sun on one side a thermal bending away from the sun occures. While the spacecraft rotates (12 rpm) this bending is not stationary and eventually leads to a complicated nutation-like motion of the spacecraft spin axis. This effect disappears at sufficiently large distances from the sun (>2.5 AU), and was absent around the first perihelion for about two month, because the boom was shaded by the spacecraft body. After the initial occurrence in December 1990, the nutation could be kept within a dead band of 0.2° (half cone) by means of e.g. an active attitude control system (CONSCAN) on board the spacecraft and thanks to the continual effort of a dedicated spacecraft control team, optimizing its operation.
FWHM and could not be resolved from the data because of long integration times. As the width of the observed distribution reflects the thermal velocity distribution, the nutation effect immediately leads to too high temperature values. A closer inspection shows that 1° of nutation (half cone) increases the temperature value by about 800 K. This again sets limits on the stability of the S/Cattitude for next-generation instruments. Accordingly, the values of the velocity components are less reliable with larger error ranges. During the perihelion passages, the nutation was kept below 0.2° by active nutation damping using the S/C-attitude control system. While during the period 1995/96 there was longer periods with zero-nutation compared to the 2000/2002 period, the lower temperature value of this period is assumed to be the more appropriate result (see Table 1). For reference, the values published by Witte et al. (1996) are included in Table 1. The discrepancy in the velocity components between those results and the new ones is due to the removal of the small systematic offset in the pointing of the telescopes and the recalibration described in the previous section stressing the importance of a precise knowledge of the pointing. 5. Determination of the density N‘ From the observed local densities the interstellar density n1 of He can be inferred only with sufficient
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Fig. 2. Summary of the parameters contributing to the density determination for the second fast latitude scan of ULS in 2000–2002: Upper panel: He densities, obtained with a latitudinal dependence of the ionization rate for ratios (R of 0.0, 0.5, and 1.0). The long-term variations in the density are smallest for ratios around R 0:5 (red curve), indicating that a reasonable result can be obtained under the assumption that 50% of the total ionization rate is latitude dependent. Middle panel: ionization rate for He particles derived from the irradiances observed from CELIAS/SEM (solid line). Effective ionization rate assuming a latitudinal dependence according to the formula for R ¼ 0:5 (red line). The points have been shifted by up to 13 days (half a solar rotation) to compensate for the longitudinal separation of ULS and SEM/SOHO, Lower panel: Latitude b of ULS and the inclination of the particle’s orbital plane relative to the ecliptic plane.
knowledge about the losses these particles experience on their way to the observer. In case of He, the predominant loss process is photo ionization by solar EUV-light (e.g. Rucinski et al., 1996). Sufficiently precise, timeresolved measurements of integral solar irradiances have become available now from the CELIAS/SEM instrument on SOHO (McMullin et al., private communication), which provides full-disk absolute solar flux measurements of the HeII (30.4 nm)-line as well as the absolute integral flux between 17 and 70 nm (Hovestadt et al., 1995). From these data the applicable ionization rates for in-flowing interstellar He can immediately be derived (McMullin et al., 2002). This allowed us for the first time to apply actual ionization rates to the Hedistributions, observed by the GAS-instrument (Fig. 2, middle panel). The resulting densities are shown in Fig. 2, upper panel by (+) symbols (blue curve). In addition to the scatter of the data, there is still a significant residual variation on a time scale of about half a year.
As it is highly unlikely that the interstellar density varies on such a short time scale, and as temporal variation of the ionization rate have been taken into account we shall attempt to explain these variations by a latitudinal dependence of the ionization rate: In principle the bulk of the observed particles moves on hyperbolic orbits in the one orbital plane which includes the position of the Sun as gravitational centre, the position of the observer (ULS) and the vector of the bulk velocity v1 . As the orbital plane of ULS accidentally is almost perpendicular to the flow direction of He, the inclination of the particle’s orbital plane ranges from 0° to 90° relative to the ecliptic plane when ULS performs its latitudinal swing from )80° to +80° solar latitude (Fig. 2, lower panel). At high inclinations, when the particles travel over the solar pole, they are illuminated predominantly by the polar hemisphere. The irradiance of this hemisphere may not be well represented by the irradiances of the equatorial hemisphere observed by the SEM-instrument from 0° latitude (Iðb ¼ 0Þ). In a first attempt to take into account a latitudinal dependence of the irradiance, we have applied a simple cosine-law in latitude to the equatorial irradiance as shown in the formula in Fig. 2. ‘‘R’’ denotes the ratio of the irradiance, (Ib ), modulated with latitude b, to an omni-directional irradiance Io , with the sum of both giving the total irradiance observed by SEM Iðb ¼ 0Þ ¼ Ib þ Io . As one can see, with R ¼ 0:5 (red curve), much of the variation seen in the case R ¼ 1 (blue curve) of a latitude-independent ionization rate is compensated, while for R ¼ 0 (green curve), the variations are clearly overcompensated. The average values obtained for R ¼ 0:5: n1 ¼ 1:4 102 cm3 and R ¼ 1:0: n1 ¼ 1:6 102 cm3 limit the range for the actual density n1 (Table 1). A very similar effect was observed during the first fast latitude scan of ULS in 1995/95. However, at that time, during solar minimum, the total ionization rate was less than half of that during 2000/2002 around solar maximum, hence the latitudinal variations accordingly were much smaller and were masked by the scatter in the data. Applying the same modelling effort as described above results in essentially the same range of densities.
6. Summary and discussion With respect to the flow vector v1 and the temperature T1 of the interstellar Helium we present the final values (Table 1), which have been obtained from observation during the 12 year mission of ULYSSES, covering a complete solar cycle. Further observations can be obtained only, if the ULYSSES mission should be extended to a third fast latitude scan. Due to a special effort, a small systematic error in the pointing determination could be removed, and after recalibration and reprocessing of all data, the values of these parameters
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have been determined with so far unprecedented accuracy. They are in extremely well agreement with values determined with completely different methods e.g. from Doppler-shift of absorption lines in the emission lines of stars (e.g. Lallement et al., 2004). It should be noted, that the flow vector of He in turn describes the motion of our solar system through the LISM. This is important information, e.g. for astronomers, because much of their knowledge is based on Doppler shifts in the light of stars, which need to be corrected for by the appropriate components of the proper motion of our solar system. With respect to the density n1 of interstellar Helium, significant progress was possible when time-resolved data of the irradiances of the solar HeII became available from the SOHO/CELIAS/SEM observations, which allowed us for the first time to apply actual ionization rates to the observed He-distributions. However, the resulting values of the densities showed a significant variation with solar latitude of the ULYSSES position. By applying a very simple model we demonstrate that these variations can be explained by a latitudinal variation of the solar irradiance. As a result the value of the interstellar density is expected to be in the range of 1.2–1.6 102 cm3 with a most likely value around 1.4 102 cm3 . The latitudinal dependence simply could be a result of the changing aspect geometry. It is known that the solar EUV-emissions are non-uniformly distributed over the solar surface and are enhanced in the activity regions in a belt close to the equator. It is assumed that these regions contribute significantly different to the ionization of a particle over the poles due to their limb proximity than of a particle at the same distance at equatorial latitudes. This definitely needs to be studied in more detail. Presently, a proposal is submitted to NASA to improve this approach by including latitudinal resolved irradiances in the analysis with the help of EIT-data from SOHO, which will enable us to replace the simple model assumptions with more realistic ionization rates.
Acknowledgements The authors are grateful to the two referees and E. Moebius for the helpful suggestions. Two of us (M.W. and D.M.) greatly benefited from stimulating discus-
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