References

1. D.H. Humes, J.M. Alvarez, R.L. O'Neal and W.H. Kinard, J. Geophys. Res. 79 (1974) 3677.

2. R.W. Fillius, C.E. Mcllwain and A. M ogro- Cam pero, Science 188 (1975) 465.

3. M.H. Acuña and N.F. Ness, J. Geophys. Res. 81 (1976) 2917.

4. T.V. Johnson, G. Morfill, and E. Grün, Geophys. Res. Lett. 7 (1980) 305.

5. G.E. Morfill, E. Grün and T.V. Johnson, Planet. Space Sei. 28 (1980) 1087.

6. E. Grün, H. Fechtig, J. Kissel, D. Linkert, D. Maas, J.A.M. McDonnell, G.E. Morfill, G. Schwehm, H.A. Zook and R.H. Giese, Astron. Astrophys. Supp. 92 (1992) 411.

7. E. Grün, H.A. Zook, M. Baguhl, A. Balogh, S.J. Bame, H. Fechtig, R. Forsyth, M.S. Hanner, M. Horányi, J. Kissel, B.A. Lindblad, D. Linkert, G. Linkert, I. Mann, J.A.M. McDonnell, G.E. Morfill, J.L. Phillips, C. Polanskey, G. Schwehm, N. Sid-dique, P. Staubach, J. Svestka and A. Taylor, Nature 362 (1993) 428.

8. M. Baguhl, E. Grün, G. Linkert, D. Linkert and N. Siddique, Planet. Space Sei. 41 (1993) 1085.

9. E. Grün, H. Fechtig, M.S. Hanner, J. Kissel, B.A. Lindblad, D. Linkert, D. Maas, G.E. Morfill and H. A. Zook, Space Science Reviews 60 (1992) 317.

10. E. Grün, M. Baguhl, D.P. Hamilton, R. Riemann, H.A. Zook, S. Dermott, H. Fechtig, B.A. Gustafson, M.S. Hanner, M. Horányi, K.K. Khurana, J. Kissel, M. Kivelson,

B.A. Lindblad, D. Linkert, G. Linkert, I. Mann, J.A.M. McDonnell, G.E. Morfill,

C. Polanskey, G. Schwehm and R. Srama, Nature 381 (1996) 395.

11. H. Krüger, E. Grün, D.P. Hamilton, M. Baguhl, S. Dermott, H. Fechtig, B.A. Gustafson, M.S. Hanner, M. Horanyi, J. Kissel, B.A. Lindblad, D. Linkert, G. Linkert, I. Mann, J.A.M. McDonnell, G.E. Morfill, C. Polanskey, R. Riemann, G. Schwehm, R. Srama and H. Zook, Planet. Space Sei. 47 (1999) 85.

12. R. Srama, J.G. Bradley, E. Grün, T.J. Ahrens, S. Auer, M. Cruise, H. Fechtig, A. Graps, 0. Havnes, A. Heck, S. Helfert, E. Igenbergs, E.K. Jeßberger, T.V. Johnson, S. Kempf, H. Krüger, P. Lamy, M. Landgraf, D. Linkert, F. Lura, J.A.M. McDonnell,

D. Möhlmann, G.E. Morfill, G.H. Schwehm, M. Stübig, J. Svestka, A.J. Tuzzolino, R. Wasch and H. A. Zook, Space Science Reviews, submitted.

13. H. Krüger, A.V. Krivov, D.P. Hamilton and E. Grün, Nature 399 (1999) 558.

14. K.-U. Thiessenhusen, H. Krüger, F. Spahn and E. Grün, learus 144 (2000) 89.

15. A.V. Krivov, H. Krüger, E. Grün, K.-U. Thiessenhusen and D. P. Hamilton, J. Geophys. Res. 107 (2002) in press.

16. A.V. Krivov, I. Wardinski, F. Spahn, H. Krüger and E. Grün, Dust on the outskirts of the Jovian system. Icarus (2002) in press.

17. J. E. Colwell, M. Horanyi and E. Grün, Science 280 (1998) 88.

18. J.E. Colwell, M. Horanyi and E. Grün, J. Geophys. Res. 103 (1998) 20023.

19. E. Grün, H. Krüger and M. Landgraf, in Cosmic Dust (eds. A. Balogh, R. Marsden and E. Smith) Springer Praxis (2001) 373.

20. H. Krüger, M. Horanyi, A.V. Krivov and A. Graps, In Jupiter: Planet, Satellites & Magnetosphere (eds. Bill McKinnon, Fran Bagena.l and Tim Dowling) Cambridge University Press (2002) submitted.

21. D.P. Hamilton and J.A. Burns, Nature 364 (1993) 695.

22. M. Horanyi, G. Morfill and E. Grün, Nature 363 (1993) 144.

23. H. Zook, E. Grün, M. Baguhl, D.P. Hamilton, G. Linkert, D. Linkert, J.-C. Liou, R. Forsyth and J.L. Phillips, Science 274 (1996) 1501.

24. E. Grün, H. Krüger, A.L. Graps, D.P. Hamilton, A. Heck, G. Linkert, H.A. Zook, S. Dermott, H. Fechtig, B.A. Gustafson, M.S. Hanner, M. Horanyi, J. Kissel, B.A. Lindblad, G. Linkert, 1. Mann, J.A.M. McDonnell, G.E. Morfill, C. Polanskey, G. Schwehm and R. Srama, J. Geophys. Res. 103 (1998) 20011.

25. H. Krüger, E. Grün, A. Graps and S. Lammers, In Proceedings of the VII. International Conference on Plasma Astrophysics and Space Physics (eds. E. Marsch, J.Büchner, I. Axford and V. Vasyliunas) Kluwer Academic Publishers 264 (1999) 247.

26. H. Krüger, E. Grün, A. Heck and S. Lammers, Planet. Space Sei 47 (1999) 1015.

27. A.L. Graps, E. Grün, H. Svedhem, H. Krüger, M. Horanyi, A. Heck and S. Lammers, Nature 405 (2000) 48.

29. J.R. Spencer, P. Sartoretti, G.E. Bellester, A.S. McEwen, J.T. Clarke and M.A. Mc-Grath, Geophys. Res. Lett. 24 (1997) 2471.

30. M. Horanyi, G. Morfill and E. Grün, J. Geophys. Res. 98 (1993) 221.

31. M. Horanyi, E. Grün and A. Heck, Geophys. Res. Lett. 24 (1997) 2175.

32. M. Horanyi, Physics of Plasmas 7(10) (2000) 3847.

33. F. L. Whipple, Astron. Astrophys. 187 (1987) 852.

34. A.S. McEwan, L. Keszthelyi, P. Geissler, D.P. Simonelli, M.H. Carr, T.V. Johnson, K.P. Klaasen, H.H. Breneman, T.J. Jones, J.M. Kaufman, K.P. Magee, D.A. Senske, M.J.S. Belton and G. Schubert, Icarus 135 (1998) 181.

35. J.R. Spencer, K.L. Jessup, M.A. McGrath, G.E. Ballester and R. Yelle, Science 288 (2000) 1208.

37. H. Krüger, E. Grün, A. Graps, D. Bindschadler, S. Dermott, H. Fechtig, B.A. Gustafson, D.P. Hamilton, M.S. Hanner, M. Horanyi, J. Kissel, B.A. Lindblad, D. Linkert, G. Linkert, I. Mann, J.A.M. McDonnell, G.E. Morfill, C. Polanskey, G. Schwehm, R. Srama and H. Zook, Planet. Space Sei. 49 (2001) 1285.

38. E. Grün, P. Krüger, S. Dermott, H. Fechtig, A. Graps, B.A. Gustafson, D.P. Hamilton, A. Heck, M. Horanyi, J. Kissel, B.A. Lindblad, D. Linkert, G. Linkert, I. Mann, J.A.M. McDonnell, G.E. Morfill, C. Polanskey, G. Schwehm, R. Srama and H.A. Zook, Geophys. Res. Lett. 24 (1997) 2171.

39. H. Krüger, A.V. Krivov and E. Grün, Planet. Space Sei. 48 (2000) 1457.

40. M. Sremcevic, A.V. Krivov and F. Spahn, Planet. Space Sei. in prep.

41. H. Iglseder, K. Uesugi and H. Svedhem, Adv. Space Res. 17 (1996) 177.

42. E. Grün, D.P. Hamilton, R. Riemann, S. Dermott, H. Fechtig, B.A. Gustafson, M.S. Hanner, A. Heck, M. Horanyi, J. Kissel, M. Kivelson, H. Krüger, B.A. Lindblad, D. Linkert, G. Linkert, I. Mann, J.A.M. McDonnell, G.E. Morfill, C. Polanskey, G. Schwehm, R. Srama and H. A. Zook, Science 274 (1996) 399.

43. M.E. Ockert-Bell, J.A. Burns, I.J. Daubar, P.C. Thomas, J. Veverka, M.J.S. Belton and K.P. Klaasen, Icarus (1999) 138.

44. J.A. Burns, M.R. Showalter, D.P. Hamilton, P.D. Nicholson, I. de Pater, M.E. Ockert-Bell and P.C. Thomas, Science 284 (1999) 1146.

CDA cruise science: Comparison of measured dust flux at 1AU with models

M. Müllera'b, BJ. Goldsworthy1, N. McBridea'b, S.F. Greena'b, J.A.M. McDonnella,b, R. Sramac, S. Kempf0 and E. Grün0

"Unit for Space Sciences and Astrophysics, University of Kent at Canterbury, CT2 7NR, UK.

bPlanetary and Space Science Research Institute, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.

°Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany

The data gathered by the Cosmic Dust Analyser (CDA) on board Cassini close to 1 AU are investigated. To compare the measured flux with models, the sensitivity of the targets is derived from calibration data. The interplanetary model by Staubach [1] underestimates the measured flux by more than one order of magnitude. Our attempt to classify the measured events as impacts on the large and the small target, indicates that there are impacts on the side walls which lead to measurable signals on the targets. This brings the measured flux closer to the model, but cannot explain the large discrepancy. It is shown that interstellar particles could fill the gap between the interplanetary flux model and the measured impact rate.

1. INTRODUCTION

In the time from April 12th and June 22nd 1999 Cassini approached the Sun from 1.2 AU to 0.74 AU. CDA was triggered 57 times. 27 of these events show clear enough signals to identify them to be due to dust impacts. CDA has two different targets [2], The charge of positive ions produced by an impact on the targets is measured by an ion collector grid in the middle of the detector. The negative ions are detected at the targets. We compare the measured impact rate on both targets with models for the interplanetary and interstellar dust flux. To do this the sensitivity of both targets needs to be determined first. Finally, we report on the attempt to classify the 27 impacts in impacts on the large and the small target.

2. CDA SENSITIVITY

To compare the dust flux measured by CDA with models it is necessary to estimate the minimum dust particle mass which can be detected by CDA. CDA is triggered if the charge measured at one of the targets, one of the grids or the multiplier, exceeds a certain threshold. The charge produced at a target due to a particle impact is usually described by a power law dependence on mass and velocity, Q, = Cm"V^ (see e.g. [3]). The coefficients a, ß and C are determined from calibration data taken with the CDA flight spare unit. If the mass and velocity in the power law are specified in units of 10"16 kg and 10 km s"1, respectively, a fit to the power law gives a= 1.25, ¡3= 4.52 and C = 8 x 10'14 C and a = 0.85, /? = 3.3 and C= 1.7 x 10"13 C for the large and the small target, respectively.

In Figure 1 it can be seen that the data only cover a narrow strip in mass-velocity space. The cut-off towards higher masses and high velocities is due to the limitations of the Van de Graaff accelerator (VdG). The cut-off towards low masses and velocities has a different slope for low and high particle masses. The cut-off at low masses can be approximated by a line of constant kinetic particle energy. In a VdG with given acceleration voltage the kinetic energy is proportional to the particle charge. Since the charge of the particle needs to be detected in the accelerator to determine its mass and velocity [4], this cut-off is given by the minimum detectable charge and is therefore not due to CDA. The slope of the cut-off at higher masses can be explained with a curve of constant charge produced at a target and shows the sensitivity of CDA. The cut-off corresponds to the target charges of 2 x 10"14 C and 5 x 10"14 C for the large and the small target, respectively.

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