JOP056: THE INTERSTELLAR FOCUSING CONE J.L. BERTAUX, E. QUEMERAIS, R. LALLEMENT, NOV. 5, 1996 ABSTRACT It is proposed to observe the gravitational focusing cone of interstellar material at the time when the Earth (and SOHO) will pass just above the cone, around December 2, 1996 (first anniversary of SOHO launch). In this privileged geometry, which occur only once per year, the cone is much brighter and conspicuous. Some species are known to exist in the cone because they have been already been observed (He, He+). Some other manifestations of the cone (dust, H, D, ... other species ) have not been detected by lackof appropriate observations, and are more speculative . There is no special requirement on the S/C for this JOP. Observations may involve SWAN, LASCO, UVCS, SUMER, EIT, CELIAS and CDS. The JOP was already presented at SWT of May 17 for a possible activation around June 1st. The opportunity of December is much more appropriate, in particular for CELIAS and LASCO (forward scattering of small dust particles). BACKGROUND It is known since 25 years that the Sun is travelling through an interstellar cloud, at a velocity Vw of 26 km/s in the direction defined by latecl =3D + 6=B0=B11=B0 , long ecl =3D - 253 =B1 1=B0. H atoms are= destroyed by the solar-wind. He atoms follow hyperbolic trajectories which all cross each other on a half straight line, opposite to Vw, starting from the Sun. This cone has been observed at 584 =C5 (resonance scattering of solar photons) beyond 1 AU, and also down to 0.3 AU. There are no observations below 0.3 AU. He atoms which are ionized become so-called He+ pick-up ions, are carried by the solar wind, but can be recognized from solar wind particles by their peculiar velocity distribution (as did several measurements in the past). Interstellar dust particles of the interstellar cloud are also penetrating the solar system. They have been detected both by Ulysses and Galileo dust particles experiments, when approaching the focusing cone of Jupiter. They should also concentrate along the focusing line, and become a conspiscuous target of observation for LASCO, because we expect an enhancement due to forward scattering in the particular geometry of December 2. However a theoretical argument says that these dust particles will be electrically charged (solar wind, photoionization) and diverted from converging to the focusing cone. Therefore, LASCO/SOHO offers a unique opportunity to discriminate which argument is right. If big dust particles (10 microns) were existing in the ISM then they could probably reach the focusing zone. A negative result will also be very important. There may also be three additional sources of H atoms in the focusing cone, which might be detected in La : 1. Dissociation of H2 interstellar molecules 2. Outgassing of interstellar dust particles 3. Neutralization of solar wind on dust particles and sputtering. Additionally, other species of interstellar origin might be detected : O, C, N, D, He3,... SUMMARY OF POTENTIAL OBSERVATIONS : Given the geometry, the focusing cone is seen in December below the South Pole of the Sun. The JOP could cover 3 to 5 days around the time of closest approach to the cone, which is known with an accuracy of 1 degree SWAN : fine scanning of the region around the Sun with SU-Z sensor + some H cell use, down to @ 3=B0. UVCS : maps of La up to 10 Rs in the South pole region. LASCO : Pictures of C1, C2, C3 at regular intervals (at least twice a day), in white light. Most important are probably C2 and C3. SUMER : Observations off the limb ; spectrum of around 584 =C5 (to see the behaviour of "mystery" twin satellite lines to 584=C5, which might be due to interstellar He atoms experiencing a strong Doppler shift). Also, La profile is derisable. EIT : pictures at 304 =C5 might reveal an enhancement of He+ ions created by ionization of He atoms of interstellar origin. CDS : Potential detection of 584 =C5 off the limb, south. CELIAS : Observation of pick-up ions. This should cover =B1 15=B0 of eclipti= c longitude around the time of focussing cone crossing, as shown by previous experiment. CAUTION : Most of 584 =C5 emission measured by CDS and SUMER will be stray light originating from disc. However, by comparison with previous observations off the limb, detection of interstellar 584 =C5 emission might be possible, or an upper limit determined. The proposed JOP leader will be Eric Qu=E9merais.