JOP129: The Gravitational Focusing Cone of Interstellar Helium (Coordinated observation of the gravitational focusing cone of interstellar helium during the rise to solar maximum conditions) Received: September 2000 Scientific Team: Coordinator: Eberhard Möbius SOHO: Eberhard Möbius, Space Science Center and Department of Physics, University of New Hampshire, Durham, NH; Lead, AMPTE SULEICA, SOHO CELIAS, Cluster CODIF John Raymond, Harvard Smithsonian Center for Astrophysics, Cambridge, MA; SOHO UVCS Donald McMullin, (Darrell Judge, Howard Ogawa), Space Science Center, University of Southern California, Los Angeles, CA; SOHO CELIAS SEM William Thompson, Goddard Space Flight Center, Greenbelt, MD, SOHO CDS Rosine Lallement (Jean-Loup Bertaux), Service d'Aèronomie du CRNS, Verrières le Buisson, France; SOHO SWAN, UV Absorption Method, Modeling EUVE: John Vallerga, (Brian Flynn), University of California, Berkeley, CA; EUV Explorer ACE/Ulysses: George Gloeckler, Department of Physics and IPST, University of Maryland, College Park, MD; ACE and Ulysses SWICS Manfred Witte, Max-Planck-Institut f¸r Aeronomie, Lindau/Harz, Germany; Ulysses GAS Geotail: Toshio Terasawa, (Hirotomo Noda), University of Tokyo, Japan, Geotail LEP Modeling: Daniel Rucinski, (Maciej Bzowski), Space Research Centre, Warsaw, Poland; Modeling of interstellar neutral gas and ionization Hans Fahr, Universität Bonn, Germany; Modeling of interstellar neutral gas, ionization, resonant scattering Sergei Chalov, Institute for Problems in Mechanics, Moscow, Russia; Modeling of pickup ion transport and acceleration Core observation period: November - December 2000 Of the components of the local interstellar medium (LISM) helium is the species for which the neutral parameters can be determined without alteration from inside the heliosphere. On the other hand He also seems to be very intriguing because of recent results that it appears to be more highly ionized than H, in spite of its higher ionization potential. Currently, this has not been reconciled yet with the local radiation environment in the LISM. Over the last fifteen years the tools for the determination of the LISM parameters, density, temperature and bulk flow vector, have been dramatically improved and diversified. The traditional method to observe the interstellar neutral He inside the heliosphere by means of resonant scattering of solar UV radiation has been augmented by direct particle measurements, using pickup ion distributions and neutral atom imaging. Yet the derivation of the interstellar parameters with all these methods relies on a detailed modeling of the gas flow through the heliosphere and ionization losses on its way. Because of various modeling steps involved in each of the methods and the need of additional information on the sunís radiation and solar wind environment results of the different methods have differed or varied to different degrees in the past. In addition, the influence of the sun and the heliosphere on the gas distribution varies naturally with the solar cycle, a subject of great interest in itself. With a unique combination of heliospheric and astronomical spacecraft it has become possible for the first time to employ all methods simultaneously, with the great advantage that they can complement each other with their capabilities. Each year in early December the Earth, and with it all earthbound spacecraft, pass through the interstellar focusing cone, whose structure depends critically on the interstellar parameters and the ionization in the heliosphere. Although combined observations have been performed in December 1998, the coming passage in December 2000 provides the opportunity for the most complete coverage. It is also close to the solar activity maximum. While the combination of the neutral gas and pickup ion observations will provide the most accurate results on the He density and flow vector, UV observations will add the most accurate account of the ionization processes. In particular, with the SOHO instrumentation the photoionization rate is measured directly and simultaneously, so that the still largely unknown electron impact ionization can be derived. How the different data sets complement each other in the comparison with the modeled interstellar gas distribution to deduce the interstellar parameters is shown in the blockdiagram in Fig. 1. In addition, the flow of interstellar hydrogen, which is efficiently coupled to the interstellar plasma and thus experiences substantial filtration and deceleration in the heliospheric interface, provides a complementary diagnostic about the coupling and thus the density of the LISM plasma. A direct comparison between hydrogen and helium has been attempted with the SWAN experiment. A non-negligible difference between the H and He velocity vectors, i.e. a deceleration of a few km/s, was observed for H, but still with substantial uncertainties. Now is the ideal time to combine the complementary observations of the two most important species of the LISM. For this reason, hydrogen and helium flow observations must be coordinated, although there is no need for strictly simultaneous measurements. The differences between H and He, once observed with high precision, will be compared with advanced models of flows through the heliospheric interface to constrain the physical processes that control the interface interactions. Observation Plan We plan to perform coordinated observations during the passage of the helium cone, with the prime time defined as the half-width of the cone, i.e. approximately from November 19 through December 21, 2000. We expect the contribution of instruments from SOHO, EUVE, ACE, Ulysses, Wind, Geotail, and (after successful launch and commissioning) also Cluster. The particle instruments on ACE, Ulysses, Wind, and Geotail, as well as SOHO SWAN, CELIAS SEM and PM will provide almost continuous coverage. It should be made sure though that spacecraft maneuvers, which require turn-off of instrumentation, should be avoided during this time period. Cluster will participate as data taking in the solar wind permits. The core optical instruments for this campaign, SOHO UVCS, CDS, SUMER, EIT and the EUVE scanner and spectrograph, which target a variety of objects, require special consideration. In the following, we provide a rationale and a brief observation plan for these instruments: SOHO UVCS: Observation of the HeI 584 Å glow south of the sun around cone passage of the Earth; center of cone: Dec 5, 2000, 1700 UT. UVCS will be the key instrument to constrain the relevant ionization rates. UVCS will measure the He I 584 flux at 4 day intervals chosen to span the Focusing Cone and extend far enough to determine the baseline flux away from the Focusing Cone. The Earth passes the axis of the cone late on Dec. 5, and the width of the cone is about 35 degrees (FWHM). Therefore, the preliminary schedule is Nov. 19, 23 & 27, Dec. 1, 5, 9, 13, 17, 21, 25, and Jan. 6, 14 & 22 (the larger spacing in January reflects the slower changes expected away from the focusing cone). All UVCS observations will be made at 9 Rsun and Position Angle 180. Typical dwell times will be 9 hours broken into 10-minute exposures. The slit width will be 150 micron. An additional 9 hours on Dec. 5 at a different grating position will be used to check the detector uniformity. Two exposures at a lower height will provide a reference stray-light spectrum, midway through each observation. The wavelength range will be approximately 940-1040 Å (470-520 Å 2nd order) and 1160-1190 Å (580-595 Å second order) in the redundant channel. Only the interplanetary Lyman beta line is expected to be detectable at about 1 Rayleigh. SOHO CDS: Full sun disk scan in He I 584 Å, HeII 304 Å; Nov 19, Dec 5, Dec 21; CDS will provide information on the spatial distribution of the source for the resonance scattering. SOHO SUMER: He I 584 Å line profile; at least Dec 5, if possible, also November 19 and December 21. In contrast to interstellar hydrogen, the shape of the solar emission line, integrated over the full disk, plays a first order role in the resonance glow for helium, because the width of the solar He I 584 Å line is significantly smaller than the Ly-alpha line. Typically, it is of the order of 40-45 km/s or 0.08-0.09 Å (e.g. Wu and Ogawa, 1986), i.e. of the same order as the Doppler shift of interstellar gas flow in the inner heliosphere. Therefore, the back-scattered signal for helium is very sensitive to the exact width and shape of the solar line. Simulations confirm this suspicion, showing that variations of the solar line can mimic changes of other parameters, such as interstellar flow velocity and temperature. In particular, discrepancies between the helium flow parameters, as deduced from optical and particle data, are very likely related to the solar line shape. Chassefiere et al (1988) have shown that including a Doppler shift of the solar line (due to a general down flow of the emitting gas in the solar atmosphere) in the modeling could help to reconcile helium temperatures deduced from optical and particle data. Clearly spectral information on the solar line is indispensible to resolve such ambiguities in the interpretation. SUMER can provide the required information. A few previous observations of He I 584 Å have been made with SUMER, which have not been fully analyzed yet: one full disk scan and a series of measurements using the stray-light above the limb. Preliminary results (Lemaire, private communication) show a width of the order of 40-45 km/s, in agreement with Wu and Ogawa (1986), with strong spatial variability across the disk. It is thus very likely that the width of the disk-integrated line varies with solar activity. Therefore, simultaneous observations of the He I 584 Å line with SUMER are extremely important for the analysis of the UVCS and EUVE data. One or a few full disk scans and/or the stray-light observations above the limb during the central week of the proposed JOP, i.e. around Dec 5th, will return the necessary information on the spectral shape and a possible Doppler shift. SOHO EIT: He II 304 Å disk images; Nov 19, 23, 27, Dec 1, 5, 9, 13, 17, 21 + 2 observations 1 month before and after. The He II 304 Å disk images provide crucial information about the spatial distribution of the photoionization rate for helium, which may influence the spatial distribution of neutral helium in the inner heliosphere. It is expected that these disk images can be worked into the EIT routine observation program. SOHO CELIAS SEM, PM: He II 304 Å absolute flux, solar wind parameters; continuous coverage SOHO SWAN: Lyman alpha skymaps; continuous coverage Other spacecraft data for joint campaign: EUVE Spectrometer and Scanner: He I 584 Å, He II 304 Å Spectrometer data; Nov 19, Dec 5, 21; Scan of He cone: 1 month before (Oct 15 or 16) and 1 month after (Jan 20 or 21) The EUVE data are crucial, since they provide the extension of the 3-dimensional spatial distribution of helium in the cone beyond the information gathered by SOHO UVCS close to the sun. In particular, EUVE will make the connection to the long history of UV back scatter observations, because of its overlap with similar information during this coordinated campaign. EUVE will provide both spectral and spatial information on the helium cone. ACE SWICS: He+ pickup ion spectra; continuous coverage ACE SWEPAM: solar wind ion and electron distributions; continuous coverage ACE MAG: magnetic field parameters; continuous coverage Ulysses GAS: continuous coverage Nov 19 - Dec 21 Ulysses SWICS: He2+ pickup ion spectra; continuous coverage WIND STICS: He+ pickup ion spectra; continuous coverage Geotail LEP: 3D distributions; continuous coverage After successful launch: Cluster CODIF: 3-dimensional He pickup ion distributions;