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 M÷bius

SOHO: 

 Eberhard M÷bius, 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'AŔronomie du
CRNS, VerriŔres 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 fŞr 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, Universitńt 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 sunÝs 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 ┼ 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 ┼ (470-520 ┼ 2nd order) and
1160-1190 ┼ (580-595 ┼ 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 ┼, HeII 304 ┼; 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 ┼ 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 ┼
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 ┼ (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 ┼ 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 ┼ 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 ┼ disk images; Nov 19, 23, 27, Dec 1, 5, 9, 13, 17, 21 + 2
observations 1 month before and after. The He II 304 ┼ 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 ┼ 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 ┼, He II 304 ┼ 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;