A New Instrument for Measuring the

1
A New Instrument for Measuring the Near-Solar
Neutral Atom Population P. Mukherjeea, T.
Zurbuchena, and F. Herrerob aDepartment of
Atmospheric, Oceanic, and Space Science
The University of Michigan, Ann Arbor, MI
48109 bDetector Systems Branch
Goddard Space Flight Center, Greenbelt, MA 20771
Mission Specifics
Enabling Technologies
Figure 3 Solar Probe will swing around the sun,
approaching as close as 4 solar radii at
periapsis. The heat-shield will always face the
sun, protecting the instruments. The proposed
instrument will be aimed to pick up the maximum
flux at periapsis as seen below. The dust cloud
surrounding the sun roughly follows 71 aspect
ratio ellipsoids of constant density, with a
profile proportional to 1/r along the ecliptic
plane.
The grating will have 50-70 nanometer slits,
which will block Lyman-alpha photons while
allowing particles through. In addition, its
likely that a significant fraction of those
particles will be ionized via tunneling
interactions with the slit walls. This behavior
will require testing.
Figure 4 As Solar Probe approaches the sun, its
velocity increases monotonically until periapsis
at 4 solar radii. In addition, it is rapidly
moving into denser regions of the dust cloud
(Fig. 3). fluxdensityvelocity The
very sharp spike in the flux at periapsis should
allow for short integration times for neutral
population measurements.

• Figure 1 The inner-source neutral density is far
  higher than the interstellar-source density in
  regions very close to the sun. Assumptions for
  these profiles are
• Interstellar neutrals follow an e-?/r falloff
  from the termination shock, where ?u/t
  t1.0810-7 s-1 is the ionization rate (Rucinski
  et al, 1996)
• Inner-source helium density taken as 5 of the
  hydrogen density (Cranmer et al, 1998) as per
  solar wind ratios as a worst-case.

Efforts to further miniaturize the design may
involve the use of this microfabricated faraday
cup array (based on Darling et al, 2002). These
individually-addressable cups are essentially
just deep-trench capacitors with very low
cup-to-cup and high cup-to-ground capacitances.
Source Expected Neutral Population
Recycled solar wind H, He, C, N, O, Ne (Gloeckler et al 2001)
Evaporating Dust H2, CO, C, O, Si, Fe (Gloeckler et al 2001)
Comets (proto-star) Significantly enhanced C, O (Gloeckler et al, 2004 ApJ)
Figure 2 Dust grains follow Keplerian orbits,
and near the sun these orbital velocities can
rise to a significant fraction of the solar wind
speed. where G is the
gravitational constant and ß is the ratio of
radiation and gravitational forces (Burns et al,
1979). We assumed circular orbits, 1µm dust
grains, and solar minimum conditions at the
ecliptic. If the neutrals come from the dust
grains, and after ionization are picked up by the
solar wind, they still have a large azimuthal
velocity component.
Open Questions Is the inner source primarily the
sungrazing comets, the asteroids, or both? Is the
velocity of the dust source important to the
neutral and/or pickup ion distributions?
Figure 6 2-D instrument diagram. The
time-of-flight region is essentially the same as
that of FIPS (Gold et al 2001) with modified
voltages and the UV grating is described in Fig
7.
Figure 5 3-D instrument view. The device is
clamped to the spacecraft on the bottom, and the
aperture is aimed 31 degrees off-axis for ideal
periapsis measurements.