Neutrals near the Sun and the inner source pickup ions

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Neutrals near the Sun and the inner source pickup
ions P. Mukherjee and T.H. Zurbuchen Departme
nt of Atmospheric, Oceanic, and Space Science,
The University of Michigan, Ann Arbor, MI 48109
Adiabatic Expansion Considerations
Density Profiles
Given the particle densities in Figures 2 and 3
and velocity components from Figure 1, we can now
take a look at the expected behavior of the
particle distributions as they adiabatically
expand out to 1 AU. Figure 6 gives us a baseline
thermal velocity at 1 AU for comparison of 1/3
the solar wind, or 150 km/sec. Then, using the
adiabatic relation
we can solve for the thermal velocity at 1 AU for
a wide variety of assumed conditions. Figure 7
demonstrates the results of these calculations.
The lambda values where the model fits the
measured data can be traced back to a given
pickup ion peak location in Figure 5. Notice
that the curves fitted to our new model require
the pickup peaks to be far closer to the Sun than
those using the standard model, which is the
entire point of this poster. The lambda values
associated with our model fall at 15Rs and 35Rs
while those of the standard model fall at 63 Rs
and 101Rs. Those correspond to peak locations
(from Figure 5) of 7.6, 12.8, 31.6, and 37 Rs
respectively, so its clear that our model
demands pickup far closer to the Sun than
currently accepted models. Luckily, three
missions currently in planning stages will
acquire data much closer to the Sun Sentinels,
Solar Orbiter, and Solar Probe. The orbital
ranges of the three spacecraft are displayed on
Figure 8 along with the density curves matching
the lambda values listed above. This author is
currently working on nanoscale ultraviolet
filters that may be of significant use on those
missions (below).
Figure 6 H distribution function and inner
source fit. The 1/e width of the inner source
distribution is approximately 0.33 Solar Wind
Speed. Thanks to Dr. George Gloeckler for this
data.
Motivation
Interplanetary neutral atoms have two major
sources the interstellar medium and the
so-called inner-source dust arising from the
asteroids and comets. These populations not only
have different compositions, but are ionized and
picked up in significantly different
environments, and yet to date have been treated
almost the same for modeling purposes. As can be
seen from Figure 1 below, there are velocity
components near the Sun that are easily ignored
farther out into the heliosphere. Since these
effects, as well as the solar wind acceleration,
are all nonlinear very close to the Sun, its
worth examining where dust will be found. Most
inner source papers to date make the assumption
that most of the dust-source neutrals are found
between 10-50 solar radii, but Krivov et al
(1998) indicates that non-negligible amounts of
dust survive to within 2-4 solar radii, depending
on their dielectric and morphological properties.

Figure 7 Modeled thermal velocities at 1 AU for
a1 and a2. Solid lines include all pickup
velocity components from Figure 1, dotted lines
include only the standard VSW component.
Figure 1 Solar wind and Alfven wave speeds in
the near solar region, computed using the
formulae below (from Hu, Kohl, Lie-Svendsen, and
Sittler papers), and azimuthal dust grain speed
calculated from standard circular Keplerian
orbit. The field-aligned speed of ions is the sum
of Up and Va, and is thus dominated by Va, while
the perpendicular velocity at injection will
depend on the azimuthal speed of the source dust.
Solar Probe (4 Rs to 5 AU)
In both cases, increasing lambda resulted in a
decrease of the peak density value and movement
of the peak outward, as is to be expected. Note
that the peak densities fell off non-linearly
while the locations moved with linear fashion as
seen in Figures 4-5 below. In addition, the a1
case uniformly resulted in lower peaks at further
radial distances.
Solar Orbiter (45 to155 Rs)
Sentinels (56 to167 Rs)
Figure 8 Particle density curves for the ?
values found in Figure 7. Notice that the a1
cases have higher peaks than the a2 cases, which
does not match Figure 4, but the reason is that
the ? values for the two cases do not match as
in previous figures. Also displayed are orbit
ranges for upcoming missions.
Figure 4
Figure 5
Conclusions
Assumed values Solar wind speed 450
km/sec Proton density at 1 AU 5 cm-3 Ratio of
pickup protons to SW protons 1E-4 Ionization
rate for H at 1 AU 7.44E-7 s-1 (Rucinski et al,
1996)
We considered inner source pickup ion populations
throughout the inner heliosphere. Close to the
Sun, the pickup process needs to account for a
pair of velocity components that are negligible
beyond a few dozen solar radii azimuthal speeds
of the dust grains, and the enhanced heating due
to increased Alfvén wave speeds. We made a model
that predicts the neutral atom and ion
populations and adiabatic cooling of the ions.
We showed that this model has solutions
consistent with inner source number densities and
thermal speeds measured at 1 AU, and that the
additional velocity components require the pickup
process to happen far closer to the Sun than
predicted by traditional models. This provides
exciting opportunities for future missions close
to the Sun.