Voyager Observations of 5 Tails in the Heliosheath

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Voyager Observations of -5 Tails in the
Heliosheath
George Gloeckler1,2, Matt Hill3, Doug Hamilton2,
Rob Decker3, Tom Krimigis3, and Len
Fisk1 1University of Michigan, Ann Arbor,
MI 2University of Maryland, College Park, MD
3Johns Hopkins University-Applied Physics
Laboratory, Laurel, MD IBEX SWT Orbital
Sciences Corporation 21839 Atlantic Blvd., Dulles
VA 20166 November 8, 2007
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LIC Composition
neutral gas
ionized gas
grains
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Observations Voyager 1 measurements of
accelerated pickup ions in the heliosheath are
used to find Interstellar neutral density ratios
for H, N, O, Ne and Ar relative to He at 100 AU
Voyager 2 measurements of energetic electrons in
the heliosheath and upstream of the termination
shock are used to estimate Electron impact
ionization rates that may account for the rapid
slow down of the solar wind Implications
resulting from Voyager and Ulysses observations
for IBEX will be discussed
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Electrons observed with Voyager Upstream of the
Termination Shock and in the Heliosheath
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Charge exchange (interstellar neutral hydrogen
density of 0.11 cm3 at the TS) and
photo-ionization will gradually slow down the
solar wind from 465 km/s at 1AU to 380 km/s If
the rapid slowdown to 320 km/s was the result of
additional mass loading from pickup ions,
electron impact ionization, at a rate of
6.5107 s1 (at 1 AU) would be required
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Do observations of electrons measure by Voyagers
support this possibility?
Voyager 1 observation of energetic electrons
upstream of the TS and in the heliosheath Electron
counting rates stay between 8 and 11
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Using a plausible extrapolation (red curve) of
the electron spectrum measured by Voyager 2 and
a maxwellian solar wind electron distribution
(blue curve) Gives the required electron impact
rate of 6.8107 s1 (at 1 AU) The combined
spectrum is consistent with all of the Voyager 2
observations
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Suprathermal Tails
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Observations in the heliosphere have revealed the
existence of suprathermal power law tails with
spectral index -5 extending to a few MeV Such
tails are always observed in the inner and outer
heliosphere and at all latitudes both in the
quiet and disturbed solar wind Observations show
that the pressure in the tails is 0.1 0.2 that
of the core particles Tail intersects the
pickup ion distribution 100 times below the
core plateau
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The Solar Wind speed distribution consists of
the bulk solar wind represented by a
maxwellian (red curve) from which density
and bulk speed are computed the halo solar
wind represented by a kappa function
(black dotted curve). The temperature of the
halo component is much higher (but the
density much lower) than that of the solar
wind the pickup ions (light blue dashed
curve obtained from model calculations) The
core distribution is the combination of the halo
solar wind and pickup ion distributions
Suprathermal tails contain 10 to 20 of the core
particle pressure
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Voyager observations in the heliosheath show a
steady, omnipresent suprathermal power law tail
with spectral index -5 extending to a few MeV
(red and blue filled circles) Heliosheath solar
wind protons have low thermal speed and a density
of 0.003 cm-3 (black curve) as reported by
Richardson Pickup protons are modeled
distributions Extrapolated 5 power law tail to
intersect the pickup ion distribution 100 times
below the core plateau
IBEX will provide information on the actual tail
distributions (3107 to 3108 cm/s) in the global
heliosheath
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The spectrum generated at the termination shock
(TS) (red curve) should be convected downstream
at the ambient solar wind speed (75-150 km/s)
without much adiabatic deceleration Black and
green curves represent estimates of pickup ions
and tails produced in the heliosheath from the TS
to 100 AU and 130 AU respectively Quiet time
Ulysses data, selected for two different speeds,
show tails comparable to the estimated tail
spectra created in the heliosheath
Observations of suprathermal tails inside the
heliosphere (with Ulysses, ACE and Cassini) are
required for proper interpretation of IBEX
obsrervations of the TS and heliosheath
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Voyager 1 oxygen velocity distribution in the
heliosheath, averaged over 2.8 years, showing
the (a) suprathermal power law tail with
spectral index -5 gently rolling over at
1.3109 cm/sec (1 MeV/nuc) (b) ACR oxygen The
sum of (a) and (b) is an excellent fit to the
data Pressure of oxygen tail is computed using
the 5 power law tail distribution with the
gentle rollover
Tail
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Roll over of spectra is clearly observed,
especially for high mass/charge ions such as
Fe Model spectra of the form dj/dE joE
1.5exp(-E/Eo ) were fit to each spectrum and
Eo was obtained for each species Because of the
gentle roll over of the spectra, best fits of
simple power laws to spectra to tails above 0.1
MeV/nuc will (e.g. Mewaldt, SHINE 2007) will
yield spectral indices equal to or larger than
1.5
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Charge of each ion species was assumed to be the
average solar wind charge state measured by
SWICS Least-squares fit to the five Eo values
gives a power law index of -1.010.13 Cutoff
energies are 2.7 MeV for protons and 0.5
MeV/nuc for iron
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Voyager 1 velocity distributions of H, He, O, N,
Ne, and Ar (also C and Fe), in the heliosheath,
averaged over 2.8 years Fits to the lower energy
data of tail distributions (5 power laws with
gentle rollover) were obtained and tail pressures
computes as previously illustrated for
oxygen Ratios (relative to He) of H, N, O, Ne
and Ar pickup ion fluxes in the heliosheath were
derived from the respective pressure ratios
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Results and implications
Neutral fractions required to make the LIC
composition the same as PSC (column 4) are lower
by about a factor of 2 for N and O, and higher
by a factor of 2 for Ne and Ar from
corresponding values of Slavin and Frisch
models For N and O this difference could be
reconciled by assuming a larger fraction in dust
than 10 For Ne and Ar the disagreement appears
to be real
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Conclusions
Electron impact ionization rates in the
heliosheath (and several AU upstream of the
Termination Shock) are comparable to the sum of
charge exchange and photo-ionization rates and
are thus important important The rapid slow down
of the solar wind several AU before the
Termination Shock is most likely due to rapid
ionization of neutral hydrogen by electron
impact Observations of pickup ion tails in the
heliosheath by Voyagers 1 and 2 give the most
accurate (least affected by systematic errors)
estimates of the neutral density ratios of H, N,
O, Ne and Ar The Voyager value of the neutral H
density (0.11cm-3) is the same as that obtain
from Ulysses measurements but the Voyager neutral
O density value is 20 higher than that derived
from Ulysses pickup oxygen measurements Observati
ons of suprathermal tails inside the heliosphere
(with Ulysses, ACE and Cassini) are required for
proper interpretation of IBEX obsrervations of
the TS and heliosheath
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Pickup Ions
Data selected for solar wind speeds and 1.3 W is the ions speed divided by the solar wind
speed) Corrected for detection efficiency,
residual background and contributions of mass 17
and 18 ions from the extended Inner
Source Number density ratios of pickup ions
(relative to O) at 4.6 AU obtained from
gaussian fits to N, O and the two Ne isotopes
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