| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set gives the best available values for ion densities,
temperatures, and velocities near Neptune derived from data
obtained by the Voyager 2 plasma experiment. All parameters are
obtained by fitting the observed spectra (current as a function
of energy) with Maxwellian plasma distributions, using a non-linear
least squares fitting routine to find the plasma parameters which,
when coupled with the full instrument response, best simulate the
data. The PLS instrument measures energy/charge, so composition is
not uniquely determined but can be deduced in some cases by the
separation of the observed current peaks in energy (assuming the
plasma is co-moving). In the upstream solar wind protons are fit
to the M-long data since high energy resolution is needed to obtain
accurate plasma parameters. In the magnetosheath the ion flux so low
that several L-long spectra (3-5) had to be averaged to increase the
signal-to-noise ratio to a level at which the data could be reliably
fit. These averaged spectra were fit using 2 proton Maxwellians with
the same velocity. The values given in the upstream magnetosheath
are the total density and the density-weighted temperature. In both
the upstream solar wind and magnetosheath full vector velocities,
densities and temperatures are derived for each fit component. In
the magnetosphere, spectra do not contain enough information to
obtain full velocity vectors, so flow is assumed to be purely
azimuthal. In some cases the azimuthal velocity is a fit parameter,
in some cases rigid corotation is assumed. In the 'outer'
magnetosphere (L>5) two distinct current peaks appear in the
spectra; these are fit assuming a composition of H+ and N+. In
the inner magnetosphere the plasma is hot and the composition is
ambiguous, although two superimposed Maxwellians are still required
to fit the data. These spectra are fit using two compositions, one
with H+ and N+ and the second with two H+ components. The N+
composition is preferred by the data provider. All fit values in
the magnetosphere come with one sigma errors. It should be noted
that no attempt has been made to account for the spacecraft
potential, which is probably about -10 V in this region and will
effect the density and velocity values. In the outbound
magnetosheath and solar wind both moment and fit values are given
for velocity, density, and thermal speed. The signal-to-noise ratio
in the M-longs is very low, especially near the magnetopause, which
can result in the analysis giving incorrect values. The L-long
spectra have too low an energy resolution to permit accurate
determinations parameters in many regions; in particular the
temperature and non-radial velocity components may be inaccurate.
Parameters
==========
Derived Parameters
------------------
Sampling Parameter Name : time
Sampling Parameter Resolution : n/a
Minimum Sampling Parameter : unk
Maximum Sampling Parameter : unk
Sampling Parameter Interval : unk
Minimum Available Sampling Interval : unk
Data Set Parameter Name : ion density
Noise Level : unk
Data Set Parameter Unit : cm**-3
Ion density: A derived parameter equaling the number of ions per
unit volume over a specified range of ion energy, energy/charge,
or energy/nucleon. Discrimination with regard to mass and or
charge state is necessary to obtain this quantity, however, mass
and charge state are often assumed due to instrument limitations.
Many different forms of ion density are derived. Some are
distinguished by their composition (N+, proton, ion, etc.)
or their method of derivation (Maxwellian fit, method of
moments). In some cases, more than one type of density will
be provided in a single dataset. In general, if more than one
ion species is analyzed, either by moment or fit, a total density
will be provided which is the sum of the ion densities. If a
plasma component does not have a Maxwellian distribution
the actual distribution can be represented as the sum of
several Maxwellians, in which case the density of each Maxwellian
is given.
Sampling Parameter Name : time
Sampling Parameter Resolution : n/a
Minimum Sampling Parameter : unk
Maximum Sampling Parameter : unk
Sampling Parameter Interval : unk
Minimum Available Sampling Interval : unk
Data Set Parameter Name : ion velocity
Noise Level : unk
Data Set Parameter Unit : km/s
Ion velocity: A derived parameter giving the average speed and
direction of motion of a plasma or plasma component. The velocity
can be obtained by taking the first moment of the distribution
function or by simulating the observations with some known
distribution function, usually a Maxwellian, to the distribution.
Velocities may be given in any of the following coordinate
systems:
RTN coordinate system: R is radially away from sun,
T is in plane of sun's equator and positive
in the direction of solar rotation
N completes right-handed system
rho, phi, z (cylindrical)
rho is radial distance from the planet's spin axis
phi is parallel to spin equator and perpendicular
to rho and positive in direction of planetary
rotation.
z completes right-handed system
parallel, perpendicular: parallel or perpendicular to the magnetic
field direction.
Measured Parameters
-------------------
Sampling Parameter Name : time
Sampling Parameter Resolution : n/a
Minimum Sampling Parameter : unk
Maximum Sampling Parameter : unk
Sampling Parameter Interval : unk
Minimum Available Sampling Interval : unk
Data Set Parameter Name : ion thermal speed
Noise Level : unk
Data Set Parameter Unit : km/s
Ion thermal speed: A measure of the velocity associated
with the temperature of the ions. It is formally defined as
the Ion Thermal Speed squared equals two times K
(Boltzmann's constant) times T (temperature of ion) divided
by M (ion mass). Each component of a plasma has a thermal speed
associated with it.
Ion Current: A measured parameter equaling the rate at which positive
charge is collected by a particle detector. The ions
contributing to this current may be restricted
by energy and/or mass. Since ion charge states may be
greater than one, this quantity generally is
greater than the corresponding ion rate.
References
==========
Richardson, J. D., M. Zhang, J. W. Belcher, and
R. L. McNutt, Jr., Low-energy ions near Neptune,
J. Geophys. Res., in press, 1991.
J. W. Belcher, H. S. Bridge, et al.,
Plasma Observations Near Neptune: Initial Results from
Voyager 2, Science, 246, 1478-1483, 1989.
Zhang, M., V. M. Vasyliunas, G. L. Siscoe, R. P. Lepping,
and N. F. Ness, Evidence for a diurnally rocking plasma
mantle at Neptune, Geophys. Res. Lett., 17, 2285-2288, 1990.
Richardson, J. D. and R. L. McNutt, Jr., Low-energy plasma
in Neptune's magnetosphere, Geophys. Res. Lett., 17,
1689-1692, 1990.
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