NASA Logo, National Aeronautics and Space Administration
NASA Space Science Data Coordinated Archive Header



Availability: Archived at NSSDC, accessible from elsewhere

Time span: 1989-08-26 to 1989-08-30


This description was generated automatically using input from the Planetary Data System.

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.

These data are available on-line from the Planetary Data System (PDS) at:

Alternate Names



  • Planetary Science: Fields and Particles

Additional Information


Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II



NameRoleOriginal AffiliationE-mail
Dr. John D. RichardsonGeneral ContactMassachusetts Institute of
[] NASA Logo -