• 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 per 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 is 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 two 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 the 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

• ==================

+-------------------------------------------------------+
| Parameter Characteristics | Value |

| 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 per Charge, or Energy per 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 Data Set. 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.

+-------------------------------------------------------+
| Parameter Characteristics | Value |

| 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 TEMPERATURE |
| Noise Level | UNK |
| Data Set Parameter Unit | EV |
+-------------------------------------------------------+

Ion Temperature: A derived Parameter giving an Indication of the Mean Energy per Ion, assuming the Shape of the Ion Energy Spectrum to be Maxwellian (i.e. highest entropy shape). Given that the Ion Energy Spectrum is not exactly Maxwellian, the Ion Temperature can be defined integrally (whereby the Mean Energy obtained by integrating under the actual Ion Energy Spectrum is set equal to the Integral under a Maxwellian, where the Temperature is a free