Physics‐based tests to identify the accuracy of solar wind ion measurements: A case study with the Wind Faraday Cups

Abstract

[1] We present techniques for comparing measurements of velocity, temperature, and density with constraints imposed by the plasma physics of magnetized bi-Maxwellian ions. Deviations from these physics-based constraints are interpreted as arising from measurement errors. Two million ion spectra from the Solar Wind Experiment Faraday Cup instruments on the Wind spacecraft are used as a case study. The accuracy of velocity measurements is determined by the fact that differential flow between hydrogen and helium should be aligned with the ambient magnetic field. Modeling the breakdown of field alignment suggests velocity uncertainties are less than 0.16% in magnitude and 3°in direction. Temperature uncertainty is found by examining the distribution of observed temperature anisotropies in high-beta solar wind intervals where the firehose, mirror, and cyclotron microinstabilities should drive the distribution to isotropy. The presence of a finite anisotropy at high beta suggests overall temperature uncertainties of 8%. Hydrogen and helium number densities are compared with the electron density inferred from observations of the local electron plasma frequency as a function of solar wind speed and year. We find that after accounting for the contribution of minor ions, the results are consistent with a systematic offset between the two instruments of 3-4%. The temperature and density methods are sensitive to non-Maxwellian features such as heat flux and proton beams and as a result are more suited to slow solar wind where these features are rare. These procedures are of general use in identifying the accuracy of observations from any solar wind ion instrument.

Electromagnetic waves and electron anisotropies downstream of supercritical interplanetary shocks

“…The SWE FCs can produce reduced ion distribution functions with up to 20 angular and 30 energy per charge bins every 92 s over an energy range of~150 eV to~8 keV (~1200 km/s proton) [Kasper et al, 2006]. We will use the results of the reduced distributions produced by SWE to compare with our results from 3DP.…”

Solar Wind Electrons Alphas and Protons (SWEAP) Investigation: Design of the Solar Wind and Coronal Plasma Instrument Suite for Solar Probe Plus

“…Since the sensor is simply a metal plate, the response of the instrument is stable with time. The FCs on Wind have a drift in response of about 0.01 %/year (Kasper et al 2006). A significant effort has been expended in an assessment of materials for SPC, solar furnace testing of materials in an optical, thermal, and radiation environment well in excess of the closest SPP solar encounters, design and fabrication of a prototype, and successful operation a prototype instrument in a flight-like environment.…”

“…Detailed properties of the solar wind such as velocity, density, and anisotropic temperatures are then determined by convolving a model velocity distribution function with a detailed instrument response function and deriving the best fit solar wind parameters (e.g. Kasper et al 2006). The AC detection process makes SPC insensitive Lower-left: Cross-section of Faraday cup Sensor Unit (FSU).…”

Solar Wind Helium Abundance as a Function of Speed and Heliographic Latitude: Variation through a Solar Cycle

“…SWE has operated nearly continuously since the launch of the spacecraft in late 1994 and has provided us with a data set spanning a solar cycle that is ideal for this study of long-term variation of A He . In particular, recent studies have shown that the Faraday CupY derived solar wind speeds are accurate to less than 1%, the absolute uncertainty in the SWE densities is less than several percent, and the drift in the density response is less than 0.1% per decade (Kasper et al 2006). For this study, we use the bi-Maxwellian hydrogen and helium data product available from the National Space Science Data Center.…”