The Plasma-Mag package consists of three parts: 1) a Faraday cup to measure the 3-D distribution function of proton and alpha components of the solar wind with a time resolution (repetition rate) of 90 milliseconds, 2) a "tophat" analyzer to give 3-D electron velocity distribution functions in 800 ms (480 points), and 3) a flux-gate magnetometer to make a vector measurement in 30 to 40 milliseconds. A much-improved time resolution is possible because Triana is a 3-axis stabilized spacecraft permitting near-continuous measurements. Previous solar-wind measurements from the spin-stabilized WIND spacecraft could only be made when the Faraday cup pointed towards the Sun.
The Faraday cup is particularly suited for precise solar-wind measurements on a stabilized spacecraft because of its large field of view (± 60 degrees). The use of multiple collectors allows the full range of the solar wind deflections (approx. ± 15 degrees in all directions) to be detected, while still allowing a full 3-D velocity-distribution function to be accommodated. The whole distribution function remains in the field of view, so that more accurate density measurements can be made, especially at high densities.
The tophat electrostatic analyzer will make measurements of the electron distribution function between 3 eV and 2 keV in a time as short as 800 milliseconds. The 3D measurement will be accomplished by electronically simulating the data sampling of an electron spectrometer on a spinning spacecraft. The instrument has a set of anodes distributed uniformly in azimuth, each with a field of view 50° x 7° in azimuth and elevation. The anode distribution around the circumference of the symmetry plane (0° elevation) provides azimuthal coverage over 2Þ¡. The coverage in elevation is between +60° and -60° above and below the plane of the anodes is accomplished by varying the potential of external deflection plates.
In addition to studies of the Sun and solar wind, the data also are used to provide early warning of solar events that might cause damage to power generation, communications, and other satellites. Together, the Plasma-Mag suite of instruments will provide a 1-hour warning to the appropriate agencies that safeguard electrical equipment on Earth and satellites in Earth orbit. Present plans include routinely giving the data to NOAA with only a 5-minute data processing delay from detection of an event at the Triana spacecraft position and reception at the ground station to the time that it is delivered.
Plasma-Mag contains a triaxal-fluxgate magnetometer that will investigate solar-wind magnetic fields with a sensitivity level of better than 0.1 nanoTesla. The Plasma-Mag instrument is an evolution of a long line of magnetometers developed at the Goddard Space Flight Center. The present design has been optimized for small size, low power, simplicity, and a very large dynamic range. Using the latest technology in sigma-delta converters it achieves a dynamic range of almost 8 orders of magnitude in field measurement capability, simplifying its implementation aboard the Triana spacecraft. The commercial-technology sensor is small and light (65 gram) and will be mounted at the end of an extensible boom remote from the spacecraft body. Although as many as 100 samples of the vector magnetic-field measurement can be acquired per second, only a fraction of these will be formatted and transmitted to Earth, the baseline being 1 sample per second.
Table 2, section 4.5, summarizes the comparison between Triana and other NASA solar missions to L-1.
