Figure L1. Simple logic diagram for determining the type of solar wind from monitor data.
1. Functions
The solar-wind monitors have three functions: (1) To distinguish among fast solar wind from coronal holes, slow interstream solar wind, and coronal mass ejections (CMEs) to allow exposure of collector panels appropriate to each type of solar-wind flow. (2) To document the properties of the solar wind collected during the mission. (3) To determine the solar-wind speed from which the optimum settings of the voltages on the concentrator can be determined.
(1) Measurements of the major elements in the solar wind indicate that their relative abundances depend on both the first-ionization potential (FIP) of the element and possibly its mass and/or charge. These relationships are a function of the type of solar wind flow, with the FIP effect, for example, being least pronounced in the wind from coronal holes, and mass/charge effects being very important in CME plasma. By the completion of the Ulysses, SOHO, and ACE missions, we expect many of the systematics of the fractionation of the solar wind to be fairly well understood. The objectives of the Genesis mission are to take the next step beyond what can be done with in-situ measurements to study the abundances of solar isotopes and rarer elements. We will use the knowledge gained from the in-situ measurements to correct for the effects of solar wind - photosphere fractionation where necessary (Section 1.D); the process will be aided considerably by having separate collection panels for the three major types of solar wind flow.
(2) The monitors will measure the density, velocity, temperature, and anisotropy of the protons, alpha particles, and electrons in the solar wind as functions of time. From those data it will be possible to fine-tune the fractionation corrections as well as to calculate the total fluences of protons and alpha particles during the time that each panel is exposed. Although we expect mass or isotope fractionation within the concentrator to be within acceptable levels, the angle-energy distributions of the solar wind measured by the ion monitor can be used with a numerical model of the concentrator to check this assumption or, in the worst case, to derive correction factors. The history compiled from the monitor data will allow interpretation of the sample analyses in terms of longer-term variations of the solar wind, such as over the solar cycle.
(3) The ion-optical properties of the concentrator can be kept nearly independent of the solar wind velocity if the voltage on the electrostatic mirror tracks the ion energy/charge ratio. Since, to a very good approximation, every ion species in the solar wind has the same bulk flow speed, only the easy-to-measure proton speed needs to be determined to control that voltage. It is also highly desirable to minimize the proton fluence on the concentrator target. That can be done by placing a retarding grid in the system with its voltage set to reject most of the protons at whatever the speed of the solar wind happens to be.
2. Technical
The solar wind monitor comprises separate sensors for positively charged ions and for electrons. Both will be spherical-section electrostatic analyzers with extensive flight heritage, most recently from the Ulysses mission. The reasons for dual analyzers are twofold. First, they provide complementary measurements which together enable reliable identification of CMEs. Second, they provide a redundant capability for measuring solar wind speed.
The measurement of solar wind speed can be done with either ion or electron measurements. Ion observations are strongly preferred due to greater accuracy and a much simpler onboard processing scheme. The solar wind speed based on ion measurements will be the primary diagnostic for distinguishing fast and slow wind and for controlling the concentrator potentials. CME identification is best done with a combination of ion and electron measurements. The single most reliable diagnostic for CMEs is the presence of counterstreaming electron fluxes at suprathermal energies (80 to 1000 eV). At the L1 point, however, a similar signature can be created by magnetic connection to the Earth's bow shock. Although the symmetry axis of the 3-D electron distribution can be used to estimate the direction of the magnetic field, measurements of the relative abundance of doubly-charged helium and the temperature of the proton distribution will help to identify the passage of a CME with confidence.
The electron spectrometer is a spherical-section electrostatic analyzer which uses an array of 7 channel electron multipliers (CEMs), plus the spacecraft spin, to map out nearly the entire unit sphere in velocity space. It will be similar to the instrument currently on Ulysses. The instrument's field of view would extend from 10o to 170o with respect to the spin axis by 30o in azimuth. We envision 24 available energy steps (analyzer plate voltages), logarithmically spaced from 1.5 eV to 1000 Ev, of which only 20 energies would be used in a given spectrum. Four out of every 5 spectra ("core + halo") would use the top 20 energies, and the remaining 1 of 5 would be a "photoelectron + core + lower halo" spectrum (necessary for accurate determination of the spacecraft potential). Two energies would be sampled per spin. For a nominal spin rate of 4 rpm, it would take 2.5 minutes to acquire a complete spectrum. Each spectrum would comprise 20 energies x 7 CEMs x 24 spin phases x 8 bits per pixel, for a total telemetry requirement of 26,880 bits per spectrum, plus a modest allocation of housekeeping words. For a 4-rpm spin this would require approximately 200 bps, including housekeeping, for a 100% instrument duty cycle. A simple, commandable calibration mode will also be incorporated.
The ion spectrometer is a spherical-section electrostatic analyzer which uses a Z-stack MCP, plus the spacecraft spin, to ensure capture of the solar wind beam over a wide range of solar wind conditions. While it is somewhat similar to the instrument currently on Ulysses, the smaller heliocentric distance and smaller range of Sun-spacecraft angle will allow a simpler and lighter design. The ion analyzer field of view is fan shaped, 40o centered on the spin axis by 10o in azimuth. We envision 40 energy steps per spectrum, chosen from an array of 180 available steps spanning 200 eV/q to 25 kev/q. The energy corresponding to peak counts (the heart of the solar wind proton beam) will be used to control the energy range of the succeeding spectrum. Every 9th spectrum would be a "search" mode, with fixed, coarser energy spacing to enable recapture of the beam in the event of a dramatic change in solar wind energy. Each spectrum would comprise 40 energies x 79 angles (combinations of spin phase and CEM) x 8 bits per pixel, for a total telemetry requirement of 25,280 bits per spectrum, plus a modest allocation of housekeeping words. Each spectrum would require 10 spacecraft spins at 4 energies per spin. For a 4-rpm spin this would require approximately 192 bps, including housekeeping, for a 100% instrument duty cycle.
Estimated Parameters of the Solar Wind Monitors
| Item | Electron Instrument | Ion Instrument |
| Box size (LxWxH)* | 25 x 20 x 20 cm | 15 x 13 x 15 cm |
| Mass* | 2.8 kg | 1.6 kg |
| Power*, average | <3.0 W | <3.0 W |
| Power*, peak | <3.0 W | <3.0 W |
| Particle species measured | 3-D electrons | 3-D protons & alphas |
| Energy range | 1.5 eV to 1000 eV | 200 eV to 25 keV |
| Telemetry rate to s/c | 200 bps | 192 bps |
| Bits/week | 1.21 x 108 | 1.16 x 108 |
| Number of CEMs/MCPs | 7 CEMs | 1 Z-stack MCP |
| Energy resolution DE/E (FWHM) | 12% | 8% |
| Polar resolution (FWHM) | 21 o | 2.5 o |
| Azimuthal Resolution (FWHM) | 9 o to 28 o | 2 to 5 o |
| Field of view (Polar x AZ) | 160 o x 30 o | 20 o x 10 o |
| Field of view, Polar | 10 o to 170o | 0 o to 20 o |
| Time to accumulate spectrum | 2.5 minutes | 2.5 minutes |
| Ordinance | 2 pyro pin cutters | 2 pyro pin cutters |
| Red tags | 4** | 4** |
| Instrument alignment with spin axis | ±1 o | ±1 o |
| Temperature: survival (off) | -30 to +60 oC | -30 to +60 oC |
| operate in spec | -20 to +50 oC | -20 to +50 oC |
| test | -25 to +50 oC | -25 to +50 oC |
| Contamination | Hydrocarbons, humidity > 50% | |
| HV operation | Ultra-clean / high vacuum | |
| Post launch outgassing | 2 weeks minimum | 2 weeks minimum |
| Bus voltage | 29.4 to 27.5 V | 29.4 to 27.5 V |
| Operational modes | 4*** | 3*** |
| * Mass, volume, & power include power supplies and analog electronics; data processing is done by the spacecraft computer | ||
| ** 1) HV arm/safe, 2) pyro arm/safe, 3) test port cover, 4) baffle cover | ||
| *** Electron Instrument: 1) Solar wind electron Mode, 2) Suprathermal electron mode, 3) Photoelectron mode, 4) CEM Cal. Mode; Ion Instrument: 1) Solar wind ion, even/odd energy mode, 2) Search/ suprathermal ion mode, 3) CEM Cal Mode. | ||
The above table summarizes the properties and requirements of the electron and ion packages. The locations of the electron and ion monitors are shown in Fig. 4-1, view C.
On-board processing is required for control of the collectors and concentrator. A crude, approximation of solar wind speed can be derived from a single ion analyzer status word: the energy step corresponding to peak counts. This would be an adequate parameter for distinguishing between fast and slow solar wind and for controlling the concentrator potentials. CME identification requires some combination of (1) counterstreaming halo electrons, (2) helium abundance, (3) proton temperature, (4) the angle corresponding to peak halo electron counts as a proxy for the magnetic field direction, (5) location of the spacecraft relative to Earth's bow shock, and perhaps other parameters. These parameters can be calculated from simple algorithms involving weighted sums (or moments) of the counts acquired in each energy-azimuth-polar angle bin. Fig. L1 illustrates a possible logic tree for calculating whether or not the spacecraft is in CME plasma. To avoid excessive shifting between collectors, there will also be a persistence criterion, such as a change from non-CME to CME collectors, or vice versa, can be triggered only by a string of (say) 4 consecutive spectra (10 minutes) each of which indicates that a change is required. It is coincidental, but fortunate, that the Genesis mission will be flown at the same phase of the solar cycle as ISEE3; this makes it possible to use ISEE3 plasma data to test the decision algorithms. During Phase B and in the course of our other solar-wind research, we will experiment with using the parameters shown in Fig. L1 and others to develop a reliable algorithm, which might include weighted voting between different indicators rather than the simpler logic of Fig. L1. Further discussion of CME identification is provided in Document G. The Genesis mission will have the capability to change the algorithm based on in-flight experience if there should be some reason to do so.
Figure L1. Simple logic diagram for determining
the type of solar wind from monitor data.
Figure L2. Perspective view of the electron monitor.
Figure L3. Perspecitve view of the ion monitor.
For more information, see the Phase A
Implementation Plan,
payload section 1.4.6.
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