The WIND/SMS Experiment:

 (GGS Logo)

Table of Contents


1. Introduction

 Solar-terrestrial physics concerns the study of the generation, flow and dissipation of mass, momentum and energy between the Sun and the Earth. The dynamic interplay of forces that link the Sun and the Earth generates and characterizes our protective near-Earth environment. The regions of space defined by the electromagnetic link include the Sun and its sphere of influence -- the heliosphere -- and the Earth and its much smaller sphere of influence -- geospace.

Global Geospace Science

The Global Geospace Science (GGS) mission is the first step in addressing the behavior of the solar-terrestrial system. GGS will use the WIND, and POLAR satellites, provided by the National Aeronatutics and Space Administration ( NASA ), and the GEOTAIL satellite, provided by the Japanese Institute of Space and Astronautical Science ( ISAS ), to perform coordinated measurements of key geospace regions. The combined data from these spacecraft and complementary ground-based observations will be used to construct quantitative models to describe how mass, momentum, and energy from the solar wind are transported across boundaries, stored and energiezed in the magnetosphere, and subsequently dissipated into the Earth's atmosphere.

The GGS mission is part of the Internationsal Solar Terrestrial Physics ( ISTP ) science initiative, whose aim is to derive the physics of the behavior of the solar-terrestrial system to predict how the Earth's atmosphere responds to changes in the solar wind. These changes cause magnetic storms, communication static, power blackouts, and navigation problems for ships and airplanes with magnetic compasses. Also, satellites can be damaged or can reenter Earth's atmosphere prematurely because of solar storms.

WIND Laboratory

The GGS WIND laboratory was launched on November 1, 1994 by a Delta II rocket from Cape Canaveral Air Station into a highly elliptical orbit beyond the Moon on the sunward side of the Earth, in a novel double-lunar swingby orbit that maintains its apogee along the earth-sun line, thereby maximizing the time it spends directly upstream of the the earth in the solar wind. The WIND orbit will have an apogee as far as 250 Earth radii and a perigee of at least 5 Earth radii. The satellite will spin at 20 revolutions per minute. From this orbit, WIND will measure properties of the solar-wind plasma before it reaches the Earth and will observe the volume of geospace called the foreshock, where turbulence is produced by particles reflected from the bow shock. Later, WIND will be inserted into a small circular orbit between the Earth and the Sun to continuously observe the solar wind an hour or so before it intercepts the magnetosphere.

(Photo) WIND checkout

The WIND laboratory comprises a cylindrical-shaped, spin stabilized spacecraft and a suite of eight instruments designed to optimize measurements of waves, fields and particle distributions. The laboratory is designed for a three-year mission life and has a total mass of 1254 kg.

(WIND Artist Rendition) Pix or (gif) Schematic

(gif) Location on Spacecraft

2. Scientific Objectives

 The observation of particle abundances of the solar wind and suprathermal ions along with the magnetic field and plasma measurement will enable us
  1. To provide the instantaneous characteristics of matter entering the Earth's magnetosphere,
  2. To determine the solar elemental abundances,
  3. To characterize physical properties of the acceleration regions in the lower corona,
  4. To study:

3. Experiment Overview

SMS instrument

The SMS instrument is designed to study the composition of the solar wind and of solar and interplanetary energetic particles on the WIND spacecraft. SMS consists of three different sensors with associated Data Processing Unit ( DPU), which are each optimized for a particular aspect of ion composition: the Solar Wind Ion Spectrometer ( SWICS), the High Mass Resolution Spectrometer ( MASS), and the Supra-thermal Ion Composition Spectrometer ( STICS). This experiment will determine the abundance, composition, and differential energy spectra of solar wind ions, and the composition, charge state and three-dimensional distribution of suprathermal ions. These ions and their abundance fluctuations provide information about events on the solar surface and about the formation of the solar wind.

(wiring gif) SMS Configuration

Measurement Objectives

All three SMS sensors employ electrostatic deflection systems in combination with time-of-flight (TOF) measurements of the impinging particles. The electrostatic deflection analyzer selects the proper entrance trajectories, traps neutrals and ultraviolet light, and serves as an energy/charge passband filter. The energy/charge is stepped once per spacecraft revolution, in a cycle of 60 spins. After exiting the deflection system, an ion enters the time-of-flight system by passing through a thin carbon foil. Secondary electrons produced when the ion exits the carbon foil are accelerated onto a microchannel plate (MCP) generating a start pulse. At the end of the flight path in SWICS and STICS, the ion stops in a solid state detector (SSD) which determines the residual energy of the ion. The secondary electrons emitted from the SSD are also acclerated to a stop MCP, generating a stop pulse. (For the MASS sensor, the ion strikes the stop MCP directly, generating a stop pulse.) Thus for STICS and SWICS, the quantities energy per charge, E/Q, time-of-flight (TOF), and energy, E, are determined for the incoming particles.

(gif) TOF technique

SWICS and STICS are aimed at different energy ranges, SWICS being devoted to solar wind, while STICS is devoted to higher energy suprathermal ions. The MASS sensor is a high resolution retarding potential mass analyzer with a quadrupolar electric field configuration, which will be able not only to measure the composition of the less abundant elements in the solar wind, but also the isotopic composition of the more abundant heavy ions.

The digital processing unit, DPU, serves all three sensors by fully handling the communications with the spacecraft. It receives rate and pulse-height data from each sensor, compresses, stores and formats it for conveying the data to the S/C telemetry. It also handles the experiment control through the commanding system and surveys the SMS housekeeping data.

4. The SWICS Sensor

 The Solar Wind Ion Composition Spectrometer (SWICS) sensor covers the energy per charge, E/Q, range 0.5 to 30 keV/charge, which fully includes the solar wind range. It is based on the heritage of the ULYSSES SWICS sensor which returned the first elemental and charge state measurements of the solar wind.The sensor can determine the omposition, charge state distribution, kinetic temperature, and speed of the more abundant solar wind ions (e.g. He, C, N, O, Ne, Mg, Si, and Fe). The sensor is designed to accept ions from 0.5 keV/charge, corresponding to 190 km/s O6+ ion, up to 30 keV/charge, corresponding to 1200 km/s Fe6+ ion.

(photo) SWICS

To cover a wide range of angles (on the rotating spacecraft) and also to allow the measurement of reasonable fraction of the thermal width of the ion distribution, the solar wind enters the instrument through an ion-optic system which accepts the solar wind within a angle of 40° and which is matched with an electrostatic analyzer. After penetrating through the analyzer and prior to their entrance into the TOF detector, the ions are accelerated by up to 30 keV/charge to raise their energy above the threshold of the solid state detector (SSD) and to improve the charge state resolution.

(gif)SWICS side view

The identification of solar wind ions is obtained by measuring the energy/charge, E/Q, the time-of-flight T, and the residual energy E_SSD deposited in the solid state detector. From this we can determine the mass, M, and the velocity, s/T, of each ion according to

M = 2*(T/s )^2 x E_SSD / a

where s is the TOF path length, and a takes into account the pulse-height defect in the SSD. The mass per charge ratio, M/Q, will be derived by combining the accurate measurement of E/Q in the electrostatic deflection system and the time-of-flight T:

M/Q = 2*( T/s )^2 x (U_acc + b E/Q)

U_acc is the acceleration voltage, and b accounts for the energy loss in the carbon foil. Thus from the measurement of M and M/Q the ionic charge Q, and finally from Q and E/Q the initial energy E can be derived. These measured quantities have been plotted on log-log scale of M vs. M/Q as shown for ULYSSES/SWICS data below.

(gif)Measured Solar Wind


ULYSSES/SWICS solar wind data, from vonSteiger and Geiss, Cosmic Winds and the Heliosphere ed. Jokipii, U.of Ariz. Press, 1995

5. The MASS Sensor

 The solar wind mass (MASS) sensor is a high resolution TOF mass spectrometer (M/dM > 100) which measure the elemental and isotopic composition of the solar wind over a wide range of solar wind bulk speeds. The instrument employs a hemispherical electrostatic preselecting deflection system, followed by a high-resolution retarding potential TOF analyzer.

(gif)MASS photo

The principle of operation of the mass analyzer is to measure the TOF of mostly singly ionized particles in a static electric harmonic potential ( V = k x ^2). In such a potential - the potential of a harmonic oscillator - the TOF depends only on M/Q:

T = c Sqrt(M/Q )

Solar wind ions within the passband of the hemispherical deflection system pass through a thin carbon foil. In passing through the foil the solar wind ions lose a fraction of their energy, are scattered, and emerge either as neutrals or singly ionized. Secondary electrons emitted from the foil will be recorded in a micro-channel-plate (MCP), while the electrostatically reflected ions are recorded in the stop-MCP.

(gif)MASS side view


MASS top view
From the TOF, the mass of the ion can be derived directly, since the mass is proportional to the square root of TOF, and independent of energy. This gives a very sharp peak for the mass, and thus solar wind isotopes heavier than He can be resolved for the first time.

(gif)MASS calibration

Calibration data showing the high mass resolution of the instrument for isotopes 12, 14, 16 and 20.

(gif)MASS SW simulation

A simulation of the solar wind showing a color log intensity plot of the expected countrate given nominal solar wind conditions. The horizontal white bands are the tips of the peaks, whereas the broad background towards longer TOF are due to the Lorentzian tails of the TOF peaks.

(gif)Dec94-July95 Solar Wind

Magnesium isotopes in the solar wind resolved for the very first time. Abundances were found to be very close to solar system abundances, indicating that neither the Sun nor the solar wind has mass fractionated very considerably from the proto-solar nebula. This constrains models of solar formation and solar wind acceleration.

6. The STICS Sensor

The Supra-Thermal Ion Composition Spectrometer (STICS) sensor is an ion telescope with a large geometrical factor of 0.05 cm^2 sr for the measurement of the energy distribution of individual charge states of various elements of suprathermal ions. It covers the energy range from 8 to 226 keV/charge. Similar to the SWICS sensor it combines the selection of incoming particles according to E/Q by electrostatic deflection in a multi-gap system with a subsequent TOF analysis and a final energy measurement in a solid state detector. Only the postacceleration is not required because the suprathermal particles carry sufficient energy to respond in the solid state detector.

(gif)STICS side view

7. The Digital Processing Unit

The Digital Processing Unit, DPU, performs all tasks necessary to communicate commands and data between the sensors and the spacecraft, which in turn is linked through telemetry to ground. The main tasks are (1) to control the SMS sensors by DPU generated and S/C routed ground commands, (2) to record the house keeping data, (3) to record and format the counting rates of the sensors, and (4) to record, classify, and format pulse-height data. Its output data are routed through the S/C telemetry. A block diagram of the DPU functions may help explain its many tasks. The interface and logic of each of sensors is also given in block diagram form for SWICS, for MASS, and for STICS.

8. Bibliography and Reader Response

Please send your comments, questions, ... concerning the WIND/SMS instrument and program to

(gif)
Rob Sheldon
Physikalisches Institut
Universität Bern
Sidlerstrasse 5
3012 Bern, Switzerland

sheldon@phim.unibe.ch
rsheldon@buasta.bu.edu

Last Update: September 5, 1995, RbS.
Coming attractions: Mass and SWICS deflection system photos