.

1.  Introduction

The solar wind experiment (SWE) for the WIND spacecraft is a
comprehensive

integrated set of instruments which can attack many outstanding problems
in

the magnetosheath, the foreshock, and the interplanetary medium. It also
has the

capability of making measurements in other flowing regions which might
become available

to it.

The \'key parameters\' (velocity, density, and temperature)of the solar
wind

ions are extracted from detailed three-dimensional measurements made by

the Faraday cup (FC) subsystem. The FC subsystem is particularly suited

for use in the interplanetary medium. Table I lists the parameters and
the expected

measurement precisions for measurement of supersonic plasma .
Furthermore,because of its stability and simplicity,the Faraday cup is
especially suitable for space weather studies

Table 1

Key Parameters from the SWE instrument

Parameter Range Precision

\*

Proton velocity 200-1250 Km sec_1 +/.-3%

(3 Components)

Proton number 0.1-200/cm_3 +/- 10%

density

Thermal speed 0-200 km sec +/-10%

Alpha/proton up to 100%

Ratio

\* Assumed incident along cup axis

The electrons in the solar wind and the foreshock electrons reflected
from the bow shock have been studied using a vector electron and ion
spectrometer (VEIS) derived from ISEE-1 instrumentation

(Ogilvie *et al.,* 1978) but with increased sensitivity which is
sufficient

to make more detailed measurements. These are highly-time-resolved,
three dimensional

snapshots of the distribution functions of ions and electrons having

energies/charge from 7 V to 24.8 kV for flowing plasmas with Mach
numbers of

unity and below The exploration of this region should include
identification of wave types and conditions for their generation so that
the changes occurring as the solar wind approaches Earth can be

incorporated in models which can then give a clear picture of the
environment just

upstream of the magnetosphere. Identification of the incoming waves
requires the

close cooperative analysis of data from different WIND experiments as
well as

those from other spacecraft .

.

2.The VEIS instrument measures three-dimensional velocity distribution
functions

for flows with Mach numbers less than unity. The electrons in the solar
wind,

magnetosheath, and magnetosphere, and ions reflected diffusely from the
bow shock

satisfy this condition. The instrument can also supply accurate
measurements of the electron heat

flux, including its direction, and the flow direction of the electrons
(see Figure 1).

The SWE strahl sensor is able to make detailed measurements of the
electron

velocity distribution along the magnetic field direction where the
distribution is

distorted, reflecting the non-local nature of those electrons

As discussed later, measurement of the entire ion velocity distributions
can be

made as the space craft rotation carries a Faraday cup with its wide
acceptance

angle of +/-60 degrees through the solar wind direction in approximately
1 s. This mode

of operation promises to extend measurement of the solar wind parameters
into a

time regime that has not yet been thoroughly explored.

2.1. STUDIES OF INTERPLANETARY SHOCKS AND INTERACTION REGIONS

In addition to studying the nature of the bow shock, the \'burst\' or
\'event\' mode

of the SWE instrument can be used to observe phenomena in the vicinity
of other

boundaries in the plasma. In this mode, all subsystems are operated at
their highest

.time resolution keeping the data storage full. A short time after a
\'trigger\' condition

is satisfied, data taking ceases so that observations both \'before\'
and \'after\' are in

the memory. The instrument then telemeters data at a reduced rate until
the memory

has been read out. Of considerable interest is the characterization of
the microscale

structure of shocks and of the interfaces between fast and slow solar
wind that

evolve into co-rotating interaction regions. Direct connections between
the SWE,

MFI, and WAVES experiments will allow complementary data triggers.

The \'event\' mode will also be used to study the structure of the
magnetopause

and bow shock, using time-tagged commands, and to make rapid
measurements of

particle properties (such as correlations) for extended periods.

2 *Global studies of the heliospheric solar wind*

Interpretation of observations from spacecraft in the distant
heliosphere (Voyager

and Pioneer) and those beyond 1 AU and out of the ecliptic (Ulysses,
Galileo,

and future missions) requires a comparison with measurements near 1 AU
where

phenomena are better understood and long-term data bases are available.
For

example, the dynamic pressure variation of the wind can move the
termination

shock by as much as 10 AU in a single year (Belcher *et al.,* 1993).
Such basic

measurements near 1 AU will be provided in part by SWE and are an
important

part of the scientific aims of the experiment.

**3. Instrument Description**

3.1. GENERAL CONFIGURATION

The instrument consists of five sensors mounted in separate boxes, a
Data Processing

Unit (DPU), and a calibrator. Figure 1 shows how these components are

mounted on the spacecraft. A Faraday cup and a triad of electrostatic
analyzers

(half the VEIS) are located at each end of a spacecraft diameter. One
group of

sensors is augmented by the strahl sensor. The DPU and calibrator are
mounted

between the two groups of sensors, near the middle of the spacecraft.
Each group

of sensors communicates with the DPU through an interface board
contained in

the VEIS housing. We describe each subsystem in more detail in the
following

sections. Table II lists some physical dimensions of the subsystem
sensors and

their relevant characteristics.

3.2. THE DPU

The DPU provides the only electrical interface with the spacecraft. It
receives and

interprets commands and controls the SWE subsystems using various
pre-defined

modes of operation. Flight software has been written for several
measurement

modes; the \'burst\' mode has already been mentioned. A description of
some of the other modes is given below.

. Table 2

Instrument characteristics

Faraday cup Energy/charge range 150 V-8.0 kV

Operating frequency \~ 200 Hz

Effective area/cup 35 cm 2

*AE/E* narrow windows 0.065

*AE/E* double windows 0.130

Maximum window width 1 kV

Equivalent geometrical factor 1.1 x 102 cm 2 sterad

Vector spectrometer Energy/charge range 7 V-24.8 kV

Analyzer FOV 7.5 degrees x 6.5degrees, ∆E/E = 0.06

Geometrical factor (per analyzer) 4.6 x 10 -4 cm 2 sterad

Minimum step dwell time 5 ms

Analyzer constant 7:1

Plate radii: inside 4.717 cm

Plate radii: outside 5.443 cm

Strahl spectrometer Energy/charge range 5 V-5 kV

Analyzer FOV 3 degrees x 30 degrees

*AE/E* 0.03

Minimum step time 30 ms

Geometrical factor (per anode) 7 x 10 -4 cm 2 sterad

Plate radii: inside 5.40 cm and 14.4 cm

Plate radii: outside 6.60 cm and 15.6 cm

New modes of operation will be developed and uploaded to the instrument

after launch. The DPU also formats data for the telemetry and can
calculate key

parameters. The interface boards transmit DPU signals to the subsystems
and

receive information from the subsystems as shown in the block diagram,
Figure 2.

The DPU uses Sandia 3300 central processing, timing, and control units.
Software

includes a default mode which, though rather general, provides a
considerable

scientific return, and other stored modes which can be activated by the
transmission

of time-tagged pointers. This flexibility means that mode changing can
be done

without the transmission of messages to the spacecraft.

3.. THE VEIS

The VEIS is an instrument designed for determining in detail the
distribution

functions of ions and electrons over the energy/charge range from 7 V to
24.8 kV.

It consists of two sets of three small electrostatic analyzers which are
identical

and employ balanced deflection through 127 degrees in a
cylindrically-symmetric layout.

The transition from electrons to ions is done by reversing the direction
of the analyzer electric field, and each is detected by individual
channel electron multipliers The field of view of each analyzer is
determined solely by a collimator at the entrance and by the sensitive
area of the detector. A mesh-covered hole in the outer plate is provided
so that light can be trapped rather than scattered inside the analyzer.
A high-transparency mesh grid placed between the exit slit and the
channel multiplier

detector prevents electric field leakage between the channeltron chamber
and the

analyzer. Other grids prevent low energy electrons formed on the
channeltron cone

from being drawn out and lost. These design features result in an
analyzer with very well

defined transmission and solid angle properties, free from
\'ghost\'and'glint' responses,

and largely immune to responses resulting from photoelectrons produced
inside the

analyzer. As indicated in Figure 3 the calculated energy response of
each analyzer is

narrow, providing a differential energy window of *AE/E ,.\~* 0.06 and a
substantial

geometric factor (4.6 x 10 -4 cm 2 sterad). The energy-angle coupling of
this proven

design is much reduced as compared with a slitless analyzer system.

These analyzers have approximately 7.5 degree x 6.5 degree fields of
view; and using

them to obtain 3-D coverage entails the assumption that each direction
of view

is representative of a larger angular region, i.e., each of the six
analyzers must

represent approximately 4pi/6 steradians. This condition is satisfied
for plasmas

having flows with Mach number M \< 1. By using analyzers looking in
opposite

senses along mutually perpendicular axes, the flow direction and other
moments

can be obtained in an elegant way. Electrons in the solar wind which
have flows with

M \< 1 can be accurately observed using such a triad, to include diffuse
ions. Although flows

with higher Mach numbers will not be missed using such a detector, to
characterize

them accurately requires the use of a larger number of
narrow-angular-response

analyzers, as wasdone on the POLAR spacecraft with the HYDRA instrument.

On a spinning spacecraft the spin causes the observations to be made
along a

curved path in velocity space. If six energy scans of 16 points are made
every 60 degrees of

spacecraft rotation, each distribution function obtained by the six
detectors contains

6 x 16 x 6 = 576 points; an example is shown in Figure 3b. With multiple
analyzers

each having rigidly-defined solid angles of acceptance, there is no
possibility that

an omni-directional background flux can be confused with the desired
directional

flux.

Two channeltron detectors are provided per analyzer, one each for
electrons and

ions; they are each equipped with a cone to match the output slit of the
analyzer. The

channeltrons are selected and qualified by established methods used
previously.

To avoid contamination and the resultant gain degradation, prelaunch
nitrogen

purging was employed. The mechanical construction of the instrument
prevents

gases emitted under vacuum from the electronics boards from passing
through to

the detector compartment. The detectors are mounted on ceramic circuit
boards

with AMPTEK A-111 charge-sensitive preamplifiers, having both digital
and analog.

pulse outputs. The latter are periodically subjected to pulse height
analysis,

to ensure that the detectors remain gain-saturated. These precautions
represent

more rigorous protection than was used for ISEE, which nonetheless
lasted for

twelve years and accumulated over 2 • E11 counts.

Figure 3 shows details of the construction of the analyzers. The voltage
supply

which provides the deflection field is bi-polar and symmetric; the
potential is

reversed between electron and ion scans, using a shunt regulator circuit
in which

the leakage current of a high voltage diode is controlled by
illuminating it with an

LED (Loidl, 1984).

3.4. THE FARADAY CUP SUBSYSTEM

The Faraday cup subsystem is used to determine distribution functions
and basic

flow parameters of the ion component of the solar wind; the sensors
operate in a

different way and have different properties from the electrostatic
analyzers of the

VEIS. Each sensor consists of a cup containing a set of planar grids and
two semicircular

collector plates onto which a selected portion of the solar wind
impinges;

the corresponding electric current due to positive ions is measured.
Figure 4(a)

shows a cross sectional view of one sensor. In order to determine the
energy of

the incoming ions and also to discriminate between the charged particles
and

the photoelectric current produced by sunlight, a selected portion of
the charged

particle flux is chopped at 200 Hz by means of a time-varying positive
potential

applied to a highly-transparent metal grid. Figure 4(b) illustrates the
principle of

operation. The time-varying potential is generated by a modulator, which
produces

a dc-biased, 200-Hz square wave. The resulting waveform can then vary
from V

to V + delta V, where V + deltaV can be as high as 8 kV and deltaV as
large as 1 kV.

The resulting chopped current from each collector is synchronously
detected and

integrated on a capacitor for a fixed time interval. The resulting
voltage is converted

to a digital signal using a logarithmic A/D converter.

The Faraday cup sensor system has at least four advantageous properties.

\(1\) Since deltaV is variable, the energy/charge bandwidth of the
detector is variable,

whereas it is fixed by geometry in the case of an electrostatic
analyzer. This

flexibility will be useful for the study of reflected ions, and for
automated modes

in which the peak of the solar wind distribution can easily be located.

\(2\) The flow direction can be determined to better than one degree by
use of the split current collectors Knowledge of the flow angle to a
high accuracy is necessary when interpreting measurements

made by other spacecraft in the magnetosphere and the magnetosheath as
well as

for determination of the properties of the bowshock and interplanetary
shocks.

\(3\) The Faraday cup is well suited to measurements at high time
resolution,

even when used on a spinning spacecraft. It has a large sensitive
acceptance angle

(approximated by a 60 degree half-angle cone, see Figure 5(c). Since we
are using

two cups facing in opposite directions at the ends of a spacecraft
diameter, the

solar wind is being measured 2/3 of the time, and variations in the full
velocity

distribution function can be observed with a time resolution of
approximately one

second.

(4)Aside from being simple and extremely stable, the Faraday cup
provides measurements of the velocity distribution functionintegrated
over directions perpendicular to the sensor\'s axis, ie. measurements of

the \'reduced\' distribution function. From measurements of the reduced
distribution

function along several different look directions, the 3-D velocity
distribution can

be characterized. Such measurements can be transmitted to the ground
with less

telemetry than would be required by a pixel-type instrument. The Faraday
cup is

also particularly suitable for absolute density determinations in the
supersonic solar

wind since it can encompass the whole distribution and has no
energy-dependent

efficiency corrections.

3.5 *The Faraday Cup Current Signals*

As discussed above, a modulated voltage selects particles whose
component of

velocity perpendicular to the modulator grid lies in a selected range.
Thus, a

mono-energetic beam with energy/charge equal to the central
energy/charge of

the modulating window would be detected if it were normally incident on
the

sensor. But if it entered the sensor at a sufficiently large angle to
the normal due,

for example, to rotation of the sensor on a spinning spacecraft, it
would not be

able to pass through the grid. To demonstrate this dependence on angle
as well as

energy/charge, we show in Figure 5 the amplitude of the chopped (or
\'modulated\')

currents resulting from a modeled solar wind distribution that has a
bulk speed of

400 km s -1 and a velocity width of 40 km s -1. The panels show the
currents vs.

azimuth angle of the spacecraft for a sequence of modulator windows
ranging from

below to above the speed of the wind.

The velocity range of each window for normally-incident protons is
indicated

in the upper right corner of each panel. The mean velocities increase
from the top

to the bottom panels of the figure. Consider first the currents measured
at 0 degrees where

the sensor is facing into the incoming flow of plasma. From the top to
the bottom

panel, the current at 0 degrees increases and then decreases as the
window velocity range

passes over the velocity distribution of the wind.

Next, consider the currents measured when the sensor axis makes an angle
to

the incoming wind. The component of wind velocity normal to the
modulator grid

will be smaller, and thus the wind can be modulated by a window of lower
voltage.

The top panel illustrates such a case. The parameters that characterize
the particle

velocity distribution can easily be extracted from such a set of data.
The spin-plane

angle from which the flow comes is determined from the azimuth angle at
which

the peak current is measured. The speed, density, and temperature of the
wind can

easily be determined by taking weighted moments of the currents measured
in each

energy window. Using a model velocity distribution, a nonlinear
least-squares fit

to the currents can be performed to extract the maximum amount of
information

from the measurements.

The elevation angle of the flow can be determined from the relative
currents

measured by the two sensors, since one sensor normal is tilted 15
degrees above the spin

plane of the spacecraft and the other normal is 15 degrees below that
plane. An alternative

technique is to make use of the relative currents from the two halves of
the collector

plate in a single cup.

The double-peaked current produced using only one window just below the
peak

of the velocity distribution can be used to obtain information about the
distribution

in a single spacecraft rotation (\~ 3 s for WIND). The angular
separation of the

peaks determines how far the window is below the bulk velocity of the
distribution,

and the width of the peaks is a measure of temperature of the
distribution. Our

simulations show that this technique can provide accurate distribution
parameters,

and this \'single-spin\', mode on WIND has been used to obtain
parameters with high time resolution.

3.4.2. *TheCurrent Measurement System*

Figure 6 shows a block diagram of the Faraday cup measurement system.
The modulator

can supply the required dc-biased, square-wave voltage waveform between

any two of 64 logarithmically-spaced voltage levels ranging from 150 to
8000 V.

The velocity resolution of a single window is ∆V/V*\~* 0.033. Normally,
we intend

to use \'double-width\' windows with a voltage waveform going from level
n to level

(n+2), thus generating velocity windows with ∆V/V *\~* 0.065. The
voltage window

will be maintained for a full revolution of the spacecraft; and in the
\'full-scan\'

mode, 31 velocity windows can be scanned in 93 seconds at a nominal
spacecraft

spin rate of 3 s/revolution -a . In a more-efficient \'tracking\' mode,
14 windows are

chosen to cover a velocity range that includes the window that had the
peak

current in the previous scan; the time for a scan in this mode is
approximately

42 s. A full-scan is taken every 30 min, to ensure that we are not
tracking a

spurious peak. A full-scan is also initiated if the maximum observed
current

is less than a pre-determined threshold.

The modulated currents from each half collector plate are measured by
independent

measurement systems. The currents are capacitively coupled to a
preamplifier

(bandwidth \~ 200 Hz, centered at 200 Hz). The preamp\'s output is fed
to three

range amplifiers connected in series. (Their gains from the first to the
third are

7, 46.5, and 46.5.) The output of each range amplifier is synchronously
detected

and the resulting dc signal is integrated on a capacitor for an integral
number of

periods of the 200 Hz modulating waveform, nominally for 30 ms. At the
end of

the integration period, a multiplexor sequentially searches for the
highest- gain

unsaturated output. That output is then logarithmically converted to a
10-bit digital

number. When combined with two additional bits specifying the range
amplifier

used, the net dynamic range of the system is 10+5. A 30 ms integration
time will

allow measurement of currents in the range from 3 x 10 -13 to 3 • 10 -8
amps.

The thermal noise level for that integration time is 3 x 10 -13 amps. In
our nominal

mode of operation, the integration time is increased to 120 ms for
angles when the

cup is looking away from the Sun.

The combined data rate from both FC sensors is 320 bits s -1. The total
times

required to obtain a spectrum in each sensor using the \'full-scan\' and
\'tracking\'

modes are 93 and 42 s, respectively. As mentioned earlier, we intend to
explore the

use of a \'single-spin\' mode which should give us information about the
velocity

distribution function in either a full- or half-spin period depending
upon whether

both cups are used to determine the longitudinal flow angle or whether
information

from a single cup with its split collector is sufficient. The fully
re-programmable

DPU allows us to adjust our data-taking modes to suit the
characteristics of the

region being observed.

3.5. THE \'STRAHL\' DETECTOR

A magnetic-field-aligned beam or strahl in the solar wind electron
velocity distribution

results partly from the velocity dependence of the cross section for
electron

coulomb collisions. As a result of the rapid fall-off in density and
hence the number

of collisions as one moves outward in the corona, electrons with
energies above

approximately 40 eV moving along the magnetic field can travel to 1 AU
with

little scattering. Thus observations close to the field direction
sometimes detect the

presence of a \'strahl\' of higher energy electrons. The properties of
this feature of

the solar wind electron distribution have been studied between 0.3 and 1
AU from

the Helios spacecraft (Marsch et *al.,* 1991), but further observations
at 1 AU over

an extended period of time will increase our knowledge of the formation
and properties

of this structure. To do so requires an angular resolution of a few
degrees, and

that measurements be performed in an angular region of size \~ 30
degrees x 30 degrees centered

on the magnetic field direction. The WIND strahl detector is a
truncated, toroidal

electrostatic analyzer with an included angle of 131 degrees (Young et
*al.,* 1987), see

Figure7. This device covers a field about +/-28 degrees in a plane
containing the spin axis.

The detectors are two channel plates. As shown in Figure 8, six anodes,
covering

approximately 5 degrees each, are used per plate. They are read out
commencing 16 degrees after

the solar direction every 31 ms until 72 degrees after the solar
direction. This sequence of

15 observations is repeated when the spacecraft has made a half
revolution and is

looking along the magnetic field in the anti-Sun direction. Although the
magnetic

field direction shows considerable fluctuations, it is within the field
of view

of this sensor approximately 60% of the total observing time. Note that
there is

no detector on which sunlight can directly fall, since the center anode
and channel

plate are absent. The outer plate of the analyzer is serrated and coated
to minimize reflected

light. In general, the proper operation of this part of the instrument
depends upon

the alignment of the spin axis of the spacecraft at right angles to the
ecliptic plane.

The channel plate technology, which is well understood, requires rigid
attention to

cleanliness and to the purging of the instrument with dry gas before
launch. Both

channeltrons and channel plates were installed after the spacecraft
thermal vacuum

test to avoid contamination.

Due to a high voltage supply failure the last available data from the
VEIS is for May 31 2000. At a SWE team meeting at UNH it was decided to
allot all of the telemetry origionally used for VEIS and STRAHL to the
output of the STRAHL analyzer. The scheme adopted is illustrated in fig
8. Its use results in improved time and angular resolution and all of
the electron data recorded since Aug 16 2002 uses this mode. The VEIS
ion mode no longer exists,and no changes of any kind were made to the
Faraday cup or its data. Table 3 and Fig8. describe the new STRAHL mode.
New software was written by G.Needel and uploaded to the DPU. The ground
software was re-written by M. Holland and all data aquired after Aug 16
2002 is in the format described Fig. 8. The spacecraft spins are divided
into 8 equal parts, each lasting about 375 mS, called sectors. During
the first of three spins, energy steps 1 through 5 are recorded. During
the next 3 spins energy steps 6 through 10 are recorded and during the
next three spins energy steps 11 through 15 are recorded. Rest periods
are included in this scheme to ensure that all steps have well-defined
start energies.

In order to replace the VEIS electron measurements the Strahl detector
is operated in a rapid sweeping mode. A full spectrum now consists of 15
energy steps and is accumulated over3

consecutive spins. (= 9+ sec.) Each spin is divided into 8 sectors with
each sector visiting 5 energy steps. There is a sixth "rest stop "
provided at the end of each sector. Each energy step consists of a 30mS
accumulation interval and a 30 mS settling interv. During each spin the
same 5 energy steps are visited 8 times.

The 12 Strahl detectors are summed over adjacent pairs to yield 6
equivalent detectors. The accumulated spectrum is stored in a dual
buffered memory area, and telemetered to the ground as soon as possible.
To synchronize data accumulation with the telemetry allocation, there
are sometimes time gaps, ranging from one to several spins, between
accumulated spectra depending on the spacecraft spin period which varies
slowly. There may also be occasions when no data is available for
transmission, but these are rare.. In these periods, pad values are
sent, At the beginning of the first spin the Strahl is commanded to the
first step,allowed to settle for 30mS,then the first 30mS accumulation
begins .After each accumulation the Strahl is commanded to the next
level. This is repeated 5 times and then the Strahl is commanded back
down to the first step of the current sequence. This pattern is repeated
8 times in all. At the end of the spin the Strahl is commanded to the
first step of the next 5 step sequence.

4.3. OTHER MODES

Other modes, include the \'burst \'mode described above, which can
capture transient phenomena; a \'tracking\' modein which a limited
number of FC energy/charge windows are chosen to track the

Center of the solar wind velocity distribution and thus yield parameters
with higher

time resolution; and a \'single-spin\' mode in which the Faraday Cup
energy/charge

window is chosen to be just below the peak of the distribution function
and measurements

are taken in a single spin as described in Section 3.

**References**

Belcher, J. W., Lazarus, A. J., McNutt, R. L., Jr., and Gordon, G. S.:
1993, \'Solar Wind Conditions

in the Outer Heliosphere and the Distance to the Termination Shock\',
*J. Geophys. Res.,* to be

published.

BeIlomo, A. and Mavretic, A.: 1978, *MIT Plasma Experiment on IMP H and
J Earth Orbited*

*Satellites,* MIT Center for Space Research Technical Report
CSR-TR-78-2, 1978.

SWE, A COMPREHENSIVE PLASMA INSTRUMENT FOR THE WIND SPACECRAFT 77

Bridge, H. S., Belcher, J. W., Butler, R. J., Lazarus, A. J., Marvetic,
A. M., Sullivan, J. D., Siscoe,

G. L., and Vasyliunas, V. M.: 1977, \'The Plasma Experiment on the 1977
Voyager Mission\',

*Space Sci. Rev.* 21, 259.

Loidl, A.: 1984, \'HV Diodes Used as Variable Resistors and Switches\',
*J. Phys. E: Sci. Instruments*

17, 357.

Marsch, E.: 1991, in R. Schwenn and E. Marsch (eds.), *Physics of the
Inner Heliosphere,* Vol. II,

Springer-Verlag, Heidelberg, p. 45.

Ogilvie, K. W., Scudder, J. D., and Doong, H.: 1978, \'The Electron
Spectrometer on ISEE-I\', *ISEE*

*Transactions on Geoscience Electronics GE-16* 261.

Scudder, J. D. and Olbert, S.: 1979a, \'A Theory of Local and Global
Processes which Affect Solar

Wind Electrons. 1. The Origin of Typical 1 AU Velocity Distribution
Functions\~Steady State

Theory\', *J. Geophys. Res.* 84, 2755.

Scudder, J. D. and Olbert, S.: 1979b, \'A Theory of Local and Global
Processes which Affect Solar

Wind Electrons. 2. Experimental Support\', *J. Geophys. Res.* 84, 6603.

Vasyliunas, V. M.: 1971, \'Deep Space Plasma Measurements, 12.1\',
*Methods of ExperimentalPhysics*

98, 49, Academic Press, New York.

Young, D. T., Ghielmetti, A. G., Shelley, E. G., Marshall, J. A., Burch,
J. L., and Booker, T. L.: 1987,

\'Experimental Tests of a Toroidal Electrostatic Analyzer\', *Rev. Sci.
Instrum.* 58, 501.

![](media/image1.jpeg){width="4.413194444444445in"
height="7.167361111111111in"}

Figure 1

![](media/image2.jpeg){width="4.490277777777778in"
height="7.185416666666667in"}

Fig. 2. A block diagram of the instrument showing the signal flow. Note
that the only interface

between the spacecraft and the instrument for signals or power is
through the DPU.

![](media/image3.jpeg){width="5.379861111111111in"
height="7.266666666666667in"}

Fig. 3.a A diagram of the vector electron and ion spectrometer showing
the response of one analyzer.

At top left we show a cross section of one analyzer and its associated
electronics. The electronics are

isolated from the detectors and are located in the \'optics\' volume.
There is another detector hidden

by the one shown. In the lower portion of the figure, we show the
response of one analyzer. The full

geometrical factor (GF) is 4.6 • 10 -4 cm 2 ster -1.

![](media/image4.jpeg){width="4.863194444444445in"
height="7.247222222222222in"}

Fig. 3b A typical reduced distribution function derived from the
electron spectrometer on ISEE 1.

We expect to obtain similar functions from the SWE instrument, except
that a gap, apparent in the

upper diagram as a shortage of observations (points) along the magnetic
field direction will be closed

by the use of the strahl detector.

![](media/image5.jpeg){width="5.242361111111111in"
height="2.8472222222222223in"}

![](media/image6.jpeg){width="5.363888888888889in"
height="2.7583333333333333in"}

i 7\"/\" \" -Fig. 4. (a) A cross section of a Faraday cup sensor. All
grids are at spacecraft ground potential except

for the modulator grid (which has a time-varying voltage applied to it)
and the suppressor grid which

is biased at - 120 V to prevent escape of secondary electrons from the
collector plates. The grounded

grids between the modulator grid and the collector plates are used to
reduce capacitively-coupled

currents due to the time-varying modulator voltage. The two outermost
grids ate used to reduce

emission of time-varying electric fields from the front of the sensorof
a particle is shown, neglecting

refraction effects due to voltages on the grids. (b) A schematic

cross section of a Faraday cup, showing the principle of operation (see
text). (c) The effective area

of the Faraday cup as a function of the incoming angle of a cold
(parallel) beam of ions. The area is

determined by internal apertures and the image of the beam on the full
collector plate (the sum of

both collector halves is used). The transparencies of the meshes have
been taken into account.

![](media/image7.jpeg){width="5.0784722222222225in"
height="6.872916666666667in"}

Fig. 5 Simulated Faraday cup currents vs azimuth angle for a 400 km s -1
bulk speed and 40 km s -1

most probable thermal speed solar wind containing 10 protons cc -1 and
0.5 c\~ aloha particles cc -1. For

any particular angle, a plot of current versus energy will have two
maxima, one for the protons and

one for the alpha particles. The energy windows in which the current
maxima occur are a function

of the angle of the incoming plasma flow to the sensor normal direction.
The currents due to alpha

particles peak in an energy window higher than shown here, but they are
just visible at large angles

in the bottom panel.

![](media/image8.jpeg){width="5.326388888888889in"
height="4.833333333333333in"}

Figure 6

![](media/image9.jpeg){width="4.954861111111111in"
height="5.996527777777778in"}

Figure 7 The Strahl instrument

+-------+--------+-------+--------+--------+--------+-------+-------+
| S     | 2      | 3     | 4      | 5      | 6      | 7     | 8     |
| ector |        |       |        |        |        |       |       |
| \`1   |        |       |        |        |        |       |       |
+-------+--------+-------+--------+--------+--------+-------+-------+
| Spin  | Steps  | St    | Steps  | Steps  | Steps  | St    | Steps |
| 1     | 1-     | eps1- | 1-     | 1-5    | 1-     | eps1- | 1-5   |
|       |        |       |        |        |        |       |       |
| Steps | 5      | 5     | 5      |        | 5      | 5     |       |
| 1to5  |        |       |        |        |        |       |       |
+-------+--------+-------+--------+--------+--------+-------+-------+
| Spin  | Steps  | Steps | Steps  | Ste    | Steps  | St    | Step  |
| 2     | 6-10   | 6-10  | 6-10   | ps6-10 | 6-10   | eps6- | s6-10 |
|       |        |       |        |        |        |       |       |
| Step  |        |       |        |        |        | 10    |       |
| s6-10 |        |       |        |        |        |       |       |
+-------+--------+-------+--------+--------+--------+-------+-------+
| Spin3 | St     | Steps | St     | St     | St     | Ste   | St    |
|       | eps11- | 11    | eps11- | eps11- | eps11- | ps11- | eps11 |
| Steps |        | -15   | 15     | 15     | 15     |       | 15    |
| 11-15 | 15     |       |        |        |        | 15    |       |
+-------+--------+-------+--------+--------+--------+-------+-------+

1 sector is 1/8 of a spin, and takes about 375 mS.1 spin takes approx
3.05 sec

Figure 8 Telemetry layout, present Strahl instrument,

.