In-situ Measurements of the Solar Wind
Marcia Neugebauer
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721-0092, USA
Abstract. In-situ measurements of the solar wind began with simple ion traps on Soviet spacecraft in 1959. It wasn’t,
however, until 1962 that the major properties of the solar wind were determined by Mariner 2. Improvements in
instrumentation since then include the use of particle detectors, extension of the observations of the solar wind velocity
vector to 2 or 3 dimensions, better time and energy resolution, greater variety of measurement locations, multipoint
measurements, electron measurements, and the use of mass spectrometers and sample returns. This review summarizes
what was learned about the solar wind from each of these improvements in instrumentation.
energy/charge of the ions that could enter the cup.
The total current measured by the Faraday cup was
the sum of the ion flux with energy/charge above the
voltage on the positive grid, electrons with energies
above 200 V which could escape from the cup, and
photoelectrons emitted from the negative grid. There
were no publishable results from Lunik 1. Lunik 2
was the most successful of the four missions,
measuring an intermittent flux of ~2×108 cm-2s-1 with
energy/charge > 15 V [4]. The direction of this flux
was not determined. The Soviet results were
consistent with, but certainly not proof of Parkers
solar wind theory. The positive-grid voltages were
higher (up to 25 and 50 V) on Lunik 3 and the Venus
probe, but only very intermittent fluxes were
detected.
INTRODUCTION
In-situ measurements of the solar wind were
preceded by both remote sensing and theoretical
predictions. An excellent review of the status of our
knowledge of and guesses about the interplanetary
medium at the beginning of the space age was given
by Parker [1] at Solar Wind Nine. Attempts to use insitu measurements to determine whether the interplanetary medium could be described by Parker's
supersonic solar wind [2] or by Chamberlain's
subsonic solar breeze [3] occupied space physicists
from 1959 to 1962. This paper reviews those first
attempts and traces the principal avenues of advances
in in-situ measurements from 1962 to the present.
The final section is a discussion of the strengths and
weaknesses of the three methods of studying the solar
wind -- in situ measurements, remote sensing, and
theory/modeling.
TABLE 1. Early attempts to measure the solar wind
Mission
Year Flux V DirecPersisttion
ence
Lunik 1
1959
√
Lunik 2
1959
√
Lunik 3
1959
√
1961
Venus pb
√
√
√
Expl. 10
1961
1961
Expl. 12
Ranger 1
1961
Ranger 2
1961
Mariner 1
1962
√
√
√
√
Mariner 2
1962
1962
Expl. 14
EARLY ATTEMPTS TO MEASURE THE
SOLAR WIND
Table 1 summarizes the early attempts to directly
measure and characterize what is now known as the
solar wind.
Not surprisingly, the first tries were made by
the Soviet Union. The Soviets launched ion traps
on four interplanetary missions. These were Faraday
cups with an inner grid at a voltage of ~200 V to
prevent the escape of photoelectrons plus an outer
grid at a positive potential to define the minimum
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
8
geomagnetic activity [8]. Mariner 2 also discovered
that the alpha-particle abundance was highly variable.
Furthermore, the two-peak spectra were inconsistent
with the protons and alphas having the same
temperature, but could be fit by assuming that the
alphas were four times hotter than the protons.
Mariner 2 also carried a magnetometer whose data
validated Parker's prediction of a spiral configuration
of the interplanetary magnetic field [9]. Mariner 2
data were also used to demonstrate the existence of
collisionless MHD shocks [10].
The first American instrument was a Faraday cup
modified by MIT to have a square-wave potential on
the positive grid; with such an arrangement the ac
current to the cup was a measure of the ion flux,
whereas, to first order, photoelectrons provided only
a dc current. This instrument was flown on Explorer
10 which, in retrospect, traveled down the flanks of
the magnetosheath passing in and out of the
magnetosphere where no ion flux was measured.
When an ion flux was detected, it came from
somewhere within a 60° field of view that included
the solar direction. Explorer 10 determined the proton
speed, density, and temperature and established that
the flow was supersonic [5]. Thus Parker's prediction
of a supersonic solar wind was confirmed, but the
persistence of the wind was still in doubt.
The next six efforts were made by American
groups flying curved-plate analyzers in which the
ions were deflected perpendicular to their direction of
motion and those ions that exited the tunnel were
recorded by an electrometer. The instrument on
Explorer 12 had neither the sensitivity nor the correct
look direction to detect the solar wind. Rocket
failures led to Rangers 1 and 2 never getting above
the ionosphere. Mariner 1 was destroyed by Range
Safety when it headed for the North Atlantic shipping
lanes. The instrument on Explorer 14 was blinded by
solar UV whenever it looked within 3° of the Sun so
that very little solar-wind data were obtained.
After a hair-raising set of failures, malfunctions,
and recoveries [6], Mariner 2 finally made it out into
interplanetary space where it measured the solar wind
nearly continuously over 3 months as the spacecraft
traveled from Earth to Venus and beyond [7]. The
Mariner 2 measurements showed that the flow was
incident from within 10° of the Sun and it blew
continuously. Figure 1 is an example of one of the
better spectra obtained by the curved-plate analyzer
on Mariner 2. It had measurably large currents in five
voltage (energy/charge) channels that defined two
spectral peaks — the first due to protons and the
second due to alpha particles. Sometimes only two or
three channels had above-background currents.
Despite the crudeness of the measurements,
Mariner 2 learned a lot about the solar wind. It
discovered that the solar wind was organized into a
series of approximately weeklong high-speed streams
which recurred at the solar rotation rate. The leading
edges of the streams were steeper than the trailing
edges, the temperature was correlated with the speed,
and the density was highest on the leading edges
where the fast wind overtook the slower wind in its
path. The average density varied with the inverse
square of the distance from the Sun. A correlation
was found between the speed and the Kp index of
FIGURE 1. One of the better energy/charge spectra
obtained by the Mariner 2 curved plate analyzer. The
current I is given in amperes.
EVOLUTION OF IN-SITU SOLAR WIND
MEASUREMENTS
In the four decades since Mariner 2 firmly
established the existence of and determined a few of
the properties of the solar wind there have been
enormous improvements in the instrumentation for
in-situ measurements. This section presents a
synopsis of some of those improvements and the
resulting increase in our knowledge of solar wind
physics. The discussion is limited to the
measurements of the low-energy charged particles
that make up the bulk of the solar wind plasma.
Important advances in the capabilities to measure
waves and energetic particles are not covered.
One early advance was the use of particle
counters rather than electrometers as sensors in
curved-plate analyzers. A succession of different
devices was used to turn the impact of a single ion or
electron into a cascade of electrons that could be
detected as a pulse. Although ac electrometers are
still used with the Faraday cup instruments (e.g., on
Voyager and Wind), the detection of individual
particles has allowed increased dynamic ranges in
ever-smaller instruments.
9
shown in Figure 2, taken from Marsch et al. [14]. In
this figure, speed increases from left to right and the
radial distance from the Sun decreases from top to
bottom. Dashed lines indicate the projection of the
interplanetary magnetic field. This figure illustrates
the increase of the temperature, of the anisotropy
(Tperpendicular/Tparallel) of the core of the distributions,
and of a high-energy tail or a secondary peak with
increasing speed and decreasing solar distance. On
the basis of early measurements of the anisotropy,
Bame et al. [15] interpreted the high perpendicular
temperature in the fast solar wind as evidence for
interplanetary heating. Later, Tu [16] explained the
radial increase of the first adiabatic invariant
(proportional to Tperpendicular/B) measured by Helios 2
on the basis of the turbulent cascade of the energy in
the Alfvén waves to higher frequencies.
The extension of the observations into two and
three dimensions (two angles plus the magnitude of
the velocity vector) led to a wealth of discoveries
about the solar wind. Systematic angular deflections
of the solar wind velocity vector were found, as
expected, at the leading edges of high-speed streams
where fast plasma pushes aside the ambient slower
plasma in its path [11]. The correlation of each
component of the vector proton velocity with the
corresponding component of the magnetic field led to
the discovery of a strong antisolar flux of Alfvén
waves, which was especially strong in the high-speed
wind [12]. It was quickly discovered that the solar
wind was not isotropic, but had different
temperatures parallel and perpendicular to the
interplanetary magnetic field [13]. Nine proton
distribution functions observed by Helios 2 are
FIGURE 2. Some contours of proton velocity distributions observed by Helios 2. Contour levels correspond to 0.8, 0.6, 0.4, 0.2,
0.1, 0.03, 0.01, 0.003, and 0.001 of the peak phase space density. The dashed line in each frame indicates the projected direction
of the interplanetary magnetic field. From [14].
10
revealed several populations of electrons, in addition
to the ever-present photoelectrons and secondary
electrons emitted from the sunward side of the
spacecraft. There is a thermal core population and a
hotter halo population which carries most of the heat
flux [18]. Especially in the high-speed solar wind,
there is a sharp tail, or strahl, of energetic electrons
which is strongly focussed along the magnetic field
[19]; the strahl electrons are thought to be a
collisionless population which has been focussed by
the conservation of the magnetic moment. Sometimes
there appears to be sort of a double strahl, with
electrons streaming in both directions along the field.
This occasional counterstreaming of suprathermal
electrons has become a popular tool for the
identification of transient solar wind from coronal
mass ejections [20], but the counterstreaming can
also be caused by magnetic connection to shocks or
other field-strength increases farther out in the solar
system [21].
An improvement in energy resolution accompanied the other advances. Contrast, for example, the
energy spectrum in Figure 3 with the 5-channel
spectrum in Figure 1. At the time the data in Figure 3
were taken the solar wind was sufficiently cold that
individual peaks of highly charged heavy ions could
be discerned above the instrumental background.
Such measurements provided the first estimates of
the plasma's ionization state, which is related to the
temperature of the solar corona.
As spacecraft communications capabilities
increased, it became possible to improve the time
resolution of observations of changes in the solar
wind. Although some spacecraft have obtained
plasma spectra at a cadence as fast as 3 seconds, even
the fastest plasma measurements are at least an order
of magnitude slower than the measurements of the
magnetic field. For example, only under highly
unusual circumstances has it been possible to observe
the particle population at the frequencies required to
study convected structures as small as the ion
gyroradius.
The decades since 1962 have also provided
opportunities for measuring the solar wind at a
variety of measurement locations. The early
eccentric Earth orbiters with apogees in the solar
wind for part of each year have been followed by
spacecraft (ISEE 3, ACE, SOHO, and Genesis) that
remain continuously in the solar wind by virtue of
their halo orbits about the Lagrangian libration point
L1. With the Helios missions it was possible to study
the solar wind as close as 0.3 AU to the Sun. The
outer heliosphere has been sampled by solar-wind
instrumentation on a series of planetary missions,
most notably Pioneers 10 and 11 and Voyagers 1 and
2. Most recently, Ulysses has obtained the first data
from the polar regions of the heliosphere. The
community has long advocated and is still working
towards extending these observations by going even
closer to the Sun than Helios (to 4 solar radii with a
solar probe) and much farther from the Sun with an
interstellar probe well instrumented to determine the
properties of the interstellar medium at a distance of
~200 AU.
Some use has been made of multipoint
measurements, whereby solar wind structures such as
shocks or discontinuities can be studied with an array
of spacecraft. In this way it should be possible to
distinguish between propagating and convected
features. NASAs planned Stereo mission is another
step forward in this regard, but arrays of at least four
spacecraft would be even better.
The study of electrons has also been crucial to
our understanding many fascinating aspects of the
solar wind. Among the earliest discoveries was that
protons and electrons do not have the same
temperature; the electron temperature is relatively
insensitive to solar wind speed, while the proton
temperature is greater/less than the electron
temperature in the fast/slow wind [17]. These results
established a clear need for multi-fluid models of the
solar wind. Although electrons do carry a heat flux
away from the Sun, the flux is lower than that
originally envisioned [17]. Further investigation
FIGURE 3. An energy/charge spectrum obtained in the
slow solar wind by the Vela 5A spacecraft. Arrows indicate
the expected positions of some iron and other ions. From
[31].
11
in a harmonic oscillator potential (potential varying
with the square of the distance) depends only on the
mass/charge of an ion and is independent of its
velocity [25].
For the purposes of planetary science, even
greater precision in the elemental and isotopic
abundances of the Sun is required. For example,
planetary scientists want to know the solar abundance
of the 17O isotope, which has yet to be measured by
spacecraft mass spectrometers. Such high precision
can only be obtained by sample return missions.
The first solar wind sample return missions were
carried out as part of the Apollo program. Samples
collected on the lunar surface returned to Earth by the
Apollo astronauts led to determination of the isotopic
abundances of helium, neon and argon [27]. The
Genesis mission [28], launched in August, 2001, is
collecting solar wind samples to be returned to Earth
in 2004. Because the Genesis collection time is about
100 times longer than the Apollo collection times and
the sample-collecting materials are much purer and
more varied, it is expected that Genesis will be able
to determine the abundances of most of the elements
in the periodic table. The systematics of ion
fractionation between the Sun and the solar wind as
determined by spacecraft mass spectrometers must be
used in interpreting the data from the Genesis
samples in terms of solar-system history.
The better energy resolution also led to the
discoveries that the alpha particles flowed away from
the Sun faster than the protons and that both the
proton and alpha particles are often double peaked, as
shown in Figure 2.
A problem with ion spectra such as that shown in
Figure 3 is that the solar wind is often too hot for the
heavy ion peaks to be resolved; at times they can be
completely swamped by the alpha particles. Thus the
motivation for ion mass spectrometers. The first
step in that direction was the use of a Wien (velocity)
filter following a curved-plate analyzer (e.g., [22]).
Later improvements, exemplified by the SWICS
instruments on Ulysses and other spacecraft [23],
included sequential electrostatic analysis (yielding
mv2/2q), time of flight analysis (v), and measurement
of total energy (mv2/2) of an ion in a solid state
detector. The requirement for triple coincidences
(between the start and stop pulses in the time-offlight section and the total-energy pulse) resulted in a
very low background. Figure 4 shows plots of the
velocity distribution functions, normalized by the
solar wind speed, for three ion species. Besides the
solar wind peak at a relative speed of 1, there is a
steep drop in interstellar pickup ions at a relative
speed of 2, and spectral tails extending to even higher
energies; the spectra for the different ion species
could not have been separated without a mass
spectrometer. This type of instrument has also been
used to study the kinetics of some of the heavy ion
species, comparing their speeds, temperatures, and
anisotropies to those of protons and alphas [24].
DISCUSSION
In-situ measurements of the solar wind have
clearly led to some wondrous discoveries and insights
that are totally beyond the realm of what s feasible
with remote sensing. By its very nature, remote
sensing is limited to the study of features visible in
some useful wavelength with an appropriate optical
depth; the observations are integrated along the line
of sight and usually have limited angular or spatial
resolution. Theory and modeling of the solar wind is
necessarily oversimplified.
In-situ observations, however, also have their
drawbacks or shortcomings. They are limited to the
study of time profiles of the properties of the
convected plasma at one, or at most a few points. As
with remote sensing, there are often problems with
the absolute calibration of the instruments (see, for
example, Russell et al. [29]). Estimates of the
uncertainty in measured plasma density are often
-30%. Determinations of the directions of the ion
beams are often off by one or a few degrees. The
greatest problem is perhaps with the determination of
the plasma temperature. Temperature is usually
defined by the second moment of the plasma
distribution, which means that the part of the
SWICS Ulysses
7
10
W
105
to3
CO
wa;
co
OH
to-1
10"
10'5
1
10
W Ion Speed/Solar Wind Speed
FIGURE 4. Phase space density versus speed relative to
the solar wind speed for several species observed by the
SWICS instrument on Ulysses. From [26], ©1999, with
kind permission of Kluwer Academic Publishers.
Newer spectrometers with improved mass
resolution operate on the principle that time of flight
12
2.
3.
4.
5.
6.
distribution farthest from the peak and closest to the
noise level is weighted the most heavily. The quoted
value of temperature is highly dependent on how far
into the noise, or into the high-energy tail, the
summation is taken. The proton temperatures
obtained by Genesis were systematically more than
20% greater than the proton temperatures obtained by
the nearby ACE spacecraft. The difference is that the
autonomous determination by Genesis summed
counts over the entire field of view, whereas the ACE
calculations were limited to the vicinity of the
spectral peak. Accurate calculation of plasma
temperature may also involve complex separation or
deconvolution of the plasma thermal motions from
instrumental smear or other contributions (see, for
example, the appendix of Neugebauer et al. [30]).
Whereas the data distributed by the US National
Space Science Data Center are extremely valuable for
a wide range of research projects, their absolute
values must be viewed with some skepticism.
I think what is important to keep in mind at the
start of this conference is how valuable the interplay
can be between in-situ measurements, remote
sensing, and theory. One example of the benefits of
combining the three methods of studying the solar
wind is the mapping of solar wind measured in-situ to
its solar sources. Such mapping typically uses
theoretical models (either MHD or potential) of the
coronal magnetic field which uses remotely sensed
magnetograph data as an inner boundary condition.
While we will continue to see great advances in each
area, the real payoff probably comes from the
synergy between the three scientific approaches.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
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