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The text of the 1997 proposal contained about 8 pages of Science Description. The LaTeX formatted version of that description is included below.

\A{D. SCIENCE}

The magnetic field, {\it one of the three major forces
of gas dynamics} (gas pressure, gravity, magnetic fields) is generally
treated by ignoring it in virtually all astrophysical settings.
This is a serious omission -- the field, which is typically
equal in importance to the other two forces in the dense interstellar medium
(ISM)\refmark{\refno, \refno}, cannot be treated as a minor
perturbation. This is akin to physicists omitting the electromagnetic
force from elementary particle theory.
Knowledge of the role of the magnetic field in the Galaxy and other
galaxies is needed to
to meet a central goal in the ``HST and Beyond'' Report, namely to
discover when and where the first heavy elements formed in the universe
(because the star formation process is related to magnetic
fields embedded in the gas).
The best way to rectify this situation is to
bring the magnetic field's role in interstellar gas dynamics into sharp
focus, via execution of a specialized space-based mission. The best
laboratories for
uncovering the roles played by the magnetic field are to be found in the disk,
star forming complexes, and diffuse material contained in the Milky
Way Galaxy.
In the following, we outline our proposed approach to advancing this
field by executing a mission capable of collecting some 10,000 times
more magnetic field structure data than has been obtained to date. This
level of advance will come only by going into space, yet no other
space mission, either past or currently envisioned, possessed or will
possess the
requisite instrumentation. M4, {\it The \underbar{M}ilky Way
\underbar{M}agnetic Field \underbar{M}apping
\underbar{M}ission}, will be the most capable small satellite
for magnetic field surveys
ever to be flown. M4 will map the polarization
of the thermal emission from magnetically-aligned dust in the ISM, over a
large fraction of the Galactic plane, with
background-limited sensitivity. It will provide a revolutionary increase in our understanding
of the interstellar magnetic field, just as the \IRAS satellite provided a
revolutionary increase in our understanding of the
distribution and properties of interstellar dust.
\noindent{\it Figure 1: IRAS 100$\mu$m sky image, showing M4
Primary Science Program surveys and viewing limits.
The M4 Primary Science Program consists of an inner Milky Way
survey, a Sco/Oph survey, and a Cirrus survey. The regions delineated with dashed black lines
identify directions unviewable from M4. ``A,''
``B,'' and ``C'' can be viewed during the Extended Mission only;
regions ``D'' and ``E'' cannot be viewed by M4 for a nominal
1 March 2001 launch.}
}
The key elements of M4 are: far-infrared focal plane
array detectors {\it already} developed for the \SIRTF mission;
an optics design
optimized for low-level linear imaging polarimetry; and, a cold telescope
design, whose critical technology
of superfluid liquid helium management in orbit has
been repeatedly demonstrated by our Co-Investigator Ball
Aerospace.
\B{D.1. Scientific Goals and Objectives}
The M4 satellite concept has been developed to answer three fundamental questions:
\item{1.} What is the magnetic field structure in the ISM of the Milky Way?
\item{2.} What role do magnetic fields play in the star formation process?
\item{3.} What magnetic field structures exist in the
infrared cirrus clouds?
\noindent
{\bf The central goal of the M4 mission is to measure
magnetic field directions in the ISM of the Milky Way Galaxy.}
This goal will be met by conducting a Primary Science Program of
three focussed surveys:
an inner Milky Way disk survey of some 1000 square degrees; a deeper
survey of some 22 square degrees of the nearby Scorpius/Ophiuchus cloud complex;
and a survey of a 100 square degree region of faint infrared cirrus.
Additionally, the M4 satellite will provide a unique platform for
Guest Investigator (GI) magnetic field surveys of extragalactic and
Galactic targets. We have planned strong support of
our GI program: reserving 25\% of the Prime mission for GI
observations, and reserving M4 MO\&DA funds for GI data analysis.
The M4 surveys will be conducted using imaging linear polarimetry with
a 20cm cooled telescope at a central wavelength of 95$\mu$m ($\lambda /
\Delta\lambda \sim 3$). The optics are diffraction limited with a beamsize
of 2$^\prime$, sampled with a pixel size of
48\arcsec, slightly smaller than Nyquist sampling. Figure 1 shows
the IRAS 100$\mu$m sky image, the three Primary Science Program
survey zones, and the outlines of the large M4 viewing zone.
\C{D.1.a. Magnetic Fields in the Dense ISM}

Galactic scale magnetic fields play roles in accelerating cosmic
rays\refmark{\refno} and mediate shock
conditions\refmark{\refno}. Within cloud complexes, the fields act to
control the star
formation rate via ambipolar diffusion of fields\refmark{\refno, \refno}
and field turbulence\refmark{\refno, \refno}.
In protostellar environments, embedded magnetic fields may regulate
anisotropic cloud core collapse and promote
the formation of disks and eventually planets.
Finally, because most of the ISM mass is in dense clouds, these may play
an important role in generating the overall Galactic magnetic field.
However at present, we have no large-scale observationally-based view of the
magnetic field in the Milky Way's dense, neutral, interstellar medium.
Much of what we do know about the structure of interstellar magnetic fields
pertains to low-density gas. For example, the polarization
of background starlight\refmark{\refno} and of synchrotron
radiation\refmark{\refno} reveals an overall spiral
pattern in the magnetic field of external disk galaxies; and optical
polarimetry shows that the local field in our own Galaxy lies predominantly
in the Galactic plane, departing from the
plane only to follow the ``bubbles" evident in HI maps\refmark{\refno}.
Yet, in the dense ISM, only far-infrared
and sub-mm polarimetry of thermal dust emission can reveal the field structure,
and very few such measurements exist, due to atmospheric
limitations\refmark{\refno}. This imbalance of information severely
restricts our physical
understanding of the interactions between magnetic fields, density, and velocity
structure in the ISM.
Existing observations have
shown that fields play important roles in many physical processes
taking place in the Galaxy. Yet, even the most basic questions
about the structural nature of the field remain unanswered, including
the very origin of
the Galactic magnetic field. How
much of a role do magnetic fields play in the evolution of H II regions,
supernova remnants, and supershells? How do fields thread star-forming
regions? Are outflows from young stars controlled or re-oriented by the
field? Are the orientations of disks around young stars influenced by
interstellar fields?
\CL{D.1.b. Optimum Sensing Technique and Wavelength: FIR Imaging Polarimetry}
Magnetic fields are quite difficult to sense. Determining field
strengths along the
line of sight in dense neutral regions requires measurement of the
Zeeman effect. For
astrophysically important dense ISM fields in the $\mu$G range, this can
require enormous quantities (e.g., days) of telescope time
{\it per point} in a map\refmark{2}.
Merely sensing the direction of the magnetic field along the line of sight
still requires Zeeman observations, again with an enormous telescope appetite.
However, in the plane of the sky, the magnetic field direction can be
sensed by linear polarimetry, whether via anisotropic absorption
of background starlight or via anisotropic thermal emission, {\it both}
produced by spinning, magnetically-aligned dust grains\refmark{\refno}.
Although linear polarimetry cannot {\it directly} reveal field strength,
it does provide a wealth of information about field geometry. Indirect
arguments are used to infer field strength from the observed (ordered)
geometry and turbulent velocities obtained from spectral line studies.
Advancing our naive understanding of the role of magnetic fields in
the ISM requires new observations. Producing
a large leap in Zeeman effect sensitivity is exceedingly unlikely,
as it requires large, dedicated ground-based telescopes
and instrumentation.
Nearly half a century ago, Hall and Hiltner discovered that the light from
many stars appears linearly polarized\refmark{\refno, \refno}. They
concluded that this was due to aligned dust grains in the diffuse ISM.
In 1970, Mathewson \& Ford published a map of the polarization of 1800
stars\refmark{11}.
These stars are viewed through interstellar material which is all rather
nearby -- most of the stars are closer than 500pc. Their map (Figure 2 is a
modern version )
shows the local magnetic field is primarily confined to the
Galactic plane, except in regions where supernova explosions have created the
``supershells" found in H I maps.
Yet the interiors of the most active star forming regions in the ISM, the
molecular clouds, are hidden from view by huge quantities of dust.
Hence, the magnetic field cannot be traced from the diffuse
into the dense ISM because of the lack of visible background stars.
\noindent{\it Figure 2: Magnetic field directions as traced by more than
4000 optical
stellar polarization measurements. The length of each line is proportional
to the degree of linear polarization, and the orientation reflects the
polarization position angle. From the Mathewson \& Ford compilation plus
recent surveys of nearby dark clouds by Klare \& Neckel\refmark{\refno} and
others.}
}
Despite this, virtually all
modern theories of grain alignment predict that the short axis of a spinning
dust grain will tend to align with the direction of the
ambient magnetic field\refmark{\refno}, except perhaps in cold, dense,
quiescent regions. This produces polarization of the thermal
emission from the dust grains along the long
axis of the grain, and perpendicular to the projected magnetic field direction.
Fifteen years ago, Cudlip et al. made the
first successful measurements of linearly polarized thermal emission from dust in the
far-infrared using a balloon-borne polarimeter\refmark{\refno}. Since then, Hildebrand and
collaborators have flown several far-infrared polarimeters on the
Kuiper Airborne Observatory (KAO)\refmark{\refno}. To enhance
polarization mapping, a 32-bolometer-array polarimeter, called STOKES, was constructed and flown on the KAO to map the polarization of thermal
dust emission in eleven of the ``brightest" molecular clouds including M17, Orion, W3, the arched filaments and dust ring associated with the Galactic Center (Figure 3), S106, W51, NGC 2024, SgrB2, DR21, and NGC
7538\refmark{\refno, \refno, \refno, \refno, \refno}.
All of the airborne observations have been limited by the
bright background presented by the partially transmitting atmosphere.
Only the brightest small central portions of
a few of the brightest sources have been detected.
The polarimetric observations have shown
the utility of the approach, while coming nowhere near fundamental
sensitivity limits. The total number of independent sky positions
measured for far-infrared polarimetry is only a few hundred,
obtained at a rate of a few tens per KAO flight.
\noindent{\it Figure 3: Comparison of far-infrared thermal dust polarization
for five regions in the Galactic Center region with radio wavelength emission.
The polarization data are from a combination of KAO (60 and 100$\mu$m) and CSO
(350$\mu$m) observations by the Hildebrand group. Radio flux halftone image
and thin contours are from
Yusef-Zadeh (1986); thick contours are 800$\mu$m emission from Lis \&
Carlstrom (1994).}
\vskip 3pt
}
The KAO efforts have shown that the range of linear polarization values
is between 0.5 -- 10\%, with a mean between 2 -- 3\%.
In order to characterize magnetic field directions,
uncertainty levels of the polarization position angles must be
below 10\deg. This level of precision, coupled to the low polarization
percentages, leads to challenging instrumental and observational
requirements. For an average 2.5\% polarized signal,
reaching 10\deg\ uncertainty requires polarimetric uncertainty
below 0.9\%. This translates to photometric signal-to-noise
(S/N) levels of 150:1.
Recent advances in SIRTF detector array technology allow
reaching space-based background limited performance in the
far-infrared. {\bf A SMEX (M4) conducting polarimetry at
the background limit would advance
magnetic field observations by four
orders of magnitude in quantity, area coverage, and sensitivity.}
\COBE has shown the Galactic dust to have temperatures
of 16-19K\refmark{\refno}. This dust emits strongly in the
submm to far-infrared.
While an M4 design wavelength closer to the emission peak
would intercept stronger signals,
no large format detectors comparable to the $32 \times 32$ MIPS arrays
operate longward of 110$\mu$m. Similarly, the silicon BIB technology
in $WIRE$ has a long wavelength cutoff shortward of 50$\mu$m, too far
off the emission peak to permit cool dust polarimetry. The best configuration
for performing magnetic
field mapping of the cool dust involves use of the \SIRTF \ MIPS arrays
near 100$\mu$m. These provide large pixel numbers, with good sensitivity
to the cool dust. The selection of a wavelength just shortward of the
emissivity peak also enhances the mapping angular resolution for a
fixed telescope aperture.
\CL{D.1.c. The M4 Primary Science Program}
Our solution to the problem of a general lack of knowledge concerning
the role of magnetic fields in the Galaxy is to
develop a short duration Small Explorer survey mission designed
to make large- and small-scale maps of the magnetic field structure
in the Milky Way using the technique of far-infrared imaging linear
polarimetry. We have identified three surveys, which together with
a robust Guest Investigator (GI) program comprise the
Primary Science Program of the M4 mission. Additionally, the hardware
design which meets the requirements of the 4 month Primary Science Program may
allow up to a 2 month Extended Mission period.
\DL{D.1.c.1. Milky Way Magnetic Field Survey}
The highest priority survey of the M4 mission is designed to answer several
questions:
\item{$\bullet$}Are the magnetic fields in dense clouds part of a
Galaxy-wide spiral pattern?
\item{$\bullet$}What is the magnetic field pattern inside and outside of the
5kpc radius molecular ring in the inner Galaxy?
\item{$\bullet$}What is the magnetic field distribution in the Galactic Center?
How does it relate to the distributions of gas and dust there?
\noindent
The M4 Milky Way survey will cover 1000 square degrees of the Galactic disk,
to $\pm$50\deg \ of Galactic longitude ($\ell$) and $\pm$5\deg \ of latitude
($b$), requiring about five weeks of M4 observing time.
The $\ell$ limits insure that the dense ISM in the inner Galaxy,
especially in the region spanned by the 5kpc ring, is well-sampled.
The $b$ limits both completely cover the bright Galactic mid-plane and
provide good latitude extent. With a
2\arcmin \ diffraction-limited beam and 48\arcsec \ pixel sampling, this survey
will contain some 6 million pixels.
The magnetic field structure map developed will be used to examine
connections between star forming regions, spiral arms, and shells, loops,
chimneys, bubbles, and worms in the disk. The ambient field in the central
100pc of the Galaxy will be mapped in unprecedented detail, completely
filling the entire region shown in Figure 3.
When combined with radio spectral line data, the 3-D structure of the Galactic
field will be traced out using the thousands of bright cloud core regions
already detected by \IRAS (see Figure 4).
This technique will help remove line-of-sight
confusion for the optically thin far-infrared dust emission and permit
testing detailed models of the structure of the Galactic magnetic field.
\noindent{\it Figure 4: M4 Milky Way survey region and example zones shown with
higher angular resolution to highlight the expected results of the survey and
how region distances will be established. The top strip shows an enlargement of the
IRAS 100$\mu$m map, with the 1000 square degree M4 survey region boxed. A portion
near 30\deg longitude is expanded in the lower left (data taken from the IRAS ISSA
data product). This is further expanded into two 20 arcmin zones, computed using
the HIRES software at Boston University from the raw IRAS data. Each of these middle
maps shows the presence of relatively bright point sources. In the lower middle
panel, polarization vectors, as would be obtained using M4 are shown. In the upper
middle panel, a small CO map was obtained at FCRAO near the point source. The
central spectrum is shown in the lower right, and contains several emission
lines indicating the presence of many clouds along this line of sight. However,
only one of the lines yields an angular distribution similar to that of the
infrared point source. That particular CO distribution is shown above the spectrum,
and contains a block identifying the M4 pixel size on this scale. Construction of the
Galactic magnetic field map will proceed via using the 1,000 - 2,000 bright
infrared sources in the survey region as test particles whose polarization
properties will be established using M4 and whose distances will be established
via correlation with dense gas spectral line surveys.}
}
\DL{D.1.c.2. Magnetic Fields in the Nearest Star-Forming Complex: Sco/Oph}
The second priority in the Primary Science Program is to
survey a nearby star forming cloud complex to address the following
questions:
\item{$\bullet$}What magnetic field patterns are found within dense clouds
and large complexes?
\item{$\bullet$}How do magnetic field patterns compare inside and outside
of clouds?
\item{$\bullet$}How does the relative mix of uniform and non-uniform magnetic
field energies change in star forming clouds, especially in
and around dense cores?
\noindent
These questions can be answered directly via a deep survey of the
Sco/Oph complex
of dark clouds found at about 125pc distance\refmark{\refno, \refno}.
This complex includes a wide range of dense ISM
properties and settings, from intermediate-mass star-cluster formation in the
$\rho$Oph cloud core\refmark{\refno}, to the eastward-extending quiescent dust streamers,
to the dark clouds affected by the ionizing radiation from the runaway
O-star $\zeta$Oph (e.g. L204), as well as many sites of single star formation.
The M4 Sco/Oph survey will provide a definitive test of the central
hypothesis of molecular cloud support -- the idea that clouds are
supported against gravity primarily by their magnetic fields and associated
waves and supersonic (but sub-Alfv\'enic) turbulence.
By combining the M4 polarization position angle information with
nonthermal line widths measured from existing data over the same region
obtained in $^{13}$CO, we will derive the relative contributions of
the parallel and perpendicular magnetic field fluctuations, {\bf b}$_\parallel$
and {\bf b}$_\perp$, and thereby estimate the total field fluctuation
amplitude {\bf b}, the static field strength {\bf B}, and the ratio
{\bf b/B}.
The M4 Sco/Oph survey will determine the spatial structure in at
least two quite different environments in the Sco/Oph complex -- the
turbulent gas in the L1688 core, which contains a young embedded stellar
cluster of more than 100 stars, and the quiescent gas in the dark ``streamers,''
including L1709, L1720, L1712, and L1755. We will test whether the
magnetic field structure is more turbulent in the cluster region than in
the streamers, as expected from models of cluster formation via fragmentation.
The Sco/Oph complex covers about 150 square degrees on the sky, but with
a fairly low filling factor (about 10-15\%). Our
M4 survey would completely map this dark material in a hybrid pointed-survey
mode to cover approximately 22 square degrees during approximately two
weeks of M4 observing time, and will yield small- to large-scale maps
composed of about 100,000 polarization pixels.
\DL{D.1.c.3. Magnetic Properties of Infrared Cirrus}
The third component of the Primary Science Program involves surveying
a representative zone of faint infrared cirrus, a component of the Galaxy
first discovered by IRAS, to answer:
\item{$\bullet$}What is the structure of the magnetic fields in infrared
cirrus clouds?
\item{$\bullet$}How do cirrus magnetic fields relate to the global,
Galactic field geometry?
\noindent
These questions can be answered via analyses of a background-limited
M4 survey of a region of cirrus emission. Because of the faintness
of the cirrus, no ground-based or airborne instrument will
ever measure the polarization produced by this material.
Additionally, no space-based mission
other than M4 will be able to map the magnetic field of the infrared cirrus.
Cirrus observations are needed to distinguish competing theories for
the origin of the Galactic field (dynamo vs. primordial), and for
obtaining a better understanding of the coupling between the disk and
the halo\refmark{\refno}. Additionally, for M4 cirrus pixels overlapping directions
with measured stellar polarizations, comparison of the polarization
percentages for the far-infrared and optical, normalized by the
optical extinction, provides a measure of the degree of grain
alignment, which is useful for distinguishing between grain models.
Cirrus structure tends to be filamentary. In some regions, the
magnetic field traced by starlight polarization lies parallel to
cirrus filaments. Goals of the M4 Cirrus survey include testing whether
this is a general characteristic, and whether the alignment persists
to all substructures in the cirrus.
Magnetic fields greatly affect the thermal conductivity along field lines,
linking the matter in flux tubes, and cooling as a unit to form the
cold, dense filaments seen\refmark{\refno}. In these regions, the field should be
parallel to the filament, and the magnetic field pressure should
be comparable to the gas pressure. However, cirrus filaments also have
substructure with departures of their orientations from the larger
structures. To what degree are magnetic fields aligned with the substructures?
For the M4 Cirrus survey, we have chosen a region that lies in the
North Polar Spur (NPS), a very large HI shell expanding at 20 km/s and
exhibiting diffuse radio synchrotron emission in a famous loop
structure\refmark{\refno} (see Fig 2).
The NPS is well-sampled in stellar polarization, it is the only structure
along the line of sight, and it has multiple small-scale structures within
its filamentary cirrus clouds.
The M4 Cirrus survey will cover some 100 square degrees, with an effective
angular resolution between 48\arcsec\ (the superresolution goal)
and 24\arcmin (using smoothing), depending on the surface
brightness of each zone of the cirrus. This survey will
require about two weeks of M4 observing time.
\DL{D.1.c.4. Guest Investigator Magnetic Field Surveys}
M4 will be the unique platform for conducting surveys of magnetic field
structure in the Milky Way and in
galaxies. In constructing the Primary Science Program, we
specifically focussed on the Galaxy. In doing so, we look to Guest
Investigators (GIs) to use M4 to conduct surveys of other galaxies
as well as of particular classes of objects within the Milky Way. With
the specific exception of M31 (see next section),
GIs will conduct M4 surveys of nearby galaxies, starburst galaxies,
high latitude clouds, specific dark clouds, Bok globules, and other
regions of interest to the community.
{\bf During the 4 month Primary Science Program of the M4 mission,
25\% of the observing time (a minimum of 3 weeks) is reserved for
GI magnetic field surveys.}
M4 will have the requisite angular resolution and sensitivity to map
effectively
nearby galaxies such as M33, M81, M82 and the Magellanic
Clouds. For these galaxies, the relationship between the magnetic
field geometry in regions of high far-infrared luminosity and the field
geometry in the remainder of the galaxy can be investigated in detail.
It will be possible, for example, to determine if the magnetic
field threading through giant molecular cloud complexes in external galaxies
is aligned
with local spiral patterns.
\DL{D.1.c.5. Extended Mission Survey Priorities}
Completion of the Primary Science Program,
containing 16-18 weeks of surveys and checkout/calibration,
requires that the instrument design lifetime be at least four months.
Our current cryogen lifetime models indicate that a period of
up to 2 months beyond the 4 month Primary Science Program may be likely.
This Extended Mission will be allocated as 50\% to GIs and 50\% to
Science Team secondary goals. These goals include a survey
of magnetic field geometry in M31, observations of a second cirrus
region, extending the $b$ and/or $\ell$ coverages of
the Milky Way survey, and surveying some of the dark clouds in
Perseus, Taurus, and/or Orion should the mission lifetime permit
viewing these regions.
\C{D.1.d. Correlative Data Sets}
The M4 Primary Science Program surveys will yield data sets
which are capable of being used to address most of the
questions listed above as well as supporting archival
and correlative studies. However, the surveys, in particular
the Milky Way survey, will be greatly enhanced by dense gase
spectroscopic data sets to assign radial
velocities and distances to the far-infrared polarimetric
features detected. In order to produce a Galactic magnetic
field direction map from the overlapped, optically thin
emission which M4 will view, systematic spectral line surveys
of dense gas in the plane of the Milky Way are needed.
A new program to obtain modern,
high quality CS and $^{13}$CO spectral line maps of the
Northern 5kpc molecular ring of the Galaxy
from the Five College Radio Astronomy Observatory will take place
over the next three years, directed by Prof. Mark Heyer.
FCRAO is about to commission a new,
state-of-the-art receiver array (SEQUOIA) which will
permit mapping the faint CS J=2-1 line with 25\arcsec \
sampling and superb signal to noise with a
modest investment in telescope time.
The ASTRO submillimeter telescope, located at the
South Pole, is expected to conduct a similar
dense gas survey of the Southern inner Galaxy region. In the
CO J=4-3 line, ASTRO has a 2$^\prime$
beam, identical to the M4 beamsize. This ASTRO survey would
be directed by Profs. Thomas Bania and James Jackson, both of
Boston University, in conjunction with the ASTRO PI, Dr. Tony Stark
of the CfA.
The data from the new 5kpc ring surveys will permit matching
dense gas properties with bright M4 polarimetric targets,
to yield radial velocities (and distances)
for those magnetic zones, without the need to obtain
new observations of single objects. The data to be obtained for these
surveys are expected to be collected and analyzed via non-NASA initiatives.
\CL{D.1.e. Other Missions and NASA OSS Themes}
No other past or planned space-based mission has conducted or will conduct
far-infrared polarimetry. \IRAS and \MSX did
not fly with polarimeters. \SIRTF will fly
without any polarimetric capability. $SOFIA$ is expected to have
far-infrared polarimetric capability -- but predominantly for
high-angular resolution work on very bright sources.
While the \ISO satellite was launched with polarimetric capability
for the PHT instrument, this capability has not yet been employed
to conduct polarimetry of any thermal dust emission and appears
unlikely to do so before cryogen exhaustion.
Nevertheless, the {\it photometric} imaging data sets from these
missions, and from the ground-based 2MASS effort, are expected
to be incorporated in correlative analyses of the M4 polarimetric
surveys. For example, high angular resolution near-, mid-, and far-infrared
imaging (2MASS, $MSX$, $ISO$, $WIRE$, $SIRTF$, and $SOFIA$) can be
used to ascertain
the detailed stellar contents of cloud cores unresolved
to M4's polarimetric observations, thus providing key insight into
tests of how star formation mode (single vs. cluster) is related
to magnetic field properties.
\WIRE observations at 12
and 25$\mu$m, to be conducted by Associate Investigators, are expected to
probe the inner Galactic plane while the primary science program
of extragalactic surveys is executed by the \WIRE Science Team.
{\bf The M4 mission speaks directly to the central goal of the NASA OSS
``Structure and Evolution of the Universe'' theme, and provides
important unique contributions to the ``Origins'' theme.}
Magnetic fields are crucial to the removal of angular momentum and
the development of disk and eventually planets around young stars.
Without knowledge of the nature of magnetic fields in regions which
are forming new stars, models of pre-planetary disks and planet
formation will remain hampered. M4 does not quite have the requisite angular
resolution to tackle these problems directly, hence its support
of Origins research is indirect. M4, in conducting the deepest, broadest
magnetic field surveys of the star forming dense gas to date, will
show the way for future SOFIA and SIM observations. The
magnetic field selected source lists and large-scale magnetic
field structure maps provided by M4 will guide selected, higher angular
resolution studies of the magnetic fields and planet-forming potential
of star forming sites in the Milky Way.
On the other hand, {\bf M4 will provide the most comprehensive,
unique insight into the
nature of the structure of the magnetic field in the ISM of
the Milky Way and other galaxies.}
No other mission, past or planned, will address questions
of magnetic field structure to the degree possible with M4.
Whether one is interested in knowning how galaxies formed and evolved,
how interstellar gas is convinced into becoming new stars, or how the
lifecycles of stars affect galaxy structure and
evolution, {\bf the magnetic field is a player in the drama}. Yet, until
we fly M4, that player will remain silent.