This web page is intended for use by participants in the M4 project only. If you have reached this page in error, please try the main M4 Site.
Click here to return to 1999 M4 Drafting Site.
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.