Explore the vast range of titles available.

Magnetic fields have an important role in the evolution of interstellar medium and star formation 1 , 2 . As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas 3 . Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H  I narrow self-absorption (HINSA) 4 , 5 towards L1544 6 , 7 —a well-studied prototypical prestellar core in an early transition between starless and protostellar phases 8 – 10 characterized by a high central number density 11 and a low central temperature 12 . A combined analysis of the Zeeman measurements of quasar H  I absorption, H  I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes 13 , 14 . An analysis of Zeeman measurements reveals that the reduction of magnetic flux relative to mass, which is necessary for star formation, seems to have occurred earlier than previously thought.

The total electron content (TEC) of the solar corona in June 2002 is calculated by three observational techniques and the results are compared. The first technique is solar rotational tomography (SRT) applied to a 14-day time series of LASCO-C2 polarized brightness images, and the other two techniques use the Cassini spacecraft radio beacon for Doppler tracking (phase delay) and ranging (group delay). While the Doppler-tracking technique has an arbitrary zero-point, it is otherwise found that the three methods produce consistent estimates of the TEC to within established uncertainties, providing an independent check on the calibrations. The verification of the accuracy of the Doppler-tracking technique enables a significant improvement to the use of spacecraft data sets in studying the heliosphere: the density component to Faraday rotation can be separated from the magnetic-field component as variable structures cross, such as coronal mass ejections and magnetohydrodynamic waves. Furthermore, we show that the unique frequency-time variable characteristics of the hydrodynamic components of waves can be studied. Based on this work, future Faraday rotation studies of variable solar phenomena will isolate the electron density changes from the magnetic-field contribution. This capability will enable advanced research into variable heliospheric magnetic fields.

Starlight linearly polarized by aligned interstellar dust grains provides the necessary data for tracing the structure of the very local interstellar magnetic field (ISMF). Two methods have been developed to recover the ISMF direction from polarized starlight, using data from an ongoing polarization survey. Both methods rely on the probability distribution function for polarized light. Method 1 calculates the ISMF direction from polarization position angles regardless of the data accuracy, while Method 2 relies on high-probability data points. The ISMF direction Bibex recovered by Method 1 corresponds to the closest ISMF to the heliosphere, traced by the center of the IBEX Ribbon arc. Method 2 reveals a new direction for the more distant ISMF, Bnew, toward l=41.1° ± 4.1° and b= 25.8° ± 3.0°, which differs by 30.4° ± 5.6° from the IBEX ISMF direction. Polarizations of filament stars that are located within 25° of a pole of Bnew, where background polarizations would be minimal, show the highest statistical probabilities of tracing the filament ISMF. The IBEX ISMF direction orders the kinematics of interstellar clouds within 15 pc, and Bnew must therefore dominate beyond 15 pc. These new data are consistent with the location of the Sun in the rim of an expanding superbubble shell associated with the evolved Loop I superbubble.

We produce a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk, demonstrating that the Galaxy is a non-axisymmetric multiarmed spiral. Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kiloparsecs, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness. The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20° to 25°.

Modern digital correlators permit the simultaneous measurement of all four Stokes parameters using auto‐ and cross‐correlation. We briefly outline the fundamental requirements and some practical details of performing such measurements and refer to two additional papers that provide overview and cover calibration issues in detail.

Astronomical magnetic fields are generally strong enough to influence the dynamics of gas in present-day galaxies, and may have played an important role in the formation and early evolution of galaxies. Yet the origin of these magnetic fields remains controversial, and observational tests that could discriminate between competing theories will challenge the capabilities of telescopes now under construction.

We review observations of the physical properties of the diffuse ISM HI components, namely the Cold and Warm Neutral Media (CNM and WNM). There is somewhat more WNM than CNM, and at least half of the WNM is not thermally stable. The CNM has typical turbulent Mach number 3. Magnetic fields in the CNM are not as large as expected from the classical flux-freezing argument; neither are magnetic fields always strong enough for the Alfven velocity to equal the turbulent velocity. Nevertheless, they are usually strong enough to put CNM clouds in the magnetically subcritical regime. We identify a probable new source of turbulence for the diffuse ISM. We discuss one very cold cloud that has considerable internal turbulence and, because of its extreme thinness 0.05 pc, a turbulent crossing time of only 5 10 super(4) yr.

Jupiter's nonthermal microwave emission, as measured by a global network of 11 radio telescopes, increased dramatically during the Shoemaker-Levy 9 impacts. The increase was wavelength-dependent, varying from ∼10 percent at 70 to 90 centimeters to ∼45 percent at 6 and 36 centimeters. The radio spectrum hardened (flattened toward shorter wavelengths) considerably during the week of impacts and continued to harden afterward. After the week of cometary impacts, the flux density began to subside at all wavelengths and was still declining 3 months later. Very Large Array and Australia Telescope images of the brightness distribution showed the enhancement to be localized in longitude and concentrated near the magnetic equator. The evidence therefore suggests that the increase in flux density was caused by a change in the resident particle population, for example, through an energization or spatial redistribution of the emitting particles.

Radio astronomical measurements of extended emission require knowledge of the beam shape and response because the measurements need correction for quantities such as beam efficiency and beamwidth. We describe a scheme that characterizes the main beam and sidelobe in all Stokes parameters employing parameters that allow reconstruction of the complete beam patterns and, also, afford an easy way to see how the beam changes with azimuth, zenith angle, and time. For the main beam in StokesI, the parameters include the beamwidth, ellipticity and its orientation, coma and its orientation, the point‐source gain, and the integrated gain (or, equivalently, the main‐beam efficiency); for the other Stokes parameters, the beam parameters include beam squint and beam squash. For the first sidelobe ring in StokesI, the parameters include an eight‐term Fourier series describing the height, radius, and radial width; for the other Stokes parameters they include only the sidelobe’s fractional polarization. We illustrate the technique by applying it to the Arecibo telescope. The main‐beam width is smaller and the sidelobe levels higher than for a uniformly illuminated aperture of the same effective area. These effects are modeled modestly well by a blocked aperture, with the blocked area equal to about 10% of the effective area (this corresponds to 5% physical blockage). In polarized emission, the effects of beam squint (difference in pointing direction between orthogonal polarizations) and squash (difference in beamwidth between orthogonal polarizations) do not correspond to theoretical expectation and are higher than expected; these effects are almost certainly caused by the blockage. The first sidelobe is highly polarized because of blockage. These polarization effects lead to severe contamination of maps of polarized emission by spatial derivatives in brightness temperature.

Modern digital cross‐correlators permit the simultaneous measurement of all four Stokes parameters. However, the results must be calibrated to correct for the polarization transfer function of the receiving system. The transfer function for any device can be expressed by its Mueller matrix. We express the matrix elements in terms of fundamental system parameters that describe the voltage transfer functions (known as the Jones matrix) of the various system devices in physical terms and thus provide a means for comparing with engineering calculations and investigating the effects of design changes. We describe how to determine these parameters with astronomical observations. We illustrate the method by applying it to some of the receivers at the Arecibo Observatory.