The Galactic distribution of magnetic fields in molecular clouds and HII regions

{Magnetic fields exist on all scales in our Galaxy. There is a controversy about whether the magnetic fields in molecular clouds are preserved from the permeated magnetic fields in the interstellar medium (ISM) during cloud formation. We investigate this controversy using available data in the light of the newly revealed magnetic field structure of the Galactic disk obtained from pulsar rotation measures (RMs).} % {We collected measurements of the magnetic fields in molecular clouds, including Zeeman splitting data of OH masers in clouds and OH or HI absorption or emission lines of clouds themselves.} % {The Zeeman data show structures in the sign distribution of the line-of-sight component of the magnetic field. Compared to the large-scale Galactic magnetic fields derived from pulsar RMs, the sign distribution of the Zeeman data shows similar large-scale field reversals. Previous such examinations were flawed in the over-simplified global model used for the large-scale magnetic fields in the Galactic disk.} % {We conclude that the magnetic fields in the clouds may still ``remember'' the directions of magnetic fields in the Galactic ISM to some extent, and could be used as complementary tracers of the large-scale magnetic structure. More Zeeman data of OH masers in widely distributed clouds are required.}Comment: Typo fixed in this new versio

The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches

A new data product from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) called Space-weather HMI Active Region Patches (SHARPs) is now available. SDO/HMI is the first space-based instrument to map the full-disk photospheric vector magnetic field with high cadence and continuity. The SHARP data series provide maps in patches that encompass automatically tracked magnetic concentrations for their entire lifetime; map quantities include the photospheric vector magnetic field and its uncertainty, along with Doppler velocity, continuum intensity, and line-of-sight magnetic field. Furthermore, keywords in the SHARP data series provide several parameters that concisely characterize the magnetic-field distribution and its deviation from a potential-field configuration. These indices may be useful for active-region event forecasting and for identifying regions of interest. The indices are calculated per patch and are available on a twelve-minute cadence. Quick-look data are available within approximately three hours of observation; definitive science products are produced approximately five weeks later. SHARP data are available at http://jsoc.stanford.edu and maps are available in either of two different coordinate systems. This article describes the SHARP data products and presents examples of SHARP data and parameters.Comment: 27 pages, 7 figures. Accepted to Solar Physic

Evidence for a dynamical ground state in the frustrated pyrohafnate Tb2Hf2O7

We report the physical properties of Tb2Hf2O7 based on ac magnetic susceptibility \chi_ac(T), dc magnetic susceptibility \chi(T), isothermal magnetization M(H), and heat capacity C_p(T) measurements combined with muon spin relaxation (\muSR) and neutron powder diffraction measurements. No evidence for long-range magnetic order is found down to 0.1 K. However, \chi_ac(T) data present a frequency-dependent broad peak (near 0.9 K at 16 Hz) indicating slow spin dynamics. The slow spin dynamics is further evidenced from the \muSR data (characterized by a stretched exponential behavior) which show persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering) but no magnetic Bragg peaks. The analysis of the diffuse scattering data reveals a dominant antiferromagnetic interaction in agreement with the negative Weiss temperature. The absence of long-range magnetic order and the presence of slow spin dynamics and persistent spin fluctuations together reflect a dynamical ground state in Tb2Hf2O7.Comment: 11 pages and 8 figure

Reversals of the solar magnetic dipole in the light of observational data and simple dynamo models

Observations show that the photospheric solar magnetic dipole usually does not vanish during the reversal of the solar magnetic field, which occurs in each solar cycle. In contrast, mean-field solar dynamo models predict that the dipole field does become zero. In a recent paper Moss et al. (2013) suggested that this contradiction can be explained as a large-scale manifestation of small-scale magnetic fluctuations of the surface poloidal field. Here we compare this interpretation with WSO (Wilcox Solar Observatory) photospheric magnetic field data in order to determine the amplitude of magnetic fluctuations required to explain the phenomenon and to compare the results with predictions from a simple dynamo model which takes these fluctuations into account. We demonstrate that the WSO data concerning the magnetic dipole reversals are very similar to the predictions of our very simple solar dynamo model, which includes both mean magnetic field and fluctuations. The ratio between the rms value of the magnetic fluctuations and the mean field is estimated to be about 2, in reasonable agreement with estimates from sunspot data. The reversal epoch, during which the fluctuating contribution to the dipole is larger than that from the mean field, is about 4 months. The memory time of the fluctuations is about 2 months. Observations demonstrate that the rms of the magnetic fluctuations is strongly modulated by the phase of the solar cycle. This gives additional support to the concept that the solar magnetic field is generated by a single dynamo mechanism rather than also by independent small-scale dynamo action. A suggestion of a weak nonaxsymmetric magnetic field of a fluctuating nature arises from the analysis, with a lifetime of about 1 year.Comment: 9 pages, 10 figures, accepted versio