Large-scale Magnetic Field Structure Research Articles

NIR polarimetry as a probe of the large-scale structure of the galactic magnetic field

AbstractH-band (1.6 μm) starlight polarimetry was used to test predictions of the large-scale symmetry of the Galactic magnetic field and to measure the Galactic magnetic pitch angle. Polarimetry was obtained with the Mimir instrument on the 1.8m Perkins Telescope outside of Flagstaff, AZ USA along a line of constant Galactic longitude for a range of Galactic latitudes. Comparison with all-sky predictions of starlight polarimetry allows significant rejection of disk anti-symmetric Galactic magnetic field geometries and favored disk symmetric geometries. The Galactic magnetic field pitch angle was also constrained to be p=–6±2° towards this direction.

A MILLIMETER-WAVE GALACTIC PLANE SURVEY WITH THE BICEP POLARIMETER

In addition to its potential to probe the Inflationary cosmological paradigm, millimeter-wave polarimetry is a powerful tool for studying the Milky Way galaxy's composition and magnetic field structure. Towards this end, presented here are Stokes I, Q, and U maps of the Galactic plane from the millimeter-wave polarimeter BICEP covering the Galactic longitude range 260 - 340 degrees in three atmospheric transmission windows centered on 100, 150, and 220 GHz. The maps sample an optical depth 1 < AV < 30, and are consistent with previous characterizations of the Galactic millimeter-wave frequency spectrum and the large-scale magnetic field structure permeating the interstellar medium. Polarized emission is detected over the entire region within two degrees of the Galactic plane and indicates that the large-scale magnetic field is oriented parallel to the plane of the Galaxy. An observed trend of decreasing polarization fraction with increasing total intensity rules out the simplest model of a constant Galactic magnetic field throughout the Galaxy. Including WMAP data in the analysis, the degree-scale frequency spectrum of Galactic polarization fraction is plotted between 23 and 220 GHz for the first time. A generally increasing trend of polarization fraction with electromagnetic frequency is found, which varies from 0.5%-1.5%at frequencies below 50 GHz to 2.5%-3.5%above 90 GHz. The BICEP and WMAP data are fit to a two-component (synchrotron and dust) model showing that the higher frequency BICEP data are necessary to tightly constrain the amplitude and spectral index of Galactic dust. Furthermore, the dust amplitude predicted by this two-component fit is consistent with model predictions of dust emission in the BICEP bands.

The large scale magnetic field structure of the spiral galaxy NGC 5775

In order to better determine the large-scale 3D-structure of magnetic fields in spiral galaxies we present a Faraday rotation analysis of the edge-on spiral galaxy NGC 5775. Deep radio-continuum observations in total power and linear polarization were performed at 8.46 GHz with the VLA and the 100-m Effelsberg telescope. They were analyzed together with archival 4.86 and 1.49 GHz VLA-data. We thus can derive rotation measures from a comparison of three frequencies and determine the intrinsic magnetic field structure. A very extended halo is detected in NGC 5775, with magnetic field lines forming an X-shaped structure. Close to the galactic disk the magnetic field is plane-parallel. The scaleheights of the radio emission esimated for NGC 5775 are comaprable with other galaxies. The rotation measure distribution varies smoothly on both sides along the major axis from positive to negative values. From the derived distribution of rotation measures and the plane-parallel intrinsic magnetic field orientation along the galactic midplane we conclude that NGC 5775 has an 'even axisymmetric' large-scale magnetic field configuration in the disk as generated by an \alpha \Omega -dynamo which is accompanied by a quadrupolar poloidal field. The magnetic field lines of the plane-parallel component are pointing 'outwards'. The observed X-shaped halo magnetic field, however, cannot be explained by the action of the disk's mean-field dynamo alone. It is probably due to the influence of the galactic wind together with the dynamo action.

Sunspots and large-scale magnetic fields at various heights

Height variation of the magnetic field structure over groups of sunspots for heights ranging from the photosphere to the source surface (R = 2.5 Ro, where Ro is the radius of the Sun) is examined. For all heights, starting from the photospheric level, groups of sunspot are shown as being independent of long-lived boundaries of large-scale structures rotating with a period shorter than the Carrington period. At heights of 1–1.5 Ro, there is a clear relation between sunspot groups and boundaries separating the head and tail sunspots in the groups (the Hale boundaries). The rotation periods of these structures are close to the Carrington period, their lifespan being less than three to five rotations. The maximal intensity of the solar magnetic field drops by two orders when height increases from H = 1 to H = 1.1 Ro. Further decrease in intensity proceeds gradually (dropping by one order from H = 1.1 to 2.5 Ro). The results obtained can be considered as evidence that large-scale magnetic field structures and long-lived boundries between them (the lines dividing polarities of the magnetic field or zero lines) all exist irrespective of sunspot fields being generated by other sources than sunspots. At the photospheric level, active regions fields are superimposed on these structures.

Magnetic field structure of Mercury

Recently planet Mercury—an unexplored territory in our solar system—has been of much interest to the scientific community due to recent flybys of the spacecraft MESSENGER that discovered its intrinsic stationary and large-scale dipole like magnetic field structure with an intensity of ∼300nT confirming Mariner 10 observations. In the present study, with the observed constraint of Mercury's atmospheric magnetic field structure, internal magnetic field structure is modeled as a solution of magnetic diffusion equation. In this study, Mercury's internal structure mainly consists of a stable stratified fluid core and the convective mantle. For simplicity, magnetic diffusivity in both parts of the structure is considered to be uniform and constant with a value represented by a suitable averages. It is further assumed that vigorous convection in the mantle disposes of the electric currents leading to a very high diffusivity in that region. Thus, in order to satisfy observed atmospheric magnetic field structure, Mercury's most likely magnetic field structure consists of a solution of MHD diffusion equation in the core and a combined multipolar (dipole and quadrupole like magnetic field structures embedded in the uniform field) solution of a current free like magnetic field structure in the mantle and in the atmosphere. With imposition of appropriate boundary conditions at the core–mantle boundary for the first two diffusion eigen modes, in order to satisfy the observed field structure, present study puts the constraint on Mercury's core radius to be ∼2000km.From the estimated magnetic diffusivity and the core radius, it is also possible to estimate the two diffusion eigen modes with their diffusion time scales of ∼8.6 and 3.7 billion years respectively suggesting that the planet inherits its present-day magnetic field structure from the solar Nebula. It is proposed that permanency of such a large-scale magnetic field structure of the planet is attained during Mercury's early evolutionary history of heavy bombardments by the asteroids and comets supporting the giant impact hypothesis for the formation of Mercury.

Solar Cycle Effects on the Dynamics of Jupiter’s and Saturn’s Magnetospheres

The giant planetary magnetospheres surrounding Jupiter and Saturn respond in quite different ways, compared to Earth, to changes in upstream solar wind conditions. Spacecraft have visited Jupiter and Saturn during both solar cycle minima and maxima. In this paper we explore the large-scale structure of the interplanetary magnetic field (IMF) upstream of Saturn and Jupiter as a function of solar cycle, deduced from solar wind observations by spacecraft and from models. We show the distributions of solar wind dynamic pressure and IMF azimuthal and meridional angles over the changing solar cycle conditions, detailing how they compare to Parker predictions and to our general understanding of expected heliospheric structure at 5 and 9 AU. We explore how Jupiter’s and Saturn’s magnetospheric dynamics respond to varying solar wind driving over a solar cycle under varying Mach number regimes, and consider how changing dayside coupling can have a direct effect on the nightside magnetospheric response. We also address how solar UV flux variability over a solar cycle influences the plasma and neutral tori in the inner magnetospheres of Jupiter and Saturn, and estimate the solar cycle effects on internally driven magnetospheric dynamics. We conclude by commenting on the effects of the solar cycle in the release of heavy ion plasma into the heliosphere, ultimately derived from the moons of Jupiter and Saturn.

Crossing Filaments

Solar filaments show the position of large scale polarity inversion lines and are used for the reconstruction of large-scale solar magnetic field structure on the basis of H{\alpha} synoptic charts for the periods when magnetographic measurements were not available. Sometimes crossing filaments are seen in H{\alpha} filtergrams. We analyze daily H{\alpha} filtergrams from the archive of Big Bear Solar Observatory for the period of 1999-2003 to find crossing and interacting filaments. A number of examples are presented and filament patterns are compared with photospheric magnetic field distributions. We have found that all crossing filaments reveal quadrupolar magnetic configurations of the photospheric field and presume the presence of null points in the corona.

Correlation between the parameters of coronal mass ejections and features of their propagation in the corona with the large-scale structure of the solar magnetic field

Correlation between the parameters of coronal mass ejections (CMEs) that are detected on the LASCO coronographs and are associated with eruptive prominences and the distances of CME axes from the coronal streamer belt has been analyzed. The deviations of CME trajectories from the radial direction have been investigated.

Dynamics of nonlinearly interacting magnetic electron drift vortex modes in a nonuniform plasma

A simulation study of dynamical evolution of nonlinearly interacting two-dimensional magnetic electron drift vortex (MEDV) modes in a nonuniform plasma is presented. Depending on the equilibrium density and temperature gradients, the system can either be stable or unstable. The unstable system reveals spontaneous generation of magnetic fields from noise level, and large-scale magnetic field structures are formed. When the system is linearly stable, one encounters MEDV mode turbulence in which there is a competition between zonons (zonal flows) and streamers. For large MEDV mode amplitudes, one encounters the formation of localized and small-scale magnetic vortices and vortex pairs with scale sizes of the order of the electron skin depth. The MEDV turbulence exhibits nonuniversal (non-Kolmogorov-type) spectra for different sets of plasma parameters. The relevance of this work to laboratory and cosmic plasmas is briefly mentioned.

Zeeman-Doppler imaging of II Peg

AbstractWe present Zeeman-Doppler images of the active K2 star II Peg for the years 2004 and 2007. The surface magnetic field was reconstructed with our new ZDI code iMap which provides a full polarized radiative transfer driven inversion to simultaneously reconstruct the surface temperature and magnetic vector field distribution. II Peg shows a remarkable large scale magnetic field structure for both years. The magnetic field is predominantly located at high latitudes and is arranged in active longitudes. A dramatic evolution in the magnetic field structure is visible for the two years, where a dominant and largely unipolar field in 2004 has changed into two distinct and large scale bipolar structures in 2007.

Relationship between the topological skeleton, current concentrations, and 3D magnetic reconnection sites in the solar atmosphere

Aims. The aim of this work is to determine the relationship between the 3D structure of the coronal magnetic field, diagnosed by the topological skeleton, and current concentrations as potential sites of 3D reconnection. Methods. We utilised the results of 3D numerical MHD simulations of an observed EUV bright point (BP) in the solar atmosphere. The simulations are based on MDI line-of-sight magnetogram data from 13 June 1998. We analysed the results of the simulations using the method of magnetic charge topology. Three different methods of reducing the magnetogram to a set of point magnetic sources are tested. Results. Observations of the BP show a rotation of one of its main magnetic source regions. Numerical simulations of this rotational motion result in a localised build-up of parallel electric current, which is dissipated by anomalous resistivity, causing 3D magnetic reconnection and BP heating. The magnetic topological structure of the simulated BP was also calculated, and a portion of the topological separatrix surface bounding the magnetic flux of the rotating source region is found to correspond to the locations of current build-up and heating. All three magnetogram reduction methods produce similar results for the large-scale magnetic field structure. Conclusions. Magnetic topology is a useful method for predicting the locations of coronal current concentrations, insofar as the results of our simulations show that strong integrated parallel electric fields are found only along topological separatrix surfaces. However, further investigation is necessary to determine exactly which parts of the reconstructed separatrices will host the electric currents. Topological magnetic field reconstructions also cast light on the location of coronal BP heating, which occurs as a result of the dissipation of the currents by 3D reconnection. The choice of the magnetogram reduction algorithm does not greatly affect the large-scale topological features of the resulting reconstructed magnetic field. Further work is required to compare these results with data for other observed BPs.

Motions and magnetic fields in filaments

Based on the developed method of jointly using data on the magnetic fields and brightness of fila- ments and coronal holes (CHs) at various heights in the solar atmosphere as well as on the velocities in the photosphere, we have obtained the following results: —The upward motion of matter is typical of filament channels in the form of bright stripes that often surround the filaments when observed in the HeI 1083 nm line. —The filament channels observed simultaneously in H α and HeI 1083 nm differ in size, emission characteris- tics, and other parameters. We conclude that by simultaneously investigating the filament channels in two spec- tral ranges, we can make progress in understanding the physics of their formation and evolution. —Most of the filaments observed in the HeI 1083 nm line consist of dark knots with different velocity distri- butions in them. A possible interpretation of these knots is offered. —The height of the small-scale magnetic field distribution near the individual dark knots of filaments in the solar atmosphere varies between 3000 and 20000 km. —The zero surface separating the large-scale magnetic field structures in the corona and calculated in the poten- tial approximation changes the inclination to the solar surface with height and is displaced in one or two days. —The observed formation of a filament in a CH was accompanied by a significant magnetic field variation in the CH region at heights from 0 to 30000 km up to the change of the predominant field sign over the entire CH area. We assume that this occurs at the stage of CH disappearance. DOI: 10.3103/S0190271708010105

Rotation Measures of Extragalactic Sources behind the Southern Galactic Plane: New Insights into the Large‐Scale Magnetic Field of the Inner Milky Way

We present new Faraday rotation measures (RMs) for 148 extragalactic radio sources behind the southern Galactic plane (253o < l < 356o, |b| < 1.5o), and use these data in combination with published data to probe the large-scale structure of the Milky Way's magnetic field. We show that the magnitudes of these RMs oscillate with longitude in a manner that correlates with the locations of the Galactic spiral arms. The observed pattern in RMs requries the presence of at least one large-scale magnetic reversal in the fourth Galactic quadrant, located between the Sagittarius- Carina and Scutum-Crux spiral arms. To quantitatively compare our measurements to other recent studies, we consider all available extragalactic and pulsar RMs in the region we have surveyed, and jointly fit these data to simple models in which the large-scale field follows the spiral arms. In the best-fitting model, the magnetic field in the fourth Galactic quadrant is directed clockwise in the Sagittarius-Carina spiral arm (as viewed from the North Galactic pole), but is oriented counter- clockwise in the Scutum-Crux arm. This contrasts with recent analyses of pulsar RMs alone, in which the fourth-quadrant field was presumed to be directed counter-clockwise in the Sagittarius- Carina arm. Also in contrast to recent pulsar RM studies, our joint modeling of pulsar and extragalactic RMs demonstrates that large numbers of large-scale magnetic field reversals are not required to account for observations.

On the dynamic nature of the prolate solar chromosphere: jet formation

Context. In spectral lines, the smoothed upper edge of the solar chromosphere is prolate in the South-North direction at the epoch of minimum solar activity and nearly spherically symmetric at the maximum phase. We attribute the effect to the dynamical nature of the upper chromosphere, which consists of a large number of small jet-like structures ascending into the corona. We could not resolve the source region of an individual jetlet, although similar but larger structures are visible, especially in EUV coronal lines. Aims. We consider the problem of the formation of an individual jet above the limb, assuming that a large number of jet-like events is responsible for the prolate solar upper chromosphere. We then assume that spicules, being the cool part of the phenomenon. behave similarly, and we will mainly concentrate the analysis on the magnetic origin of the event. Methods. Image processing is used to reveal the displacement of magnetic field tubes filled with coronal plasma and jet formation due to field aligned motion above the null point created in the corona by the emerging magnetic bipole. Results. The growth of the bipole leads to a reconnection of the field lines and to a specific plasma motion in the vicinity of the null point that results in a plasma flow along the spine line of the 3D null. We assume that similar but smaller processes could happen very often at a smaller scale in the chromosphere, near emerging magnetic ephemeral regions, forming numerous jetlets in the upper chromosphere. As the field aligned motion is guided by the magnetic field, at the epoch of low activity the large-scale structure of the polar magnetic field and the one of the quiet equatorial region is sufficiently different to explain the prolateness of the chromosphere.

Supernova remnant G292.2-0.5, its pulsar, and the Galactic magnetic field

The extended low-brightness Galactic radio source G292.2–0.5 is one of the few supernova remnants (SNRs) showing a likely association with a young pulsar. New observations of the remnant with the Australia Telescope Compact Array yield a distance of 8.4 kpc determined from H i absorption measurements, and the first detection of linear polarization. The polarization was studied at two frequencies near 5 GHz, revealing a high mean rotation measure, approximately +800 rad m−2, strikingly similar to that of the pulsar. This similarity, and the compatibility of the pulsar distance estimate with the new SNR distance, now provides overwhelming evidence that the pulsar is indeed embedded within the SNR, and that both were presumably born in the same supernova event. The ratio of rotation measure to pulsar dispersion measure yields a value of −1.4 μG (towards us) for the (density-weighted) average line-of-sight component of magnetic field for the 8.4-kpc path-length to the SNR and pulsar. The unusually high rotation measure, together with the large distance over which it has accumulated, argues that this field is a persistent feature on a large scale that outweighs smaller-scale fluctuations and reversals. The 8.4-kpc path-length lies almost wholly within the Carina spiral arm of our Galaxy and thus this portion of the arm possesses an average clockwise field of 1.4 μG. We interpret other evidence to suggest that the clockwise field extends for at least a further 8.5 kpc along the same arm, in the region where it is usually referred to as the Sagittarius arm. Observations such as these provide a powerful tool for exploring the large-scale structure of the Galactic magnetic field in relation to the spiral-arm structure.

Two‐dimensional Magnetohydrodynamic Models of the Solar Corona: Mass Loss from the Streamer Belt

The equations of magnetohydrodynamics (MHD) are used to study an axially symmetric model of the large-scale solar corona, extending from the coronal base to 15 solar radii. We use a uniform heating of the inner corona to investigate the energy output when the magnetic field is given as a dipole at the coronal base. The heat input produces a large-scale magnetic field structure similar to that found by Pneuman and Kopp, with coronal holes in the polar regions and a helmet streamer around the equator. We pay special attention to the energy balance in the system, and find that the role of heat conduction is important in determining the thermal structure in magnetically closed regions. Insufficient energy loss to the transition region leads to a high temperature inside the closed region. In the coronal holes a solar wind is accelerated into interplanetary space, and the temperature is lower. As the difference in pressure scale height along open and closed flux tubes is large, the helmet streamer does not relax to a steady state; it opens periodically to eject mass into interplanetary space. These mass ejections may contribute significantly to the mass and energy flux in the solar wind.

Multi-wavelength observations of a flare spray and CME of 1 October 2001

Using the Multi-channel Infrared Solar Spectrograph (MISS) at the Purple Mountain Observatory (PMO), we obtained the Halpha, Ca II 8542 Angstrom, He I 10 830 Angstrom spectra and slit-jaw Halpha images of a large dramatic flare spray near the southwest solar limb on 1 October 2001. For this event, we also got Extreme-Ultraviolet (EUV) images at 171 Angstrom from TRACE and 195 Angstrom from EIT/SOHO, coronal images from the C2 and C3/LASCO coronagraph on SOHO, soft X-ray (SXR) images from the Soft X-ray Telescope (SXT) and hard X-ray (HXR) flux from the Hard X-ray Telescope (HXT) onboard Yohkoh, X-ray flux from GOES, and Halpha images and radio-frequency spectrograms from the National Astronomical Observatories of China (NAOC). The complex eruption event lasted about 40 min and was composed of two flaring and three distinct mass ejecting processes. The preliminary analysis leads to the following results: (1) the three peak-times on the HXR light curves correspond to the interactions and reconnections of coronal loops shown on the SXR images and are consistent with the flaring times in Halpha; (2) the first mass ejection process of this event is apparently related to the loop interaction and magnetic opening and reconnection, while the second mass ejection, i.e., the large flare spray, is associated with the second flare. There are bright fine arcs connecting the onset location of the spray and the footpoints of the flare loops. The rising speeds the three ejections in the sky plane are all 296 km s(-1) and uniform in the early phase; (3) comparisons show that the flare spray has nearly the same shape in Halpha, 171 Angstrom, 195 Angstrom and SXR images, which implies that plasma with temperatures differs by a factor of more than 100 can exist in the same spray and move by about 2 x 10(5) km with the same shape; (4) we infer from the height-time curves of the spray and CME that the observed CME by LASCO is closely related to the flare spray. However, we could not conclude what the trigger of the CME is due to the lack of more direct and convincing evidence, such as images from the LASCO C1 coronagraph. Meanwhile, the CME is accelerated at about 2 R. presumably due to change in the large-scale magnetic field structure; (5) in the initial phase of the spray, all the Halpha, Ca II 8542 Angstrom and He I 10 830 Angstrom spectra show unusually broadened profiles and large blue-shift velocities, and the maximum of the line-of-sight velocity reaches 286 km s(-1).

Simulation‐based Investigation of a Model for the Interaction between Stellar Magnetospheres and Circumstellar Accretion Disks

We examine, parametrically, the interaction between the magnetosphere of a rotating, young stellar object (YSO) and a circumstellar accretion disk using 2.5-D (cylindrically symmetric) numerical magnetoydrodynamic simulations. The interaction drives a collimated outflow, and we find that the jet formation mechanism is robust. For variations in initial disk density of a factor of 16, variations of stellar dipole strength of a factor of 4, and for various initial conditions with respect to the disk truncation radius and the existence of a disk field, outflows with similar morphologies were consistently produced. Secondly, the system is self-regulating, where the outflow properties depend relatively weakly on the parameters above. The large scale magnetic field structure rapidly evolves to a configuration that removes angular momentum from the disk at a rate that depends most strongly on the field and weakly on the rotation rate of the foot-points of the field in the disk and the mass outflow rate. Third, the simulated jets are episodic, with the timescale of jet outbursts identical to the timescale of magnetically induced oscillations of the inner edge of the disk. To better understand the physics controlling these disk oscillations, we present a semi-analytical model and confirm that the oscillation period is set by the spin down rate of the disk inner edge. Finally, our simulations offer strong evidence that it is indeed the interaction of the stellar magnetosphere with the disk, rather than some primordial field in the disk itself, that is responsible for the formation of jets from these systems.

Solar energetic particles inside a coronal mass ejection event observed with the ACE spacecraft

In this work, solar flare energetic particle fluxes (E e ⩾38 keV) observed by the EPAM experiment aboard ACE are utilized as diagnostics of the large-scale structure and topology of the interplanetary magnetic field (IMF) embedded within a well-identified interplanetary coronal mass ejection (ICME). The still controversial question of whether the detected ICME structure has been detached from the solar corona or is still magnetically anchored to it is addressed. The observation of two impulsive solar flare electron events inside the ICME suggests that field lines in this ICME are rooted at the Sun. From the time evolution of the angular distributions of the particle intensities we infer that the observations are consistent with the magnetic topology of a magnetic bottle between a magnetic mirror located at the Sun and a magnetic constriction upstream from ACE formed by the convergence of open field lines that reflects the outgoing electrons. The magnetic mirror strength is calculated in one case based upon the local IMF observations and the electron event onset characteristics. A magnetic field enhancement observed by ACE in the downstream region of the CME-driven shock is identified as the agent responsible for the mirroring of the energetic electrons.

Zonal structure and meridional drift of large-scale solar magnetic fields

Digitized synoptic charts of photospheric magnetic fields were analyzed for the past 4 incomplete solar activity cycles (1969–2000). The zonal structure and cyclic evolution of large-scale solar magnetic fields were investigated using the calculated values of the radial Br, |Br|, meridional Bθ, |Bθ|, and azimuthal Bφ, |Bφ| components of the solar magnetic field averaged over a Carrington rotation (CR). The time–latitude diagrams of all 6 parameters and their correlation analysis clearly reveal a zonal structure and two types of the meridional poleward drift of magnetic fields with the characteristic times of travel from the equator to the poles equal to ∼16–18 and ∼2–3 years. A conclusion is made that we observe two different processes of reorganization of magnetic fields in the Sun that are related to generation of magnetic fields and their subsequent redistribution in the process of emergence from the field generation region to the solar surface. Redistribution is supposed to be caused by some external forces (presumably, by sub-surface plasma flows in the convection zone).