New magnetic field measurements of β Cephei stars and slowly pulsating B stars

We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B‐type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B (hereafter SPB) stars, along with a small number of normal B‐type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three β Cephei stars and two stars suspected to be β Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3σ has been diagnosed in two normal B‐type stars, the nitrogen‐rich early B‐type star HD 52089 and in the B5 IV star HD 153716. Roughly one third of β Cephei stars have detected magnetic fields: Out of 13 β Cephei stars studied to date with FORS 1, four stars possess weak magnetic fields, and out of the sample of six suspected β Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: Roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non‐magnetic pulsating stars in the H‐R diagram. We find that their domains in the H‐R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i ‐values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-precision magnetic field measurements of Ap and Bp stars

In this paper we describe a new approach for measuring the mean longitudinal magnetic field and net linear polarization of Ap and Bp stars. As was demonstrated by Wade et al., least-squares deconvolution (LSD; Donati et al.) provides a powerful technique for detecting weak Stokes V , Q and U Zeeman signatures in stellar spectral lines. These signatures have the potential to apply strong new constraints to models of stellar magnetic field structure. Here we point out two important uses of LSD Stokes profiles. First, they can provide very precise determinations of the mean longitudinal magnetic field. In particular, this method allows one frequently to obtain 1 σ error bars better than 50 G, and smaller than 20 G in some cases. This method is applicable to both broad- and sharp-lined stars, with both weak and strong magnetic fields, and effectively redefines the quality standard of longitudinal field determinations. Secondly, LSD profiles can in some cases provide a measure of the net linear polarization, a quantity analogous to the broad-band linear polarization recently used to derive detailed magnetic field models for a few stars (e.g. Leroy et al.). In this paper we report new high-precision measurements of the longitudinal fields of 14 magnetic Ap/Bp stars, as well as net linear polarization measurements for four of these stars, derived from LSD profiles.     

Automatic detection and correction algorithms for magnetic saturation in the SMFT/HSOS longitudinal magnetograms

A longitudinal magnetic field often suffers the saturation effect in a strong magnetic field region when the measurement is performed at a single-wavelength point and linear calibration is adopted.In this study,we develop a method that can judge the threshold of saturation in Stokes V/I observed by the Solar Magnetic Field Telescope(SMFT)and correct it automatically.The procedure is to first perform the second-order polynomial fit to the Stokes V/I vs.I/I_m(I_m is the maximum value of Stokes I)curve to estimate the threshold of saturation,then reconstruct Stokes V/I in a strong field region to correct for saturation.The algorithm is demonstrated to be effective by comparing with the magnetograms obtained by the Helioseismic and Magnetic Imager(HMI).The accuracy rate of detection and correction for saturation is~99.4%and~88%respectively among 175 active regions.The advantages and disadvantages of the algorithm are discussed.     

Fractal properties of solar magnetic fields

We study the spatial properties of solar magnetic fields using data from the Solar Vector Magnetograph of the Marshall Space Flight Center (MSFC) (FeI 5250.2 Å) and SOHO/MDI longitudinal magnetic field measurements (Ni 6767.8 Å) (96-min full-disk maps). Our study is focused on two objects: the fractal properties of sunspots and the fractal properties of the spatial magnetic field distribution of active and quiet regions considered as global structures. To study the spatial structure of sunspots, we use a well-known method of determining the fractal dimension based on an analysis of the perimeter—area relation. To analyze the fractal properties of the spatial magnetic field distribution over the solar surface, we use a technique developed by Higuchi. We have revealed the existence of three families of self-similar contours corresponding to the sunspot umbra, penumbra, and adjacent photosphere. The fractal coefficient has maxima near the umbra—penumbra and penumbra—photosphere boundaries. The fractal dependences of the longitudinal and transverse magnetic field distributions are similar, but the fractal numbers themselves for the transverse fields are larger than those for the longitudinal fields approximately by a factor of 1.5. The fractal numbers decrease with increasing mean magnetic field strength, implying that the magnetic field distribution is more regular in active regions.     

On the possibility of investigation of the magnetic field structure in subphotospheric layers of the Sun according to observations of sunspot torsional oscillations

A method of investigation of the magnetic field structure in subphotospheric layers of the Sun has been developed. The method is based on observations of the torisonal oscillations of single sunspots. Characteristics of the torsional oscillations have been obtained from observations of the longitudinal magnetic field and radial velocities of seven single sunspots in the photospheric line Fe I λ5253 Å. The parameters of the torsional oscillations and magnetic tubes in the deep layers have been determined. The radius of the cross section of a magnetic flux tube forming a sunspot is greatest near the Sun’s surface and is approximately equal to the radius of a sunspot umbra. Down to the deeper layers, it decreases quite quickly. The longitudinal electric current appearing in the magnetic tube changes direction. The typical time of the current changes is determined by the period of the torsional oscillations. The intensity of the longitudinal magnetic field in the tube increases with depth. The Alfven wave velocity averaged over the length of a magnetic tube is tens or hundreds of times less than this velocity in a sunspot umbra. It decreases with an increase in the period of oscillations. A decrease in the Alfven wave velocity leads to an increase in the twisting angle of magnetic field lines.     

无力场模型黑子上空的纵向磁场梯度

本文按常α无力场模型计算了1980年10月23日Boulder 2744活动区前导黑子的纵向磁场随高度的变化,并与用CIV 1548谱线观测得到的色球一日冕过渡区的磁场资料相结合,求得CIV 1548发射区的有效高度。这些结果与文献[4]中对同一黑子用势场模型推求的结果有很大差别。从而表明,势场和无力场在某些方面导致的结果是极不相同的。鉴于观测已表明活动区上空存在电流的事实,在活动区磁场的模拟中,特别是在强扭曲活动区磁场的计算中,应当避免采用势场,而尽可能采用无力场模型。     

Magnetic field of CP stars in the Ori OB1 association. I. HD35456, HD35881, HD36313 A,HD36526

We present the results of magnetic field measurements of four chemically peculiar (CP) stars, the members of the Orion stellar association OB1. Observations were carried out with the circular polarization analyzer at the Main Stellar Spectrograph of the 6-m telescope. All the studied stars refer to the subtype of Bp stars with weak helium lines. Canadian astronomer E. F. Borra detected a magnetic field in three of them (HD35456, HD36313, and HD36526) from the Balmer line magnetometer observations. HD35881 was observed for the first time for the purpose to search for a magnetic field. We obtained the following results: HD35456 is a magnetic star with longitudinal field variation range from +300 to +650 G and a period of 4.9506 days; HD35881 is possible a new magnetic star, the longitudinal component of which varies from?1 to +1 kGwith a period of 0.6998 days, however, a small number of lines broadened by rotation does not allow us to conduct measurements more accurately; HD36313 is a binary star with the components similar in brightness, the primary component is a magnetic star with broad lines, the magnetic field of the secondary component (the star with narrow lines) was not detected. Measurements in the Hβ hydrogen line showed the variations of the longitudinal component from ?1.5 to +2 kG with a period of 1.17862 days; a strong longitudinal field was detected in HD36526 (from 0 to +3000 G) varying with a rotation period of the star of 3.081 days. In all the cases, we observe considerable discrepancies with the data on magnetic fields of these objects obtained earlier.     

Measurements of mean longitudinal magnetic fields in the Of?p stars HD 108 and HD 191612

Using polarimetric spectra obtained with the SOFIN spectrograph installed at the Nordic Optical Telescope, we detect a longitudinal magnetic field 〈B_z〉 = –168±35 G in the Of?p star HD 108. This result is in agreement with the longitudinal magnetic field measurement of the order of –150 G recently reported by the MiMeS team. The measurement of the longitudinal magnetic field in the Of?p star HD 191612 results in 〈B_z〉 = +450±153 G. The only previously published magnetic field measurement for this star showed a negative longitudinal magnetic field 〈B_z〉 = –220±38 G, indicating a change of polarity over ∼100 days. Further, we report the detection of distinct Zeeman features in the narrow Ca II and Na I doublet lines for both Of?p stars, hinting at the possible presence of material around these stars. The origin of these features is not yet clear and more work is needed to investigate how magnetic fields interact with stellar wind dynamics (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fine structure of solar magnetic fields

The problem of inequality of results of longitudinal magnetic fields measured in the magnotosensetive lines FeI 5250 and FeI 5247 is investigated. The observed ratio of longitudinal components of magnetic field H ₁₁(5250)/H ₁₁(5247) is compared to the calculated for different combinations of magnetic field elements. The calculations have been made with standard model photosphere for quiet regions. It was concluded that the underestimated value of magnetic field obtained with the FeI 5250 line is explained by the presence of unresolved elements with kilogauss magnetic fields.     

On the development of magnetic fields in active regions

Transverse and longitudinal magnetic field scans together with K₂₃₂ spectroheliograms that cover the early phases of active region formation reveal the following:

1. The new active region forms near the periphery of an old magnetic region. There is evidence that the new region forms an interrelated system with the old magnetic structures on the sun.
2. Noticeable changes in the background magnetic field are seen nearly 3 days prior to the appearance of the sunspot. Magnetic hills of the longitudinal component appear along with bright localized K₂₃₂ emission. Subsequently the K₂₃₂ emission spreads along the boundary of one or two adjacent supergranules and at the time of sunspot formation occupies the whole supergranular cell.
3. Transverse fields with strengths of 100–150 gauss form closed regions in the area of the longitudinal component hills, in the very early phases of the region. These fields stretch and link up the two areas later, at which time the peak transverse fields with values near 250 gauss coincide with the zero line of the longitudinal field. When subsequently the spots appear in the new region, the transverse fields are located about the hills of the longitudinal field. The total field vectors just prior to sunspot formation are pressed to the surface. These are inclined about 45° to the surface after the spot appears. The findings indicate that the magnetic field of a new region emerges from the sub-photospheric layers. It is highly likely that the dynamics of a supergranule influences only the emergence of the magnetic field into the upper layers of the solar atmosphere.

     

Regular versus chaotic motion of charged particles in non-neutral current sheets

Charged particle motion in reconnecting current sheets (CS) can be both regular and chaotic, depending on the values of transverse (perpendicular to the CS plane) and longitudinal (parallel to the electric field inside the CS) components of the magnetic field. The non-zero transverse field gives rise to chaos, whereas a sufficiently large longitudinal field tends to stabilize the motion. The longitudinal field change in time may be the cause of different regimes of electron acceleration in solar flares and magnetospheric substorms.     

Line formation in microturbulent magnetic fields

The formation of Zeeman lines in Gaussian microturbulent magnetic fields is considered assuming LTE. General formulae are derived for the local mean values of the transfer matrix elements. The cases of one-dimensional (longitudinal), isotropic, and two-dimensional (transversal) magnetic microturbulence are studied in some detail. Asymptotic formulae are given for small mean as well as for small microturbulent magnetic fields. Characteristic effects of magnetic microturbulence on the transfer coefficients are: (i) the broadening of the frequency contours, although only for the case of longitudinal Zeeman effect and longitudinal magnetic microturbulence this effect can be described analogous to Doppler broadening, (ii) the appearance of a pseudo-Zeeman structure for nonlongitudinal magnetic microturbulence, (iii) the reduction of maximal values of circular polarization, and (iv) the appearance of characteristic linear polarization effects due to the anisotropy of the magnetic microturbulence.Line contours and polarization of Zeeman triplets are computed for Milne-Edddington atmospheres. It is shown that magnetic intensification due to microturbulent magnetic fields may be much more efficient than that due to regular fields. The gravity center of a Zeeman line observed in circularly polarized light remains a reasonable measure of the line of sight component of the mean magnetic field for a line strength47-1. For saturated lines, the gravity center distance depends significantly on the magnetic microturbulence and its anisotropy. The influence of magnetic microturbulence on the ratio of longitudinal field magnetographic signals shows that unique conclusions about the magnetic microstructure can be drawn from the line ratio measurement only in combination with further spectroscopic data or physical reasoning.     

The role of photospheric magnetic field in the development of solar flares

A statistical investigation has been made about the flare-process in relation to the photospheric magnetic field and configuration. It is understood from the analysis that the flare energy bears a linear relationship with the rate of change of flux of the longitudinal component of photospheric magnetic field.     

First detection of a magnetic field in the fast rotating runaway Oe star ζ Ophiuchi

The star ζ Ophiuchi is one of the brightest massive stars in the northern hemisphere and was intensively studied in various wavelength domains. The currently available observational material suggests that certain observed phenomena are related to the presence of a magnetic field. We acquired spectropolarimetric observations of ζ Oph with FORS 1 mounted on the 8‐m Kueyen telescope of the VLT to investigate if a magnetic field is indeed present in this star. Using all available absorption lines, we detect a mean longitudinal magnetic field 〈B_z〉_all = 141 ± 45 G, confirming the magnetic nature of this star. We review the X‐ray properties of ζ Oph with the aim to understand whether the X‐ray emission of ζ Oph is dominated by magnetic or by wind instability processes (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Neutrino pair emission by a magnetized stellar plasma

The decay of a plasmon into two neutrinos in the presence of an intense magnetic field has been studied by Canutoet al. (1970). They suggest that one of the principal longitudinal plasmon modes, which occurs only in magnetized plasmas, would cause certain magnetic stars to cool more rapidly than their unmagnetized counterparts. We show here that this mechanism is inoperative since the plasmon mode involved cannot be excited in the direction parallel to the magnetic field as considered by Canutoet al. Moreover, for ω_c/ω_p?1, we show that the other principal longitudinal plasmon mode considered earlier by Adamset al. (1963) (which is largely independent of the magnetic field) dominates the plasmon-neutrino decay cooling of magnetic stars.     

Fractal Brownian surface and distribution of longitudinal magnetic fields in two solar active regions

The distribution of photospheric longitudinal magnetic fields in a solar active region can be considered to be a natural surface in a mathematical sense. Just as with landscapes on the Earth, the surface may be a fractal Brownian surface (FBS). A method suggested in the paper can manifest whether the surface is an FBS or not. The method has been applied to the longitudinal magnetic fields in AR 5988 on March 24, 1990 and AR 6233 on August 30, 1990, in which the observational characteristics are quite different. The testing results indicate that the distributions of longitudinal magnetic field in both regions are not in agreement with the FBS model     

On the longitudinal extent of the polar cusp

Variations of the longitudinal extent of the polar cusp are studied in relation to the orientation of the interplanetary magnetic field. In cases when the vertical component of the solar wind magnetic field is positive, the polar cusp is shown to be restricted to a relatively localized region at 12 ± 2 LMT.     

Longitudinal Structure of the Double Magnetic Cycle

The longitudinal structure of the double magnetic cycle of solar activity has been investigated using Stanford magnetograph data from 1976 to 1996 (cycles 21 and 22). It is shown that both high-frequency and low-frequency sources of the magnetic field are present in the data and that they reach their maxima in longitudinal zones that are separated by 20°. Some longitudinal zones are found to exist in which the high-frequency component is dominant and other longitudinal zones are found in which the low-frequency component is dominant.     

Analysis and results of cooperative magnetographic measurements. I. Correction,comparison and discussion of measurements

On June 24, 1983 cooperative magnetographic measurements were made with the vectormagnetographs of the Sayan Observatory (Irkutsk) and the Potsdam Solar Observatory “Einsteinturm” and with the longitudinal magnetograph of the Ondrejov Observatory. Additionally, the maximum field strengths in the sunspot were measured with the photographic method. Using the photographically measured field strengths as reference values we provide a method to eliminate the influences of stray light in a time series of vectormagnetograms. A comparison of nearly simultaneous magnetograms shows a good correspondence in general. The deviations of the zero points as well as the scales are comparable with the results of other authors. Regarding the magnetic field distribution the magnetograms reflect a substantial nonsymmetric structure in the spot under study. The magnetic field lines tend to concentrate into several flux tube clusters. In one region of the penumbra we find a magnetic field with nearly longitudinal character in close neighbourhood of a strong, nearly horizontal flux tube bundle. This indicates a strong nonradial horizontal field gradient in the penumbra.     

A note on chromospheric fine structure at active region polarity boundaries

High resolution H filtergrams from Big Bear Solar Observatory reveal that some filamentary features in active regions have fine structure and hence magnetic field transverse to the gross structure and the zero longitudinal field line. These features are distinct from the usual active region filament, in which fine structure, magnetic field and filament are all parallel to the zero longitudinal field line. The latter occur on boundaries between regions of weaker fields while the former occur at boundaries between regions of stronger field.