Coherence time scales of long-period ULF variations of magnetic field in vicinity of the night-side magnetopause

A large fraction of major flares occur in active regions that exhibit a δ configuration. The formation and disintegration of δ configurations is very important in understanding the evolution of photospheric magnetic fields. In this paper we study the relationship between the change in δ spot structures and associated major flares. We present a new observational result that part of penumbral segments in the outer δ spot structure decay rapidly after major flares; meanwhile, the neighboring umbral cores and/or inner penumbral regions become darker. Using white-light (WL) observations from the Transition Region and Coronal Explorer (TRACE), we study the short-term evolution of δ spots associated with seven major flares, including six X-class flares and one M-class flare. The rapid changes, which can be identified in the time profiles of WL mean intensity are permanent, not transient, and thus are not due to flare emission. The co-aligned magnetic field observations obtained with the Michelson Doppler Imager (MDI) show substantial changes in the longitudinal magnetic field associated with the decaying penumbrae and darkened central areas. For two events for which vector magnetograms were available, we find that the transverse field associated with the penumbral decay areas decreased while it increased in the central darkened regions. Both events also show an increase in the magnetic shear after the flares. For all the events, we find that the locations of penumbral decay are related to flare emission and are connected by prominent TRACE postflare loops. To explain these observations, we propose a reconnection picture in which the two components of a δ spot become strongly connected after the flare. The penumbral fields change from a highly inclined to a more vertical configuration, which leads to penumbral decay. The umbral core and inner penumbral region become darker as a result of increasing longitudinal and transverse magnetic field components.

We investigate the effect of different dust flow velocities and two dimensional magnetic fields on the combined Kelvin-Helmholtz instability (KHI) and Rayleigh-Taylor instability (RTI) of two superimposed incompressible dusty fluids. A single fluid reduced dusty magnetohydrodynamic model is obtained for the three component magnetized incompressible dusty plasma, and it is assumed that a plane interface of infinite boundaries separates heavy and light dusty fluids at z = 0. The general dispersion relations for RT and KH configurations are obtained using appropriate boundary conditions and discussed in the case of equal and different dust fluid flow velocities. In the case of the stable RT configuration, we find that the condition of RTI depends upon both longitudinal and transverse magnetic field components and relative dust flow velocity. In the case of the KH configuration, the effect of magnetic field and relative dust flow velocity is observed and it is shown that dust flow velocity must be larger than a particular value of Alfven speed in order to excite KHI. Numerical calculations have been performed to illustrate the linear growth rates of RTI and KHI in both magnetized and unmagnetized dusty plasmas. We observe that the magnetic field has stabilizing whereas the dust Atwood number has destabilizing influence on the growth rate of RTI. The different dust flow velocities have destabilizing whereas magnetic field has stabilizing influence on the growth rate of KHI in dusty plasmas. The dust Mach number yields a stabilizing influence on the linear growth rate of KHI.

Simple self-consistent models for ‘non-neutral’ current sheets are considered. Characteristics of high-temperature turbulent current sheets (HTCS) with a small transverse component of magnetic field are determined for conditions in the solar corona. The energy output of such an HTCS is much larger than that of a neutral sheet. This makes it possible to consider the HTCS as an energy source not only in long-lived X-ray loops but also in flaring loops during the ‘not’ or ‘main’ phase of a flare. In this case, the magnetic reconnection velocity agrees with the observed velocity of the loop rise. Thus, these phenomena can be interpreted as a result of magnetic reconnection, for example, between new flux emerging from under the photosphere and an ‘old’ magnetic field. The role of a longitudinal magnetic field in a current sheet is less important for HTCS. As a result of the compression of a longitudinal field, there appears an electric current circulating around the sheet. This current may induce strong Joule heating, if the compression is large. This ‘additional’ heating is realized because of the annihilation of the main component, not the longitudinal component of magnetic field. The effect is small for HTCS, but may be significant for preflare current sheets.

We adapt the line‐ratio method to magnetographic measurements. The method yields information on inhomogeneous magneitc fields without resolving their fine structure. Supplementing the magnetic field measurements, each day several polarization calibrations were made to eliminate the influence of the instrument. Modified theoretical calibration curves served for the transformation into inclination angles and magnetic field strength. The circular polarization along some scans across two sunspots practically shows the same distributions in both wings. The linear polarization in these selected scans attained only about half as large values as the circular polarization. The maxima were found in the steep parts of the V distributions. Comparing the two lines a similar behaviour was found, but also differences of about 0.5 percent, which may be significant.The inclination angles determined by means of calibration curves also seem to be more accurate than those calculated from transverse and longitudinal magnetic field components. As an evidence the results in sunspots and faculae are quoted. The values separately determined for the lines scattered only by 3 respectively 7 degrees from the mean values. In the darkest points of an umbra practically accordance of inclination and heliocentric angle was obtained, i.e., the magnetic vector was directed perpendicular to the surface. From the amount of the circular polarization in faculae comparing it with results of model calculations the filling factors were determined. Our results arranged between 3 and 11 percent nearly agree with the values by SOLANKI and STENFLO (1984).The line‐ratio method in the version adapted to our magnetograph yielded from the V ratios in sunspots an average of 1880 G, but from the Q0 ratios an average of 2240 G. In faculae both values are practically identical and run to 1980 G. These results show that the line‐ratio method is superior to the old reduction procedure of magnetographic measurements.

The drawn vacuum arc begins with a bridge column formed from the explosion of a liquid metal bridge after separating the contacts. The expansion process of the initial arc has an effect on arc motion or diffusion. In order to deeply study the influence of transverse magnetic field (TMF) and axial magnetic field (AMF) on the drawn arc characteristics in the initial expansion stage, in this article, arc ignition mode and appearance under two groups of contacts with different AMF and TMF components were experimentally studied. It was found that simultaneously reducing TMF and AMF components would end the expansion stage earlier, and the fluctuation of the arc column pressure during the expansion was larger. When TMF and AMF components were further reduced, the arc needs to take a longer time to extend across the slot gap due to insufficient Ampere force. When TMF was basically unchanged and AMF was reduced, the arc burned more intense, the diameter of the arc column was smaller, and the arc would merge earlier in multipoint ignition mode. The characteristics of arc voltage, pressure, and duration of expansion stage were analyzed, founding that the arc with a moderate TMF and AMF components had a smaller rising slope of arc voltage, a faster reducing speed of arc pressure, and a smaller range of the duration of expansion stage. In addition, the variation of magnetic field strength in different ignition modes and contact structures during the arc expansion stage was simulated to explain the mechanism of arc behavior.

The Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station (HSOS) has conducted continuous observations of solar vector magnetic fields for nearly four decades, and while the primary optical system remains unchanged, critical components—including filters, polarizers, and detectors—have undergone multiple upgrades and replacements. Maintaining data consistency is essential for reliable long-term studies of magnetic field evolution and solar activity, as well as current helicity. In this study, we systematically analyze background bias and noise levels in SMFT observations from 1988 to 2019. Our dataset comprises 12,281 vector magnetograms of 1484 active regions. To quantify background bias, we computed mean values of Stokes Q/I, U/I and V/I over each entire magnetogram. The background bias of Stokes V/I is small for the whole dataset. The background biases of Stokes Q/I and U/I fluctuate around zero during 1988–2000. From 2001 to 2011, however, the fluctuations in the background bias of both Q/I and U/I become significantly larger, exhibiting mixed positive and negative values. Between 2012 and 2019, the background biases shift to predominantly positive values for both Stokes Q/I and U/I parameters. To address this issue, we propose a potential method for removing the background bias and further discuss its impact on the estimation of current helicity. For each magnetogram, we quantify measurement noise by calculating the standard deviation (σ) of the longitudinal (Bl) and transverse (Bt) magnetic field components within a quiet-Sun region. The noise levels for Bl and Bt components were approximately 15 Gauss (G) and 87 G, respectively, during 1988–2011. Since 2012, these values decreased significantly to ∼6 G for Bl and ∼55 G for Bt, likely due to the installation of a new filter.

Microwave polarization diagnostics of solar current sheets with transverse component of magnetic field

Kinetic effects of the dynamics of protons in plasmoids with a non-zero longitudinal (By) magnetic field component in a current sheet (CS) of a geomagnetic tail are considered. The results of modeling proton dynamics and a description of the mechanism of emergence of “north-south” density asymmetry are presented. The mechanism that is possibly responsible for maintaining the longitudinal magnetic field component is described. The obtained parameters are evaluated and the results are compared with observations of the Cluster mission.

The motion of vacuum arc has an important effect on the performance of triggered vacuum switch with multirod (MTVS), and the motion characteristics of vacuum arc are closely influenced by the amplitude and profile of the magnetic flux density in the MTVS. A 3-D simulation model of the MTVS is built up, and the distribution of magnetic field of MTVS is calculated and analyzed. The simulation results show that the magnetic field in the interred gap has transverse component and longitudinal component. The simulation results also show that the transverse magnetic field is dominant when the MTVS is conducted by one single vacuum arc channel; however, the longitudinal magnetic field (LMF) is dominant when all the gap of the MTVS is conducted by vacuum arc. When the current flowing through the MTVS is up to 100 kA, the transverse component of magnetic field is up to 2.96 mT/kA, and the maximum value of the longitudinal component is up to 3.8 mT/kA, which meets the critical value of the LMF, which can keep the vacuum arc spreading. The influence of the structure parameters and the conductivity of vacuum arc on the magnetic field distribution are also analyzed. In addition, the high current capacity was tested, and the motion of vacuum arc in the MTVS has been investigated by the use of a high-speed camera. The experimental results agree well with the simulation results.

We report on the temperature variation of spin-orbit torques in perpendicularly magnetized W/CoFeB bilayers. Harmonic Hall voltage measurements in perpendicularly magnetized CoFeB reveal increased longitudinal and transverse effective magnetic field components at low temperatures. The damping-like spin-orbit torque reaches an efficiency of 0.55 at 19 K. Scanning transmission electron microscopy and X-ray reflectivity measurements indicate that considerable interface mixing between W and CoFeB may be responsible for strong spin-orbit interactions.

The presence of the additional transverse or longitudinal magnetic field component (Bv or Bz) in the gun region has been earlier reported to result in a significant reduction of noise in an injected-beam crossed-field device, without any significant deterioration in beam transmission. The additional magnetic field component is present throughout the gun region. Some theoretical work on the beam formation and noise transport does not seem to indicate the reduction in noise as observed experimentally.Experimental results carried out on crossed-field beam analyzers with titled cathodes also result in a significant reduction in noise without any significant deterioration of beam transmission. The function of the tilt seems somewhat similar to that in a magnetron injection gun. This indicates some modification of the noise phenomenon originating at the cathode due to the presence of an additional magnetic field component resulting from either cathode tilt or arising externally.

How magnetic field structure changes with eruptive events (e.g., flares and CMEs) has been a long-standing problem in solar physics. Here we present the analysis of eruption-associated changes in the magnetic inclination angle, the transverse component of magnetic field and the Lorentz force. The analysis is based on an observation of the X3.4 flare on Dec.13 2006 and a numerical simulation of a solar eruption made by Yuhong Fan. Both observation and simulation show that (1) the magnetic inclination angle in the decayed peripheral penumbra increases, while that in the central area close to flaring polarity inversion line (PIL) deceases after the flare; (2) the transverse component of magnetic field increases at the lower altitude near flaring PIL after the flare. The result suggests that the field lines at flaring neutral line turn to more horizontal near the surface, that is in agreement with the prediction of Hudson, Fisher & Welsch (2008).

The influence of the momentum addition, which may be associated with the average or fluctuation transverse component of the magnetic field or others, on the acceleration the solar wind or stellar wind is studied in a local streamtube. The results show that the larger the momentum addition the stronger the acceleration of the wind. For example, if the typical transverse magnetic field is about 0.1 of the longitudinal field, the velocity of the solar wind at 1 AU may be increased by 40%. The coronal hole may be considered as a streamtube, the presence of a high stream from the coronal hole may be explained by the existence of an average or fluctuation transverse magnetic field in the streamtube. A similar conclusion may be applied to the polar region, where the velocity of the solar wind will be larger than elsewhere as if there is a transverse component of magnetic field, as well as to the stellar wind. The influence of other parameters on the acceleration of the solar wind is also discussed. From the viewpoint of the solar wind mechanism, the present paper shows that the momentum addition in the subsonic flow region can increase the velocity of the solar wind at 1 AU.

AC loss characteristics of multifilamentary NbTi AC wires under AC external magnetic field were experimentally and theoretically investigated. The authors measured AC transport current losses of AC wires with various twist pitches under the AC external magnetic field which included longitudinal and transverse components. It was shown by the measurement that the AC transport current losses decreased with decreasing the twist pitches when the external and self longitudinal fields were in the same direction. The measured dependence of the AC losses on the twist pitches and the applied longitudinal field component were well explained by their previously derived model. They also theoretically estimated the AC losses caused by the longitudinal and azimuthal field components. It was found that the AC losses can be substantially reduced by properly selecting the twist pitch and the external longitudinal field component.

Investigation on the evolution and distribution of plasma in magnetic field assisted laser-induced plasma micro-machining