Distribution Of Weak Magnetic Fields Research Articles

A New Signal of the Solar Magnetic Cycle: Opposite Shifts of Weak Magnetic Field Distributions in the Two Hemispheres

AbstractWe study the asymmetric distribution of weak photospheric magnetic field values in the two hemispheres separately using synoptic maps from Solar Dynamics Observatory/Helioseismic and Magnetic Imager, Synoptic Optical Long‐term Investigations of the Sun/Vector SpectroMagnetograph, and Wilcox Solar Observatory during Solar Cycles 21–24. We calculate the weak‐field asymmetry (shift) by fitting the distributions of weak‐field values to a shifted Gaussian. Hemispheric shifts derived from the three data sets agree very well and increase systematically when reducing the spatial resolution of the map. Shifts of the Northern and Southern Hemispheres are typically opposite to each other. Shifts follow the evolution of the trailing flux and have a strong solar cycle variation with maxima in the early to mid declining phase of the solar cycle. The sign of the hemispheric weak‐field shift is always the same as the polarity of the polar field in the respective hemisphere and solar cycle. We also find that shifts in the south are systematically larger in absolute value than in the north.

Hanle effect in the CN violet system with LTE modeling

Context. Weak entangled magnetic fields with mixed polarity occupy the main part of the quiet Sun. The Zeeman effect diagnostics fails to measure such fields because of cancellation in circular polarization. However, the Hanle effect diagnostics, accessible through the second solar spectrum, provides us with a very sensitive tool for studying the distribution of weak magnetic fields on the Sun. Aims. Molecular lines are very strong and even dominate in some regions of the second solar spectrum. The CN B 2 Σ −X 2 Σ system is one of the richest and most promising systems for molecular diagnostics and well suited for the application of the differential Hanle effect method. The aim is to interpret observations of the CN B 2 Σ − X 2 Σ system using the Hanle effect and to obtain an estimation of the magnetic field strength. Methods. We assume that the CN molecular layer is situated above the region where the continuum radiation is formed and employ the single-scattering approximation. Together with the Hanle effect theory this provides us with a model that can diagnose turbulent magnetic fields. Results. We have succeeded in fitting modeled CN lines in several regions of the second solar spectrum to observations and obtained a magnetic field strength in the range from 10–30 G in the upper solar photosphere depending on the considered lines.

Sample vibration technique for HTS SQUID microscope

We have developed a scanning HTS SQUID microscope which provides fine images of very weak magnetic field distributions. To achieve a high spatial resolution, the microscope should have high signal to noise (S/N) ratio. We have investigated a sample vibration technique which modulates the magnitude of detected magnetic field of the sample to achieve a higher S/N ratio. The sample stage was vibrated perpendicular to the plane of the SQUID with a frequency of 380 Hz by using a piezo device. The displacement of the stage was about 40 μm. The SQUID output was connected to a lock-in amplifier, and magnetic field signal was obtained using lock-in detection. In order to evaluate performance of the sample vibration technique, we measured magnetic field distributions of laser printed patterns. Observed image showed that improved S/N ratio of the SQUID microscope led to a higher spatial resolution.

Three channel non-force magnetic SQUID microscope

The development of scanning magnetic microscope (SMM) without appreciable applied forces or magnetic excitations on specimens are presented. The magnetic microscope is intended to measure weak magnetic field distributions near the object surface at micron and sub-micron scales. Specifically, the SMM consists of three measurement units with respective output channels.

Does the large-scale solar magnetic field distribution really reflect the convective velocity fields?

We have tried to decide whether the typical circular cellular-like features, which are striking during some intervals in the large-scale distribution of weak magnetic fields measured with low resolution, are related to large-scale convective motions. Two scales of such patterns were found and their morphological, kinematical and evolutionary behaviour was estimated. Their slower and overall rotation is also demonstrated in comparison with the rotation of highly averaged sunspot and magnetic fields. It is difficult to explain all the observed characteristics as random, or due to the method of field measurement and map construction used. We also discuss the change of their magnetic field polarities with the solar polar field reversal.

Large-Scale Distribution of Solar Magnetic Fields.

Solar magnetograms covering a period of years were used to study the distribution of weak magnetic fields on the solar surface. The semiregular pattern of these background fields is the result of the expansion, weakening, and stretching by differential rotation of magnetic fields of old active regions and their interaction with neighboring fields, and of the continuing development of magnetic fields of new regions within the pattern. The net result is a slowly changing pattern of background fields which occupies at times during the solar cycle more than 50 per cent of the surface area of the Sun, as seen with the 23" resolution of the magnetograph. The interaction of nearby magnetic fields takes the form of the apparent attraction of features of the same polarity and the apparent repulsion of features of opposite polarity. Sometimes weak magnetic features covering a large area apparently disappear over a period of a few rotations. It is not clear from the observations what the mechanism of this disappearance could be. It is clear from the period near minimum that active regions are concentrated in complexes of activity whose location and development are clearly defined. The largest of these complexes, consisting of many active regions, result in the formation of Unipolar Magnetic Regions (UMR). The polar fields are discussed from the standpoint of the poleward drift of the UMR's.