Helicity Patterns of the Active Regions Connected by Transequatorial Loops

Using photospheric vector magnetograms of the Huairou Solar Observing Station and coronal X-ray images from the Yohkoh Soft X-Ray Telescope, we calculate the helicity patterns of 43 pairs of active regions and the chirality of 50 pairs of opposite magnetic polarity regions that are connected by transequatorial loops (TLs). To make the results more convincing, two helicity proxies including the local current helicity h _c and the force-free factor α _best are computed. The results, which are similar for both parameters, are as follows: (1) Current helicity of the active regions pairs connected by transequatorial loops have no obvious regularity: About 50% of the active region pairs carry the same current helicity sign and about 50% of them have the opposite. (2) If we consider the magnetic polarity pairs connected by the TLs, the result is almost the same as that for the active region pairs, with a little more than half of them showing the same chirality. We also make linear force-free extrapolations for 33 TLs and determine their force-free parameter α by comparing extrapolated field lines to X-ray images of the TLs. Out of the 19 cases when the footpoints of the TLs have the same current helicity sign, we find that the sign of α of the TLs is the same as the sign of the current helicity in the footpoints in 12 cases, whereas it is of opposite sign in 4 cases, and in 3 cases the TLs were found to be potential.     

<Emphasis Type="Italic">In Situ</Emphasis> Signatures of Interchange Reconnection between Magnetic Clouds and Open Magnetic Fields: A Mechanism for the Erosion of Polar Coronal Holes?

We outline a method to determine the direction of solar open flux transport that results from the opening of magnetic clouds (MCs) by interchange reconnection at the Sun based solely on in-situ observations. This method uses established findings about i) the locations and magnetic polarities of emerging MC footpoints, ii) the hemispheric dependence of the helicity of MCs, and iii) the occurrence of interchange reconnection at the Sun being signaled by uni-directional suprathermal electrons inside MCs. Combining those observational facts in a statistical analysis of MCs during solar cycle 23 (period 1995 – 2007), we show that the time of disappearance of the northern polar coronal hole (1998 – 1999), permeated by an outward-pointing magnetic field, is associated with a peak in the number of MCs originating from the northern hemisphere and connected to the Sun by outward-pointing magnetic field lines. A similar peak is observed in the number of MCs originating from the southern hemisphere and connected to the Sun by inward-pointing magnetic field lines. This pattern is interpreted as the result of interchange reconnection occurring between MCs and the open field lines of nearby polar coronal holes. This reconnection process closes down polar coronal hole open field lines and transports these open field lines equatorward, thus contributing to the global coronal magnetic field reversal process. These results will be further constrainable with the rising phase of solar cycle 24.     

A New Explanation for the Origin of Trans-equatorial Loops based on a Dynamo Model

Trans-equatorial loops (TLs) are one of the distinct magnetic structures in the solar corona and have a close relationship to solar activity. We present a systematic study of the origin of TLs linking with the Babcock?–?Leighton dynamo process based on the model of Chatterjee, Nandy, and Choudhuri (Astron. Astrophys. 427, 1019, 2004). We propose that TLs are visible signatures of poloidal field lines across the equator. The cycle variation of TL lengths obtained by the connectivities of poloidal field lines happens to be roughly in agreement with what one gets by considering the positions of sunspots. This explains why this cycle variation of TL lengths was found to be in conformity with Spörer’s law. The active regions always make the poloidal field configuration favorable to form TLs, which causes the conformity. The formation of TLs is a three-dimensional problem, which will require three-dimensional dynamo models for full investigation.     

High-Resolution Studies of the Solar Polar Magnetic Fields

We present high-resolution studies of the solar polar magnetic fields near sunspot maximum in 1989 and towards sunspot minimum in 1995. We show that, in 1989, the polar latitudes were covered by several unipolar regions of both polarities. In 1995, however, after the polar field reversal was complete, each pole exhibited only one dominant polarity region.Each unipolar region contains magnetic knots of both polarities but the number count of the knots of the dominant polarity exceeds that of the opposite polarity by a ratio of order 4:1, and it is rare to find opposite polarity pairs, i.e., magnetic bipoles.These knots have lifetimes greater than 7 hours but less than 24 hours. We interpret the longitudinal displacement of the knots over a 7-hour period as a measure of the local rotation rate. This rotation rate is found to be generally consistent with Snodgrass' (1983) magnetic rotation law.In an attempt to obtain some insight into the operation of the solar dynamo, sketches of postulated subsurface field configurations corresponding to the observed surface fields at these two epochs of the solar cycle are presented.     

Solar Modulation of Cosmic Rays during the Declining and Minimum Phases of Solar Cycle 23: Comparison with Past Three Solar Cycles

We study solar modulation of galactic cosmic rays (GCRs) during the deep solar minimum, including the declining phase, of solar cycle 23 and compare the results of this unusual period with the results obtained during similar phases of the previous solar cycles 20, 21, and 22. These periods consist of two epochs each of negative and positive polarities of the heliospheric magnetic field from the north polar region of the Sun. In addition to cosmic-ray data, we utilize simultaneous solar and interplanetary plasma/field data including the tilt angle of the heliospheric current sheet. We study the relation between simultaneous variations in cosmic ray intensity and solar/interplanetary parameters during the declining and the minimum phases of cycle 23. We compare these relations with those obtained for the same phases in the three previous solar cycles. We observe certain peculiar features in cosmic ray modulation during the minimum of solar cycle 23 including the record high GCR intensity. We find, during this unusual minimum, that the correlation of GCR intensity is poor with sunspot number (correlation coefficient R=?0.41), better with interplanetary magnetic field (R=?0.66), still better with solar wind velocity (R=?0.80) and much better with the tilt angle of the heliospheric current sheet (R=?0.92). In our view, it is not the diffusion or the drift alone, but the solar wind convection that is the most likely additional effect responsible for the record high GCR intensity observed during the deep minimum of solar cycle 23.     

Variability of Extended Bipolar Regions

Using NSO/Kitt Peak synoptic charts from 1975 to 2003, we group the main solar magnetic fields into two categories: one for active regions (ARs) and the other for extended bipolar regions (EBRs). Comparing them, we find that there exist three typical characteristics in the variability of EBRs: First, there exists a correlation between ARs and EBRs. The phase of EBR flux has a delay nearly two CRs. Second, we find that the EBR flux has two prominent periods at 1.79 years and 3.21 years. The 1.79-year period seems to only belong to large-scale magnetic features. Lastly, the North – South asymmetry of EBR flux is not very significant on a time scale of one solar cycle. However, during solar maxima, its dominance is found to shift from one hemisphere to the other.     

North–South Asymmetry in the Distribution of Solar Background Magnetic Field

The aim of this article is to investigate how the background magnetic field of the Sun behaves in different hemispheres. We used SOHO/MDI data obtained during a period of eight years from 2003 to 2011 to analyze the intensity distribution of the background magnetic field over the solar surface. We find that the background fields of both polarities (signs) are more intense in the southern than in the northern hemisphere. Mixed polarities are observed in the vicinity of the equator. In addition to the main field, a weaker field of opposite polarity is always present in the polar regions. In the declining phase of the cycle, the main field dominates, but at the minimum and in the rising phase of the cycle, it is gradually replaced by the growing stronger secondary field.     

Evolution of vector magnetic fields and the August 27 1990 X-3 flare

This paper studies the evolution of vector magnetic fields in the active region Boulder No. 6233 during an 11-hour observing period and its relationship to an X-3 flare on August 27, 1990.We observed the evolution of magnetic fields, which includes magnetic shear build-up, directly in high-resolution vector magnetograph movies. The magnetic shear is observed to be built up in two ways: (1) shear motion between two poles of opposite magnetic polarities and (2) direct collision of two poles of opposite polarities. When two magnetic elements of opposite polarities are canceling, the field lines are observed to turn from direct connection (potential) to a sheared configuration during the process.An X-3 flare occurred at 2100 UT. The vector magnetic structure showed an unexpected pattern of changes during and after the flare. The shear (defined as the angle between the measured transverse field and the calculated potential field) in the area covering two major footpoints increased rapidly coinciding with the burst of GOES X-ray flux. While the flare faded away in about one hour, the high shear status dropped slowly for the remainder of the observing period. Immediately after the flare, new flux emerged more rapidly and the flow speed of several magnetic elements increased near the flare footpoints.In this active region and a few other flare-productive regions we have studied recently, we always find rapid and complicated flow motions near the sites where flares occur. Photospheric flows appear to be another important factor for the production of flares.     

Real and virtual unipolar regions

Difficulties in relating magnetograph measurements to the actual solar magnetic field are discussed. After a brief review both of problems inherent in the nature of the measurements and of sources of instrumental error, we show that field measurements taken within the photosphere can map out large-scale regions of a single magnetic polarity even though these regions contain no footpoints of large-scale magnetic structures, but instead only aggregates of small, unresolved bipoles. This may occur wherever the density of unresolved bipoles has a preferred orientation and a spatial variation along the direction of that orientation. We call these regionsvirtual unipolar regions, as they are not connected to regions of opposite polarity by field loops or lines passing through the corona. Investigation of these regions shows that they can arise at widely separated locations, and that they may evolve into real unipolar magnetic regions which are connected to the chromospheric and coronal fields. These results can explain a number of puzzling aspects of magnetograph observations of the solar background magnetic field.     

Solar cycle variation of large-scale coronal structures

A green line intensity variation is associated with the interplanetary and photospheric magnetic sector structure. This effect depends on the solar cycle and occurs with the same amplitude in the latitude range 60° N–60° S. Extended longitudinal coronal structures are suggested, which indicate the existence of closed magnetic field lines over the neutral line, separating adjacent regions of opposite polarities on the photospheric surface.Supported by an ESRO/NASA fellowhip.On leave from Torino University, Italy; now at Istituto di Fisica, Universita di Torino, Italy.     

Statistical study of the north–south asymmetry of the solar differential rotation based on various solar structures during 1966–1985

The time variation and latitude dependence of the solar rotation are found using observational data on Hα filaments and compact magnetic features with different polarities during solar activity cycles 20 and 21 (1966–1985). Statistical analysis of the observational data shows that there is a north–south asymmetry in the rotation, both for the Hα filaments and for compact magnetic features (structures) with negative and positive polarities. The N-S asymmetry in the differential rotation of the Hα filaments and the compact magnetic features with both polarities shows up quite distinctly in solar activity cycles 20 and 21, but the asymmetry for the compact magnetic features with positive polarity is comparatively lower in cycle 21. The confidence level is lower the compact magnetic features with positive polarity than for the compact magnetic features with negative polarity.     

Comparisons of interplanetary type III storm footpoints with solar features

The trajectories of 38 type III storms in the interplanetary medium have been deduced from ISEE-3 radio observations and extrapolated back to the Sun to determine the Carrington coordinates of their footpoints. The analysis assumes radial motion of the solar wind, and the trajectories are projected radially back toward the surface for the last few solar radii. To identify the storm sources, the footpoints were compared to a variety of solar features: to the large-scale neutral line at the base of the current sheet, to active regions, to the small-scale neutral lines and Hα filaments which trace out active regions, and to coronal holes. Most of the footpoints were found to lie near active regions, in agreement with metric storm locations. There is a weak correlation with Hα filaments, no apparent association with the current sheet, and an anticorrelation with coronal holes. There is a small excess of storms in the leading half of magnetic sectors.     

The Rotation Rates of Solar Magnetic Fields During Solar Cycles 21 – 23

Employing the synoptic maps of the photospheric magnetic fields from the beginning of solar cycle 21 to the end of 23, we first build up a time – longitude stackplot at each latitude between ±35°. On each stackplot there are many tilted magnetic structures clearly reflecting the rotation rates, and we adopt a cross-correlation technique to explore the rotation rates from these tilted structures. Our new method avoids artificially choosing magnetic tracers, and it is convenient for investigating the rotation rates of the positive and negative fields by omitting one kind of field on the stackplots. We have obtained the following results. i) The rotation rates of the positive and negative fields (or the leader and follower polarities, depending on the hemispheres and solar cycles) between latitudes ±35° during solar cycles 21–23 are derived. The reversal times of the leader and follower polarities are usually not consistent with the years of the solar minimum, nevertheless, at latitudes ±16°, the reversal times are almost simultaneous with them. ii) The rotation rates of the three solar cycles averaged over each cycle are calculated separately for the positive, negative and total fields. The latitude profiles of rotation of the positive and negative fields exhibit equatorial symmetries with each other, and those of the total fields lie between them. iii) The differences in rotation rates between the leader and follower polarities are obtained. They are very small near the equator, and increase as latitude increases. In the latitude range of 5° – 20°, these differences reach 0.05 deg day⁻¹, and the mean difference for solar cycle 22 is somewhat smaller than cycles 21 and 23 in these latitude regions. Then, the differences reduce again at latitudes higher than 20°.     

Observations of the Polar Magnetic Fields During the Polarity Reversals of Cycle 22

The Mount Wilson synoptic magnetic data for the period September 1987 through March 1996 are completely revised and used to provide polar plots of the solar magnetic fields for both hemispheres. This period, from Carrington rotations 1793 to 1906, covers the reversals of the polar magnetic fields in cycle 22. Comparison of our plots with the presently available H filtergrams for this period shows that the polarity boundaries are consistent in these two data sets where they overlap. The Mount Wilson plots show that the polar field reversals involve a complex sequence of events. Although the details differ slightly, the basic patterns are similar in each hemisphere. First the old polarity becomes isolated at the pole, then shortly thereafter, the isolation is broken, and the polar field includes unipolar regions of both polarities. The old polarity then reclaims the polar region, but when the isolation of this field is established for a second time, it declines in both area and strength. We take the reversal to be complete when the old polarity field is no longer observed in the Mount Wilson plots. With this criterion we find that the polar field reversal is completed in the north by CR 1836, i.e., by December 1990, and in the south by CR 1853, i.e., March 1992.     

Current Helicity and Twist as Two Indicators of the Mirror Asymmetry of Solar Magnetic Fields

A comparison between the two tracers of magnetic field mirror asymmetry in solar active regions – twist and current helicity – is presented. It is shown that for individual active regions these tracers do not possess visible similarity but averaging by time over the solar cycle, or by latitude, reveals similarities in their behavior. The main property of the data set is antisymmetry over the solar equator. Considering the evolution of helical properties over the solar cycle we find signatures of a possible sign change at the beginning of the cycle, though more systematic observational data are required for a definite confirmation. We discuss the role of both tracers in the context of solar dynamo theory.     

Characteristics of active-region sources of solar wind near solar maximum

Previous studies of the source regions of solar wind sampled by ACE and Ulysses showed that some solar wind originates from open magnetic flux rooted in active regions. These solar wind sources were labeled active-region sources when the open flux was from a strong field region with no corresponding coronal hole in the NSO He 10830 Å synoptic coronal-hole maps. Here, we present a detailed investigation of several of these active-region sources using ACE and Ulysses solar wind data, potential field models of the corona, and solar imaging data. We find that the solar wind from these active-region sources has distinct signatures, e.g., it generally has a higher oxygen charge state than wind associated with helium-10830 Å coronal-hole sources, indicating a hotter source region, consistent with the active region source interpretation. We compare the magnetic topology of the open field lines of these active-region sources with images of the hot corona to search for corresponding features in EUV and soft X-ray images. In most, but not all, cases, a dark area is seen in the EUV and soft X-ray image as for familiar coronal-hole sources. However, in one case no dark area was evident in the soft X-ray images: the magnetic model showed a double dipole coronal structure consistent with the images, both indicating that the footpoints of the open field lines, rooted deep within the active region, lay near the separatrix between loops connecting to two different opposite polarity regions.     

Formation Mechanism of Soft X-Ray Transient Trans-Equatorial Loop System

The Soft X-ray Telescope (SXT) onboard Yohkoh often observed large-scale coronal loops connecting two active regions situated in opposite hemispheres. These are the trans-equatorial loop systems (TLSs). The formation mechanism of TLSs is not yet known. We analyzed a TLS observed simultaneously with Yohkoh/SXT and a coronagraph (SOHO/LASCO-C1). SOHO/LASCO-C1 observed loop expansion and eruption at the west solar limb. Yohkoh/SXT observed a rising motion (chromospheric evaporation) of hot and dense plasmas from the active regions located at the footpoints of the loop. Important results of our analyses are that (1) the loop eruption and the rising motion of the plasmas were simultaneous, (2) the TLS had a cusp-like appearance, and (3) the highest temperature region of the TLS was located above the bright loop seen in soft X rays. These observational results (loop expansion, eruption, and chromospheric evaporation) suggest that this bright (high-density) TLS was created by the same mechanism by which a solar flare occurs, namely, magnetic reconnection. In this paper, we propose a formation mechanism of the TLS that forms between two independent active regions.     

Photospheric Magnetic Field of the Sun: Two Patterns of the Longitudinal Distribution

Longitudinal distributions of the photospheric magnetic field studied on the basis of National Solar Observatory (Kitt Peak) data (1976 – 2003) displayed two opposite patterns during different parts of the 11-year solar cycle. Helio-longitudinal distributions differed for the ascending phase and the maximum of the solar cycle on the one hand and for the descending phase and the minimum on the other, depicting maxima around two diametrically opposite Carrington longitudes (180° and 0°/360°). Thus the maximum of the distribution shifted its position by 180° with the transition from one characteristic period to the other. Two characteristic periods correspond to different situations occurring in the 22-year magnetic cycle of the Sun, in the course of which both global magnetic field and the magnetic field of the leading sunspot in a group change their sign. During the ascending phase and the maximum (active longitude 180°) polarities of the global magnetic field and those of the leading sunspots coincide, whereas for the descending phase and the minimum (active longitude 0°/360°) the polarities are opposite. Thus the observed change of active longitudes may be connected with the polarity changes of Sun’s magnetic field in the course of 22-year magnetic cycle.     

Using a Chandra Source-Finding Algorithm to Automatically Identify Solar X-ray Bright Points

We describe adapting a method that is used to find point sources in Chandra X-ray telescope data for use in finding solar X-ray bright points. The algorithm allows selected pixels to be excluded from the source-finding, thus excluding saturated pixels (from flares and/or active regions). For Chandra data the noise is determined by photon-counting statistics, whereas solar telescopes typically integrate a flux. Thus, the calculated signal-to-noise ratio is incorrect, but we find that we can scale the number to get reasonable results. We compare our source-finding to previous Yohkoh results and find a similar number of bright points. Finally, we analyze three sets of data from Hinode, representing different parts of the decline to minimum of the solar cycle. Although these preliminary results are based on a small sample, we see no dependence on the solar cycle.     

Common Characteristics of the Active Regions of Strong Proton Flares

Common characteristics of nine active regions with strong proton flares in the 22nd solar activity cycle have been presented. Results show that the typical morphology of these active regions is a -type sunspot with a single multiple structure, in which there are many umbras with different magnetic polarities, packed tightly by a single penumbra. In these active regions, the rotating directions of the sunspot groups are nearly independent of their position on the solar disk. When the angle of rotation approaches the positive or the negative maximum, proton flares may occur in these active regions. After proton flares, sunspot groups rotate in the inverse direction because of the slack in the flux rope.