Transequatorial Loops Research Articles

Reconstructing Spherical Nonlinear Force‐free Field in the Solar Corona

We present a spherical nonlinear force-free field (NFFF) reconstructing method based on the photospheric vector magnetograms. The importance of this method is its ability to reveal the NFFF configurations, which is necessary for understanding the physical mechanisms of the initiation of the large-scale solar eruptions, such as coronal mass ejections and sympathetic flares. Using smooth continuous functions, the basic NFFF-governing partial differential equations in spherical coordinates reduce to a set of tractable ordinary differential equations. The numerical scheme used in this paper is similar to the recent local nonlinear force-free one developed by Song and coworkers. To illustrate this method, we give two test examples. One is to compute a well-known NFFF analytical solution given by Low & Lou. The other is for two active regions NOAA 10486 and NOAA 10488 observed on 2003 October 29. The results show that the transequatorial magnetic loops are revealed and coincided with some EUV Imaging Telescope loops.

Transequatorial magnetic flux loops on the Sun as a possible new source of geomagnetic storms

Some intense geomagnetic storms have no clear solar activity several days prior to their onset (SSC). On the other hand, some of such storms are associated with an unmistakable indication of magnetic flux ropes in interplanetary space. We found that some of such SSC geomagnetic storms occurred several days after the passage of transequatorial loops on the central part of the solar disk; transequatorial magnetic loops connect sunspot groups between the northern and southern hemispheres and at times show some brightening activity for a day or so. Since transequatorial loops must have the IMF Bz component, we examined them as a new possible source of geomagnetic storms, in addition to solar flares and filament disappearances.

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 hc and the force-free factor αbest 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.

Coronal Magnetic Connectivity and EUV Dimmings

Coronal dimming can be considered to be a disk signature of front-side coronal mass ejections (CMEs) (Thompson et al.: 2000, Geophys. Res. Lett.27, 1431). The study of the magnetic connectivity associated with coronal dimming can shed new light on the magnetic nature of CMEs. In this study, four major flare-CME events on 14 July 2000, 28 October 2003, 7 November 2004, and 15 January 2005 are analyzed. They were all halo CMEs associated with major flare activity in complex active regions (ARs) and produced severe space weather consequences. To explore the magnetic connectivity of these CMEs, global potential-field extrapolations based on the composite synoptic magnetograms from the Michelson Doppler Imager onboard the Solar and Heliospheric Observatory are constructed, and their association with coronal dimming is revealed by the Extreme ultraviolet Imaging Telescope. It is found that each flare-CME event involved interaction of more than ten sets of magnetic-loop systems. These loop systems occupied over 50% of all identified loop systems in the visible hemisphere and covered a wide range of solar longitudes and latitudes. We categorize the loop systems as active-region loops (ARLs), AR-interconnecting loops (ARILs) including transequatorial loops (TLs), and long arcades (LAs) straddling filament channels. A recurring pattern, the saddle-field configuration (SFC), consisting of ARILs, is found to be present in all four major flare-CME events. The magnetic connectivity revealed by this work implies that intercoupling and interaction of multiple flux-loop systems are required for a major CME. For comparison, a simple flare-CME event of 12 May 1997 with a relatively simple magnetic configuration is chosen. Even for this simple flare-CME event, we find that multiple flux-loop systems are also present.

Some statistical properties of transequatorial loops

Yohkoh soft X-ray telescope brought plenty of high quality images, it provides a good chance to research coronal loops, especially for transequatorial loops (TLs). In this paper, we focus on the statistical results of TLs including static properties and dynamic properties. There are two types of classification about TLs: according to configuration and according to magnetic polarities of footpoints, respectively. The footpoints of TLs never root in sunspot, in a general way, they exist in moderately strong field. The mean separation value of TLs is close to 30° and the separation value varies with solar cycle. The helicity patterns of active regions connected by TLs are discussed, the mean twist value of TLs is close to zero. The formation of TLs is generally thought to be caused by magnetic reconnection, the relationship of TLs eruption with flare and CME is introduced.

A Statistical Study of Transequatorial Loops

We identify 356 transequatorial loops (TLs) from the data set of Yohkoh Soft X-Ray Telescope (SXT) in the period of solar cycles 22 and 23. The classification of the TLs can be made on two bases. One is according to the magnetic polarities of the footpoints of TLs, and the other is according to the number of TLs in the same region. Based on the first criterion, TLs fall into two categories: PTLs in which the magnetic polarities of the footpoints are the same as the preceding polarities and FTLs in which the footpoint polarities are the same as the following polarities of active regions, respectively. It is found that PTLs have a preference; about 66% of the TLs are PTLs, and this preference of PTLs is independent of the solar cycle. The percentage of FTLs is about 34%. Based on the second criterion, TLs are also divided into two categories: the number of TLs in a region is either single (STLs) or multiple (MTLs). In addition, we find that the number of TLs, PTLs, and FTLs have good correlations with solar cycle indices. By comparing the number of TLs and the number of active regions in each year, we obtain the ratio between them. The separation of footpoints and their yearly variations are calculated, and we find that our result is consistent with sporer's law.

Magnetic Fields and Large-Scale SXR and EUV Coronal Structures

An overview is presented of large-scale coronal structures as observed in soft X-rays (SXR) and extreme ultraviolet (EUV) wavelengths in the context of their magnetic properties. These structures include large-scale interconnecting and trans-equatorial loops, coronal streamers, coronal holes, filaments and filament channels. Since the general appearance of the corona and its structures change with evolving underlying fields, evolutionary trends and solar cycle dependence of these coronal structures are discussed as well.

Large-scale source regions of earth-directed coronal mass ejections

Based on SOHO/MDI, EIT, Yohkoh/SXT, Hα , and other relevant observations, we analyzed all the earth-directed halo coronal mass ejections (CMEs) in the interval from Mar. 1997 to Dec. 2003. A total of 288 earth-directed CMEs were studied and their associated surface activity events identified. Unlike the previous studies that often attributed a surface activity event or a given active region to a CME source region, this statistical analysis puts emphasis on the large-scale magnetic structures of CMEs, in which the CME-associated surface activity takes place. All the CMEs are found to be associated with large-scale source structures. The identified large-scale structures can be grouped into four different categories: extended bipolar regions (EBRs), transequatorial magnetic loops, transequatorial filaments and their associated magnetic structures, and long filaments along the boundaries of EBRs. The relative percentages of their associated CMEs are 36%, 40%, 13%, and 11%, respectively. The analysis indicates that CMEs are intrinsically associated with source magnetic structures on a large spatial scale.

Early signatures of large-scale field line opening

A fast halo-type coronal mass ejection (CME) associated with a two-ribbon flare, GOES class M 1.3, was observed on February 8, 2000. Soft X-ray and EUV images revealed several loop ejections and one wave-like moving front that started from a remote location, away from the flare core region. A radio type-II burst was observed near the trajectory of the moving soft X-ray front, although association with the CME itself cannot be ruled out. Large-scale dimmings were observed in EUV and soft X-rays, both in the form of disappearing transequatorial loops. We can pinpoint the time and the location of the first large-scale field-line opening by tracing the electron propagation paths above the active region and along the transequatorial loop system, in which large-scale mass depletion later took place. The immediate start of a type-IV burst (interpreted as an upward moving structure) which was located over a soft X-ray dimming region, confirms that the CME had lifted off .W e compare these signatures with those of another halo CME event observed on May 2, 1998, and discuss the possible connections with the magnetic breakout model.

Transequatorial Connections: Loops or Magnetic Separators?

We provide a brief overview of properties of transequatorial loops (TLs) and show that some TLs fit the description of a loop, while others appear to be the magnetic separators.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The association of transequatorial loops in the solar corona with coronal mass ejection onset

It has been shown that transequatorial loops can disappear in association with the onset of a coronal mass ejection (CME) (Khan & Hudson 2000). We extend this result by considering a larger sample of transequatorial loop systems (TLS) to investigate their associated flaring and CME activity. We find 10 of a total 18 TLS considered here to be associated with flaring and CME onset originating from a connected active region. A total 33 cases of flaring and associated CME onset are observed from these 10 systems during their lifetime. We observe the influence of this activity on the TLS in each case. In contrast to the Khan & Hudson result, we find evidence that transequatorial loop eruption leading to soft X-ray brightening equivalent in temperature to a B-class flare is equally as common as dimming in the corona. Consequently we conclude that the scenario observed by Khan & Hudson is not universal and that other types of CME-TLS association occur. It was found that for transequatorial loops that were associated with CMEs the asymmetry in longitude was larger than for those that were not associated to a CME by 10degrees. In addition, the extent in latitude (as a measure of the loop length) was nearly twice as large for those TLS associated with CMEs than those that were not. The asymmetry in latitude was actually on average larger for those TLS not associated with CMEs, than for those that were. This suggests that differential rotation is not a major contributor to the production of CMEs from transequatorial loops. Instead it is more likely for a CME to be produced if the loop is long, and if there is a large asymmetry in longitude. The implications of these results for CME onset prediction are discussed.

What can we learn from lfff magnetic extrapolations

Observations of the Sun is done, up to now, in 2D and magnetic field is measured mainly in the photosphere. Magnetic extrapolation techniques allow us nevertheless to have a 3D view of the magnetic field. Different methods are available today. We present in this paper interesting and exploitable results obtained with linear force-free-field methods (lfff). The α parameter is assumed to be constant in the whole volume of computations. The results concern the global potential field in the corona (e.g. example of transequatorial loops), the magnetic topology of flaring active regions (e.g. flares and bald patch regions), twisted magnetic fields in filaments, and magnetic reconnection in emerging active region (e.g. Ellerman Bombs). We discuss on the limits of the validity of the methods.

Statistical Evidence for Sympathetic Flares

Sympathetic flares are a pair of flares that occur almost simultaneously in different active regions, not by chance, but because of some physical connection. In this paper statistical evidence for the existence of sympathetic flares is presented. From GOES X-ray flare data, we have collected 48 pairs of near simultaneous flares whose positional information and Yohkoh soft X-ray telescope images are available. To select the active regions that probably have sympathetic flares, we have estimated the ratio R of actual flaring overlap time to random-coincidence overlap time for 38 active region pairs. We have then compared the waiting-time distributions for the two different groups of active region pairs (R > 1 and R 1. This is the first time such strong statistical evidence has been found for the existence of sympathetic flares. To examine the role of interconnecting coronal loops, we have also conducted the same analysis for two subgroups of the R > 1 group: one with interconnecting X-ray loops and the other without. We do not find any statistical evidence that the subgroup with interconnecting coronal loops is more likely to produce sympathetic flares than the subgroup without. For the subgroup with loops, we find that sympathetic flares favor active region pairs with transequatorial loops.

CMEs: How do the puzzle pieces fit together?

This review consists of questions to participants in the S-RAMP Symposium (S3) on CMEs and Coronal Holes, as well as to a few others, and their responses in a “town meeting” format (originally conducted on Hugh Hudson's website). Here we deal only with CMEs. The questions we ask aim at probing the weaknesses of existing models and highlighting controversies, thereby providing guidance toward a more complete view of solar eruptions. Topics covered include: the “solar flare myth”, flux ropes, new phenomena (EIT waves, dimmings, global brightenings), helicity and sigmoids, and transequatorial loops (as sources of CMEs). Although this is a review, we're more concerned here with what is not known than what is already agreed upon. We asked people to speculate freely in advance of the observational, analytical, and theoretical work that will provide definitive answers—this is not the standard Scientific Method at work!

Another View of the EIT Wave Phenomenon

This paper investigates three different EIT waves. The events occurred on 1997 November 3 at 10:31 UT, on 1998 January 27 at 22:19 UT, and on 1998 June 13 at 15:23 UT and were observed with EIT on board the Solar and Heliospheric Observatory (SOHO) using the 195 A filter that contains an Fe XII line. The three events show the following similar structures: A flare in an active region. A bright front that can lie at the same location for several hours. A first dimming located between the bright front and the flare. A second dimming located between the bright front and a magnetic dipole located in the opposite hemisphere. A third dimming located in an active region located in the opposite hemisphere. Brightenings or dimmings of transequatorial loops that connect the flaring active region to an active region located in the opposite hemisphere. We propose that the appearance of the above features is strongly related to magnetic field topology. Most of the EIT waves are associated with a coronal mass ejection (CME). Moreover, the presence of a flare is not a necessary condition to produce an EIT wave since only eruptive flares that are associated with CMEs are associated with a wave. Therefore, we suggest that the EIT-wave phenomenon is more closely related to the magnetic field evolution involved in CMEs than to wave propagation driven by solar flares.

Homologous sudden disappearances of transequatorial interconnecting loops in the solar corona

We have found a remarkable sequence of homologous disappearances of transequatorial X‐ray loops linking active regions. Each disappearance was closely associated with a major flare and coronal mass ejection (CME). In each case the flarings precede the disappearances and the CMEs. Mass estimates for the X‐ray loops are similar to CME masses. This, the timing of the disappearances, their morphology, and the homology of the events in the sequence, provide direct evidence for a new class of CME origins in the low corona. We also briefly report observations of features which we infer to be the soft X‐ray counterparts of shock waves emanating from the flare region. The inferred shocks appeared to play a vital role in the disappearances. Our results suggest that flare‐generated shock waves may destabilize large transequatorial loops, causing them to erupt.

Transequatorial Loops in the Solar Corona

Using X-ray coronal images, full-disk longitudinal magnetograms, and vector magnetograms of active regions, we study active regions connected across the solar equator. We survey the Yohkoh data set between 1991 October and 1998 December and find 87 transequatorial loop systems (TLSs). We classify these loops in four different categories and study a separation between regions, their rotation rates, and the sign of the current helicity (chirality) of the magnetic fields. We find that approximately one-third of all active regions on the Sun exhibit transequatorial loops. The fraction of TLSs is solar-cycle independent. Transequatorial loops may develop between existing active regions or between mature regions and new magnetic flux shortly after flux emergence. Observations suggest, however, that formation of TLSs is not a random process—a connection between two areas may exist well before the reconnection takes place. We find that the reconnected regions have approximately the same rotation rate and tend to appear on certain longitudes, similar to the complexes of activity. In most cases transequatorial interconnected regions have the same handedness of their magnetic field.

Cme Associated with Transequatorial Loops and a Bald Patch Flare

We study a flare which occurred on 3 November 1997 at 10:31 UT in the vicinity of a parasitic polarity of AR 8100. Using SOHO/EIT 195 A observations, we identify the brightening of thin transequatorial loops connecting AR 8100 and AR 8102, and dimmings located between the two active regions. Difference images highlight the presence of a loop-like structure rooted near the flare location usually called an EIT wave. The corona] magnetic field derived from potential extrapolations from a SOHO/MDI magnetogram shows that the topology is complex near the parasitic polarity. There, a ‘bald patch’ (defined as the locations where the magnetic field is tangent to the photosphere) is present. We conclude that the flare was a ‘bald patch flare’. Moreover, the extrapolation confirms that there is a large coronal volume filled with transequatorial field lines interconnecting AR 8100 and AR 8102, and overlaying the bald patch.

Long transequatorial interconnecting loops of the new solar cycle

We study two long transequatorial loops connecting high-latitude regions of the new solar cycle. These loops (with lengths of 47 and 61 heliographic degrees) provide evidence that the upper length limit of 37° found by Chase et al. (1976) from Skylab data was determined simply by the typical distances between northern and southern active regions during the period of Skylab observations. We find strong support for the idea that these long interconnecting loops originate through reconnection of field lines extending from the two active regions towards and beyond the equator, and confirm the earlier finding by Canfield, Pevtsov, and McClymont (1996) that only field lines from active regions with the same chirality reconnect. As we are not aware of any longitudinal (E–W) loops of comparable lengths, we suggest that it is mainly the solar differential rotation which drives the reconnection of latitudinal (N–S) field lines.

Evidence for Large-Scale Solar Magnetic Reconnection from Radio and X-Ray Measurements

Utilizing Yohkoh Soft X-ray Telescope and Nancay radioheliograph data, we present, for the first time, observations of expanding twisted X-ray loops and a series of nonthermal radio bursts that follow the loop expansion in time and space up to ∼12′ distance. The loops were produced during a long-duration C4.7 flare close to disk center on 1994 October 25 at 1049 UT. The series of radio bursts were observed on the southern hemisphere above a weak positive-polarity region. The Kitt Peak magnetogram shows the existence of a weak negative-polarity region on the northern hemisphere at the same heliolongitude. Simultaneously with the nonthermal radio bursts, we observed the appearance of two remote X-ray brightenings and subsequent formation of two coronal holes above these weak (quiet) magnetic regions of opposite polarity, which strongly suggest the involvement of these remote regions in the event. During the 6 hr-long gradual phase of the flare, new X-ray loop connections developed among the active region and the remote quiet regions. A nonthermal radio continuum emission originating from the active region was also observed. We propose that the series of radio bursts, two remote X-ray brightenings, and new coronal loop connections were all signatures of a large-scale reconnection process between the expanding twisted flare loops and overlying transequatorial loops connecting quiet-Sun regions. The reconnection was only partial; the external part of the overlying large-scale fields were pushed out in the solar wind by the expanding twisted loops, leading to the formation of the coronal holes. The interaction between the active region and the large-scale fields seemed to be active during the entire gradual phase of the flare. This scenario may also explain the measurement of high-energy electrons in the interplanetary medium from 74° south heliolatitude as observed by Ulysses. © 1996. The American Astronomical Society. All rights reserved.