Motion of sunspot magnetic fields and its relation to solar flares

The magnetic polarity distributions in sunspot groups which produced solar proton flares have been analyzed. It is shown that the fluid motion in sunspot groups and below may be responsible for the origin of inverted or unusual polarity distributions, since rotating motion in these spot groups is often observed. Since such motion seems to produce twisting of magnetic field lines above sunspot groups, the origin of solar flares seems to be closely dependent on instability associated with this twisting of sunspot field lines in the chromosphere and the lower corona.     

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.     

质子活动与太阳黑子群

本文对太阳活动第２１周、２２周（１９７６年—１９９２年间）９７个质子活动区进行统计分析，包括活动区的面积、型别、磁结构、半影纤维等，结果表明：７５％的质子耀斑产生于面积为５００≤Ｓｐ≤３０００单位的黑子群中；耀斑爆发前一天及后一天活动区面积有显著减少；质子活动区含δ复杂磁结构的占７０％；具有半影旋涡形态的质子活动区中，约７７％的耀斑发生在旋涡黑子出现以后。     

Structure and evolution of magnetic fields associated with solar eruptions

This paper reviews the studies of solar photospheric magnetic field evolution in active regions and its relationship to solar flares. It is divided into two topics, the magnetic structure and evolution leading to solar eruptions and rapid changes in the photospheric magnetic field associated with eruptions. For the first topic, we describe the magnetic complexity, new flux emergence, flux cancelation, shear motions, sunspot rotation and magnetic helicity injection, which may all contribute to the storage and buildup of energy that trigger solar eruptions. For the second topic, we concentrate on the observations of rapid and irreversible changes of the photospheric magnetic field associated with flares, and the implication on the restructuring of the three-dimensional magnetic field. In particular, we emphasize the recent advances in observations of the photospheric magnetic field, as state-of-the-art observing facilities(such as Hinode and Solar Dynamics Observatory) have become available. The linkages between observations, theories and future prospectives in this research area are also discussed.     

Energetic electrons associated with solar flares and their relation to type I noise activity

The generation of energetic electrons is always associated with the solar flares which occur within the sunspot groups that are highly active in emitting type I noise storms. The number of the solar flares which are associated with the distinct electron events observed at the earth tends to increase in association with the westward movement of these active groups. This tendency is not contradictory to the close association between electron producing solar flares and type I active regions if we take into account the limited directivity of type I noise storms associated with these sunspot groups.The acceleration of the energetic electrons associated with solar flares seems to be closely related to the type I active regions where the enormous numbers of suprathermal electrons exist and play a role in generating these radio noise storms.NAS-NRC Associate with NASA.     

An essay on sunspots and solar flares

The presently prevailing theories of sunspots and solar flares rely on the hypothetical presence of magnetic flux tubes beneath the photosphere and the two subsequent hypotheses, their emergence above the photosphere and explosive magnetic reconnection, converting magnetic energy carried by the flux tubes for solar flare energy.In this paper, we pay attention to the fact that there are large-scale magnetic fields which divide the photosphere into positive and negative (line-of-sight) polarity regions and that they are likely to be more fundamental than sunspot fields, as emphasized most recently by McIntosh (1981). A new phenomenological model of the sunspot pair formation is then constructed by considering an amplification process of these largescale fields near their boundaries by shear flows, including localized vortex motions. The amplification results from a dynamo process associated with such vortex flows and the associated convergence flow in the largescale fields.This dynamo process generates also some of the familiar “force-free” fields or the “sheared” magnetic fields in which the magnetic field-aligned currents are essential. Upward field-aligned currents generated by the dynamo process are carried by downward streaming electrons which are expected to be accelerated by an electric potential structure; a similar structure is responsible for accelerating auroral electrons in the magnetosphere. Depending on the magnetic field configuration and the shear flows, the current-carrying electrons precipitate into different geometrical patterns, causing circular flares, umbral flares, two-ribbon flares, etc. Thus, it is suggested that “low temperature flares” are directly driven by the photospheric dynamo process.     

Sunspot and Auroral Activity During Maunder Minimum

The extremely low sunspot activity during the period of the Maunder minimum 1645–1715 was confirmed by group sunspot numbers, a new sunspot index constructed by Hoyt and Schatten (1998a,b). Neither sunspots nor auroral data time behavior indicate the presence of 11-year solar cycles as stated by Eddy (1976). The evidence for solar cycles was found in the butterfly diagram, constructed from observations made at Observatoire de Paris. After Clivier, Boriakoff, and Bounar (1998) the solar cycles were reflected also in geomagnetic activity. Results are supported by the variation of cosmogenic isotopes ¹⁰Be and ¹⁴C. The majority of the observed 14 naked-eye sunspots occurred on days when telescopic observations were not available. A part of them appeared in the years when no spot was allegedly observed. Two-ribbon flares appear in plages with only very small or no sunspots. Some of these flares are geoactive. Most aurorae (90%), which were observed during the Maunder minimum, appeared in years when no spot was observed. Auroral events as a consequence of proton flares indicate that regions with enhanced magnetic field can occur on the Sun when these regions do not produce any sunspots.     

Longitudinal distribution of PCA sources on the sun

For 181 PCA's recorded during the years 1956–1969 the association with flares is studied. Both the number of events which cannot be associated with any flare on the visible hemisphere, as well as the longitude distribution of identified proton flares, lead to the conclusion that 25–30% of PCA's are caused by flares behind the western solar limb. PCA's of this kind are mostly small. During the investigated years no PCA > 13 dB and possibly no PCA > 8.5 dB were caused by flares behind the limb, while hardly 60% of PCA's < l dB had their origin on the visible hemisphere. While the sources of GLE's and of PCA's in general, are centered around 50°W which corresponds to the average curvature of the magnetic field lines in interplanetary space, the strongest PCA's (> 8.5 dB) show an anomalous longitude distribution centered around ∼ 20°W. It is suggested that this anomaly may be a consequence of the fact that in strong PCA events the kinetic energy density of protons below 100 MeV becomes comparable to the magnetic energy density in space, thus leading to a ‘straightening’ of the magnetic field lines.     

The Clustering Properties of Active Regions During the First Part of Solar Cycle 23

In this paper we study the longitudinal distribution of solar magnetic regions, using the synoptic magnetic maps from Kitt Peak National Observatory, the active region data from Solar Geophysical Data and the Hobservations from Prairie View Solar Observatory. The clusters of activity were identified by comparing the positions of sunspot groups between successive Carrington rotations. We have found that a large percentage of active regions was involved in the clustering process (40–50%, if we only take into account clusters with a minimum lifetime of 4 rotations). The nests followed the differential rotation of the solar surface, within an intrinsic spread. A remarkable feature of sunspot nests detected in our study is their high degree of complexity, with a large number of nests being organized in diverging, converging, or parallel structures. Of the flares which occurred during the time interval of interest, the great majority originated from the sunspot nests; the distribution of the flares between these nests was not uniform, revealing active and quiet nests. A high flaring rate was recorded at the intersection points of diverging or converging nests, suggesting that these points represent violent interactions of magnetic fluxes. The complexes were in continuous interaction, which impacts their properties and future evolution. The behavior of the nests indicate that they are maintained by repeated injection of magnetic flux rather than by the evolution of the surface magnetic fields.     

SID flares and sunspot morphology

The degree of association between geoeffective (SID producing) flares (hereafter called SID flares) and sunspot morphology is examined. It is found that: (1) the frequency of SID flares associated with sunspot groups is linear function of sunspot area and rate of change in area; (2) the SID flare intensity is dependent on the sunspot area and on the magnetic morphology (field geometry); (3) the probability of a sunspot group being magnetically complex (henceforth called complex ratio) is a linear function of spot area, the larger this area the more likely a group is in the βγ or δ magnetic class; (4) the complex ratio exhibits the greatest degree of association to SID flare frequency. We conclude from these results that a higher frequency of D-region ionizing flares (emitting a soft X-ray flux >2 × 10^?3 erg cm^?2 s^?1) is likely to accompany the disk transit of large area, complex spot groups. This combination of morphological factors reflects a shearing of the associated force-free magnetic field, with accumulation of free magnetic energy to power SID flares. Mutual polarity intrusion would be one observational signature of the pre-flare energy storing process.     

Testing and Improving a Set of Morphological Predictors of Flaring Activity

Efficient prediction of solar flares relies on parameters that quantify the eruptive capability of solar active regions. Several such quantitative predictors have been proposed in the literature, inferred mostly from photospheric magnetograms and/or white-light observations. Two of them are the Ising energy and the sum of the total horizontal magnetic field gradient. The former has been developed from line-of-sight magnetograms, while the latter uses sunspot detections and characteristics, based on continuum images. Aiming to include these parameters in an automated prediction scheme, we test their applicability on regular photospheric magnetic field observations provided by the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). We test their efficiency as predictors of flaring activity on a representative sample of active regions and investigate possible modifications of these quantities. The Ising energy appears to be an efficient predictor, and the efficiency is even improved if it is modified to describe interacting magnetic partitions or sunspot umbrae. The sum of the horizontal magnetic field gradient appears to be slightly more promising than the three variations of the Ising energy we implement in this article. The new predictors are also compared with two very promising predictors: the effective connected magnetic field strength and the total unsigned non-neutralized current. Our analysis shows that the efficiency of morphological predictors depends on projection effects in a nontrivial way. All four new predictors are found useful for inclusion in an automated flare forecasting facility, such as the Flare Likelihood and Region Eruption Forecasting (FLARECAST), but their utility, among others, will ultimately be determined by the validation effort underway in the framework of the FLARECAST project.     

Large-Scale Structure of the Sunspot Activity and the Background Magnetic Field Formation

Large-scale distribution of the sunspot activity of the Sun has been analyzed by using a technique worked out previously (Erofeev, 1997) to study long-lived, non-axisymmetric magnetic structures with different periods of rotation. Results of the analysis have been compared with those obtained by analyzing both the solar large-scale magnetic field and large-scale magnetic field simulated by means of the well-known flux transport equation using the sunspot groups as a sole source of new magnetic flux in the photosphere. A 21-year period (1964–1985) has been examined.The rotation spectra calculated for the total time interval of two 11-year cycles indicate that sunspot activity consists of a series of discrete components (modes) with different periods of rotation. The largest-scale component of the sunspot activity reveals modes with 27-day and 28-day periods of rotation situated, correspondingly, in the northern and southern hemispheres of the Sun, and two modes with rotation periods of about 29.7 days situated in both hemispheres. Such a modal structure of the sunspot activity agrees well with that of the large-scale solar magnetic field. Moreover, the magnetic field distribution simulated with the flux transport equation also reveals the same modal structure. However, such an agreement between the large-scale solar magnetic field and both the sunspot activity and simulated magnetic field is unstable in time; so, it is absent in the northern hemisphere of the Sun during solar cycle No. 20. Thus the sources of magnetic flux responsible for formation of the large-scale, rigidly rotating magnetic patterns appear to be closely connected, but are not identical with the discrete modes of the sunspot activity.     

Relationship between magnetic field evolution and flaring sites in AR 6659 in June 1991

During the international campaign of June 1991, the active region AR 6659 produced six very large, long-duration flares (X10/12) during its passage across the solar disk. We present the characteristics of four of them (June 4, 6, 9, 15). Precise measurements of the spot motions from Debrecen and Tokyo white-light pictures are used to understand the fragmentation of the main sunspot group with time. This fragmentation leads to a continuous restructuring of the magnetic field pattern while rapid changes are evidenced due to fast new flux emergence (magnetograms of MFSC, Huairou). The first process leads to a shearing of the field lines along which there is energy storage; the second one is the trigger which causes the release of energy by creating a complex topology. We conjecture that these two processes with different time scales are relevant to the production of flares.     

A Statistical Study of Rapid Sunspot Structure Change Associated with Flares

We reported recently some rapid changes of sunspot structure in white-light(WL) associated with major flares.We extend the study to smaller events and present here results of a statistical study of this phenomenon.In total,we investigate 403 events from 1998 May 9 to 2004 July 17,including 40 X-class,174 M-class,and 189 C-class flares.By monitoring the structure of the flaring active regions using the WL observations from the Transition Region and Coronal Explorer(TRACE),we find that segments in the outer sunspot structure decayed rapidly right after many flares;and that,on the other hand,the central part of sunspots near the flare-associated magnetic neutral line became darkened.These rapid and permanent changes are evidenced in the time profiles of WL mean intensity and are not likely resulted from the flare emissions.Our study further shows that the outer sunspot structure decay as well as the central structure darkening are more likely to be detected in larger solar flares.For X-class flares,over 40% events show distinct sunspot structure change.For M-and C-class flares,this percentage drops to 17% and 10%,respectively.The results of this statistical study support our previously proposed reconnection picture,i.e.,the flare-related magnetic fields evolve from a highly inclined to a more vertical configuration.     

Energetic particles from the sun

This paper discusses solar cosmic ray phenomena and related topics from the solar physical point of view. Basic physics of the solar atmosphere and solar flare phenomena are, therefore, considered in some detail. Since solar cosmic rays are usually produced by solar flares, we must first understand the processes and mechanism of solar flares, especially the so-called proton flares, in order to understand the acceleration mechanism of solar cosmic rays and their behaviour in both the solar atmosphere and interplanetary space. For this reason, detailed discussion is given on various phenomena associated with solar flares, proton flare characteristics, and the mechanism of solar flares.Since the discovery of solar cosmic rays by Forbush, the interplanetary space has been thought of as medium in which solar cosmic rays propagate. In this paper, the propagation of solar cosmic rays in this space is, therefore, discussed briefly by referring to the observed magnetic properties of this space. Finally, some problems related to the physics of galactic cosmic rays are discussed.Astrophysics and Space Science Review Paper.     

Solar Flare Occurrence Rate and Probability in Terms of the Sunspot Classification Supplemented with Sunspot Area and Its Changes

We investigate the solar flare occurrence rate and daily flare probability in terms of the sunspot classification supplemented with sunspot area and its changes. For this we use the NOAA active region data and GOES solar flare data for 15 years (from January 1996 to December 2010). We consider the most flare-productive 11 sunspot classes in the McIntosh sunspot group classification. Sunspot area and its changes can be a proxy of magnetic flux and its emergence/cancellation, respectively. We classify each sunspot group into two sub-groups by its area: ??Large?? and ??Small??. In addition, for each group, we classify it into three sub-groups according to sunspot area changes: ??Decrease??, ??Steady??, and ??Increase??. As a result, in the case of compact groups, their flare occurrence rates and daily flare probabilities noticeably increase with sunspot group area. We also find that the flare occurrence rates and daily flare probabilities for the ??Increase?? sub-groups are noticeably higher than those for the other sub-groups. In case of the (M+X)-class flares in the ??Dkc?? group, the flare occurrence rate of the ??Increase?? sub-group is three times higher than that of the ??Steady?? sub-group. The mean flare occurrence rates and flare probabilities for all sunspot groups increase with the following order: ??Decrease??, ??Steady??, and ??Increase??. Our results statistically demonstrate that magnetic flux and its emergence enhance the occurrence of major solar flares.     

Solar neutrino in relation to solar activity

Here we have carried out a power-spectrum analysis of solar nuclear gamma-ray (NGR) flares observed by SMM and HINOTORI satellites. The solar NGR flares show a periodicity of 152 days, confirming the existence of a 152–158 days periodicity in the occurrence of solar activity phenomena and also indicating that the NGR flares are a separate class of solar flares. The power-spectrum analysis of the daily sunspot areas on the Sun for the period 1980–1982 shows a peak around 159 days while sunspot number data do not show any periodicity (Verma and Joshi, 1987). Therefore, only sunspot area data should be treated as an indicator of solar activity and not the daily sunspot number data.     

Sunspots with the Strongest Magnetic Fields

The strongest observed solar magnetic fields are found in sunspot umbrae and associated light bridges. We investigate systematic measurements of approximately 32 000 sunspot groups observed from 1917 through 2004 using data from Mt. Wilson, Potsdam, Rome and Crimea observatories. Isolated observations from other observatories are also included. Corrections to Mt. Wilson measurements are required and applied. We found 55 groups (0.2%) with at least one sunspot with one magnetic field measurement of at least 4000 G including five measurements of at least 5000 G and one spot with a record field of 6100 G. Although typical strong-field spots are large and show complex structure in white light, others are simple in form. Sometimes the strongest fields are in light bridges that separate opposite polarity umbras. The distribution of strongest measured fields above 3 kG appears to be continuous, following a steep power law with exponent about −9.5. The observed upper limit of 5 – 6 kG is consistent with the idea that an umbral field has a more or less coherent structure down to some depth and then fragments. We find that odd-numbered sunspot cycles usually contain about 30% more total sunspot groups but 60% fewer >3 kG spots than preceding even-numbered cycles.     

On the association of predominant polarity of North-South component of IMF in the GSE system near 1 AU with solar polar magnetic fields during 1967–2020

The skewness of the monthly distribution of GSE latitudinal angles of Interplanetary Magnetic Field (IMF) observed near the Earth (Sk) is found to show anti-correlation with sunspot activity during the solar cycles 20–24. Sk can be considered as a measure of the predominant polarity of north-south component of IMF (B_z component) in the GSE system near 1 AU. Sk variations follow the magnitude of solar polar magnetic fields in general and polarity of south polar fields in particular during the years 1967–2020. Predominant polarity of Sk is found to be independent of the heliographic latitude of Earth. Sk basically reflects the variations of the solar dipolar magnetic field during a sunspot cycle. It is also found that IMF sector polarity variation is not a good indicator of the magnitude changes in solar polar magnetic fields during a sunspot cycle. This is possibly due to the influence of non-dipolar components of the solar magnetic field and the associated north-south asymmetries in the heliospheric current sheet.     

Proton activity of the Sun in current solar cycle 24

We present a study of seven large solar proton events in the current solar cycle 24(from 2009 January up to the current date). They were recorded by the GOES spacecraft with the highest proton fluxes being over 200 pfu for energies 10 Me V. In situ particle measurements show that:(1) The profiles of the proton fluxes are highly dependent on the locations of their solar sources, namely flares or coronal mass ejections(CMEs), which confirms the "heliolongitude rules" associated with solar energetic particle fluxes;(2) The solar particle release(SPR) times fall in the decay phase of the flare emission, and are in accordance with the times when the CMEs travel to an average height of 7.9 solar radii; and(3) The time differences between the SPR and the flare peak are also dependent on the locations of the solar active regions. The results tend to support the scenario of proton acceleration by the CME-driven shock,even though there exists a possibility of particle acceleration at the flare site, with subsequent perpendicular diffusion of accelerated particles in the interplanetary magnetic field. We derive the integral time-of-maximum spectra of solar protons in two forms: a single power-law distribution and a power law roll-over with an exponential tail. It is found that the unique ground level enhancement that occurred in the event on 2012 May 17 displays the hardest spectrum and the largest roll-over energy which may explain why this event could extend to relativistic energies.