Sunspot Rotation Research Articles

High-Resolution Observation of Blowout Jets Regulated by Sunspot Rotation

High-Resolution Observation of Blowout Jets Regulated by Sunspot Rotation

Temporal Behaviors of Magnetic Helicity Injections by Self and Mutual Sunspot Rotations

Magnetic helicity is a physical parameter used to quantify the complexity of magnetic fields, providing an indication of the energy state in the coronal magnetic structure. We investigate the temporal evolution of magnetic helicity and its relationship to the occurrence of a variety of flares in the solar active region NOAA 12297, which was well observed using the Solar Dynamics Observatory/Helioseismic and Magnetic Imager in 2015 March. The active region produced many M-class flares and an X-class flare in two distinctive areas, both of which had a similar magnetic evolution, i.e., the opposite polarity of an emerging flux developed beside a preexisting sunspot, but exhibited flares with different magnitudes and frequencies. We derived the spatiotemporal evolution of the magnetic helicity injections and evaluated how spinning and braiding helicity injections evolved with time in the two areas. In one area, we observed a remarkable evolution, in which a negative spinning helicity injection in the preexisting sunspot increased in a positive helicity system, followed by the occurrence of the X-class flare. The negative helicity injection was clearly caused by the flux emergence that developed along the outer edge of the preexisting sunspot. The other area showed positive braiding helicity injections, with spinning helicity injections fluctuating concurrently with flux emergence, changing their signs several times, i.e., variable energy, and helicity input. The observed temporal behaviors of the helicity injections may explain different types of flare occurrences in the regions.

MHD Simulation of Homologous Eruptions from Solar Active Region 10930 Caused by Sunspot Rotation

The relationship between solar eruption and sunspot rotation has been widely reported, and the underlying mechanism needs to be studied. Here we performed a full 3D MHD simulation using a data-constrained approach to study the mechanism of flare eruptions in active region (AR) NOAA 10930, which is characterized by continuous sunspot rotation and homologous eruptions. We reconstructed the potential magnetic field from the magnetogram of Hinode/SOT as the initial condition and drove the MHD system by applying continuous sunspot rotation at the bottom boundary. The key magnetic structure before the major eruptions and the preformed current sheet were derived, which is responsible for the complex MHD evolution with multiple stages. The major eruptions were triggered directly by fast reconnection in the preformed current sheet above the main polarity inversion line between the two major magnetic polarities of the AR. Furthermore, our simulation shows the homologous eruption successfully. It has reasonable consistency with observations in relative strength, energy release, X-ray and Hα features, and time interval of eruptions. In addition, the rotation angle of the sunspot before the first eruption in the simulation is also close to the observed value. Our simulation offers a scenario different from many previous studies based on ideal instabilities of a twisted magnetic flux rope and shows the importance of sunspot rotation and magnetic reconnection in efficiently producing homologous eruptions by continuous energy injection and impulsive energy release in a recurrent way.

Sunspot Rotation in High- and Low-Flaring Active Regions

Sunspot rotations are closely linked with flaring activity. They are thought to contribute to the accumulation of helicity in magnetic flux tubes and to triggering magnetic reconnection in large solar flares. This link to solar flares has led to sunspot rotations being used as a parameter in solar flare prediction methods, but analysis for long-period observations of rotations in the literature is scarce. In this study, the rotation profiles of sunspots in a selection of six active regions are studied over time periods of 5 – 10 days to measure how sunspot rotation varies as active regions develop. The active regions are divided into two categories: high-flaring groups, which produced at least one X-class flare, and low-flaring regions that had little flaring activity. Comparison of the rotation profiles in these regions showed that young complex sunspot groups exhibit faster angular velocities and more frequent changes in rotation than older single-spot groups and, although the most rotating groups were also the most flare-productive, sudden changes in rotation were found to not definitively indicate an imminent eruption.

A Double-decker Filament Formation Driven by Sunspot Rotation and Magnetic Reconnection

In this paper, through analyzing data from the Solar Dynamics Observatory (SDO) and the Global Oscillation Network Group (GONG), we present a study on the formation of a double-decker filament in NOAA Active Region 12665 from 2017 July 8 to 14. We find that magnetic reconnection occurs between two smaller filaments to form a longer filament. According to the evolution of the leading sunspot, it is obvious that the sunspot experiences a continuous rotation around its umbra. During the period from 03:00 UT on July 11 to 10:00 UT on July 14, the average speed of sunspot rotation is about 3.°7 hr–1. The continuous rotation of sunspot stretches the filament and results in the formation of a reversed S-shaped filament. Due to the motion of the magnetic field and internal magnetic reconnection, the filament splits into two branches and forms a double-decker filament structure. In the process of filament separation, internal magnetic reconnection can also accelerate the filament separation. Nonlinear force-free field extrapolation indicates that there are two magnetic flux ropes, which are consistent with the observed results. Eventually, the upper filament erupts and produces an M-class flare and a halo coronal mass ejection.

Understanding the Initiation of the M2.4 Flare on 2017 July 14

Abstract We present both the observation and the magnetohydrodynamics (MHD) simulation of the M2.4 flare (SOL2017-07-14T02:09) of NOAA active region (AR) 12665 with a goal to identify its initiation mechanism. The observation by the Atmospheric Image Assembly (AIA) on board the Solar Dynamics Observatory (SDO) shows that the major topology of the AR is a sigmoidal configuration associated with a filament/flux rope. A persistent emerging magnetic flux and the rotation of the sunspot in the core region were observed with Magnetic Imager (HMI) on board the SDO on the timescale of hours before and during the flare, which may provide free magnetic energy needed for the flare/coronal mass ejection (CME). A high-lying coronal loop is seen moving outward in AIA EUV passbands, which is immediately followed by the impulsive phase of the flare. We perform an MHD simulation using the potential magnetic field extrapolated from the measured pre-flare photospheric magnetic field as initial conditions and adopting the observed sunspot rotation and flux emergence as the driving boundary conditions. In our simulation, a sigmoidal magnetic structure and an overlying magnetic flux rope (MFR) form as a response to the imposed sunspot rotation, and the MFR rises to erupt like a CME. These simulation results in good agreement with the observation suggest that the formation of the MFR due to the sunspot rotation and the resulting torus and kink instabilities were essential to the initiation of this flare and the associated coronal mass ejection.

A Semi-Automatic Method to Measure the Rotation of Sunspots

Sunspots have been observed to undergo rotation about their umbral centre. This is typically a slow rotation, with even the fastest sunspot rotations only reaching angular velocities of a few degrees per hour. This rotation may inject magnetic energy into the Sun’s atmosphere, which can be stored in the coronal magnetic field and later released in eruptive events such as solar flares and coronal mass ejections. To usefully investigate rotating sunspots long periods of data need to be analysed, often of the order of several days, to build up a bulk rotation profile for the sunspot over time. This article outlines a semi-automated approach for analysing series of solar continuum data to extract the rotation profile of a sunspot as it transits across the solar disc. Moving towards an automated approach is vital for generating large, unbiased statistical samples of rotating sunspots in order to understand their contribution to solar activity. Existing methods typically focus on sunspots near disc centre for short time periods, neglecting much of the rotation history of the sunspot. The method is tested on six sunspots observed in continuum data from the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO). These have been chosen to test the method for a range of different types of sunspots, including well-behaved sunspots, shape-changing sunspots, fast rotators, non-rotators, and interacting sunspots. The rotation profiles are compared by eye to animations of the sunspot from the data and are in acceptable visual agreement with the observed bulk rotation of the sunspot for all of the cases, except for the one which contains two sunspots in a shared penumbra. The method is also tested against sunspot rotations in active region (AR) 11158 that have been reported in the literature. While the results compare to some degree, the method outlined in this article reports lower rotations than those reported in the literature. Some of this discrepancy can be attributed to selection bias by the approaches in the literature, where only features that undergo larger rotation are tracked in sunspots that exhibit non-uniform rotation. The method also provides uncertainties on the calculated rotation profile which can be broken down to allow the principal sources of error to be identified. For the test sunspots in this article, the dominant source of uncertainty is the resolution of the SDO/HMI instrument.

Structure and Evolution of an Inter–Active Region Large-scale Magnetic Flux Rope

Abstract Magnetic flux rope (MFR) has been recognized as the key magnetic configuration of solar eruptions. While pre-eruption MFRs within the core of solar active regions (ARs) have been widely studied, those existing between two ARs, i.e., the intermediate ones in weak-field regions, were rarely studied. There are also major eruptions that occurred in such intermediate regions and study of the MFR there will help us understand the physics mechanism underlying the eruptions. Here, with a nonlinear force-free field reconstruction of solar coronal magnetic fields, we tracked the five-day evolution covering the full life of a large-scale inter-AR MFR forming between ARs NOAA 11943 and 11944, which is closely cospatial with a long sigmoidal filament channel and an eruptive X1.2 flare occurring on 2014 January 7. Through topological analysis of the reconstructed 3D magnetic field, it is found that the MFR begins to form early on 2014 January 6; then with its magnetic twist degree continuously increasing for over 30 hr, it becomes highly twisted with field lines winding numbers approaching six turns, which might be the highest twisting degree in extrapolated MFRs that have been reported in the literature. The formation and strength of the MFR are attributed to a continuous sunspot rotation of AR 11944 and flux cancellation between the two ARs. The MFR and its associated filaments exhibit no significant change across the flare time, indicating it is not responsible for the flare eruption. After the flare, the MFR slowly disappears, possibly due to the disturbance by the eruption.

Hard X-Ray Emission from an Activated Flux Rope and Subsequent Evolution of an Eruptive Long-duration Solar Flare

Abstract In this paper, we present a comprehensive study of the evolutionary phases of a major M6.6 long duration event with special emphasize on its pre-flare phase. The event occurred in NOAA 12371 on 2015 June 22. A remarkable aspect of the event was an active pre-flare phase lasting for about an hour during which a hot EUV coronal channel was in the build-up stage and displayed cospatial hard X-ray (HXR) emission up to energies of 25 keV. This is the first evidence of the HXR coronal channel. The coronal magnetic field configuration based on nonlinear-force-free-field modeling clearly exhibited a magnetic flux rope (MFR) oriented along the polarity inversion line (PIL) and cospatial with the coronal channel. We observed significant changes in the AR’s photospheric magnetic field during an extended period of ≈42 hr in the form of rotation of sunspots, moving magnetic features, and flux cancellation along the PIL. Prior to the flare onset, the MFR underwent a slow rise phase (≈14 km s−1) for ≈12 minutes, which we attribute to the faster build-up and activation of the MFR by tether-cutting reconnection occurring at multiple locations along the MFR itself. The sudden transition in the kinematic evolution of the MFR from the phase of slow to fast rise (≈109 km s−1 with acceleration ≈110 m s−2) precisely divides the pre-flare and impulsive phase of the flare, which points toward the feedback process between the early dynamics of the eruption and the strength of the flare magnetic reconnection.

The proper motion of sunspots umbra in the rising phase of Cycle 24

The differential rotation is the difference in the rate of rotation of each latitude in the Sun indicating that the Sun is not a solid body. The differential rotation is the result of the interaction between rotation and convection and causes dynamo circulation that affects the cycle of solar activity. In this research, we measured the coordinates of 304 sunspots umbra as tracer in the rising phase of the Solar Cycle 24 to obtain the differential rotation equation. Coordinates of sunspots were measured using AIA images at a wavelength of 4500 Å from SDO (Solar Dynamic Observatory) with JHelioviewer software. The areas of sunspots were measured using HMI (Helioseismic Magnetic Imager) images with ImageJ software. From the measurement, we derived the differential rotation equation and the relation of sidereal rotation and area of sunspots umbra. The differential rotation equation obtained in this research is ω(B) = (14.27 ± 0.01) - (0.78 ± 0.10) sin2B (°/day). There is a difference between the differential rotation equation of the northern and southern hemispheres that indicates the asymmetry between different hemispheres. We got the sidereal rotation of sunspots with area < 5 MH is 0.70% higher than sunspots with area > 15 MH.

Identification of Pre-flare Processes and Their Possible Role in Driving a Large-scale Flux Rope Eruption with Complex M-class Flare in the Active Region NOAA 12371

In this article, we study the origin of precursor flare activity and investigate its role towards triggering the eruption of a flux rope which resulted into a dual-peak M-class flare (SOL2015-06-21T02:36) in the active region NOAA 12371. The flare evolved in two distinct phases with peak flux levels of M2.1 and M2.6 at an interval of $\approx$54 min. The active region exhibited striking moving magnetic features (MMFs) along with sunspot rotation. Non-linear force free field (NLFFF) modelling of the active region corona reveals a magnetic flux rope along the polarity inversion line in the trailing sunspot group which is observationally manifested by the co-spatial structures of an active region filament and a hot channel identified in the 304 and 94 \AA\ images, respectively, from the Atmospheric Imaging Assembly (AIA). The active region underwent a prolonged phase of flux enhancement followed by a relatively shorter period of flux cancellation prior to the onset of the flare which led to the build up and activation of the flux rope. Extreme ultra-violet (EUV) images reveal localised and structured pre-flare emission, from the region of MMFs, adjacent to the location of the main flare. Our analysis reveals strong, localised regions of photospheric currents of opposite polarities at the precursor location, thereby making the region susceptible to small-scale magnetic reconnection. Precursor reconnection activity from this location most likely induced a slipping reconnetion towards the northern leg of the hot channel which led to the destabilization of the flux rope. The application of magnetic virial theorem suggests that there was an overall growth of magnetic free energy in the active region during the prolonged pre-flare phase which decayed rapidly after the hot channel eruption and its successful transformation into a halo coronal mass ejection (CME).

Trigger mechanisms of the major solar flares

AbstractSolar flares, suddenly releasing a large amount of magnetic energy, are one of the most energetic phenomena on the Sun. For the major flares (M- and X-class flares), there exist strong-gradient polarity-inversion lines in the pre-flare photospheric magnetograms. Some parameters (e.g., electric current, shear angle, free energy) are used to measure the magnetic non-potentiality of active regions, and the kernels of major flares coincide with the highly non-potential regions. Magnetic flux emergence and cancellation, shearing motion, and sunspot rotation observed in the photosphere are deemed to play an important role in the energy buildup and flare trigger. Solar active region 12673 produced many major flares, among which the X9.3 flare is the largest one in solar cycle 24. According to the newly proposed block-induced eruption model, the block-induced complex structures built the flare-productive active region and the X9.3 flare was triggered by an erupting filament due to the kink instability.

Evolution of Photospheric Vector Magnetic Field Associated with Moving Flare Ribbons as Seen by GST

Abstract The photospheric response to solar flares, also known as coronal back reaction, is often observed as sudden flare-induced changes in the vector magnetic field and sunspot motions. However, it remains obscure whether evolving flare ribbons, the flare signature closest to the photosphere, are accompanied by changes in vector magnetic field therein. Here we explore the relationship between the dynamics of flare ribbons in the chromosphere and variations of magnetic fields in the underlying photosphere, using high-resolution off-band Hα images and near-infrared vector magnetograms of the M6.5 flare on 2015 June 22 observed with the 1.6 m Goode Solar Telescope. We find that changes of photospheric fields occur at the arrival of the flare ribbon front, thus propagating analogously to flare ribbons. In general, the horizontal field increases and the field lines become more inclined to the surface. When ribbons sweep through regions that undergo a rotational motion, the fields transiently become more vertical with decreased horizontal field and inclination angle, and then restore and/or become more horizontal than before the ribbon arrival. The ribbon propagation decelerates near the sunspot rotation center, where the vertical field becomes permanently enhanced. Similar magnetic field changes are discernible in magnetograms from the Helioseismic and Magnetic Imager (HMI), and an inward collapse of coronal magnetic fields is inferred from the time sequence of nonlinear force-free field models extrapolated from HMI magnetograms. We conclude that photospheric fields respond nearly instantaneously to magnetic reconnection in the corona.

Energy Build-up and Triggering Leading to a M1.5 Flare on 1 August 2014

The energy storage and trigger mechanisms of solar flares are important for understanding of solar activity. We analyzed multi-wavelength observations of a M1.5 flare on 1 August 2014, in active region NOAA 12127 (SOL2014-08-01T18:13). There are evident large scale sunspot rotations in positive magnetic field of the main energy release region before the eruption; the rotations contain both clockwise and counter-clockwise directions. Injection of magnetic helicity from the photosphere prior to the flare. The sign of the helicity injection is reversed after the flare. It is found that both persistent larger scale ($\approx$ one day) and impulse smaller scale ($\approx$ one to two hours) magnetic-flux emergences are associated with the flare. We conclude that larger-scale flux emergence, helicity injection and sunspot rotation contribute to the energy build up, while the small-scale magnetic-flux emergence plays crucial role in triggering the flare.

The Formation of a Sunspot Penumbra Sector in Active Region NOAA 12574

Abstract We present a particular case of the formation of a penumbra sector around a developing sunspot in the active region NOAA 12574 on 2016 August 11 by using the high-resolution data observed by the New Solar Telescope at the Big Bear Solar Observatory and the data acquired by the Helioseismic and Magnetic Imager and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory satellite. Before the new penumbra sector formed, the developing sunspot already had two umbrae with some penumbral filaments. The penumbra sector gradually formed at the junction of two umbrae. We found that the formation of the penumbra sector can be divided into two stages. First, during the initial stage of penumbral formation, the region where the penumbra sector formed always appeared blueshifted in a Dopplergram. The area, mean transverse magnetic field strength, and total magnetic flux of the umbra and penumbra sector all increased with time. The initial penumbral formation was associated with magnetic emergence. Second, when the penumbra sector appeared, the magnetic flux and area of the penumbra sector increased after the umbra’s magnetic flux and area decreased. These results indicate that the umbra provided magnetic flux for penumbral development after the penumbra sector appeared. We also found that the newly formed penumbra sector was associated with sunspot rotation. Based on these findings, we suggest that the penumbra sector was the result of the emerging flux that was trapped in the photosphere at the initial stage of penumbral formation, and when the rudimentary penumbra formed, the penumbra sector developed at the cost of the umbra.

Successive X-class Flares and Coronal Mass Ejections Driven by Shearing Motion and Sunspot Rotation in Active Region NOAA 12673

Abstract We present a clear case study on the occurrence of two successive X-class flares, including a decade-class flare (X9.3) and two coronal mass ejections (CMEs) triggered by shearing motion and sunspot rotation in active region NOAA 12673 on 2017 September 6. A shearing motion between the main sunspots with opposite polarities began on September 5 and lasted even after the second X-class flare on September 6. Moreover, the main sunspot with negative polarity rotated around its umbral center, and another main sunspot with positive polarity also exhibited a slow rotation. The sunspot with negative polarity at the northwest of the active region also began to rotate counterclockwise before the onset of the first X-class flare, which is related to the formation of the second S-shaped structure. The successive formation and eruption of two S-shaped structures were closely related to the counterclockwise rotation of the three sunspots. The existence of a flux rope is found prior to the onset of two flares by using nonlinear force-free field extrapolation based on the vector magnetograms observed by Solar Dynamics Observatory/Helioseismic and Magnetic Image. The first flux rope corresponds to the first S-shaped structures mentioned above. The second S-shaped structure was formed after the eruption of the first flux rope. These results suggest that a shearing motion and sunspot rotation play an important role in the buildup of the free energy and the formation of flux ropes in the corona that produces solar flares and CMEs.

Probing Subsurface Flows in NOAA Active Region 12192: Comparison with NOAA 10486

Abstract NOAA Active Region (AR) 12192 is the biggest AR observed in solar cycle 24 so far. This was a long-lived AR that survived for four Carrington rotations (CRs) and exhibited several unusual phenomena. We measure the horizontal subsurface flows in this AR in multiple rotations using the ring-diagram technique of local helioseismology and the Global Oscillation Network Group (GONG+) Dopplergrams, and we investigate how different was the plasma flow in AR 12192 from that in AR 10486. Both regions produced several high M- and X-class flares, but they had different coronal mass ejection (CME) productivity. Our analysis suggests that these ARs had unusually large horizontal flow amplitude with distinctly different directions. While meridional flow in AR 12192 was poleward that supports the flux transport to poles, it was equatorward in AR 10486. Furthermore, there was a sudden increase in the magnitude of estimated zonal flow in shallow layers in AR 12192 during the X3.1 flare; however, it reversed direction in AR 10486 with the X17.2 flare. These flow patterns produced strong twists in horizontal velocity with depth in AR 10486 that persisted throughout the disk passage, as opposed to AR 12192, which produced a twist only after the eruption of the X3.1 flare that disappeared soon after. Our study indicates that the sunspot rotation combined with the reorganization of magnetic field in AR 10486 was not sufficient to decrease the flow energy even after several large flares that might have triggered CMEs. Furthermore, in the absence of sunspot rotation in AR 12192, this reorganization of magnetic field contributed significantly to the substantial release of flow energy after the X3.1 flare.

The Eruption of a Small-scale Emerging Flux Rope as the Driver of an M-class Flare and of a Coronal Mass Ejection

Abstract Solar flares and coronal mass ejections are the most powerful explosions in the Sun. They are major sources of potentially destructive space weather conditions. However, the possible causes of their initiation remain controversial. Using high-resolution data observed by the New Solar Telescope of Big Bear Solar Observaotry, supplemented by Solar Dynamics Observatory observations, we present unusual observations of a small-scale emerging flux rope near a large sunspot, whose eruption produced an M-class flare and a coronal mass ejection. The presence of the small-scale flux rope was indicated by static nonlinear force-free field extrapolation as well as data-driven magnetohydrodynamics modeling of the dynamic evolution of the coronal three-dimensional magnetic field. During the emergence of the flux rope, rotation of satellite sunspots at the footpoints of the flux rope was observed. Meanwhile, the Lorentz force, magnetic energy, vertical current, and transverse fields were increasing during this phase. The free energy from the magnetic flux emergence and twisting magnetic fields is sufficient to power the M-class flare. These observations present, for the first time, the complete process, from the emergence of the small-scale flux rope, to the production of solar eruptions.

Sunspot rotation and magnetic transients associated with flares in NOAA AR 11429

We analyze sunspot rotation and magnetic transients in NOAA AR 11429 during two X-class (X5.4 and X1.3) flares using data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. A large leading sunspot with positive magnetic polarity rotated counterclockwise. As expected, the rotation was significantly affected by the two flares. Magnetic transients induced by the flares were clearly evident in the sunspots with negative polarity. They were moving across the sunspots with speed of order 3 – 7 km s−1. Furthermore, the trend of magnetic flux evolution in these sunspots exhibited changes associated with the flares. These results may shed light on understanding the evolution of sunspots.

SUNSPOT ROTATION AS A DRIVER OF MAJOR SOLAR ERUPTIONS IN THE NOAA ACTIVE REGION 12158

ABSTRACT We studied the development conditions of sigmoid structure under the influence of the magnetic non-potential characteristics of a rotating sunspot in the active region (AR) 12158. Vector magnetic field measurements from the Helioseismic Magnetic Imager and coronal EUV observations from the Atmospheric Imaging Assembly reveal that the erupting inverse-S sigmoid had roots at the location of the rotating sunspot. The sunspot rotates at a rate of 0°–5° h−1 with increasing trend in the first half followed by a decrease. The time evolution of many non-potential parameters had a good correspondence with the sunspot rotation. The evolution of the AR magnetic structure is approximated by a time series of force-free equilibria. The non-linear force-free field magnetic structure around the sunspot manifests the observed sigmoid structure. Field lines from the sunspot periphery constitute the body of the sigmoid and those from the interior overlie the sigmoid, similar to a flux rope structure. While the sunspot was rotating, two major coronal mass ejection eruptions occurred in the AR. During the first (second) event, the coronal current concentrations were enhanced (degraded), consistent with the photospheric net vertical current; however, magnetic energy was released during both cases. The analysis results suggest that the magnetic connections of the sigmoid are driven by the slow motion of sunspot rotation, which transforms to a highly twisted flux rope structure in a dynamical scenario. Exceeding the critical twist in the flux rope probably leads to the loss of equilibrium, thus triggering the onset of the two eruptions.