Polarity Sunspot Research Articles

Analysis of photospheric magnetic fields in AR 12546: a case study

We investigate high-resolution observations with the spectropolarimeter (SP) aboard the Hinode satellite of the Solar Optical Telescope (SOT) of a positive polarity sunspot of an active region (AR) (NOAA 12546). We present a case study for the properties of the thermal, magnetic field, and plasma flows as a function of the optical depth from the inversion of the observed Stokes profiles, covering a wide field of view area. Particular attention is paid to the examination of the net circular polarization (NCP) and area asymmetry of spectral lines in sunspots. We detect a large red-shifted velocity of 10 km sec−1 localized with the presence of a strong magnetic field corresponding to the NCP best fit of the inverted profiles. In addition, the comparison between the observed and calculated NCPs or Stokes V area asymmetries of spectral lines fitted well for most pixels in the field of view region, with a significant indication of a single-component inversion. We study the vertical gradients of temperature, magnetic strength, inclination field, and LOS velocity in the deeper and higher layers of the photosphere. The difference in the penumbral temperature between the two atmospheric layers is −130 K. The penumbral regions reveal a magnetic field gradient of Delta B sim -0.5 to −0.9 G km−1. The inclination gradient in the limb-side penumbra is between (Delta gamma /Delta z) = -9 times 10^{-3} deg km−1 to -3.5 times 10^{-2} deg km−1. In particular, we find a higher positive inclination gradient in the disk center-side penumbra (Delta gamma /Delta z)= 0.2 - 1.5 deg text{ km}^{-1}. The velocity gradient is (Delta V_{LOS}/Delta log tau )= -0.13 to −0.30 km sec−1 in the limb-side penumbra, and the disk center-side penumbra is given by (Delta V_{LOS}/Delta log tau )= 0.11 to 0.29 km sec−1.

Comparative Solar Minima Using the McIntosh Archive

AbstractThe McIntosh Archive consists of four solar cycles (SC20–23) of synoptic maps of solar features (magnetic polarities, coronal holes, filaments, sunspots, plage) that were produced by Patrick McIntosh and his cartographers. In this paper we present the full Archive for the first time and find a hemispheric pattern of coronal hole polarity reflecting the Hale sunspot polarity pattern. In addition, we present a supplemental set of related maps created for the 2019–2020 solar minimum in support of the Whole Heliosphere and Planetary Interactions (WHPI) interval. To facilitate this addition, we cross calibrate EUV and He‐I 10,830 Å coronal holes and study differences between the WHPI solar minimum and the earlier minima in the main Archive. We find, first, that the visibility of coronal holes in EUV introduces a bias toward larger counts of low‐latitude coronal holes near solar minimum. However, the estimated lifetimes (in Carrington Rotations) of coronal hole complexes are unaffected by this fragmentation, and we find the longest‐lived low‐latitude coronal holes occurred during the 2008–2009 solar minimum, with 2019–2020 solar minimum lifetimes not far behind. In this way, we consider the large‐scale organization of solar magnetic fields between solar minima and study each solar minimum in the context of its solar cycle.

Inferring Quadrupolar Dynamo Mode from Sunspot Statistics

Observations of long-term north-south asymmetry in solar activity demand the equator-symmetric (quadrupolar) mode be present in the solar magnetic field in line with the dominant antisymmetric (dipolar) mode. This paper proposes treating the sunspot area as a proxy for subsurface toroidal magnetic flux to infer the quadrupolar mode of the solar dynamo from sunspot data. Toroidal pseudo-fluxes (PF) in the northern and southern hemispheres are defined as a signed sunspot area with plus or minus sign prescribed to them in accord with the Hale's sunspot polarity rules. Statistical correlation analysis and wavelet analysis of so-defined PFs reveal quadrupolar oscillations with a period of about 16 yr and amplitude of about 0.17 relative to the amplitude of the dominant 22-yr dipolar mode.

Reconstructing solar magnetic fields from historical observations

Context. We apply our recently developed method to reconstruct synoptic maps of the photospheric magnetic field from observations of chromospheric plages and the magnetic polarity of sunspots. Here, we apply the method to an extended time interval from 1915 to 1985. Aims. Systematic magnetographic observations of the solar photospheric magnetic field were initiated as recently as the 1970s and the lack of earlier observations limits our ability to study and understand the long-term evolution of the Solar global field. This study is aimed at creating synoptic maps of magnetic fields for the pre-magnetograph era and using these maps as input for modern simulation models to investigate the long-term (centennial) evolution of the Sun’s global magnetic fields. Methods. We reconstructed active Solar regions by identifying chromospheric plages from Ca II K line synoptic maps and assigning magnetic polarities based on the observed polarity of sunspots. We used a surface flux transport (SFT) model to simulate the evolution of the photospheric magnetic field from the reconstructed active regions. We used the potential field source surface (PFSS) model to determine the amount of open magnetic flux from the reconstruction and from magnetographic observations. We also reconstructed the coronal field during two eclipses and compared the result with eclipse drawings. Results. We successfully reconstructed the photospheric magnetic field from 1915 to 1985. The number and total magnetic flux of the reconstructed active regions shows a realistic cyclic behavior that mostly follows the evolution of the sunspot number, even on relatively short timescales. The polar field strengths of cycles 19 and 20 do not reflect the evolution of the sunspot number very accurately, which may be related to problems related to the calcium data during cycle 19 and the long data gap during cycle 20. The polarity of polar fields and the amount of open field both at high and low latitudes all demonstrate the expected cyclic behavior. The agreement of the modeled coronal structure with eclipse drawings in 1922 and 1923 is fair.

A New Small Satellite Sunspot Triggering Recurrent Standard and Blowout Coronal Jets

Abstract In this paper, we report a detailed analysis of recurrent jets originated from a location with emerging, canceling, and converging negative magnetic field at the east edge of NOAA active region AR11166 from 2011 March 9 to 10. The event presented several interesting features. First, a satellite sunspot appeared and collided with a pre-existing opposite polarity magnetic field and caused a recurrent solar jet event. Second, the evolution of the jets showed blowout-like nature and standard characteristics. Third, the satellite sunspot exhibited a motion toward the southeast of AR11166 and merged with the emerging flux near the opposite polarity sunspot penumbra, which afterward, due to flux convergence and cancellation episodes, caused recurrent jets. Fourth, three of the blowout jets associated with coronal mass ejections (CMEs), were observed from the field of view of the Solar Terrestrial Relations Observatory. Fifth, almost all the blowout jet eruptions were accompanied with flares or with more intense brightening in the jet base region, while almost standard jets did not manifest such obvious features during eruptions. The most important feature, the blowout jets, were inclined to faster and larger scales than the standard jets. The standard jets instead were inclined to be relatively longer-lasting. The obvious shearing and twisting motions of the magnetic field may be interpreted as due to the shearing and twisting motions for a blowout jet eruption. The statistical results show that ~30% of the blowout jets directly developed into CMEs. This suggests that the blowout jets and CMEs should have a close relationship.

The association of the Hale sector boundary with RHESSI solar flares and active longitudes

Context. The heliospheric magnetic field (HMF) is structured into large sectors of positive and negative polarity. The parts of the boundary between these sectors where the change in polarity matches that of the leading-to-following sunspot polarity in that solar hemisphere, are called Hale sector boundaries (HSB). Aims. We investigate the flare occurrence rate near HSBs and the association between HSBs and active longitudes. Methods. Previous work determined the times HSBs were at solar central meridian, using the detection of the HMF sector boundary crossing at the Earth. In addition to this, we use a new approach which finds the HSB locations at all times by determining them from potential field source surface (PFSS) extrapolations of photospheric magnetograms. We use the RHESSI X-ray flare list for comparison to the HSB as it provides accurate flare locations over 14 years, from February 2002 to February 2016, covering both Cycles 23 and 24. For the active longitude positions we use previously published work based on sunspot observations. Results. We find that the two methods of determining the HSB generally agree and that 41% (Cycle 23) and 47% (Cycle 24) of RHESSI flares occur within 30° of the PFSS determined-HSB. The behaviour of the HSBs varies over the two Cycles studied, and as expected they swap in hemisphere as the Cycles change. The HSBs and active longitudes do overlap but not consistently. They often move at different rates relative to each other (and the Carrington solar rotation rate) and these vary over each Cycle. The HSBs provide a useful additional activity indicator, particularly during periods when active longitudes are difficult to determine.

Observing Kelvin\u2013Helmholtz instability in solar blowout jet

Kelvin–Helmholtz instability (KHI) is a basic physical process in fluids and magnetized plasmas, with applications successfully modelling e.g. exponentially growing instabilities observed at magnetospheric and heliospheric boundaries, in the solar or Earth’s atmosphere and within astrophysical jets. Here, we report the discovery of the KHI in solar blowout jets and analyse the detailed evolution by employing high-resolution data from the Interface Region Imaging Spectrograph (IRIS) satellite launched in 2013. The particular jet we focus on is rooted in the surrounding penumbra of the main negative polarity sunspot of Active Region 12365, where the main body of the jet is a super-penumbral structure. At its maximum, the jet has a length of 90 Mm, a width of 19.7 Mm, and its density is about 40 times higher than its surroundings. During the evolution of the jet, a cavity appears near the base of the jet, and bi-directional flows originated from the top and bottom of the cavity start to develop, indicating that magnetic reconnection takes place around the cavity. Two upward flows pass along the left boundary of the jet successively. Next, KHI develops due to a strong velocity shear (∼204 km s−1) between these two flows, and subsequently the smooth left boundary exhibits a sawtooth pattern, evidencing the onset of the instability.

Multiwavelength observations of a flux rope formation by series of magnetic reconnection in the chromosphere

Using high-resolution observations from the 1.6 m New Solar Telescope (NST) operating at the Big Bear Solar Observatory (BBSO), we report direct evidence of merging/reconnection of cool H$\alpha$ loops in the chromosphere during two homologous flares (B- and C-class) caused by a shear motion at the footpoint of two loops. The reconnection between these loops caused the formation of an unstable flux rope which showed counterclockwise rotation. The flux rope could not reach the height of torus instability and failed to form a coronal mass ejection. The HMI magnetograms revealed rotation of the negative/positive (N1/P2) polarity sunspots in the opposite directions, which increased the right and left-handed twist in the magnetic structures rooted at N1/P2. Rapid photospheric flux cancellation (duration$\sim$20-30 min, rate$\approx$3.44$\times$10$^{20}$ Mx h$^{-1}$) was observed during and even after the first B6.0 flare and continued until the end of the second C2.3 flare. The RHESSI X-ray sources were located at the site of the loop's coalescence. To the best of our knowledge, such a clear interaction of chromospheric loops along with rapid flux cancellation has not been reported before. These high-resolution observations suggest the formation of a small flux rope by a series of magnetic reconnection within chromospheric loops associated with very rapid flux cancellation.

Magnetic Flux Emergence and Decay Rates for Preceder and Follower Sunspots Observed with HMI

Abstract We quantify the emergence and decay rates of preceder (p) and follower (f) sunspots within 10 active regions from 2010 to 2014 using Space-weather Helioseismic Magnetic Imager Active Region Patch data. The sunspots are small to mid-sized regions and contain a signed flux within a single polarity sunspot of . The net unsigned flux within the regions, including plage, ranges from . Rates are calculated with and without intensity contours to differentiate between sunspot formation and flux emergence. Signed flux emergence rates, calculated with intensity contours, for the p (f) spots average hr−1, while decay rates are hr−1. The mean, signed flux emergence rate of the regions, including plage, is hr−1, for a mean peak flux of . Using a synthesis of these results and others reported previously, there is a clear trend for larger flux regions to emerge faster than smaller ones. Observed emergence rates ( , Mx hr−1) scale with total signed peak flux, as a power law with an exponent of 0.36, i.e., . The observed rates may assist in constraining the boundary and initial conditions in simulations which already demonstrate increased rates for flux tubes with higher buoyancy and twist, or in the presence of a strong upflow. Overall, the observed emergence rates are smaller than those in simulations, which may indicate a slower rise of the flux in the interior than what is captured in simulations.

FORMATION AND ERUPTION OF A SMALL FLUX ROPE IN THE CHROMOSPHERE OBSERVED BY NST,IRIS, ANDSDO

Using high-resolution images from 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO), we report the direct evidence of chromospheric reconnection at the polarity inversion line (PIL) between two small opposite polarity sunspots. Small jet-like structures (with velocities of ~20-55 km/s) were observed at the reconnection site before the onset of the first M1.0 flare. The slow rise of untwisting jets was followed by the onset of cool plasma inflow (~10 km/s) at the reconnection site, causing the onset of a two-ribbon flare. The reconnection between two sheared J-shaped cool H$\alpha$ loops causes the formation of a small twisted flux rope (S shaped) in the chromosphere. In addition, Helioseismic and Magnetic Imager (HMI) magnetograms show the flux cancellation (both positive and negative) during the first M1.0 flare. The emergence of negative flux and cancellation of positive flux (with shear flows) continue until the successful eruption of the flux rope. The newly formed chromospheric flux rope becomes unstable and rises slowly with the speed of ~108 km/s during a second C8.5 flare that occurred after ~3 hours of the first M1.0 flare. The flux rope was destroyed by repeated magnetic reconnection induced by its interaction with the ambient field (fan-spine toplology) and looks like an untwisting surge (~170 km/s) in the coronal images recorded by Solar Dynamic Observatory/Atmospheric Imaging Assembly (SDO/AIA). These observations suggest the formation of a chromospheric flux rope (by magnetic reconnection associated with flux cancellation) during the first M1.0 flare and its subsequent eruption/disruption during the second C8.5 flare.

ON POLAR MAGNETIC FIELD REVERSAL AND SURFACE FLUX TRANSPORT DURING SOLAR CYCLE 24

As each solar cycle progresses, remnant magnetic flux from active regions (ARs) migrates poleward to cancel the old-cycle polar field. We describe this polarity reversal process during Cycle 24 using four years (2010.33--2014.33) of line-of-sight magnetic field measurements from the Helioseismic and Magnetic Imager. The total flux associated with ARs reached maximum in the north in 2011, more than two years earlier than the south; the maximum is significantly weaker than Cycle 23. The process of polar field reversal is relatively slow, north-south asymmetric, and episodic. We estimate that the global axial dipole changed sign in October 2013; the northern and southern polar fields (mean above 60$^\circ$ latitude) reversed in November 2012 and March 2014, respectively, about 16 months apart. Notably, the poleward surges of flux in each hemisphere alternated in polarity, giving rise to multiple reversals in the north. We show that the surges of the trailing sunspot polarity tend to correspond to normal mean AR tilt, higher total AR flux, or slower mid-latitude near-surface meridional flow, while exceptions occur during low magnetic activity. In particular, the AR flux and the mid-latitude poleward flow speed exhibit a clear anti-correlation. We discuss how these features can be explained in a surface flux transport process that includes a field-dependent converging flow toward the ARs, a characteristic that may contribute to solar cycle variability.

The Persistence of Apparent Non-Magnetostatic Equilibrium in NOAA 11035

AbstractNOAA 11035 was a highly sheared active region that appeared in December 2009 early in the new activity cycle. The leading polarity sunspot developed a highly unusual feature in its penumbra, an opposite polarity pore with a strong magnetic field in excess of 3500 G along one edge, which persisted for several days during the evolution of the region. This region was well observed by both space- and ground-based observatories, including Hinode, FIRS, TRACE, and SOHO. These observations, which span wavelength and atmospheric regimes, provide a complete picture of this unusual feature which may constitute a force-free magnetic field in the photosphere which is produced by the reconnection of magnetic loops low in the solar atmosphere.

VELOCITY AND MAGNETIC FIELD DISTRIBUTION IN A FORMING PENUMBRA

We present results from the analysis of high-resolution spectropolarimetric and spectroscopic observations of the solar photosphere and chromosphere, obtained shortly before the formation of a penumbra in one of the leading polarity sunspots of NOAA active region 11490. The observations were performed at the Dunn Solar Telescope of the National Solar Observatory on 2012 May 28, using the Interferometric Bidimensional Spectrometer. The data set is comprised of a 1 hr time sequence of measurements in the Fe I 617.3 nm and Fe I 630.25 nm lines (full Stokes polarimetry) and in the Ca II 854.2 nm line (Stokes I only). We perform an inversion of the Fe I 630.25 nm Stokes profiles to derive magnetic field parameters and the line-of-sight (LOS) velocity at the photospheric level. We characterize chromospheric LOS velocities by the Doppler shift of the centroid of the Ca II 854.2 nm line. We find that, before the formation of the penumbra, an annular zone of 3''-5'' width is visible around the sunspot. In the photosphere, we find that this zone is characterized by an uncombed structure of the magnetic field although no visible penumbra has formed yet. We also find that the chromospheric LOS velocity field showsmore » several elongated structures characterized by downflow and upflow motions in the inner and outer parts of the annular zone, respectively.« less

Multiwavelength Study of a Solar Eruption from AR NOAA 11112 I. Flux Emergence, Sunspot Rotation and Triggering of a Solar Flare

We analyse the multiwavelength observations of an M2.9/1N flare that occurred in the active region (AR) NOAA 11112 in the vicinity of a huge filament system on 16 October 2010. SDO/HMI magnetograms reveal the emergence of a bipole (within the existing AR) 50 hours prior to the flare event. During the emergence, both the positive and negative sunspots in the bipole show translational as well as rotational motion. The positive polarity sunspot shows the significant motion/rotation in the south-westward/clockwise direction and continuously pushing/sliding the surrounding opposite polarity field region. On the other hand, the negative polarity sunspot moves/rotates in the westward/anticlockwise direction. The positive polarity sunspot rotates ~70 deg. within 30 hours, whereas negative polarity ~20 deg. within 10 hours. SDO/AIA 94 {\AA} EUV images show the emergence of a flux tube in the corona consistent with the emergence of the bipole in HMI. A high speed plasmoid ejection (speed~1197 km/s) was observed during the flare impulsive phase, which suggests the magnetic reconnection of the emerged positive polarity sunspot with the surrounding opposite polarity field region. The entire AR shows the positive helicity injection before the flare event. Moreover, the newly emerging bipole reveal the signature of negative (left-handed) helicity. These observations provide the unique evidences of the emergence of twisted flux tube from below the photosphere to coronal heights triggering a flare mainly due to the interaction between the emerged positive polarity sunspot and a nearby negative polarity sunspot by the shearing motion of the emerging positive sunspot towards the negative one. Our observations also strongly support the idea that the rotation is most likely attributed to the emergence of twisted magnetic fields, as proposed by recent models.

EVOLUTION OF SPINNING AND BRAIDING HELICITY FLUXES IN SOLAR ACTIVE REGION NOAA 10930

The line-of-sight magnetograms from Solar Optical Telescope Narrowband Filter Imager observations of NOAA Active Region 10930 have been used to study the evolution of spinning and braiding helicities over a period of five days starting from 2006 December 9. The north (N) polarity sunspot was the follower and the south (S) polarity sunspot was the leader. The N-polarity sunspot in the active region was rotating in the counterclockwise direction. The rate of rotation was small during the first two days of observations and it increased up to 8 Degree-Sign hr{sup -1} on the third day of the observations. On the fourth and fifth days it remained at 4 Degree-Sign hr{sup -1} with small undulations in its magnitude. The sunspot rotated about 260 Degree-Sign in the last three days. The S-polarity sunspot did not complete more than 20 Degree-Sign in five days. However, it changed its direction of rotation five times over a period of five days and injected both the positive and negative type of spin helicity fluxes into the corona. Through the five days, both the positive and negative sunspot regions injected equal amounts of spin helicity. The total injected helicity is predominantly negative in sign. However, the signmore » of the spin and braiding helicity fluxes computed over all the regions were reversed from negative to positive five times during the five-day period of observations. The reversal in spinning helicity flux was found before the onset of the X3.4-class flare, too. Though, the rotating sunspot has been observed in this active region, the braiding helicity has contributed more to the total accumulated helicity than the spinning helicity. The accumulated helicity is in excess of -7 Multiplication-Sign 10{sup 43} Mx{sup 2} over a period of five days. Before the X3.4-class flare that occurred on 2006 December 13, the rotation speed and spin helicity flux increased in the S-polarity sunspot. Before the flare, the total injected helicity was larger than -6 Multiplication-Sign 10{sup 43} Mx{sup 2}. The observed reversal in the sign of spinning and braiding helicity fluxes could be the signature of the emergence of a twisted flux tube, which acquires the writhe of an opposite sign. The magnetic cloud associated with the ejected mass has carried about -7 Multiplication-Sign 10{sup 41} Mx{sup 2} of helicity. A time integration of helicity flux of about 1.2 hr integrated backward in time of the observation of the coronal mass ejection is sufficient for this event.« less

Evidence for magnetic flux cancelation leading to an ejective solar eruption observed byHinode,TRACE,STEREO, andSoHO/MDI

[NASA ADS]

OBSERVATION OF KINK INSTABILITY DURING SMALL B5.0 SOLAR FLARE ON 2007 JUNE 4

Using multi-wavelength observations of SoHO/MDI, SOT-Hinode/blue-continuum (4504 \AA), G-band (4305 \AA), Ca II H (3968 \AA) and TRACE 171 \AA, we present the observational signature of highly twisted magnetic loop in AR 10960 during the period 04:43 UT-04:52 UT at 4 June, 2007. SOT-Hinode/blue-continuum (4504 \AA) observations show that penumbral filaments of positive polarity sunspot have counter-clock wise twist, which may be caused by the clock-wise rotation of the spot umbrae. The coronal loop, whose one footpoint is anchored in this sunspot, shows strong right-handed twist in chromospheric SOT-Hinode/Ca II H (3968 \AA) and coronal TRACE 171 \AA\, images. The length and the radius of the loop are $L\sim$80 Mm and $a\sim$4.0 Mm respectively. The distance between neighboring turns of magnetic field lines (i.e. pitch) is estimated as $\approx$ 10 Mm. The total twist angle, $\Phi\sim$12$\pi$ (estimated for the homogeneous distribution of the twist along the loop), is much larger than the Kruskal -Shafranov instability criterion. We detected clear double structure of the loop top during 04:47-04:51 UT on TRACE 171 \AA \ images, which is consistent with simulated kink instability in curved coronal loops (T{\"o}r{\"o}k et al. 2004). We suggest, that the kink instability of this twisted magnetic loop triggered B5.0 class solar flare, which occurred between 04:40 UT and 04:51 UT in this active region.

A new dynamo pattern revealed by solar helical magnetic fields

Abstract A previously unobservable mirror asymmetry of the solar magnetic field – a key ingredient of the dynamo mechanism which is believed to drive the 11-year activity cycle – has recently been measured. This was achieved through systematic monitoring of solar active regions carried out for more than 20 years at observatories in Mees, Huairou and Mitaka. In this Letter we report on detailed analysis of vector magnetic field data, obtained at Huairou Solar Observing Station in China. Electric current helicity (the product of current and magnetic field components in the same direction) was estimated from the data and a latitude–time plot of solar helicity during the last two solar cycles has been produced. We find that like sunspots helicity patterns propagate equatorwards, but unlike sunspot polarity helicity in each solar hemisphere does not change sign from cycle to cycle, thus confirming the theory. There are, however, two significant time–latitudinal domains in each cycle when the sign briefly inverts. Our findings shed new light on stellar and planetary dynamos and are yet to be included in the theory.

EVOLUTION OF SOLAR MAGNETIC FIELD AND ASSOCIATED MULTIWAVELENGTH PHENOMENA: FLARE EVENTS ON 2003 NOVEMBER 20

We analyze H-alpha images, soft X-ray profiles, magnetograms, extreme ultra-violet images and radio observations of two homologous flare events (M1.4/1N and M9.6/2B) on 20 November 2003 in the active region NOAA 10501 and study properties of reconnection between twisted filament systems, energy release and associated launch of coronal mass ejections (CMEs). During both events twisted filaments observed in H-alpha approached each other and initiated the flare processes. However, the second event showed the formation of cusp as the filaments interacted. The rotation of sunspots of opposite polarities, inferred from magnetograms likely powered the twisted filaments and injection of helicity. Along the current sheet between these two opposite polarity sunspots, the shear was maximum, which could have caused the twist in the filament. At the time of interaction between filaments, the reconnection took place and flare emission in thermal and non-thermal energy ranges attained the maximum. The radio signatures revealed the opening of field lines resulting from reconnection. The H-alpha images and radio data provide the inflow speed leading to reconnection and the scale size of particle acceleration region. The first event produced a narrow and slow CME, whereas the later one was associated with a fast full halo CME. The halo CME signatures observed between Sun and Earth using white-light and scintillation images and in-situ measurements indicated the magnetic energy utilized in the expansion and propagation. The magnetic cloud signature at the Earth confirmed the flux rope ejected at the time of filament interaction and reconnection.

ON THE ORIGIN OF THE ASYMMETRIC HELICITY INJECTION IN EMERGING ACTIVE REGIONS

To explore the possible causes of the observed asymmetric helicity flux in emerging active regions between the leading and following polarities reported in a recent study by Tian & Alexander, we examine the subsurface evolution of buoyantly rising ?-shaped flux tubes using three-dimensional, spherical-shell anelastic MHD simulations. We find that due to the asymmetric stretching of the ?-shaped tube by the Coriolis force, the leading side of the emerging tube has a greater field strength, is more buoyant, and remains more cohesive compared to the following side. As a result, the magnetic field lines in the leading leg show more coherent values of local twist ? ? (? ? B) ? B/B 2, whereas the values in the following leg show large fluctuations and are of mixed sign. On average, however, the field lines in the leading leg do not show a systematically greater mean twist compared to the following leg. Due to the higher rise velocity of the leading leg, the upward helicity flux through a horizontal cross section at each depth in the upper half of the convection zone is significantly greater in the leading polarity region than that in the following leg. This may contribute to the observed asymmetric helicity flux in emerging active regions. Furthermore, based on a simplified model of active region flux emergence into the corona by Longcope & Welsch, we show that a stronger field strength in the leading tube can result in a faster rotation of the leading polarity sunspot driven by torsional Alfv?n waves during flux emergence into the corona, contributing to a greater helicity injection rate in the leading polarity of an emerging active region.