Flux Emergence Events Research Articles

Nested active regions anchor the heliospheric current sheet and stall the reversal of the coronal magnetic field

During the solar cycle, the Sun's magnetic field polarity reverses due to the emergence, cancellation, and advection of magnetic flux towards the rotational poles. Flux emergence events occasionally cluster together, although it is unclear if this is due to the underlying solar dynamo or simply by chance. Regardless of the cause, we aim to characterise how the reversal of the Sun's magnetic field and the structure of the solar corona are influenced by nested flux emergence. From the spherical harmonic decomposition of the Sun's photospheric magnetic field, we identified times when the reversal of the dipole component stalls for several solar rotations. Using observations from sunspot cycle 23 to present, we located the nested active regions responsible for each stalling and explored their impact on the coronal magnetic field using potential field source surface extrapolations. Nested flux emergence has a more significant impact on the topology of the coronal magnetic field than isolated emergences as it produces a coherent (low spherical harmonic order) contribution to the photospheric magnetic field. The heliospheric current sheet, which separates oppositely directed coronal magnetic fields, can become anchored above nested active regions due to the formation of strong opposing magnetic fluxes. Further flux emergence, cancellation, differential rotation, and diffusion, then effectively advects the heliospheric current sheet and shifts the dipole axis. Nested flux emergence can restrict the evolution of the heliospheric current sheet and impede the reversal of the coronal magnetic field. The sources of the solar wind can be more consistently identified around nested active regions because the magnetic field topology remains self-similar for multiple solar rotations. This highlights the importance of identifying and tracking nested active regions to guide the remote-sensing observations of modern heliophysics missions.

Characterizing the Umbral Magnetic Knots of δ-Sunspots

Delta (δ) spots are active regions (ARs) in which positive and negative umbrae share a penumbra. They are known to be the source of strong flares. We introduce a new quantity, the degree of δ (Doδ), to measure the fraction of umbral flux participating in the δ-configuration and to isolate the dynamics of the magnetic knot, i.e., adjacent umbrae in the δ-configuration. Using Helioseismic and Magnetic Imager data, we analyze 19 δ-spots and 11 β-spots in detail, as well as 120 δ-spots in less detail. We find that δ-regions are not in a δ-configuration for the entire time but spend 55% of their observed time as δ-spots with an average, maximum Doδ of 72%. Compared to β-spots, δ-spots have 2.6× the maximum umbral flux, 1.9× the flux emergence rate, 2.6× the rotation, and 72× the flare energy. On average, the magnetic knots rotate 17° day−1, while the β-spots rotate 2° day−1. Approximately 72% of the magnetic knots present anti-Hale or anti-Joy tilts, contrasting starkly with only 9% of the β-spots. A positive correlation exists between ϕ Doδ and the flare energy emitted by that region. The δ-spots obey the hemispheric current helicity rule 64% of the time. A total of 84% of the δ-spots are formed by single flux emergence events, and 58% have a quadrupolar magnetic configuration. The δ-spot characteristics are consistent with the formation mechanism signatures as follows: 42% with the kink instability or Sigma effect, 32% with multisegment buoyancy, 16% with collisions, and two ARs that are unclassified but consistent with a rising O-ring.

Emergence of Internetwork Magnetic Fields through the Solar Atmosphere

Abstract Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they emerge all over the Sun, these small-scale fields bring a substantial amount of flux, and therefore energy, to the solar surface. Because of this, IN fields are crucial for understanding the quiet Sun (QS) magnetism. However, they are weak and produce very small polarization signals, which is the reason why their properties and impact on the energetics and dynamics of the solar atmosphere are poorly known. Here we use coordinated, high-resolution, multiwavelength observations obtained with the Swedish 1 m Solar Telescope and the Interface Region Imaging Spectrograph (IRIS) to follow the evolution of IN magnetic loops as they emerge into the photosphere and reach the chromosphere and transition region. We studied in this paper three flux emergence events having total unsigned magnetic fluxes of 1.9 × 1018, 2.5 × 1018, and 5.3 × 1018 Mx. The footpoints of the emerging IN bipoles are clearly seen to appear in the photosphere and to rise up through the solar atmosphere, as observed in Fe i 6173 Å and Mg i b2 5173 Å magnetograms, respectively. For the first time, our polarimetric measurements taken in the chromospheric Ca ii 8542 Å line provide direct observational evidence that IN fields are capable of reaching the chromosphere. Moreover, using IRIS data, we study the effects of these weak fields on the heating of the chromosphere and transition region.

The Formation of an Atypical Sunspot Light Bridge as a Result of Large-scale Flux Emergence

Abstract We use a combination of full-disk data from the Solar Dynamics Observatory and high-resolution data from the Dunn Solar Telescope (DST) to study the formation, structure, and evolution of an atypical light bridge (LB) in a regular sunspot. The LB results from the emergence of magnetic flux with one footpoint rooted in a pore outside the parent sunspot that appears about 17 hr before the LB. The pore has a polarity opposite to that of the sunspot and recedes from it at a speed of about 0.4 km s−1. This is accompanied by the development of an elongated magnetic channel in the outer penumbra that triggers the formation of the LB when it reaches the inner penumbral boundary. The LB is a nearly horizontal structure with a field strength of about 1.2 kG that exhibits long-lived photospheric blueshifts of about 0.85 km s−1 along its entire length. The emergence of the LB leads to dynamic surges in the chromosphere and transition region about 13 minutes later. We derive the photospheric and chromospheric structure of the LB in the DST data from spectral line parameters and inversions of He i at 1083 nm, Si i at 1082.7 nm, Ca ii IR at 854 nm, and H α at 656 nm and speckle-reconstructed imaging at 700 and 430 nm. The LB shows an elongated filamentary shape in the photosphere without lateral extrusions. The thermal inversion of Ca ii IR reveals the LB to be about 600–800 K hotter than the umbra. Different sections of the LB are elevated to heights between 400 and 700 km. Our results indicate that LB formation is part of a flux emergence event with the LB envelope reaching a height of about 29 Mm before dissolving after about 13 hr. We conclude that the existence of persistent, large-scale photospheric blueshifts in LBs is the most likely criterion for distinguishing between flux emergence events and overturning convection in field-free umbral intrusions.

On the Magnetic Nature of an Exploding Granule as Revealed by Sunrise/IMaX

Abstract We study the photospheric evolution of an exploding granule observed in the quiet Sun at high spatial (∼0.″3) and temporal (31.5 s) resolution by the imaging magnetograph Sunrise/IMaX in 2009 June. These observations show that the exploding granule is cospatial to a magnetic flux emergence event occurring at mesogranular scale (up to ∼12 Mm2 area). Using a modified version of the SIR code for inverting the IMaX spectropolarimetric measurements, we obtain information about the magnetic configuration of this photospheric feature. In particular, we find evidence of highly inclined emerging fields in the structure, carrying a magnetic flux content up to ∼4 × 1018 Mx. The balance between gas and magnetic pressure in the region of flux emergence, compared with a very quiet region of the Sun, indicates that the additional pressure carried by the emerging flux increases the total pressure by about 5% and appears to allow the granulation to be modified, as predicted by numerical simulations. The overall characteristics suggest that a multipolar structure emerges into the photosphere, resembling an almost horizontal flux sheet. This seems to be associated with exploding granules. Finally, we discuss the origin of such flux emergence events.

Light Bridge Brightening and Plasma Ejection Driven by a Magnetic Flux Emergence Event

Abstract Observations with the Goode Solar Telescope (GST) are presented here showing that the emergence of 1.91 × 1018 Mx of new magnetic flux occurred at the edge of a filamentary light bridge (LB). This emergence was accompanied by brightness enhancement of a photospheric overturning convection cell (OCC) at the endpoints of the emerging magnetic structure. We present an analysis of the origin and the dynamics of this event using high-resolution GST Fe i 1564.85 nm vector magnetic field data, TiO photospheric, and Hα chromospheric images. The emerged structure was 1.5 × 0.3 Mm in size at the peak of development and lasted for 17 minutes. Doppler observations showed presence of systematic upflows before the appearance of the magnetic field signal and downflows during the decay phase. Changes in the orientation of the associated transverse fields, determined from the differential angle, suggest the emergence of a twisted magnetic structure. A fan-shaped jet was observed to be spatially and temporally correlated with the endpoint of the OCC intruding into the LB. Our data suggest that the emerging fields may have reconnected with the magnetic fields in the vicinity of the LB, which could lead to the formation of the jet. Our observation is the first report of flux emergence within a granular LB with evidence in the evolution of vector magnetic field, as well as photosphere convection motions, and supports the idea that the impulsive jets above the LB are caused by magnetic reconnection.

On the Origin of the Magnetic Energy in the Quiet Solar Chromosphere

Abstract The presence of magnetic field is crucial in the transport of energy through the solar atmosphere. Recent ground-based and space-borne observations of the quiet Sun have revealed that magnetic field accumulates at photospheric heights, via a local dynamo or from small-scale flux emergence events. However, most of this small-scale magnetic field may not expand into the chromosphere due to the entropy drop with height at the photosphere. Here we present a study that uses a high-resolution 3D radiative MHD simulation of the solar atmosphere with non-gray and non-LTE radiative transfer and thermal conduction along the magnetic field to reveal that (1) the net magnetic flux from the simulated quiet photosphere is not sufficient to maintain a chromospheric magnetic field (on average), (2) processes in the lower chromosphere, in the region dominated by magnetoacoustic shocks, are able to convert kinetic energy into magnetic energy, (3) the magnetic energy in the chromosphere increases linearly in time until the rms of the magnetic field strength saturates at roughly 4–30 G (horizontal average) due to conversion from kinetic energy, (4) and that the magnetic features formed in the chromosphere are localized to this region.

Energetics of magnetic transients in a solar active region plage

Context. Densely packed coronal loops are rooted in photospheric plages in the vicinity of active regions on the Sun. The photospheric magnetic features underlying these plage areas are patches of mostly unidirectional magnetic field extending several arcsec on the solar surface. Aims. We aim to explore the transient nature of the magnetic field, its mixed-polarity characteristics, and the associated energetics in the active region plage using high spatial resolution observations and numerical simulations. Methods. We used photospheric Fe I 6173 Å spectropolarimetric observations of a decaying active region obtained from the Swedish 1-m Solar Telescope (SST). These data were inverted to retrieve the photospheric magnetic field underlying the plage as identified in the extreme-ultraviolet emission maps obtained from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). To obtain better insight into the evolution of extended unidirectional magnetic field patches on the Sun, we performed 3D radiation magnetohydrodynamic simulations of magnetoconvection using the MURaM code. Results. The observations show transient magnetic flux emergence and cancellation events within the extended predominantly unipolar patch on timescales of a few 100 s and on spatial scales comparable to granules. These transient events occur at the footpoints of active region plage loops. In one case the coronal response at the footpoints of these loops is clearly associated with the underlying transient. The numerical simulations also reveal similar magnetic flux emergence and cancellation events that extend to even smaller spatial and temporal scales. Individual simulated transient events transfer an energy flux in excess of 1 MW m−2 through the photosphere. Conclusions. We suggest that the magnetic transients could play an important role in the energetics of active region plage. Both in observations and simulations, the opposite-polarity magnetic field brought up by transient flux emergence cancels with the surrounding plage field. Magnetic reconnection associated with such transient events likely conduits magnetic energy to power the overlying chromosphere and coronal loops.

Observations of solar small-scale magnetic flux-sheet emergence

Aims. Two types of flux emergence were recently discovered in numerical simulations: magnetic loops and magnetic sheet emergence. While magnetic loop emergence has been documented well in recent years using high-resolution full Stokes data from ground-based telescopes as well as satellites, magnetic sheet emergence is still an understudied process. We report here on the first clear observational evidence of a magnetic sheet emergence and characterise its development. Methods. Full Stokes spectra from the Hinode spectropolarimeter were inverted with the Stokes Inversion based on Response functions (SIR) code to obtain solar atmospheric parameters such as temperature, line-of-sight velocities, and full magnetic field vector information. Results. We analyse a magnetic flux emergence event observed in the quiet-Sun internetwork. After a large-scale appearance of linear polarisation, a magnetic sheet with horizontal magnetic flux density of up to 194 Mx cm−2 hovers in the low photosphere spanning a region of 2–3 arcsec. The magnetic field azimuth obtained through Stokes inversions clearly shows an organised structure of transversal magnetic flux density emerging. The granule below the magnetic flux sheet tears the structure apart leaving the emerged flux to form several magnetic loops at the edges of the granule. Conclusions. A large amount of flux with strong horizontal magnetic fields surfaces through the interplay of buried magnetic flux and convective motions. The magnetic flux emerges within 10 minutes and we find a longitudinal magnetic flux at the foot points of the order of ∼1018 Mx. This is one to two orders of magnitude larger than what has been reported for small-scale magnetic loops. The convective flows feed the newly emerged flux into the pre-existing magnetic population on a granular scale.

A Tale of Two Emergences: Sunrise II Observations of Emergence Sites in a Solar Active Region

Abstract In 2013 June, the two scientific instruments on board the second Sunrise mission witnessed, in detail, a small-scale magnetic flux emergence event as part of the birth of an active region. The Imaging Magnetograph Experiment (IMaX) recorded two small ( ) emerging flux patches in the polarized filtergrams of a photospheric Fe i spectral line. Meanwhile, the Sunrise Filter Imager (SuFI) captured the highly dynamic chromospheric response to the magnetic fields pushing their way through the lower solar atmosphere. The serendipitous capture of this event offers a closer look at the inner workings of active region emergence sites. In particular, it reveals in meticulous detail how the rising magnetic fields interact with the granulation as they push through the Sun’s surface, dragging photospheric plasma in their upward travel. The plasma that is burdening the rising field slides along the field lines, creating fast downflowing channels at the footpoints. The weight of this material anchors this field to the surface at semi-regular spatial intervals, shaping it in an undulatory fashion. Finally, magnetic reconnection enables the field to release itself from its photospheric anchors, allowing it to continue its voyage up to higher layers. This process releases energy that lights up the arch-filament systems and heats the surrounding chromosphere.

EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. III. THE PATH TO THE TRANSITION REGION

ABSTRACT We study, for the first time, the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above. Such events occurred in a flux emerging region in NOAA 11850 on 2013 September 25. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft was carried out. Simultaneous observations of the chromospheric Hα 656.28 nm and Ca ii 854.2 nm lines, plus the photospheric Fe i 630.25 nm line, were made with the CRISP spectropolarimeter at the Spitzer Space Telescope (SST) reaching a spatial resolution of 0.″14. At the same time, IRIS was performing a four-step dense raster of the emerging flux region, taking slit jaw images at 133 (C ii, transition region), 140 (Si iv, transition region), 279.6 (Mg ii k, core, upper chromosphere), and 283.2 nm (Mg ii k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far- and near-ultraviolet, of which we have used the Si iv 140.3 and the Mg ii k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona.

STATISTICAL ANALYSIS OF THE HORIZONTAL DIVERGENT FLOW IN EMERGING SOLAR ACTIVE REGIONS

Solar active regions (ARs) are thought to be formed by magnetic fields from the convection zone. Our flux emergence simulations revealed that a strong horizontal divergent flow (HDF) of unmagnetized plasma appears at the photosphere before the flux begins to emerge. In our earlier study, we analyzed HMI data for a single AR and confirmed presence of this precursor plasma flow in the actual Sun. In this paper, as an extension of our earlier study, we conducted a statistical analysis of the HDFs to further investigate their characteristics and better determine the properties. From SDO/HMI data, we picked up 23 flux emergence events over a period of 14 months, the total flux of which ranges from 10^{20} to 10^{22} Mx. Out of 23 selected events, 6 clear HDFs were detected by the method we developed in our earlier study, and 7 HDFs detected by visual inspection were added to this statistic analysis. We found that the duration of the HDF is on average 61 minutes and the maximum HDF speed is on average 3.1 km s^{-1}. We also estimated the rising speed of the subsurface magnetic flux to be 0.6-1.4 km s^{-1}. These values are highly consistent with our previous one-event analysis as well as our simulation results. The observation results lead us to the conclusion that the HDF is rather a common feature in the earliest phase of AR emergence. Moreover, our HDF analysis has capability of determining the subsurface properties of emerging fields that cannot be directly measured.

EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. I. SPECTROPOLARIMETRIC OBSERVATIONS AND SIMULATIONS

We study a granular-sized magnetic flux emergence event that occurred in NOAA 11024 in July 2009. The observations were made with the CRISP spectropolarimeter at the Swedish 1 m Solar Telescope achieving a spatial resolution of 0.14". Simultaneous full Stokes observations of the two photospheric Fe I lines at 630.2 nm and the chromospheric Ca II 854.2 nm line allow us to describe in detail the emergence process across the solar atmosphere. We report here on 3D semi-spherical bubble events, where instead of simple magnetic footpoints, we observe complex semi-circular feet straddling a few granules. The most characteristic signature in these events is the observation of a dark bubble in filtergrams taken in the wings of the Ca II 854.2 nm line. We can infer how the bubble rises through the solar atmosphere as we see it progressing from the wings to the core of Ca II 854.2 nm. In the photosphere, the magnetic bubble shows mean upward Doppler velocities of 2 km/s. In about 3.5 minutes it travels some 1100 km to reach the mid chromosphere, implying an average ascent speed of 5.2 km/s. To aid the interpretation of the observations, we carry out 3D numerical simulations of the evolution of a horizontal, untwisted magnetic flux sheet injected in the convection zone, using the Bifrost code. The computational domain spans from the upper convection zone to the lower corona. In the modeled chromosphere the rising flux sheet produces a large, cool, magnetized bubble. We compare this bubble with the observed ones and find excellent agreement, including similar field strengths and velocity signals in the photosphere and chromosphere, temperature deficits, ascent speeds, expansion velocities, and lifetimes.

Granular-Scale Elementary Flux Emergence Episodes in a Solar Active Region

We analyse data from Hinode spacecraft taken over two 54-minute periods during the emergence of AR 11024. We focus on small-scale portions within the observed solar active region and discover the appearance of very distinctive small-scale and short-lived dark features in Ca ii H chromospheric filtergrams and Stokes I images. The features appear in regions with close-to-zero longitudinal magnetic field, and are observed to increase in length before they eventually disappear. Energy release in the low chromospheric line is detected while the dark features are fading. Three complete series of these events are detected with remarkably similar properties, i.e. lifetime of ≈ 12 min, maximum length and area of 2 – 4 Mm and 1.6 – 4 Mm2, respectively, and all with associated brightenings. In time series of magnetograms a diverging bipolar configuration is observed accompanying the appearance of the dark features and the brightenings. The observed phenomena are explained as evidencing elementary flux emergence in the solar atmosphere, i.e. small-scale arch filament systems rising up from the photosphere to the lower chromosphere with a length scale of a few solar granules. Brightenings are explained as being the signatures of chromospheric heating triggered by reconnection of the rising loops (once they have reached chromospheric heights) with pre-existing magnetic fields, as well as being due to reconnection/cancellation events in U-loop segments of emerging serpentine fields. The characteristic length scale, area and lifetime of these elementary flux emergence events agree well with those of the serpentine field observed in emerging active regions. We study the temporal evolution and dynamics of the events and compare them with the emergence of magnetic loops detected in quiet Sun regions and serpentine flux emergence signatures in active regions. The physical processes of the emergence of granular-scale magnetic loops seem to be the same in the quiet Sun and active regions. The difference is the reduced chromospheric emission in the quiet Sun attributed to the fact that loops are emerging in a region of lower ambient magnetic field density, making interactions and reconnection less likely to occur. Incorporating the novel features of granular-scale flux emergence presented in this study, we advance the scenario for serpentine flux emergence.

Statistical Study on the Nature of Solar-Flux Emergence

We studied 101 flux emergence events ranging from small ephemeral regions to large emerging flux regions that were observed with the Hinode Solar Optical Telescope filtergram. We investigated how the total magnetic flux of the emergence event controls the nature of emergence. To determine the modes of emergences, horizontal velocity fields of the global motion of the magnetic patches in the flux emerging sites were measured by local correlation tracking. Between two main polarities of the large emerging flux regions with more than around 2 $\times$ 10$^{19}$ Mx, there were converging flows of anti-polarity magnetic patches. On the other hand, small ephemeral regions showed no converging flow, but a simple diverging pattern. When we looked into the detailed features in the emerging sites, irrespective of the total flux and the spatial size, all of the emergence events were observed to consist of single or multiple elementary emergence unit(s). The typical size of unitary emergence is 4 Mm, and consistent with simulation results. From a statistical study of the flux emergence events, the maximum spatial distance between two main polarities, the magnetic flux growth rate and the mean separation speed were found to follow the power-law functions of the total magnetic flux with indices of 0.27, 0.57, and $-$0.16, respectively. From a discussion on the observed power-law relations, we obtained a physical view of solar flux emergence, in which the emerging magnetic fields float and evolve while balancing to the surrounding turbulent atmosphere.

PHOTOSPHERIC SIGNATURES OF GRANULAR-SCALE FLUX EMERGENCE AND CANCELLATION AT THE PENUMBRAL BOUNDARY

We studied flux emergence events of sub-granular scale in a solar active region. New Solar Telescope (NST) of Big Bear Solar Observatory made it possible to clearly observe the photospheric signature of flux emergence with very high spatial (0".11 at 7057{\AA}) and temporal (15 s) resolution. From TiO observations with the pixel scale of 0".0375, we found several elongated granule-like features (GLFs) stretching from the penumbral filaments of a sunspot at a relatively high speed of over 4 km s-1. After a slender arched darkening appeared at a tip of a penumbral filament, a bright point (BP) developed and quickly moved away from the filament forming and stretching a GLF. The size of a GLF was approximately 0.5" wide and 3" long. The moving BP encountered nearby structures after several minutes of stretching, and a well-defined elongated shape of a GLF faded away. Magnetograms from SDO/HMI and NST/IRIM revealed that those GLFs are photospheric indicators of small-scale flux emergence, and their disappearance is related to magnetic cancellation. From two well-observed events, we describe detailed development of the sub-structures of GLFs, and different cancellation processes that each of the two GLFs underwent.

Small-Scale Flux Emergence Observed Using Hinode/SOT

The aim of this paper is to determine the flux emergence rate due to small-scale magnetic features in the quiet Sun using high-resolution Hinode SOT NFI data. Small-scale magnetic features are identified in the data using two different feature identification methods (clumping and downhill); then three methods are applied to detect flux emergence events. The distribution of the intranetwork peak emerged fluxes is determined. When combined with previous emergence results, from ephemeral regions to sunspots, the distribution of all fluxes are found to follow a power-law distribution which spans nearly seven orders of magnitude in flux (1016 – 1023 Mx) and 18 orders of magnitude in frequency. The power-law fit to all these data is of the form $$\frac{\mathrm{d}N}{\mathrm{d}\Psi} = \frac{n_0}{\Psi_0}\frac{\Psi}{\Psi _0}^{-2.7},$$ where Ψ0=1016 Mx and is used to predict a global flux emergence rate of ≈ 450 Mx cm−2 day−1 from all features with fluxes of 1016 Mx or more. Since the slope of all emerged fluxes is less than −2, this implies that most of the new flux that is fed into the solar atmosphere is from small-scale emerging events. This suggests that the rate of flux emergence is independent of the solar cycle and is equivalent to a global rate of flux emergence of more than a few times 1025 Mx day−1. The single power-law distribution over all emerged fluxes implies a scale-free dynamo, therefore indicating that a turbulent dynamo may act throughout the convection zone. Moreover, from the slope of the emerging flux distribution the (turbulent?) dynamo producing small-scale features produces considerably more flux than the active-region dynamo at the tachocline.

THE DETECTION OF NUMEROUS MAGNETIC SEPARATORS IN A THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC MODEL OF SOLAR EMERGING FLUX

Magnetic separators in three-dimensional (3D) magnetic fields are believed to be often associated with locations of magnetic reconnection. In this preliminary study, we investigate this relationship using data from a numerical resistive 3D MHD experiment of a solar flux emergence event. For the first time separators are detected in complex magnetic fields resulting from a 3D resistive MHD model of flux emergence. Two snapshots of the model, taken from different stages of its evolution, are analyzed. Numerous separators are found in both snapshots, and their properties, including their geometry, length, relationship to the magnetic null points, and integrated parallel electric field are studied. The separators reside at the junctions between the emerging flux, the overlying field, and two other flux domains that are newly formed by reconnection. The long separators, which connect clusters of nulls that lie either side of the emerging flux, pass through spatially localized regions of high parallel electric field and correspond to local maxima in integrated parallel electric field. These factors indicate that strong magnetic reconnection takes place along many of the separators, and that separators play a key role during the interaction of emerging and overlying flux.

MULTIWAVELENGTH OBSERVATIONS OF SMALL-SCALE RECONNECTION EVENTS TRIGGERED BY MAGNETIC FLUX EMERGENCE IN THE SOLAR ATMOSPHERE

The interaction between emerging magnetic flux and the pre-existing ambient field has become a "hot" topic for both numerical simulations and high-resolution observations of the solar atmosphere. The appearance of brightenings and surges during episodes of flux emergence is believed to be a signature of magnetic reconnection processes. We present an analysis of a small-scale flux emergence event in NOAA 10971, observed simultaneously with the Swedish 1-m Solar Telescope on La Palma and the \emph{Hinode} satellite during a joint campaign in September 2007. Extremely high-resolution G-band, H$\alpha$, and \ion{Ca}{2} H filtergrams, \ion{Fe}{1} and \ion{Na}{1} magnetograms, EUV raster scans, and X-ray images show that the emerging region was associated with chromospheric, transition region and coronal brightenings, as well as with chromospheric surges. We suggest that these features were caused by magnetic reconnection at low altitude in the atmosphere. To support this idea, we perform potential and linear force-free field extrapolations using the FROMAGE service. The extrapolations show that the emergence site is cospatial with a 3D null point, from which a spine originates. This magnetic configuration and the overall orientation of the field lines above the emerging flux region are compatible with the structures observed in the different atmospheric layers, and remain stable against variations of the force-free field parameter. Our analysis supports the predictions of recent 3D numerical simulations that energetic phenomena may result from the interaction between emerging flux and the pre-existing chromospheric and coronal field.

Small-scale flux emergence events observed by Sunrise/IMaX

AbstractThanks to the unprecedented combination of high spatial resolution (0″.2) and high temporal cadence (33 s) spectropolarimetric measurements, the IMaX magnetograph aboard the Sunrise balloon-borne telescope is revealing new insights about the plasma dynamics of the all-pervasive small-scale flux concentrations in the quiet Sun. We present the result of a case study concerning the appearance of a bipole, with a size of about 4″ and a flux content of 5 × 1017 Mx, with strong signal of horizontal fields during the emergence. We analyze the data set using the SIR inversion code and obtain indications about the three-dimensional shape of the bipole and its evolution with time.