Emergence Of New Flux Research Articles

The Solar Internetwork. III. Unipolar versus Bipolar Flux Appearance

Abstract Small-scale internetwork (IN) magnetic fields are considered to be the main building blocks of quiet Sun magnetism. For this reason, it is crucial to understand how they appear on the solar surface. Here, we employ a high-resolution, high-sensitivity, long-duration Hinode/NFI magnetogram sequence to analyze the appearance modes and spatiotemporal evolution of individual IN magnetic elements inside a supergranular cell at the disk center. From identification of flux patches and magnetofrictional simulations, we show that there are two distinct populations of IN flux concentrations: unipolar and bipolar features. Bipolar features tend to be bigger and stronger than unipolar features. They also live longer and carry more flux per feature. Both types of flux concentrations appear uniformly over the solar surface. However, we argue that bipolar features truly represent the emergence of new flux on the solar surface, while unipolar features seem to be formed by the coalescence of background flux. Magnetic bipoles appear at a faster rate than unipolar features (68 as opposed to 55 Mx cm−2 day−1), and provide about 70% of the total instantaneous IN flux detected in the interior of the supergranule.

The magnetic properties of photospheric magnetic bright points with high-resolution spectropolarimetry

ABSTRACTMagnetic bright points (MBPs) are small-scale magnetic elements ubiquitous across the solar disc, with the prevailing theory suggesting that they form due to the process of convective collapse. Employing a unique full Stokes spectropolarimetric data set of a quiet Sun region close to disc centre obtained with the Swedish Solar Telescope, we look at general trends in the properties of magnetic bright points. In total we track 300 MBPs in the data set and we employ NICOLE inversions to ascertain various parameters for the bright points such as line-of-sight magnetic field strength and line-of-sight velocity, for comparison. We observe a bimodal distribution in terms of maximum magnetic field strength in the bright points with peaks at ∼480 G and ∼1700 G, although we cannot attribute the kilogauss fields in this distribution solely to the process of convective collapse. Analysis of muram simulations does not return the same bimodal distribution. However, the simulations provide strong evidence that the emergence of new flux and diffusion of this new flux play a significant role in generating the weak bright point distribution seen in our observations.

Small Coronal Holes Near Active Regions as Sources of Slow Solar Wind

Abstract We discuss the nature of the small areas of rapidly diverging, open magnetic flux that form in the strong unipolar fields at the peripheries of active regions (ARs), according to coronal extrapolations of photospheric field measurements. Because such regions usually have dark counterparts in extreme-ultraviolet (EUV) images, we refer to them as coronal holes, even when they appear as narrow lanes or contain sunspots. Revisiting previously identified “AR sources” of slow solar wind from 1998 and 1999, we find that they are all associated with EUV coronal holes; the absence of well-defined He i 1083.0 nm counterparts to some of these holes is attributed to the large flux of photoionizing radiation from neighboring AR loops. Examining a number of AR-associated EUV holes during the 2014 activity maximum, we confirm that they are characterized by wind speeds of ∼300–450 km s−1, O7+/O6+ ratios of ∼0.05–0.4, and footpoint field strengths typically of order 30 G. The close spacing between ARs at sunspot maximum limits the widths of unipolar regions and their embedded holes, while the continual emergence of new flux leads to rapid changes in the hole boundaries. Because of the highly nonradial nature of AR fields, the smaller EUV holes are often masked by the overlying canopy of loops, and may be more visible toward one solar limb than at central meridian. As sunspot activity declines, the AR remnants merge to form much larger, weaker, and longer-lived unipolar regions, which harbor the “classical” coronal holes that produce recurrent high-speed streams.

EVOLUTION AND DYNAMICS OF ORPHAN PENUMBRAE IN THE SOLAR PHOTOSPHERE: ANALYSIS FROM MULTI-INSTRUMENT OBSERVATIONS

We investigate the dynamics and magnetic properties of orphan penumbrae observed in the solar photosphere to understand the formation process of such structures. We observed two orphan penumbrae in active region NOAA 11089 during a coordinated observing campaign carried out in 2010 July, involving the Hinode/Solar Optical Telescope (SOT) and Dutch Open Telescope (DOT), benefiting also from continuous observations acquired by the SDO satellite. We follow their evolution during about three days. The two structures form in different ways: one seems to break off the penumbra of a nearby sunspot, the other is formed through the emergence of new flux. Then they fragment while evolving. The SDO Helioseismic and Magnetic Imager measurements indicate the presence of strong line-of-sight motions in the regions occupied by these orphan penumbrae, lasting for several hours and decreasing with time. This is confirmed by SOT spectro-polarimetric measurements of the Fe I 630.2 nm pair. The latter also show that Stokes parameters exhibit significant asymmetries in the orphan penumbral regions, typical of an uncombed filamentary structure. The orphan penumbrae lie above polarity inversion lines, where peculiar plasma motions take place with velocities larger than ±3 km s{sup –1}. The vector magnetic field in these regions ismore » highly inclined, with the average magnetic field strength decreasing with time. The DOT observations in the Hα line and SDO Atmospheric Imaging Assembly measurements in the He II 30.4 nm line indicate that there is no counterpart for the orphan penumbrae at midchromospheric heights or above. Our findings suggest that in at least one of the features investigated the emerging flux may be trapped in the low atmospheric layers by the overlying pre-existing fields, forming these filamentary structures.« less

Emerging Active Regions Studied with Ring‐Diagram Analysis

We study the temporal variation of subsurface flows associated with emerging active regions, focusing on four regions in detail. Two of them, AR 10314 and AR 10488, emerge near disk center and the other two, AR 10365 and AR 10375, are older regions where new flux emerges during their disk passage. We measure the horizontal subsurface flows from high-resolution Global Oscillation Network Group (GONG) data using ring-diagram analysis and derive the vertical flow component. Before flux emergence, we find upflows in AR 10314, while the other emerging region, AR 10488, shows mainly weak vertical flows. Both aging regions, AR 10365 and AR 10375, initially show downflows, as expected from already established regions. When new flux emerges, the weaker one of the two, AR 10365, shows upflows, while AR 10375 shows an even stronger downflow. In strong active regions, such as AR 10375 and AR 10488, strong downflows are present after the region has been established. In all four regions, the transition occurs on timescales of about one to two days. As a control experiment, we repeat the analysis for the same locations as those of the four active regions in 53 Carrington rotations and find that it is unlikely that the temporal variations of the vertical velocity are caused by systematics such as a projection effect. We then search our data set for emerging regions with similar characteristics to AR 10314 and AR 10488, i.e., emergence near disk center and large flux increase. From an analysis of 13 emerging regions, we conclude that there is a small preference for upflows before the emergence of new flux and for a transition toward downflows after flux emergence.

Solar Cycle Prediction Using Precursors and Flux Transport Models

We study the origin of the predictive skill of some methods to forecast the strength of solar activity cycles. A simple flux transport model for the azimuthally averaged radial magnetic field at the solar surface is used, which contains a source term describing the emergence of new flux based on observational sunspot data. We consider the magnetic flux diffusing over the equator as a predictor, since this quantity is directly related to the global dipole field from which a Babcock-Leighton dynamo generates the toroidal field for the next activity cycle. If the source is represented schematically by a narrow activity belt drifting with constant speed over a fixed range of latitudes between activity minima, our predictor shows considerable predictive skill with correlation coefficients up to 0.95 for past cycles. However, the predictive skill is completely lost when the actually observed emergence latitudes are used. This result originates from the fact that the precursor amplitude is determined by the sunspot activity a few years before solar minimum. Since stronger cycles tend to rise faster to their maximum activity (known as the Waldmeier effect), the temporal overlapping of cycles leads to a shift of the minimum epochs that depends on the strength of the following cycle. This information is picked up by precursor methods and also by our flux transport model with a schematic source. Therefore, their predictive skill does not require a memory, i.e., a physical connection between the surface manifestations of subsequent activity cycles.

Radio and Hard X‐Ray Imaging Observations of the M5.7 Flare of 2002 March 14

We describe a flare of GOES class M5.7 that was observed simultaneously by RHESSI (Ramaty High Energy Solar Spectroscopic Imager) and NoRH (Nobeyama Radio Heliograph). The flare occurred in AR 9866 located near the disk center. The hard X-ray (HXR), microwave, EIT, and TRACE 195 A observations indicate that the flaring region consisted of a complex of multiple loops. In the microwave domain the source morphology, the timing, the polarization characteristics, and the photospheric magnetic fields clearly indicate that it is of a class characterized as a double configuration, meaning two systems of magnetic flux, each consisting of many smaller loops. The observations suggest the existence of a small loop system created by the emergence of new flux, which interacts with an old flux system, and of a remote flare site that is observed primarily in radio. The former is the main flare site where we observe microwave, HXR, and EUV emissions. In HXR there are two main identifiable loop systems. The first is an elongated one filled with energetic electrons primarily emitting lower energy (12-25 keV) HXR with a colocated microwave source; this source has distinct footpoints at higher X-ray energies. The second loop system is implied by compact HXR sources in opposite magnetic polarities separated by a distance greater than the length of the first loop system. Spectroscopic analysis of the RHESSI data shows that the spectrum can be fitted with a thick-target model with a thermal component and a broken power-law component of the electron energy distribution. This model is used to address the thermal/nonthermal and radio/HXR electron number problems.

Magnetic Field, Hα, andRHESSIObservations of the 2002 July 23 Gamma‐Ray Flare

In this paper we examine two aspects of the 2002 July 23 gamma-ray flare by using multiwavelength observations. First, the data suggest that the interaction of the erupted field with an overlying large-scale coronal field can explain the offset between the gamma-ray and the hard X-ray sources observed in this event. Second, we pay attention to rapid and permanent changes in the photospheric magnetic field associated with the flare. MDI and BBSO magnetograms show that the following magnetic flux had rapidly decreased by 1 × 1020 Mx immediately after the flare, while the leading polarity was gradually increasing for several hours after the flare. Our study also suggests that the changes were most probably associated with the emergence of new flux and the reorientation of the magnetic field lines. We interpret the magnetograph and spectral data for this event in terms of the tether-cutting model.

Nature of the magnetic carpet – I. Distribution of magnetic fluxes

The distribution of magnetic fluxes found in the solar magnetic carpet, the photospheric magnetic field of the quiet-Sun, is investigated. A total of 27 500 concentrations are studied with fluxes ranging between a few times 10 1 7 and a few times 10 2 0 Mx. A histogram of the fluxes shows that there are more small fluxes and more large fluxes than anticipated by an exponential distribution. However, there are significantly fewer large and small fluxes than a power-law distribution fitted to the middle of the range. Thus, the fluxes are not distributed in the form of either an exponential or a power-law distribution, as previously suggested. Instead, the Weibull distribution, which involves both a power law and an exponential, is found to provide both a statistically good fit to the data and a physically reasonable prediction for the total absolute flux density. The best-fitting Weibull distribution has a Kolmogorov-Smirnov D statistic of approximately 0.07 - well below the 5 per cent significance level and the Weibull distribution predicts the observed total absolute flux densities to within better than 92 per cent. Physically, the observed distribution of fluxes can be explained as being made up of the following three elements: (i) emergence of new flux over all scales gives rise to an initial exponential distribution (observed); (ii) fragmentation and partial cancellation produce smaller and smaller fluxes thus creating excess small fluxes; (iii) excess large fluxes are created by coalescence and an additional (possibly significant) injection of flux from remnants of active regions. Finally, the equations of 'magneto-chemistry' are used to derive suitable forms for the rate of emergence, cancellation, coalescence and fragmentation consistent with a Weibull distribution of fluxes.

Prominence eruptions and coronal mass ejections triggered by newly emerging flux

Using a simple model for the onset of solar eruptions, we investigate how an existing magnetic configuration containing a flux rope evolves in response to new emerging flux. Our results show that the emergence of new flux can cause a loss of ideal MHD equilibrium under certain circumstances, but the circumstances which lead to eruption are much richer and more complicated than one might expect given the simplicity of the model. The model results suggest that the actual circumstances leading to an eruption are sensitive not only to the polarity of the emerging region, but also to several other parameters, such as the strength, distance, and area of the emerging region. It has been suggested by various researchers that the emergence of new flux with an orientation which allows reconnection with the preexisting flux (a process sometimes referred to as tether cutting) will generally lead to destabilization of the coronal or prominence magnetic field. Although our results can replicate such behavior for certain restricted classes of boundary conditions, we find that, in general, there is no simple, universal relation between the orientation of the emerging flux and the likelihood of an eruption.

A model of the Solar Magnetic Carpet

There are four key processes that dictate the behaviorof the magnetic flux concentrations that form the so-called `magnetic carpet' of the quiet photosphere. These processes are emergence, cancellation, coalescence, and fragmentation. Rates of emergence have been estimated from observations, but the rates of cancellation, coalescence, and fragmentation are much more difficult to determine observationally. A model is set up to simulate an area of magnetic carpet in the quiet Sun. In the model there are three imposed parameters: the rate of emergence of new flux, the distribution of emerged flux and the rate of fragmentation of flux concentrations. The rate of cancellation and the rate of coalescence are deduced from the model. From the simulations it is estimated that the average emergence rate of new flux in the quiet Sun must be between 6×10−6 and 10− 5 Mx cm−2 s−1 to maintain an absolute flux density of between 2.5 and 3 G. For this rate of emergence a fragmentation rate of more than 12×10−5 s−1 is required to produce the observed exponential index for the number density of flux concentrations. This is equivalent to each fragment canceling more than once every 200 minutes. The rate of cancellation is calculated from the model and is found naturally to be equivalent to the rate of emergence. However, it is found that the frequency of cancellation is much greater than the frequency of emergence. In fact, it is likely that there are several orders of magnitude more cancellation events than emergence events. This implies that flux is injected in relatively large concentrations whereas cancellation occurs though the disappearance of many small concentrations.

Acceleration of the fast solar wind by the emergence of new magnetic flux

Recent observations have shown that small magnetic loops are continuously emerging within supergranules in the solar photosphere. The subsequent reconnection of this emerging flux with field lines which open into the corona should define the Poynting's vector and mass flux into the corona. These two quantities uniquely determine the final energy flow and speed of the steady, fast solar wind. It is also pointed out that if the consequence of the emergence of new flux and its reconnection is lateral displacements of coronal magnetic field lines, this process should increase the magnetic field energy in the corona in a definable manner. The resulting dissipation of this energy, by work done on the plasma or by Joule heating, defines the spatial distribution for the heating of the corona. A simple model is constructed that yields profiles for the acceleration of the fast solar wind and for coronal temperatures that are reasonable.

Bright Points and Subflares in Ultraviolet Lines and X‐Rays

We have analyzed an active region which was observed in H.alpha (Multichannel Subtractive Double Pass Spectrograph), in UV lines (SMM/UVSP), and in X-rays (SMM/HXIS). In this active region there were only a few subflares and many small bright points visible in UV and in X-rays. Using an extrapolation based on the Fourier transform, we have computed magnetic field lines connecting different photospheric magnetic polarities from ground-based magnetograms. Along the magnetic inversion lines we find two different zones: (1) a high-shear region (> 70 deg) where subflares occur, and (2) a low-shear region along the magnetic inversion line where UV bright points are observed. In these latter regions the magnetic topology is complex with a mixture of polarities. According to the velocity field observed in the Si IV lamda.1402 line and the extrapolation of the magnetic field, we notice that each UV bright point is consistent with emission from low-rising loops with downflows at both ends. We notice some hard X-ray emissions above the bright-point regions with temperatures up to 8 x 10(exp 6) K, which suggests some induced reconnection due to continuous emergence of new flux. This reconnection is also enhanced by neighboring subflares.

Emerging flux and X-class flares in NOAA 6555

The active region NOAA 6555 had several locations of highly sheared magnetic field structure, yet, only one of them was the site for all the five X-class flares during its disk passage in March 1991. The pre-flare observations of high-resolution Hα filtergrams, vector magnetograms and Hβ Dopplergrams of the 2B/X5.3 flare on 25 March 1991 show that the flaring site was characterized by a new rising ‘emerging flux region’ (EFR) near the highly sheared magnetic field configuration. The polarity axis of the emerging flux was nearly perpendicular to the pre-existing magnetic neutral line. The location of the EFR was the site of initial brightening in Hα. The post-flare magnetograms show higher magnetic shear at the flare location compared to the post-flare magnetograms, which might indicate that the EFR was sheared at the time of its emergence. As the new EFR coincided with the occurrence of the flare, we suggest that it might have triggered the observed flare. Observations from Big Bear Solar Observatory and Marshall Space Flight Center also show that there was emergence of new flux at the same location prior to two other X-class flares. We find that out of five observed X-class flares in NOAA 6555, at least in three cases there are clear signatures of flare-related flux emergence. Therefore, it is concluded that EFRs might play an important role in destabilizing the observed sheared magnetic structures leading to large X-class flares of NOAA 6555.

Basic magnetic field configurations for filament channels and filaments

The three-dimensional magnetic structure of solar filament channels and filaments is considered. A simple analytical potential model of a filament channel is setup with line sources representing the overlying arcades and point sources the flux of the filament. A possible explanation of the distinct upper and lower bounds of a filament is given. A more detailed numerical force-free model with discrete flux sources is then developed and the effect of magnetic shear on the separatrix surface explored. Dextral channels are shown to exist for a wider range of negative values of the force-free alpha and sinistral channels for positive values of alpha. Potential models of a variety of coronal structures are then considered. The bending of a filament is modelled and a method of determining the horizontal component of a filament's magnetic field is proposed. Next, the observed opposite skew of arcades lying above switchbacks of polarity inversion lines is shown to be produced by a local flux imbalance at the corner of the switchback. Then, the magnetic structure of a particular filament in a filament channel is modelled using observations from a photospheric magnetogram. It is shown that dips in the filaments magnetic field could result from opposite polarity fragments lying below the filament. Finally, the formation of a specific filament channel and filament is modelled. The formation of the channel is shown to be due to the emergence of new flux in a sheared state. It is shown that convergence and reconnections between the new flux and old remnant flux is required for the filament to form. The field lines that represent the filament form a thin vertical sheet of flux. The changing angle of inclination of the sheet gives the appearance of twist. The method of formation is then generalised to other cases and it is shown that a hemispheric pattern consistent with the results of Martin et al. (1995) can be obtained.

3-D reconnection related to new emerging flux

We present evidences that emergence of new flux in the lower atmosphere leads to magnetic reconnection of field lines. In a first phase the phenomenon is observed in the chromosphere by the formation of dark filaments (arch filament system) which are overlaid by bright loops visible in soft X-rays. Different types of event appear according to the magnetic field configuration and the amount of energy involved. 3-D modelling of the photospheric magnetic field provides a new tool for understanding reconnection in real configurations. The observed chromospheric and coronal loops are good diagnostics for the modelling. We document our statement by examples obtained during coordinated campaigns with the Hα Multichannel Subtractive Double Pass spectrographs-MSDP (Pic du Midi and Tenerife) and the Yohkoh instruments.

Heating and activity of the solar corona: 1. Boundary shearing of an initially homogeneous magnetic field

To contribute to the understanding of heating and dynamic activity in boundary‐driven, low‐beta plasmas such as the solar corona, we investigate how an initially homogeneous magnetic field responds to random large‐scale shearing motions on two boundaries, by numerically solving the dissipative MHD equations, with resolutions ranging from 243 to 1363. We find that even a single application of large‐scale shear, in the form of orthogonal sinusoidal shear on two boundaries, leads to the formation of tangential discontinuities (current sheets). The formation time scales logarithmically with the resistivity and is of the order of a few times the inverse shearing rate for any reasonable resistivity, even though no mathematical discontinuity would form in a finite time in the limit of vanishing resistivity. The reason for the formation of the current sheets is the interlocking of two magnetic flux systems. Reconnection in the current sheets is necessary for the field lines to straighten out. The formation of current sheets causes a transition to a very dynamic plasma state, where reconnection drives supersonic and super‐Alfvénic jet flows and where these, in turn, cause the formation of smaller‐scale current sheets. A statistically steady state level for the average Poynting flux and the average Joule dissipation is reached after a few correlation times, but both boundary work and Joule dissipation are highly fluctuating in time and space and are only weakly correlated. Strong and bursty Joule dissipation events are favored when the volume has a large length/diameter ratio and is systematically driven for periods longer than the Alfvèn crossing time. The understanding of the reason for the current sheet formation allows a simple scaling law to be constructed for the average boundary work. Numerical experiments over a range of parameter values, covering over 3 orders of magnitude in average dissipation, obey the scaling law to within a factor of 2. The heating rate depends on the boundary velocity amplitude and correlation time, the Alfvén speed, and the initial magnetic field strength but appears to be independent of the resistivity because of the formation of a hierarchy of current sheets. Estimates of the photospheric boundary work on the solar coronal magnetic field using the scaling law are consistent with estimates of the required coronal heating rates. We therefore conclude that the work supplied to the solar corona as a consequence of the motion of the magnetic foot points in the solar photosphere and the emergence of new flux is a significant contributor to coronal heating and flaring and that it quite plausibly is the dominant one.

Flows, Evolution of Magnetic Fields, and Flares

AbstractThis paper reviews observations on the evolution of magnetic fields and flows in active regions which produce major flares. It includes the following topics: (1) Relationship between magnetic shear and flares; (2) Relationship between electric currents and flares; (3) Flows in active regions, particularly the emergence of new flux inside sheared penumbrae, and the mixed magnetic polarity nature of this kind of flux emergence; and (4) Changes of magnetic structure immediately before and after major solar flares; in particular, I will describe some recent findings that shear may increase after major flares.

The flare-associated magnetic changes in an active region

The associations of flares to flux emergence and cancellation have been further examined and clarified with the aid of complete time sequences of vector magnetograms of an active region for a 4-day period around the central meridian passage. It is found that the emergence of new flux and its driven flux cancellation with existing flux is a wholly inseparable, elementary process in the active region, favorable for flare occurence. The early discovery of structures magnetique evolutive (Martres et al., 1968) is confirmed and identified to be the net result of this process

A high spatial resolution study of microwave flare precursors

We provide several examples of preflare changes from VLA high resolution microwave data, which show that in general the active region increases in intensity and polarization over a period of several tens of minutes prior to the onset of a flare in that region. However, this phenomenon by itself does not appear to be sufficient to trigger a flare. We show that a few minutes (≲ 10 min) just before the flare onset something else happens, usually in the form of one of the following three features: (1) sudden change of polarization of the flaring region; (2) change of orientation of the neutral plane separating one polarity from another in a bipolar region; and (3) appearance of new sources in the immediate vicinity of some pre-existing structure of the active region. All three features are consistent with the emergence of new flux, which interacts with a pre-existing region to form a neutral or current sheet. The formation of the latter is ultimately responsible for triggering the onset of a flare.