Active Region Scale Research Articles

A Comprehensive Radiative Magnetohydrodynamics Simulation of Active Region Scale Flux Emergence from the Convection Zone to the Corona

We present a comprehensive radiative magnetohydrodynamic simulation of the quiet Sun and large solar active regions. The 197 Mm wide simulation domain spans from 18(10) Mm beneath the photosphere to 113 Mm in the solar corona. Radiative transfer assuming local thermal equilibrium, optically thin radiative losses, and anisotropic conduction transport provide the necessary realism for synthesizing observables to compare with remote-sensing observations of the photosphere and corona. This model self-consistently reproduces observed features of the quiet Sun, emerging and developed active regions, and solar flares up to M class. Here, we report an overview of the first results. The surface magneto-convection yields an upward Poynting flux that is dissipated in the corona and heats the plasma to over 1 MK. The quiescent corona also presents ubiquitous propagating waves, jets, and bright points with sizes down to 2 Mm. Magnetic flux bundles emerge into the photosphere and give rise to strong and complex active regions with over 1023 Mx magnetic flux. The coronal free magnetic energy, which is approximately 18% of the total magnetic energy, accumulates to approximately 1033 erg. The coronal magnetic field is clearly non-force-free, as the Lorentz force needs to balance the pressure force and viscous stress as well as drive magnetic field evolution. The emission measure from to provides a comprehensive view of the active region corona, such as coronal loops of various lengths and temperatures, mass circulation by evaporation and condensation, and eruptions from jets to large-scale mass ejections.

Signature of the turbulent component of the solar dynamo on active region scales and its association with flaring activity

ABSTRACT It is a challenging problem to obtain observational evidence of the turbulent component of solar dynamo operating in the convective zone because the dynamo action is hidden below the photosphere. Here we present results of a statistical study of flaring active regions (ARs) that produced strong solar flares of an X-ray class X1.0 and higher during a time period that covered solar cycles 23 and 24. We introduced a magneto-morphological classification of ARs, which allowed us to estimate the possible contribution of the turbulent component of the dynamo into the structure of an AR. We found that in 72 per cent of cases, flaring ARs do not comply with the empirical laws of the global dynamo (frequently they are not bipolar ARs or, if they are, they violate the Hale polarity law, the Joy law, or the leading sunspot prevalence rule). This can be attributed to the influence of the turbulent dynamo action inside the convective zone on spatial scales of typical ARs. Thus, it appears that the flaring is governed by the turbulent component of the solar dynamo. The contribution into the flaring from these AR ‘violators’ (irregular ARs) is enhanced during the second maximum and the descending phase of a solar cycle, when the toroidal field weakens and the influence of the turbulent component becomes more pronounced. These observational findings are in consensus with a concept of the essential role of non-linearities and turbulent intermittence in the magnetic fields generation inside the convective zone, which follows from dynamo simulations.

The Coronal Global Evolutionary Model: Using HMI Vector Magnetogram and Doppler Data to Determine Coronal Magnetic Field Evolution

Abstract The Coronal Global Evolutionary Model (CGEM) provides data-driven simulations of the magnetic field in the solar corona to better understand the build-up of magnetic energy that leads to eruptive events. The CGEM project has developed six capabilities. CGEM modules (1) prepare time series of full-disk vector magnetic field observations to (2) derive the changing electric field in the solar photosphere over active-region scales. This local electric field is (3) incorporated into a surface flux transport model that reconstructs a global electric field that evolves magnetic flux in a consistent way. These electric fields drive a (4) 3D spherical magnetofrictional (SMF) model, either at high resolution over a restricted range of solid angles or at lower resolution over a global domain to determine the magnetic field and current density in the low corona. An SMF-generated initial field above an active region and the evolving electric field at the photosphere are used to drive (5) detailed magnetohydrodynamic (MHD) simulations of active regions in the low corona. SMF or MHD solutions are then used to compute emissivity proxies that can be compared with coronal observations. Finally, a lower-resolution SMF magnetic field is used to initialize (6) a global MHD model that is driven by an SMF electric field time series to simulate the outer corona and heliosphere, ultimately connecting Sun to Earth. As a demonstration, this report features results of CGEM applied to observations of the evolution of NOAA Active Region 11158 in 2011 February.

Flare stars across the H-R diagram: a clue to the origin of the corona

AbstractKepler observations show that starspots and superflares are present in A stars. An analysis of Kepler short-cadence data shows that the relative number of A/F flare stars is only a factor of four smaller than K/M flare stars, which can be explained as a selection effect. The average maximum flare amplitude does not depend much on spectral type, which is to be expected if the size of the active region scales in proportion to the stellar radius. The presence of starspots and superflares in A stars suggests that these stars have magnetic fields. However, X-ray observations show that A stars do not possess coronae. I therefore conclude that convection in the stellar envelope is a necessary condition for the formation of the corona. A magnetic field may be necessary to enable coronal heating.

Spatio-Temporal Scaling of Turbulent Photospheric Line-of-Sight Magnetic Field in Active Region NOAA 11158

We study structure and dynamics of the turbulent photospheric magnetic field in the active region NOAA 11158 by characterizing the spatial and temporal scaling properties of the line-of-sight (LOS) component. Using high-resolution high-cadence LOS magnetograms from SDO/HMI, we measured power-law exponents $\alpha$ and $\beta$ describing wavenumber- ($k$) and frequency-domain ($f$) Fourier power spectra, respectively, and investigated their evolution during the passage of the active region through the field of view of HMI. Flaring active region NOAA 11158 produces an average one-dimensional spatial power spectral density that follows approximately a $k^{-2}$ power law -- a spectrum that suggests parallel MHD fluctuations in an anisotropic turbulent medium. In addition, we found that values of $\alpha$ capture systematic changes in the configuration of LOS photospheric magnetic field during flaring activity in the corona. Position-dependent values of the temporal scaling exponent $\beta$ showed that, on average, the core of the active region scales with $\beta >$ 3 surrounded by a diffusive region with an approximately $f^{-2}$-type spectrum. Our results indicate that only about 1 - 3 \% of the studied LOS photospheric magnetic flux displays $\beta\approx\alpha$, implying that Taylor's hypothesis of frozen-in-flow turbulence is typically invalid for this scalar in presence of turbulent photospheric flows. In consequence, both spatial and temporal variations of the plasma and magnetic field must be included in a complete description of the turbulent evolution of active regions.

Large scale reconstruction of the solar coronal magnetic field

It is now becoming necessary to access the global magnetic structure of the solar low corona at a large scale in order to understand its physics and more particularly the conditions of energization of the magnetic fields and the multiple connections between distant active regions (ARs) which may trigger eruptive events in an almost coordinated way. Various vector magnetographs, either on board spacecraft or ground-based, currently allow to obtain vector synoptic maps, composite magnetograms made of multiple interactive ARs, and full disk magnetograms. We present a method recently developed for reconstructing the global solar coronal magnetic field as a nonlinear force-free magnetic field in spherical geometry, generalizing our previous results in Cartesian geometry. This method is implemented in the new code XTRAPOLS, which thus appears as an extension of our active region scale code XTRAPOL. We apply our method by performing a reconstruction at a specific time for which we dispose of a set of composite data constituted of a vector magnetogram provided by SDO/HMI, embedded in a larger full disk vector magnetogram provided by the same instrument, finally embedded in a synoptic map provided by SOLIS. It turns out to be possible to access the large scale structure of the corona and its energetic contents, and also the AR scale, at which we recover the presence of a twisted flux rope in equilibrium.

NUMERICAL SIMULATIONS OF ACTIVE REGION SCALE FLUX EMERGENCE: FROM SPOT FORMATION TO DECAY

We present numerical simulations of active region scale flux emergence covering a time span of up to 6 days. Flux emergence is driven by a bottom boundary condition that advects a semi-torus of magnetic field with $1.7 \times 10^{22}$ Mx flux into the computational domain. The simulations show that, even in the absense of twist, the magnetic flux is able the rise through the upper $15.5$ Mm of the convection zone and emerge into the photosphere to form spots. We find that spot formation is sensitive to the persistence of upflows at the bottom boundary footpoints, i.e. a continuing upflow would prevent spot formation. In addition, the presence of a torus-aligned flow (such flow into the retrograde direction is expected from angular momentum conservation during the rise of flux ropes through the convection zone) leads to a significant asymmetry between the pair of spots, with the spot corresponding to the leading spot on the Sun being more axisymmetric and coherent, but also forming with a delay relative to the following spot. The spot formation phase transitions directly into a decay phase. Subsurface flows fragment the magnetic field and lead to intrusions of almost field free plasma underneath the photosphere. When such intrusions reach photospheric layers, the spot fragments. The time scale for spot decay is comparable to the longest convective time scales present in the simulation domain. We find that the dispersal of flux from a simulated spot in the first two days of the decay phase is consistent with self-similar decay by turbulent diffusion.

THE RISE OF ACTIVE REGION FLUX TUBES IN THE TURBULENT SOLAR CONVECTIVE ENVELOPE

We use a thin flux tube model in a rotating spherical shell of turbulent convective flows to study how active region scale flux tubes rise buoyantly from the bottom of the convection zone to near the solar surface. We investigate toroidal flux tubes at the base of the convection zone with field strengths ranging from 15 kG to 100 kG at initial latitudes ranging from 1 degree to 40 degrees with a total flux of 10^22 Mx. We find that the dynamic evolution of the flux tube changes from convection dominated to magnetic buoyancy dominated as the initial field strength increases from 15 kG to 100 kG. At 100 kG, the development of Omega-shaped rising loops is mainly controlled by the growth of the magnetic buoyancy instability. However, at low field strengths of 15 kG, the development of rising Omega-shaped loops is largely controlled by convective flows, and properties of the emerging loops are significantly changed compared to previous results in the absence of convection. With convection, rise times are drastically reduced (from years to a few months), loops are able to emerge at low latitudes, and tilt angles of emerging loops are consistent with Joy's Law for initial field strengths of greater than or equal to 40 kG. We also examine other asymmetries that develop between the leading and following legs of the emerging loops. Taking all the results together, we find that mid-range field strengths of approximately 40 - 50 kG produce emerging loops that best match the observed properties of solar active regions.

Magnetic Fields in the Solar Convection Zone

AbstractActive regions on the solar surface are generally thought to originate from a strong toroidal magnetic field generated by a deep seated solar dynamo mechanism operating at the base of the solar convection zone. Thus the magnetic fields need to traverse the entire convection zone before they reach the photosphere to form the observed solar active regions. Understanding this process of active region flux emergence is therefore a crucial component for the study of the solar cycle dynamo. This article reviews studies with regard to the formation and rise of active region scale magnetic flux tubes in the solar convection zone and their emergence into the solar atmosphere as active regions.

How to improve the maps of magnetic helicity injection in active regions?

Magnetic helicity, a topological quantity which measures the twist, the writhe and the shear of a magnetic field, has recently appeared as a key quantity to understand some mechanisms of the solar activity such as Coronal Mass Ejections and flare onset. It is thus becoming of major importance to be able to compute magnetic helicity in active regions. Computing photospheric maps of the injection of magnetic helicity provides new spatial information that helps us to understand basic properties of solar activity, such as where and how magnetic helicity is injected. Several helicity flux density maps have been published for different active regions. Unfortunately, the classical helicity flux density is not a correct physical quantity and it does induce spurious signals (fake polarities) which mask the real injection of helicity. To map the real helicity injection, the knowledge of the complete connectivity of the field lines is fundamental. Even without the connectivity, improved helicity flux density maps can be derived. They have fake polarities which are lower by more than a factor 10 than the previous incorrect maps. Rather than a mixture of negative and positive injection patterns, they show almost unipolar injection on the active region scale. This leads to a completely new way of understanding the dynamics of active regions, in the frame of magnetic helicity studies.

Coronal Loop Widths and Pressure Scale Heights

The scale heights of stratification and the widths of steady solar coronal loops exhibit properties unexplained by standard theory: observed scale heights are often much greater than static theory predicts, while the nearly-constant widths of loop emission signatures defy theoretical expectations for large flux tubes in stratified media. In this work we relate the cross-sectional profile of a coronal flux tube to its density scale height in steady-state plasma flow regimes. Steady flows may shorten or lengthen the scale height according to how the tube cross-sectional area varies with arclength. In a near-potential corona the flux tubes are expected to be sufficiently expansive in many active regions for scale heights to be increased by steady flows. On the other hand, cases where scale lengths are actually increased to observed sizes form a small part of the solution space, close to regimes where density profiles reverse. Therefore, although steady flows are the only steady process known to be capable of extending scale heights significantly, they are not expected to be not responsible for the majority of extended active region scale heights.

What is the spatial distribution of magnetic helicity injected in a solar active region?

Context. Magnetic helicity is suspected to play a key role in solar phenomena such as flares and coronal mass ejections. Several investigations have recently computed the photospheric flux of magnetic helicity in active regions. The derived spatial maps of the helicity flux density, called GA, have an intrinsic mixed-sign patchy distribution. Aims. Pariat et al. (2005) recently showed thatGA is only a proxy of the helicity flux density, which tends to create spurious polarities. They proposed a better proxy, Gθ. We investigate here the implications of this new approach on observed active regions. Methods. The magnetic data are from MDI/SoHO instrument and the photospheric velocities are computed by local correlation tracking. Maps and temporal evolution of GA and Gθ are compared using the same data set for 5 active regions. Results. Unlike the usual GA maps, most of our Gθ maps show almost unipolar spatial structures because the nondominant helicity flux densities are significantly suppressed. In a few cases, the Gθ maps still contain spurious bipolar signals. With further modelling we infer that the real helicity flux density is again unipolar. On time-scales larger than their transient temporal variations, the time evolution of the total helicity fluxes derived from GA and Gθ show small differences. However, unlike GA, with Gθ the time evolution of the total flux is determined primarily by the predominant-signed flux while the nondominant-signed flux is roughly stable and probably mostly due to noise. Conclusions. Our results strongly support the conclusion that the spatial distribution of helicity injected into active regions is much more coherent than previously thought: on the active region scale the sign of the injected helicity is predominantly uniform. These results have implications for the generation of the magnetic field (dynamo) and for the physics of both flares and coronal mass ejections.

Magnetic Fields in the Solar Convection Zone

Active regions on the solar surface are generally thought to originate from a strong toroidal magnetic field generated by a deep seated solar dynamo mechanism operating at the base of the solar convection zone. Thus the magnetic fields need to traverse the entire convection zone before they reach the photosphere to form the observed solar active regions. Understanding this process of active region flux emergence is therefore a crucial component for the study of the solar cycle dynamo. This article reviews studies with regard to the formation and rise of active region scale magnetic flux tubes in the solar convection zone and their emergence into the solar atmosphere as active regions.

Post-Eruptive Flare Energy Release as Detected on AU Mic by EUVE

The long-duration emission arising after the impulsive rise and decay in a flaring event observed by the Extreme Ultraviolet Explorer on the red dwarf star AU Mic is discussed. The decay of the intensity in the Deep Survey 65–190 Å band and in the Fe XVIII line during this prolonged event is 10 times slower than the time of radiative cooling of coronal loops with the typical for the flare plasma density. The temporal behavior of the emission measure is determined for both the 65–190 Å band and the Fe XVIII line fluxes. The total energy emitted in the 1–2000 Å region over nearly 12 hrs is 3 · 1035 ergs. We first point out some difficulties with earlier explanations proposed for this event; we then propose the following physical model: the source of the prolonged emission is a system of high coronal loops, the size of which is more than the active region scale, but less than the stellar radius. Such systems are observed in soft X-rays during large solar flares after coronal mass ejections. Some additional post-flare energy input into this high coronal loop system can be caused by reconnection in a vertical current sheet, and this post-eruptive energy release provides prolonged and intensive EUV emission.Apparently, we are faced here with new kind of the surface activity on late-type stars which is intermediate between impulsive flares on red dwarfs and long-duration, powerful events the subgiants components of the RS CVn binaries.

Post-flare Coronal Activity on AU Mic Detected by EUVE

Data of long-duration emission arising after the impulsive rise and decay in a flaring event on the red dwarf star AU Mic are discussed. Intensive EUV emission in the band 65-190 Å was registered by the Extreme Ultraviolet Explorer (EUVE) after both impulses during half a day. A similar behavior of the flux in the Fe XVIII 93.9 Å line is detected after the first powerful impulse. The decay of the intensity in the 65-190 Å band and in the Fe XVIII line during this prolonged event is 10 times slower than the time of radiative cooling of coronal loops with a typical flare plasma density. Some difficulties with two explanations of this event proposed earlier are discussed. Explanation (i) - the radiation of dense loops at main phase of the flare, and (ii) - the emission of the low-dense plasma of coronal transients (CME). The temporal behavior of the emission measure is determined for both the 65-190 Å band and the Fe XVIII line fluxes. The total energy emitted in the 1-2000 Å region for the long-duration event lasting almost 12 hours is 3 · 1035 ergs. The following physical model is proposed to explain the prolonged event (Fig. 1): the source of emission is the system of high coronal loops, the size of which is more than the active region scale, but is less than the stellar radius. The temperature of the plasma in the loops decreases from 107 K slowly, during a few hours. The densities in these loops are in the range 1013 cm−3 to 5 · 109 cm−3. Such systems, when the plasma therein becomes cool, are observed in the Hα line during large solar flares (for instance, June 15, 1991) after CME. Some additional post-flare energy input into this high coronal loop systems can be caused by the reconnection in vertical current sheet, and this post-eruptive energy release provides prolonged and intensive EUV emission.Apparently, we are faced here with a new kind of surface activity on late-type stars which is intermediate between impulsive flares on red dwarfs and long-duration, powerful events on subgiants, which are components of RS CVn binaries.The full version of this contribution will be published in Astronomicheskij Zhurnal (Astronomy Reports), 1995, Vol. 72.