Mesogranular Scales Research Articles

Novel Approach to Forecasting Photospheric Emergence of Active Regions

One key aspect of understanding the solar dynamo mechanism and the evolution of solar magnetism is to properly describe the emergence of solar active regions. In this Letter, we describe the Lagrangian photospheric flows dynamics during a simulated flux emergence that produces an active region formed by pores. We analyze the lower photospheric flow organization prior, during and following the rise of an active region, uncovering the repelling and attracting photospheric structures that act as sources and sinks for magnetic element transport. Our results show that around 10 hr before the simulated emergence, considerable global changes are taking place on mesogranular scales indicated by an increase of the number of regions acting as a source to the multiple and scattered emergences of small-scale magnetic flux. At the location of active region’s appearance, the converging flows become weaker and there is an arising of a diverging region 8 hr before the emergence time. Our study also indicates that the strong concentration of magnetic field affects the flow dynamics beyond the area of the actual simulated pores, leading to complex and strongly diverging flows in the neighboring regions. Our findings suggest that the Lagrangian analysis is a powerful tool to describe the changes in the photospheric flows due to magnetic flux emergence.

Identifying Acoustic Wave Sources on the Sun. I. Two-dimensional Waves in a Simulated Photosphere

Abstract The solar acoustic oscillations are likely stochastically excited by convective dynamics in the solar photosphere, though few direct observations of individual source events have been made and their detailed characteristics are still unknown. Wave source identification requires measurements that can reliably discriminate the local wave signal from the background convective motions and resonant modal power. This is quite challenging as these noise contributions have amplitudes several orders of magnitude greater than the sources and the propagating wave fields they induce. In this paper, we employ a high-temporal-frequency filter to identify sites of acoustic emission in a radiative magnetohydrodynamic simulation. The properties of the filter were determined from a convolutional neural network trained to identify the two-dimensional acoustic Green’s function response of the atmosphere, but once defined, it can be directly applied to an image time series to extract the signal of local wave excitation, bypassing the need for the original neural network. Using the filter developed, we have uncovered previously unknown properties of the acoustic emission process. In the simulation, acoustic events are found to be clustered at mesogranular scales, with peak emission quite deep, about 500 km below the photosphere, and sites of very strong emission can result from the interaction of two supersonic downflows that merge at that depth. We suggest that the method developed, when applied to high-resolution high-cadence observations, such as those forthcoming with the Daniel K. Inouye Solar Telescope, will have important applications in chromospheric wave studies and may lead to new investigations in high-resolution local helioseismology.

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.

Reconstruction of Horizontal Plasma Motions at the Photosphere from Intensitygrams: A Comparison Between DeepVel, LCT, FLCT, and CST

Direct measurements of plasma motions in the photosphere are limited to the line-of-sight component of the velocity. Several algorithms have therefore been developed to reconstruct the transverse components from observed continuum images or magnetograms. We compare the space and time averages of horizontal velocity fields in the photosphere inferred from pairs of consecutive intensitygrams by the LCT, FLCT, and CST methods and the DeepVel neural network in order to identify the method that is best suited for generating synthetic observations to be used for data assimilation. The Stein and Nordlund (Astrophys. J. Lett. 753, L13, 2012) magnetoconvection simulation is used to generate synthetic SDO/HMI intensitygrams and reference flows to train DeepVel. Inferred velocity fields show that DeepVel performs best at subgranular and granular scales and is second only to FLCT at mesogranular and supergranular scales.

Convectively Driven Sinks and Magnetic Fields in the Quiet-Sun

Abstract We study the relation between mesogranular flows, convectively driven sinks and magnetic fields using high spatial resolution spectropolarimetric data acquired with the Imaging Magnetograph eXperiment on board Sunrise. We obtain the horizontal velocity flow fields of two quiet-Sun regions (31.2 × 31.2 Mm2) via local correlation tracking. Mesogranular lanes and the central position of sinks are identified using Lagrange tracers. We find sinks per Mm2 in the two observed regions. The sinks are located at the mesogranular vertices and turn out to be associated with (1) horizontal velocity flows converging to a central point and (2) long-lived downdrafts. The spatial distribution of magnetic fields in the quiet-Sun is also examined. The strongest magnetic fields are preferentially located at sinks. We find that 40% of the pixels with longitudinal components of the magnetic field stronger than 500 G are located in the close neighborhood of sinks. In contrast, the small-scale magnetic loops detected by Martínez González et al. in the same two observed areas do not show any preferential distribution at mesogranular scales. The study of individual examples reveals that sinks can play an important role in the evolution of quiet-Sun magnetic features.

Families of Granules, Flows, and Acoustic Events in the Solar Atmosphere from Hinode Observations

We investigate the relationship between trees of fragmenting granules (TFG), horizontal and vertical flows, and acoustic events (AE) in the photospheric network. AE are spatially concentrated and short-duration locations of acoustic energy flux. We performed observations at disk center of a 2D field of view (FOV) with high spatial and temporal resolutions provided by the Solar Optical Telescope onboard Hinode. Line profiles of Fe i 557.6 nm were recorded by the Narrow-band Filter Imager on an 80″×36″ FOV during five hours with a cadence of 22 seconds and 0.08″ pixel size. Vertical velocities were derived at two atmospheric levels allowing the determination of the energy flux at the acoustic frequency of 3.3 mHz. Families of granules and horizontal velocities were obtained from local correlation tracking (LCT) after segmentation and labeling of either continuum intensities or granular Doppler shifts. AE exhibit durations in the range 0.25 to 1 hour compatible with the lifetime of families (80 % do not last more than two hours). High-energy AE have the shortest lifetimes. We found that most AE occur in intergranular lanes located in or close to the boundaries between different families (called inter families) in regions with predominantly downward vertical motions and horizontal converging flows. In contrast, diverging flows are observed inside families, with a few AE in the intergranules. At the beginning of the sequence, when families are not yet detected, the distribution of AE is not uniform and is already organized at spatial lengths related to the mesogranular scale, with maximum contribution in the range 5″ to 10″, fully compatible with the scale of the maximum contribution of families in the TFG space. Although all sizes and durations seem to exist for families, their number decreases with increasing size and lifetime.

Multiscale Magnetic Underdense Regions on the Solar Surface: Granular and Mesogranular Scales

The Sun is a non-equilibrium dissipative system subjected to an energy flow which originates in its core. Convective overshooting motions create temperature and velocity structures which show a temporal and spatial evolution. As a result, photospheric structures are generally considered to be the direct manifestation of convective plasma motions. The plasma flows on the photosphere govern the motion of single magnetic elements. These elements are arranged in typical patterns which are observed as a variety of multiscale magnetic patterns. High resolution magnetograms of quiet solar surface revealed the presence of magnetic underdense regions in the solar photosphere, commonly called voids, which may be considered a signature of the underlying convective structure. The analysis of such patterns paves the way for the investigation of all turbulent convective scales from granular to global. In order to address the question of magnetic structures driven by turbulent convection at granular and mesogranular scales we used a "voids" detection method. The computed voids distribution shows an exponential behavior at scales between 2 and 10 Mm and the absence of features at 5-10 Mm mesogranular scales. The absence of preferred scales of organization in the 2-10 Mm range supports the multiscale nature of flows on the solar surface and the absence of a mesogranular convective scale.

Spatial structure of gravity waves in the solar photosphere

Observations of the Fe I line are used to simulate spatial and temporal pressure variations in the solar photosphere. The local internal gravity waves, which are essentially structures that are quasi-periodic in space (on granular and mesogranular scales) and time and propagate along inclined paths at subsonic velocities, are isolated by appropriate filtering. The phase and group velocities of the wave trains are orthogonal; their z-projections are of the opposite sign.

Magnetic field emergence in mesogranular-sized exploding granules observed with sunrise/IMaX data

We report on magnetic field emergences covering significant areas of exploding granules. The balloon-borne mission SUNRISE provided high spatial and temporal resolution images of the solar photosphere. Continuum images, longitudinal and transverse magnetic field maps and Dopplergrams obtained by IMaX onboard SUNRISE are analyzed by Local Correlation Traking (LCT), divergence calculation and time slices, Stokes inversions and numerical simulations are also employed. We characterize two mesogranular-scale exploding granules where $\sim$ 10$^{18}$ Mx of magnetic flux emerges. The emergence of weak unipolar longitudinal fields ($\sim$100 G) start with a single visible magnetic polarity, occupying their respective granules' top and following the granular splitting. After a while, mixed polarities start appearing, concentrated in downflow lanes. The events last around 20 min. LCT analyses confirm mesogranular scale expansion, displaying a similar pattern for all the physical properties, and divergence centers match between all of them. We found a similar behaviour with the emergence events in a numerical MHD simulation. Granule expansion velocities are around 1 \kms while magnetic patches expand at 0.65 \kms. One of the analyzed events evidences the emergence of a loop-like structure. Advection of the emerging magnetic flux features is dominated by convective motion resulting from the exploding granule due to the magnetic field frozen in the granular plasma. Intensification of the magnetic field occurs in the intergranular lanes, probably because of being directed by the downflowing plasma.

On the Relation between Surge Activities and a Photospheric Horizontal Velocity Field

A photospheric horizontal velocity field was observed in an active region, NOAA 8375, which showed high H$\alpha $ surge activities on 1998 November 6. From analyses of H$\alpha $ images, the photospheric horizontal velocity field, and the magnetic field in and around the active region, we found the following: (1) Meso-granular or supergranular photospheric horizontal motions were found to be suppressed in the active region where the magnetic filed strength was high. No systematic large-scale flows were found carrying the moving satellite spots with them. (2) Roots of H$\alpha $ surges were situated near a magnetic neutral line, where photospheric horizontal flow converged. Anti-polar magnetic fields on both sides of the neutral line were pushed together by the photospheric horizontal velocity field of meso-granular scales. Magnetic cancellation or magnetic reconnection in the chromospheric and/or photospheric layers at a magnetic neutral line accelerated by photospheric horizontal flows may be the origin of the observed H$\alpha $ surge activities.

The Horizontal Magnetic Flux of the Quiet‐Sun Internetwork as Observed with theHinodeSpectro‐Polarimeter

Observations of very quiet Sun using the Solar Optical Telescope/Spectro-Polarimeter (SOT/SP) aboard the Hinode spacecraft reveal that the quiet internetwork regions are pervaded by horizontal magnetic flux. The spatial average horizontal apparent flux density derived from wavelength-integrated measures of Zeeman-induced linear polarization is -->BTapp = 55 Mx cm −2, as compared to the corresponding average vertical apparent flux density of -->| BLapp| = 11 Mx cm −2. Distributions of apparent flux density are presented. Magnetic fields are organized on mesogranular scales, with both horizontal and vertical fields showing voids of reduced flux density of a few granules spatial extent. The vertical fields are concentrated in the intergranular lanes, whereas the stronger horizontal fields are somewhat separated spatially from the vertical fields and occur most commonly at the edges of the bright granules. High-S/N observations from disk center to the limb help to constrain possible causes of the apparent imbalance between -->| BLapp| and -->BTapp, with unresolved structures of linear dimension on the surface smaller by at least a factor of 2 relative to the SOT/SP angular resolution being one likely cause of this discrepancy. Other scenarios for explaining this imbalance are discussed. The horizontal fields are likely the source of the seething fields of the quiet Sun discovered by Harvey et al. The horizontal fields may also contribute to the hidden turbulent flux suggested by studies involving Hanle effect depolarization of scattered radiation.

Solar Small‐Scale Magnetoconvection

Magnetoconvection simulations on mesogranule and granule scales near the solar surface are used to study the effect of convective motions on magnetic fields: the sweeping of magnetic flux into downflow lanes, the twisting of magnetic field lines, and the emergence and disappearance of magnetic flux tubes. From weak seed fields, convective motions produce highly intermittent magnetic fields in the intergranular lanes that collect over the boundaries of the underlying mesogranular scale cells. Instances of both emerging magnetic flux loops and magnetic flux disappearing from the surface occur in the simulations. We show an example of a flux tube collapsing to kilogauss field strength and a case of flux disappearance due to submergence of the flux. We note that observed Stokes profiles of small magnetic structures are severely distorted by telescope diffraction and seeing, so caution is needed in interpreting low-resolution vector magnetograms of small-scale magnetic structures.

DOT tomography of the solar atmosphere

We use G-band and Ca ii H image sequences from the Dutch Open Telescope (DOT) to study magnetic elements that appear as bright points in internetwork parts of the quiet solar photosphere and chromosphere. We find that many of these bright points appear recurrently with varying intensity and horizontal motion within longer-lived magnetic patches. We develop an algorithm for detection of the patches and find that all patches identified last much longer than the granulation. The patches outline cell patterns on mesogranular scales, indicating that magnetic flux tubes are advected by granular flows to mesogranular boundaries. Statistical analysis of the emergence and disappearance of the patches points to an average patch lifetime as long as (about nine hours), which suggests that the magnetic elements constituting strong internetwork fields are not generated by a local turbulent dynamo.

Spatial pattern of photospheric granules in a sunspot neighborhood

Based on high-resolution (∼0.3 arcsec) observations, we studied the behavior of solar granulation in the neighborhood of a sunspot. The bright granules’ spatial distribution and the granules’ surface density as a function of distance from the center of the sunspot umbra were determined. Bright granules distribute delimiting cells of dimensions in the mesogranular scale. The mean diameter of these cells does not show significant variation with the variation of the magnetic field of the sunspot. The granules’ surface density does not show significant variation with distance to the sunspot umbra. Both results point to a very weak, if any, influence of the sunspot magnetic field at distances greater than 20 arcsec.

Solar granulation properties derived from three different time series

We need automatic pattern recognition algorithms to extract a large statistical sample of granules from high spatial and temporal resolution images series of photospheric fields. In this paper, we present the new Two-level Structure Tracking (TST) algorithm, based on a two-level representation of granulation, which allows us to monitor the characteristics of identified photospheric structures during their duration. TST is also able to retrieve horizontal velocity maps from measured granule displacements. Direct comparison of the results from different works describing granular evolution is often not possible as discrepancies stemming from the use of different procedures or from different data cannot be distinguished. Here, three different solar granulation broadband time series, acquired at THEMIS in July 1999, at NSO-DST in October 1996, and at SVST in June 1995, with different spatial and temporal resolutions, are coherently analyzed via the same TST procedure, allowing direct comparison of the results. Among the obtained results, we confirm the dynamical heterogeneity of photospheric small scale structures pointed out by granular lifetime histograms, characterized by a stretched exponential function. Furthermore, by monitoring the breaking-up of a granule and the movements of its neighbours and of its fragments, we found evidence of how an exploding granule may produce a divergence signal on spatial and temporal mesogranular scales.

Solar Surface Magnetoconvection

Magnetoconvection simulations on meso-granule and granule scales near the solar surface are used to study small scale dynamo activity, the emergence and disappearance of magnetic flux tubes, and the formation and evolution of micropores.From weak seed fields, convective motions produce highly intermittent magnetic fields in the intergranular lanes which collect over the boundaries of the underlying meso-granular scale cells. Instances of both emerging magnetic flux loops and magnetic flux disappearing from the surface occur in the simulations. We show an example of a flux tube collapsing to kG field strength and discuss how the nature of flux disappearance can be investigated. Observed Stokes profiles of small magnetic structures are severely distorted by telescope diffraction and seeing.Because of the strong stratification, there is little recycling of plasma and field in the surface layers. Recycling instead occurs by exchange with the deep layers of the convection zone. Plasma and field from the surface descend through the convection zone and rise again toward the surface. Because only a tiny fraction of plasma rising up from deep in the convection zone reaches the surface due to mass conservation, little of the magnetic energy resides in the near surface layers. Thus the dynamo acting on weak incoherent fields is global, rather than a local surface dynamo.

Modulation of Acoustic Waves by Solar Convection

Amplitude and phase of an acoustic oscillation in the solar convection zone vary in response to the local variation of the speed of sound and the convection velocity. Such wave modulation is considered by means of a two-dimensional periodic model, with alternating vertical channels of hot rising and cool sinking gas. According to this model, vertically propagating waves show only amplitude modulation. For low wave frequencies the amplitude is larger in the upflow channels, for high frequencies it is larger in the downflow channels. The transition occurs at a frequency for which the vertical wavelength is approximately equal to the horizontal period of the model. Waves with an inclined propagation direction show a similar amplitude modulation but, in addition, a modulation of their phase. The present results are compared with recent observational studies. There is evidence that wave modulation indeed occurs on the Sun, on the granular as well as on the mesogranular scale, in addition to the episodic amplitude enhancement that has been interpreted in terms of local acoustic events.

On the nature of 'exploding' granules and granule fragmentation

view Abstract Citations (76) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Nature of ``Exploding'' Granules and Granule Fragmentation Rast, Mark Peter Abstract The morphological evolution of solar granulation is dominated by granule expansion and fragmentation. 'Exploding' granules undergo these processes in a particularly vigorous manner, rapidly expanding to a large size, darkening in the center, and splitting by the formation of dark interior radially directed lanes. We argue that such events can be better understood if granulation is viewed as downflow-dominated surface-driven convection rather than as a collection of more deeply driven upflowing thermal plumes. Regions of maximum granular upflow lie not in the centers of the granules but along their sides, immediately adjacent to the intergranular downflow lanes. These upflows occur primarily in response to the buoyancy and pressure gradient forces induced in proximity to the strongly driven downflow plumes. The upflows are thus dynamically linked to the downflow sites, and granular expansion results in a weakening to the central flow. Radiative losses can then exceed the advected heat supply in the granule center, with the fluid cooling until buoyancy forces becomes sufficient to trigger the formation of a new downflow plume there. Lateral propagation proceeds as neighboring flows are distributed, with propagation preferentially occurring in directions predisposed to weak upflow by the strength and shape of the downflows defining the granule boundary. Thus the radially oriented structures seen in observations of some fragmenting granules may be formed. Finally, the strong downflow plumes initiated in the solar photosphere entrain surrounding material as they descend. With depth this more weakly downflowing material establishes a connectivity which is strikingly of mesogranular scale. This may help to explain the observed correlation between the spatial distribution of exploding granules and mesogranular flows, and suggests that both mesogranulation and supergranulation are secondary manifestations of granulation itself. Publication: The Astrophysical Journal Pub Date: April 1995 DOI: 10.1086/175576 Bibcode: 1995ApJ...443..863R Keywords: Expansion; Fragmentation; Morphology; Solar Convection (Astronomy); Solar Granulation; Advection; Buoyancy; Photosphere; Plumes; Pressure Distribution; Radiative Heat Transfer; Solar Physics; CONVECTION; SUN: GRANULATION full text sources ADS |

Simulating exploding granules and mesogranular flows

Cellular convective motion at the solar surface can be simulated by a suitable distribution of axisymmetric sources /1/. With this model we represent randomly distributed exploding granules or mesogranules. The effect of a given velocity field on the magnetic field is modelled by inserting test particles (“corks”) and following their resultant motions. An important question raised by the observations is whether mesogranular flows are generated entirely by exploding granules which recur in approximately the same location or whether there is a persistent underlying circulation. Our model calculations suggest that a combination of systematic cellular motion on a mesogranular scale and recurrent exploding granules located near the mesogranular centers is compatible with observed magnetic structures. We also generate randomly-distributed exploders not tethered to mesogranular sites, and the resulting cork patterns do not resemble those observed on the solar surface. Finally we introduce a large-scale persistent supergranular flow which transports the mesogranules and exploders towards the supergranular network and obtain patterns not unlike those seen on the Sun.

Vertical flows of supergranular and mesogranular scale observed on the sun with OSO 8

A program of observations was carried out in order to study the penetration of supergranular flows over a broad range of heights in the solar atmosphere. Steady Doppler velocities are determined from observations of a Si II spectral line using the Ultraviolet Spectrometer on the Orbiting Solar Observatory 8 (OSO 8) satellite and Fe I and Mg I lines with the diode-array instrument on the vacuum telescope at Sacramento Peak Observatory (SPO). The heights of formation of these spectral lines span about 1400 km or nearly 11 density scale heights from the photosphere to the middle chromosphere. Steady vertical flows on spatial scales typical of supergranulation and mesogranulation have been detected in the middle chromosphere with OSO 8. The patterns of intensity and steady velocity of granular scale are reproducible in successive data sets. The patterns appear to evolve slowly over the 9 hr period spanned by six orbits.