Quiet Sun Research Articles

What Can DKIST/DL-NIRSP Tell Us about Quiet-Sun Magnetism?

Abstract Quiet-Sun regions cover most of the Sun's surface; their magnetic fields contribute significantly to solar chromospheric and coronal heating. However, characterizing the magnetic fields of the quiet Sun is challenging due to their weak polarization signal. The 4 m Daniel K. Inouye Solar Telescope (DKIST) is expected to improve our understanding of quiet-Sun magnetism. In this paper, we assess the diagnostic capability of the Diffraction Limited Near Infrared Spectropolarimeter (DL-NIRSP) instrument on DKIST for the energy transport processes in the quiet-Sun photosphere. To this end, we synthesize high-resolution, high-cadence Stokes profiles of the Fe i 630 nm lines using a realistic magnetohydrodynamic simulation, degrade them to emulate the DKIST/DL-NIRSP observations, and subsequently infer the vector magnetic and velocity fields. For the assessment, we first verify that a widely used flow tracking algorithm, the Differential Affine Velocity Estimator for Vector Magnetograms, works well for estimating the large-scale (>200 km) photospheric velocity fields with these high-resolution data. We then examine how the accuracy of the inferred velocity depends on the temporal resolution. Finally, we investigate the reliability of the Poynting flux estimate and its dependence on the model assumptions. The results suggest that the unsigned Poynting flux, estimated with existing schemes, can account for about 71.4% and 52.6% of the reference ground truth at log τ = 0.0 and log τ = − 1 . However, the net Poynting flux tends to be significantly underestimated. The error mainly arises from the underestimated contribution of the horizontal motion. We discuss the implications for DKIST observations.

Hinode EIS: Updated In-flight Radiometric Calibration

Abstract We present an update to the in-flight radiometric calibration of the Hinode Extreme Ultraviolet Imaging Spectrometer (EIS), revising and extending our previous studies. We analyze full-spectral EIS observations of quiet Sun (QS) and active regions (ARs) from 2007 until 2022. Using CHIANTI version 10, we adjust the EIS relative effective areas for a selection of dates with emission measure analyses of off-limb QS. We find generally good agreement (within typically ±15%) between measured and expected line intensities. We then consider selected intensity ratios for all of the dates and apply an automatic fitting method to adjust the relative effective areas. To constrain the absolute values from 2010 and later, we force agreement between EIS and Solar Dynamics Observatory Atmospheric Imaging Assembly 193 Å observations. The resulting calibration, with an uncertainty of about ±20%, is then validated in various ways, including flare line ratios from Fe xxiv and Fe xvii, emission measure analyses of cool AR loops, and several density-dependent line ratios.

On the Existence of Long-period Decayless Oscillations in Short Active Region Loops

Abstract Decayless kink oscillations, characterized by their lack of decay in amplitude, have been detected in coronal loops of varying scales in active regions, the quiet Sun, and coronal holes. Short-period (<50 s) decayless oscillations have been detected in short loops (< 50 Mm) within active regions. Nevertheless, long-period decayless oscillations in these loops remain relatively unexplored and crucial for understanding the wave modes and excitation mechanisms of decayless oscillations. We present the statistical analysis of decayless oscillations from two active regions observed by the Extreme Ultraviolet Imager onboard Solar Orbiter. The average loop length and period of the detected oscillations are 19 Mm and 151 s, respectively. We find 82 long-period and 23 short-period oscillations in these loops. We do not obtain a significant correlation between loop length and period. We discuss the possibility of different wave modes in short loops, although standing waves cannot be excluded from possible wave modes. Furthermore, a different branch exists for active region short loops in the loop length versus period relation, similar to decayless waves in short loops in the quiet Sun and coronal holes. The magnetic fields derived from MHD seismology, based on standing kink modes, show lower values for multiple oscillations compared to previous estimates for long loops in active regions. Additionally, the comparison of period distributions in short loops across different coronal regions indicates that different excitation mechanisms may trigger short-period kink oscillations in active regions compared to the quiet Sun and coronal holes.

Magnetic topology of quiet-Sun Ellerman bombs and associated ultraviolet brightenings

Context. Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower atmosphere of the quiet Sun. Recent work has shown that a small percentage of them can occur co-spatially and co-temporally with ultraviolet (UV) brightenings in the transition region. Aims. We aim to understand how the magnetic topologies associated with closely occurring QSEBs and UV brightenings can facilitate energy transport and connect these events. Methods. We used high-resolution Hβ observations from the Swedish 1-m Solar Telescope (SST) and detected QSEBs using k-means clustering. We obtained the magnetic field topology from potential field extrapolations using spectro-polarimetric data in the photospheric Fe I 6173 Å line. To detect UV brightenings, we used coordinated and co-aligned data from the Interface Region Imaging Spectrograph (IRIS) and imposed a threshold of 5σ above the median background on the (IRIS) 1400 Å slit-jaw image channel. Results. We identify four distinct magnetic configurations that associate QSEBs with UV brightenings, including a simple dipole configuration and more complex fan-spine topologies with a 3D magnetic null point. In the fan-spine topology, the UV brightenings occur near the 3D null point, while QSEBs can be found close to the footpoints of the outer spine, the inner spine, and the fan surface. The height of the 3D null varies between 0.2 Mm and 2.6 Mm, depending on the magnetic field strength in the region. Some QSEBs and UV brightenings, though occurring close to each other, are not topologically connected with the same reconnection process. The energy released during QSEBs falls in the range 1023–1024 ergs. Conclusions. This study shows that magnetic connectivity and topological features, such as 3D null points, are crucial in linking QSEBs in the lower atmosphere with UV brightenings in the transition region.

Spectral resolution effects on the information content in solar spectra

When interpreting spectropolarimetric observations of the solar atmosphere, wavelength variations in the emergent intensity and polarization translate into information on the depth stratification of physical parameters such as the temperature, velocity, and magnetic field. Resolving the fine details in the shapes of the spectral lines and their polarization gives us the capability to resolve small-scale depth variations in these physical parameters. With the advent of large-aperture solar telescopes and the development of state-of-the-art instrumentation, the requirements on spectral resolution have become a prominent question. We aim to quantify how the information content contained in a representative set of polarized spectra of photospheric spectral lines depends on the spectral resolution and spectral sampling of that spectrum. We used a state-of-the-art numerical simulation of a sunspot and the neighboring quiet Sun photosphere to synthesize polarized spectra of magnetically sensitive neutral iron lines. We then applied various degrees of spectral degradation to the synthetic spectra and analyzed the impact on its dimensionality using principal component analysis and the wavelength power spectrum using wavelet decomposition. Finally, we applied the Stokes Inversion based on Response functions (SIR) code to the degraded synthetic data to assess the effect of spectral resolution on the inferred parameters. We find that the dimensionality of the Stokes spectra and the power contained in the small spectral scales significantly change with the spectral resolution. We find that regions with strong magnetic fields where convection is suppressed have more homogeneous atmospheres and produce less complex Stokes profiles. On the other hand, regions with strong gradients in the physical quantities give rise to more complex Stokes profiles that are more affected by spectral degradation. The degradation also makes the inversion problem more ill-defined, so inversion models with a larger number of free parameters overfit and give wrong estimates. The impact of spectral degradation in the interpretation of solar spectropolarimetric observations depends on multiple factors, including the spectral resolution, noise level, line spread function (LSF) shape, complexity of the solar atmosphere, and degrees of freedom in our inversion methods. To mitigate this impact, incorporating a good estimation of the LSF into the inversion process is recommended. Having a finely sampled spectrum may be more beneficial than achieving a higher signal-to-noise ratio per wavelength bin. Considering the inclusion of different spectral lines that can counter these effects, and calibrating the effective degrees of freedom in modeling strategies, are also important considerations. These strategies are crucial for the accurate interpretation of such observations and have the potential to offer more cost-effective solutions.

A BCool survey of stellar magnetic cycles

The magnetic cycle on the Sun consists of two consecutive 11-yr sunspot cycles and exhibits a polarity reversal around sunspot maximum. Although solar dynamo theories have progressively become more sophisticated, the details as to how the dynamo sustains magnetic fields are still the subject of research. Observing the magnetic fields of Sun-like stars can bring useful insights to contextualise the solar dynamo. With the long-term spectropolarimetric monitoring of stars, the BCool survey studies the evolution of surface magnetic fields to understand how dynamo-generated processes are influenced by key ingredients, such as mass and rotation. Here, we focus on six Sun-like stars with masses between 1.02 and 1.06 M$_ and with rotation periods of 3.5-21 d (or 0.3-1.8 in Rossby numbers), a practical sample with which to study magnetic cycles across distinct activity levels. We analysed high-resolution spectropolarimetric data collected with ESPaDOnS, Narval, and Neo-Narval between 2007 and 2024 within the BCool programme. We measured longitudinal magnetic field from least-squares deconvolution line profiles and we inspected its long-term behaviour with both a Lomb-Scargle periodogram and a Gaussian process. We then applied Zeeman-Doppler imaging to reconstruct the large-scale magnetic field geometry at the stellar surface for different epochs. Two of our slow rotators, namely HD\,9986 and HD\,56124 (P$_ rot exhibit repeating polarity reversals in the radial or toroidal field component on shorter timescales than the Sun (5 to 6\,yr). HD\,73350 (P$_ rot has one polarity reversal in the toroidal component and HD\,76151 (P$_ rot $=17\,d) may have short-term evolution (2.5\,yr) modulated by the long-term (16\,yr) chromospheric cycle. Our two fast rotators, HD 166435 and HD 175726 (P$_ rot $=3-5\,d), manifest complex magnetic fields without an evident cyclic evolution. Our findings indicate the potential dependence of the magnetic cycles' nature on the stellar rotation period. For the two stars with likely cycles, the polarity reversal timescale seems to decrease with a decreasing rotation period or Rossby number. These results represent important observational constraints for dynamo models of solar-like stars.

Effects of light, electromagnetic fields and water on biological rhythms.

The circadian rhythm controls a wide range of functions in the human body and is required for optimal health. Disruption of the circadian rhythm can produce inflammation and initiate or aggravate chronic diseases. The modern lifestyle involves long indoor hours under artificial lighting conditions as well as eating, working, and sleeping at irregular times, which can disrupt the circadian rhythm and lead to poor health outcomes. Seasonal solar variations, the sunspot cycle and anthropogenic electromagnetic fields can also influence biological rhythms. The possible mechanisms underlying these effects are discussed, which include resonance, radical-pair formation in retina cryptochromes, ion cyclotron resonance, and interference, ultimately leading to variations in melatonin and cortisol. Intracellular water, which represents a coherent, ordered phase that is sensitive to infrared light and electromagnetic fields, may also respond to solar variations and man-made electromagnetic fields. We describe here various factors and underlying mechanisms that affect the regulation of biological rhythms, with the aim of providing practical measures to improve human health.

A Study of Cosmic Ray Variability During a Solar Magnetic Cycle (Solar Cycles 23 and 24)

A Study of Cosmic Ray Variability During a Solar Magnetic Cycle (Solar Cycles 23 and 24)

Solar shape variations across cycles 24 and 25: Observations from 2010 to 2023

The longest continuous time-series of solar oblateness measurements, initiated in 2010 and still ongoing, has been obtained from data collected by the Helioseismic Magnetic Imager (HMI) aboard NASA's Solar Dynamics Observatory (SDO). Based on HMI data, we developed two methods for determining the solar oblateness at 617.33\,nm in the continuum . The first method involves determining solar oblateness using HMI solar disk images and limb observations from twenty-three SDO satellite roll calibration maneuvers between 2010 and 2023 . Through meticulous analysis of these observation sequences, we obtained a precise measurement of solar oblateness using this technique, yielding a value of 9.02 (pm 0.72)times 10$^ $ (6.28pm 0.50\,kilometers), unaffected by brightness contamination from sunspots and magnetically induced excess emission. We also verified the polarization independence of light, showing consistent HMI solar oblateness measurements across Stokes states. Interestingly, our solar oblateness time-series, based on HMI solar disk images and limb observations, seems to be in anti-phase with solar activity. The second method we used relies on determining solar oblateness from HMI helioseismic inference of internal rotation. With this approach, we obtained a solar oblateness of 8.40 (pm 0.02)times 10$^ $ (5.85pm 0.01 kilometers) with a variation in phase with solar activity (0.05times 10$^ $ (0.04\,kilometers at 1\,sigma ) over an 11--year sunspot cycle). This outcome is troubling as it conflicts with our results obtained from the HMI solar limb observations

Center-to-limb Variations in Solar Plage Using IRIS Observations

The center-to-limb variations (CLV) of transition region line Gaussian fit parameters in solar plage are reported for the first time. The Si iv 1402.77 Å line observed by the Interface Region Imaging Spectrograph is used. The spectral intensity increases linearly from the disk center to the solar limb. Similarly, the nonthermal velocity also increases linearly from 23.6 (at the disk center) to 30.9 km s−1 (at the solar limb). On the other hand, the Doppler velocity decreases from 8.9 ± 1.0 at the disk center to 0.0 km s−1 at the limb. This CLV pattern in solar plages is consistent with the CLV pattern reported in the quiet Sun (QS). However, the average values of the parameters in the solar plage are significantly higher than in the QS. The intensity and nonthermal velocity increase linearly with the magnetic field at the disk center, while the Doppler velocity does not depend on the magnetic field. Due to the line-of-sight effect, the plasma column depth increases toward the solar limb, which leads to a linear increase in the spectral intensity. Further, the increasing plasma column depth toward the solar limb adds more and more unresolved motions, and as a result, the nonthermal velocity increases from the disk center to the solar limb. In the solar plages, the higher plasma density due to the strong magnetic field leads to higher intensity and nonthermal velocity compared to the QS and coronal hole.

An Evaluation of Antarctic Ice Core Nitrate Records as a Proxy for Solar Activity

AbstractNitrate (NO3−) deposition in polar ice sheets archives valuable information on past solar activity. However, interpretation of Antarctic ice core NO3− records as a proxy for past solar activity remains challenging due to multiple sources and processes controlling NO3− variability in ice core records. Here, we present a new high‐resolution ice core NO3− record (1905–2005 CE) from coastal Dronning Maud Land, East Antarctica, to investigate the solar signal and other forcing factors/processes in controlling ice core NO3− variability. Our record exhibits significant periodicity in the range of 8–12 years frequency band during 1940–2005 CE, apparently identified as the signal of ∼11 year sunspot cycle; however, such signal was not detected in the previous interval during 1905–1940 CE. To address the discontinuous and/or obscured signals in the present ice core record and inconsistency among various Antarctica ice core records, we extended our investigations to 10 ice core NO3− records from various regions of Antarctica. Analysis of seven records for the common interval from 1738 to 1990 CE reveals dominant periodicities of 8–12 years, indicating solar forcing as a primary driver, followed by precipitation modulated by El Niño‐Southern Oscillation and Pacific Decadal Oscillation. Further, our investigation reveals that the solar signal extracted from multiple records becomes undetectable when mean annual hemispheric sunspot numbers larger than 140, suggesting this is a threshold limit for detecting the solar signal. These findings will improve our present understanding of ice core NO3− records as a proxy for past solar activity.

Magnetohydrodynamic Instabilities of Double Magnetic Bands in a Shallow-water Tachocline Model. II. Teleconnection Between High- and Low-latitude Bands and Across Equator

The “extended solar cycle” indicates that there are two deeply seated toroidal magnetic field bands in each hemisphere. Both bands migrate equatorward as a sunspot cycle progresses. Here, we examine the consequences of global MHD instability of this migrating double-band system in tachocline on the latitudinal structure of unstable modes, which are essentially MHD Rossby waves. We find that latitude-location, latitude-separation, and the amplitude of the bands strongly influence the latitudinal structure and growth rates of the unstable modes of both symmetries about the equator. These properties can lead to “teleconnections” between low- and high-latitudes in each hemisphere and across the equator. High-latitude bands can destabilize low-latitude bands that would otherwise be stable. Stronger high-latitude bands lead to strong interactions between low and high latitude in each hemisphere, but inhibit cross-equatorial band-interaction. Strong cross-equatorial interactions of modes can synchronize cycle minima in north and south. Symmetric and antisymmetric modes of similar amplitudes can lead to substantial asymmetries between north and south. As a solar cycle progresses, excited MHD Rossby waves go through a sequence of changes in latitude structure and growth rate, while maintaining strong links in latitude. These changes and links are theoretical evidence of teleconnections between widely separated latitudes and longitudes in the Sun, which may explain many of the evolving surface magnetic patterns observed as a solar cycle progresses. The wider the separation between high- and low-latitude bands, the earlier the cross-equatorial teleconnection starts in a cycle, and hence the earlier the cycle starts declining.

Spatial distributions of extreme-ultraviolet brightenings in the quiet Sun

Spatial distributions of extreme-ultraviolet brightenings in the quiet Sun

Large solar energetic particles and solar radio emissions during Cycle 25. A comparative analysis of trends and characteristics with cycles 23 and 24

Solar energetic particles (SEPs) affect space weather in both the heliosphere and on Earth. The present study investigates the occurrences of large SEP events focusing on their influence on the Earth, as well as their correlation with solar radio bursts (SRBs). The velocity dispersion analysis (VDA) is used to calculate the release times of large SEP events from their launch locations, as well as their apparent path length that connects them to interplanetary magnetic fields lines. According to the study, 122 large SEP events impacted the planet Earth from 1997 to 2024. The comparison of occurrence rates with previous solar cycles (SCs) suggests that SC 25 will peak with greater solar activity than the cycle 24 in terms of large SEP occurrences since large SEP events correlate well with the pattern of the sunspot cycle progression. In general, a few (35 out of 122) large SEP events are typically released from the Sun at times that coincide with the peak of associated solar flares and the onsets of corresponding SRBs indicating no delay while the rest have delayed in the release. The projected apparent lengths (L) range from 1.0 to 3.0 AU, with L exceeding 1.5 AU due to particle scattering and launch site pitch angles. The majority (115/122) of SEP occurrences are accelerated by shock waves from solar flares, CMEs, and fast plasma flow in the magnetic reconnection regions. Relevant SRBs for space weather study as they precede large SEP events diagnose the properties of particle populations propelled by solar flares and CMEs. This study finds that 90% of the large SEP events are preceded by solar radio emissions of type II, III and IV; and WAVES/STEREO revealed ∼76% of SRBs have continuation in IP medium indicating the dynamics of associated shocks and electron beams traveling along open and quasi-open magnetic field lines. Thus, SRB monitoring continues to be a valuable tool for studying space weather and understanding physical phenomena in the solar corona and IP medium, such as particle populations that cause large SEP occurrences.

Mg II h&k spectra of an enhanced network region simulated with the MURaM-ChE code

Context. The Mg II h&k lines are key diagnostics of the solar chromosphere. They are sensitive to the temperature, density, and nonthermal velocities in the chromosphere. The average Mg II h&k line profiles arising from previous 3D chromospheric simulations are too narrow compared to observations. Aims. We study the formation and properties of the Mg II h&k lines in a model atmosphere. We also compare the average spectrum, peak intensity, and peak separation of Mg II k with a representative observation taken by the Interface Region Imaging Spectrograph (IRIS). Methods. We use a model based on the recently developed nonequilibrium version of the radiative magneto-hydrodynamics code MURaM, the MURaM Chromospheric Extension (MURaM-ChE), in combination with forward modeling using the radiative transfer code RH1.5D to obtain synthetic spectra. Our model resembles an enhanced network region created using an evolved MURaM quiet Sun simulation and adding an imposed large-scale bipolar magnetic field similar to that in the public Bifrost snapshot of a bipolar magnetic feature. Results. The line width and the peak separation of the spatially averaged spectrum of the Mg II h&k lines from the MURaM-ChE simulation are close to a representative observation of the quiet Sun, which also includes network fields. However, we find the synthesized line width to be still slightly narrower than in the observation. We find that velocities in the chromosphere play a dominant role in the broadening of the spectral lines. While the average synthetic spectrum also shows a good match to the observations in the pseudo continuum between the two emission lines, the peak intensities are higher in the modeled spectrum. This discrepancy may be due in part to the larger magnetic flux density in the simulation than in the considered observations, but could also be a result of the 1.5D radiative transfer approximation. Conclusions. Our findings show that strong maximum-velocity differences or turbulent velocities in the chromosphere and lower atmosphere are necessary to reproduce the observed line widths of chromospheric spectral lines.

Seismic differences between solar magnetic cycles 23 and 24 for low-degree modes

Solar magnetic activity follows regular cycles of about 11 years with an inversion of polarity in the poles every sim 22 years. This changing surface magnetism impacts the properties of the acoustic modes. The acoustic mode frequency shifts are a good proxy of the magnetic cycle. In this Letter we investigate solar magnetic activity cycles 23 and 24 through the evolution of the frequency shifts of low-degree modes (ell = 0, 1, and 2) in three frequency bands. These bands probe properties between 74 and 1575 km beneath the surface. The analysis was carried out using observations from the space instrument Global Oscillations at Low Frequency and the ground-based Birmingham Solar Oscillations Network and Global Oscillation Network Group. The frequency shifts of radial modes suggest that changes in the magnetic field amplitude and configuration likely occur near the Sun's surface rather than near its core. The maximum shifts of solar cycle 24 occurred earlier at mid and high latitudes (relative to the equator) and about 1550 km beneath the photosphere. At this depth but near the equator, this maximum aligns with the surface activity but has a stronger magnitude. At around 74 km deep, the behaviour near the equator mirrors the behaviour at the surface, while at higher latitudes, it matches the strength of cycle 23.

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.

Solar internetwork magnetic fields: Statistical comparison between observations and MHD simulations

Context. Although the magnetic fields in the quiet Sun account for the majority of the magnetic energy in the solar photosphere, inferring their exact spatial distribution, origin, and evolution poses an important challenge because the signals lie at the limit of today’s instrumental precision. This severely hinders and biases our interpretations, which are mostly made through nonlinear model-fitting approaches. Aims. Our goal is to directly compare simulated and observed polarization signals in the Fe I 6301 Å and 6302 Å spectral lines in the very quiet Sun, the so-called solar internetwork (IN). This way, we aim to constrain the mechanism responsible for the generation of the quiet Sun magnetism while avoiding the biases that plague other diagnostic methods. Methods. We used three different three-dimensional radiative magneto-hydrodynamic simulations representing different scenarios of magnetic field generation in the internetwork: small-scale dynamo, decay of active regions, and horizontal flux emergence. We synthesized Stokes profiles at different viewing angles and degraded them according to the instrumental specifications of the spectro-polarimeter (SP) on board the Hinode satellite. Finally, we statistically compared the simulated spectra to the Hinode/SOT/SP observations at the appropriate viewing angles. Results. Of the three simulations, the small-scale dynamo best reproduced the statistical properties of the observed polarization signals. This is especially prominent for the disk center viewing geometry, where the agreement is excellent. Moving toward more inclined lines of sight, the agreement worsens slightly. Conclusions The agreement between the small-scale dynamo simulation and observations at the disk center suggests that small-scale dynamo action plays an important role in the generation of quiet Sun magnetism. However, the magnetic field around 50 km above the continuum layer in this simulation does not reproduce observations as well as at the very base of the photosphere.

Automated detection of exploding granules with SDO/HMI data

Context. Exploding granules on the solar surface play a major role in the dynamics of the outer part of the convection zone, especially in the diffusion of the magnetic field. Aims. We aim to develop an automated procedure able to investigate the location and evolution of exploding granules over the solar surface and to get rid of visual detection. Methods. We used sequences of observations of intensity and Doppler velocity, as well as magnetograms, provided by the Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory. The automated detection of the exploding granules was performed by applying criteria on either three or two parameters: the granule area, the amplitude of the velocity field divergence, and, at the disc centre, the radial Doppler velocity. Our analyses show that granule area and divergence amplitudes are sufficient to detect the largest exploding granules; thus, we can automatically detect them, not only at the disc centre, but across the whole solar surface. Results. Using a 24-hour-long observation sequence, we have demonstrated the important contribution of the most dynamic exploding granules in the diffusion of the magnetic field in the quiet Sun. Indeed, we have shown that the most intense exploding granules are sufficient to build a large part of the photospheric network. We have also applied our procedure on Hinode observations to locate the exploding granules relative to trees of fragmenting granules (TFGs). We conclude that, during a first phase of about 300 minutes after the birth of a TFG, exploding granules are preferentially located on its edge. Finally, we also show that the distribution of exploding granules is homogeneous (at the level of our measurement errors) over the solar surface without a significant dependency on latitude.

Umbral flashes and their association with running penumbral waves: a study using MAST Ca ii 8542 Å narrow-band observations

ABSTRACT Umbral flashes (UFs) are one of the most dynamic phenomena observed in the sunspot umbra at the chromospheric heights. In this paper, we present spectroscopic observations of UFs in the Ca ii 8542 Å line recorded by a narrow-band imager working with the Multi-Application Solar Telescope (MAST). The deduced data are analysed to obtain various properties of the UFs occuring at different locations inside the umbral boundary. An intensity enhancement of up to 30% or more was observed at the location of UFs, with a periodicity $\approx$3 min. The line-of-sight (LOS) velocity of UFs was estimated using bisector application to the emission profile resulting from the removal of mean umbral and the mean quiet Sun (QS) line profiles. The emission profiles resulting from removing the mean umbral profile were observed to better represent the emission component of the UF line profile. Both up-flows and down-flows of the order $\approx$5 km s$^{-1}$ were associated with the UFs with an average up-flow of $\approx$1 km s$^{-1}$. Out of all UFs analysed, 31% were observed to be associated with down-flows in case of removal of the mean umbral profile from the UF line profile. We observed multiple radially propagating LOS velocity disturbances ($\approx$20–40 km s$^{-1}$) in the penumbra, which might be associated with the UFs, even though we could not establish a one-to-one correspondence. The horizontally propagating LOS velocity disturbances could produce the visual effect of running penumbral waves, which produce intensity fluctuations in intensity images when observed at the line-centre wavelength. The simultaneous photospheric HMI observations showed no distinct intensity or velocity signatures corresponding to the UFs observed in the chromospheric Ca ii 8542 Å line.