Author name code: gosic ADS astronomy entries on 2022-09-14 author:"Gosic, Milan" ------------------------------------------------------------------------ Title: Unipolar versus Bipolar Internetwork Flux Appearance Authors: Gosic, Milan; Katsukawa, Yukio; Bellot Rubio, L. R.; Del Toro Iniesta, Jose Carlos; Cheung, Mark; Orozco Suárez, David Bibcode: 2022cosp...44.2513G Altcode: Small-scale internetwork (IN) magnetic fields are considered to be the main building blocks of the quiet Sun magnetism. It is therefore of paramount importance to understand how these fields are generated on the solar surface. To shed new light on this open question, we studied the appearance modes and spatio-temporal evolution of individual IN magnetic elements inside one supergranular cell. For that purpose, we employed a high-resolution, high-sensitivity, long-duration Hinode/NFI magnetogram sequence. 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, live longer and carry more flux than unipolar features. About $70$% of the total instantaneous IN flux detected inside the supergranule is in the form of bipoles. Both types of flux concentrations are uniformly distributed over the solar surface. However, bipolar features appear (randomly oriented) at a faster rate than unipolar features (68 as opposed to 55~Mx~cm$^{-2}$~day$^{-1}$). Our results lend support to the idea that bipolar features may be the signature of local dynamo action, while unipolar features seem to be formed by coalescence of background flux. Title: Emergence of internetwork magnetic fields into the chromosphere and transition region Authors: Gosic, Milan; De Pontieu, Bart; Bellot Rubio, L. R.; Sainz Dalda, A.; Esteban Pozuelo, Sara Bibcode: 2022cosp...44.2511G Altcode: Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they are spread all over the Sun, they may hold a significant fraction of the total magnetic energy stored in the photosphere. Therefore, it is crucial to understand their role in the quiet Sun magnetism and impact on the energetics and dynamics of the solar atmosphere. To provide new insights into this topic, we studied three flux emergence events and followed them as they emerge into the photosphere and reach the chromosphere and transition region. We used coordinated, high-resolution, multiwavelength observations obtained with the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). SST Fe I 6173 and Mg I b$_2$ 5173 magnetograms show the footpoints of the IN bipoles emerging at the solar surface and rising toward the upper solar atmosphere. For the first time, our spectropolarimetric measurements in the Ca II 8542 \AA\/ line provide direct observational evidence that IN fields are capable of reaching the chromosphere. IRIS observations reveal another important piece of information - small-scale IN loops can even reach transition region heights, and locally heat the upper solar atmosphere. Title: The Solar Internetwork. III. Unipolar versus Bipolar Flux Appearance Authors: Gošić, M.; Bellot Rubio, L. R.; Cheung, M. C. M.; Orozco Suárez, D.; Katsukawa, Y.; del Toro Iniesta, J. C. Bibcode: 2022ApJ...925..188G Altcode: 2021arXiv211103208G 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. Title: Emergence of Internetwork Magnetic Fields through the Solar Atmosphere Authors: Gošić, M.; De Pontieu, B.; Bellot Rubio, L. R.; Sainz Dalda, A.; Pozuelo, S. Esteban Bibcode: 2021ApJ...911...41G Altcode: 2021arXiv210302213G Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they emerge all over the Sun, these small-scale fields bring a substantial amount of flux, and therefore energy, to the solar surface. Because of this, IN fields are crucial for understanding the quiet Sun (QS) magnetism. However, they are weak and produce very small polarization signals, which is the reason why their properties and impact on the energetics and dynamics of the solar atmosphere are poorly known. Here we use coordinated, high-resolution, multiwavelength observations obtained with the Swedish 1 m Solar Telescope and the Interface Region Imaging Spectrograph (IRIS) to follow the evolution of IN magnetic loops as they emerge into the photosphere and reach the chromosphere and transition region. We studied in this paper three flux emergence events having total unsigned magnetic fluxes of 1.9 × 1018, 2.5 × 1018, and 5.3 × 1018 Mx. The footpoints of the emerging IN bipoles are clearly seen to appear in the photosphere and to rise up through the solar atmosphere, as observed in Fe I 6173 Å and Mg I b2 5173 Å magnetograms, respectively. For the first time, our polarimetric measurements taken in the chromospheric Ca II 8542 Å line provide direct observational evidence that IN fields are capable of reaching the chromosphere. Moreover, using IRIS data, we study the effects of these weak fields on the heating of the chromosphere and transition region. Title: Expected spectropolarimetric observables in the lower solar atmosphere from 3D radiative MHD models Authors: Sainz Dalda, A.; Gosic, M.; Martinez-Sykora, J. Bibcode: 2020AGUFMSH0010019S Altcode: No abstract at ADS Title: Lagrangian chaotic saddles and objective vortices in solar plasmas Authors: Chian, Abraham C. -L.; Silva, Suzana S. A.; Rempel, Erico L.; Bellot Rubio, Luis R.; Gošić, Milan; Kusano, Kanya; Park, Sung-Hong Bibcode: 2020PhRvE.102f0201C Altcode: We report observational evidence of Lagrangian chaotic saddles in plasmas, given by the intersections of finite-time unstable and stable manifolds, using an ≈22 h sequence of spacecraft images of the horizontal velocity field of solar photosphere. A set of 29 persistent objective vortices with lifetimes varying from 28.5 to 298.3 min are detected by computing the Lagrangian averaged vorticity deviation. The unstable manifold of the Lagrangian chaotic saddles computed for ≈11 h exhibits twisted folding motions indicative of recurring vortices in a magnetic mixed-polarity region. We show that the persistent objective vortices are formed in the gap regions of Lagrangian chaotic saddles at supergranular junctions. Title: Chromospheric response to emergence of internetwork magnetic fields Authors: Gosic, M.; De Pontieu, B.; Bellot Rubio, L.; Sainz Dalda, A. Bibcode: 2020AGUFMSH0010006G Altcode: Internetwork (IN) magnetic fields are weak, short-lived, but highly dynamic magnetic structures that emerge all over the Sun. They bring an enormous amount of magnetic flux and energy to the solar surface. Therefore, IN fields are of paramount importance for maintenance of the QS magnetism. Since these fields are ubiquitous, they may have a substantial impact on the energetics and dynamics of the solar atmosphere. In this work, we use coordinated, high-resolution, multiwavelength observations obtained with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1 m Solar Telescope (SST) to follow the evolution of IN magnetic loops as they emerge into the photosphere. The footpoints of the emerging IN bipoles are clearly visible as they appear in the photosphere and rise up through the solar atmosphere, as seen in SST magnetograms taken in the Fe I 6173 Å and Mg I b2 5173 Å lines, respectively. Our polarimetric measurements, taken in the Ca II 8542 Å line, provide the first direct observational evidence that IN fields are capable of reaching the chromosphere. Moreover, using IRIS data, we describe in detail how individual IN bipoles affect the dynamics and energetics of the chromosphere and transition region. Title: Chromospheric and TR diagnostics in a large scale numerical simulation of flux emergence: Synthetic vs Real observables Authors: Hansteen, V. H.; De Pontieu, B.; Testa, P.; Gosic, M.; Martinez-Sykora, J. Bibcode: 2020AGUFMSH0010021H Altcode: Field stored just below or rising to the photosphere will break through the surface and enter the upper atmosphere once the gradient of the subphotospheric field strength becomes sufficiently large. Opposite polarity flux bundles will reconnect in the photosphere and above, to form steadily longer loops that expand into the outer solar atmosphere, forming the corona. Some of the emerging flux is likely due to a local dynamo, but also the direct emergence of large scale magnetic structures from below is important, even in the quiet Sun. A significant proportion of this field likely reaches the chromosphere and may leave imprint on chromospheric dynamics and energetics. Using large scale numerical models (72x72x60) Mm and the high resolution spectra and slit jaw images from IRIS, as well as photospheric data from Hinode/SOT, and SDO/HMI we study the interactions between the magnetic flux caught in the granular flow field and the chromosphere and chromospheric field above. We will compare synthetic observables of the photospheric Fe I 617.3 nm line, the chromospheric Mg II h&k lines, and the transition region Si IV lines, with their observational counterparts. We will also generate synthetic ALMA band 3 images. The comparison of synthetic observational data will let us draw conclusions as to the validity of the numerical modeling and the importance of flux emergence for the dynamics and energetics of the outer solar atmosphere. Title: High-resolution observations of the solar photosphere, chromosphere, and transition region. A database of coordinated IRIS and SST observations Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson, M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.; Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques, V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.; Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.; Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.; Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski, M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P. Bibcode: 2020A&A...641A.146R Altcode: 2020arXiv200514175R NASA's Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe I lines and spectrally resolved imaging in the chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes. Title: Erratum: Supergranular turbulence in the quiet Sun: Lagrangian coherent structures Authors: Chian, Abraham C. -L.; Silva, Suzana S. A.; Rempel, Erico L.; Gošić; , Milan; Bellot Rubio, Luis R.; Kusano, Kanya; Miranda, Rodrigo A.; Requerey, Iker S. Bibcode: 2019MNRAS.489..707C Altcode: 2019MNRAS.tmp.2225C No abstract at ADS Title: Supergranular turbulence in the quiet Sun: Lagrangian coherent structures Authors: Chian, Abraham C. -L.; Silva, Suzana S. A.; Rempel, Erico L.; Gošić; , Milan; Bellot Rubio, Luis R.; Kusano, Kanya; Miranda, Rodrigo A.; Requerey, Iker S. Bibcode: 2019MNRAS.488.3076C Altcode: 2019MNRAS.tmp.1841C; 2019arXiv190408472C The quiet Sun exhibits a wealth of magnetic activities that are fundamental for our understanding of solar magnetism. The magnetic fields in the quiet Sun are observed to evolve coherently, interacting with each other to form prominent structures as they are advected by photospheric flows. The aim of this paper is to study supergranular turbulence by detecting Lagrangian coherent structures (LCS) based on the horizontal velocity fields derived from Hinode intensity images at disc centre of the quiet Sun on 2010 November 2. LCS act as transport barriers and are responsible for attracting/repelling the fluid elements and swirling motions in a finite time. Repelling/attracting LCS are found by computing the forward/backward finite-time Lyapunov exponent (FTLE), and vortices are found by the Lagrangian-averaged vorticity deviation method. We show that the Lagrangian centres and boundaries of supergranular cells are given by the local maximum of the forward and backward FTLE, respectively. The attracting LCS expose the location of the sinks of photospheric flows at supergranular junctions, whereas the repelling LCS interconnect the Lagrangian centres of neighbouring supergranular cells. Lagrangian transport barriers are found within a supergranular cell and from one cell to other cells, which play a key role in the dynamics of internetwork and network magnetic elements. Such barriers favour the formation of vortices in supergranular junctions. In particular, we show that the magnetic field distribution in the quiet Sun is determined by the combined action of attracting/repelling LCS and vortices. Title: Evolution of bipolar internetwork magnetic fields Authors: Gosic, Milan; De Pontieu, Bart; Bellot Rubio, Luis Ramon Bibcode: 2019AAS...23431102G Altcode: Internetwork (IN) magnetic fields can be found inside supergranular cells all over the solar surface. Thanks to their abundance and appearance rate, IN fields are considered to be an essential contributor to the magnetic flux and energy budget of the solar photosphere, and may also play a major role in the energy budget of the chromosphere and transition region. Therefore, it is crucial to understand how IN magnetic fields appear, evolve, interact with the preexisting magnetic structures, and what impact they have on the upper solar atmosphere. Here, we analyze spatio-temporal evolution of IN magnetic bipolar structures, i.e, loops and clusters, employing multi-instrument (IRIS and SST), multi-wavelength observations of IN regions with the highest sensitivity and resolution possible. For the first time, our observations allow us to describe in detail how IN bipoles emerge in the photosphere and even reach the chromosphere. We estimate the field strengths of these IN magnetic structures both in the photosphere and the chromosphere, using full Stokes measurements in Fe I 6173 Å, Mg I b2 5173 Å, and Ca II 8542 Å. Employing the IRIS FUV and NUV spectra, we show that IN fields contribute to the chromospheric and transition region heating through interaction with the preexisting ambient fields. Title: On the Origin of the Magnetic Energy in the Quiet Solar Chromosphere Authors: Martínez-Sykora, Juan; Hansteen, Viggo H.; Gudiksen, Boris; Carlsson, Mats; De Pontieu, Bart; Gošić, Milan Bibcode: 2019ApJ...878...40M Altcode: 2019arXiv190404464M The presence of magnetic field is crucial in the transport of energy through the solar atmosphere. Recent ground-based and space-borne observations of the quiet Sun have revealed that magnetic field accumulates at photospheric heights, via a local dynamo or from small-scale flux emergence events. However, most of this small-scale magnetic field may not expand into the chromosphere due to the entropy drop with height at the photosphere. Here we present a study that uses a high-resolution 3D radiative MHD simulation of the solar atmosphere with non-gray and non-LTE radiative transfer and thermal conduction along the magnetic field to reveal that (1) the net magnetic flux from the simulated quiet photosphere is not sufficient to maintain a chromospheric magnetic field (on average), (2) processes in the lower chromosphere, in the region dominated by magnetoacoustic shocks, are able to convert kinetic energy into magnetic energy, (3) the magnetic energy in the chromosphere increases linearly in time until the rms of the magnetic field strength saturates at roughly 4-30 G (horizontal average) due to conversion from kinetic energy, (4) and that the magnetic features formed in the chromosphere are localized to this region. Title: Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach Authors: Sainz Dalda, Alberto; de la Cruz Rodríguez, Jaime; De Pontieu, Bart; Gošić, Milan Bibcode: 2019ApJ...875L..18S Altcode: 2019arXiv190408390S Inversion codes allow the reconstruction of a model atmosphere from observations. With the inclusion of optically thick lines that form in the solar chromosphere, such modeling is computationally very expensive because a non-LTE evaluation of the radiation field is required. In this study, we combine the results provided by these traditional methods with machine and deep learning techniques to obtain similar-quality results in an easy-to-use, much faster way. We have applied these new methods to Mg II h and k lines observed by the Interface Region Imaging Spectrograph (IRIS). As a result, we are able to reconstruct the thermodynamic state (temperature, line-of-sight velocity, nonthermal velocities, electron density, etc.) in the chromosphere and upper photosphere of an area equivalent to an active region in a few CPU minutes, speeding up the process by a factor of 105 - 106. The open-source code accompanying this Letter will allow the community to use IRIS observations to open a new window to a host of solar phenomena. Title: Convection-driven Generation of Ubiquitous Coronal Waves Authors: Aschwanden, Markus J.; Gošic, Milan; Hurlburt, Neal E.; Scullion, Eamon Bibcode: 2018ApJ...866...73A Altcode: We develop a new method to measure the 3D kinematics of the subphotospheric motion of magnetic elements, which is used to study the coupling between the convection-driven vortex motion and the generation of ubiquitous coronal waves. We use the method of decomposing a line-of-sight magnetogram from MDI/SDO into unipolar magnetic charges, which yields the (projected) 2D motion [x(t), y(t)] and the (half) width evolution w(t) of an emerging magnetic element from an initial depth of d ≲ 1500 km below the photosphere. A simple model of rotational vortex motion with magnetic flux conservation during the emergence process of a magnetic element predicts the width evolution, i.e., w(t)/w 0 = [B(t)/B 0]-1/2, and an upper limit of the depth variation d(t) ≤ 1.3 w(t). While previous 2D tracing of magnetic elements provided information on advection and superdiffusion, our 3D tracing during the emergence process of a magnetic element is consistent with a ballistic trajectory in the upward direction. From the estimated Poynting flux and lifetimes of convective cells, we conclude that the Coronal Multi-channel Polarimeter-detected low-amplitude transverse magnetohydrodynamic waves are generated by the convection-driven vortex motion. Our observational measurements of magnetic elements appear to contradict the theoretical random-walk braiding scenario of Parker. Title: Emergence of internetwork magnetic fields through the solar atmosphere Authors: Gosic, Milan; De Pontieu, Bart; Bellot Rubio, L. R. Bibcode: 2018cosp...42E1261G Altcode: Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they emerge all over the Sun, these small-scale fields bring a substantial amount of flux, and therefore energy, to the solar surface. Because of this, IN fields are crucial for understanding the quiet Sun magnetism. However, they are weak and produce very small polarization signals, which is the reason why their properties and impact on the energetics and dynamics of the solar atmosphere are largely unknown. Here we use coordinated IRIS and SST observations of IN regions at high spatial and temporal resolution. They give us the opportunity to follow the evolution of IN magnetic loops as they emerge into the photosphere. For the first time, our polarimetric measurements provide a direct observational evidence of IN fields reaching the chromosphere. Moreover, we show that IN magnetic loops contribute to the chromospheric and transition region heating through interaction with preexisting ambient fields. Title: Transport of Internetwork Magnetic Flux Elements in the Solar Photosphere : Signatures of Large-Scale Flows and their Effect on Transport Statistics Authors: Agrawal, Piyush; Rast, Mark; Gosic, Milan; Rempel, Matthias; Bellot Rubio, Luis Bibcode: 2018tess.conf21704A Altcode: The motions of small-scale magnetic <span class="s1" flux elements in the solar photosphere can provide some measure of the Lagrangian properties of the convective <span class="s1" flow. Measurements of these motions have been critical in estimating the turbulent diffusion coef<span class="s1" ficient in <span class="s1" flux-transport dynamo models and in determining the Alfvén wave excitation spectrum for coronal heating models. We examine the motions of internetwork <span class="s1" flux elements in Hinode<span class="s1" /Narrowband Filter Imager magnetograms and study the scaling of their mean squared displacement and the shape of their displacement probability distribution as a function of time. We <span class="s1" find that the mean squared displacement scales super-diffusively with a slope of about 1.48. Super-diffusive scaling has been observed in other studies for temporal increments as small as 5 s, increments over which ballistic scaling would be expected. Using high-cadence MURaM simulations, we show that the observed super-diffusive scaling at short increments is a consequence of random changes in barycenter positions due to <span class="s1" flux evolution. We also <span class="s1" find that for long temporal increments, beyond granular lifetimes, the observed displacement distribution deviates from that expected for a diffusive process, evolving from Rayleigh to Gaussian. This change in distribution can be modeled analytically by accounting for supergranular advection along with granular motions. These results complicate the interpretation of magnetic element motions as strictly advective or diffusive on short and long timescales and suggest that measurements of magnetic element motions must be used with caution in turbulent diffusion or wave excitation models. We propose that passive tracer motions in measured photospheric <span class="s1" flows may yield more robust transport statistics. Title: Emergence of internetwork magnetic fields through the solar atmosphere Authors: Gosic, Milan; De Pontieu, Bart; Bellot Rubio, Luis Bibcode: 2018tess.conf21701G Altcode: Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they emerge all over the Sun, these small-scale fields bring a substantial amount of flux, and therefore energy, to the solar surface. Because of this, IN fields are crucial for understanding the quiet Sun magnetism. However, they are weak and produce very small polarization signals, which is the reason why their properties and impact on the energetics and dynamics of the solar atmosphere are largely unknown. Here we use coordinated IRIS and SST observations of IN regions at high spatial and temporal resolution. They give us the opportunity to follow the evolution of IN magnetic loops as they emerge into the photosphere. For the first time, our polarimetric measurements provide a direct observational evidence of IN fields reaching the chromosphere. Moreover, we show that IN magnetic loops contribute to the chromospheric and transition region heating through interaction with preexisting ambient fields. Title: Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields Authors: Gošić, M.; de la Cruz Rodríguez, J.; De Pontieu, B.; Bellot Rubio, L. R.; Carlsson, M.; Esteban Pozuelo, S.; Ortiz, A.; Polito, V. Bibcode: 2018ApJ...857...48G Altcode: 2018arXiv180207392G The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by the magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper, we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for ∼3 minutes in the photosphere and ∼12 minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg II h and k lines, we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating. Title: Investigating the Response of Loop Plasma to Nanoflare Heating Using RADYN Simulations Authors: Polito, V.; Testa, P.; Allred, J.; De Pontieu, B.; Carlsson, M.; Pereira, T. M. D.; Gošić, Milan; Reale, Fabio Bibcode: 2018ApJ...856..178P Altcode: 2018arXiv180405970P We present the results of 1D hydrodynamic simulations of coronal loops that are subject to nanoflares, caused by either in situ thermal heating or nonthermal electron (NTE) beams. The synthesized intensity and Doppler shifts can be directly compared with Interface Region Imaging Spectrograph (IRIS) and Atmospheric Imaging Assembly (AIA) observations of rapid variability in the transition region (TR) of coronal loops, associated with transient coronal heating. We find that NTEs with high enough low-energy cutoff ({E}{{C}}) deposit energy in the lower TR and chromosphere, causing blueshifts (up to ∼20 km s-1) in the IRIS Si IV lines, which thermal conduction cannot reproduce. The {E}{{C}} threshold value for the blueshifts depends on the total energy of the events (≈5 keV for 1024 erg, up to 15 keV for 1025 erg). The observed footpoint emission intensity and flows, combined with the simulations, can provide constraints on both the energy of the heating event and {E}{{C}}. The response of the loop plasma to nanoflares depends crucially on the electron density: significant Si IV intensity enhancements and flows are observed only for initially low-density loops (<109 cm-3). This provides a possible explanation of the relative scarcity of observations of significant moss variability. While the TR response to single heating episodes can be clearly observed, the predicted coronal emission (AIA 94 Å) for single strands is below current detectability and can only be observed when several strands are heated closely in time. Finally, we show that the analysis of the IRIS Mg II chromospheric lines can help further constrain the properties of the heating mechanisms. Title: Persistent magnetic vortex flow at a supergranular vertex Authors: Requerey, Iker S.; Cobo, Basilio Ruiz; Gošić, Milan; Bellot Rubio, Luis R. Bibcode: 2018A&A...610A..84R Altcode: 2017arXiv171201510R Context. Photospheric vortex flows are thought to play a key role in the evolution of magnetic fields. Recent studies show that these swirling motions are ubiquitous in the solar surface convection and occur in a wide range of temporal and spatial scales. Their interplay with magnetic fields is poorly characterized, however.
Aims: We study the relation between a persistent photospheric vortex flow and the evolution of a network magnetic element at a supergranular vertex.
Methods: We used long-duration sequences of continuum intensity images acquired with Hinode and the local correlation-tracking method to derive the horizontal photospheric flows. Supergranular cells are detected as large-scale divergence structures in the flow maps. At their vertices, and cospatial with network magnetic elements, the velocity flows converge on a central point.
Results: One of these converging flows is observed as a vortex during the whole 24 h time series. It consists of three consecutive vortices that appear nearly at the same location. At their core, a network magnetic element is also detected. Its evolution is strongly correlated to that of the vortices. The magnetic feature is concentrated and evacuated when it is caught by the vortices and is weakened and fragmented after the whirls disappear.
Conclusions: This evolutionary behavior supports the picture presented previously, where a small flux tube becomes stable when it is surrounded by a vortex flow.

A movie attached to Fig. 2 is available at https://www.aanda.org Title: Occurrence and persistence of magnetic elements in the quiet Sun Authors: Giannattasio, F.; Berrilli, F.; Consolini, G.; Del Moro, D.; Gošić, M.; Bellot Rubio, L. Bibcode: 2018A&A...611A..56G Altcode: 2018arXiv180103871G Context. Turbulent convection efficiently transports energy up to the solar photosphere, but its multi-scale nature and dynamic properties are still not fully understood. Several works in the literature have investigated the emergence of patterns of convective and magnetic nature in the quiet Sun at spatial and temporal scales from granular to global.
Aims: To shed light on the scales of organisation at which turbulent convection operates, and its relationship with the magnetic flux therein, we studied characteristic spatial and temporal scales of magnetic features in the quiet Sun.
Methods: Thanks to an unprecedented data set entirely enclosing a supergranule, occurrence and persistence analysis of magnetogram time series were used to detect spatial and long-lived temporal correlations in the quiet Sun and to investigate their nature.
Results: A relation between occurrence and persistence representative for the quiet Sun was found. In particular, highly recurrent and persistent patterns were detected especially in the boundary of the supergranular cell. These are due to moving magnetic elements undergoing motion that behaves like a random walk together with longer decorrelations ( 2 h) with respect to regions inside the supergranule. In the vertices of the supegranular cell the maximum observed occurrence is not associated with the maximum persistence, suggesting that there are different dynamic regimes affecting the magnetic elements. Title: Transport of Internetwork Magnetic Flux Elements in the Solar Photosphere Authors: Agrawal, Piyush; Rast, Mark P.; Gošić, Milan; Bellot Rubio, Luis R.; Rempel, Matthias Bibcode: 2018ApJ...854..118A Altcode: 2017arXiv171101290A The motions of small-scale magnetic flux elements in the solar photosphere can provide some measure of the Lagrangian properties of the convective flow. Measurements of these motions have been critical in estimating the turbulent diffusion coefficient in flux-transport dynamo models and in determining the Alfvén wave excitation spectrum for coronal heating models. We examine the motions of internetwork flux elements in Hinode/Narrowband Filter Imager magnetograms and study the scaling of their mean squared displacement and the shape of their displacement probability distribution as a function of time. We find that the mean squared displacement scales super-diffusively with a slope of about 1.48. Super-diffusive scaling has been observed in other studies for temporal increments as small as 5 s, increments over which ballistic scaling would be expected. Using high-cadence MURaM simulations, we show that the observed super-diffusive scaling at short increments is a consequence of random changes in barycenter positions due to flux evolution. We also find that for long temporal increments, beyond granular lifetimes, the observed displacement distribution deviates from that expected for a diffusive process, evolving from Rayleigh to Gaussian. This change in distribution can be modeled analytically by accounting for supergranular advection along with granular motions. These results complicate the interpretation of magnetic element motions as strictly advective or diffusive on short and long timescales and suggest that measurements of magnetic element motions must be used with caution in turbulent diffusion or wave excitation models. We propose that passive tracer motions in measured photospheric flows may yield more robust transport statistics. Title: Chromospheric Heating Driven by Cancellations of Internetwork Magnetic Flux Authors: Gosic, M.; de la Cruz Rodriguez, J.; De Pontieu, B.; Bellot Rubio, L.; Esteban Pozuelo, S.; Ortiz-Carbonell, A. N. Bibcode: 2017AGUFMSH41C..02G Altcode: The heating of the solar chromosphere remains to be one of the most important questions in solar physics. It is believed that this phenomenon may significantly be supported by small-scale internetwork (IN) magnetic fields. Indeed, cancellations of IN magnetic flux can generate transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by magnetic reconnection of IN field lines. Using high resolution, multiwavelength, coordinated observations recorded with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1-m Solar Telescope (SST), we analyzed cancellations of IN flux and their impact on the energetics and dynamics of the quiet Sun atmosphere. From their temporal and spatial evolution, we determine that these events can heat locally the upper atmospheric layers. However, employing multi-line inversions of the Mg II h & k lines, we show that cancellations, although occurring ubiquitously over IN regions, are not capable of sustaining the total radiative losses of the quiet Sun chromosphere. Title: Chromospheric heating due to internetwork magnetic flux cancellations Authors: Gosic, Milan; de la Cruz Rodriguez, Jaime; De Pontieu, Bart; Bellot Rubio, Luis; Ortiz, Ada; Esteban Pozuelo, Sara Bibcode: 2017SPD....4810404G Altcode: The heating of the solar chromosphere is one of the most intriguing unanswered problems in solar physics. It is believed that this phenomenon may significantly be supported by small-scale internetwork (IN) magnetic fields. Indeed, cancellations of IN magnetic flux patches might be an efficient way to transport flux and energy from the photosphere to the chromosphere. Because of this, it is essential to determine where they occur, the rates at which they proceed, and understand their influence on the chromosphere. Here we study the spatial and temporal evolution of IN cancelling patches using high resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1-m Solar Telescope (SST). Employing multi-line inversions of the Mg II h&k lines we show that cancelling events, while occurring ubiquitously over IN regions, produce clear signatures of heating in the upper atmospheric layers. Using the RADYN code we determine the energy released due to cancellations of IN elements and discuss about their impact on the dynamics and energetics of the solar chromosphere. Title: Flux appearance and disappearance rates in the solar internetwork Authors: Gosic, Milan; Bellot Rubio, Luis; Del Toro Iniesta, Jose Carlos; Orozco Suarez, David; Katsukawa, Yukio Bibcode: 2016SPD....4740105G Altcode: The solar internetwork contains weak and highly dynamic magnetic fields that are essential to understanding the solar magnetism at small spatial and temporal scales. Therefore, it is important to determine how these fields are maintained on the solar surface. Using unique Hinode observations, we follow the evolution of individual magnetic elements in the interior of two supergranular cells at the disk center. From up to 38 hr of continuous measurements, we show that magnetic flux appears in internetwork regions at a rate of 120±3 Mx cm-2 day-1 (3.7±0.4 × 1024 Mx day-1 over the entire solar surface). Flux disappears from the internetwork at a rate of 125±6 Mx cm-2 day-1 (3.9±0.5 × 1024 Mx day-1) through fading of magnetic elements, cancellation between opposite-polarity features, and interactions with network patches, which converts internetwork elements into network features. The removal of flux from supergranules occurs mainly through fading and interactions with network, at nearly the same rate of about 50 Mx cm-2 day-1. Our results demonstrate that the sources and sinks of internetwork magnetic flux are well balanced, reflecting the steady-state nature of the quiet Sun. Using the instantaneous flux appearance and disappearance rates, we successfully reproduce, for the first time, the temporal evolution of the total unsigned flux in the interior of supergranular cells. Title: The Solar Internetwork. II. Flux Appearance and Disappearance Rates Authors: Gošić, M.; Bellot Rubio, L. R.; del Toro Iniesta, J. C.; Orozco Suárez, D.; Katsukawa, Y. Bibcode: 2016ApJ...820...35G Altcode: 2016arXiv160205892G Small-scale internetwork magnetic fields are important ingredients of the quiet Sun. In this paper we analyze how they appear and disappear on the solar surface. Using high resolution Hinode magnetograms, we follow the evolution of individual magnetic elements in the interior of two supergranular cells at the disk center. From up to 38 hr of continuous measurements, we show that magnetic flux appears in internetwork regions at a rate of 120 ± 3 Mx cm-2 day-1 (3.7 ± 0.4 × 1024 Mx day-1 over the entire solar surface). Flux disappears from the internetwork at a rate of 125 ± 6 Mx cm-2 day-1 (3.9 ± 0.5 × 1024 Mx day-1) through fading of magnetic elements, cancelation between opposite-polarity features, and interactions with network patches, which converts internetwork elements into network features. Most of the flux is lost through fading and interactions with the network, at nearly the same rate of about 50 Mx cm-2 day-1. Our results demonstrate that the sources and sinks of internetwork magnetic flux are well balanced. Using the instantaneous flux appearance and disappearance rates, we successfully reproduce the time evolution of the total unsigned flux in the two supergranular cells. Title: Super-diffusion versus competitive advection: a simulation Authors: Del Moro, D.; Giannattasio, F.; Berrilli, F.; Consolini, G.; Lepreti, F.; Gošić, M. Bibcode: 2015A&A...576A..47D Altcode: 2015arXiv150105444D Context. Magnetic element tracking is often used to study the transport and diffusion of the magnetic field on the solar photosphere. From the analysis of the displacement spectrum of these tracers, it has recently been agreed that a regime of super-diffusivity dominates the solar surface. Quite habitually this result is discussed in the framework of fully developed turbulence.
Aims: However, the debate whether the super-diffusivity is generated by a turbulent dispersion process, by the advection due to the convective pattern, or even by another process is still open, as is the question of the amount of diffusivity at the scales relevant to the local dynamo process.
Methods: To understand how such peculiar diffusion in the solar atmosphere takes place, we compared the results from two different data sets (ground-based and space-borne) and developed a simulation of passive tracers advection by the deformation of a Voronoi network.
Results: The displacement spectra of the magnetic elements obtained by the data sets are consistent in retrieving a super-diffusive regime for the solar photosphere, but the simulation also shows a super-diffusive displacement spectrum: its competitive advection process can reproduce the signature of super-diffusion.
Conclusions: Therefore, it is not necessary to hypothesize a totally developed turbulence regime to explain the motion of the magnetic elements on the solar surface. Title: The solar internetwork Authors: Gosic, Milan Bibcode: 2015PhDT.......305G Altcode: No abstract at ADS Title: The Solar Internetwork. I. Contribution to the Network Magnetic Flux Authors: Gošić, M.; Bellot Rubio, L. R.; Orozco Suárez, D.; Katsukawa, Y.; del Toro Iniesta, J. C. Bibcode: 2014ApJ...797...49G Altcode: 2014arXiv1408.2369G The magnetic network (NE) observed on the solar surface harbors a sizable fraction of the total quiet Sun flux. However, its origin and maintenance are not well known. Here we investigate the contribution of internetwork (IN) magnetic fields to the NE flux. IN fields permeate the interior of supergranular cells and show large emergence rates. We use long-duration sequences of magnetograms acquired by Hinode and an automatic feature tracking algorithm to follow the evolution of NE and IN flux elements. We find that 14% of the quiet Sun (QS) flux is in the form of IN fields with little temporal variations. IN elements interact with NE patches and modify the flux budget of the NE either by adding flux (through merging processes) or by removing it (through cancellation events). Mergings appear to be dominant, so the net flux contribution of the IN is positive. The observed rate of flux transfer to the NE is 1.5 × 1024 Mx day-1 over the entire solar surface. Thus, the IN supplies as much flux as is present in the NE in only 9-13 hr. Taking into account that not all the transferred flux is incorporated into the NE, we find that the IN would be able to replace the entire NE flux in approximately 18-24 hr. This renders the IN the most important contributor to the NE, challenging the view that ephemeral regions are the main source of flux in the QS. About 40% of the total IN flux eventually ends up in the NE. Title: Pair separation of magnetic elements in the quiet Sun Authors: Giannattasio, F.; Berrilli, F.; Biferale, L.; Del Moro, D.; Sbragaglia, M.; Bellot Rubio, L.; Gošić, M.; Orozco Suárez, D. Bibcode: 2014A&A...569A.121G Altcode: 2014arXiv1409.1010G The dynamic properties of the quiet Sun photosphere can be investigated by analyzing the pair dispersion of small-scale magnetic fields (i.e., magnetic elements). By using 25 h-long Hinode magnetograms at high spatial resolution (0.3 arcsec), we tracked 68 490 magnetic element pairs within a supergranular cell near the disk center. The computed pair separation spectrum, calculated on the whole set of particle pairs independently of their initial separation, points out what is known as a super-diffusive regime with spectral index γ = 1.55 ± 0.05, in agreement with the most recent literature, but extended to unprecedented spatial and temporal scales (from granular to supergranular). Furthermore, for the first time, we investigated here the spectrum of the mean square displacement of pairs of magnetic elements, depending on their initial separation r0. We found that there is a typical initial distance above (below) which the pair separation is faster (slower) than the average. A possible physical interpretation of such a typical spatial scale is also provided. Title: Flux emergence in the solar internetwork and its contribution to the network Authors: Gosic, Milan; Katsukawa, Yukio; Orozco Suarez, David; Bellot Rubio, L. R. Bibcode: 2014cosp...40E1055G Altcode: Network and internetwork magnetic fields are believed to play a crucial role in the energy budget of the solar atmosphere. Therefore, it is essential to understand how they are maintained on the solar surface. Using high resolution Hinode/NFI magnetograms at disk center, we automatically follow quiet Sun magnetic elements from their appearance to disappearance. From up to 40 hours of continuous measurements, we derive the flux emergence rate in the solar internetwork to be around 40 Mx cm(-2) day(-1) . We show that internetwork fields appearing in the interior of individual supergranular cells contribute flux to the surrounding network at a rate of 2×10(19) Mx h(-1) . In only 10-20 hours, internetwork elements can transfer as much flux as resides in network features, establishing them as the most important source of flux for the network and the quiet Sun flux budget. Title: Diffusion of Solar Magnetic Elements up to Supergranular Spatial and Temporal Scales Authors: Giannattasio, F.; Del Moro, D.; Berrilli, F.; Bellot Rubio, L.; Gošić, M.; Orozco Suárez, D. Bibcode: 2013ApJ...770L..36G Altcode: 2013arXiv1305.4006G The study of spatial and temporal scales on which small magnetic structures (magnetic elements) are organized in the quiet Sun may be approached by determining how they are transported on the solar photosphere by convective motions. The process involved is diffusion. Taking advantage of Hinode high spatial resolution magnetograms of a quiet-Sun region at the disk center, we tracked 20,145 magnetic elements. The large field of view (~50 Mm) and the long duration of the observations (over 25 hr without interruption at a cadence of 90 s) allowed us to investigate the turbulent flows at unprecedented large spatial and temporal scales. In the field of view an entire supergranule is clearly recognizable. The magnetic element displacement spectrum shows a double-regime behavior: superdiffusive (γ = 1.34 ± 0.02) up to granular spatial scales (~1500 km) and slightly superdiffusive (γ = 1.20 ± 0.05) up to supergranular scales. Title: Turbulent diffusion on the solar photosphere through 24-hour continuous observations of magnetic elements Authors: Giannattasio, F.; Berrilli, F.; Del Moro, D.; Bellot Rubio, L.; Orozco Suarez, D.; Gosic, M. Bibcode: 2012AGUFMSH13A2242G Altcode: Solar atmosphere is a unique laboratory for the study of turbulent flows under extreme conditions (e.g. very high Reynolds numbers). The turbulent nature of the flow may be approached by determining how magnetic flux elements are transported on the solar surface, and measuring the spatio-temporal scales on which these small magnetic structures are organized. The process involved is diffusion. Several works explored this topic, both by simulations and observations, and the results are often contradictory, ranging from fully-developed turbulent scenarios to normal-diffusive motions. We analyze 24-hour continuous Hinode SOT observations of a supergranular region (for the first time these long scales are explored), studying the evolution of the mutual distance between magnetic element pairs and its scaling laws, in order to investigate the diffusion process. We find a super-diffusive behavior, with a gamma index depending on the spatial scale selected. Title: Evolution of internetwork magnetic fields inside supergranular cells Authors: Gosic, Milan; Katsukawa, Yukio; Bellot Rubio, Luis; Orozco Suarez, David Bibcode: 2012cosp...39..657G Altcode: 2012cosp.meet..657G To understand the formation of small-scale magnetic fields in the quiet Sun and their contribution to the solar activity, it is essential to investigate the properties of internetwork magnetic fields. Using Hinode/NFI magnetograms of very high sensitivity (7 Mx/cm^{2}), spatial resolution (0.16 arcsec/pixel), and cadence (90 s), we follow the evolution of magnetic fields inside of a supergranular cell located at disk center. In 5 hours of continuous measurements covering an area of 20.8 × 23.2 arcsec^{2}, we manually track 2415 magnetic elements from appearance to disappearance and derive their physical properties. The average values of the magnetic flux, effective diameter, lifetime, and horizontal velocity are 3 × 10^{17} Mx, 0.5 Mm, 17 min, and 2 km/s, respectively. We also investigate how the physical parameters of the individual elements vary as a function of time, flux, and spatial position. Using this unique data set, we determine with unprecedented accuracy the flux emergence and disappearance rate in the solar internetwork. Title: On the solar abundance of indium Authors: Vitas, N.; Vince, I.; Lugaro, M.; Andriyenko, O.; Gošić, M.; Rutten, R. J. Bibcode: 2008MNRAS.384..370V Altcode: 2008MNRAS.tmp...25V; 2007arXiv0711.2166V The generally adopted value for the solar abundance of indium is over six times higher than the meteoritic value. We address this discrepancy through numerical synthesis of the 451.13-nm line on which all indium abundance studies are based, both for the quiet Sun and the sunspot umbra spectrum, employing standard atmosphere models and accounting for hyperfine structure and Zeeman splitting in detail. The results, as well as a re-appraisal of indium nucleosynthesis, suggest that the solar indium abundance is close to the meteoritic value, and that some unidentified ion line causes the 451.13-nm feature in the quiet-Sun spectrum. Title: Program of Telluric Lines Monitoring Authors: Vince, I.; Kos, P.; Latkovic, O.; Martinovic, N.; Gosic, M.; Stojadinovic, J. Bibcode: 2006SerAJ.173..101V Altcode: A new observational program of telluric lines monitoring was introduced at Belgrade Astronomical Observatory. The ultimate goal of this program is to investigate the properties of Earth's atmosphere through modeling the observed profiles of telluric lines. The program is intend to observe infrared molecular oxygen lines that were selected according to spectral sensitivity of the available CCD camera. In this paper we give the initial and the final selection criteria for spectral lines included in the program, the description of equipment and procedures used for observations and reduction, a review of preliminary observational results with the estimated precision, and a short discussion on the comparison of the theoretical predictions and the measurements.