Plage Regions Research Articles

Solar Vortex Tubes. III. Vorticity and Energy Transport

This study investigated the mechanisms of vorticity generation and the role of vortex tubes in plasma heating and energy transport. Vortex tubes were identified using the instantaneous vorticity deviation technique in the MURaM data set of a simulated solar plage region of the solar photosphere. Within 3D kinetic vortex tubes, the misalignment of the magnetic pressure and the inverse of the density gradient, rather than baroclinic effects, primarily drive vorticity within the tubes. During their lifetime, vortices become less dense as the Lorentz force pushes plasma outwards against pressure gradients. In the simulated upper photosphere, the Lorentz force contributes to adiabatic cooling and heating by expanding or compressing the plasma around the vortex tubes. In turn, vortex motion affects the magnetic field, enhancing current generation and intensifying the Lorentz force, which may further increase adiabatic cooling and heating. Moreover, our results confirm that vortices can significantly boost viscous and ohmic heating on intergranular scales in the photosphere. They generate more magnetic than kinetic energy, with energy transport by Poynting flux notably nonuniform and dominant at the vortex boundaries. This creates energy circulation in which the net upwards Poynting flux can enhance chromospheric plasma heating and support chromospheric temperatures.

On the Nature of Nonthermal Broadening of Spectral Lines Observed by IRIS

The origin of nonthermal broadening in solar spectra is one of the long-standing questions in solar physics. Various processes have been invoked—including unresolved flows, waves, and turbulent processes—but definitive answers are lacking. To investigate the physical processes responsible for nonthermal broadening, we examine its relation with the angle between the magnetic field and the line of sight in three different closed-field regions above plage regions at different locations on the solar disk. We obtained the nonthermal width of transition-region Si iv 1403 Å spectra observed in active regions by the Interface Region Imaging Spectrograph, after subtraction of the thermal and instrumental line broadening. To investigate the dependence of the measured broadening on the viewing angle between the line of sight and magnetic field direction, we determined the magnetic field direction at transition-region heights using nonlinear force-free extrapolations based on the observed photospheric vector magnetic field taken by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We found that the nonthermal broadening shows a correlation with downward motion (redshifts) and alignment between the magnetic field and the observer’s line-of-sight direction. Based on the observed correlations, we suggest that velocity gradients within plasma flowing down along the magnetic field may lead to a significant portion of the observed nonthermal broadening of transition-region spectral lines in closed fields above plage regions.

A multi-instrument study of UV bursts and associated surges in AR 12957

The relationship between UV bursts and solar surges is complex, with these events sometimes being observed together and sometimes being observed independently. Why this sporadic association exists is unknown; however, it likely relates to the physical conditions at the site of the energy release that drives these events. Here, we aim to better understand the relationship between UV bursts and solar surges through a multi-instrument analysis of several associated events that occurred around the trailing sunspot in AR $12957$. We used data from Solar Orbiter, the Solar Dynamics Observatory (SDO), and the Interface Region Imaging Spectrograph (IRIS) to achieve our aims. These data were sampled on 3 March 2022 between $09$:$30$:$30$ UT and $11$:$00$:$00$ UT, during which time a coordinated observing campaign associated with the Slow Solar Wind Connection Solar Orbiter Observing Plan (SOOP) took place. Numerous small-scale negative polarity magnetic magnetic features (MMFs) were observed to move quickly (potentially up to $3.3$ km s$^ $) away from a sunspot until they collided with a more stable positive polarity plage region around $7$ Mm away. Several UV bursts were identified in IRIS slit-jaw imager (SJI) $1400$ data co-spatial to where these opposite polarity fields interacted, with spatial scales ($2$ Mm$<$) and lifetimes ($20<$ minutes) larger than typical values for such events. Two surges were also observed to occur at these locations, with one being short ($5$ Mm) and hot (bright in the IRIS SJI images), whilst the other was a cooler (dark in coronal imaging channels), longer surge that appeared to fill an active region loop. Magnetic reconnection between the negative polarity MMFs around the sunspot and the positive polarity plage region appears to be the driver of these events. Both the speed of the MMFs and the locally open magnetic topology of the plage region could possibly have been important for forming the surges.

Mapping the Longitudinal Magnetic Field in the Atmosphere of an Active Region Plage from the Inversion of the Near-ultraviolet CLASP2.1 Spectropolarimetric Data

We apply the HanleRT Tenerife Inversion Code to the spectropolarimetric observations obtained by the Chromospheric Layer Spectropolarimeter. This suborbital space experiment measured the variation with wavelength of the four Stokes parameters in the near-ultraviolet spectral region of the Mg ii h and k lines over a solar disk area containing part of an active region plage and the edge of a sunspot penumbra. We infer the stratification of the temperature, the electron density, the line-of-sight velocity, the microturbulent velocity, and the longitudinal component of the magnetic field from the observed intensity and circular polarization profiles. The inferred model atmosphere shows larger temperature and electron density in the plage and the superpenumbra regions than in the quiet regions. The shape of the plage region in terms of its brightness is similar to the pattern of the inferred longitudinal component of the magnetic field in the chromosphere, as well as to that of the overlying moss observed by the Atmospheric Imaging Assembly in the 171 Å band, which suggests a similar magnetic origin for the heating in both the plage and the moss region. Moreover, this heating is particularly significant in the regions with larger inferred magnetic flux. In contrast, in the superpenumbra, the regions with larger electron density and temperature are usually found in between these regions with larger magnetic flux, suggesting that the details of the heating mechanism in the chromosphere of the superpenumbra may be different from those in the plage, but with the magnetic field still playing a key role.

Solar Chromospheric Heating by Magnetohydrodynamic Waves: Dependence on the Inclination of the Magnetic Field

A proposed mechanism for solar chromospheric heating is magnetohydrodynamic waves propagating upward along magnetic field lines and dissipating their energy in the chromosphere. In particular, compressible magnetoacoustic waves may contribute to the heating. Theoretically, the components below the cutoff frequency cannot propagate into the chromosphere; however, the cutoff frequency depends on the inclination of the magnetic field lines. In this study, using high-temporal cadence spectral data of IRIS and Hinode Solar Optical Telescope spectropolarimeter in plages, we investigated the dependence of the low-frequency waves on magnetic field properties and quantitatively estimated the amount of energy dissipation in the chromosphere. The following results were obtained: (a) The amount of energy dissipated by the low-frequency component (3–6 mHz) increases with the inclination of the field below 40°, whereas it decreases as a function of the inclination of the field above 40°. (b) The amount of the energy is enhanced toward 104 W m−2, which is the energy required for heating in the chromospheric plage regions when the magnetic field is higher than 600 G and inclined more than 40°. (c) In the photosphere, the low-frequency component has much more power in the magnetic field inclined more and weaker than 400 G. The results suggest that the observed low-frequency components can bring the energy along the magnetic field lines and that only a specific range of the inclination angles of the field and the strength of the field may allow the low-frequency component to bring a sufficient amount of the energy into the chromosphere.

Modeling Stellar Ca ii H and K Emission Variations: Spot Contribution to the S-index

The S-index is a measure of emission in the Ca ii H and K lines and is a widely used proxy of stellar magnetic activity. It has been assumed until now that the S-index is mainly affected by bright plage regions in the chromosphere. In particular, the effect of starspots on the S-index has been neglected. In this study, we revisit this assumption. For this, we analyze high-resolution observations of sunspots recorded in the Ca ii H spectral line at the Swedish 1 m Solar Telescope and determine the contrast of spots with respect to the quiet surroundings. We find that the Ca ii H line core averaged over whole sunspots (including superpenumbrae) is brighter than in the quiet surroundings and that the spot contrast in the line core is comparable to the facular contrast. This allows us to get a first estimate of the influence of spots on the S-index. We show that spots increase the S-index. While this increase is quite small for the Sun, it becomes significantly larger for more active stars. Further, we show that the inclusion of the contribution of spots to the S-index strongly affects the relationship between the S-index and stellar disk area coverages by spots and faculae, and present the new relations.

Magnetic Fields in Solar Plage Regions: Insights from High-sensitivity Spectropolarimetry

Plage regions are patches of concentrated magnetic field in the Sun’s atmosphere where hot coronal loops are rooted. While previous studies have shed light on the properties of plage magnetic fields in the photosphere, there are still challenges in measuring the overlying chromospheric magnetic fields, which are crucial to understanding the overall heating and dynamics. Here, we utilize high-sensitivity, spectropolarimetric data obtained by the 4 meter Daniel K. Inouye Solar Telescope to investigate the dynamic environment and magnetic field stratification of an extended, decaying plage region. The data show strong circular polarization signals in both plage cores and surrounding fibrils. Notably, weak linear polarization signals clearly differentiate between plage patches and the fibril canopy, where they are relatively stronger. Inversions of the Ca II 8542 Å spectra show an imprint of the fibrils in the chromospheric magnetic field, with typical field strength values ranging from ∼200 to 300 G in fibrils. We confirm the weak correlation between field strength and cooling rates in the lower chromosphere. Additionally, we observe supersonic downflows and strong velocity gradients in the plage periphery, indicating dynamical processes occurring in the chromosphere. These findings contribute to our understanding of the magnetic field and dynamics within plages, emphasizing the need for further research to explore the expansion of magnetic fields with height and the three-dimensional distribution of heating rates in the lower chromosphere.

Quiet solar corona: daily images at 8.8–10.7 cm wavelengths

We discuss results of test observations of the 3–6 GHz range array of the Siberian Radio Heliograph (SRH). A method for calibrating brightness temperatures of images was verified using measurements of the brightness temperature of the quiet Sun at a minimum between solar activity cycles 20 and 21 known in the literature. The obtained time dependences of the integral solar flux at 2.8 GHz are similar to those measured at the Dominion Radio Astrophysical Observatory (DRAO), but the absolute values of SRH fluxes are lower relative to the DRAO fluxes by 10–15 %.
 The spectral density of the solar microwave flux at a frequency of 2.8 GHz, the so-called F10.7 index, is one of the main solar activity indices used as input parameters in models of Earth’s ionosphere. The paper considers the relationship between total radio fluxes and changes in the structure of sources on the solar disk during an interval of 50 days. During the period of daily observations from September 1 to October 20, 2021, the number of active regions on the disk changed several times, and the integral flux density at 2.8 GHz changed up to 1.5 times. We determine the relative contributions to the integral flux of bremsstrahlung of near-limb brightenings and plage regions, as well as bremsstrahlung in magnetic fields of active regions. The measured brightness temperatures of SRH radio maps are compared to the model temperatures calculated from observations of extreme ultraviolet emission (EUV) with the AIA/SDO telescope. The results of the analysis can be used to organize regular measurements of the corrected solar activity proxy index F10.7 at SRH, in which the contribution of gyroresonance emission is excluded.

Спокойная корона Солнца: ежедневные изображения на длинах волн 8.8–10.7 см

We discuss results of test observations of the 3–6 GHz range array of the Siberian Radio Heliograph (SRH). A method for calibrating brightness temperatures of images was verified using measurements of the brightness temperature of the quiet Sun at a minimum between solar activity cycles 20 and 21 known in the literature. The obtained time dependences of the integral solar flux at 2.8 GHz are similar to those measured at the Dominion Radio Astrophysical Observatory (DRAO), but the absolute values of SRH fluxes are lower relative to the DRAO fluxes by 10–15 %.
 The spectral density of the solar microwave flux at a frequency of 2.8 GHz, the so-called F10.7 index, is one of the main solar activity indices used as input parameters in models of Earth’s ionosphere. The paper considers the relationship between total radio fluxes and changes in the structure of sources on the solar disk during an interval of 50 days. During the period of daily observations from September 1 to October 20, 2021, the number of active regions on the disk changed several times, and the integral flux density at 2.8 GHz changed up to 1.5 times. We determine the relative contributions to the integral flux of bremsstrahlung of near-limb brightenings and plage regions, as well as bremsstrahlung in magnetic fields of active regions. The measured brightness temperatures of SRH radio maps are compared to the model temperatures calculated from observations of extreme ultraviolet emission (EUV) with the AIA/SDO telescope. The results of the analysis can be used to organize regular measurements of the corrected solar activity proxy index F10.7 at SRH, in which the contribution of gyroresonance emission is excluded.

Understanding the Relationship between Solar Coronal Abundances and F10.7 cm Radio Emission

Sun-as-a-star coronal plasma composition, derived from full-Sun spectra, and the F10.7 radio flux (2.8 GHz) have been shown to be highly correlated (r = 0.88) during solar cycle 24. However, this correlation becomes nonlinear during increased solar magnetic activity. Here we use cotemporal, high spatial resolution, multiwavelength images of the Sun to investigate the underlying causes of the nonlinearity between coronal composition (FIP bias) and F10.7 solar index correlation. Using the Karl G. Jansky Very Large Array, Hinode/EIS (EUV Imaging Spectrometer), and the Solar Dynamics Observatory, we observed a small active region, AR 12759, throughout the solar atmosphere from the photosphere to the corona. The results of this study show that the magnetic field strength (flux density) in active regions plays an important role in the variability of coronal abundances, and it is likely the main contributing factor to this nonlinearity during increased solar activity. Coronal abundances above cool sunspots are lower than in dispersed magnetic plage regions. Strong magnetic concentrations are associated with stronger F10.7 cm gyroresonance emission. Considering that as the solar cycle moves from minimum to maximum, the sizes of sunspots and their field strength increase with the gyroresonance component, the distinctly different tendencies of radio emission and coronal abundances in the vicinity of sunspots is the likely cause of saturation of Sun-as-a-star coronal abundances during solar maximum, while the F10.7 index remains well correlated with the sunspot number and other magnetic field proxies.

Tomography of a Solar Plage with the Tenerife Inversion Code

We apply the Tenerife Inversion Code (TIC) to the plage spectropolarimetric observations obtained by the Chromospheric LAyer SpectroPolarimeter (CLASP2). These unprecedented data consist of full Stokes profiles in the spectral region around the Mg ii h and k lines for a single slit position, with around two thirds of the 196″ slit crossing a plage region and the rest crossing an enhanced network. A previous analysis of these data had allowed us to infer the longitudinal component of the magnetic field by applying the weak-field approximation (WFA) to the circular polarization profiles, and to assign the inferred magnetic fields to different layers of the solar atmosphere based on the results of previous theoretical radiative transfer investigations. In this work, we apply the recently developed TIC to the same data. We obtain a stratified model atmosphere that fits the intensity and circular polarization profiles at each position along the spectrograph slit and we compare our results for the longitudinal component of the magnetic field with the previously obtained WFA results, highlighting the generally good agreement in spite of the fact that the WFA is known to produce an underestimation when applied to the outer lobes of the Mg ii h and k circular polarization profiles. Finally, we use the inverted model atmospheres to give a rough estimation of the energy that could be carried by Alfvén waves propagating in the chromosphere in the plage and network regions, showing that it is sufficient to compensate for the estimated energy losses in the chromosphere of solar active regions.

Evidence for the Operation of the Hanle and Magneto-optical Effects in the Scattering Polarization Signals Observed by CLASP2 across the Mg ii h and k Lines

Radiative transfer investigations of the solar Mg ii h and k resonance lines around 280 nm have shown that, while their circular polarization (Stokes V) signals arise from the Zeeman effect, the linear polarization profiles (Stokes Q and U) are dominated by the scattering of anisotropic radiation and the Hanle and magneto-optical (MO) effects. Using the unprecedented observations of the Mg ii and Mn i resonance lines obtained by the Chromospheric LAyer Spectro-Polarimeter (CLASP2), here we investigate how the linear polarization signals at different wavelengths (i.e., at the center, and at the near and far wings of the k line) vary with the longitudinal component of the magnetic field (B L ) at their approximate height of formation. The B L is estimated from the V signals in the aforementioned spectral lines. Particular attention is given to the following quantities that are expected to be influenced by the presence of magnetic fields through the Hanle and MO effects: the sign of the U signals, the total linear polarization amplitude (LP) and its direction (χ) with respect to a reference direction. We find that at the center and near wings of the k line, the behavior of these quantities is significantly different in the observed quiet and plage regions, and that both LP and χ seem to depend on B L . These observational results are indicative of the operation of the Hanle effect at the center of the k line and of the MO effects at the near wings of the k line.

Rapid blue- and redshifted excursions in Hα line profiles synthesized from realistic 3D magnetohydrodynamic simulations

Context. Rapid blue- and redshifted excursions (RBEs and RREs) may play an important role in mass-loading and heating the solar corona, but their nature and origin are still debatable. Aims. We aim to model these features to learn more about their properties, formation, and origin. Methods. We created a realistic three-dimensional (3D) magnetohydrodynamic model of a solar plage region. Synthetic Hα spectra were generated and the spectral signatures of these features identified. The magnetic field lines associated with these events were traced, and the underlying dynamic was studied. Results. The model reproduces many properties of RBEs and RREs well, such as spatial distribution, lateral movement, length, and lifetimes. Synthetic Hα line profiles, similarly to observed ones, show a strong blue- or redshift as well as asymmetries. These line profiles are caused by the vertical component of velocities higher than 30 − 40 km s−1, which mostly appear in the height range 2 − 4 Mm. By tracing magnetic field lines, we show that the vertical velocity that causes the appearance of RBEs or RREs is always associated with the component of velocity perpendicular to the magnetic field lines. Conclusions. The study confirms the hypothesis that RBEs and RREs are signs of Alfvénic waves with, in some cases, a significant contribution from slow magneto-acoustic modes.

Non-LTE formation of the Fe I 6173 Å line in the solar atmosphere

The current analysis is dedicated to a detailed investigation of the non-local thermodynamic equilibrium (NLTE) effects influencing the formation of the Fe I 6173 Å line, which is widely used by many instruments, including the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and the Polarimetric and Helioseismic Imager on board the Solar Orbiter. We synthesize the Stokes profiles in a snapshot of a three-dimensional magnetohydrodynamic simulation of the solar photosphere under both LTE and NLTE conditions. The simulation cube contains a sunspot and a plage region around it. The LTE and NLTE Stokes profiles formed in different features are compared and analysed. NLTE effects are evident in both intensity and polarization profiles. For the 6173 Å line, UV overionization is the dominant NLTE mechanism, and scattering effects are much less important. In addition to Fe, an NLTE treatment of Si, Mg, and Al is necessary to set the right photon density in the UV. This is found to further enhance the LTE departures compared to the case where Fe alone is treated in NLTE. These effects in the Stokes profiles survive even when the profiles are averaged spatially or sampled on a coarse wavelength grid such as that used by the SDO/HMI and other magnetographs. The deviations from the LTE profiles are stronger in the Fe I 6173 Å compared to the 6301 Å–6302 Å lines because in the latter case, line scattering compensates the effect of UV overionization. Based on the nature of departures from LTE, treating the 6173 Å line in LTE will likely result in an overestimation of temperature and an underestimation of the magnetic field strength.

Full-disc Ca ii K observations—A window to past solar magnetism

Full-disc observations of the Sun in the Ca iiK line provide one of the longest collections of solar data. First such observations were made in 1892 and since then various sites around the world have carried out regular observations, with Kodaikanal, Meudon, Mt Wilson, and Coimbra being some of the most prominent ones. By now, Ca iiK observations from over 40 different sites allow an almost complete daily coverage of the last century. Ca iiK images provide direct information on plage and network regions on the Sun and, through their connection to solar surface magnetic field, offer an excellent opportunity to study solar magnetism over more than a century. This makes them also extremely important, among others, for solar irradiance reconstructions and studies of the solar influence on Earth’s climate. However, these data also suffer from numerous issues, which for a long time have hampered their analysis. Without properly addressing these issues, Ca iiK data cannot be used to their full potential. Here, we first provide an overview of the currently known Ca iiK data archives and sources of the inhomogeneities in the data, before discussing existing processing techniques, followed by a recap of the main results derived with such data so far.

The Circular Polarization of the Mn 1 Resonance Lines around 280 nm for Exploring Chromospheric Magnetism

We study the circular polarization of the Mn i resonance lines at 279.56, 279.91, and 280.19 nm (hereafter, UV multiplet) by means of radiative transfer modeling. In 2019, the CLASP2 mission obtained unprecedented spectropolarimetric data in a region of the solar ultraviolet including the Mg ii h and k resonance lines and two lines of a subordinate triplet, as well as two Mn i resonance lines. The first analysis of such data, in particular those corresponding to a plage region, allowed the inference of the longitudinal magnetic field from the photosphere to the upper chromosphere just below the transition region. This was achieved by applying the weak field approximation to the circular polarization profiles of the Mg ii and Mn i lines. While the applicability of this approximation to the Mg ii lines was already demonstrated in previous works, this is not the case for the Mn i UV multiplet. These lines are observed as absorptions between the Mg ii h and k lines, a region whose intensity is shaped by their partial frequency redistribution effects. Moreover, the only Mn i stable isotope has nuclear spin I = 5/2 and thus hyperfine structure must be, a priori, taken into account. Here we study the generation and transfer of the intensity and circular polarization of the Mn i resonance lines accounting for these physical ingredients. We analyze their sensitivity to the magnetic field by means of their response function, and we demonstrate the applicability of the weak field approximation to determine the longitudinal component of the magnetic field.

Spatio-temporal analysis of chromospheric heating in a plage region

Context. Our knowledge of the heating mechanisms that are at work in the chromosphere of plage regions remains highly unconstrained from observational studies. While many heating candidates have been proposed in theoretical studies, the exact contribution from each of them is still unknown. The problem is rather difficult because there is no direct way of estimating the heating terms from chromospheric observations. Aims. The purpose of our study is to estimate the chromospheric heating terms from a multi-line high-spatial-resolution plage dataset, characterize their spatio-temporal distribution and set constraints on the heating processes that are at work in the chromosphere. Methods. We used nonlocal thermodynamical equilibrium inversions in order to infer a model of the photosphere and chromosphere of a plage dataset acquired with the Swedish 1-m Solar Telescope (SST). We used this model atmosphere to calculate the chromospheric radiative losses from the main chromospheric cooler from H I, Ca II, and Mg II atoms. In this study, we approximate the chromospheric heating terms by the net radiative losses predicted by the inverted model. In order to make the analysis of time-series over a large field of view computationally tractable, we made use of a neural network which is trained from the inverted models of two non-consecutive time-steps. We have divided the chromosphere in three regions (lower, middle, and upper) and analyzed how the distribution of the radiative losses is correlated with the physical parameters of the model. Results. In the lower chromosphere, the contribution from the Ca II lines is dominant and predominantly located in the surroundings of the photospheric footpoints. In the upper chromosphere, the H I contribution is dominant. Radiative losses in the upper chromosphere form a relatively homogeneous patch that covers the entire plage region. The Mg II also peaks in the upper chromosphere. Our time analysis shows that in all pixels, the net radiative losses can be split in a periodic component with an average amplitude of amp̅Q = 7.6 kW m−2 and a static (or very slowly evolving) component with a mean value of −26.1 kW m−2. The period of the modulation present in the net radiative losses matches that of the line-of-sight velocity of the model. Conclusions. Our interpretation is that in the lower chromosphere, the radiative losses are tracing the sharp lower edge of the hot magnetic canopy that is formed above the photosphere, where the electric current is expected to be large. Therefore, Ohmic current dissipation could explain the observed distribution. In the upper chromosphere, both the magnetic field and the distribution of net radiative losses are room-filling and relatively smooth, whereas the amplitude of the periodic component is largest. Our results suggest that acoustic wave heating may be responsible for one-third of the energy deposition in the upper chromosphere, whereas other heating mechanisms must be responsible for the rest: turbulent Alfvén wave dissipation or ambipolar diffusion could be among them. Given the smooth nature of the magnetic field in the upper chromosphere, we are inclined to rule out Ohmic dissipation of current sheets in the upper chromosphere.

An ALMA Observation of Time Variations in Chromospheric Temperature of a Solar Plage Region

Nanoflares and the shock formation of magnetohydrodynamic waves in the solar chromosphere have been considered as key physical mechanisms of the heating of the chromosphere and corona. To investigate candidates of their signature in the mm-wavelength, a tiny active region located on the solar disk was observed with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm, coordinated with observatories on orbit including Hinode SOT spectro-polarimeter in the Cycle 4 solar campaign (19 March 2017). ALMA’s spatial resolution was moderate, far from the best performance, but it provided stable conditions that are suitable to investigate temporal variations in the mm-wavelength. We determined that the noise level is less than 20 K (σ) over 1 hour in the 20-s cadence time series of synthesized ALMA images. The time variations with amplitudes above the noise level were observed throughout the field of view, but variations exceeding 200 K, corresponding to energy input to the chromosphere on the order of 1020-22 erg, were localized in two locations. One location was on the polarity inversion line, where tiny concentrated magnetic patches exist in weak field and a tiny magnetic flux may be emergent. The other location was at the outer edge of a bipolar magnetic region, which was under development with a successive series of magnetic flux emergence. This observation suggests that nanoflare-class energy inputs in the chromosphere can occur associated with emerging flux activities.

Solar Hαexcess during Solar Cycle 24 from full-disk filtergrams of the Chromospheric Telescope

Context.The chromospheric Hαspectral line is a strong line in the spectrum of the Sun and other stars. In the stellar regime, this spectral line is already used as a powerful tracer of stellar activity. For the Sun, other tracers, such as Ca IIK, are typically used to monitor solar activity. Nonetheless, the Sun is observed constantly in Hαwith globally distributed ground-based full-disk imagers.Aims.The aim of this study is to introduce the imaging Hαexcess and deficit as tracers of solar activity and compare them to other established indicators. Furthermore, we investigate whether the active region coverage fraction or the changing Hαexcess in the active regions dominates temporal variability in solar Hαobservations.Methods.We used observations of full-disk Hαfiltergrams of the Chromospheric Telescope and morphological image processing techniques to extract the imaging Hαexcess and deficit, which were derived from the intensities above or below 10% of the median intensity in the filtergrams, respectively. These thresholds allowed us to filter for bright features (plage regions) and dark absorption features (filaments and sunspots). In addition, the thresholds were used to calculate the mean intensityImeanE/Dfor Hαexcess and deficit regions. We describe the evolution of the Hαexcess and deficit during Solar Cycle 24 and compare it to the mean intensity and other well established tracers: the relative sunspot number, the F10.7 cm radio flux, and the Mg IIindex. In particular, we tried to determine how constant the Hαexcess and number density of Hαexcess regions are between solar maximum and minimum. The number of pixels above or below the intensity thresholds were used to calculate the area coverage fraction of Hαexcess and deficit regions on the Sun, which was compared to the imaging Hαexcess and deficit and the respective mean intensities averaged for the length of one Carrington rotation. In addition, we present the Hαexcess and mean intensity variation of selected active regions during their disk passage in comparison to the number of pixels of Hαexcess regions.Results.The Hαexcess and deficit follow the behavior of the solar activity over the course of the cycle. They both peak around solar maximum, whereby the peak of the Hαdeficit is shortly after the solar maximum. Nonetheless, the correlation of the monthly averages of the Hαexcess and deficit is high with a Spearman correlation ofρ = 0.91. The Hαexcess is closely correlated to the chromospheric Mg IIindex with a correlation of 0.95. The highest correlation of the Hαdeficit is found with the F10.7 cm radio flux, with a correlation of 0.89, due to their peaks after the solar activity maximum. Furthermore, the Hαdeficit reflects the cyclic behavior of polar crown filaments and their disappearance shortly before the solar maximum. We investigated the mean intensity distribution for Hαexcess regions for solar minimum and maximum. The shape of the distributions for solar minimum and maximum is very similar, but with different amplitudes. Furthermore, we found that the area coverage fraction of Hαexcess regions and the Hαexcess are strongly correlated with an overall Spearman correlation of 0.92. The correlation between the Hαexcess and the mean intensity of Hαexcess regions is 0.75. The correlation of the area coverage fraction and the mean intensity of Hαexcess regions is in general relatively low (ρ = 0.45) and only for few active regions is this correlation above 0.7. The weak correlation between the area coverage fraction and mean intensity leaves us pessimistic that the degeneracy between these two quantities can be broken for the modeling of unresolved stellar surfaces.

Power distribution of oscillations in the atmosphere of a plage region

Context. Joint observations of the Atacama Large Millimeter/Submillimeter Array (ALMA) with other solar observatories can provide a wealth of opportunities for understanding the coupling between different layers of the solar atmosphere. Aims. We present a statistical analysis of the power distribution of oscillations in a plage region in active region NOAA AR12651, which was observed jointly with ALMA, the Interface Region Imaging Spectrograph (IRIS), and the Solar Dynamics Observatory (SDO). Methods. We employ coordinated ALMA Band 6 (1.25 mm) brightness temperature maps, IRIS slit-jaw images in the 2796 Å passband, and observations in six passbands (1600 Å, 304 Å, 131 Å, 171 Å, 193 Å, and 211 Å) from the Atmospheric Imaging Assembly (AIA) on board SDO. We perform Lomb-Scargle transforms to study the distribution of oscillation power by means of dominant period maps and power maps. We study the spatial association of oscillations through the atmosphere, with a focus on the correlation of the power distribution of ALMA oscillations with others. Results. We do not observe any significant association of ALMA oscillations with IRIS and AIA oscillations. While the global behavior of the dominant ALMA oscillations shows a similarity with that of the transition region and coronal passbands of AIA, the ALMA dominant period maps and power maps do not show any correlation with those from the other passbands. The spatial distribution of dominant periods and power in different period intervals of ALMA oscillations is uncorrelated with those of any other passbands. Conclusions. We speculate that the non-association of ALMA oscillations with those of IRIS and AIA is due to significant variations in the height of formation of the millimeter continuum observed by ALMA. Additionally, the fact that ALMA directly maps the brightness temperature, in contrast to the intensity observations by IRIS and AIA, can result in the very different intrinsic nature of the ALMA oscillations compared to the IRIS and AIA oscillations.