Transition Region Temperatures Research Articles

On the Response of the Transition Region and the Corona to Rapid Excursions in the Chromosphere

Spicules are the thin hair/grass-like structures that are prominently observed at the chromospheric solar limb. It is believed that fibrils and rapid blueshifted and redshifted excursions (RBEs and RREs; collectively referred to as REs) correspond to on-disk counterparts of type I spicules and type II spicules, respectively. Our investigation focuses on observing the response of these REs alongside similar spectral features in the chromosphere, transition region (TR), and corona, utilizing space–time plots derived from coordinated observations from the Swedish 1 m Solar Telescope/Hα, Interface Region Imaging Spectrograph (IRIS), and Solar Dynamics Observatory. Our analysis reveals upflowing REs, promptly reaching temperatures characteristic of the TR and corona, indicating a multithermal nature. Similarly, downflowing features exhibiting similar spectral signatures over the disk display plasma motion from the corona to chromospheric temperatures, demonstrating a multithermal nature. In addition to distinct upflows and downflows, we observe sequential upflow and downflow along the same path, depicting a distinctive parabolic trajectory in space–time plots of observations sampling TR and various coronal passbands. Similar to isolated upflows and downflows, these REs also exhibit a multithermal nature throughout their trajectory. Furthermore, our results reveal a more intricate motion of the REs in which both upflow and downflow coexist at the same spatial location. On a different note, our analysis, utilizing coordinated IRIS spectral observations, shows spatiotemporal redshifts/downflows in both the TR and chromosphere, suggesting that at least subsets of the strong redshifts/downflows observed in TR temperature spectra result from the return from the upper atmosphere flow of plasma in the form of bundles of spicules or features exhibiting similar spectra.

Is It Possible to Detect Coronal Mass Ejections on Solar-type Stars through Extreme-ultraviolet Spectral Observations?

Stellar coronal mass ejections (CMEs) from host stars are an important factor that affects the habitability of exoplanets. Although their solar counterparts have been well observed for decades, it is still very difficult to find solid evidence of stellar CMEs. Using the spectral line profile asymmetry caused by the Doppler shift of erupting plasma, several stellar CME candidates have been identified from spectral lines formed at chromospheric or transition region temperatures of the stars. However, a successful detection of stellar CME signals based on the profile asymmetries of coronal lines is still lacking. It is unclear whether we can detect such signals. Here we construct an analytical model for CMEs on solar-type stars and derive an expression of stellar extreme-ultraviolet (EUV) line profiles during CME eruptions. For different instrumental parameters, exposure times, CME propagation directions, and stellar activity levels, we synthesized the corresponding line profiles of Fe ix λ171.07 and Fe xv λ284.16. Further investigations provide constraints on the instrumental requirements for successful detection and characterization of stellar CMEs. Our results show that it is possible to detect stellar CME signals and infer their velocities based on spectral profile asymmetries using an EUV spectrometer with a moderate spectral resolution and signal-to-noise ratio. Our work provides important references for the design of future EUV spectrometers for stellar CME detection and the development of observation strategies.

Nonthermal Motions and Atmospheric Heating of Cool Stars

The magnetic processes associated with the nonthermal broadening of optically thin emission lines appear to carry enough energy to heat the corona and accelerate the solar wind. We investigate whether nonthermal motions in cool stars exhibit the same behavior as on the Sun by analyzing archival stellar spectra taken by the Hubble Space Telescope, and full-disk Solar spectra taken by the Interface Region Imaging Spectrograph. We determined the nonthermal velocities by measuring the excess broadening in optically thin emission lines formed in the stellar atmosphere; the chromosphere, the transition region, and the corona. Assuming the nonthermal broadening is caused by the presence of Alfvén waves, we also determined the associated wave energy densities. Our results show that with a nonthermal velocity of ∼23 km s−1 the Sun-as-a-star results are in very good agreement with values obtained from spatially resolved solar observations. The nonthermal broadening in our sample shows a correlation to stellar rotation, with the strength of the nonthermal velocity decreasing with decreasing rotation rate. Finally, the nonthermal velocity in cool Sun-like stars varies with atmospheric height or temperature of the emission lines, and peaks at transition region temperatures. This points toward a solar-like Alfvén wave-driven heating in stellar atmospheres. However, the peak is at a lower temperature in some cool stars suggesting that other magnetic processes such as flaring events could also dominate.

Spatial and Temporal Analysis of Quiescent Coronal Rain over an Active Region

The solar corona produces coronal rain, hundreds of times colder and denser material than the surroundings. Coronal rain is known to be deeply linked to coronal heating, but its origin, dynamics, and morphology are still not well understood. The leading theory for its origin is thermal instability (TI) occurring in coronal loops in a state of thermal nonequilibrium (TNE), the TNE-TI scenario. Under steady heating conditions, TNE-TI repeats in cycles, leading to long-period EUV intensity pulsations and periodic coronal rain. In this study, we investigate coronal rain on the large spatial scales of an active region (AR) and over the long temporal scales of EUV intensity pulsations to elucidate its distribution at such scales. We conduct a statistical study of coronal rain observed over an AR off limb with Interface Region Imaging Spectrograph and Solar Dynamics Observatory imaging data, spanning chromospheric to transition region (TR) temperatures. The rain is widespread across the AR, irrespective of the loop inclination, and with minimal variation over the 5.45 hr duration of the observation. Most rain has a downward (87.5%) trajectory; however, upward motions (12.5%) are also ubiquitous. The rain dynamics are similar over the observed temperature range, suggesting that the TR emission and chromospheric emission are colocated on average. The average clump widths and lengths are similar in the SJI channels and wider in the AIA 304 Å channel. We find ubiquitous long-period EUV intensity pulsations in the AR. Short-term periodicity is found (16 minutes) linked to the rain appearance, which constitutes a challenge to explain under the TNE-TI scenario.

Multifluid Simulations of Upper-chromospheric Magnetic Reconnection with Helium–Hydrogen Mixture

Our understanding of magnetic reconnection (MR) under chromospheric conditions remains limited. Recent observations have demonstrated the important role of ion–neutral interactions in the dynamics of the chromosphere. Furthermore, the comparison between the spectral profiles and synthetic observations of reconnection events suggests that current MHD approaches appear to be inconsistent with observations. First, collisions and multithermal aspects of the plasma play a role in these regions. Second, hydrogen and helium ionization effects are relevant to the energy balance of the chromosphere. This work investigates the multifluid multispecies (MFMS) effects on MR in conditions representative of the upper chromosphere using the multifluid Ebysus code. We compare an MFMS approach based on a helium–hydrogen mixture with a two-fluid MHD model based on hydrogen only. The simulations of MR are performed in a Lundquist number regime high enough to develop plasmoids and instabilities. We study the evolution of the MR and compare the two approaches including the structure of the current sheet and plasmoids, the decoupling of the particles, the evolution of the heating mechanisms, and the composition. The presence of helium species leads to more efficient heating mechanisms than the two-fluid case. This scenario, which is out of reach of the two-fluid or single-fluid models, can reach transition region temperatures starting from upper-chromospheric thermodynamic conditions, representative of a quiet Sun scenario. The different dynamics between helium and hydrogen species could lead to chemical fractionation and, under certain conditions, enrichment of helium in the strongest outflows. This could be of significance for recent observations of helium enrichment in the solar wind in switchbacks and coronal mass ejections.

Small-scale loops heated to transition region temperatures and their chromospheric signatures in the simulated solar atmosphere

Context. Recent observations have revealed loop-like structures at very small scales visible in observables that sample the transition region (TR) and even coronal temperatures. These structures are referred to as either ‘unresolved fine structures’, ‘dynamic cool loops’, ‘miniature hot loops’ or ‘campfires’ depending on the observables in which they are detected. Their formation remains unclear. Aims. Realistic magnetohydrodynamic simulations and forward synthesis of spectral lines are used to investigate how these features occur. Methods. Computations were carried out using the MURaM code to generate model atmospheres. The synthetic Hα and Si IV spectra are calculated at two angles (μ = 1, μ = 0.66) using the Multi3D code. We traced magnetic field lines in the model and examined the evolution of the underlying field topology. Results. The synthetic Hα Dopplergrams reveal loops that evolve dramatically within a few minutes. The synthetic Hα line profiles show observed asymmetries and Doppler shifts in the line core. However, they also show strong emission peaks in the line wings, even at the slanted view. The synthetic Si IV emission features partly coincide with structures visible in Hα Dopplergrams and partly follow separate magnetic field threads. Some are even visible in the emission measure maps for the lg(T/K) = [5.8, 6.2] temperature interval. The emission areas trace out the magnetic field lines rooted in opposite polarities in a bipolar region. Conclusions. The model shows that a loop-like structure in a bipolar system with footpoints undergoing rapid movement and shuffling can produce many small-scale recurrent events heated to high temperatures. It demonstrates that heating to different temperatures occurs and can be confined to a small part of the loop, at the location where resistive and viscous heating increases. The model largely reproduces the observed features in terms of size, lifetime and morphology in chromospheric, TR and coronal observables. The morphology and evolution of the resulting observable features can vary depending on the viewing angle.

Large Ion-neutral Drift Velocities and Plasma Heating in Partially Ionized Coronal Rain Blobs

In this paper we present a numerical study of the dynamics of partially ionized coronal rain blobs. We use a two-fluid model to perform a high-resolution 2D simulation that takes into account the collisional interaction between the charged and neutral particles contained in the plasma. We follow the evolution of a cold plasma condensation as it falls through an isothermal vertically stratified atmosphere that represents the much hotter and lighter solar corona. We study the consequences of the different degrees of collisional coupling that are present in the system. On the one hand, we find that at the dense core of the blob there is a very strong coupling and the charged and neutral components of the plasma behave as a single fluid, with negligible drift velocities (of a few cm s−1). On the other hand, at the edges of the blob the coupling is much weaker and larger drift velocities (of the order of 1 km s−1) appear. In addition, frictional heating causes large increases of temperature at the transition layers between the blob and the corona. For the first time we show that such large drift velocities and temperature enhancements can develop as a consequence of ion-neutral decoupling associated to coronal rain dynamics. This can lead to enhanced emission coming from the plasma at the coronal rain-corona boundary, which possesses transition region temperature.

First Flight of the EUV Snapshot Imaging Spectrograph (ESIS)

The Extreme-ultraviolet Snapshot Imaging Spectrograph (ESIS) launched on a sounding rocket from White Sands Missile Range on 2019 September 30. ESIS is a computed tomography imaging spectrograph (CTIS) designed to map emission line profiles across a wide field of view, revealing the structure and dynamics of small-scale transient events that are prevalent at transition region temperatures. In this paper, we review the ESIS instrument, mission, and data captured. We demonstrate how this unique data set can be interpreted qualitatively and further present some initial quantitative inversions of the data. Using a multiplicative algebraic reconstruction technique, we combine information from all four ESIS channels into a single spatial–spectral cube at every exposure. We analyze two small explosive events in the O v 629.7 Å spectral line with jets near ±100 km s−1 that evolve on 10 s timescales and vary significantly over small spatial scales. Intriguingly, each of these events turns out to be a bimodal (red+blue) jet with outflows that are asymmetric and unsynchronized. We also present a qualitative analysis of a small jetlike eruption captured by ESIS and draw comparisons to previously observed mini-filament eruptions. In 5 minutes of observing time, ESIS captured the spatial and temporal evolution of tens of these small events across the ∼11.′5 field of view, as well as several larger extended eruptions, demonstrating the advantage of CTIS instruments over traditional slit spectrographs in capturing the spatial and spectral information of dynamic solar features across large fields of view.

A 2D Model for Coronal Bright Points: Association with Spicules, UV Bursts, Surges, and EUV Coronal Jets

Coronal bright points (CBPs) are ubiquitous structures in the solar atmosphere composed of hot small-scale loops observed in extreme-ultraviolet (EUV) or X-rays in the quiet Sun and coronal holes. They are key elements to understanding the heating of the corona; nonetheless, basic questions regarding their heating mechanisms, the chromosphere underneath, or the effects of flux emergence in these structures remain open. We have used the Bifrost code to carry out a 2D experiment in which a coronal-hole magnetic null-point configuration evolves perturbed by realistic granulation. To compare with observations, synthetic SDO/AIA, Solar Orbiter EUI-HRI, and IRIS images have been computed. The experiment shows the self-consistent creation of a CBP through the action of stochastic granular motions alone, mediated by magnetic reconnection in the corona. The reconnection is intermittent and oscillatory, and it leads to coronal and transition-region temperature loops that are identifiable in our EUV/UV observables. During the CBP lifetime, convergence and cancellation at the surface of its underlying opposite polarities takes place. The chromosphere below the CBP shows a number of peculiar features concerning its density and the spicules in it. The final stage of the CBP is eruptive: Magnetic flux emergence at the granular scale disrupts the CBP topology, leading to different ejections, such as UV bursts, surges, and EUV coronal jets. Apart from explaining observed CBP features, our results pave the way for further studies combining simulations and coordinated observations in different atmospheric layers.

Prevalence of Thermal Nonequilibrium over an Active Region

Recent observations have shown that besides the characteristic multimillion degree component, the corona also contains a large amount of cool material called coronal rain, whose clumps are 10–100 times cooler and denser than the surroundings and are often organized in larger events, termed showers. Thermal instability (TI) within a coronal loop in a state of thermal nonequilibrium (TNE) is the leading mechanism behind the formation of coronal rain but no investigation on showers exists to date. In this study, we conduct a morphological and thermodynamic multiwavelength study of coronal rain showers observed in an active region (AR) off-limb with IRIS and the Solar Dynamics Observatory, spanning chromospheric to transition region and coronal temperatures. Rain showers were found to be widespread across the AR over the 5.45 hr observing time, with an average length, width, and duration of 27.37 ± 11.95 Mm, 2.14 ± 0.74 Mm, and 35.22 ± 20.35 minutes, respectively. We find a good correspondence between showers and the cooling coronal structures consistent with the TNE–TI scenario, thereby properly identifying coronal loops in the “coronal veil”, including the strong expansion at low heights and an almost zero expansion in the corona. This agrees with previous work suggesting that the observed zero expansion in the EUV is due to specific cross-field temperature distribution. We estimate the total number of showers to be 155 ± 40, leading to a TNE volume of 4.56 ± [3.71] × 1028 cm3, i.e., on the same order of the AR volume. This suggests a prevalence of TNE over the AR indicating strongly stratified and high-frequency heating on average.

Coronal condensation as the source of transition-region supersonic downflows above a sunspot

Context. Plasma loops or plumes rooted in sunspot umbrae often harbor downflows with speeds of 100 km s−1. These downflows are supersonic at transition region temperatures of ∼0.1 MK. The source of these flows is not well understood. Aims. We aim to investigate the source of sunspot supersonic downflows (SSDs) in active region 12740 using simultaneous spectroscopic and imaging observations. Methods. We identified SSD events from multiple raster scans of a sunspot by the Interface Region Imaging Spectrograph, and we calculated the electron densities, mass fluxes, and velocities of these SSDs. The extreme-ultraviolet (EUV) images provided by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and the EUV Imager onboard the Solar Terrestrial Relations Observatory were employed to investigate the origin of these SSDs and their associated coronal rain. Results. Almost all the identified SSDs appear at the footpoints of sunspot plumes and are temporally associated with the appearance of chromospheric bright dots inside the sunspot umbra. Dual-perspective EUV imaging observations reveal a large-scale closed magnetic loop system spanning the sunspot region and a remote region. We observed that the SSDs are caused by repeated coronal rain that forms and flows along these closed magnetic loops toward the sunspot. One episode of coronal rain clearly indicates that reconnection near a coronal X-shaped structure first leads to the formation of a magnetic dip. Subsequently, hot coronal plasma catastrophically cools from ∼2 MK in the dip region via thermal instability. This results in the formation of a transient prominence in the dip, from which the cool gas mostly slides into the sunspot along inclined magnetic fields under the gravity. This drainage process manifests as a continuous rain flow, which lasts for ∼2 h and concurrently results in a nearly steady SSD event. The total mass of condensation (1.3 × 1014 g) and condensation rate (1.5 × 1010 g s−1) in the dip region were found to be sufficient to sustain this long-lived SSD event, which has a mass transport rate of 0.7 − 1.2 × 1010 g s−1. Conclusions. Our results demonstrate that coronal condensation in magnetic dips can result in the quasi-steady sunspot supersonic downflows.

Properties of the C ii 1334 Å Line in Coronal Hole and Quiet Sun as Observed by IRIS

Abstract Coronal holes (CHs) have subdued intensity and net blueshifts when compared to the quiet Sun (QS) at coronal temperatures. At transition region temperatures, such differences are obtained for regions with identical absolute photospheric magnetic flux density (∣B∣). In this work, we use spectroscopic measurements of the C ii 1334 Å line from the Interface Region Imaging Spectrograph, formed at chromospheric temperatures, to investigate the intensity, Doppler shift, line width, skew, and excess kurtosis variations with ∣B∣. We find the intensity, Doppler shift, and linewidths to increase with ∣B∣ for CHs and QS. The CHs show deficit in intensity and excess total widths over QS for regions with identical ∣B∣. For pixels with only upflows, CHs show excess upflows over QS, while for pixels with only downflows, CHs show excess downflows over QS that cease to exist at ∣B∣ ≤ 40. Finally, the spectral profiles are found to be more skewed and flatter than a Gaussian, with no difference between CHs and QS. These results are important in understanding the heating of the atmosphere in CH and QS, including solar wind formation, and provide further constraints on the modeling of the solar atmosphere.

Effects of erupting magnetic flux rope on running penumbral waves

Context. It is widely known that solar flares have a substantial impact on the low atmosphere but the matter of how they affect sunspot waves and oscillations is generally unknown. In addition, there are ongoing debates on whether the flare-induced photospheric changes are due to the momentum conservation with coronal mass ejections or to magnetic reconnection. Aims. To shed light on the so-called “back reaction” of solar eruptions, we investigated how running penumbral waves (RPWs) at one foot of an erupting magnetic flux rope (MFR) respond to the rope buildup and subsequent erosion. Methods. We used UV/EUV images from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO) to explore the changing behaviors of RPWs in response to the MFR evolution, as well as 135-s vector magnetograms from the SDO Helioseismic and Magnetic Imager to analyze the changes in photospheric magnetic field during the eruption. Results. During the rope buildup stage, the western foot of the rope, which is completely enclosed by a hooked ribbon, expands rapidly and consequently ends up overlapping a sunspot penumbra. This converts the original penumbral field into the rope field, which is associated with a transient increase in electric currents flowing through the ribbon-swept penumbral region. During the rope erosion stage, the rope foot shrinks as the eastern section of the hooked ribbon slowly sweeps the same penumbral region, where the rope field is converted into flare loops. This conversion induces mixed effects on the photospheric field inclination but heats up the low atmosphere at the footpoints of these flare loops to transition-region temperatures, therefore resulting in the post-eruption RPWs with an enhanced contrast in the 1600 Å passband and an extended bandwidth to low frequencies at 3–5 mHz, compared with the pre-eruption RPWs that peak at 6 mHz. Conclusions. This observation clearly demonstrates that it is the magnetic reconnection in the corona that impacts the low atmosphere and leads to the changing behaviors of RPWs, which, in turn, offer a new window onto diagnosing flare reconnections.

ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages

Abstract We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate a plage from its surrounding regions than other methods commonly used. We caution that isolating a plage from its immediate surroundings must be done with care in order to successfully mitigate statistical biases that, for instance, can impact quantitative comparisons between different chromospheric observables. Using this methodology, our analysis suggests that λ = 1.25 mm free–free emission in plage regions observed with the Atacama Large Millimeter/submillimeter Array (ALMA)/Band6 may not form in the low chromosphere as previously thought, but rather in the upper chromospheric parts of dynamic plage features (such as spicules and other bright structures), i.e., near geometric heights of transition-region temperatures. We investigate the high degree of similarity between chromospheric plage features observed in ALMA/Band6 (at 1.25 mm wavelengths) and the Interface Region Imaging Spectrograph (IRIS)/Si iv at 1393 Å. We also show that IRIS/Mg ii h and k are not as well correlated with ALMA/Band6 as was previously thought, and we discuss discrepancies with previous works. Lastly, we report indications of chromospheric heating due to propagating shocks supported by the ALMA/Band6 observations.

Relation of Coronal Rain Originating from Coronal Condensations to Interchange Magnetic Reconnection

Abstract Using extreme-ultraviolet images, we recently proposed a new and alternative formation mechanism for coronal rain along magnetically open field lines due to interchange magnetic reconnection. In this paper we report coronal rain at chromospheric and transition region temperatures originating from the coronal condensations facilitated by reconnection between open and closed coronal loops. For this, we employ the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory. Around 2013 October 19, a coronal rain along curved paths was recorded by IRIS over the southeastern solar limb. Related to this, we found reconnection between a system of higher-lying open features and lower-lying closed loops that occurs repeatedly in AIA images. In this process, the higher-lying features form magnetic dips. In response, two sets of newly reconnected loops appear and retract away from the reconnection region. In the dips, seven events of cooling and condensation of coronal plasma repeatedly occur due to thermal instability over several days, from October 18 to 20. The condensations flow downward to the surface as coronal rain, with a mean interval between condensations of ∼6.6 hr. In the cases where IRIS data were available we found the condensations to cool all the way down to chromospheric temperatures. Based on our observations we suggest that some of the coronal rain events observed at chromospheric temperatures could be explained by the new and alternative scenario for the formation of coronal rain, where the condensation is facilitated by interchange reconnection.

A multi-diagnostic spectral analysis of penumbral microjets

Context. Penumbral microjets (PMJs) are short-lived, jet-like objects found in the penumbra of sunspots. They were first discovered in chromospheric lines and have later also been shown to exhibit signals in transition region (TR) lines. Their origin and manner of evolution is not yet settled. Aims. We perform a comprehensive analysis of PMJs through the use of spectral diagnostics that span from photospheric to TR temperatures to constrain PMJ properties. Methods We employed high-spatial-resolution Swedish 1-m Solar Telescope observations in the Ca II 8542 Å and H α lines, IRIS slit-jaw images, and IRIS spectral observations in the Mg II h & k lines, the Mg II 2798.75 Å & 2798.82 Å triplet blend, the C II 1334 Å & 1335 Å lines, and the Si IV 1394 Å & 1403 Å lines. We derived a wide range of spectral diagnostics from these and investigated other secondary phenomena associated with PMJs. Results. We find that PMJs exhibit varying degrees of signal in all of our studied spectral lines. We find low or negligible Doppler velocities and velocity gradients throughout our diagnostics and all layers of the solar atmosphere associated with these. Dark features in the inner wings of H α and Ca II 8542 Å imply that PMJs form along pre-existing fibril structures. We find evidence for upper photospheric heating in a subset of PMJs through emission in the wings of the Mg II triplet lines. There is little evidence for ubiquitous twisting motion in PMJs. There is no marked difference in onset-times for PMJ brightenings in different spectral lines. Conclusions. PMJs most likely exhibit only very modest mass-motions, contrary to earlier suggestions. We posit that PMJs form at upper photospheric or chromospheric heights at pre-existing fibril structures.

The Multi-slit Approach to Coronal Spectroscopy with the Multi-slit Solar Explorer (MUSE)

Abstract The Multi-slit Solar Explorer (MUSE) is a proposed mission aimed at understanding the physical mechanisms driving the heating of the solar corona and the eruptions that are at the foundation of space weather. MUSE contains two instruments, a multi-slit extreme ultraviolet (EUV) spectrograph and a context imager. It will simultaneously obtain EUV spectra (along 37 slits) and context images with the highest resolution in space (0.″33–0.″4) and time (1–4 s) ever achieved for the transition region (TR) and corona. The MUSE science investigation will exploit major advances in numerical modeling, and observe at the spatial and temporal scales on which competing models make testable and distinguishable predictions, thereby leading to a breakthrough in our understanding of coronal heating and the drivers of space weather. By obtaining spectra in four bright EUV lines (Fe ix 171 Å, Fe xv 284 Å, Fe xix 108Å, Fe xxi 108 Å) covering a wide range of TR and coronal temperatures along 37 slits simultaneously, MUSE will be able to “freeze” the evolution of the dynamic coronal plasma. We describe MUSE’s multi-slit approach and show that the optimization of the design minimizes the impact of spectral lines from neighboring slits, generally allowing line parameters to be accurately determined. We also describe a Spectral Disambiguation Code to resolve multi-slit ambiguity in locations where secondary lines are bright. We use simulations of the corona and eruptions to perform validation tests and show that the multi-slit disambiguation approach allows accurate determination of MUSE observables in locations where significant multi-slit contamination occurs.

Thermal Bremsstrahlung of Local Sources Over Solar Spots Based on Microwave Observations

The paper presents the results of a study of microwave thermal bremsstrahlung of local sources above sunspots. Special attention is paid to the transition from thermal bremsstrahlung to the dominant gyroresonance radiation in the high-frequency part of the spectrum of 12–20 GHz (1.5–2.5 cm). At high frequencies, for which the magnitude of the magnetic field is not enough to generate gyroresonance radiation, thermal bremsstrahlung is the main factor. The presented calculations demonstrate an increase in brightness in an extraordinary mode and a decrease in its intensity in an ordinary mode, as well as corresponding examples of observations. The possibilities and advantages of determination of the field with one polarization corresponding to ordinary radiation are revealed. The studied spectral-polarization features of the radiation, estimates of the effective radiation height, and the characteristic features of the change in the sizes of sources with the wave frequency will make it possible to correctly interpret the observations and to improve the accuracy of magnetic field measurement and the diagnostics of the transition region temperature.

Plasma Flows in the Cool Loop Systems

Abstract We study the dynamics of low-lying cool loop systems for three data sets as observed by the Interface Region Imaging Spectrograph. Radiances, Doppler shifts, and line widths are investigated in and around observed cool loop systems using various spectral lines formed between the photosphere and transition region (TR). Footpoints of the loop threads are either dominated by blueshifts or redshifts. The cospatial variation of velocity above the blueshifted footpoints of various loop threads shows a transition from very small upflow velocities ranging from (−1 to +1) −1 in the Mg ii k line (2796.20 Å; formation temperature: ) to the high upflow velocities from (−10 to −20) −1 in Si iv. Thus, the transition of the plasma flows from redshift (downflows) to blueshift (upflows) is observed above the footpoints of these loop systems in the spectral line C ii (1334.53 Å; log(T/ ) = 4.3) lying between Mg ii k and Si iv (1402.77 Å; ). This flow inversion is consistently observed in all three sets of the observational data. The other footpoint of the loop system always remains redshifted, indicating downflowing plasma. The multispectral line analysis in the present paper provides a detailed scenario of the plasma flow’s inversions in cool loop systems leading to the mass transport and their formation. The impulsive energy release due to small-scale reconnection above the loop footpoint seems to be the most likely cause for sudden initiation of the plasma flows evident at TR temperatures.

Quiet-Sun and Coronal Hole in Mg II k Line as Observed by IRIS

Abstract Coronal hole (CH) regions are dark in comparison to the quiet Sun (QS) at coronal temperatures. However, at chromospheric and transition region temperatures, the QS and CHs are hardly distinguishable. In this study, we have used the Mg ii 2796.35 Å spectral line recorded by the Interface Region Imaging Spectrometer (IRIS) to understand the similarities and differences in the QS and CH at chromospheric levels. Our analysis reveals that the emission from Mg ii k3 and k2v that originates in the chromosphere is significantly lower in CH than in QS for the regions with similar magnetic field strength. The wing emissions of Mg ii k that originates from the photospheric layer, however, do not show any difference between QS and CH. The difference in Mg ii k3 intensities between QS and CH increases with increasing magnetic field strength. We further studied the effects of spectral resolution on these differences and found that the difference in the intensities decreases with decreasing spectral resolution. For a resolution of 11 Å, the difference completely disappears. These findings are not only important for mass and energy supply from the chromosphere to the corona but also provides essential ingredients for the modeling of the solar spectral irradiance for the understanding of the Sun–climate relationships.