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The ultraviolet emission from the solar transition region is dominated by dynamic, low-lying magnetic loops. The enhanced spatial and temporal resolution of the solar observation satellite Interface Region Imaging Spectrograph (IRIS) has made it possible to study these structures in fine detail. IRIS has observed ‘transient brightenings’ in these loops, associated with strong excess line broadenings1,2 providing important clues to the mechanisms that heat the solar atmosphere. However, the physical origin of the brightenings is debated. The line broadenings have been variously interpreted as signatures of nanoflares3, magneto-hydrodynamic turbulence4, plasmoid instabilities5 and magneto-acoustic shocks6. Here we use IRIS slit-jaw images and spectral data, and the Atmospheric Imaging Assembly of the Solar Dynamics Observatory spacecraft, to show that the brightenings are consistent with magnetic-reconnection-mediated impulsive heating at field-line braiding sites in multi-stranded transition-region loops. The spectroscopic observations present evidence for preferential heating of heavy ions from the transition region and we show that this is consistent with ion cyclotron turbulence caused by strong currents at the reconnection sites. Time-dependent differential emission measure distributions are used to determine the heating frequency7–9 and to identify pockets of faintly emitting ‘super-hot’ plasma. The observations we present and the techniques we demonstrate open up a new avenue of diagnostics for reconnection-mediated energy release in solar plasma. Solar imaging and spectral data indicate that impulsive heating through magnetic reconnection in transition region loops is responsible for observed transient brightenings, consistent with ion cyclotron turbulence due to strong currents at the reconnection sites.

The transition region in the solar atmosphere bridges the lower temperature, higher density chromosphere at the base with the tenuous, high temperature plasma in the corona. It undergoes a change in temperature from 25000K to about 600000K. Plasma conditions mean that fundamentally different models for line emission are required for the corona and chromosphere. The modelling framework of the corona is remarkably straightforward by comparison, and the tendency for modellers and observers is to use coronal modelling in the transition region as well. However, discrepancies with observations become apparent when using the modelling for lines emitted by low charge ions. To investigate more suitable modelling for ions which form in the transition region, the coronal approximation is extended by successively including atomic processes which become more important further down. Firstly, the effects of higher density are added to the modelling of carbon and oxygen, to account for its effects on electron impact ionisation and dielectronic recombination. New calculations are made for electron impact ionisation from long-lived metastable levels, which become populated at these densities. Other atomic processes not normally included in the coronal approximation are also added to the modelling of carbon and oxygen. These include processes induced by the radiation field, photo-ionisation and -excitation, and collisions involving neutral and ionised hydrogen and helium, principally charge transfer. The same models are built for other elements routinely observed in the atmosphere: these are nitrogen, neon, magnesium, silicon and sulphur. For these models an approximation is used for ionisation rates from metastable levels. All of the models are run in steady state equilibrium, to allow an assessment of how much atomic processes alter the modelling separately from dynamic events taking place in the plasma. To test how the new models compare with observations, the resulting ion balances from the models are used in conjunction with differential emission measure modelling to predict line intensities in the transition region. Observations of the quiet Sun show that the models resolve a number of the discrepancies found when using the coronal approximation for emission from low charge ions., Science and Technologies Facilities Council Doctoral Training Programme; University of Cambridge Isaac Newton Studentship; Cambridge Philosophical Society research studentship

For accurate electron density diagnostics in the solar transition region, the principle of electron density diagnostics are discussed by lines from Si VIII ion for kappa and Maxwellian distributions By observed line ratio of the Si VIII 1440.50 to 1445.75 A lines in quiet sun and active region, the electron density is discussed for any observed line ratio, and results are consistent with reported values in literature. The relationships between line ratio and electron density for the kappa and Maxwellian distributions are also discussed, in the case of lower and higher electron density limits, and results indicate that different distributions have no effect on relationships between the line ratio and the electron density at lower or higher electron density limit. This discussion is significant for accurate electron density diagnostics in the solar transition region, which will be important for study on coronal heating and acceleration of solar wind.

In this paper, we review observational aspects of three common small-scale energetic events in the solar transition region (TR), namely: TR explosive events, ultraviolet bursts and jets. These events are defined in either (both) spectral or (and) imaging data. The development of multiple instruments capable of observing the TR has allowed researchers to gain numerous insights into these phenomena in recent years. These events have provided a proxy to study how mass and energy are transported between the solar chromosphere and the corona. As the physical mechanisms responsible for these small-scale events might be similar to the mechanisms responsible for large-scale phenomena, such as flares and coronal mass ejections, analysis of these events could also help our understanding of the solar atmosphere from small to large scales. The observations of these small-scale energetic events demonstrate that the TR is extremely dynamic and is a crucial layer in the solar atmosphere between the chromosphere and the corona., review paper to be published in Journal Solar-Terrestrial Physics

The plasma contributing to emission from the Sun between the cool chromosphere ($\le 10^4$K) and hot corona ($\ge 10^6$K) has been subjected to many different interpretations. Here we look at the magnetic structure of this transition region (TR) plasma, based upon the implications of CLASP2 data of an active region recently published by Ishikawa et al., and earlier IRIS and SDO data of quiet regions. Ishikawa et al. found that large areas of sunspot plages are magnetically unipolar as measured in the cores of \ion{Mg}{2} resonance lines, formed in the lower transition region under low plasma-$\beta$ conditions. Here we show that IRIS images in the line cores have fibrils which well aligned with the overlying coronal loop segments seen in the 171 \AA{} channel of SDO. When the TR emission in active regions arise from plasma magnetically and thermally connected to the corona, then the line cores can provide the first credible magnetic boundary conditions for force-free calculations extended to the corona. We also re-examine IRIS images of dynamic TR cool loops previously reported as a major contributor to transition region emission from the quiet Sun. Dynamic cool loops contribute only a small fraction of the total TR emission from the quiet Sun., Comment: 10 Pages, 5 figures, 1 table

Spectroscopic observations at extreme- and far-ultraviolet wavelengths have revealed systematic upflows in the solar transition region and corona. These upflows are best seen in the network structures of the quiet Sun and coronal holes, boundaries of active regions, and dimming regions associated with coronal mass ejections. They have been intensively studied in the past two decades because they are likely to be closely related to the formation of the solar wind and heating of the upper solar atmosphere. We present an overview of the characteristics of these upflows, introduce their possible formation mechanisms, and discuss their potential roles in the mass and energy transport in the solar atmosphere. Although past investigations have greatly improved our understanding of these upflows, they have left us with several outstanding questions and unresolved issues that should be addressed in the future. New observations from the Solar Orbiter mission, the Daniel K. Inouye Solar Telescope, and the Parker Solar Probe will likely provide critical information to advance our understanding of the generation, propagation, and energization of these upflows.

On the basis of the two-layer model, we investigate the reflection of linear torsional Alfven waves propagating in a thin magnetic flux tube from the solar transition region. As distinguished from Hollweg (Astrophys. J. 277, 392, 1984), the density jump across the transition region modeled by a sharp boundary is taken into account. A new expression for the determination of the reflection coefficient is proposed. Weakly damping Alfven modes with periods from a few tens of seconds to a few minutes can quite effectively penetrate from the chromosphere to the corona and vice versa.

The solar photosphere, chromosphere and corona are known to rotate differentially as a function of latitude. To date, it is unclear if the solar transition region also rotates differentially. In this paper, we investigate differential rotational profile of solar transition region as a function of latitude, using solar full disk (SFD) images at 30.4 nm wavelength recorded by Extreme Ultraviolet Imager (EUVI) onboard Solar Terrestrial Relations Observatory (STEREO) space mission for the period from 2008 to 2018 (Solar Cycle 24). Our investigations show that solar transition region rotates differentially. The sidereal rotation rate obtained at +/- 5 degree equatorial band is quite high (~ 14.7 degree/day), which drops to ~ 13.6 degree/day towards both polar regions. We also obtain that the rotational differentiality is low during the period of high solar activity (rotation rate varies from 14.86 to 14.27 degree/day) while it increases during the ascending and the descending phases of the 24th solar cycle (rotation rate varies from 14.56 to 13.56 degree/day in 2008 and 14.6 to 13.1 degree/day in 2018). Average sidereal rotation rate (over SFD) follows the trend of solar activity (maximum ~ 14.97 degree/day during the peak phase of the solar activity, which slowly decreases to minimum ~ 13.9 degree/day during ascending and the descending phases of the 24th solar cycle). We also observe that solar transition region rotates less differentially than the corona., Comment: 8 pages, 6 figures, 2 tables, Accepted for publication in MNRAS

Small scale transients occur in the Solar corona at much higher frequencies than flares and play a significant role in coronal dynamics. Here we study three well-identified transients discovered by Hi-C and also detected by the EUV channels of Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO). We use 0-D enthalpy-based hydrodynamical simulations and produce synthetic light curves to compare with AIA observations. We have modeled these transients as loops of ~ 1.0~Mm length depositing energies ~ 10^23 ergs in ~ 50 seconds. The simulated synthetic light curves show reasonable agreement with the observed light curves. During the initial phase, conduction flux from the corona dominates over the radiation, like impulsive flaring events. Our results further show that the time-integrated net enthalpy flux is positive, hence into the corona. The fact that we can model the observed light curves of these transients reasonably well by using the same physics as those for nanoflares, microflares, and large flares, suggests that these transients may have a common origin., Comment: Published in ApJ, 16 pages, 13 figures, and 3 tables

The identifications of Fe VII emission lines in the wavelength range 193-197 {\AA} are discussed in the light of new measurements of laboratory spectra and atomic data calculations. This region is of importance to studies of solar spectra from the EUV Imaging Spectrometer (EIS) on board the Hinode spacecraft, which has its peak sensitivity at these wavelengths. Ten lines are measured, arising from seven fine structure levels in the $3p^53d^3$ configuration. Two lines have not previously been reported and lead to new experimental energies for the $(a^2D)^3F_{2,3}$ levels. Updated experimental energies are obtained for the remaining levels. The new atomic model is used to compute theoretical values for the two density diagnostic ratios $\lambda$196.21/$\lambda$195.39 and $\lambda$196.21/$\lambda$196.06, and densities are derived from EIS spectra of coronal loop footpoints., Comment: Accepted by ApJ; 10 pages; 4 figures

In this paper, we report the observed temporal correlation between extreme-violet (EUV) emission and magneto-acoustic oscillations in a EUV moss region, which is the footpoint region only connected by magnetic loops with million-degree plasma. The result is obtained from a detailed multi-wavelength data analysis to the region with the purpose of resolving fine-scale mass and energy flows that come from the photosphere, pass through the chromosphere and finally heat solar transition region or the corona. The data set covers three atmospheric levels on the Sun, consisting of high-resolution broad-band imaging at TiO 7057 \AA\ and the line of sight magnetograms for the photosphere, high-resolution narrow-band images at Helium \textsc{i} 10830 \AA\ for the chromosphere and EUV images at 171 \AA\ for the corona. We report following new phenomena: 1) Repeated injections of chromospheric material shown as 10830 \AA\ absorption are squirted out from inter-granular lanes with the period of $\sim$ 5 minutes. 2) EUV emissions are found to be periodically modulated with the similar periods of $\sim$ 5 minutes. 3) Around the injection area where 10830 \AA\ absorption is enhanced, both EUV emissions and the strength of magnetic field are remarkably stronger. 4) The peaks on the time profile of the EUV emissions are found to be in sync with oscillatory peaks of the stronger magnetic field in the region. These findings may give a series of strong evidences supporting the scenario that coronal heating is powered by magneto-acoustic waves., Comment: 8 pages, 4 figures

We analyze the evolution of shock waves in high-resolution 3D radiative MHD simulations of the quiet Sun and their synthetic emission characteristics. The simulations model the dynamics of a 12.8x12.8x15.2 Mm quiet-Sun region (including a 5.2 Mm layer of the upper convection zone and a 10 Mm atmosphere from the photosphere to corona) with an initially uniform vertical magnetic field of 10 G, naturally driven by convective flows. We synthesize the Mg II and C II spectral lines observed by the IRIS satellite and EUV emission observed by the SDO/AIA telescope. Synthetic observations are obtained using the RH1.5D radiative transfer code and temperature response functions at both the numerical and instrumental resolutions. We found that the Doppler velocity jumps of the C II 1334.5 A IRIS line and a relative enhancement of the emission in the 335 A SDO/AIA channel are the best proxies for the enthalpy deposited by shock waves into the corona (with Kendall's $��$ correlation coefficients of 0.59 and 0.38, respectively). The synthetic emission of the lines and extreme ultraviolet passbands are correlated with each other during the shock wave propagation. All studied shocks are mostly hydrodynamic (i.e., the magnetic energy carried by horizontal fields is $\leq{}$2.6% of the enthalpy for all events) and have Mach numbers > 1.0-1.2 in the low corona. The study reveals the possibility of diagnosing energy transport by shock waves into the solar corona, as well as their other properties, by using IRIS and SDO/AIA sensing observations., 31 pages, 8 figures, 2 tables

We investigate the relative contributions from the transition region and corona of coronal loops observed by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO). Using EBTEL (Enthalpy-Based Thermal Evolution of Loops) hydrodynamic simulations, we model loops with multiple lengths and energy fluxes heated randomly by events drawn from power-law distributions with different slopes and minimum delays between events to investigate how each of these parameters influences observable loop properties. We generate AIA intensities from the corona and transition region for each realization. The variations within and between models generated with these different parameters illustrate the sensitivity of narrowband imaging to the details of coronal heating. We then analyze the transition region and coronal emission from a number of observed active regions and find broad agreement with the trends in the models. In both models and observations, the transition region brightness is significant, often greater than the coronal brightness in all six "coronal" AIA channels. We also identify an inverse relationship, consistent with heating theories, between the slope of the differential emission measure (DEM) coolward of the peak temperature and the observed ratio of coronal to transition region intensity. These results highlight the use of narrowband observations and the importance of properly considering the transition region in investigations of coronal heating., Comment: Accepted for publication in ApJ. 16 pages, 10 figures, 4 tables

In the context of the solar atmosphere, we re-examine the role of of neutral and ionized species in dissipating the ordered energy of intermediate-mode MHD waves into heat. We solve conservation equations for the hydrodynamics and for hydrogen and helium ionization stages, along closed tubes of magnetic field. First, we examine the evolution of coronal plasma under conditions where coronal heating has abruptly ceased. We find that cool ($< 10^5$K) structures are formed lasting for several hours. MHD waves of modest amplitude can heat the plasma through ion-neutral collisions with sufficient energy rates to support the plasma against gravity. Then we examine a calculation starting from a cooler atmosphere. The calculation shows that warm ($> 10^4 $) K long ($>$ several Mm) tubes of plasma arise by the same mechanism. We speculate on the relevance of these solutions to observed properties of the Sun and similar stars whose atmospheres are permeated with emerging magnetic fields and stirred by convection. Perhaps this elementary process might help explain the presence of ``cool loops'' in the solar transition region and the production of broad components of transition region lines. The production of ionized hydrogen from such a simple and perhaps inevitable mechanism may be an important step towards finding the more complex mechanisms needed to generate coronae with temperatures in excess of 10$^6$K, independent of a star's metallicity., 13 pages, 9 figures

For accurate spectral diagnostics of the solar plasma in the transition region, the averaged collision strength of the κ distribution of electron energies is discussed for non-Maxwellian distribution, and the Maxwellian averaged collision strengths and the non-Maxwellian averaged collision strengths for κ = 2, 3 and 5 from 104.4 to 106.0 K are calculated for Carbon II doublet in the transition region. Results are in agreement with those from the CHIANTI database, and the main error comes from the approximation of the collision strength. This calculation will be significant for spectral diagnostics of the solar plasma for non-Maxwellian distribution of the electron energies.

Two competing fundamental hypotheses are usually postulated in the solar coronal heating problem: heating by nanoflares and heating by waves. In the latter it is assumed that acoustic and magnetohydrodynamic disturbances whose amplitude grows as they propagate in a medium with a decreasing density come from the convection zone. The shock waves forming in the process heat up the corona. In this paper we draw attention to yet another very efficient shock wave generation process that can be realized under certain conditions typical for quiet regions on the Sun. In the approximation of stationary dissipative hydrodynamics we show that a shock wave can be generated in the quiet solar chromosphere–corona transition region by the fall of plasma from the corona into the chromosphere. This shock wave is directed upward, and its dissipation in the corona returns part of the kinetic energy of the falling plasma to the thermal energy of the corona. We discuss the prospects for developing a quantitative nonstationary model of the phenomenon.

The mechanism of the solar corona heating still remains unexplained, almost 80 years after the discovery of the million degree hot solar corona. Observations show that the temperature increases more than one order of magnitude in the transition region (TR), the boundary between the solar chromosphere and the solar corona. We are giving a detailed magnetohydrodynamic (MHD) calculation of the height dependence of the temperature and solar wind velocity. The temperature and solar wind velocity profiles are calculated for static frequency dependent spectral density of incoming MHD waves, no time dependent computer simulations have been performed. In our calculation we take into account only Alfven wave (AW) polarization. The other modes (slow- and fast-magnetosonic waves) do not create cooling. A self-consistent calculation of MHD wave propagation through a static background of fully ionized hydrogen plasma in weak magnetic field is performed. Heated by the MHD waves, the background plasma temperature increases leading to strong plasma viscosity increase, which results in more efficient MHD wave absorption. Within this calculation, the width of the TR is also evaluated by maximal value of the logarithmic derivative of the temperature. Comparison of the calculated temperature profile with the available observational data show qualitative agreement and this gives the final answer to the problem of the solar corona heating. There are no alternative explanations of the narrow width of the TR. In such a way, after more than 70 years we have returned to the original Alfven idea [Alfven, H. 1947, MNRAS, 107, 211] that the solar corona is heated by AW.The mechanism of the solar corona heating still remains unexplained, almost 80 years after the discovery of the million degree hot solar corona. Observations show that the temperature increases more than one order of magnitude in the transition region (TR), the boundary between the solar chromosphere and the solar corona. We are giving a detailed magnetohydrodynamic (MHD) calculation of the height dependence of the temperature and solar wind velocity. The temperature and solar wind velocity profiles are calculated for static frequency dependent spectral density of incoming MHD waves, no time dependent computer simulations have been performed. In our calculation we take into account only Alfven wave (AW) polarization. The other modes (slow- and fast-magnetosonic waves) do not create cooling. A self-consistent calculation of MHD wave propagation through a static background of fully ionized hydrogen plasma in weak magnetic field is performed. Heated by the MHD waves, the background plasma temperature increas...

For accurate spectral diagnostics in the solar transition region, we discuss the electron energies for non-Maxwellian distributions both for and Druyvesteyn distributions. We analyze the difference between the κ and the Druyvesteyn distributions with the Maxwellian distribution and derive the expressions for the averaged collision strengths for the κ and the Druyvesteyn distributions. This discussion will be significant for spectral diagnostics of the electron density and temperature in the solar transition region.

Magnetic reconnection is a fundamental plasma process that plays a critical role not only in energy release in the solar atmosphere, but also in fusion, astrophysical, and other space plasma environments. One of the challenges in explaining solar observations in which reconnection is thought to play a critical role is to account for the transition of the dynamics from a slow quasi-continuous phase to a fast and impulsive energetic burst of much shorter duration. Despite the theoretical progress in identifying mechanisms that might lead to rapid onset, a lack of observations of this transition has left models poorly constrained. High-resolution spectroscopic observations from NASA's Interface Region Imaging Spectrograph (IRIS) now reveal tell-tale signatures of the abrupt transition of reconnection from a slow phase to a fast, impulsive phase during UV bursts or explosive events in the Sun's atmosphere. Our observations are consistent with numerical simulations of the plasmoid instability, and provide evidence for the onset of fast reconnection mediated by plasmoids and new opportunities for remote-sensing diagnostics of reconnection mechanisms on the Sun., 9 pages, 5 figures, accepted for publication in Astrophysical Journal

The conditions for accurate electron density diagnostics in the solar transition region are discussed, and result shows that lines from Si viii can provide an excellent tool for electron density diagnostics of the emitting plasma. For the Si viii 1440.50 A and 1445.75 A lines, the principle of the electron density diagnostics is discussed for any intensity ratio. By the observed intensity ratio, the diagnostic results of the electron density for the quiet sun and the active region are calculated, and results indicate that in the quiet sun, the averaged electron density is $\log (N_{e}) = 8.63$ ; while in the active region, the averaged density gets the maximum $\log (N_{e}) = 8.86$ in the active region (B), and gets the minimum $\log (N_{e}) = 8.38$ in the active region (E), where the electron density is in the unit of cm−3. Finally, the relationship of intensity ratio and electron density is discussed, in the case of lower and higher electron density limits. This discussion is significant in the electron density diagnostics, which will be important for study on coronal heating and acceleration of solar wind.

The sharp almost step like increase the temperature in the transition region (TR) between chromosphere and solar corona is well-known from decades; for first time we are giving a detailed magnetohydrodynamic (MHD) calculation of the height dependence of the temperature. The width of the transition region is evaluated by maximal value of the logarithmic derivative of the temperature. At fixed heating, only MHD can give such a narrow width and in such sense, even the qualitative agreement with the observational data, gives the final verdict what the heating mechanism of the solar corona is. Static profiles of the temperature and wind velocity are calculated for static frequency dependent spectral density of the incoming MHD waves; no time dependent computer simulations. At fixed spectral density of MHD waves, the MHD calculation predicts height dependence of the non-thermal broadening of spectral lines and its angular dependence. For illustration is used one dimensional approximation of completely ionized hydrogen plasma in weak magnetic field, but it is considered that the width of the TR is weakly dependent with respect of further elaboration. The analyzed MHD calculation is a numerical confirmation of the qualitative concept of self-induced opacity of the plasma with respect to MHD waves. The plasma viscosity strongly increases with the temperature. Heated by MHD waves, plasma increases the wave absorption and this positive feedback leads to further heating. The static temperature profile is a result of a self-consistent calculation of propagation of MHD wave through the static background of wind and temperature profile. The numerical method allows consideration of incoming MHD waves with an arbitrary spectral density. Further elaboration of the method are briefly discussed: influence of second viscosity in the chromospheric part of the TR, influence of the magnetic field on the coronal side of the TR and investigation of such type effects on the width of the TR.

Assuming the thin flux tube approximation, we introduce an analytical model that contemplates the presence of: a non-isothermal temperature; a varying magnetic field and a non-uniform stratified medium in hydrostatic equilibrium due to a constant gravity acceleration. This allows the study of slow magnetoacoustic cut-off periods across the solar transition region, from the base of the solar chromosphere to the lower corona. The used temperature profile approaches the VAC solar atmospheric model. The periods obtained are consistent with observations. Similar to the acoustic cut-off periods, the resulting magnetoacoustic gravity ones follow the sharp temperature profile, but shifted towards larger heights; in other words, at a given height the magnetoacoustic cut-off period is significantly lower than the corresponding acoustic one. Along a given longitude of an inclined thin magnetic tube, the greater its inclination the softener the temperature gradient it crosses. Changes in the magnetic field intensity do not significantly modify the periods at the coronal level but modulate the values below the transition region within periods between $\sim [2\,- 6]\,$min. Within the limitations of our model, we show that monochromatic oscillations of the solar atmosphere are the atmospheric response at its natural frequency to random or impulsive perturbations, and not a consequence of the forcing from the photosphere., Comment: 7 pages, 6 figures

The measured average velocities in solar and stellar spectral lines formed at transition region temperatures have been difficult to interpret. However, realistic three-dimensional radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere are able to reproduce the observed dominant line shifts and may thus hold the key to resolve these issues. Our new 3D rMHD simulations aim to shed light on how mass flows between the chromosphere and corona and on how the coronal mass is maintained. Passive tracer particles, so-called corks, allow the tracking of parcels of plasma over time and thus the study of changes in plasma temperature and velocity not only locally, but also in a co-moving frame. By following the trajectories of the corks, we can investigate mass and energy flows and understand the composition of the observed velocities. Our findings show that most of the transition region mass is cooling. The preponderance of transition region redshifts in the model can be explained by the higher percentage of downflowing mass in the lower and middle transition region. The average upflows in the upper transition region can be explained by a combination of both stronger upflows than downflows and a higher percentage of upflowing mass. The most common combination at lower and middle transition region temperatures are corks that are cooling and traveling downward. For these corks, a strong correlation between the pressure gradient along the magnetic field line and the velocity along the magnetic field line has been observed, indicating a formation mechanism that is related to downward propagating pressure disturbances. Corks at upper transition region temperatures are subject to a rather slow and highly variable but continuous heating process., Comment: 13 pages, 10 figures, online movie

Low-lying loops have been discovered at the solar limb in transition region temperatures by the Interface Region Imaging Spectrograph (IRIS). They do not appear to reach coronal temperatures, and it has been suggested that they are the long-predicted unresolved fine structures (UFS). These loops are dynamic and believed to be visible during both heating and cooling phases. Making use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope, we study how these loops impact the solar chromosphere. We show for the first time that there is indeed a chromospheric signal of these loops, seen mostly in the form of strong Doppler shifts and a conspicuous lack of chromospheric heating. In addition, we find that several instances have a inverse Y-shaped jet just above the loop, suggesting that magnetic reconnection is driving these events. Our observations add several puzzling details to the current knowledge of these newly discovered structures; this new information must be considered in theoretical models., Comment: 5 pages, 3 figures, 2 movies; accepted for publication in A&A Letters

Time-Distance ‘travel time’ perturbations (as inferred from wave phase) are calculated relative to the quiet-Sun as a function of wave orientation and field inclination in a uniform inclined magnetic field. Modelling indicates that the chromosphere-corona Transition Region (TR) profoundly alters travel times at inclinations from the vertical θ for which the ramp-reduced acoustic cutoff frequency ω c cosθ is similar to the wave frequency ω. At smaller inclinations phase shifts are much smaller as the waves are largely reflected before reaching the TR. At larger inclinations, the shifts resume their quiet-Sun values, although with some resonant oscillatory behaviour. Changing the height of the TR in the model atmosphere has some effect, but the thickness and temperature jump do not change the results substantially. There is a strong correspondence between travel-time shifts and the Alfven flux that emerges at the top of the modelled region as a result of fast/Alfven mode conversion. We confirm that the TR transmission coefficient for Alfven waves generated by mode conversion in the chromosphere is far larger (typically 30 % or more) than for Alfven waves injected from the photosphere.

Context. Suprathermal tails are a common feature of solar wind electron velocity distributions, and are expected in the solar corona. From the corona, suprathermal electrons can propagate through the steep temperature gradient of the transition region towards the chromosphere, and lead to non-Maxwellian electron velocity distribution functions (VDFs) with pronounced suprathermal tails. Aims. We calculate the evolution of a coronal electron distribution through the transition region in order to quantify the suprathermal electron population there. Methods. A kinetic model for electrons is used, which is based on solving the Boltzmann-Vlasov equation for electrons including Coulomb collisions with both ions and electrons. Initial and chromospheric boundary conditions are Maxwellian VDFs with densities and temperatures based on a background fluid model. The coronal boundary condition has been adopted from earlier studies of suprathermal electron formation in coronal loops. Results. The model results show the presence of strong suprathermal tails in transition region electron VDFs, starting at energies of a few 10 eV. Above electron energies of 600 eV, electrons can traverse the transition region essentially collision-free. Conclusions. The presence of strong suprathermal tails in transition region electron VDFs shows that the assumption of local thermodynamic equilibrium is not justified there. This has a significant impact on ionization dynamics, as is shown in a companion paper.

Recent IRIS observations have revealed a prevalence of intermittent small-scale jets with apparent speeds of 80 - 250 km s$^{-1}$, emanating from small-scale bright regions inside network boundaries of coronal holes. We find that these network jets appear not only in coronal holes but also in quiet-sun regions. Using IRIS 1330A (C II) slit-jaw images, we extract several parameters of these network jets, e.g. apparent speed, length, lifetime and increase in foot-point brightness. Using several observations, we find that some properties of the jets are very similar but others are obviously different between the quiet sun and coronal holes. For example, our study shows that the coronal-hole jets appear to be faster and longer than those in the quiet sun. This can be directly attributed to a difference in the magnetic configuration of the two regions with open magnetic field lines rooted in coronal holes and magnetic loops often present in quiet sun. We have also detected compact bright loops, likely transition region loops, mostly in quiet sun. These small loop-like regions are generally devoid of network jets. In spite of different magnetic structures in the coronal hole and quiet sun in the transition region, there appears to be no substantial difference for the increase in foot-point brightness of the jets, which suggests that the generation mechanism of these network jets is likely the same in both regions., 16 pages, 6 Figures, Solar Physics (in press)

Solar radio emission provides valuable information on the structure and dynamics of the solar atmosphere above the temperature minimum. We review the background and most recent observational and theoretical results on the quiet Sun and active region studies, covering the entire radio range from millimeter to decameter wavelengths. We examine small- and large-scale structures, at short and long time scales, as well as synoptic aspects. Open questions and challenges for the future are also identified. Solar Physics

The origin of the solar wind is one of the most important unresolved prob- lems in space and solar physics. We report here the first spectroscopic signatures of the nascent fast solar wind on the basis of observations made by the EUV Imaging Spectrometer (EIS) on Hinode in a polar coronal hole, in which patches of blueshift are clearly present on dopplergrams of coronal emission lines with a formation temperature of lg(T/K)=6.0. The corresponding upflow is associated with open field lines in the coronal hole, and seems to start in the solar transition region and becomes more prominent with increasing temperature. This temperature-dependent plasma outflow is interpreted as evidence of the nascent fast solar wind in the polar coronal hole. The patches with significant upflows are still isolated in the upper transition region but merge in the corona, in agreement with the scenario of solar wind outflow being guided by expanding magnetic funnels.

We use UV and extreme-UV emission lines observed in quiet regions on the solar disk with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument and the Extreme Ultraviolet Imaging Spectrometer (EIS) to determine the electron temperature in solar transition region plasmas. Prominent emission lines of O IV and O VI are present in the solar spectrum, and the measured intensity line ratios provide electron temperatures in the range of log T = 5.6-6.1. We find that the theoretical O IV and O VI ion formation temperatures are considerably lower than our derived temperatures. The line ratios expected from a plasma in ionization equilibrium are larger by a factor of about 2-5 than the measured line ratios. A careful cross-calibration of SUMER and EIS has been carried out, which excludes errors in the relative calibration of the two instruments. We checked for other instrumental and observational effects, as well as line blending, and can exclude them as a possible source of the discrepancy between theoretical and observed line ratios. Using a multi-thermal quiet-Sun differential emission measure changes the theoretical line ratio by up to 28% which is not sufficient as an explanation. We also explored additional excitation mechanisms. Photoexcitation from photospheric blackbody radiation, self-absorption, and recombination into excited levels cannot be a possible solution. Adding a second Maxwellian to simulate the presence of non-thermal, high-energy electrons in the plasma distribution of velocities also did not solve the discrepancy.

Using the correlation between the radiance or Doppler velocity and the extrapolated magnetic field, we determined the emission heights of a set of solar transition region lines in an equatorial coronal hole and in the surrounding quiet Sun region. We found that for all of the six lower-transition-region lines, the emission height is about 4–5 Mm in the equatorial coronal hole, and around 2 Mm in the quiet Sun region. This result confirms the previous findings that plasma with different temperature can coexist at the same layer of transition region. In the quiet Sun region, the emission height of the upper-transition-region line Ne VIII is almost the same that of the lower-transition-region line, but in the coronal hole, it is twice as high. This difference reveals that the outflow of Ne VIII is a signature of solar wind in the coronal hole and is just a mass supply to the large loops in the quiet Sun.

Nonresonant wave-particle interactions are studied within the framework of quasi-linear theory for the reduced velocity distribution functions of coronal ions. Our parametric study shows that in collisionless low-β plasma, Alfven waves at low-frequencies (with ω Ωi, where Ωi is ion gyrofrequency) can heat ions perpendicularly to the direction of the mean magnetic field. Consequently, a temperature anisotropy can be achieved by the ions, whereby the heavy species are heated more strongly, by a factor of the mass ratio, than the protons. Yet in the lower corona, such wave-induced features will be destroyed by collisions that are still strong there. Although the coronal plasma β is small, Alfven waves may efficiently heat the nonresonant ions only if their energy is relatively large. The heuristic values of wave energy needed for strong heating are, however, much larger than the ones assumed in previous models and obtained from recent observations. Nevertheless, heating of the ions by low-frequency Alfven waves can contribute to raising the temperature of the lower solar transition region.

Electron beams accelerated during solar flares carry electric currents which should be neutralized by so-called return currents. Both the electron beam and return current modify the electron distribution function in the solar transition region and low corona. Thus, they influence the intensities of the spectral lines formed in these layers. Synthetic spectra for the solar flare atmosphere are computed from model conditions and the possibilities of diagnostics of the return current from the EUV and X-ray line spectra are discussed.

Doppler-shift properties of the solar transition region (TR) and low corona are investigated in relation to the underlying chromospheric supergranular network, with particular regard to the role of the magnetic field. EUV line properties were obtained from a large raster scan of the solar chromosphere, transition region, and corona acquired by the SUMER spectrometer on board SOHO. The observed region includes an equatorial coronal hole, as well as surrounding quiet-Sun areas. The chromospheric supergranular network pattern is enhanced using a filter defined previously by Aiouaz and coworkers applied to a Lyman continuum image. The filtered continuum image and Dopplergrams are used to produce dispersion plots. We find correlations between the chromospheric network, and the N IV (765.15 Å), O IV (790.19 Å), S V (786.50 Å), O V (760.45 Å) Doppler shifts in quiet Sun and coronal hole. It is established that the maximum inflow (redshift) at transition region temperatures appears statistically toward the center of the network lanes in the quiet-Sun areas and toward the boundary of the network lanes in the coronal hole. Furthermore, while the strong redshifts in the TR lines complement spatially blueshifts from the low corona (Ne VIII 770.41 Å) in a puzzle-like pattern, bidirectional flows (i.e., cospatial strong red- and blueshifts in the TR/Corona) appear predominantly in the coronal hole. The bidirectional flows congregate almost systematically at boundaries of magnetic field concentrations seen in SOHO MDI magnetograms. These results support a coherent interpretation of almost 30 years of unexplained statistical Doppler-shift variations in the transition region and corona for quiet Sun and coronal hole. The proposed scenario involves reconnections between the strong network magnetic field and continuously advected weak field from the supergranular cell interior.

The non-Maxwellian $\kappa$-distributions have been detected in the solar transition region and flares. These distributions are characterized by a high-energy tail and a near-Maxwellian core and are known to have significant impact on the resulting optically thin spectra arising from collisionally dominated astrophysical plasmas. We developed the KAPPA package (http://kappa.asu.cas.cz) for synthesis of such line and continuum spectra. The package is based on the freely available CHIANTI database and software, and can be used in a similar manner. Ionization and recombination rates together with the ionization equilibria are provided for a range of $\kappa$ values. Distribution-averaged collision strengths for excitation are obtained by an approximate method for all transitions in all ions available within CHIANTI. The validity of this approximate method is tested by comparison with direct calculations. Typical precisions of better than 5% are found, with all cases being within 10%. Tools for calculation of synthetic line and continuum intensities are provided and described. Examples of the synthetic spectra and SDO/AIA responses to emission for the $\kappa$-distributions are given., Comment: The Astrophysical Journal Supplement Series, accepted

I investigate dynamical correlations between the lower transition region and the upper transition region using spectra from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The measured quantity is the Doppler shift (mostly solar radial) along the line of sight. The purpose is to shed light on the fraction of the lower transition region that resides in cool structures not physically associated with higher temperature regions and the fraction that can be described by a classical transition region with a physical connection from the chromosphere into the corona. "Transition region" in this context means the transition region sufficiently bright in ultraviolet spectral lines to be observed. I find strong Doppler shift correlations between two different lower region lines from C IV and S V, but much weaker correlations between these lines and a line of Ne VIII formed well into the upper transition region. I conclude that most of the lower transition region that is observable because of its brightness arises in cool loop structures. The results also favor a scenario in which the lower transition region is heated and cooled transiently on timescales less than or perhaps comparable to the SUMER exposure times of 21 s for the data analyzed in this work.

This is the first in a series of papers on simulation results of plasma dynamics with protons and neutral hydrogen, taking into account the transport processes of proton-neutral hydrogen collision, heat conduction, recombination, and ionization that are important in the solar transition region as well as in the chromosphere. We investigate the collision process of two current loops with counterhelicity in the upper chromosphere where the density of protons and neutral hydrogen are comparable. We found that the two-fluid dynamics of protons and neutral hydrogen is different from that of a single-fluid MHD approximation. We found that about 79% of the magnetic field energy can dissipate to heat protons, and then bidirectional jets occur of protons coupled well with hydrogen. The obtained results may be applicable to the footpoint heating of coronal loops and the explosive events that are strongly localized high-velocity jets in the transition region.

Using time series data of spectral lines originating from a wide range of temperatures in the solar transition region, above a polar coronal hole, from SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on SoHO (Solar and Heliospheric Observatory), we report on the detection of very long (≈170 min) periodic intensity fluctuations, above the limb. Our data also reveal long periodicities (10–90 min), previously observed with other SoHO instruments. With the acoustic cut-off frequency implying a maximum allowable period of ≈90 min, it is unclear whether these intensity fluctuations are due to waves or are the result of a recurrent magnetic reconnection process.

The radiance and Doppler-velocity maps of the emission lines of Si II, C IV, and Ne VIII obtained in a quiet region of the Sun by SUMER (Solar Ultraviolet Measurements of Emitted Radiation) are correlated with the vertical component, Bz, of the magnetic field vector as extrapolated, by means of a force-free field model, from the photospheric magnetic field measured by MDI (Michelson Doppler Imager). It is found that, with increasing vertical height, each of the correlation coefficients initially increases to a maximum value before it decreases again. The height corresponding to this maximum is called the correlation height. For the data sets selected from a quiet-Sun region, the correlation heights of Si II and C IV are near 2 Mm, and for Ne VIII near 4 Mm. At their correlation heights, the averaged square root of the radiance of the emission lines of Si II and C IV, considered as a proxy of the plasma density, has a linear relationship with . This result supports the empirical concept that the solar transition region is very thin and still affected by frozen-in convection. A way for improvement of such studies is also outlined.

We explore the formation of the He. 10 830 Angstrom line in a flaring atmosphere, with special attention to the nonthermal effects of an electron beam. Using non-LTE calculations we obtain the line profiles from different model atmospheres. Without the nonthermal effects, the line changes from weak absorption in a cool atmosphere to emission in a hot and condensed atmosphere, as expected. However, the presence of an electron beam can significantly change the line strength, producing much stronger absorption and emission in these two cases. We find that in the nonthermal case, the collisional ionization of He. followed by recombinations becomes an important process in populating the triplet levels corresponding to the He I 10 830 Angstrom line. These results suggest that the He I 10 830 Angstrom line is also a potential diagnostic tool for nonthermal effects in solar or stellar flares.

The sun is covered by a network of supergranular cells. The convective motion of these cells leads to the formation of strong magnetic fields at the cell boundaries. At larger heights in the solar transition region and low corona, this magnetic field geometry expands rapidly within a short distance, and forms the magnetic structure of "coronal funnels". This field line geometry represents a magnetic mirror, and since the plasma density strongly increases with depth in the transition region, the electron velocity distribution function (VDF) can develop a loss cone. Within such a coronal funnel, the plasma frequency can have smaller values than the electron cyclotron frequency, ωp < Ωe. These are the necessary conditions for the generation of X-mode waves through the electron cyclotron maser mechanism. Since there is some observational evidence for radio emission from the supergranular network, it is of interest to investigate the possibility of this plasma wave generation in a quiet stellar atmosphere in detail. In this paper, a kinetic model is used to calculate the electron VDF in a coronal funnel. A method is derived to determine wave growth rates from the electron VDF. Its application on the coronal funnel VDF indeed results in X-mode wave growth. However, it is also found that wave absorption by higher-order resonances at larger heights in the atmosphere plays an important role.

We present, for the first time, blinker phenomena being associated with brightenings in pre-existing coronal loops registered by the Extreme-ultraviolet Imaging Telescope (EIT) in Fe XII 195 A. The brightenings occur during the emergence of new magnetic flux as registered by the Big Bear Solar Observatory (BBSO) magnetograph. The blinkers were identified using simultaneous observations obtained with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrngraph. In light of the new observational results, we present one possible theoretical interpretation of the blinker phenomenon. We suggest that the blinker activity we observe is triggered by interchange reconnection, serving to provide topological connectivity between newly emerging flux and pre-existing flux. The EIT images show the existence of loop structures prior to the onset of the blinker activity. Based on the available spatial resolution the blinker occurs within, or nearby, an existing coronal loop. The temperature interfaces created in the reconnection process between the cool plasma of the newly emerging loop and the hot plasma of the existing loop are what we suggest to causes the observed activity seen in both the SUMER and CDS data. As the temperature interfaces propagate with the characteristic speed of a conduction front, they heat up the cool chromospheric plasma to coronal temperatures, an increasing volume of which brightens at transition region temperatures. We believe this new interpretation gives further qualitative understanding about the evolution of newly emerging flux on the Sun. This also provides new insight into the dynamic nature of the solar transition region.

The widths of spectral lines in the ultraviolet (UV) and extreme ultraviolet (EUV) spectral regions that are formed in the solar transition region and corona are usually greater than the optically thin widths due to thermal Doppler broadening calculated under the assumption of ionization equilibrium. Although opacity can explain the widths of some lines, there are a host of optically thin lines for which the excess widths are attributed to nonthermal motions. Interest in these motions for coronal heating theories has led to the measurement and comparison of spectral line profiles/widths throughout the solar UV and EUV spectrum. We find that for the quiet Sun the widths of some optically allowed lower transition region lines, deduced from spectra obtained by the Solar Ultraviolet Measurements of Ultraviolet Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft, are considerably larger than predicted from simply scaling previously measured wavelengths of other lines from the same ion. For example, the O III lines of the multiplet near 834 A are considerably wider than predicted from the previously measured (from Skylab) width of the optically thin O III 1666.15 A intersystem line. The excess widths are not due to nonthermal motions, as these are already included in the width of the 1666.15 A line. In this paper, we analyze the widths of some prominent optically allowed lines and discuss possible causes for discrepancies with previous measurements of intersystem lines.

Robert Wilson had broad scientific interests that ranged from laboratory and solar plasmas to quasars. In his early career at Harwell he made fundamental studies of the plasma device Zeta, and discovered a group of extreme ultraviolet emission lines that were also present but unidentified in the solar spectrum. Subsequent work at Culham showed that these were due to highly ionized iron. He initiated a programme of observations using stabilized Skylark rockets that led to the discovery of many important emission lines in the solar ultraviolet spectrum and gave new information on the structure of the solar transition region. He was the founder of ultraviolet astronomy in the UK and led the team that provided the S2½68 instrument on the TD–1 satellite. Ultraviolet astronomy became his main interest after his move to University College London. He was the intellectual driving force that eventually led to the outstandingly successful International Ultraviolet Explorer (IUE), which was the first satellite to be easily accessible to the astronomical community. His interests in astronomy were diverse and, using results from both TD–1 and the IUE, he made significant contributions through his studies of the interstellar medium, hot stars, binary X–ray sources and quasars.

The oblique interaction of the solar chromospheric rotational or Alfven discontinuity with the transition region, described as a contact discontinuity, is studied. The appearance of the refracted magnetohydrodynamic shock wave going through the solar corona is shown. This wave makes the dissipation of the magnetic field energy inside the coronal plasma possible and may cause explosive events.

The aim of the present paper is to explore the role of anomalous resistivity on the dynamics of magnetic reconnection in a 2D environment of relevance to the solar transition region. We adopt an ad hoc but explicit form of the anomalous resistivity, motivated by a streaming instability, in which the resistivity jumps suddenly as the electron drift velocity exceeds some fraction of the mean electron thermal speed. Experiments have been conducted to investigate the impact of various critical speeds and arbitrary scaling constants of the resistivity level on the time-dependent evolution of the magnetic reconnection process. The specific threshold value is found to influence the dynamics of the reconnection, with higher values providing a localised on-off effect of patchy diffusion. For a given normalised value of the anomalous resistivity, the amount of Joule heating released scales inversely with the threshold value. The total energy release is found to be above the lower limit of "quiet" Sun nano-flares required to maintain a hot corona. The reconnection events discussed here may be important to the energy balance of the solar transition region and overlying corona, as already suggested in earlier work based on SUMER observations.

For temperature diagnostics of plasma, using the silicon spectral lines emitted from the solar transition region, under the optically-thin conditions, we discuss temperature diagnostics of the quiet sun in some typical features. For the silicon IV 112.8325 nm and 140.2770 nm spectral lines, using the observed intensity ratio, we calculate the temperature of faint cell center, average cell center, average quiet sun, average network, bright network and very bright network of the quiet sun, and results are in good agreement with those predicted at the [Formula: see text] ionization equilibrium temperature of formation of the silicon IV, and we discuss the temperature when the observed intensity varies from 0.05 to 0.2. This investigation will be significant for temperature diagnostics of plasma under the optically-thin conditions.

Solar plasma that exists at around 105 K, which has traditionally been referred to as the solar transition region, is probably in a dynamic and fibril state with a small filling factor. Its origin is as yet unknown, but we suggest that it may be produced primarily by one of five different physical mechanisms, namely: the heating of cool spicular material; the containment of plasma in low-lying loops in the network; the thermal linking of cool and hot plasma at the feet of coronal loops; the heating and evaporating of chromospheric plasma in response to a coronal heating event; and the cooling and draining of hot coronal plasma when coronal heating is switched off. We suggest that, in each case, a blinker could be produced by the granular compression of a network junction, causing subtelescopic fibril flux tubes to spend more of their time at transition-region temperatures and so to increase the filling factor temporarily.

Two decades ago it was discovered that emission lines from the solar transition region from the chromosphere to the corona are composed by two Gaussian components: a narrow core and a broad second component contributing up to 25% to the total intensity. New observations with SUMER/SOHO allow for the first time statistically significant results on the widths and Doppler shifts of the broad second components. Evidence will be presented that the two components of the line profile correspond to two radically different physical regimes. Of course, the results for the dominant line core fit into the well known properties of transition region lines, e.g. the redshifts. In contrast, the second broad components, which occur only in the bright network, show different properties. Most striking is the fact that large parts of the network show blueshifts in the second components, while the core is redshifted at the same location. The results suggest that the second component is formed in Gabriel-type coronal funnels and that these are heated in the same manner as open field regions, i.e. the solar wind. However, the analysis of the Doppler shifts shows that these funnels should not be magnetically open, but rather the footpoint regions of large coronal loops spanning across several network elements.

We present an analysis of observations and theory of selected transition-region phenomena, concentrating on small scale jet-like structures known as spicules and macrospicules. We examine a number of mechanisms that may be responsible for their formation and conclude that Alfven waves could provide the necessary acceleration through the ponderomotive force and dissipation for heating forming a beam or jet like structure. In applying the Alfven wave model we make no fundamental distinction between spicules and macrospicules. In this respect we consider them to be manifestations of the same phenomenon on different scales. We predict that the most effective Alfven waves have frequencies around 1 Hz and amplitudes of 1 V m−1. The resulting plasma jet sets up plasma conditions suitable for creating rotating structures which are also observed.