Solar Transition Research Articles

Investigating the Transition Region Explosive Events and Their Relationship to Network Jets

Abstract Recent imaging observations with the Interface Region Imaging Spectrograph (IRIS) have revealed prevalent intermittent jets with apparent speeds of 80–250 km s−1 from the network lanes in the solar transition region (TR). Additionally, spectroscopic observations of the TR lines have revealed the frequent presence of highly non-Gaussian line profiles with enhanced emission at the line wings, often referred to as explosive events (EEs). Using simultaneous imaging and spectroscopic observations from IRIS, we investigate the relationship between EEs and network jets. We first identify EEs from the Si iv 1393.755 Å line profiles in our observations, then examine related features in the 1330 Å slit-jaw images. Our analysis suggests that EEs with double peaks or enhancements in both wings appear to be located at either the footpoints of network jets or transient compact brightenings. These EEs are most likely produced by magnetic reconnection. We also find that EEs with enhancements only at the blue wing are mainly located on network jets, away from the footpoints. These EEs clearly result from the superposition of the high-speed network jets on the TR background. In addition, EEs showing enhancement only at the red wing of the line are often located around the jet footpoints, which is possibly caused by the superposition of reconnection downflows on the background emission. Moreover, we find some network jets that are not associated with any detectable EEs. Our analysis suggests that some EEs are related to the birth or propagation of network jets, and that others are not connected to network jets.

Ireland’s Transition towards a Low Carbon Society: The Leadership Role of Higher Education Institutions in Solar Photovoltaic Niche Development

Ireland is currently considered a laggard within Europe in relation to decarbonisation of its society, with future projections estimating increases in greenhouse gas emissions up to and beyond 2030. To accelerate Ireland’s transition towards a low-carbon society, there is a need for leadership in deployment and experimentation of low carbon technologies. As Higher Education Institutions (HEI) currently play a major role in generation of human capital and the associated impact on societal development, HEIs are ideal locations to focus resources in terms of deployment and experimentation of decarbonisation technologies to demonstrate best practice for further replication within wider society. To guide Irish HEIs in this regard, a novel integrated approach titled ‘Higher Education Accelerating Development for Sustainability’ (HEADS) has been developed and applied to the sector. The HEADS approach utilises the perspectives of quantitative systems analysis, sociotechnical analysis, and living lab learning to inform HEIs of their potential roles within national sustainability transitions. Applied to solar photovoltaic transitions in Ireland, the HEADS approach has identified HEIs as vital locations to deploy low-carbon technologies due to their amplification effect in signalling to wider society the attractiveness of these technologies.

Space Weather Events, Hurricanes, and Earthquakes in Mexico in September 2017

AbstractIn the interval of 4–10 September 2017, the Sun presented multiple solar flares from active region AR 2673. There were also coronal mass ejections that interacted with the Earth's magnetosphere. This solar activity produced several space weather events. These events were observed with ground‐based instruments of the Mexican Space Weather Service. The Mexican Array RadioTelescope detected highly perturbed solar transits associated with Type I radio emissions from active regions. The Compact Astronomical Low‐frequency, Low‐cost Instrument for Spectroscopy in Transportable Observatories‐Mexican Array RadioTelescope station detected several radio bursts including a Type III associated with the X8.2 flare on 10 September. The magnetometer detected variations reaching a regional K index of 8.3 during the geomagnetic storm. The ionosphere over Mexico was disturbed by different space weather phenomena with the dominant effects of the geomagnetic storm. We used total electron content data to study latitudinal and longitudinal ionospheric effects in this interval. The cosmic rays monitor detected a Forbush decrease associated also with the geomagnetic storm. This low‐latitude instrumental network in Mexico allowed estimating the regional response to space weather events. Coincidentally with the space weather events referred above, there were also two other types of natural hazards affecting the country at that moment, the hurricane Katia category 2 in the Gulf of Mexico, and two major earthquakes (7 and 19 September 2018). The conjunction of these natural phenomena were close to creating a worst‐case scenario in terms of civil protection reaction.

Emission of solar chromospheric and transition region features related to the underlying magnetic field

Context. The emission of the upper atmosphere of the Sun is closely related to magnetic field concentrations at the solar surface. Aims. It is well established that this relation between chromospheric emission and magnetic field is nonlinear. Here we investigate systematically how this relation, characterised by the exponent of a power-law fit, changes through the atmosphere, from the upper photosphere through the temperature minimum region and chromosphere to the transition region. Methods. We used spectral maps from the Interface Region Imaging Spectrograph (IRIS) covering Mg II and its wings, C II, and Si IV together with magnetograms and UV continuum images from the Solar Dynamics Observatory. After a careful alignment of the data we performed a power-law fit for the relation between each pair of observables and determine the power-law index (or exponent) for these. This was done for different spatial resolutions and different features on the Sun. Results. While the correlation between emission and magnetic field drops monotonically with temperature, the power-law index shows a hockey-stick-type variation: from the upper photosphere to the temperature-minimum it drops sharply and then increases through the chromosphere into the transition region. This is even seen through the features of the Mg II line, this is, from k1 to k2 and k3. It is irrespective of spatial resolution or whether we investigate active regions, plage areas, quiet Sun, or coronal holes. Conclusions. In accordance with the general picture of flux–flux relations from the chromosphere to the corona, above the temperature minimum the sensitivity of the emission to the plasma heating increases with temperature. Below the temperature minimum a different mechanism has to govern the opposite trend of the power-law index with temperature. We suggest four possibilities, in other words, a geometric effect of expanding flux tubes filling the available chromospheric volume, the height of formation of the emitted radiation, the dependence on wavelength of the intensity-temperature relationship, and the dependence of the heating of flux tubes on the magnetic flux density.

On the electron density diagnostics of silicon viii ion in solar transition region

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.

Tracing the evolution of radiation-MHD simulations of solar and stellar atmospheres in the Lagrangian frame

Context. Radiation magnetohydrodynamics (radiation-MHD) simulations have become a standard tool for investigating the physics of solar and stellar atmospheres. Aims. The aim of this paper is to present a method that allows the efficient and accurate analysis of flows in such simulations in the Lagrangian frame. Methods. This paper presents a method that allows the construction of pathlines given a seed point that can be chosen freely at any location and at any time during the simulation where the simulation state is stored. The method is based on passive tracer particles. Through injection of particles in expanding regions the occurrence of particle-free volumes is avoided, even in the case of strongly compressive flows. Results. The method was implemented in the solar and stellar atmosphere simulation code Bifrost. It is efficient and accurate. As examples I present an analysis of a gas parcel in the convection zone and a particle in the solar transition region.

Slow magnetoacoustic gravity waves in an equilibrium stratified solar atmosphere: cut-off periods through the transition region

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 solar atmospheric model of G18. 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 softer 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.

Disentangling flows in the solar transition region

Context. The measured average velocities in solar and stellar spectral lines formed at transition region temperatures have been difficult to interpret. The dominant redshifts observed in the lower transition region naturally leads to the question of how the upper layers of the solar (and stellar) atmosphere can be maintained. Likewise, no ready explanation has been made for the average blueshifts often found in upper transition region lines. 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. Aims. These 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. These simulations give new insights into the coupling of various atmospheric layers and the origin of Doppler shifts in the solar transition region and corona. Methods. The 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. Results. 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. Conclusions. Corks are shown to be an essential tool in 3D rMHD models in order to study mass and energy flows. We have shown that most transition region plasma is cooling after having been heated slowly to upper transition region temperatures several minutes before. Downward propagating pressure disturbances are identified as one of the main mechanisms responsible for the observed redshifts at transition region temperatures.

CTR Wilson Meeting 2016

CTR Wilson Meeting 2016

Solar Energy Potential Assessment on Rooftops and Facades in Large Built Environments Based on LiDAR Data, Image Processing, and Cloud Computing. Methodological Background, Application, and Validation in Geneva (Solar Cadaster)

The paper presents the core methodology for assessing solar radiation and energy production on building rooftops and vertical facades (still rarely considered) of the inner-city. This integrated tool is based on the use of LiDAR, 2D and 3D cadastral data. Together with solar radiation and astronomical models it calculates the global irradiance for a set of points located on roofs, ground and facades. Although the tool takes simultaneously roofs, ground and facades, different methods of shadow casting are applied. Shadow casting on rooftops is based on image processing techniques. On the other hand, the assessment on facade involves first to create and interpolate points along the facades and then to implement a point-by-point shadow casting routine. The paper is structured in five parts: (i) state of the art on the use of 3D GIS and automated processes in assessing solar radiation in the built environment, (ii) overview on the methodological framework used in the paper, (iii) detailed presentation of the method proposed for solar modelling and shadow casting, in particular by introducing an innovative approach for modelling the Sky View Factor (SVF), (iv) demonstration of the solar model introduced in this paper through applications in Geneva’s building roofs (solar cadaster) and facades, (v) validation of the solar model in some Geneva’s spots, focusing especially on two distinct comparisons: solar model versus fisheye catchments on partially inclined surfaces (roof component); solar model versus photovoltaic simulation tool PVSyst on vertical surfaces (facades). Concerning the roof component, validation results emphasize global sensitivity related to the density of light sources on the sky vault to model the SVF. The low dense sky model with 145 light sources gives satisfying results, especially when processing solar cadasters in large urban areas, thus allowing to save computation time. In the case of building facades, introducing weighting factor in SVF calculation leads to outputs close to those obtained by PVSyst. Such good validation results make the proposed model a reliable tool to: (i) automatically process solar cadaster on building rooftops and facades at large urban scales, (ii) support solar energy planning and energy transition policies.

Chromospheric counterparts of solar transition region unresolved fine structure loops

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.

Flare Prediction Using Photospheric and Coronal Image Data

The precise physical process that triggers solar flares is not currently understood. Here we attempt to capture the signature of this mechanism in solar image data of various wavelengths and use these signatures to predict flaring activity. We do this by developing an algorithm that [1] automatically generates features in 5.5 TB of image data taken by the Solar Dynamics Observatory of the solar photosphere, chromosphere, transition region, and corona during the time period between May 2010 and May 2014, [2] combines these features with other features based on flaring history and a physical understanding of putative flaring processes, and [3] classifies these features to predict whether a solar active region will flare within a time period of $T$ hours, where $T$ = 2 and 24. We find that when optimizing for the True Skill Score (TSS), photospheric vector magnetic field data combined with flaring history yields the best performance, and when optimizing for the area under the precision-recall curve, all the data are helpful. Our model performance yields a TSS of $0.84 \pm 0.03$ and $0.81 \pm 0.03$ in the $T$ = 2 and 24 hour cases, respectively, and a value of $0.13 \pm 0.07$ and $0.43 \pm 0.08$ for the area under the precision-recall curve in the $T$ = 2 and 24 hour cases, respectively. These relatively high scores are similar to, but not greater than, other attempts to predict solar flares. Given the similar values of algorithm performance across various types of models reported in the literature, we conclude that we can expect a certain baseline predictive capacity using these data. This is the first attempt to predict solar flares using photospheric vector magnetic field data as well as multiple wavelengths of image data from the chromosphere, transition region, and corona.

Magnetohydrodynamic calculation of the temperature and wind velocity profile of the solar transition region. Preliminary results.

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.

Formation of Cool and Warm Jets by Magnetic Flux Emerging from the Solar Chromosphere to Transition Region

Abstract In the solar atmosphere, jets are ubiquitous at various spatial-temporal scales. They are important for understanding the energy and mass transports in the solar atmosphere. According to recent observational studies, the high-speed network jets are likely to be intermittent but continual sources of mass and energy for the solar wind. Here, we conduct a 2D magnetohydrodynamics simulation to investigate the mechanism of these network jets. A combination of magnetic flux emergence and horizontal advection is used to drive the magnetic reconnection in the transition region between a strong magnetic loop and a background open flux. The simulation results show that not only a fast warm jet, much similar to the network jets, is found, but also an adjacent slow cool jet, mostly like classical spicules, is launched. Differing from the fast warm jet driven by magnetic reconnection, the slow cool jet is mainly accelerated by gradients of both thermal pressure and magnetic pressure near the outer border of the mass-concentrated region compressed by the emerging loop. These results provide a different perspective on our understanding of the formation of both the slow cool jets from the solar chromosphere and the fast warm jets from the solar transition region.

Temperature diagnostics of plasma under optically-thin conditions

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.

Probing the solar transition region: current status and future perspectives

The solar transition region (TR) is the temperature regime from roughly 0.02 MK to 0.8 MK in the solar atmosphere. It is the transition layer from the collisional and partially ionized chromosphere to the collisionless and fully ionized corona. The TR plays an important role in the mass and energy transport in both the quiet solar atmosphere and solar eruptions. Most of the TR emission lines fall into the spectral range of far ultraviolet and extreme ultraviolet ( 400 Å–1600 Å). Imaging and spectroscopic observations in this spectral range are the most important ways to obtain information about the physics of the TR. Static solar atmosphere models predict a very thin TR. However, recent high-resolution observations indicate that the TR is highly dynamic and inhomogeneous. I will summarize some major findings about the TR made through imaging and spectroscopic observations in the past 20 years. These existing observations have demonstrated that the TR may be the key to understanding coronal heating and origin of the solar wind. Future exploration of the solar TR may need to focus on the upper TR, since the plasma in this temperature regime (0.1 MK–0.8 MK) has not been routinely imaged before. High-resolution imaging and spectroscopic observations of the upper TR will not only allow us to track the mass and energy from the lower atmosphere to the corona, but also help us to understand the initiation and heating mechanisms of coronal mass ejections and solar flares.

κ distribution of carbon II doublet in the solar transition region

For accurate spectral diagnostics of the solar plasma in the transition region, the averaged collision strength of the κdistribution 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.

A new approach for modelling chromospheric evaporation in response to enhanced coronal heating

We proposed that the use of an approximate "jump condition" at the solar transition region permits fast and accurate numerical solutions of the one dimensional hydrodynamic equations when the corona undergoes impulsive heating. In particular, it eliminates the need for the very short timesteps imposed by a highly resolved numerical grid. This paper presents further examples of the applicability of the method for cases of non-uniform heating, in particular, nanoflare trains (uniform in space but non-uniform in time) and spatially localised impulsive heating, including at the loop apex and base of the transition region. In all cases the overall behaviour of the coronal density and temperature shows good agreement with a fully resolved one dimensional model and is significantly better than the equivalent results from a 1D code run without using the jump condition but with the same coarse grid. A detailed assessment of the errors introduced by the jump condition is presented showing that the causes of discrepancy with the fully resolved code are (i) the neglect of the terms corresponding to the rate of change of total energy in the unresolved atmsophere, (ii) mass motions at the base of the transition region and (iii) for some cases with footpoint heating, an over-estimation of the radiative losses in the transition region.

Typical solar radiation year construction using k-means clustering and discrete-time Markov chain

Daily solar radiation (DSR) fluctuation and transition rules affect the design of the energy storage system and online control strategy of solar energy utilisation systems. However, the current synthesis methods for the typical meteorological year do not emphasise such features of DSR. To overcome this shortcoming, this study presents an innovative synthesis method for a Typical Solar Radiation Year (TSRY) based on k-means clustering and discrete-time Markov chain (DTMC). The historical distributions of clear-sky ratio (CSR) in four representative regions were analysed, and a six-dimensional feature vector that represents the DSR fluctuations based on CSR was defined. Then, based on the feature vector, k-means clustering was used to cluster the historical DSR into four types. Subsequently, a DTMC-based model was built for transition rule estimation among the four types of solar radiation. Finally, the TSRY was established based on the clustering categories and transition rules among them. The innovative synthesis method was also verified in this study. Results for the four regions showed that the average error of the synthesised TSRY has maximum and minimum values of 10% and 6% in all seasons, respectively, compared with historical data. The proposed method could represent DSR fluctuation and transition characteristics of certain regions and could also be extended to other regions.

VLF modal interference distance and nighttime D region VLF reflection height for west‐east and east‐west propagation paths to Fiji

AbstractVery low frequency (VLF) signals from navigational transmitters propagate through the Earth‐ionosphere waveguide formed by the Earth and the lower conducting ionosphere and show the pronounced minima during solar terminator transition between transmitter and receiver. Pronounced amplitude minima observed on 19.8 kHz (NWC transmitter) and 24.8 kHz (NLK transmitter) signals recorded at Suva (18.149°S, 178.446°E), Fiji, during 2013–2014, have been used to estimate the VLF modal interference distance (DMS) and nighttime D region VLF reflection height (hN). The NWC transmitter signal propagates mostly in west‐east direction, and the NLK transmitter follows a transequatorial path propagating significantly in the east‐west direction. The values of DMS calculated using midpath terminator speed are 2103 ± 172 km and 2507 ± 373 km for these paths having west‐east and east‐west components of VLF subionospheric propagation, respectively, which agree with previously published results and within 10% with theoretical values. We have also compared the DMS estimated using a terminator time method with that calculated using terminator speed for a particular day and found both the values to be consistent. The hN values were found to be maximum during winter of Southern Hemisphere for NWC signal and winter of Northern Hemisphere for NLK signal VLF propagation paths to Suva. The hN also shows significant day‐to‐day and seasonal variabilities with a maximum of about 10 km and 23 km for NWC and NLK signal propagation paths, respectively, which could be due to the atmospheric gravity waves associated with solar terminator transition, as well as meteorological factors such as strong lightnings.