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The term “ultraviolet (UV) burst” is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016–2017.

The long-term study of the temporal variation of the rotation period of the solar photosphere, chromosphere, and corona has been widely undertaken. To date it is unclear whether the temporal variation of the rotation period of the solar transition region has a systematic periodicity. In this article we perform a study on the temporal variation of the rotation period of the solar transition region. For this purpose, we use the Lyman α line emission at a wavelength of 121.56 nm corresponding to the solar transition region from the year 1965 to 2019, covering four complete solar cycles (i.e., Cycles 21, 22, 23, 24) as well as descending and ascending phases of Cycles 20 and 25, respectively. An autocorrelation analysis depicts that the average sidereal rotation period of the transition region (from 1965 to 2019) is 24.8 days. Furthermore, we find that a significant periodicity of about 12 years exists in the temporal variation of the sidereal rotation period of the solar transition region. The results indicate that this periodicity is closely linked to the 11-year Schwabe cycle. A cross-correlation analysis between the time series of the sidereal rotation period and sunspot numbers (as a function of lag in years) exhibits a positive correlation between these aforementioned parameters. From this result, we can state that the sidereal rotation period of the solar transition region leads the solar activity by about six months. This correlation again proves the periodicity of about 11 years in the rotation period of the transition region which is closely linked to the 11-year Schwabe cycle. Furthermore, long-term variation of rotation periods also demonstrates a decreasing trend from 1965 to 2019, which is similar to that in the sunspot numbers. From this long-term study, it seems that solar activity is largely driven by solar rotation.

We study the temporal variation of solar rotation profiles based on solar irradiance at 93.5 nm and solar radio flux at 10.7 cm originating from the transition region and lower corona, respectively. The autocorrelation technique is used to calculate the period in periodic time series data. The sidereal rotation periods for normalized and detrended data are studied for 2011 – 2021. The sidereal rotation periods for solar irradiance and radio flux for 2011 – 2021 vary from 22.75 to 26.17 days and 19.42 to 28.14 days, respectively. The mean of the sidereal rotation periods for solar irradiance and radio flux are 24.76 and 23.76 days, respectively. The mean sidereal rotation period for solar irradiance is higher than the mean sidereal rotation period for solar radio flux. The sidereal rotation period for solar irradiance is greater than or equal to the sidereal rotation period for solar radio flux for almost all the years between 2011 and 2021. It is found that the lower corona rotates faster than the transition region during 2011 – 2021, i.e., the lower corona is found to be moving 4% faster than the transition region during 2011 – 2021. We found a linear relationship between the normalized daily irradiance and radio flux with a correlation coefficient of 0.986. Using cross-correlation analysis, we investigated a phase relationship between solar irradiance and radio flux and found no time lag between solar irradiance and radio flux.

The empirical investigation of the magnetic field in the outer solar atmosphere is a very important challenge in astrophysics. To this end, we need to identify, measure and interpret observable quantities sensitive to the magnetism of the upper chromosphere, transition region and corona. This paper provides an overview of the physics and diagnostic potential of spectropolarimetry in permitted spectral lines of the ultraviolet solar spectrum, such as the Mg ii h and k lines around 2800 Å, the hydrogen Lyman- α line at 1216 Å, and the Lyman- α line of He ii at 304 Å. The outer solar atmosphere is an optically pumped vapor and the linear polarization of such spectral lines is dominated by the atomic level polarization produced by the absorption and scattering of anisotropic radiation. Its modification by the action of the Hanle and Zeeman effects in the inhomogeneous and dynamic solar atmosphere needs to be carefully understood because it encodes the magnetic field information. The circular polarization induced by the Zeeman effect in some ultraviolet lines (e.g., Mg ii h & k ) is also of diagnostic interest, especially for probing the outer solar atmosphere in plages and more active regions. The few (pioneering) observational attempts carried out so far to measure the ultraviolet spectral line polarization produced by optically pumped atoms in the upper chromosphere, transition region and corona are also discussed. We emphasize that ultraviolet spectropolarimetry is a key gateway to the outer atmosphere of the Sun and of other stars.

Both coronal plumes and network jets are rooted in network lanes. The relationship between the two, however, has yet to be addressed. For this purpose, we perform an observational analysis using images acquired on 2015 December 4 with the Atmospheric Imaging Assembly (AIA) 171 Å passband to follow the evolution of coronal plumes, the observations taken by the Interface Region Imaging Spectrograph (IRIS) at slit-jaw 1330 Å to study the network jets, and the line-of-sight magnetograms taken by the Helioseismic and Magnetic Imager (HMI) to overview the photospheric magnetic features in the regions. Four regions in the network lanes are identified and labeled R1–R4, which are abundant with network jets. R1 and R2 are associated with coronal plumes that could be clearly seen for at least five days, while coronal plumes are hardly seen in R3 and R4 for at least three days. Furthermore, while magnetic features in all these regions are dominated by positive polarity, they are more compact (suggesting stronger convergence) in R1 and R2 than that in R3 and R4. We develop an automated method to identify and track the network jets in the regions. We find that the network jets rooted in R1 and R2 are higher and faster than those in R3 and R4, indicating that network regions producing stronger coronal plumes also tend to produce more dynamic network jets. We suggest that the stronger convergence in R1 and R2 might provide a condition for faster shocks and/or more small-scale magnetic reconnection events that power more dynamic network jets and coronal plumes.

Solar ultraviolet (UV) bursts are small-scale compact brightenings in transition region images. The spectral profiles of transition region lines in these bursts are significantly enhanced and broadened, often with chromospheric absorption lines such as Ni ii 1335.203 and 1393.330 Å superimposed. We investigate the properties of several UV bursts using a coordinated observation of the Interface Region Imaging Spectrograph (IRIS), Solar Dynamics Observatory (SDO), and Hinode on February 7, 2015. We have identified 12 UV bursts, and 11 of them reveal small blueshifts of the Ni ii absorption lines. However, the Ni ii lines in one UV burst exhibit obvious redshifts of ~20 km s−1, which appear to be related to the cold plasma downflows observed in the IRIS slit-jaw images. We also examine the three-dimensional magnetic field topology using a magnetohydrostatic model, and find that some UV bursts are associated with magnetic null points or bald patches. In addition, we find that these UV bursts reveal no obvious coronal signatures from the observations of the Atmospheric Imaging Assembly (AIA) on board SDO and the EUV Imaging Spectrometer (EIS) on board Hinode.

Observations of small-scale brightenings in the low solar atmosphere can provide valuable constraints on possible heating and heat transport mechanisms. We present a method for the detection and analysis of brightenings, and demonstrate its application to time-series imagery of the Interface Region Imaging Spectrograph (IRIS) in the extreme ultraviolet (EUV). The method is based on spatio-temporal band-pass filtering, adaptive thresholding and centroid tracking, and records an event’s spatial position, duration, total brightness and maximum brightness. Spatial area, brightness, and position are also recorded as functions of time throughout the event’s lifetime. Detected brightenings can fragment, or merge, over time – thus the number of distinct regions constituting a brightening event is recorded over time, and the maximum number of regions recorded as N frag , which is a simple measure of an event’s coherence or spatial complexity. A test is made on a synthetic datacube composed of a static background based on IRIS data, Poisson noise and ≈ 10 4 randomly-distributed, moving, small-scale Gaussian brightenings. Maximum brightness, total brightness, area, and duration follow power-law distributions, and the results show the range over which the method can successfully extract information. The test shows that the recorded maximum brightness of an event is a reliable measure for the brightest and most accurately detected events, with an error of 6%. Event area, duration and speed are generally underestimated by around 15% and have an uncertainty of 20–30%. The total brightness is underestimated by 30%, and has an uncertainty of 30%. Applying this detection method to real IRIS quiet-sun data spanning 19 minutes over a 54.40 ″ × 55.23 ″ field of view (FOV) yields 2997 detections, 1340 of these detections either remain un-fragmented or fragment to two distinct regions at least once during their lifetime ( N frag ≤ 2 ), equating to an event density of 3.96 × 10 − 4 arcsec −2  s −1 . The method will be used for a future large-scale statistical analysis of several quiet-sun (QS) data sets from IRIS, other EUV imagers, and other types of data including H α and visible photospheric imagery.

Solar Ultraviolet bursts (UBs) associated with flux emergence are expected to help understand the physical processes of the flux emergence itself. In the present study, we analyse imaging and spectroscopic observations of a special group of UBs (including twelve of them) occurring in the joint footpoint regions of multiple transition region loops above the flux emerging regions. Consistent with previous studies of common UBs, we found that the spectral characteristics of this group of UBs are varied. Our results show that the responses of UBs in Ni ii, NUV continuum, Mg ii h and O i are originated from locations differ from that emits Si iv. The imaging data show that UBs have connections with the dynamics in the transition region loops. Brightenings starting from UB-regions and propagating along loops can be seen in SJ 1400/1330 Å and AIA 304 Å images and the corresponding time-space images. The apparent velocities are tens of kilometers per second in AIA 304 Å. For symmetry, the brightenings can propagate from the UB-regions towards opposite directions with similar apparent velocities in some cases. Given that these UBs are magnetic reconnection phenomena, we suggest that the propagating brightenings are the signals of the plasma flows resulted from heatings in the UB-regions.

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 Å and 1445.75 Å 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(Ne)=8.63\\( (N_e) = 8.63\\); while in the active region, the averaged density gets the maximum log(Ne)=8.86\\( (N_e) = 8.86\\) in the active region (B), and gets the minimum log(Ne)=8.38\\( (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.

We construct two classes of the magnetohydrostatic equilibria of the axisymmetric flux tubes with twisted magnetic fields in the stratified solar atmosphere that span from the photosphere to the transition region. We built the models by incorporating specific forms of the gas pressure and poloidal current in the Grad-Shafranov equation. This model gives both closed and open field structure of the flux tube. The other open field model we construct is based on the self-similar formulation, where we have incorporated specific forms of the gas pressure, poloidal current and two different shape functions. We study the homology of the parameter space that is consistent with the solar atmosphere and find that the estimation of the magnetic structure inside the flux tubes is consistent with the observation and simulation results of the magnetic bright points.