Coronal Bright Points Research Articles

Characteristics of Nanoflare Heating in a Coronal Bright Point

We have obtained constraints on the nanoflare energy distribution and timing for the heating of a coronal bright point. Observations of the bright point were made using the Extreme Ultraviolet Imaging Spectrometer on Hinode in slot mode, which collects a time series of monochromatic images of the region leading to unambiguous temperature diagnostics. The Enthalpy-Based Thermal Evolution of Loops model was used to simulate nanoflare heating of the bright point and generate a time series of synthetic intensities. The nanoflare heating in the model was parameterized in terms of the power-law index α of the nanoflare energy distribution, which is ∝ E −α ; average nanoflare frequency f; and the number N of magnetic strands making up the observed loop. By comparing the synthetic and observed light curves, we inferred the region of the model parameter space (α, f, N) that was consistent with the observations. Broadly, we found that N and f are inversely correlated with one another, while α is directly correlated with either N or f. These correlations are likely a consequence of the region requiring a certain fixed energy input, which can be achieved in various ways by trading off among the different parameters. We also find that a value of α > 2 generally gives the best match between the model and observations, which indicates that the heating is dominated by low-energy events. Our method of using monochromatic images, focusing on a relatively simple structure, and constraining nanoflare parameters on the basis of statistical properties of the intensity provides a versatile approach to better understand the nature of nanoflares and coronal heating.

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

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

Automatic detection of small-scale EUV brightenings observed by the Solar Orbiter/EUI

Context. Accurate detections of frequent small-scale extreme ultraviolet (EUV) brightenings are essential to the investigation of the physical processes heating the corona. Aims. We detected small-scale brightenings, termed campfires, using their morphological and intensity structures as observed in coronal EUV imaging observations for statistical analysis. Methods. We applied a method based on Zernike moments and a support vector machine (SVM) classifier to automatically identify and track campfires observed by Solar Orbiter/Extreme Ultraviolet Imager (EUI) and Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA). Results. This method detected 8678 campfires (with length scales between 400 km and 4000 km) from a sequence of 50 High Resolution EUV telescope (HRIEUV) 174 Å images. From 21 near co-temporal AIA images covering the same field of view as EUI, we found 1131 campfires, 58% of which were also detected in HRIEUV images. In contrast, about 16% of campfires recognized in HRIEUV were detected by AIA. We obtain a campfire birthrate of 2 × 10−16 m−2 s−1. About 40% of campfires show a duration longer than 5 s, having been observed in at least two HRIEUV images. We find that 27% of campfires were found in coronal bright points and the remaining 73% have occurred out of coronal bright points. We detected 23 EUI campfires with a duration greater than 245 s. We found that about 80% of campfires are formed at supergranular boundaries, and the features with the highest total intensities are generated at network junctions and intense H I Lyman-α emission regions observed by EUI/HRILya. The probability distribution functions for the total intensity, peak intensity, and projected area of campfires follow a power law behavior with absolute indices between 2 and 3. This self-similar behavior is a possible signature of self-organization, or even self-organized criticality, in the campfire formation process.

Moving structures in ultraviolet bright points: Observations from Solar Orbiter/EUI

Context. Moving structures have been detected in coronal bright points and in a solar flare in active regions that are bidirectional, symmetrical, simultaneous, and quasi-periodic. These could be regarded as observational evidence of plasma outflows via magnetic reconnection. Aims. We explored pairs of moving structures in fifteen ultraviolet bright points (UBPs), which were observed in the quiet Sun or inside a small active region on 19 November 2020. Methods. The UBPs were measured by the High Resolution (HRI) Telescopes of the Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter (SolO) in two passbands, HRIEUV 174 Å and HRILyα 1216 Å. The pairs of moving structures are identified in time-distance slices along curved slits of UBPs and their quasi-periods are determined from the fast Fourier transform and wavelet analysis methods. Results. Moving structures observed in ten UBPs, starting from their bright cores and propagating toward two ends, are interpreted as diverging motions of bidirectional moving structures. These moving structures are also characterized by simultaneity and symmetry and in the case of seven UBPs, they exhibit quasi-periodicity. Moving structures seen in another five UBPs, originating from double ends, moving closer, and merging together are manifested as converging motions. A sympathetic UBP induced by the primary UBP is observed at the edge of a small active region and their moving structures also show the converging motion. Conclusions. The diverging motions of bidirectional moving structures could be generated by outflows after magnetic reconnections. The converging motions of two moving structures might be caused by inflows through the magnetic reconnection or could also be interpreted as upflows driven by the chromospheric evaporation.

Decayless Oscillations in Solar Coronal Bright Points

Decayless kink oscillations of solar coronal loops (or decayless oscillations for short) have attracted great attention since their discovery. Coronal bright points (CBPs) are mini-active regions and consist of loops with a small size. However, decayless oscillations in CBPs have not been widely reported. In this study, we identified this kind of oscillations in some CBPs using 171 Å images taken by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. After using the motion magnification algorithm to increase oscillation amplitudes, we made time–distance maps to identify the oscillatory signals. We also estimated the loop lengths and velocity amplitudes. We analyzed 23 CBPs and found 31 oscillation events in 16 of them. The oscillation periods range from 1 to 8 minutes (on average about 5 minutes), and the displacement amplitudes have an average value of 0.07 Mm. The average loop length and velocity amplitude are 23 Mm and 1.57 km s−1, respectively. Relationships between different oscillation parameters are also examined. Additionally, we performed a simple model to illustrate how these subpixel oscillation amplitudes (less than 0.4 Mm) could be detected. Results of the model confirm the reliability of our data processing methods. Our study shows for the first time that decayless oscillations are common in small-scale loops of CBPs. These oscillations allow for seismological diagnostics of the Alfvén speed and magnetic field strength in the corona.

Eruptions from coronal bright points: A spectroscopic view by IRIS of a mini-filament eruption, QSL reconnection, and reconnection-driven outflows

Context. Our study investigates a mini-filament eruption associated with cancelling magnetic fluxes. The eruption originates from a small-scale loop complex commonly known as a coronal bright point (CBP). The event is uniquely recorded in both the imaging and spectroscopic data taken with the Interface Region Imaging Spectrograph (IRIS). Aims. The investigation aims to gain a better understanding of the physical processes driving these ubiquitous small-scale eruptions. Methods. We analysed IRIS spectroscopic and slit-jaw imaging observations as well as images taken in the extreme-ultraviolet channels of the Atmospheric Imaging Assembly (AIA) and line-of-sight magnetic-field data from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory. As the observations can only indicate the possible physical processes at play, we also employed a non-linear force-free field (NLFFF) relaxation approach based on the HMI magnetogram time series. This allowed us to further investigate the evolution of the magnetic-field structures involved in the eruption process. Results. We identified a strong small-scale brightening as a micro-flare in a CBP, recorded in emission from chromospheric to flaring plasmas. The mini-eruption is manifested via the ejection of hot (CBP loops) and cool (mini-filament) plasma recorded in both the imaging and spectroscopic data. The micro-flare is preceded by the appearance of an elongated bright feature in the IRIS slit-jaw 1400 Å images, located above the polarity inversion line. The micro-flare starts with an IRIS pixel size brightening and propagates bi-directionally along the elongated feature. We detected, in both the spectral and imaging IRIS data and AIA data, strong flows along and at the edges of the elongated feature; we believe that these represent reconnection outflows. Both edges of the elongated feature that wrap around the edges of the erupting MF evolve into a J-type shape, creating a sigmoid appearance. A quasi-separatrix layer (QSL) is identified in the vicinity of the polarity inversion line by computing the squashing factor, Q, in different horizontal planes of the NLFFF model. Conclusions. This CBP spectro-imaging study provides further evidence that CBPs represent downscaled active regions and, as such, they may make a significant contribution to the mass and energy balance of the solar atmosphere. They are the sources of all range of typical active-region features, including magnetic reconnection along QSLs, (mini-)filament eruptions, (micro-)flaring, reconnection outflows, etc. The QSL reconnection site has the same spectral appearance as the so-called explosive events identified by strong blue- and red-shifted emission, thus providing an answer to an outstanding question regarding the true nature of this spectral phenomenon.

Synchronization of Small-scale Magnetic Features, Blinkers, and Coronal Bright Points

Abstract We investigate the relationship between different transients such as blinkers detected in images taken at 304 Å, extreme ultraviolet coronal bright points (ECBPs) at 193 Å, X-ray coronal bright points (XCBPs) at 94 Å on the Atmospheric Imaging Assembly, and magnetic features observed by the Helioseismic and Magnetic Imager during 10 yr of solar cycle 24. An automatic identification method is applied to detect transients, and the YAFTA algorithm is used to extract the magnetic features. Using 10 yr of data, we detect in total 7,483,827 blinkers, 2,082,162 ECBPs, and 1,188,839 XCBPs, respectively, with their birth rate of about 1.1 × 10−18 m−2 s−1, 3.8 × 10−19 m−2 s−1, and 1.5 × 10−19 m−2 s−1. We find that about 80% of blinkers are observed at the boundaries of supergranules, and 57% (34%) are associated with ECBPs (XCBPs). We further find that about 61%–80% of transients are associated with the isolated magnetic poles in the quiet Sun and that the normalized maximum intensities of the transients are correlated with the photospheric magnetic flux of poles via a power law. These results conspicuously show that these transients have a magnetic origin and their synchronized behavior provides further clues toward the understanding of the coupling among the different layers of the solar atmosphere. Our study further reveals that the appearance of these transients is strongly anticorrelated with the sunspots’ cycle. This finding can be relevant for a better understanding of solar dynamo and magnetic structures at different scales during the solar cycle.

First observations from the SPICE EUV spectrometer on Solar Orbiter

Aims. We present first science observations taken during the commissioning activities of the Spectral Imaging of the Coronal Environment (SPICE) instrument on the ESA/NASA Solar Orbiter mission. SPICE is a high-resolution imaging spectrometer operating at extreme ultraviolet (EUV) wavelengths. In this paper we illustrate the possible types of observations to give prospective users a better understanding of the science capabilities of SPICE. Methods. We have reviewed the data obtained by SPICE between April and June 2020 and selected representative results obtained with different slits and a range of exposure times between 5 s and 180 s. Standard instrumental corrections have been applied to the raw data. Results. The paper discusses the first observations of the Sun on different targets and presents an example of the full spectra from the quiet Sun, identifying over 40 spectral lines from neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur, magnesium, and iron. These lines cover the temperature range between 20 000 K and 1 million K (10 MK in flares), providing slices of the Sun’s atmosphere in narrow temperature intervals. We provide a list of count rates for the 23 brightest spectral lines. We show examples of raster images of the quiet Sun in several strong transition region lines, where we have found unusually bright, compact structures in the quiet Sun network, with extreme intensities up to 25 times greater than the average intensity across the image. The lifetimes of these structures can exceed 2.5 hours. We identify them as a transition region signature of coronal bright points and compare their areas and intensity enhancements. We also show the first above-limb measurements with SPICE above the polar limb in C III, O VI, and Ne VIII lines, and far off limb measurements in the equatorial plane in Mg IX, Ne VIII, and O VI lines. We discuss the potential to use abundance diagnostics methods to study the variability of the elemental composition that can be compared with in situ measurements to help confirm the magnetic connection between the spacecraft location and the Sun’s surface, and locate the sources of the solar wind. Conclusions. The SPICE instrument successfully performs measurements of EUV spectra and raster images that will make vital contributions to the scientific success of the Solar Orbiter mission.

Spatio-temporal bands of coronal bright points and their relation to solar torsional oscillations

ABSTRACT Using automatic processing of daily fits files from SOHO_EIT, we studied the temporal evolution of the latitudinal distribution of bright coronal points from 1995 to 2020. The study revealed homogeneous and inhomogeneous components of the latitudinal distribution. The homogeneous component has cyclical variations with a significant phase shift compared to the number of sunspots. The residual inhomogeneous component manifests itself as migration of the centre of increased density from high to low latitudes. The bands of the residual density of coronal bright points (CBP) reflect solar torsional oscillations. It should be associated with the strengthening of the local alpha-omega dynamo. However, the cyclical nature of the number of CBP is explained by the diffusion of large-scale magnetic fluxes.

Segmentation of Coronal Features to Understand the Solar EUV and UV Irradiance Variability III. Inclusion and Analysis of Bright Points

The study of solar irradiance variability is of great importance in heliophysics, Earth’s climate, and space weather applications. These studies require careful identifying, tracking and monitoring of features in the solar photosphere, chromosphere, and corona. Do coronal bright points contribute to the solar irradiance or its variability as input to the Earth atmosphere? We studied the variability of solar irradiance for a period of 10 years (May 2010 – June 2020) using the Large Yield Radiometer (LYRA), the Sun Watcher using APS and image Processing (SWAP) on board PROBA2, and the Atmospheric Imaging Assembly (AIA), and applied a linear model between the segmented features identified in the EUV images and the solar irradiance measured by LYRA. Based on EUV images from AIA, a spatial possibilistic clustering algorithm (SPoCA) is applied to identify coronal holes (CHs), and a morphological feature detection algorithm is applied to identify active regions (ARs), coronal bright points (BPs), and the quiet Sun (QS). The resulting segmentation maps were then applied on SWAP images, images of all AIA wavelengths, and parameters such as the intensity, fractional area, and contribution of ARs/CHs/BPs/QS features were computed and compared with LYRA irradiance measurements as a proxy for ultraviolet irradiation incident to the Earth atmosphere. We modeled the relation between the solar disk features (ARs, CHs, BPs, and QS) applied to EUV images against the solar irradiance as measured by LYRA and the F10.7 radio flux. A straightforward linear model was used and corresponding coefficients computed using a Bayesian method, indicating a strong influence of active regions to the EUV irradiance as measured at Earth’s atmosphere. It is concluded that the long- and short-term fluctuations of the active regions drive the EUV signal as measured at Earth’s atmosphere. A significant contribution from the bright points to the LYRA irradiance could not be found.

ALMA small-scale features in the quiet Sun and active regions

Aims. The main aim of the present analysis is to decipher (i) the small-scale bright features in solar images of the quiet Sun and active regions obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and (ii) the ALMA correspondence of various known chromospheric structures visible in the Hα images of the Sun. Methods. Small-scale ALMA bright features in the quiet Sun region were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz) and in an active region using interferometric ALMA measurements (3 mm, 100 GHz). With the single-dish observations, a full-disk solar image is produced, while interferometric measurements enable the high-resolution reconstruction of part of the solar disk, including the active region. The selected quiet Sun and active regions are compared with the Hα (core and wing sum), EUV, and soft X-ray images and with the magnetograms. Results. In the quiet Sun region, enhanced emission seen in the ALMA is almost always associated with a strong line-of-sight magnetic field. Four coronal bright points were identified, while other small-scale ALMA bright features are most likely associated with magnetic network elements and plages. In the active region, in 14 small-scale ALMA bright features randomly selected and compared with other images, we found five good candidates for coronal bright points, two for plages, and five for fibrils. Two unclear cases remain: a fibril or a jet, and a coronal bright point or a plage. A comparison of the Hα core image and the 3 mm ALMA image of the analyzed active region showed that the sunspot appears dark in both images (with a local ALMA radiation enhancement in sunspot umbra), the four plage areas are bright in both images and dark small Hα filaments are clearly recognized as dark structures of the same shape also in ALMA.

Sympathetic Standard and Blowout Coronal Jets Observed in a Polar Coronal Hole

Abstract We present the sympathetic eruption of a standard and a blowout coronal jet originating from two adjacent coronal bright points (CBP1 and CBP2) in a polar coronal hole, using soft X-ray and extreme-ultraviolet observations respectively taken by the Hinode and the Solar Dynamics Observatory. In the event, a collimated jet with obvious westward lateral motion first launched from CBP1, during which a small bright point appeared around CBP1's east end, and magnetic flux cancellation was observed within the eruption source region. Based on these characteristics, we interpret the observed jet as a standard jet associated with photospheric magnetic flux cancellation. About 15 minutes later, the westward-moving jet spire interacted with CBP2 and resulted in magnetic reconnection between them, which caused the formation of the second jet above CBP2 and the appearance of a bright loop system in between the two CBPs. In addition, we observed the writhing, kinking, and violent eruption of a small kink structure close to CBP2's west end but inside the jet base, which made the second jet brighter and broader than the first one. These features suggest that the second jet should be a blowout jet triggered by the magnetic reconnection between CBP2 and the spire of the first jet. We conclude that the two successive jets were physically connected to each other rather than a temporal coincidence, and this observation also suggests that coronal jets can be triggered by external eruptions or disturbances, as well as internal magnetic activities or magnetohydrodynamic instabilities.

On the Nature of Propagating Intensity Disturbances in Polar Plumes during the 2017 Total Solar Eclipse

Abstract The propagating intensity disturbances (PIDs) in plumes are still poorly understood, and their identity (magnetoacoustic waves or flows) remains an open question. We investigate PIDs in five plumes located in the northern polar coronal hole observed during the 2017 total solar eclipse. Three plumes are associated with coronal bright points, jets, and macrospicules at their base (active plumes), and the other two plumes are not (quiet plumes). The electron temperature at the base of the plumes is obtained from the filter ratio of images taken with the X-ray Telescope on board Hinode and the passband ratio around 400 nm from an eclipse instrument, the Diagnostic Coronagraph Experiment. The phase speed (v r ), frequency (ω), and wavenumber (k) of the PIDs in the plumes are obtained by applying a Fourier transformation to the spacetime (r − t plane) plots in images taken with the Atmospheric Imaging Assembly (AIA) in three different wavelength channels (171, 193, and 211 Å). We found that the PIDs in the higher-temperature AIA channels, 193 and 211 Å, are faster than that of the cooler AIA 171 Å channel. This tendency is more significant for the active plumes than the quiet ones. The observed speed ratio (∼1.3) between the AIA 171 and 193 Å channels is similar to the theoretical value (1.25) of a slow magnetoacoustic wave. Our results support the idea that PIDs in plumes represent a superposition of slow magnetoacoustic waves and plasma outflows that consist of dense cool flows and hot coronal jets.

The Photospheric Footpoints of Solar Coronal Hole Jets

Abstract We study the photospheric footpoints of a set of 35 coronal jets in a coronal hole as observed by Hinode/EIS. We use SDO/AIA data to coalign the spectroscopic EIS data with SDO/HMI line-of-sight magnetograms and calculate the plane-of-sky flow field using local correlation tracking (LCT) on SDO/HMI white light images. The jets are put into categories according to the changes observed in the photospheric magnetic flux at the footpoints of the coronal bright point where the jets originate: flux cancellation, complex flux changes (flux appearance/emergence and cancellation), and no flux changes. We also present three jets in detail. Observed magnetic flux evolution, LCT flow field structure and location of the jet footpoints at supergranular boundaries do not support the flux emergence scenario used in most jet simulations and are also not consistent with a rotational photospheric driver. Detailed numerical jet simulations using our observed photospheric features, in particular converging flows and flux cancellation do not currently exist, although such models would provide a realistic eruptive event scenario.

The chromospheric component of coronal bright points

Context. We investigate the chromospheric counterpart of small-scale coronal loops constituting a coronal bright point (CBP) and its response to a photospheric magnetic-flux increase accompanied by co-temporal CBP heating. Aims. The aim of this study is to simultaneously investigate the chromospheric and coronal layers associated with a CBP, and in so doing, provide further understanding on the heating of plasmas confined in small-scale loops. Methods. We used co-observations from the Atmospheric Imaging Assembly and Helioseismic Magnetic Imager on board the Solar Dynamics Observatory, together with data from the Fast Imaging Solar Spectrograph taken in the Hα and Ca II 8542.1 Å lines. We also employed both linear force-free and potential field extrapolation models to investigate the magnetic topology of the CBP loops and the overlying corona, respectively. We used a new multi-layer spectral inversion technique to derive the temporal variations of the temperature of the Hα loops (HLs). Results. We find that the counterpart of the CBP, as seen at chromospheric temperatures, is composed of a bundle of dark elongated features named in this work Hα loops, which constitute an integral part of the CBP loop magnetic structure. An increase in the photospheric magnetic flux due to flux emergence is accompanied by a rise of the coronal emission of the CBP loops, that is a heating episode. We also observe enhanced chromospheric activity associated with the occurrence of new HLs and mottles. While the coronal emission and magnetic flux increases appear to be co-temporal, the response of the Hα counterpart of the CBP occurs with a small delay of less than 3 min. A sharp temperature increase is found in one of the HLs and in one of the CBP footpoints estimated at 46% and 55% with respect to the pre-event values, also starting with a delay of less than 3 min following the coronal heating episode. The low-lying CBP loop structure remains non-potential for the entire observing period. The magnetic topological analysis of the overlying corona reveals the presence of a coronal null point at the beginning and towards the end of the heating episode. Conclusions. The delay in the response of the chromospheric counterpart of the CBP suggests that the heating may have occurred at coronal heights.

Energetics of Solar Coronal Bright Points

Abstract The several-million-degree, low-density quiet solar corona requires a total energy-loss flux of about 3 × 105 erg cm−2 s−1. Solar coronal bright points (CBPs) are ubiquitous in the quiet Sun. They may release magnetic energy to heat the solar corona, but their contribution to the energy flux has not been determined yet. We used an automatic identification and tracking method for CBPs, which was developed based on the support vector machine classifier and Zernike moments of extreme ultraviolet (EUV) observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. We applied a spatial synthesis differential emission measure method and a Vertical-Current Approximation Nonlinear Force-Free Field technique to extract the thermal and magnetic energetics of the CBPs, respectively. By analyzing 7.5 yr (within the solar cycle 24) of AIA observations, we show that the average thermal energy and magnetic free energy of 140,000 CBPs are positively correlated with sunspots. However, the number of CBPs and sunspots are highly anti-correlated. We calculate a total energy-loss flux (sum of the radiative and conductive loss flux) of about (4.84 ± 1.60) × 103 erg cm−2 s−1 for the system of CBPs. Therefore, it is about 1.61% ± 0.53% of the total energy-loss flux of quiet corona. By extending the distribution of the magnetic Poynting flux and energy-loss flux for CBPs to nanoflares, the total magnetic Poynting flux and total energy-loss flux are obtained to be in the range of 1.48 × 105 to 1.57 × 106 and 3.86 × 104 to 2.35 × 105 erg cm−2 s−1, respectively.

Eruptions from coronal hole bright points: Observations and non-potential modelling

Context. We report on the third part of a series of studies on eruptions associated with small-scale loop complexes named coronal bright points (CBPs). Aims. A single case study of a CBP in an equatorial coronal hole with an exceptionally large size is investigated to expand on our understanding of the formation of mini-filaments, their destabilisation, and the origin of the eruption triggering the formation of jet-like features recorded in extreme ultraviolet (EUV) and X-ray emission. We aim to explore the nature of the so-called micro-flares in CBPs associated with jets in coronal holes and mini coronal mass ejections in the quiet Sun. Methods. Co-observations from the Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory as well as GONG Hα images are used together with a non-linear force free field (NLFFF) relaxation approach, where the latter is based on a time series of HMI line-of-sight magnetograms. Results. A mini-filament (MF) that formed beneath the CBP arcade about 3−4 h before the eruption is seen in the Hα and EUV AIA images to lift up and erupt triggering the formation of an X-ray jet. No significant photospheric magnetic flux concentration displacement (convergence) is observed and neither is magnetic flux cancellation between the two main magnetic polarities forming the CBP in the time period leading to MF lift-off. The CBP micro-flare is associated with three flare kernels that formed shortly after the MF lift-off. No observational signature is found for magnetic reconnection beneath the erupting MF. The applied NLFFF modelling successfully reproduces both the CBP loop complex as well as the magnetic flux rope that hosts the MF during the build-up to the eruption. Conclusions. The applied NLFFF modelling is able to clearly show that an initial potential field can be evolved into a non-potential magnetic field configuration that contains free magnetic energy in the region that observationally hosts the eruption. The comparison of the magnetic field structure shows that the magnetic NLFFF model contains many of the features that can explain the different observational signatures found in the evolution and eruption of the CBP. In the future, it may eventually indicate the location of destabilisation that results in the eruptions of flux ropes.

Observational evidence of magnetic reconnection in a coronal bright point

Magnetic reconnection is considered to be the fundamental process by which magnetic energy is converted into plasma or particle kinetic energy. Magnetic reconnection is a widely applied physics model to explain the solar eruption events, such as coronal bright points (CBPs). Meanwhile, it is an usual way of the solar physics research to look for the observational evidences of magnetic reconnection in the solar eruption events in order to support the model. In this paper, we have explored the evidences of magnetic reconnection in a CBP observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) at NOAA No. 11163 on 2011 March 5. Our observations show that this event is a small-scale loop system in active regions that have similar size as a traditional CBP and it might shed light on the physics of a traditional CBP. This CBP is bright in all nine AIA wavelengths and displays a flaring development with three bursts intermittently. Each burst exhibits a pair of bi-directional jets almost along a line. They originate from the same position (CBP core), then move in the opposite directions. Our findings are well consistent with the magnetic reconnection process by which the bi-directional plasma outflows are produced and radiate the bi-directional jets detected by SDO/AIA. These facts further support the conclusion that the CBP is produced by the magnetic reconnection process.

Regular and Oscillatory Motion of Coronal Bright Points

Long-term automatic tracking of coronal bright points has made it possible to study their proper motions using data from the AIA 193 A channel of SDO/AIA. Fits files for 95 visually long-lived bright points uniformly distributed in latitude have been chosen for processing. A specially developed program automatically fixes the heliographic coordinates of the centroid of the chosen coronal bright points in a series of Fits files. It was found that the proper motions of all the coronal bright points studied here have a distinct oscillatory character. The periods of the oscillations range from 5 to 60 h with an average of 20 h. Some tendencies in the latitudinal dependence of the oscillatory characteristics have been found. A possible explanation of these oscillations in terms of the propagation of helical waves along the magnetic loops that form the structure of the coronal bright points is pointed out. Linear trends in the latitudinal and longitudinal components of the motion of the coronal points are used to determine the rotational velocity and meridional migration of the coronal bright points. An analysis of the rotational velocities confirms the differential character of the latitudinal dependence. The problem of determining the direction and magnitude of the meridional migration is discussed.

Identifying 8 mm Radio Brightenings During the Solar Activity Minimum

Strong solar radio brightenings have been extensively studied in the past, and their correlation to the sunspots and active regions are already well known. But even when the Sun is ostensibly quiet, there is practically always some activity that can be detected in the radio domain. In this article we investigate these semi-active features at 8 mm using the radio telescope at Aalto University Metsähovi Radio Observatory. The observations were made between May and September 2019 when the solar activity was very low, and for our detailed study we chose dates when no active regions were identified on the solar surface by the National Oceanic and Atmospheric Administration. The brightness temperature of these radio regions during this quiescent period of solar activity is at maximum approximately 250 K above the quiet-Sun level. We compared our millimeter data with data taken in extreme ultraviolet, and we found that these weak radio brightenings are mostly related to coronal hole features and magnetic bright points. We also found that there are two different categories of bright points: those with and without flux tube structure. The formation of the weak radio brightenings is comparable to the stronger radio brightenings: the rising fluxes from the weak photospheric features can be detected as a radio source.