Two types of dynamic cool coronal structures observed with STEREO and Hirtode

Solar coronal loops show significant plasma motions during their formation and eruption stages. Dynamic cool coronal structures, on the other hand, are often observed to propagate along coronal loops. We report on the discovery of two types of dynamic cool coronal structures, and characterize their fundamental properties. Using the EUV 304 A images from the Extreme UltraViolet Imager （EUVI） telescope on the Solar TErrestrial RElation Observatory （STEREO） and the Ca Ⅱ filtergrams from the Solar Optical Telescope （SOT） instrument on Hinode, we study the evolution of an EUV arch and the kinematics of cool coronal structures. The EUV 304A observations show that a missile-like plasmoid moves along an arch-shaped trajectory, with an average velocity of 31 km s^- 1. About three hours later, a plasma arch forms along the trajectory, subsequently the top part of the arch fades away and disappears; meanwhile the plasma belonging to the two legs of the arch flows downward to the arch＇s feet. During the arch formation and disappearance, SOT Ca Ⅱ images explore dynamic cool coronal structures beneath the arch. By tracking these structures, we classify them into two types. Type I is thread- like in shape and flows downward with a greater average velocity of 72 km s-l; finally it combines with a loop fibril at a chromospheric altitude. Type Ⅱ is shape-transformable and sometimes rolling as it flows downward with a smaller velocity of 37 km s-1, then disappears insularly in the chromosphere. It is suggested that the two types of structures are possibly controlled by different magnetic configurations.     

Measurements of spectral lines observed with Hinode/EIS as implications for coronal heating

We have measured the line widths and nonthermal velocities in 12 solar regions using high resolution EUV data taken by Hinode/EIS. We find that there exists a positive correlation between the intensity and nonthermal velocity for the Fe xII emission line as well as some other lines. The correlation coefficients decrease from the disk center to the limb. However, the nonthermal velocities of a particular spectral line do not vary much in different regions, so they are considered isotropic. In particular, we find that for a coronal loop structure, the largest widths and nonthermal velocities occur at the footpoints, where outflows appear. Based on these observational results, we discuss several physical processes responsible for coronal heating.     

What SDO tells us about structure and evolution of coronal bright points

Using magnetograms and coronal images from two instruments on board the Solar Dynamics Observatory (SDO), we study structure and evolution of a limited number of coronal bright points (CBPs). Our results show that the relation between CBPs and their magnetic footpoints is not simple. In some cases, CBP may appear as a bright portion of a larger loop (with clearly identifiable footpoints), and in some cases, an isolated CBP may develop between magnetic poles, which might not be the closest ones to each other or which might not be involved in the magnetic flux cancellation. We suggest that the magnetic connectivity responsible for formation of isolated coronal bright points is governed by the orientation of the large‐scale magnetic field. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurements of spectral lines observed with Hinode/EIS as implications for coronal heating

We have measured the line widths and nonthermal velocities in 12 solar regions using high resolution EUV data taken by Hinode/EIS. We find that there exists a positive correlation between the intensity and nonthermal velocity for the Fe XII emission line as well as some other lines. The correlation coefficients decrease from the disk center to the limb. However, the nonthermal velocities of a particular spectral line do not vary much in different regions, so they are considered isotropic. In particular, we find that for a coronal loop structure, the largest widths and nonthermal velocities occur at the footpoints, where outflows appear. Based on these observational results, we discuss several physical processes responsible for coronal heating.     

An Electron Density Model above the Sunspot from a Mapping of NOAA 7260 at 17 GHz

The brightness temperature distribution of microwave emission in a solar active region generally shows a ring structure, with a dip at the centre. However, no dip was found in the Nobeyama Radioheliograph left handed circular polarization (LCP) image on 1992 August 18; instead, there was a peak. This is a completely LCP source with zero right-handed circular polarization (RCP). We examine this structure in terms of the joint effect of gyroresonance and bremsstrahlung mechanism with a raised electron density above the central part of the sunspot, and the commonly assumed temperature and vertical dipole magnetic field models. The raised electron density is found to be 1.4 × 1011 cm-3 at the chromosphere base.     

Review of Hinode results

Hinode is an observatory‐style satellite, carrying three advanced instruments being designed and built to work together to explore the physical coupling between the photosphere and the upper layers for understanding the mechanism of dynam‐ ics and heating. The three instruments aboard are the Solar Optical Telescope (SOT), which can provide high‐precision photometric and polarimetric data of the lower atmosphere in the visible light (388–668 nm) with a spatial resolution of 0.2–0.3 arcseconds, the X‐Ray Telescope (XRT) which takes a wide field of full sun coverage X‐ray images being capable of diagnosing the physical condition of coronal plasmas, and the EUV Imaging Spectrometer (EIS) which observes the upper transition region and coronal emission lines in the wavelength ranges of 17–21 nm and 25–29 nm. Since first‐light observations in the end of October 2006, Hinode has been continuously providing unprecedented high‐quality solar data. We will present some new findings of the sun with Hinode, focusing on those from SOT (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Study of the Hinode Observations of the Bipolar Moving Magnetic Features in Sunspot Penumbrae

By using the monochromatic images and magnetograms obtained with the satellite Hinode, 35 pairs of bipolar moving magnetic features (MMFs) in sunspot penumbrae are studied in the following three aspects: the morphological characteristics, velocities of motion and responses in low atmospheric layers. Then the following conclusions are drawn. (1) The bipolar MMFs appear in pairs of positive and negative polarities, are located in the midst of the approximately vertical magnetic fields in spot penumbrae, and move toward the outer boundaries of penumbrae. This indirectly justifies that the bipolar MMFs originate in the horizontal magnetic fields of penumbrae. In the time intervals of 2-8 hours and at the same positions, there appear the bipolar MMFs with similar morphologial characteristics and velocities of motion. This povides an evidence which supports the model of magnetic lines in the shape of sea serpent. (2) In the process of motion of bipolar MMFs there may appear brightenings in the photospere and chromosphere, and this implies that the middle and low layers of solar atmosphere are heated by the bipolar MMFs. (3) The sites of occurrence of bipolar MMFs and the distribution of penumbral magnetic field agree with the structural characteristics of uncombed sunspot penumbrae.     

Small-Scale Brightenings in the UV Continuum of an M9.1 Solar Flare

We analyze an M9.1 two-ribbon solar flare which occurred on 2004 July 22 us- ing the TRACE white-light and 1700A~。images,the RHESSI,and the SOHO/MDI data.We find many small-scale fast-varying brightenings that appeared in the white-light and 1700A~。images along the flare ribbons.Some of them underwent rapid motions in weak magnetic field regions.We identify these short-lived brightenings as UV continuum enhancement.Our preliminary result shows that the brightenings are closely related to the HXR emission.They have a lifetime of 30-60 s and a typical size of about 1″-2″.The intensity enhancement is about 150-200 times the mean value of the quiet-Sun.According to previous works,we infer that the 1700A~。enhancement may be dominated by the increased emission of 1680 A con- tinuum coming from the temperature minimum region.The impulsive feature in the 1700 A~。light curves of the small-scale brightenings may be due to the irradiation of the impulsive CIV line intensity caused by the bombardment of non-thermal electron beams.     

Fe xiixii emission lines in solar active regions observed by the RES-C spectroheliograph on the CORONAS-I mission

Theoretical line intensity ratios involving Fe  xii transitions in the 186–201 Å wavelength range are compared with observational data for five solar active regions, obtained by the RES-C spectroheliograph on the CORONAS-I mission. Generally good agreement is found between theory and observation, hence resolving discrepancies previously found in the comparison of calculations with active region and subflare spectra from the Solar EUV Rocket Telescope and Spectrograph ( SERTS ). However, the Fe  xii 190.06- and 201.12-Å lines are blended with Fe  x 190.04 Å and Fe  xiii 201.13 Å, respectively. In addition, a weak feature at ∼197 Å, tentatively identified as Fe  xii 196.87 Å, does not appear to be due to this ion.     

Study of white-light flares observed by Hinode

White-light flares are considered to be the most energetic flaring events that are observable in the optical broad-band continuum of the solar spectrum. They have not been commonly observed. Observations of white-light flares with sub-arcsecond resolution have been very rare. The continuous high resolution observations of Hinode provide a unique opportunity to systematically study the white-light flares with a spatial resolution around 0.2 arcsec. We surveyed all the flares above GOES magnitude C5.0 since the launch of Hinode in 2006 October. 13 of these kinds of flares were covered by the Hinode G-band observations. We analyzed the peak contrasts and equivalent areas (calculated via integrated excess emission contrast) of these flares as a function of the GOES X-ray flux, and found that the cut-off visibility is likely around M1 flares under the observing limit of Hinode. Many other observational and physical factors should affect the visibility of white-light flares; as the observing conditions are improved, smaller flares are likely to have detectable white-light emissions. We are cautious that this limiting visibility is an overestimate, because G-band observations contain emissions from the upper atmosphere.Among the 13 events analyzed, only the M8.7 flare of 2007 June 4 had near-simultaneous observations in both the G-band and the blue continuum. The blue continuum had a peak contrast of 94% vs. 175% in G-band for this event. The equivalent area in the blue continuum is an order of magnitude lower than that in the G-band. Very recently, Jess et al.studied a C2.0 flare with a peak contrast of 300% in the blue continuum. Compared to the events presented in this letter, that event is probably an unusual white-light flare: a very small kernel with a large contrast that can be detected in high resolution observations.     

Transition region and coronal plasmas: instrumentation and spectral analysis

The plasma conditions in the solar atmosphere and, in particular, in coronal holes are summarized, before space-borne instrumentation for observing these regions in vacuum-ultraviolet light is briefly introduced with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) as example. Spectroscopic measurements of small plasma jets are then analyzed in detail. Magnetic reconnection is thought to be responsible for heating the corona of the Sun as well as accelerating the solar wind by converting magnetic energy into thermal and kinetic energies. The continuous outflow of the fast solar wind from coronal holes on ‘open’ field lines, which reach out into interplanetary space, then requires many reconnection events of very small scale sizes – most of them probably below the resolution capabilities of present-day instruments. Our observations of such an event have been obtained with the Solar and Heliospheric Observatory (SOHO) providing both high-resolution imaging and spectral information for structural and dynamical studies. We find whirling or rotating motions as well as jets with acceleration along their propagation paths in close spatial and temporal vicinity to the coronal jet. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observation of multiple sausage oscillations in cool post-flare loop

Using simultaneous high spatial (1.3 arcsec) and temporal (5 and 10 s) resolution Hα observations from the 15 cm Solar Tower Telescope at Aryabhatta Research Institute of Observational Sciences (ARIES), we study the oscillations in the relative intensity to explore the possibility of sausage oscillations in the chromospheric cool post-flare loop. We use the standard wavelet tool, and find the oscillation period of ≈587 s near the loop apex, and ≈349 s near the footpoint. We suggest that the oscillations represent the fundamental and the first harmonics of the fast-sausage waves in the cool post-flare loop. Based on the period ratio   P ₁/ P ₂∼1.68  , we estimate the density scaleheight in the loop as ∼17 Mm. This value is much higher than the equilibrium scaleheight corresponding to Hα temperature, which probably indicates that the cool post-flare loop is not in hydrostatic equilibrium. Seismologically estimated Alfvén speed outside the loop is  ∼300–330  km s⁻¹  . The observation of multiple oscillations may play a crucial role in understanding the dynamics of lower solar atmosphere, complementing such oscillations already reported in the upper solar atmosphere (e.g. hot flaring loops).     

Periodic variations in the coronal green line intensity and their connection with the white-light coronal structures

We present an analysis of short time-scale intensity variations in the coronal green line as obtained with high time resolution observations. The observed data can be divided into two groups. The first one shows periodic intensity variations with a period of 5 min. the second one does not show any significant intensity variations. We studied the relation between regions of coronal intensity oscillations and the shape of white-light coronal structures. We found that the coronal green-line oscillations occur mainly in regions where open white-light coronal structures are located.     

The anomalous intensities of helium lines in a coronal hole

Observations made at the quiet Sun-centre with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instruments on the Solar and Heliospheric Observatory ( SOHO ) have shown that the intensities of the resonance lines of He  i and He  ii are significantly larger than predicted by emission measure distributions found from other transition region lines. The intensities of the helium lines are observed to be lower in coronal holes than in the quiet Sun. Any theory proposed to account for the behaviour of the helium lines must explain the observations of both the quiet Sun and coronal holes. We use observations made with SOHO to find the physical conditions in a polar coronal hole. The electron pressure is found using the C  iii 1175-Å and N  iii 991.5-Å lines, as the C  iii line at 977.0 Å becomes optically thick in some regions at high latitudes. The mean electron pressure is a factor of ≃2 lower than that at the quiet Sun-centre. The mean coronal electron temperature is     . The helium lines are enhanced with respect to other transition region lines but by factors which are ≃ 30 per cent smaller than at the quiet Sun-centre. The mean ratios of the intensities of the He  i 537.0- and 584.3-Å lines and of the He  i and He  ii 303.8-Å lines vary little with the type of region studied. These ratios are compared with those predicted by models of the transition region, taking into account the radiative transfer in the helium lines. No significant variation is found in the relative abundances of carbon and silicon.     

Spectropolarimetery of umbral fine structures from Hinode: evidence for magnetoconvection

We present spectropolarimetric analysis of umbral dots and a light bridge fragment that show dark lanes in G -band images. Umbral dots show upflow as well as associated positive Stokes V area asymmetry in their central parts. Larger umbral dots show downflow patches in their surrounding parts that are associated with negative Stokes V area asymmetry. Umbral dots show weaker magnetic field in central part and higher magnetic field in peripheral area. Umbral fine structures are much better visible in total circularly polarized light than in continuum intensity. Umbral dots show a temperature deficit above dark lanes. The magnetic field inclination shows a cusp structure above umbral dots and a light bridge fragment. We compare our observational findings with 3D magnetohydrodynamic simulations.