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The ionization degree of hydrogen is crucial in the physics of the plasma in the solar chromosphere. It specifically limits the range of plasma temperatures that can be determined from the Hα line. Given that the chromosphere greatly deviates from the local thermodynamic equilibrium (LTE) condition, precise determinations of hydrogen ionization require the solving of the full set of non-LTE radiative transfer equations throughout the atmosphere, which is usually a formidable task. In many cases, it is still necessary to obtain a quick estimate of hydrogen ionization without having to solve for the non-LTE radiative transfer. Here, we present a simple method to meet this need. We adopt the assumption that the photoionizing radiation field changes little over time, even if physical conditions change locally. With this assumption, the photoionization rate can be obtained from a published atmosphere model and can be used to determine the degree of hydrogen ionization when the temperature and electron density are specified. The application of our method indicates that in the chromospheric environment, plasma features contain more than 10% neutral hydrogen at temperatures lower than 17,000 K but less than 1% neutral hydrogen at temperatures higher than 23,000 K, implying that the hydrogen temperature determined from the Hα line is physically plausible if it is lower than 20,000 K, but may not be real, if it is higher than 25,000 K. We conclude that our method can be readily exploited to obtain a quick estimate of hydrogen ionization in plasma features in the solar chromosphere.

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. We used co-observations from the AIA and HMI/SDO, together with data from the Fast Imaging Solar Spectrograph taken in the Halpha and Ca II 8542 lines. We used a new multi-layer spectral inversion technique to derive the temporal variations of the temperature of the Halpha loops (HLs). 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 Halpha 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 Halpha 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. The delay in the response of the chromospheric counterpart of the CBP suggests that the heating may have occurred at coronal heights., 14 pages, 14 figures, accepted for publication in A&A

As one of the most violent eruptions on the Sun, flares are believed to be powered by magnetic reconnection. The fundamental physics involving the release, transfer and deposition of energy have been studied extensively. Taking advantage of the unprecedented resolution provided by the 1.6-m Goode Solar Telescope, here we show a sudden rotation of vector magnetic fields, about 12$^{\circ}$-20$^{\circ}$ counterclockwise, associated with a flare. Unlike the permanent changes reported previously, the azimuth-angle change is transient and co-spatial/temporal with H$\alpha$ emission. The measured azimuth angle becomes closer to that in potential fields suggesting untwist of flare loops. The magnetograms were obtained in the near infrared at 1.56~$\mu$m, which is minimally affected by flare emission and no intensity profile change was detected. We believe that these transient changes are real and discuss the possible explanations in which the high energy electron beams or $Alfv\acute{e}n$ waves play a crucial role., Comment: http://rdcu.be/Epb8

It is not yet fully understood how magnetohydrodynamic waves in the interior and atmosphere of the Sun are excited. Traditionally, turbulent convection in the interior is considered to be the source of wave excitation in the quiet Sun. Over the last few decades, acoustic events observed in the intergranular lanes in the photosphere have emerged as a strong candidate for a wave excitation source. Here we report our observations of wave excitation by a new type of event: rapidly changing granules. Our observations were carried out with the Fast Imaging Solar Spectrograph in the H$\alpha$ and Ca II 8542 $\unicode{xC5}$ lines and the TiO 7057 $\unicode{xC5}$ broadband filter imager of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. We identify granules in the internetwork region that undergo rapid dynamic changes such as collapse (event 1), fragmentation (event 2), or submergence (event 3). In the photospheric images, these granules become significantly darker than neighboring granules. Following the granules' rapid changes, transient oscillations are detected in the photospheric and chromospheric layers. In the case of event 1, the dominant period of the oscillations is close to 4.2 min in the photosphere and 3.8 min in the chromosphere. Moreover, in the Ca II$-$0.5 $\unicode{xC5}$ raster image, we observe repetitive brightenings in the location of the rapidly changing granules that are considered the manifestation of shock waves. Based on our results, we suggest that dynamic changes of granules can generate upward-propagating acoustic waves in the quiet Sun that ultimately develop into shocks., Comment: 11 pages, 13 figures, Accepted for publication in A&A

Solar rotational tomography (SRT) is an important method to reconstruct the physical parameters of the three-dimensional solar corona. Here we propose an approach to apply the filtered backprojection (FBP) algorithm to the SRT. The FBP algorithm is generally not suitable for SRT due to the several issues with solar extreme ultraviolet (EUV) observations, in particular a problem caused by missing data because of the unobserved back side of corona hidden behind the Sun. We developed a method to generate a modified sinogram which resolves the blocking problem. The modified sinogram is generated by combining the EUV data at two opposite sites observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). We generated the modified sinogram for about one month in 2019 February and reconstructed the three-dimensional corona under the static state assumption. In order to obtain the physical parameters of the corona, we employed a DEM inversion method. We tested the performance of the FBP algorithm with the modified sinogram by comparing the reconstructed data with the observed EUV image, electron density models, previous studies of electron temperature, and an observed coronagraph image. The results illustrate that the FBP algorithm reasonably reconstructs the bright regions and the coronal holes, and can reproduce their physical parameters. The main advantage of the FBP algorithm is that it is easy to understand and computationally efficient. Thus, it enables us to easily probe the inhomogeneous coronal electron density and temperature distribution of the solar corona., Comment: Accepted for ApJ

The coronagraph is an instrument enables the investigation of faint features in the vicinity of the Sun, particularly coronal mass ejections (CMEs). So far coronagraphic observations have been mainly used to determine the geometric and kinematic parameters of CMEs. Here, we introduce a new method for the determination of CME temperature using a two filter (4025 A and 3934 A) coronagraph system. The thermal motion of free electrons in CMEs broadens the absorption lines in the optical spectra that are produced by the Thomson scattering of visible light originating in the photosphere, which affects the intensity ratio at two different wavelengths. Thus the CME temperature can be inferred from the intensity ratio measured by the two filter coronagraph system. We demonstrate the method by invoking the graduated cylindrical shell (GCS) model for the 3 dimensional CME density distribution and discuss its significance., 7 pages, 4 figures, published in J. of Korean Astronomical Society (JKAS)

Light bridges (LBs) are relatively bright structures that divide sunspot umbrae into two or more parts. Chromospheric LBs are known to be associated with various activities including fan-shaped jet-like ejections and brightenings. Although magnetic reconnection is frequently suggested to be responsible for such activities, not many studies present firm evidence to support the scenario. We carry out magnetic field measurements and imaging spectroscopy of a LB where fan-shaped jet-like ejections occur with co-spatial brightenings at their footpoints. We study LB fine structure and magnetic field changes using TiO images, Near-InfraRed Imaging Spectropolarimeter, and Halpha data taken by the 1.6~m Goode Solar Telescope. We detect magnetic flux emergence in the LB that is of opposite polarity to that of the sunspot. The new magnetic flux cancels with the pre-existing flux at a rate of 5.6x10^18 Mx/hr. Both the recurrent jet-like ejections and their base brightenings are initiated at the vicinity of the magnetic flux cancellation, and show apparent horizontal extension along the LB at a projected speed of up to 18.4km/s to form a fan-shaped appearance. Based on these observations, we suggest that the fan-shaped ejections may have resulted from slipping reconnection between the new flux emerging in the LB and the ambient sunspot field., Comment: 24pages, 11figures, accepted by the ApJ

The solar chromosphere can be observed well through strong absorption lines. We infer the physical parameters of chromospheric plasmas from these lines using a multilayer spectral inversion. This is a new technique of spectral inversion. We assume that the atmosphere consists of a finite number of layers. In each layer the absorption profile is constant and the source function varies with optical depth with a constant gradient. Specifically, we consider a three-layer model of radiative transfer where the lowest layer is identified with the photosphere and the two upper layers are identified with the chromosphere. The absorption profile in the photosphere is described by a Voigt function, and the profile in the chromosphere by a Gaussian function. This three-layer model is fully specified by 13 parameters. Four parameters can be fixed to prescribed values, and one parameter can be determined from the analysis of a satellite photospheric line. The remaining 8 parameters are determined from a constrained least-squares fitting. We applied the multilayer spectral inversion to the spectral data of the Hα and the Ca II 854.21 nm lines taken in a quiet region by the Fast Imaging Solar Spectrograph (FISS) of the Goode Solar Telescope (GST). We find that our model successfully fits most of the observed profiles and produces regular maps of the model parameters. The combination of the inferred Doppler widths of the two lines yields reasonable estimates of temperature and nonthermal speed in the chromosphere. We conclude that our multilayer inversion is useful to infer chromospheric plasma parameters on the Sun.

The umbral oscillations of velocity are commonly observed in the chromosphere of a sunspot. Their sources are considered to be either the external p-mode driving or the internal excitation by magnetoconvection. Even though the possibility of the p-mode driving has been often considered, the internal excitation has been rarely investigated. We report the identification of the oscillation patterns that may be closely related to the events of internal excitation from the observations of velocity oscillations in the temperature minimum region of two sunspots. The velocities were determined from the spectral data of the Fe I 5435 A line, a magnetically insensitive line, taken with the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. As a result, we identified 4 oscillation patterns of 2.0 * 10^3 km coherent size that were clearly identified for about 7.9 minutes with oscillation amplitude of 9.3 * 10^-2 km/s. The power of the oscillations in these centers was concentrated in the 3 minute band. All the oscillation centers were located above the umbral dots undergoing noticeable morphological and dynamical changes that may be regarded as an observable signature of small-scale magnetoconvection inside the umbrae. Our results support the notion that magnetoconvection associated with umbral dots inside sunspots can drive the 3 minute umbral oscillations., Comment: Accepted for publication in the Astrophysical Journal

Recently, spiral wave patterns (SWPs) have been detected in 3 minute oscillations of sunspot umbrae, but the nature of this phenomenon has remained elusive. We present a theoretical model that interprets the observed SWPs as the superposition of two different azimuthal modes of slow magnetoacoustic waves driven below the surface in an untwisted and non-rotating magnetic cylinder. We apply this model to SWPs of the line-of-sight (LOS) velocity in a pore observed by the Fast Imaging Solar Spectrograph installed at the 1.6 m Goode Solar Telescope. One- and two-armed SWPs were identified in instantaneous amplitudes of LOS Doppler velocity maps of 3 minute oscillations. The associated oscillation periods are about 160 s, and the durations are about 5 minutes. In our theoretical model, the observed spiral structures are explained by the superposition of non-zero azimuthal modes driven 1600 km below the photosphere in the pore. The one-armed SWP is produced by the slow-body sausage (m = 0) and kink (m = 1) modes, and the two-armed SWP is formed by the slow-body sausage (m = 0) and fluting (m = 2) modes of the magnetic flux tube forming the pore., Comment: 10 pages, 5 figures, Published for publication in The Astrophysical Journal Letters

The noise embedded in the raw data in one scientific discipline has sometimes been proven to be a valuable signal for another discipline, and there are examples throughout science history. The solar images taken by the solid state detectors (CCDs) of the Sun monitoring satellites are usually cleaned by removing the traces of cosmic rays on the raw CCD data files. Thus, while applying the method of removing the cosmic ray traces, we may be able to estimate the cosmic ray flux by counting the number of traces. The net cosmic ray flux is the sum of galactic cosmic rays and solar-originating particles. The latter are seen as highly transient flux changes related to solar eruptions. We can identify this kind of “cosmic ray” event from the association with phenomena revealed in processed solar images, and we show this using the data of SOHO/extreme ultraviolet imaging telescope (EIT). On the other hand, the estimated cosmic ray flux in the steady state is anticorrelated with solar cycle sunspot number. The profiles of estimated solar cosmic ray flux showing significant increase are found to be strongly correlated with the ground neutron monitor ground level enhancements. Additionally, the profile of estimated cosmic ray flux is consistent with that of the GOES P6 channel. It indicates that the particles with energy higher than 80 MeV may mainly produce the tracks on CCD of EIT. In conclusion, the raw solar images are valuable data for estimating both long-term cosmic ray variations and transient solar particles events.

We study chromospheric oscillations including umbral flashes and running penumbral waves in a sunspot of active region NOAA 11242 using scanning spectroscopy in Hα and Ca ii 8542 A with the Fast Imaging Solar Spectrograph (FISS) at the 1.6 meter New Solar Telescope at the Big Bear Solar Observatory. A bisector method is applied to spectral observations to construct chromospheric Doppler-velocity maps. Temporal-sequence analysis of these shows enhanced high-frequency oscillations inside the sunspot umbra in both lines. Their peak frequency gradually decreases outward from the umbra. The oscillation power is found to be associated with magnetic-field strength and inclination, with different relationships in different frequency bands.

We observed solar prominences with the Fast Imaging Solar Spectrograph (FISS) at the Big Bear Solar Observatory on 30 June 2010 and 15 August 2011. To determine the temperature of the prominence material, we applied a nonlinear least-squares fitting of the radiative transfer model. From the Doppler broadening of the Hα and Ca ii lines, we determined the temperature and nonthermal velocity separately. The ranges of temperature and nonthermal velocity were 4000 – 20 000 K and 4 – 11 km s−1. We also found that the temperature varied much from point to point within one prominence.

We report a different type of three-minute chromospheric oscillations above a sunspot in association with a small-scale impulsive event in a light bridge. During our observations, we found a transient brightening in the light bridge. The brightening was composed of elementary bursts that may be a manifestation of fast repetitive magnetic reconnections in the light bridge. Interestingly, the oscillations in the nearby sunspot umbra were impulsively excited when the intensity of the brightening reached its peak. The initial period of the oscillations was about 2.3 minutes and then gradually increased to 3.0 minutes with time. In addition, we found that the amplitude of the excited oscillations was twice the amplitude of oscillations before the brightening. Based on our results, we propose that magnetic reconnection occurring in a light bridge can excite of oscillations in the nearby sunspot umbra., Comment: 13 pages, 4 figures, accepted for publication in ApJL

For high resolution spectral observations of the Sun – particularly its chromosphere, we have developed a dual-band echelle spectrograph named Fast Imaging Solar Spectrograph (FISS), and installed it in a vertical optical table in the Coude Lab of the 1.6 meter New Solar Telescope at Big Bear Solar Observatory. This instrument can cover any part of the visible and near-infrared spectrum, but it usually records the Hα band and the Ca ii 8542 A band simultaneously using two CCD cameras, producing data well suited for the study of the structure and dynamics of the chromosphere and filaments/prominences. The instrument does imaging of high quality using a fast scan of the slit across the field of view with the aid of adaptive optics. We describe its design, specifics, and performance as well as data processing

We analyzed chromospheric events and their connection to oscillation phenomena and photospheric dynamics. The observations were done with the New Solar Telescope of Big Bear Solar Observatory using a broad-band imager at the wavelength of a TiO band and FISS spectrograph scanning Ca ii and Hα spectral lines. The event in Ca ii showed strong plasma flows and propagating waves in the chromosphere. The movement of the footpoints of flux tubes in the photosphere indicated flux tube entanglement and magnetic reconnection as a possible cause of the observed brightening and waves propagating in the chromosphere. An upward propagating train of waves was observed at the site of the downflow event in Hα. There was no clear relationship between photospheric waves and the Ca ii and Hα events. Our observations indicate that chromospheric waves that were previously thought to originate from the photosphere may be generated by some events in the chromosphere as well.

We analyzed transient Ca ii H brightening associated with small-scale canceling magnetic features (CMFs) in the quiet Sun near disk center using Ca ii H filter images and Na D1 magnetograms of Hinode/SOT. We found that in most Ca ii brightening events related to CMFs the Ca ii H intensity peaks after the magnetic flux cancellation. Moreover, the brightening tends to appear as a pair of bright points of similar size and brightness overlying a magnetic bipole. Then a new opposite polarity fragment moves to them and cancels out. These results imply that magnetic reconnection takes place there and is responsible for CMFs.

This paper presents a generic method to automatically segment and characterize solar filaments from various Hα full-disk solar images, obtained by different solar observatories, with different dynamic ranges and statistical properties. First, a cascading Hough circle detector is designed to find the center location and radius of the solar disks. Second, polynomial surface fitting is adopted to correct unbalanced luminance. Third, an adaptive thresholding method is put forward to segment solar filaments. Finally, for each piece of a solar filament, its centroid location, area, and length are characterized, in which morphological thinning and graph theory are used for identifying the main skeletons of filaments. To test the performance of the proposed methods, a dataset composed of 125 Hα images is considered. These images were obtained by four solar observatories from January 2000 to May 2010, one image per month. Experimental results show that the accuracy rate is above 95% as measured by filament number and above 99% as measured by filament area, indicating that only a few tiny filaments are not detected.

The photospheric response to solar flares, also known as coronal back reaction, is often observed as sudden flare-induced changes in vector magnetic field and sunspot motions. However, it remains obscure whether evolving flare ribbons, the flare signature closest to the photosphere, are accompanied by changes in vector magnetic field therein. Here we explore the relationship between the dynamics of flare ribbons in the chromosphere and variations of magnetic fields in the underlying photosphere, using high-resolution off-band H-alpha images and near-infrared vector magnetograms of the M6.5 flare on 2015 June 22 observed with the 1.6 m Goode Solar Telescope. We find that changes of photospheric fields occur at the arrival of the flare ribbon front, thus propagating analogously to flare ribbons. In general, the horizontal field increases and the field lines become more inclined to the surface. When ribbons sweep through regions that undergo a rotational motion, the fields transiently turn more vertical with decreased horizontal field and inclination angle, and then restore and/or become more horizontal than before the ribbon arrival. The ribbon propagation decelerates near the sunspot rotation center, where the vertical field becomes permanently enhanced. Similar magnetic field changes are discernible in magnetograms from the Helioseismic and Magnetic Imager (HMI), and an inward collapse of coronal magnetic fields is inferred from the time sequence of non-linear force-free field models extrapolated from HMI magnetograms. We conclude that photospheric fields respond nearly instantaneously to magnetic reconnection in the corona., 15 pages, 9 figures, accepted to ApJ

The study of pores, small penumbraless sunspots, can give us a chance to understand how strong magnetic fields interact with convective motions in the photosphere. For a better understanding of this interaction, we investigate the temporal variation of several tiny pores smaller than 2''. These pores were observed by the Solar Optical Telescope on board Hinode on 2006 December 29. We have analyzed the high-resolution spectropolarimetric (SP) data and the G-band filtergrams taken during the observation. Magnetic flux density and Doppler velocities of the pores are estimated by applying the center-of-gravity method to the SP data. The horizontal motions in and around the pores are tracked by adopting the nonlinear affine velocity estimator method to the G-band filter images. As a result, we found the following. (1) The darkness of the pores is positively correlated with the magnetic flux density. (2) Downflows always exist inside and around the pores. (3) The speed of downflows inside the pores is negatively correlated with their darkness. (4) The pores are surrounded by strong downflows. (5) Brightness changes of the pores are correlated with the divergence of mass flow (correlation coefficient >0.9). (6) The pores in the growing phase are associated with the converging flow pattern and the pores in the decay phase with the diverging flow pattern. Our results support the idea that a pore grows as the magnetic flux density increases due to the convergence of ambient mass flow and it decays with the decrease of the flux density due to the diverging mass flow.

With the ever increasing influx of high resolution images of the solar surface obtained at a multitude of wavelengths, various processes occurring at small spatial scales have become a greater focus of our attention. Complex small-scale magnetic fields have been reported that appear to have enough stored to heat the chromosphere. While significant progress has been made in understanding small-scale phenomena, many specifics remain elusive. We present here a detailed study of a single event of disappearance of a magnetic dipole and associated chromospheric activity. Based on New Solar Telescope H$\alpha$ data and {\it Hinode} photospheric line-of-sight magnetograms and Ca II H images we report the following. 1) Our analysis indicates that even very small dipoles (elements separated by about 0\arcsec.5 or less) may reach the chromosphere and trigger non-negligible chromospheric activity. 2) Careful consideration of the magnetic environment where the new flux is deposited may shed light on the details of magnetic flux removal from the solar surface. We argue that the apparent collision and disappearance of two opposite polarity elements may not necessarily indicate their cancellation (i.e., reconnection, emergence of a "U" tube or submergence of $ \Omega $ loops). In our case, the magnetic dipole disappeared by reconnecting with overlying large-scale inclined plage fields. 3) Bright points seen in off-band H$\alpha$ images are very well-correlated with the Ca II H bright points, which in turn are co-spatial with G-band bright points. We further speculate that, in general, H$\alpha$ bright points are expected be co-spatial with photospheric BPs, however, a direct comparison is needed to refine their relationship.

We investigate how plasma structures in the solar chromosphere and corona can extend to altitudes much above hydrostatic scale heights from the solar surface even under the force of gravity. Using a simple modified form of equation of motion in the vertical direction, we argue that there are two extreme ways of non-hydrostatic support: dynamical support and magnetic support. If the vertical acceleration is downward and its magnitude is a significant fraction of gravitational acceleration, non-hydrostatic support is dynamical in nature. Otherwise non-hydrostatic support is static, and magnetic support by horizontal magnetic fields is the only other possibility. We describe what kind of observations are needed in the clarification of the nature of non-hydrostatic support. Observations available so far seem to indicate that spicules in the quiet regions and dynamic fibrils in active regions are dynamically supported whereas the general chromosphere as well as prominences is magnetically supported. Moreover, it appears that magnetic support is required for plasma in some coronal loops as well. We suspect that the identification of a coronal loop with a simple magnetic flux tube might be wrong in this regard.

Many solar prominences end their lives in eruptions or abrupt disappearances that are associated with dynamical or thermal instabilities. Such instabilities are important because they may be responsible for energy transport and conversion. We present a clear observation of a streaming kink-mode Kelvin-Helmholtz Instability (KHI) taking place in a solar prominence using the H alpha Lyot filter installed at the New Vacuum Solar Telescope, Fuxian-lake Solar Observatory in Yunnan, China. On one side of the prominence, a series of plasma blobs floated up from the chromosphere and streamed parallel to the limb. The plasma stream was accelerated to about 20-60 km s(-1) and then undulated. We found that 2 ''- and 5 ''-size vortices formed, floated along the stream, and then broke up. After the 5 ''-size vortex, a plasma ejection out of the stream was detected in the Solar Dynamics Observatory/Atmospheric Imaging Assembly images. Just before the formation of the 5 ''-size vortex, the stream displayed an oscillatory transverse motion with a period of 255 s with the amplitude growing at the rate of 0.001 s(-1). We attribute this oscillation of the stream and the subsequent formation of the vortex to the KHI triggered by velocity shear between the stream, guided by the magnetic field and the surrounding media. The plasma ejection suggests the transport of prominence material into the upper layer by the KHI in its nonlinear stage.

The existence and behavior of vertical fine structures of plasma—threads and knots—are a significant observational clue to understanding the magnetic structure and dynamics of quiescent prominences on the quiet Sun. Based on the equation of motion in ideal MHD, we reason that the non-hydrostatic support of plasma against gravity in general requires either the motion of plasma with a high value of downward acceleration (dynamical support) or the role of horizontal magnetic fields (magnetic support). By carefully tracking the motion of several bright threads seen in a hedgerow prominence observed by the Solar Optical Telescope aboard Hinode, we confirm that these threads are essentially static and stable, which negates the dynamic support. The application of the Kippenhahn-Schluter solution suggests that they may be supported by sagged magnetic field lines with a sag angle of about 43°. We also track several bright descending knots and find that their descending speeds range from 10 to 30 km s–1, with a mean value of 16 km s–1, and their vertical accelerations from –0.10 to 0.10 km s–2, with a mean of practically zero. This finding suggests that these knots are basically supported by horizontal magnetic fields against gravity even when they descend, and the complex variations of their descending speeds should be attributed to small imbalances between gravity and the force of magnetic tension. Furthermore, some knots are observed to impulsively get accelerated downward from time to time. We conjecture that these impulsive accelerations are a result of magnetic reconnection and the subsequent interchange of magnetic configuration between a knot and its surrounding structure. It is proposed that this process of reconnection and interchange not only initiates the descending motion of the knots, but also allows knots to keep falling long distance through the medium permeated by horizontal magnetic fields.

Highest resolution observations made with the new 1.6 m aperture solar telescope in Big Bear Solar Observatory during this time of historic inactivity on the Sun reveal new insights into the small-scale dynamics of the Sun's photosphere. The telescope's unprecedented resolution enabled us to observe that the smallest scale photospheric magnetic field seems to come in isolated points in the dark intergranular lanes, rather than the predicted continuous sheets confined to the lanes, and the unexpected longevity of the bright points implies a deeper anchoring than predicted. Further, we demonstrated for the first time that the photospheric plasma motion and magnetic fields are in equipartition over a wide dynamic range, and both cascade energy to ever-smaller scales according to classical Kolmogorov turbulence theory. Finally, we discovered tiny jet-like features originating in the dark lanes that surround the ubiquitous granules that characterize the solar surface.

Magnetic reconnection is a process in which field-line connectivity changes in a magnetized plasma. On the solar surface, it often occurs with the cancellation of two magnetic fragments of opposite polarity. Using the 1.6 m New Solar Telescope, we observed the morphology and dynamics of plasma visible in the H? line, which is associated with a canceling magnetic feature (CMF) in the quiet Sun. The region can be divided into four magnetic domains: two pre-reconnection and two post-reconnection. In one post-reconnection domain, a small cloud erupted, with a plane-of-sky speed of 10 km s?1, while in the other one, brightening began at points and then tiny bright loops appeared and subsequently shrank. These features support the notion that magnetic reconnection taking place in the chromosphere is responsible for CMFs.

Recent theoretical studies suggest that the nonlinearity of three-minute velocity oscillations at each atmospheric level can be quantified by the two independent parameters—the steepening parameter and the velocity amplitude parameter. For the first time, we measured these two parameters at different atmospheric levels by analyzing a set of spectral lines formed at different heights of sunspots ranging from the temperature minimum to the transition region. The spectral data were taken by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope, and by the Interface Region Imaging Spectrograph. As a result, from the wavelet power spectra of the velocity oscillations at different heights, we clearly identified the growth of the second harmonic oscillations associated with the steepening of the velocity oscillation, indicating that higher-frequency oscillations of periods of 1.2 to 1.5 minutes originate from the nonlinearity of the three-minute oscillations in the upper chromosphere. We also found that the variation of the measured nonlinearity parameters is consistent with the theoretical expectation that the nonlinearity of the three-minute oscillations increases with height, and shock waves form in the upper chromosphere. There are, however, discrepancies as well between theory and observations, suggesting the need to improve both theory and the measurement technique.

Photospheric magnetic flux cancellation on the Sun is generally believed to be caused by magnetic reconnection occurring in the low solar atmosphere. Individual canceling magnetic features are observationally characterized by the rate of flux cancellation. The specific cancellation rate, defined as the rate of flux cancellation divided by the interface length, gives an accurate estimate of the electric field in the reconnecting current sheet. We have determined the specific cancellation rate using the magnetograms taken by the Solar Optical Telescope (SOT) aboard the Hinode satellite. The specific rates determined with SOT turned out to be systematically higher than those based on the data taken by the Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory. The median value of the specific cancellation rate was found to be 8 × 106 G cm s-1—a value four times that obtained from the MDI data. This big difference is mainly due to a higher angular resolution and better sensitivity of the SOT, resulting in magnetic fluxes up to five times larger than those obtained from the MDI. The higher rates of flux cancellation correspond to either faster inflows or stronger magnetic fields of the reconnection inflow region, which may have important consequences for the physics of photospheric magnetic reconnection.

A necessary condition for the formation of a filament is magnetic helicity. In the present paper we seek the origin of magnetic helicity of intermediate filaments. We observed the formation of a sinistral filament at the boundary of a decaying active region using full-disk H images obtained from Big Bear Solar Observatory. We have measured the rate of helicity injection during the formation of the filament using full-disk 96 minute-cadence magnetograms taken by SOHO MDI. As a result we found that 1) no significant helicity was injected around the region (polarity inversion line; PIL) of filament formation and 2) negative helicity was injected in the decaying active region. The negative sign of the injected helicity was opposite to that of the filament helicity. On the other hand, at earlier times when the associated active region emerged and grew, positive helicity was intensively injected. Our results suggest that the magnetic helicity of the intermediate filament may have originated from the helicity accumulated during the period of the growth of its associated active region.

Aims. Predicted and observed signatures of magnetic reconnection in the solar chromosphere are discussed. Methods. Compressible reconnection scalings for the Sweet-Parker current sheet in the chromosphere are derived, which generalize earlier treatments by including both the Lorentz force and the pressure gradient in the reconnection outflow as possible drivers of reconnection jets. The theoretical scalings were tested against the measured speed, density, and temperature of an extreme-ultraviolet jet, observed with TRACE. Model predictions for the reconnection jet parameters as functions of height above the solar photosphere were computed. Results. The predicted variation of reconnection jet parameters with height appears to correspond to the observed wide range of parameters of chromospheric jet-like features.

There has been some controversy as to whether the magnetic fields of vertical threads seen in quiescent prominences are predominantly vertical or horizontal. We report finding special patterns of flow in a quiescent prominence observed by the Solar Optical Telescope aboard Hinode. This prominence is a small hedgerow prominence composed of many vertical threads. To one side of it, we found a pattern of persistent horizontal flows of Hα-emitting plasma. These flows originated from a region in the chromosphere, rose to coronal heights, and then extended horizontally for a long distance until they reached the main body of the prominence. In the higher altitudes the flows either moved across vertical threads or lifted them up, while in the lower altitudes they often formed bright blobs of plasma and shed them, resulting in a sudden change of flow direction from horizontal to vertical. The observed persistent horizontal flows support a configuration of initially horizontal magnetic fields, and our results appear to be consistent with the traditional theory that vertical threads in quiescent prominences are stacks of plasma supported against gravity by the sagging of initially horizontal magnetic field lines.

The magnetic diffusivity in the solar photosphere is determined by applying a new method to the magnetic induction equation. The magnetic field evolution is specified by a time sequence of high-resolution magnetograms of plage regions, taken by Hinode/SOT and SOHO/MDI. The mean value of magnetic diffusivity determined from SOT magnetograms with the smallest pixel size of 116 km is about -->0.87 ? 0.08 km2 s?1. This is the smallest value that has been empirically determined so far. High-resolution and full-disk MDI magnetograms with the pixel sizes of 440 and 1400 km yielded larger values of -->4.4 ? 0.4 and -->18 ? 7.4 km2 s?1, respectively. The measured diffusivity values at different length scales are consistent with a turbulent cascade that ends at a resistive dissipation scale of about 30 km. The results suggest that turbulent magnetic diffusivity should be taken into account in the analysis of the observed rate of magnetic flux cancellation in the photosphere.

The measurements of magnetic twist of EUV coronal loops for 14 loops are presented here. EUV coronal loops are thin, EUV-emitting structures of hot plasma tracing magnetic field lines in the corona. The constriction of plasma into a loop without dispersion may be explained if the magnetic field of the loop is twisted. On the basis of this idea, Chae and Moon developed a method of determining magnetic twist of coronal loops by analyzing coronal images and photospheric magnetograms together. By applying this method to as many coronal loops observed by TRACE 171 A as possible, we attempt to determine a statistically meaningful value of magnetic twist of coronal loops for the first time. We have selected a number of conspicuous loops which are bright enough and well separated from other adjacent loops on TRACE EUV images. We have constructed and examined coronal magnetic fields of selected active regions containing the loops with consideration of the projection effect, and we can identify 14 coronal loops whose magnetic field lines are well represented by a linear force-free field. We have found that these loops have absolute twist values from 0.22π to 1.73π, which suggests that the absolute winding number of EUV coronal loop may be mostly less than one turn. Our results support the idea that EUV coronal loops may be a product of magnetic reconnection of magnetic flux tubes.

Solar flares are sudden flashes of brightness on the Sun and are often associated with coronal mass ejections and solar energetic particles which have adverse effects in the near Earth environment. By definition, flares are usually referred to bright features resulting from excess emission. Using the newly commissioned 1.6~m New Solar Telescope at Big Bear Solar Observatory, here we show a striking "negative" flare with a narrow, but unambiguous "dark" moving front observed in He I 10830 \AA, which is as narrow as 340 km and is associated with distinct spectral characteristics in H-alpha and Mg II lines. Theoretically, such negative contrast in He I 10830 \AA\ can be produced under special circumstances, by nonthermal-electron collisions, or photoionization followed by recombination. Our discovery, made possible due to unprecedented spatial resolution, confirms the presence of the required plasma conditions and provides unique information in understanding the energy release and radiative transfer in astronomical objects.

Magnetic helicity is now regarded as an important physical quantity in understanding solar magnetic activities. We have studied the injection of magnetic helicity through the photosphere of four active regions during the long periods of their lifetimes. The rate of helicity injection was determined as a function of time in each active region by applying the local correlation tracking (LCT) method to full-disk, 96 minute cadence magnetograms taken by SOHO MDI. As a result, we have found that most helicity was intensively injected while active regions emerged and grew, suggesting that magnetic flux emergence may be the major process of helicity injection. In all the active regions studied, the rate of helicity injection during flux emergence was initially low, increased and stayed high for a while, and then became low again, while magnetic flux steadily increased at a more or less constant rate all the time. Flux cancellation, as a minor process, resulted in some loss of coronal magnetic helicity, or inverse helicity injection. The contribution of differential rotation to helicity injection was found to be insignificant in these active regions. It was also found that the magnetic helicity budget of an active region strongly depends on its average magnetic flux.

A massive two-ribbon flare and its source magnetic field region were well captured by the Solar Optical Telescope (SOT) on board Hinode in the Ca II H spectral line and by the Spectro-Polarimeter of SOT, respectively. Using the high-resolution Hinode data sets, we compare the spatial distribution of the local magnetic reconnection rate and the energy release rate along the ribbons with that of G-band kernels that serve as a proxy for the primary energy release. The G-band kernels spatially coincide with the maximum of both modeled quantities, which gives strong support for the reconnection model. We also investigate the magnitude scaling correlation between the ribbon separation speed Vr and magnetic field strength Bn at four 2 minute time bins around the maximum phase of the flare. It is found that Vr is weakly and negatively correlated with Bn. An empirical relation of Vr ∝ Bn−0.15 is obtained at the flare peak time with an correlation coefficient ~–0.33. The correlation is weaker at other time bins.

Magnetic reconnection in the temperature minimum region of the solar photosphere, observationally manifested as canceling magnetic features, is considered. Flux pile-up reconnection in the Sweet-Parker current sheet is analyzed. It is shown that the standard Sweet-Parker reconnection rate in the photosphere is too slow to explain the observed cancellation. Flux pile-up reconnection scalings, however, are shown to be in agreement with the speeds of canceling magnetic fragments, magnetic fields in the fragments, and the rates of magnetic flux cancellation, derived from SOHO MDI data. Pile-up factors in the range between 1 and 5 and local reconnecting magnetic fields of a few hundred G are calculated for the analyzed canceling features. The analysis shows that flux pile-up is a likely mechanism for adjusting the local parameters of reconnecting current sheets in the photosphere and for sustaining the reconnection rates that are determined by large-scale supergranular flows. The upward mass flux in the reconnection jet, associated with a large canceling feature, is predicted to exceed 1015 g hr-1. Hence, cancellation in a few photospheric patches over several hours can lead to the formation of a solar filament in the corona.

Magnetic helicity is a useful quantity in characterizing the magnetic systems of solar active regions. The purpose of the present work is to check for consistency between the local correlation tracking (LCT) method used to measure helicity injection through the photosphere, and the linear force-free field (LFFF) method used to determine helicity in the corona, based on the principle of helicity conservation in the solar corona. We have calculated the amount of magnetic helicity injected through the photosphere during the first disk passage of AR 10696 using the LCT method initially described by Chae. We have also measured the coronal magnetic helicity as a function of time using the LFFF method. With a value for the force-free α, the coronal field is constructed from the extrapolation of the Solar and Heliospheric Observatory (SOHO) MDI magnetograms, then compared with the coronal loops in the EUV images taken by the SOHO EIT. The force-free α that best fits the loops is used to calculate the coronal helicity. From a careful comparison of different helicity measurements during each time interval, we have reached the core conclusion that our measurements follow the helicity conservation principle with an uncertainty of ~15% and hence support the consistency between the two different methods with the same amount of uncertainty.

There is a controversy about how features protruding laterally from filaments, called barbs, are magnetically structured. On 2004 August 3, we observed a filament that had well-developed barbs. The observations were performed using the 10 inch refractor of the Big Bear Solar Observatory. A fast camera was employed to capture images at five different wavelengths of the Hα line and successively record them on the basis of frame selection. The terminating points of the barbs were clearly discernable in the Hα images without any ambiguity. The comparison of the Hα images with the magnetograms taken by SOHO MDI revealed that the termination occurred above the minor polarity inversion line dividing the magnetic elements of the major polarity and those of the minor polarity. There is also evidence that the flux cancellation proceeded on the polarity inversion line. Our results together with similar other recent observations support the idea that filament barbs are cool matter suspended in local dips of magnetic field lines, formed by magnetic reconnection in the chromosphere.

【In this paper, we present preliminary feasibility studies on three types of solar observation payloads for future Korean Science and Technology Satellite (STSAT) programs. The three candidates are (1) an UV imaging telescope, (2) an UV spectrograph, and (3) an X-ray spectrometer. In the case of UV imaging telescope, the most important constraint seems to be the control stability of a satellite in order to obtain a reasonably good spatial resolution. Considering that the current pointing stability estimated from the data of the Far ultraviolet Imaging Spectrograph (FIMS) onboard the Korean STSAT-1, is around 1 arc minutes/sec, we think that it is hard to obtain a spatial resolution sufficient for scientific research by such an UV Imaging Telescope. For solar imaging missions, we realize that an image stabilization system, which is composed of a small guide telescope with limb sensor and a servo controller of secondary mirror, is quite essential for a very good pointing stability of about 0.1 arcsec. An UV spectrograph covering the solar full disk seems to be a good choice in that there is no risk due to poor pointing stability as well as that it can provide us with valuable UV spectral irradiance data valuable for studying their effects on the Earth's atmosphere and satellites. The heritage of the FIMS can be a great advantage of developing the UV spectrograph. Its main disadvantage is that two major missions are in operation or scheduled. Our preliminary investigations show that an X-ray spectrometer for the full disk Sun seems to be the best choice among the three candidates. The reasons are : (1) high temporal and spectral X-ray data are very essential for studying the acceleration process of energetic particles associated with solar flares, (2) we have a good heritage of X-ray detectors including a rocket-borne X-ray detector, (3) in the case of developing countries such as India and Czech, solar X-ray spectrometers were selected as their early stage satellite missions due to their poor pointing stabilities, and (4) there is no planned major mission after currently operating Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) mission. Finally, we present a preliminary design of a solar X-ray spectrometer covering soft X-ray (2 keV) to gamma ray (10 MeV).】

I present a method to correct solar magnetograms for a non-uniform detector pattern that depends on the sense of light polarization. The application to Big Bear Solar Observatory magnetograph observations demonstrates that the determination of a flat pattern can be done at an accuracy imposed by the photon noise in the raw magnetograms.

In the present study, we have investigated morphology and evolution of small-scale Ha dynamic features on the quiet sun by analyzing video magnetograms and high resolution Ha images simultaneously taken for 5 hours at Big Bear Solar Observatory on April 18, 1997. From comparisons between time sequential longitudinal magnetograms and H images covering , several small-scale H dynamic features have been observed at a site of magnetic flux cancellation. A close relationship between such features and cancelling magnetic fluxes has been revealed temporarily and spatially. Our results support that material injection by chromospheric magnetic reconnect ion may be essential in supporting numerous small-scale H dynamical absorption features, being in line with recent observational studies showing that material injection by chromospheric magnetic reconnect ion is essential for the formation of solar filaments.

Observations have indicated that magnetic reconnect ion may occur frequently in the photosphere and chromosphere as well as in the solar corona. The observed features include cancelling magnetic features seen in photospheric magnetograms, and different kinds of small-scale activities such as UV explosive events and EUV jets. By integrating the observed parameters of these features with the Sweet-Parker reconnect ion theory, an attempt is made to clarify the nature of chromospheric magnetic reconnection. Our results suggest that magnetic reconnect ion may be occurring at many different levels of the photosphere and chromosphere without a preferred height and at a faster speed than is predicted by the Sweet-Parker reconnect ion model using the classical value of electric conductivity. Introducing an anomalous magnetic diffusivity 10-100 times the classical value is one of the possible ways of explaining the fast reconnect ion as inferred from observations.

Recent observations have shown that coronal magnetic fields in the northern (southern) hemisphere tend to have negative (positive) magnetic helicity. There has been controversy as to whether this hemispheric pattern is of surface or sub-surface origin. A number of studies have focused on clarifying the effect of the surface differential rotation on the change of magnetic helicity in the corona. Meanwhile, recent observational studies reported the existence of transient shear flows in active regions that can feed magnetic helicity to the corona at a much higher rate than the differential rotation does. Here we propose that such transient shear flows may be driven by the torque produced by either the axial or radial expansion of the coronal segment of a twisted flux tube that is rooted deeply below the surface. We have derived a simple relation between the coronal expansion parameter and the amount of helicity transferred via shear flows. To demonstrate our proposition, we have inspected Yohkoh soft X-ray images of NOAA 8668 in which strong shear flows were observed. As a result, we found that the expansion of magnetic fields really took place in the corona while transient shear flows were observed in the photosphere, and the amount of magnetic helicity change due to the transient shear flows is quantitatively consistent with the observed expansion of coronal magnetic fields. The transient shear flows hence may be understood as an observable manifestation of the pumping of magnetic helicity out of the interior portions of the field lines driven by the expansion of coronal parts as was originally proposed by Parker (1974).

Our previous study has shown that the formation of a reverse S-shaped filament in NOAA active region 8668 was closely associated with a large canceling magnetic feature. In the present paper we investigate the response of the upper atmosphere at the region of this canceling magnetic feature. The UV and EUV data taken by the Transition Region and Coronal Explorer (TRACE) reveal that a series of jets and small eruptions took place there during the formation of the prominence. Plasma in each jet originated from a single site of flux cancellation and moved in opposite directions at a transverse speed of 80-250 km s-1 across the plane of the sky. Plasma eruptions showing complex morphology and dynamics started from two or more sites of flux cancellation and appear to have the same physical origin as the jets. The two filter ratio technique indicates that the EUV-emitting plasma in the jets and eruptions have transition-region temperatures of (2-3) × 105 K. It is also found from emission-measure analysis that the electron density is (0.7-1.9) × 1010 cm-3 and that each jet carries plasma mass of (1.7-4.6) × 1013 g and each eruption carries additional mass of (9-25) × 1013 g. Our results are consistent with the current pictures that (1) flux cancellation observed in the photosphere is a consequence of magnetic reconnection occurring in the chromosphere and (2) that a series of such magnetic reconnection events is able to supply the mass necessary for the formation of a solar prominence.

Fast Imaging Solar Spectrograph of the 1.6 Meter New Solar Telescope at Big Bear Solar Observatory.- FISS Observations of Vertical Motion of Plasma in Tiny Pores.- Velocities and Temperatures of an Ellerman Bomb and Its Associated Features.- Connection Between Chromospheric Events and Photospheric Dynamics.- Chromospheric Sunspot Oscillations in Halpha and Ca II 8542 A.- Doppler Shifts of the Halpha Line and the Ca II 854.2 nm Line in a Quiet Region of the Sun Observed with the FISS/NST.- Temperature of Solar Prominences Obtained with the Fast Imaging Solar Spectrograph on the 1.6 m New Solar Telescope at the Big Bear Solar Observatory.- Active Region Coronal Rain Event Observed by the Fast Imaging Solar Spectrograph on the NST.

Several recent papers have presented new observational results indicating that many coronal loops in active regions are nearly isothermal. It is expected that quiet-Sun loops may have similar thermal structures, since quiet-Sun differential emission measures look similar to those in active regions. In the quiet Sun, it is well known from observations that the nonthermal velocity inferred from the excess broadening of a line over thermal broadening reaches a peak of about 30 km s-1 around 3 × 105 K and then decreases with temperature, having a value of about 20 km s-1 at 1 × 106 K. In the present work, we make the assumption that the observed nonthermal velocities are a manifestation of magnetohydrodynamic (MHD) turbulence and present a model of static, isobaric coronal loops heated by turbulence. Instead of solving the MHD equations, we adopt simple energy spectra in MHD turbulence and infer the heating rate as a function of temperature from the observed nonthermal velocities. By solving the steady state energy equation of a loop in which temperature monotonically increases with height, we obtain the following results: (1) The heating rate is predominantly near the footpoints and decreases with the loop arc length. (2) There is a critical temperature above which the loop cannot be maintained in a steady state. (3) The loop is denser and is more isothermal than uniformly heated loops, being compatible with recent observations. (4) The theoretical differential emission measures are in good agreement with the empirical values at temperatures above 105 K. Below this temperature, we still have a large discrepancy. (5) It is possible to explain the observed strong correlation between intensity and nonthermal velocity of a spectral line in the quiet Sun. Our results support the idea that quasi-statically driven MHD turbulence of the direct current (DC) type in the stratified medium (transition region and corona) is a viable mechanism for coronal heating.

In this paper, we investigate impulsive variations of the magnetic helicity change rate associated with eruptive solar flares (three X class flares and one M class flare) accompanying halo coronal mass ejections. By analyzing four sets of 1 minute cadence full-disk magnetograms taken by the Michelson Doppler Imager on board the Solar and Heliospheric Observatory, we have determined the rates of magnetic helicity transport due to horizontal photospheric motions. We have found that magnetic helicity of the order of 1041 Mx2 was impulsively injected into the corona around the flaring peak time of all the flares. We also found that there is a positive correlation between the impulsively injected magnetic helicity and the X-ray peak flux of the associated flare. The impulsive helicity variations are attributed to horizontal velocity kernels localized near the polarity inversion lines. Finally, we report that there is a close spatial proximity between the horizontal velocity kernels and Hα bright points.

We present observational evidence that the occurrence of homologous flares in an active region is physically related to the injection of magnetic helicity by horizontal photospheric motions. We have analyzed a set of 1 minute cadence magnetograms of NOAA AR 8100 taken over a period of 6.5 hours by Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO). During this observing time span, seven homologous flares took place in the active region. We have computed the magnetic helicity injection rate into the solar atmosphere by photospheric shearing motions, and found that a signicant amount of magnetic helicity was injected during the observing period. In a strong M4.1 flare, the magnetic helicity injection rate impulsively increased and peaked at the same time as the X-ray flux did. The flare X-ray flux integrated over the Xray emission time strongly correlates with the magnetic helicity injected during the flaring interval. The integrated X-ray flux is found to be a logarithmically increasing function of the injected magnetic helicity. Our results suggest that injection of helicity and abrupt increase of helicity magnitude play a signicant role in flare triggering.

Solar observations support that magnetic reconnect ion ubiquitously occurs in the chromosphere as well as in the corona. It is now widely accepted that coronal magnetic reconnect ion is fast reconnect ion of the Petschek type, and is the main driver of solar flares. On the other hand, it has been thought that the traditional Sweet-Parker model may describe chromospheric reconnect ion without difficulty, since the electric conductivity in the chromoshphere is much lower than that in the corona. However, recent observations of cancelling magnetic features have suggested that chromospheric reconnect ion might proceed at a faster rate than the Sweet-Parker model predicts. We have applied the Sweet-Parker model and Petschek model to a well-observed cancelling magnetic feature. As a result, we found that the inflow speed of the Sweet-Parker reconnect ion is too small to explain the observed converging speed of the feature. On the other hand, the inflow speeds and outflow speeds of the Petschek reconnect ion are well compatible with observations. Moreover, we found that the Sweet-Parker type current sheet is subject to the ion-acoustic instability in the chromosphere, implying the Petschek mechanism may operate there. Our results strongly suggest that chromospheric reconnect ion is of the Petschek type.