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Slow magnetoacoustic waves in a static background provide a seismological tool to probe the solar atmosphere in the analytic frame. By analyzing the spatiotemporal variation of the electron number density of plume structure in coronal holes above the limb for a given temperature, we find that the density perturbations accelerate with supersonic speeds in the distance range from 1.02 to 1.23 solar radii. We interpret them as slow magnetoacoustic waves propagating at about the sound speed with accelerating subsonic flows. The average sonic height of the subsonic flows is calculated to be 1.27 solar radii. The mass flux of the subsonic flows is estimated to be 44.1$\%$ relative to the global solar wind. Hence, the subsonic flow is likely to be the nascent solar wind. In other words, the evolution of the nascent solar wind in plumes at the low corona is quantified for the first time from imaging observations. Based on the interpretation, propagating density perturbations present in plumes could be used as a seismological probe of the gradually accelerating solar wind., Comment: Accepted for publication in ApJL, 11 pages, 5 figures

We analyze ground-based chromospheric data acquired at a high temporal cadence of 2 s in wings of the H$\alpha$ spectral line using Goode Solar Telescope (GST) operating at the Big Bear Solar Observatory. We inspected a 30 minute long H$\alpha$-0.08~nm data set to find that rapid blue-shifted H$\alpha$ excursions (RBEs), which are a cool component of type II spicules, experience very rapid morphological changes on the time scales of the order of 1 second. Unlike typical reconnection jets, RBEs very frequently appear \textit{in situ} without any clear evidence of H$\alpha$ material being injected from below. Their evolution includes inverted "Y", "V", "N", and parallel splitting (doubling) patterns as well as sudden formation of a diffuse region followed by branching. We also find that the same feature may undergo several splitting episodes within about 1 min time interval., Comment: 11 pages, 4 figures

Hα surges (i.e. cool/dense collimated plasma ejections) may act as a guide for a propagation of magnetohydrodynamic waves. We report a high-resolution observation of a surge observed with 1.6 m Goode Solar Telescope (GST) on 2009 August 26, from 18:20 UT to 18:45 UT. Characteristics of plasma motions in the surge are determined with the normalizing radial gradient filter and the Fourier motion filter. The shape of the surge is found to change from a ‘C’ shape to an inverse ‘C’ shape after a formation of a cusp, a signature of reconnection. There are apparent upflows seen above the cusp top and downflows below it. The upflows show rising and rotational motions in the right-hand direction, with the rotational speed decreasing with height. Near the cusp top, we find a transverse oscillation of the surge, with the period of∼2 min. There is no change of the oscillation phase below the cusp top, but above the top a phase change is identified, giving a vertical phase speed about 86 km s−1. As the height increases, the initial amplitude of the oscillation increases, and the oscillation damping time decreases from 5.13 to 1.18 min. We conclude that the oscillation is a propagating kink wave that is possibly excited by the repetitive spontaneous magnetic reconnection.

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.

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.

Quasi-periodic oscillations in the speed profile of coronal mass ejections (CMEs) in the radial distance range 2-30 solar radii are studied. We considered the height-time data of the 307 CMEs recorded by the Large Angle and Spectrometric Coronagraph (LASCO) during 2005 January-March. In order to study the speed-distance profile of the CMEs, we have used only 116 events for which there are at least 10 height-time measurements made in the LASCO field of view. The instantaneous CME speed is estimated using a pair of height-time data points, providing the speed-distance profile. We found quasi-periodic patterns in at least 15 speed-distance profiles, where the speed amplitudes are larger than the speed errors. For these events we have determined the speed amplitude and period of oscillations. The periods of quasi-periodic oscillations are found in the range 48-240 minutes, tending to increase with height. The oscillations have similar properties as those reported by Krall et al., who interpreted them in terms of the flux-rope model. The nature of forces responsible for the motion of CMEs and their oscillations are discussed.

We report the first simultaneous observation of X-ray plasma ejection (XPE), coronal mass ejection (CME), and type II solar radio burst on 1999 October 26. First, an XPE was observed from 21:12 UT to 21:24 UT in the Yohkoh SXT field of view (1.1 to 1.4 R ☉). The XPE was accelerated with a speed range from 190 to 410 km s–1 and its average speed is about 290 km s–1. Second, the associated CME was observed by the Mauna Loa Mk4 coronameter (1.1-2.8 R ☉) from 21:16 UT. The CME front was clearly identified at 21:26 UT and propagated with a deceleration of about –110 m s–2. Its average speed is about 360 km s–1. At the type II burst start time (21:25 UT), the height of the CME front is around 1.7 R ☉ and its speed is about 470 km s–1. Third, a type II solar radio burst was observed from 21:25 UT to 21:43 UT by the Culgoora solar radio spectrograph. The burst shows three emission patches during this observing period and the emission heights of the burst are estimated to be about 1.3 R ☉ (21:25 UT), 1.4 R ☉ (21:30 UT), and 1.8 R ☉ (21:40 UT). By comparing these three phenomena, we find that: (1) kinematically, while the XPE shows acceleration, the associated CME front shows deceleration; (2) there is an obvious height difference (0.3 R ☉) between the CME front and the XPE front around 21:24 UT and the formation height of the type II burst is close to the trajectory extrapolated from the XPE front; (3) both speeds of the XPE and the CME are comparable with each other around the starting time of the type II burst. Considering the formation height and the speed of the type II burst, we suggest that its first emission is due to the coronal shock generated by the XPE and the other two emissions are driven by the CME flank interacting with the high-density streamer.

We present a morphological study of the 2004 August 18 solar eruption that occurred in the active region NOAA 10656 near the west limb using extreme-ultraviolet (EUV) data from the Transition Region and Coronal Explorer (TRACE), Halpha filtergram of Big Bear Solar Observatory, white light images of Mauna Loa Solar Observatory (MLSO), hard X-ray (HXR) data of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and microwave data of the Owens Valley Solar Array. In this event, we have an excellent set of observations for tracing the early evolution of the coronal mass ejection (CME) from a flux rope emergence to its propagation into space as a well-connected series of events thanks to the coronameter's field of view (FOV) down to 1.1 solar radius in an overlap with that of the TRACE. This data set reveals continuously evolving EUV, Halpha, and WL features that suggest the rise of a small, low-lying loop, its writhing motion, break of the kinked loop at its crossing point, and transformation of the ejecta to the CME. The HXR and microwave sources are found in varying locations with a complicated temporal dependence, which, we interpret, is due to two successive flares in the event.more » The first flare appears to be associated with the rise of the small loop, which then triggers the second flare. During the second flare a HXR coronal source is detected at the crossing point of the kinked loop, and more intriguingly, the kinked loop apparently breaks at the crossing point of the two legs, which indicates a magnetic reconnection at the X-point configuration. After the break of the kinked UV loop, a CME structure shows up in the MLSO FOV, and propagates away from the Sun. It is concluded that this CME occurred due to the kink instability.« less

To investigate the relationship between the speed of a coronal mass ejection (CME) and the magnetic energy released during its eruption, we have compared the initial speed of CMEs (V CME) and the two parameters of their associated magnetic clouds (MC), magnetic flux (F MC), and magnetic helicity per unit length (|H MC|/L), for 34 pairs of CMEs and MCs. The values of these parameters in each MC have been determined by fitting the magnetic data of the MC to the linear force-free cylindrical model. As a result, we found that there are positive correlations between V 2 CME and F MC, and between V 2 CME and |H MC|/L. It is also found that the kinetic energy of CMEs (E CME) is correlated with F MC and |H MC|/L of the associated MC. In contrast, we found no significant correlation between V MC2 and F MC, nor between V MC2 and |H MC|/L. Our results support the notion that the eruption of a CME is related to the magnetic helicity of the source active region.

Frontside halo coronal mass ejections (CMEs) are generally considered as potential candidates for producing geomagnetic storms, but there was no definite way to predict whether they will hit the Earth or not. Recently Moon et al. suggested that the degree of CME asymmetries, as defined by the ratio of the shortest to the longest distances of the CME front measured from the solar center, be used as a parameter for predicting their geoeffectiveness. They called this quantity a direction parameter, D, as it suggests how much CME propagation is directed to Earth, and examined its forecasting capability using 12 fast halo CMEs. In this paper, we extend this test by using a much larger database (486 frontside halo CMEs from 1997 to 2003) and more robust statistical tools (contingency table and statistical parameters). We compared the forecast capability of this direction parameter to those of other CME parameters, such as location and speed. We found the following results: (1) The CMEs with large direction parameters (D ≥ 0.4) are highly associated with geomagnetic storms. (2) If the direction parameter increases from 0.4 to 1.0, the geoeffective probability rises from 52% to 84%. (3) All CMEs associated with strong geomagnetic storms (Dst ≤ − 200 nT) are found to have large direction parameters (D ≥ 0.6). (4) CMEs causing strong geomagnetic storms (Dst ≤ − 100 nT), in spite of their northward magnetic field, have large direction parameters (D ≥ 0.6). (5) Forecasting capability improves when statistical parameters (e.g., "probability of detection—yes" and "critical success index") are employed, in comparison with the forecast solely based on the location and speed of CMEs. These results indicate that the CME direction parameter can be an important indicator for forecasting CME geoeffectiveness.

This paper presents a multiwavelength study of the M8.0 flare and its associated fast halo CME that originated from a bipolar active region NOAA 10759 on 2005 May 13. The source active region has a conspicuous sigmoid structure at TRACE 171 A channel as well as in the SXI soft X-ray images, and we mainly concern ourselves with the detailed process of the sigmoid eruption as evidenced by the multiwavelength data ranging from Halpha, WL, EUV/UV, radio, and hard X-rays (HXRs). The most important finding is that the flare brightening starts in the core of the active region earlier than that of the rising motion of the flux rope. This timing clearly addresses one of the main issues in the magnetic eruption onset of sigmoid, namely, whether the eruption is initiated by an internal tether-cutting to allow the flux rope to rise upward or a flux rope rises due to a loss of equilibrium to later induce tether cutting below it. Our high time cadence SXI and Halpha data shows that the first scenario is relevant to this eruption. As other major findings, we have the RHESSI HXR images showing a change of the HXR source from a confined footpoint structure to an elongated ribbon-like structure after the flare maximum, which we relate to the sigmoid-to-arcade evolution. Radio dynamic spectrum shows a type II precursor that occurred at the time of expansion of the sigmoid and a drifting pulsating structure in the flare rising phase in HXR. Finally type II and III bursts are seen at the time of maximum HXR emission, simultaneous with the maximum reconnection rate derived from the flare ribbon motion in UV. We interpret these various observed properties with the runaway tether-cutting model proposed by Moore et al. in 2001., 10 pages, 10 figures, The Astrophysical Journal, accepted July, 2007

The phenomenon of band splitting in type II bursts can be a unique diagnostic for the magnetic field in the corona, which is, however, inevitably sensitive to the ambient density. We apply this diagnostic to the CME-flare event on 2004 August 18, for which we are able to locate the propagation of the type II burst and determine the ambient coronal electron density by other means. We measure the width of the band splitting on a dynamic spectrum of the bursts observed with the Green Bank Solar Radio Burst Spectrometer (GBSRBS), and convert it to the Alfven Mach number under the Rankine-Hugoniot relation. We then determine the Alfven speed and magnetic field strength using the coronal background density and shock speed measured with the MLSO/MK4 coronameter. In this way we find that the shock compression ratio is in the range of 1.5-1.6, the Alfvenic Mach number is 1.4-1.5, the Alfven speed is 550-400 km s-1, and finally the magnetic field strength decreases from 1.3 to 0.4 G while the shock passes from 1.6 to 2.1 R☉. The magnetic field strength derived from the type II spectrum is finally compared with the potential field source surface (PFSS) model for further evaluation of this diagnostic.

In this study, we have investigated the kinematic properties of flare-associated X-ray plasma ejections. First, we obtained the speed profiles of well-observed several events and compared them with the GOES soft X-ray flux profiles as well as the HXT hard X-ray flux profiles of their associated flares. Second, we have estimated the Alfven speed at the observing height of X-ray plasma ejections in order to find whether the X-ray plasma ejection is a reconnection outflow as predicted by standard magnetic reconnection model. Finally, we have estimated the representative speeds of all 137 X-ray plasma ejections and then compared them with the speeds of the coronal mass ejections (CMEs). Our main results are as follows: (1) X-ray plasma ejections usually initially accelerate and then constantly propagate or slowly decelerate; (2) for several well-observed examples, the speed profiles of X-ray plasma ejections are similar to those of the hard X-ray emission profiles; (3) the speed of an X-ray plasma ejection ranges from 30 to 1300 km s-1, with a mean speed of 230 km s-1, and the speed of a CME ranges from 150 to 2000 km s-1 with a mean value of 530 km s -1; (4) there is no statistical correlation between the speeds of X-ray plasma ejections and the corresponding CME speeds; (5) an X-ray plasma ejection seems to have a much shorter acceleration duration (less than 10 minutes) than that of a CME (larger than 30 minutes). On the basis of these results, we suggest that the majority of X-ray plasma ejections are not likely to be the X-ray counterpart of CMEs but outflows generated by magnetic reconnection, at least from the kinematical point of view.

We have examined the physical characteristics of very fast coronal mass ejections (CMEs) and their geoeffective parameters. For this we consider SOHO LASCO CMEs whose speeds are larger than 1300 km s-1. By examining all SOHO EIT and SOHO LASCO images of the CMEs, we selected 38 front-side very fast CMEs and then examined their associations with solar activity such as X-ray flares and type II bursts. As a result, we found that among these front-side fast CMEs, 25 are halo (or full halo) CMEs with span of 360°, 12 are partial halo CMEs with span greater than 130°, and only one is a broadside CME, with a span of 53°. There are 13 events that are shock-deflected CMEs: six are full halo CMEs, and seven are partial halo CMEs. It is found that about 60% (23/38) CMEs were ejected from the western hemisphere. We also note that these very fast CMEs have very high associations with other solar activities: all the CMEs are associated with X-ray flares (X-12, M-23, C-3), and about 80% of the CMEs (33/38) were accompanied by type II bursts. For the examination of CME geoeffectiveness, we select 12 halo CMEs whose longitudes are less than 40°, which are thought to be the most plausible candidates of geoeffective CMEs. Then we examine the relation between their CME physical parameters (mass, column density, location of an associated flare, and direction) and the Dst index. In particular, a CME direction parameter, which is defined as the maximum ratio of its shorter front from solar disk center and its longer one, is proposed as a new geoeffective parameter. Its major advantage is that it can be directly estimated from coronagraph observation. It is found that while the location of the associated flare has a poor correlation with the Dst index, the new direction parameter has a relatively good correlation. In addition, the column density of a CME also has a comparable good correlation with the Dst index. Noting that the CME column density is strongly affected by the direction of a CME, our results imply that the CME direction seems to be the most important parameter that controls the geoeffectiveness of very fast halo CMEs.

We have made a comprehensive statistical study of the relationship between flare-associated X-ray plasma ejections and coronal mass ejections (CMEs). For this we considered all flare-mode data in Yohkoh SXT observations from 1999 April to 2001 March and then selected 279 limb flares seen at longitudes greater than 60°. For these events, we identified whether there were associated X-ray plasma ejections or not. We found that about half (137/279) of the flares have X-ray plasma ejections, and we present a comprehensive list of these with their event times and speeds. We then determined whether there was an association between the flares with plasma ejections and CMEs detected by the Solar and Heliospheric Observatory LASCO instrument, on the basis of temporal and spatial proximity. It is found that about 69% (95/137) of the X-ray plasma ejections are associated with CMEs and that about 84% (119/142) of the events without plasma ejections do not have related CMEs. The associations are found to increase with flare strength and duration. We find that X-ray plasma ejections occur nearly simultaneously with the hard X-ray flare peak, supporting the idea that the X-ray plasma ejections are tightly associated with the flaring process. When the CMEs are extrapolated into the Yohkoh field of view for 43 selected, well-observed events, it is found that about 80% of the CMEs preceded X-ray plasma ejections, by approximately 20 minutes on average. Our results show that X-ray plasma ejections usually do not represent the early signature of a CME's leading edge but are closely associated with CMEs.

It is well known that while flare-associated coronal mass ejections (CMEs) show higher speeds and little acceleration in the corona, filament-associated CMEs have lower speeds and large accelerations. In this paper, we examine three flare-associated CMEs with relatively large accelerations as counterexamples of the former tendency. The estimated accelerations are all larger than 45 m s-2 below 15 R☉. By analyzing SOHO EIT, SOHO LASCO, and GOES data, we attempt to find out what kind of physical characteristics control such strong accelerations. The first event is the 1999 July 9 event associated with a C1.1 flare. Considering the fact that its CME appearance, seen in the LASCO running difference imagery, is quite similar to the shape of a helmet streamer, we speculate that its eruption is related to the destabilization of a helmet streamer, which may induce the weak X-ray flare. The second event is the 1999 August 17 event associated with a C2.6 flare. The CME speed abruptly increased from 232 to 909 km s-1 for 1 hr, and the strong acceleration is coincident with the occurrence of a subsequent flare/CME. The third event is the 2000 November 24 event associated with a C4.1 flare. The CME speed first decreased and then constantly accelerated for 3 hr. The start of such an acceleration is also coincident with a subsequent CME/flare event. For the last two CME events, the Lorentz forces acting on the subsequent events may play an important role in accelerating CMEs. Our results show that large accelerations of flare-associated CMEs, as counterexamples of the two classes of CMEs, seem to be caused by other solar activities, such as helmet streamer disruptions or subsequent CMEs/flares.

Solar flare ribbons provide an important clue to the magnetic reconnection process and associated magnetic field topology in the solar corona. We detected a large-scale secondary flare ribbon of a circular shape that developed in association with two successive M-class flares and one CME. The ribbon revealed interesting properties such as 1) a quasi-circular shape and enclosing the central active region; 2) the size as large as 500\arcsec\, by 650\arcsec\,, 3) successive brightenings in the clockwise direction at a speed of \kms{160} starting from the nearest position to the flaring sunspots, 4) radial contraction and expansion in the northern and the southern part, respectively at speeds of $\leq$ \kms{10}. Using multi-wavelength data from \textit{SDO}, \textit{RHESSI}, XRT, and Nobeyama, along with magnetic field extrapolations, we found that: 1) the secondary ribbon location is consistent with the field line footpoints of a fan-shaped magnetic structure that connects the flaring region and the ambient decaying field; 2) the second M2.6 flare occurred when the expanding coronal loops driven by the first M2.0 flare encountered the background decayed field. 3) Immediately after the second flare, the secondary ribbon developed along with dimming regions. Based on our findings, we suggest that interaction between the expanding sigmoid field and the overlying fan-shaped field triggered the secondary reconnection that resulted in the field opening and formation of the quasi-circular secondary ribbon. We thus conclude that interaction between the active region and the ambient large-scale fields should be taken into account to fully understand the entire eruption process., Accepted for publication in the ApJ, (20 pages, 13 figures)

We analyze the high-energy particle emission from the Sun in two extreme solar particle events, in which protons are accelerated to relativistic energies and can cause a significant signal even in the ground-based particle detectors. Analysis of a relativistic proton event is based on modeling of the particle transport and interaction, from a near-Sun source through the solar wind and the Earth’s magnetosphere and atmosphere to a detector on the ground. This allows us to deduce the time profile of the proton source at the Sun and compare it with observed electromagnetic emissions. The 2 May 1998 event is associated with flare and coronal mass ejection (CME) well observed by the Nan¸cay Radioheliograph, so that the images of radio sources are available. For the 2 November 2003 event, there are available the low-corona images of the CME liftoff obtained at the Mauna Loa Solar Observatory. Those complementary data sets are analyzed jointly with the broadband dynamic radio spectra, EUV images and other data available for both events. We find a common scenario for both eruptions, including the flare’s dual impulsive phase, the CME-launch-associated decimetric-continuum burst, and the late, low-frequency type III radio bursts at the time of the relativistic proton injection into the interplanetary medium. The analysis supports the idea that the two considered events start with emission of relativistic protons previously accelerated during the flare and CME launch, then trapped in large-scale magnetic loops and later released by the expanding CME.

It is generally believed that fast coronal mass ejections (CMEs) can generate their associated shocks, which are characterized by faint structures ahead of CMEs in white-light coronagraph images. In this study, we examine whether the observational stand-off distance ratio, defined as the CME stand-off distance divided by its radius, can be explained by bow shock theories. Of 535 SOHO/LASCO CMEs (from 1996 to 2015) with speeds greater than 1000 km s−1 and angular widths wider than 60°, we select 18 limb CMEs with the following conditions: (1) their Alfvenic Mach numbers are greater than one under Mann's magnetic field and Saito's density distributions; and (2) the shock structures ahead of the CMEs are well identified. We determine observational CME stand-off distance ratios by using brightness profiles from LASCO-C2 observations. We compare our estimates with theoretical stand-off distance ratios from gasdynamic (GD) and magnetohydrodynamic (MHD) theories. The main results are as follows. Under the GD theory, 39% (7/18) of the CMEs are explained in the acceptable ranges of adiabatic gamma (γ) and CME geometry. Under the MHD theory, all the events are well explained when we consider quasi-parallel MHD shocks with γ = 5/3. When we use polarized brightness (pB) measurements for coronal density distributions, we also find similar results: 8% (1/12) under GD theory and 100% (12/12) under MHD theory. Our results demonstrate that the bow shock relationships based on MHD theory are more suitable than those based on GD theory for analyzing CME-driven shock signatures.

For 478 centrally located sunspots observed in the optical continuum with Solar Dynamics Observatory/Helioseismic Magnetic Imager, we perform seismological diagnostics of the physical parameters of umbral photospheres. The new technique is based on the theory of slow magnetoacoustic waves in a non-isothermally stratified photosphere with a uniform vertical magnetic field. We construct a map of the weighted frequency of three-minute oscillations inside the umbra and use it for the estimation of the Alfven speed, plasma-beta, and mass density within the umbra. We find the umbral mean Alfven speed ranges between 10.5 and 7.5 km s−1 and is negatively correlated with magnetic field strength. The umbral mean plasma-beta is found to range approximately between 0.65 and 1.15 and does not vary significantly from pores to mature sunspots. The mean density ranges between (1–6) × 10−4 kg m−3 and shows a strong positive correlation with magnetic field strength.

It is well-known that light bridges inside a sunspot produce small-scale plasma ejections and transient brightenings in the chromosphere, but the nature and origin of such phenomena are still unclear. Utilizing the high-spatial and high temporal resolution spectral data taken with the Fast Imaging Solar Spectrograph and the TiO 7057 A broadband filter images installed at the 1.6 meter New Solar Telescope of Big Bear Solar Observatory, we report arcsecond-scale chromospheric plasma ejections (1.7 arc) inside a light bridge. Interestingly, the ejections are found to be a manifestation of upwardly propagating shock waves as evidenced by the sawtooth patterns seen in the temporal-spectral plots of the Ca II 8542 A and H-alpha intensities. We also found a fine-scale photospheric pattern (1 arc) diverging with a speed of about 2 km/s two minutes before the plasma ejections, which seems to be a manifestation of magnetic flux emergence. As a response to the plasma ejections, the corona displayed small-scale transient brightenings. Based on our findings, we suggest that the shock waves can be excited by the local disturbance caused by magnetic reconnection between the emerging flux inside the light bridge and the adjacent umbral magnetic field. The disturbance generates slow-mode waves, which soon develop into shock waves, and manifest themselves as the arcsecond-scale plasma ejections. It also appears that the dissipation of mechanical energy in the shock waves can heat the local corona., 27 pages, 9 figures, accepted for publication in ApJ

We investigate triggering, activation, and ejection of a solar eruptive prominence that occurred in a multi-polar flux system of active region NOAA 11548 on 2012 August 18 by analyzing data from AIA on board SDO, RHESSI, and EUVI/SECCHI on board STEREO. Prior to the prominence activation, we observed striking coronal activities in the form of a blowout jet which is associated with rapid eruption of a cool flux rope. Further, the jet-associated flux rope eruption underwent splitting and rotation during its outward expansion. These coronal activities are followed by the prominence activation during which it slowly rises with a speed of ~12 km/s while the region below the prominence emits gradually varying EUV and thermal X-ray emissions. From these observations, we propose that the prominence eruption is a complex, multi-step phenomenon in which a combination of internal (tether-cutting reconnection) and external (i.e., pre-eruption coronal activities) processes are involved. The prominence underwent catastrophic loss of equilibrium with the onset of the impulsive phase of an M1.8 flare suggesting large-scale energy release by coronal magnetic reconnection. We obtained signatures of particle acceleration in the form of power law spectra with hard electron spectral index (delta ~ 3) and strong HXR footpoint sources. During the impulsive phase, a hot EUV plasmoid was observed below the apex of the erupting prominence that ejected in the direction of the prominence with a speed of ~177 km/s. The temporal, spatial and kinematic correlations between the erupting prominence and the plasmoid imply that the magnetic reconnection supported the fast ejection of prominence in the lower corona., 17 pages, 10 figures, The Astrophysical Journal

We explore the similarity and difference of the quasi-periodic pulsations (QPPs) observed in the decay phase of solar and stellar flares at X-rays. We identified 42 solar flares with pronounced QPPs, observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and 36 stellar flares with QPPs, observed with X-ray Multi Mirror Newton observatory (XMM-Newton). The Empirical Mode Decomposition (EMD) method and least-square fit by a damped sine function were applied to obtain the periods (P) and damping times (τ ) of the QPPs. We found that (1) the periods and damping times of the stellar QPPs are 16.21±15.86 min and 27.21±28.73 min, while those of the solar QPPs are 0.90±0.56 and 1.53±1.10 min, respectively. (2) The ratio of the damping times to the periods (τ/P) observed in the stellar QPPs (1.69±0.56) are statistically identical to those of solar QPPs (1.74±0.77). (3) The scalings of the QPP damping time with the period are well described by the power law in both solar and stellar cases. The power indices of the solar and stellar QPPs are 0.96±0.10 and 0.98±0.05, respectively. This scaling is consistent with the scalings found for standing slow magnetoacoustic and kink modes in solar coronal loops. Thus, we propose that the underlying mechanism responsible for the stellar QPPs is the natural magnetohydrodynamic oscillations in the flaring or adjacent coronal loops, as in the case of solar flares.

We present multi-wavelength analysis of a quasi-periodic pulsation (QPP) observed in the hard X-ray, radio, and extreme-ultraviolet (EUV) channels during an M1.9 flare occurred on 23-24 September 2011. The non-thermal hard X-ray emission in 25-50 keV observed by RHESSI shows five distinct impulsive peaks of decaying amplitude with a period of about three minutes. Similar QPP was observed in the microwave emission recorded by the Nobeyama Radioheliograph and Polarimeter in the 8.8, 15, 17 GHz channels. Interestingly, the 3-min QPP was also observed in the metric and decimetric radio frequencies (25-180, 245, 610 MHz) as repetitive type III bursts. Multi-wavelength observations from the SDO/AIA, Hinode/SOT, and STEREO/SECCHI suggest a fan-spine topology at the eruption site, associated with the formation of a quasi-circular ribbon during the flare. A small filament was observed below the fan-loops before the flare onset. The filament rose slowly and interacted with the ambient field. This behaviour was followed by an untwisting motion of the filament. Two different structures of the filament showed $\sim$3-min periodic alternate rotation in the clockwise and counterclockwise directions. The 3-min QPP was found to highly correlate with 3-min oscillations in a nearby sunspot. We suggest that the periodic reconnection (modulated either by sunspot slow-mode wave or by untwisting filament) at a magnetic null-point most likely causes the repetitive particle acceleration, generating the QPP observed in hard X-ray, microwave and type III radio bursts., Comment: ApJ, 14 pages, 10 figures

We studied temporal changes of morphological and magnetic properties of a succession of four confined flares followed by an eruptive flare using the high-resolution New Solar Telescope (NST) operating at the Big Bear Solar Observatory (BBSO), Helioseismic and Magnetic Imager (HMI) magnetograms and Atmospheric Image Assembly (AIA) EUV images provided by Solar Dynamics Observatory (SDO). From the NST/Halpha and the SDO/AIA~304 A observations we found that each flare developed a jet structure that evolved in a manner similar to evolution of the blowout jet : 1) an inverted-Y shape jet appeared and drifted away from its initial position; 2) jets formed a curtain-like structure that consisted of many fine threads accompanied with subsequent brightenings near the footpoints of the fine threads; and finally 3) the jet showed a twisted structure visible near the flare maximum. Analysis of the HMI data showed that both the negative magnetic flux and the magnetic helicity have been gradually increasing in the positive polarity region indicating the continuous injection of magnetic twist before and during the series of flares. Based on these results, we suggest that the continuous emergence of twisted magnetic flux played an important role in producing a successive flares and developing a series of blowout jets., 34 pages, 10 figures, accepted for publication in ApJ

We present the transient brightening of a coronal loop and an associated fine-scale magnetic discontinuity detected in the photosphere. Utilizing the high-resolution data taken with the Fast Imaging Solar Spectrograph and InfraRed Imaging Magnetograph of the New Solar Telescope at Big Bear Solar Observatory, we detect a narrow lane of intense horizontal magnetic field representing a magnetic discontinuity. It was visible as a dark lane partially encircling a pore in the continuum image, and was located near one of the footpoints of a small coronal loop that experienced transient brightenings. The horizontal field strength gradually increased before the loop brightening, and then rapidly decreased in the impulsive phase of the brightening, suggesting the increase of the magnetic non-potentiality at the loop footpoint and the sudden release of magnetic energy via magnetic reconnection. Our results support the nanoflare theory that coronal heating events are caused by magnetic reconnection events at fine-scale magnetic discontinuities.

It was theoretically demonstrated that a shock propagating in the solar atmosphere can overtake another and merge with it. We provide clear observational evidence that shock merging does occur quite often in the chromosphere of sunspots. Using Hα imaging spectral data taken by the Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope at the Big Bear Soar Observatory, we construct time–distance maps of line-of-sight velocities along two appropriately chosen cuts in a pore. The maps show a number of alternating redshift and blueshift ridges, and we identify each interface between a preceding redshift ridge and the following blueshift ridge as a shock ridge. The important finding of ours is that two successive shock ridges often merge with each other. This finding can be theoretically explained by the merging of magneto-acoustic shock waves propagating with lower speeds of about 10 km s−1 and those propagating at higher speeds of about 16–22 km s−1. The shock merging is an important nonlinear dynamical process of the solar chromosphere that can bridge the gap between higher-frequency chromospheric oscillations and lower-frequency dynamic phenomena such as fibrils.

We report decaying quasi-periodic intensity oscillations in the X-ray (6-12 keV) and extreme ultraviolet (EUV) channels (131, 94, 1600, 304 \AA) observed by the Fermi GBM (Gamma-ray Burst Monitor) and SDO/AIA, respectively, during a C-class flare. The estimated period of oscillation and decay time in the X-ray channel (6-12 keV) was about 202 s and 154 s, respectively. A similar oscillation period was detected at the footpoint of the arcade loops in the AIA 1600 and 304 \AA channels. Simultaneously, AIA hot channels (94 and 131 \AA) reveal propagating EUV disturbances bouncing back and forth between the footpoints of the arcade loops. The period of the oscillation and decay time were about 409 s and 1121 s, respectively. The characteristic phase speed of the wave is about 560 km/s for about 115 Mm loop length, which is roughly consistent with the sound speed at the temperature about 10-16 MK (480-608 km/s). These EUV oscillations are consistent with the SOHO/SUMER Doppler-shift oscillations interpreted as the global standing slow magnetoacoustic wave excited by a flare. The flare occurred at one of the footpoints of the arcade loops, where the magnetic topology was a 3D fan-spine with a null-point. Repetitive reconnection at this footpoint could cause the periodic acceleration of non-thermal electrons that propagated to the opposite footpoint along the arcade and precipitating there, causing the observed 202-s periodicity. Other possible interpretations, e.g. the second harmonics of the slow mode are also discussed., Comment: ApJ (in press), 13 pages, 6 figures

Vertical transverse oscillations of a coronal magnetic rope, observed simultaneously in the 171 A and 304 A bandpasses of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO), are detected. The oscillation period is about 700 s and the displacement amplitude is about 1 Mm. The oscillation amplitude remains constant during the observation. Simultaneous observation of the rope in the bandpasses corresponding to the coronal and chromospheric temperatures suggests that it has a multi-thermal structure. Oscillatory patterns in 171 A and 304 A are coherent, which indicates that the observed kink oscillation is collective, in which the rope moves as a single entity. We interpret the oscillation as a fundamental standing vertically polarized kink mode of the rope, while the interpretation in terms of a perpendicular fast wave could not be entirely ruled out. In addition, the arcade situated above the rope and seen in the 171 A bandpass shows an oscillatory motion with the period of about 1000 s.

The 2011 August 9 flare is one of the largest X-ray flares of sunspot cycle 24, but spatial information is rather limited due to its position close to the western limb. This paper presents information about the location of high-energy electrons derived from hard X-ray and microwave spectra obtained with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Korean Solar Radio Burst Locator (KSRBL), respectively. The KSRBL microwave spectrum shows significant fluxes at low frequencies, implying that the high-energy electrons reside in a coronal volume highly concentrated at strong magnetic fields, and rapidly expanding with decreasing magnetic fields. After a simple modeling of the microwave spectrum, we found that the microwave source should be located above the inner pair of magnetic poles in a large quadrupolar configuration. The time-dependent evolution of the magnetic field distribution and total nonthermal energy derived from the microwave spectra is also consistent with the standard picture of multiple magnetic reconnections recurring at a magnetic null point that forms above the magnetic quadrupoles and moves up with time.

It is still a mystery how the solar chromosphere can stand high above the photosphere. The dominant portion of this layer must be dynamically supported, as is evident by the common occurrence of jets such as spicules and mottles in quiet regions, and fibrils and surges in active regions. Hence, revealing the driving mechanism of these chromospheric jets is crucial for our understanding of how the chromosphere itself exists. Here, we report our observational finding that fibrils in the superpenumbra of a sunspot are powered by sunspot oscillations. We find patterns of outward propagation that apparently originate from inside the sunspot, propagate like running penumbral waves, and develop into the fibrils. Redshift ridges seen in the time-distance plots of velocity often merge, forming a fork-like pattern. The predominant period of these shock waves increases, often jumping with distance, from 3 minutes to 10 minutes. This short-to-long period transition seems to result from the selective suppression of shocks by the falling material of their preceding shocks. Based on our results, we propose that the fibrils are driven by slow shock waves with long periods that are produced by the merging of shock waves with shorter periods propagating along the magnetic canopy.

We have investigated a supra-arcade structure associated with an M1.6 flare, which occurred on the south-east limb on 2010 November 4. It is observed in EUV with the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, microwaves at 17 and 34 GHz with the Nobeyama Radioheliograph (NoRH), and soft X-rays of 8-20 keV with RHESSI. Interestingly, we found exceptional properties of the supra-arcade thermal plasma from the AIA 131 A and the NoRH: (1) plasma upflows along large coronal loops and (2) enhancing microwave emission. RHESSI detected two soft X-ray sources, a broad one in the middle of the supra-arcade structure and a bright one just above the flare-arcade. We estimated the number density and thermal energy for these two source regions during the decay phase of the flare. In the supra-arcade source, we found that there were increases of the thermal energy and the density at the early and last stages, respectively. On the contrary, the density and thermal energy of the source on the top of the flare-arcade decreases throughout. The observed upflows imply that there is continuous energy supply into the supra-arcade structure from below during the decay phase of the flare. It is hard to explain by the standard flare model in which the energy release site is located high in the corona. Thus, we suggest that a potential candidate of the energy source for the hot supra-arcade structure is the flare-arcade, which has exhibited a predominant emission throughout.

To better understand a preferred magnetic field configuration and its evolution during coronal mass ejection (CME) events, we investigated the spatial and temporal evolution of photospheric magnetic fields in the active region NOAA 9236 that produced eight flare-associated CMEs during the time period of 2000 November 23-26. The time variations of the total magnetic helicity injection rate and the total unsigned magnetic flux are determined and examined not only in the entire active region but also in some local regions such as the main sunspots and the CME-associated flaring regions using SOHO/MDI magnetogram data. As a result, we found that (1) in the sunspots, a large amount of positive (right-handed) magnetic helicity was injected during most of the examined time period, (2) in the flare region, there was a continuous injection of negative (left-handed) magnetic helicity during the entire period, accompanied by a large increase of the unsigned magnetic flux, and (3) the flaring regions were mainly composed of emerging bipoles of magnetic fragments in which magnetic field lines have substantially favorable conditions for making reconnection with large-scale, overlying, and oppositely directed magnetic field lines connecting the main sunspots. These observational findings can also be well explained by some MHD numerical simulations for CME initiation (e.g., reconnection-favored emerging flux models). We therefore conclude that reconnection-favored magnetic fields in the flaring emerging flux regions play a crucial role in producing the multiple flare-associated CMEs in NOAA 9236.

The presence of a penumbra is one of the main properties of a mature sunspot, but its formation mechanism has been elusive due to a lack of observations that fully cover the formation process. Utilizing the New Solar Telescope at the Big Bear Solar Observatory, we observed the formation of a partial penumbra for about 7 hr simultaneously at the photospheric (TiO; 7057 A) and the chromospheric (Hα – 1 A) spectral lines with high spatial and temporal resolution. From this uninterrupted, long observing sequence, we found that the formation of the observed penumbra was closely associated with flux emergence under the pre-existing chromospheric canopy fields. Based on this finding, we suggest a possible scenario for penumbra formation in which a penumbra forms when the emerging flux is constrained from continuing to emerge, but rather is trapped at the photospheric level by the overlying chromospheric canopy fields.

We report white-light observations of a fast magnetosonic wave associated with a coronal mass ejection observed by STEREO/SECCHI/COR1 inner coronagraphs on 2011 August 4. The wave front is observed in the form of density compression passing through various coronal regions such as quiet/active corona, coronal holes, and streamers. Together with measured electron densities determined with STEREO COR1 and Extreme UltraViolet Imager (EUVI) data, we use our kinematic measurements of the wave front to calculate coronal magnetic fields and find that the measured speeds are consistent with characteristic fast magnetosonic speeds in the corona. In addition, the wave front turns out to be the upper coronal counterpart of the EIT wave observed by STEREO EUVI traveling against the solar coronal disk; moreover, stationary fronts of the EIT wave are found to be located at the footpoints of deflected streamers and boundaries of coronal holes, after the wave front in the upper solar corona passes through open magnetic field lines in the streamers. Our findings suggest that the observed EIT wave should be in fact a fast magnetosonic shock/wave traveling in the inhomogeneous solar corona, as part of the fast magnetosonic wave propagating in the extended solar corona.

We examine the relationship between the high-frequency (425 MHz) type II radio burst and the associated white-light coronal mass ejection (CME) that occurred on 2011 February 13. The radio burst had a drift rate of 2.5 MHz s–1, indicating a relatively high shock speed. From SDO/AIA observations we find that a loop-like erupting front sweeps across high-density coronal loops near the start time of the burst (17:34:17 UT). The deduced distance of shock formation (0.06 Rs) from the flare center and speed of the shock (1100 km s–1) using the measured density from SDO/AIA observations are comparable to the height (0.05 Rs, from the solar surface) and speed (700 km s–1) of the CME leading edge observed by STEREO/EUVI. We conclude that the type II burst originates even in the low corona (