Su-Chan Bong

New geoeffective parameters of very fast halo coronal mass ejections

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.

Magnetic Reconnection During the Two-phase Evolution of a Solar Eruptive Flare

We present a detailed multi-wavelength analysis and interpretation of the evolution of an M7.6 flare that occurred near the southeast limb on 2003 October 24. Pre-flare images at TRACE 195 A show that the bright and complex system of coronal loops already existed at the flaring site. The X-ray observations of the flare taken from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft reveal two phases of the flare evolution. The first phase is characterized by the altitude decrease of the X-ray looptop (LT) source for ~11 minutes. Such a long duration of the descending LT source motion is reported for the first time. The EUV loops, located below the X-ray LT source, also undergo contraction with similar speed (~15 km s–1) in this interval. During the second phase the two distinct hard X-ray footpoint (FP) sources are observed which correlate well with UV and Hα flare ribbons. The X-ray LT source now exhibits upward motion as anticipated from the standard flare model. The RHESSI spectra during the first phase are soft and indicative of hot thermal emission from flaring loops with temperatures T > 25 MK at the early stage. On the other hand, the spectra at high energies (e 25 keV) follow hard power laws during the second phase (γ = 2.6-2.8). We show that the observed motion of the LT and FP sources can be understood as a consequence of three-dimensional magnetic reconnection at a separator in the corona. During the first phase of the flare, the reconnection releases an excess of magnetic energy related to the magnetic tensions generated before a flare by the shear flows in the photosphere. The relaxation of the associated magnetic shear in the corona by the reconnection process explains the descending motion of the LT source. During the second phase, the ordinary reconnection process dominates describing the energy release in terms of the standard model of large eruptive flares with increasing FP separation and upward motion of the LT source.

Low coronal observations of metric type II associated CMEs by MLSO coronameters

Aims. We have investigated the relationship between coronal mass ejections (CMEs) and coronal type II radio bursts by using type II associated CMEs whose low coronal observations by MLSO MK coronameters (1.08-2.85 solar radii for MK4) were available. Methods. For this we considered all type II burst data at 17:00 UT to 22:00 UT from 1996 to 2003, and then compared them with CME images that were obtained during the same MLSO (Mauna Loa Solar Observatory) observing periods. As a result, we selected 19 type II associated CMEs whose kinematics are well identified. A relationship between CMEs and type IIs has been examined in terms of spatial and temporal closeness without any extrapolation of CME kinematics as well as in terms of CME-streamer interaction. Results. We found that: (1) except one event, all the metric type II events occur simultaneously or after the CME appearance in MK field of view within 30 min, mostly within 10 min after; (2) the distribution of height difference between the CME front and type II formation shows that there are double peaks, one at the CME fronts and the other at about 1 solar radius behind the front; (3) about half of the events (9/19) are identified to have CME-streamer interaction (seven streamer deflection and two overlapping), and the interaction heights are very similar to those of type II formation as well as their interaction times are nearly coincident with those of type II starting; (4) for the other events (10/19), the CME front heights at the starting time of type IIs are comparable to the heights of type II formation. Conclusions. Our low coronal observations of type II associated CMEs suggest that CME front and/or CME-streamer interaction at CME flank are two main mechanisms to generate type II bursts.

Relationship between multiple type II solar radio bursts and CME observed by STEREO/SECCHI

Aims. Two or more type II bursts are occasionally observed in close time sequence during solar eruptions, which are known as multiple type II bursts. The origin of the successive burst has been interpreted in terms of coronal mass ejections (CMEs) and/or flares. Detailed investigations of the relationship between CMEs and the bursts enable us to understand the nature of the multiple type II bursts. In this study, we examine multiple type II bursts and compare their kinematics with those of a CME occurring near the time of the bursts. Methods. To do this, we selected multiple type II bursts observed by the Culgoora radiospectrographs and a limb CME detected in the low corona field of view (1.4−4 Rs) of a STEREO/SECCHI instrument on December 31, 2007. To determine the 3D kinematics of the CME, we applied the stereoscopic technique to the STEREO/SECCHI data. Results. Our main results are as follows: (1) the multiple type II bursts occurred successively at ten minute intervals and displayed various emission structures and frequency drifting rates; (2) near the time of the bursts, the CME was observed by STEREO and SOHO simultaneously, but no evidence of other CMEs was detected; (3) inspection of the 3D kinematics of the CME using the stereoscopic observation by STEREO/SECCHI revealed that the CME propagated along the eastward radial direction as viewed from the Earth; (4) very close time and height associations were found between the CME nose and the first type II burst, and between CME-streamer interaction and the second type II burst. Conclusions. On the basis of these results, we suggest that a single shock in the leading edge of the CME could be the source of the multiple type II bursts and support the notion that the CME nose and the CME-streamer interaction are the two main mechanisms able to generate the bursts.

Spatio-spectral Maximum Entropy Method. I. Formulation and Test

The spatio-spectral maximum entropy method (SSMEM) has been developed by Komm and coworkers in 1997 for use with solar multifrequency interferometric observation. In this paper we further improve the formulation of the SSMEM to establish it as a tool for astronomical imaging spectroscopy. We maintain their original idea that spectral smoothness at neighboring frequencies can be used as an additional a priori assumption in astrophysical problems and that this can be implemented by adding a spectral entropy term to the usual maximum entropy method (MEM) formulation. We, however, address major technical difficulties in introducing the spectral entropy into the imaging problem that are not encountered in the conventional MEM. These include calculation of the spectral entropy in a generally frequency-dependent map grid, simultaneous adjustment of the temperature variables and Lagrangian multipliers in the spatial and spectral domain, and matching the solutions to the observational constraints at a large number of frequencies. We test the performance of the SSMEM in comparison with the conventional MEM.

ESTIMATION OF SPICULE MAGNETIC FIELD USING OBSERVED MHD WAVES BY THE HINODE SOT

Using the MHD coronal seismology technique, we estimated the magnetic field for three spicules observed in 2008 June. For this study, we used the high resolution Ca II H line (3968.5 ˚A) images observed by the Hinode SOT and considered a vertical thin flux tube as a spicule model. To our knowledge, this is the first attempt to estimate the spicule magnetic field using the Hinode observation. From the observed oscillation properties, we determined the periods, amplitudes, minimum wavelengths, and wave speeds. We interpreted the observed oscillations as MHD kink waves propagating through a vertical thin flux tube embedded in a uniform field environment. Then we estimated spicule magnetic field assuming spicule densities. Major results from this study are as follows : (1) we observed three oscillating spicules having durations of 5-7 minutes, oscillating periods of 2-3 minutes, and transverse displacements of 700-1000 km. (2) The estimated magnetic field in spicules is about 10-18 G for lower density limit and about 43-76 G for upper density limit. (3) In this analysis, we can estimate the minimum wavelength of the oscillations, such as 60000 km, 56000 km, and 45000 km. This may be due to the much longer wavelength comparing with the height of spicules. (4) In the first event occurred on 2008 June 03, the oscillation existed during limited time (about 250 s). This means that the oscillation may be triggered by an impulsive mechanism (like low atmospheric reconnection), not continuous. Being compared with the ground-based observations of spicule oscillations, our observation indicates quite different one, i.e., more than one order longer in wavelength, a factor of 3-4 larger in wave speed, and 2-3 times longer in period.

A Study of Flare-associated X-Ray Plasma Ejections. III. Kinematic Properties

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.

Pre-Flare Activity and Magnetic Reconnection during the Evolutionary Stages of Energy Release in a Solar Eruptive Flare

In this paper, we present a multi-wavelength analysis of an eruptive white-light M3.2 flare that occurred in active region NOAA 10486 on 2003 November 1. The excellent set of high-resolution observations made by RHESSI and the TRACE provides clear evidence of significant pre-flare activities for ~9 minutes in the form of an initiation phase observed at EUV/UV wavelengths followed by an X-ray precursor phase. During the initiation phase, we observed localized brightenings in the highly sheared core region close to the filament and interactions among short EUV loops overlying the filament, which led to the opening of magnetic field lines. The X-ray precursor phase is manifested in RHESSI measurements below ~30 keV and coincided with the beginning of flux emergence at the flaring location along with early signatures of the eruption. The RHESSI observations reveal that both plasma heating and electron acceleration occurred during the precursor phase. The main flare is consistent with the standard flare model. However, after the impulsive phase, an intense hard X-ray (HXR) looptop source was observed without significant footpoint emission. More intriguingly, for a brief period, the looptop source exhibited strong HXR emission with energies up to ~50-100 keV and significant non-thermal characteristics. The present study indicates a causal relation between the activities in the pre-flare and the main flare. We also conclude that pre-flare activities, occurring in the form of subtle magnetic reorganization along with localized magnetic reconnection, played a crucial role in destabilizing the active region filament, leading to a solar eruptive flare and associated large-scale phenomena.

SIMULTANEOUS OBSERVATION OF A HOT EXPLOSION BY NST AND IRIS

We present the first simultaneous observations of so-called “hot explosions” in the cool atmosphere of the Sun made by the New Solar Telescope (NST) of Big Bear Solar Observatory and the Interface Region Imaging Spectrograph (IRIS) in space. The data were obtained during the joint IRIS-NST observations on 2014 July 30. The explosion of interest started around 19:20 UT and lasted for about 10 minutes. Our findings are as follows: (1) the IRIS brightening was observed in three channels of slit-jaw images, which cover the temperature range from 4000 to 80,000 K; (2) during the brightening, the Si iv emission profile showed a double-peaked shape with highly blue and redshifted components ( and 80 km s−1); (3) wing brightening occurred in Hα and Ca ii 8542 Å bands and related surges were observed in both bands of the NST Fast Imaging Solar Spectrograph (FISS) instrument; (4) the elongated granule, seen in NST TiO data, is clear evidence of the emergence of positive flux to trigger the hot explosion; (5) the brightening in Solar Dynamics Observatory/Atmospheric Imaging Assembly 1600 Å images is quite consistent with the IRIS brightening. These observations suggest that our event is a hot explosion that occurred in the cool atmosphere of the Sun. In addition, our event appeared as an Ellerman bomb (EB) in the wing of Hα, although its intensity is weak and the vertical extent of the brightening seems to be relatively high compared with the typical EBs.

Intensity and Doppler velocity oscillations in pore atmospheres

We have investigated chromospheric traveling features running across two merged pores from their centers at speeds of about 55 km s−1, in the active region AR 11828. The pores were observed on 2013 August 24 by using high-time, spatial, and spectral resolution data from the Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope. We infer a line-of-sight (LOS) velocity by applying the lambdameter method to the Ca ii 8542 A band and Hα band, and investigate intensity and LOS velocity changes at different wavelengths and different positions at the pores. We find that they have three-minute oscillations, and the intensity oscillation from the line center (0.0