Kwangsu Ahn

HIGHEST RESOLUTION OBSERVATIONS OF THE QUIETEST SUN

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 Suns photosphere. The telescopes 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.

A 60 GHz Wideband Phased-Array LNA With Short-Stub Passive Vector Generator

This letter presents 60 GHz 5 b phased-array low noise amplifier (LNA) implemented in 90 nm CMOS for a short range wireless application. The design consists of common source two-stage LNA, short-stub vector generator, I/Q modulator with 5 b digital to analog converter and differential to single amplifier. All the proposed amplifiers are designed using a transformer coupled method which results in wideband operation to meet the frequency requirement of the standard. The proposed circuit provides 360 phase controllability over 50-70 GHz band while achieving 12.5 dB gain, 6.55 dB NF and consuming 50 mA from 1.2 V. The measured rms phase error is in the phase accuracy limitation (differential phase array) and gain variation is quite low (<; 0.92 dB) over 5 b control range.

Patterns of Flows in an Intermediate Prominence Observed by Hinode

The investigation of plasma flows in filaments/prominences gives us clues to understanding their magnetic structures. We studied the patterns of flows in an intermediate prominence observed by Hinode/SOT. By examining a time series of Hα images and Caii H images, we have found horizontal flows in the spine and vertical flows in the barb. Both of these flows have a characteristic speed of 10–20 km s −1 . The horizontal flows displayed counterstreaming. Our detailed investigation revealed that most of the moving fragments in fact reversed direction at the end point of the spine near a footpoint close to the associated active region. These returning flows may be one possible explanation of the well-known counterstreaming flows in prominences. In contrast, we have found vertical flows—downward and upward—in the barb. Most of the horizontal flows in the spine seem to switch into vertical flows when they approach the barb, and vice versa. We propose that the net force resulting from a small deviation from magnetohydrostatic equilibrium, where magnetic fields are predominantly horizontal, may drive these patterns of flow. In the prominence studied here, the supposed magnetohydrostatic configuration is characterized by magnetic field lines sagging with angles of 13 ◦ and 39 ◦ in the spine and the barb, respectively.

Nasmyth focus instrumentation of the New Solar Telescope at Big Bear Solar Observatory

The largest solar telescope, the 1.6-m New Solar Telescope (NST) has been installed and is being commissioned at Big Bear Solar Observatory (BBSO). It has an off-axis Gregorian configuration with a focal ratio of F/52. Early in 2009, first light scientific observations were successfully made at the Nasmyth focus, which is located on the east side of the telescope structure. As the first available scientific instruments for routine observation, Nasmyth focus instrumentation (NFI) consists of several filtergraphs offering high spatial resolution photometry in G-band 430 nm, Ha 656 nm, TiO 706 nm, and covering the near infrared 1083 nm, 1.6 μm, and 2.2 μm. With the assistance of a local correlation tracker system, diffraction limited images were obtained frequently over a field-of-view of 70 by 70 after processed using a post-facto speckle reconstruction algorithm. These data sets not only serve for scientific analysis with an unprecedented spatial resolution, but also provide engineering feedback to the NST operation, maintenance and optimization. This paper reports on the design and the implementation of NFI in detail. First light scientific observations are presented and discussed.

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.

New Solar Telescope Observations of Magnetic Reconnection Occurring in the Chromosphere of the Quiet Sun

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.

High-resolution observations of flare precursors in the low solar atmosphere

Solar flares are generally believed to be powered by free magnetic energy stored in the corona1, but the build up of coronal energy alone may be insufficient to trigger the flare to occur2. The flare onset mechanism is a critical but poorly understood problem, insights into which could be gained from small-scale energy releases known as precursors. These precursors are observed as small pre-flare brightenings in various wavelengths3–13 and also from certain small-scale magnetic configurations such as opposite-polarity fluxes14–16, where the magnetic orientation of small bipoles is opposite to that of the ambient main polarities. However, high-resolution observations of flare precursors together with the associated photospheric magnetic field dynamics are lacking. Here we study precursors of a flare using the unprecedented spatiotemporal resolution of the 1.6-m New Solar Telescope, complemented by new microwave data. Two episodes of precursor brightenings are initiated at a small-scale magnetic channel17–20 (a form of opposite-polarity flux) with multiple polarity inversions and enhanced magnetic fluxes and currents, lying near the footpoints of sheared magnetic loops. Microwave spectra corroborate that these precursor emissions originate in the atmosphere. These results provide evidence of low-atmospheric small-scale energy release, possibly linked to the onset of the main flare. Magnetic energy powers explosive flares on the Sun. Now, observations of unprecedented resolution identify the precursors of such flares in the lower solar atmosphere. These findings will help to constrain theoretical models of flare formation.

OSCILLATORY BEHAVIOR IN THE QUIET SUN OBSERVED WITH THE NEW SOLAR TELESCOPE

Surface photometry of the quiet Sun has achieved an angular resolution of 01 with the New Solar Telescope at Big Bear Solar Observatory, revealing that a disproportionate fraction of the oscillatory events appear above observed bright point-like structures. During the tracking of these structures, we noted that the more powerful oscillatory events are cospatial with them, indicating that observed flux tubes may be the source of many observed oscillatory events.

CAPABILITY OF THE FAST IMAGING SOLAR SPECTROGRAPH ON NST/BBSO FOR OBSERVING FILAMENTS/PROMINENCES AT THE SPECTRAL LINES Hα, Ca II 8542, AND Ca II K

Spectral line profiles of filaments/prominences to be observed by the Fast Imaging Solar Spectrograph (FISS) are studied. The main spectral lines of interests are Hα, Ca II 8542, and Ca II K. FISS has a high spectral resolving power of 2×10 5 , and supports simultaneous dual-band recording. This instrument will be installed at the 1.6m New Solar Telescope (NST) of Big Bear Solar Observatory, which has a high spatial resolution of 0.065˝ at 500nm. Adopting the cloud model of radiative transfer and using the model parameters inferred from pre-existing observations, we have simulated a set of spectral profiles of the lines that are emitted by a filament on the disk or a prominence at the limb. Taking into account the parameters of the instrument, we have estimated the photon count to be recorded by the CCD cameras, the signal-to-noise ratios, and so on. We have also found that FISS is suitable for the study of multi-velocity threads in filaments if the spectral profiles of Ca II lines are recorded together with Ha lines.

CHARACTERISTIC LENGTH OF ENERGY-CONTAINING STRUCTURES AT THE BASE OF A CORONAL HOLE

An essential parameter for models of coronal heating and fast solar wind acceleration that rely on the dissipation of MHD turbulence is the characteristic energy-containing length ?? of the squared velocity and magnetic field fluctuations (u 2 and b 2) transverse to the mean magnetic field inside a coronal hole (CH) at the base of the corona. The characteristic length scale directly defines the heating rate. We use a time series analysis of solar granulation and magnetic field measurements inside two CHs obtained with the New Solar Telescope at Big Bear Solar Observatory. A data set for transverse magnetic fields obtained with the Solar Optical Telescope/Spectro-Polarimeter on board the Hinode spacecraft was utilized to analyze the squared transverse magnetic field fluctuations . Local correlation tracking was applied to derive the squared transverse velocity fluctuations u 2. We find that for u 2 structures, the Batchelor integral scale ? varies in a range of 1800-2100?km, whereas the correlation length and the e-folding length L vary between 660 and 1460?km. Structures for yield ? 1600?km, 640?km, and L 620?km. An averaged (over ?, , and L) value of the characteristic length of u 2 fluctuations is 1260?? 500?km, and that of is 950?? 560?km. The characteristic length scale in the photosphere is approximately 1.5-50?times smaller than that adopted in previous models (3-30?? 103?km). Our results provide a critical input parameter for current models of coronal heating and should yield an improved understanding of fast solar wind acceleration.