Kiyoto Shibasaki

THE HARD X-RAY TELESCOPE (HXT) FOR THE SOLAR-A MISSION*

The Hard X-ray Telescope (HXT) is a Fourier-synthesis imager; a set of spatially-modulated photon count data are taken from 64 independent subcollimators and are Fourier-transformed into an image by using procedures such as the maximum entropy method (MEM) or CLEAN. The HXT takes images of solar flares simultaneously in four energy bands, nominally 15 (or 19)–24, 24–35, 35–57, and 57–100 keV, with an ultimate angular resolution as fine as ∼ 5 arc sec and a time resolution 0.5 s. Each subcollimator has a field of view wider than the solar disk. The total effective area of the collimator/detector system reaches ∼ 70 cm2, about one order of magnitude larger than that of the HINOTORI hard X-ray imager. Thanks to these improvements, HXT will for the first time enable us to take images of flares at photon energies above ∼ 30 keV. These higher-energy images will be compared with lower-energy ones, giving clues to the understanding of nonthermal processes in solar flares, i.e., the acceleration and confinement of energetic electrons. It is of particular importance to specify the acceleration site with regard to the magnetic field figuration in a flaring region, which will be achieved by collaborative observations between HXT and the Soft X-ray Telescope on board the same mission.

Evidence for Alfvén Waves in Solar X-ray Jets

Coronal magnetic fields are dynamic, and field lines may misalign, reassemble, and release energy by means of magnetic reconnection. Giant releases may generate solar flares and coronal mass ejections and, on a smaller scale, produce x-ray jets. Hinode observations of polar coronal holes reveal that x-ray jets have two distinct velocities: one near the Alfvén speed (∼800 kilometers per second) and another near the sound speed (200 kilometers per second). Many more jets were seen than have been reported previously; we detected an average of 10 events per hour up to these speeds, whereas previous observations documented only a handful per day with lower average speeds of 200 kilometers per second. The x-ray jets are about 2 × 103 to 2 × 104 kilometers wide and 1 × 105 kilometers long and last from 100 to 2500 seconds. The large number of events, coupled with the high velocities of the apparent outflows, indicates that the jets may contribute to the high-speed solar wind.

Continuous plasma outflows from the edge of a solar active region as a possible source of solar wind

The Sun continuously expels a huge amount of ionized material into interplanetary space as the solar wind. Despite its influence on the heliospheric environment, the origin of the solar wind has yet to be well identified. In this paper, we report Hinode X-ray Telescope observations of a solar active region. At the edge of the active region, located adjacent to a coronal hole, a pattern of continuous outflow of soft-x-ray–emitting plasmas was identified emanating along apparently open magnetic field lines and into the upper corona. Estimates of temperature and density for the outflowing plasmas suggest a mass loss rate that amounts to ∼1/4 of the total mass loss rate of the solar wind. These outflows may be indicative of one of the solar wind sources at the Sun.

Spatially resolved microwave pulsations of a flare loop

A microwave burst with quasi-periodic pulsations was studied with high spatial resolution using observations with the Nobeyama Radioheliograph (NoRH). We found that the time profiles of the microwave emission at 17 and 34 GHz exhibit quasi-periodic (with two well defined periods P1 = 14-17 s and P2 = 8-11 s) variations of the intensity at different parts of an observed flaring loop. Detailed Fourier analysis shows the P1 spectral component to be dominant at the top, while the P2 one near the feet of the loop. The 14-17 s pulsations are synchronous at the top and in both legs of the loop. The 8-11 s pulsations at the legs are well correlated with each other but the correlation is not so obvious with the pulsations at the loop top. For this P2 spectral component, a definite phase shift, P2/4 ≈ 2.2 s, between pulsations in the northern leg and loop top parts of the loop have been found. The length of the flaring loop is estimated as L = 25 Mm (≈34 �� ) and its average width at half intensity at 34 GHz as about 6 Mm (≈8 �� ). Microwave diagnostics shows the loop to be filled with a dense plasma with the number density n0 ≈ 10 11 cm −3 , penetrated by the magnetic field changing from B0 ≈ 100 G near the loop top up to B0 ≈ 200 G near the north footpoint. A comparative analysis of different MHD modes of the loop demonstrates the possibility of the simultaneous existence of two modes of oscillations in the loop: the global sausage mode, with the period P1 = 14-17 s and the nodes at the footpoints, and a higher harmonics mode (possibly with the radial wave number l > 1), with P2 = 8-11 s.

Loop-Top Nonthermal Microwave Source in Extended Solar Flaring Loops

Nobeyama Radioheliograph (NoRH) microwave data provide us with unique information about the radio brightness distribution along flaring loops. In particular, it has been found that for several events with extended looplike sources well resolved with NoRH, the brightness maximum at 17 and 34 GHz is located at the top of the corresponding flaring loops. The detailed analysis of these events strongly suggests that the distribution of mildly relativistic electrons along an extended flaring loop must be highly inhomogeneous: accelerated electrons are concentrated in the upper part of the loop. This finding imposes important new constraints on the acceleration/injection mechanisms and on the kinetics of high-energy particles in solar flares.

Behavior of Solar Cycles 23 and 24 Revealed by Microwave Observations

Using magnetic and microwave butterfly diagrams, we compare the behavior of solar polar regions to show that (1) the polar magnetic field and the microwave brightness temperature during solar minimum substantially diminished during the cycle 23/24 minimum compared to the 22/23 minimum. (2) The polar microwave brightness temperature (Tb) seems to be a good proxy for the underlying magnetic field strength (B). The analysis indicates a relationship, B = 0.0067Tb - 70, where B is in G and Tb in K. (3) Both the brightness temperature and the magnetic field strength show north-south asymmetry most of the time except for a short period during the maximum phase. (4) The rush-to-the-pole phenomenon observed in the prominence eruption (PE) activity seems to be complete in the northern hemisphere as of 2012 March. (5) The decline of the microwave brightness temperature in the north polar region to the quiet-Sun levels and the sustained PE activity poleward of 60oN suggest that solar maximum conditions have arrived at the northern hemisphere. The southern hemisphere continues to exhibit conditions corresponding to the rise phase of solar cycle 24.

THREE-MINUTE OSCILLATIONS ABOVE SUNSPOT UMBRA OBSERVED WITH THE SOLAR DYNAMICS OBSERVATORY/ATMOSPHERIC IMAGING ASSEMBLY AND NOBEYAMA RADIOHELIOGRAPH

Three-minute oscillations over a sunspots umbra in AR 11131 were observed simultaneously in UV/EUV emission by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and in radio emission by the Nobeyama Radioheliograph (NoRH). We use 24 hr series of SDO and 8 hr series of NoRH observations to study spectral, spatial, and temporal variations of pulsations in the 5-9 mHz frequency range at different layers of the solar atmosphere. High spatial and temporal resolution of SDO/AIA in combination with long-duration observations allowed us to trace the variations of the cutoff frequency and spectrum of oscillations across the umbra. We found that higher frequency oscillations are more pronounced closer to the umbras center, while the lower frequencies concentrate on the peripheral parts. We interpreted this discovery as a manifestation of variation of the magnetic field inclination across the umbra at the level of temperature minimum. Possible implications of this interpretation for the diagnostics of sunspot atmospheres are discussed.Three-minute oscillations over sunspots umbra in AR 11131 were observed simultaneously in UV/EUV emission by SDO/AIA and in radio emission by Nobeyama Radioheliograph (NoRH). We use 24-hours series of SDO and 8-hours series of NoRH observations to study spectral, spatial and temporal variations of pulsations in the 5-9 mHz frequency range at different layers of the solar atmosphere. High spatial and temporal resolution of SDO/AIA in combination with long-duration observations allowed us to trace the variations of the cut-off frequency and spectrum of oscillations across the umbra. We found that higher frequency oscillations are more pronounced closer to the umbras center, while the lower frequencies concentrate to the peripheral parts. We interpreted this discovery as a manifestation of variation of the magnetic field inclination across the umbra at the level of temperature-minimum. Possible implications of this interpretation for the diagnostics of sunspot atmospheres is discussed.

Microwave Detection of Umbral Oscillation in NOAA Active Region 8156: Diagnostics of Temperature Minimum in Sunspot

A radio brightness oscillation of 3 minutes at 17 GHz was detected in a compact radio source associated with a sunspot umbra in the NOAA active region 8156. We interpret the radio brightness oscillation by the density and temperature fluctuations due to upward-traveling acoustic waves through the third harmonic gyroresonance layer (2000 G). SOHO/SUMER observed the same active region close to the time of radio observations by the Nobeyama Radioheliograph. Transition region line observations by SUMER showed both velocity and intensity oscillations with the period of 3 minutes, and the oscillation was interpreted as an upward-traveling acoustic wave. We applied the value of the density and temperature fluctuations deduced from the SUMER experiment to the gyroresonance emission in the transition region and found good agreement with the detected radio brightness oscillation. The origin of the 3 minute oscillation is attributed to the resonant excitation of the cutoff frequency mode of the temperature plateau around the temperature minimum without assuming the chromospheric cavity. We can estimate the temperature of the temperature minimum region in the umbra from the measured frequency.

Multi-mode quasi-periodic pulsations in a solar flare

Context. Quasi-periodic pulsations (QPP) of the electromagnetic radiation emitted in solar and stellar flares are often detected in microwave, white light, X-ray, and gamma-ray bands. Mechanisms for QPP are intensively debated in the literature. Previous studies revealed that QPP may manifest non-linear, non-stationary and, perhaps, multi-modal processes operating in flares. Aims. We study QPP of the microwave emission generated in an X3.2-class solar flare on 14 May, 2013, observed with the Nobeyama Radioheliograph (NoRH), aiming to reveal signatures of the non-linear, non-stationary, and multi-modal processes in the signal. Methods. The NoRH correlation signal obtained at the 17 GHz intensity has a clear QPP pattern. The signal was analysed with the Hilbert-Huang transform (HHT) that allows one to determine its instant amplitude and frequency, and their time variation. Results. It was established that the QPP consists of at least three well-defined intrinsic modes, with the mean periods of 15, 45, and 100 s. All the modes have quasi-harmonic behaviour with different modulation patterns. The 100 s intrinsic mode is a decaying oscillation, with the decay time of 250 s. The 15 s intrinsic mode shows a similar behaviour, with the decay time of 90 s. The 45 s mode has a wave-train behaviour. Conclusions. Dynamical properties of detected intrinsic modes indicate that the 100 s and 15 s modes are likely to be associated with fundamental kink and sausage modes of the flaring loop, respectively. The 100 s oscillation could also be caused by the fundamental longitudinal mode, while this interpretation requires the plasma temperature of about 30 million K and hence is not likely. The 45 s mode could be the second standing harmonics of the kink mode.

Pulsations of Microwave Emission and Flare Plasma Diagnostics

We consider the modulation of nonthermal gyrosynchrotron emission from solar flares by the ballooning and radial oscillations of coronal loops. The damping mechanisms for fast magnetoacoustic modes are analyzed. We suggest a method for diagnosing the plasma of flare loops that allows their main parameters to be estimated from peculiarities of the microwave pulsations. Based on observational data obtained with the Nobeyama Radioheliograph (17 GHz) and using a technique developed for the event of May 8, 1998, we determined the particle density n≈3.7×1010 cm−3, the temperature T≈4×107 K, and the magnetic field strength B≈220 G in the region of flare energy release. A wavelet analysis for the solar flare of August 28, 1999, has revealed two main types of microwave oscillations with periods P1≈7, 14 s and P2≈2.4 s, which we attribute to the ballooning and radial oscillations of compact and extended flare loops, respectively. An analysis of the time profile for microwave emission shows evidence of coronal loop interaction. We determined flare plasma parameters for the compact (T≈5.3×107 K, n≈4.8≈1010 cm−3, B≈280 G) and extended (T≈2.1≈107 K, n≈1.2≈1010 cm−3, B≈160 G) loops. The results of the soft X-ray observations are consistent with the adopted model.