Convection,helioseismology and solar energy:personal reminiscence

This article is a brief history of my life from childhood and describes how I became interested in astronomy. Starting from researches using radiative transfer as a main tool, I gradually expanded my research field to hydrodynamics （particularly convection, turbulence, pulsation, waves and helioseismology）, magnetohydrodynamics and chaotic systems. My recent interest is to develop a sustainable society using solar energy.

Acoustic streaming and Sun's meridional circulation

A vast number of physical processes involving oscillations of a bounded viscous fluid are relevantly influenced by acoustic streaming. When this happens a steady circulation of fluid develops in a thin boundary adjacent to the interface. Some examples are refracted sound waves, a fluid inside a spherical cavity undergoing torsional oscillations or a pulsating liquid droplet. Steady streaming around circular interfaces consists of a hemispherically symmetric recirculation of fluid from the equatorial plane to the polar axes closely resembling the meridional circulation pattern observed in the Sun＇s convection zone that determines the solar cycle. In this paper, it is argued that the acoustic pulsations exhibited by the Sun would lead to acoustic streaming in the boundary of the convection zone. A simple estimation using a typical dominant frequency of 3 mHz and the observed surface oscillation amplitude yields a steady streaming velocity Us - 10 m s^-1, which is on the order of the meridional circulation velocity observed in the Sun＇s convection zone.

Magnetic non-potentiality on the quiet Sun and the filigree

From the observed vector magnetic fields by the Solar Optical Telescope/ Spectro-Polarimeter aboard the satellite Hinode, we have examined whether or not the quiet Sun magnetic fields are non-potential, and how the G-band filigrees and Ca II network bright points （NBPs） are associated with the magnetic non-potentiality. A sizable quiet region in the disk center is selected for this study. The new findings by the study are as follows. （1） The magnetic fields of the quiet region are obviously non-potential. The region-average shear angle is 40°, the average vertical current is 0.016A m^-2, and the average free magnetic energy density, 2.7× 10^2erg cm^-3. The magnitude of these non-potential quantities is comparable to that in solar active regions. （2） There are overall correlations among current helicity, free magnetic energy and longitudinal fields. The magnetic non-potentiality is mostly concentrated in the close vicinity of network elements which have stronger longitudinal fields. （3） The filigrees and NBPs are magnetically characterized by strong longitudinal fields, large electric helicity, and high free energy density. Because the selected region is away from any enhanced network, these new results can generally be applied to the quiet Sun. The findings imply that stronger network elements play a role in high magnetic non-potentiality in heating the solar atmosphere and in conducting the solar wind.

A study of the relation between intensity oscillations and magnetic field parameters in a sunspot: Hinode observations

We present properties of intensity oscillations of a sunspot in the photo- sphere and chromosphere using G band and Ca u H filtergrams from Hinode. Intensity power maps as function of magnetic field strength and frequency reveal reduction of power in the G band with an increase in photospheric magnetic field strength at all frequencies. In Ca II H, however, stronger fields exhibit more power at high frequen- cies, particularly in the 4.5-8.0 mHz band. Power distributions in different locations of the active region show that the oscillations in Ca II H exhibit more power compared to that of the G band. We also relate the power in intensity oscillations with differ- ent components of the photospheric vector magnetic field using near simultaneous spectro-polarimetric observations of the sunspot from the Hinode spectropolarime- ter. The photospheric umbral power is strongly anti-correlated with the magnetic field strength and its line-of-sight component but there is a good correlation with the trans- verse component. A reversal of this trend is observed in the chromosphere except at low frequencies （V≤ 1.5 mHz）. The power in sunspot penumbrae is anti-correlated with the magnetic field parameters at all frequencies （1.0 ≤ v ≤ 8.0 mHz） in both the photosphere and chromosphere, except that the chromospheric power shows a strong correlation in the frequency range 3-3.5 mHz.

Studies of grand minima in sunspot cycles by using a flux transport solar dynamo model

We propose that grand minima in solar activity are caused by simultane- ous fluctuations in the meridional circulation and the Babcock-Leighton mechanism for the poloidal field generation in the flux transport dynamo model. We present the following results： （a） fluctuations in the meridional circulation are more effective in producing grand minima; （b） both sudden and gradual initiations of grand minima are possible; （c） distributions of durations and waiting times between grand minima seem to be exponential; （d） the coherence time of the meridional circulation has an effect on the number and the average duration of grand minima, with a coherence time of about 30 yr being consistent with observational data. We also study the occurrence of grand maxima and find that the distributions of durations and waiting times between grand maxima are also exponential, like the grand minima. Finally we address the question of whether the Babcock-Leighton mechanism can be operative during grand minima when there are no sunspots. We show that an a-effect restricted to the upper portions of the convection zone can pull the dynamo out of the grand minima and can match various observational requirements if the amplitude of this a-effect is suitably fine-tuned.

What does the Sun tell and hint now?

Some historical records, which have held since the beginning of modern solar activity cycles, are being broken by the present Sun： cycle 23 records the longest cycle length and fall time; latitudes of high-latitude sunspots belonging to a new cycle around the minimum time of the cycle are statistically the lowest at present, compared with those of other cycles; there are only one or no sunspots in a month appearing at high latitudes for 58 months, which is the first time that such a long duration has been observed. The solar dynamo is believed to be slowing down due to： （1） the minimum smoothed monthly mean sunspot number is the smallest since cycle 16 onwards, and even probably among all modern solar cycles; and （2） once the time interval between the first observations of two neighboring sunspot groups is larger than 14 d, it should be approximately regarded as an observation of no sunspots on the visible solar disk, called a spotless event. Spotless events occur with the highest frequency around the minimum time of cycle 24, and the longest spotless event also appears around the minimum time for observations of the Sun since cycle 16. Cycle 24 is expected to have the lowest level of sunspot activity from cycle 16 onwards and even probably for all of the modern solar cycles.

A Statistical Study of Rapid Sunspot Structure Change Associated with Flares

We reported recently some rapid changes of sunspot structure in white-light （WL） associated with major flares. We extend the study to smaller events and present here results of a statistical study of this phenomenon. In total, we investigate 403 events from 1998 May 9 to 2004 July 17, including 40 X-class, 174 M-class, and 189 C-class flares. By monitoring the structure of the flaring active regions using the WL observations from the Transition Region and Coronal Explorer （TRACE）, we find that segments in the outer sunspot structure decayed rapidly right after many flares; and that, on the other hand, the central part of sunspots near the flare-associated magnetic neutral line became darkened. These rapid and permanent changes are evidenced in the time profiles of WL mean intensity and are not likely resulted from the flare emissions. Our study further shows that the outer sunspot structure decay as well as the central structure darkening are more likely to be detected in larger solar flares. For X-class flares, over 40% events show distinct sunspot structure change. For M- and C-class flares, this percentage drops to 17% and 10%, respectively. The results of this statistical study support our previously proposed reconnection picture, i.e., the flare-related magnetic fields evolve from a highly inclined to a more vertical configuration.

Solar flare forecasting based on sequential sunspot data

It is widely believed that the evolution of solar active regions leads to solar flares. However, information about the evolution of solar active regions is not employed in most existing solar flare forecasting models. In the current work, a short- term solar flare forecasting model is proposed, in which sequential sunspot data, in- cluding three days of information about evolution from active regions, are taken as one of the basic predictors. The sunspot area, the Mclntosh classification, the mag- netic classification and the radio flux are extracted and converted to a numerical for- mat that is suitable for the current forecasting model. Based on these parameters, the sliding-window method is used to form the sequential data by adding three days of information about evolution. Then, multi-layer perceptron and learning vector quanti- zation are employed to predict the flare level within 48 h. Experimental results indicate that the performance of the proposed flare forecasting model works better than previ- ous models.

Phase analysis of sunspot group numbers on both solar hemispheres

Cross-correlation analysis and wavelet transform methods are proposed to investigate the phase relationship between the monthly sunspot group numbers in the solar northern and southern hemispheres. It is found that （1） the monthly sunspot group numbers in the northern hemisphere begin two months earlier than those in the southern one, which should lead to phase asynchrony between them but with a slight effect; （2） the Schwabe cycle length for the monthly sunspot group numbers in the two hemispheres obviously differs from each other, and the mean Schwabe cycle length of the monthly sunspot group numbers in the northern hemisphere is slightly larger than that in the southern one; （3） the monthly sunspot group numbers in the northern hemisphere precede those in the southern hemisphere during the years of about 1874- 1927, after which, the southern hemisphere leads the northern hemisphere in the years 1928-1964, and then the northern hemisphere leads in time till the present.

Characterizing motion types of G-band bright points in the quiet Sun

We study the motion of G-band bright points （GBPs） in the quiet Sun to obtain the characteristics of different motion types. A high resolution image sequence taken with the Hinode/Solar Optical Telescope （SOT） is used, and GBPs are automat- ically tracked by segmenting 3D evolutional structures in a space-time cube. After putting the GBPs that do not move during their lifetimes aside, the non-stationary GBPs are categorized into three types based on an index of their motion type. Most GBPs that move in straight or nearly straight lines are categorized as a straight mo- tion type, a few moving in rotary paths into rotary motion, and the others fall into a motion type we called erratic. The mean horizontal velocities are 2.18±0.08 km s-1, 1.63±0.09km s^-1 and 1.33±0.07 km s^-1 for straight, erratic and rotary motion types, respectively. We find that a GBP drifts at a higher and constant velocity during its whole life if it moves in a straight line. However, it has a lower and variational velocity if it moves on a rotary path. The diffusive process is ballistic-, super- and sub-diffusion for straight, erratic and rotary motion types, respectively. The corresponding diffusion index （γ） and coefficients （K） are 2.13±0.09 and 850±37km^2 s^-1, 1.82±0.07 and 331 ±24 km^2 s^-1, and 0.73±0.19 and 13±9 km^2 s^-1. In terms of direction of motion, it is homogeneous and isotropic, and usually persists between neighboring frames, no matter what motion type a GBP is classified as.

The relationships of solar flares with both sunspot and geomagnetic activity

The relationships between solar flare parameters （total importance, time duration, flare index, and flux） and sunspot activity （R z ） as well as those between geomagnetic activity （aa index） and the flare parameters can be well described by an integral response model with the response time scales of about 8 and 13 months, respectively. Compared with linear relationships, the correlation coefficients of the flare parameters with R z , of aa with the flare parameters, and of aa with R z based on this model have increased about 6%, 17%, and 47% on average, respectively. The time delays between the flare parameters with respect to R z , aa to the flare parameters, and aa to R z at their peaks in a solar cycle can be predicted in part by this model （82%, 47%, and 78%, respectively）. These results may be further improved when using a cosine filter with a wider window. It implies that solar flares are related to the accumulation of solar magnetic energy in the past through a time decay factor. The above results may help us to understand the mechanism of solar flares and to improve the prediction of the solar flares.

Signature of high-order azimuthal MHD body modes in sunspot's low atmosphere

The five-minute oscillations inside sunspots appear to be the absorption of the solar p-mode. It is a potential tool to probe a sunspot＇s sub-structure. We studied the collective properties of five-minute oscillations in the power and phase distribution at the sunspot＇s umbra-penumbra boundary. The azimuthal distributions of the power and phase of five-minute oscillations enclosing a sunspot＇s umbra were obtained with images taken with the Solar Dynamics Observatory/Atmospheric Imaging Assembly （SDO/AIA）. The azimuthal modes were quantified with periodogram analysis and justified with significance tests. The azimuthal nodal structures in an approximately ax- ially symmetric sunspot AR 11131 （2010 Dec 08） were investigated. Mode numbers ra = 2, 3, 4, 7, 10 were obtained in both 1700 A and 1600A bandpasses. The 1600A channel also revealed an extra mode at m = 9. In the upper atmosphere （304 A）, fewer modes were detected at m = 3, 4, 7. The azimuthal modes in the sunspot＇s low atmo- sphere could be interpreted as high-order azimuthal MHD body modes. They were detected in the power and phase of the five-minute oscillations in sunspot AR 11131 with SDO/AIA data. Fewer modes were detected in the sunspot＇s upper atmosphere.

Magnetic cycles of Sun-like stars with different levels of coronal and chromospheric activity——comparison with the Sun

The atmospheric activity of the Sun and Sun-like stars is analyzed involving observations from the HK-project at the Mount Wilson Observatory, the California and Carnegie Planet Search Program at the Keck and Lick Observatories and the Magellan Planet Search Program at the Las Campanas Observatory. We show that for stars of E G and K spectral classes, the cyclic activity, similar to the 11-yr solar cycle, is different： it becomes more prominent in K-stars. Comparative study of Sun-like stars with different levels of chromospheric and coronal activity confirms that the Sun belongs to stars with a low level of chromospheric activity and stands apart among these stars by its minimum level of coronal radiation and minimum level of variations in photospheric flux.

Short-Term Period Variation of Relative Sunspot Numbers

We use wavelet transform to analyze the daily relative sunspot number series over solar cycles 10-23. The characteristics of some of the periods shorter than - 600-day are discussed. The results exhibit not only the variation of some short periods in the 14 solar cycles but also the characteristics and differences around solar peaks and valley years. The short periodic components with larger amplitude such as ～27, ～ 150 and ～360-day are obvious in some solar cycles, all of them are time-variable, also their lengths and amplitudes are variable and intermittent in time. The variable characteristics of the periods are rather different in different solar cycles.

Period ratios for standing kink and sausage modes in magnetized structures with siphon flow on the Sun

Standing oscillations with multiple periods have been found in a number of atmospheric struc- tures on the Sun. The ratio of the period of the fundamental to twice the one of its first overtone, P1/2192, is important in applications of solar magneto-seismology. We examine how field-aligned flows impact P1/2P2 of standing modes in solar magnetic cylinders. For coronal loops, the flow effects are significant for both fast kink and sausage modes. For kink modes, they reduce P1/2P2 by up to 17% relative to the static case even when the density contrast between the loop and its surroundings approaches infinity. For sausage modes, the reduction in P1/2P2 due to flow is typically ≤ 5.5% compared with the static case. However, the threshold aspect ratio, only above which can trapped sausage modes be supported, may increase dramatically with the flow magnitude. For photospheric tubes, the flow effect on P1/2P2 is not as strong. However, when applied to sausage modes, introducing field-aligned flows offers more possibilities in interpreting the multiple peri- ods that have recently been measured. We conclude that field-aligned flows should be taken into account to help better understand what causes the departure of P1/2P2 from unity.

Low-dimensional chaos and fractal properties of long-term sunspot activity

Two primary solar-activity indicators- sunspot numbers （SNs） and sunspot areas （SAs） in the time interval from November 1874 to December 2012 - are used to determine the chaotic and fractal properties of solar activity. The results show that （1） the long-term solar activity is governed by a low-dimensional chaotic strange attractor, and its fractal motion shows a long-term persistence on large scales; （2） both the fractal dimension and maximal Lyapunov exponent of SAs are larger than those of SNs, implying that the dynamical system of SAs is more chaotic and complex than SNs; （3） the predictions of solar activity should only be done for short- to mid-term behaviors due to its intrinsic complexity; moreover, the predictability time of SAs is obviously smaller than that of SNs and previous results.

Proton activity of the Sun in current solar cycle 24

We present a study of seven large solar proton events in the current solar cycle 24（from 2009 January up to the current date）. They were recorded by the GOES spacecraft with the highest proton fluxes being over 200 pfu for energies 〉10 Me V. In situ particle measurements show that：（1） The profiles of the proton fluxes are highly dependent on the locations of their solar sources, namely flares or coronal mass ejections（CMEs）, which confirms the ＂heliolongitude rules＂ associated with solar energetic particle fluxes;（2） The solar particle release（SPR） times fall in the decay phase of the flare emission, and are in accordance with the times when the CMEs travel to an average height of 7.9 solar radii; and（3） The time differences between the SPR and the flare peak are also dependent on the locations of the solar active regions. The results tend to support the scenario of proton acceleration by the CME-driven shock,even though there exists a possibility of particle acceleration at the flare site, with subsequent perpendicular diffusion of accelerated particles in the interplanetary magnetic field. We derive the integral time-of-maximum spectra of solar protons in two forms： a single power-law distribution and a power law roll-over with an exponential tail. It is found that the unique ground level enhancement that occurred in the event on 2012 May 17 displays the hardest spectrum and the largest roll-over energy which may explain why this event could extend to relativistic energies.

太阳表面震波能量与磁场强度的关系

过去的一系列研究表明在太阳光球表面,随着纵向磁场的增强,对应区域所观测到的震波能量随之降低.为了进一步研究震波和磁场的关系,使用日震和磁成像仪(Helioseismic and Magnetic Imager,HMI)的数据,通过对视向磁场进行势场外推得到磁场总强度,与震波能量进行关联.对一定总磁场区域的p模式震波能量拟合可以得到其随总磁场强度而变化的关系.研究证实了震波能量与磁场总强度有更强关联.不同活动区的结果表明p模式震波能量随着磁场总强度增加而下降的趋势在500 Gs以下非常接近,在500 Gs至1000 Gs范围内有略微差别.并且p模式震波能量的对数与总磁场强度有较强的线性相关性.对比迈克尔逊多普勒成像仪(Michelson Doppler Imager,MDI)和HMI得到的结果时发现,虽然MDI观测的磁场结果整体会比HMI观测结果偏大,但归一化p模式震波能量与磁场强度的关系分析对比结果相差不大.

日震学研究进展

日震学旨在通过对太阳震荡的观测和分析来研究太阳内部的结构和动力学。回顾了日震学产生和发展的历史过程,简单介绍了太阳震荡的基本性质和部分观测仪器,综述了全球日震学与局部日震学的研究方法和近年来取得的部分重要进展,指出了日震学研究是理解太阳内部结构不可替代的工具,其研究成果具有极其重要的意义。

Automated lfare forecasting using a statistical learning technique

We present a new method for automatically forecasting the occurrence of solar flares based on photospheric magnetic measurements. The method is a cascading combination of an ordinal logistic regression model and a support vector machine classifier. The predictive variables are three photospheric magnetic parameters, i.e., the total unsigned magnetic flux, length of the strong-gradient magnetic polarity inversion line, and total magnetic energy dissipation. The output is true or false for the occurrence of a certain level of flares within 24 hours. Experimental results, from a sample of 230 active regions between 1996 and 2005, show the accuracies of a 24- hour flare forecast to be 0.86, 0.72, 0.65 and 0.84 respectively for the four different levels. Comparison shows an improvement in the accuracy of X-class flare forecasting.