Determination of the Topology Skeleton of Magnetic Fields in a Solar Active Region

Magnetic topology has been a key to the understanding of magnetic energy release mechanism. Based on observed vector magnetograms, we have determined the threedimensional （3D） topology skeleton of the magnetic fields in the active region NOAA 10720. The skeleton consists of six 3D magnetic nulls and a network of corresponding spines, fans, and null-null lines. For the first time, we have identified a spiral magnetic null in Sun＇s corona. The magnetic lines of force twisted around the spine of the null, forming a ＇magnetic wreath＇ with excess of free magnetic energy and resembling observed brightening structures at extraultraviolet （EUV） wavebands. We found clear evidence of topology eruptions which are referred to as catastrophic changes of topology skeleton associated with a coronal mass ejection （CME） and an explosive X-ray flare. These results shed new lights on the structural complexity and its role in explosive magnetic activity. The concept of flux rope has been widely used in modelling explosive magnetic activity, although their observational identity is rather obscure or, at least, lacking of necessary details up to date. We suggest that the magnetic wreath associated with the 3D spiral null is likely an important class of the physical entity of flux ropes.

Prediction of solar activities:Sunspot numbers and solar magnetic synoptic maps

The evolution of solar magnetic fields is significant for understanding and predicting solar activities.And our knowledge of solar magnetic fields largely depends on the photospheric magnetic field.In this paper,based on the spherical harmonic expansion of the photospheric magnetic field observed by Wilcox Solar Observatory,we analyze the time series of spherical harmonic coefficients and predict Sunspot Number as well as synoptic maps for Solar Cycle 25.We find that solar maximum years have complex short-period disturbances,and the time series of coefficient g_(7)~0 is nearly in-phase with Sunspot Number,which may be related to solar meridional circulation.Utilizing Long Short-Term Memory networks(LSTM),our prediction suggests that the maximum of Solar Cycle 25 is likely to occur in June 2024 with an error of 8 months,the peak sunspot number may be 166.9±22.6,and the next solar minimum may occur around January 2031.By incorporating Empirical Mode Decomposition,we enhance our forecast of synoptic maps truncated to Order 5,validating their relative reliability.This prediction not only addresses a gap in forecasting the global distribution of the solar magnetic field but also holds potential reference value for forthcoming solar observation plans.

Magnetohydrodynamic process in solar activity

Magnetohydrodynamics is one of the major disciplines in solar physics. Vigorous magnetohydrodynamic process is taking place in the solar convection zone and atmosphere. It controls the generating and structuring of the solar mag- netic fields, causes the accumulation of magnetic non-potential energy in the solar atmosphere and triggers the explosive magnetic energy release, manifested as vi- olent solar flares and coronal mass ejections. Nowadays detailed observations in solar astrophysics from space and on the ground urge a great need for the studies of magnetohydrodynamics and plasma physics to achieve better understanding of the mechanism or mechanisms of solar activity. On the other hand, the spectac- ular solar activity always serves as a great laboratory of magnetohydrodynamics. In this article, we reviewed a few key unresolved problems in solar activity studies and discussed the relevant issues in solar magnetohydrodynamics.

Observations of the solar corona during the total solar eclipse on 21 August 2017

The Sun’s tenuous outer atmosphere,the corona,emits only about one-millionth as much light as the solar surface.In ancient times the corona was observed by human beings only during total solar eclipses,when the strong emission of visible light from the photosphere was completely blocked by the Moon.In 1931,

The Kinematic Theory of Solar Dynamo

Generation of the Sun's magnetic fields by self-inductive processes in the solar electrically conducting interior, the solar dynamo theory, is a fundamentally important subject in astrophysics. The kinematic dynamo theory concerns how the magnetic fields are produced by kinematically possible flows without being constrained by the dynamic equation. We review a number of basic aspects of the kinematic dynamo theory, including the magnetohydrodynamic approximation for the dynamo equation, the impossibility of dynamo action with the solar differential rotation, the Cowling's anti-dynamo theorem in the solar context, the turbulent alpha effect and recently constructed three-dimensional interface dynamos controlled by the solar tachocline at the base of the convection zone.

Mapping the magnetic field in the solar corona through magnetoseismology

Magnetoseismology,a technique of magnetic field diagnostics based on observations of magnetohydrodynamic(MHD)waves,has been widely used to estimate the field strengths of oscillating structures in the solar corona.However,previously magnetoseismology was mostly applied to occasionally occurring oscillation events,providing an estimate of only the average field strength or one-dimensional distribution of field strength along an oscillating structure.This restriction could be eliminated if we apply magnetoseismology to the pervasive propagating transverse MHD waves discovered with the Coronal Multi-channel Polarimeter(CoMP).Using several CoMP observations of the Fe xiii 1074.7 nm and 1079.8 nm spectral lines,we obtained maps of the plasma density and wave phase speed in the corona,which allow us to map both the strength and direction of the coronal magnetic field in the plane of sky.We also examined distributions of the electron density and magnetic field strength,and compared their variations with height in the quiet Sun and active regions.Such measurements could provide critical information to advance our understanding of the Sun's magnetism and the magnetic coupling of the whole solar atmosphere.

Microwave diagnostics of magnetic field strengths in solar flaring loops

We have performed microwave diagnostics of the magnetic field strengths in solar flaring loops based on the theory of gyrosynchrotron emission.From Nobeyama Radioheliograph observations of three flare events at 17 and 34 GHz,we obtained the degree of circular polarization and the spectral index of microwave flux density,which were then used to map the magnetic field strengths in post-flare loops.Our results show that the magnetic field strength typically decreases from ~800 G near the loop footpoints to~100 G at a height of 10-25 Mm.Comparison of our results with magnetic field modeling using a flux rope insertion method is also discussed.Our study demonstrates the potential of microwave imaging observations,even at only two frequencies,in diagnosing the coronal magnetic field of flaring regions.

An empirical method to correct the saturation seen in the SOHO/MDI magnetograms

The magnetic field of the umbrae is sometimes found to be saturated in the magnetograms taken by the Michelson Doppler Imager （MDI） onboard the Solar and Heliospheric Observatory （SOHO）. It is suggested that the combination of the low intensity of sunspot umbrae and the limitation of the 15-bit onboard numerical data acquisition leads to this saturation. In this paper, we propose to use the MDI＇s intensity data to correct this saturation. This method is based on the well-established relationship between the continuum intensity and the magnetic field （the so-called I-B relationship）. A comparison between the corrected magnetic field and the data taken by the Stokes-Polarimeter of the Solar Optical Telescope （SOT/SP） onboard Hinode shows a reasonable agreement, suggesting that this correction is effective.

Origin and structures of solar eruptions Ⅱ: Magnetic modeling

The topology and dynamics of the three-dimensional magnetic field in the solar atmosphere govern various solar eruptive phenomena and activities, such as flares, coronal mass ejections, and filaments/prominences. We have to observe and model the vector magnetic field to understand the structures and physical mechanisms of these solar activities. Vector magnetic fields on the photosphere are routinely observed via the polarized light, and inferred with the inversion of Stokes profiles. To analyze these vector magnetic fields, we need first to remove the 180° ambiguity of the transverse components and correct the projection effect. Then, the vector magnetic field can be served as the boundary conditions for a force-free field modeling after a proper preprocessing. The photospheric velocity field can also be derived from a time sequence of vector magnetic fields.Three-dimensional magnetic field could be derived and studied with theoretical force-free field models, numerical nonlinear force-free field models, magnetohydrostatic models, and magnetohydrodynamic models. Magnetic energy can be computed with three-dimensional magnetic field models or a time series of vector magnetic field. The magnetic topology is analyzed by pinpointing the positions of magnetic null points, bald patches, and quasi-separatrix layers. As a well conserved physical quantity,magnetic helicity can be computed with various methods, such as the finite volume method, discrete flux tube method, and helicity flux integration method. This quantity serves as a promising parameter characterizing the activity level of solar active regions.

Application of the three-dimensional telegraph equation to cosmic-ray transport

An analytical solution to the three-dimensional telegraph equation is presented. This equation has recently received some attention but so far the treatment has been one-dimensional. By using the structural similarity to the Klein-Gordon equation, the telegraph equation can be solved in closed form. Illustrative examples are used to discuss the qualitative differences from the diffusion solution. A comparison with a numerical test-particle simulation reveals that some features of an intensity profile can be better explained using the telegraph approach.

Statistical study of co-rotating interaction region properties with STEREO and ACE observations

We analyzed the data on co-rotating interaction regions （CIRs） measured by the Advanced Composition Explorer （ACE） and Solar TErrestrial RElations Observatory （STEREO） from 2007 to 2010. The CIRs were observed by STEREO B （STB）, ACE and STEREOA （STA） one after another, and a total of 28 CIRs were identified in this work. Since the same characteristics of CIRs were detected by these three spacecraft at three different locations and times, these data can help us to study the evolutions of CIRs. For a single event, the properties of CIRs observed by the three spacecraft were quite different and could be explained by spatial or temporal variations. For all these 28 CIRs, STA and STB observed similar mean parameters, such as peak magnetic field strength （offset 11%）, peak and change in solar wind speed （offset 3% and 10% respectively）, peak proton temperature （offset 14%） and peak perpendicular pressure （offset 15%）. Surprisingly, STA detected much higher （41%） peak density of protons than STB.

Recent advances in solar storm studies in China

"Solar storm" has been commonly accepted by academic community and the public as a very popular scientific term. It is avivid description of violent ejections of a huge amount of magnetized plasma from the Sun as strong flare/CMEs, which sweepover into interplanetary space, develop, and affect our space environment. The solar storm could bring us disastrous spaceweather, destroy crucial technology, and cause a large-scale blackout. It is one of the natural disasters faced by modern humanbeings. Here we first briefly summarize the observational features of solar storms and introduce some key issues, and then wefocus on major advances in observational studies. We mainly introduce the efforts made by the Chinese scientists and comment on the challenges and opportunities that they are facing. In this era when scientific breakthroughs in solar storm studiescrucially depend on space-borne devices and large-aperture ground-based telescopes, the Chinese solar research communityneeds to develop its own major observational facilities and improve space weather forecasting abilities.

Effect of Volcanic Activities and CO_2 Concentration on Summer Temperature in Middle Stratosphere,North Hemisphere

Successive filtration and comparison show that the stratosphere air temperature in 10 hPa-layer of the Northern Hemisphere （NH） in July continuously increases,which is associated to the increases in greenhouse gases mostly CO2,volcanic activities,and solar activity,demonstrating the follows.（a） The increase in CO2 concentration is largely consistent with that of the stratosphere air temperature in 10 hPa-layer of the NH in July.However,the increase in the air temperature is not in a linear pattern,during which several cooling events interrupt.The cooling events between late 1960s and late 1970s are remarkable ones and so is the one before mid 1990s.Analysis shows that these events are induced by volcanic activities and solar activity.（b） The CO2-free variation in the stratosphere air temperature in 10 hPa-layer of the NH is consistent with that of the solar magnetic index.The wave crests and wave troughs of the two curves are consistent in phase,and the curve of solar magnetic index leads the other slightly.In other words,when the solar magnetic pole is southward,a warming in the NH stratosphere corresponds;and on the contrary,the northward solar magnetic pole corresponds to a cooling event.The variation in solar magnetic polarity strongly impacts the variation in the stratosphere temperature.