Solar Impulsive Hard X-Ray Emission and Two-Stage Electron Acceleration

Heating and acceleration of electrons in solar impulsive hard X-ray （HXR） flares are studied according to the two-stage acceleration model developed by Zhang for solar ^3Herich events. It is shown that electrostatic H-cyclotron waves can be excited at a parallel phase velocity less than about the electron thermal velocity and thus can significantly heat the electrons （up to 40 MK） through landau resonance. The preheated electrons with velocities above a threshold are further accelerated to high energies in the flare-acceleration process. The flareproduced electron spectrum is obtained and shown to be thermal at low energies and power law at high energies. In the non-thermal energy range, the spectrum can be double power law if the spectral power index is energy dependent or related. The electron energy spectrum obtained by this study agrees quantitatively with the result derived from the Reuven Ramaty High Energy Solar Spectroscopic Imager （RHESSI） HXR observations in the flare of 2002 July 23. The total flux and energy flux of electrons accelerated in the solar flare also agree with the measurements.

Investigation of the possible source for the solar energetic particle event on 2017 September 10

According to the solar proton data observed by Geostationary Operational Environmental Satellites （GOES）, ground-based neutron monitors on Earth and near-relativistic electron data measured by the ACE spacecraft, the onset times of protons with different energies and near-relativistic electrons have been estimated and compared with the time of solar soft and hard X-ray and radio burst data. The results show that first arriving relativistic and non-relativistic protons and electrons may have been accelerated by the concurrent flare. The results also suggest that release times of protons with different energies may be different, and the protons with lower energy may have been released earlier than those with higher energy. Some protons accelerated by concurrent flares may be further accelerated by the shock driven by the associated CME.

Is the enhancement of type Ⅱ radio bursts during CME interactions related to the associated solar energetic particle event?

We investigated 64 pairs of interacting-CME events identified from simultaneous observations by the SOHO and STEREO spacecraft from January 2010 to August 2014, to examine the relationship between large SEP events in the energy range of ～25 to～60 MeV and properties of the interacting CMEs.We found that during CME interactions, the large SEP events in this study were all generated by CMEs with the presence of enhanced type Ⅱ radio bursts, which also have wider longitudinal distributions compared to events without a type Ⅱ radio burst or its enhancement(almost always associated with small SEP events).It seems that the signature of type Ⅱ radio burst enhancement is a good discriminator between large SEP and small or no SEP event producers during CME interactions. The type Ⅱ radio burst enhancement is more likely to be generated by CME interactions, with the main CME having a larger speed(v), angular width(WD), mass(m) and kinetic energy(Ek), and taking over the preceding CMEs. The preceding CMEs in these instances have higher v, WD, m and Ekthan those in CME pairs missing type Ⅱ radio bursts or enhancements. Generally, the values of these properties in the type-Ⅱ-enhanced events are typically higher than the corresponding non-type-Ⅱ or non-type-Ⅱ-enhanced cases for both the main and preceding CMEs. Our analysis also revealed that the intensities of associated SEP events correlate negatively with the intersection height of the two CMEs. Moreover, the overlap width of two CMEs is typically larger in type-Ⅱ-enhanced events than in non-type-Ⅱ or non-type-Ⅱ-enhanced events. Most type-Ⅱ-enhanced events and SEP events are coincident and are almost always made by the fast and wide main CMEs that sweep fully over relatively slower and narrower preceding CMEs. We suggest that a fast CME with enough energy completely overtaking a relatively narrower preceding CME, especially at low height, can drive a more energetic shock signified by the enhanced type Ⅱ radio bursts. The shock may accelerate ambient particles(likely provided by the preceding CME) and lead to large SEP events more easily.

Solar Energetic Particle Event of 2005 January 20:Release Times and Possible Sources

Based on cosmic ray data obtained by neutron monitors at the Earth＇s surface, and data on near-relativistic electrons measured by the WIND satellite, as well as on solar X-ray and radio burst data, the solar energetic particle （SEP） event of 2005 January 20 is studied. The results show that this event is a mixed event where the flare is dominant in the acceleration of the SEPs, the interplanetary shock accelerates mainly solar protons with energies below 130 MeV, while the relativistic protons are only accelerated by the solar flare. The interplanetary shock had an obvious acceleration effect on relativistic electrons with energies greater than 2 MeV. It was found that the solar release time for the relativistic protons was about 06：41 UT, while that for the near-relativistic electrons was about 06：39 UT. The latter turned Out to be about 2 rain later than the onset time of the interplanetary type HI burst.

Large solar energetic particle event that occurred on 2012 March 7 and its VDA analysis

On 2012 March 7, the STEREO Ahead and Behind spacecraft, along with near-Earth spacecraft （e.g. SOHO, Wind） situated between the two STEREO spacecraft, observed an extremely large global solar energetic particle （SEP） event in Solar Cycle 24. Two successive coronal mass ejections （CMEs） have been detected close in time. From the multi-point in-situ observations, it can be found that this SEP event was caused by the first CME, but the second one was not involved. Using velocity dispersion analysis （VDA）, we find that for a well magnetically connected point, the energetic protons and electrons are released nearly at the same time. The path lengths to STEREO-B （STB） for protons and electrons have a distinct difference and deviate remarkably from the nominal Parker spiral path length, which is likely due to the presence of interplanetary magnetic structures situated between the source and STB. Also, the VDA method seems to only obtain reasonable results at well-connected locations and the inferred release times of energetic particles in different energy channels are similar. We suggest that good-connection is crucial for obtaining both an accurate release time and path length simultaneously, agreeing with the modeling result of Wang ＆ Qin （2015）.

Numerical simulations of solar energetic particle event timescales associated with ICMEs

Recently, S. W. Kahler studied the timescales of solar energetic particle （SEP） events asso- ciated with coronal mass ejections （CMEs） from analysis of spacecraft data. They obtained different timescales for SEP events, such as TO, the onset time from CME launch to SEP onset, TR, the rise time from onset to half the peak intensity （0.5/p）, and TD, the duration of the SEP intensity above 0.5Ip. In this work, we solve the transport equation for SEPs considering interplanetary coronal mass ejection （ICME） shocks as energetic particle sources. With our modeling assumptions, our simulations show similar results to Kahler＇s analysis of spacecraft data, that the weighted average of TD increases with both CME speed and width. Moreover, from our simulation results, we suggest TD is directly dependent on CME speed, but not dependent on CME width, which were not found in the analysis of observational data.

Flare evolution and polarization changes in fine structures of solar radio emission in the 2013 April 11 event

The measurement of positions and sizes of radio sources in observations is important for un- derstanding of the flare evolution. For the first time, solar radio spectral fine structures in an M6.5 flare that occurred on 2013 April 11 were observed simultaneously by several radio instruments at four different observatories： Chinese Solar Broadband Radio Spectrometer at Huairou （SBRS/Huairou）, Ondrejov Radio Spectrograph in the Czech Republic （ORSC/Ondrejov）, Badary Broadband Microwave Spectropolarimeter （BMS/Irkutsk）, and spectrograph/IZMIRAN （Moscow, Troitsk）. The fine structures included microwave zebra patterns （ZPs）, fast pulsations and fiber bursts. They were observed during the flare brightening lo- cated at the tops of a loop arcade as shown in images taken by the extreme ultraviolet （EUV） telescope onboard NASA＇s satellite Solar Dynamics Observatory （SDO）. The flare occurred at 06：58-07：26 UT in solar active region NOAA 11719 located close to the solar disk center. ZPs appeared near high frequency boundaries of the pulsations, and their spectra observed in Huairou and Ondrejov agreed with each other in terms of details. At the beginning of the flare＇s impulsive phase, a strong narrowband ZP burst occurred with a moderate left-handed circular polarization. Then a series of pulsations and ZPs were observed in almost unpolarized emission. After 07：00 UT a ZP appeared with a moderate right-handed polarization. In the flare decay phase （at about 07：25 UT）, ZPs and fiber bursts become strongly right-hand polarized. BMS/Irkutsk spectral observations indicated that the background emission showed a left-handed circular polarization （similar to SBRS/Huairou spectra around 3 GHz）. However, the fine structure appeared in the right-handed polarization. The dynamics of the polarization was associated with the motion of the flare ex- citer, which was observed in EUV images at 171 A and 131 A by the SDO Atmospheric Imaging Assembly （AIA）. Combining magnetograms observed by the SDO Helioseismic and Magnetic Imager （HMI） with the homologous assumption of EUV flare brightenings and ZP bursts, we deduced that the observed ZPs correspond to the ordinary radio emission mode. However, future analysis needs to verify the assumption that zebra radio sources are really related to a closed magnetic loop, and are located at lower heights in the solar atmosphere than the source of pulsations.

Modeling the 2017 September 10 solar energetic particle event using the iPATH model

On 2017 September 10,a fast coronal mass ejection(CME)erupted from the active region(AR)12673,leading to a ground level enhancement(GLE)event at Earth.Using the 2D improved Particle Acceleration and Transport in the Heliosphere(iPATH)model,we model the large solar energetic particle(SEP)event of 2017 September 10 observed at Earth,Mars and STEREO-A.Based on observational evidence,we assume that the CME-driven shock experienced a large lateral expansion shortly after the eruption,which is modeled by a double Gaussian velocity profile in this simulation.We apply the in-situ shock arrival times and the observed CME speeds at multiple spacecraft near Earth and Mars as constraints to adjust the input model parameters.The modeled time intensity profiles and fluence for energetic protons are then compared with observations.Reasonable agreements with observations at Mars and STEREO-A are found.The simulated results at Earth differ from observations of GOES-15.However,the simulated results at a heliocentric longitude 20°west to Earth fit reasonably well with the GOES observation.This can be explained if the pre-event solar wind magnetic field at Earth is not described by a nominal Parker field.Our results suggest that a large lateral expansion of the CME-driven shock and a distorted interplanetary magnetic field due to previous events can be important in understanding this GLE event.

The Role of Nuclei-Nuclei Interactions in the Production of Gamma-ray Lines in Solar Flares

Dramatic extensions of experimental possibilities (spacecraft RHESSI, CORONAS-F and others) in solar gamma-ray astronomy call for urgent, detailed theoretical consideration of a set of physical problems of solar activity and solar-terrestrial relationships that earlier may have only been outlined. Here we undertake a theoretical analysis of issues related to the production of gamma-radiation in the processes of interactions of energetic (accelerated) heavy and middle nuclei with the nuclei of the solar atmosphere (the so-called i-j interactions). We also make an estimate of the contribution of these interactions to the formation of nuclear and isotopic abundances of the solar atmosphere in the range of light and rare elements. The analysis is carried out for solar flares in the wide range of their intensities. We compare our theoretical estimates with RHESSI observations for the flare of 2002 July 23. It was shown that the 24Mg gamma-ray emission in this event was produced by the newly generated Mg nuclei. With a high probability, the gamma-ray line emission of 28Si nuclei from this flare was generated by the same processes.

The Influence of Ion-Acoustic Turbulence on the Electron Acceleration in the Reconnecting Current Sheet

Through solving the single electron equation of motion and the Fokker Planck equation including the terms of electric field strength and ion-acoustic turbulence, we study the influence of the ion-acoustic wave on the electron acceleration in turbulent reconnecting current sheets. It is shown that the ion-acoustic turbulence which causes plasma heating rather than particle acceleration should be considered. With typical parameter values, the acceleration time scale is around the order of 10-6 s, the accelerated electrons may have approximately a power-law distribution in the energy range 20-100 keV and the spectral index is about 3-10, which is basically consistent with the observed hard X-ray spectra in solar flares.

Extreme space weather events caused by super active regions during solar cycles 21-24

Extreme space weather events including≥X5.0 flares,ground level enhancement(GLE)events and super geomagnetic storms(Dst≥-250 nT)caused by super active regions(SARs)during solar cycles 21-24 were studied.The total number ofX5.0 solar flares was 62,among which 41 were X5.0-X9.9 flares and 21 were≥X10.0 flares.We found that 83.9%of the≥X5.0 flares were produced by SARs;78.05%of the X5.0-X9.9 and 95.24%of the≥X10.0 solar flares were produced by SARs;46 GLEs were registered during solar cycles 21-24,and 25 GLEs were caused by SARs,indicating that 54.3%of the GLEs were caused by SARs;24 super geomagnetic storms were recorded during solar cycles 21-24,and 12 of them were caused by SARs,namely 50%of the super geomagnetic storms were caused by SARs.We ascertained that only 29 SARs produced≥X5.0 flares,15 SARs generated GLEs and 10 SARs triggered super geomagnetic storms.Of the 51 SARs,only 33 SARs produced at least one extreme space weather event,while none of the other 18 SARs could trigger an extreme space weather event.There were only four SARs and each of them generated not only a≥X5.0 flare,but also a GLE event and a super geomagnetic storm.Most of the extreme space weather events caused by the SARs appeared during solar cycles 22 and 23,especially for GLE events and super geomagnetic storms.The longitudinal distributions of source locations for the extreme space weather events caused by SARs were also studied.

太阳和太阳系等离子体研究专辑:序言

宇宙中超过99.9%的可见物质处于等离子体状态,等离子天体物理是天体物理的重要分支,为理解天体系统的形成、演化及爆发现象提供着重要的理论基础.专辑通过14篇文章系统介绍了中国科学院紫金山天文台等离子天体物理团队在太阳和太阳系等离子体方面的研究成果,希望能帮助读者全面了解太阳与日球等离子体物理研究的重要进展及存在的问题.

太阳中微子问题

文中从中微子物理学、太阳中微子的探测、标准太阳模型的建立等方面对太阳中微子问题的提出进行了回顾。各类太阳中微子探测器测量结果不同程度的偏低，以及不同类探测器（如Kamiokande和Homestake）测量结果之间的矛盾，使得人们对太阳中微子的研究表现出浓厚的兴趣。对太阳中微子问题可从粒子物理和天体物理两个方面进行研究。文中分别对这两个研究领域中提出的企图解决太阳中微子问题的模型作了简要评述。

Polar Coronal Holes During Solar Cycles 22 and 23

Data from the Solar Wind Ion Composition Spectrometer （SWICS） on Ulysses and synoptic maps from Kitt Peak are used to analyze the polar coronal holes of solar activity cycles 22 and 23 （from 1990 to end of 2003）. In the beginning of the declining phase of solar cycles 22 and 23, the north polar coronal holes （PCHs） appear about one year earlier than the ones in the south polar region. The solar wind velocity and the solar wind ionic charge composition exhibit a characteristic dependence on the solar wind source position within a PCH. From the center toward the boundary of a young PCH, the solar wind velocity decreases, coinciding with a shift of the ionic charge composition toward higher charge states. However, for an old PCH, the ionic charge composition does not show any obvious change, although the latitude evolution of the velocity is similar to that of a young PCH.

The energetic relationship among geoeffective solar flares, associated CMEs and SEPs

Major solar eruptions （flares, coronal mass ejections （CMEs） and solar energetic particles （SEPs）） strongly influence geospace and space weather. Currently, the mechanism of their influence on space weather is not well understood and requires a detailed study of the energetic relationship among these eruptive phenomena. From this perspective, we investigate 30 flares （observed by RHESSI）, followed by weak to strong geomagnetic storms. Spectral analysis of these flares suggests a new power-law relationship （r - 0.79） between the hard X-ray （HXR） spectral index （before flarepeak） and linear speed of the associated CME observed by LASCO/SOHO. For 12 flares which were followed by SEP enhancement near Earth, HXR and SEP spectral analysis reveals a new scaling law （r - 0.9） between the hardest X-ray flare spectrum and the hardest SEP spectrum. Furthermore, a strong correlation is obtained between the linear speed of the CME and the hardest spectrum of the corresponding SEP event （r - 0.96）. We propose that the potentially geoeffective flare and associated CME and SEP are well-connected through a possible feedback mechanism, and should be regarded within the framework of a solar eruption. Owing to their space weather effects, these new results will help improve our current understanding of the Sun-Earth relationship, which is a major goal of research programs in heliophysics.

与高能质子共生的两类太阳微波爆发(英)

在分析了近年来对太阳射电爆发与高能质子观测的基础上指出，既非II型也非米波IV型而是强微波爆发几乎总是同高能质子共生的；这一结果否定了以前长期所持的观点。同高能质子共生的微波爆发可分成两类：强脉冲型和强微波IV型，前者共生的被俘质子或相互作用质子要多于逃逸质子，后者则常共生有更多的逃逸质子．作者对每种情况中质子的有效加速过程进行了考虑，并对强微波爆发为何几乎总是有高能质子共生的缘由作了解释．

Influence of coronal holes on CMEs in causing SEP events

The issue of the influence of coronal holes （CHs） on coronal mass ejections （CMEs） in causing solar energetic particle （SEP） events is revisited. It is a continuation and extension of our previous work, in which no evident effects of CHs on CMEs in generating SEPs were found by statistically investigating 56 CME events. This result is consistent with the conclusion obtained by Kahler in 2004. We extrapolate the coronal magnetic field, define CHs as the regions consisting of only open magnetic field lines and perform a similar analysis on this issue for 76 events in total by extending the study interval to the end of 2008. Three key parameters, CH proximity, CH area and CH relative position, are involved in the analysis. The new result confirms the previous conclusion that CHs did not show any evident effect on CMEs in causing SEP events.

A review of solar type Ⅲ radio bursts

Solar type III radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the solar system. Consequently, they provide information on electron acceleration and transport, and the conditions of the background ambient plasma they travel through. We review the observational properties of type III bursts with an emphasis on recent results and how each property can help identify attributes of electron beams and the ambient background plasma. We also review some of the theoretical aspects of type III radio bursts and cover a number of numerical efforts that simulate electron beam transport through the solar corona and the heliosphere.

Dependence of large SEP events with different energies on the associated flares and CMEs

To investigate the dependence of large gradual solar energetic particle（SEP） events on the associated flares and coronal mass ejections（CMEs）, the correlation coefficients（CCs） between peak intensities of E 〉 10 MeV（I10）, E 〉 30 MeV（I30） and E 〉 50 MeV（I50） protons and soft X-ray（SXR） emission of associated flares and the speeds of associated CMEs in the three longitudinal areas W0–W39, W40–W70（hereafter the well connected region） and W71–W90 have been calculated.Classical correlation analysis shows that CCs between SXR emission and peak intensities of SEP events always reach their largest value in the well connected region and then decline dramatically in the longitudinal area outside the well connected region, suggesting that they may contribute to the production of SEPs in large SEP events. Both classical and partial correlation analyses show that SXR fluence is a better parameter describing the relationship between flares and SEP events. For large SEP events with source location in the well connected region, the CCs between SXR fluence and I10, I30 and I50 are0.58±0.12, 0.80±0.06 and 0.83±0.06 respectively, while the CCs between CME speed and I10, I30 and I50 are 0.56±0.12, 0.52±0.13 and 0.48±0.13 respectively. The partial correlation analyses show that in the well connected region, both CME shock and SXR fluence can significantly affect I10, but SXR peak flux makes no additional contribution. For E 〉 30 MeV protons with source location in the well connected region, only SXR fluence can significantly affect I30, and the CME shock makes a small contribution to I30, but SXR peak flux makes no additional contribution. For E 〉 50 MeV protons with source location in the well connected region, only SXR fluence can significantly affect I50, but both CME shock and SXR peak flux make no additional contribution. We conclude that these findings provide statistical evidence that for SEP events with source locations in the well connected region, a CME shock is only an effective accelerator for E 〈 30 MeV protons. However, flares are not only effective accelerators for E 〈 30 MeV protons, but also for E 〉 30 MeV protons, and E 〉 30 MeV protons may be mainly accelerated by concurrent flares.

Dependence of E ≥ 100 MeV protons on the associated flares and CMEs

To investigate the possible solar source of high-energy protons, correlation coefficients between the peak intensities of E ≥ 100 MeV protons, I100, and the peak flux and fluence of solar soft X-ray（SXR） emission, and coronal mass ejection（CME） linear speed in the three longitudinal areas W0-W39, W40-W70 and W71-W90 have been calculated respectively. Classical correlation analysis shows that the correlation coefficients between CME speeds and I100 in the three longitudinal areas are0.28±0.21, 0.35±0.21 and 0.04±0.30 respectively. The classical correlation coefficients between I100 and SXR peak flux in the three longitudinal areas are 0.48±0.17, 0.72±0.13 and 0.02±0.30 respectively, while the correlation coefficients between I100 and SXR fluence in the three longitudinal areas are 0.25±0.21, 0.84±0.07 and 0.10±0.30 respectively. Partial correlation analysis shows that for solar proton events with source location in the well connected region（W40-W70）, only SXR fluence can significantly affect the peak intensity of E ≥ 100 MeV protons, but SXR peak flux has little influence on the peak intensities of E ≥ 100 MeV protons; moreover, CME speed has no influence on the peak intensities of E ≥ 100 MeV protons. We conclude that these findings provide statistical evidence that E ≥ 100 MeV protons may be mainly accelerated by concurrent flares.