Some statistical properties of surges

All surges observed at the Swedish Astrophysical Station in Anacapri from July 1, 1957 until December 31, 1967 have been studied statistically. In 1958 the Southern hemisphere was most active in producing surges, but thereafter the Northern hemisphere has dominated the activity with increasing rate. The average for the whole period shows that the Northern part of the sun has been twice as active as the Southern part. The latitude distribution for surges has varied with the solar cycle with a pattern similar to the butterfly diagram for sunspots. The polar surges were most frequent in the beginning of the new cycle. The importance distribution is also dependent of the solar cycle, as a higher percentage of surges of importance 1 are found during the solar minimum. Surges of higher importance tend to have longer durations, and they are more often associated with flares. The association rate between surges and radio emission is dependent on both the solar cycle and the importance of the surge. During years with high activity, more than 20% of all surges were followed within 5 min from their start by radio emission, compared to 4% during solar minimum. 17% of surges of importance 2 are closer related to radio emission, but only 10% of surges of importance 1. On the whole, 11% of the surges occurred almost simultaneously with radio bursts. Bursts in discrete frequencies were registered in connection with 8% of the surges, while 11% were followed within 5 min from their start by bursts in spectral observations. All these surges were isolated, i.e. no flares have been reported to occur during their lifetimes. This is also the case for the 18 surges covered by X-ray observations, three of which were closer related to X-ray bursts.     

Energetic particles from the sun

This paper discusses solar cosmic ray phenomena and related topics from the solar physical point of view. Basic physics of the solar atmosphere and solar flare phenomena are, therefore, considered in some detail. Since solar cosmic rays are usually produced by solar flares, we must first understand the processes and mechanism of solar flares, especially the so-called proton flares, in order to understand the acceleration mechanism of solar cosmic rays and their behaviour in both the solar atmosphere and interplanetary space. For this reason, detailed discussion is given on various phenomena associated with solar flares, proton flare characteristics, and the mechanism of solar flares.Since the discovery of solar cosmic rays by Forbush, the interplanetary space has been thought of as medium in which solar cosmic rays propagate. In this paper, the propagation of solar cosmic rays in this space is, therefore, discussed briefly by referring to the observed magnetic properties of this space. Finally, some problems related to the physics of galactic cosmic rays are discussed.Astrophysics and Space Science Review Paper.     

Mass motions associated with solar flares

Mass motions are a principal means by which components of solar flares can be distinguished. Typical patterns of mass motions in H are described for chromospheric flare ribbons, remote chromospheric flare patches, flare loops, flaring arches, surges, erupting filaments and some expanding coronal features. Interrelationships between these phenomena are discussed and illustrations of each are presented.     

The acceleration of electrons and ions in solar flares

We find that gamma-ray line (GRL) emissions start later than the hard X-ray (HXR) emissions during impulsive and extended solar flares. Starting delay is more in the case of extended solar flares suggesting a slow acceleration of electrons and ions, in comparison to impulsive solar flares which indicate different acceleration mechanism for impulsive and extended solar flares. We further infer that during solar flares, electrons and ions are accelerated simultaneously and the delay between HXR and GRL emissions results mainly due to differences in acceleration times of electrons and ions to attain energies required for producing HXR emissions for electrons and GRL emissions for ions. Therefore, we are of view that a single step acceleration mechanism may work in solar flares.     

The possible role of energetic electrons in the production of surges

Energetic electrons, which play a major role in the explosive phases of flares, are proposed as the energy source for the production of surges. Flare data from a two-year interval are analyzed to show that the probability of having surges associated with flares is greater when there are accompanying decimeter type III bursts or impulsive 8800 MHz bursts. The model of chromospheric heating by impulsive electrons proposed by Hudson is examined and shown to provide an adequate explanation for the origin of flare surges. The proposed surge model is consistent with the temporal evolution of the flare-surge event and the required surge energy. Surges not accompanied by flares can also probably be explained by the model.     

Are solar flares a result of a sudden conversion of magnetic energy stored prior to their onset?

It has long been believed that solar flares result from a sudden conversion of magnetic energy stored prior to their onset. However, it is difficult to prove such an idea without knowing both the rate of energy input and the rate of energy output in the flare region. In spite of the fact that a similar mechanism has long been contemplated, magnetospheric substorms are found to be directly driven by an enhanced dynamo process. The results suggest also that the presence of magnetic energy in the magnetotail does not mean that it can be consumed for substorms. Implications of this finding for solar flares are discussed.     

On the power-law distributions of X-ray fluxes from solar flares observed with GOES

The power-law frequency distributions of the peak flux of solar flare X-ray emission have been studied extensively and attributed to a system having self-organized criticality(SOC).In this paper,we first show that,so long as the shape of the normalized light curve is not correlated with the peak flux,the flux histogram of solar flares also follows a power-law distribution with the same spectral index as the powerlaw frequency distribution of the peak flux,which may partially explain why power-law distributions are ubiquitous in the Universe.We then show that the spectral indexes of the histograms of soft X-ray fluxes observed by GOES satellites in two different energy channels are different:the higher energy channel has a harder distribution than the lower energy channel,which challenges the universal power-law distribution predicted by SOC models and implies a very soft distribution of thermal energy content of plasmas probed by the GOES satellites.The temperature(T) distribution,on the other hand,approaches a power-law distribution with an index of 2 for high values of T.Hence the application of SOC models to the statistical properties of solar flares needs to be revisited.     

Waiting Time Distribution of Coronal Mass Ejections

Inspired by the finding that the large waiting time of solar flares presents a power-law distribution, we investigate the waiting time distribution (WTD) of coronal mass ejections (CMEs). SOHO/LASCO CME observations from 1996 to 2003 are used in this study. It is shown that the observed CMEs have a similar power-law behavior to the flares, with an almost identical power-law index. This strongly supports the viewpoint that solar flares and CMEs are different manifestations of the same physical process. We have also investigated separately the WTDs of fast-type and slow-type CMEs and found that their indices are identical, which imply that both types of CME may originate from the same physical mechanism.     

High energetic solar proton flares on the declining phase of solar cycle 22

The year 1991 is a part of the declining phase of the solar cycle 22, during which high energetic flares have been produced by active regions NOAA/USAF 6659 in June. The associated solar proton events have affected the Earth environment and their proton fluxes have been measured by GOES space craft. The evaluation of solar activity during the first half of June 1991, have been carried out by applying a method for high energetic solar flares prediction on the flares of June 1991. The method depends on cumulative summation curves according to observed H-alpha flares, X-ray bursts, in the active region 6659 during one rotation when the energetic solar flares of June 1991 have occurred. It has been found that the steep trend of increased activity sets on several tens of hours prior to the occurrence of the energetic flare, which can be used, together with other methods, for forecasts of major flares. All the used data at the present work are received from NOAA, Boulder, Colorado, USA.     

Generation of resonance transition emissions in the solar atmosphere

We derive formulas for the radio flux generated in solar flares by the resonance transition mechanism. This mechanism is shown to produce the observed decimeter-wave emission in continuum radio bursts at a level of small-scale irregularities of ～10^?6–10^?7. Thus, an analysis of continuum decimeter emission offers a unique opportunity to study small-scale turbulence in solar flares.     

Thermal Evolution and Radiative Output of Solar Flares Observed by the EUV Variability Experiment (EVE)

This paper describes the methods used to obtain the thermal evolution and radiative output during solar flares as observed by the Extreme ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory (SDO). How EVE measurements, due to the temporal cadence, spectral resolution and spectral range, can be used to determine how the thermal plasma radiates at various temperatures throughout the impulsive and gradual phase of flares is presented and discussed in detail. EVE can very accurately determine the radiative output of flares due to pre- and in-flight calibrations. Events are presented that show that the total radiated output of flares depends more on the flare duration than the typical GOES X-ray peak magnitude classification. With SDO observing every flare throughout its entire duration and over a large temperature range, new insights into flare heating and cooling as well as the radiative energy release in EUV wavelengths support existing research into understanding the evolution of solar flares.     

Heating of the solar flare plasma by high energy electrons

Heating of the ambient plasma by high energy electrons in solar flares is discussed. It is shown that for large flares the heating is enough to produce a thermal plasma of a temperature up to a few times of 10⁷K rapidly in the initial phase of the flares. Thus thermal bremsstrahlung in addition to non-thermal bremsstrahlung should be considered for the X-ray emission of solar flares in the initial phase.NAS-NRC Resident Research Associate.     

Large solar wind disturbances during late May and early June 1991

Solar wind data from the Ames experiment aboard the Pioneer Venus Orbiter, coincident with a period of unprecedented solar activity that began at the end of May, 1991, within the highly active earlier portion of 1991, are summarized and discussed. Some comparison is made with corresponding data from Earth. Some particularly large, strong shocks and solar ejecta were observed at Venus. The solar longitude of Venus, relative to associated flares, varied over a wide range, for a series of flares that produced X-rays that saturated the GOES X-ray counters. Some of the disturbances at Venus must be due to CMEs with longitudinal extents up to 40–50 deg.     

Detailed analysis of flares,magnetic fields and activity in the sunspot group of Sept. 13–26, 1963

We analyze large-scale H-alpha movies of the large spot group of Sept. 13–26, 1963, together with radio, ionospheric and magnetic field data as well as white light pictures. The evolution of the group and associated magnetic fields is followed, and the positions of solar flares relative to the fields are noted, along with their morphology. Although the magnetic field is deformed in time, characteristic field structures may be traced through the deformation as the seat of recurrent homologous flares.We find that most flares are homologous, and some are triggered by disturbances elsewhere in the region. We note events produced by surges falling back to the surface, and one flare initiated by a bright bead seen to fly across the region. In almost every case of an isolated type III radio burst, a corresponding H-alpha brightening could be found, but not all flares produced bursts. Flares close to the sunspots are most likely to produce radio bursts. Flare surface waves in the region all travel out to the west, because of more open magnetic field structure there. In one case (Sept. 25) a wave is turned back by the closed field structure to the east.In almost all cases the time association of radio or ionospheric events is with the beginning of the flare or with the flash phase.Several morphological classes of flares are noted as recurrent types.     

Solar neutrino in relation to solar activity

Here we have carried out a power-spectrum analysis of solar nuclear gamma-ray (NGR) flares observed by SMM and HINOTORI satellites. The solar NGR flares show a periodicity of 152 days, confirming the existence of a 152–158 days periodicity in the occurrence of solar activity phenomena and also indicating that the NGR flares are a separate class of solar flares. The power-spectrum analysis of the daily sunspot areas on the Sun for the period 1980–1982 shows a peak around 159 days while sunspot number data do not show any periodicity (Verma and Joshi, 1987). Therefore, only sunspot area data should be treated as an indicator of solar activity and not the daily sunspot number data.     

Characteristics of gamma-ray line flares as observed in hard X-ray emissions and other phenomena

Observations of gamma-ray lines from solar flares by SMM demonstrated that energetic protons and heavy ions are accelerated during the impulsive phase. In order to understand the acceleration mechanism for gamma-ray producing protons and heavy ions, we have studied the characteristics of the flares from which gamma-ray lines were observed by SMM In order to identify the characteristics unique to the gamma-ray line flares, we have also studied intense hard X-ray flares with no gamma-ray line emissions. We have found the following characteristics: 1) Most of the gamma-ray line flares produced intense radio bursts of types II and IV. 2) For most of the gamma-ray line flares, the time profiles of high-energy (? 300 keV) hard X-rays are delayed by order of several seconds with respect to those of low-energy hard X-rays. The delay times seem to be correlated with the spatial sizes of the flares. 3) In Hα importance, the gamma-ray line flares range from sub-flares to importance-3 flares. 4) The hard X-ray spectra of the gamma-ray line flares are generally flatter (harder) than those of flares with no gamma-ray line emission. From these characteristics, we conclude that the first-order Fermi acceleration operating in a flare loop is likely to be the acceleration mechanism for energetic protons and heavy ions as well as relativistic electrons.     

Energetic electrons associated with solar flares and their relation to type I noise activity

The generation of energetic electrons is always associated with the solar flares which occur within the sunspot groups that are highly active in emitting type I noise storms. The number of the solar flares which are associated with the distinct electron events observed at the earth tends to increase in association with the westward movement of these active groups. This tendency is not contradictory to the close association between electron producing solar flares and type I active regions if we take into account the limited directivity of type I noise storms associated with these sunspot groups.The acceleration of the energetic electrons associated with solar flares seems to be closely related to the type I active regions where the enormous numbers of suprathermal electrons exist and play a role in generating these radio noise storms.NAS-NRC Associate with NASA.     

Forced reconnection by nonlinear magnetohydrodynamic waves

Forced magnetic reconnection induced by magnetohydrodynamic (MHD) waves may account for the triggering of explosive solar activities such as flares. Reconnection in a neutral sheet plasma can be driven by the ponderomotive force associated with nonlinear MHD waves accompanying plasma vortex motion. The nonlinear stage of forced reconnection by MHD waves is simulated with a MHD particle-code: Some conditions for fast reconnection are discussed with applications to solar flares.     

耀斑氦线的观测和研究

介绍了耀斑各波区（从ＥＵＶ到红外）氦线观测的进展，从中阐明氦线观测和研究在提供太阳耀斑物理参数，了解耀斑动力学过程，电场，能量平衡及高能粒子产生和传输方面的特殊意义，并分析了这个领域目前已经取犁研究结果和今后研究中在观测技术和理论分析两个方面尚需解决的关键问题。     

Very High-Resolution Solar X-Ray Imaging Using Diffractive Optics

This paper describes the development of X-ray diffractive optics for imaging solar flares with better than 0.1 arcsec angular resolution. X-ray images with this resolution of the ???10?MK plasma in solar active regions and solar flares would allow the cross-sectional area of magnetic loops to be resolved and the coronal flare energy release region itself to be probed. The objective of this work is to obtain X-ray images in the iron-line complex at 6.7?keV observed during solar flares with an angular resolution as fine as 0.1 arcsec ?C over an order of magnitude finer than is now possible. This line emission is from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma at temperatures in excess of ???10 MK. It provides information on the flare morphology, the iron abundance, and the distribution of the hot plasma. Studying how this plasma is heated to such high temperatures in such short times during solar flares is of critical importance in understanding these powerful transient events, one of the major objectives of solar physics. We describe the design, fabrication, and testing of phase zone plate X-ray lenses with focal lengths of ???100 m at these energies that would be capable of achieving these objectives. We show how such lenses could be included on a two-spacecraft formation-flying mission with the lenses on the spacecraft closest to the Sun and an X-ray imaging array on the second spacecraft in the focal plane ???100 m away. High-resolution X-ray images could be obtained when the two spacecraft are aligned with the region of interest on the Sun. Requirements and constraints for the control of the two spacecraft are discussed together with the overall feasibility of such a formation-flying mission.