On the generation of high-energy particles in solar flares

This paper discusses the relationship between some characteristics of microwave type IV radio bursts and solar cosmic ray protons of MeV energy. It is shown that the peak flux intensity of those bursts is almost linearly correlated with the MeV proton peak flux observed by satellites near the Earth and that protons and electrons would be accelerated simultaneously by a similar mechanism during the explosive phase of solar flares.Brief discussion is given on the propagation of solar cosmic rays in the solar envelope after ejection from the flare regions.     

Stellar and solar flares

Some ideas are developed concerning solar flares which have been presented earlier by the author (Schatzman, 1966a). Emphasis is laid on the problem of energy transport; from the energy supply to the region of the optical flare, on the storage of low energy cosmic ray particles in a magnetic bottle before the beginning of the optical flare, and the mechanism which triggers both the optical flare, and the production of high-energy cosmic rays. The relation between solar and stellar flares is considered.Lecture given at Goddard Space Flight Center, November 4, 1966.     

Relationship of ground level enhancements with solar,interplanetary and geophysical parameters

Cosmic rays registered by Neutron Monitor on the surface of the Earth are believed to originate from outer space, and sometimes also from the exotic objects of the Sun. Whilst the intensities of the cosmic rays are observed to be enhanced with sudden, sharp and short-lived increases, they are termed as ground level enhancements (GLEs). They are the occurrences in solar cosmic ray intensity variations on short-term basis, so different solar factors erupted from the Sun can be responsible for causing them. In this context, an attempt has been made to determine quantitative relationships of the GLEs having peak increase >5% with simultaneous solar, interplanetary and geophysical factors from 1997 through 2006, thereby searching the responsible factors which seem to cause the enhancements. Results suggest that GLE peaks might be caused by solar energetic particle fluxes and solar flares. The proton fluxes which seemed to cause GLE peaks were also supported by their corresponding fluences. For most of the flares, the time integrated rising portion of the flare emission refers to the strong portion of X-ray fluxes which might be the concern to GLE peak. On an average, GLE peak associated X-ray flux (0.71×10⁻⁴ w/m²) is much stronger than GLE background associated X-ray flux (0.11×10⁻⁶ w/m²). It gives a general consent that the GLE peak is presumably caused by the solar flare. Coronal mass ejection alone does not seem to cause GLE. Coronal mass ejection presumably causes geomagnetic disturbances characterized by geomagnetic indices and polarities of interplanetary magnetic fields.     

Solar energetic particles and wide-band continuum storms from metric to hectometric frequencies

In association with solar flares accompanying type IV radio bursts of U-shaped spectrum, solar cosmic rays (MeV) and energetic electrons (keV) were generated. After acceleration, they were first stored in or near the flare regions and then gradually emitted into outer space. It seems that the streams of keV electrons generated the continuum radio emissions from metric to hectometric frequencies while passing through the outer coronal regions.     

Solar and stellar flares

Short-lived increases in the brightness of many red dwarfs have been observed for the last 30 yr, and a variety of more or less exotic models have been proposed to account for such flares. Information about flares in the Sun has progressed greatly in recent years as a result of spacecraft experiments, and properties of coronal flare plasma are becoming increasingly better known. In this paper, after briefly reviewing optical, radio and X-ray observations of stellar flares, we show how a simplified model which describes conductive plus radiative cooling of the coronal flare plasma in solar flares has been modified to apply to optical and X-ray stellar flare phenomena. This model reproduces many characteristic features of stellar flares, including the mean UBV colors of flare light, the direction of flare decay in the two-color diagram, precursors, Stillstands, secondary maxima, lack of sensitivity of flare color to flare amplitude, low flux of flare X-rays, distinction between so-called spike flares and slow flares, Balmer jumps of as much as 6–8, and emission line redshifts up to 3000 km s^–1. In all probability, therefore, stellar flares involve physical processes which are no more exotic (and no less!) than those in solar flares. Advantages of observing stellar flares include the possibilities of (i) applying optical diagnostics to coronal flare plasma, whereas this is almost impossible in the Sun, and (ii) testing solar flare models in environments which are not generally accessible in the solar atmosphere.     

Solar circumstances at the time of the cosmic ray increase on January 28, 1967

Solar circumstances have been evaluated for January 28, 1967, the date of an observed ground level enhancement of cosmic rays which was not preceded by observation of a suitably great Hα flare. On the visible solar hemisphere, a bright subflare at S23° E19° occurred in appropriate time association with the cosmic ray event, and was accompanied by weak X-ray enhancement and radio frequency emission. If this flare, alone, or in combination with other minor flares observed on the visible hemisphere on January 28 was the source of the energetic cosmic rays recorded on that date, then current thinking regarding the characteristics of cosmic ray flares must be modified. An initial study of probable circumstances on the invisible hemisphere did not lead to the immediate recognition of amajor center of activity as the probable source of a cosmic ray flare. Further evaluation of all centers of activity on the invisible hemisphere identified one region, McMath Plage No. 8687, 64° beyond the west limb, as the most plausible, possible site for the cosmic ray flare on January 28, 1967. The location of this region is in accord with the source-position deduced in Lockwood's analysis (1968) of the cosmic ray event. This center of activity could not have been more than 5 days old on January 28, 1967. The interval of major activity in the region was confined primarily to the invisible hemisphere. The occurrence of an ‘isolated’ major flare in the region on February 13, 1967 is discussed. The present study exemplifies the partial nature of solar observations which are limited to the visible hemisphere. The possible role of exceptional geomagnetic calm, 1963–1967, in permitting atypical cosmic ray enhancements, as on January 28, 1967, is mentioned.     

Relationship between high-energy physical phenomena on the sun and in astrophysics

High energy phenomena on the surface of the Sun are manifestations of part of the solar dynamo cycle. Convection and magnetic field give rise to unstable, twisted flux loops that become solar flares when the resistive tearing mode proceeds to the nonlinear limit. If such twisted flux loops did not dissipate rapidly due to an enhanced resistivity, then the dynamo would not work. The act of dissipation leads to intense heating and acceleration leading to X-rays and accelerated particles. The particles in turn give rise to hard X-rays, gamma rays, neutrons, and solar cosmic rays. In high-energy astrophysics such phenomena occur in accretion disks around compact objects like black holes in quasars and SS433. The resulting acceleration may explain the observed extremely high-energy cosmic rays of up to 10²⁰ eV and the high-energy gamma rays of 10¹² to 10¹⁵ eV. These high energies are more readily explained by acceleration E parallel to B as opposed to stochastic shock acceleration. The anisotropy and localization of gamma rays from solar flares potentially may indicate which mechanism is prevalent.     

太阳射电爆发的起因：耀斑或／和日冕物质抛射

本文分析了近二十年来的地面和空间太阳有关观测资料,得出太阳射电爆发的起因为耀斑和／ 或日冕物质抛射（ＣＭＥ） 而不仅仅是耀斑,这将有利于更深刻地了解太阳射电爆发和共生高能现象的物理过程     

Cosmic-Ray Variations During the Two Greatest Bursts of Solar Activity in the 23rd Solar Cycle

During two extreme bursts of solar activity in March–April 2001 and October–November 2003, the ground-based neutron monitor network recorded a series of outstanding events distinguished by their magnitude and unusual peculiarities. The important changes that lead to increased activity initiated not with the sunspot appearance, but with the large-scale solar magnetic field reconfiguration. A series of strong and moderate magnetic storms and powerful proton events (including ground-level enhancements, GLE) were registered during these periods. The largest and most productive in the 23rd solar cycle, active region 486, generated a significant series of solar flares among which the 4 November 2003 flare (X28/3B) was the most powerful X-ray solar event ever observed. The fastest arrival of the interplanetary disturbance from the Sun (after August 1972) and the highest solar wind velocity and IMF intensity were recorded during these events. Within 1 week, three GLEs of solar cosmic rays were registered by the neutron monitor network (28 and 29 October and 2 November 2003). In this work, we perform a tentative analysis of a number of the effects seen in cosmic rays during these two periods, using the neutron monitor network and other relevant data.     

The release of energetic particles from the Sun

A model is developed to account for the release of solar cosmic rays from the Sun. The solar atmosphere out to 3–5 solar radii above the photosphere is permeated with magnetic field lines which trap low rigidity ( 50 MV) flare particles. Plasma heated by the flare process disturbs the trapping field, and not until the disturbance reaches 3–5 solar radii can the low rigidity flare particles have access to interplanetary space. If the plasma is not heated sufficiently to overcome the coronal field, flare particles are trapped, efficiently. Subsequent leakage of these particles into interplanetary space forms corotating streams. Reference is made to satellite observations of solar electromagnetic radiation and charged particles.     

Gamma-ray,neutron, and hard X-ray studies and requirements for a high-energy solar physics facility

The requirements for future high-resolution spatial, spectral, and temporal observations of hard X-rays, gamma rays and neutrons from solar flares are discussed in the context of current high-energy flare observations. There is much promise from these observations for achieving a deep understanding of processes of energy release, particle acceleration and particle transport in a complicated environment such as the turbulent and highly magnetized atmosphere of the active Sun.     

Note on the characteristics of sunspot groups which produce solar proton flares

Solar proton flares are associated with sunspot groups which show an unusual distribution of magnetic polarities. Furthermore, the gradient of the magnetic field is very large before the onset of these flares. The importance of polar cap absorptions, which is proportional to the integral flux of solar cosmic rays, tends to increase as the gradient of the magnetic field becomes greater. It is shown that the formation of such gradients is associated with the rotating motion of sunspot groups. Hence, the sunspot groups which show a reversed polarity distribution are very effective for the production of solar proton flares.NASA Associate with University of Maryland.     

太阳耀斑研究的新进展(英)

近年来对太阳耀斑的研究取得了重要的进展。一些新的发现主要来自高分辨率的观测，特别是来自"阳光"卫星的结果。综述的范围包括太阳耀斑中磁重联的新证据、硬X射线源（包括所谓的超热源）的分类、X射线喷流的发现、环－环相互作用的证据以及对耀斑大气动力学过程的新认识等。基于这些新的知识，讨论了有关耀斑模型的一些问题。     

High abnormal rotation rates of sunspots and flare productivity

Solar activity, such as flares and CMEs, affect the interplanetary medium, and Earth’s atmosphere. Therefore, to understand the Space Weather, we need to understand the mechanisms of solar activity. Towards this end, we use 1135 events of solar H_α flares and the positional data of sunspots from the archive of Solar Geophysical Data (SGD) for the period January–April, 2000 and compute the abnormal rotation rates that lead to high flare productivity. We report that the occurrence of 5 or more flares in a day in association with a given sunspot group can be defined as high flare productivity and the sunspots that have an abnormal rotation rates of ～4–10 deg day^?1 trigger high flare productivity. Further, in order to compare the flare productivity expressed as the strength of the flux emitted, especially the soft X-ray (SXR) flares in the frequency range of 1–8 Å, we compute the flare index of SXR flares and find that 8 out of 28 active regions used in this study satisfy the requirement for being flare productive. This enables us to conclude that the high rotation rates of sunspots are an important mechanism to understand the flare productivity, especially numerical flare productivity that includes flares of all class.     

On the role of energetic particles in solar flares

The importance of energetic particles in the generation of solar flares and related phenomena has been underestimated if not completely neglected. A reexamination of their role in the light of recent observations carried out during the last solar maximum by a number of experiments on SMM and Hinotori satellites points out the continuous and violent evolution of the solar atmosphere. Most observed features can be better explained by the old idea that particles are trapped in magnetic loops above active regions where they are first heated and then accelerated by absorbing part of the wave energy flowing upwards continuously from the convection zone. Their catastrophic release into the chromosphere as a consequence of an instability in the region such as chromospheric heating or due to the emergence of new magnetic flux is considered as being the flare proper. Since the trapping of the particles involves the generation of resonant waves, a reassessment of the isotopic overabundance problem as well as a search for these waves in interplanetary space are proposed.     

The physics of particle acceleration at the heliospheric termination shock

Cosmic rays are ubiquitous in space, and the essential similarity of their energy spectra in many different regions places significant general constraints on the mechanisms for their acceleration and confinement. Diffusive shock acceleration is at present the most successful acceleration mechanism proposed, and, together with transport in Kolmogorov turbulence, can account for the universal specta. A unique laboratory for studying the acceleration and transport of charged particles is the outer heliosphere, including the solar wind termination shock and heliosheath.

A widely accepted paradigm for the transport and acceleration of energetic particles in the heliosphere has evolved over the last few decades. This picture has successfully explained many features of the modulation of galactic cosmic rays and the transport and acceleration of anomalous cosmic rays at the solar-wind termination shock. Recent Voyager observations near and beyond the termination shock have revealed new, and in some cases, unexpected phenomena which have led to questions concerning the established paradigm. The physical interpretation of the observations requires a blunt termination shock, rapid inward motion of the shock and temporal variations over time scales ranging from hours to 22 years. Incorporation of these into the physics has promise of explaining most, if not, all of the observed phenomena while retaining the advantages of the termination shock paradigm for both galactic and anomalous cosmic rays.     

Transient perturbations and their effects in the heliosphere, the geo-magnetosphere, and the Earth’s atmosphere: Space weather perspective

We discuss the effects of certain dynamic features of space environment in the heliosphere, the geo-magnetosphere, and the earth’s atmosphere. In particular, transient perturbations in solar wind plasma, interplanetary magnetic field, and energetic charged particle (cosmic ray) fluxes near 1 AU in the heliosphere have been discussed. Transient variations in magnetic activity in geo-magnetosphere and solar modulation effects in the heliosphere have also been studied. Emphasis is on certain features of transient perturbations related to space weather effects. Relationships between geomagnetic storms and transient modulations in cosmic ray intensity (Forbush decreases), especially those caused by shock-associated interplanetary disturbances, have been studied in detail. We have analysed the cosmic ray, geomagnetic and interplanetary plasma/field data to understand the physical mechanisms of two phenomena namely, Forbush decrease and geomagnetic storms, and to search for precursors to Forbush decrease (and geomagnetic storms) that can be used as a signature to forecast space weather. It is shown that the use of cosmic ray records has practical application for space weather predictions. Enhanced diurnal anisotropy and intensity deficit of cosmic rays have been identified as precursors to Forbush decreases in cosmic ray intensity. It is found that precursor to smaller (less than 5%) amplitude Forbush decrease due to weaker interplanetary shock is enhanced diurnal anisotropy. However, larger amplitude (greater than 5%) Forbush decrease due to stronger interplanetary shock shows loss cone type intensity deficit as precursor in ground based intensity record. These precursors can be used as inputs for space weather forecast.     

Hydrodynamic models of solar and stellar flares

This paper discusses the hydrodynamic modeling of flaring plasma confined in magnetic loops and its objectives within the broader scope of flare physics. In particular, the Palermo-Harvard model is discussed along with its applications to the detailed fitting of X-ray light curves of solar flares and to the simulation of high-resolution Ca xix spectra in the impulsive phase. These two approaches provide complementary constraints on the relevant features of solar flares. The extension to the stellar case, with the fitting of the light curve of an X-ray flare which occurred on Proxima Centauri, demonstrates the feasibility of using this kind of model for stars too. Although the stellar observations do not provide the wealth of details available for the Sun, and, therefore, constrain the model more loosely, there are strong motivations to pursue this line of research: the wider range of physical parameters in stellar flares and the possibility of studying further the solar-stellar connection.     

Use of Simultaneous EUV and Soft X-ray Measurements for Diagnostics of the Solar Flare Radiation Danger

The fluxes of extreme ultraviolet (EUV) and soft X-ray emission are key parameters for modelling the ionosphere and upper atmosphere. A new aspect is considered in using these fluxes for diagnostics and short-term prediction of proton radiation danger from the flare. The EUV (λ < 105 nm) and soft X-ray (0.1–0.8 nm) fluxes were compared for two types of solar flares. The first type is followed by a strong enhancement in solar energetic (E >10 MeV) proton flux, the second is not followed by any enhancement in proton flux. It was discovered that the flare UV flux was considerably higher for flares with protons than for those without protons. Soft X-ray fluxes were approximately equal in both cases. An excess of EUV emission in proton flares grows with increasing proton flux. An analytic expression was found for the growth in proton flux as a function of the excess of EUV radiation at a given X-ray flux. These results can be used in predicting flare radiation danger.     

Turbulence In The Solar Atmosphere: Manifestations And Diagnostics Via Solar Image Processing

Intermittent magnetohydrodynamical turbulence is most likely at work in the magnetized solar atmosphere. As a result, an array of scaling and multi-scaling image-processing techniques can be used to measure the expected self-organization of solar magnetic fields. While these techniques advance our understanding of the physical system at work, it is unclear whether they can be used to predict solar eruptions, thus obtaining a practical significance for space weather. We address part of this problem by focusing on solar active regions and by investigating the usefulness of scaling and multi-scaling image-processing techniques in solar flare prediction. Since solar flares exhibit spatial and temporal intermittency, we suggest that they are the products of instabilities subject to a critical threshold in a turbulent magnetic configuration. The identification of this threshold in scaling and multi-scaling spectra would then contribute meaningfully to the prediction of solar flares. We find that the fractal dimension of solar magnetic fields and their multi-fractal spectrum of generalized correlation dimensions do not have significant predictive ability. The respective multi-fractal structure functions and their inertial-range scaling exponents, however, probably provide some statistical distinguishing features between flaring and non-flaring active regions. More importantly, the temporal evolution of the above scaling exponents in flaring active regions probably shows a distinct behavior starting a few hours prior to a flare and therefore this temporal behavior may be practically useful in flare prediction. The results of this study need to be validated by more comprehensive works over a large number of solar active regions. Sufficient statistics may also establish critical thresholds in the values of the multi-fractal structure functions and/or their scaling exponents above which a flare may be predicted with a high level of confidence. Based on the author's contributed talk “Manifestations and Diagnostics of Turbulence in the Solar Atmosphere”, presented at the Solar Image Processing Workshop II, Annapolis, Maryland, USA, 3–5 November 2004.