Stereoscopic measurements of flares from PHOBOS and GOES

A unique approach to observing the Sun stereoscopically in soft X-rays was afforded by the PHOBOS mission to Mars during 1989. Concurrent measurements of two flares from two widely separated spacecraft allowed us to obtain estimates of each flare's height above the solar surface. The requirement was that the flare had to be over the limb as observed by one spacecraft and on the visible disk as viewed by the other. The first flare occurred on March 4, when the active region was beyond the east limb as observed by GOES (at Earth), but on the disk as viewed by PHOBOS (at Mars). The second flare, on March 15, was on the disk for GOES, but beyond the west limb for PHOBOS. We believe that the same extraordinary active region, 5395, was responsible for both events. Soft X-ray photometers on each spacecraft contained two broad-band channels. The two-channel data were used to computeflare (assumed isothermal) plasma temperatures. The sharply peaked flare on March 4 indicated essentially identical maximum electron temperatures ( 13 Mk) at both spacecraft, confirming that the hottest plasma was indeed concentrated at the highest (unocculted) part of the loop. However, in the case of the long-duration March 15 flare, whose loop was in apparent upwards motion, the partially occulted flare indicated substantially cooler temperatures. This finding suggests that the hot core of this flare may have been below the limb, or that the partially occulted flux originated not from post-flare loops but from an independent, higher X-ray arch. The PHOBOS and GOES X-ray photometers were intercompared in July 1988, soon after the PHOBOS launch, to establish relative calibration parameters.     

Localization of the source of flare X-ray emission during the eclipse of 7 March, 1970

An occultation of X-ray emission from a solar flare occurred during the eclipse of 7 March, 1970 and was observed by an NRL instrument aboard the OSO-5 satellite. Ionization chamber photometers covering the wavelength ranges 0.5–3 Å, 1–8 Å, and 8–16 Å provided flux measurements once every 15 s providing a spatial resolution of 20 arc sec at the solar surface. Within this limitation the X-ray flare was observed to be confined within a region 136 000 km in one dimension.However, the measurements indicate the existence of a denser core 54 000 km wide in the direction of advance of the Moon's limb. Comparison of these results with X-ray photographs of flare regions are made and a model for the development of the soft X-ray flare is proposed.     

Soft X-ray and microwave observations of hot regions in solar flares

Hot regions in solar flares produce X-radiation and microwaves by thermal processes. Recent X-ray data make it possible to specify the temperature and emission measure of the soft X-ray source, by using, for instance, a combination of the 1–8 Å (peak response at about 2 keV) and the 0.5–3 Å (peak response at about 5 keV) broad-band photometers. The temperatures and emission measures thus derived satisfactorily explain the radio fluxes, within systematic errors of about a factor of 3. Comparison of 15 events with differing parameters shows that a hot solar flare region has an approximately isothermal temperature distribution. The time evolution of the correlation in a single event shows that the hot material originates in the chromosphere, rather than the corona. The density must lie between 10¹⁰ and 2 × 10¹¹ cm^–3. For an Importance 1 flare, this implies a stored energy of roughly 2 x 10³⁰-10²⁹ ergs. A refinement of the data will enable us to choose between conductive and radiative cooling models.     

Interpretation of the observed motions of hard X-ray sources in solar flares

In connection with the RHESSI satellite observations of solar flares, which have revealed new properties of hard X-ray sources during flares, we offer an interpretation of these properties. The observed motions of coronal and chromospheric sources are shown to be the consequences of three-dimensional magnetic reconnection at the separator in the corona. During the first (initial) flare phase, the reconnection process releases an excess of magnetic energy related predominantly to themagnetic tensions produced before the flare by shear plasma flows in the photosphere. The relaxation of a magnetic shear in the corona also explains the downward motion of the coronal source and the decrease in the separation between chromospheric sources. During the second (main) flare phase, ordinary reconnection dominates; it describes the energy release in the terms of the “standard model” of large eruptive flares accompanied by the rise of the coronal source and an increase in the separation between chromospheric sources.     

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。     

The super-hot thermal component in the decay phase of solar flares

Solar X-ray observations from balloons and from the SMM and HINOTORI spacecraft have revealed evidence for a super-hot thermal component with a temperature of 3 × 10⁷ K in many solar flares, in addition to the usual 10–20 × 10⁶ K soft X-ray flare plasma. We have systematically studied the decay phase of 35 solar flare X-ray events observed by ISEE-3 during 1980. Based on fits to the continuum X-ray spectrum in the 4.8–14 keV range and to the intensity of the 1.9 Å feature of iron lines, we find that 15 (about 43%) of the analyzed events have a super-hot thermal component in the decay phase of the flare. In this paper the important properties of the super-hot thermal component in the decay phase are summarized. It is found that an additional input of energy is required to maintain the super-hot thermal components. Finally, it is suggested that the super-hot thermal component in the decay phase is created through the reconnection of the magnetic field during the decay phase of 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.     

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.     

The Dynamics of High-Temperature Plasma in the Solar Corona according to SPIRIT Observations in Mg XII 8.42 Å Line

The spatial-distribution dynamics of the hot coronal plasma with T ～ 10 MK during a period of high solar activity is studied. We analyze images of the NOAA 9830 active region and its surroundings obtained during the second half of February 2002 with the SPIRIT spectroheliograph in the Mg XII 8.42-Å line and simultaneously on the SOHO satellite with the EIT instrument and on the TRACE satellite in the 195-Å channel. As shown by a multiwavelength analysis, a high-temperature plasma is concentrated in the corona near the apices of magnetic loops, it has long lifetimes (up to several days), and its dynamics is complex and bears no direct relation to flare activity. During the flares, conspicuous increases are observed in the X-ray flux and the emission measure for temperatures of ～5–15 MK. Our analyses of the time variations in emission during a flare suggest that hot plasma is heated by fluxes of accelerated electrons.     

Multi-Wavelength Analysis of High-Energy Electrons in Solar Flares: A Case Study of the August 20, 2002 Flare

A multi-wavelength spatial and temporal analysis of solar high-energy electrons is conducted using the August 20, 2002 flare of an unusually flat (γ₁ = 1.8) hard X-ray spectrum. The flare is studied using RHESSI, Hα, radio, TRACE, and MDI observations with advanced methods and techniques never previously applied in the solar flare context. A new method to account for X-ray Compton backscattering in the photosphere (photospheric albedo) has been used to deduce the primary X-ray flare spectra. The mean electron flux distribution has been analysed using both forward fitting and model-independent inversion methods of spectral analysis. We show that the contribution of the photospheric albedo to the photon spectrum modifies the calculated mean electron flux distribution, mainly at energies below ∼100 keV. The positions of the Hα emission and hard X-ray sources with respect to the current-free extrapolation of the MDI photospheric magnetic field and the characteristics of the radio emission provide evidence of the closed geometry of the magnetic field structure and the flare process in low altitude magnetic loops. In agreement with the predictions of some solar flare models, the hard X-ray sources are located on the external edges of the Hα emission and show chromospheric plasma heated by the non-thermal electrons. The fast changes of Hα intensities are located not only inside the hard X-ray sources, as expected if they are the signatures of the chromospheric response to the electron bombardment, but also away from them.     

Positional measurements on the eruptive loop prominence of 27 April 1981 and comparison with the X-ray sources

Using the Hα observational data from Yunnan Observatory, we have made position measurements on the eruptive loop prominence of 27 April 1981, and have compared the results with the positions of X-ray sources obtained by the hard X-ray telescope (SXT) on board the HINOTORI satellite. From the results of measurement and comparison, it is suggested that 1) The two mounds A and C at 0830 UT are extensions of two ribbons in the flare near the limb, which started before 0758 UT. 2) The central positions of two X-ray sources at 0756 UT are just situated at the top of the mound A and the mound C, respectively. The Hα footpoint corresponding to the main source of X-rays was behind the solar limb. The second source of X-rays corresponds to C₁ and C₂. 3) The X-ray sources were probably located near the footpoints of loops.     

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.     

Our New View of the Solar Corona from Yohkoh and Soho

The Yohkoh satellite has now been orbiting the Earth for about 6 years and during this time it has revealed a number of new features in the solar corona. These include the discovery of X-ray jets and active region transient brightenings, as well as observations of hard X-ray sources above and at the feet of soft X-ray flare loops. SOHO, the newest solar space mission, is not orbiting the Earth, but is in fact orbiting the Sun and has been at the Earth's L1 point for about 2 years. During this time it too has identified some new and interesting characteristics of both the solar corona, the photosphere and the solar interior. For example, studies of high resolution MDI/SOHO magnetograms indicate that the magnetic carpet may play an important role in the heating of the coronal. Also polar plume studies from EIT/SOHO data suggest that they may be a possible source for the fast solar wind. Together these two missions have dominated solar research throughout the 90s and are expected to continue to do so during the rise from solar minimum to the next solar maximum. This revised version was published online in July 2006 with corrections to the Cover Date.     

Yohkoh observations of the creation of high-temperature plasma in the flare of 16 December 1991

Yohkoh observations of an impulsive solar flare which occurred on 16 December, 1991 are presented. This flare was a GOES M2.7 class event with a simple morphology indicative of a single flaring loop. X-ray images were taken with the Hard X-ray Telescope (HXT) and soft X-ray spectra were obtained with the Bragg Crystal Spectrometer (BCS) on board the satellite. The spectrometer observations were made at high sensivity from the earliest stages of the flare, are continued throughout the rise and decay phases, and indicate extremely strong blueshifts, which account for the majority of emission in Caxix during the initial phase of the flare. The data are compared with observations from other space and ground-based instruments. A balance calculation is performed which indicates that the energy contained in non-thermal electrons is sufficient to explain the high temperature plasma which fills the loop. The cooling of this plasma by thermal conduction is independently verified in a manner which indicates that the loop filling factor is close to 100%. The production of superhot plasma in impulsive events is shown to differ in detail from the morphology and mechanisms appropriate for more gradual events.     

Solar cycle variations in the growth and decay of sunspot groups

We analysed the combined Greenwich (1874–1976) and Solar Optical Observatories Network (1977–2011) data on sunspot groups. The daily rate of change of the area of a spot group is computed using the differences between the epochs of the spot group observation on any two consecutive days during its life-time and between the corrected whole spot areas of the spot group at these epochs. Positive/negative value of the daily rate of change of the area of a spot group represents the growth/decay rate of the spot group. We found that the total amounts of growth and decay of spot groups whose life times ≥2 days in a given time interval (say one-year) well correlate to the amount of activity in the same interval. We have also found that there exists a reasonably good correlation and an approximate linear relationship between the logarithmic values of the decay rate and area of the spot group at the first day of the corresponding consecutive days, largely suggesting that a large/small area (magnetic flux) decreases in a faster/slower rate. There exists a long-term variation (about 90-year) in the slope of the linear relationship. The solar cycle variation in the decay of spot groups may have a strong relationship with the corresponding variations in solar energetic phenomena such as solar flare activity. The decay of spot groups may also substantially contribute to the coherence relationship between the total solar irradiance and the solar activity variations.     

Energies of Electrons Accelerated in Turbulent Reconnecting Current Sheets in Solar Flares

Yohkoh observations strongly suggest that electron acceleration in solar flares occurs in magnetic reconnection regions in the corona above the soft X-ray flare loops. Unfortunately, models for particle acceleration in reconnecting current sheets predict electron energy gains in terms of the reconnection electric field and the thickness of the sheet, both of which are extremely difficult to measure. It can be shown, however, that application of Ohm's law in a turbulent current sheet, combined with energy and Maxwell's equations, leads to a formula for the electron energy gain in terms of the flare power output, the magnetic field strength, the plasma density and temperature in the sheet, and its area. Typical flare parameters correspond to electron energies between a few tens of keV and a few MeV. The calculation supports the viewpoint that electrons that generate the continuum gamma-ray and hard X-ray emissions in impulsive solar flares are accelerated in a large-scale turbulent current sheet above the soft X-ray flare loops.     

Evidence of Magnetic Reconnection in Solar Flares and a Unified Model of Flares

The solar X-ray observing satellite Yohkoh has discovered various new dynamic features in solar flares and corona, e.g., cusp-shaped flare loops, above-the-loop-top hard X-ray sources, X-ray plasmoid ejections from impulsive flares, transient brightenings (spatially resolved microflares), X-ray jets, large scale arcade formation associated with filament eruption or coronal mass ejections, and so on. It has soon become clear that many of these features are closely related to magnetic reconnection. We can now say that Yohkoh established (at least phenomenologically) the magnetic reconnection model of flares. In this paper, we review various evidence of magnetic reconnection in solar flares and corona, and present unified model of flares on the basis of these new Yohkoh observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Energetics and Propagation of Coronal Mass Ejections in Different Plasma Environments

Based on previous work, we investigate the propagation of CMEs in a more realistic plasma environment than the isothermal atmosphere, and find that it is a slightly faster reconnection for flux ropes to break free. The average Alfven Mach number MA for the inflow into the reconnection site has to be at least 0.013 in order to give a plausible eruption (compared to MA = 0.005 for the isothermal atmosphere). Taking MA = 0.1, we find that the energy output and the electric field induced inside the current sheet match the temporal behavior inferred from the energetic, long duration, CME-associated X-ray events. The results indicate that catastrophic loss of equilibrium in the coronal magnetic field provides the most promising mechanism for major solar eruptions, and that the more energetic the eruption is, the earlier the associated flare peaks. The variation of the output power with the background field strength revealed by our calculations implies the poor correlation between slow CMEs and solar flares. Th