Partial reconstruction of the initial conditions for streams of energetic electrons associated with a solar type III burst

A simple test of the applicability of quasilinear theory is presented to be applied to measured velocity distributions of energetic electrons associated with solar type III bursts or similar particle streams: it consists in taking the time integral of these distributions after removing the background. If quasilinear diffusion of weak plasma turbulence applies to the particle dynamics and if all other losses are neglected, the time integral of the distributions injected at the source of the streaming particles is recovered. In particular, if these distributions are approximated by hot Maxwellian ones, direct estimates of the initial temperatures are retrieved. Further analysis might even permit an experimental evaluation of the relative effects of collision and scattering losses.     

Polarization of a periodic solar microwave burst

No fluctuations in polarization have been found during a 7 GHz solar burst showing 17 s periodic pulses in intensity. Polarization effects can be produced by the propagation media in the active centre, which are not affected directly by the burst source, but situated more deeply than the observed heights at that microwave frequency.     

Millisecond spikes in a short decimetric solar radio burst

The characteristics of the millisecond spikes with short duration and weak flux density which were observed with high time resolution (1 ms) at 1420, 2000 and 2840 MHz during the great type IV solar radio burst of 30 July 1990 are introduced in detail in this article. The time profiles of the spikes are statistically analyzed and the parameters of the spike source are also estimated.     

Source parameters of a solar radio microwave burst

Choosing a representative solar radio microwave burst, a typical burst which contains a precursor and an impulsive burst followed by a slowly decaying component, as an example, we have researched its radio emission mechanism and calculated the two important theoretical characteristic parameters, intensity and dimension, of its various sources.     

Fractionation of nitrogen isotopes in solar energetic particles

Abstract In a further step to assess processes leading to the complicated secular trend of the isotopic composition of N implanted in lunar regolith, we investigate mechanisms fractionating solar energetic particles (SEPs). We conclude that such mechanisms are likely to occur, most probably producing an enrichment of ¹⁵N over ¹⁴N in SEPs over the photospheric abundance ratio. Simultaneously, ²²Ne is enriched over ²⁰Ne but to a lesser extent. An enrichment of the heavy Ne isotope is observed in the suprathermal solar particles, implanted in the lunar regolith. Hence, the now well-established difference between the isotopic composition of suprathermal Ne and solar wind Ne in the lunar regolith might be taken as evidence for the validity of this model. The present-day fluxes of energetic particles produced in impulsive flare events, capable to produce such isotopic fractionations are, however, orders of magnitude below the required amounts to explain the lunar observations. The details of the secular variation of the N isotopic composition remain an enigma.     

Propagation of energetic electron streams in solar flares

The microscopic stability of an electron stream flowing down to the photosphere from the corona is examined. It is found that, while a power-law distribution is stable in the low-density corona, it is unstable against the generation of magnetized electron plasma waves in the high-density photosphere. The scattering of these energetic electrons may alter their radiation signatures.     

Power-law distribution for solar energetic proton events

Analyses of the time-integrated fluxes of solar energetic particle events during the period 1965–1990 show that the differential distribution of events with flux F is given by a power law, with indices between 1.2 and 1.4 depending on energy. The power law represents a good fit over three to four orders of magnitude in fluence. Similar power-law distributions have been found for peak proton and electron fluxes, X-ray flares and radio and type III bursts. At fluences greater than 10⁹ cm^–2, the slope of the distribution steepens and beyond 10¹⁰ cm^–2 the power-law index is estimated to be 3.5. At energies greater than 10 MeV, the slope of the distribution was found to be essentially independent of solar cycle, when the active years of solar cycles 20, 21, and 22 were analysed. The results presented are the first for a complete period of 27 years, covering nearly 3 complete solar cycles. Other new aspects of the results include the invariance of the exponent with solar cycle and also with integral energy.     

A study of energetic solar flare X-rays

A new series of solar flare energetic X-ray events has been detected by an ionization chamber on the OGO-I and OGO-III satellites in free space. These X-rays lie in the range 10–50 keV, and a study has been made of their relationship to 3 and 10 cm radio bursts and with the emission of electrons and protons observed in space. The onset times, times of maximum intensity and total duration are very similar for the radio and X-ray emission. Also, the average decay is similar and usually follows an exponential type behavior. However, this good correlation applies most often to the flash phase of flares, whereas subsequent surges of activity from the same eruption may produce microwave emission or further X-ray bursts not closely correlated. An approximate proportionality is found between the total energy content of the X-rays and of the 3 and 10 cm integrated radio fluxes. These measurements suggest that the X-ray and microwave emission have a common energizing process which determines the time profile of both. The recording of electrons greater than 40 keV by the Interplanetary Monitoring Probe (IMP satellite) has been found to correlate very well with flares producing X-ray and microwave emission provided the propagation path to the sun is favorable. There is evidence that the acceleration of solar protons may not be closely associated with the processes responsible for the production of microwaves, X-rays, and interplanetary electrons.The OGO ionization chamber responds to energies (10–50 keV) intermediate between the soft X-rays giving SID disturbances (1–10 keV) and energetic quanta previously measured with balloons (50–500 keV). Proposed source mechanisms should be capable of covering this range of energies including the most energetic quanta occasionally observed.     

Narrow-band split frequency decimeter solar burst

A wide-band digital decimetric spectroscope is in the process of development at INPE (Brazil), in conjunction with a 9-m diameter polar mounted antenna. Initially, this spectroscope was operating over a narrow-band 1.6 ± 0.05 MHz in an analogue mode. Here we report a slow drifting 5.0 MHz s^-1, narrow-band 1630-1580 MHz, split frequency solar burst lasting for about 15 s, observed on 15 June, 1991. The separation between the split components is 30 MHz, and the upper split frequency component is more intense than the lower split frequency component. These observed characteristics favour the hypothesis of conversion of plasma waves by combinational scattering on upgoing ion-sound waves in a magnetic loop. Existing strong electron density gradients across the magnetic field in the source region will reduce the free-free absorption of radiation at the fundamental frequency. In order to explain the observed temporal characteristics of the upper and lower split frequency components, the radiation has to propagate at some angle to the electron density gradient in the source region. Estimated physical parameters of the source and exciters are as follows: (i) maximum source size (height) 2 × 10⁷ cm; (ii) velocity and length of ion-sound pulse 3.1 × 10⁶ cm s^-1 and 5 × 10⁷ cm, respectively, and (iii) velocity of the exciter of plasma waves (electron or proton beam) 5 × 10⁸ cm s^-1.On leave from the Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia.     

Direct observations of low-energy solar electrons associated with a type III solar radio burst

A highly anisotropic packet of solar electron intensities was observed on 6 April 1971 with a sensitive electrostatic analyzer array on the Earth-orbiting satellite IMP-6. The anisotropies of intensities at electron energies of several keV were factors 10 favoring the expected direction of the interplanetary magnetic lines of force from the Sun. The directional, differential intensities of solar electrons were determined over the energy range 1–40 keV and peak intensities were 10² cm^–2 s^–1 sr^–1 eV^–1 at 2–6 keV. This anisotropic packet of solar electrons was detected at the sattelite for a period of 4200 s and was soon followed by isotropic intensities for a relatively prolonged period. This impulsive emission was associated with the onsets of an optical flare, soft X-ray emission and a radio noise storm at centimeter wavelengths on the western limb of the Sun. Simultaneous measurements of a type III radio noise burst at kilometric wavelengths with a plasma wave instrument on the same satellite showed that the onsets for detectable noise levels ranged from 500 s at 178 kHz to 2700 s at 31.1 kHz. The corresponding drift rate requires a speed of 0.15c for the exciting particles if the emission is at the electron plasma frequency. The corresponding electron energy of 6 keV is in excellent agreement with the above direct observations of the anisotropic electron packet. Further supporting evidence that several-keV solar electrons in the anisotropic packet are associated with the emission of type III radio noise beyond 50R is provided by their time-of-arrival at Earth and the relative durations of the radio noise and the solar electron packet. Electron intensities at E 45 keV and the isotropic intensities of lower-energy solar electrons are relatively uncorrelated with the measurements of type III radio noise at these low frequencies. The implications of these observations relative to those at higher frequencies, and heliocentric radial distances 50R , include apparent deceleration of the exciting electron beam with increasing heliocentric radial distance.Research supported in part by the National Aeronautics and Space Administration under contracts NAS5-11039 and NAS5-11074 and grant NGL16-001-002 and by the Office of Naval Research under contract N000-14-68-A-0196-0003.     

Propagation and confinement of energetic electrons in solar flares

The propagation, cofinement and total energy of energetic (>25 keV) electrons in solar flares are examined through a brief review of the following hard X-ray measurements: (1) spatially resolved observations obtained by imaging instruments; (2) stereoscopic observations of partially occulted sources providing radial (vertical) spatial resolution; and (3) directivity of the emission measured through stereoscopic observations and the center-to-limb variation of the occurrence frequency of hard X-ray flares. The characteristics of the energetic electrons are found to be quite distinct in impulsive and gradual hard X-ray flares. In impulsive flares the non-thermal electron spectrum seems to extend down to 2 keV indicating that the total energy of non-thermal electrons is much larger than that assumed in the past.     

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.     

Propagation and confinement of energetic electrons in solar flares

Solar Physics - The propagation, cofinement and total energy of energetic (>25 keV) electrons in solar flares are examined through a brief review of the following hard X-ray measurements:...     

The magnetic field strength and energy flux of energetic electrons in a microwave burst source region

The observations of a microwave burst with multiple impulses on 1993 Oct 2, 073940–074100 UT are analysed. This event consists of multiple impulses superimposed on a slowly varying burst background. Our formula for coronal magnetic field diagnostics was used here for the first time to derive the field strength and information on the energetic electrons. The results are: 1) The mean spectral index of the impulsive component in the optically thin part is less than that of the slow background by 1 (a harder spectrum). The mean brightness temperature at 19.6 GHz of the former is 6 times that of the latter. 2) The mean magnetic strengths of the impulse and slow burst regions are 158 G and 531 G, respectively. The time variation in the slow burst region is saddle-shaped, being 50% lower in the middle than at the beginning and end. 3) The column density NL and number density N of energetic electrons in the impulsive component are 4% and 8% of those of the slow component, but the energy flux and emission coefficient are 100% and 800% greater. The two components appear to be produced by two different electron groups with different energy distributions in two different regions.     

Variations of type III burst parameters during a decametric solar storm

An analysis has been made of type III bursts recorded during a decametric solar storm observed from July 29 to August 16, 1975 with the UTR-2 antenna (Kharkov, IRE Acad. Sci. Ukr. SSR). The bursts were recorded with a dynamic spectrograph and radiometers at 25.0, 20.0, 16.7, and 12.5 MHz. Daily observations have yielded histograms of the type III burst distribution with respect to the frequency drift rate in three subbands between 25.0 and 12.5 MHz. During the middle stage of the storm the drift rate was about twice as high as at the onset and the final stage of the storm. Abrupt changes in the mean frequency drift rate were registered some two to three days after the active region McMath 13790 had come onto the limb and also before it disappeared behind the solar disk. Sudden changes in the drift rates of the type III bursts were accompanied by sudden changes of their mean duration. The rather long burst durations observed at 25.0 MHz at the beginning and the end of the radio storm coincided with such at the twice lower frequency, i.e. 12.5 MHz, during the period when an increased drift rate was observed.Similar variations of type III burst parameters can be interpreted in the framework of the plasma mechanism of burst generation in the corona, assuming that at the middle stage of the storm the bursts observed in the 25.0–12.5 MHz range were emitted at the fundamental whereas when the emitting region was near the limb the bursts received corresponded to the second harmonic of the Langmuir oscillations in the range of 12.5 to 6.25 MHz excited at greater heights.     

Spectral development of a solar X-ray burst observed on OSO-7

The UCSD solar X-ray instrument on the OSO-7 satellite observes X-ray bursts in the 2–300 keV range with 10.24 s time resolution. Spectra obtained from the proportional counter and scintillation counter are analyzed for the event of November 16, 1971, at 0519 UT in terms of thermal (exponential spectrum) and non-thermal (power law) components. The energy content of the approximately 20 × 10⁶K thermal plasma increased with the 60 s duration hard X-ray burst which entirely preceded the 5 keV soft X-ray maximum. If the hard X-rays arise by thick target bremsstrahlung, the nonthermal electrons above 10 keV have sufficient energy to heat the thermally emitting plasma. In the thin target case the collisional energy transfer from non-thermal electrons suffices if the power law electron spectrum is extrapolated below 10 keV, or if the ambient plasma density exceeds 4 × 10¹⁰ cm^–3.Formerly at UCSD.     

Electrons and protons in long-lived streams of energetic solar particles

In 1966 and 1967 many long-lived streams of low-energy solar electrons and protons were observed near Earth. These streams were sometimes associated with bright flares which occurred many hours earlier and sometimes no individual flare could be found. In the latter case the particles are evidently to be associated in a general way with solar active centers as Fan et al. (1968) have done. The long-lived solar events discussed here include energetic storm particles, delayed events and fluxes associated with solar active regions. It is suggested here that these are all probably the same basic phenomena viewed in somewhat different ways depending on the age of the region and its location on the solar disc. These events are usually associated with a depression in the sea-level neutron intensity and one or more sudden commencements or sudden impulses. Both electrons and protons are present in these events but in several cases electrons were not detected. The most unusual feature is that when both particle species are present, the electron flux is centered several hours before the proton flux.     

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.