Impulsive Phase Of Solar Flares Research Articles

Impulsive phase of solar flares. I - Characteristics of high energy electrons

The variation along a magnetic field line of the energy and pitch angle distribution of high energy electrons injected into a cold hydrogen plasma containing either an open or closed magnetic field structure was investigated. The problem is formulated as a time independent Fokker-Planck Equation for the electron number distribution as a function of the electron energy, electron pitch angle, and the structure of the global magnetic field. Simple analytic solution valid in the small pitch angle regime and for slowly varying magnetic field is presented. For the more general situation a numerical code for solving the Fokker-Planck Equation was used and it was found that the analytic expression agrees well with the numerical results to values of the pitch angle much larger than expected. For most practical applications, one many confidently use the analytic expression instead of having to resort to lengthy numerical computations. These results are useful in the study of the nonthermal models of the impulsive phase of solar flares.

OSO-8 observations of the impulsive phase of solar flares in the transition-zone and corona

Several solar flares have been observed from their onset in C IV λ 1548.2 and 1–8 A X-rays using instruments aboard OSO-8. In addition, microwave and Hα flare patrol data have been obtained for this study. The impulsive brightening in C IV is frequently accompanied by redshifts, interpreted as downflows, of the order of 80 km s-1. The maximum soft X-ray intensity usually arrives several minutes after the maximum C IV intensity. The most energetic C IV event studied shows a small blueshift just before reaching maximum intensity, and estimates of the mass flux associated with this upflow through the transition-zone are consistent with the increase of mass in the coronal loops as observed in soft X-rays. This event had no observable microwave burst, suggesting that electron beams did not play a major role in the chromospheric and transition-zone excitation. Lastly, our observations suggest that the frequent occurrence of violent dynamical processes at the onset of the flare are associated with the initial energy release mechanism.

Observations of the hard X-ray spectrum of the impulsive phase of solar flares

view Abstract Citations (35) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of the hard X-ray spectrum of the impulsive phase of solar flares. Elcan, M. J. Abstract The impulsive solar hard X-ray spectrum provides a direct diagnostic on the impulsive energy release of the flare. The data from the hard X-ray (14-342 keV) spectrometer on OSO 7 have been examined for events covering a wide energy range to determine the photon spectrum more precisely. This resulted in a sample of 38 events. The data have been fitted with either single- or double-power laws, and with an exponential spectrum, as from thermal bremsstrahlung. The chi-squared statistic between two-parameter fits strongly favors the exponential fit, particularly at peak intensity when the widest energy range is sampled. This result supports a single-temperature thermal source of the impulsive solar hard X-rays with temperatures in the range 4.7-31 keV and emission measures in the range from 8.4 x 10 to the 43rd to 6.0 x 10 to the 46th per cu cm. The temperature reaches a single maximum and then rapidly decays, but the emission measure usually increases throughout, even after temperature maximum, for data with 10-s resolution. Publication: The Astrophysical Journal Pub Date: December 1978 DOI: 10.1086/182840 Bibcode: 1978ApJ...226L..99E Keywords: Solar Flares; Solar Temperature; Solar X-Rays; X Ray Spectra; Bremsstrahlung; Oso-7; Plasma Diagnostics; Thermal Energy; Solar Physics; Solar Flares:X-Ray Spectra full text sources ADS |

The Alfv�n-Carlquist double-layer theory of solar flares

We use the Vlasov equations for ions and electrons to develop a theory of a double layer in which there are both free and trapped electrons and ions. We find the equations which replace the Langmuir condition and the Bohm conditions and by numerically solving the resultant differential equation we find for particular choices of distribution functions the potential distribution in the layer. We discuss the applicability of this theory to solar flares, and show that conditions in solar flares may be such that double layers can exist for which the free particles have a power-law energy distribution. These particles will be accelerated in a double layer and may in this way account for the production of high-energy particles during the impulsive phase of solar flares.

Hα, hard X-ray, and microwave emissions in the impulsive phase of solar flares

I have studied the observational relationship between the location of flare sites in active regions and three other observables, viz., Hα line width, hard X-ray burst parameters, and peak microwave fluxes. Results suggest that the strength of the magnetic field plays a role in governing the magnitudes of these emissions. Qualitative relationships are derived on the assumption of proportionality between the spectral maximum frequency of the associated microwave burst and the field strength in the microwave source.

The nonequilibrium ionization of solar flare coronal plasma and the emergent X-ray spectrum

view Abstract Citations (38) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The nonequilibrium ionization of solar flare coronal plasma and the emergent X-ray spectrum. Shapiro, P. R. ; Moore, R. T. Abstract Consequences of a lack of equilibrium between the ionization and recombination of ions in the coronal plasma responsible for the thermal X-ray emission spectrum of solar flares are considered. A model of an impulsively heated nonequilibrium flare plasma is investigated in which a preflare coronal loop is 'instantaneously' heated, with rapid thermalization of the loop electrons occurring at a temperature of the order of 100 million K. It is shown that since the plasma is out of ionization equilibrium during the first few seconds before the onset of some energy-releasing instability, the emergent X-ray spectrum is a superposition of the thermal bremsstrahlung spectrum of hot electrons and the X-ray line and enhanced two-photon continuous spectra of the nonequilibrium ionization structure. Soft X-ray emission from this plasma is therefore burstlike, jumping to a significantly higher level when the electron temperature jumps and decaying dramatically as the gas becomes more ionized. The plasma model is used in a detailed calculation of the ionization structure of and the X-ray emission from the coronal flare plasma during the impulsive phase of solar flares. Publication: The Astrophysical Journal Pub Date: October 1977 DOI: 10.1086/155609 Bibcode: 1977ApJ...217..621S Keywords: Nonequilibrium Ionization; Plasma Temperature; Solar Corona; Solar Flares; Solar X-Rays; Ion Recombination; Radiant Flux Density; Temperature Effects; X Ray Spectra; Solar Physics full text sources ADS |

High Energy Gamma-Ray Radiation Above 300 KeV Associated with Solar Activity

The present status of our knowledge concerning the production of gamma-ray lines and continuum during the impulsive phase of solar flares is reviewed. Our data in this field is based solely on the OSO-7 observations made in 1972, August 4 and August 7. The experimental data will be reviewed. These observations along with theoretical work of Ramaty and Lingenfelter (1973a, b) and the charged secondary observations of the Chicago group (Anglin et al., 1973) lead to the investigation of different hypothetical models to explain the production of neutral and charged secondaries in the solar atmosphere. At the present time it is not possible to rule out the preflare and postflare accumulation models if all the data is considered. We will discuss the outstanding experimental questions to be answered in future investigations.

Rise Time of Hard X-Ray Bursts

SummaryA study was made of the hard X-ray component in the impulsive phase of solar flares. In 36 randomly chosen events the value for the slope in the differential electron power spectrum E−δ electrons cm−2 s−1 keV−1, was related to the 20–32 keV spike rise time (e-folding) as trise = 0.56 exp (0.88δ) in the thin target model and tfrise = 0.10 exp (0.88δ) in the thick target picture. In the thin target model, the above empirical relation would imply that the acceleration of electrons can be longer when the acceleration rate is smaller. An alternative interpretation would be that an impulsive hard X-ray burst is a superposition of two components emitted from thin and thick targets; when the former predominates, the duration is longer and the photon spectral index is larger, while when the latter predominates, the duration is shorter and the photon spectral index is smaller; 3 ≲δ ≲4 is required (Figure 1). The uncertainty in δ is 0.5 while that in the rise time is 1 s.

Rise Time in 20-32 keV Impulsive X-Radiation

A study was made of the hard X-ray component in the impulsive phase of solar flares. In 36 randomly chosen events the value for the slope in differential electron power spectrum, E -6 electrons cm 2 1 keV -1, was related to the keV spike rise time (e-folding) as tr8se = 0.56 exp (0.888) in the thin-target model and 4ise = 0.10 exp (0.888) in the thick-target picture. In the thin-target model, the above empirical relation would imply that the acceleration of electrons can last longer when the acceleration rate is smaller. An alternative interpretation would be that an impulsive hard X-ray burst is a superposition of two components emitted from thin and thick targets; when the former predominates the duration is longer and the photon spectral index is larger, and vice versa. Subject headings: flares, solar - X-rays, solar

X-radiation (E > 10keV), H? and microwave emission during the impulsive phase of solar flares

A study has been made of the variation in hard (E⩾ 10 keV) X-radiation, Hα and microwave emission during the impulsive phase of solar flares. Analysis shows that the rise-time in the 20–30-keV X-ray spike depends on the electron hardness, i.e., trise ∼ exp (0.87 δ). The impulsive phase is also marked by an abrupt, very intense increase in Hα emission in one or more knots of the flare. Properties of these Hα kernels include: (1) a luminosity several times greater than the surrounding flare, (2) an intensity rise starting about 20–30 s before, peaking about 20–25 s after, and lasting about twice as long as the hard spike, (3) an effective diameter of 3000–6000 km for class ≤ 1 flares, representing less than 1/8-1/2 of the main flare, (4) a location lower in the chromosphere than the remaining flare, (5) essentially no expansion prior to the hard spike, (6) a position within 6000 km of the boundary separating polarities, usually forming on both sides of the neutral line near both feet of the same tube of force, (7) a shape often resembling isogauss contours of the photospheric field indicated on magnetograms and (8) total radiated energy less than l/50 that of the hard electrons. Correspondingly, impulsive microwave events are characterized by: (1) the detection of a burst at 8800 MHz for every X-ray spike ifthe number of electrons above 100 keV is greater than 1033, (2) great similarity in burst structure with 20–32 keV X-rays but only at f > 5000 MHz, (3) typical low frequency burst cutoff between 1400–3800 MHz, and (4) maximum emission at f > 7500 MHz. Finally the Hα, X-ray and microwave data are combined to present a picture of the impulsive phase consistent with the above observations.