Observations and interpretation of solar flares at microwave frequencies

We study the association of type III bursts related to He flares in different magnetic environments in the period 1970-1981. Special attention is paid to flares which partly cover a major spot umbra (Z-flares). In particular we consider the location of the spots in the active regions and the magnetic field intensities of spots covered by a ribbon. The association rate with type lII bursts decreases to 17% when the flare is located inside the bipolar pattern of a large active region, compared with an association rate of 54% when the flare is situated outside it. The association rate increases with the magnetic field intensity of the spot covered by Ha emission; this is most clearly revealed for the flares occurring outside the bipolar pattern of active regions. Ninety-three percent of the flare-associated type III burst were accompanied by 10 cm radio bursts. For the most general case in which a flare is developing anywhere in an active region, the association with type III bursts generation increases with the increasing magnetic field intensity of the main spot of the group.

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“…This examines the relationship of 9 type III bursts with energetic (>~ 100 keV) electrons (see the review by Crannell et al, 1988).…”

Section: Introductionmentioning

confidence: 99%

The role of the magnetic field intensity and geometry in the type III burst generation

Zlobec

Ruždjak²,

Vršnak³

et al. 1990

Sol Phys

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“…Maser emission occurs in narrow bands close to the cyclotron frequency Q = eB/m e and its harmonics sCl and its coherence can account for the high brightness temperatures associated with the observed microwave bursts (Melrose and Dulk 1982a, b;Crannell et al 1988). Due to spatial gradients in the field, the waves quickly propagate out of the region in which they can be amplified, thereby producing elementary bursts of short duration (Melrose and Dulk 1984).…”

Section: (A) the Loss-lone-driven Cyclotron Masermentioning

confidence: 99%

“…1. Introduction Millisecond bursts of microwave emission observed during some solar flares are found to exhibit characteristic properties of high brightness temperatures (up to ~ 10 18 K), high degrees of circular polarisation (up to ~ 100%), narrow bandwidths (-1%) and fine temporal structure (1-10 ms) (Slottje 1978(Slottje , 1980Crannell et al 1988). The emission mechanism most widely invoked to account for these properties of microwave spike bursts is the loss-cone-driven electron cyclotron maser (e.g., Wu and Lee 1979;Melrose and Dulk 1982a,b;Melrose et al 1984;Dulk 1985;McKean et al 1989).…”

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confidence: 99%

Propagation Effects on the Cyclotron Maser Mechanism for Solar Microwave Spike Bursts

Kuncic

Robinson

1993

Publ. Astron. Soc. Aust.

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The loss-cone-driven electron cyclotron maser instability is widely believed to be responsible for millisecond bursts of intense microwave emission often observed during solar flares. However, the maser radiation is strongly absorbed as it propagates outward from the corona and existing analytical models predict that this absorption should be sufficiently strong to prevent observable levels of the radiation from escaping, except under highly restrictive conditions. In order to address the problem of how microwave spike bursts can be observed at all, we present a numerical ray-tracing analysis which incorporates emission, propagation and absorption of fundamental cyclotron maser radiation in a realistic model of a coronal flux loop. It is found that the radiation can escape to a potential observer and that the physical conditions under which escape occurs are more restrictive for fundamental emission in the extraordinary (x)-mode than in the ordinary (0)-mode. Escaping radiation in the x-mode is found to be highly directional and chiefly observable toward the center of the solar disk, while escaping 0-mode radiation is found to emerge from the corona over a much wider range of directions, with some cases corresponding to observable radiation near the solar limb.

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“…For example, the range of required magnetic fields for the thick‐target model was smaller than for the thin‐target and multithermal models, suggesting that this might be the better model [ Lu and Petrosian , 1989]. However, some correlations seem to depend on the duration of the x‐ray event [ Kosugi et al , 1988], such that the thick‐target model was preferred for the impulsive flares, while extended flares were better represented by electrons trapped in the corona.…”

Section: Energy Input: the Impulsive Phasementioning

confidence: 99%

Solar X‐Ray Physics

Bornmann

1991

Reviews of Geophysics

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This is a review of the research in solar x‐ray phenomena that has been done by American scientists in the last four years. This research ranges from new instrumentation, such as multilayer mirrors and high spectral resolution detectors, to refined theories and observations of the flare process. Contributions have been made in our understanding of x‐ray bright points, solar active regions, solar flares, and flare‐associated phenomena. The hard x‐ray spectra during the impulsive phase of solar flares have been examined in greater detail and found, among other things, to fit a double power law. Models for the hard x‐ray emissions have focused largely on electron acceleration by field‐aligned currents, although other models such as proton beams, waves, and neutral beams have also been investigated. Correlations between the hard x‐ray emission and radio, ultraviolet, and gamma‐ray emissions have been used to provide additional constraints on the flare models. The process known as chromospheric evaporation, which is seen as blueshifts of the soft x‐ray line profiles, has been explored through further analysis of the observations and the development of new models. New atomic calculations have been performed so that further information about the plasma properties during the decay phase of solar flares can be derived from the soft x‐ray line emissions. Spatially resolved observations have been used to identify new x‐ray features such as flaring arches and giant post‐flare arches. Other topics of investigation included the energy in solar flares and the long‐term variations in solar indices, the 155‐day periodicity of solar flares, and the distribution of flares on the solar disk. Finally, associations have been made between the solar x‐ray emission and the phenomena of coronal mass ejections and solar energetic particles, as well as the effect of this radiation on the earth's upper atmosphere.

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Observations and interpretation of solar flares at microwave frequencies

Cited by 22 publications

References 62 publications

The role of the magnetic field intensity and geometry in the type III burst generation

The role of the magnetic field intensity and geometry in the type III burst generation

Propagation Effects on the Cyclotron Maser Mechanism for Solar Microwave Spike Bursts

Solar X‐Ray Physics

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