Absence Of Flare Research Articles

Non-thermal particles in the solar atmosphere during flare developments and in the absence of flares

Non-thermal particles play a major role in the active Sun since they contain a large amount of the energy released during flares and since dekakeV electrons are produced in the absence of flares in association with active regions. These particles are detected either in the interplanetary space or through the radiations they produce in the solar atmosphere during flares (radio, X-ray/γ-ray bursts) and outside flares (e.g. radio noise storms). We focus on the particles interacting at the Sun. After a brief discussion on emission processes, we discuss some evidences for the fragmentation of the non- thermal energy release during flares and present some observations relevant to the onset of noise storms. New results obtained on the more energetic flares are then summarized: relative amount of accelerated electrons and ions, particle transport from the acceleration site to the emitting regions, hard X-ray and γ-ray coronal contributions from partially trapped particles, high energy (≥ 10 MeV) radiation from relativistic electrons and 100 MeV-1 GeV ions.

Enhanced X-ray emission above 3.5 keV in active regions in the absence of flares

We demonstrate that even in the absence of flares there are very often volumes of hot plasma in the corona above active regions with temperatures in excess of 10 million degrees. Characteristics of this hot plasma and its time variations seem to be different in active regions of different phase of development. These hot plasma regions are sources of very weak, but clearly recognizable, X-ray emission above 3.5 keV. Long-lived X-ray brightenings, 104 times weaker than a flare, but lasting up to 10 hr occur predominantly along the H∥ = 0 line, apparently low in the corona. After major flares, long-lived X-ray emission is also radiated from tops of arches extending high into the corona. Some other long-lived sources, far from the H∥ = 0 line, may be associated with newly emerging flux. Short-lived X-ray sources, with fluxes ranging from subflare levels to 10−3 times the flare flux, last for 2 to more than 30 min and are probably microflares. They seem to be most frequent in growing young active regions and appear often in areas with newly emerging flux.

Preflare conditions, changes and events

Prefiare conditions, changes and events are loosely categorized as distinct, evolutionary or statistical. Distinct preflare phenomena are those for which direct physical associations with flares are implied. Also, they are not known to occur in a like manner during the absence of flares. These include the early stage of filament eruptions within active centers, preflare vortical structures, some transient X-ray emitting features, 5303 A accelerating coronal arches, and increases in circular polarization at cm wavelengths. Evolutionary preflare changes are considered to be any long-term effect that may be related to the flare build-up even though the same changes may occur in the absence of flares. This category covers the development of current sheets or strongly sheared magnetic fields, evolving magnetic features, emerging flux regions, the development of satellite fields around sunspots, the evolution of reverse polarity field configurations, the merging of adjacent active centers, sunspot motions and the development of velocity patterns. Statistical preflare changes logically include both distinct and evolutionary preflare changes. However, in addition, there are preflare conditions and events that are not necessarily linked to the flare in either a direct physical or indirect evolutionary way. Such parameters or events that may only be statistically significant are certain magnetic field properties, the brightness of active centers at various wavelengths, the previous occurrence of flares and subflares, increased turbulence in filaments and certain radio events.

Non-thermal processes during the ?build-up? phase of solar flares and in absence of flares

Hard X-ray and radio observations lead to the conclusion that production of non-thermal electrons is a common phenomenon of the active Sun. A preliminary analysis of three hard X-ray bursts observed with the OGO-5 satellite and the radio observations reported in the literature indicates that non-thermal particles are present in the flare region prior to the impulsive (flash) phase and also during the gradual rise and fall (GRF) bursts which are usually explained in terms of purely “thermal” radiation. The principal difference between the non-thermal electrons observed before the flash phase and during the flash phase appears to be in their total number rather than in the hardness of their energy spectrum. This indicates that the basic characteristics of the two acceleration processes are probably similar although the total energy converted into non-thermal electrons is considerably larger in the flash phase than in the build-up phase. Transient absorbing Hα features and filament activations are discussed in terms of their ability to produce energetic particle events and magnetic energy release.

The absence of flares in ?3835 and the heating of the chromosphere

Simultaneous observations of flares in Hα and a band 15 A wide centered on 3835 A show no change whatever in 3835 A at the time of several flares, although the chromospheric network is easily visible. Flares are therefore transparent in this wavelength.

Solar Non-thermal Radio Emissions in the Absence of Flares

IN this article we present observations which indicate an association between some isolated groups of metric type III bursts and transient Doppler-shifted Hα absorbing features in the chromosphere. This suggests the acceleration or release of fast charged particles by chromospheric phenomena in the absence of flares.

Treatment of Camera Lenses with Low Reflecting Films

An f : 2 photographic lens having five separated elements was treated with evaporated fluorite to decrease the reflection from all ten air-glass surfaces. Photographs were taken with the lens under carefully controlled conditions before and after it was treated. The effective speed of the lens was found to be almost doubled by the treatment. A slight increase in contrast resulted in photographs taken under normal lighting conditions and a large increase in contrast resulted in those taken under adverse lighting conditions. An increase in detail was observed, because of the absence of flare, and the ghost images usually observed under adverse lighting conditions were eliminated.