Storm Radiation Research Articles

Spectrum of Solar Type I Continuum Noise Storm in the 50–80 MHz Band and Plasma Characteristics in the Associated Source Region

Continuum observations of a solar noise storm in the frequency range of 50 - 80 MHz observed with the Gauribidanur radio spectrograph during 2000 September, 26 & 27, are presented here. The radio spectral index of the noise storm continuum in the band 50 - 80 MHz is found to be ~3.65 during the above period. The Noise Storm continuum radiation is explained as a consequence of the non-thermal, plasma emission mechanism. The beam-density of suprathermal electrons is estimated for the coronal plasma near the source region of storm radiation. Supplementary evidence for the density-estimate is provided by way of analysing the imaging data from the SXT on-board the Yohkoh spacecraft, and the LASCO, MDI, and EIT on board the SoHO spacecraft.

Polarization features of solar radio emission and possible existence of current sheets in active regions

We show that it is possible to account for the polarization features of solar radio emission provided the linear mode coupling theory is properly applied and the presence of current sheets in the corona is taken into account. We present a schematic model, including a current sheet that can explain the polarization features of both the low frequency slowly varying component and the bipolar noise storm radiation; the two radiations face similar propagation conditions through a current sheet and hence display similar polarization behavior. We discuss the applications of the linear mode coupling theory to the following types of solar radio emission: the slowly varying component, the microwave radio bursts, metric type U bursts, and bipolar noise storms.

New observations of solar noise storm radiation at decameter wavelengths

We report multifrequency observations of storm continuum and other radio bursts. Based on their positional study and their correlation with other coronal and photospheric features, we deduce that the storm source is located in the magnetic field lines lying above a single bipolar active region. Energetic electrons trapped in the magnetic structures above the spots must be responsible for the storm radiation. We show that spontaneous emission of Langmuir waves by anisotropic distributions can explain both storm continuum and bursts self-consistently. Whenever the collisional damping (ν c ) is more than the growth (- υ A ), there is a steady emission responsible for the continuum, and whenever ν c = -υ A (which may be satisfied randomly) there is a sudden jump in T b giving rise to bursts. The number density of energetic particles required to explain the storm continuum at 73.8, 50, and 30.9 MHz frequencies is estimated to lie in the limits n b /n e ∼ 10−10−10−9 in the context of the present observations. The brightness spectrum of the storm continuum is computed and compared with observations.

Particle beams as a source of noise storm depression?

The February 5, 1986 flare-related radio continuum depression is studied, compared with other noise storm depression events and discussed in the framework of current type I storm models. The influence of flare plasma flow or shocks and of superthermal electrons on noise storm radiation is considered. The presence of fast drifting emission features just before and during the decrease of the intensity, the association between the depression onset and the microwave burst maximum, the simultaneous appearance of the intensity minimum over a broad spectral range as well as preflare evidence of an interconnection of the flare site and the noise storm source are arguments for a preference of the role of beams of superthermal electrons. We distinguish abrupt and slow depressions (Figure 5). The abrupt depressions are in agreement with Melrose's (1980) predictions. Slow depressions can only be understood by invoking the diffusion of super-thermal electrons through the magnetic field carrying the storm source.

Filament eruption and storm radiation at meter-decameter wavelengths

We report the study of a weak noise storm observed by the Clark Lake multifrequency radioheliograph at four frequencies. The noise storm onset was associated with a filament eruption and a gradual rise and fall in soft X-rays. We compare the noise storm emission with related emissions in other wavelengths to develop a composite scenario of the event. Using the properties of the quiet corona inferred from the simultaneously observed quiet-Sun radiation, we estimate the brightness temperature of the storm continuum, which seems to be consistent with the observations reported in Solar Geophysical Data. Superthermal particles with a temperature that is ten times the coronal electron temperature and a density of ∼ 10−3 times the coronal density are adequate to explain the observed radiation. Since the noise storm observations were made at four frequencies, we were able to obtain a brightness temperature spectrum of the storm radiation. If the storm radiation is affected in the same way as the quiet-Sun emission by inhomogeneities, the observed spectrum can be interpreted as due to propagation effects. Since the Clark Lake instrument can observe both quiet Sun and weak bursts simultaneously, we were able to estimate the propagation effects from the quiet-Sun observations and use it to correct the brightness temperature of storm radiation.

A note on the emission mechanism of storm radiation

A new mechanism has been proposed for the continuum and burst components of solar storm radiation by Genkin, Erukhimov, and Levin (1989a, b). In this paper, we point out that while bursts can be explained by the proposed mechanism of scattering on plasma turbulence generated density fluctuations, the continuum cannot be explained by sattering on thermal ion density fluctuations. The reason is, under the same coronal conditions, second harmonic emissions will dominate over the fundamental emission due to scattering on thermal ion density fluctuations in contradiction to observations. We also note that the range of plasma wave densities needed for this mechanism may not be realistic for the case of plasma wave generation due to loss cone instability. It is further argued that coalescence of plasma waves with low-frequency waves still seems to be the plausible mechanism.

Variability of metric emission from the sun

Full-disk observations of the sun obtained with the VLA at 317 MHz have been analyzed, with special attention being given to the short period variability of the meter-wavelength sources. A reduction in brightness is noted over coronal holes at 90 cm. Thermal emission models for the slowly varying component are investigated, along with a nonthermal emission mechanism for storm radiation at meter wavelengths. The results indicate that the thermal model encounters difficulties in accommodating short period changes over a few tens of minutes, and that the nonthermal mechanism encounters difficulty in accounting for the variability of low brightness temperatures of less than 10 to the 6th K. A gyrotron synchrotron emission mechanism has been ruled out.

The spectrum and position of solar noise storms at decameter wavelengths

Decametric storm radiation during the period July–August 1970 has been observed simultaneously with a high sensitivity spectrograph at Arecibo Observatory and with the log-periodic, swept-frequency array of the Clark Lake Radio Observatory. The observations complement each other; different types of fine structure emissions can be easily identified on the spectrograph records and their position can be determined from the swept-frequency recordings. We study the relative positions of the different emissions which have been observed during the storms. Four distinct sources appeared to be present. The continuum emission, the type I bursts and the flare-related type III's were all emitted at different locations. The storm type III bursts, type IIIb's and drift pairs overlapped in position, but appeared at different locations than the previously mentioned sources.

Decameter storm radiation, I

A log-periodic array, 3 km long in the E-W direction is in operation at the Clark Lake Radio Observatory. The solar brightness distribution is swept once per second in the 65-20 MHz frequency range. The analysis of the interferometer records allows the determination of one dimensional solar burst positions, to an accuracy of 0.1 R⊙ at 60 MHz and 0.3 R⊙ at 30 MHz, approximately.

Decameter storm radiation, II

Decameter storm radiation, II

Radio Bursts from the Solar Corona

III, type II, and moving type IV bursts-and the role they play in the flare phe­ nomenon. Considerable progress has been made in the understanding of these bursts during the last decade, owing to the introduction of interplanetary and new forms of ground-ba sed observa tions, and to developments in plasma theory. By so restricting the field, we have omitted some important t opic s , notably the whole class of microwave bursts and their relation to X-ray emission, a sub­ ject wel l covered in reviews by Takakura (1967, 19 69); also omitted are the deci­ meter wave bursts and the long-wave continuum and storm radiation from sta­ tionary sources in the corona, still bare ly understood. Dur ing the period under review books dealing extensively with the subject of solar bursts have been published

Cyclotron Radiation From Electron Streams As The Origin Of Solar Type I Noise Storms

In this report, cyclotron radiation from electron streams gyrating in some spot-field configurations in the corona is proposed to be the origin of solar type I noise storm radiation. In order to investigate whether one or both of the characteristic waves

The Position and Movement on the Solar Disk of Sources of Radiation at a Frequency of 97 Mc/s. II. Noise Storms

Using the equipment described in Part I of this series, the apparent position and polarization of the sources of solar radiation at 97 Mc/s have been measured at some time during about 30 noise storms occurring between May 1949 and July 1950, and the progress of the source daring about six of these has been observed in some detail. The best criterion available for the chance of a storm occurring is shown to be the size of the largest visible individual sunspot. The evidence suggests that the storm radiation is associated with this spot only and not with the rest of the group, that the spot magnetic field is the important factor, and that the direction of rotation of the circular polarization of the storm radiation depends on the magnetic polarity of the associated spot, being usually left-handed when the largest spot is a north-seeking pole and right-handed when it is a south pole. Deviations observed between the apparent positions of radio source and visible sunspot on the solar disk are explained in terms of a high origin of the storm radiation in the solar corona, the observed heights varying from 0.3 to 1.0 photospheric radii above the Sun's visible surface.