Coronal Condensation Research Articles

A small-scale energy release observed on the limb

The D3 emission lines observed on the emerging flux loops and mound prominences above the limb were locally found to consist of two components, i.e., the main and the broadened components. A number of Doppler shifted emission lines, separated distinctly from the main body of the emission lines were also detected, which we call the separated components. On the sequential spectrograms, the lifetimes of these broadened and separated components were obtained and their histograms were made. We present the results of fitting the observed emission lines with the sum of two gaussian profiles in two simple cases under two different assumptions. In each assumption, it is found that the line-of-sight velocities of the broadened components with lifetimes shorter than 10 min monotonously increase with time and we attribute this behaviour to a single ejection of mass from the parent structures. In the broadened components with lifetimes longer than 10 min, the acceleration turns on and off alternately, each with a duration of several minutes. We attribute this behaviour to multiple ejections of mass. Finally, it is shown that the corona over the active region is supplied with enough mass for the formation of a coronal condensation if the ejected mass diffuses out high into the corona and some heating mechanisms for the mass switch on.

VLA observations of a radio plage at centimeter wavelengths

VLA observations of a solar plage region at 6 and 20 cm wavelengths are presented. The high frequency 6 cm emission correlates well with the associated sunspots, whereas 20 cm emission shows good correlation with the Hα plage. Large temperature variations over a period of one day are observed in the plage associated component without any significant changes in the sunspots. The dominant emission mechanisms at 6 and 20 cm are found to be gyroresonance radiation and bremstrahlung respectively. It is concluded that the coronal condensation above the chromospheric Hα plage has an electron density of ∼5 × 109 cm−3 and it extends to a height of ∼5 × 104 km.

Interpretation of patterns of drifting Zebra stripes

The earlier interpretation of the Zebra patterns by the concept of double resonance is examined for a realistic magneto-hydrostatic model of a coronal condensation over a bipolar spot group. It appears that the frequency drift of the Zebra stripes over a longer time can scarcely be explained by a change of state of the plasma as done till now. The proposed modified mechanism is based on a helical beam of electrons forming fronts that excite radiation at places of double resonance. Considering the field and density configuration conditions are found which produce drifting Zebra stripes. Structures of longer duration and absorptions arise from more or less overlapping stripe segments, which are excited by the fronts following one another in a beam of sufficient great length. The second stripe system of other drift direction found by Slottje (1972a) arises from lateral splitting of the beam.

Chromospheric evaporation in a well-observed compact flare

Hudson and Ohki (1972) pointed out that the increase of the soft X-ray emission measure during flares might be accounted for in two different ways, either by 'coronal condensation', or by what they termed 'chromospheric rarefaction', now more commonly called 'chromospheric evaporation'. They ruled out coronal condensation on the basis of cornal mass content arguments. Moore et al. (1980) found it highly probable that the bulk of the mass of the soft X-ray emitting plasma is supplied during the rise phase by chromospheric evaporation from the feet of the soft X-ray loops. On the other hand, Cheng et al. (1981) argued that chromospheric evaporation is not important as a source of soft X-ray plasma. The present investigation is concerned with an event in which direct chromospheric observations contradict the conclusions reached by Cheng et al. Up to now chromospheric evaporation has always been an inference, without compelling positive evidence. In the current investigation, observations are considered which constitute such evidence.

Measuring electron density in coronal active regions

The new coronameter described in this paper, now in service at Pic du Midi Observatory, has been designed for the study of coronal condensations with a 30″ spatial resolution. The instrument associates measurements of the K-corona polarized light with simultaneous pictures of the coronal structures as seen in the light of the green emission line of Fe xiv (λ5303 A). It has allowed us to engage in an extensive program of observation devoted to the study of electron density in active coronal regions. As an example we present results concerning a coronal condensation observed on 1980 February 15, which is some hours before the time of the India-Kenya total eclipse.

VLA Observations of Solar Active Regions - Part Two - Solar Bursts

view Abstract Citations (21) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Observations of Solar Active Regions. II. Solar Bursts Lang, K. R. ; Willson, R. F. ; Felli, M. Abstract The VLA has been used at wavelengths of 6 cm and 20 cm to detect seven solar bursts from two active regions with flux densities between 0. 1 and 6.0 sfu. The regions of burst energy release were resolved with angular sizes between 6" and 12" and brightness temperatures between 1 and 2 × 107 K. The angular sizes of the radio bursts are comparable to those of the soft X-ray kernels, and the brightness temperatures are comparable to the temperatures of the plasma which gives rise to soft X-ray emission. This suggests that we are detecting the radio wavelength counterpart of the flares seen at soft X-ray wavelengths. The two solar bursts observed at 6 cm wavelength occurred within a plage-associated "coronal condensation" in regions away from sunspots and near magnetic neutral lines. The Hα emission associated with one burst was, within the uncertainties of measurement, spatially coincident with the 6 cm burst. The five solar bursts detected at 20 cm wavelength were composed of individual events which had a duration of between 20 s and 60 s, but our time resolution of 10 s precluded the detection of components with shorter duration. Although thermal energies normally detected at soft X-ray wavelengths are inferred from their brightness temperatures, the train of events which make up the 20 cm bursts are similar to the elementary flare bursts detected at hard X-ray wavelengths in other active regions. The observed bursts at 20 cm wavelength can be explained by either the bremsstrahlung or the gyroresonant radiation of a hot, dense plasma with an electron temperature Te ≲ 2 × 107 K and an electron density of Ne ≍ 1010 cm-3. The gyroradiation easily explains the high observed circular polarization while indicating magnetic field strengths of H 90 G in the emitting region; while the bremsstrahlung requires polarization due to propagation effects at frequencies near the gyrofrequency in magnetic fields of strength H ≍ 500 G. We also discuss the implications of VLA observations for studies of the physical nature of the burst emitting sources. Publication: The Astrophysical Journal Pub Date: July 1981 DOI: 10.1086/159042 Bibcode: 1981ApJ...247..338L full text sources ADS |

Magnetic fields in a coronal condensation

Using the potential field model, we have calculated the magnetic structure of the coronal condensation, which appeared on the east limb of the solar disk at the 22 September 1968 eclipse. The comparison between calculated magnetic field geometry and observed contour of the coronal condensation shows general agreement. From this analysis, we see that the magnetic field extending to the lower coronal region is approximately the potential field during the steady period of an active region. The observed contour of the coronal condensation may be considered as the projection of the magnetic tubes of the active region on the plane perpendicular to the line of sight. The matter flows along the magnetic field lines.

Manifestation of pulsation instability in solar radio emission preceding proton flares

The investigation of solar radio emission fluctuations at the wavelength λ ∼- 3 cm led to the discovery of a visible increase in pulsations with periods of about 30–120 min prior to proton flares. These pulsations were observed before all (seven) proton flares included in our cycle of observations from 1969 to 1974. The phenomenon was not found to occur before non-proton flares. The assumption is made that the observed pulsations are a manifestation of pre-flare instability in coronal structures. Estimations have been made for fluctuations of the gyro-resonance radiation from the regions above spots associated with the magnetic field variations when a groove instability of a coronal condensation is developed. They are in good agreement with the observational data. The discovered manifestation of the pre-flare instability in fluctuations of the solar radio emission open new ways to study the flare development and to predict geo-effective phenomena on the Sun.

A summary report on the coordinated optical, radio, geomagnetic and ionospheric observations of the solar eclipse of September 22, 1968 in Sinkiang

This paper summarizes the main results obtained from the coordinated optical, radio, geomagnetic and ionospheric observations of the solar eclipse in Sinkiang. 1. 1. Photographs of the corona at various wavelengths were taken during totality. The brightness distribution and polarization were photometrically studied, and the electron density and temperature of the eqatorial zone for the range 1.1–3.5 R ⊙ were derived. 2. 2. Good photographs of a coronal condensation region were taken; Through photometric measurement its brightness distribution and polarization were studied. It was shown that the maximum electron density in the center of the condensation is about 7.7 × 10 9 cm −3 whixh was 7–8 times than in the sorrounding undisturbed corona. The deviation angles of the direction of polarization in the condensation region were systematically greater than in the sorrounding undisturbed corona. 3. 3. From the analysis of the radio observations at wavelength 3.2, 11.1 and 21 cm we derived the following parameters of the local sources: flux density, one-dimensional size, height, brightness temperature, magnetic field and spectrum. The polarization observation at wavelength 3.2 cm showed that the sunspot No. 332 and the polarization source associated with it have one to one correspondence in their nucleus-penumbra formation. The smallest separation between the double nuclei of the polarization source was about 2.″5. The electron density distribution and the temperature distribution in the chromosphere-corona transition region for two sources were derived from the observed spectrum. 4. 4. The analysis of the eclipse effect on the geomagnetic field showed that this effect was less than 5 γ. In order to explain this effect the electric-current system at about 110 km in the E-layer was calculated. 5. 5. The eclipse changes in the critical frequency ƒ; 0 E of the E-layer were observed the maximum decrease of the ƒ; 0 E caused by the eclipse was about 0.8 MHz. Based on the “classical” method of eclipse analysis, the relative intensity distribution of the ionizing radiation sources on the Sun were derived, and the correlation between the ionizing radiation sources and the radio-optical activity was discussed.

Energy Balance and Structure Of Active Regions

With the advent of radio and space astronomy it became necessary to extend the definition of a center of activity or active region (AR) originally proposed by L. d’Azambuja. At IAU Symposium 35, K.O. Klepenheuer (1968) defined an AR as “The totality of all observable phenomena preceding, accompanying and following the birth of sunspots, including radio-, X-, EUV-, and particle emission.” The recognition that there are other short-lived bipolar features with a distribution similar to that of active regions (ephemeral active regions) by Harveyet al. (1975) and their identification with coronal bright points by Golub et al. (1975) suggests that the definition will have to be extended further. Active regions manifest themselves in the photosphere as sunspots and faculae; in the chromosphere as the plage and its structures; in the corona as a coronal enhancement with a complex, often loop-like internal structure. (The termenhancementwas Introduced by Billings. The original termpermanent coronal condensation, introduced by Waldemeler, only referred to the very bright enhancements and was, moreover, often confused with hissporadic coronal condensations, a flareassodated phenomena. The termcoronal active regionhas, recently also been used for the coronal extension of the AR.) In keeping with the aims of this symposium the stress of this review will be on the chromosphere and corona. Active regions are especially Important as the site of most flare-associated phenomena. Here we shall be concerned with flares only as they affect the overall energy balance. Our concern is with the “quiet” active regions that cause the slowly varying components of solar activity and provide the ambiance within which solar flares occur.

The coronal condensation observed at the 1973 eclipse

The flash spectrograms obtained at the June 30, 1973 eclipse contain the monochromatic images of a coronal condensation in three coronal lines of Fexiv 5303, Fex 6374 and Fexi 7892 and Hα line. The assumption of the axially-symmetric distribution of the emissivity in the coronal lines allows us to find the density and temperature structure of the coronal condensation. While the electron density in the central axis of the condensation is about ten times as high as that of the normal corona at each height, the temperature is not so high (T⩽2.3×106K). This seems to be a representative nature of a coronal active region in the post maximum phase of activity. It is found that there exists a cool and dense core (T = 106K, N e =6 × 109 cm-3 at 17000 km) at the lower part of the coronal condensation, which is in a close geometrical coincidence with the small active prominence protruding from the underlying plage region.

The Interpretation of the Forbidden Emission Lines from a Coronal Condensation

The Interpretation of the Forbidden Emission Lines from a Coronal Condensation

Absolute abundances and distribution of material versus density and temperature in a coronal condensation

Data derived from coronal eclipse spectra in the visible range are analyzed in terms of electron density and temperature in a coronal condensation using the method recently developed by Jefferies, Orrall and Zirker. A mean density and a certain amount of material are associated to each temperature. Abundances relative to Fe are derived for Ni, Cr, Mn, S, Ca. The absolute abundance of Fe is found to be 2 × 10−5. No absolute calibration is required. It appears that there must exist a non-negligible amount of ‘cold’ material (T e < 106K) in the corona, which cannot be observed in the visible spectral range.

Theoretical model of flares and prominences

A theoretical model of flare which explains observed quantities in Ha, EUV, soft X-ray and flare-associated solar wind is presented. It is assumed that large mass observed in the soft X-ray flare and the solar wind comes from the chromosphere by the process like evaporation while flare is in progress. From mass and pressure balance in the chromosphere and the corona, the high temperature in the soft X-ray flare is shown to be attained by the larger mass loss to the solar wind compared with the mass remained in the corona, in accord with observations. The total energy of 10 a2 erg, the electron density of l013'5 cm -a in Hc~ flare, the temperature of the X-ray flare of 107.3K and the time to attain maximum Ha brightness (600 s) are derived consistent with observations. It is shown that the top height of the Ha flare is located about 1000 km lower than that of the active chromosphere because of evaporation. So-called limb flares are assigned to either post-flare loops, surges or rising prominences. The observed small thickness of the Ha flare is interpreted by free streaming and/or heat conduction. Applications are suggested to explain the maximum temperature of a coronal condensation and the formation of quiescent prominences.

Analysis of EUV Observations of a Coronal Active Region Made During the 7 March 1970 Eclipse

(Monthly Notices Roy. Astron. Soc.). Rocket observations of the EUV solar spectrum obtained during the total eclipse of 7 March 1970 showed the presence of a large coronal condensation on the NE limb (Gabriel et al., 1971). The condensation shows the existence of loop structure, which defines the local magnetic fields, and is apparent in forbidden lines including those of Si viii, Si ix, S xi, Fe ix, Fe xi and Fe xii which lie between 1200 Å and 2050 Å (Jordan, 1971). These lines are formed in the temperature range 9.3 × 105K to 2.0 × 106K. The photographic observations of the active region show clearly that the spatial distribution of material varies considerably with its temperature.

Coronal prominences on the disk observed on 29 October 1972

A group of coronal prominences appeared over a large sunspot group near the central meridian on October 29, 1972. The bright points, which were most clearly seen on the Hα-image taken at the line center, are interpreted as the points of inflow of the prominence matter into the penumbra. The amount of mass transported by the largest prominence among the group is found to share a considerable fraction of the total mass which may be contained in the permanent coronal condensation over the spot group. The observational data for the microwave emission from the spot group is discussed in relation to this phenomenon.

On the Aller's admixture radiation effect during the compression process in the solar corona and generation of coronal formations

The cooling due to Aller's admixture radiation in the energy balance equation describing the coronal gas condensation is taken into account. It is shown that the compression mechanism does not apply for the explanation of the quiescent prominences formation but is quite suitable for definite kinds of active prominences and flares of coronal origin. Two possible variants of magnetic field before and in the process of compression within the coronal element and beyond it are considered.

On the long-term behaviour of the circular polarization from coronal condensation radio emission at 4.3 cm wavelength

The circular polarization from coronal condensations at λ = 4.3 cm correspond to the extraordinary mode of propagation, due to the contribution of preceding spots' polarities, being usually left-handed. The fewer cases of right-handed polarization are normally associated to an excess of sunspot plages in the southern hemisphere, thus making it difficult to give evidence for magnetoionic coupling phenomena as a general rule.

Solar Superthermal Event Deduced from X-Ray Lines

view Abstract Citations (5) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Solar Superthermal Event Deduced from X-Ray Lines Blake, Richard L. ; House, Lewis L. Abstract improved identifications are given for the ionization stages of iron that contributed to the 1.91 A emission feature in solar spectra obtained by the NRL group in a 1966 rocket flight. in addition, it is shown that the observed emission requires a two-component model of the coronal condensation in which, over some 100 seconds, one component must have either departures from iaxwellian velocity distributions or differences in ionization and electron temperatures. Publication: The Astrophysical Journal Pub Date: June 1971 DOI: 10.1086/150970 Bibcode: 1971ApJ...166..423B full text sources ADS |

The origin of interplanetary sectors from radio observations

The interplanetary sectors have been correlated to observations of solar coronal active centers and condensations in the metric wavelengths. We have found that (1) a sector boundary is always located to the west of a coronal condensation, and (2) the effect of active centers is to displace systematically the boundary toward the east, thus enlarging the sector. A physical interpretation is discussed.