共查询到20条相似文献，搜索用时 444 毫秒
1.
Using microwave observations made with the Nobeyama radioheliograph (=1.76 cm), we have studied temporal variations of sunspot-associated sources in the circularly polarized component. For all three cases of well-developed and rather stable sunspots we found nearly harmonic oscillations with periods in a range of 120–220 s. In one case of an unstable and quickly devolving active region, the fluctuations appear to be irregular with no dominant period. Sunspot-associated solar radio sources are known to be generated by cyclotron radiation of thermal electrons in magnetic tubes of sunspots at the level of the lower solar corona or chromosphere–corona transition region (CCTR). At the wavelength of 1.76 cm, the polarized emission arises in a layer where the magnetic field is B=2000 G (assuming the emission generated at the third harmonic of electron gyrofrequency). We suggest that the observed effect is a manifestation of the well-known 3-min oscillations observed in the chromosphere and photosphere above sunspots. The observed effects are believed to be a result of resonance oscillation of MHD waves inside a magnetic tube. Radio observations of this phenomenon open a new tool for studying regions of reflection of MHD waves near CCTR level. The method is very sensitive both to the height of the CCTR and magnetic fields above sunspots. Thus, detection of oscillations of the height of the transition region even with an amplitude of a few km are possible. The use of a spectrum of one of the observed sources obtained with the radio telescope RATAN-600 allows us to conclude that oscillations in magnetic field strength of about 4 G could be responsible for the effect and are reliably registered. The appearance of the famous 5-min oscillations in the solar atmosphere was also registered in some spectra of radio oscillations. 相似文献
2.
V. E. Abramov-Maksimov G. F. Vyalshin G. B. Gelfreikh V. I. Shatilov 《Solar physics》1996,164(1-2):333-343
In the present paper we present the results of measurement of magnetic fields in some sunspots at different heights in the solar atmosphere, based on simultaneous optical and radio measurements. The optical measurements were made by traditional photographic spectral observations of Zeeman splitting in a number of spectral lines originating at different heights in the solar photosphere and chromosphere. Radio observations of the spectra and polarization of the sunspot - associated sources were made in the wavelength range of 2–4 cm using large reflector-type radio telescope RATAN-600. The magnetic field penetrating the hot regions of the solar atmosphere were found from the shortest wavelength of generation of thermal cyclotron emission (presumably in the third harmonic of electron gyrofrequency). For all the eight cases under consideration we have found that magnetic field first drops with height, increases from the photosphere to lower chromosphere, and then decreases again as we proceed to higher chromosphere and chromosphere-corona transition region. Radio measurements were found to be well correlated with optical measurements of magnetic fields for the same sunspot. An alternative interpretation implies that different lines used for magnetic field measurements refer to different locations on the solar surface. If this is the case, then the inversion in vertical gradients of magnetic fields may not exist above the sunspots. Possible sources of systematic and random errors are also discussed. 相似文献
3.
Simultaneous observations made at several wavelengths in microwave range using the high spatial resolution of radiotelescope RATAN-600 make it possible to develop methods of measuring the magnetic fields in the solar corona and the chromosphere. In this paper we develop a method of measuring the magnetic fields from thermal bremsstrahlung and demonstrate it, using observations of a flocculus (plage) during August 1–3, 1977. The observations show that the flocculus under investigation possessed bipolar magnetic structure with peak to peak amplitude of magnetic field strength of about 40 G at the level of the upper chromosphere and the transition region (with a r.m.s. error of 5.7 G for favourable conditions). The radio astronomical map of the magnetic field is in agreement with the Mt. Wilson magnetic field map to within the experimental error. It follows that the average longitudinal magnetic field above the flocculus does not drop significantly with height above the photosphere up to the CCTR (chromosphere-corona transition region). An analysis of the spectra of polarized radio emission also gives an opportunity to determine the temperature gradient in the CCTR (which proved to amount to about 1000 K km-1 and to follow their variation with height. 相似文献
4.
Keizo Kai 《Solar physics》1978,56(2):417-427
Two-dimensional, high-resolution observations of about 30 moving type IV bursts allow us to compare the polarization structure of the radio sources high in the corona with the distribution of magnetic fields measured at the photospheric level. Left- and right-handed circularly polarized moving type IV bursts are associated with active regions dominated by magnetic fields of plus and minus polarity respectively. The result suggests that the polarity of magnetic fields within the type IV source which moves high in the corona ( 1R
above the photosphere) is closely related to the polarity of local magnetic fields at the photosphere. The above relation between the sense of polarization and the polarity of magnetic field is contrary to what would be expected from the generally accepted synchroton hypothesis. One way of resolving this conflict is to postulate that the magnetic field within the radio source has the opposite polarity to that of the ambient magnetic fields. 相似文献
5.
Leonid V. Yasnov 《Solar physics》2014,289(4):1215-1225
A number of authors claimed that radio sources above the neutral line of the magnetic field in solar active regions are due to non-thermal emission. This study shows that the thermal mechanism explains the radio emission from such sources. Models similar to those used for interpreting cyclotron lines were used in this study. Such models account for a steep decline in the spectrum at high frequencies and a low degree of polarization. The magnetic field between the two sunspots with an anti-parallel magnetic field has a lower gradient than the field above the sunspots. This, combined with the possibly high temperature in coronal loops connecting the sunspots, leads to the following conclusions. The optical thickness of the gyroresonance layers is increased and leads to more effective radiation at a harmonic number of 4 or 5. The lower gradient of the field between the sunspots also results in more rapid growth of emission intensity with increasing wavelength in this region than in the regions immediately above the sunspots. Additionally, the spatial averaging of the source structure due to the antenna beam pattern leads to a decrease in the degree of polarization in the region between the sunspots. 相似文献
6.
A. C. Riddle 《Solar physics》1969,6(2):251-254
The slowly varying component of solar microwave emission is associated with plage and sunspot regions seen optically. Under the assumption that the microwave emission originates radially above the associated optical feature the height of the microwave emission region above the photosphere can be calculated.For 99 regions seen at 9.1 cm during the International Quiet Sun Years 1964–1965 the average height was 8000 km. This result compares with 18000 km measured for the same wavelength emission during 1960. A change of height in this direction could be the result of the changing strength of the magnetic field associated with regions seen during the two periods. 相似文献
7.
V. N. Borovik G. B. Gelfreikh V. M. Bogod A. N. Korzhavin A. Krüger J. Hildebrandt S. Urpo 《Solar physics》1989,124(1):157-166
Recently recognized solar millimetre-wave off-limb sources are interpreted as a special phenomenon of long-duration post- and inter-flare emission at coronal altitudes. We present, for the first time, information about the brightness and polarization spectrum in the centimetre range for one such event of September 22, 1980 by means of RATAN-600 observations.The brightness temperatures observed favour the interpretation of the bulk of the emission by thermal optically thin bremsstrahlung. The degree of polarization measured (p 0.1–0.2 in the range 7.5–15 GHz) implies quite strong magnetic fields of about 300 ± 100 G at a height z > 3 × l04km above the photosphere and indicates a possible contribution of gyromagnetic radiation and/or optically thick bremsstrahlung at longer wavelengths. 相似文献
8.
In the first part of this communication we briefly summarize the results of the first observation of linear polarization in the microwave emission above a solar active region obtained with the Westerbork Synthesis Radio Telescope, taking advantage of the very narrow bandwidths of a multi-channel spectral line receiver. The intensity of the Stokes parameterU, measured at several points close to the line of zero circular polarization, showed a clear sinusoidal trend as a function of 2, in accordance to what is expected from Faraday rotation (Alissandrakis and Chiuderi Drago, 1994). Combining the measured period of the Faraday rotation with the observed deplacement of the depolarization line with respect to the photospheric neutral line, the height above the photosphere of the depolarization point and the value of the electron density and the magnetic field at this point are computed. Although the calculations are done in the very simplified assumptions of a bipolar magnetic field and of a density following hydrostatic equilibrium, they represent the first estimate of the coronal magnetic field in an active region, far from sunspots.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994. 相似文献
9.
Daily surveys of the solar disc made at 2.8 cm wavelength over the period 1–13 November 1981, complemented by magnetograms and H filtergrams, are used to examine the relationship between sources of the slowly varying component of solar radio emission and properties of their host regions. Two classes of source are noted: diffuse and compact. Sources are designated compact when smaller than 40. The diffuse sources may be explained in terms of free–free thermal emission from trapped plasma in loops overlying the active region. The great majority of compact sources can be accounted for in terms of gyroresonance from thermal electrons in the strong magnetic fields overlying sunspots. A small minority are less amenable to this explanation. They are associated with magnetic complexity and dynamism, lie close to magnetic polarity reversals, and could be non-thermal. Microwave sources are an evolutionary feature common to all but the smallest active regions. 相似文献
10.
We present observations of active region radio emission at 6.16 cm wavelength, obtained with an angular resolution of 3 by 10 arc using the Westerbork Synthesis Radio Telescope (WSRT) during the action interval May 20–27, 1980 of the Solar Maximum Year (SMY). We present maps in both total intensity (I) and circular polarization (V) of three regions (Hale numbers 16850, 16863, and 16864) and provide a detailed comparison of these maps with on- and off-band H pictures and with magnetograms. The strongest sources were associated with neutral lines and soft X-ray arcades. We present evidence that these neutral lines were characterized by having their two opposite polarities close to each other, implying a high magnetic field gradient, and by their association with arch filament systems. The sunspot associated radio sources had a relatively simple structure in region 16850; however for the large spots of regions 16864 and 16863 the emission had a patchy appearance with a tendency of the peaks to lie over the penumbra. In the V maps we observed for the first time two islands, polarized in the sense of the ordinary mode, which were located inside the sunspot associated sources and were associated with intrusions of opposite polarity field into the penumbra. These structures can be accounted for if the electron temperature along the line of sight is not a monotonically increasing function of height, but has a maximum near the second harmonic level. Finally we give a detailed analysis of observations of the inversion of the sense of circular polarization in region 16863. We find that the large scale structure of the magnetic field can be approximated by a dipole with its axis inclined by 11° with respect to the photosphere and with a dipole moment of about 2 × 1031 cgs units; the depolarization line is located at a height of 0.16–0.19 R above the dipole, where the estimated intensity of the magnetic field is 10–20 G. 相似文献
11.
It is pointed out that, because of the large Faraday rotation an outlet of linear polarization from the photosphere of a white dwarf is hampered. In accordance with this fact it is proposed to distinguish two types of magnetic white dwarfs. The first type (its representative is Grw 70°8247) has a linear polarization which is comparable in magnitude with the circular one. Polarization of radiation from the white dwarfs of the first type cannot arise in the photosphere. It arises in the corona of the star either as a result of cyclotron emission of hot electrons (T～106 K) or as a result of scattering of slightly polarized emission from the photosphere in the corona. For the first type dwarfs such magnetic fields are required thatω B ωopt, i.e.B(1?3)×108G. The white dwarfs of the second type (its representative is G 99-37) have their linear polarization much smaller than the circular one. Polarization of these white dwarfs can arise as a result of the transfer of radiation in the nonisothermal photosphere. Magnetic fields required for the second type can be much smaller:B cos γ=(1?10)×106 G. It is shown that the photospheric model allows to obtain the quantitative accordance of the theory with all the observational data for G 99-37 and is not in accordance with the data for Grw 70°8247, at the same time the model with cyclotron emission from the corona explains the magnitude of both linear and circular polarization and their wavelength dependence for Grw 70°8247. 相似文献
12.
The microwave circular polarization of the active region (AR) NOAA 7260 on 21–23 August 1992 is analyzed. Two-dimensional images at 1.76 cm with spatial resolution of =10 from the Nobeyama radioheliograph and one-dimensional scans at 9 wavelengths in the range of 1.81–3.43 cm and =16.3–31.1 from the radio telescope RATAN-600 were used. An inversion of the sense of circular polarization through the wavelength range was recorded on 22 August. It is shown that both the wavelength and the time dependence of the inversion are consistent with quasi-transverse (QT) propagation of the radiation in the solar corona. From this, the strength of the coronal magnetic field in the active region was found to be H=20–65 G at a height of h= (5.7–8.7)×109 cm above the photosphere on 22 and 30 August and 125 G at the lower height of (3.7–6.4)×109 cm on 23 August. We present a new technique, based on the radio mapping (in both Stokes I and V) of an AR undergoing circular polarization inversion; applying this method to the Nobeyama data we obtained, for the first time, a magnetogram of the coronal magnetic field. For AR 7260 we found values in the range of 70–100 G at heights of (4–6)×109 cm on 23 August, adopting a constant value of NL (where N is the electron density and L is the scale of the coronal field divergence) of 2.5×1018 cm–2. We compare our results with force-free extrapolations of the photospheric magnetic field from a MSFC magnetogram obtained on 20 August. 相似文献
13.
A study of the circular polarization structure of solar active regions has been made from data obtained at 3.5 mm wavelength, using the 36 ft diameter radio telescope of the National Radio Astronomy Observatory at Kitt Peak, Arizona. The angular resolution of the telescope at this wavelength is 1.2. All important active regions observed at 3.5 mm are bipolar in nature; the degree of polarization ranges from 1 to about 2%. These oppositely polarized components correspond with the Mt. Wilson magnetic regions of opposite polarity; the line of zero polarization delineates the neutral line between the regions of opposite polarity on magnetograms. The longitudinal magnetic fields at the level of 3.5 mm emission computed from the degree of polarization are found to be several hundred gauss. 相似文献
14.
C. E. Alissandrakis F. Borgioli F. Chiuderi Drago M. Hagyard K. Shibasaki 《Solar physics》1996,167(1-2):167-179
The solar active region (AR) 7530 was observed at 6 cm on July 3 and 4, 1993 with the Westerbork Synthesis Radio Telescope, using a multi-channel receiver with very narrow bandwidth. We compare the radio data with Yohkoh SXT observations and with the magnetic field extrapolated from the Marshall vector magnetograms in the force-free and current-free approximations. The comparison with soft X-rays shows that, although a general agreement exists between the shape of the radio intensity map and the X-ray loops, the brightness temperature, T
b, obtained using the parameters derived from the SXT is much lower than that observed. The comparison with the extrapolated photospheric fields shows instead that they account very well for the observed T
b above the main sunspots, if gyroresonance emission is assumed. In the observation of July 4 an inversion and strong suppression of the circular polarization was clearly present above different portions of the AR, which indicates that particular relationships exist between the electron density and the magnetic field in the region where the corresponding lines of sight cross the field quasi-perpendicularly. The extrapolated magnetic field at a much higher level ( 1010 cm), satisfies the constraints required by the wave propagation theory all over the AR. However, a rather low electron density is derived. 相似文献
15.
Observations of radio emission at 3.3 mm wavelength associated with magnetic fields in active regions are reported. Results of more than 200 regions during the years 1967–1968 show a strong correlation between peak enhanced millimeter emission, total flux of the longitudinal component of photospheric magnetic fields and the number of flares produced during transit of active regions. For magnetic flux greater than 1021 maxwells flares will occur and for flux of 1023 maxwells the sum of the H flare importance numbers is about 40. The peak millimeter enhancement increases with magnetic flux for regions which subsequently flared. Estimates of the magnetic energy available and the correlation with flare production indicate that the photospheric fields and probably chromospheric currents are responsible for the observed pre-flare heating and provide the energy of flares.This work was supported in part by NASA Contract No. NAS2-7868 and in part by Company funds of The Aerospace Corporation. 相似文献
16.
The source positions of solar radio bursts of spectral types I, III(U) and III(J) and V observed by the Culgoora radioheliograph are found to lie almost radially above soft X-ray loops on pictures taken by the S-056 telescope aboard Skylab. The radio source positions and the X-ray loops occur near magnetic loops on computed potential field maps. However, the magnetic induction required to explain the radio observations is much greater than the computed potential field value at that height. Dense current-carrying magnetic flux tubes emanating from active regions on the Sun and extending to 1.5R
above the photosphere provide a satisfactory model for the radio bursts. 相似文献
17.
The possibility of measuring magnetic fields of solar active regions at coronal heights up to 1010 cm by observing the inversion of circular polarization of local sources at microwaves is demonstrated. The observations by the radiotelescope RATAN-600 were accomplished with the angular resolution 17–34 in the wavelength range 2–4 cm. It is found that the inversion of polarization occured within a core of local source situated above the largest sunspot of Mc Math 14822. The inversion was followed during the period of June 30–July 3, 1977. The measured coronal magnetic field of 16 G is found to be at the height 12 × 109 cm. This measured field proves to agree with a simulated potential structure of Mc Math 14822 coronal magnetic field. Our analysis of the inversion has been based on the theory of interaction (coupling) of the ordinary and extraordinary wave modes in the region of quasi-transverse propagation. 相似文献
18.
Kiyoto Shibasaki Franca Chiuderi-Drago Mauro Melozzi Cornelis Slottje Ester Antonucci 《Solar physics》1983,89(2):307-321
Hale region 16898 was observed by the Westerbork Synthesis Radio Telescope at 6 cm and by the Ultraviolet Spectrometer and Polarimeter and the X-Ray Spectrometer on the Solar Maximum Mission satellite. Optical pictures of the same active region were taken at Sacramento Peak, Big Bear, and Meudon Observatories. The radio emission mechanisms are identified by comparing radio data with ultraviolet and soft X-ray data. The height of the radio sources and the magnetic field strength at that height are deduced. A radio source above a large sunspot shows a crescent shaped depression of circular polarization and a high brightness temperature. The emission mechanism is identified as gyroresonance at the second and the third harmonic layers and it is found that the second harmonic layer, where the magnetic field strength is 900 G, must be in the corona. An extended loop-like source connecting the leading and the following part of the active region as well as the sources associated with small spots are mainly due to thermal free-free emission by hot and dense plasma which is also observed in ultraviolet and soft X-ray radiation. The calculated radio brightness temperature, using the physical parameters deduced from the ultraviolet and soft X-ray line intensities, agrees with the observed brightness temperature. The height of the low brightness temperature sources above the small spots is 6000 ± 3000 km and that above the large spot is less than 3000 km: the source above the large spot does not show any shift relative to the sunspot due to the projection effect. Very strong radio emission was found which was associated with the merging of a group of small spots into the large sunspot. In the same day, warm ( 106 K) and dense matter was present above the large spot. Evidence for nonthermal emission is presented. 相似文献
19.
The magnetic field above two unrelated active regions on 11 and 12 September, 1974 has been studied using magnetograms obtained in C I 9111, Fe I 8688, Ca II 8542, and H. In C I 9111, originating low in the photosphere, the fields are strong and sharply defined. In Ca II 8542 and H they are very diffuse, with significant diffuseness also in Fe I 8688, due to the spreading of the field with height to form almost horizontal magnetic canopies over regions free of field at lower levels.Within a region between two small sunspots some 140 Mm apart, the canopy height found is typically 300–400 km. Within a small superpenumbra, the canopy height is 150–250 km. In extensive areas surrounding the active regions, over one-half the canopy bases are less than 400–500 km above the
c = 1 level, and over 80% less than 700 km.Arguments are given that the chromospheric fibrils (e.g., in H), taken to delineate the field configuration, are not due primarily to lateral variations in field but rather to differences in density or excitation of gas across the lines of force.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 相似文献
20.
V. M. Bogod V. Garimov G. B. Gelfreikh K. R. Lang R. F. Willson J. N. Kile 《Solar physics》1995,160(1):133-149
Solar radio and microwave sources were observed with the Very Large Array (VLA) and the RATAN-600, providing high spatial resolution at 91 cm (VLA) and detailed spectral and polarization data at microwave wavelengths (1.7 to 20 cm - RATAN). The radio observations have been compared with images from the Soft X-ray Telescope (SXT) aboard theYohkoh satellite and with full-disk phoptospheric magnetic field data from the Kislovodsk Station of the Pulkovo Observatory. The VLA observations at 91 cm show fluctuating nonthermal noise storm sources in the middle corona. The active regions that were responsible for the noise storms generally had weaker microwave emission, fainter thermal soft X-ray emission, as well as less intense coronal magnetic fields than those associated with other active regions on the solar disk. The noise storms did, however, originate in active regions whose magnetic fields and radiation properties were evolving on timescales of days or less. We interpret these noise storms in terms of accelerated particles trapped in radiation belts above or near active regions, forming a decimetric coronal halo. The particles trapped in the radiation belts may be the source of other forms of nonthermal radio emission, while also providing a reservoir from which energetic particles may drain down into lower-lying magnetic structures.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994. 相似文献