Determination and analysis of coronal hole radio spectra

Solar radio maps obtained by our group and others over a wide wavelength range (millimeter to meter) and over a considerable time span (1973–1978) have allowed us to compute the radio spectrum of an average coronal hole, i.e., the brightness temperature inside a coronal hole normalized by the brightness temperature of the quiet Sun outside the coronal hole measured at several different radio wavelengths. This radio spectrum can be used to obtain the changes of the quiet Sun atmosphere inside coronal holes and also as an additional check for coronal hole profiles obtained by other methods. Using a standard solar atmosphere and a computer program which included ray tracing, we have tried to reproduce the observed radio spectrum by computing brightness temperatures at many different wavelengths for a long series of modifications in the electron density, neutral particle density and temperature profiles of the standard solar atmosphere. This analysis indicates that inside an average coronal hole the following changes occur: the upper chromosphere expands by about 20% and its electron density and temperature decrease by about 10%. The transition zone experiences the largest change, expanding by a factor of about 6, its electron density decreases by a similar factor, and its temperature decreases by about 50%. Finally in the corona the electron density decreases by about 20% and the temperature by about 15%.     

Microwave observations of compact radio sources during solar eclipses on RT-22

Based on the observations of solar eclipses performed on the RT-22 radio telescope at CrAO in the wavelength range 2.0–3.5 cm, we consider the fine spatial structure of the microwave emission from the quiet Sun. We have established that the positions of compact radio sources with a typical size of about 7″.0 and coronal bright points coincide. The mean radio flux exceeds the level of the quiet Sun by 0.28 sfu. The brightness temperatures increase with wavelength and lie within the range 0.3–2.7 MK. Evidence for a nonthermal nature of the emission from compact radio sources has been obtained.     

Slowly varying component spectrum of the solar radio emission at millimetre wavelengths

This paper deals with the observed data on the solar S-component sources at millimetre wavelengths. The observations were made in 1968 and 1969 using the 22-m radio telescope of the Crimean Astrophysical Observatory at six wavelengths: 2, 4, 6, 8, 13 and 17 mm. The enhanced intensity of the solar active region in comparison with the quiet Sun level varies proportionally to ^–2 if the wavelength is within the range of 2 ÷ 6 mm. In the wavelength band of 6 ÷ 17 mm almost flat spectra of the solar S-component sources is observed. Assuming the bremsstrahlung mechanism of the radio emission for the quiet Sun and the solar active regions an attempt has been made to treat the above presented data. It appears that the most probable explanation of the 2 ÷ 6 mm spectrum is that the S-component sources are opaque. In the 6 ÷ 17 mm wavelength band there are two possibilities: the active region may be either transparent or opaque. But in the last case the source brightness temperature must be proportional to ². Some differences in the spectra of the sources, identified with flocculi and with bipolar sunspot groups, were mentioned. The cold regions (as compared with the quiet Sun) were observed up to = 2 mm and identified with the filaments. However, its visibility falls when the wavelength decreases.     

Brightness temperature distribution in solar corona based on RATAN-600 observations of the maximum phase of the March 29, 2006 solar eclipse

We describe the technique and results of modelling the solar radio emission during the maximum phase of the solar eclipse of March 29, 2006 on the RATAN-600. The aim of modelling is to refine the brightness temperature of the solar corona at the distances up to two solar radii from the center of the optical disk of the Sun. We obtained the distribution of brightness temperature in the vicinity of the coronal hole above the solar North Pole at the wavelength of 13 cm. The results of modelling showed that brightness temperatures of the coronal hole at the distances greater than 1.02 R_C (here R_C is the radius of the optical disk of the Sun) is substantially lower than the expected average brightness temperature of a typical coronal hole, and that of the quiescent Sun (below 30000 K) at the wavelength of 13 cm. The classical Baumbach-Allen formula for electron density in a spherically symmetric corona agrees with the results of observations starting at distances of (1.4–1.5) R_C.     

Brightness temperatures of the quiet sun and new moon at the 6 mm wavelength

Absolute brightness temperatures and brightness temperature ratios of a quiet region near the center of the solar disk and the central region of the new moon were measured simultaneously at the 6 mm wavelength. The measured quiet sun/new moon brightness temperature ratios and reported central brightness temperatures of the new moon confirm the measured brightness temperature of the quiet sun at the 6 mm wavelength.Reported central brightness temperatures of the new moon are tabulated and graphed as a function of frequency and wavelength. The equation of a linear regression line for the reported measurements is given for estimating the brightness temperature of the new moon at any millimeter wavelength. Estimated brightness temperatures of the new moon and measured quiet sun/new moon ratios are used to estimate solar brightness temperatures at several millimeter wavelengths. The solar brightness temperatures, the regression line, and the Van de Hulst theoretical model are presented graphically as a function of frequency and wavelength. The regression line equation is given for estimating solar brightness temperatures at any wavelength in the 6 to 1 mm wavelength interval and is solved for the wavelength of the measured ratios.Reported solar brightness temperatures in the millimeter wavelength region are tabulated. The measured temperatures in the 6 to 1 mm wavelength interval and a linear regression line are presented graphically as a function of frequency and wavelength. The regression line equation is given and solved for the solar brightness temperatures at the 6 mm wavelength.This work supported by the U.S. Air Force under Contract No. F04701-69-C-0066.     

Low‐frequency heliographic observations of the quiet Sun corona

We present new results of heliographic observations of quiet‐Sun radio emission fulfilled by the UTR‐2 radio telescope. The solar corona investigations have been made close to the last solar minimum (Cycle 23) in the late August and early September of 2010 by means of the two‐dimensional heliograph within 16.5–33 MHz. Moreover, the UTR‐2 radio telescope was used also as an 1‐D heliograph for one‐dimensional scanning of the Sun at the beginning of September 2010 as well as in short‐time observational campaigns in April and August of 2012. The average values of integral flux density of the undisturbed Sun continuum emission at different frequencies have been found. Using the data, we have determined the spectral index of quiet‐Sun radio emission in the range 16.5–200 MHz. It is equal to –2.1±0.1. The brightness distribution maps of outer solar corona at frequencies 20.0 MHz and 26.0 MHz have been obtained. The angular sizes of radio Sun were estimated. It is found that the solar corona at these frequencies is stretched‐out along equatorial direction. The coefficient of corona ellipticity varies slightly during above period. Its mean magnitudes are equal to ≈ 0.75 and ≈ 0.73 at 20.0 MHz and 26.0 MHz, respectively. The presented results for continuum emission of solar corona conform with being ones at higher frequencies. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The quiet sun brightness temperature at 408 MHz

The flux of the radio quiet Sun and the brightness temperature at 408 MHz (73 cm) are derived from measurements with the E-W Nançay interferometer and the E-W arm of the Medicina North Cross. It is shown that the lowest envelopes, which defined the radio quiet Sun, correspond to transits of extended coronal holes across the disk of the Sun.     

Observations of the continuum radio emission from the undisturbed Sun at a wavelength of 8.7 meters

We have mapped the continuum emission from the undisturbed Sun at a wavelength of 8.7 m during 1981–1985 using the large decameter-wave radiotelescope at Gauribidanur, India with a resolution of 26 #x00D7; 38 arc min. During the period August 6–30, 1983, the Sun was exceptionally quiet at meter and decameter wavelengths, and we were able to make maps on several consecutive days. On these days the position of the centroid of the radio Sun agreed quite closely with the center of the optical Sun indicating that there is very little or no contribution from active regions. But the observed peak brightness temperature varied from 100 000 to 700 000 K. The half-power widths of the brightness distribution were in the range of 3 to 4R ^⊙. The variations of the brightness temperature and the half-power widths are not correlate. It is therefore suggested that the variations of the brightness temperature are not caused by uniform density variations or due to scattering by an irregular corona.     

A model for solar oscillations at cm and mm wavelengths

It is suggested that large scale oscillations of the Sun give rise to the periodic fluctuations observed in the quiet Sun at radio wavelengths. These fluctuations would be most readily observed from regions where there is a large gradient of electron temperature with electron density, and are thus more readily observed at cm wavelengths than at millimetre wavelengths.     

Fine structure of the Sun at 1.3 cm wavelength

The two-element interferometer at Hat Creek Observatory was used at 1.3 cm wavelength to study the fine structure of the radio emissive regions on the Sun. Observations of the quiet Sun at 1.3 cm show sudden changes in the fringe amplitude and phase, lasting for typically about 5–8 min. Assuming that these events are identical in nature, a plot of peak amplitude vs the projected baseline at the time of the event suggests emission from a region of angular size of about 10″. The corresponding brightness temperature is 50000 K. It is possible that these events may be related to the appearance and disappearance of groups of spicules or mottles.     

Microwave radiation of solar coronal holes: Results of CrAO RI RT-22 observations in 2003–2006

Results of the identification of solar coronal holes (CH) with radio brightness depressions and excesses are presented, which were obtained as a result of RT-22 observations at 2.0–3.5 cm in 2004–2006. Microwave radiation inhomogeneities in CHs, quiet SUN, and small sunspots were studied using RT-22 observations of May 31, 2003; October 3, 2005; and March 29, 2006 eclipses.     

A model of the chromosphere and transition zone. Radio and UV emission of these layers

A model of the chromosphere and transition zone for quiet regions of the Sun is given.The height dependence of temperature and density are different for the borders of the supergranules and for their inner parts. The difference is caused by the enhanced mechanical energy flux at the places of strengthened magnetic field.The average dependence of density on height is given by eclipse data, the corresponding values for temperatures are obtained by a trial and error method using calculation of solar radio emission for different wavelengths and comparison with observations.The adopted model is consistent with observational data on the solar visual, radio and UV emission. This model also accounts for the brightness distribution on spectroheliograms taken in H wings, K₂₃₂ Caii and in several UV spectral lines.     

The distribution of the 9 cm radio emission over the solar disk during the sunspot minimum

The brightness distribution of the quiet sun on 9.1 cm wavelength is determined from the Stanford pencil-beam radioheliograms for three periods in the recent solar activity minimum, centred at July 15 and September 15, 1964 and at April 5, 1965. The brightness maxima near the limbs are not symmetric with respect to the central meridian, but are situated at 76 °E and 66 °W longitude, respectively. On the disk, the brightness temperature is likewise distributed asymmetrically, but the direction of this asymmetry changes as the new cycle starts. The asymmetries are tentatively explained by assuming the presence of inhomogeneous streamers in the solar atmosphere, which are tilted by the solar rotation. The eastward shift of the limb maxima with respect to the optical disk is confirmed by the 21 cm-heliograms obtained at Fleurs near Sydney, Australia.     

The Brightness Temperature of the Quiet Solar Chromosphere at 2.6 mm

The absolute brightness temperature of the Sun at millimeter wavelengths is an important diagnostic of the solar chromosphere. Because the Sun is so bright, measurement of this property usually involves the operation of telescopes under extreme conditions and requires a rigorous performance assessment of the telescope. In this study, we establish solar observation and calibration techniques at 2.6 mm wavelength for the Nobeyama 45 m telescope and accurately derive the absolute solar brightness temperature. We tune the superconductor–insulator–superconductor (SIS) receiver by inducing different bias voltages onto the SIS mixer to prevent saturation. Then, we examine the linearity of the receiver system by comparing outputs derived from different tuning conditions. Furthermore, we measure the lunar filled beam efficiency of the telescope using the New Moon, and then derive the absolute brightness temperature of the Sun. The derived solar brightness temperature is \(7700 \pm 310~\mbox{K}\) at 115 GHz. The telescope beam pattern is modeled as a summation of three Gaussian functions and derived using the solar limb. The real shape of the Sun is determined via deconvolution of the beam pattern from the observed map. Such well-calibrated single-dish observations are important for high-resolution chromospheric studies because they provide the absolute temperature scale that is lacking from interferometer observations.     

The radius of the Sun at centimeter waves and the brightness distribution across the disk

We report observations of the solar radio radius at wavelengths between 1.2 and 11 cm performed with the Bonn 100 m-telescope. In combination with former measurements of the centre-to-limb variation of the solar brightness these observations are discussed in terms of atmospheric models. We consider the solar disk to be covered by arches at low latitudes, while at the poles coronal holes are located. The temperature dependence on height is taken from EUV-line intensities, hydrostatic equilibrium is adopted, spicules are assumed to be responsible for the relatively low brightening. The interpretation of our measurements demands certain values of the brightness temperature of spicules as a function of wavelength within a modest interval.     

Solar radio emission at 1.2 mm wavelength

Some properties of solar active regions at 1.2 mm wavelength are discussed. Equatorial and polar brightness distributions of the quiet Sun at 1.2 mm wavelength are also presented.     

明安图射电频谱日像仪图像的强度定标方法

明安图射电频谱日像仪(Mingantu Spectral Radioheliograph, MUSER)能够在0.4--15GHz超宽频带内实现高时间、高空间、高频率分辨率的太阳射电成像. 而射电亮温度是描述太阳物理过程的一个重要的参数, 在研究不同射电辐射机制、太阳磁场以及太阳爆发过程中非热粒子加速等问题上有着非常重要的作用, 因此必须对MUSER观测的图像进行亮温度定标. 将介绍一种适用于射电日像仪图像强度定标的方法. 太阳射电图像中包含着太阳圆盘的结构信息, 利用射电日像仪短基线的可视度函数拟合第一类贝塞尔函数, 可以得到图像中宁静太阳圆盘的射电半径和强度, 再利用瑞利-金斯定律和每天的太阳射电流量可以计算得到每天图像的定标因子$G_c$, 从而实现对MUSER图像强度的定标. 将该方法应用到MUSER的实际观测数据中, 包括宁静太阳和有太阳射电爆发等不同的情况, $G_c$的误差基本不超过10%, 得到的宁静太阳亮温度与其他宁静太阳的结果具有较高的相关性, 表明了此方法的可行性和有效性.     

The brightness distribution of the sun at 2.8cm wavelength

Maps of the Sun at 2.8 cm wavelength, observed with the 100-m telescope in Effelsberg, on 1972, October 31 and 1973, February 10, are discussed. The brightness distribution over the Sun is, with the exception of active regions, approximately constant and nearly sharp-edged. Regions of small enhancements in the radiation can be identified with zones of weak activity in the solar magnetograms. Optical filaments could also be seen in absorption at 2.8 cm wavelength, the optical depth being of the order of 0.2. Several active regions have been analysed with respect to their position relative to their optical counterparts and to their brightness temperature: The latter was found to reach almost 10⁶K in one case. No noticeable limb brightening could be observed; if any exists, it should be smaller than 5%. Likewise no noticeable elliptical deformation of the Sun's disk could be found. The geometrical thickness of the coronal layer, contributing to the undisturbed radiation at 2.8 cm was estimated to be a maximum of 4000 km.     

Radio emission from the quiet sun and local sources at wavelengths 5, 10.7, 12, and 95 cm from observations of the January 4, 2011 eclipse in Crimea

The radio radii of the Sun at wavelengths of 5, 10.7, 12, and 95 cm have been determined from eclipse observations as R₅ ?? (1.0 ± 0.015)R _??, R _10,12 = (1.05 ± 0.003)R _??, and R ₉₅ = (1.2 ± 0.02)R _??. The bright-ness temperatures of quiet solar disk areas at these wavelengths have turned out to be Td ₅ = (22 ± 2) × 10³, Td ₁₀ = (44 ± 3) × 10³, Td ₁₂ = (47 ± 3) × 10³, and Td ₉₅ = (1000 ± 30) × 10³ K. There were local sources of radio emission with angular sizes from 1.9 to 2.4 arcmin and brightness temperatures from 80 × 10³ to 1.75 × 10⁶ K above sunspot groups at short wavelengths of 5, 10.7, and 12 cm. The radio flux from the local sources at 95 cm turned out to be below the detection threshold of 1.0 × 10^?22 W m^?2 Hz^?1. Comparison of the values obtained with the results of observations of another eclipse on August 1, 2008, occurred at the epoch of minimum of the 11-year solar cycle has shown that the radio radius of the Sun at 10.7 and 12 cm increased from 1.016 R _?? to 1.05 ± 0.003R _??, the height of the emitting layer at these wavelengths moved from 11 × 10³ km to (30 ± 7) × 10³ K, and the brightness temperature of the quiet Sun rose from (35.8 ± 0.4) × 10³ K to (44 ± 3) × 10³ K at 10.7 cm and from (37.3 ± 0.4) × 10³ K to (47 ± 3) × 10³ K at 12 cm. Consequently, the parameters of the solar atmosphere changed noticeably in 2 years in connection with the beginning of the new solar cycle 24. The almost complete absence of local sources at the longest wavelength of 95 cm suggests that the magnetic fields of the sunspot groups on January 4, 2011, were weak and did not penetrate to the height from where their emission could originate. If this property is inherent in most sunspot groups of cycle 24, then it can be responsible for its low flare activity.     

Solar limb brightening at 3.1 cm

The existence of limb brightening at the equator of the quiet Sun at 3.1 cm is derived from a statistical analysis of scans of the solar disk taken during 1968.A ratio of 0.7% is found between the flux emitted by the equatorial limb enhancements and that of a disk of uniform brightness temperature.