Observations of the 7 March, 1970 total solar eclipse at wavelengths of 3.2 and 8.3 mm

The 7 March, 1970 total solar eclipse was observed at wavelengths of 3.2 and 8.3 mm; the object being to use the knife edge of the Moon as it passed across the Sun to improve angular resolution on the Sun. This in turn would provide a radial brightness distribution of the Sun with an angular resolution of a few seconds of arc.Excellent eclipse curves were obtained at 3 mm; however, some external interference marred the 8 mm record near totality.The 8 mm brightness distribution is subject to some uncertainty, but tends to show limb brightening. The 3 mm brightness distribution shows a well defined complex limb brightening within about 1 arc min of the optical limb. The maximum brightening is approximately 30% above the average disc temperature.     

The coarse structure of the solar atmosphere

Observations of the quiet sun at wavelengths from 3 Å to 75 cm show (with two exceptions: the Ovi line at 1032 Å and possibly the continuum at 1.2 mm) either no limb brightening or less than had been supposed. On the other hand, the brightness temperature is observed to increase with wavelength in the millimeter and centimeter range. If this increase is due to greater visibility of hot overlying material, that material ought to be evident at the limb at shorter wavelengths, resulting in limb brightening. The only possible explanation for the absence of limb brightening at almost all wavelengths is that the emitting surface is rough at all wavelengths, with a scale of roughness approximately equal to the scale height at each temperature. Contradictions with existing models, along with the additional observations required for an improved model are discussed.     

Peering over the Sun's pole: behavior of its rotational vortex

A new method for measuring spectroscopically the rotation at the Sun's poles is described. Using solar CO lines at 4.666 µm, infrared spectra are recorded at a fixed limb distance of 4.8 arc sec while progressing along an arc ±5.7 deg from the Sun's rotational pole. Since the poles dip twice a year to about 7 arc sec from the limb, our observations can range either side of and through the vortex axis. Advantages to this technique are: (1) a low disturbing signal from supergranules owing to their superposition at the limb, (2) no ‘limb shift’ error since limb distance is constant and the CO lines have no known limb shift, (3) emphasis is on the quiet Sun since the CO molecule is confined there, (4) negligible scattered light in the IR (<1%), and (5) the improved seeing afforded by the IR. Although any definitive determination of solar rotation requires observations over an extended time span, our preliminary results suggest two features peculiar to the extreme pole: (1) the occasional apparent cessation of rotation, (2) some sort of singularity, again occasional, producing a sharp velocity signal (a vortex?) within 1 deg of the pole.     

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.     

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.     

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.     

Time variability and structure of quiet Sun sources at 6 cm wavelength

Using the Westerbork Synthesis Radio Telescope (WSRT) we produced a synthesized map of a quiet Sun region on June 15, 1976, and studied the structure and time variability of the quiet emitting regions at 6 cm wavelength with a spatial resolution of 6 arc sec. Comparison of the 12hr synthesis map with Ca⁺ K filtergram shows that bright and dark features on the 6 cm quiet Sun synthesized map correspond to the chromospheric networks and cells observed in Ca⁺ K. All 6 cm bright features lie over bright Ca⁺ K network elements. The reverse correlation is not true, that is, not all bright Ca⁺ K network features have their 6 cm counterparts. Comparison with the photospheric magnetogram shows that about 72% of the photospheric magnetic field enhancements (¦B¦ 5 G) are coincident with 6 cm emissive regions. Only one 6 cm feature could be positively identified with a bipolar magnetic structure. This implies that no more than 20–25% of the 6 cm emitting features could be associated with X-ray bright points. Intercomparison of our 12hr two-dimensional synthesis map, a 4hr two-dimensional synthesis map (around meridian) and the one-dimensional fan beam scans of the quiet Sun region at 6 cm, along with the Ca⁺ K filtergram and photospheric magnetogram shows that: (1) All of the 15 time-varying elements at 6 cm were located on Ca⁺ K networks; (2) about 40% of the 15 time varying elements at 6 cm are coincident with enhancements of the photospheric magnetogram; (3) individual time-varying sources have minimum source size (FWHM) of 15 arc sec and maximum brightness temperature of 10⁵ K; (4) the life time of the time varying sources varies from a few minutes to several tens of minutes; (5) the intensity of the sources varies by factors of 2 to 7 over time periods of 1 min to tens of minutes; and (6) the sources tend to disappear for periods of up to tens of minutes and to reappear at the same locations.     

Spectral radio observations of a solar eclipse

Measurements were made of the 7 March, 1970 solar eclipse by the AFCRL Sagamore Hill Radio Observatory in Hamilton, Massachusetts, on the wavelengths of 0.86, 1.95, 3.4, 6.0, 11.1, 21.2, 49.5, and 122.5 cm. Near-total obscuration (m=0.96) occurred at eclipse maximum. Source flux spectra for the intense sources located in McMath plages 10 618(SE), 10617(NE), and 10 607(NW) show gyro-resonance spectral peaking, whereas the less intense bremsstrahlung emission is observed for the weaker sources in plages 10 614 and 10 619. Associated one-dimensional source sizes for these regions vary from 0.8 arc min (at 3.4 cm) to 5.4 arc min (at 49.5 cm); with sizes at a particular wavelength increasing with intensity of the source. An estimated flux spectrum of the undisturbed radio Sun for 7 March, 1970 is given and compared to the spectrum for the solar minimum of 1964. In plage 10 607 a weak halo emission was isolated from the intense emission from the central source over the spot. The measure of emission from the halo above plage 10 607 was calculated to be 7 × 10²⁷ electron²/cm⁵.     

Multifrequency radio observations of coronal holes,filaments and cavities on RATAN–600

On the basis of multifrequency solar radio observations made on RATAN–600 radiotelescope with high spatial resolution at nine wavelengths in the 2–32–wavelength range is shown that filaments and cavities are well detected on the solar scans at short centimeter wavelengths as the regions of low radio brightness with angular dimensions of 25′–80′ in E—W direction. The tendency of decreasing radio sizes for cavities and filaments from 2.0 to 8.0 cm is observed. The coronal hole (CH) is more contrast in the range of 8–32 cm. The radio size of CH in E—N direction increases from 2′ (at 8.2) to 5′.0 (at 31.6 cm). The spectra of the brightness temperature of CH and the quiet Sun are obtained. The brightness temperature of CH is twice lower than that of the quiet Sun at wavelength of 31.6 cm.     

EUV Full-Sun Imaging and Pointing Calibration of the SOHO Coronal Diagnostic Spectrometer

Although the field of view of the Normal Incidence Spectrometer (NIS) of the Coronal Diagnostic Spectrometer (CDS) is 4×4 arc min, it is possible to observe the full solar disk by forming a mosaic of images taken in succession. This paper describes just such a study which has been used to collect images of the Sun simultaneously in six wavelengths between 304 Ú and 630 Ú, and with a temperature coverage between 5×10⁴ K and 2.5×10⁶ K. A representative sample of the resulting images is presented. These data can be used to explore the origin of solar EUV variability, and examine large-scale solar features. Another use of these data is to calibrate the pointing of the CDS Offset Pointing System (OPS), by comparing them against the SOHO Extreme ultraviolet Imaging Telescope (EIT) full-disk images taken at the same time. Many joint observations are made with CDS and other SOHO instruments, and calibration of the pointing is crucial to the co-pointing of the instruments, and to the analyses of these data. Coalignment is done by fitting to a cross-correlation function, using an IDL procedure which can be applied to any CDS/NIS data set. The accuracy of an individual coalignment can be demonstrated to be in the range 1–2 arc sec. The overall accuracy of the OPS calibration is ±5 arc sec, mainly attributable to measurement error in the actuator positions. An onboard Spartan Intermediate Sun Sensor of the Lockheed design, which was intended to provide greater pointing accuracy, exhibits a time-varying calibration, possibly due to a gradual loss of sensitivity.     

Radar measurement of small-scale surface texture: Syrtis major

Recent radar observations of Syrtis Major have shown it to be smooth and relatively homogeneous when sensed at centimeter wavelengths λ. There is a gradual decrease in surface roughness east to west across the basin, which correlates with an apparent decrease in small (< 1 km diameter) crater density. Root mean square surface slopes σ range from more than 1.5–2.0° in the east to less than 0.5° along the western margin at λ = 12.6 cm. The surface appears somewhat rougher at λ = 3.6 cm; a σ ∝ λ^?0.3 dependence is inferred. Radar reflectivity increases from about 5% to about 12% across Syrtis Major, being greatest near the western margin. High-resolution (8 to 20 m/pixel) Viking images suggest that an increasing amount of resurfacing has occurred in western Syrtis Major compared with the eastern parts. The radar, infrared, and optical imaing evidence are consistent with resurfacing by geologically recent, low-viscosity lavas which were subsequently mantled by thin layers of aeolian material. Each data set may be taken as a unique source of scale-dependent information on surface materials and properties. From comparison of radar-derived surface roughness with image-derived crater density curves we conclude that processes other than cratering control the surface texture on 0.03- to 100-m surface scales.     

Absolute redshifts in the C iv 1548 Å line in the transition region of the quiet Sun

Observations with the UVSP instrument on the SMM spacecraft were made at the polar limb and disk center for the accurate determination of Doppler shifts of the Civ 1548 Å emission line formed at 10⁵ K in the transition region of the quiet Sun. Individual data points representing 3 arc sec square pixels yield both redshifts and blueshifts, but the mean values from four different days of observations are toward the red. The mean redshifts are in the range 4–8 km s^-1 and are produced by nearly vertically directed flows; the uncertainty associated with the mean values correspond to ±0.5 km s^-1. The redshift increases with brightness of the Civ line.     

High spatial resolution microwave observations of the Sun

Over the past decade two large arrays — the Westerbork Synthesis Radio Telescope (WSRT) and the Very Large Array (VLA) built primarily for sidereal radio astronomy have been used for solar radio astronomical studies with spatial resolution of a few seconds of arc. In this review, we discuss some results obtained at Maryland using these instruments.The quiet Sun observations made with the WSRT have premitted us to produce synthesized maps of supergranulation network at 6 cm wavelength. The brightness temperatures of typical network elements and cells are respectively 2.5 × 10⁴ K and 1.5 × 10⁴ K; thus the contrast is 1.71 which compares with 1.31 for Ca ⁺ K and 20 for L networks. Limb profiles in both equatorial and polar directions have been obtained; limb brightening is observed at both west and south limbs, peak limb temperature being about 40% higher than disk temperature. We have produced synthesized maps of disk filaments which correspond well to H disk filaments and regions of reduced emission in He i 10 830 Å spectroheliograms. Using the WSRT synthesized maps at 6 cm, we have compared the structure of a sunspot associated source with model computations. Using a new method of analysis we have been able to map the vertical as well as the horizontal component of the sunspot magnetic field at specific locations in the low corona. Using the VLA, we have mapped coronal loops at 20 cm; the radio emission is attributed to bremsstrahlung near the loop footpoints whereas gyroresonance process dominates near the loop top. Using the VLA, we have carried out simultaneous observations of a microwave burst at 2 and 6 cm. The 6 cm burst source is apparently located near the top of a flaring loop, while the 2 cm emission originates from the loop footpoints. The 6 cm emission is attributed to gyrosynchrotron radiation of thermal electons in the bulk heated plasma at 4 × 10⁷ K, while the 2 cm emission is due to nonthermal particles released and accelerated during the flare process. A DC electric field flare model appears to explain the observed delay between the peaks at the two wavelengths. From the delay, the strength of the electric field in the flaring region is estimated.     

Airborne total eclipse observation of the extreme solar limb at 400 μm

The total solar eclipse of February 26, 1979 was monitored at far infrared wavelengths from the NASA Lear Jet Observatory flying at 12.9 km in the eclipse shadow. The resultant eclipse curve for radiation within a bandwidth of 20 cm^–1 centered upon 25 cm^–1 (400 m) was measured and analysed at an equivalent angular resolution of 1 arc sec over a 100 arc sec region adjacent to the limb to provide information on the intensity distribution of continuum radiation close to this limb. The curve has been compared to predictions derived from models of the solar atmosphere for the specific geometry of this eclipse, and is shown to match most closely that derived from a uniform distribution of radiation across the disk. This is in distinct contrast to the result obtained in the only other comparable experiment, carried out over Africa in 1973 from a supersonic Concorde aircraft, in which an intense but narrow spike of far infrared radiation at the extreme solar limb was inferred from the data. The absence also in the present observations of the significant limb brightening predicted by the HSRA model (in which homogeneity within the source region is assumed) is in substantial agreement with lower resolution results from mountain altitudes. This result is interpreted as further evidence for the presence in the Sun's lower chromosphere of significant inhomogeneity with a scale size of at least 1000 km at this depth.     

The Mg/Ne abundance ratio in a recently emerged flux region observed by CDS

Evidence for the existence of the FIP-effect in the transition region is presented here based on recent observations from the Coronal Diagnostic Spectrometer (CDS) on-board the Solar and Heliospheric Observatory (SOHO). Observations of an emerging flux region in lines of Mgv–vii and Nevi–vii reveal differences in the relative Mg/Ne abundance of a factor of 9.2 between two transition region brightenings separated by less than 1 arc min on the Sun. The lower abundance ratio is approximately equal to the photospheric Mg/Ne value and is associated with a small loop-like feature in the central, hottest part of the active region. The higher abundance ratio is found in spike-like structures at the edge of the active region. A density diagnostic of Oiv is used to derive an electron number density of 10^11.3 cm^-3 for the low Mg/Ne brightening, while a Mgvii diagnostic gives a density of 10^9.2 cm for the high Mg/Ne brightening.     

Radio emission from the Sun at 843 MHz

The Molonglo Observatory Synthesis Telescope (MOST) has recently been modified to permit observations of the Sun. With a collecting area of 18000 m², MOST makes high-sensitivity measurements in right-hand circular polarisation over a 3 MHz bandwidth at 843 MHz. The maximum baseline of the multi-element interferometer is 1600 m, so that one-dimensional spatial resolution as fine as 32 arc sec is available. A resistor array produces simultaneously a set of 64 beams separated by 22 arc sec, which may be offset electronically to cover the entire Sun in a few seconds. Observations may be made with a beam shape corresponding to either a multiplying or an adding interferometer. By exploiting the technique of Earth-rotation synthesis the telescope may be used to make two-dimensional maps of the Sun at the time of the austral solstice with a synthesized beamwidth of 43 × 110 arc sec. This paper describes the instrument and the procedures used to make various types of solar observations, and exhibits some of the first data collected.     

Lunar heat flow and regolith structure inferred from interferometric observations at a wavelength of 49.3 cm

We discuss observations of the Moon at a wavelength of 49.3 cm made with the Owens Valley Radio Observatory Interferometer. These observations have been fit to models in order to estimate the lunar dielectric constant, the equatorial subsurface temperature, the latitude dependence of the subsurface temperature, and the subsurface temperature gradient. The models are most consistent with a dielectric constant of 2.52 ± 0.01 (formal errors), an equatorial subsurface temperature of 249_?5⁺⁸K, and a change in the subsurface temperature with latitude (ψ), which is proportional to cos^0.38ψ. Since the temperature of the Moon has been measured by the Apollo Lunar Heat Flow Experiment, we have been able to use our determination of the equatorial temperature to estimate the error in the flux density calibration scale at 49.3cm (608 MHz). This results in a correction factor of 1.03 ± 0.04, which must be applied to the flux density scale. This factor is much different from 1.21 ± 0.09 estimated by Muhleman et al. (1973) from the brightness temperature of Venus and apparently indicates that the observed decrease in the brightness temperature of Venus at long wavelengths is a real effect.The estimates of the temperature gradient, which are based on the measurement of limb darkening, are small and negative (temperature decreases with depth) and may be insignificantly different from zero since they are only as large as their formal errors. We estimate that a temperature gradient in excess of 0.6K/m at 10m depth would have been observed. Thus, a temperature gradient like that measured in situ at the Apollo 15 and 17 landing sites in the upper 2m of the regolith is not typical of the entire lunar frontside at the 10m depths where the 49.3 cm wavelength emission originates. This result may indicate that the mean lunar heat flow is lower than that measured at the Apollo landing sites, that the thermal conductivity is greater at 10m depth than it is at 2m depth, or that the radio opacity is greater at 10m depth than at 2m depth. The negative estimates of the temperature gradient indicate that the Moon appeared limb bright and might be explained by scattering of the emission from boulders or an interface with solid rock. The presence of solid rock at 10m depths will probably cause heat flows like those measured by Apollo to be unobservable by our interferometric method at long wavelengths, since it will cause both the thermal conductivity and radio opacity of the regolith to increase. Thus, our data may be most consistent with a change in the physical properties of the regolith to those of solid rock or a mixture of rock and soil at depths of 7 to 16m. Our results show that future radio measurements for heat flow determinations must utilize wavelengths considerably shorter than 50 cm (25 cm or less) to avoid the rock regions below the regolith.     

The Solar Oscillations Investigation - Michelson Doppler Imager

The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun by measuring the photospheric manifestations of solar oscillations. Characteristics of the modes reveal the static and dynamic properties of the convection zone and core. Knowledge of these properties will improve our understanding of the solar cycle and of stellar evolution. Other photospheric observations will contribute to our knowledge of the solar magnetic field and surface motions. The investigation consists of coordinated efforts by several teams pursuing specific scientific objectives.The instrument images the Sun on a 1024² CCD camera through a series of increasingly narrow spectral filters. The final elements, a pair of tunable Michelson interferometers, enable MDI to record filtergrams with a FWHM bandwidth of 94 m. Normally 20 images centered at 5 wavelengths near the Ni I 6768 spectral line are recorded each minute. MDI calculates velocity and continuum intensity from the filtergrams with a resolution of 4 over the whole disk. An extensive calibration program has verified the end-to-end performance of the instrument.To provide continuous observations of the longest-lived modes that reveal the internal structure of the Sun, a carefully-selected set of spatial averages are computed and downlinked at all times. About half the time MDI will also be able to downlink complete velocity and intensity images each minute. This high rate telemetry (HRT) coverage is available for at least a continuous 60-day interval each year and for 8 hours each day during the rest of the year. During the 8-hour HRT intervals, 10 of the exposures each minute can be programmed for other observations, such as measurements in MDI's higher resolution (1.25) field centered about 160 north of the equator; meanwhile, the continuous structure program proceeds during the other half minute. Several times each day, polarizers will be inserted to measure the line-of-sight magnetic field.MDI operations will be scheduled well in advance and will vary only during the daily 8-hour campaigns. Quick-look and summary data, including magnetograms, will be processed immediately. Most high-rate data will be delivered only by mail to the SOI Science Support Center (SSSC) at Stanford, where a processing pipeline will produce 3 Terabytes of calibrated data products each year. These data products will be analyzed using the SSSC and the distributed resources of the co-investigators. The data will be available for collaborative investigations.The MDI Engineering Team leaders include: D. Akin, B. Carvalho, R. Chevalier, D. Duncan, C. Edwards, N. Katz, M. Levay, R. Lindgren, D. Mathur, S. Morrison, T. Pope, R. Rehse, and D. Torgerson.     

Centimeter-wave radio emission of a high-latitude prominence

The results of observations of the radio emission of a high-latitude prominence located in the NE part of the solar limb are discussed. Observations were performed on the radio telescope RATAN-600 using its Northeastern sector and Southern sector with a periscope during the maximum phase (0.998) of the solar eclipse of March 29, 2006. The prominence was studied in the wavelength interval 1.03 ÷ 5.0 cm. The absence of the background illumination from the solar disk allowed us to study the parameters of the radio emission of the high-latitude prominence (? = 45°, NE limb of the Sun). Observations of the solar limb at the time of the maximum phase made it possible to record very small radio fluxes from the prominence, which amounted to 0.05 ÷ 0.01 s.f.u. in the wavelength interval from 1.84 to 5.0 cm. The position of the maximum of the radio emission of the prominence coincides, according to the results of observations performed on both sectors, with the summit of the prominence as seen on the solar image taken in the He II 304 Å line (SOHO, ? = 45°, NE limb of the Sun). The degree of polarization is P ≈ 7 ÷ 16% at 1.88 ÷ 5.0 cm. If interpreted in terms of the thermal mechanism considered here, polarized emission corresponds to a magnetic-field strength of H ? (550 ÷ 100) G in the prominence region.     

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.