Quiet Sun Level Research Articles

Calculated brightness temperatures of solar structures compared with ALMA and Metsähovi measurements

AbstractThe Atacama Large Millimeter/submillimeter Array (ALMA) allows for solar observations in the wavelength range of 0.3–10 mm, giving us a new view of the chromosphere. The measured brightness temperature at various frequencies can be fitted with theoretical models of density and temperature versus height. We use the available ALMA and Metsähovi measurements of selected solar structures (quiet sun (QS), active regions (AR) devoid of sunspots, and coronal holes (CH)). The measured QS brightness temperature in the ALMA wavelength range agrees well with the predictions of the semiempirical Avrett–Tian–Landi–Curdt–Wülser (ATLCW) model, better than previous models such as the Avrett–Loeser (AL) or Fontenla–Avrett–Loeser model (FAL). We scaled the ATLCW model in density and temperature to fit the observations of the other structures. For ARs, the fitted models require 9%–13% higher electron densities and 9%–10% higher electron temperatures, consistent with expectations. The CH fitted models require electron densities 2%–40% lower than the QS level, while the predicted electron temperatures, although somewhat lower, do not deviate significantly from the QS model. Despite the limitations of the one‐dimensional ATLCW model, we confirm that this model and its appropriate adaptations are sufficient for describing the basic physical properties of the solar structures.

Solar Observations with Single-Dish INAF Radio Telescopes: Continuum Imaging in the 18 – 26 GHz Range

We present a new solar radio imaging system implemented through the upgrade of the large single-dish telescopes of the Italian National Institute for Astrophysics (INAF), not originally conceived for solar observations.During the development and early science phase of the project (2018 – 2020), we obtained about 170 maps of the entire solar disk in the 18 – 26 GHz band, filling the observational gap in the field of solar imaging at these frequencies. These solar images have typical resolutions in the 0.7 – 2 arcmin range and a brightness temperature sensitivity <10 K. Accurate calibration adopting the Supernova Remnant Cas A as a flux reference provided typical errors <3% for the estimation of the quiet-Sun level components and active regions flux measurements.As the first early scientific result of the project, we present a catalog of radio continuum solar imaging observations with Medicina 32-m and SRT 64-m radio telescopes, including the multi-wavelength identification of active regions, their brightness and spectral characterization. The interpretation of the observed emission as thermal bremsstrahlung components combined with gyro-magnetic variable emission paves the way for the use of our system for long-term monitoring of the Sun. We also discuss useful outcomes both for solar physics (e.g., study of the chromospheric network dynamics) and space weather applications (e.g., flare precursors studies).

Identifying 8 mm Radio Brightenings During the Solar Activity Minimum

Strong solar radio brightenings have been extensively studied in the past, and their correlation to the sunspots and active regions are already well known. But even when the Sun is ostensibly quiet, there is practically always some activity that can be detected in the radio domain. In this article we investigate these semi-active features at 8 mm using the radio telescope at Aalto University Metsähovi Radio Observatory. The observations were made between May and September 2019 when the solar activity was very low, and for our detailed study we chose dates when no active regions were identified on the solar surface by the National Oceanic and Atmospheric Administration. The brightness temperature of these radio regions during this quiescent period of solar activity is at maximum approximately 250 K above the quiet-Sun level. We compared our millimeter data with data taken in extreme ultraviolet, and we found that these weak radio brightenings are mostly related to coronal hole features and magnetic bright points. We also found that there are two different categories of bright points: those with and without flux tube structure. The formation of the weak radio brightenings is comparable to the stronger radio brightenings: the rising fluxes from the weak photospheric features can be detected as a radio source.

Solar polarization observations at 3 and 13 mm

AbstractThe first solar radio polarization observations at 3 mm (86 GHz) and 13 mm (22 GHz) were made at Aalto University Metsähovi Radio Observatory in Finland in spring 2019. This was the first time that 3 mm (86 GHz) and 13 mm (1.3 cm) solar polarization observations were made with the same radio telescope. In this paper, we describe the observing system and data analysis, and present the first results. We also compare our data with polarization observations made with the Nobeyama Radioheliograph and find that the results are consistent. Additionally, we determine the quiet Sun level brightness temperature at 3 mm (86 GHz) using the New Moon for reference, and obtain a mean value of 6,310 K.

Modelling solar irradiance from ground-based photometric observations

Total solar irradiance (TSI) has been monitored from space since 1978, i.e. for about four solar cycles. The measurements show a prominent variability in phase with the solar cycle, as well as fluctuations on timescales shorter than a few days. However, the measurements were done by multiple and usually relatively short-lived missions. The different absolute calibrations of the individual instruments and the unaccounted for instrumental trends make estimates of the possible long-term trend in the TSI highly uncertain. Furthermore, both the variability and the uncertainty are strongly wavelength-dependent. While the variability in the UV irradiance is clearly in-phase with the solar cycle, the phase of the variability in the visible range has been debated. In this paper, we aim at getting an insight into the long-term trend of TSI since 1996 and the phase of the solar irradiance variations in the visible part of the spectrum. We use independent ground-based full-disc photometric observations in Ca II K and continuum from the Rome and San Fernando observatories to compute the TSI since 1996. We follow the empirical San Fernando approach based on the photometric sum index. We find a weak declining trend in the TSI of $ {-7.8}_{-0.8}^{+4.9}\times 1{0}^{-3}$ Wm−2 y−1 between the 1996 and 2008 activity minima, while between 2008 and 2019 the reconstructed TSI shows no trend to a marginally decreasing (but statistically insignificant) trend of $ {-0.1}_{-0.02}^{+0.25}\times 1{0}^{-3}$ Wm−2 y−1. The reference TSI series used for the reconstruction does not significantly affect the determined trend. The variation in the blue continuum (409.2 nm) is rather flat, while the variation in the red continuum (607.1 nm) is marginally in anti-phase, although this result is extremely sensitive to the accurate assessment of the quiet Sun level in the images. These results provide further insights into the long-term variation of the TSI. The amplitude of the variations in the visible is below the uncertainties of the processing, which prevents an assessment of the phase of the variations.

Measurements of the Quiet-Sun Level Brightness Temperature at 8 mm

Defining the solar brightness temperature accurately at millimeter wavelengths has always been challenging. One of the main reasons has been the lack of a proper calibration source. New Moon was used earlier as a calibration source. We carried out a new extensive set of observations at 8 mm using the New Moon for calibration. The solar and Moon observations were made using the 14-meter radiotelescope operated by the Aalto University Metsähovi Radio Observatory in Finland. In this article, we present our method for defining the brightness temperature of the quiet-Sun level (QSL). Based on these observations, we found 8100~mbox{K} pm 300~mbox{K} to be the mean value for the QSL temperature. This value is between the values that were reported in earlier studies.

Sizes of solar active regions at 8 mm

BackgroundWe investigate the sizes of 86 non‐flaring solar active regions with respect to the brightness temperature at 8 mm and active region magnetic field strength.Materials and MethodsThe data were collected in AprilŰSeptember, 2014, and AprilŰJuly, 2015 at Aalto University, Metsähovi Radio Observatory with the 14‐m radio telescope. The observations were made at the maximum of the solar cycle 24. We used active region magnetic field strength values measured with the Helioseismic and Magnetic Imager instrument aboard the Solar Dynamics Observatory.ResultsWe calculated that the mean active region size in percentage of the full solar disk is 3.7 ś 0.86%. In addition, the mean temperature of selected active regions is around 109% of the quiet Sun level and the average magnetic field strength of the studied active region is 1,400 G.ConclusionsOur results show correlation between active region sizes, the total brightness temperature of the Sun, and the active region magnetic field strengths.

THE TOTAL SOLAR IRRADIANCE CLIMATE DATA RECORD

ABSTRACT We present the composite measurements of total solar irradiance (TSI) as measured by an ensemble of space instruments. The measurements of the individual instruments are put on a common absolute scale, and their quality is assessed by intercomparison. The composite time series is the average of all available measurements. From 1984 April to the present the TSI shows a variation in phase with the 11 yr solar cycle and no significant changes of the quiet-Sun level in between the three covered solar minima.

Solar Radius at Sub-Terahertz Frequencies

AbstractThe visible surface of the Sun, or photosphere, is defined as the solar radius in the optical spectrum range located at 696,000 km (Cox et al. (Ed. 2015)). However, as the altitude increases, the dominant electromagnetic radiation is emitted at other frequencies. Our aim is to measure the solar radius at frequencies of 212 GHz and 405 GHz through out a solar cycle and, therefore, the altitude where these emissions are generated and that variation along the years. Also we tried to verify the the radius dependence on the solar activity cycle, which can be a good indicator of the changes that occur in the atmosphere structure. For this, we used data obtained by the Submillimetric Solar Telescope (SST) created from daily scans made by SST from 1990 to 2015. From these scans a 2D map of the solar disk was constructed. The solar radius is then determined by adjusting a circumference to the points where the brightness is half of the quiet Sun level, which is set as the most common temperature value in the solar map, i.e., the mode of the temperature distribution. Thus, we determined the solar radius at 212 and 405 GHz and the altitude of the emissions respectively. For 212 GHz, we obtained a radius of 976.5”±8” (707±4 Mm), whereas for 405 GHz, we obtained 975.0”±8” (707±5 Mm). optical spectrum range

UNUSUAL POLAR CONDITIONS IN SOLAR CYCLE 24 AND THEIR IMPLICATIONS FOR CYCLE 25

ABSTRACT We report on the prolonged solar-maximum conditions until late 2015 at the north-polar region of the Sun indicated by the occurrence of high-latitude prominence eruptions (PEs) and microwave brightness temperature close to the quiet-Sun level. These two aspects of solar activity indicate that the polarity reversal was completed by mid-2014 in the south and late 2015 in the north. The microwave brightness in the south-polar region has increased to a level exceeding the level of the Cycle 23/24 minimum, but just started to increase in the north. The north–south asymmetry in the polarity reversal has switched from that in Cycle 23. These observations lead us to the hypothesis that the onset of Cycle 25 in the northern hemisphere is likely to be delayed with respect to that in the southern hemisphere. We find that the unusual condition in the north is a direct consequence of the arrival of poleward surges of opposite polarity from the active region belt. We also find that multiple rush-to-the-pole episodes were indicated by the PE locations that lined up at the boundary between opposite-polarity surges. The high-latitude PEs occurred in the boundary between the incumbent polar flux and the insurgent flux of opposite polarity.

Corrections of Sun-as-a-star p-mode frequencies for effects of the solar cycle

Solar p-mode frequencies vary with solar activity. It is important to take this into account when comparing the frequencies observed from epochs that span different regions of the solar cycle. We present details of how to correct observed p-mode frequencies for the effects of the solar cycle. We describe three types of correction. The first allows mode frequencies to be corrected to a nominal activity level, such as the canonical quiet-Sun level. The second accounts for the effect on the observed mode frequencies, powers, and damping rates of the continually varying solar cycle and is pertinent to frequencies obtained from very long data sets. The third corrects for Sun-as-a-star observations not seeing all components of the modes. Suitable combinations of the three correction procedures allow the frequencies obtained from different sets of data to be compared and enable activity-independent inversions of the solar interior. As an example of how to apply the corrections we describe those used to produce a set of definitive Sun-as-a-star frequencies.

An Interpretation of the Coronal Holes’ Visibility in the Millimeter Wavelength Range

Various observations indicate that coronal holes generally appear as low brightness temperature regions (LTRs) in the centimeter and millimeter wavelength ranges. However, within their borders local enhancements of radiation, that is, high brightness temperature regions (HTRs), often occur. The theory behind the described behavior is not fully understood and therefore we analyze full-disk solar images obtained at a wavelength of 8 mm at Metsahovi Radio Observatory and compare them with data simultaneously taken in other wavelength ranges. The observational finding that the average brightness temperature of coronal holes is not much different from the quiet-Sun level (with localized deviations toward higher and lower intensities on the order of a few percent) is compared with theoretical models of the thermal bremsstrahlung radiation originating in the solar chromosphere, transition region, and corona. Special attention is devoted to the interpretation of the localized enhancements of radiation observed inside coronal holes at millimeter wavelengths. The main conclusion is that the most important contribution to the brightness temperature comes from an increased density in the transition region and low corona (i.e., at the heights where the temperature is below 106 K). This can explain both the LTRs and HTRs associated with coronal holes.

THE PROMINENCE – CORONA AND THE FILAMENT – CORONA TRANSITION REGION: IS THERE ANY DIFFERENCE?

The ratio between the Extreme Ultraviolet emission of the prominence–corona transition region and that of the quiet Sun (QS) transition region is measured using observations from the CDS and SUMER instruments on board the SOHO Satellite. These results are compared with those obtained in an earlier paper, analysing the same prominence as a filament on the disk. Theoretical models predict a difference in the emission of the prominence–corona transition region when it is observed at the limb and on the disk as a filament; the aim of the present work is to provide an observational check of this difference. SUMER and CDS data provide fairly good agreement if the prominence intensity measured by SUMER is compared with the average quiet-Sun intensity, measured near the disk center; the prominence intensity relative to the average quiet-Sun level measured on the same rasters results in disagreement with CDS, due to the smaller size of the disk portion and to the very strong limb brightening present in SUMER rasters. The relative prominence to quiet-Sun intensity ratio varies from 0.2 to 0.4, depending on the line formation temperature. This value leads to a discrepancy with the results obtained in a previous study when the same prominence was observed as a filament. This discrepancy indicates that the prominence–corona transition region emission is different when emitted by different sides of the prominence.

A multi-wavelength study of solar coronal-hole regions showing radio enhancements

We observed 17 GHz microwave-enhanced regions in equatorial coronal holes (ECH) together with extreme-ultraviolet (EUV), far-ultraviolet (FUV) and visible emissions in a search for temperature increases which might explain the bright spots in radio wavelengths. The ultraviolet (UV) observations span a wide range of formation temperatures (8000 K to 630 000 K). Increased UV emission was observed at the approximate location of the radio enhancements, but unlike the radio brightening, the UV emission did not exceed the mean quiet sun level. However, there were two observations showing increased H brightness in radio enhancements above mean quiet sun levels. No H bright spots were detected in ECHs outside of radio enhancement regions. The ECH H bright spots were caused by bright brils, bright points and a lack of dark brils. Since the 17 GHz and H enhancements are co-spatial, have equal integrated normalized enhanced emission and brightness temperatures, the observations suggest that the radio enhancements are caused by increased bril radio emission. In addition, increased Fe xii EUV emission was recorded at the location of some well-dened radio enhancements, which were the bases of coronal plumes. Since the radio brightness temperature is much lower than the Fe xii formation temperature, the radio and EUV enhancements are likely both related to the presence of concentrated magnetic flux, but do not arise from the same physical layer.

Some Features of Manifestations of Coronal Holes in Microwave Emission

Abstract Using observational data from the SSRT, we have investigated the behavior of four mid- and low-latitude coronal holes. The coronal holes under study appeared as regions of decreased emission intensity relative to the quiet Sun level totally or partially coincident with holes in EUV emission. Difference in the electron density and temperature in different holes or different parts of a hole can account for the diversity of manifestations of coronal holes. Estimates showed that the influence of these parameters is strongest in the wavelength range of 2–5 cm. Depending on particular values of the coronal electron density and temperature, coronal holes are either indistinguishable from the background of the quiet Sun, or show up as regions of decreased or increased microwave emission.

A broadband spectrometer for decimeter and microwave radio bursts

Observations of solar microwave bursts with high temporal and spectral resolution have shown interesting fine structures (FSs) of short duration and small bandwidth which are usually superimposed on the smooth continuum. These FSs are very intense (up to 1015 K) and show sometimes a high degree of circular polarization (up to 100%). They are believed to be generated by electron cyclotron maser emission (ECME) in magnetic loops. Another type are the microwave type III bursts, which are drifting microwave FSs, and are probably the signatures of travelling electron beams in the solar atmosphere. The exact emission mechanisms for these phenomena, in particular the source configuration, the plasma parameters and the distribution of radiating electrons are not clear. For a detailed study of these problems new observations of intensity and polarization with high resolution in time and in frequency in decimeter and microwave wavebands are essential. In order to investigate these features in greater detail, spectrometers with high temporal and spectral resolution are being developed by the solar radio astronomy community of China (Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Astronomical Observatory (YAO), and Nanjing University (NJU)). The frequency range from 0.7 to about 12 GHz is covered by about five spectrometers in frequency ranges of 0.7–1.4 GHz, 1–2 GHz, 2.4–3.6 GHz, 4.9–7.3 GHz, and 8–12 GHz, respectively. The radiospectrometers will form a combined type of swept-frequency and multi-channel receivers. The main characteristics of the solar radio spectrometers are: frequency resolution: 1–10 MHz; temporal resolution: 1–10 ms; sensitivity: better than 2% of the quiet-Sun level. We pay special attention to the sensitivity and the accuracy of polarization. Now, the 1–2 GHz radiospectrometer is being set up. The full system will be set up in 3–4 years.

Calcium plage intensity and solar irradiance variations

We have established a statistical relation between the facular contribution to the solar irradiance and the intensity of the associated calcium plage. For the solar irradiance data the ACRIM measurements were used. The quiet-Sun level of the irradiance was determined as a function of the time for the period studied. A sample of plages in the period of the solar activity minimum was selected, during the periods when no spots were present on the solar disc. We have expressed the dependence studied through the parameter C p in the ‘proxy’ PFI concept. The parameter C p could be related to the plage intensity (I) as C p = 0.006I + 0.003. The mean value of the parameter C p ranged between 0.015 and 0.017 depending on the choice of samples.

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 λ−2 if the wavelength λ 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 λ2. 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 λ = 2 mm and identified with the filaments. However, its visibility falls when the wavelength decreases.

THE SOLAR NOISE BURST OF NOVEMBER 12, 1960

The intense burst of solar radio noise which accon~panied the now wellknown flare of November 12, 1960, has been observed a t the discrete frequencies of 2800 Mc/s and 48 Mc/s a t the Radio Observatory of the Radio and Electrical Engineering Division of the National Research Council. These observations will be presented and the essential information noted. Figure 1 shows a reconstruction of the high-frequency burst while Fig. 2 shows a portion of the low-frequency record. 2800 Mc/s event.-When the solar patrol observations were begun a t 12h 4 P U.T., the solar flux was found to be iilcreasiilg slightly, and by 13h 20 U.T. an increase of 11 units of flux had been recorded in excess of the daily level of 147 flux units (where 1 unit of flux is equal to watts per square meter per cycle per second). At this time, 13h 20m, the increase became more rapid and showed a noticeable departure from the residual receiver noise; by 13h 23 the flux had reached 50 flux units, by 13h 24+ the flux had reached 100 units, and by 13h 27 a level of 3000 units. The intense part of the burst shows a number of oscillations, with a peak value of flux of 5500 units a t 13h 4P. The burst declined rapidly from this peak value to a level of 300 units a t 14h OGm. This was follo\ved by an enhanced post burst increase and the occurrence of a number of superi~rlposed relatively srnaller bursts. At lgh OOm the quiet sun level was reached and no further activity was recorded during the relnaining two hours of observations. 48 Alc/s event.-A portion of the 48 Mc/s film record is shown in Fig. 2 (note that the times recorded on the film are E.S.T. and are 1 minute fast). This record has the output of the 2800 Mc/s radiometer si~nultaneously recorded as a single curve which inoves upwards for increasing signal strength. The sloping curve across the upper track represents a portion of the 2800 Mc/s record as the burst level increases from 85 flux units to 160 flux units. In Fig. 1 these levels are shown as broken lines. The time, 13h 24 27' a t which the 85-flux level is reached has been taken as the high-frequency burst commencement. The 48 ~\/Ic/s burst started abruptly a t 13h 27 36' and in relation to the somewhat arbitrarily chosen starting time for the higher frequency burst,

Time relations between centimeter wavelength bursts and solar Hα flares.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Time relations between centimeter wavelength bursts and solar Hα flares. Edelson, S. ; Coates, R. J. ; Santini, N. ; McCullough, T. F. Abstract Since March 1959 the Naval Research Laboratory has been making Ha spectroheliograms of the sun at 6-second intervals concomitantly with centimeter solar radio observations. Flare light-curves derived from these rapid-sequence films have been compared directly with the observed flux curves of the associated radio bursts. Current measurements of time differences between the Ha maximum intensity and the 10-cm peak flux indicate that the events may be classified according to the 10-cm excitation level which exists before the flux peak. In the first type, the 10-cm radio flux rises rapidly to a peak from the quiet sun level in less than 2 minutes, and always precedes the Ha maxima. In 6o per cent of the events of this class, the 10-cm burst peaks occur 2 to 10 seconds before the Ha maxima; 35 per cent have time differences of 10 to 20 seconds; and 5 per cent lead the Ha by 20 to 30 seconds. In the second type, 10-cm excitation exists for more than 4 minutes before the time of burst peak flux. Five per cent of the events of this class have 10-cm peaks following the Ha maxima by o to 10 seconds, while the great majority have 10-cm peaks preceding the flare maxima with the following time distribution: 30 per cent lead by 0 to 10 seconds, 35 per cent lead by 10 to 20 seconds, 25 per cent lead by 20 to 30 seconds, and 5 per cent lead by 30 to 40 seconds. There is evidence that several events of the second type may be associated with more than one source. U.S. Naval Research Laboratory Washington, D. C. Publication: The Astronomical Journal Pub Date: 1959 DOI: 10.1086/107994 Bibcode: 1959AJ.....64Q.330E full text sources ADS |