Figure - available from: Solar Physics
This content is subject to copyright. Terms and conditions apply.
Histogram of AR mean diameters in the 18 – 23 GHz (K1-band, left panel) and 23 – 26 GHz (K2-band, right panel) frequency ranges.
Source publication
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 i...
Citations
... In recent years, other radio telescopes were used to perform solar observations, such as the Medicina and Sardinia radio telescopes. These telescopes have multiple receivers which covers a wide range of frequencies [20]. Although the observations made through radio bands usually suffer from many technical problems related to accurate equipment calibrations over wide frequency ranges. ...
... The Cagliari Astronomical Observatory used the Medicina 32 m and Sardinia Radio Telescope (SRT) 64 m antennas for solar imaging observations and obtained approximately 170 maps of the entire solar disk in the 18-26 GHz band, with beamwidths of 2 1 and 1 5 for the 18 GHz and 26 GHz solar observations, respectively, when using the Medicina 32 m antenna. The beamwidths of the 18 GHz, 24 GHz, and 25 GHz solar observations with the SRT 64 m antenna were 1 02, 0 78, and 0 75, respectively (Prandoni et al. 2017;Pellizzoni et al. 2022). When correlated by two antennas (a nulling interferometer) to compensate for the quiet solar background radiation, the solar flare can be considered a point-source signal as compared to the solar disk, and the nulling interferometer can largely reduce fluctuations in the overall quiet solar flux density (Luthi et al. 2005). ...
In this paper, we present the design and implementation of a two-element interferometer operating in the millimeter-wave band (39.5–40 GHz) for observing solar radio emissions through nulling interference. The system is composed of two 50 cm aperture Cassegrain antennas installed on a common equatorial mount, with a separation of 230 wavelengths. The cross-correlation of the received signals effectively cancels out the quiet solar component of the high flux density (∼3000 sfu) that reduces the detection limit due to atmospheric fluctuations. The system performance is as follows: the noise factor of the analog front end in the observation band is less than 2.1 dB, system sensitivity is approximately 12.4 K (∼34 sfu) with an integration time constant of 0.1 ms (default), the frequency resolution is 153 kHz, and the dynamic range is ≥30 dB. Through actual testing, the nulling interferometer observes a quiet Sun with a low level of output fluctuations (up to 50 sfu) and has a significantly lower radiation flux variability (up to 190 sfu) than an equivalent single-antenna system, even under thick cloud cover. As a result, this new design can effectively improve observation sensitivity by reducing the impact of atmospheric and system fluctuations during observation.
... The 14-meter radio telescope at the MRO Observatory of Aalto University, Finland, has a beam width of 2.4 arcmins when observing an 8 mm signal (Kallunki & Tornikoski (2018)).The Cagliari Astronomical Observatory used the Medicina 32 m and SRT 64 m antennas for solar imaging observations and obtained approximately 170 maps of the entire solar disk in the 18GHz∼26 GHz band, with beam widths of 2.1 arcmins and 1.5 arcmins for 18 GHz and 26 GHz solar observations, respectively, when using the Medicina 32 m antenna. The beam widths of the 18 GHz, 24 GHz and 25 GHz solar observations with the SRT 64 m antenna are 1.02 arcmin, 0.78 arcmins, and 0.75 arcmins, respectively (Pellizzoni et al. (2022); Prandoni et al. (2017)).When correlated by two antennas (nulling interferometer) to compensate for the quiet solar background radiation, the solar flare can be considered a point source signal compared to the solar disk, and the nulling interferometer can largely reduce the fluctuations in the overall quiet solar flux density (Luthi et al. (2005)).The sensitivity of the nulling interferometer is higher than that of the single-antenna radiometer with the same antenna diameter. The nulling interferometer detects the burst of small flares on the sun with two small antennas with beam widths covering the whole sun; however, if each antenna is installed separately, the effective baseline distance varies with the sun's time angle (Koda et al. (2016)).Therefore, the two antennas must be mounted in the same plane to ensure a stable effective baseline. ...
In this paper, we present the design and implementation of a two-element interferometer working in the millimeter wave band (39.5 GHz - 40 GHz) for observing solar radio emissions through nulling interference. The system is composed of two 50 cm aperture Cassegrain antennas mounted on a common equatorial mount, with a separation of 230 wavelengths. The cross-correlation of the received signals effectively cancels the quiet solar component of the large flux density (~3000 sfu) that reduces the detection limit due to atmospheric fluctuations. The system performance is obtained as follows: the noise factor of the AFE in the observation band is less than 2.1 dB, system sensitivity is approximately 12.4 K (~34 sfu) with an integration time constant of 0.1 ms (default), the frequency resolution is 153 kHz, and the dynamic range is larger than 30 dB. Through actual testing, the nulling interferometer observes a quiet sun with a low level of output fluctuations (of up to 50 sfu) and has a significantly lower radiation flux variability (of up to 190 sfu) than an equivalent single-antenna system, even under thick cloud cover. As a result, this new design can effectively improve observation sensitivity by reducing the impact of atmospheric and system fluctuations during observation.
Context. One of the most important objectives of solar physics is to gain a physical understanding of the solar atmosphere, whose structure can also be described in terms of the density ( N ) and temperature ( T ) distributions of the atmospheric matter. Several multi-frequency analyses have shown that the characteristics of these distributions are still under debate, especially for outer coronal emission.
Aims. We aim to constrain the T and N distributions of the solar atmosphere through observations in the centimetric radio domain. We employed single-dish observations from two of the INAF radio telescopes at the K -band frequencies (18–26 GHz). We investigated the origin of the significant brightness temperature ( T B ) detected up to the upper corona (at an altitude of ∼800 mm with respect to the photospheric solar surface).
Methods. To probe the physical origin of the atmospheric emission and to constrain instrumental biases, we reproduced the solar signal by convolving specific 2D antenna beam models. We performed an analysis of the solar atmosphere by adopting a physical model that assumes the thermal bremsstrahlung as the emission mechanism, with specific T and N distributions. We compared the modelled T B profiles with those observed by averaging solar maps obtained at 18.3 and 25.8 GHz during the minimum of solar activity (2018–2020).
Results. We probed any possible discrepancies between the T and N distributions assumed from the model and those derived from our measurements. The T and N distributions are compatible (within a 25% of uncertainty) with the model up to ∼60 mm and ∼100 mm in altitude, respectively.
Conclusions. Our analysis of the role of the antenna beam pattern on our solar maps proves the physical nature of the atmospheric emission in our images up to the coronal tails seen in our T B profiles. Our results suggest that the modelled T and N distributions are in good agreement (within 25% of uncertainty) with our solar maps up to altitudes of ≲100 mm. A subsequent, more challenging analysis of the coronal radio emission at higher altitudes, together with the data from satellite instruments, will require further multi-frequency measurements.
Context. The Sun is an extraordinary workbench, on which several fundamental astronomical parameters can be measured with high precision. Among these parameters, the solar radius R ⊙ plays an important role in several aspects, for instance, in evolutionary models. Moreover, it conveys information about the structure of the different layers that compose the solar interior and its atmosphere. Despite the efforts to obtain accurate measurements of R ⊙ , the subject is still debated, and measurements are puzzling and/or lacking in many frequency ranges.
Aims. We determine the mean, equatorial, and polar radii of the Sun ( R c , R eq , and R pol ) in the frequency range 18.1 − 26.1 GHz. We employed single-dish observations from the newly appointed Medicina Gavril Grueff Radio Telescope and the Sardinia Radio Telescope (SRT) in five years, from 2018 to mid-2023, in the framework of the SunDish project for solar monitoring.
Methods. Two methods for calculating the radius at radio frequencies were employed and compared: the half-power, and the inflection point. To assess the quality of our radius determinations, we also analysed the possible degrading effects of the antenna beam pattern on our solar maps using two 2D models (ECB and 2GECB). We carried out a correlation analysis with the evolution of the solar cycle by calculating Pearson’s correlation coefficient ρ in the 13-month running means.
Results. We obtained several values for the solar radius, ranging between 959 and 994 arcsec, and ρ , with typical errors of a few arcseconds. These values constrain the correlation between the solar radius and solar activity, and they allow us to estimate the level of solar prolatness in the centimeter frequency range.
Conclusions. Our R ⊙ measurements are consistent with the values reported in the literature, and they provide refined estimates in the centimeter range. The results suggest a weak prolateness of the solar limb ( R eq > R pol ), although R eq and R pol are statistically compatible within 3 σ errors. The correlation analysis using the solar images from the Grueff Radio Telescope shows (1) a positive correlation between solar activity and the temporal variation in R c (and R eq ) at all observing frequencies, and (2) a weak anti-correlation between the temporal variation of R pol and solar activity at 25.8 GHz.
