The Sun environment and its phenomenology can be exploited as a natural multi-disciplinary laboratory for a wide range of scientific, technological and educational applications. The solar physics science offers a rich interdisciplinary ground on astrophysics, plasma physics, nuclear physics and fundamental physics (see e.g. [1]). Furthermore, the magnetic and radiative activity of our star has an enormous impact on planetary magnetospheres and ionospheres ranging from subtle climate dependencies to severe radiation phenomena affecting operations and safety of our technologies on Earth (see e.g. [2]).

All these plasma processes (e.g. magnetic reconnections, shocks, particle acceleration [3]) contribute to a complex and not fully understood radio emission picture needing spatially-resolved and time-resolved scientific data in order to fully explore their nature, complementing the wealth of existing information in the optical/UV/IR domain.

Compared to the emission in other spectral ranges (e.g. the EUV), the radio quiet Sun has the advantage of being understood as originating mostly from thermal bremsstrahlung in local thermodynamic equilibrium. It can thus be used as a powerful diagnostic of the physical conditions in a wide range of atmospheric layers (see e.g. [11]). In particular, mapping the brightness temperature of the free-free continuum radio emission in the centimetre and millimetre range is an effective tool to characterise the vertical structure and physical parameters of the chromosphere (see e.g. [4]).

As the opacity increases with the wavelength of observation, the effective height of formation moves from the temperature minimum (at sub-millimiter waves) to the low corona (mostly visible at meter waves). Among these extreme wavelengths, the chromosphere is visible in our range and it hosts most of active solar features linking the photosphere to the corona with a temperature rise of still unclear origin. Solar atmospheric models are critically constrained by actual temperature brightness in K-band (see e.g. [13]).

In perspective, our observing project will be suitable for detailed measurements of the chromospheric brightness temperature of the quiet Sun, the sunspot umbrae, and active regions, contributing to Space Weather monitoring networks and forecast along the solar cycle.

For these scientific applications, smart single-dish radio mapping of the solar disk is more suitable than interferometric observations, especially at high-frequencies (see e.g. [5][6][12]). In fact, synthesis images of the full solar disk through interferometric networks cannot be easily obtained in the frequency range 10-30 GHz on relatively large sources, with the exception of dedicated short-baseline facilities, as for example the Nobeyama Radioheliograph which covers a complementary longitude range with respect to INAF radio-telescopes. The availability of recent imaging observations in the neighbor bands (e.g. with LOFAR in the decimetric range [25], and with ALMA in the millimetric bands [14]) enriches and empowers the exploitation of K-band observations.

Up to now we accomplished preliminary tests for solar and near-sun pointing using both Medicina and SRT radio telescopes. SRT requires additional hardware setup and tests (DDT proposal, March 2019) before the validation of solar observations in spectro-polarimetric mode. On the other hand, with the 32-m Medicina radio telescope, we recently performed a first pioneering single-dish solar monitoring campaign in K-band (total power only) [9][10], in the January-November 2018 time frame, totalling about 30 observing sessions (mostly in suitable weather conditions). Preliminary brightness temperatures and spectra of the observed quiet-Sun structures and active regions are provided in the so far poorly known and transitional 18-26 GHz frequency range [10]. A typical resulting image of the solar disk by Medicina 32m total-power observations is represented in Figure 1. On average, such images reach a sensitivity of about 5 mJy (1 K). These early observations allowed us to improve and refine the optimal setup of the radio-telescope for solar observations, implementing first in the procedure a self-calibration routine (assuming a rough brightness estimate for the quiet Sun in K-band obtained from the literature) and then assessing the feasibility of an absolute calibration procedure (through the observation of the very bright Cas A SNR). This will allows us to appreciate very small brightness fluctuations and in perspective will provide helpful scientific insights on the structure of the solar chromosphere as a baseline model for the monitoring of brightness anomalies (e.g. active regions and flares).

Pioneering campaigns with the Medicina-32m (full-disk continuum monitoring) and SRT (spectro-polarimentric observations NAPA/ToO on selected regions) will set the basis for the establishment of the Italian radio-telescopes network as a non-dedicated solar imaging facility, closing different gaps that presently exist in the worldwide solar monitoring scenario and empowering the present capabilities by introducing state-of-the-art techniques.

In particular, a long-term monitoring campaign with Medicina could allow us to achieve the following major goals:

• Accurate measurement of the brightness temperature of the quiet Sun component (QS) in the presently poorly known 20-26 GHz range. The available measurements described in the literature around this range are scattered due to the difficulty to separate the pure quiet Sun component from the active regions (see Figure 3). Single-dish observations offer a suitable spatial resolution combined with adequate sensitivity for the disentanglement (from the QS regions) of both compact and weaker/scattered/larger active components. Present information suggests a possible spectral change (flattening?) in the quiet solar chromosphere in K-band that could be investigated and assessed by our observations [21]. The temperature rise in the chromosphere and the corona is expected to cause limb brightening at radio wavelength. Thus, observations of the radio brightness variation from the disk center to the limb provide a good test for solar atmospheric models (see e.g. [13]). The limb brightening shows solar activity dependence due to the change of structure of the upper atmosphere. The radio limb is enhanced when active regions are closer to the limb. The fact of being near the minimum of solar activity cycle (with a reduced presence of active regions) will ease the center-limb-variations measurements for the QS modelling.
• Characterize the flux density of the active regions and coronal holes and their spectral properties and evolution. Disentanglement and quantification of the thermal (free-free and gyro-resonance) and non-thermal emission components from radio active regions is an open issue (see e.g. [24][26]) that could be investigated through multi-wavelength observations and analysis. These could provide information on the formation process of these features hosting major flares and determining the slowly varying component of the Sun over the 11-year cycle. In particular, multi-wavelength observations of strong emission over sunspots groups can be used to test in detail the magnetic field model extrapolated from measurements at the photosphere (see [23]). Detection and monitoring parameters of each peculiar radio region in the solar disk can be published nearly real-time on a dedicated publicly available web site (developed ad hoc) and coupled with the available information from other radio observatories (e.g. LOFAR [25], ALMA [14], NoBH [18], MRO [27]). This shared information will contribute to the multi-wavelength assessment of the physical model of active regions and their possible geoeffective processes. It is worth noting that in addition to the spatial coincidence of bright radio features with the chromospheric network, it is important to measure the intensity and size of the bright features as a function of the frequency. We should note that interferometric/synthesis observations cannot easily measure the background level (and then only rms intensity fluctuation measurements are possible), which instead can be provided by our single-dish measurements. Magnetic network elements, similar to those responsible for the enhanced radio emission in bright points, have been suggested as the origin for coronal hole brightenings. Krissinel et al. (see e.g. [15]) proposed that a difference in electron density and temperature in different holes and in different parts of a hole can account for the variable brightness temperatures. They also noted that the influence of these parameters is strongest in C and K bands, where we can operate.
• Significant spectral variations and/or the formation of a local maximum in the microwave spectrum of solar active regions are an important factor in predicting powerful flares (within 24-48 hours from the event) as suggested by RATAN-600 observations (see e.g. [16]). A frequent monitoring of the solar disk with Medicina could provide a valuable forecasting probe for flare events, contributing to the precious Space Weather warning/alert network. This service could reserve to the Medicina station a relevant role in international Space Weather forecasting. A monitoring campaign including two frequency at the low/high edge of the K-band could provide spectral index estimates for the active regions, and trigger anomalous spectral variations (e.g. sudden flattening) as a signature anticipating the flaring process. Empirical and physical models of spectral behaviour prodromal of an upcoming flare can be statistically investigated through our proposal that will be helpful to refine trigger/alert conditions for multi-wavelength NAPA/ToO observations aimed at flare detection.

The study of peculiar large-scale structures, such as coronal holes, loop systems, filaments, streamers and the coronal plateau, will benefit from systematic long-term observations. For example, interesting solar disk features as polar brightening could be investigated through our radio monitoring in K-band (see. e.g. [17][18]). The radio brightness in polar regions seems anti-correlated with solar activity as suggested by NoRH images at 17 GHz in the epoch range 1992-2013 (see e.g. [11][19]). This effect could be related to the fact that around solar activity minimum, polar regions are dominated by strong unipolar magnetic field that may enhance their brightness. For systematic studies of solar cycle dependence of polar brightening (as a probe of magnetic field variations during a solar cycle), long-term full-disk observations are required, and ideally covered by Medicina starting from the present phase of minimum solar activity.

On longer time scales, the total K-band radio flux integrated over the whole disk is known to be an excellent index of solar activity. As suggested by Shibasaki et al. [18] radio butterfly diagrams show very well not only the migration of active regions toward the equator, but also the long-term behavior of polar brigthening. Selhorst et al. [20] demonstrated that the statistics of the number of active regions (NAR) observed at 17 GHz with NoRH, and used as solar activity index, is more sensitive to weaker magnetic fields than those necessary to form sunspot. NAR minima are shorter than those of the sunspot number (SSN) and other activity index. This could reflect the presence of active regions generated by faint magnetic fields or spotless regions, which are a considerable fraction of the counted active regions. It is thus important that such observations continue for the decades to come.

Long-term diachronic observations of the solar disk in K-band are manifestly a key element of solar radio science, and Medicina 32m could have a leading role in this context. In a first phase of a radio monitoring campaign we could mostly focus on the assessment of the temperature brightness of the quiet Sun component, exploiting the present minimum of solar activity. Major radio active regions (commonly present even in this solar phase) can be then detected and monitored on a weekly basis contributing to the multi-wavelength assessment of their spectral and variability parameters, including Space Weather forecasting issues.

The Medicina results about solar active regions monitoring in the radio continuum could be coupled with the information form other solar observatories and trigger high-resolution spectro-polarimetric NAPA/ToO with other instrumentation in selected regions (see Coordinated Observations).

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