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Coronal holes on the Sun have been observed at individual frequencies for quite a long time in the wavelength range from radio to X-rays. Observations in a wide range of radio frequencies are carried out with the RATAN-600 radio telescope. An analysis of long-term spectral observations of the RATAN-600 radio telescope showed that the emission spectrum of coronal holes is radically different from the spectrum of active formations above sunspots, but, differing markedly from the spectrum of the quiet Sun, it also has similarities with it. It has been established that the radio emission of coronal holes has an adiabatic spectrum and does not contain noticeable coherent radiation, that is, recombination radio lines and lines of the fine structure of hydrogen and other elements.

Radio observations play a very important role in understanding the structure of the solar atmosphere. In this paper the quiet sun component of the solar radio emission has been investigated using data obtained from the Solar Indices Bulletin, National Geophysical Data Centre. By statistical method, the quiet sun component is estimated for 84 successive basic periods containing three solar rotations each using data obtained at different frequencies. From the quiet sun component we estimate the brightness temperature in each observing frequency.

The heating of the solar chromosphere and corona to the observed high temperatures, imply the presence of ongoing heating that balances the strong radiative and thermal conduction losses expected in the solar atmosphere. It has been theorized for decades that the required heating mechanisms of the chromospheric and coronal parts of the active regions, quiet-Sun, and coronal holes are associated with the solar magnetic fields. However, the exact physical process that transport and dissipate the magnetic energy which ultimately leads to the solar plasma heating are not yet fully understood. The current understanding of coronal heating relies on two main mechanism: reconnection and MHD waves that may have various degrees of importance in different coronal regions. In this review we focus on recent advances in our understanding of MHD wave heating mechanisms. First, we focus on giving an overview of observational results, where we show that different wave modes have been discovered in the corona in the last decade, many of which are associated with a significant energy flux, either generated in situ or pumped from the lower solar atmosphere. Afterwards, we summarise the recent findings of numerical modelling of waves, motivated by the observational results. Despite the advances, only 3D MHD models with Alfvén wave heating in an unstructured corona can explain the observed coronal temperatures compatible with the quiet Sun, while 3D MHD wave heating models including cross-field density structuring are not yet able to account for the heating of coronal loops in active regions to their observed temperature.

The quiet Sun is the region of the solar surface outside of sunspots, pores, and plages. In continuum intensity it appears dominated by granular convection. However, in polarized light the quiet Sun exhibits impressive magnetic activity on a broad range of scales, from the 30,000 km of supergranular cells down to the smallest magnetic features of about 100 km resolvable with current instruments. Quiet Sun fields are observed to evolve in a coherent way, interacting with each other as they are advected by the horizontal photospheric flows. They appear and disappear over surprisingly short time scales, bringing large amounts of magnetic flux to the solar surface. For this reason they may be important contributors to the heating of the chromosphere. Peering into such fields is difficult because of the weak signals they produce, which are easily affected, and even completely hidden, by photon noise. Thus, their evolution and nature remain largely unknown. In recent years the situation has improved thanks to the advent of high-resolution, high-sensitivity spectropolarimetric measurements and the application of state-of-the-art Zeeman and Hanle effect diagnostics. Here we review this important aspect of solar magnetism, paying special attention to the techniques used to observe and characterize the fields, their evolution on the solar surface, and their physical properties as revealed by the most recent analyses. We identify the main open questions that need to be addressed in the future and offer some ideas on how to solve them.

In this article we review small-scale dynamo processes that are responsible for magnetic field generation on scales comparable to and smaller than the energy carrying scales of turbulence. We provide a review of critical observation of quiet Sun magnetism, which have provided strong support for the operation of a small-scale dynamo in the solar photosphere and convection zone. After a review of basic concepts we focus on numerical studies of kinematic growth and non-linear saturation in idealized setups, with special emphasis on the role of the magnetic Prandtl number for dynamo onset and saturation. Moving towards astrophysical applications we review convective dynamo setups that focus on the deep convection zone and the photospheres of solar-like stars. We review the critical ingredients for stellar convection setups and discuss their application to the Sun and solar-like stars including comparison against available observations.

We detect and analyze the oscillatory behavior of waves using a coronal seismology tool on sequences of coronal images. We study extreme-ultraviolet image sequences of active and quiet Sun regions and of coronal holes we identify 3- and 5-minute periodicities. In each studied region the 3- and 5-minute periodicities are similarly frequent. The number of pixels exhibiting a 3-minute periodicity is between 6 % – 8 % and those pixels exhibiting a 5-minute periodicity is between 5 % – 9 % of the total number of observed pixels. Our results show 3-minute oscillations along coronal loop structures but do not show 5-minute oscillations along these same loop structures. The number of pixels exhibiting 3- and 5-minute periodicities in one type of region (active Sun, quiet Sun, and coronal holes) is roughly the same for all observed regions, leading us to infer that the 3- and 5-minute oscillations are the result of a global mechanism.

Coronal holes are large-scale structures in the solar atmosphere that feature a reduced temperature and density in comparison to the surrounding quiet Sun and are usually associated with open magnetic fields. We perform a differential emission measure analysis on the 707 non-polar coronal holes in the Collection of Analysis Tools for Coronal Holes (CATCH) catalog to derive and statistically analyze their plasma properties (i.e. temperature, electron density, and emission measure). We use intensity filtergrams of the six coronal EUV filters from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory , which cover a temperature range from ≈ 10 5.5 to 10 7.5 K . Correcting the data for stray and scattered light, we find that all coronal holes have very similar plasma properties with an average temperature of 0.94 ± 0.18 MK , a mean electron density of ( 2.4 ± 0.7 ) × 10 8 cm − 3 , and a mean emission measure of ( 2.8 ± 1.6 ) × 10 26 cm − 5 . The temperature distribution within the coronal holes was found to be largely uniform, whereas the electron density shows a 30 to 40% linear decrease from the boundary towards the inside of the coronal hole. At distances greater than 20″ ( ≈ 15 Mm ) from the nearest coronal hole boundary, the density also becomes statistically uniform. The coronal hole temperature may show a weak solar-cycle dependency, but no statistically significant correlation of plasma properties with solar-cycle variations could be determined throughout the observed period between 2010 and 2019.

Nonthermal radio transients from the quiet Sun have been recently discovered and it has been hypothesized using rough calculations that they might be important for coronal heating. It is well realized that energy calculations using coherent emissions are often subject to poorly constrained parameters and hence have large uncertainties. However, energy estimates using observations in the extreme ultraviolet (EUV) and soft X-ray bands are routinely done and the techniques are pretty well established. This work presents the first attempt to identify the EUV counterparts of these radio transients and then use them to estimate the energy deposited into the corona during the event. I show that the group of radio transients studied here is associated with a brightening observed in the EUV waveband and is produced by an energy release of ≈ 10 25  ergs. The fact that the flux density of the radio transient is only ≈ 2  mSFU suggests that it might be possible to do large statistical studies in the future for understanding the relationship between these radio transients and other EUV and X-ray counterparts, as well as for understanding their importance in coronal heating.

Observations of the Sun at millimeter and submillimeter wavelengths offer a unique probe into the structure, dynamics, and heating of the chromosphere; the structure of sunspots; the formation and eruption of prominences and filaments; and energetic phenomena such as jets and flares. High-resolution observations of the Sun at millimeter and submillimeter wavelengths are challenging due to the intense, extended, low-contrast, and dynamic nature of emission from the quiet Sun, and the extremely intense and variable nature of emissions associated with energetic phenomena. The Atacama Large Millimeter/submillimeter Array (ALMA) was designed with solar observations in mind. The requirements for solar observations are significantly different from observations of sidereal sources and special measures are necessary to successfully carry out this type of observations. We describe the commissioning efforts that enable the use of two frequency bands, the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum interferometric-imaging observations of the Sun with ALMA. Examples of high-resolution synthesized images obtained using the newly commissioned modes during the solar-commissioning campaign held in December 2015 are presented. Although only 30 of the eventual 66 ALMA antennas were used for the campaign, the solar images synthesized from the ALMA commissioning data reveal new features of the solar atmosphere that demonstrate the potential power of ALMA solar observations. The ongoing expansion of ALMA and solar-commissioning efforts will continue to enable new and unique solar observing capabilities.

The magnetic field of the low corona above quiet Sun regions is extremely challenging to observe directly, and the topology is difficult to discern from extreme ultraviolet (EUV) image data due to the lack of distinct loops that are present in, for example, active regions. We aim to show that the velocity field of faint propagating disturbances (PD) observed on-disk in the quiet corona can be interpreted in terms of the underlying magnetic topology. The PD are observed in Atmospheric Imaging Assembly/Solar Dynamics Observatory (AIA/SDO) time series in three channels: 304, 171, and 193 Å corresponding to the high chromosphere, transition region/low corona, and the corona, respectively. An established Time-Normalised Optical Flow method enhances the PD and applies a Lucas–Kanade algorithm to gain their velocity field. From the velocity field, we identify the source and sink locations of the PDs, and compare these locations between channels and with the underlying photospheric network. Source regions tend to be located above the photospheric network, and sink regions with the internetwork. Sink regions in the internetwork suggest either that closed field can be concentrated rather than evenly distributed in the internetwork, or that fieldlines opening into the corona can sometimes be concentrated above internetwork regions. We find regions of almost exact alignment between channels, and other regions where similar-shaped structures are offset by a few pixels between channels. These are readily interpreted as vertical or non-vertical alignment of the magnetic field relative to the observer viewing from above. Regions of isolated source regions in the cold (304 Å) or hotter (171 and 193 Å) channels can be interpreted in terms of the magnetic topology, but support for this is weaker. These results offer support for the future use of PD velocity fields as a coronal constraint on magnetic extrapolation models.