Societal dependence on space-based services demands major advances in predicting the impacts of eruptive solar events. Millimeter-wavelength observations offer uniquely direct access to the time-dependent physical conditions in the atmospheric layers of the Sun where these events originate. A facility capable of full-disk, high-cadence, multi-frequency imaging would provide a transformative view of the Sun and its influence on the heliosphere. AtLAST is ideally suited to deliver this capability, and to establish a European leadership role in advancing the scientific foundations that will enable reliable, operational space-weather forecasting for the first time.

Clette recently showed that F _10.7 systematically approaches a quiet Sun daily value of 67 solar flux units (sfu) at solar minima as the number of spotless days on the Sun increases. Previously, a floor of ∼2.8 nT had been proposed for the solar wind (SW) magnetic field strength (B). F _10.7 , which closely tracks the Sun's unsigned photospheric magnetic flux, and SW B exhibit different relationships to their floors at 11 yr solar minima during the last ∼50 yr. While F _10.7 approaches 67 sfu at each minimum, the corresponding SW B is offset above ∼2.8 nT by an amount approximately proportional to the solar polar field strength—which varied by a factor of ∼2.5 during this interval. This difference is substantiated by ∼130 yr of reconstructed F _10.7 (via the range of the diurnal variation of the East-component (rY) of the geomagnetic field) and SW B (based on the interdiurnal variability geomagnetic activity index). For the last ∼60 yr, the contribution of the slow SW to SW B has exhibited a floor-like behavior at ∼2 nT, in contrast to the contributions of coronal mass ejections and high-speed streams that vary with the solar cycle. These observations, as well as recent SW studies based on Parker Solar Probe and Solar Dynamics Observatory data, suggest that (1) the Sun has a small-scale turbulent dynamo that is independent of the 11 yr sunspot cycle; and (2) the small-scale magnetic fields generated by this nonvarying turbulent dynamo maintain a constant open flux carried to the heliosphere by the Sun's floor-like slow SW.

Extreme solar radio bursts can impact several areas of human activity, but there remain many gaps in our understanding of what leads to their occurrence.This paper discusses such events, focusing on three different emission mechanisms, which correspond to three different frequency ranges.Bright decimetric bursts in particular are dangerous because they could affect the use of global navigation systems for landing commercial passenger aircraft.The properties of extreme bursts are discussed, and shortcomings in our understanding of such events are presented.

The solar corona and upper chromosphere represent an important and unique opportunity for studying universal physical processes occurring in astrophysical plasmas.In particular, energy stored in the magnetic field in the solar atmosphere above active regions is a key driver of solar activity including particle acceleration and transport in solar flares and coronal mass ejections, some of which can have a profound effect on Earth.Yet, quantitative measurements of the coronal and chromospheric magnetic field is currently in its infancy.However, a number of important diagnostics of coronal and chromospheric magnetic fields have recently been developed that must be exploited in the next decade, and these include well-understood techniques offered through ultra-broadband imaging spectropolarimetry at radio wavelengths.To capitalize on these developments requires the superior broadband microwave imaging for which the Frequency Agile Solar Radiotelescope (FASR) has been strongly endorsed by several past decadal surveys.

FOXSI is a direct-imaging, hard X-ray (HXR) telescope optimized for solar flare observations. It detects hot plasma and energetic electrons in and near energy release sites in the solar corona via bremsstrahlung emission, measuring both spatial structure and particle energy distributions. It provides two orders of magnitude faster imaging spectroscopy than previously available, probing physically relevant timescales (<1s) never before accessible to address fundamental questions of energy release and efficient particle acceleration that have importance far beyond their solar application (e.g., planetary magnetospheres, flaring stars, accretion disks). FOXSI measures not only the bright chromospheric X-ray emission where electrons lose most of their energy, but also simultaneous emission from electrons as they are accelerated in the corona and propagate along magnetic field lines. FOXSI detects emission from high in the tenuous corona, where previous instruments have been blinded by nearby bright features and will fully characterizes the accelerated electrons and hottest plasmas as they evolve in energy, space, and time to solve the mystery of how impulsive energy release leads to solar eruptions, the primary drivers of space weather at Earth, and how those eruptions are energized and evolve.

The question of why the solar corona is much hotter than the visible solar surface still puzzles solar researchers.Most theories of coronal heating involve a tight coupling between the coronal magnetic field and the associated thermal structure.This coupling is based on two facts: (i) the magnetic field is the main source of free energy in the corona and (ii) heat transfer by charged particles preferentially happens along the magnetic field, while is strongly suppressed across it.However, much of the information about coronal heating is derived from the analysis of extreme ultraviolet (EUV) emission, which is not explicitly sensitive to the magnetic field.Rather, the EUV emission, dominated by ions' spectral lines, is sensitive to elemental abundances, which are known to vary in space and time in the ever-changing corona.Quantification of the abundances requires EUV spectroscopic measurements, which are not routinely available.Thus, to interpret EUV data from narrowband imagers, a pre-defined set of coronal abundances is used, which introduces poorly quantified uncertainties.This White Paper emphasizes the new utility of complementing the EUV data with spatially resolved microwave data to determine at the same time the relationship of the magnetic field with plasma parameters, and the composition of the solar corona.The microwave emission from the corona is formed by two emission processes: free-free and gyroresonant emissions.The former is only weakly sensitive to the elemental abundances, and the latter not at all, which allows the radio emission to serve as a standard against which to measure abundance-related emissivity differences.The radio mechanisms also have the advantage that they are both explicitly sensitive to the magnetic field.Therefore, joint microwave and EUV observations can be used to measure both elemental abundances, thermal structure, and the coronal magnetic field.This task of understanding the abundance problem and constraining the poorly known Fe/H abundance ratio requires high spatial and spectral resolutions in both data sets-EUV and microwave.

Understanding the nature of energetic particles in the solar atmosphere is one of the most important outstanding problems in heliophysics. Flare-accelerated particles compose a huge fraction of the flare energy budget; they have large influences on how events develop; they are an important source of high-energy particles found in the heliosphere; and they are the single most important corollary to other areas of high-energy astrophysics. Despite the importance of this area of study, this topic has in the past decade received only a small fraction of the resources necessary for a full investigation. For example, NASA has selected no new Explorer-class instrument in the past two decades that is capable of examining this topic. The advances that are currently being made in understanding flare-accelerated electrons are largely undertaken with data from EOVSA (NSF), STIX (ESA), and NuSTAR (NASA Astrophysics). This is despite the inclusion in the previous Heliophysics decadal survey of the FOXSI concept as part of the SEE2020 mission, and also despite NASA's having invested heavily in readying the technology for such an instrument via four flights of the FOXSI sounding rocket experiment. Due to that investment, the instrumentation stands ready to implement a hard X-ray mission to investigate flare-accelerated electrons. This white paper describes the scientific motivation for why this venture should be undertaken soon.

Particle acceleration and particle transport are ubiquitous in astrophysics.The Sun offers an astrophysical laboratory to study these and other fundamental processes in minute detail.The physical context on the Sun involves solar flares and coronal mass ejections (CMEs).These complex coupled phenomena require comprehensive and complementary observations to disentangle the relevant physical mechanisms at work.Radio observations are emphasized here because they are positioned to make unique and innovative contributions to these important problems, especially in the current era when hard X-ray and gamma-ray spectral imaging capability is lacking.In particular, the transformative technique of ultra-broadband radio imaging spectroscopy is discussed, which serves as the observational basis for new insights into particle acceleration and transport.These include the dynamic measurement of coronal magnetic fields, the measurement of the spatiotemporal evolution of the electron distribution function, and the observation of flares and CMEs as coupled systems.

The solar corona, even during the most quiescent periods, is extremely dynamic and hosts numerous small-scale transient phenomena.The latter is thought to be one of the most probable mechanisms to heat the solar corona to million degrees.Many studies have been done to try to detect and characterize the transients happening in the non-flaring, quiescent solar corona.However, most of these studies probe the thermal aspects of these emissions, although there has been a long-standing, while largely unproven, belief that these transients may have a significant nonthermal component-a key element in Gene Parker's "nanoflare" coronal heating scenario.While it has long been realized that radio observations are an excellent probe to detect and ultimately characterize the nonthermal component of the transients, at present, the instrumentation available does not have sufficient image fidelity, dynamic range, and resolution for measuring the weak transients against the solar disk.Recently, instruments such as the Expanded Owens Valley Solar Array (EOVSA), Jansky Very Large Array (VLA), and Murchison Widefield Array (MWA), which realize this technical requirement partially have produced very interesting results that suggest nonthermal emissions from the quiet solar corona.However, due to various reasons mentioned in this white paper, such studies are mainly limited to small flares and the brightest population of these weak transients against the quiet sun disk.Hence, to unleash the full potential of radio observations and revolutionize our understanding on this topic, we need a new instrument that produces data suitable for high-resolution, high-sensitivity, high-fidelity snapshot radio imaging over a wide frequency band.Using a simulated 200-element array configuration with a dense u-v coverage, based on thermal and nonthermal properties of these nanoflare-like transients extrapolated from their more energetic "cousins", we demonstrate that achieving this goal is indeed highly likely with such a next-generation solar radio facility as the Frequency Agile Solar Radiotelescope.

Identification of the mechanisms responsible for heating the solar chromosphere and corona remains an outstanding problem, one of great relevance to late-type stars as well.There has been tremendous progress in the past decade, largely driven by new instruments, new observations, and sophisticated modeling efforts.Despite this progress, gaps remain.With dynamic broadband imaging spectropolarimetry, radio observations enable key diagnostics of the 3D distribution of temperature, density, and magnetic field of the solar atmosphere from the chromosphere to mid-coronal heights, and their connection to the solar wind outflow, as a function of time.These measurements can be made from tiny microflare/nanoflare scales up to global scales.We briefly discuss the need for radio coverage of the 3D solar atmosphere and discuss the requirements.

The solar chromosphere is heated to temperatures higher than predicted by radiative equilibrium. This excess heating is greater in active regions where the magnetic field is stronger. We aim to investigate the magnetic topology associated with an area of enhanced millimeter (mm) brightness temperatures in a solar active region mapped by the Atacama Large Millimeter/submillimeter Array (ALMA) using spectropolarimetric co-observations with the 1-m Swedish Solar Telescope (SST). We used Milne-Eddington inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions, and a magnetohydrostatic extrapolation to obtain constraints on the three-dimensional stratification of temperature, magnetic field, and radiative energy losses. We compared the observations to a snapshot of a magnetohydrodynamics simulation and investigate the formation of the thermal continuum at 3 mm using contribution functions. We find enhanced heating rates in the upper chromosphere of up to $\sim 5\rm\,kW\,m^{-2}$, where small-scale emerging loops interact with the overlying magnetic canopy leading to current sheets as shown by the magnetic field extrapolation. Our estimates are about a factor of two higher than canonical values, but they are limited by the ALMA spatial resolution ($\sim 1.2^{\prime\prime}$). Band 3 brightness temperatures reach about $\sim10^{4}\,$K in the region, and the transverse magnetic field strength inferred from the non-LTE inversions is on the order of $\sim 500\,$G in the chromosphere. We are able to quantitatively reproduce many of the observed features, including the integrated radiative losses in our numerical simulation. We conclude that the heating is caused by dissipation in current sheets. However, the simulation shows a complex stratification in the flux emergence region where distinct layers may contribute significantly to the emission in the mm continuum.

Quiescent filaments appear as absorption features on the solar disk when observed in chromospheric lines and at continuum wavelengths in the millimeter (mm) range. Active region (AR) filaments are their small-scale, low-altitude analogues, but they could not be resolved in previous mm observations. This spectral diagnostic can provide insight into the details of the formation and physical properties of their fine threads, which are still not fully understood. Here, we shed light on the thermal structure of an AR filament using high-resolution brightness temperature ( T b ) maps taken with ALMA Band 6 complemented by simultaneous IRIS near-UV spectra, Hinode/SOT photospheric magnetograms, and SDO/AIA extreme-UV images. Some of the dark threads visible in the AIA 304 Å passband and in the core of Mg ii resonance lines have dark ( T b < 5,000 K) counterparts in the 1.25 mm maps, but their visibility significantly varies across the filament spine and in time. These opacity changes are possibly related to variations in temperature and electron density in filament fine structures. The coolest T b values ( < 5,000 K) coincide with regions of low integrated intensity in the Mg ii h and k lines. ALMA Band 3 maps taken after the Band 6 ones do not clearly show the filament structure, contrary to the expectation that the contrast should increase at longer wavelengths based on previous observations of quiescent filaments. The ALMA maps are not consistent with isothermal conditions, but the temporal evolution of the filament may partly account for this.

Motivated by dark coronal lanes in 284 Å extreme-ultraviolet (EUV) observations from the Extreme-ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory (SOHO/EIT), we construct and optimize an atmosphere model of the active region (AR) 8535 sunspot by adding a cool and dense component in the volume of plasma along open field lines determined using the potential-field source-surface (PFSS) extrapolation. Our model qualitatively reproduces the observed reduced microwave brightness temperature in the northern part of the sunspot in Very Large Array (VLA) observations from 13 May 1999 and provides a physical explanation for the coronal dark lanes. We propose the application of this method to other sunspots with such observed dark regions in the EUV or soft X-rays and with concurrent microwave observations to determine the significance of open field regions. The connection between open fields and the resulting plasma temperature and density change is of relevance for slow solar wind source investigations.

By direct measurements of the gas temperature, the Atacama Large Millimeter/submillimeter Array (ALMA) has yielded a new diagnostic tool to study the solar chromosphere. Here, we present an overview of the brightness-temperature fluctuations from several high-quality and high-temporal-resolution (i.e. 1 and 2 s cadence) time series of images obtained during the first 2 years of solar observations with ALMA, in Band 3 and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz), respectively. The various datasets represent solar regions with different levels of magnetic flux. We perform fast Fourier and Lomb–Scargle transforms to measure both the spatial structuring of dominant frequencies and the average global frequency distributions of the oscillations (i.e. averaged over the entire field of view). We find that the observed frequencies significantly vary from one dataset to another, which is discussed in terms of the solar regions captured by the observations (i.e. linked to their underlying magnetic topology). While the presence of enhanced power within the frequency range 3–5 mHz is found for the most magnetically quiescent datasets, lower frequencies dominate when there is significant influence from strong underlying magnetic field concentrations (present inside and/or in the immediate vicinity of the observed field of view). We discuss here a number of reasons which could possibly contribute to the power suppression at around 5.5 mHz in the ALMA observations. However, it remains unclear how other chromospheric diagnostics (with an exception of H α line-core intensity) are unaffected by similar effects, i.e. they show very pronounced 3-min oscillations dominating the dynamics of the chromosphere, whereas only a very small fraction of all the pixels in the 10 ALMA datasets analysed here show peak power near 5.5 mHz. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’.

Aims. We aim to investigate the temperature enhancements and formation heights of solar active-region brightenings such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric observations in the millimeter (mm) continuum provided by the Atacama Large Millimeter/submillimeter Array (ALMA). Methods. We examined 3 mm signatures of heating events identified in Solar Dynamics Observatory observations of an active region and compared the results with synthetic spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the contribution from the corona to the mm brightness using differential emission measure analysis. Results. We report the null detection of EBs in the 3 mm continuum at ∼1.2″ spatial resolution, which is evidence that they are sub-canopy events that do not significantly contribute to heating the upper chromosphere. In contrast, we find the active region to be populated with multiple compact, bright, flickering mm-bursts – reminiscent of UVBs. The high brightness temperatures of up to ∼14 200 K in some events have a contribution (up to ∼7%) from the corona. We also detect FAF-like events in the 3 mm continuum. These events show rapid motions of > 10 kK plasma launched with high plane-of-sky velocities (37 − 340 km s−1) from bright kernels. The mm FAFs are the brightest class of warm canopy fibrils that connect magnetic regions of opposite polarities. The simulation confirms that ALMA should be able to detect the mm counterparts of UVBs and small flares and thus provide a complementary diagnostic for localized heating in the solar chromosphere.

In this work we use solar observations with the ALMA radio telescope at the wavelength of 1.21 mm. The aim of the analysis is to improve understanding of the solar chromosphere, a dynamic layer in the solar atmosphere between the photosphere and corona. The study has an observational and a modeling part. In the observational part full-disc solar images are analyzed. Based on a modified FAL atmospheric model, radiation models for various observed solar structures are developed. Finally, the observational and modeling results are compared and discussed.

The authors of this report met on 28-30 March 2018 at the New Jersey Institute of Technology, Newark, New Jersey, for a 3-day workshop that brought together a group of data providers, expert modelers, and computer and data scientists, in the solar discipline. Their objective was to identify challenges in the path towards building an effective framework to achieve transformative advances in the understanding and forecasting of the Sun-Earth system from the upper convection zone of the Sun to the Earth's magnetosphere. The workshop aimed to develop a research roadmap that targets the scientific challenge of coupling observations and modeling with emerging data-science research to extract knowledge from the large volumes of data (observed and simulated) while stimulating computer science with new research applications. The desire among the attendees was to promote future trans-disciplinary collaborations and identify areas of convergence across disciplines. The workshop combined a set of plenary sessions featuring invited introductory talks and workshop progress reports, interleaved with a set of breakout sessions focused on specific topics of interest. Each breakout group generated short documents, listing the challenges identified during their discussions in addition to possible ways of attacking them collectively. These documents were combined into this report-wherein a list of prioritized activities have been collated, shared and endorsed.

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.