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Using photospheric vector magnetograms obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and a magnetic connectivity-based method, we compute the magnetic helicity and free magnetic energy budgets of a simple bipolar solar active region (AR) during its magnetic flux emergence phase which lasted $\sim$47 hrs. The AR did not produce any coronal mass ejections (CMEs) or flares with an X-ray class above C1.0 but it was the site of 60 jet events during its flux emergence phase. The helicity and free energy budgets of the AR were below established eruption-related thresholds throughout the interval we studied. However, in addition to their slowly-varying evolution, each of the time profiles of the helicity and free energy budgets showed discrete localized peaks, with eight pairs of them occurring at times of jets emanating from the AR. These jets featured larger base areas and longer durations than the other jets of the AR. We estimated, for the first time, the helicity and free magnetic energy changes associated with these eight jets which were in the ranges of $0.5-7.1 \times 10^{40}$ Mx$^2$ and $1.1-6.9 \times 10^{29}$ erg, respectively. Although these values are one to two orders of magnitude smaller than those usually associated with CMEs, the relevant percentage changes were significant and ranged from 13% to 76% for the normalized helicity and from 9% to 57% for the normalized free magnetic energy. Our study indicates that occasionally jets may have a significant imprint in the evolution of helicity and free magnetic energy budgets of emerging active regions., Comment: A&A (Letters), in press

The R-value is a measure of the strength of photospheric magnetic Polarity Inversion Lines (PILs) in Active Regions (ARs). This work investigates the possibility of a relation between R-value variations and the occurrence of X-class flares in ARs, not in the solar photosphere, as usual, but above it in regions, closer to where flares occur. The modus operandi is to extrapolate the Solar Dynamic Observatory's (SDO) Helioseismic and Magnetic Imager (HMI) magnetogram data up to a height of 3.24 Mm above the photosphere and then compute the R-value based on the extrapolated magnetic field. Recent studies have shown that certain flare-predictive parameters such as the horizontal gradient of the vertical magnetic field and magnetic helicity may improve flare prediction lead times significantly if studied at a specific height range above the photosphere, called the Optimal Height Range (OHR). Here we define the OHR as a collection of heights where a sudden but sustained increase in R-value is found. For the eight case studies discussed in this paper, our results indicate that it is possible for OHRs to exist in the low solar atmosphere (between 0.36 - 3.24 Mm), where R-value spikes occur 48-68 hrs before the first X-class flare of an emerging AR. The temporal evolution of R-value before the first X-class flare for an emerging AR is also found to be distinct from that of non-flaring ARs. For X-class flares associated with non-emerging ARs, an OHR could not be found., Comment: 25 pages, 15 figures, 13 tables

We estimate the near-Sun axial magnetic field of a coronal mass ejection (CME) on 10 March 2022. Solar Orbiter's in situ measurements, 7.8 degrees east of the Sun-Earth line at 0.43 AU, provided a unique vantage point, along with the WIND measurements at 0.99 AU. We determine a single power-law index from near-Sun to L1, including in situ measurements from both vantage points. We tracked the temporal evolution of the instantaneous relative magnetic helicity of the source active region (AR), NOAA AR 12962. By estimating the helicity budget of the pre-and post-eruption AR, we estimated the helicity transported to the CME. Assuming a Lundquist flux-rope model and geometrical parameters obtained through the graduated cylindrical shell (GCS) CME forward modelling, we determined the CME axial magnetic field at a GCS-fitted height. Assuming a power-law variation of the axial magnetic field with heliocentric distance, we extrapolated the estimated near-Sun axial magnetic field to in situ measurements at 0.43 AU and 0.99 AU. The net helicity difference between the post-and pre-eruption AR is $(-7.1 \pm 1.2) \times 10^{41} \mathrm{Mx^{2}}$, which is assumed to be bodily transported to the CME. The estimated CME axial magnetic field at a near-Sun heliocentric distance of 0.03 AU is 2067 $\pm$ 405 nT. From 0.03 AU to L1, a single power-law falloff, including both vantage points at 0.43 AU and 0.99 AU, gives an index $-1.23 \pm 0.18$. We observed a significant decrease in the pre-eruptive AR helicity budget. Extending previous studies on inner-heliospheric intervals from 0.3 AU to $\sim$1 AU, referring to estimates from 0.03 AU to measurements at $\sim$1 AU. Our findings indicate a less steep decline in the magnetic field strength with distance compared to previous studies, but they align with studies that include near-Sun in situ magnetic field measurements, such as from Parker Solar Probe.

Context. Relative field line helicity (RFLH) is a recently developed quantity which can approximate the density of relative magnetic helicity. Aims. This paper aims to determine whether RFLH can be used as an indicator of solar eruptivity. Methods. Starting from magnetographic observations from the Helioseismic and Magnetic Imager instrument onboard the Solar Dynamic Observatory of a sample of seven solar active regions (ARs), which comprises over 2000 individual snapshots, we reconstruct the AR's coronal magnetic field with a widely-used non-linear force-free method. This enables us to compute RFLH using two independent gauge conditions for the vector potentials. We focus our study around the times of strong flares in the ARs, above the M class, and in regions around the polarity inversion lines (PILs) of the magnetic field, and of RFLH. Results. We find that the temporal profiles of the relative helicity that is contained in the magnetic PIL follow those of the relative helicity that is computed by the accurate volume method for the whole AR. Additionally, the PIL relative helicity can be used to define a parameter similar to the well-known parameter R (Schrijver 2007), whose high values are related with increased flaring probability. This helicity-based R-parameter correlates well with the original one, showing in some cases even higher values, and additionally, it experiences more pronounced decreases during flares. This means that there exists at least one parameter deduced from RFLH, that has important value as a solar eruptivity indicator., Comment: accepted by Astronomy & Astrophysics

In a previous work we analysed the white-light coronal brightness as a function of elongation and time from Wide-Field Imager (WISPR) observations on board the Parker Solar Probe (PSP) mission when PSP reached a minimum heliocentric distance of ~ 28 Rs. We found 4-5 transient outflows per day over a narrow wedge in the PSP orbital plane, which is close to the solar equatorial plane. However, the elongation versus time map (J-map) analysis supplied only lower limits on the number of released density structures due to the small spatial-scales of the transient outflows and line-of-sight integration effects. In this work we place constraints on the properties of slow solar wind transient mass release from the entire solar equatorial plane. We simulated the release and propagation of transient density structures in the solar equatorial plane for four scenarios: (1) periodic release in time and longitude with random speeds; (2) corotating release in longitude, periodic release in time with random speeds; (3) random release in longitude, periodic release in time and speed; and (4) random release in longitude, time, and speed. The simulations were used in the construction of synthetic J-maps, which are similar to the observed J-map. The four considered scenarios have similar ranges (35-45 for the minimum values and 96-127 for the maximum values) of released density structures per day from the solar equatorial plane and consequently from the streamer belt, given its proximity to the solar equatorial plane during the WISPR observation. Our results also predict that density structures with sizes in the range 2-8 Rs, covering 1-20 % of the perihelion could have been detectable by PSP in situ observations during that interval., Comment: A&A, 2023, in press

Using vector magnetograms from the HMI/SDO and a magnetic connectivity-based method, we calculate the instantaneous relative magnetic helicity and free magnetic energy budgets for several days in two solar active regions (ARs), AR11890 and AR11618, both with complex photospheric magnetic field configurations. The ARs produced several major eruptive flares while their photospheric magnetic field exhibited primarily flux decay in AR11890 and primarily flux emergence in AR11618. Throughout much of their evolution both ARs featured substantial budgets of free magnetic energy and of both positive and negative helicity. In fact, the imbalance between the signed components of their helicity was as low as in the quiet Sun and their net helicity eventually changed sign 14-19 hours after their last major flare. Despite such incoherence, the eruptions occurred at times of net helicity peaks that were co-temporal with peaks in the free magnetic energy. The losses associated with the eruptive flares in the normalized free magnetic energy were in the range 10-60%. For the helicity, changes ranged from 25% to the removal of the entire excess helicity of the prevailing sign, leading a roughly zero net helicity, but with significant equal and opposite budgets of both helicity senses. The removal of the slowly varying background component of the free energy and helicity timeseries revealed that all eruption-related peaks of both quantities exceeded the 2$\sigma$ levels of their detrended timeseries. There was no eruption when only one or none of these quantities exceeded its 2$\sigma$ level. Our results indicate that differently evolving ARs may produce major eruptive flares even when, in addition to the accumulation of significant free magnetic energy budgets, they accumulate large amounts of both negative and positive helicity without a strong dominance of one handedness over the other., Comment: 17 pages, 11 figures, accepted for publication in Astronomy & Astrophysics (abbreviated abstract)

We present the first full-disk solar images obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 7 (0.86 mm; 347 GHz). In spite of the low spatial resolution (21"), several interesting results were obtained. During our observation, the sun was practically devoid of active regions. Quiet Sun structures on the disk are similar to those in Atmospheric Imaging Assembly (AIA) images at 1600 A and 304 A, after the latter are smoothed to the ALMA resolution, as noted previously for Band 6 (1.26 mm) and Band 3 (3 mm) images; they are also similar to negative H$\alpha$ images of equivalent resolution. Polar coronal holes, which are clearly seen in the 304 A band and small H$\alpha$ filaments, are not detectable at 0.86 mm. We computed the center-to-limb variation (CLV) of the brightness temperature, $T_b$, in Band 7, as well as in Bands 6 and 3, which were obtained during the same campaign, and we combined them to a unique curve of $T_b(\log\mu_{100})$, where $\mu_{100}$ is the cosine of the heliocentric angle reduced to 100 GHz. Assuming that the absolute calibration of the Band 3 commissioning observations is accurate, we deduced a brightness temperature at the center of the disk of 6085 K for Band 7, instead of the value of 5500 K, extrapolated from the recommended values for Bands 3 and 6. More importantly, the $T_b(\log\mu_{100})$ curve flattens at large values of $\mu_{100}$, and so does the corresponding $T_e(\log\tau_{100})$ at large $\tau_{100}$. This is probably an indication that we are approaching the temperature minimum., Comment: Astronomy and Astrophysics, accepted

We report the first detection of metric radio emission from a surge, observed with the Nan\c{c}ay Radioheliograph (NRH), STEREO and other instruments. The emission was observed during the late phase of the M9 complex event SOL2010-02-012T11:25:00, described in a previous publication and was associated with a secondary energy release, also observed in STEREO 304 {\AA} images: there was no detectable soft X-ray emission. Triangulation of the STEREO images allowed the identification of the surge with NRH sources near the central meridian. The radio emission of the surge occurred in two phases and consisted of two sources, one located near the base of the surge, apparently at or near the site of energy release, and another in the upper part of the surge; these were best visible in the frequency range of 445.0 to about 300MHz, whereas a spectral component of different nature was observed at lower frequencies. Sub-second time variations were detected in both sources during both phases, with 0.2-0.3s a delay of the upper source with respect to the lower, suggesting superluminal velocities. This effect can be explained if the emission of the upper source was due to scattering of radiation from the source at the base of the surge. In addition, the radio emission showed signs of pulsations and spikes. We discuss possible emission mechanisms for the slow time variability component of the lower radio source. Gyrosynchrotron emission reproduced fairly well the characteristics of the observed total intensity spectrum at the start of the second phase of the event, but failed to reproduce the high degree of the observed circular polarization as well the spectra at other instances. On the other hand, type IV-like plasma emission from the fundamental could explain the high polarization and the fine structure in the dynamic spectrum; moreover, it gives projected radio source positions on the plane of the sky, as seen from STEREO-A, near the base of the surge. Taking everything into consideration, we suggest type IV-like plasma emission with a low intensity gyrosynchrotron component as the most plausible mechanism., Comment: Ref: AA/2022/43169, accepted for publication in Astronomy & Astrophysics, the ABSTRACT has been shortened due to the 1920 Character Limit

Aims. We investigate the role of the accumulation of both magnetic helicity and magnetic energy in the generation of coronal mass ejections (CMEs) from emerging solar active regions (ARs). Methods. Using vector magnetic field data obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we calculate the magnetic helicity and magnetic energy injection rates as well as the resulting accumulated budgets in 52 emerging ARs from the start time of magnetic flux emergence until they reach heliographic longitude of 45$^{\circ}$ West (W45). Results. Seven of the ARs produced CMEs while 45 did not. In a statistical sense, the eruptive ARs accumulate larger budgets of both magnetic helicity and energy than the noneruptive ones over intervals that start from flux emergence start time and end (i) at the end of flux emergence phase, and (ii) when the AR produces its first CME or crosses W45, whichever happens first. We found magnetic helicity and energy thresholds of $9 \times 10^{41}$ Mx$^2$ and $2 \times 10^{32}$ erg, respectively, which, if crossed, ARs are likely to erupt. The segregation, in terms of accumulated magnetic helicity and energy budgets, of the eruptive ARs from the noneruptive ones is violated in one case when an AR erupts early in its emergence phase and in six cases with noneruptive ARs exhibiting large magnetic helicity and energy budgets. Decay index calculations may indicate that these ARs did not erupt because the overlying magnetic field provided stronger or more extended confinement than in eruptive ARs. Conclusions. Our results indicate that emerging ARs tend to produce CMEs when they accumulate significant budgets of both magnetic helicity and energy. Any study of their eruptive potential should place magnetic helicity on equal footing with magnetic energy., Comment: 13 pages, 11 figures, accepted for publication in Astronomy & Astrophysics

We have studied a complex metric radio event which originated in a compact flare, observed with the ARTEMIS-JLS radiospectro-graph on February 12, 2010. The event was associated with a surge observed at 195 and 304 {\AA} and with a coronal mass ejection observed by instruments on-board STEREO A and B near the East and West limbs respectively. On the disk the event was observed at 10 frequencies by the Nancay Radioheliograph, in Ha by the Catania observatory, in soft x-rays by GOES SXI and Hinode XRT and in hard x-rays by RHESSI. We combined these data, together with MDI longitudinal magnetograms, to get as complete a picture of the event as possible. Our emphasis is on two type-II bursts that occurred near respective maxima in the GOES light curves. The first, associated with the main peak of the event, showed an impressive F-H structure, while the emission of the second consisted of three well-separated bands with superposed pulsations. Using positional information for the type-IIs from the NRH and triangulation from STEREO A and B, we found that the type IIs were associated neither with the surge nor with the disruption of a nearby streamer, but rather with an EUV wave probably initiated by the surge. The fundamental-harmonic structure of the first type II showed a band split corresponding to a magnetic field strength of 18G, a frequency ratio of 1.95 and a delay of 0.23-0.65s of the fundamental with respect to the harmonic; moreover it became stationary shortly after its start and then drifted again. The pulsations superposed on the second type II were broadband and had started before the burst. In addition, we detected another pulsating source, also before the second type II, polarized in the opposite sense; the pulsations in the two sources were out of phase and hence hardly detectable in the dynamic spectrum. The pulsations had a measurable reverse frequency drift of about 2/s., Comment: Accepted for Publication in A&A, Article Number: aa41672-21

Using Atacama Large Millimeter/submillimeter Array (ALMA) observations of the quiet Sun at 1.26 and 3 mm, we study spatially resolved oscillations and transient brightenings, i.e. small, weak events of energy release. Both phenomena may have a bearing on the heating of the chromosphere. At 1.26 mm, in addition to power spectra of the original data, we degraded the images to the spatial resolution of the 3 mm images and used fields of view of equal area for both data sets. The detection of transient brightenings was made after the oscillations were removed. At both frequencies we detected p-mode oscillations in the range 3.6-4.4 mHz. In the corrected data sets, the oscillations at 1.26 and 3 mm showed brightness temperature fluctuations of ~1.7-1.8% with respect to the average quiet Sun, corresponding to 137 and 107 K, respectively. They represented a fraction of 0.55-0.68 of the full power spectrum and their energy density at 1.26 mm was 0.03 erg cm$^{-3}$. We detected 77 transient brightenings at 1.26 mm and 115 at 3 mm. Although the majority of the 1.26 mm events occurred in cell interior, their occurrence rate per unit area was higher than that of the 3 mm events. The computed low-end energy of the 1.26 mm transient brightenings ($1.8 \times 10^{23}$ erg) is among the smallest ever reported, irrespective of the wavelength of observation. However, their power per unit area is smaller than that of the 3 mm events, probably due to the detection of many weak 1.26 mm events. We also found that ALMA bright network structures corresponded to dark mottles/spicules seen in broadband H$\alpha$ images from the GONG network., Comment: A&A, accepted for publication (abbreviated abstract)

Context. Magnetic helicity is a physical quantity of great importance in the study of astrophysical and natural plasmas. Although a density for helicity cannot be defined, a good proxy for it is field line helicity. The appropriate quantity for use in solar conditions is relative field line helicity (RFLH). Aims. This work aims to study in detail the behaviour of RFLH, for the first time, in a solar active region (AR). Methods. The target active region is the large, eruptive AR 11158. In order to compute RFLH and all other quantities of interest we use a non-linear force-free reconstruction of the AR coronal magnetic field of excelent quality. Results. We find that the photospheric morphology of RFLH is quite different than that of the magnetic field or of the electrical current, and this is not sensitive to the chosen gauge in the computation of RFLH. The value of helicity experiences a large decrease, 25% of its pre-flare value, during an X-class flare of the AR, a change that is also depicted in the photospheric morphology of RFLH. Moreover, the area of this change coincides with the area that encompasses the flux rope, the magnetic structure that later erupted. Conclusions. The use of RFLH can provide important information about the value and location of the magnetic helicity expelled from the solar atmosphere during eruptive events., Comment: accepted by Astronomy & Astrophysics

Aims: Our goal is to develop methodologies to seamlessly track transient solar wind flows viewed by coronagraphs or heliospheric imagers from rapidly varying viewpoints. Methods: We constructed maps of intensity versus time and elongation (J-maps) from Parker Solar Probe (PSP) Wide-field Imager (WISPR) observations during the fourth encounter of PSP. From the J-map, we built an intensity on impact-radius-on-Thomson-surface map (R-map). Finally, we constructed a latitudinal intensity versus time map (Lat-map). Our methodology satisfactorily addresses the challenges associated with the construction of such maps from data taken from rapidly varying viewpoint observations. Results: Our WISPR J-map exhibits several tracks, corresponding to transient solar wind flows ranging from a coronal mass ejection (CME) down to streamer blobs. The latter occurrence rate is about 4-5 per day, which is similar to the occurrence rate in a J-map made from $\sim1$ AU data obtained with the Heliospheric Imager-1 (HI-1) on board the Solar Terrestrial Relations Observatory Ahead spacecraft (STEREO-A). STEREO-A was radially aligned with PSP during the study period. The WISPR J-map tracks correspond to angular speeds of $2.28 \pm 0.7$$^{\circ}$/hour ($2.49 \pm 0.95$$^{\circ}$/hour), for linear (quadratic) time-elongation fittings, and radial speeds of about 150-300 km s$^{-1}$. The analysis of the Lat-map reveals a bifurcating streamer, which implies that PSP was flying through a slightly folded streamer during perihelion. Conclusions: We developed a framework to systematically capture and characterize transient solar wind flows from space platforms with rapidly varying vantage points. The methodology can be applied to PSP WISPR observations as well as to upcoming observations from instruments on board the Solar Orbiter mission., Comment: A&A, in press

A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation) . Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images. Our key conclusion is that the differentiation of pre-eruptive configurations in terms of SMAs and MFRs seems artificial. Both observations and modeling can be made consistent if the pre-eruptive configuration exists in a hybrid state that is continuously evolving from an SMA to an MFR. Thus, the 'dominant' nature of a given configuration will largely depend on its evolutionary stage (SMA-like early-on, MFR-like near the eruption)., Comment: Space Science Reviews, accepted for publication

Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate? (2) How do solar transients drive heliospheric variability? (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere? (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans (SOOPs), resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime., Comment: 20 pages, 1 figure, accepted by Astronomy & Astrophysics

We investigate the formation times of eruptive magnetic flux ropes relative to the onset of solar eruptions, which is important for constraining models of coronal mass ejection (CME) initiation. We inspected uninterrupted sequences of 131 \AA\ images that spanned more than eight hours and were obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) to identify the formation times of hot flux ropes that erupted in CMEs from locations close to the limb. The appearance of the flux ropes as well as their evolution toward eruptions were determined using morphological criteria. Two-thirds (20/30) of the flux ropes were formed well before the onset of the eruption (from 51 minutes to more than eight hours), and their formation was associated with the occurrence of a confined flare. We also found four events with preexisting hot flux ropes whose formations occurred a matter of minutes (from three to 39) prior to the eruptions without any association with distinct confined flare activity. Six flux ropes were formed once the eruptions were underway. However, in three of them, prominence material could be seen in 131 \AA\ images, which may indicate the presence of preexisting flux ropes that were not hot. The formation patterns of the last three groups of hot flux ropes did not show significant differences. For the whole population of events, the mean and median values of the time difference between the onset of the eruptive flare and the appearance of the hot flux rope were 151 and 98 minutes, respectively. Our results provide, on average, indirect support for CME models that involve preexisting flux ropes; on the other hand, for a third of the events, models in which the ejected flux rope is formed during the eruption appear more appropriate., Comment: A&A, in press

Incoherent solar radio radiation comes from the free-free, gyroresonance, and gyrosynchrotron emission mechanisms. Free-free is primarily produced from Coulomb collisions between thermal electrons and ions. Gyroresonance and gyrosynchrotron result from the acceleration of low-energy electrons and mildly relativistic electrons, respectively, in the presence of a magnetic field. In the non-flaring Sun, free-free is the dominant emission mechanism with the exception of regions of strong magnetic fields which emit gyroresonance at microwaves. Due to its ubiquitous presence, free-free emission can be used to probe the non-flaring solar atmosphere above temperature minimum. Gyroresonance opacity depends strongly on the magnetic field strength and orientation; hence it provides a unique tool for the estimation of coronal magnetic fields. Gyrosynchrotron is the primary emission mechanism in flares at frequencies higher than 1-2 GHz and depends on the properties of both the magnetic field and the accelerated electrons, as well as the properties of the ambient plasma. In this paper we discuss in detail the above mechanisms and their diagnostic potential., Comment: Frontiers in Astronomy and Space Sciences, accepted for publication

ALMA observations of the Sun at mm-$\lambda$ offer a unique opportunity to investigate the temperature structure of the solar chromosphere. In this article we expand our previous work on modeling the chromospheric temperature of the quiet Sun, by including measurements of the brightness temperature in the network and cell interiors, from high resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We also examine the absolute calibration of ALMA full-disk images. We suggest that the brightness temperature at the center of the solar disk in Band 6 is $\sim440$ K above the value recommended by White et al. (2017) and we give improved results for the electron temperature variation of the average quiet Sun with optical depth, as well as the derived spectrum at the center of the disk. We found that the electron temperature in the network is considerably lower than predicted by model F of Fontenla et al. (1993) and that of the cell interior considerably higher than predicted by model A. Depending upon the network/cell segregation scheme, the electron temperature difference between network and cell at $\tau=1$ (100 GHz) is from $\sim$660 to $\sim$1550 K, compared to $\sim$3280 K predicted by the models; similarly, the $T_e$ ratio is from $\sim$1.10, to 1.24, against $\sim$1.55 of the model prediction. We also found that the network/cell $T_e(\tau)$ curves diverge as $\tau$ decreases, indicating an increase of contrast with height and possibly a steeper temperature rise in the network than in the cell interior., Comment: Accepted, Astronomy & Astrophysics

Using ALMA observations, we performed the first systematic survey for transient brightenings (i.e. weak, small-scale episodes of energy release) in the quiet solar chromosphere at 3 mm. Our dataset included images of six 87'' x 87'' regions of the quiet Sun obtained with angular resolution of a few arcsec at a cadence of 2 s. The transient brightenings were detected as weak enhancements above the average intensity after we removed the effect of the p-mode oscillations. A similar analysis, over the same regions, was performed for simultaneous 304 and 1600 \AA\ data obtained with the Atmospheric Imaging Assembly. We detected 184 3 mm transient brightening events with brightness temperatures from 70 K to more than 500 K above backgrounds of $\sim 7200-7450$ K. Their mean duration and maximum area were 51.1 s and 12.3 Mm$^2$, respectively, with a weak preference of appearing at network boundaries rather than in cell interiors. Both parameters exhibited power-law behavior with indices of 2.35 and 2.71, respectively. Only a small fraction of ALMA events had either 304 or 1600 \AA\ counterparts but the properties of these events were not significantly different from those of the general population except that they lacked their low-end energy values. The total thermal energies of the ALMA transient brightenings were between $1.5 \times 10^{24}$ and $9.9 \times 10^{25}$ erg and their frequency distribution versus energy was a power law with an index of 1.67. We found that the power per unit area provided by the ALMA events could account for only 1\% of the chromospheric radiative losses (10\% of the coronal ones). Therefore, their energy budget falls short of meeting the requirements for the heating of the upper layers of the solar atmosphere and this conclusion does not change even if we use the least restrictive criteria possible for the detection of transient brightenings., Comment: A&A, accepted for publication (abbreviated abstract)

We studied chromospheric oscillations using Atacama Large millimeter and sub-millimeter Array (ALMA) time-series of interferometric observations of the quiet Sun obtained at 3 mm with a 2-s cadence and a spatial resolution of a few arcsec. The same analysis, over the same fields of view and for the same intervals, was performed for simultaneous Atmospheric Imaging Assembly (AIA) image sequences in 1600 A. Spatially-resolved chromospheric oscillations at 3 mm, with frequencies of $ 4.2 +- 1.7$ mHz are observed in the quiet Sun, in both cell and network. The coherence length-scale of the oscillations is commensurate with the spatial resolution of our ALMA observations. Brightness-temperature fluctuations in individual pixels could reach up to a few hundred K, while the spatially averaged power spectral densities yield rms in the range ~ 55-75 K, i.e., up to ~ 1 % of the averaged brightness temperatures and exhibit a moderate increase towards the limb. For AIA 1600 A, the oscillation frequency is 3.7 +- 1.7 mHz. The relative rms is up to 6 % of the background intensity, with a weak increase towards disk center (cell, average). ALMA 3 mm time-series lag AIA 1600 A by ~ 100 s, which corresponds to a formation-height difference of ~ 1200 km. The ALMA oscillations that we detected exhibit higher amplitudes than those derived from the lower (~ 10 arcsec) resolution observations at 3.5 mm by White et al. (2006). Chromospheric oscillations are, therefore, not fully resolved at the length-scale of the chromospheric network, and possibly not even at the spatial resolution of our ALMA observations. Any study of transient brightenings in the mm-domain should take into account the oscillations., Comment: A&A, accepted for publication (abbreviated abstract)

We present an overview of high resolution quiet Sun observations, from disk center to the limb, obtained with the Atacama Large mm and sub-mm Array (ALMA) at 3 mm. Seven quiet Sun regions were observed with resolution of up to 2.5" by 4.5". We produced both average and snapshot images by self-calibrating the ALMA visibilities and combining the interferometric images with full disk solar images. The images show well the chromospheric network, which, based on the unique segregation method we used, is brighter than the average over the fields of view of the observed regions by $\sim 305$ K while the intranetwork is less bright by $\sim 280$ K, with a slight decrease of the network/intranetwork contrast toward the limb. At 3 mm the network is very similar to the 1600 \AA\ images, with somewhat larger size. We detected for the first time spicular structures, rising up to 15" above the limb with a width down to the image resolution and brightness temperature of $\sim$ 1800 K above the local background. No trace of spicules, either in emission or absorption, was found on the disk. Our results highlight ALMA's potential for the study of the quiet chromosphere., Comment: Astronomy and Astrophysics (Letters), in press

The Square Kilometre Array (SKA) will be the largest radio telescope ever built, aiming to provide collecting area larger than 1 km$^2$. The SKA will have two independent instruments, SKA-LOW comprising of dipoles organized as aperture arrays in Australia and SKA-MID comprising of dishes in South Africa. Currently the phase-1 of SKA, referred to as SKA1, is in its late design stage and construction is expected to start in 2020. Both SKA1-LOW (frequency range of 50-350 MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350-1050, 950-1760, and 4600-15300 MHz, respectively) are important for solar observations. In this paper we present SKA's unique capabilities in terms of spatial, spectral, and temporal resolution, as well as sensitivity and show that they have the potential to provide major new insights in solar physics topics of capital importance including (i) the structure and evolution of the solar corona, (ii) coronal heating, (iii) solar flare dynamics including particle acceleration and transport, (iv) the dynamics and structure of coronal mass ejections, and (v) the solar aspects of space weather. Observations of the Sun jointly with the new generation of ground-based and space-borne instruments promise unprecedented discoveries., Comment: Accepted for publication in Advances in Space Research

The phenomena observed at the Sun have a variety of unique radio signatures that can be used to diagnose the processes in the solar atmosphere. The insights provided by radio obervations are further enhanced when they are combined with observations from space-based telescopes. This special issue demonstrates the power of combination methodology at work and provides new results on i) type I solar radio bursts and thermal emission to study active regions; ii) type II and IV bursts to better understand the structure of coronal mass ejections; iii)~non-thermal gyro-synchrotron and/or type III bursts to improve characterization of particle acceleration in solar flares. The ongoing improvements in time, frequency, and spatial resolutions of ground-based telescopes reveal new levels of solar phenomena complexity and pose new questions.

We measured the center-to-limb variation of the brightness temperature, $T_b$, from ALMA full-disk images at two frequencies and inverted the solution of the transfer equation to obtain the electron temperature, $T_e$ as a function of optical depth, $\tau$. The ALMA images are very similar to AIA images at 1600\AA. The brightness temperature at the center of the disk is 6180 and 7250 K at 239 and 100 GHz respectively, with dispersions of 100 and 170 K. Plage regions stand out clearly in the 239/100 GHz intensity ratio, while faculae and filament lanes do not. The solar disk radius, reduced to 1 AU, is $961.1\pm2.5$ arcsec and $964.1\pm4.5$ arcsec at 239 and 100 GHz respectively. A slight but statistically significant limb brightening is observed at both frequencies. The inversion of the center-to-limb curves shows that $T_e$ varies linearly with the logarithm of optical depth for $0.34<\tau_{100\,GHz}<12$, with a slope $d\ln T_e/d\tau=-608$ K. Our results are 5% lower than predicted by the average quiet sun model C of Fontenla et al. (1993), but do not confirm previous reports that the mm-$\lambda$ solar spectrum is better fitted with models of the cell interior., Comment: Accepted for publication in Astronomy and Astrophysics

We analyzed spectral and imaging data from the Interface Region Imaging Spectrograph (IRIS), images from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO), and magnetograms from the Helioseismic and Magnetic Imager (HMI) aboard SDO. We report observations of small flaring loops in the penumbra of a large sunspot on July 19, 2013. Our main event consisted of a loop spanning ~ 15 arcsec, from the umbral-penumbral boundary to an opposite polarity region outside the penumbra. It lasted approximately 10 minutes with a two minute impulsive peak and was observed in all AIA/SDO channels, while the IRIS slit was located near its penumbral footpoint. Mass motions with an apparent velocity of ~ 100 km/s were detected beyond the brightening, starting in the rise phase of the impulsive peak; these were apparently associated with a higher-lying loop. We interpret these motions in terms of two-loop interaction. IRIS spectra in both the C II and Si IV lines showed very extended wings, up to about 400 km/s, first in the blue (upflows) and subsequently in the red wing. In addition to the strong lines, emission was detected in the weak lines of Cl I, O I and C I as well as in the Mg II triplet lines. Absorption features in the profiles of the C II doublet, the Si IV doublet and the Mg h and k lines indicate the existence of material with a lower source function between the brightening and the observer. We attribute this absorption to the higher loop and this adds further credibility to the two-loop interaction hypothesis. We conclude that the absorption features in the C II, Si IV and Mg II profiles originate in a higher-lying, descending loop; as this approached the already activated lower-lying loop, their interaction gave rise to the impulsive peak, the very broad line profiles and the mass motions., Comment: 2017, Astronomy and Astrophysics, in press. Article movies could be found in http://doi.org/10.5281/zenodo.557094

We study the characteristics of moving type IV radio bursts which extend to the hectometric wavelengths (interplanetary type IV or type IV IP bursts) and their relationship with energetic phenomena on the Sun. Our dataset comprises 48 interplanetary type IV bursts observed with Wind/WAVES in the 13.825 MHz-20 kHz frequency range. The dynamic spectra of the RSTN, the Nancay Decametric Array (DAM), the ARTEMIS-IV, the Culgoora, Hiraiso and Izmiran Radio Spectrographs were used to track the evolution of the events in the low corona. These were supplemented with SXR flux data from the GOES and CME data from the SOHO/LASCO. Positional information of the coronal bursts was obtained by the NRH. We examined the relationship of the type IV events with coronal radio bursts, CMEs and SXR flares. The majority of the events (45) were characterized as compact; their duration was on average 106 minutes. This type of events was, mostly, associated with M-and X-class flares (40 out of 45) and fast CMEs; 32 of these events had CMEs faster than 1000 km/s. Furthermore, in 43 compact events the CME was, possibly, subjected to reduced aerodynamic drag as it was propagating in the wake of a previous CME. A minority (three) of long lived type IV IP bursts was detected, with duration from 960 minutes to 115 hours. These events are referred to as extended or long duration and appear to replenish their energetic electron content, possibly from electrons escaping from the corresponding coronal type IV bursts. The latter were found to persist on the disk, for tens of hours to days. Prominent among them was the unusual interplanetary type IV burst of 18--23 May 2002, which is the longest event in the Wind/WAVES catalog. The three extended events were, usually, accompanied by a number of flares, of GOES class C in their majority, and of CMEs, many of which were slow and narrow., Comment: 51 pages (including a 27 page Table) 14 Figures Keywords: Radio bursts. Dynamic spectrum. Meter and longer wavelengths. Coronal mass ejections

We investigate the conditions under which small scale energy release events in the low corona gave rise to strong interplanetary (IP) type III bursts. We analyze observations of three tiny events, detected by the Nan\c cay Radio Heliograph (NRH), two of which produced IP type IIIs. We took advantage of the NRH positioning information and of the high cadence of AIA/SDO data to identify the associated EUV emissions. We measured positions and time profiles of the metric and EUV sources. We found that the EUV events that produced IP type IIIs were located near a coronal hole boundary, while the one that did not was located in a closed magnetic field region. In all three cases tiny flaring loops were involved, without any associated mass eruption. In the best observed case the radio emission at the highest frequency (435 MHz) was displaced by ~55" with respect to the small flaring loop. The metric type III emission shows a complex structure in space and in time, indicative of multiple electron beams, despite the low intensity of the events. From the combined analysis of dynamic spectra and NRH images we derived the electron beam velocity as well as the height, ambient plasma temperature and density at the level of formation of the 160 MHz emission. From the analysis of the differential emission measure derived from the AIA images we found that the first evidence of energy release was at the footpoints and this was followed by the development of flaring loops and subsequent cooling. We conclude that even small energy release events can accelerate enough electrons to give rise to powerful IP type III bursts. The proximity of the electron acceleration site to open magnetic field lines facilitates the escape of the electrons into the interplanetary space. The offset between the site of energy release and the metric type III location warrants further investigation., Comment: Accepted for publication in Astronomy & Astrophysics

We use data at 131, 171, and 304 A from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) to search for hot flux ropes in 141 M-class and X-class solar flares that occurred at solar longitudes equal to or larger than 50 degrees. Half of the flares were associated with coronal mass ejections (CMEs). The goal of our survey is to assess the frequency of hot flux ropes in large flares irrespective of their formation time relative to the onset of eruptions. The flux ropes were identified in 131 A images using morphological criteria and their high temperatures were confirmed by their absence in the cooler 171 and 304 A passbands. We found hot flux ropes in 45 of our events (32% of the flares); 11 of them were associated with confined flares while the remaining 34 were associated with eruptive flares. Therefore almost half (49%) of the eruptive events involved a hot flux rope configuration. The use of supplementary Hinode X-Ray Telescope (XRT) data indicates that these percentages should be considered as lower limits of the actual rates of occurrence of hot flux ropes in large flares., Comment: Astrophysical Journal, in press (acceptance date: 2015 June 16)

We study selected properties of Solar Energetic Particle (SEP) events as inferred from their associated radio emissions. We used a catalogue of 115 SEP events that consists of entries of proton intensity enhancements at one AU, with complete coverage over solar cycle 23, based on high-energy (~68 MeV) protons from SOHO/ERNE and we calculated the proton release time at the Sun using velocity dispersion analysis (VDA). After an initial rejection of cases with unrealistic VDA path lengths, we assembled composite radio spectra for the remaining events using data from ground-based and space-borne radio-spectrographs. For every event we registered the associated radio emissions and we divided the events in groups according to their associated radio emissions. The proton release was found to be most often accompanied by both type III and II radio bursts, but a good association percentage was also registered in cases accompanied by type IIIs only. The worst association was found for the cases with type II only association. These radio association percentages support the idea that both flare- and shock-resident particle release processes are observed in high-energy proton events. In cases of type III-associated events we extended our study to the timings between the type III radio emission, the proton release, and the electron release as inferred from VDA based on Wind/3DP 20-646 keV data. Typically, the protons are released after the start of the associated type III bursts and simultaneously or before the release of energetic electrons. For the cases with type II radio association we found that the distribution of the proton release heights had a maximum at ~2.5 Rs. Most (69%) of the flares associated to our SEP events were located at the western hemisphere, with a peak within the well-connected region of 50-60 deg western longitude., Comment: 17 pages, 11 figures, accepted for publication in A&A

The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere - a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases., Comment: 73 pages, 21 figures ; Space Science Reviews (accepted December 10th, 2015); accepted version

The Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) was initiated in 2014 in connection with two ALMA development studies. The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful new tool, which can also observe the Sun at high spatial, temporal, and spectral resolution. The international SSALMONetwork aims at coordinating the further development of solar observing modes for ALMA and at promoting scientific opportunities for solar physics with particular focus on numerical simulations, which can provide important constraints for the observing modes and can aid the interpretation of future observations. The radiation detected by ALMA originates mostly in the solar chromosphere - a complex and dynamic layer between the photosphere and corona, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. Potential targets include active regions, prominences, quiet Sun regions, flares. Here, we give a brief overview over the network and potential science cases for future solar observations with ALMA., Comment: 15 pages, 3 figures, to appear in Advances in Solar Physics (special issue of Advances in Space Research, AdSR), proceedings of the 14th European Solar Physics Meeting (ESPM-14, Dublin, Ireland, September 2014); (resubmitted version after minor revision)

We present the association rates between solar energetic particles (SEPs) and the radio emission signatures in the corona and IP space during the entire solar cycle 23. We selected SEPs associated with X and M-class flares from the visible solar hemisphere. All SEP events are also accompanied by coronal mass ejections. Here, we focus on the correlation between the SEP events and the appearance of radio type II, III and IV bursts on dynamic spectra. For this we used the available radio data from ground-based stations and the Wind/WAVES spacecraft. The associations are presented separately for SEP events accompanying activity in the eastern and western solar hemisphere. We find the highest association rate of SEP events to be with type III bursts, followed by types II and IV. Whereas for types III and IV no longitudinal dependence is noticed, these is a tendency for a higher SEP-association rate with type II bursts in the eastern hemisphere. A comparison with reports from previous studies is briefly discussed., Comment: http://adsabs.harvard.edu.ezproxy.obspm.fr/abs/2013CEAB...37..541M

Context: Metric type II bursts are the most direct diagnostic of shock waves in the solar corona. Aims: There are two main competing views about the origin of coronal shocks: that they originate in either blast waves ignited by the pressure pulse of a flare or piston-driven shocks due to coronal mass ejections (CMEs). We studied three well-observed type II bursts in an attempt to place tighter constraints on their origins. Methods: The type II bursts were observed by the ARTEMIS radio spectrograph and imaged by the Nan\c{c}ay Radioheliograph (NRH) at least at two frequencies. To take advantage of projection effects, we selected events that occurred away from disk center. Results: In all events, both flares and CMEs were observed. In the first event, the speed of the shock was about 4200 km/s, while the speed of the CME was about 850 km/s. This discrepancy ruled out the CME as the primary shock driver. The CME may have played a role in the ignition of another shock that occurred just after the high speed one. A CME driver was excluded from the second event as well because the CMEs that appeared in the coronagraph data were not synchronized with the type II burst. In the third event, the kinematics of the CME which was determined by combining EUV and white light data was broadly consistent with the kinematics of the type II burst, and, therefore, the shock was probably CME-driven. Conclusions: Our study demonstrates the diversity of conditions that may lead to the generation of coronal shocks., Comment: 13 pages, 14 figures. "Astronomy and Astrophysics", in press

The Solar Radiospectrograph of the University of Athens (ARTEMIS-IV) is in operation at the Thermopylae Satellite Communication Station since 1996. The observations extend from the base of the Solar Corona (650 MHz) to about 2 Solar Radii (20 MHz) with time resolution 1/10-1/100 sec. The instruments recordings, being in the form of dynamic spectra, measure radio flux as a function of height in the corona; our observations are combined with spatial data from the Nancay Radioheliograph whenever the need for 3D positional information arises. The ARTEMIS-IV contribution in the study of solar radio bursts is two fold- Firstly, in investigating new spectral characteristics since its high sampling rate facilitates the study of fine structures in radio events. On the other hand it is used in studying the association of solar bursts with interplanetary phenomena because of its extended frequency range which is, furthermore, complementary to the range of the WIND/WAVES receivers and the observations may be readily combined. This reports serves as a brief account of this operation. Joint observations with STEREO/WAVES and LOFAR low frequency receivers are envisaged in the future.

In this report we present a complex metric burst, associated with the 14 July 2000 major solar event, recorded by the ARTEMIS-IV radio spectrograph at Thermopylae. Additional space-borne and Earth-bound observational data are used, in order to identify and analyze the diverse, yet associated, processes during this event. The emission at metric wavelengths consisted of broad-band continua including a moving and a stationary type IV, impulsive bursts and pulsating structures. The principal release of energetic electrons in the corona was 15 20 min after the start of the flare, in a period when the flare emission spread rapidly eastwards and a hard X-ray peak occurred. Backward extrapolation of the CME also puts its origin in the same time interval, however, the uncertainty of the extrapolation does not allow us to associate the CME with any particular radio or X-ray signature. Finally, we present high time and spectral resolution observations of pulsations and fiber bursts, together with a preliminary statistical analysis., Comment: Solar Physics, v. 204, Issue 1/2, p. 165-177 (2001)

The results of the microwave observations of the Sun made with the RATAN-600 have shown the existence of many types of spectral peculiarities in polarized emission of active regions, which produce powerful flares. These phenomena happen at microwaves and reflect inhomogeneous structure of magnetic field in magnetospheres of flaring active regions in wide range of heights above the photosphere. In this presentation we demonstrate an analysis of the AR 9415 during all the period of its passage across the solar disk. Results of the study point out to existence of different scenarios of circular polarization variations in the radio wave band. Here, we separated the phenomenon of the cyclotron emission passage through the quasi-transverse magnetic field (QT-region) and several effects connected with flare activity of active region. New observational data are presented and compared with the data of several observatories: SSRT, NoRH, MDI SOHO, GOES and MEES. The preliminary interpretation of the phenomena are given., Comment: 26 pages, 13 Postscript figures for better resolution pics, see http://www.sao.ru/hq/sun/pdf/paper9415.pdf