arXiv subjects

A. Nindos

Publications and source records attributed to A. Nindos.

At least 19 recordsLinked to original sources

arXiv2025

ALMA Observations of Solar Spicules in a Polar Coronal Hole

We report observations of solar spicules at millimeter wavelengths (mm-$\lambda$) using the Atacama Large Millimeter/submillimeter Array (ALMA). These are supplemented by observations in optical (O), ultraviolet (UV) and extreme ultraviolet (EUV) wavelengths. The observations were made on 2018 December 25 of the northern polar coronal hole. ALMA obtained time-resolved imaging observations at wavelengths of 3 mm (100 GHz; 2~s cadence) and 1.25~mm (239 GHz; $\approx 2$~min cadence) with an angular resolution of $2.2" \times 1.3"$ and $1.5" \times 0.7"$, respectively. Spicules observed at mm-$\lambda$ are easily seen low in the chromosphere whereas spicules in UV bands are seen to extend higher. The spicules observed at mm-$\lambda$ are seen in absorption against coronal EUV emission, allowing us to estimate the column depth of neutral hydrogen. Spicular emission at mm-$\lambda$, due to thermal free-free radiation, allows us to estimate the electron number density as a function of height. We find that spicule densities, inferred from the mm-$\lambda$ data are uniquely insensitive to assumptions regarding the temperature of plasma in spicules. We suggest that the upward mass flux carried by spicules is unlikely to play a significant role in the mass budget of the solar corona and solar wind, and the transport of hot material into the corona by spicules may not play a significant role in coronal heating. However, the possibility that electric currents, fast kink and torsional waves, or other wave modes carried by spicules may play a role in transporting energy into the solar corona cannot be excluded.

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arXiv2024

Magnetic helicity and energy budgets of jet events from an emerging solar active region

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.

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arXiv2023

On the use of relative field line helicity as an indicator for solar eruptivity

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.

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arXiv2023

Magnetic Helicity and Free Magnetic Energy as Tools to Probe Eruptions in two Differently Evolving Solar Active Regions

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$σ$ levels of their detrended timeseries. There was no eruption when only one or none of these quantities exceeded its 2$σ$ 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.

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arXiv2022

A first look at the submillimeter Sun with ALMA

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$α$ images of equivalent resolution. Polar coronal holes, which are clearly seen in the 304 A band and small H$α$ 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 $T_b(\logμ_{100})$, where $μ_{100}$ 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 $T_b(\logμ_{100})$ curve flattens at large values of $μ_{100}$, and so does the corresponding $T_e(\logτ_{100})$ at large $τ_{100}$. This is probably an indication that we are approaching the temperature minimum.

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arXiv2022

First detection of metric emission from a solar surge

We report the first detection of metric radio emission from a surge, observed with the Nançay 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 Å 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.

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arXiv2022

Magnetic helicity and energy of emerging solar active regions and their erruptivity

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.

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arXiv2021

Multi-wavelength Observations of a Metric Type-II Event

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 Å 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.

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arXiv2021

ALMA observations of the variability of the quiet Sun at millimeter wavelengths

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$α$ images from the GONG network.

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arXiv2021

Relative field line helicity of a large eruptive solar active region

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.

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arXiv2020

Tracking solar wind flows from rapidly varying viewpoints by the Wide-field Imager for Parker Solar Probe

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.

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arXiv2020

Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections

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).

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arXiv2020

When do solar erupting hot magnetic flux ropes form?

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 Å 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 Å 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.

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arXiv2020

Incoherent Solar Radio Emission

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.

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arXiv2020

Modeling the quiet Sun cell and network emission with ALMA

ALMA observations of the Sun at mm-$λ$ 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 $τ=1$ (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 $T_e(τ)$ curves diverge as $τ$ decreases, indicating an increase of contrast with height and possibly a steeper temperature rise in the network than in the cell interior.

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arXiv2020

Transient brightenings in the quiet Sun detected by ALMA at 3 mm

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 Å 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 Å 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.

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arXiv2019

Observations of solar chromospheric oscillations at 3 mm with ALMA

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.

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arXiv2018

First high-resolution look at the quiet Sun with ALMA at 3 mm

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 Å 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.

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