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Our understanding of magnetic reconnection (MR) under chromospheric conditions remains limited. Recent observations have demonstrated the important role of ion–neutral interactions in the dynamics of the chromosphere. Furthermore, the comparison between the spectral profiles and synthetic observations of reconnection events suggests that current MHD approaches appear to be inconsistent with observations. First, collisions and multithermal aspects of the plasma play a role in these regions. Second, hydrogen and helium ionization effects are relevant to the energy balance of the chromosphere. This work investigates the multifluid multispecies (MFMS) effects on MR in conditions representative of the upper chromosphere using the multifluid Ebysus code. We compare an MFMS approach based on a helium–hydrogen mixture with a two-fluid MHD model based on hydrogen only. The simulations of MR are performed in a Lundquist number regime high enough to develop plasmoids and instabilities. We study the evolution of the MR and compare the two approaches including the structure of the current sheet and plasmoids, the decoupling of the particles, the evolution of the heating mechanisms, and the composition. The presence of helium species leads to more efficient heating mechanisms than the two-fluid case. This scenario, which is out of reach of the two-fluid or single-fluid models, can reach transition region temperatures starting from upper-chromospheric thermodynamic conditions, representative of a quiet Sun scenario. The different dynamics between helium and hydrogen species could lead to chemical fractionation and, under certain conditions, enrichment of helium in the strongest outflows. This could be of significance for recent observations of helium enrichment in the solar wind in switchbacks and coronal mass ejections.

The CLASP2 (Chromospheric LAyer SpectroPolarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg II h & k spectral region around 2800 Angstroms along a 200 arcsecond slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg II h & k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semi-empirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry., 14 pages, 5 figures, to be published in the Astrophysical Journal (ApJ), initially submitted May 2022, revised submission July 2022

This work aims to provide an accurate description and calculations of collision frequencies in conditions relevant to the solar atmosphere. To do so, we focus on the detailed description of the collision frequency in the solar atmosphere based on a classical formalism with Chapman–Cowling collision integrals, as described by Zhdanov. These collision integrals allow linking the macroscopic transport fluxes of multifluid models to the kinetic scales involved in the Boltzmann equations. In this context, the collision frequencies are computed accurately while being consistent at the kinetic level. We calculate the collision frequencies based on this formalism and compare them with approaches commonly used in the literature for conditions typical of the solar atmosphere. To calculate the collision frequencies, we focus on the collision integral data provided by Bruno et al., which is based on a multicomponent hydrogen–helium mixture used for conditions typical for the atmosphere of Jupiter. We perform a comparison with the classical formalism of Vranjes & Krstic and Leake & Linton. We highlight the differences obtained in the distribution of the cross sections as functions of the temperature. Then, we quantify the disparities obtained in numerical simulations of a 2.5D solar atmosphere by calculating collision frequencies and ambipolar diffusion. This strategy allows us to validate and assess the accuracy of these collision frequencies for conditions typical of the solar atmosphere.

Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they emerge all over the Sun, these small-scale fields bring a substantial amount of flux, and therefore energy, to the solar surface. Because of this, IN fields are crucial for understanding the quiet Sun (QS) magnetism. However, they are weak and produce very small polarization signals, which is the reason why their properties and impact on the energetics and dynamics of the solar atmosphere are poorly known. Here we use coordinated, high-resolution, multiwavelength observations obtained with the Swedish 1 m Solar Telescope and the Interface Region Imaging Spectrograph (IRIS) to follow the evolution of IN magnetic loops as they emerge into the photosphere and reach the chromosphere and transition region. We studied in this paper three flux emergence events having total unsigned magnetic fluxes of 1.9 × 1018, 2.5 × 1018, and 5.3 × 1018 Mx. The footpoints of the emerging IN bipoles are clearly seen to appear in the photosphere and to rise up through the solar atmosphere, as observed in Fe I 6173 A and Mg I b2 5173 A magnetograms, respectively. For the first time, our polarimetric measurements taken in the chromospheric Ca II 8542 A line provide direct observational evidence that IN fields are capable of reaching the chromosphere. Moreover, using IRIS data, we study the effects of these weak fields on the heating of the chromosphere and transition region. © 2021 Institute of Physics Publishing. All rights reserved., IRIS is a NASA Small Explorer Mission developed and operated by LMSAL with mission operations executed at NASA Ames Research Center and major contributions to downlink communications funded by the ESA and the Norwegian Space Centre. M.G., B.D.P., and A.S.D. are supported by NASA contract NNG09FA40C (IRIS). The work of L.B.R. and S.E.P. was supported by the Spanish Ministerio de Economia and Competitividad through grants ESP201347349-C6-1-R and ESP2016-77548-C5-1-R, including a percentage from European Regional Development Fund (FEDER) funds. L.B.R. acknowledges financial support from the State Agency for Research of the Spanish Ministerio de Ciencia e Innovacion through project RTI2018-096886-B-C5 (including FEDER funds) and through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709). The Swedish 1 m Solar Telescope is operated on the island of La Palma by the Institute for Solar Physics of Stockholm University in the Spanish Observatory del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. The Institute for Solar Physics is supported by a grant for research infrastructures of national importance from the Swedish Research Council (registration No. 2017-00625). This research has made use of NASA's Astrophysics Data System.

NASA's Interface Region Imaging Spectrograph (IRIS) provides high resolution observations of the solar atmosphere through UV spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1-m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectro-polarimetric imaging in photospheric Fe I lines and spectrally-resolved imaging in the chromospheric Ca II 8542 A, H-alpha, and Ca II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plage, quiet Sun, and coronal holes., Accepted for publication in Astronomy & Astrophysics (July 21, 2020)

Context. Heating signatures from small-scale magnetic reconnection events in the solar atmosphere have proven to be difficult to detect through observations. Numerical models that reproduce flaring conditions are essential in understanding how nanoflares may act as a heating mechanism of the corona. Aims. We study the effects of non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN simulations of nanoflare heated loops to investigate the diagnostic potential of chromospheric emission from small-scale events. Methods. The Mg II h and k, Ca II H and K, Ca II 854.2 nm, and Hα and Hβ chromospheric lines were synthesised from various RADYN models of coronal loops subject to electron beams of nanoflare energies. The contribution function to the line intensity was computed to better understand how the atmospheric response to the non-thermal electrons affects the formation of spectral lines and the detailed shape of their spectral profiles. Results. The spectral line signatures arising from the electron beams highly depend on the density of the loop and the lower cutoff energy of the electrons. Low-energy (5 keV) electrons deposit their energy in the corona and transition region, producing strong plasma flows that cause both redshifts and blueshifts of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons deposit their energy in the lower transition region and chromosphere, resulting in increased emission from local heating. Our results indicate that effects from small-scale events can be observed with ground-based telescopes, expanding the list of possible diagnostics for the presence and properties of nanoflares.

Solar and stellar flares are the most intense emitters of X-rays and extreme ultraviolet radiation in planetary systems1,2. On the Sun, strong flares are usually found in newly emerging sunspot regions3. The emergence of these magnetic sunspot groups leads to the accumulation of magnetic energy in the corona. When the magnetic field undergoes abrupt relaxation, the energy released powers coronal mass ejections as well as heating plasma to temperatures beyond tens of millions of kelvins. While recent work has shed light on how magnetic energy and twist accumulate in the corona4 and on how three-dimensional magnetic reconnection allows for rapid energy release5,6, a self-consistent model capturing how such magnetic changes translate into observable diagnostics has remained elusive. Here, we present a comprehensive radiative magnetohydrodynamics simulation of a solar flare capturing the process from emergence to eruption. The simulation has sufficient realism for the synthesis of remote sensing measurements to compare with observations at visible, ultraviolet and X-ray wavelengths. This unifying model allows us to explain a number of well-known features of solar flares7, including the time profile of the X-ray flux during flares, origin and temporal evolution of chromospheric evaporation and condensation, and sweeping of flare ribbons in the lower atmosphere. Furthermore, the model reproduces the apparent non-thermal shape of coronal X-ray spectra, which is the result of the superposition of multi-component super-hot plasmas8 up to and beyond 100 million K. A state-of-the-art magnetohydrodynamic simulation of a solar flare from emergence to eruption is able to reproduce observations at visible, UV and X-ray wavelengths, and suggests that non-thermal particles at high energy may play a less prominent role than usually assumed in flare models.

In the lower solar atmosphere, the chromosphere is permeated by jets known as spicules, in which plasma is propelled at speeds of 50 to 150 kilometers per second into the corona. The origin of the spicules is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with Alfv\'enic waves that help drive the solar wind. We compare magnetohydrodynamic simulations of spicules with observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules are shown to occur when magnetic tension is amplified and transported upward through interactions between ions and neutrals or ambipolar diffusion. The tension is impulsively released to drive flows, heat plasma (through ambipolar diffusion), and generate Alfv\'enic waves., Comment: 28 pages, 10 figures

This work is supported by NASA contract NNG09FA40C (IRIS). The Interface Region Imaging Spectrograph (IRIS) is a NASA small explorer mission that provides high-resolution spectra and images of the Sun in the 133 – 141 nm and 278 – 283 nm wavelength bands. The IRIS data are archived in calibrated form and made available to the public within seven days of observing. The calibrations applied to the data include dark correction, scattered light and background correction, flat fielding, geometric distortion correction, and wavelength calibration. In addition, the IRIS team has calibrated the IRIS absolute throughput as a function of wavelength and has been tracking throughput changes over the course of the mission. As a resource for the IRIS data user, this article describes the details of these calibrations as they have evolved over the first few years of the mission. References to online documentation provide access to additional information and future updates. Publisher PDF

In addition to their jet-like dynamic behaviour, spicules usually exhibit strong transverse speeds, multi-stranded structure and heating from chromospheric to transition region temperatures. In this work we first analyse \textit{Hinode} \& \textit{IRIS} observations of spicules and find different behaviours in terms of their Doppler velocity evolution and collective motion of their sub-structure. Some have a Doppler shift sign change that is rather fixed along the spicule axis, and lack coherence in the oscillatory motion of strand-like structure, matching rotation models or long wavelength torsional Alfv��n waves. Others exhibit a Doppler shift sign change at maximum displacement and coherent motion of their strands, suggesting a collective MHD wave. By comparing with an idealised 3-D MHD simulation combined with radiative transfer modelling, we analyse the role of transverse MHD waves and associated instabilities in spicule-like features. We find that Transverse Wave Induced Kelvin-Helmholtz (TWIKH) rolls lead to coherence of strand-like structure in imaging and spectral maps, as seen in some observations. The rapid transverse dynamics and the density and temperature gradients at the spicule boundary lead to ring-shaped \ion{Mg}{2} k and \ion{Ca}{2} H source functions in the transverse cross-section, potentially allowing IRIS to capture the KHI dynamics. Twists and currents propagate along the spicule at Alfv��nic speeds, and the temperature variations within TWIKH rolls produce sudden appearance / disappearance of strands seen in Doppler velocity and in \ion{Ca}{2} H intensity. However, only a mild intensity increase in higher temperature lines is obtained, suggesting there is an additional heating mechanism at work in spicules., Accepted for publication in The Astrophysical Journal

The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by the magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper, we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for ∼3 minutes in the photosphere and ∼12 minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg ii h and k lines, we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating.© 2018. The American Astronomical Society. All rights reserved., RIS is a NASA small explorer mission developed and operated by LMSAL with mission operations executed at NASA Ames Research center and major contributions to downlink communications funded by ESA and the Norwegian Space Centre. M.G. was supported by NASA grant NNX16AC34G. J.dl.C.R. is supported by grants from the Swedish Research Council (2015-03994), the Swedish National Space Board (128/15), and the Swedish Civil Contingencies Agency (MSB). This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (SUNMAG, grant agreement 759548). B.D.P. was supported by NASA grant NNX11AN98G and NASA contracts NNG09FA40C (IRIS). The work of L.B.R. and S.E.P. was supported by the Spanish Ministerio de Economia and Competitividad through grants ESP2013-47349-C6-1-R and ESP2016-77548-C5-1-R, including a percentage from European FEDER funds. Image reconstruction was performed at IAA-CSIC supercomputing facilities. The Swedish 1 m Solar Telescope is operated by the Institute for Solar Physics of Stockholm University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. M.C. was supported by the Research Council of Norway through its Centres of Excellence scheme, project number 262622, and through grants of computing time from the Programme for Supercomputing. This research has made use of NASA's Astrophysics Data System.

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

We present joint observations of the Sun by the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph (IRIS). The observations were made of a solar active region on 2015 December 18 as part of the ALMA science verification effort. A map of the Sun's continuum emission of size $2.4' \times 2.3'$ was obtained by ALMA at a wavelength of 1.25 mm (239 GHz) using mosaicing techniques. A contemporaneous map of size $1.9'\times 2.9'$ was obtained in the Mg II h doublet line at 2803.5\AA\ by IRIS. Both mm/submm$-\lambda$ continuum emission and ultraviolet (UV) line emission are believed to originate from the solar chromosphere and both have the potential to serve as powerful and complementary diagnostics of physical conditions in this poorly understood layer of the solar atmosphere. While a clear correlation between mm-$\lambda$ brightness temperature $T_B$ and the Mg II h line radiation temperature $T_{rad}$ is observed the slope is $, Comment: 8 pages, 2 figures

Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this paper we exploit the high-resolution instruments Interface Region Imaging Spectrograph (IRIS) and the new CHROMIS Fabry-Perot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in so-called UV bursts in the low solar atmosphere. The Si IV 1403A UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca II K 3934A line now provide compelling evidence for the presence of plasmoids, by revealing highly dynamic and rapidly moving brightenings that are smaller than 0.2 arcsec and that evolve on timescales of order seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere., Comment: Accepted for publication in Astrophysical Journal Letters. Movies are available at http://folk.uio.no/rouppe/plasmoids_chromis/

Various solar features can be seen on maps of the Sun in the mm and sub-mm wavelength range. The recently installed Atacama Large Millimeter/submillimeter Array (ALMA) is capable of observing the Sun in that wavelength range with an unprecedented spatial, temporal and spectral resolution. To interpret solar observations with ALMA the first important step is to compare ALMA maps with simultaneous images of the Sun recorded in other spectral ranges. First we identify different structures in the solar atmosphere seen in the optical, IR and EUV parts of the spectrum (quiet Sun (QS), active regions (AR), prominences on the disc, magnetic inversion lines (IL), coronal holes (CH) and coronal bright points (CBPs)) in a full disc solar ALMA image. The second aim is to measure the intensities (brightness temperatures) of those structures and compare them with the corresponding QS level. A full disc solar image at 1.21 mm obtained on December 18, 2015 during a CSV-EOC campaign with ALMA is calibrated and compared with full disc solar images from the same day in H\alpha, in He I 1083 nm core, and with SDO images (AIA at 170 nm, 30.4 nm, 21.1 nm, 19.3 nm, and 17.1 nm and HMI magnetogram). The brightness temperatures of various structures are determined by averaging over corresponding regions of interest in the ALMA image. Positions of the QS, ARs, prominences on the disc, ILs, CHs and CBPs are identified in the ALMA image. At 1.21 mm ARs appear as bright areas (but sunspots are dark), while prominences on the disc and CHs are not discernible from the QS background, although having slightly less intensity than surrounding QS regions. ILs appear as large, elongated dark structures and CBPs correspond to ALMA bright points. These results are in general agreement with sparse earlier measurements at similar wavelengths. The identification of CBPs represents the most important new result., Comment: 9 pages, 3 figures

The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that utilizes the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions we derive quiet-Sun values at disk center of 7300 K at lambda=3 mm and 5900 K at lambda=1.3 mm. These values have statistical uncertainties of order 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of order 25 arcsec, the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range., Comment: Solar Physics, accepted: 24 pages, 13 figures

The Interface Region Imaging Spectrograph (IRIS) provides an unparalleled opportunity to explore the (thermal) interface between the chromosphere, transition region, and the coronal plasma observed by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). The SDO/AIA observations of coronal loop footpoints show strong recurring upward propagating signals—"propagating coronal disturbances" (PCDs) with apparent speeds of the order of 100–120 km s−1. That signal has a clear signature in the slit-jaw images of IRIS in addition to identifiable spectral signatures and diagnostics in the Mg iih (2803 A) line. In analyzing the Mg iih line, we are able to observe the presence of magnetoacoustic shock waves that are also present in the vicinity of the coronal loop footpoints. We see there is enough of a correspondence between the shock propagation in Mg iih, the evolution of the Si iv line profiles, and the PCD evolution to indicate that these waves are an important ingredient for PCDs. In addition, the strong flows in the jet-like features in the IRIS Si iv slit-jaw images are also associated with PCDs, such that waves and flows both appear to be contributing to the signals observed at the footpoints of PCDs.

The Sun's outer atmosphere, or corona, is heated to millions of degrees, considerably hotter than its surface or photosphere. Explanations for this enigma typically invoke the deposition in the corona of nonthermal energy generated by magnetoconvection. However, the coronal heating mechanism remains unknown. We used observations from the Solar Dynamics Observatory and the Hinode solar physics mission to reveal a ubiquitous coronal mass supply in which chromospheric plasma in fountainlike jets or spicules is accelerated upward into the corona, with much of the plasma heated to temperatures between ~0.02 and 0.1 million kelvin (MK) and a small but sufficient fraction to temperatures above 1 MK. These observations provide constraints on the coronal heating mechanism(s) and highlight the importance of the interface region between photosphere and corona.

We present the results of 1D hydrodynamic simulations of coronal loops that are subject to nanoflares, caused by either in situ thermal heating or nonthermal electron (NTE) beams. The synthesized intensity and Doppler shifts can be directly compared with Interface Region Imaging Spectrograph (IRIS) and Atmospheric Imaging Assembly (AIA) observations of rapid variability in the transition region (TR) of coronal loops, associated with transient coronal heating. We find that NTEs with high enough low-energy cutoff (EC) deposit energy in the lower TR and chromosphere, causing blueshifts (up to approximately 20 kilometers per second) in the IRIS Si IV lines, which thermal conduction cannot reproduce. The EC threshold value for the blueshifts depends on the total energy of the events (approximately 5 kiloelectronvolts for 1024 ergs, up to 15 kiloelectronvolts for 1025 ergs). The observed footpoint emission intensity and flows, combined with the simulations, can provide constraints on both the energy of the heating event and EC. The response of the loop plasma to nanoflares depends crucially on the electron density: significant Si IV intensity enhancements and flows are observed only for initially low-density loops (less than 10 (sup 9) per cubic centimeter). This provides a possible explanation of the relative scarcity of observations of significant moss variability. While the TR response to single heating episodes can be clearly observed, the predicted coronal emission (AIA 94 Angstroms) for single strands is below current detectability and can only be observed when several strands are heated closely in time. Finally, we show that the analysis of the IRIS Mg II chromospheric lines can help further constrain the properties of the heating mechanisms.

Recently a second type of spicules was discovered at the solar limb with the Solar Optical Telescope (SOT) onboard the Japanese Hinode spacecraft. These previously unrecognized type II spicules are thin chromospheric jets that are shorter-lived (10-60 s) and that show much higher apparent upward velocities (of order 50-100 km/s) than the classical spicules. Since they have been implicated in providing hot plasma to coronal loops, their formation, evolution and properties are important ingredients for a better understanding of the mass and energy balance of the low solar atmosphere. Here we report on the discovery of the disk counterparts of type II spicules using spectral imaging data in the Ca II 854.2 nm and Halpha lines with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish Solar Telescope (SST) in La Palma. We find rapid blueward excursions in the line profiles of both chromospheric lines that correspond to thin, jet-like features that show apparent velocities of order 50 km/s. These blueward excursions seem to form a separate absorbing component with Doppler shifts of order 20 and 50 km/s for the Ca II 854.2 nm and Halpha line respectively. We show that the appearance, lifetimes, longitudinal and transverse velocities and occurrence rate of these rapid blue excursions on the disk are very similar to those of the type II spicules at the limb. A detailed study of the spectral line profiles in these events suggests that plasma is accelerated along the jet, and plasma is being heated throughout the short lifetime of the event., Accepted for publication in Astrophysical Journal. See http://www.astro.uio.no/~rouppe/rbe/ for better resolution figures and movies

We present the results of numerical simulations of wave-induced magnetic reconnection in a model of the solar atmosphere. In the magnetic field geometry we study in this article, the waves, driven by a monochromatic piston and a driver taken from Hinode observations, induce periodic reconnection of the magnetic field, and this reconnection appears to help drive long-period chromospheric jets. By synthesizing observations for a variety of wavelengths that are sensitive to a wide range of temperatures, we shed light on the often confusing relationship between the plethora of jet-like phenomena in the solar atmosphere, e.g., explosive events, spicules, blinkers, and other phenomena thought to be caused by reconnection., 13 pages, 22 figures. Submitted to The Astrophysical Journal

We present the results of numerical simulations of shock wave-driven jets in the solar atmosphere. The dependence of observable quantities like maximum velocity and deceleration on parameters such as the period and amplitude of initial disturbances and the inclination of the magnetic field is investigated. Our simulations show excellent agreement with observations, and shed new light on the correlation between velocity and deceleration and on the regional differences found in observations.

We present high spatial and high temporal resolution observations of the quiet Sun in H-alpha obtained with the Swedish 1-m Solar Telescope on La Palma. We observe that many mottles, jet-like features in the quiet Sun, display clear up- and downward motions along their main axis. In addition, many mottles show vigorous transverse displacements. Unique identification of the mottles throughout their lifetime is much harder than for their active region counterpart, dynamic fibrils. This is because many seem to lack a sharply defined edge at their top, and significant fading often occurs throughout their lifetime. For those mottles that can be reliably tracked, we find that the mottle tops often undergo parabolic paths. We find a linear correlation between the deceleration these mottles undergo and the maximum velocity they reach, similar to what was found earlier for dynamic fibrils. Combined with an analysis of oscillatory properties, we conclude that at least part of the quiet Sun mottles are driven by magnetoacoustic shocks. In addition, the mixed polarity environment and vigorous dynamics suggest that reconnection may play a significant role in the formation of some quiet Sun jets., Comment: 12 pages, 4 figures. ApJ Letters, in press

Aims. This paper aims to look at the propagation of synthetic photospheric oscillations from a point source into a two-dimensional non-magnetic solar atmosphere. It takes a particular interest in the leakage of 5-min global oscillations into the atmosphere, and aims to complement efforts on the driving of chromospheric dynamics (e.g. spicules and waves) by 5-min oscillations. Methods. A model solar atmosphere is constructed based on realistic temperature and gravitational stratification. The response of this atmosphere to a wide range of adiabatic periodic velocity drivers is numerically investigated in the hydrodynamic approximation. Results. The findings of this modelling are threefold. Firstly, high-frequency waves are shown to propagate from the lower atmosphere across the transition region experiencing relatively low reflection and transmitting energy into the corona. Secondly, it is demonstrated that driving the upper solar photosphere with a harmonic piston driver at around the 5 min period may generate three separate standing modes with similar periods in the chromosphere and transition region. In the cavity formed by the chromosphere and bounded by regions of low cut-off period at the photospheric temperature minimum and the transition region this is caused by reflection, while at either end of this region in the lower chromosphere and transition region the standing modes are caused by resonant excitation. Finally, the transition region becomes a guide for horizontally propagating surface waves for a wide range of driver periods, and in particular at those periods which support chromospheric standing waves. Crucially, these findings are the results of a combination of a chromospheric cavity and resonant excitation in the lower atmosphere and transition region.

We study the dynamical interaction of the solar chromosphere with the transition region in mossy and non-mossy active-region plage. We carefully align image sequences taken with the Transition Region And Coronal Explorer (TRACE) in the ultraviolet passbands around 1550, 1600, and 1700 A and the extreme ultraviolet passbands at 171 and 195 A. We compute Fourier phase-difference spectra that are spatially averaged separately over mossy and non-mossy plage to study temporal modulations as a function of temporal frequency. The 1550 versus 171 A comparison shows zero phase difference in non-mossy plage. In mossy plage, the phase differences between all UV and EUV passbands show pronounced upward trends with increasing frequency, which abruptly changes into zero phase difference beyond 4-6 mHz. The phase difference between the 171 and 195 A sequences exhibits a shallow dip below 3 mHz and then also turns to zero phase difference beyond this value. We attribute the various similarities between the UV and EUV diagnostics that are evident in the phase-difference diagrams to the contribution of the C IV resonance lines in the 1550 and 1600 A passbands. The strong upward trend at the lower frequencies indicates the presence of upward-traveling disturbances. It points to correspondence between the lower chromosphere and the upper transition region, perhaps by slow-mode magnetosonic disturbances, or by a connection between chromospheric and coronal heating mechanisms. The transition from this upward trend to zero phase difference at higher frequencies is due to the intermittent obscuration by fibrils that occult the foot points of hot loops, which are bright in the EUV and C IV lines, in oscillatory manner., Comment: 14 pages, 6 figures

Advanced 3D radiative MHD simulations now reproduce many properties of the outer solar atmosphere. When including a domain from the convection zone into the corona, a hot chromosphere and corona are self-consistently maintained. Here we study two realistic models, with different simulated area, magnetic field strength and topology, and numerical resolution. These are compared in order to characterize the heating in the 3D-MHD simulations which self-consistently maintains the structure of the atmosphere. We analyze the heating at both large and small scales and find that heating is episodic and highly structured in space, but occurs along loop shaped structures, and moves along with the magnetic field. On large scales we find that the heating per particle is maximal near the transition region and that widely distributed opposite-polarity field in the photosphere leads to a greater heating scale height in the corona. On smaller scales, heating is concentrated in current sheets, the thicknesses of which are set by the numerical resolution. Some current sheets fragment in time, this process occurring more readily in the higher-resolution model leading to spatially highly intermittent heating. The large scale heating structures are found to fade in less than about five minutes, while the smaller, local, heating shows time scales of the order of 2 minutes in one model and 1 minutes in the other, higher-resolution, model., 20 pages, accepted by ApJ

Ellerman bombs are transient brightenings of the extended wings of the solar Balmer lines in emerging active regions. We describe their properties in the ultraviolet lines sampled by the Interface Region Imaging Spectrograph (IRIS), using simultaneous imaging spectroscopy in H$\alpha$ with the Swedish 1-m Solar Telescope (SST) and ultraviolet images from the Solar Dynamics Observatory for Ellerman bomb detection and identification. We select multiple co-observed Ellerman bombs for detailed analysis. The IRIS spectra strengthen the view that Ellerman bombs mark reconnection between bipolar kilogauss fluxtubes with the reconnection and the resulting bi-directional jet located within the solar photosphere and shielded by overlying chromospheric fibrils in the cores of strong lines. The spectra suggest that the reconnecting photospheric gas underneath is heated sufficiently to momentarily reach stages of ionization normally assigned to the transition region and the corona. We also analyze similar outburst phenomena that we classify as small flaring arch filaments and ascribe to higher-located reconnection. They have different morphology and produce hot arches in million-Kelvin diagnostics., Comment: Accepted for publication in the Astrophysical Journal. 19 pages, 18 figures, see http://folk.uio.no/gregal/onlinematerial/eb3 for movies

The Mg II h&k doublet are two of the primary spectral lines observed by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These lines are tracers of the magnetic and thermal environment that spans from the photosphere to the upper chromosphere. We use a double gaussian model to fit the Mg II h profile for a full-Sun mosaic dataset taken 24-Aug-2014. We use the ensemble of high-quality profile fits to conduct a statistical study on the variability of the line profile as it relates the magnetic structure, dynamics, and center-to-limb viewing angle. The average internetwork profile contains a deeply reversed core and is weakly asymmetric at h2. In the internetwork, we find a strong correlation between h3 wavelength and profile asymmetry as well h1 width and h2 width. The average reversal depth of the h3 core is inversely related to the magnetic field. Plage and sunspots exhibit many profiles which do not contain a reversal. These profiles also occur infrequently in the internetwork. We see indications of magnetically aligned structures in plage and network in statistics associated with the line core, but these structures are not clear or extended in the internetwork. The center-to-limb variations are compared with predictions of semi-empirical model atmospheres. We measure a pronounced limb darkening in the line core which is not predicted by the model. The aim of this work is to provide a comprehensive measurement baseline and preliminary analysis on the observed structure and formation of the Mg II profiles observed by IRIS., Accepted for publication

In recent years realistic 3D numerical models of the solar atmosphere have become available. The models attempt to recreate the solar atmosphere and mimic observations in the best way, in order to make it possible to couple complicated observations with physical properties such as the temperatures, densities, velocities, and magnetic fields. We here present a study of synthetic spectra created using the Bifrost code in order to assess how well they fit with previously taken solar data. A study of the synthetic intensity, nonthermal line widths, Doppler shifts, and correlations between any two of these three components of the spectra first assuming statistical equilibrium is made, followed by a report on some of the effects nonequilibrium ionization will have on the synthesized spectra. We find that the synthetic intensities compare well with the observations. The synthetic observations depend on the assumed resolution and point-spread function (PSF) of the instrument, and we find a large effect on the results, especially for intensity and nonthermal line width. The Doppler shifts produce the reported persistent redshifts for the transition region (TR) lines and blueshifts for the upper TR and corona lines. The nonthermal line widths reproduce the well-known turnoff point around (2–3) × 105 K, but with much lower values than those observed. The nonthermal line widths tend to increase with decreasing assumed instrumental resolution, also when nonequilibrium ionization is included. Correlations between the nonthermal line width of any two TR line studies as reported by Chae et al. are reproduced, while the correlations of intensity to line width are reproduced only after applying a PSF to the data. Doppler shift correlations reported by Doschek for the TR lines and correlations of Doppler shift to nonthermal line width of the Fe xii 19.5 line reported by Doschek et al. are reproduced. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

We use coordinated observations with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1 m Solar Telescope to identify the disk counterpart of type II spicules in upper-chromospheric and transition region (TR) diagnostics. These disk counterparts were earlier identified through short-lived asymmetries in chromospheric spectral lines: rapid blue- or red-shifted excursions (RBEs or RREs). We find clear signatures of RBEs and RREs in Mg II h & k, often with excursions of the central h3 and k3 absorption features in concert with asymmetries in co-temporal and co-spatial Hα spectral profiles. We find spectral signatures for RBEs and RREs in C II 1335 and 1336 Å and Si IV 1394 and 1403 Å spectral lines and interpret this as a sign that type II spicules are heated to at least TR temperatures, supporting other recent work. These C II and Si IV spectral signals are weaker for a smaller network region than for more extended network regions in our data. A number of bright features around extended network regions observed in IRIS slit-jaw imagery SJI 1330 and 1400, recently identified as network jets, can be clearly connected to Hα RBEs and/or RREs in our coordinated data. We speculate that at least part of the diffuse halo around network regions in the IRIS SJI 1330 and 1400 images can be attributed to type II spicules with insufficient opacity in the C II and Si IV lines to stand out as single features in these passbands. © American Astronomical Society. All rights reserved.

Aims. To study the interaction of the solar chromosphere with the transition region, in particular active-region jets in the transition region and their relation to chromospheric fibrils. Methods. We carefully align image sequences taken simultaneously in Civ with the Transition Region and Coronal Explorer and in Hα with the Swedish 1-m Solar Telescope. We examine the temporal evolution of “dynamic fibrils”, i.e., individual short-lived active-region chromospheric jet-like features in Hα. Results. All dynamic fibrils appear as absorption features in Hα that progress from the blue to the red wing through the line, and often show recurrent behavior. Some of them, but not all, appear also as bright features in Civ which develop at or just beyond the apex of the Hα darkening. They tend to best resemble the Hα fibril at +700 mA half a minute earlier. Conclusions. Dynamic chromospheric fibrils observed in Hα regularly correspond to transition-region jets observed in the ultraviolet. This correspondence suggests that some plasma associated with dynamic fibrils is heated to transition-region temperatures.

We present high-resolution G-band observations (obtained with the Swedish 1 m Solar Telescope) of the rapid temporal variability of faculae, which occurs on granular timescales. By combining these observations with magnetoconvection simulations of a plage region, we show that much of this variability is not intrinsic to the magnetic field concentrations that are associated with faculae, but rather a phenomenon associated with the normal evolution and splitting of granules. We also show examples of facular variability caused by changes in the magnetic field, with movies of dynamic behavior of the striations that dominate much of the facular appearance at 01 resolution. Examples of these dynamics include merging, splitting, rapid motion, apparent fluting, and possibly swaying.

The formation of jets such as dynamic fibrils, mottles, and spicules in the solar chromosphere is one of the most important, but also most poorly understood, phenomena of the Sun's magnetized outer atmosphere. We use extremely high-resolution observations from the Swedish 1-m Solar Telescope combined with advanced numerical modeling to show that in active regions these jets are a natural consequence of upwardly propagating slow mode magnetoacoustic shocks. These shocks form when waves generated by convective flows and global p-mode oscillations in the lower lying photosphere leak upward into the magnetized chromosphere. We find excellent agreement between observed and simulated jet velocities, decelerations, lifetimes and lengths. Our findings suggest that previous observations of quiet sun spicules and mottles may also be interpreted in light of a shock driven mechanism., ApJ Letters, in press

There are now many observations of waves in the solar corona with periods around 5 minutes. The source of these waves is uncertain, although global p-modes in the photosphere are an obvious candidate, given the similarity of the dominant periods. However, p-modes are traditionally considered evanescent in the upper photosphere, and it has been unclear how they could propagate through the chromosphere into the corona. Using a numerical model, we show that photospheric oscillations with periods around 5 minutes can actually propagate into the corona so long as they are guided along an inclined magnetic flux tube. The nonverticality of the flux tube increases the acoustic cutoff period to values closer to the dominant periods of the photospheric oscillations, thus allowing tunneling or even direct propagation into the outer atmosphere. The photospheric oscillations develop into shocks, which drive chromospheric spicules and reach the corona. We suggest that Transition Region and Coronal Explorer (TRACE) observations of propagating magnetoacoustic waves in the corona represent these shocked and tunneled photospheric oscillations. We also explore how seismology of these waves could be exploited to determine the connectivity between photosphere and corona.

Solar observations at a resolution of 0.2 arc seconds show the reconnection and relaxation of magnetic braids in a coronal active region, leading to the dissipation of sufficient energy to heat the structures to about 4,000,000 K. The Sun's outer atmosphere, or corona, is millions of degrees hotter than its surface. The search for mechanisms capable of transferring energy from the Sun's interior to the periphery on such a scale has identified two strong candidates: wave heating is thought to heat the corona to 1.5 million K, and the reconnection and unravelling of magnetic braids has been proposed as the means of boosting the temperature towards 4 million K. New evidence to support that latter mechanism is provided by a five-minute series of images obtained by a high-resolution camera on-board a sounding rocket. The images reveal fine-scale braiding to a resolution of about 150 km in a coronal active region, and the images are consistent with energy production sufficient for the observed heating. It is now apparent that there are at least two heating mechanisms in the Sun’s outer atmosphere, or corona1,2,3,4,5. Wave heating may be the prevalent mechanism in quiet solar periods and may contribute to heating the corona to 1,500,000 K (refs 1, 2, 3). The active corona needs additional heating to reach 2,000,000–4,000,000 K; this heat has been theoretically proposed6,7,8,9,10,11,12 to come from the reconnection and unravelling of magnetic ‘braids’. Evidence favouring that process has been inferred13,14, but has not been generally accepted because observations are sparse and, in general, the braided magnetic strands that are thought1,2,3,15,16,17 to have an angular width of about 0.2 arc seconds have not been resolved10,18,19,20. Fine-scale braiding has been seen21,22 in the chromosphere but not, until now, in the corona. Here we report observations, at a resolution of 0.2 arc seconds, of magnetic braids in a coronal active region that are reconnecting, relaxing and dissipating sufficient energy to heat the structures to about 4,000,000 K. Although our 5-minute observations cannot unambiguously identify the field reconnection and subsequent relaxation as the dominant heating mechanism throughout active regions, the energy available from the observed field relaxation in our example is ample for the observed heating.

Although there are now many observations showing the presence of oscillations in the corona, almost no observational studies have focused on the bright upper transition region (TR) emission (the so-called moss) above active region plage. Here we report on a wavelet analysis of observations (made with the Transition Region and Coronal Explorer) of strong (~5%-15%) intensity oscillations in the upper TR footpoints of hot coronal loops. They show a range of periods from 200 to 600 s, typically persisting for 4-7 cycles. These oscillations are not associated with sunspots, as they usually occur at the periphery of plage regions. A preliminary comparison to photospheric vertical velocities (using the Michelson Doppler Imager on board the Solar and Heliospheric Observatory) reveals that some upper TR oscillations show a correlation with p-modes in the photosphere. In addition, a majority of the upper TR oscillations are directly associated with upper chromospheric oscillations observed in Hα, i.e., periodic flows in spicular structures. The presence of such strong oscillations at low heights (of the order of 3000 km) provides an ideal opportunity to study the propagation of oscillations from photosphere and chromosphere into the TR and corona. It can also help us understand the magnetic connectivity in the chromosphere and TR and shed light on the source of chromospheric mass flows such as spicules.

The possible mechanism of generation of spicules by Alfvenic waves is studied in dissipative MHD where dissipation is mainly caused by ion-neutral collision damping, as suggested by Haerendel (1992). Ion-neutral damping becomes non-negligible at the high cyclic frequencies involved, typically greater than 0.1 Hz, and the potential role played by this effect in both forming and supporting solar spicules is investigated. The propagation of high frequency Alfven waves on vertically open solar magnetic flux tubes is considered. The flux tubes are taken to be axisymmetric and initially untwisted with the field strength declining from 1600 G in the photosphere to 10−40 G in the corona. Their propagation is investigated by numerically solving a set of fully nonlinear, dissipative 1.5D MHD equations with the waves being generated by a continuous sinusoidal driver introduced into the equation of angular momentum in the low atmosphere of the Sun. Spicule-like structures with heights of up to 7000 km were formed. The formation was found to be caused by the impact of a series of slow shocks generated by the continuous interaction between the upward propagating driven wave train and the downward propagating train of waves created by reflection off the transition region and aided by the increased thermal pressure gradient caused by Joule heating due to ion-neutral collisions. The adiabatic results suggest that ion-neutral damping may not support spicules as described by Haerendel (1992). However, the effect is highly sensitive to the level of ionisation and therefore to the energy balance. Including the effects of thermal conduction and radiation may well lead to different results and thus it would be premature to dismiss the mechanism completely at this point. In addition, the relatively high chromospheric temperatures obtained, even at frequencies for which ion-neutral damping and heating might be expected to be unimportant, suggest intriguing possibilities for combining the mechanism with others that are better able to recreate spicule dynamics but suffer from unrealistically low temperatures.

The discovery of active region moss, i.e., dynamic and bright upper transition region (TR) emission at chromospheric heights above active region plage, provides a powerful diagnostic to probe the structure, dynamics, energetics, and coupling of the magnetized solar chromosphere and TR. Here we present an observational study of the interaction of the chromosphere with the upper TR, by studying correlations (or lack thereof) between emission at varying temperatures: from the low chromosphere (Ca II K line), to the middle and upper chromosphere (H?), to the low TR (C IV ?1550 at 0.1 MK) and the upper TR (Fe IX/X ?171 at 1 MK and Fe XII ?195 at 1.5 MK). We use several data sets at high cadence (24-42 s) obtained with the Swedish Vacuum Solar Telescope (SVST, La Palma) and the Transition Region and Coronal Explorer (TRACE). This correlation analysis from low chromosphere to upper TR in active region plage quantifies and considerably expands on previous studies. Our results elucidate various issues, such as (1) how the heating mechanisms of the chromosphere and lower and upper TR are related (if at all), (2) how important heating of spicular jets is for the energy balance of the lower TR, (3) which timescales dominate the dynamic behavior of the active region TR, and (4) whether the spatial and temporal variability of moss can be used as a diagnostic for coronal heating.

We report here for the first time observations of prominence velocities over a wide range of temperatures and with a high time cadence. Our study of ultraviolet movies of prominences reveals that multithermal features with speeds of 5-70 km/s perpendicular to the line of sight are common in the prominences which showed traceable motions. These speeds are noticeably higher than the typical speeds of 5-20 km/s observed in \Ha\ data from "quiet" prominences and are more typical of "activated" prominences in which speeds of up to 40 km/s have been reported. The observations were performed using five separate datasets taken by the Solar and Heliospheric Observatory's Coronal Diagnostic Spectrometer (SOHO/CDS) in its wide slit overlap program mode in lines from He approx.I, O approx. V, and Mg approx. IX and a separate prominence observation taken with both the Transition Region and Coronal Explorer (TRACE) in its 1216 and 1600 \A\ bands and in \Ha\ by the Swedish Vacuum Solar Telescope (SVST) at La Palma. The movies were taken with cadences $> 1$ image per minute and were made simultaneously or near-simultaneously in spectral lines formed at two or more temperatures.

High-resolution observations of a quiet-Sun region made on 1998 May 23 with the Swedish Vacuum Solar Telescope (SVST) are presented. I study the dynamics of magnetic elements on scales ≤1'' using Fe I λ6302 magnetograms and cotemporal G-band λ4305, Ca II K line λ3933, and Hα λ6563 filtergrams. This 3 hr long data set, obtained under excellent seeing conditions, reveals several cases in which flux concentrations that have recently emerged in the internetwork are rapidly (within 10-15 minutes) dispersed after emergence. The magnetic flux, initially quite concentrated (~05), seems to get spread out (to several arcseconds) and/or shredded as a result of granular action, until it falls below the noise level in the magnetograms and becomes undetectable. These magnetic flux concentrations are much smaller than ephemeral regions, in both spatial extent and magnetic flux (of order 5 × 1017 Mx). They do not seem to get cancelled by opposite polarity and typically are not associated with G-band bright points or Ca II K line brightenings. These events may be related to the horizontal internetwork fields (HIFs) described by Lites and colleagues.

The heating of the outer solar atmospheric layers, i.e., the transition region and corona, to high temperatures is a long standing problem in solar (and stellar) physics. Solutions have been hampered by an incomplete understanding of the magnetically controlled structure of these regions. The high spatial and temporal resolution observations with the Interface Region Imaging Spectrograph (IRIS) at the solar limb reveal a plethora of short, low lying loops or loop segments at transition-region temperatures that vary rapidly, on the timescales of minutes. We argue that the existence of these loops solves a long standing observational mystery. At the same time, based on comparison with numerical models, this detection sheds light on a critical piece of the coronal heating puzzle., Published in Science on October 17 and can be found on: http://www.sciencemag.org/content/346/6207/1255757.full?sid=23ef14f7-7090-4270-8eb2-35af3def0fa6 17 pages, 5 figures. Movies are available at http://folk.uio.no/viggoh/download/science_movies/

A dominant constituent in the dynamic chromosphere are spicules. Spicules at the limb appear as relatively small and dynamic jets that are observed to everywhere stick out. Many papers emphasize the important role spicules might play in the energy and mass balance of the chromosphere and corona. However, many aspects of spicules remain a mystery. In this Letter we shed more light on the multi-threaded nature of spicules and their torsional component. We use high spatial, spectral and temporal resolution observations from the Swedish 1-m Solar Telescope in the H{\alpha} spectral line. The data targets the limb and we extract spectra from spicules far out from the limb to reduce the line-of-sight superposition effect. We discover that many spicules display very asymmetric spectra with some even showing multiple peaks. To quantify this asymmetry we use a double Gaussian fitting procedure and find an average velocity difference between the single Gaussian components to be between 20-30 km s$^{-1}$ for a sample of 57 spicules. We observe that spicules show significant sub-structure where one spicule consists of many 'threads'. We interpret the asymmetric spectra as line-of-sight superposition of threads in one spicule and therefore have a measure for a perpendicular flow inside spicules which will be important for future numerical model to reproduce. In addition we show examples of {\lambda}-x-slices perpendicular across spicules and find spectral tilts in individual threads providing further evidence for the complex dynamical nature of spicules., Comment: Accepted by APJ Letters