arXiv subjects
Publications and source records attributed to Robert J. Rutten.
The solar spectrum conveys most of our diagnostics to find out how our star works. They must be understood for utilization, but solar spectrum formation is complex because the interaction of matter and radiation within the solar atmosphere suffers non-local control in space, wavelength, and time. These complexities are summarized and illustrated with classic literature. They combine in chromospheric spectrum formation.
Cornelis ("Kees") de Jager, co-founder of "Solar Physics", passed away in 2021. He was an exemplary human being, a great scientist, and had large impact on our field. In this tribute we first briefly summarize his life and career and then describe some of his solar activities, from his PhD thesis on the hydrogen lines in 1952 to the book on cycle-climate relations completed in 2020.
This contribution to "Solar Magnetic Variability and Climate" reviews small-scale magnetic features on the solar surface, in particular the strong-field but tiny magnetic concentrations that constitute network and plage and represent most magnetism outside sunspots and filaments. Where these are mostly of the same polarity, as in active-region plage, their occurrence varies with the activity variations measured by the sunspot number, but when they appear bipolar-mixed on small scales they can also result from granular-scale dynamo action that does not vary with the cycle.
I present the appearance of "Solar Orbiter campfires" in simultaneous images from the Solar Dynamics Observatory where most are visible although less sharp. I also show such features elsewhere in the SDO database. I show some in detail and discuss their nature.
Even in quiet areas underneath coronal holes the solar chromosphere contains ubiquitous heating events. They tend to be small scale and short lived, hence difficult to identify. Here we do not address their much-debated contribution to outer-atmosphere heating, but their aftermaths. We performed a statistical analysis of high-resolution observations in the Balmer H-alpha line to suggest that many slender dark H-alpha fibrils spreading out from network represent cooling gas that outlines tracks of preceding rapid type II spicule events or smaller similar but as yet unresolved heating agents in which the main gas constituent, hydrogen, ionizes at least partially. Subsequent recombination then causes dark H-alpha fibrils enhanced by nonequilibrium overopacity. We suggest that the extraordinary fibrilar appearance of the H-alpha chromosphere around network results from intermittent, frequent small-scale prior heating.
This is an overview of non-equilibrium aspects of the formation of solar continua and lines affecting the contributions by magnetic network and plage to spectrally resolved solar irradiance. After a brief summary of these contributions and a compact refresher of solar spectrum formation, the emphasis is on graphical exposition. Major obstacles for simulation-based irradiance studies are how to cope with NLTE scattering in the violet and ultraviolet line haze and how to cope with retarded hydrogen opacities in infrared and mm radiation.
Ellerman bombs are transient brightenings in the wings of H-alpha 6563 Å that pinpoint photospheric sites of magnetic reconnection in solar active regions. Their partial visibility in the 1600 Å and 1700 Å continua registered routinely by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) offers a unique opportunity to inventory such magnetic-field disruptions throughout the AIA database if a reliable recipe for their detection can be formulated. This is done here. We improve and apply an H-alpha Ellerman bomb detection code to ten data sets spanning viewing angles from solar disc centre to the limb. They combine high-quality H-alpha imaging spectroscopy from the Swedish 1-m Solar Telescope with simultaneous AIA imaging around 1600 Å and 1700 Å. A trial grid of brightness, lifetime and area constraints is imposed on the AIA images to define optimal recovery of the 1735 Ellerman bombs detected in H-alpha. The best results when optimising simultaneously for recovery fraction and reliability are obtained from 1700 Å images by requiring 5-sigma brightening above the average 1700 Å nearby quiet-Sun intensity, lifetime above one minute, area of 1-18 AIA pixels. With this recipe 27% of the AIA detections are H-alpha-detected Ellerman bombs while it recovers 19% of these (of which many are smaller than the AIA resolution). Better yet, among the top 10% AIA 1700 Å detections selected with combined brightness, lifetime and area thresholds as many as 80% are H-alpha Ellerman bombs. Automated selection of the best 1700 Å candidates therefore opens the entire AIA database for detecting most of the more significant photospheric reconnection events. This proxy is applicable as flux-dynamics tell-tale in studying any Earth-side solar active region since early 2010 up to the present.
The term "ultraviolet (UV) burst" is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016-2017.
I have proposed that long Halpha fibrils are caused by heating events of which the tracks are afterwards outlined by contrails of cooling gas with extraordinary Halpha opacity and yet larger opacity at the ALMA wavelengths. Here I detail the radiative transfer background.
In H-alpha most of the solar surface is covered by dense canopies of long opaque fibrils, but predictions for quiet-Sun observations with ALMA have ignored this fact. Comparison with Ly-alpha suggests that the large opacity of H-alpha fibrils is caused by hot precursor events. Application of a recipe that assumes momentary Saha-Boltzmann extinction during their hot onset to millimeter wavelengths suggests that ALMA will observe H-alpha-like fibril canopies, not acoustic shocks underneath, and will yield data more interesting than if these canopies were transparent.
We describe a new quiet-Sun phenomenon which we call "Quiet-Sun Ellerman-like Brightenings" (QSEB). QSEBs are similar to Ellerman bombs (EB) in some respects but differ significantly in others. EBs are transient brightenings of the wings of the Balmer H-alpha line that mark strong-field photospheric reconnection in complex active regions. QSEBs are similar but smaller and less intense Balmer-wing brightenings that occur in quiet areas away from active regions. In the H-alpha wing we measure typical lengths of less than 0.5 arcsec, widths of 0.21 arcsec, and lifetimes of less than a minute. We discovered them using high-quality H-alpha imaging spectrometry from the Swedish 1-m Solar Telescope (SST) and show that in lesser-quality data they cannot be distinguished from more ubiquitous facular brightenings, nor in the ultraviolet diagnostics currently available from space platforms. We add evidence from concurrent SST spectropolarimetry that QSEBs also mark photospheric reconnection events, but in quiet regions on the solar surface.
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$α$ 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.
We use high-resolution imaging spectroscopy with the Swedish 1-m Solar Telescope (SST) to study the transient brightenings of the wings of the Balmer Halpha line in emerging active regions that are called Ellerman bombs. Simultaneous sampling of Ca II 854.2 nm with the SST confirms that most Ellerman bombs occur also in the wings of this line, but with markedly different morphology. Simultaneous images from the Solar Dynamics Observatory (SDO) show that Ellerman bombs are also detectable in the photospheric 170 nm continuum, again with differing morphology. They are also observable in 160 nm SDO images, but with much contamination from C IV emission in transition-region features. Simultaneous SST spectropolarimetry in Fe I 630.1 nm shows that Ellerman bombs occur at sites of strong-field magnetic flux cancelation between small bipolar strong-field patches that rapidly move together over the solar surface. Simultaneous SDO images in He II 30.4 nm, Fe IX 17.1 nm, and Fe XIV 21.1 nm show no clear effect of the Ellerman bombs on the overlying transition region and corona. These results strengthen our earlier suggestion, based on Halpha morphology alone, that the Ellerman bomb phenomenon is a purely photospheric reconnection phenomenon.
Ellerman bombs are short-lived brightenings of the outer wings of Halpha that occur in active regions with much flux emergence. We point out fads and fallacies in the extensive Ellerman bomb literature, discuss their appearance in various spectral diagnostics, and advocate their use as indicators of field reconfiguration in active-region topography using AIA 1700 A images.
Type-II solar spicules appear as long, thin, highly dynamic strands of field-tied matter that feed significant mass and energy to the corona and solar wind. A recent result is that they exhibit torsional Alfvén waves in addition to accelerating outflows and swaying motions due to transverse Alfvénic waves. I summarize this finding and then re-interpret older observations in its light: the striking similarity of near-limb scenes in the outer blue and red wings of Hα, and the tilts of absorption lines with respect to emission lines in eclipse spectra taken in 1973.
The overall structure and the fine structure of the solar photosphere outside active regions are largely understood, except possibly important roles of a turbulent near-surface dynamo at its bottom, internal gravity waves at its top, and small-scale vorticity. Classical 1D static radiation-escape modelling has been replaced by 3D time-dependent MHD simulations that come closer to reality. The solar chromosphere, in contrast, remains ill-understood although its pivotal role in coronal mass and energy loading makes it a principal research area. Its fine structure defines its overall structure, so that hard-to-observe and hard-to-model small-scale dynamical processes are the key to understanding. However, both chromospheric observation and chromospheric simulation presently mature towards the required sophistication. The open-field features seem of greater interest than the easier-to-see closed-field features.
High-resolution imaging-spectroscopy movies of solar active region NOAA 10998 obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope show very bright, rapidly flickering, flame-like features that appear intermittently in the wings of the Balmer H-alpha line in a region with moat flows and likely some flux emergence. They show up at regular H-alpha blue-wing bright points that outline magnetic network, but flare upward with much larger brightness and distinct "jet" morphology seen from aside in the limbward view of these movies. We classify these features as Ellerman bombs and present a morphological study of their appearance at the unprecedented spatial, temporal, and spectral resolution of these observations. The bombs appear along magnetic network with footpoint extents up to 900km. They show apparent travel away from the spot along the pre-existing network at speeds of about 1 km/s. The bombs flare repetitively with much rapid variation at time scales of seconds only, in the form of upward jet-shaped brightness features. These reach heights of 600-1200km and tend to show blueshifts; some show bi-directional Doppler signature, and some seem accompanied with an H-alpha surge. They are not seen in the core of H-alpha due to shielding by overlying chromospheric fibrils. The network where they originate has normal properties. The morphology of these jets strongly supports deep-seated photospheric reconnection of emergent or moat-driven magnetic flux with pre-existing strong vertical network fields as the mechanism underlying the Ellerman bomb phenomenon.
The introduction to this review summarizes chromosphere observation in two figures. The first part showcases the historical emphasis on the eclipse chromosphere in the development of NLTE line formation theory and criticizes 1D modeling. The second part advertises recent breakthroughs after many decades of standstill. The third part discusses what may or should come next.