High-resolution imagery shows the quiet-Sun chromosphere as a web of filamentary structures: H-alpha mottles and fibrils, and slender upright network features in Ca II H and Lyman-alpha that Rutten (2007) called 'straws' and described as 'hot and dense from unidentified mass and energy filling'. Rutten conjectured four structure types (hot upright straws; p-mode-loaded mottles; cool cell-spanning mottles; and hard-to-observe short weak-field near-network loops postulated by Schrijver & Title 2003) and called for comprehensive multi-dimensional radiative-dynamic models treating chromospheric cylinders and sheets with the full LTE-NLTE-PRD radiative physics of the classical 1D semi-empirical models. Later work resolved parts of this program: magnetoacoustic shocks from leaked p-modes were confirmed as the driver of dynamic fibrils and quiet-Sun mottles; non-equilibrium hydrogen ionization was computed in radiation-MHD simulations; 3D radiative-MHD codes and 3D non-LTE transfer with partial redistribution became available; first direct observations of low-lying 'unresolved fine structure' loops were reported; and the rapidly heated Type II spicules were discovered and identified with the straws. What remains open is the identification of the mechanism(s) that heat and mass-fill the hot network fine structure and their contribution to the transition-region/coronal mass and energy budget, the prevalence and connectivity role of the low-lying loops, and a simultaneously coupled multi-dimensional model validated against the multiwavelength appearance of the chromosphere.

Background

The solar chromosphere is the complex, dynamic layer of the Sun's atmosphere between the photosphere (the visible surface, near 5800 K) and the multi-million-degree corona. In the classical one-dimensional semi-empirical models (the VAL series of Vernazza, Avrett & Loeser 1981 and the FAL series of Fontenla, Avrett & Loeser 1993) it is a roughly horizontal slab: temperatures dip to about 4000 K, rise to about 10410^4 K, and then jump through the thin transition region to coronal values near 10610^6 K. High-resolution imaging has always contradicted this slab picture. In the core of the hydrogen Balmer line H-alpha the quiet Sun shows a web of dark elongated features called mottles (rooted in the photospheric magnetic network) and fibrils (jutting from active regions); at the limb the same family appears as jets called spicules. In the violet line cores of Ca II H&K the network is covered with bright grains and short bright stalks. The Dutch Open Telescope (DOT), with image reconstruction, additionally revealed near the limb long, thin, bright, mostly upright Ca II H features jutting out from the network in 'hedge rows', which Rutten called 'straws', and sounding-rocket Lyman-alpha (Ly-alpha, the hydrogen 1216 Angstrom resonance line) imagery showed hedge rows of straws roughly twice as high and optically thick. Rutten (2007) synthesized such imagery into a four-type taxonomy of the filamentary structures between photosphere and corona: Type 1 = hot, dense, upright straws opening into coronal plasma, 'hot and dense from unidentified mass and energy filling'; Type 2 = mottles loaded by magnetoacoustic shocks fed by photospheric p-mode oscillations leaking along inclined magnetic field, a mechanism proposed by De Pontieu, Erdélyi & James (2004) (p-modes are the Sun's global five-minute acoustic oscillations); Type 3 = cool, opaque mottles spanning the internetwork cells; Type 4 = short, weak-field near-network loops postulated by Schrijver & Title (2003) from 'magnetic carpet' modeling (the pervasive mixed-polarity carpet of small photospheric magnetic flux), which lack both the photospheric evacuation (the Wilson depression that makes other structures visible as bright points) and the mass loading that would give them any brightness signature. Three radiative-physics terms are needed: LTE (local thermodynamic equilibrium) means atomic level populations follow the local temperature; NLTE means they must instead be solved jointly with the radiative-transfer equations; and PRD (partial redistribution) describes the frequency correlation between absorbed and re-emitted photons in strong resonance lines such as Ly-alpha and Ca II H&K, an effect that must be treated to model chromospheric line cores correctly. Rutten conjectured that chromospheric modeling must be upgraded from 1D slab geometry to chromospheric cylinders and sheets embedded in coronal or upper-photospheric surroundings, with transition-region sheaths, treated with the full VAL/FAL radiative physics from high-density LTE through NLTE and PRD up to coronal conditions, and fed dynamically by p-mode pistoning, convective 'squirting', siphon flows (pressure-gradient-driven flows along a magnetic loop), and conductive energy input from above; the ionization balances and level populations inside the structures (e.g., Ca II populations and the neutral-hydrogen n=2n=2 population that controls H-alpha opacity) were identified as the key uncomputed quantities. The subsequent literature reframed this taxonomy. Hansteen et al. (2006) and De Pontieu et al. (2007) established the shock-driven dynamic fibrils and codified 'Type I' spicules (p-mode/shock driven, 3-7 minute periodicities), while discovering 'Type II' spicules: rapid, thin (widths under 200 km) jets that form in tens of seconds, seem to be heated to at least transition-region temperatures, and whose on-disk manifestations De Pontieu et al. (2007) explicitly identified with Rutten's straws. The Interface Region Imaging Spectrograph (IRIS), launched 2013, added ultraviolet Mg II, C II and Si II/IV spectroscopy of these structures, and 3D radiative-MHD codes such as Bifrost (a widely used solar radiative magnetohydrodynamics simulation code), coupled to 3D non-LTE radiative transfer with non-equilibrium ionization and PRD, replaced 1D semi-empirical models as the community's standard tool (Carlsson, De Pontieu & Hansteen 2019; Leenaarts 2020). Rutten's own companion review (Rutten 2007, 'Observing the Solar Chromosphere') and the review of Tsiropoula et al. (2012) document the observational phenomenology of these jets. The unresolved questions are what physically fills and heats the hot network fine structure, how prevalent the hidden low-lying loops are, and whether any coupled model can reproduce the entire multiwavelength appearance.

Problem Statement

Determine the physical nature of the quiet-Sun chromosphere's filamentary fine structure between the photosphere and the corona, as posed by Rutten (2007): (i) identify the mechanism(s) behind the 'unidentified mass and energy filling' of the hot, dense, upright network structures ('straws': the slender bright Ca II H features at and near the limb and the Ly-alpha hedge rows; in the modern framework largely corresponding to the rapidly heated Type II spicules) - what supplies their mass from below, what heats them to transition-region temperatures, and what their contribution is to the transition-region and coronal mass and energy budget; (ii) establish observationally the prevalence and network-connectivity role of the short, weak-field, near-network loops (Rutten's Type 4, postulated by Schrijver & Title 2003, now called unresolved fine structure loops), whose existence has been directly reported (Hansteen et al. 2014) but whose lack of evacuation and mass-loading signatures makes their prevalence and connectivity hard to establish; and (iii) construct and validate comprehensive multi-dimensional radiative-dynamic models of these chromospheric filaments - cylinders and sheets with transition-region sheaths, treated with the full radiative physics of the classical VAL/FAL models (high-density LTE through NLTE with partial redistribution, optically thick to optically thin, up to coronal contact) and driven dynamically by p-mode pistoning, convective flows, siphoning, and conductive energy input from above - reproducing and validated against multiwavelength imagery (H-alpha, Ca II H&K, Ly-alpha, and UV transition-region lines), thereby also fixing the ionization balances and level populations (e.g., Ca II fractional populations, the H I n=2n=2 population) inside the structures. The three parts are jointly required and share one objective: a single validated physical account of what fills, heats, and bounds the quiet chromosphere's fine structure; an answer that treats only one part resolves only that part, not the task as a whole. Each part is resolved by a documented, source-grounded physical account - for (i), identification of the heating and mass-supply mechanism(s) with estimates of their mass and energy budget contribution; for (ii), an observational assessment of the loops' prevalence and network-connectivity role; and for (iii), a specified multi-dimensional model design together with the criterion by which it is validated - rather than by delivering the named artifact (a completed simulation or a new observation); the drivers listed in (iii) are illustrative of the class of dynamics to be treated. Two auxiliary predictions of the source paper - the five-minute modulation of mottle endpoints and Dopplershifts from p-mode mass loading, and the importance of non-equilibrium hydrogen ionization in dynamic chromospheres - have since been substantially confirmed or realized and are excluded from the scope of this task; this exclusion does not remove the requirement in (iii) that the model compute the ionization balances and level populations it names.

References

  1. Robert J. Rutten. On the Nature of the Solar Chromosphere. ASP Conference Series 354, 276 (2007). arXiv:astro-ph/0701379, https://arxiv.org/abs/astro-ph/0701379 (source paper)
  2. Robert J. Rutten. Observing the Solar Chromosphere. ASP Conference Series 368, 27 (2007). arXiv:astro-ph/0703637, https://arxiv.org/abs/astro-ph/0703637
  3. J. E. Vernazza, E. H. Avrett, R. Loeser. Structure of the solar chromosphere. III. Models of the EUV brightness components of the quiet-Sun. The Astrophysical Journal Supplement Series 45, 635 (1981). DOI 10.1086/190731, https://doi.org/10.1086/190731
  4. J. M. Fontenla, E. H. Avrett, R. Loeser. Energy balance in the solar transition region. III. Helium emission in hydrostatic, constant-abundance models with diffusion. The Astrophysical Journal 406, 319 (1993). DOI 10.1086/172443, https://doi.org/10.1086/172443
  5. C. J. Schrijver, A. M. Title. The Magnetic Connection between the Solar Photosphere and the Corona. The Astrophysical Journal 597, L165-L168 (2003). DOI 10.1086/379870, https://doi.org/10.1086/379870
  6. B. De Pontieu, R. Erdélyi, S. P. James. Solar chromospheric spicules from the leakage of photospheric oscillations and flows. Nature 430, 536-539 (2004). DOI 10.1038/nature02749, https://doi.org/10.1038/nature02749
  7. V. H. Hansteen, B. de Pontieu, L. Rouppe van der Voort, M. van Noort, M. Carlsson. Dynamic Fibrils are driven by Magnetoacoustic Shocks. The Astrophysical Journal 647, L73-L76 (2006). arXiv:astro-ph/0607332, https://arxiv.org/abs/astro-ph/0607332
  8. L. H. M. Rouppe van der Voort, B. de Pontieu, V. H. Hansteen, M. Carlsson, M. van Noort. Magnetoacoustic shocks as driver of quiet Sun mottles. The Astrophysical Journal 660, L169-L172 (2007). arXiv:astro-ph/0703535, https://arxiv.org/abs/astro-ph/0703535
  9. B. De Pontieu, S. W. McIntosh, V. H. Hansteen, M. Carlsson, C. J. Schrijver, T. D. Tarbell, A. M. Title, R. A. Shine, Y. Suematsu, S. Tsuneta, Y. Katsukawa, K. Ichimoto, T. Shimizu, S. Nagata. A Tale of Two Spicules: The Impact of Spicules on the Magnetic Chromosphere. Publications of the Astronomical Society of Japan 59, S655 (2007). DOI 10.1093/pasj/59.sp3.s655, arXiv:0710.2934, https://arxiv.org/abs/0710.2934
  10. J. Leenaarts, M. Carlsson, V. Hansteen, R. J. Rutten. Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere. Astronomy & Astrophysics 473, 625-632 (2007). DOI 10.1051/0004-6361:20078161, arXiv:0709.3751, https://arxiv.org/abs/0709.3751
  11. Philip G. Judge, Mats Carlsson. On the solar chromosphere observed at the limb with Hinode. The Astrophysical Journal 719, 469-473 (2010). DOI 10.1088/0004-637x/719/1/469, arXiv:1004.1398, https://arxiv.org/abs/1004.1398
  12. Philip G. Judge, Bart de Pontieu, Scott McIntosh, Kosovare Olluri. The Connection of Type II Spicules to the Corona. The Astrophysical Journal 746, 158 (2012). DOI 10.1088/0004-637x/746/2/158, arXiv:1112.6174, https://arxiv.org/abs/1112.6174
  13. G. Tsiropoula, K. Tziotziou, I. Kontogiannis, M. S. Madjarska, J. G. Doyle, Y. Suematsu. Solar Fine-Scale Structures. I. Spicules and Other Small-Scale, Jet-Like Events at the Chromospheric Level: Observations and Physical Parameters. Space Science Reviews 169, 181-244 (2012). DOI 10.1007/s11214-012-9920-2, https://doi.org/10.1007/s11214-012-9920-2
  14. V. Hansteen, B. De Pontieu, M. Carlsson, J. Lemen, A. Title, P. Boerner, N. Hurlburt, T. D. Tarbell, J. P. Wuelser, T. M. D. Pereira, E. E. De Luca, L. Golub, S. McKillop, K. Reeves, S. Saar, P. Testa, H. Tian, C. Kankelborg, S. Jaeggli, L. Kleint, J. Martinez-Sykora. The unresolved fine structure resolved: IRIS observations of the solar transition region. Science 346, 1255757 (2014). DOI 10.1126/science.1255757, https://doi.org/10.1126/science.1255757
  15. H. Tian, E. E. DeLuca, S. R. Cranmer, B. De Pontieu, H. Peter, J. Martinez-Sykora, L. Golub, S. McKillop, K. K. Reeves, M. P. Miralles, P. McCauley, S. Saar, P. Testa, M. Weber, N. Murphy, J. Lemen, A. Title, P. Boerner, N. Hurlburt, T. D. Tarbell, J. P. Wuelser, L. Kleint, C. Kankelborg, S. Jaeggli, M. Carlsson, V. Hansteen, S. W. McIntosh. Prevalence of small-scale jets from the networks of the solar transition region and chromosphere. Science 346, 1255711 (2014). DOI 10.1126/science.1255711, arXiv:1410.6143, https://arxiv.org/abs/1410.6143
  16. D. J. Schmit, Bart De Pontieu. What Is the Source of Quiet Sun Transition Region Emission? The Astrophysical Journal 831, 158 (2016). DOI 10.3847/0004-637x/831/2/158, arXiv:1608.07620, https://arxiv.org/abs/1608.07620
  17. Juan Martínez-Sykora, Bart De Pontieu, Viggo H. Hansteen, Luc Rouppe van der Voort, Mats Carlsson, Tiago M. D. Pereira. On the generation of solar spicules and Alfvénic waves. Science 356, 1269-1272 (2017). DOI 10.1126/science.aah5412, arXiv:1710.07559, https://arxiv.org/abs/1710.07559
  18. G. Chintzoglou, B. de Pontieu, J. Martinez-Sykora, T. M. D. Pereira, A. Vourlidas, S. Tun Beltran. Bridging the Gap: Capturing the Ly-alpha Counterpart of a Type-II Spicule and Its Heating Evolution with VAULT2.0 and IRIS Observations. The Astrophysical Journal 857, 73 (2018). DOI 10.3847/1538-4357/aab607, https://doi.org/10.3847/1538-4357/aab607
  19. Tanmoy Samanta, Hui Tian, Vasyl Yurchyshyn, Hardi Peter, Wenda Cao, Alphonse Sterling, Robertus Erdélyi, Kwangsu Ahn, Song Feng, Dominik Utz, Dipankar Banerjee, Yajie Chen. Generation of solar spicules and subsequent atmospheric heating. Science 366, 890-894 (2019). DOI 10.1126/science.aaw2796, arXiv:2006.02571, https://arxiv.org/abs/2006.02571
  20. Mats Carlsson, Bart De Pontieu, Viggo H. Hansteen. New View of the Solar Chromosphere. Annual Review of Astronomy and Astrophysics 57, 189-226 (2019). DOI 10.1146/annurev-astro-081817-052044, https://doi.org/10.1146/annurev-astro-081817-052044
  21. Jorrit Leenaarts. Radiation hydrodynamics in simulations of the solar atmosphere. Living Reviews in Solar Physics 17, 3 (2020). DOI 10.1007/s41116-020-0024-x, https://doi.org/10.1007/s41116-020-0024-x
  22. B. De Pontieu, V. Polito, V. Hansteen, P. Testa, K. K. Reeves, P. Antolin, D. Nobrega-Siverio, A. Kowalski, J. Martinez-Sykora, M. Carlsson, S. W. McIntosh, W. Liu, A. Daw, C. C. Kankelborg. A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS). Solar Physics 296, 84 (2021). DOI 10.1007/s11207-021-01826-0, arXiv:2103.16109, https://arxiv.org/abs/2103.16109
  23. Quentin Noraz, Mats Carlsson, Guillaume Aulanier. Chromosphere of the quiet sun - I. Shock and current-sheet dynamics and heating. Astronomy & Astrophysics 705, A86 (2026). DOI 10.1051/0004-6361/202556375, arXiv:2511.01858, https://arxiv.org/abs/2511.01858
  24. Jin Li, Lei Ni, Robertus Erdélyi, Jun Lin, Xiaoli Yan, Guanchong Cheng. Radiative MHD Studies of Solar Spicules: Driving Mechanisms and the Role of Background Magnetic Field (2026). arXiv:2605.24374, https://arxiv.org/abs/2605.24374