Solar spicules — thin, jet-like plasma structures rooted in the chromosphere — have been proposed for decades as major carriers of mass and energy into the million-degree solar corona, but whether their heated plasma constitutes bulk injection that materially feeds the coronal energy and mass budgets has never been quantitatively established. Building on the detection of localized ~1 MK emission measure co-spatial and co-temporal with spicules, the open problem is twofold and joint: (1) determine whether spicule-heated plasma contributes substantially to coronal mass and energy budgets; and (2) determine, so far as the currently available observations and simulations permit, whether the outward-propagating coronal disturbances associated with spicules are field-aligned plasma flows, slow-mode magnetoacoustic waves (including shock-steepened compressive waves), or conduction/heating fronts — a discrimination that bears directly on the proposed causal chain from spicule generation, via Alfvénic waves and electric currents, to in-loop dissipation. Neither question is settled by current imaging-only data; both can be pursued with the coordinated multi-wavelength observations and multi-dimensional radiative magnetohydrodynamic simulations already available, while the spectrally resolved coronal spectroscopy expected from MUSE and Solar-C EUVST would make the determination definitive.

Background

The solar corona is maintained at about 11–22 million kelvin (MK), roughly a thousand times hotter than the photosphere, and identifying the process that transports the required energy and mass upward — the coronal heating problem — has stood for some eighty years. Spicules are ubiquitous, short-lived (tens of seconds to a few minutes), thin (~100–200 km) jets of cool chromospheric plasma that shoot upward along magnetic field lines through the transition region (TR, the thin layer where temperature rises from ~10,000 K to ~1 MK). 'Type-II' spicules, identified by De Pontieu et al. (2007) from Hinode/Solar Optical Telescope data, form a distinct, faster, more dynamic subclass: they live ~50–150 s, reach 30–100 km s−1^{-1} upward speeds, and appear to fade rather than fall, suggesting heating of their plasma while in flight. On the solar disk their counterparts appear as rapid blue-shifted excursions (RBEs) in chromospheric spectral lines and as 'network jets' in TR lines observed by IRIS, the Interface Region Imaging Spectrograph, a NASA satellite spectrograph imaging the chromosphere and TR in ultraviolet passbands; the co-pointing Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory images the corona in extreme-ultraviolet channels. De Pontieu et al. (2011) argued from coordinated Hinode and Solar Dynamics Observatory observations — chromospheric imaging and extreme-ultraviolet spectroscopy from Hinode, coronal imaging from AIA — that type-II spicules eject plasma that is heated to at least coronal temperatures, potentially dominating the corona's mass and energy supply, but this claim was contested on grounds of insufficient resolution and possible contamination of coronal channels by much cooler (~0.1 MK) TR plasma. Radiative magnetohydrodynamic (MHD) simulations — which self-consistently treat magnetic fields, plasma radiation, and thermal conduction — by Martínez-Sykora et al. (2017, 2018) reproduce type-II spicules driven by magnetic reconnection and ambipolar diffusion, launch Alfvénic waves (transverse magnetic-oscillation waves that carry energy along field lines without compressing the plasma) and electric currents into the corona, and predict dissipation that heats spicular plasma to TR and coronal temperatures while producing synthetic propagating coronal disturbances (PCDs): quasi-periodic bright ridges moving outward along coronal loops in space-time diagrams at roughly the sound speed (~100 km s−1^{-1}). Samanta et al. (2019) showed observationally that spicule-producing jets are driven by photospheric magnetic flux cancellation and are followed by heating signatures. The central observational difficulty is thermodynamic: coronal imager passbands are contaminated by TR emission, so the amount of genuinely ~1 MK plasma associated with a spicule can only be inferred from a differential emission measure (DEM) analysis — an inversion yielding the emission measure (EM; the column density of emitting plasma squared-integrated over temperature bins) as a function of temperature. Bose et al. (2025) performed the most stringent DEM analysis to date, using two independent inversion algorithms (basis-pursuit/L1L^1 and Tikhonov/L2L^2) on coordinated IRIS and AIA observations of a quiet-Sun region and a coronal hole (a region of open magnetic field). They found ubiquitous small-scale ~1 MK EM enhancements, 2–3 times the background, in clear spatio-temporal coherence with spicules and network jets, with a lower temperature threshold near 500,000 K — the strongest evidence yet that spicular plasma genuinely reaches lower-coronal temperatures. However, their space-time analysis shows the associated coronal signals propagating outward at plane-of-sky speeds of ~15–45 km s−1^{-1}, which is equally consistent with field-aligned mass flows viewed obliquely, with slow-mode magnetoacoustic waves (compressive waves propagating near the sound speed), or with thermal conduction or heating fronts — and imaging data alone cannot distinguish these. Moreover, as the authors state explicitly, the paper 'does not attempt to answer whether spicules play any role in heating the corona as a whole': quantifying the global energetic and mass impact requires broader coverage and spectrally resolved coronal diagnostics that no current instrument combination provides. Existing dataset limitations compound the ambiguity: IRIS raster cadence under-samples short-lived spicules, single-Gaussian fits to the Si IV line profile may miss high-velocity components in the line wings, radiative-transfer effects in the optically thick Mg II line complicate Doppler interpretations, and DEM inversions are unreliable below log⁡T[K]≈5.6\log T[\mathrm{K}]\approx5.6. The decisive capabilities — high-resolution, high-cadence coronal spectroscopy covering entire loop lengths and their footpoints — are expected from the upcoming NASA MUSE (Multi-slit Solar Explorer, planned launch ~2027) and JAXA-led Solar-C EUVST (the EUV High-Throughput Spectroscopic Telescope, planned ~2028) missions, coordinated with IRIS and ground-based telescopes such as the Daniel K. Inouye Solar Telescope (DKIST, a 4-m aperture solar telescope on Maui) and the Swedish 1-m Solar Telescope (SST, on La Palma); as of 2026 these missions are still under development. Several post-2025 studies have advanced partial, environment-, mechanism-, or model-specific pieces of this problem — radiative-MHD studies of spicule formation mechanisms (Li et al., 2026), shock-driven routes to propagating coronal disturbances (Chaurasiya et al., 2026), coronal-hole mass- and wave-energy-flux estimates from simulations combined with observations (Ni et al., 2026), rapid-blue-excursion mass-flux statistics (Rahangdale, 2026), and Alfvénic-fluctuation measurements in polar spicules (Tajfirouze et al., 2026) — but each is confined to a single environment, formation mechanism, or model, or infers the disturbances from reduced models or imaging rather than resolved coronal spectroscopy; none therefore delivers the spicule-driven mass and energy flux across the representative coronal environments, stated relative to each environment's supply requirement, and none settles the propagating-disturbance mechanism, which remains attributed variously to shocks, compressive waves, and bulk flows. Both deliverables consequently remain open.

Problem Statement

Determine the role of type-II spicules in the mass and energy balance of the solar corona, as left open by Bose et al. (2025). This comprises two jointly linked questions that together define one scientific objective: (1) establish whether the ~1 MK plasma associated with spicules, demonstrated locally by differential-emission-measure analysis, corresponds to bulk mass and energy injection that contributes substantially to coronal mass and energy budgets — where 'substantially' denotes a spicule-driven mass and energy flux that supplies at least of order ten percent of an environment's coronal mass and energy supply requirement — requiring quantitative estimates of the spicule-driven mass and energy flux into the corona over representative coronal environments (at least quiet Sun and coronal holes, and active regions where the data permit), with defensible uncertainties, stated as a fraction of the coronal energy and mass supply requirements of each environment, rather than localized emission-measure detections; and (2) determine, so far as the currently available observations and the published multi-dimensional radiative MHD simulations permit, which of the candidate mechanisms produces the spicule-associated propagating coronal disturbances — field-aligned plasma flows, slow-mode magnetoacoustic waves (including shock-steepened compressive waves), or thermal conduction/heating fronts — a discrimination that bears on the proposed causal chain in which spicule generation launches Alfvénic waves and electric currents that dissipate within coronal loops to produce the observed disturbances. Coordinated, high-cadence, high-resolution, spectrally resolved coronal observations covering entire loop lengths and their footpoints — of the kind expected from MUSE and Solar-C EUVST, together with IRIS and ground-based telescopes such as DKIST and SST — along with quantitative comparison against multi-dimensional radiative MHD simulations, constitute the obvious next step toward settling both questions. Resolving these questions requires determining, from the currently available coordinated observations and the published multi-dimensional radiative MHD simulations, how far the spicule-driven mass and energy flux accounts for each representative environment's coronal supply requirement, and what the available data establish about which candidate mechanism produces the observed propagating disturbances.

References

  1. Souvik Bose, Jayant Joshi, Paola Testa, Bart De Pontieu, 'On the Million-degree Signature of Spicules', The Astrophysical Journal Letters 983, L7 (2025), DOI: 10.3847/2041-8213/adc30d, arXiv:2503.08887, https://doi.org/10.3847/2041-8213/adc30d
  2. Bart De Pontieu, Scott McIntosh, Viggo H. Hansteen, Mats Carlsson, Carolus J. Schrijver, Theodore D. Tarbell, Alan M. Title, Richard A. Shine, Yoshinori Suematsu, Saku Tsuneta, Yukio Katsukawa, Kiyoshi Ichimoto, Toshifumi Shimizu, Shin'ichi 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, https://doi.org/10.1093/pasj/59.sp3.s655
  3. B. De Pontieu, S. W. McIntosh, M. Carlsson, V. H. Hansteen, T. D. Tarbell, P. Boerner, J. Martinez-Sykora, C. J. Schrijver, A. M. Title, 'The Origins of Hot Plasma in the Solar Corona', Science 331, 55 (2011), DOI: 10.1126/science.1197738, https://doi.org/10.1126/science.1197738
  4. 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 (2017), DOI: 10.1126/science.aah5412, arXiv:1710.07559, https://doi.org/10.1126/science.aah5412
  5. Bart De Pontieu, Ineke De Moortel, Juan Martinez-Sykora, Scott McIntosh, 'Observations and numerical models of solar coronal heating associated with spicules', The Astrophysical Journal Letters 845, L18 (2017), DOI: 10.3847/2041-8213/aa7fb4, arXiv:1710.06790, https://doi.org/10.3847/2041-8213/aa7fb4
  6. Juan Martínez-Sykora, Bart De Pontieu, Ineke De Moortel, Viggo H. Hansteen, Mats Carlsson, 'Impact of Type II Spicules in the Corona: Simulations and Synthetic Observables', The Astrophysical Journal 860, 116 (2018), DOI: 10.3847/1538-4357/aac2ca, arXiv:1805.06475, https://doi.org/10.3847/1538-4357/aac2ca
  7. 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 (2019), DOI: 10.1126/science.aaw2796, arXiv:2006.02571, https://doi.org/10.1126/science.aaw2796
  8. Ravi Chaurasiya, Sankalp Srivastava, Piyali Chatterjee, Sahel Dey, Robertus Erdélyi, Ankala Raja Bayanna, 'On the Relationship between Solar Spicules and Propagating Coronal Disturbances: The Role of Shocks', The Astrophysical Journal 1005, 111 (2026), DOI: 10.3847/1538-4357/ae75eb, arXiv:2606.02516, https://doi.org/10.3847/1538-4357/ae75eb
  9. Lei Ni, Jun Lin, Tanmoy Samanta, Guanchong Cheng, Yifu Wang, Robert Erdelyi, 'Energy and mass transport associated with impulsive spicular flows in solar coronal holes' (2026), arXiv:2602.19953, https://arxiv.org/abs/2602.19953
  10. Govindswaroop Rahangdale, 'Statistical Study of Rapid Blue Excursions as Mass Conduits in Solar Atmosphere' (2026), arXiv:2602.22811, https://arxiv.org/abs/2602.22811
  11. 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
  12. Edris Tajfirouze, Christopher H. K. Chen, Richard J. Morton, Peter R. Young, 'Spatially Coherent and Intermittent Alfvénic Fluctuations in Solar Polar Spicules' (2026), arXiv:2606.30299, https://arxiv.org/abs/2606.30299