{"id":"7574dbda-d478-4ebc-b5b6-a653a57d0038","arxiv_id":"2411.19340","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"The authors couple the COCONUT coronal model to the EUHFORIA heliospheric model with time-varying boundary conditions and show that CME structures propagate smoothly from the Sun to Earth in six test simulations.","lead":"This paper builds a two-part computer model that follows a coronal mass ejection from the Sun's surface to Earth by feeding the output of a coronal simulation into a heliospheric forecast model in real time. The result is a testbed for more accurate space weather predictions that include the CME's early evolution in the corona.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"One-way coupling at 21.5 R_sun is asserted but never verified; if the boundary is locally sub-Alfvénic or subsonic during the CME passage, the L1 profiles are boundary artifacts rather than genuine propagation.","rationale":"The reader's conditional verdict already captures the main risks. I isolate the unverified super-Alfvénic/supersonic condition as the single most load-bearing concern because the paper itself flags it as 'crucial', yet provides no Mach-number diagnostics. The temporal-interpolation issue is real but secondary: a 144 s cadence is short compared with the multi-hour crossing of the sheath and ejecta, and the paper does present consistency checks between interface and Earth profiles. The lack of observed-event validation is an external-validity limitation, not a correctness risk in the numerical coupling itself. The static-boundary tail of the forecast is less critical because the CME has already entered the EUHFORIA domain by the time the boundary becomes static. The proposed test directly settles whether the one-way coupling is physically valid; if it fails, the central claim of genuine corona-to-L1 propagation collapses, and if it passes, the coupling concept is substantially strengthened. I therefore keep the reader's conditional verdict unchanged while sharpening the condition that would make it acceptable.","tokens_in":31718,"tokens_out":8805,"duration_ms":90175,"concrete_test":"For each of the 600 saved coupling files per simulation, at every grid cell on R=21.5 R_sun, compute the ideal MHD characteristic speeds normal to the boundary from the local density, velocity, and magnetic field. In particular, evaluate v_r - c_f, where c_f is the fastest magnetosonic speed for propagation along the radial direction, and require v_r - c_f > 0 for all cells and all times, with the same check for the Alfvén and slow speeds. Report the minimum value, the affected boundary area, and the time interval over which the condition fails. If failures occur, rerun one representative EUHFORIA case with a characteristic-based non-reflecting boundary condition and compare the L1 B_z, density, and velocity profiles with Fig. 12; material differences would demonstrate that the current boundary prescription, not the coronal propagation, controls the predicted arrival profiles.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section 3.1.1 states that R_b=21.5 R_sun was chosen because the solar wind is expected to be supersonic and super-Alfvénic, and that it is 'crucial to ensure the solar wind is not sub-Alfvénic in COCONUT'. The paper never actually verifies this for the July 2019 full-MHD solution. Full-MHD COCONUT with Q_H proportional to |B| produces a bimodal wind, and the slow streamer-belt plasma near the equatorial plane at 0.1 AU is exactly where sub-Alfvénic conditions could arise. The condition must also hold in every boundary snapshot, including the sheath and ejecta crossing the interface: inside the CME, B and density are strongly perturbed, so the local Alfvén and fast-magnetosonic speeds differ from the ambient wind. If any radial characteristic speed becomes inward (v_r - c_f < 0), prescribing all eight MHD variables in the EUHFORIA ghost cells is an overspecified boundary problem that can generate spurious reflected waves. The smooth transition at the interface and the Earth profiles in Figs. 10 and 12 would then be partly produced by the boundary treatment rather than by genuine coronal propagation. Because the central claim is that the coupled chain transmits the true CME structure from the corona to L1, this unverified characteristic-speed condition is the most load-bearing assumption in the paper.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents a time-dependent coupling between the coronal MHD model COCONUT and the heliospheric model EUHFORIA. Six CME simulations are run in COCONUT (three Titov-Démoulin and three RBSL flux ropes) for about 24 hours; the plasma state on the spherical surface at 21.5 R_sun is saved every ~144 s and used via linear temporal interpolation as a time-varying inner boundary for EUHFORIA. The authors show smooth transitions of density, temperature, velocity, and magnetic field across the model interface, qualitatively consistent magnetic and thermodynamic profiles at L1 across the six cases, and a power-law decay of the magnetic field amplitude with heliocentric distance (exponent about -1.5). They conclude that the dynamically coupled COCONUT+EUHFORIA chain constitutes a new space weather forecasting tool that can predict flux-rope CME characteristics at L1, including a pre-formed sheath.","tokens_in":32000,"tokens_out":8315,"duration_ms":67007,"significance":"If the coupling is physically sound, this is a valuable step: it replaces the ad-hoc insertion of CME models at 0.1 AU with a disturbance that has evolved self-consistently through the corona, and it is the first time-dependent COCONUT-EUHFORIA linkage. The study benefits from six test cases covering two independent flux-rope models; the CME parameters are inputs, not fitted to the outputs; the self-similar expansion exponent is derived from simulation points, not assumed; and the paper explicitly acknowledges several limitations (fixed photospheric field, sequential workflow, high computational cost). The main caveat is that the central claim of a forecasting tool is supported only by idealized test cases with arbitrarily chosen CME parameters and no comparison to in-situ observations; the 'forecasting tool' wording in the abstract and conclusion is stronger than the presented evidence.","major_comments":[{"comment":"The one-way coupling assumption is asserted but never verified. The paper states at 21.5 R_sun that 'we expect a supersonic and super-Alfvénic solar wind' and that 'it is crucial to ensure that the solar wind is not sub-Alfvénic in COCONUT', but no Mach-number or characteristic-speed diagnostic is shown for the July 2019 full-MHD solution or for any of the time-dependent runs. The full-MHD COCONUT wind is bimodal, and the slow streamer-belt plasma near the equatorial plane is exactly where sub-Alfvénic conditions could occur; moreover, during the passage of the sheath and ejecta across the interface, B and density are strongly perturbed, so the condition must hold in every boundary snapshot, not only in the ambient wind. If any radial characteristic speed becomes inward, prescribing all eight MHD variables in the EUHFORIA ghost cells is an overspecified boundary problem that can generate spurious reflected waves, making the smooth transition and the Earth profiles partly numerical artifacts rather than genuine propagation. Please add a quantitative verification, e.g., time- and space-dependent Alfvén and fast-mode Mach numbers at R_b=21.5 R_sun for all six simulations, and show that all radial characteristics are outward throughout the CME passage.","section":"§3.1.1, §4.2, §4.4"},{"comment":"The live coupling covers only about one seventh of the forecast phase. The text states that eleven days are used for relaxation and seven days for the forecast, but the coupling files cover only 1/7 of that forecast phase; for the remaining six days the last available boundary map is used. Since the COCONUT runs stop after about 24 hours, the CME front has crossed the interface within the live period, but the subsequent evolution of the solar wind boundary (including the CME wake and the persistent high-speed stream) is frozen. The Earth profiles in Fig. 12 are therefore mostly the result of propagation of the injected disturbance with a static boundary condition, not of a continuously coupled chain. The authors should either extend the COCONUT simulations through the full forecast phase, or explicitly quantify the time interval during which the boundary is live and discuss how the frozen boundary after day one affects the late-time Earth profiles and the 'dynamically coupled' claim.","section":"§4.4"}],"minor_comments":[{"comment":"The sentence 'the maximum density ranges between 6×10^10 m^-3 and 4×10^8 m^-3' contradicts Fig. 12, whose density panel shows values on the order of 10^8 m^-3 (approximately 6×10^8 to 4×10^8 m^-3); the subsequent comparison with COCONUT values of 1.6–2.2×10^10 m^-3 should be corrected accordingly.","section":"§4.5"},{"comment":"In the discussion of the RBSL simulations, the text states that 'the radial velocity varies from 1263 km/s (TDm_2) to 1804 km/s (TDm_3)'; the parenthetical labels should be RBSL_2 and RBSL_3, respectively.","section":"§4.2"},{"comment":"The initial magnetic flux for RBSL_3 is given as 18×10^20 Mx in the text but as 15×10^20 Mx in Table 1; please state which value was used and correct the inconsistency, since the amplitude ordering of the results is attributed to the flux.","section":"§4.1 and Table 1"},{"comment":"The reference 'cf. Fig. 4.3' should be 'cf. Fig. 9'.","section":"§4.5"},{"comment":"The smooth transition across the interface is assessed visually from Fig. 10; a quantitative measure, such as the relative difference of each variable across the interface for the six cases, would strengthen the claim that the boundary treatment does not distort the transmitted physics.","section":"§4.4"},{"comment":"The linear temporal interpolation between boundary files saved every ~144 s is described, but the possible distortion of a sharp CME front by this interpolation is not assessed; a brief estimate of the front displacement per file interval relative to the grid resolution would be useful.","section":"§3.2"}],"recommendation":"major_revision","confidential_remarks":"The paper is a proof-of-concept of a coupled corona-heliosphere chain, and the technical implementation appears sound in outline. The main risk is the unverified super-Alfvénic/supersonic condition at the interface, which is load-bearing for the central claim; the requested Mach-number verification is straightforward and should be feasible with the existing simulation outputs. The 'forecasting tool' wording in the abstract and conclusion is stronger than the evidence, which covers only synthetic test cases with no in-situ validation; I would encourage the editor to require a softened claim or an explicit statement that this is a demonstrator. The inconsistent RBSL_3 flux value (15 vs 18×10^20 Mx) should be fixed before publication."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThis paper reports something genuinely new: the first time-dependent coupling of the COCONUT coronal model to EUHFORIA, passing full MHD variables across a 21.5 R_sun interface so that flux rope CMEs evolve in the corona before entering the heliosphere. The authors run six test cases with two CME models, and the qualitative transfer of structure is convincing—the sheath, the flux rope field pattern, and the S-shape of RBSL all survive the interface, and the Earth profiles are consistent with what enters EUHFORIA. That is a real step forward for this code family, and the paper is honest about its exploratory status.\n\nCredit where due: the interpolation and file handling are described in enough detail to reproduce, the authors flag the four-point fit caveat themselves, and the abstract's claim about prediction is appropriately hedged in the body (they say a real event test is future work). The full MHD COCONUT wind reproduces a bi-modal structure, which strengthens the relevance.\n\nThe soft spots are mostly about what is not checked. The biggest is the interface condition. The paper asserts the wind at 21.5 R_sun is supersonic and super-Alfvénic, but never computes the local Alfvén and fast-mode speeds in any boundary snapshot. That condition has to hold not just in the ambient wind but inside the CME sheath and ejecta as they cross. If any radial characteristic speed drops below zero, prescribing all eight MHD variables in the ghost cells is overspecified and can generate reflected waves. The smooth transition they show could then be partly a boundary artifact. This is a load-bearing assumption for the whole coupling concept, and it is easy to check—they should show Mach numbers as a function of time at the interface.\n\nSecond, the live boundary data cover only about one day of a seven-day forecast; the rest uses a static, rotated boundary. That undermines the phrase \"forecast\" and limits what the L1 profiles tell us about long-distance coupled propagation. Third, there is no comparison with observed events. The paper is framed as a proof of concept, so that is acceptable, but the title and abstract sell it as a forecasting tool. I'd soften that. Finally, there are small internal inconsistencies—the RBSL_3 flux is 15 in Table 1 and 18 in the text, and some figure labels (Vr in kg/s) are wrong. These are fixable.\n\nOverall: this deserves a serious referee. The coupling is a useful engineering contribution and the paper is largely clear. I'd ask for the characteristic-speed check, at least one event validation or a clearly stated re-scoping as a method paper, and a longer overlapping boundary period before acceptance.","headline":"Genuinely new time-dependent COCONUT-EUHFORIA coupling; the demonstration is plausible but the unverified super-Alfvénic boundary condition and short live-boundary window keep it from being a validated forecast.","tokens_in":32622,"tokens_out":2888,"would_cite":true,"duration_ms":25036,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper reports the first time-dependent coupling of the coronal MHD model COCONUT to the heliospheric forecast tool EUHFORIA, allowing flux-rope CMEs to be followed continuously from the solar surface to Earth.","keywords":["coronal mass ejections","space weather forecasting","MHD simulation","flux rope","solar corona","heliosphere","COCONUT","EUHFORIA"],"falsifier":"During a coupled run, compute the local Alfvén and sound Mach numbers over the whole $21.5\\,R_\\odot$ sphere for the full CME passage; if any patch is sub-Alfvénic or subsonic, the one-way boundary assumption is violated, and the smooth transition at the interface would have to be treated as a boundary artifact rather than genuine propagation.","tokens_in":31470,"feed_emoji":"🌞","tokens_out":10561,"duration_ms":79228,"temperature":0.7,"pith_summary":"This paper establishes that the coronal model COCONUT and the heliospheric forecast tool EUHFORIA can be joined into a single time-dependent chain that carries a flux-rope coronal mass ejection (a coherent, twisted magnetic structure) from the solar surface to Earth. Instead of injecting an idealized CME model at 0.1 AU as standard EUHFORIA does, the chain saves the evolving magnetic field, velocity, density, and temperature on a 21.5-solar-radius sphere in COCONUT and feeds those maps into EUHFORIA as a continuously updated inner boundary. For six test cases spanning two flux-rope models, the solar wind in EUHFORIA behaves as a direct extension of the coronal wind, and the CME's thermodynamic and magnetic signatures, including a pre-formed sheath, cross the interface smoothly and reach L1 with their main structural features preserved. The paper concludes that this coupled chain is a new space-weather forecasting tool that can predict flux-rope CME characteristics at L1, with the amplitudes at Earth set by the flux rope's initial properties at the Sun.","feed_headline":"Sun-to-Earth CME tracking now runs on one coupled model chain","feed_subtitle":"Time-dependent boundary maps keep flux-rope CMEs structurally intact from the corona all the way to L1.","key_machinery":"The load-bearing mechanism is the time series of coupling files: COCONUT is run in time-accurate mode with a flux rope inserted at the photosphere, and every twenty iterations (about 144 seconds of physical time) the three magnetic-field components, three velocity components, temperature, and density are interpolated from COCONUT's unstructured mesh onto a 360-by-180 structured grid at the $21.5\\,R_\\odot$ interface. A modified EUHFORIA identifies the two boundary files bracketing the current simulation time and linearly interpolates between them to update its inner-boundary ghost cells at every step. This one-way transfer is valid only because the interface flow is assumed to be supersonic and super-Alfvénic, so no signal can propagate back toward the Sun. The full-MHD version of COCONUT supplies the background solar wind, and the TDm and RBSL flux-rope models provide the coronal disturbances whose imprints are carried through the boundary.","core_discovery":"The central claim is that a coronal MHD simulation and a heliospheric MHD simulation can be dynamically coupled at 21.5 solar radii through a time series of boundary maps, so that a CME's evolution in the low corona is not lost when it enters the heliosphere. In the six test cases, the heliospheric solar wind in EUHFORIA matches the coronal solar wind across the interface, and the disturbances created by flux-rope propagation in COCONUT continue to evolve in EUHFORIA with a smooth transition. Comparing the magnetic field components at 10 solar radii, at the 21.5-solar-radius interface, at 68 solar radii, and at Earth, the authors find that the transient magnetic structures expand self-similarly, with the field amplitude falling roughly as a power law with fitted exponent near $-1.5$, while the overall profile shapes, including sign changes in the $B_y$ and $B_z$ components, persist to Earth. The pre-formed sheath that develops ahead of the flux rope in the corona is inserted through the boundary and continues to grow in the heliosphere, which the authors argue should make sheath predictions closer to observations than models that only form a sheath from 0.1 AU onward.","pith_inferences":["I would expect the same boundary-file coupling scheme to transfer to other heliospheric MHD codes, because the interface data are standard MHD quantities; the only requirement is that the receiving code can ingest time-dependent inner-boundary maps.","A numerical test the paper does not report is a convergence check on output cadence: rerunning one case with files saved every ~14 seconds instead of ~144 seconds would show whether linear temporal interpolation samples the sharp CME front adequately.","The persistent high-speed stream in the CME wake appears to be an artifact of keeping the photospheric magnetic field fixed during the run; allowing the solar surface to evolve with a time series of magnetograms should dissipate that stream and would change the late-time L1 profiles.","If the super-Alfvénic assumption holds for faster CMEs in solar-maximum conditions, the chain should work in more active epochs; if it does not, a two-way or overlapping-domain coupling would become necessary."],"forward_implications":["Forecasts of CME arrival at Earth will now include whatever acceleration, deflection, heating, and sheath formation happened in the corona, rather than starting from an idealized injection at 0.1 AU.","The initial flux-rope parameters (model type, magnetic flux, geometry) become the dominant control on the predicted L1 profiles, so observational determination of those parameters is required for event-specific forecasts.","Because the magnetic field profiles at Earth retain the sign-change structure seen at 10 solar radii, the geoeffective $B_z$ orientation can in principle be traced back to the solar-source flux-rope configuration.","The chain provides a test bed for studying sheath and magnetic-ejecta evolution with distance, including comparison with multi-spacecraft measurements of how CME magnetic fields decay outward."],"supporting_citations":[{"why":"presents the EUHFORIA model and its standard WSA-based inner boundary that this work modifies.","marker":"Pomoell & Poedts 2018"},{"why":"introduces the COCONUT coronal solver whose implicit scheme and unstructured grid enable the time-accurate runs used here.","marker":"Perri et al. 2022"},{"why":"documents the TDm flux-rope implementation in COCONUT and its propagation, which the coupled runs build on.","marker":"Linan et al. 2023"},{"why":"provides the RBSL flux-rope implementation in COCONUT and the force-free field construction used for the RBSL cases.","marker":"Guo et al. 2024a"},{"why":"supplies the full-MHD heating formulation and the July 2, 2019 solar wind used as the background state.","marker":"Baratashvili et al. 2024"},{"why":"defines the modified Titov-Démoulin flux-rope model whose parameters set the TDm CME cases.","marker":"Titov et al. 2014"},{"why":"defines the Regularized Biot-Savart Laws method that produces the force-free RBSL flux ropes.","marker":"Titov et al. 2018"},{"why":"provides the observational magnetic-field decay with distance to which the self-similar expansion fit is compared.","marker":"Winslow et al. 2015"}],"fun_headline_variants":["Coupled COCONUT-EUHFORIA chain tracks CMEs from Sun to Earth","Dynamic coronal-heliospheric coupling keeps CME structures to Earth","One coupled model chain takes CMEs from corona to Earth","Flux-rope CMEs survive Sun-to-Earth trip in new coupled simulations","Corona-to-heliosphere coupling preserves CME shape and amplitude to L1"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The entire transfer of information hinges on the flow at the 21.5-solar-radius interface remaining supersonic and super-Alfvénic throughout the CME passage, so that the one-way boundary condition loses nothing, and on the boundary maps saved every ~144 seconds with linear time interpolation capturing the sharp CME front without distortion.","fun_headline_variants_meta":{"raw":{"variants":["Coupled COCONUT-EUHFORIA chain tracks CMEs from Sun to Earth","Dynamic coronal-heliospheric coupling keeps CME structures to Earth","One coupled model chain takes CMEs from corona to Earth","Flux-rope CMEs survive Sun-to-Earth trip in new coupled simulations","Corona-to-heliosphere coupling preserves CME shape and amplitude to L1"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001703,"raw_usage":{"total_tokens":6850,"prompt_tokens":1157,"completion_tokens":5693,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":773,"completion_tokens_details":{"reasoning_tokens":5590}},"tokens_in":773,"tokens_out":5693,"duration_ms":35972,"temperature":1.0,"reasoning_tokens":5590,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T10:16:09.993516+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"During a coupled run, compute the local Alfvén and sound Mach numbers over the whole $21.5\\,R_\\odot$ sphere for the full CME passage; if any patch is sub-Alfvénic or subsonic, the one-way boundary assumption is violated, and the smooth transition at the interface would have to be treated as a boundary artifact rather than genuine propagation.","supporting_citations":[{"cited_title":"2023, Astronomy & Astrophysics, 675, A101","cited_arxiv_id":null,"evidence_quote":"documents the TDm flux-rope implementation in COCONUT and its propagation, which the coupled runs build on."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"defines the modified Titov-Démoulin flux-rope model whose parameters set the TDm CME cases."},{"cited_title":"S., Downs, C., Miki´c, Z., et al","cited_arxiv_id":null,"evidence_quote":"defines the Regularized Biot-Savart Laws method that produces the force-free RBSL flux ropes."},{"cited_title":"M., Lugaz, N., Philpott, L","cited_arxiv_id":null,"evidence_quote":"provides the observational magnetic-field decay with distance to which the self-similar expansion fit is compared."}],"review_version":1}