ForceFreeStates - BUG - Fixed-boundary zero-crossing check can reject genuine Newcomb crossings

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
julia
Domain
backend, testing-qa

Research direction

Start with src/ForceFreeStates/FixedBoundaryStability.jl:71-86 and compare its crossing check with the Fortran behavior documented in GPEC#292. Then run a fixed-boundary Solov'ev q0 scan at default and tightened tolerances, including a pole case. Done means Julia finds the same genuine crossings at both tolerance settings while still rejecting the pole.

Written by the indexing model from the issue text.

Description

check_for_zero_crossings! (src/ForceFreeStates/FixedBoundaryStability.jl:71-86) is a line-for-line port of the Fortran ode_output_monitor test. When crit changes sign between two accepted ODE steps, it interpolates u linearly to the estimated root and accepts the crossing only if the midpoint value lies between the endpoints and |crit_mid| < 0.5*min(|crit|,|crit_prev|). Accepted steps can be long where crit is strongly nonlinear, so a genuine Newcomb zero crossing can fail this test and be discarded.

In the Fortran code this defect is reproduced and documented in GPEC#292: for fixed-boundary Solov'ev cases near q0 ≈ 1.03, DCON misses crossings at its default tolerances and finds them only after the tolerances are tightened. The Julia check has the same logic, so the same failure is expected, but I have not yet run the Julia q0 scan.

Simply relaxing the magnitude test is not the fix, because the sign change can also come from a pole of crit, which must not be counted. A robust check brackets the root, for example by bisection on the integrator's dense output, and distinguishes a zero (|crit| decreasing towards the bracket point) from a pole (|crit| growing).

Acceptance: a fixed-boundary Solov'ev q0 scan where Julia reports the same crossings at default and at tightened tolerances, with a pole case that is still rejected.

Chris&AI

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Merged PRs (30d)
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