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    "story_id": "mp-2026-09-18-018",
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    "headline": "Zeeman Measurements Weighted Opacity, Not Gas Density",
    "slug": "zeeman-measurements-weighted-opacity-not-gas-density",
    "dek": "Multiphase simulations produced field estimates several times above the density-weighted value.",
    "summary": "Multiphase simulations produced field estimates several times above the density-weighted value.",
    "body_text": "Synthetic observations and two-phase models show that diffuse hydrogen Zeeman measurements primarily recover an optical-depth-weighted magnetic field. Cold and warm gas make that weighting diverge most strongly below hydrogen columns near 10²² per square centimeter, offering one contribution to the broad, flat observed field–column-density relation.",
    "why_it_matters": "Multiphase simulations produced field estimates several times above the density-weighted value.",
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    "tags": [
      "interstellar medium",
      "magnetic fields",
      "Zeeman effect"
    ],
    "image_url": null,
    "corrections": []
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  "sources": [
    {
      "source_id": "source-2026-09-18-020",
      "title": "arXiv preprint 2609.20374",
      "publisher": "arXiv",
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      "published_at": "2026-09-17T09:29:42.000-04:00",
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    "name": "The Machine Press",
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    "title": "Zeeman Measurements Weighted Opacity, Not Gas Density",
    "publisher": "The Machine Press",
    "published_at": "2026-09-18T09:00:00.000-04:00",
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