boundary-minimum-variance

Boundary crossing: minimum-variance (LMN) analysis

A guided version of the IDL SPEDAS minimum-variance crib sheet — the standard way to find the orientation of a magnetic boundary from single-spacecraft field data and rotate the field into the boundary-normal (L, M, N) frame.

When to use

• "Find the magnetopause/bow-shock normal for this crossing."
• "Rotate B into LMN for this current sheet / discontinuity."
• "Is this a well-determined boundary normal?" (the reliability question MVA exists to answer.)

Tool chain (all already exist)

load_data_source → browse_data_parameters → fetch_data_product → analyze_minvar_coordinates → (render_tplot), in a create_spedas_analysis_bundle.

Procedure

1. Bundle & scope the crossing. create_spedas_analysis_bundle(...). Pick a window tight around the single boundary crossing — MVA assumes one planar structure; a window spanning two boundaries or lots of turbulence gives a meaningless normal.

2. Fetch the field. Choose a magnetometer dataset appropriate to the mission/region and confirm the vector variable with browse_data_parameters. Then fetch_data_product(source_type=..., dataset_id=..., parameters=[<B vector>], start, stop, output_dir=<bundle>/data). Use the highest cadence that keeps the crossing well-sampled.

3. Run MVA. analyze_minvar_coordinates(input_file=<B csv>, output_dir=<bundle>/data, vector_cols=[<Bx>,<By>,<Bz>], time_col="time"). It returns:

   • eigenvalues (λ_max ≥ λ_int ≥ λ_min),
   • eigenvectors (maximum=L, intermediate=M, minimum=N),
   • normal_vector (= the minimum-variance eigenvector N),
   • intermediate_to_min_ratio (λ_int / λ_min),
   • and a rotated_file with B in LMN.

4. Judge reliability — do not skip this. The boundary normal is only trustworthy when the minimum eigenvalue is well-separated from the intermediate one:

   • λ_int / λ_min ≳ 5–10 → well-determined normal.
   • ratio ≲ 2–3 → the normal is poorly constrained; report it as unreliable and consider a different window, a longer/shorter interval, or multi-spacecraft timing instead.
   • Always report the ratio alongside the normal so the result is self-documenting.