Struct

`space::irbem::T89FixedParameters`

cheatah-space v0.1.0-alpha — Biome Standard 0.6.5-alpha

The T89 parameters that are the same in every Kp bin.

Tsyganenko (1989) §3 and §4 name these explicitly as held constant during the fit, "from a priori considerations and preliminary test runs": the §3 group because the data cannot separate them from the free parameters they trade against (the paper's own example is that gamma_t and gamma_rc "yield nearly the same effects" on the dayside, so one of the two had to be pinned), the §4 group because the closure sheets contribute too little of the total field to constrain their geometry.

They are gathered into one constexpr object rather than scattered as literals so that the evaluator below reads as the paper's equations and every magic number has a name and a citation.

Unit testIrbemT89.FixedParametersAreThePublishedOnes

Constants & variables

var double l_y #

L_y, R_E — the dawn-dusk scale of the current sheet's transverse bending, eq.

(11). Fixed at 10 R_E "in accordance with results of Fairfield (1980) and Gosling et al. (1986)".

var double d_x #

D_x, R_E — the sunward scale over which the truncation factor W falls off, eq. (13).

var double l_rc #

L_RC, R_E — the scale distance of the ring-current thickness profile h_RC, eq. (13).

var double l_t #

L_T, R_E — the scale distance of the tail thickness profile h_T, eq. (13).

var double gamma_t #

gamma_T, R_E — how much thicker the tail sheet is on the dayside than the nightside, eq.

(13). The one of the two thickness increments the paper pinned; gamma_RC stayed free.

var double delta #

delta, R_E^-1 — the rate at which the tail sheet thickens towards its flanks, eq. (13).

var double gamma_1 #

gamma_1, R_E — the amplitude of the far-tail thickening h_1 that removes the spurious B_z reversals the paper reports beyond x ~ -15 R_E, eq.

(13).

var double r_t #

R_T, R_E — the distance of the two planar closure current sheets from the GSM equatorial plane, §4.

They sit at z = ∓R_T, i.e. outside the modelling region.

var double x_0c #

x_0c, R_E — where the closure sheets' own truncation factor W_c falls off, eq. (19).

var double l_xc2 #

L_xc^2, R_E^2 — the squared sunward scale of W_c, eq.

(19). Squared because that is how the paper states it and how it enters: never as L_xc.

var double d_yc2 #

D_yc^2, R_E^2 — the squared dawn-dusk scale of W_c, eq. (19). Squared, as above.

var double hinge_scale2 #

The 16 inside eq.

(11)'s square root: (4 R_E)^2, the scale over which the sheet's hinge is rounded off. Named rather than inlined so the equation reads as printed.

var double h1_offset #

The 16 in eq.

(13)'s h_1, R_E — the tailward distance at which the far-tail thickening is half-grown.

var double h1_scale2 #

The 36 in eq. (13)'s h_1: (6 R_E)^2, the squared scale over which it grows.