Appendix C: Magnetic Coordinate Definitions and Nomenclature

Transcription

1 Appendix C: Magnetic Coordinate Definitions and Nomenclature This appendix presents definitions of various magnetic coordinates. The intent is not only to collect the definitions in one place, but also to note some of the problems involved with using them and to attempt to standardize some of the nomenclature. Of particular importance are the ambiguities introduced by parameters such as McIlwain s L m which require the use of a magnetic moment fixed in time. C.1 Physical Constants Table 64 lists several fundamental parameters which are either constant or are often treated as constants. Table 64. List of fundamental physical parameters. Parameter, value, definition Comments M 8 1 A-m arth magnetic dipole moment. M can be computed for a given epoch from the IGRF coefficients using M where 4π = R S = ( g ) + ( g ) + ( h ) S k M R S 4π.1 G-R = = arth magnetic dipole parameter. Sometimes referred to as the dipole moment. = π Permeability of free space Wb/A-m R 671. km arth mean radius. NOT: this is the value used for the IGRF normalizations, and does not correspond to any officially recognized radius. 1

2 C. Mapping Transformations and Definitions C..1 Magnetic field intensity One of the simplest coordinates is the magnetic field intensity. Strictly speaking, to be used as a coordinate, this should be m, the intensity at the particle s mirror point. Thus, for directional data the magnetic field intensity should be mapped to the mirror point using the relation or sin α1 sin α = (C1) 1 m local = (C) sin α For omnidirectional data, or if the pitch angle is not given, we assume locally-mirroring particles, and is simply calculated for the ephemeris point. C.. Normalized magnetic field intensity / / is simply the magnetic field intensity above normalized by the magnetic intensity at the equator. However, there is ambiguity about the definition of. Several versions of are encountered: (1) The minimum value of encountered on a magnetic field line. Generally this is obtained while one is integrating along the field line to calculate the second adiabatic invariant. () An approximate (or average) value defined by k = (C) L where k is the arth s magnetic dipole parameter (not the magnetic moment). () An approximate (or average) value where k is fixed at.1165 G-R. This is the definition used with the A8/AP8 particle maps, and the one used by McIlwain in defining his L m parameter. ecause of this ambiguity, it is recommended that the term be reserved for the McIlwain definition (case () above), i.e.:.1165 G-R = (C4) Lm

3 When mapping pitch angle data to the equator or calculating the equatorial pitch angle of a particle, definition (1) above should be used. In this case, we recommend using the term min or eq to distinguish from case (). Note, however, that there are a few cases (e.g., Shabansky orbits) where a field line may have multiple minima in. C.. quatorial pitch angle α The equatorial pitch angle can easily be computed by sin α = sin α (C5) min local For locally mirroring particles (or for omnidirectional fluxes) 1 α = sin ; = (C6) 1 min local local min sin α Again, in this case the true min should be used instead of, if possible. Another expression [Schulz and Lanzerotti, 1974] is: y k Φ Y( y) = = RKL * π K (C7) where y = sin( α ) * Two approximations are available for Y(y): Y y = T y + T T y y+ y y 1/ ( ) (1 ) ( 1)( ln ) T where 1 = 1+ ln( + ) π T1 = (C7a) or Y( y) y y /4 = + (C7b)

4 C..4 Magnetic latitude λm The magnetic latitude is usually calculated from the geodetic latitude, longitude, and altitude using a tilted dipole representation of the arth s magnetic field (a magnetic longitude can also be calculated). It is not usually specified whether the magnetic latitude refers to a centered or offset dipole (which does not really make a large difference). Some formulations (e.g., Pfitzer s INVARM routine) use a fixed dipole tilt angle which is strictly valid only for one particular epoch. The ONRA-DSP subroutine get_terms calculates the dipole moment at epoch (variable b), the direction cosines of the dipole tilt (variables alpha and beta), and the eccentric dipole offset (x, y, z). Another version of the magnetic latitude, designated λ g, is available: min 4 cos λg = (C8) 6 cos λ g or M 4 cos λ g = (C9) 6 L cos λ g This can be considered to be a generalized magnetic latitude which can be used with non-dipolar fields. Note that these relations define λ g implicitly and require an inverse procedure to obtain λ g as a function of or /. Roberts [1964], as described by Cabrera and Lemaire [7], provides a simple procedure for performing this calculation. C..5 Invariant latitude Λ The invariant latitude is used mainly for low altitude phenomena such as the aurora. Its definition is cos Λ= (C1) L 1 C..6 First adiabatic invariant (magnetic moment) μ The first adiabatic invariant captures the particle s gyration around the field line and is given by sin p p αspc = = (C11) m m spc where p is the particle momentum ( p is the momentum perpendicular to the magnetic field). 4

5 C..7 Second adiabatic invariant The second adiabatic invariant captures the bounce motion of a particle along a magnetic field line. It is calculated by integrating between particle mirror points A and A, and hence is a function of the particle pitch angle. The true second invariant is given by mv ds (C1) A A More widely used is the parameter I: I A s () 1 ds mv m A (C1) Recently the Kaufmann K parameter has become more popular: K = I 8m A [ () m ] A 1/ = s ds m (C14) Again, all of these quantities require integration along a field line through some model magnetic field. C..8 Third adiabatic invariant Φ The third or flux invariant is equal to the flux of enclosed by the surface =constant and captures the drift motion of a trapped particle drifting around the arth. It requires integrating around an entire drift shell: Φ= dl A = da S (C15) Φ has units of G-R or Webers (Wb). C..9 Roederer s L * In order to arrive at a parameter which would have more physical significance than Φ in labeling a particle drift shell, Roederer developed L*, defined as π k M L* = = ΦR ΦR (C16) 5

6 where M is the arth magnetic dipole moment, is the permeability of free space, and k is the magnetic dipole parameter: k M = (C17) 4π Note that for L* to be an adiabatic invariant, k or M must be assumed to be constant, i.e., not functions of time. C..1 McIlwain s Lm McIlwain s L m has become one of the most used and misunderstood parameters in magnetospheric physics. Strictly speaking, L m labels a magnetic field line and is approximately the radial distance at which that field line intersects the magnetic equator. L m approximately labels a drift shell, although this approximation gets worse as radial distance increases (Roederer s L* does label a drift shell). L m is defined as a function of and the second invariant I: Lm = f( I, ) and is generally approximated using a function given by McIlwain or another developed by Hilton. oth of these functions use a constant based on the arth dipole moment. Hilton recommended using the dipole moment for the epoch for which the calculations are being made, but this practice leads to a loss of adiabatic invariance. L m should always be calculated with k =.1165 G-R, the value originally used by McIlwain. Note that this value for k corresponds to an epoch in the mid 195s. C..11 Invariant radius Rinv and invariant altitude hinv The invariant radius is defined by R = L cos λ (C18) inv m g R inv is approximately the radial distance of a point in space with L m and λ g. Polar plots of particle flux in R inv -λ g space are often used to provide a physical picture of the spatial distribution of the flux. Closely related to R inv is the invariant altitude h inv discussed by Cabrera and Lemaire [7]: h = ( R 1) R (C19) inv inv oth R inv and h inv are ideal for mapping fluxes at low altitudes where the flux is controlled more by neutral atmospheric density (and hence altitude) than by the magnetic field. 6

7 C. Nomenclature Table 65 provides a proposed nomenclature which should be followed in order to avoid confusion and ambiguity. Table 65. List of quantities. Magnetic field intensity Approximate minimum magnetic field intensity along a field line (C4) min Minimum magnetic field intensity along a field line h inv Invariant Altitude (C19) I Alternate form of second adiabatic invariant (C1) Omnidirectional flux Second adiabatic invariant (C1) j Directional flux j 9, j perp Directional flux perpendicular to magnetic field K Kaufmann K parameter, related to second adiabatic invariant (C14) k arth magnetic dipole parameter (C17) L* Roederer L parameter (C16) L m McIlwain L parameter M arth magnetic dipole moment R arth mean radius R inv Invariant Radius (C18) Φ Third adiabatic invariant (C1) Λ Invariant latitude (C1) α Particle pitch angle α quatorial pitch angle First adiabatic invariant (particle magnetic moment) (C11) λ g Generalized magnetic latitude (C8) Dipole magnetic latitude λ m 7