perihelion is the point in the earth's orbit nearest to the sun.
perihelion is the point in the earth's orbit nearest to the sun.
point in the earth's orbit nearest to the sun?
perihelion
apsis (; plural apsides , greek: ἁψῖδες; "orbit") denotes either of the two extreme points (i.e., the farthest or nearest point) in the orbit of a planetary body about its primary body (or simply, "the primary"). the plural term, "apsides," usually implies both apsis points (i.e., farthest and nearest); apsides can also refer to the distance of the extreme range of an object orbiting a host body. for example, the apsides of earth's orbit of the sun are two: the "apsis" for earth's farthest point from the sun, dubbed the aphelion; and the "apsis" for earth's nearest point, the perihelion (see top figure). (the term "apsis", a cognate with "apse", comes via latin from greek).
apsis
however, for particles which mirror at safe altitudes, (in yet a further level of approximation) the fact that the field generally increases towards the center of the earth means that the curvature on the side of the orbit nearest the earth is somewhat greater than on the opposite side, so that the orbit has a slightly non-circular, with a (prolate) cycloidal shape, and the guiding center slowly moves perpendicular both to the field line and to the radial direction. the guiding center of the cyclotron orbit, instead of moving exactly along the field line, therefore drifts slowly east or west (depending on the sign of the charge of the particle), and the local field line connecting the two mirror points at any moment, slowly sweeps out a surface connecting them as it moves in longitude. eventually the particle will drift entirely around the earth, and the surface will be closed upon itself. these drift surfaces, nested like the skin of an onion, are the surfaces of constant "l"  in the mcilwain coordinate system. they apply not only for a perfect dipole field, but also for fields that are approximately dipolar. for a given particle, as long as only the lorentz force is involved, "b"  and "l"  remain constant and particles can be trapped indefinitely. use of "(b,l)"  coordinates provides us with a way of mapping the real, non-dipolar terrestrial or planetary field into coordinates that behave essentially like those of a perfect dipole. the "l"  parameter is traditionally labeled in earth-radii, of the point where the shell crosses the magnetic equator, of the equivalent dipole. "b"  is measured in gauss.
l-shell