2. Coordinate system

2.1. The E-xyz system

Fig. 2.2 illustrates the basic coordinate system which is internally used in Cosmos. The x-axis is directed to the longitude 0 and latitude 0. The y-axis is to the 90 degrees east and the z-axis to the north. The Earth is expressed by a complete sphere. The origin is at the center of the Earth. This coordinate system is abbreviated as the E-xyz system.

Fig. 2.2 The basic (interal) coordinate system (left Fig.) and observation plains (right Fig.)

2.2. The detector system and the primary system

You may setup several observation levels, at each of which you can record particle information passing through it. The plane of the levels may be horizontal or perpendicular to the primary particle direction. Since detectors are normally placed horizontally, we call the horizontal recording system the “detector system”. (See Fig. 2.2 right). From the figure the user might misunderstand that partiles are recorded when they cross a plane which is tangential to the surface of a sphere (this was acutually true once in older versions), but it is the surface of a sphere, i.e., particles are recorded when they cross this spherical surface. However, the x-y plane of the coordinate system is tangetial as it is the rectangular system. The x-axis of the system is directed to the magnetic east (default), the y to the magnetic north and the z to the vertical.

The coordinate system whose x-y plane is perpendicular to the primary is called the “primary system”. Hence the z-axis is the primary direction (upward is positive), the x-axis is directed to the vector product of the z and the vertical direction, the y to the vector product of the z and the x-axis direction.

In both cases, the origin is assumed to be the location of a specified observation place. If the primary direction is the vertical, both systems are almost identical, as far as near z-axis particles are concerned.