The RTN system (see top part of Figure 101) has the axis along the radial direction, positive from the Sun to the s/c, the component perpendicular to the plane formed by the rotation axis of the Sun and the radial direction, i.e., , and the component resulting from the vector product .The Solar Ecliptic reference system SE, is shown (see bottom part of Figure 101) in the configuration used for Helios magnetic field data, i.e., s/c centered, with the axis positive towards the Sun, and the axis lying in the ecliptic plane and oriented opposite to the orbital motion. The third component is defined as . However, solar wind velocity is given in the Sun-centered SE system, which is obtained from the previous one after a rotation of around the axis.
Sometimes, studies are more meaningful if they are performed in particular reference systems which result to be rotated with respect to the usual systems, in which the data are provided in the data centers, for example RTN or SE. Here we will recall just two reference systems commonly used in data analysis.
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