Soft X-ray images of the solar corona obtained during the last 20 years have systematically shown an intimate relationship between intense emitting structures and magnetic fields (Vaiana and Rosner 1978). The magnetic field confines a 106 K plasma, which is an intense source of soft X-ray photons. Therefore, it is natural to expect the bright X-ray structures to follow the field's geometry. But this relationship does not seem to be just geometrical. It is generally believed that the energy

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... sary to heat the plasma comes from the dissipation of magnetic stresses, which are continually being re-generated by subphotospheric convective motions. However, there is still great uncertainty about the precise physical mechanisms involved in the production and release of the magnetic energy.