[en] Energetic electrons and protons stored on a coronal magnetic loop develop a ''loss-cone'' distribution in pitch-angle which is unstable. The particles are lost quickly unless the resulting waves are damped. Under coronal conditions Landau damping of the waves stabilizes the particles if the field lines are sufficiently curved. Solar cosmic rays (electrons >0.5 MeV, protons >4 MeV) may be confined for hours on solar magnetic loops if the field curvature within these loops is roughly 10⁴ km. However, the instability quickly destroys any distribution of electrons with unique pitch-angles such as is invoked for some models of nonthermal X-ray bursts