2 DYNAMICAL MODELS FOR SOLAR SPICULES

Solar spicules are luminous jets that erupt up to 10 000 km from the s urface of the Sun and have diameters, all along their length, of only about 150-200 km. We first review some recent observations made at the H alpha wavelength. According to our models, a solar spicule is a sel f-channelled proton beam emitted by a magnetic element and surrounded by a cold sheath. The beam originates in a self-excited dynamo that ex ploits a v x B field and that could be situated either below or above the element, where v is the local plasma velocity and B is the local m agnetic flux density. In falling back, the sheath provides a return cu rrent of protons that cancels the outgoing current. We discuss the cha nnelling of charged-particle beams of very large cross-section and pro pose velocity channelling, which is apparently a new concept. We assum e a steady state and a hydrogen plasma.