Weakly damped Alfvén waves as drivers of solar chromospheric spicules

THE solar chromosphere, which separates the photosphere (temperature T ~ 6,400 K) from the hotter solar corona (T ~ 10⁶ K), has a very inhomogeneous structure which is strongly influenced by the magnetic field. In the lowest 2,000 km of chromospheric altitude, the density falls by six orders of magnitude, and the temperature stays below 8,000 K. Above this altitude the transition to the corona is extremely irregular. It is dominated by spicules: thin (< 1,000 km) protrusions of cool chromospheric material which extend, with speeds of 25 km s^-1 and for durations of 5-10 min, up to 10,000 km into the corona with little change in density. Their origin is not yet understood, and I suggest here that the force that propels them against gravity may be the transfer of momentum from upward-moving Alfvén waves. In the upper chromosphere the ionized plasma component is collisionally coupled to the neutral gas, but the coupling is not perfect, so that the neutral material can acquire a net velocity with respect to the ionized component. This process is known to damp the Alfvén waves¹², but I show that it can also, as the chromosphere peters out, transfer enough momentum to local volumes to create and drive the spicules.