Chromospheric Spicules driven by Alfvén waves

Bart De Pontieu

Spicules are jets of chromospheric nature which protrude into the corona. During their lifetime of five to ten minutes, they reach heights of 5000 to 10000 km above the photosphere. The spicular plasma, flowing with upward velocities of 20 to 30 km/s, follows a non-ballistic motion. Both the temperature (5000 to 15000 K) and the density (3*10¹⁰ - 1.5*10¹¹ cm^-3) show little variation along the length of a spicule.

We study the damping, due to collisions between ions and neutrals, of Alfvén waves in the partially ionized chromosphere. This type of damping of upward traveling Alfvén waves with frequencies between 0.2 and 0.6 Hz, can cause not only significant heating but also upward motion of the upper chromospheric plasma. In addition the upward force and heating associated with this type of damping can sustain an already formed spicule [Haerendel, 1992; De Pontieu and Haerendel, 1997]. In two independent numerical models (WKB, MHD) we study the time-dependent evolution of a flux tube in the solar atmosphere on the base of which a train of upward traveling Alfvén waves (with a frequency of 0.5 Hz) is launched. The results of our models indicate that ion-neutral damping of Alfvén waves can also explain the formation of spicules in a self-consistent way. Structures are formed in which the plasma is propelled upward with velocities of 20 to 30 km/s, reaching heights, densities and temperatures typical of spicules. The ion-neutral damping provides not only upward momentum but, equally important, also heating; whereas the interaction of non-linearly formed slow shocks with the transition region provides additional upward momentum [De Pontieu and Haerendel, 1994; De Pontieu, 1996]. The first phase of spicular formation for pulsed wave input is shown in the figure which gives the logarithm of the density as a function of height above the photosphere (1000 km) and as a function of time (s). The average upward trend due to the wave damping is visible, as are the subsequent interactions of slow shocks with the transition region between the dense chromosphere and thin corona.

De Pontieu, B. and Haerendel, G.: Numerical simulation of chromospheric spicules driven by weakly-damped Alfvén waves, in Solar Coronal Structures, IAU Colloq. 144, ed.: Rusin V., Heinzel P. and Vial J.-C., VEDA, Bratislava, 323, 1994.

De Pontieu B.: Chromospheric spicules driven by Alfvén waves, Ph D Thesis, University of Ghent, 1996.

De Pontieu, B. and Haerendel, G.: Weakly damped Alfvén waves as drivers for spicules, submitted to Astronomy and Astrophysics, 1997.

Haerendel G.: Weakly damped Alfvén waves as drivers of solar chromospheric spicules, Nature, 360, 241, 1992.