Modelling and Observations of Chromospheric Spicules
Spicules are
dynamic jets propelled upwards at speeds of ~20 km/s
from the photosphere into the low
magnetized solar atmosphere. They
carry a mass flux of 100 times that of
the solar wind up to low
coronal heights. Despite their ubiquity
they have been a significant
challenge to both observers and
theorists, mostly because of their
dynamic nature and small size, which
until recently were both close
to observational limits.
I will present a brief overview of
observations and theoretical
modelling of spicules. The main focus of the talk will be recent
work on a synthesis of very high
resolution observations of spicules
in active region plage
using the Swedish Solar Telescope (SST,
formation. These recent results indicate
that some spicules are
quasi-periodic with dominant periods (~5
minutes) that are similar to
those of the normally evanescent photospheric 5 minute oscillations.
Using a set of numerical models we show
that photospheric
oscillations can propagate into the low
atmosphere as long as they
are guided along a magnetic flux tube
that is inclined away from
the vertical. The leaked photospheric oscillations develop
non-linearly into shocks at low chromospheric heights because of
the density decrease with height. The
upward traveling shocks and
resulting rebound shocks of the chromosphere lead to periodic upward
chromospheric
flows, which we identify as the quasi-periodic spicules
that are observed in active region plage.
Our theoretical approach includes 1D
hydrodynamic simulations of
a rigid flux tube expanding with height,
but we also show recent
results of 2D MHD simulations that
illustrate, e.g., the role played
by mode coupling around the plasma \beta
= 1 surface. In addition, we
present results from the 1D RADYN code
which we use to study the
influence of non-LTE radiative
transfer on the spectral diagnostics
and shock formation in spicules.
Bart De Pontieu