  We investigate the sequence of events leading to the solar X1 flare
SOL2014-03-29T17:48. Because of the unprecedented joint observations of an
X-flare with the ground-based Dunn Solar Telescope and the spacecraft IRIS,
Hinode, RHESSI, STEREO, and SDO, we can sample many solar layers from the
photosphere to the corona. A filament eruption was observed above a region of
previous flux emergence, which possibly led to a change in magnetic field
configuration, causing the X-flare. This was concluded from the timing and
location of the hard X-ray emission, which started to increase slightly less
than a minute after the filament accelerated. The filament showed Doppler
velocities of $\sim$2-5 km s$^{-1}$ at chromospheric temperatures for at least
one hour before the flare occurred, mostly blueshifts, but also redshifts near
its footpoints. 15 minutes before the flare, its chromospheric Doppler shifts
increased to $\sim$6-10 km s$^{-1}$ and plasma heating could be observed,
before it lifted off with at least 600 km s$^{-1}$, as seen in IRIS data.
Compared to previous studies, this acceleration ($\sim$3-5 km s$^{-2}$) is very
fast, while the velocities are in the common range for coronal mass ejections.
An interesting feature was a low-lying twisted second filament near the
erupting filament, which did not seem to participate in the eruption. After the
flare ribbons started on each of the second filament's sides, it seems to have
untangled and vanished during the flare. These observations are some of the
highest resolution data of an X-class flare to date and reveal some small-scale
features yet to be explained.
