90 Encyclopedia of the Solar System
FIGURE 18 Soft X-ray and EUV
images of flare loops and flare
arcades with bipolar structure.
Yohkoh/SXT observed flares
(March 18, 1999, 16:40 UT, and
June 7, 2000, 14:49 UT) with
“candle-flame”-like cusp geometry
during ongoing reconnection,
whileTRACEsees postflare loops
once they cooled down to 1–2 MK,
when they already relaxed into a
near-dipolar state. Examples are
shown for a small flare (the April
19, 2001, 13:31 UT, GOES class
M2 flare), and for two large flares
with long arcades, seen at the limb
(September 30, 1998, 14:30 UT)
and on the disk (the July 14, 2000,
10:59 UT, X5.7 flare). (Courtesy of
Yohkoh/ISAS andTRACE/NASA.)because of the chromospheric opacities and partial ion-
ization. The third stage of chromospheric evaporation has
been extensively explored with hydrodynamic simulations,
in particular to explain the observed Doppler shifts in soft
X-ray lines, while application of spatial models to imaging
data is quite sparse. Also certain types of slow-drifting radio
bursts seem to contain information on the motion of chro-
mospheric evaporation fronts. The fourth stage of postflare
loop cooling is now understood to be dominated by thermal
conduction initially and by radiative cooling later on. How-
ever, spatiotemporal temperature modeling of flare plasmas
(Fig. 19) has not yet been fitted to observations in detail.6.5 Particle Acceleration and Kinematics
Particle acceleration in solar flares is mostly explored by
theoretical models because neither macroscopic nor mi-
croscopic electric fields are directly measurable by remote-
sensing methods. The motion of particles can be described
in terms of acceleration by parallel electric fields, drift