The Sun 85
FIGURE 13 Two global wave
events observed withSoHO/EIT
195 A, on April 7, 1997 (top row) ̊
and May 12, 1997 (bottom row).
The intensity images (right) were
recorded before the eruption,
while the difference images (left
and middle) show differences
between the subsequent images,
enhancing emission measure
increases (white areas) and
dimming (black areas). (Courtesy
of Yi-Ming Wang.)in the solar corona range fromF= 3 × 105 erg cm−^2 s−^1 in
quiet-Sun regions toF≈ 107 erg cm−^2 s−^1 in active regions.
Two main groups of DC (direct current) and AC (alternating
current) models involve as a primary energy source chromo-
spheric footpoint motion or upward leaking Alfv ́en waves,
which are dissipated in the corona by magnetic reconnec-
tion, current cascades, MHD turbulence, Alfv ́en resonance,
resonant absorption, or phase mixing. There is also strong
observational evidence for solar wind heating by cyclotron
resonance, while velocity filtration seems not to be con-
sistent with EUV data. Progress in theoretical models has
mainly been made by abandoning homogeneous fluxtubes,
but instead including gravitational scale heights and more
realistic models of the transition region, and taking advan-
tage of numerical simulations with 3D MHD codes (by
Boris Gudiksen and Aake Nordlund). From the observa-
tional side we can now unify many coronal small-scale phe-
nomena with flare-like characteristics, subdivided into mi-
croflares (in soft X-rays) and nanoflares (in EUV) solely by
their energy content. Scaling laws of the physical parame-
ters corroborate their unification. They provide a physical
basis to understand the frequency distributions of their pa-
rameters and allow estimation of their energy budget for
coronal heating. Synthesized data sets of microflares and
nanoflares in EUV and soft X-rays have established that
these impulsive small-scale phenomena match the radiative