  The giant flare of December 27, 2004 from SGR 1806-20 represents one of the
most extraordinary events captured in over three decades of monitoring the
gamma-ray sky. One measure of the intensity of the main peak is its effect on
X- and gamma-ray instruments. RHESSI, an instrument designed to study the
brightest solar flares, was completely saturated for ~0.5 s following the start
of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the
time of the giant flare, however, allowed RHESSI a unique view of the giant
flare event, including the precursor, the main peak decay, and the pulsed tail.
Since RHESSI was saturated during the main peak, we augment these observations
with Wind and RHESSI particle detector data in order to reconstruct the main
peak as well. Here we present detailed spectral analysis and evolution of the
giant flare. We report the novel detection of a relatively soft fast peak just
milliseconds before the main peak, whose timescale and sizescale indicate a
magnetospheric origin. We present the novel detection of emission extending up
to 17 MeV immediately following the main peak, perhaps revealing a
highly-extended corona driven by the hyper-Eddington luminosities. The spectral
evolution and pulse evolution during the tail are presented, demonstrating
significant magnetospheric twist and evolution during this phase. Blackbody
radii are derived for every stage of the flare, which show remarkable agreement
despite the range of luminosities and temperatures covered. Finally, we place
significant upper limits on afterglow emission in the hundreds of seconds
following the giant flare.
