  The bulk of solar flare emission originates from very compact sources located
in the lower solar atmosphere and seen in various wavelength ranges: near
optical, UV, EUV, soft and hard X-rays, and gamma-ray emission, yet very few
spatially resolved imaging observations to determine the structure of these
regions exist. We investigate the above-the-photosphere heights of hard X-ray
(HXR), EUV and white-light continuum sources in the low atmosphere and the
corresponding densities at these heights. Simultaneous EUV/continuum images
from SDO and HXR RHESSI images are compared to study a well observed gamma-ray
limb flare. Using RHESSI X-ray visibilities we determine the height of the HXR
sources as a function of energy above the photosphere. Co-aligning AIA/SDO and
HMI/SDO images with RHESSI we infer, for the first time, the heights and
characteristic densities of HXR, EUV and continuum sources in a flaring
footpoint. 35-100 keV HXR sources are found at heights between 1.7 and 0.8 Mm
above the photosphere, below the white-light continuum emission which appears
at heights 1.5-3 Mm, and the peak of EUV emission originating near 3 Mm. The
EUV emission locations are consistent with energy deposition from low energy
electrons of ~12 keV occurring in the top layers of the fully ionized
chromosphere/low corona and not by >20 keV electrons that produce HXR
footpoints in the lower neutral chromosphere. The maximum of white-light
emission appears between the HXR and EUV emission, presumably in the transition
between ionized and neutral atmospheres suggesting free-bound and free-free
continuum emission. We note that the energy deposited by low energy electrons
is sufficient to explain the energetics of optical and UV emissions.
