  We report on the detection of the gamma-ray emission above 100 MeV from the
solar flare of September 29, 2022, by Fermi LAT with simultaneous coverage in
HXR by Solar Orbiter STIX. The Solar Orbiter-Earth separation was 178$^{\circ}$
at the time of the flare as seen from Earth, with Solar Orbiter observing the
east limb. Based on STIX imaging, the flare was located 16$^{\circ}$ behind the
eastern limb as seen from Earth. The STIX and GBM non-thermal emission and the
LAT emission above 100 MeV all show similarly shaped time profiles, and the
Fermi profiles peaked only 20 seconds after the STIX signal from the main flare
site, setting this flare apart from all the other occulted flares observed by
Fermi LAT. The radio spectral imaging based on the Nan\c{c}ay Radioheliograph
and ORFEES spectrograph reveal geometries consistent with a magnetic structure
that connects the parent active region behind the limb to the visible disk. We
studied the basic characteristics of the gamma-ray time profile, in particular,
the rise and decay times and the time delay between the gamma-ray and HXR peak
fluxes. We compared the characteristics of this event with those of four Fermi
LAT behind-the-limb flares and with an on-disk event and found that this event
is strikingly similar to the impulsive on-disk flare. Based on multiwavelength
observations, we find that the gamma-ray emission above 100 MeV originated from
ions accelerated in the parent active region behind the limb and was
transported to the visible disk via a large magnetic structure connected to the
parent active region behind the limb. Our results strongly suggest that the
source of the emission above 100 MeV from the September 29, 2022 flare cannot
be the CME-driven shock.
