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We present the Fermi-LAT observations of the behind-the-limb (BTL) flare ofJuly 17, 2021 and the joint detection of this flare by STIX onboard SolarOrbiter. The separation between Earth and the Solar Orbiter was 99.2$^{\circ}$at 05:00 UT, allowing STIX to have a front view of the flare. The location ofthe flare was ~S20E140 in Stonyhurst heliographic coordinates making this themost distant behind-the-limb flare ever detected in $>$100 MeV gamma-rays. TheLAT detection lasted for $\sim$16 minutes, the peak flux was $ 3.6 \pm 0.8 $(10$^{-5}$) ph cm$^{-2}$ s$^{-1}$ with a significance $>$15$\sigma$. A coronalwave was observed from both STEREO-A and SDO in extreme ultraviolet (EUV) withan onset on the visible disk in coincidence with the LAT onset. A complex typeII radio burst was observed by GLOSS also in coincidence with the onset of theLAT emission indicating the presence of a shock wave. We discuss the relationbetween the time derivative of the EUV wave intensity profile at 193\angstrom\as observed by STEREO-A and the LAT flux to show that the appearance of thecoronal wave at the visible disk and the acceleration of protons as traced bythe observed $>$100 MeV gamma-ray emission are coupled. We also report how thiscoupling is present in the data from 3 other BTL flares detected by Fermi-LATsuggesting that the protons driving the gamma-ray emission of BTL solar flaresand the coronal wave share a common origin.

The coupling of an EUV coronal wave and ion acceleration in a Fermi-LAT behind-the-limb solar flare