On analogues of modular units for the moduli stack of Drinfeld shtukas

We construct certain rational functions on the moduli stack of Drinfeld shtukas. These rational functions are analogues of modular units for elliptic curves. Their divisors are supported on horocycles. We introduce the notion of toy shtukas, and use their moduli space to describe the space of principal horospherical divisors generated by the divisors of these rational functions. This gives an explicit version of the Manin-Drinfeld theorem for function fields.