  In disordered elastic systems, driven by displacing a parabolic confining
potential adiabatically slowly, all advance of the system is in bursts, termed
avalanches. Avalanches have a finite extension in time, which is much smaller
than the waiting-time between them. Avalanches also have a finite extension
$\ell$ in space, i.e. only a part of the interface of size $\ell$ moves during
an avalanche. Here we study their spatial shape $\left< S(x)\right>_{\ell}$
given $\ell$, as well as its fluctuations encoded in the second cumulant
$\left< S^{2}(x)\right>_{\ell}^{{\rm c}}$. We establish scaling relations
governing the behavior close to the boundary. We then give analytic results for
the Brownian force model, in which the microscopic disorder for each degree of
freedom is a random walk. Finally, we confirm these results with numerical
simulations. To do this properly we elucidate the influence of discretization
effects, which also confirms the assumptions entering into the scaling ansatz.
This allows us to reach the scaling limit already for avalanches of moderate
size. We find excellent agreement for the universal shape, its fluctuations,
including all amplitudes.
