A uniquely located, statistically prominant radiowave scintillation enhancement has been identified in nighttime auroral-zone data. The enhancement occurs whenever the propagation path coincides with an L-shell in the region of diffuse auroral particle precipitation. It is hypothesized that this enhancement is a purely geometric effect due to sheet-like ionospheric irregularity structures aligned along L-shells. This hypothesis is confirmed by spaced-receiver data showing that the diffraction pattern is elongated along the geomagnetic east-west direction when the scintillation enhancment occurs.