By deploying a small broadband antenna and spectrum analyzer in the auroral zone, one can easily detect at least three different kinds of naturally occuring radio signals emitted by Earth's aurora: A broadband emission below 1 MHz, called auroral hiss; a broadband emission at 1.5-4.5 MHz, called MF-burst; and narrowband emissions near 2.8 and 4.2 MHz, called auroral roar. This talk focusses on this last type of emissions, which are believed to originate when electrostatic upper hybrid waves excited by the auroral electrons at harmonics of the electron gyrofrequency mode-convert to O-mode and propagate through the ionosphere-atmosphere boundary to reach the ground. The evidence for this hypothesis will be presented. An interesting feature of these emissions is their fine-frequency structure: they are composed of multiple discrete features as narrow as a few Hertz separated by hundreds of Hertz to several kilohertz. Direct detection of electrostatic Langmuir and upper hybrid waves have played a key role suggesting that this fine structure is a natural consequence of the inhomogeneity of the ionospheric plasma in the source region of the waves. The effort to understand the fine structure of auroral radio emissions thus provides an excellent example of the interplay of ground-based and in-situ observations to solve an outstanding space physics problem.