Langmuir Waves Upstream of Interplanetary Shocks: Dependence on Shock and Plasma Parameters

We examine several hundred interplanetary shocks observed by the Wind spacecraft to determine which plasma environments and shock parameters are favorable for the production of upstream Langmuir waves. Langmuir wave generation is a necessary condition for producing type II radio bursts, which are a primary signature of interplanetary coronal mass ejections. We split a list of roughly 250 interplanetary shocks into two populations, based on the presence or absence of upstream Langmuir waves. Our results indicate that the dominant factor in Langmuir wave production is the shock magnetic compression from the upstream to the downstream region. Other parameters, including the de Hoffmann-Teller velocity and the shock Mach number, have no discernable effect. Given these results, and assuming that Langmuir waves are created by electron beams accelerated by a fast-Fermi process, we draw the tentative conclusion that it is the fraction of incident electrons that are reflected that controls the presence or absence of upstream Langmuir waves, rather than the maximum beam energy.