| We study collective modes in an ultracold neutral plasma (UNP), as observed in recent experiments via microwave excitation of plasma electrons. The absence sharp boundary in freely expanding UNPs requires different approaches than used to understand the response of confined hot plasmas, but suggests an analogy to the behavior of hot stars. We present extensive molecular dynamics simulations that reproduce experimental observations, upon a proper inclusion of temperature and charge imbalance effects. We elucidate the nature of these modes, based on a hydrodynamical theory. Upon proper understanding such modes can serve as diagnostic tool to monitor the plasma temperature and other properties, such as the degree of neutrality during the expansion of ultracold plasmas. |
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