Analysis of electron distributions associated with the source of auroral roar

In January of 2003, the HIgh Bandwidth Auroral Rocket (HIBAR) passed through two separate regions of strong upper hybrid emission in which the upper hybrid frequency was twice the electron cyclotron frequency (f_uh = 2 f_ce) [\textit{Samara et~al},2004]. These emissions are believed to be the source of HF auroral roar wave emission often observed on ground based receivers. The current theoretical model implies that the electro-magnetic waves observed on the ground are produced through mode conversion of strong emission of upper hybrid waves near 2 and 3 times the cyclotron frequency. The cyclotron maser theory predicts that the Z mode wave undergoes significant growth when the local upper hybrid frequency is just below ( ∼1%) twice the electron cyclotron frequency and with the appropriately unstable distribution of energetic electrons [\textit{Yoon et~al.}, 1996,1998,2000]. Though roar is frequently observed from the ground, the source region has rarely been identified in-situ. Analysis of the in-situ electron distributions from HIBAR show qualitative agreement with the theoretical distributions used by \textit{Yoon et~al.},1998. HIBAR encountered three distinctly separate regions where f_uh ∼ 2 f_ce, two of these regions include strong upper hybrid emission, while the third is void of upper hybrid wave activity. The measured particle distributions demonstrate that, in the two regions with wave emission, the electron cyclotron maser instability condition is achieved and that, in the third region without wave emission, the instability condition fails. \begin{thebibliography}{} \bibitem{samara:04} Samara, M., J.~LaBelle, C.~A. Kletzing, and S.~R. Bounds, Rocket observations of structured upper hybrid wave at f_uh}=2f{ce, \textit{Geophys. Res. Lett.}, submitted August 2004. \bibitem{yoon:96} Yoon, P.~H., A.~T. Weatherwax, and T.~J. Rosenberg, Lower ionospheric cyclotron maser theory: A possible source of 2f_ce and 3f_ce auroral radio emissions, \textit{J. Geophys. Res.}, \textit{101}, 27,015--27,025, 1996. \bibitem{yoon:98} Yoon, P.~H., A.~T. Weatherwax, and T.~J. Rosenberg, On the generation of auroral radio emission at harmonics of the lower ionospheric electron cyclotron frequency: X, O and Z mode maser calculations, \textit{J. Geophys. Res.}, \textit{103}, 4071--4078, 1998. \bibitem{yoon:00} Yoon, P.~H., A.~T. Weatherwax, and J.~LaBelle, Discrete electrostatic eigenmodes associated with ionospheric density structure: generation of auroral roar fine frequency structure, \textit{J. Geophys. Res.}, \textit{105}, 27,589--27,596, 2000.