1. 1 Sounding rocket measurements of decameter structures in the cusp K. Oksavik 1, J. Moen 1,2, D. A. Lorentzen 1, F. Sigernes 1, T. Abe 3, Y. Saito 3, and M. Lester 4 1) UNIS, Longyearbyen, Norway 2) Department of Physics, Univ. Oslo, Oslo, Norway 3) ISAS, Japan Aerospace Exploration Agency, Japan 4) Department of Physics & Astronomy, Univ. Leicester, UK SD2011

2. 2 ICI-2 Sounding Rocket Launched at 10:35:10 UT on 05 December 2008 Rocket instrumentation: –Four-Needle Langmuir Probe (4-NLP): Absolute Ne –Electric Field Wave Experiment (EFW): E-field (AC and DC) –Low Energy Particle spectrometer (LEP-ESA): Electrons 0.01-10 keV Ground optics, EISCAT and SuperDARN Photo: Martin Langteigen

3. 3 The ICI-2 trajectory

4. 4 Comparing rocket and EISCAT measurements during flight ICI-2 reveals fine structure in the electron density

5. 5 ICI-2 flew through the F-region Several minutes were spent in the F-region An auroral form was intersected on the up-leg, and the cusp aurora was intersected on the down-leg

6. 6 ICI-2 flew through an area of HF backscatter in the cusp On its down-leg ICI-2 intersected the poleward boundary of HF backscatter and cusp aurora

7. 7 Decameter scale irregularities were observed 10-20% electron density gradients over a distance of a few tens of meters

8. 8 Encountering HF backscatter Highly structured Ne Inverted-V signatures Fluctuating electron flux Gradients in Ne at all spatial scales (10 m, 100 m, and 1000 m) Gradient Drift instability is stable on the poleward side of the blob, and unstable on the equatorward side [Ossakow and Chaturvedi, 1979]

9. 9 Analysis of the entire flight: Kelvin-Helmholtz Instability (KHI) Highest growth rate around 200-230 s 1-5 min. growth time of 4-6 km irregularities

10. 10 Analysis of the entire flight: Gradient Drift Instability (GDI) Many occurrences of decameter scale gradients Growth times often between 10 s and 1 min. Gradient Drift Instability is dominant!

11. 11 Conclusions ICI-2 made the first direct observation of HF backscatter targets (decameter scale plasma density irregularities) Km-scale irregularities were most likely modulated by auroral particle precipitation onto which plasma instabilities can operate [Kelley et al., 1998] The plasma gradient on the poleward cusp boundary was stable to GDI growth, while the equatorward boundary was unstable GDI alone, working on km scale gradients, can explain the generation of HF backscatter targets, but other instability processes may also contribute The growth rate of the KHI mechanism was too slow to explain any of the observed plasma irregularities