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Sounding rocket measurements of decameter structures in the cusp

K. Oksavik1, J. Moen1,2 , D. A. Lorentzen1, F. Sigernes1, T. Abe3, Y. Saito3, and

M. Lester4

1) UNIS, Longyearbyen, Norway2) Department of Physics, Univ. Oslo, Oslo, Norway3) ISAS, Japan Aerospace Exploration Agency, Japan 4) Department of Physics & Astronomy, Univ. Leicester, UK

SD2011

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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

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The ICI-2 trajectory

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Comparing rocket and EISCAT measurements during flight

ICI-2 reveals fine structure in the electron density

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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

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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

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Decameter scale irregularities were observed

10-20% electron density gradients over a distance of a few tens of meters

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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]

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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

LVKH 2.0

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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!

xN

NV

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0GD

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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