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SuperDARN and reversed flow events in the cusp

K. Oksavik1, J. Moen1,2 , E. H. Rekaa2, H. C. Carlson3, and M. Lester4

1) UNIS, Longyearbyen, Norway2) Department of Physics, Univ. Oslo, Oslo, Norway3) CASS, Utah State University, USA 4) Department of Physics & Astronomy, Univ. Leicester, UK

SD2011

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Background on RFE

Reversed flow events (RFEs):• First seen with the EISCAT

Svalbard radar• Longitudinally elongated,

segments of transiently enhanced ion flow, in the direction opposite to the background flow

• For both RFE and background flow: |Vlos| > 250 m/s (but in opposite directions)

• Size: >400 km in longitude• Form one by one (not in pairs)

Rinne et al. [2007]

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Three events will be studied

• Solar wind is stable:– By positive– Bz negative

• SD coverage:– 700-800 data points

– VPC = 50-60 kV

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RFE seen by SuperDARN

• The RFE flow channel (red color) is located in an area of sunward flow (blue color) in the postnoon sector

• This is the first report of RFE in SD data!

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Several examples of RFEs

• RFEs seem to occur in areas of high backscatter power and wide Doppler spectra

• The backscatter power / spectral width is often highest along one edge of RFEs, and low along the other edge

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Time development of RFEs

• Events 1 and 2 were part of an interval of prolonged activity

• A transition took place at 09:05 UT

• The poleward edge of event 3 coincided with wide Doppler spectra and growing backscatter power

• Are RFE important for the formation of irregularities?

1 2 3

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Formation of plasma irregularities via flow shears

• The KH growth rate is [Keskinen et al., 1988]:

• Most ∆VLOS are small, but some reach 800 m/s (i.e. growth time of 300 s)

• A 42˚ angle between the radar beam and the large-scale flow decreases the net growth time to 200 s

• Rinne et al. [2007] found examples of L = 12 km (i.e. growth time of 60 s)

• KH may be efficient at producing irregularities!

LV /19.0 ∆VLOS Neighboring

range gates

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Offset between SD and optics

There appears to be an offset in latitude between RFE and the aurora

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Flow vorticity and FACs

[Sofko et al., 1995] HPP|| ΣBΣΣJ vEvB

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Determination of ground range

• The clockwise flow reversal (i.e. upward FAC) aligns with the aurora, if we shift the Hankasalmi radar echoes equatorward by 140 km

• Consistent with other studies [Yeoman et al., 1997]

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Conclusions• We have documented several examples of

RFE channels seen by SuperDARN• The location of RFE flow shears relative to

auroral arcs shows that the SD ground range was overestimated by 140 km

• RFEs may be important for the formation of plasma irregularities via flow shear instabilities (Kelvin Helmholtz)

• The large SuperDARN dataset offers new opportunities to learn more about RFEs

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From June 01, 2011New address:

Department of Physics and Technology, University of Bergen, P.B. 7803, NO-5020 Bergen, NORWAY.

E-mail: kjellmar.oksavik@uib.no