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Mesoscale transient flow channels observed in the cusp ionosphere by the EISCAT Svalbard Radar

Mesoscale transient flow channels observed in the cusp ionosphere by the EISCAT Svalbard Radar. by Y. Rinne J. Moen H. C. Carlson K. Oksavik. Outline. The Fast Scan Mode at the EISCAT Svalbard Radar (ESR) SP-NO Data base Occurrence of Reversed Flow Events (RFEs) Discussion

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Mesoscale transient flow channels observed in the cusp ionosphere by the EISCAT Svalbard Radar

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  1. Mesoscale transient flow channels observed in the cusp ionosphere by the EISCAT Svalbard Radar by Y. Rinne J. Moen H. C. Carlson K. Oksavik

  2. Outline • The Fast Scan Mode at the EISCAT Svalbard Radar (ESR) • SP-NO Data base • Occurrence of Reversed Flow Events (RFEs) • Discussion • Conclusion

  3. The radar sweeps azimut sectors in a windshield wiper mode, centered at the location of the cusp

  4. Large spatial coverage decreases the spatial resolution

  5. SP-NO Fast Scan Data base • We gathered data during january and december 2001 • The analyzed data set spans 11 days (36 hours) and consists of 767 scans • Data was gathered beween 9-15MLT

  6. Definition of Reversed Flow Events (RFEs) • A Reversed Flow Event is an elongated segment of enhanced ion flow in the opposite direction of the background flow • The RFE has to be evident in more than one radar beam direction/azimuth (this criterion should eliminate bad measurements). • The line-of-sight ion velocity inside the RFE must be greater than |250| m/s for at least one scan. • The RFE has to have a longitudinal extent of at least 400-600 km in the radar field– of-view. • The RFE has to stay in clear contrast to the background, i.e. the background flow must exhibit uniform opposite velocities higher than |250| m/s in the area surrounding the RFE. • The RFE has to be embedded within the background flow for at least one scan (this criterion avoids large-scale convection reversals being detected as RFE).

  7. Occurrence of RFEs • RFEs occured 16% of the time throughout the dataset • Their average lifetime was 19 minutes • They exceeded the field of view in length (> 400-600km) and are around 100-200km wide • No preference was found for the IMF Bz polarity • It may seem as if flow channels are as well IMF By independent  RFEs appear to be a regular feature of the active cusp

  8. RFE occurrences versus MLT

  9. 86% of the RFEs were characterized by reversed ion flow with respect to the direction of the background flow, i.e. in opposite direction of magnetic tension

  10. RFEs as a posible signature of Southwood FTEs? • RFEs cannot be interpreted as the centre flux in the Southwood model due to the direction of the ion flow beeing opposite to the magnetic tension force • If they should be interpreted as the return flow, two channels are expected to appear simultaneously

  11. Summary • RFEs seem be regular feature of the cusp ionosphere, occuring at least 16% of the time in the current data set • Their average lifetime is 19 minutes • They occur near the cusp inflow region in the MLT range from 11:45 to 12:45MLT in association with enhanced plasma flow, and seem to be L-shell aligned • The occurrence of RFEs is independend of IMF Bz (and By) • 86% of the RFEs were characterized by ion flow opposite to the magnetic tension force, meaning that they cannot be interpreted as FTE centre flux according to Southwood • RFEs coexist but never appear simultaneously • RFEs cannot be attributed the same FTE as claimed by Oksavik et al. (2004)

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