Yvonne Rinne, Departement of Physics, University of Oslo

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

by

Y. Rinne J. MoenH. C. CarlsonK. Oksavik

Yvonne Rinne, Departement of Physics, University of Oslo

Outline

• The Fast Scan Mode at the EISCAT Svalbard Radar (ESR)

• SP-NO Data base• Occurrence of Reversed Flow

Events (RFEs)• Discussion• Conclusion

Yvonne Rinne, Departement of Physics, University of Oslo

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

Yvonne Rinne, Departement of Physics, University of Oslo

Large spatial coverage decreases the spatial resolution

Yvonne Rinne, Departement of Physics, University of Oslo

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

Yvonne Rinne, Departement of Physics, University of Oslo

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).

Yvonne Rinne, Departement of Physics, University of Oslo

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

Yvonne Rinne, Departement of Physics, University of Oslo

RFE occurrences versus MLT

Yvonne Rinne, Departement of Physics, University of Oslo

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

Yvonne Rinne, Departement of Physics, University of Oslo

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

Yvonne Rinne, Departement of Physics, University of Oslo

Yvonne Rinne, Departement of Physics, University of Oslo

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)