1. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 1 Short-period Doppler shift variations in the polar cap: ULF waves or something else? G. Scoular, P.V. Ponomarenko, J.P. St.-Maurice University of Saskatchewan Saskatoon, Canada

2. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 2 Abstract Daytime Pc3-4 waves (10-50 mHz) are generated at the bowshock and propagate through the inner magnetosphere to the ground as Alfven waves in the “closed” field line geometry. These waves have also been detected in the polar cap, but their propagation mode for the “open” field lines remains largely unknown. To address this issue, we ran a pilot study of ionospheric ULF wave signatures in the northern polar cap. For this purpose we analysed Doppler shift variations in the HF ground scatter echoes across the 5-60 mHz frequency range as measured by the PolarDARN radars at Rankin Inlet and Inuvik. Previous ground magnetometer studies showed that the high-latitude Pc3-4 waves exhibit relatively long spatial coherence, distinct band-limited spectral shape, and occurrence/power maximum near MLT noon. In contrast, our observations revealed a dominance of Doppler shift variations that exhibit low spatial coherence, a featureless power-law spectrum and no connection to the ground magnetic field variations, while their power has two diurnal peaks before and after 12 MLT. This rather unexpected outcome of our studies has forced us to look for alternative explanations of the observed Doppler shift variations.

3. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 3 Outline Previous results and motivation Measurement setup and data processing Experimental results Problems with interpretation and alternative mechanisms Conclusions

4. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 4 Bow shock Magnetopause ? ? Polar cap Pc3-4 waves (10-50 mHz) generated at bow shock Compressional waves Propagation to auroral and cusp latitudes occurs through field aligned MHD waves. Propagation from ionosphere to ground as E&M wave Observed on ground in polar cap but propagation mechanism is unknown Motivation: Polar cap ULF Waves

5. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 5 Lower Latitude Observations (TIGER)

6. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 6 Lower Latitude Observations (cont.) Pc3-4 are detected by radars and magnetometers ULF signals consist of: –band bimited enhancement –power law background

7. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 7 Experiment Objectives Investigate seasonal and diurnal trends Analyze spectral shape Determine propagation properties using multiple radar beams In uv ik Rankin Inlet

8. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 8 Observations and Data Processing Oscillations are coherent in range so we can –take median across range gates to generate timeseries –perform Fourier analysis Rankin Inlet beam 7 – April 17, 2008

9. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 9 Diurnal trend Bad stats Occurrence analysis “by eye” Power Spectra (m/s) 2 /Hz

10. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 10 Magnetometer Observations EW and NS components show only a single peak

11. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 11 Coherence Along the Line of Sight Range Gates Phase (degrees) Coherence 1 10 100 Frequency, mHz 1.0 0.8 0.6 0.4 0.2 0 100 0 -100 Good coherence at low frequencies

12. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 12 Coherence between beams Phase (degrees) Coherence 1 10100 Frequency, mHz Poor Coherence! Random Phase! ?

13. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 13 Inuvik Rankin Inlet Inuvik Rankin Inlet Different scales along and across line-of sight

14. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 14 Black: 5:00-6:0 MLT Red: 8:45-10:15 MLT Blue: 13:00-14:30 MLT S  f - 1 PSD, (m/s) 2 /Hz ?

15. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 15 Magnetometer-Radar Coherence Phase (degrees) Coherence 110 100 Frequency, mHz 1.0 0.8 0.6 0.4 0.2 0 100 0 -100 ?

16. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 16 Possible Causes Pc3-4 waves Gravity waves Infrasound waves (C. Wilson, UoAF) –Compressional waves –Low spatial coherence –No E&M component –Auroral activity as possible source

17. 31 May 2011SuperDARN Workshop, 29 May - 3June 2011, Hanover, US 17 Conclusions There are plasma velocity oscillations in polar ionosphere with T~10-100s Maximum amplitude is observed at local morning and early afternoon Low spatial coherence, L<100km No E&M component propagating to ground Alternative sources of the observed waves