1 / 17

Short-period Doppler shift variations in the polar cap: ULF waves or something else?

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. Abstract.

masao
Télécharger la présentation

Short-period Doppler shift variations in the polar cap: ULF waves or something else?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 SuperDARN 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. SuperDARN 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 SuperDARN Workshop, 29 May - 3June 2011, Hanover, US

  4. Motivation: Polar cap ULF Waves Magnetopause Bow shock • 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 Polar cap ? ? SuperDARN Workshop, 29 May - 3June 2011, Hanover, US

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

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

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

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

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

  12. Coherence between beams ? Poor Coherence! Coherence Random Phase! Phase (degrees) 1 10 100 Frequency, mHz SuperDARN Workshop, 29 May - 3June 2011, Hanover, US

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

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

  15. Magnetometer-Radar Coherence ? 1.0 0.8 0.6 0.4 0.2 0 Coherence 100 0 -100 Phase (degrees) 1 10 100 Frequency, mHz SuperDARN 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 SuperDARN 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 SuperDARN Workshop, 29 May - 3June 2011, Hanover, US

More Related