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

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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

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

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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.

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

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Outline

• Previous results and motivation

• Measurement setup and data processing

• Experimental results

• Problems with interpretation and alternative mechanisms

• Conclusions

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

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Bow

shoc

k

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

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

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Lower Latitude Observations (TIGER)

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

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Lower Latitude Observations (cont.)

• Pc3-4 are detected by radars and magnetometers

• ULF signals consist of:

– band bimited enhancement

– power law background

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

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Experiment Objectives

• Investigate seasonal and diurnal trends

• Analyze spectral shape

• Determine propagation properties using multiple radar beams

Inuvik

Inuvik

Rankin Inlet

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

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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

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

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Diurnal trend

Bad stats

Bad stats

• Occurrence analysis “by eye”

• Power Spectra

(m/s)2/Hz

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

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Magnetometer Observations

• EW and NS components show only a single peak

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

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Coherence Along the Line of Sight

Ran

ge G

ates

Pha

se (

degr

ees)

Coh

eren

ce

1 10 100Frequency, mHz

1.0

0.8

0.6

0.4

0.2

0

100

0

-100

Good coherence at low frequencies

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

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Coherence between beams

Pha

se

(deg

rees

)C

oher

ence

1 10 100Frequency, mHz

Poor Coherence!

Random Phase!?

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

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Inuvik

RankinInlet

Inuvik

Rankin Inlet

Different scales along and across line-of sight

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

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Black: 5:00-6:0 MLTRed: 8:45-10:15 MLTBlue: 13:00-14:30 MLT

S f -1

PS

D,

(m/s

)2 /H

z

?

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

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Magnetometer-Radar Coherence

Phase (degrees)

Coherence

1 10 100Frequency, mHz

1.0

0.8

0.6

0.4

0.2

0

100

0

-100 ?

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

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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

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

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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