short-period doppler shift variations in the polar cap: ulf waves or something else?
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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. Abstract. - PowerPoint PPT PresentationTRANSCRIPT
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