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.

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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

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


Abstract

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


Outline

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


Motivation polar cap ulf waves

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


Lower latitude observations tiger

Lower Latitude Observations (TIGER)

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


Lower latitude observations cont

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


Experiment objectives

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


Observations and data processing

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


Diurnal trend

Diurnal trend

  • Occurrence analysis “by eye”

  • Power Spectra

(m/s)2/Hz

Bad stats

Bad stats

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


Magnetometer observations

Magnetometer Observations

  • EW and NS components show only a single peak

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


Coherence along the line of sight

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


Coherence between beams

Coherence between beams

?

Poor Coherence!

Coherence

Random Phase!

Phase (degrees)

1 10100

Frequency, mHz

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


Different scales along and across line of sight

Different scales along and across line-of sight

Inuvik

Inuvik

Rankin Inlet

Rankin

Inlet

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


Short period doppler shift variations in the polar cap ulf waves or something else

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


Magnetometer radar coherence

Magnetometer-Radar Coherence

?

1.0

0.8

0.6

0.4

0.2

0

Coherence

100

0

-100

Phase (degrees)

110 100

Frequency, mHz

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


Possible causes

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


Conclusions

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


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