PHYSICS AND ENGINEERING PHYSICS

PHYSICS AND ENGINEERING PHYSICS Spectral widths of F-region PolarDARN echoes, a statistical assessment A.V. Koustov, S. Toderian and P.T. Jayachandran (UNB)

Outline 1. Motivation 2. Narrow spectra 3. Spectral width vs flow angle 4. Spectral width vs ExB magnitude 5. Conclusions

Motivation 1. Information on spectral width has been of limited use. In recent past, interest to the width has almost evaporated 2. New FITACF approach changed the typical values of the width (dramatically) 3. It has been suggested that F-region spectral widths should be considered in “Hz” rather than “m/s” 4. Limited data on the spectral width in the polar cap, on open magnetic flux lines. Nisitani et al. 2004 reported occurrence of “narrow” echoes in the polar cap

FoVs of the Northern hemisphere SuperDARN radars as of January 2011 Polar cap echoes can be detected by all auroral zone radars as well as by Inuvik (INV) and Rankin Inlet (RKN).

Auroral radars and RKN widths in m/s Villain et al., 2002: Auroral radars RKN, Jan 2010 RKN radar and the auroral zone radars show comparable widths in the polar cap (>80 MLAT).

RKN echo width, 1/s December 2010 June 2010 noon noon Midnight gap Gates 26-30 Over Resolute Midnight gap Gates 20-22 There is MLT variation: echoes are broader during pre-noon hours

Issue #1: Nature of “narrow” PolarDARN echoes

Typical RKN echo width in units of 1/s. Beams 4,5 Range: 945 km Range: 1215 km Range: 1755 km As we consider larger ranges, narrow component becomes dominating. Data were combined over 0-24 MLT

Typical RKN echo width in units of 1/s, beams 5,6, prenoon sector, 1 hour slot Gates 10-11

Typical RKN echo width in units of 1/s, beams 5,6, prenoon sector Gates 14-15

Typical RKN echo width in units of 1/s, beams 5,6, prenoon sector Gates 26-27, over Resolute

Typical RKN echo width in units of 1/s. Beams 4,5 800 MLAT Range=1125 ResBay Max widths are at ~ 1000-1200 km. Decrease towards the Pole. Summer echoes are more narrow, by ~10-15 (1/s). INV shows similar trends

Nature of narrow spectra Hypothesis 1: Narrow component is due to mixed scatter (GS contamination) Hypothesis 2: Narrow component is due to a special type of irregularities that exist only in the polar cap. (One opinion is that they are associated with polar cap patches)

John Gelleta tracings for RKN midnight noon Elev=20 Elev=10 Echoes at 1000-1500 km can be either ½ hop F region or 1&1/2 hop E region. E region GS is possible

Testing GS hypothesis #1, 1 RKN echo width, 1/s f<11.2 MHz Gates 21-22, all MLTs Gates 21-22, midnight There are lots of narrow echoes in the midnight sector as well as in the noon sector. This is NOT what is expected from the GS contamination hypothesis as GS is more frequent at noon hours Gates 21-22, noon

Spectral width over RB in Dec 2010 Typical width is ~ 27 1/s, red vertical line

Spectral width over RB in Dec 2010 Typical width is ~ 22 1/s

Spectral width over RB in Dec 2010 Typical width is ~ 15 1/s, very narrow echoes

Testing GS hypothesis #1, 1 There are lots of narrow echoes in the midnight sector as well as in the noon sector. This is NOT what is expected from the GS contamination hypothesis as GS is more frequent at noon hours It seems to me that narrow echoes are a significant portion of all echoes when RKN looks along the flow (noon and late evening sectors)

Testing GS hypothesis, feature 2 Typical RKN echo width in units of 1/s Range: 1350 km Echoes in central beams are, statistically speaking, more narrow. GS is more frequent in central beams. Signals are expected to be stronger affected by GS in central beams.

Testing GS hypothesis, feature 3 RKN velocity and CADI l-o-s velocity for narrow and broad echoes N=22139 Bad 18% Good 55.0% Noon, <50m/s Midnight, <50m/s All echoes The velocity of narrow echoes often disagrees with the CADI ExB component (up to 500 m/s) along RKN beam 5.

Outline Introduction Objectives PolarDARN Echo Occurrence CADI Observations Summary RKN LOS-Velocity Significantly Decreases at Noon noon PolarDARN HF Echo Occurrence Near Winter Magnetic Noon ● M. Ghezelbash, A. V. Koustov, D. Mori, D. André 13

Joint KODIAK-Inuvik observations in “common” echo regions but from different ranges: Testing GS hypothesis, 4 INV: F region, KOD: F region Wkod<Winv, by 10 Hz

For the time being: We cannot make for sure a statement on the nature of “narrow” echoes regularly detected by the PolarDARN radars, but it seems as many of these are GS contaminated ionospheric signals

Issue #2: Does spectral width of PolarDARN echoes depend on the flow angle of observations? • - CADI at Res Bay • measures ExB • magnitude and • direction • Consider only points • for which CADI and RKN • velocities are consistent

Spectral widths for simultaneous INV and RKN echoes detected over RB All flow angles are represented. Widths are comparable, overall. Winter: Both narrow and broad components at RKN have smaller widths

RKN spectral width varies with the ExB azimuth. Tendencies are difficult to infer Lowest ExB Largest ExB

Issue #3: Does spectral width (in Hz) of RKN (INV) echoes depend on ExB magnitude? • 3 data sets were compiled: • Joint RKN-RB CADI • Joint INV-RB CADI • Joint RKN-INV-RB CADI

Spectral width (in Hz) vs CADI ExB magnitude RKN Range: 1350 km S: Slope=0.14 (1/s)/(mV/m) r: Corr. Coeff.=0.14

Spectral width (in Hz) vs CADI ExB magnitude INV Range: ~1500 km S: Slope=0.19 (1/s)/(mV/m) r: Corr. Coeff.=0.08

Conclusions for Spectral Width • Significant portion of PolarDARN F region echoes has very low spectral widths (<10 Hz) • Systematic decrease of the width with range (going towards the Pole) • No flow angle dependence • Weak (statistical) increase with ExB magnitude, but correlation coefficient of the tendency is low • Exact single reason for occurrence of “narrow” echoes is still unknown 6-dB threshold

Thank you for attention

PHYSICS AND ENGINEERING PHYSICS