Magnus Ivarsen (University of Saskatchewan, Saskatoon, Canada) will deliver a talk on the topic "The motion and internal structure of radar echo clusters from the auroral E-region ionosphere".

Abstract:

Advances in radar imaging have yielded the ICEBEAR 3D dataset: point-source echo locations produced by electrojet turbulence in the auroral E-region. This talk describes two recent analysis techniques, which a) track echo clusters as they move through the field of view, and b) characterize the static structure factor of the discrete echo point set.

a) Echoes are clustered with DBSCAN (Ester et al. 1996) and associated across frames by solving the assignment problem (Kuhn 1955) with a kinematic prediction. The echo source regions are electric field enhancements frozen into the large-scale convection, and cluster trajectories therefore measure the E×B drift, information the Doppler shifts withhold, since Farley-Buneman phase speeds saturate near the ion-acoustic speed.

b) Second-order statistics of mesospheric winds have been recovered from specular meteor radar by differencing line-of-sight Doppler across echo pairs, with the echo positions treated as incidental (Vierinen et al. 2019; Poblet et al. 2023). We present the complementary measurement, in which positions themselves carry the statistics. In the auroral electrojets, coherent backscatter requires the local electron drift to exceed the ion-acoustic speed. Echo occurrences therefore constitute a threshold sample, an excursion set, of the ionospheric electric field, and subsequently records that field's spatial spectrum. We compute the static structure factor from binned pairwise echo separations using Bartlett's isotropic estimator with its recently completed bias and variance theory (Hawat et al. 2023), evaluated in a co-moving frame, supplied by the tracking, that removes advective smearing. The spectra tend to be scale-free with indices near -5/3 across 1 to 100 km, and space-ground conjunctions with Swarm and GNSS extend the agreement along the flux tube over almost four decades in wavenumber.

We discuss the important implication: together, a) and b) constitute a measurement of the amplitude, direction, and internal structure of the auroral electric field. We close with two future prospects for imaging meteor observations: the structure factor of meteor trail echo positions measures the radar point-spread function empirically and thereby calibrates the auroral E-region structure factor spectra; and, lastly, tracking long-lived meteor trails across frames could yield two wind components per trail.

References:

Ester et al., Proc. KDD-96, 226 (1996).
Kuhn, Nav. Res. Logist. Q. 2, 83 (1955).
Vierinen et al., Earth Space Sci. 6, 1171 (2019).
Poblet et al., J. Geophys. Res. Atmos. 128, e2022JD038092 (2023).
Hawat et al., Stat. Comput. 33, 61 (2023).