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Horizontal electric fields from flow of auroral O+(2P) ions at sub-second temporal resolution

Horizontal electric fields from flow of auroral O+(2P) ions at sub-second temporal resolution
Horizontal electric fields from flow of auroral O+(2P) ions at sub-second temporal resolution
Electric fields are a ubiquitous feature of the ionosphere and are intimately linked with aurora through particle precipitation and field-aligned currents. They exhibit order-of-magnitude changes on temporal and spatial scales of seconds and kilometres which are not easy to measure; knowing their true magnitude and temporal variability is important for a theoretical understanding of auroral processes. We present a unique method to estimate ionospheric electric fields in the region close to (km-scale) a dynamic auroral arc by solving the continuity equation for the metastable O+(2P) ions, which emit as they move under the influence of electric fields during their 5 s lifetime. The main advantage of this optical method is the increase in temporal resolution over other methods such as ground-based radars. Simultaneous measurements of emission at 732.0 nm (from the O+(2P) ions), and prompt emissions at 673.0 nm (N2) and 777.4 nm (O), all at high spatial (100 m) and temporal (0.05 s) resolution, are used in the solution of the continuity equation, which gives the dynamic changes of the O+ion population at 10 all heights in a 3D volume close to the magnetic zenith. Perspective effects are taken into account by a new geometric method, which is based on an accurate estimate of the magnetic zenith position. The emissions resulting from the metastable ions are converted to brightness images by projecting onto the plane of the ground, which are compared with the measured images. The flow velocity of the ions is a free parameter in the solution of the continuity equation; the value that minimizes the difference between the modelled and observed images is the extracted flow velocity at each time step. We demonstrate the method with 15 an example event during the passage of a brightening arc feature, lasting about 10 s, in which the inferred electric fields vary between 20 and 120 mV/m. These inferred electric fields are compared with SuperDARN measurements, which have an average value of 30 mV /m. An excellent agreement is found in the magnitude and direction of the background electric field; an increase in magnitude during the brightening of the arc feature supports theories of small-scale auroral arc formation and electrodynamics.
0992-7689
845–859
Tuttle, Sam Arthur
5c4af2b4-ed89-4aee-83af-e4aed700f937
Lanchester, Betty
e864533e-eea0-471f-a3f9-7c70c25be55b
Gustavsson, Bjorn J.
f8ee0667-051d-4ca8-b028-f29a7330b58d
Whiter, Daniel
9a30d7b6-ea41-44fb-bd52-3ff1964eca5c
Ivchenko, Nickolay
99752978-8a44-4c2e-8c4a-984575a88f8f
Fear, Robert
8755b9ed-c7dc-4cbb-ac9b-56235a0431ab
Lester, Mark
a6e8f6c8-5112-4837-8609-d8b9fe1f1692
Tuttle, Sam Arthur
5c4af2b4-ed89-4aee-83af-e4aed700f937
Lanchester, Betty
e864533e-eea0-471f-a3f9-7c70c25be55b
Gustavsson, Bjorn J.
f8ee0667-051d-4ca8-b028-f29a7330b58d
Whiter, Daniel
9a30d7b6-ea41-44fb-bd52-3ff1964eca5c
Ivchenko, Nickolay
99752978-8a44-4c2e-8c4a-984575a88f8f
Fear, Robert
8755b9ed-c7dc-4cbb-ac9b-56235a0431ab
Lester, Mark
a6e8f6c8-5112-4837-8609-d8b9fe1f1692

Tuttle, Sam Arthur, Lanchester, Betty, Gustavsson, Bjorn J., Whiter, Daniel, Ivchenko, Nickolay, Fear, Robert and Lester, Mark (2020) Horizontal electric fields from flow of auroral O+(2P) ions at sub-second temporal resolution. Annales Geophysicae, 38 (4), 845–859. (doi:10.5194/angeo-38-845-2020).

Record type: Article

Abstract

Electric fields are a ubiquitous feature of the ionosphere and are intimately linked with aurora through particle precipitation and field-aligned currents. They exhibit order-of-magnitude changes on temporal and spatial scales of seconds and kilometres which are not easy to measure; knowing their true magnitude and temporal variability is important for a theoretical understanding of auroral processes. We present a unique method to estimate ionospheric electric fields in the region close to (km-scale) a dynamic auroral arc by solving the continuity equation for the metastable O+(2P) ions, which emit as they move under the influence of electric fields during their 5 s lifetime. The main advantage of this optical method is the increase in temporal resolution over other methods such as ground-based radars. Simultaneous measurements of emission at 732.0 nm (from the O+(2P) ions), and prompt emissions at 673.0 nm (N2) and 777.4 nm (O), all at high spatial (100 m) and temporal (0.05 s) resolution, are used in the solution of the continuity equation, which gives the dynamic changes of the O+ion population at 10 all heights in a 3D volume close to the magnetic zenith. Perspective effects are taken into account by a new geometric method, which is based on an accurate estimate of the magnetic zenith position. The emissions resulting from the metastable ions are converted to brightness images by projecting onto the plane of the ground, which are compared with the measured images. The flow velocity of the ions is a free parameter in the solution of the continuity equation; the value that minimizes the difference between the modelled and observed images is the extracted flow velocity at each time step. We demonstrate the method with 15 an example event during the passage of a brightening arc feature, lasting about 10 s, in which the inferred electric fields vary between 20 and 120 mV/m. These inferred electric fields are compared with SuperDARN measurements, which have an average value of 30 mV /m. An excellent agreement is found in the magnitude and direction of the background electric field; an increase in magnitude during the brightening of the arc feature supports theories of small-scale auroral arc formation and electrodynamics.

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More information

Accepted/In Press date: 8 June 2020
e-pub ahead of print date: 14 July 2020
Published date: 2020

Identifiers

Local EPrints ID: 442306
URI: http://eprints.soton.ac.uk/id/eprint/442306
ISSN: 0992-7689
PURE UUID: a60714bb-c9a3-4ce0-a30a-5697bb68d4c4
ORCID for Daniel Whiter: ORCID iD orcid.org/0000-0001-7130-232X
ORCID for Robert Fear: ORCID iD orcid.org/0000-0003-0589-7147

Catalogue record

Date deposited: 13 Jul 2020 16:30
Last modified: 18 Feb 2021 17:23

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