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 respectively 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 (kilometre 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 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 them onto the plane of the ground, and the projected images are then 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 minimises the difference between the modelled and observed images is the extracted flow velocity at each time step. We demonstrate the method with 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−1. These inferred electric fields are compared with SuperDARN measurements, which have an average value of 30 mV m−1. 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.

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

Electric fields are a ubiquitous feature of the ionosphere and are intimately linked with aurora through particle precipitation and field-aligned currents. We present a unique method to estimate ionospheric electric fields beside a dynamic auroral feature 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. 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 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 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−1. These inferred electric fields are compared with SuperDARN measurements, which have an average value of 30 mV m−1. An excellent agreement is found in 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.

Electron heating by HF pumping of high-latitude ionospheric F-region plasma near magnetic zenith

High-frequency electromagnetic pumping of ionospheric F-region plasma at high and mid latitudes gives the strongest plasma response in magnetic zenith, antiparallel to the geomagnetic field in the Northern Hemisphere. This has been observed in optical emissions from the pumped plasma turbulence, electron temperature enhancements, filamentary magnetic field-aligned plasma density irregularities, and in self-focusing of the pump beam in magnetic zenith. We present results of EISCAT (European Incoherent SCATter association) Heating-induced magnetic-zenith effects observed with the EISCAT UHF incoherent scatter radar. With heating transmitting a left-handed circularly polarized pump beam towards magnetic zenith, the UHF radar was scanned in elevation in steps of 1.0 and 1.5∘ around magnetic zenith. The electron energy equation was integrated to model the electron temperature and associated electron heating rate and optimized to fit the plasma parameter values measured with the radar. The experimental and modelling results are consistent with pump wave propagation in the L mode in magnetic zenith, rather than in the O mode.

Electron heating by HF-pumping of high-latitude ionospheric F-region plasma near magnetic zenith

High frequency electromagnetic pumping of ionospheric F-region plasma at high and mid latitudes gives the strongest plasma response in magnetic zenith, antiparallel to the geomagnetic field in the northern hemisphere. This has been observed in optical emissions from the pumped plasma turbulence, electron temperature enhancements, filamentary magnetic field-aligned plasma density irregularities, and in self-focusing of the pump beam in magnetic zenith. We present results of EISCAT (European Incoherent SCATter association) Heating-induced magnetic-zenith effects observed with the EISCAT UHF incoherent scatter radar. With Heating transmitting a left-handed circularly polarised pump beam towards magnetic zenith, the UHF radar was scanned in elevation in steps of 1.0° and 1.5° around magnetic zenith. The electron energy equation was integrated to model the electron temperature and associated electron heating rate and optimized to fit the plasma parameter values measured with the radar. The experimental and modeling results are consistent with pump wave propagation in the L mode in magnetic zenith, rather than in the O mode.

Evidence of <i>L</i>-mode electromagnetic wave pumping of ionospheric plasma near geomagnetic zenith

The response of ionospheric plasma to pumping by powerful HF (high frequency) electromagnetic waves transmitted from the ground into the ionosphere is the strongest in the direction of geomagnetic zenith. We present experimental results from transmitting a left-handed circularly polarized HF beam from the EISCAT (European Incoherent SCATter association) Heating facility in magnetic zenith. The CASSIOPE (CAScade, Smallsat and IOnospheric Polar Explorer) spacecraft in the topside ionosphere above the F-region density peak detected transionospheric pump radiation, although the pump frequency was below the maximum ionospheric plasma frequency. The pump wave is deduced to arrive at CASSIOPE through L-mode propagation and associated double (O to Z, Z to O) conversion in pump-induced radio windows. L-mode propagation allows the pump wave to reach higher plasma densities and higher ionospheric altitudes than O-mode propagation so that a pump wave in the L-mode can facilitate excitation of upper hybrid phenomena localized in density depletions in a larger altitude range. L-mode propagation is therefore suggested to be important in explaining the magnetic zenith effect. Keywords. Space plasma physics (active perturbation experiments)