B. Gustavsson

Optical ring formation and ionization production in high‐power HF heating experiments at HAARP

[1] Observations of HF-induced artificial optical emissions at the 3.6 MW HAARP facility show unexpected features not seen at the previous 960 kW level. Optical emissions often form a bright rayed ring near the 10% power contour surrounding a central disk with a sharp edge near the 50% power contour. Artificial bottomside layers in ionograms and positive perturbations in total electron content suggest that the bullseye optical patterns are associated with localized enhancements in plasma density below the main F layer. Ray tracing shows transmitter power concentrates in an annular structure consistent with the optical observations. Estimated ionization rates are well within the power available from the transmitter and agree well with the observed intensity of N + 2 427.8 nm emissions. We conclude that the optical bullseye patterns are a refraction phenomenon and an indicator of ionization production within the transmitter beam.

HF radio wave acceleration of ionospheric electrons: Analysis of HF‐induced optical enhancements

The shape of the HF-pump modified electron energy distribution has long been a central question in the field of ionospheric high-frequency radio wave modification experiments. Here we present estimates of the enhanced differential electron flux, from 1.9 to 100 eV based on optical multiwavelength (6300, 5577, 8446, and 4278 A) data and 930 MHz incoherent scatter radar measurements of ion temperature, electron temperature and concentration. According to our estimate, the electron energy distribution is depleted in the energy range of 2 to approximately 4 eV, probably caused by electron excitation of vibrational states in N2. At the higher energies the electron energy distribution has a nonthermal tail. Further we show that the altitude variations of the four optical emissions should differ both in altitude coverage and center altitude.

Tomographic inversion for ALIS noise and resolution

In this report the problems of resolution and noise sensitivity of tomographic reconstructions from ground-based multistation imaging of aurora with the auroral large imaging system (ALIS) are considered. ALIS is a ground-based grid of high-performance CCD-imaging stations at high latitudes. For evaluation of the resolution and noise sensitivity of current tomographic reconstruction procedures a full model simulation of the ALIS system is presented. The results show that relative errors are typically in the range 0.05 – 0.1 for typical noise levels in measurements of aurora. A general method to estimate resolution in a tomographic imaging system is developed and used to give estimates of the horizontal and vertical resolution. Its current limitations and future perspective are briefly discussed. A method to retrieve feasible tomographic reconstructions from a few image projections with variable noise level are outlined.

Rise and fall of electron temperatures: Ohmic heating of ionospheric electrons from underdense HF radio wave pumping

Here we present electron temperature variations observed with incoherent scatter radar during a European Incoherent Scatter Scientific Association Heating experiment with high-frequency (HF) radio wave transmission at frequencies above the peak ionospheric critical frequency. The electron temperature increased from 2000 K up to 2800 K during the HF transmission periods. During the experiment both pump frequency and polarization were altered between pump pulses. The observed temperature variation is compared with numerical solutions to the electron energy equation with ohmic heating modeling the effect of the radio wave heating of the plasma. Agreement between observations and model is found to be good.

Conditional integration of Incoherent Scattering in relation to flickering aurora

[1] In this report we present incoherent scatter radar (ISR) observations of ionospheric response to precipitation causing flickering aurora. Flickering aurora is caused by electron precipitation with modulations at frequencies higher than 5 Hz. To resolve the variation at these short time-scales with ISR we have integrated together pulses at the same phase of the optical intensity variation observed with high-speed narrow field-of-view imaging in white light to determine the intensity variation in the field aligned direction, which is also the direction of the beam of the EISCAT Svalbard Radar (ESR). Further we show that the 3% modulation in ISR back-scattered power can be explained with electron heating by temporally modulated electron precipitation and electron cooling in collisions with ions and neutrals.

Temporal and spatial evolution of auroral electron energy spectra in a region surrounding the magnetic zenith

We present a new method for estimating the spatial and temporal evolution of the auroral electron energy spectrum at subkilometer and subsecond scales using optical and incoherent scatter radar data. This method is applied to an event on 12 December 2006 when a thin auroral arc that exhibits subkilometer structuring is observed. The energy spectrum and resultant emission rates are estimated for a 10 s period when the arc was in the field of view of the optical instrumentation. Modeled images of the observed aurora are produced using the estimated emission rates and compared with the optical observations of the aurora. We find the modeled images reproduce the structure and dynamics of the observed aurora to within the uncertainties of the models used. The brightness underestimate of about 30% can be explained by the underestimate of the energy flux from the radar measurements.

Multisite Optical Imaging of Artificial Ionospheric Plasmas

Artificial ionospheric plasmas are formed on the bottom side of the natural ionospheric F region during high-power high-frequency (HF) heating experiments and descend to altitudes as low as 140 km before disappearing. Optical emissions produced during these events often exhibit bulls-eye structures, where the artificial plasma is thought to form a central spot that diverts or blocks HF waves to form an empty ring of emissions from the natural ionosphere at higher altitudes. We present multisite image data showing that, in some cases, both the spot and ring represent distinct artificial plasma layers.

Relative brightness of the O+(2D-2P) doublets in low-energy aurorae

The ratio of the emission line doublets from O+ at 732.0 nm (I 732) and 733.0 nm (I 733) has been measured in auroral conditions of low-energy electron precipitation from Svalbard (7820 north, 1583 east). Accurate determination of R = I 732/I 733 provides a powerful method for separating the density of the O+ levels in modeling of the emissions from the doublets. A total of 383 spectra were included from the winter of 2003-2004. The value obtained is R = I 732/I 733 = 1.38 ± 0.02, which is higher than theoretical values for thermal equilibrium in fully ionized plasma, but is lower than reported measurements by other authors in similar auroral conditions. The continuity equations for the densities of the two levels are solved for different conditions, in order to estimate the possible variations of R. The results suggest that the production of ions in the two levels from O (3 P 1) and O (3 P 2) does not follow the statistical weights, unlike astrophysical calculations for plasmas in nebulae. The physics of auroral impact ionization may account for this difference, and therefore for the raised value of R. In addition, the auroral solution of the densities of the ions, and thus of the value of R, is sensitive to the temperature of the neutral atmosphere. Although the present work is a statistical study, it shows that it is necessary to determine whether there are significant variations in the ratio resulting from non-equilibrium conditions, from auroral energy deposition, large electric fields, and changes in temperature and composition.

Electron Energy Spectrum and Auroral Power Estimation from Incoherent Scatter Radar Measurements

Differential energy flux of electrons precipitating into the high-latitude ionosphere can be estimated from incoherent scatter radar observations of the ionospheric electron density profile. We present a method called ELSPEC for electron spectrum estimation from incoherent scatter radar measurements, which is based on integration of the electron continuity equation and spectrum model selection by means of the Akaike information criterion. This approach allows us to use data with almost arbitrary time resolutions, enables spectrum estimation with dense energy grids, avoids noise amplifications in numerical derivatives, and yields statistical error estimates for all the output parameters, including the number and energy fluxes and upward field-aligned currents carried by the precipitating electrons. The technique is targeted for auroral energies, 1–100 keV, which ionize the atmosphere mainly between 80 and 150 km altitudes. We validate the technique by means of a simulation study, which shows that Maxwellian, kappa, and mono-energetic spectra, as well as combinations of those, can be reproduced. Comparison study for two conjugate satellite measurements to the EISCAT UHF radar are shown, for Reimei and Swarm, showing an agreement with the results. Finally, an example of a 2-hr measurement by the EISCAT radar is shown, during which we observe a variety of precipitation characteristics, from soft background precipitation to mono-energetic spectra with peak energies up to 60 keV. The upward field-aligned current varies from 0 to 10 μAm−2 and the total energy flux from 0 to 250 mWm−2.

Radar observations of thermal plasma oscillations in the ionosphere

Incoherent scatter radar observations of ionospheric plasmas rely on echoes from electron density fluctuations with properties governed by the dispersion relations for ion-acoustic and Langmuir waves. Radar observations of echoes associated with Langmuir waves (plasma lines) from thermal plasma are weak and only a few near-thermal level measurements have been reported. Plasma line echoes are typically only observed with existing radars only when the Langmuir waves are enhanced by suprathermal electrons. A new observation technique has been developed which is sensitive enough to allow observations of these echoes without the presence of suprathermal electrons up to at least 1000 km. This paper presents recent observations from the Arecibo Observatory 430-MHz incoherent scatter radar which show plasma-line echoes during the night when no suprathermal enhancement is expected to be present. The observations are compared with theory, and the results are found to be in agreement with classical incoherent scatter theory for thermal plasmas. The theoretical ratio of the ion line and plasma line power spectral density is within approximately 3 dB of the predicted value. The finding adds a new observational capability, allowing electron density to also be observed at night using the plasma line well into the top side of the ionosphere, increasing the accuracy of the electron density measurement.