Two new approaches to spatial interpolation with inherent sidelobe suppression for imaging riometers

Absorption images as obtained by imaging riometers such as IRIS are usually created by interpolating between absorption values for individual beams. For IRIS, the locations of the beam centres serve as grid points for subsequent linear interpolation. Although generally producing good results, the fact that the actual shape of the imaging beams is not considered, potentially introduces errors and can lead to misinterpretations. In this paper, two alternative interpolation methods are introduced. Method A is based on measuring the similarity between simulated reception of individual point sources and actually received data. Method B uses a mathematical model of the sky brightness distribution parametrised by the received data. All interpolation methods are applied to power data, as opposed to absorption data, in order to avoid any errors that might be introduced by intermediate processing steps, especially QDC (quiet-day curve) generation. We apply all methods to synthetically generated test data as well as to three exemplary real datasets which are also compared to a calculated sky brightness distribution obtained from a skymap

Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption

Space weather events may adversely affect high frequency (HF) radio propagation, hence the ability to provide now-casting and forecasting of HF radio absorption is key for industries that rely on HF communications. This paper presents methods of assimilating 30 MHz radio absorption measurements into two types of ionospheric polar cap absorption (PCA) model to improve their performance as nowcasting tools. Type 1 models calculate absorption as m times the square root of the flux of solar protons above an energy threshold, Et. Measurements from 14 riometers during 94 solar proton events (1995-2010) are assimilated by optimising the day and night values of m by linear regression. Further non-linear optimisations are demonstrated in which parameters such as  Et are also optimised and additional terms characterise local time and seasonal variations. These optimisations reduce RMS errors by up to 36%. Type 2 models incorporate altitude profiles of electron and neutral densities and electron temperatures. Here the scale height of the effective recombination coefficient profile in the D-region is optimised by regression. Furthermore, two published models of the rigidity cut-off latitude (CL) are assessed by comparison with riometer measurements. A small improvement in performance is observed by introducing a three-hour lag in the geomagnetic index Kp in the CL models. Assimilating data from a single riometer in the polar cap reduces RMS errors below 1 dB with less than 0.2 dB bias. However, many high-latitude riometers now provide absorption measurements in near real time and we demonstrate how these data may be assimilated by fitting a low-order spherical harmonic function to both the measurements and a PCA model with optimised parameters

A GPU-accelerated finite-difference time-domain scheme for electromagnetic wave interaction with plasma

A GPU-accelerated Finite-Difference Time-Domain (FDTD) scheme for the simulation of radio-frequency (RF) wave propagation in a dynamic, magnetized plasma is presented. This work builds on well-established FDTD techniques with the inclusion of new time advancement equations for the plasma fluid density and temperature. The resulting FDTD formulation is suitable for the simulation of the time-dependent behaviour of an ionospheric plasma due to interaction with an RF wave and the excitation of plasma waves and instabilities. The stability criteria and the dependence of accuracy on the choice of simulation parameters are analyzed and found to depend on the choice of simulation grid parameters. It is demonstrated that accelerating the FDTD code using GPU technology yields significantly higher performance, with a dual-GPU implementation achieving a rate of node update almost two orders of magnitude faster than a serial implementation. Optimization techniques such as memory coalescence are demonstrated to have a significant effect on code performance. The results of numerical tests performed to validate the FDTD scheme are presented, with a good agreement achieved when the simulation results are compared to both the predictions of plasma theory and to the results of the Tech-X® VORPAL 4.2.2 software that was used as a benchmark

Decoding solar wind–magnetosphere coupling

We employ a new NARMAX (Nonlinear Auto-Regressive Moving Average with eXogenous inputs) code to disentangle the time-varying relationship between the solar wind and SYM-H. The NARMAX method has previously been used to formulate a Dst model, using a preselected solar wind coupling function. In this work, which uses the higher-resolution SYM-H in place of Dst, we are able to reveal the individual components of different solar wind-magnetosphere interaction processes as they contribute to the geomagnetic disturbance. This is achieved with a graphics processing unit (GPU)-based NARMAX code that is around 10 orders of magnitude faster than previous efforts from 2005, before general-purpose programming on GPUs was possible. The algorithm includes a composite cost function, to minimize overfitting, and iterative reorthogonalization, which reduces computational errors in the most critical calculations by a factor of ∼106. The results show that negative deviations in SYM-H following a southward interplanetary magnetic field (IMF) are first a measure of the increased magnetic flux in the geomagnetic tail, observed with a delay of 20–30 min from the time the solar wind hits the bow shock. Terms with longer delays are found which represent the dipolarization of the magnetotail, the injections of particles into the ring current, and their subsequent loss by flowout through the dayside magnetopause. Our results indicate that the contribution of magnetopause currents to the storm time indices increase with solar wind electric field, E = v × B. This is in agreement with previous studies that have shown that the magnetopause is closer to the Earth when the IMF is in the tangential direction

Simulation of high energy tail of electron distribution function

This report presents Monte Carlo simulations of the electron energy distribution for alow ionized plasma interacting with the F-region neutral gas. The results show a depletion in theelectron distribution above 2 eV between 10 and 80 %, decreasing with altitude. The depletion ismainly due to electron energy loss to . This micro-physical energy transfer model gives goodagreement with optical observations of enhanced emissions from at 6300Å and EISCATUHF measurements of electron cooling during HF radio wave heating experiments. Someimplications for incoherent scatter spectra are derived. The results suggest that a weak(approximately 1000 times weaker than the ion-line) and wide (2 MHz) peak around +-1 MHz fromthe ion-line in the EISCAT VHF incoherent scatter spectrum should be a consequence of theelectron-neutral interaction

Two-dimensional numerical simulation of O-mode to Z-mode conversion in the ionosphere

Experiments in the illumination of the F region of the ionosphere via radio frequency waves polarized in the ordinary mode (O-mode) have revealed that the magnitude of artificial heating-induced effects depends strongly on the inclination angle of the pump beam, with a greater modification to the plasma observed when the heating beam is directed close to or along the magnetic zenith direction. Numerical simulations performed using a recently developed finite-difference time-domain (FDTD) code are used to investigate the contribution of the O-mode to Z-mode conversion process to this effect. The aspect angle dependence and angular size of the radio window for which conversion of an O-mode pump wave to the Z-mode occurs is simulated for a variety of plasma density profiles including 2-D linear gradients representative of large-scale plasma depletions, density-depleted plasma ducts, and periodic field-aligned irregularities. The angular shape of the conversion window is found to be strongly influenced by the background plasma profile. If the Z-mode wave is reflected, it can propagate back toward the O-mode reflection region leading to resonant enhancement of the electric field in this region. Simulation results presented in this paper demonstrate that this process can make a significant contribution to the magnitude of electron density depletion and temperature enhancement around the resonance height and contributes to a strong dependence of the magnitude of plasma perturbation with the direction of the pump wave

Excitation of Plasma Irregularities in the F Region of the Ionosphere by Powerful HF Radio Waves of X‐Polarization

We discuss theoretically the excitation of artificial plasma irregularities in the auroral ionosphere by high-frequency X-mode radio wave. This is done via a two-step process. As a first step we adopt the thermal self-focusing instability excited in the F region of the ionosphere under the action of a strong high-frequency (HF) radio wave. This instability causes the formation of perturbations of the electron temperature and plasma concentration across the magnetic field. In addition, the plasma becomes depleted in the regions of the electron temperature enhancements and vice versa, since the gradients of plasma concentration and the electron temperature have opposite signs. In such conditions the temperature gradient instability comes into play. As a second step we consider plasma and electron temperature inhomogeneities that appear due to this instability to be responsible for the generation of irregularities with transverse sizes smaller than the typical scales of the self-focusing instability. Alternative mechanisms such as excitation of the gradient-drift and the current-convective instabilities, which are often attributed to the generation of plasma irregularities in the F region and can contribute to the formation of artificial irregularities in the case of X-mode heating, are also discussed

Substorm induced energetic electron precipitation:morphology and prediction

The injection, and subsequent precipitation, of 20 to 300 keV electrons during substorms is modeled using parameters of a typical substorm found in the literature. When combined with onset timing from, for example, the SuperMAG substorm database, or the Minimal Substorm Model, it may be used to calculate substorm contributions to energetic electron precipitation in atmospheric chemistry and climate models. Here the results are compared to ground-based data from the Imaging Riometer for Ionospheric Studies riometer in Kilpisjärvi, Finland, and the narrowband subionospheric VLF receiver at Sodankylä, Finland. Qualitatively, the model reproduces the observations well when only onset timing from the SuperMAG network of magnetometers is used as an input and is capable of reproducing all four categories of substorm associated riometer spike events. The results suggest that the different types of spike event are the same phenomena observed at different locations, with each type emerging from the model results at a different local time, relative to the center of the injection region. The model's ability to reproduce the morphology of spike events more accurately than previous models is attributed to the injection of energetic electrons being concentrated specifically in the regions undergoing dipolarization, instead of uniformly across a single-injection region