Hasan Bahcivan

Estimating the vector electric field using monostatic, multibeam incoherent scatter radar measurements

An algorithm has been developed to image the local structure in the convection electric field using multibeam incoherent scatter radar (ISR) data. The imaged region covers about 4° in magnetic latitude and 8° in magnetic longitude for the specific geometry considered (that of the Poker Flat ISR). The algorithm implements the Lagrange method of undetermined multipliers to regularize the underdetermined problem posed by the radar measurements. The error on the reconstructed image is estimated by mapping the mathematical form to a Bayesian estimate and observing that the Lagrangian method determines an effective a priori covariance matrix from a user-defined regularization metric. There exists a unique solution when the average measurement error is smaller than the average measurement amplitude. The algorithm is tested using synthetic and real data and appears surprisingly robust at estimating the divergence of the field. Future applications include imaging the current systems surrounding auroral arcs in order to distinguish physical mechanisms.

Magnetic aspect sensitivity of high‐latitude E region irregularities measured by the RAX‐2 CubeSat

The second Radio Aurora Explorer (RAX-2) satellite has completed more than 30 conjunction experiments with the Advanced Modular Incoherent Scatter Radar chain of incoherent scatter radars in Alaska and Resolute Bay, Canada. Coherent radar echoing occurred during four of the passes: three when E region electron drifts exceeded the ion acoustic speed threshold and one during HF heating of the ionosphere by the High Frequency Active Auroral Research Program heater. In this paper, we present the results for the first three passes associated with backscatter from natural irregularities. We analyze, in detail, the largest drift case because the plasma turbulence was the most intense and because the corresponding ground-to-space bistatic scattering geometry was the most favorable for magnetic aspect sensitivity analysis. A set of data analysis procedures including interference removal, autocorrelation analysis, and the application of a radar beam deconvolution algorithm mapped the distribution of E region backscatter with 3 km resolution in altitude and ∼0.1° in magnetic aspect angle. To our knowledge, these are the highest resolution altitude-resolved magnetic aspect sensitivity measurements made at UHF frequencies in the auroral region. In this paper, we show that despite the large electron drift speed of ∼1500 m/s, the magnetic aspect sensitivity of submeter scale irregularities is much higher than previously reported. The root-mean-square of the aspect angle distribution varied monotonically between 0.5° and 0.1° for the altitude range 100–110 km. Findings from this single but compelling event suggest that submeter scale waves propagating at larger angles from the main E×B flow direction (secondary waves) have parallel electric fields that are too small to contribute to E region electron heating. It is possible that anomalous electron heating in the auroral electrojet can be explained by (a) the dynamics of those submeter scale waves propagating in the E×B direction (primary waves) or (b) the dynamics of longer wavelengths.

Magnetic Sensor Calibration and Residual Dipole Characterization for Application to Nanosatellites

In this paper, we describe the techniques used to calibrate the individual (non-integrated) magnetometers onboard the Radio Aurora Explorer (RAX) satellite and describe a technique to characterize the residual dipole of small satellites. Magnetometer calibration is a critical part of RAX’s magnetic-based attitude determination system. Our technique uses vector knowledge of the local magnetic field, a three-axis Helmholtz cage, and a leastsquares regression. Using this calibration, we can also quantify magnetometer uncertainty. The satellite residual dipole, in addition to affecting the onboard magnetic measurements, can be a dominant disturbance torque for nanosatellites. We present a method to characterize the residual dipole of nanosatellites that uses redundant external magnetic measurements, a least-squares minimization, and does not require large magnetically clean facilities. These techniques are used on RAX but can be extended to other fields of study involving magnetic measurements.

Radio Aurora Explorer: A Mission Overview

The primary objective of the Radio Aurora Explorer mission is to study plasma instabilities that lead to magnetic field aligned irregularities of electron density in the lower polar ionosphere (80–...

Direct measurement of lower thermospheric neutral density using multifrequency incoherent scattering

Incoherent scatter (IS) is sensitive to collisional properties of the ion gas when the mean free path is close to the radar wave number. However, it has been traditionally difficult to infer the rate of collisions from IS measurements because of ambiguities in the theory for measurements at a single wave number (k). We demonstrate that multifrequency measurements to achieve diversity in k can allow for direct inference of the composition-weighted ion-neutral collision frequency in the upper mesosphere and lower thermosphere. By direct, we mean that no significant constraints are imposed on the interpretation of the IS spectra and that interpretation relies only on the IS formalism (rather than a steady state ion-momentum equation, for example). The technique is demonstrated using measurements from the European Incoherent Scatter VHF and UHF radar systems. This technique can be used to investigate neutral atmosphere variations as well as the validity of collision models commonly used in the IS formalism.