M. Zettergren

MICA sounding rocket observations of conductivity‐gradient‐generated auroral ionospheric responses: Small‐scale structure with large‐scale drivers

A detailed, in situ study of field-aligned current (FAC) structure in a transient, substorm expansion phase auroral arc is conducted using electric field, magnetometer, and electron density measurements from the Magnetosphere-Ionosphere Coupling in the Alfven Resonator (MICA) sounding rocket, launched from Poker Flat, AK. These data are supplemented with larger-scale, contextual measurements from a heterogeneous collection of ground-based instruments including the Poker Flat incoherent scatter radar and nearby scanning doppler imagers and filtered all-sky cameras. An electrostatic ionospheric modeling case study of this event is also constructed by using available data (neutral winds, electron precipitation, and electric fields) to constrain model initial and boundary conditions. MICA magnetometer data are converted into FAC measurements using a sheet current approximation and show an up-down current pair, with small-scale current density and Poynting flux structures in the downward current channel. Model results are able to roughly recreate only the large-scale features of the field-aligned currents, suggesting that observed small-scale structures may be due to ionospheric feedback processes not encapsulated by the electrostatic model. The model is also used to assess the contributions of various processes to total FAC and suggests that both conductance gradients and neutral dynamos may contribute significantly to FACs in a narrow region where the current transitions from upward to downward. Comparison of Poker Flat Incoherent Scatter Radar versus in situ electric field estimates illustrates the high sensitivity of FAC estimates to measurement resolution.

Auroral ionospheric F region density cavity formation and evolution: MICA campaign results

Auroral ionospheric F region density depletions observed by PFISR (Poker Flat Incoherent Scatter Radar) during the MICA (Magnetosphere-Ionosphere Coupling in the Alfven Resonator) sounding rocket campaign are critically examined alongside complementary numerical simulations. Particular processes of interest include cavity formation due to intense frictional heating and Pedersen drifts, evolution in the presence of structured precipitation, and refilling due to impact ionization and downflows. Our analysis uses an ionospheric fluid model which solves conservation of mass, momentum, and energy equations for all major ionospheric species. These fluid equations are coupled to an electrostatic current continuity equation to self-consistently describe auroral electric fields. Energetic electron precipitation inputs for the model are specified by inverting optical data, and electric field boundary conditions are obtained from direct PFISR measurements. Thus, the model is driven in as realistic a manner as possible. Both incoherent scatter radar (ISR) data and simulations indicate that the conversion of the F region plasma to molecular ions and subsequent recombination is the dominant process contributing to the formation of the observed cavities, all of which occur in conjunction with electric fields exceeding ∼90 mV/m. Furthermore, the cavities often persist several minutes past the point when the frictional heating stops. Impact ionization and field-aligned plasma flows modulate the cavity depth in a significant way but are of secondary importance to the molecular generation process. Informal comparisons of the ISR density and temperature fits to the model verify that the simulations reproduce most of the observed cavity features to a reasonable level of detail.

Ion Upflow Dependence on Ionospheric Density and Solar Photoionization

Motivated by rocket observations showing a variety of different ionospheric responses to precipitation, this paper explores the influence of the background ionospheric density on upflow resulting from auroral precipitation. Simulations of upflow driven by auroral precipitation were conducted using a version of the Varney et al. (2014) model driven by precipitation characterized by observations made during the 2012 Magnetosphere-Ionosphere Coupling in the Alfven resonator rocket mission and using a variety of different initial electron density profiles. The simulation results show that increased initial density before the onset of precipitation leads to smaller electron temperature increases, longer ionospheric heating timescales, weaker ambipolar electric fields, lower upflow speeds, and longer upflow timescales but larger upflow fluxes. The upflow flux can increase even when the ambipolar electric field strength decreases due to the larger number of ions that are accelerated. Long-term observations from the European Incoherent Scatter (EISCAT) Svalbard radar taken during the International Polar Year support the effects seen in the simulations. This correlation between ionospheric density and ion upflows emphasizes the important role of photoionization from solar ultraviolet radiation, which the EISCAT observations show can increase ionospheric density by as much as an order of magnitude during the summer months.

Including sheath effects in the interpretation of planar retarding potential analyzer’s low-energy ion data

The interpretation of planar retarding potential analyzers (RPA) during ionospheric sounding rocket missions requires modeling the thick 3D plasma sheath. This paper overviews the theory of RPAs with an emphasis placed on the impact of the sheath on current-voltage (I-V) curves. It then describes the Petite Ion Probe (PIP) which has been designed to function in this difficult regime. The data analysis procedure for this instrument is discussed in detail. Data analysis begins by modeling the sheath with the Spacecraft Plasma Interaction System (SPIS), a particle-in-cell code. Test particles are traced through the sheath and detector to determine the detectors response. A training set is constructed from these simulated curves for a support vector regression analysis which relates the properties of the I-V curve to the properties of the plasma. The first in situ use of the PIPs occurred during the MICA sounding rocket mission which launched from Poker Flat, Alaska in February of 2012. These data are presented as a case study, providing valuable cross-instrument comparisons. A heritage top-hat thermal ion electrostatic analyzer, called the HT, and a multi-needle Langmuir probe have been used to validate both the PIPs and the data analysis method. Compared to the HT, the PIP ion temperature measurements agree with a root-mean-square error of 0.023 eV. These two instruments agree on the parallel-to-B plasma flow velocity with a root-mean-square error of 130 m/s. The PIP with its field of view aligned perpendicular-to-B provided a density measurement with an 11% error compared to the multi-needle Langmuir Probe. Higher error in the other PIPs density measurement is likely due to simplifications in the SPIS model geometry.

Reconstruction of Fine Scale Auroral Dynamics

We present a feasibility study for a high-frame-rate Short-baseline auroral tomographic imaging system useful for estimating parametric variations in the precipitating electron number flux spectrum of dynamic auroral events. Of particular interest are auroral substorms, which are characterized by spatial variations of order 100 m and temporal variations of order 10 ms. These scales are thought to be produced by dispersive Alfvén waves in the near-Earth magnetosphere. The auroral tomography system characterized in this paper reconstructs the auroral volume emission rate, to estimate the characteristic energy and location in the direction perpendicular to the geomagnetic field of peak electron precipitation flux, using a distributed network of precisely synchronized ground-based cameras. As the observing baseline decreases, the tomographic inverse problem becomes highly ill-conditioned; as the sampling rate increases, the signal-to-noise ratio degrades and synchronization requirements become increasingly critical. Our approach to these challenges uses a physics-based auroral model to regularize the poorly observed vertical dimension. Specifically, the vertical dimension is expanded in a low-dimensional basis, consisting of eigenprofiles computed over the range of expected energies in the precipitating electron flux, while the horizontal dimension retains a standard orthogonal pixel basis. Simulation results show typical characteristic energy estimation error less than 30% for a 3-km baseline achievable within the confines of the Poker Flat Research Range, using GPS-synchronized electron-multiplying charge-coupled device cameras with broadband BG3 optical filters that pass prompt auroral emissions.

Unmanned aerial vehicle based passive radar agile sensing for computerized ionospheric tomography

This paper aims to address the technical challenges associated with traditional ground-based system by introducing a novel approach of using unmanned aerial vehicle (UAV) to perform tomographic sensing. This reconstruction algorithm uses the most recent development in auroral ionospheric modelling as a synthetic prior combined with an innovative reconstruction approach involving a multiscale Natural Pixel decomposition and a MAP-based penalty weighted least-squared (PWLR) Signal Reconstruction. This proposed algorithm will not only improve the existing CIT techniques by providing high resolution reconstructions of the space plasma density, but also generate a new array of research opportunities for modern space tomographic imaging.