Russell B. Cosgrove

Coupled electrodynamics in the nighttime midlatitude ionosphere

We describe a new, self-consistent scenario in which sporadic-E doublets, height bands of upward-displaced F-layer profiles, F-region plasma depletions, and radar backscatter plumes, are all manifestations of a coupled electrodynamical response by the nighttime midlatitude ionosphere to the presence of a traveling ionospheric disturbance (TID). We show that the response consists of (1) formation of image plasma structure in the E region, (2) initiation of a Hall-current-driven polarization process by the E-region plasma structure, and (3) mapping of the polarization electric field to the F region, where it strengthens the electrical properties of the TID that initiated the E-region processes. This scenario provides ready answers for several, hitherto puzzling, questions and a basis for new directions on this research topic.

Coupling of the Perkins instability and the sporadic E layer instability derived from physical arguments

[i] Tsunoda and Cosgrove [2001] recently pointed out that the F layer and sporadic E (E s ) layers in the nighttime midlatitude ionosphere must be considered electrodynamically as a coupled system in light of the presence of a Hall polarization process in E s layers [Haldoupis et al., 1996; Tsunoda, 1998; Cosgrove and Tsunoda, 2001, 2002a] and the fact that kilometer-scale electric fields map efficiently between the E and F regions. They further noted the apparent presence of positive feedback between processes in those regions. Cosgrove and Tsunoda [2002b, 2003] have since shown that E s layers are unstable with properties not unlike those of the Perkins instability in the F region [Perkins, 1973], motivating the idea that the two instabilities may couple. Finally, Cosgrove and Tsunoda [2004] derived the linear growth rate for the coupled system of a E s layer and the F layer, thus realizing a unified formalism for the Perkins and E s layer (E s L) instabilities. They found that the growth rate was significantly enhanced by the coupling. However, the growth rate computed in Cosgrove and Tsunoda [2004] was expressed only as the largest eigenvalue of a very complex 3 × 3 matrix. In this paper we present a physical interpretation of the E-F coupled-layer (EFCL) instability, and derive the condition for maximal coupling. We obtain a circuit model for the coupled-layer system that provides a physical interpretation for the wavelength dependence of electric field mapping between layers, and allows quantitative predictions. Using the circuit model we derive a rule of thumb for computing the two growth rates of the coupled system from the isolated Perkins and E s L instability growth rates. We compare the result with the exact computation of Cosgrove and Tsunoda 120041.

Polarization electric fields sustained by closed‐current dynamo structures in midlatitude sporadic E

The existence of anomalously large midlatitude E region electric fields, occurring at night when sporadic-E ( Es) is present, has now been verified experimentally. The only known source mechanism requires that the Hall current driven by the polarization electric field be sustained in some way. While current closure in the F layer is possible, we show that closure through nearby dynamo structures that involve Es patches may be more likely. To illustrate the concept, we have considered an analytically tractable problem in which two dynamo regions have formed, one above and one below an ion-convergent wind-shear node. We show that the polarization Hall current generated in each region can close through the other region via field-aligned currents. With this kind of closure, we further show that the polarization electric field can approach the idealized Cowling-conductivity-enhanced field strength, which would be large enough to account for the observations that have been made by rocket and radar.

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.

GEM‐CEDAR challenge: Poynting flux at DMSP and modeled Joule heat

Poynting flux into the ionosphere measures the electromagnetic energy coming from the magnetosphere. This energy flux can vary greatly between quiet times and geomagnetic active times. As part of the Geospace Environment Modeling-coupling energetics and dynamics of atmospheric regions modeling challenge, physics-based models of the 3-D ionosphere and ionospheric electrodynamics solvers of magnetosphere models that specify Joule heat and empirical models specifying Poynting flux were run for six geomagnetic storm events of varying intensity. We compared model results with Poynting flux values along the DMSP-15 satellite track computed from ion drift meter and magnetic field observations. Although being a different quantity, Joule heat can in practice be correlated to incoming Poynting flux because the energy is dissipated primarily in high latitudes where Poynting flux is being deposited. Within the physics-based model group, we find mixed results with some models overestimating Joule heat and some models agreeing better with observed Poynting flux rates as integrated over auroral passes. In contrast, empirical models tend to underestimate integrated Poynting flux values. Modeled Joule heat or Poynting flux patterns often resemble the observed Poynting flux patterns on a large scale, but amplitudes can differ by a factor of 2 or larger due to the highly localized nature of observed Poynting flux deposition that is not captured by the models. In addition, the positioning of modeled patterns appear to be randomly shifted against the observed Poynting flux energy input. This study is the first to compare Poynting flux and Joule heat in a large variety of models of the ionosphere.

Does a localized plasma disturbance in the ionosphere evolve to electrostatic equilibrium? Evidence to the contrary

Electrostatic equilibrium must be achieved through electromagnetic evolution. From an initial state with non-zero neutral wind localized along the geomagnetic field, and with all other plasma and electromagnetic perturbations initially zero, evolution progresses from plasma velocity, to electric field, to magnetic field, where the last step can launch an Alfven wave that transmits the electromagnetic disturbance along geomagnetic field lines. Without the Alfven wave the disturbance does not map along geomagnetic field lines, and there is no semblance of electrostatic equilibrium. This paradigm is essentially the traditional magnetosphere/ionosphere coupling paradigm, except addressed to smaller scale, local ionospheric phenomena. However, Alfven waves have not been thoroughly studied in the context of the partially ionized, collisional ionospheric plasma: and so the full effects predicted by this modeling paradigm are not known. In this work we adopt the two-fluid equations and investigate whether the ionosphere supports Alfven-type waves that can transmit disturbances along geomagnetic field lines, and perform a wave analysis of the “lumped circuit” parameters normally used to characterize the ionosphere under electrostatic equilibrium. We find that under the wave analysis: (1) the Pedersen conductivity is severely modified and has a negative real part at short wavelengths; (2) the mapping distance for electric fields is significantly modified, and there is a non-negligible wavelength along the geomagnetic field; and (3) the load admittance seen by a localized dynamo is strongly reactive, causing a phase offset between electric field and current, as compared with when the load is electrostatic.