The Active Magnetosphere and Planetary Electrodynamics Response Experiment uses magneticfield data from the Iridium constellation to derive the global Birkeland current distribution every 10 min. We examine cases in which the interplanetary magnetic field (IMF) rotated from northward to southward resulting in onsets of the Birkeland currents. Dayside Region 1/2 currents, totaling~25% of the final current, appear within 20 min of the IMF southward turning and remain steady. Onset of nightside currents occurs 40 to 70 min after the dayside currents appear. Thereafter, the currents intensify at dawn, dusk, and on the dayside, yielding a fully formed Region 1/2 system~30 min after the nightside onset. The results imply that the dayside Birkeland currents are driven by magnetopause reconnection, and the remainder of the system forms as magnetospheric return flows start and progress sunward, ultimately closing the Dungey convection cycle.

[1] Every day, billions of microgram-sized-extraterrestrial particles enter and ablate in the upper layers of the Earth's atmosphere, depositing their mass in the mesosphere and lower thermosphere (MLT). This evaporated meteoric mass is the source of global layers of neutral metal atoms, sporadic E layers of metal ions, and meteoric smoke particles. Because their kinetic energy is insufficient to produce detectable optical emissions, these particles can only be observed using sensitive radars, which detect the plasma (i.e., electrons) either immediately surrounding the meteoroid (head-echo), or left behind along its path (trailecho). Here we show that observed short-scale temporal features in the radar returned signal from the meteor headecho are explained by differential ablation of the chemical constituents. These results represent the first observation of this mass-loss process, indicating that this is the main mechanism through which the meteoric mass of micronsized particles is deposited in the MLT.

[1] Radar data of non-specular meteor trails shows two clear and consistent features: (1) non-specular meteor trails are observed from a narrower altitude range than are head echoes and (2) an approximately 20 ms delay between meteor head echoes and trail radar scatter. This paper shows that both features can result from meteor trail plasma instability. Simulations have demonstrated that trails often develop Farley-Buneman/gradient-drift (FBGD) waves which become turbulent and generate field aligned irregularities (FAI). Plasma stability analysis shows that trails are only unstable within a limited altitude range, matching the observed altitudes of non-specular trails to within 1 -2 km. The simulations show that instability develops into turbulence in $20 ms and appears to be the only meteor trail process that can explain both the observed delay between head and trail echoes and generate coherent scatter at both UHF and VHF wavelengths.

Abstract. Currents flowing in the Earth's ionospheric electrojets often develop Farley-Buneman (FB) streaming instabilities and become turbulent. The resulting electron density irregularities cause these regions to readily scatter VHF and UHF radar signals. Many of the observed characteristics of these radar measurements result from the nonlinear behavior of this plasma. This paper describes a set of high-resolution, 2-D, fully kinetic simulations of electric field driven turbulence in the electrojet. These show the saturated amplitude of the waves; coupling between linearly growing modes and damped modes; the evolution of the system from dominance by shorter (1 m-5 m) to longer (10 m-200 m) wavelength modes; and the propagation of the dominant modes at phase velocities that lie below the linearly predicted phase velocity and close to but slightly above the acoustic velocity. These simulations reproduce many of the observational characteristics of type 1 waves. They provide information useful in accurately modeling FB turbulence and demonstrate the significant progress we have made in simulating the electrojet.

Abstract.Radars frequently detect meteor trails created by the ablation of micro-meteoroids between 70 and 120 km altitude in the atmosphere. Plasma simulations show that density gradients a.t the edges of meteor trails drive gradientdrift instabilities which develop into waves with perturbed electric fields often exceeding hundreds of mV/m. These waves create an anomalous cross-field diffusion that can exceed the cross-field (2_ B) ambipolar diffusion by an order of magnitude. The characteristics of the instabilities and anomalous diffusion depend on the trail altitude, latitude, and density gradient. A simple relation defines the minimum altitude at which meteor trail density gradients drive plasma instabilities and anomalous diffusion. These results impact a number of meteor radar studies, including those that use diffusion rates to determine trail altitude, and atmospheric temperature.

Abstract. Using analytical models and kinetic simulations, this paper shows that weakly ionized meteor trails near the geomagnetic equator evolve through three distinct stages. First, a large electric field is generated perpendicular to both the geomagnetic field and the trail. Second, plasma density waves grow asymmetrically across the trail. Third, turbulence develops in the trails. Throughout this process, the electron E x B-drift velocity plays an essential role in controlling the motion of the trail. These plasma dynamics have important implications for the interpretation of meteor radar echoes.

Abstract. Low frequency electrostatic waves in the lower parts of the ionosphere are studied by a comparison of observations by instrumented rockets and of results from numerical simulations. Particular attention is given to the spectral properties of the waves. On the basis of a good agreement between the observations and the simulations, it can be argued that the most important nonlinear dynamics can be accounted for in a 2-D numerical model, referring to a plane perpendicular to a locally homogeneous magnetic field. It does not seem necessary to take into account turbulent fluctuations or motions in the neutral gas component. The numerical simulations explain the observed strongly intermittent nature of the fluctuations: secondary instabilities develop on the large scale gradients of the largest amplitude waves, and the small scale dynamics is strongly influenced by these secondary instabilities. We compare potential variations obtained at a single position in the numerical simulations with two point potential-difference signals, where the latter is the adequate representation for the data obtained by instrumented rockets. We can demonstrate a significant reduction in the amount of information concerning the plasma turbulence when the latter signal is used for analysis. In particular we show that the bicoherence estimate is strongly affected. The conclusions have implications for studies of low frequency ionospheric fluctuations in the E and F regions by instrumented rockets, and also for other methods relying on difference measurements, using two probes with large separation. The analysis also resolves a long standing controversy concerning the supersonic phase velocities of these cross-field instabilities being observed in laboratory experiments.

[1] A head echo is the radar reflection from the plasma immediately surrounding a meteoroid upon its entry into the Earth's atmosphere; analysis of these plasmas can help determine a parent meteoroid's inherent properties, such as mass. In the past, meteoroid mass was calculated using head echo velocity and deceleration data by assuming momentum conservation between the meteoroid and air molecule. We refer to such masses as ''dynamical masses.'' This method, however, can only be used to determine meteoroid mass if either the meteoroid radius or density is assumed. In this paper, we expound upon a new method for determining a meteoroid's mass by utilizing our new spherical scattering theory. This theory allows us to use head echo measurements to calculate head echo plasma density. Then, by using the plasma density in an established formula that estimates the ratio of unionized to ionized material produced by an ablating meteoroid, we can determine a meteoroid's mass. We refer to such masses as ''scattering masses.'' We show that our new mass determination method applies to head echoes detected simultaneously at VHF and UHF and verify that the meteoroid mass is the same at both frequencies. We conclude with a comparison between dynamical and scattering masses and show that in general, these methods agree to within an order of magnitude.