J. D. Huba

Sami2 is Another Model of the Ionosphere (SAMI2): A new low‐latitude ionosphere model

A new low-latitude ionospheric model has been developed at the Naval Research Laboratory: Sami2 is Another Model of the Ionosphere (SAMI2). SAMI2 treats the dynamic plasma and chemical evolution of seven ion species (H + , He + , N + , O + , N + 2 , NO + , and O + 2 ) in the altitude range ∼ 100 km to several thousand kilometers. The ion continuity and momentum equations are solved for all seven species; the temperature equation is solved for H + , He + , O + , and the electrons. SAMI2 models the plasma along the Earths dipole field from hemisphere to hemisphere, includes the E x B drift of a flux tube (both in altitude and in longitude), and includes ion inertia in the ion momentum equation for motion along the dipole field line. The final point is relevant for plasma dynamics at very high altitudes where ion inertia can be important. For example, we have found that ion sound waves, which are supported by ion inertia, may be generated in the topside ionosphere (> 1000 km) at sunrise and sunset [Huba et al., 2000b]. The neutral species are specified using tile Mass Spectrometer Incoherent Scatter model (MSIS86) and the Horizontal Wind Model (HWM93). In this paper we describe in detail the SAMI2 model and present representative results from the model.

Preliminary study of the CRRES magnetospheric barium releases

Preliminary theoretical and computational analyses of the Combined Release and Radiation Effects Satellite (CRRES) magnetospheric barium releases are presented. The focus of the studies is on the evolution of the diamagnetic cavity which is formed by the barium ions as they expand outward, and on the structuring of the density and magnetic field during the expansion phase of the releases. Two sets of simulation studies are discussed. The first set is based upon a two-dimensional ideal MHD code and provides estimates of the time and length scales associated with the formation and collapse of the diamagnetic cavity. The second set uses a nonideal MHD code; specifically, the Hall term is included. This additional term is critical to the dynamics of sub-Alfvenic plasma expansions, such as the CRRES barium releases, because it leads to instability of the expanding plasma. We performed detailed simulations of the G4 and G10 releases. In both cases the expanding plasma rapidly structured: the G4 release structured at time t ≲ 3 s and developed scale sizes ∼ 1-2 km, while the G10 release structured at time t ≲ 22 s and developed scale sizes ∼ 10-15 km. We also find that the diamagnetic cavity size is reduced from those obtained from the ideal MHD results because of the structure. On the other hand, the structuring allows the formation of plasma blobs which appear to free stream across the magnetic field; thus, the barium plasma can propagate to larger distances transverse to the magnetic field than the case where no structuring occurs. Finally, we also discovered a new normal mode of the system which may be excited at the leading edge of the expanding barium plasma. This mode is a magnetic drift wave which propagates azimuthally around the barium cloud in the frequency range Ωi ≪ ω ≪ Ωe.

Lightning driven EMP in the upper atmosphere

Large lightning discharges can drive electromagnetic pulses that cause breakdown of the neutral atmosphere between 80 and 95 km leading to order of magnitude increases in the plasma density. The increase in the plasma density leads to increased reflection and absorption, and limits the pulse strength that propagates higher into the ionosphere.

An improved coupling model for the lithosphere‐atmosphere‐ionosphere system

In our previous model for the lithosphere-atmosphere-ionosphere coupling, the background magnetic field was assumed to be perpendicular to the horizontal plane. In the present paper, we improve the calculation of currents in the atmosphere by solving the current density J directly from the current continuity equation ∇  •  J  =  0. The currents in the atmosphere can be solved for any arbitrary angle of magnetic field, i.e., any magnetic latitude. In addition, a large ratio (~10) of Hall to Pedersen conductivities is used to generate a large Hall electric field. The effects of atmospheric currents and electric fields on the ionosphere with lithosphere current source located at magnetic latitudes of 7.5°, 15°, 22.5°, and 30° are obtained. For upward (downward) atmospheric currents flowing into the ionosphere, the simulation results show that the westward (eastward) electric fields dominate. At magnetic latitude of 7.5° or 15°, the upward (downward) current causes the increase (decrease) of total electron content (TEC) near the source region, while the upward (downward) current causes the decrease (increase) of TEC at magnetic latitude of 22.5°or 30°. The dynamo current density required to generate the same amount of TEC variation in the improved model is found to be smaller by a factor of 30 as compared to that obtained in our earlier paper. We also calculate the ionosphere dynamics with imposed zonal westward and eastward electric field based on SAMI3 code. It is found that the eastward (westward) electric field may trigger one (two) plasma bubble(s) in the nighttime ionosphere.

The Kelvin‐Helmholtz instability: Finite Larmor radius magnetohydrodynamics

A preliminary theoretical and computational study of the Kelvin-Helmholtz instability in an inhomogeneous plasma is presented using finite Larmor radius magnetohydrodynamic (FLR MHD) theory. We show that FLR effects (1) can increase or decrease the linear growth rate, (2) cause the nonlinear evolution to be asymmetric, and (3) allow plasma ‘blobs’ to detach from the boundary layer. The asymmetric growth and nonlinear evolution depend on the sign of B · Ω where B is the magnetic field and Ω = ▽ × V is the vorticity. The simulation results are qualitatively consistent with the hybrid simulations of Thomas and Winske (1991, 1993) and Thomas (1995). These results suggest that FLR MHD can capture important physical processes on length scales approaching the ion Larmor radius.

Equatorial spread F modeling: Multiple bifurcated structures, secondary instabilities, large density ‘bite-outs,’ and supersonic flows

references therein]. These numerical studies have shed light on a number of processes affecting the evolution of ESF: the role of a conducting E-layer, an inhomogeneous neutral wind, seeding conditions, molecular ions, and the interaction of multiple bubbles. Despite the progress made by these studies there are still a number of observations that have not been reported in simulation studies. [4] In this Letter we present new results of the onset and evolution of ESF bubbles using a 2D simulation code (NRLESF2) recently developed at the Naval Research Laboratory. The simulation study shows for the first time multiple bifurcations, secondary instabilities, density ‘biteouts’ of over three orders of magnitude, and supersonic flows within low density channels (V ’ few km/s). These results are consistent with radar and satellite measurements, and all-sky optical images.

Theoretical study of the ionospheric Weddell Sea Anomaly using SAMI2

[1] The ionospheric Weddell Sea Anomaly (WSA) was first reported more than five decades ago based on ionosonde data near the Antarctica peninsula. The WSA is an ionospheric structure characterized by a larger nighttime electron density than daytime density. Recent satellite observations indicate that the WSA can extend from South America and Antarctica to the central Pacific. The major physical mechanisms that have been suggested for the WSA formation are an equatorward neutral wind, an electric field, the photoionization, and the downward diffusion from the plasmasphere. On the basis of the theoretical modeling performed in this study using the SAMI2 model, an equatorward neutral wind is identified as the major cause of the WSA, while the downward flux from the plasmasphere provides an additional plasma source to enhance or maintain the density of the anomalous structure.

CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge for systematic assessment of ionosphere/thermosphere models: NmF2, hmF2, and vertical drift using ground‐based observations

[1] Objective quantification of model performance based on metrics helps us evaluate the current state of space physics modeling capability, address differences among various modeling approaches, and track model improvements over time. The Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) Electrodynamics Thermosphere Ionosphere (ETI) Challenge was initiated in 2009 to assess accuracy of various ionosphere/thermosphere models in reproducing ionosphere and thermosphere parameters. A total of nine events and five physical parameters were selected to compare between model outputs and observations. The nine events included two strong and one moderate geomagnetic storm events from GEM Challenge events and three moderate storms and three quiet periods from the first half of the International Polar Year (IPY) campaign, which lasted for 2 years, from March 2007 to March 2009. The five physical parameters selected were NmF2 and hmF2 from ISRs and LEO satellites such as CHAMP and COSMIC, vertical drifts at Jicamarca, and electron and neutral densities along the track of the CHAMP satellite. For this study, four different metrics and up to 10 models were used. In this paper, we focus on preliminary results of the study using ground-based measurements, which include NmF2 and hmF2 from Incoherent Scatter Radars (ISRs), and vertical drifts at Jicamarca. The results show that the model performance strongly depends on the type of metrics used, and thus no model is ranked top for all used metrics. The analysis further indicates that performance of the model also varies with latitude and geomagnetic activity level.

Self-consistent modeling of equatorial dawn density depletions with SAMI3

[1] Large-scale, dawn density depletions in the equatorial ionosphere have been observed by instruments on the STPSat1 and CHAMP satellites. The Naval Research Laboratory (NRL) ionosphere model SAMI3 (Sami3 is Also a Model of the Ionosphere) is used to study this new phenomenon using a self-consistent electric field. Two empirical Horizontal Wind Models (HWM) are used in the simulation study: HWM93 and HWM07. Dawn density depletions are found using HWM07 but not with HWM93. The cause of the depletions is a post-midnight enhancement of the eastward electric field that generates an upward plasma drift. This drift lifts low density plasma to high altitudes (i.e., ~600 km). We compare our model results to remote sensing data and to in situ satellite data.

Ground and Space-Based Measurement of Rocket Engine Burns in the Ionosphere

On-orbit firings of both liquid and solid rocket motors provide localized disturbances to the plasma in the upper atmosphere. Large amounts of energy are deposited to ionosphere in the form of expanding exhaust vapors which change the composition and flow velocity. Charge exchange between the neutral exhaust molecules and the background ions (mainly O+) yields energetic ion beams. The rapidly moving pickup ions excite plasma instabilities and yield optical emissions after dissociative recombination with ambient electrons. Line-of-sight techniques for remote measurements rocket burn effects include direct observation of plume optical emissions with ground and satellite cameras, and plume scatter with UHF and higher frequency radars. Long range detection with HF radars is possible if the burns occur in the dense part of the ionosphere. The exhaust vapors initiate plasma turbulence in the ionosphere that can scatter HF radar waves launched from ground transmitters. Solid rocket motors provide particulates that become charged in the ionosphere and may excite dusty plasma instabilities. Hypersonic exhaust flow impacting the ionospheric plasma launches a low-frequency, electromagnetic pulse that is detectable using satellites with electric field booms. If the exhaust cloud itself passes over a satellite, in situ detectors measure increased ion-acoustic wave turbulence, enhanced neutral and plasma densities, elevated ion temperatures, and magnetic field perturbations. All of these techniques can be used for long range observations of plumes in the ionosphere. To demonstrate such long range measurements, several experiments were conducted by the Naval Research Laboratory including the Charged Aerosol Release Experiment, the Shuttle Ionospheric Modification with Pulsed Localized Exhaust experiments, and the Shuttle Exhaust Ionospheric Turbulence Experiments.