Joseph D. Huba

Sub‐Alfvénic plasma expansion

A large ion Larmor radius plasma undergoes a particularly robust form of Rayleigh–Taylor instability when sub‐Alfvenically expanding into a magnetic field. Results from an experimental study of this instability are reported and compared with theory, notably a magnetohydrodynamic (MHD) treatment that includes the Hall term, a generalized kinetic lower‐hybrid drift theory, and with computer simulations. Many theoretical predictions are confirmed while several features remain unexplained. New and unusual features appear in the development of this instability. In the linear stage there is an onset criterion insensitive to the magnetic field, initial density clumping (versus interchange), linear growth rate much higher than in the ‘‘classic’’ MHD regime, and dominant instability wavelength of order of the plasma density scale length. In the nonlinear limit free‐streaming flutes, apparent splitting (bifurcation) of flutes, curling of flutes in the electron cyclotron sense, and a highly asymmetric expansion are ...

Stability of sub-Alfvenic plasma expansions

A comprehensive theoretical treatment of the linear stability of a sub‐Alfvenic plasma expansion is developed. The analysis is similar to those performed for the lower‐hybrid‐drift instability and the drift cyclotron instability. In addition to the diamagnetic drift (Vdi) that drives these instabilities, the gravitational drift (Vg) caused by the deceleration of the plasma shell, and the Pedersen drift (VP) caused by ion–neutral collisions and neutral gas flow, are included. The emphasis of the paper is on the instability driven by the gravitational drift. The theory is fully kinetic and includes finite‐beta effects (i.e., electromagnetic coupling and electron ∇B drift‐wave resonances), collisional effects (electron–ion, electron–neutral, and ion–neutral collisions), and neutral gas flow, effects that have not been considered to date. The analysis is carried out in a slab geometry although the applications are to spherical expansions. The main conclusions are as follows. In the strong drift limit (Vg>vi a...

Laboratory laser-produced astrophysical-like plasmas

Laser-produced plasmas have many properties similar to, or which can be scaled to, those encountered in space, magnetospheric, ionospheric, and astrophysical situations. We describe several such experiments performed with the PHAROS III Nd-laser facility at NRL.

Hall Magnetohydrodynamics - A Tutorial

Over the past fifteen years it has become increasingly clear that Hall magnetohydrodynamics plays a crucial role in many space and laboratory plasma processes: magnetic reconnection, sub-Alfenic plasma expansions, and plasma opening switches to name a few. Hall magnetohydrodynamics is important for plasma dynamics on length scales less than the ion inertial scale length but greater than the electron inertial length. On these scales the ion and electron motions are decoupled; the electrons remain frozen to the magnetic field but the ions are not. In this paper we provide a basic overview of Hall magnetodydrodynamics with an emphasis on numerical methods. We also provide several concrete examples of Hall dynamics: whistler waves, Hall drift waves, plasma opening switch dynamics, and three dimensional magnetic reconnection.

Theory and simulation of a high-frequency magnetic drift wave

A theoretical and computational study of a high‐frequency electromagnetic drift mode are presented. A fluid theory based upon the resistive magnetohydrodynamic (MHD) equations, which include the Hall term, as well as a kinetic theory based upon the Vlasov equation, are developed. The fluid theory is valid in the regime ω<ωlh (where ωlh is the lower hybrid frequency), and yields an approximate dispersion equation ω≂kyV2A/LnΩi −iηk2y, where ky is the wave number transverse to B0 and the density gradient, VA is the Alfven velocity, Ln is the density gradient scale length, and η is the resistivity. This dispersion equation is valid in the limit Ln≪VA/Ωi. Using kinetic theory it is shown that the drift mode transitions to the lower hybrid mode in the limit ω≳ωlh. The simulation study is based upon the modified MHD equations and the nonlocal nature of the mode is investigated. Applications to sub‐Alfvenic plasma expansions, electromagnetic waves in the Earth’s magnetosphere, and plasma switches are discussed.

Lifetime of a depression in the plasma density over Jicamarca produced by space shuttle exhaust in the ionosphere

When the space shuttle orbiting maneuver subsystem (OMS) engines burn in the ionosphere, a plasma density depression, or “hole,” is produced. Charge exchange between the exhaust molecules and the ambient O+ ions yields molecular ion beams that eventually recombine with electrons. The resulting plasma hole in the ionosphere can be studied with ground-based, incoherent scatter radars (ISRs). This type of ionospheric modification is being studied during the Shuttle Ionospheric Modification with Pulsed Localized Exhaust (SIMPLEX) series of experiments over ISR systems located around the globe. The SIMPLEX 1 experiment occurred over Jicamarca, Peru, in the afternoon on October 4, 1997, during shuttle mission STS 86. An electron density depression was produced at 359 km altitude at the midpoint of a magnetic field line. The experiment was scheduled when there were no zonal drifts of the plasma so the modified field line remained fixed over the 50 MHz Jicamarca radar. The density depression was filled in by plasma flowing along the magnetic field line with a time constant of 4.5 min. The density perturbation had completely vanished 20 min after the engine burn. The experimental measurements were compared with two models: (1) SAMI2, a fully numerical model of the F region, and (2) an analytic representation of field-aligned transport by ambipolar diffusion. The computed recovery time from each model is much longer than the observed recovery time. The theory of ambipolar diffusion currently used in ionospheric models seems to be inadequate to describe the SIMPLEX 1 observations. Several possible sources for this discrepancy are discussed. The SIMPLEX 1 active experiment is shown to have the potential for testing selected processes in ionospheric models.

Nonlocal theory of the Rayleigh–Taylor instability in the limit of unmagnetized ions

A nonlocal theory of the Rayleigh‐Taylor instability is developed that applies to both magnetized and unmagnetized ion systems. The distinction between these two regimes is the time scale of the instability: for γ Ωi the ions can be considered to be unmagnetized, where γ is the growth rate and Ωi is the ion gyrofrequency. Both analytical and numerical results are presented that contrast the behavior of the Rayleigh‐Taylor instability in these two regimes. In the long wavelength limit (kyLn≪1, where ky is the wavenumber and Ln is the density gradient scale length), it is found that the growth rate of the Rayleigh‐Taylor instability in the unmagnetized ion limit is γ≂(kyg/A)1/2, where A is the Atwood number; this is quite different from the growth rate in the magnetized ion limit which is γ=(kygA)1/2. Another distinction between the two limits is that in the unmagnetized ion regime the fastest growing mode is not the lowest‐order mode; again this is ...

Self‐consistent generation of MSTIDs within the SAMI3 numerical model

In this study, we use the three-dimensional, physics-based numerical model, SAMI3 (Sami3 is Another Model of the Ionosphere), to self-consistently generate nighttime, electrified, medium-scale traveling ionospheric disturbances (MSTIDs) at midlatitudes. These are the first numerical simulations to use the fundamental, physics-based equations in a full flux tube model for the self-consistent generation of MSTIDs. We show that a random perturbation results in the development of modes consistent with the Perkins instability and that the growth rate of a specified k perturbation agrees well with linear theory. We also present synthetic observations of MSTIDs: total electron content, integrated 630.0 nm airglow emission, E × B drift, and electron density. The modeling results show the signature of the instability in the geomagnetic conjugate hemisphere, which has been previously observed experimentally. The qualitative descriptions of the E × B drift and electron density profiles of the MSTIDs provided by SAMI3 are found to be consistent with experimental studies of MSTIDs found in the literature.

Self‐generation of magnetic fields by sheared flows in weakly ionized plasmas

A sheared, relative ion–neutral flow can generate a magnetic field in an unmagnetized, weakly ionized plasma. The field generation term is ∂B/∂t=(mec/e)∇×νen(Vi−Vn) where νen is the electron–neutral collision frequency, Vi is the ion fluid velocity, and Vn is the neutral fluid velocity. The time period over which the field grows is limited by diffusion, convection, or collisional relaxation of the relative drift. Since the field generation term scales as νen/Ωe relative to the other terms in the field induction equation, the maximum field generated is found from Ωe≂few νen so that Bmax≂few (mec/e)νen. Both analytical and numerical results are presented. The computational results are based upon a two‐dimensional (2‐D) magnetohydrodynamic (MHD) code which includes the following terms: ion–neutral drag, gravity, resistivity, recombination, the Hall term, and the shear‐driven source term. The theory is applied to the generation of magnetic fields in an unmagnetized planetary ionosphere, such as Venus, and to ...

Hemispheric Daytime Ionospheric Response to Intense Solar Wind Forcing

We investigate the ionospheric response to events where the z-component of the interplanetary magnetic field, B 2 , becomes large and negative for several hours, associated with the largest geomagnetic storms over the prior solar maximum period (2000-2004). We compute the average vertical total electron content (TEC) in the broad region covering 1200-1600 local time and ±40 degrees geomagnetic latitude (dipole), using data from the global network of Global Positioning System (GPS) receivers. In several cases, we find approximately a two-fold increase in total electron content within 2-3 hours of the time when the southward-B solar wind impinged on the magnetopause. We also analyze daytime super-satellite TEC data from the GPS receiver on the CHAMP satellite orbiting at approximately 400 km altitude, and find that for the October 30, 2003 storm at mid-latitudes the TEC increase is nearly one order of magnitude relative to the TEC just prior to the B southward onset. The geomagnetic storm-time phenomenon of prompt penetration electric fields into the ionosphere from enhanced magnetospheric convection is the most likely cause of these TEC increases, at least for certain of the events, resulting in eastward directed electric fields at the equator. The resulting dayside vertical ExB drift of plasma to higher altitudes, while solar photons create more plasma at lower altitudes, results in a daytime super-fountain effect that rapidly changes the plasma structure of the entire dayside ionosphere. This phenomenon has major practical space weather implications.