Ronald H. Miller

Interaction of ring current and radiation belt protons with ducted plasmaspheric hiss: 1. Diffusion coefficients and timescales

Protons that are converted into the inner magnetosphere in response to enhanced magnetic activity can resonate with ducted plasmaspheric hiss in the outer plasmasphere via an anomalous Doppler-shifted cyclotron resonance. Plasmaspheric hiss is a right-hand-polarized electromagnetic emission that is observed to fill the plasmasphere on a routine basis. When plasmaspheric hiss is confined within field-aligned ducts or guided along density gradients, wave normal angles remain largely below 45°. This allows resonant interactions with ions at typical ring current and radiation belt energies to take place. Such field-aligned ducts have been observed both within the plasmasphere (Kozyra et al., 1987a; Koons, 1989) and in regions outside of the plasmasphere (Chan and Holzer, 1976). Wave intensities are estimated using statistical information from studies of detached plasma regions (Chan et al., 1974). Diffusion coefficients are presented for a range of L shells and proton energies for a fixed wave distribution. Harmonic resonances in the range n = ±100 are considered in order to include interactions between hiss at 100 Hz to 2 kHz frequencies, and protons in the energy range between ∼10 keV and 1000 keV. Diffusion timescales are estimated to be of the order or tens of days and comparable to or shorter than lifetimes for Coulomb decay and charge exchange losses over most of the energy and spatial ranges of interest.

Multicenter Trial of Sotalol for Suppression of Frequent, Complex Ventricular Arrhythmias: A Double-Blind, Randomized, Placebo-Controlled Evaluation of Two Doses

Sotalol is a unique beta-adrenergic blocking agent with additional actions characteristic of Vaughn-Williams class III antiarrhythmic agents in experimental models. To test the efficacy of sotalol to suppress ventricular arrhythmias, a 6 week parallel, placebo-controlled out-patient study of two doses (320 and 640 mg/day, in two divided doses) was performed in four hospitals in 56 patients with chronic premature ventricular complexes at a frequency of 30/h or more (mean +/- SE, 528 +/- 60/h) on 48 hour ambulatory electrocardiographic recording. During a placebo week, no change occurred in arrhythmia frequency (532 +/- 76/h). Subsequent sotalol therapy significantly reduced median arrhythmia frequency in patients receiving both low (n = 19) and high (n = 18) doses compared with that in patients receiving placebo (by 77 and 83%, respectively, versus 6%; p less than 0.001). Twenty-two (59%) of 37 sotalol-treated patients, 11 in each group, reached the prospectively defined criterion of efficacy (greater than or equal to 75% arrhythmia reduction) versus 2 (11%) of 19 placebo control patients (p less than 0.001). Sotalol reduced the median frequency of couplets by 94% (p less than 0.0001) and that of runs by 89% (p less than 0.0007). The electrocardiographic effects of sotalol included reductions in heart rate (by 17 to 27%) and increases in the QTc (by 6 to 9%) and PR (by 6%) intervals. Ejection fraction was unchanged. The most common adverse side effect was fatigue, but drug discontinuation was required in only three patients taking 640 mg/day. No proarrhythmic events or biochemical abnormalities were observed. In summary, sotalol displays significant antiarrhythmic activity of moderately high degree with good tolerance in doses of both 320 and 640 mg/day. Its antiarrhythmic actions are distinguished from those reported for other beta-blockers by its effects on the QTc interval and its moderately high degree of antiarrhythmic activity.

Kinetic simulation of plasma flows in the inner magnetosphere

A one-dimensional hybrid particle code is used to study the interactions between upflowing thermal ions from conjugate ionospheres. The simulation model allows for multiple species, convection of plasmaspheric flux tubes, and Coulomb self-collisions which conserve momentum and energy locally. The model incorporates a variable-flux boundary condition where the flux, at the boundaries, approaches zero as the plasmasphere fills and equilibrium conditions are reached. The effects of two important processes on plasmaspheric refilling have been considered. The first includes convection of the plasmaspheric flux tube. The second is the interaction of ionospheric thermal plasma and particle injection from an external source. Particle injection seems to play an important role in the evolution of the total particle distribution on the early timescales (t 8 days) the thermal plasma from the ionosphere dominates the particle distribution.

The dynamics of low-β plasma clouds as simulated by a three-dimensional, electromagnetic particle code

The dynamics of low-β plasma clouds moving perpendicular to an ambient magnetic field in vacuum and in a background plasma is simulated by means of a three-dimensional, electromagnetic, and relativistic particle simulation code. The simulations show the formation of the space charge sheaths at the sides of the cloud with the associated polarization electric field which facilitate the cross-field propagation, as well as the sheaths at the front and rear end of the cloud caused by the larger ion Larmor radius, which allows ions to move ahead and lag behind the electrons as they gyrate. Results on the cloud dynamics and electromagnetic radiation include the following: (1) In a background plasma, electron and ion sheaths expand along the magnetic field at the same rate, whereas in vacuum the electron sheath expands much faster than the ion sheath. (2) Sheath electrons are accelerated up to relativistic energies. This result indicates that artificial plasma clouds released in the ionosphere or magnetosphere may generate optical emissions (aurora) as energetic sheath electrons scatter in the upper atmosphere. (3) The expansion of the electron sheaths is analogous to the ejection of high-intensity electron beams from spacecraft: a stagnation point is reached where the expansion is halted, allowing only a fraction of the electrons to escape. These electrons may have energies exceeding the initial beam energy (sheath electrostatic potential energy). (4) Second-order and higher-order sheaths are formed which extend out into the ambient plasma. These sheaths are curved, the curvature increasing with the order of the sheath. (5) The formation of the sheaths and the polarization field reduces the forward momentum of the cloud. Furthermore, as the cloud moves across the field, sheath particles are peeled off and left behind, while fresh particles continue to drift into the sheaths in order to maintain these. This process requires continued energy supply which further decelerates the cloud. (6) The coherent component of the particle gyromotion (the particles are born with a coherent component at t=0) is damped in time as the particles establish a forward directed drift velocity. In addition, particles undergo orbital phase mixing associated with the establishment of the polarization electric field. For a dense cloud in vacuum the damping occurs very quickly, while for clouds in a background plasma the polarization fields are shorted, leaving the particles to gyrate as single particles. In this case, the coherent component is almost undamped. (7) The coherent particle gyrations generate electromagnetic radiation. The simulations indicate that artificial plasma clouds released in space will radiate close to the cloud electron or ion cyclotron/upper hybrid frequencies.

A Coulomb collision algorithm for weighted particle simulations

A binary Coulomb collision algorithm is developed for weighted particle simulations employing Monte Carlo techniques. Charged particles within a given spatial grid cell are pair-wise scattered, explicitly conserving momentum and implicitly conserving energy. A similar algorithm developed by Takizuka and Abe (1977) conserves momentum and energy provided the particles are unweighted (each particle representing equal fractions of the total particle density). If applied as is to simulations incorporating weighted particles, the plasma temperatures equilibrate to an incorrect temperature, as compared to theory. Using the appropriate pairing statistics, a Coulomb collision algorithm is developed for weighted particles. The algorithm conserves energy and momentum and produces the appropriate relaxation time scales as compared to theoretical predictions. Such an algorithm is necessary for future work studying self-consistent multi-species kinetic transport.

Interaction of ring current and radiation belt protons with ducted plasmaspheric hiss: 2. Time evolution of the distribution function

The evolution of the bounce-averaged ring current/radiation belt proton distribution is simulated during resonant interactions with ducted plasmaspheric hiss. The plasmaspheric hiss is assumed to be generated by ring current electrons and to be damped by the energetic protons. Thus energy is transferred between energetic electrons and protons using the plasmaspheric hiss as a mediary. The problem is not solved self-consistently. During the simulation period, interactions with ring current electrons (not represented in the model) are assumed to maintain the wave amplitudes in the presence of damping by the energetic protons, allowing the wave spectrum to be held fixed. Diffusion coefficients in pitch angle, cross pitch angle/energy, and energy were previously calculated by Kozyra et al. (1994) and are adopted for the present study. The simulation treats the energy range, E ≥ 80 keV, within which the wave diffusion operates on a shorter timescale than other proton loss processes (i.e., Coulomb drag and charge exchange). These other loss processes are not included in the simulation. An interesting result of the simulation is that energy diffusion maximizes at moderate pitch angles near the edge of the atmospheric loss cone. Over the simulation period, diffusion in energy creates an order of magnitude enhancement in the bounce-averaged proton distribution function at moderate pitch angles. The loss cone is nearly empty because scattering of particles at small pitch angles is weak. The bounce-averaged flux distribution, mapped to ionospheric heights, results in elevated locally mirroring proton fluxes. OGO 5 observed order of magnitude enhancements in locally mirroring energetic protons at altitudes between 350 and 1300 km and invariant latitudes between 50° and 60° (Lundblad and Soraas, 1978). The proton distributions were highly anisotropic in pitch angle with nearly empty loss cones. The similarity between the observed distributions and those resulting from this simulation raises the possibility that interactions with plasmaspheric hiss play a role in forming and maintaining the characteristic zones of anisotropic proton precipitation in the subauroral ionosphere. Further assessment of the importance of this process depends on knowledge of the distribution in space and time of ducted plasmaspheric hiss in the inner magnetosphere.

Pitch angle scattering of cometary ions into monospherical and bispherical distributions

Low frequency magnetic fluctuations generated by solar wind/cometary ion distributions cause pitch-angle scattering of cometary ions. The nature of this pitch angle scattering is examined by means of one-dimensional hybrid simulations, in which newborn ions are created at a constant rate, and for various injection angles, {alpha}, relative to the magnetic field. Pitch angle scattering in the quasi-perpendicular regime ({alpha} > 60{degree}) results in a bispherical velocity distribution while in the quasi-parallel regime (0 {le} {alpha} {le} 60{degree}), pitch angle scattering can result in either a mono or bispherical distribution depending of the injection angle and intial beam velocity.

Pitch‐angle scattering of cometary ions: Computer simulations

Pitch-angle evolution of newborn cometary ion is studied by means of one-dimensional electromagnetic hybrid computer simulations of homogeneous plasmas. Newborn ions are injected into the simulations at a constant rate, with a velocity relative to the solar wind which makes an angle {alpha} with respect to the ambient magnetic field. The simulations are done with relatively weak ion injection rates commensurate with those in the distant environment of comet Halley. In response to the linear temporal growth of the fluctuating magnetic field energy, the injected ions pitch-angle scatter toward isotropy in both the quasi-parallel (0{degree} {le} {alpha} {approx} {le} 60{degree}) and quasi-perpendicular(60{degree} < {alpha} {le} 90{degree}) regimes. For the injection of cometary oxygen ions the simulations show pitch angle scattering rates that are essentially independent of the solar wind ion/cometary ion relative speed, and increase as the square root of the injection rate. Furthermore, the oxygen ion pitch angle scattering rate at perpendicular injection is approximately twice that in the quasi-parallel regime, in qualitative agreement with observations at comet Halley.

Self‐consistent electrostatic potential due to trapped plasma in the magnetosphere

A steady state solution for the self-consistent electrostatic potential due to a plasma confined in a magnetic flux tube is considered. A steady state distribution function is constructed for the trapped particles from the constants of the motion, in the absence of waves and collisions. Using Liouvilles theorem, the particle density along the geomagnetic field is determined and found to depend on the local magnetic field, self-consistent electric potential, and the equatorial plasma distribution function. A hot anisotropic magnetospheric plasma in steady state is modeled by a bi-Maxwellian at the equator. The self-consistent electric potential along the magnetic field is calculated assuming quasineutrality, and the potential drop is found to be approximately equal to the average kinetic energy of the equatorially trapped plasma. The potential is compared with that obtained by Alfven and Falthammar [1963].

The directional dependence of magnetic fluctuations generated by cometary ion pickup

The properties of low frequency magnetic fluctuations generated by cometary ion pickup are examined by means of one-dimensional hybrid simulations, in which newborn ions are created at a constant rate. The helicity and direction of propagation of magnetic fluctuations are investigated for various cometary ion injection angles, {alpha}, relative to the solar wind magnetic field. The parameter {eta} represents the relative contribution of wave energy density propagating in the direction away from the comet, parallel to the beam. For small (quasi-parallel) injection angles, {alpha}{approximately} 0{degree} and {eta} is of order unity, while for larger (quasi-perpendicular) angles, {alpha}{approximately} 90{degree} and {eta} is about 0.5. At intermediate angles, {alpha}{approximately} 60{degree}, {eta} can vary between 0 and 1, depending on the wave number. The wave properties are consistent with the instabilities (right-hand cyclotron resonant modes at small {alpha}, left-hand Alfven/ion cyclotron modes at large a) expected from linear theory. Consequences of these results for particle acceleration are also discussed.