W. Allan

Are low‐latitude Pi2 pulsations cavity/waveguide modes?

The plasmasphere seems the most likely magnetospheric region in which compressional hydromagnetic waves may be trapped to form cavity modes. It has been suggested that substorm-associated Pi 2 magnetic pulsations have a cavity mode character at low latitudes. Recent detailed observations of night-side Pi2 events at low latitudes have also suggested a cavity or waveguide character. We apply a well-tried numerical hydromagnetic wave coupling model at low latitudes, and compare the model output with these observations. We find that the qualitative amplitude and phase characteristics of the model magnetic fields in latitude and longitude fit well with the observations, provided the ionospheric boundary condition at low latitudes gives negligible rotation of magnetic field components between ionosphere and ground. The comparison supports the idea that low-latitude Pi 2 pulsations are cavity/waveguide modes. It also suggests that the magnetic H component observed at the ground near the equator may be a combination of the radial and compressional components above the ionosphere.

Magnetotail waveguide: Fast and Alfvén waves in the plasma sheet boundary layer and lobe

Numerical simulations of MHD wave propagation and coupling in a realistic magnetotail are presented. Fast mode waves are observed to disperse and couple resonantly to Alfven waves over a broad layer rather than on an isolated field line. Indeed, the layer is likely to be so broad as to include the entire tail lobe as well as the plasma sheet boundary layer (PSBL). It appears that small k y modes (where k y is the cross-tail wave number) will provide the most efficient coupling as they will tend to propagate along the magnetotail field lines rather than across them and out of the tail boundaries. (Moreover, it is only small k y fast modes that will be able to penetrate the lobe.) Alfven waves in the PSBL are shown to phase mix rapidly resulting in strong field-aligned currents with an equatorward phase motion. These properties are in agreement with observations of optical auroral emissions. The lobe Alfven waves do not phase mix, and are not expected to excite optical emissions. They may, however, provide a significant ponderomotive force and could account for the transport of oxygen ions from the ionosphere into the distant tail lobes.

Shipboard determinations of the distribution of 13C in atmospheric methane in the Pacific

Measurements of the mixing ratio and δ 13 C in methane (δ 13 CH 4 ) are reported from large, clean air samples collected every 2.5° to 5° of latitude on four voyages across the Pacific between New Zealand and the West Coast of the United States in 1996 and 1997. The data show that the interhemispheric gradient for δ 13 CH 4 was highly dependent on season and varied from 0.5‰ in November 1996 with an estimated annual mean of 0.2-0.3‰. The seasonal cycles in δ 13 CH 4 reveal three distinct latitude bands differentiated by phase. Maxima occur in January-February for the extratropical Southern Hemisphere, in September-October for the tropics, and in June-July for the extratropical Northern Hemisphere. The data are compared with results from a three-dimensional transport and atmospheric chemistry model that simulates the observed latitudinal structure of either δ 13 CH 4 or the methane mixing ratio well, but not both simultaneously. The requirement that a methane source-sink budget be consistent with both types of data clearly imposes stricter constraints than arise from either mixing ratio or isotopic data alone. The seasonal δ 13 CH 4 data in the extratropical Southern Hemisphere are used to estimate a value for the net fractionation in the CH 4 sink of 12-15‰, which is larger than can be explained by current laboratory measurements of a kinetic isotope effect for the OH + CH 4 reaction and soil sink processes. The hypothesis that the discrepancy is caused by competitive reaction of active chlorine with methane in the marine boundary layer is discussed.

Hydromagnetic wave propagation and coupling in a magnetotail waveguide

For some time the magnetotail has been considered as a possible region where hydromagnetic waves can propagate as waveguide modes. Recently, attention has turned to the magnetospheric flanks as waveguides, and much useful insight has been gained into propagation of fast waveguide modes there, and the structure of the field line resonances they can drive. We return to the magnetotail and investigate hydromagnetic wave propagation and coupling in a magnetotail waveguide. This problem is significantly different from the flank waveguide as the ambient magnetic field is directed along the waveguide rather than across. Field line resonances of the flank type are not possible in the lobe waveguide. We describe a numerical simulation of a model waveguide in which the Alfven speed decreases across the waveguide to the central plasma sheet. The waveguide is stimulated by a short compressional perturbation located in the far tail. The cross-tail spatial structure is chosen to give relatively weak coupling between fast and Alfven modes so that phase and group velocities of uncoupled fast modes can be used to interpret the results. We find that the perturbation propagates dispersively down the waveguide in the form of fast waveguide modes. Fourier components with small parallel wavenumber contain most of the energy, and propagate relatively slowly toward the “Earth.” These act as moving sources which launch Alfven waves continuously earthward. The wave dispersion relations are such that the waveguide modes couple with Alfven waves only in a limited region of the transverse Alfven speed gradient. The Alfven waves travel at the local Alfven speed along each field line, so that as they travel the wave on a given field line becomes increasingly out of phase with waves on adjacent field lines. The phase mixing in our model is novel in that it includes the effects of transverse gradients in both Alfven frequency and parallel wavenumber which tend to cancel each other out. Nevertheless, the phase-mixing process leads to increasingly fine transverse structure as the waves progress down the waveguide. The results are likely to be applicable in regions such as the plasma sheet boundary layer and the plasma mantle.

Phase mixing and phase motion of Alfvén waves on tail‐like and dipole‐like magnetic field lines

The time-dependent phase structure of Alfven waves on open and closed field lines is studied. In accord with previous observations we find that Alfven waves on near-Earth closed field lines exhibit a poleward phase motion unless they are close to the plasmapause, in which case the motion may be equatorward. Alfven waves generated on tail-like closed or open field lines threading the plasma sheet boundary layer have received much less attention but may be shown to have an equatorward phase motion [Liu et al., 1995]. Phase mixing in the magnetotail proves to have a much richer behavior than that on near-Earth (dipole-like) closed field lines as not only the Alfven frequency varies across the background field lines but the field-aligned wavenumber varies too. The two contributions tend to cancel each other partially for typical tail equilibria. Observations are given of a double oval configuration showing long-period pulsations on the poleward portion of this oval. Equatorward phase motion is observed and supports the theory presented here. These observations illustrate that Pc5 pulsation activity can be much richer than previously thought and can occur at locations not in the dipole-like region, as is usually supposed. The concepts presented in this paper provide a powerful framework with which to interpret observations related to auroral arcs, substorms, and magnetospheric equilibria.

Structure, phase motion, and heating within Alfvén resonances

An analytical investigation of Alfven resonances is presented for the cases when energy is dissipated by (1) resistivity (η) within the body of the plasma, and (2) finite Pedersen conductivity (Σ p ) within the ionospheric boundaries. Universal functions for both solutions are compared, and we find that the structure and phase motion of the resonant fields is fairly insensitive to the details of dissipation. We also identify a curious property of resonances : the spatially integrated (but not time-averaged) dissipation rate is independent of time for whichever dissipation process we consider. It appears that resonances have a remarkably robust structure.

Active chlorine in the remote marine boundary layer: Modeling anomalous measurements of δ13C in methane

Measurements of δ13C in methane in the marine boundary layer (MBL) of the extratropical Southern Hemisphere imply a kinetic isotope fractionation much larger than would be expected if the hydroxyl radical were the only tropospheric methane sink. We use a simple chemical box model to show that the assumption of a MBL active chlorine (Cl•) sink can explain these anomalous observations provided there is a seasonal cycle in the Cl• sink with a summer-winter concentration difference ∼ 6 · 10³ cm−3. The required summer maximum and yearly mean Cl• concentrations are plausible, and imply a global Cl• sink strength for methane of < 5 Tg yr−1. Choice of a Cl• sink seasonal cycle linked to the nonsinusoidal dimethyl sulfide seasonal cycle gives the observed fractionation with a smaller yearly mean Cl• concentration than equivalent sinusoidal Cl• cycles.

Large-m waves generated by small-m field line resonances via the nonlinear Kelvin-Helmholtz instability

Recently, ultralow-frequency waves with large azimuthal wavenumber (large m) have been observed on similar L shells and with the same (or similar) frequencies as small-m field line resonances (FLRs). The large-m waves appeared to the west of the small-m FLRs and had westward phase propagation while the small-m FLRs had tailward phase propagation. We propose an extension to an earlier waveguide model to explain these observations. We suggest that small-m tailward propagating waveguide modes drive the small-m FLRs. Phase mixing within these FLRs allows the development of the nonlinear Kelvin-Helmholtz (K-H) instability near the resonant field lines. Phase-mixing scale lengths are limited by ionospheric dissipation, and we show that realistic ionospheric Pedersen conductivities result in the dominance of a single zero-frequency K-H wave in each small-m FLR region having m consistent with observation of the large-m disturbances. K-H growth rates are significant, but not large enough to disrupt the small-m FLRs. We propose that unstable ion distributions amplify the seed K-H waves as the ions drift westward. This leads to observable large-m drift waves at or beyond the westward limits of the small-m FLR regions.

Alfvén wave dissipation via electron energization

[1] It has recently been noted that FAST (Fast Auroral SnapshoT) mission data of auroral current systems associated with Alfven waves has an electron kinetic energy flux density that is similar to the Poynting vector [Chaston et al., 2002]. In Ultra-Low-Frequency (ULF) wave theory, which considers global wave modes of the magnetosphere (with frequency 1-5 mHz), the traditional dissipation mechanism is taken to be Joule heating associated with ionospheric currents that are fed by the Poynting vector. Hence FAST observations indicate that electron acceleration can supply an additional sink of energy that is of similar magnitude to the traditional one. In this letter we use typical Pe5 parameters to estimate the importance of electron acceleration as a sink of Alfven wave energy for these global waves. We find that the electron dynamics must be treated nonlinearly, and that electron acceleration drains a similar amount of energy from the Alfven wave fields as ionospheric dissipation, and for some events may actually exceed the latter.

Are two-fluid effects relevant to ULF pulsations?

It has been suggested recently that the traditional single-fluid MHD description of ULF pulsations is wrong and that a two-fluid model (which includes electron inertia) is required [Bellan, 1994]. If this claim is correct, it suggests most previous studies are inadequate and of questionable value. We have examined the appropriate equations and find that two-fluid effects are not important for typical ULF pulsations. Specifically, the time taken to develop spatial scales similar to the electron inertia length far exceeds the lifetime of the waves. We argue that the singular normal modes of single-fluid MHD are important properties of the magnetospheric system that should be studied. They may be employed to calculate the evolution of waves and are also a useful mathematical limit of the more realistic dissipative normal modes.