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A common observation made by the toroidal imaging mass-angle spectrograph on the polar orbiting Polar Spacecraft is the acceleration of ionospheric O+ and H+ to similar velocities on open field lines in the low-latitude boundary layer (LLBL) and cusp regions. A detailed study of nine events made in the midaltitude cusp region is presented here and the acceleration can be split into two regions. On open field lines in the LLBL both H+ and O+ undergo rapid upward acceleration within approximately 0.75° invariant latitude. The acceleration aH+/ao+ ≈ 2 is inconsistent with the proposed electrostatic acceleration. Intense electrostatic waves often associated with the acceleration is consistent with energy transfer from the faster H+ to the slower O+ population via two-stream instabilities. In the cusp region injected magnetosheath plasma is a possible free energy source for the acceleration of ionospheric ion populations: The component temperature ratio T‖/T⊥. of the upgoing, mirrored injected magnetosheath ions is generally lower than the downgoing ions consistent with a two-stream instability driven energy transfer from the magnetosheath to the ionospheric ion populations.

Observations of cusp ion velocity dispersions made by the TIMAS instrument on the Polar spacecraft in the mid-altitude cusp during intervals of northward interplanetary magnetic field (IMF) reveal a clear ordering with magnetic local time (MLT). Between 1100 and 1300 MLT the injected ion velocity increased with invariant latitude for 78% of the cusp crossings, between 0900 and 1100 MLT this percentage reduced to 35% and between 1300 and 1500 MLT the percentage reduced to 57%. In contrast similar observations made during intervals of southward IMF revealed no MLT dependency. Between 0900 and 1500 MLT the injected ion velocity increased with invariant latitude for only 17% of the observed cusp crossings. We suggest that the difference in the MLT dependency between northward and southward IMF can be best explained by the different characteristics of polar convection patterns for sub-solar and lobe reconnection.

During the hour following the bow shock of the outbound leg of Giotto's encounter with comet P/Grigg-Skjellerup, asymmetric cometary ion distributions were observed in velocity space. Various explanations for the observed asymmetry include sudden changes in the magnetic field or magnetic mirroring. Several workers have noted that the parallel Alfvenic Mach number was close to unity during this interval, a result of the near orthogonal orientation of the interplanetary magnetic field with respect to the solar wind. Newly formed cometary ions would be implanted into the upstream diffusion gap and would therefore be more likely to diffuse through resonant interactions with downstream propagating waves. Starting with this observation, a simple finite difference representation of Boltzmann's equation has been utilized to model the velocity space distributions. The results of the simulation show that the asymmetry is a consequence of the geometry of the implanted ion distribution and the lifetime of the cometary ions in the solar wind and that no other mechanisms are needed to explain the observations.

One of the principal effects of an outgassing comet is to introduce additional mass into the solar wind flow. This mass-loading process results in the generation of Alfven waves via ion-cyclotron resonance. In the case of the Halley encounter this source of “new” fluctuations was introduced into a low speed solar wind 0.89 AU from the sun. Previous in situ observations in low speed flows at this distance have described the fluctuations as turbulently evolved or “aged.” The spectral characteristics and time dependent anisotropies of these new cometary fluctuations have been effectively explained by a conservation-of-energy description of the cometary ion pickup process. An alternative description has been proposed whereby turbulent cascading generates the observed spectrum, implying that the fluctuations evolve significantly in the time between their generation and their observation. Analysis of the rugged invariants and, in particular, the Alfven ratio suggests that throughout the interaction region upwind from the comet the cometary ion pickup generated fluctuations became less evolved as the bow shock was approached. Hence turbulent cascading of the fluctuation energy is unlikely to be an effective description of the observations or to modify the spectral characteristics of these fluctuations significantly in the upwind interaction region.

It is well known that most of the Alfven waves generated in the solar wind by cometary ion pickup are propagating upstream in the solar wind reference frame. However, the magnitude of the small proportion propagating downstream is extremely important because it controls the amount of ion acceleration driven by the second order Fermi process. The velocity diffusion coefficient is approximately proportional to the intensity of the weaker component of the upstream or downstream propagating waves. In previous papers we developed a new technique to calculate the wave spectrum produced by pickup ions which successfully accounted for the total wave spectrum quantitatively. The theory also gives the spectrum of upstream and downstream waves separately. We set out to test the theory by obtaining the upstream and downstream proportions of the wave energy from the data. We have used analyses based on Elsasser variables and the use of the Poynting vector and find that while the downstream proportion is small, it is much larger than expected, and its spectral shape is different from our theoretical prediction. The observations reveal that the spectrum of the cometary ion generated downstream waves is similar to that of the upstream waves, that is, a power law spectrum with a spectral index of −2 above the wavenumber corresponding to the cometary ion gyrofrequency. Since the weaker component is stronger than expected over a wide range of frequencies (and thus resonant velocities), the acceleration process will be enhanced and may be sufficient to explain the discrepancies between theory and observation which still exist. The mechanism responsible for the generation of the increased amount of downstream waves could be nonlinear induced scattering of the upstream waves by the solar wind protons.

A numerical simulation study of the ultra-low frequency (ULF) H-component magnetic field at the Earth's surface arising from a perturbation ionospheric Hall current has been developed. The Hall current system is driven by field-aligned currents (FACs) associated with shear Alfvén field line resonances (FLRs) driven by fast mode global cavity oscillations. The ionospheric phase mixing of the Hall current manifests itself in a number of ways in the ground field, these are: (i) Smoothing the spectral maxima of the ground signal: (ii) Loss in clarity of the harmonic structure of the spectra: (iii) A small increase in the damping rate of the ULF wave at the resonance latitude and (iv) small localised minimum in the spectra at the resonance latitude.