Dr Murray Sciffer

It is often stated that high-frequency radars experience a loss of backscatter during geomagnetic storm events. The occurrence of backscatter during 25 intense geomagnetic storms was examined using data from the Bruny Island and Kodiak radars and a superposed epoch analysis. It was found that while a reduction of backscatter occurred in the middle to far ranges, there was an increase in the amount of backscatter from close range following storm onset. Ray tracing showed that an enhanced charge density in the E region can reduce the chance of F region and increase the chance of E region backscatter. It was also shown that reduction in backscatter cannot be explained by D region absorption. Using a normalized SYM-H value, percentage time through recovery phase can be estimated during storm progression which allows a prediction of backscatter return in real time that accounts for varying storm recovery phase duration.

We use test particle simulations incorporating an MHD model of ULF wave propagation in the magnetosphere with realistic ionosphere boundary conditions to study electron energization in the dayside outer Van Allen radiation belt, referenced to in situ particle and wave observations. On 7 January 2011 the THEMIS spacecraft detected 3 and 4¿5 mHz waves simultaneous with flux enhancement of >10¿keV electrons during the early recovery phase of a moderate geomagnetic storm. We find that internal energization of equatorially mirroring electrons via nonresonant ULF wave-particle interactions can explain these observations. The wave poloidal components cause radial drift of electrons, increasing (decreasing) their kinetic energy as they move inward (outward). Electrons with initial kinetic energies of a few keV can be energized to double these values within an hour by interaction with the 3 mHz waves. The energization rate is somewhat less for the 4¿5 mHz waves. An increase in the ionospheric conductance decreases the power of the fast mode wave, reducing radial drift velocities and hence decreasing the rate of energization. The fast mode poloidal field varies with radial distance and longitude, and this also affects energization. Electrons which drift outward encounter a region where the toroidal field due to the field line resonance becomes dominant and produces strong azimuthal drift. These electrons become trapped in an L-shell range just outward of the resonance region and are not energized.

©2015. American Geophysical Union. All Rights Reserved. The diffusion and energization of electrons in the equatorial plane of Earth's magnetosphere by ULF waves under different ionosphere boundary conditions are examined. Using test-particle simulations and considering intervals of weak geomagnetic activity, we find that the highest energization and minimum diffusion rates correspond to nightside ionosphere conditions. Conversely, the highest diffusion rates and minimum energization are seen for a perfectly reflecting ionosphere boundary. The maximum energies gained under dayside conditions, when Hall conductivity is included, are slightly greater than the maximum energy for similar conditions without Hall conductivity. The diffusion rates for dayside ionosphere conditions with only Pedersen conductivity are greater than the diffusion rates when Hall conductivity is included. These findings show that ULF wave-particle interactions in Earth's magnetosphere depend on the ionosphere conductance.

©2014. American Geophysical Union. All Rights Reserved. Localized fast flows that impinge on the inner magnetosphere from the plasma sheet are observed to oscillate on time scales of minutes. The compression ahead of these flows will launch fast mode waves, while the velocity shears at the edges of these flows directly excite shear Alfvén waves. These waves, which are coupled by gradients in the Alfvén speed, have been suggested as a source for the Pi1 and Pi2 waves that are observed at both high and low latitudes in the ionosphere. A new three-dimensional simulation of the propagation of ULF waves in the dipolar region of the magnetosphere has been developed to study these coupled wave modes. This model includes a height-resolved ionospheric conductivity so that ionospheric fields can be more realistically determined, as well as a direct calculation of ground magnetic fields to compare with ground magnetometers using an inductive ionosphere model. Results from this model show that a plasmaspheric resonance can be set up by waves with periods about 1 min and that field line resonances can be excited both inside and outside the plasmasphere. The use of the inductive ionosphere model leads to the conclusion that even a uniform Hall conductivity can break the dawn-dusk symmetry of the convection pattern. Waves from a source at 10 Earth radii reach the ionosphere with time delays between high and low latitudes of tens of seconds, with implications for the timing of substorm phenomena observed by spacecraft and by ground magnetometers and radars.

© 2015 American Geophysical Union. Quarter-wave modes are standing shear Alfvén waves supported along geomagnetic field lines in space. They are predicted to be generated when the ionosphere has very different conductance between the north compared with the south ionosphere. Our previous observation reported that the resonant frequency is sometimes very low around the dawn terminator and suggested these were due to quarter-wave modes. In this paper, we examine the resonance structure that provides further evidence of the presence of quarter-wave modes. Data from three magnetometers in New Zealand were analyzed. Four events are discussed which show extraordinarily low eigenfrequencies, wide resonance widths, and strong damping when the ionosphere above New Zealand was in darkness while the conjugate northern hemisphere ionosphere was sunlit. Later in the morning, the eigenfrequencies and resonance widths changed to normal daytime values. The wide resonance width and the strong damping of the quarter-wave modes arise from strong energy dissipation in the dark side ionosphere. One event exhibited field line resonance structure continuously through a transition from very low frequency to the normal daytime values. The frequency change began when the dawn terminator passed over New Zealand and finished 1h later when the ratio of the interhemispheric ionospheric conductances decreased and reached ~5. These observations are strong evidence of the presence of quarter-wave modes and mode conversion from quarter- to half-wave resonances. These experimental results were compared with the ULF wave fields obtained from a 2.5-dimensional simulation model.

The plasmapause is a highly dynamic boundary between different magnetospheric particle populations and convection regimes. Some of the most important space weather processes involve wave-particle interactions in this region, but wave properties may also be used to remote sense the plasmasphere and plasmapause, contributing to plasmasphere models. This paper discusses the use of existing ground magnetometer arrays for such remote sensing. Using case studies we illustrate measurement of plasmapause location, shape and movement during storms; refilling of flux tubes within and outside the plasmasphere; storm-time increase in heavy ion concentration near the plasmapause; and detection and mapping of density irregularities near the plasmapause, including drainage plumes, biteouts and bulges. We also use a 2D MHD model of wave propagation through the magnetosphere, incorporating a realistic ionosphere boundary and Alfvén speed profile, to simulate ground array observations of power and cross-phase spectra, hence confirming the signatures of plumes and other density structures.