F. W. Menk

The resonance structure of low latitude Pc3 geomagnetic pulsations

The spectral difference in ULF wave amplitude between closely spaced meridional ground stations may be used to measure the eigenfrequency of magnetospheric field lines (Baransky et al., 1985). A more reliable technique based on the crossphase spectrum has been used to identify eigenfrequencies and study the temporal evolution of local field line resonances. Pc3 (22-100 mHz) pulsations recorded with two pairs of low latitude ground stations have been specifically examined

Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances

It is well known that the resonant frequency of geomagnetic field lines is determined by the magnetic field and plasma density. We used cross-phase and related methods to determine the field line resonance frequency across 2.4≤<L≤4.5 in the Northern Hemisphere at 78°–106° magnetic longitude and centered on L=2.8 in the Southern Hemisphere at 226° magnetic longitude, for several days in October and November 1990. The temporal and spatial variation in plasma mass density was thus determined and compared with VLF whistler measurements of electron densities at similar times and locations. The plasma mass loading was estimated and found to be low, corresponding to 5–10% He+ on the days examined. The plasma mass density is described by a law of the form (R/Req)−p, where p is in the range 3–6 but shows considerable temporal variation, for example, in response to changes in magnetic activity. Other features that were observed include diurnal trends such as the sunrise enhancement in plasma density at low latitudes, latitude-dependent substorm refilling effects, shelves in the plasma density versus L profile, and a longitudinal asymmetry in plasma density. We can also monitor motion of the plasmapause across the station array. Properties of the resonance were examined, including the resonance size, Q, and damping. Finally, we note the appearance of fine structure in power spectra at these latitudes, suggesting that magnetospheric waveguide or cavity modes may be important in selecting wave frequencies.

MRI-guided prostate radiation therapy planning: Investigation of dosimetric accuracy of MRI-based dose planning

BACKGROUND AND PURPOSE Dose planning requires a CT scan which provides the electron density distribution for dose calculation. MR provides superior soft tissue contrast compared to CT and the use of MR-alone for prostate planning would provide further benefits such as lower cost to the patient. This study compares the accuracy of MR-alone based dose calculations with bulk electron density assignment to CT-based dose calculations for prostate radiotherapy. MATERIALS AND METHODS CT and whole pelvis MR images were contoured for 39 prostate patients. Plans with uniform density and plans with bulk density values assigned to bone and tissue were compared to the patients gold standard full density CT plan. The optimal bulk density for bone was calculated using effective depth measurements. The plans were evaluated using ICRU point doses, dose volume histograms, and Chi comparisons. Differences in spatial uniformity were investigated for the CT and MR scans. RESULTS The calculated dose for CT bulk bone and tissue density plans was 0.1±0.6% (mean±1 SD) higher than the corresponding full density CT plan. MR bulk bone and tissue density plans were 1.3±0.8% lower than the full density CT plan. CT uniform density plans and MR uniform density plans were 1.4±0.9% and 2.6±0.9% lower, respectively. Paired t-tests performed on specific points on the DVH graphs showed that points on DVHs for all bulk electron density plans were equivalent with two exceptions. There was no significant difference between doses calculated on Pinnacle and Eclipse. The dose distributions of six patients produced Chi values outside the acceptable range of values when MR-based plans were compared to the full density plan. CONCLUSIONS MR-alone bulk density planning is feasible provided bone is assigned a density, however, manual segmentation of bone on MR images will have to be replaced with automatic methods. The major dose differences for MR bulk density plans are due to differences in patient external contours introduced by the MR couch-top and pelvic coil.

Low latitude geomagnetic field line resonance: Experiment and modeling

Geomagnetic field line resonances may be identified in ground magnetometer data by comparing the difference in amplitude and phase of signals recorded at two closely spaced sites or by examining the latitudinal variation in polarization properties across a more extended array. These two methods give comparable results for values of the resonant frequency and width at low latitudes (L < 3). We have also found an upper limit for the damping factor, γ∼0.07 at L=1.8, by applying a damped simple harmonic oscillator model. The field line resonance structure observed in 5 weeks of data showed only one resonant frequency at L=1.8 but up to four harmonies concurrently at L=2.8. An early local morning decrease in eigenfrequency was usually present at L=1.8. This is attributed to dynamic heavy ion mass loading effects in the ionosphere where the plasma density increases around dawn. The observed eigenfrequencies were used to evaluate two plasma density models. Calculations using a combined IRI-90 and diffusive equilibrium (DE) model gave eigenfrequencies which are considerably smaller than the experimentally observed values at both L=1.8 and L=2.8. Furthermore, the calculated harmonic spacings at L=2.8 do not agree with the experimental values, although the diurnal trends were successfully modeled using the IRI-DE plasma description. The low-latitude plasma density model described by Bailey (1983) yields eigenfrequencies which show good agreement with the experimentally observed values at both latitudes.

Field line resonances and waveguide modes at low latitudes: 1. Observations

Field line resonances (FLRs) are an important mechanism for the generation of Pc3–4 (∼7–100 mHz) geomagnetic pulsations. There is considerable observational evidence for the existence of FLRs at middle latitudes, both in satellite and ground data. However, the low-latitude regions are less accessible for such studies, and consequently many aspects of low-latitude FLRs are not well understood. A temporary 12-station magnetometer array spanning eastern Australia from L= 1.3–2.0 was used to investigate the variation in Pc3–4 power with latitude, the nature and low-latitude limit of FLRs, and properties of spectral components below the local resonant frequency. Examples are presented for representative days. Power spectra are remarkably similar over this range of latitudes and often exhibit a multitude of peaks separated by ∼3–5 mHz. Using cross-phase techniques, we find that the resonant frequency increases with decreasing latitude to L∼1.6, then decreases at lower latitudes. This is due to the effect of ionospheric heavy ions at low altitudes. The characteristic size of the resonances is L∼0.15, the resonance Q is ∼2 at L=2.0 and 1.3–1.4 at L=1.3, and the normalized damping factor γ/ωR∼0.2–0.4. The low-latitude detection limit of FLRs depends on a number of factors, but on a day examined in detail it was L∼1.4. For signals below the local resonant frequency, amplitude decreased with latitude at ∼3 dB/0.1 L. Interstation phase delays are not consistent with the time of flight of radially propagating fast-mode waves in the equatorial plane, although a peak occurs in the region where the Alfven velocity peaks. We conclude that these results are consistent either with modulation of the incoming fast-mode waves or the existence of cavity or waveguide modes which drive discrete forced oscillations of low-latitude field lines across a range of frequencies, and which couple to the local FLR where the frequencies match.

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.

Discrete Field Line Resonances and the Alfvén Continuum in the Outer Magnetosphere

Using Pc5 pulsation events observed with the IMAGE magnetometer array, we demonstrate that mHz frequency field line resonances (FLRs) represent local enhancements in the background Alfven continuum of field line eigenfrequencies. By comparing resonance profiles for a typical event with the continuum frequency profile determined using cross-phase techniques, we show that pulsation frequencies as low as 1-2 mHz can couple to FLRs on high latitude closed field lines. We also suggest that the transition to open field lines on days of higher geomagnetic activity may lead to a breakdown in pulsation characteristics at the highest latitudes of the IMAGE array. In addition, we show evidence of the U-shaped diurnal variation in field line eigenfrequency and suggest that this is primarily due to field line stretching, especially on the magnetosphere flanks.

Field line resonances and waveguide modes at low latitudes: 2. A model

The power spectra of magnetometer data recorded at low latitudes show a remarkable similarity over the latitudinal range, 1.3<L<2.0. Power spectra from magnetometer sites along the east coast of Australia show a similar fine structure where adjacent power maxima are spaced between 3 and 5 mHz apart. The H component data also show a superposed feature of the power spectrum that varies with latitude according to that expected for field line resonance. In order to explain these data, the propagation of fast mode, ULF wave energy through a one-dimensional model of the magnetosphere is examined. The model features an inner boundary at 1.1 R E , an input wave spectrum at the outer boundary (10 R E ) that varies as 1/f, and a realistic Alfven velocity that depends on the radial coordinate. The model includes a plasmapause, and in the inner plasmasphere, the Alfven velocity reaches a maximum at L=1.6 then decreases as the larger plasma density of the upper ionosphere is approached. The azimuthal wave number and background magnetic field also depend on the radial coordinate, and a realistic variation of the field line resonant frequency is included. This model can reproduce important features of observed low-latitude ULF power spectra and indicates that the interaction between waveguide and field line resonance modes is important in understanding low-latitude data.

A coordinated ground-based and IMAGE satellite study of quiet-time plasmaspheric density profiles

Cold plasma mass density profiles in the plasmasphere have been determined for the geomagnetically quiet day of 19th August 2000 using the cross-phase technique applied to ground-based magnetometer data from the SAMNET, IMAGE and BGS magnetometer arrays. Cross-phase derived mass densities have been compared to electron densities derived from both ground-based VLF receiver measurements, and the IMAGE satellite RPI. The cross-phase results are in excellent agreement with both the VLF and IMAGE observational results, thus validating the cross-phase technique during quiet times. This is the first such coordinated multi-instrument study, and has enabled very few heavy ions to be inferred in the plasmasphere for L > 3.45 on this day. The observational results were compared to plasma mass densities from the SUPIM model and were found to be in excellent agreement. IMAGE EUV data also verified the existence of azimuthal structure in the outer quiet-time plasmasphere.

Properties and sources of low and very low latitude Pi2 pulsations

Data from a low-latitude ground array of 10 fluxgate magnetometer stations spanning a geomagnetic latitude range from 38°N through the equator to 47°S, with a geomagnetic longitudinal extent from 185°E to 227°E covering ∼3 hours in magnetic local time, and an additional high-latitude station (65°N) are used to study the spatial and conjugate properties of Pi2 geomagnetic pulsation wave frequency, amplitude, phase, and polarization. Using fast fourier transform and pure state vector analysis techniques, multistation observations show that individual frequency elements of the Pi2 waves are constant with latitude and longitude, but the dominant frequency may vary. In fact, the higher frequencies often dominate at low latitudes, and the lower frequencies often dominate at higher latitudes. The D component amplitude shows a clearly decreasing trend to a near-zero minimum at the equator, while the H component amplitude does not show a simple behavior. The H component phase is nearly constant at low latitudes, while the high-latitude station may be out of phase or in phase with the low-latitude stations for different events. The D component phase is constant in each hemisphere with a 180° phase difference between hemispheres. Azimuthal wave numbers m obtained from interstation phase differences are small, typically |m| < 3, and show a dominance of westward propagation at most longitudes. It is concluded that the main features of low-latitude Pi2 properties are most easily explained by magnetospheric cavity or wave guide mode resonances, and field line resonances are most likely responsible for the high-latitude Pi2s. This supports the idea that midlatitude and low-latitude Pi2s are two-source phenomena.