Emeritus Professor Fred Menk

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.

Purpose To validate automatic substitute computed tomography CT (sCT) scans generated from standard T2-weighted (T2w) magnetic resonance (MR) pelvic scans for MR-Sim prostate treatment planning. Patients and Methods A Siemens Skyra 3T MR imaging (MRI) scanner with laser bridge, flat couch, and pelvic coil mounts was used to scan 39 patients scheduled for external beam radiation therapy for localized prostate cancer. For sCT generation a whole-pelvis MRI scan (1.6 mm 3-dimensional isotropic T2w SPACE [Sampling Perfection with Application optimized Contrasts using different flip angle Evolution] sequence) was acquired. Three additional small field of view scans were acquired: T2w, T2*w, and T1w flip angle 80° for gold fiducials. Patients received a routine planning CT scan. Manual contouring of the prostate, rectum, bladder, and bones was performed independently on the CT and MR scans. Three experienced observers contoured each organ on MRI, allowing interobserver quantification. To generate a training database, each patient CT scan was coregistered to their whole-pelvis T2w using symmetric rigid registration and structure-guided deformable registration. A new multi-atlas local weighted voting method was used to generate automatic contours and sCT results. Results The mean error in Hounsfield units between the sCT and corresponding patient CT (within the body contour) was 0.6 ± 14.7 (mean ± 1 SD), with a mean absolute error of 40.5 ± 8.2 Hounsfield units. Automatic contouring results were very close to the expert interobserver level (Dice similarity coefficient): prostate 0.80 ± 0.08, bladder 0.86 ± 0.12, rectum 0.84 ± 0.06, bones 0.91 ± 0.03, and body 1.00 ± 0.003. The change in monitor units between the sCT-based plans relative to the gold standard CT plan for the same dose prescription was found to be 0.3% ± 0.8%. The 3-dimensional ¿ pass rate was 1.00 ± 0.00 (2 mm/2%). Conclusions The MR-Sim setup and automatic sCT generation methods using standard MR sequences generates realistic contours and electron densities for prostate cancer radiation therapy dose planning and digitally reconstructed radiograph generation.

©2015. American Geophysical Union. All Rights Reserved. Ultralow frequency (ULF) plasma waves propagate through the magnetosphere and ionosphere where they can alter the Doppler velocity of HF radar echoes. Data from the two Tasman International Geospace Environment Radars and the fluxgate and induction coil magnetometers located on Macquarie Island (54.5S, 158.95E geographic) over 2006-2009 show that ULF wave signatures are common. Using coincident radar and magnetometer data selection criteria, 194 events representing a total of 233.4 h were identified. The majority of ULF signatures seen in the radar data were detected between 06 and 12 UT (15 and 21 LT). Using the Maximum Entropy Method, the spectral content showed favored frequencies of 1.6, 2.1, 2.9, and 3.3 mHz but no obvious variation of frequency with latitude. Most of the observed frequencies were in the range 1-4 mHz. A class of Doppler velocity signatures that appeared as a zigzag shape in the radar range:time plots were identified and may be related to the expected plasmapause latitudes. Regularly spaced, favored frequencies appeared in the ground magnetometer data during the afternoon, night, and morning sectors for those days where ULF wave events were observed in the radar data.

Ionization of the Earth's atmosphere by sunlight forms a complex, multilayered plasma environment within the Earth's magnetosphere, the innermost layers being the ionosphere and plasmasphere. The plasmasphere is believed to be embedded with cylindrical density structures (ducts) aligned along the Earth's magnetic field, but direct evidence for these remains scarce. Here we report the first direct wide-angle observation of an extensive array of field-aligned ducts bridging the upper ionosphere and inner plasmasphere, using a novel ground-based imaging technique. We establish their heights and motions by feature tracking and parallax analysis. The structures are strikingly organized, appearing as regularly spaced, alternating tubes of overdensities and underdensities strongly aligned with the Earth's magnetic field. These findings represent the first direct visual evidence for the existence of such structures.

© 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 purpose of this study was to investigate performance of the couch and coil mounts designed for MR-simulation prostate scanning using data from ten volunteers. Volunteers were scanned using the standard MR scanning protocol with the MR coil directly strapped on the external body and the volunteer lying on the original scanner table. They also were scanned using a MR-simulation table top and pelvic coil mounts. MR images from both setups were compared in terms of body contour variation and image quality effects within particular organs of interest. Six-field conformal plans were generated on the two images with assigned bulk density for dose calculation. With the MR-simulation devices, the anterior skin deformation was reduced by up to 1.7 cm. The hard tabletop minimizes the posterior body deformation which can be up to 2.3 cm on the standard table, depending on the weight of volunteer. The image signal-to-noise ratio reduced by 14% and 25% on large field of view (FOV) and small FOV images, respectively, after using the coil mount; the prostate volume contoured on two images showed difference of 1.05 ± 0.66 cm³. The external body deformation caused a mean dose reduction of 0.6 ± 0.3 Gy, while the coverage reduced by 22% ± 13% and 27% ± 6% in V98 and V100, respectively. A dedicated MR simulation setup for prostate radiotherapy is essential to ensure the agreement between planning anatomy and treatment anatomy. The image signal was reduced after applying the coil mount, but no significant effect was found on prostate contouring.

To clinically implement MRI simulation or MRI-alone treatment planning requires comprehensive end-to-end testing to ensure an accurate process. The purpose of this study was to design and build a geometric phantom simulating a human male pelvis that is suitable for both CT and MRI scanning and use it to test geometric and dosimetric aspects of MRI simulation including treatment planning and digitally reconstructed radiograph (DRR) generation. A liquid filled pelvic shaped phantom with simulated pelvic organs was scanned in a 3T MRI simulator with dedicated radiotherapy couch-top, laser bridge and pelvic coil mounts. A second phantom with the same external shape but with an internal distortion grid was used to quantify the distortion of the MR image. Both phantoms were also CT scanned as the gold-standard for both geometry and dosimetry. Deformable image registration was used to quantify the MR distortion. Dose comparison was made using a seven-field IMRT plan developed on the CT scan with the fluences copied to the MR image and recalculated using bulk electron densities. Without correction the maximum distortion of the MR compared with the CT scan was 7.5 mm across the pelvis, while this was reduced to 2.6 and 1.7 mm by the vendor's 2D and 3D correction algorithms, respectively. Within the locations of the internal organs of interest, the distortion was <1.5 and <1 mm with 2D and 3D correction algorithms, respectively. The dose at the prostate isocentre calculated on CT and MRI images differed by 0.01% (1.1 cGy). Positioning shifts were within 1 mm when setup was performed using MRI generated DRRs compared to setup using CT DRRs. The MRI pelvic phantom allows end-to-end testing of the MRI simulation workflow with comparison to the gold-standard CT based process. MRI simulation was found to be geometrically accurate with organ dimensions, dose distributions and DRR based setup within acceptable limits compared to CT.

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.

Methods: A custom-designed pelvic-shaped phantom was scanned by systematically increasing the anterior body-tocoil (BTC) distance from 30 to 90mm. The image quality near the organs of interest was determined in order to characterize the relationship between image quality and BTC distance at the critical organ structures. The half intensity reduction (HIR) was calculated to determine the sensitivity of each organ structure to the BTC distance change.

We present a technique for the automatic detection of Pc5 (150 s to 600 s period) ultralow frequency (ULF) pulsations in ground and ionospheric backscatter from the Super Dual Auroral Radar Network (SuperDARN). This new technique enables rapid identification and visualization of ULF oscillations over the very wide geographical coverage of SuperDARN. The technique detects ULF oscillations using the Lomb-Scargle periodogram method, which provides a natural test for periodic behavior against the null hypothesis of a pure noise distribution. This does not require any interpolation across data gaps and is thus appropriate for SuperDARN data. We demonstrate the application of the technique to SuperDARN data for March 2014 and find that Pc5 pulsations are frequently observed by multiple radars simultaneously. A preliminary investigation using data from all Northern Hemisphere SuperDARN radars indicates that Pc5 pulsation activity occurs most often on the nightside of the magnetosphere, with a mean frequency of about 2 mHz. Key PointsTechnique for automatic detection of ULF oscillations in SuperDARN dataRapid visualization of the presence and spatial extent of Pc5 pulsation activityPc5 ULF oscillations observed most often on the nightside of the magnetosphere

Low-frequency, wide-field radio telescopes such as the Murchison Widefield Array (MWA) enable the dense spatial sampling of the ionosphere and plasmasphere on regional scales. For a physically compact array such as the MWA, the refractive shifts in the positions of celestial sources in the synthesised radio images are proportional to spatial gradients in the total electron content (TEC) transverse to the line of sight. By measuring the angular position shifts of celestial radio sources, one can probe waves and disturbances in the intervening plasma. Radio telescopes differ fundamentally from other techniques for measuring plasma fluctuations in that they are sensitive to TEC gradients/differences rather than absolute TEC. This makes them sensitive specifically to fluctuations about the ambient density, and therefore powerful probes of plasma density waves and irregularities.

Purpose: A method that could enable dose calculations to be performed using magnetic resonance (MR) images for conventional treatment planning and adaptive planning using MR-accelerator systems would be to apply bulk electron densities to the MR images. However currently bone must be manually segmented making this impractical. This work develops and tests an atlas-based method to automatically segment bone on pelvic MR images for dose calculations. Methods: An MR whole-pelvic atlas was created using manually delineated scans from 39 patients. Atlas-based pelvic-bone auto-segmentations were then created for 25 patient scans using deformable image registration of the atlas to each patients scan with a leave-one-out atlas approach. These and corresponding expert manual segmentations were compared using the Dice similarity coefficient. Bone was assigned a density of 1.19 g/cm3 and all other tissues a water equivalent density. Treatment plans were generated on the whole-pelvis MR images and doses compared for the manual and auto-segmented bone plans. Results: The average Dice coefficient was 0.83 (standard deviation = 0.05). The average manual bone volume was 834.6 cm3 compared to the atlas based average volume of 840.0 cm3, with a mean difference of 5.4 cm3 (0.64%). The average ICRU point dose calculated on the MR images using the atlas-based bone segmentations was 0.2% lower (standard deviation 0.3%) than the dose calculated using the manual bone segmentations. Conclusions: The atlas based method for auto-segmentation of pelvic bone enables MR-based prostate radiotherapy dose calculations for treatment and adaptive planning. © 2011, American Association of Physicists in Medicine. All rights reserved.

The accuracy of dosimetry measurements using a-Si EPIDs is affected by their structural characteristics particularly due to the presence of the Gd2O2S phosphor layer. Our previous measurements have shown that modification of the structure to direct detection configuration by removal of the phosphor layer can improve the imager properties for transit dosimetry applications. In this study a research dedicated Varian a-Si EPID has been changed to direct detection configuration and evaluated for transit dosimetry measurements using 7 prostate and 9 IMRT fields with a 20 cm thick phantom in the beam by comparison to a MatriXX detector array. The EPID images were converted to dose using a calibrated 0.6 cc ionization chamber. Gamma evaluation (3%, 3 mm criteria) of the results for for all points greater than 10% of the maximum dose showed that the fraction of points with a Gamma index less than 1 was at least 94.6% in head and neck fields and 99.3% in prostate fields. The mean Gamma was 0.360 and 0.288 for head and neck and prostate fields, respectively. In conclusion, The EPID results are very close to the reference dosimeter and the direct EPID appears to be a promising device for online patient dosimetry applications for the future. © 2010, American Association of Physicists in Medicine. All rights reserved.