Pulsating aurora is a common phenomenon generally believed to occur mainly in the aftermath of a substorm, where dim long‐period pulsating patches appear. The study determines the temporal and spatial evolution of pulsating events using two THEMIS all‐sky imager stations, at Gillam (66.18 magnetic latitude, 332.78 magnetic longitude, magnetic midnight at 0634 UT) and Fort Smith, (67.38 magnetic latitude, 306.64 magnetic longitude, magnetic midnight at 0806 UT) along roughly the same invariant latitude. Parameters have been calculated from a database of 74 pulsating aurora events from 119 days of good optical data within the period from September 2007 through March 2008 as identified with the Gillam camera. It is shown that the source region of pulsating aurora drifts or expands eastward, away from magnetic midnight, for premidnight onsets and that the spatial evolution is more complicated for postmidnight onsets, which has implications for the source mechanism. The most probable duration of a pulsating aurora event is roughly 1.5 h, while the distribution of possible event durations includes many long (several hours) events. This may suggest that pulsating aurora is not strictly a substorm recovery phase phenomenon but rather a persistent, long‐lived phenomenon that may be temporarily disrupted by auroral substorms. Observations from the Gillam station show that in fact, pulsating aurora is quite common with the occurrence rate increasing to around 60% for morning hours, with 69% of pulsating aurora onsets occurring after substorm breakup.
[1] In this study, we present the first in situ continuous measurements of electron flux modulations in the near-equatorial magnetosphere correlated with pulsating aurora (PA) observations. The contested conjecture that the source of these electrons originates near the equator, made decades ago using sounding rocket data, has now been confirmed using this data. We compared the frequencies of equatorial electron flux pulsations and PA luminosity fluctuations at their ionospheric footprint, using simultaneous satellite-and ground-based data from GOES 13 and Time History of Events and Macroscale Interactions during Substorms (THEMIS) instrumentation. Observations of PA were obtained on 15 March 2008 using a THEMIS All-Sky Imager (ASI) located in northern Canada. The field line footprint of the geostationary GOES 13 satellite, mapped down to the ionosphere at 100 km, falls within the field-of-view of the ASI. We examined electron flux data from the Magnetospheric Electron Detector (MAGED) on GOES 13, in the energy range of 30 to 50 keV, by computing an array of the correlation coefficients between the pixel luminosity for each individual pixel of the ASI images and the flux measurements at the satellite. The results reveal a direct correlation between diffuse luminosity fluctuation periods near the ground and particle pulsation periods. The time variance between the two data sets was examined in order to explore the validity of the equatorial source region premise. The resulting time lag of < 1 s in the PA measurements is consistent with this claim. We also report on a preliminary quantification of the loss cone using the MAGED telescope response functions.
The management of radiation injuries following a catastrophic event where large numbers of people may have been exposed to life-threatening doses of ionizing radiation will rely critically on the availability and use of suitable biodosimetry methods. In vivo electron paramagnetic resonance (EPR) tooth dosimetry has a number of valuable and unique characteristics and capabilities that may help enable effective triage. We have produced a prototype of a deployable EPR tooth dosimeter and tested it in several in vitro and in vivo studies to characterize the performance and utility at the state of the art. This report focuses on recent advances in the technology, which strengthen the evidence that in vivo EPR tooth dosimetry can provide practical, accurate, and rapid measurements in the context of its intended use to help triage victims in the event of an improvised nuclear device. These advances provide evidence that the signal is stable, accurate to within 0.5 Gy, and can be successfully carried out in vivo. The stability over time of the radiation-induced EPR signal from whole teeth was measured to confirm its long-term stability and better characterize signal behavior in the hours following irradiation. Dosimetry measurements were taken for five pairs of natural human upper central incisors mounted within a simple anatomic mouth model that demonstrates the ability to achieve 0.5 Gy standard error of inverse dose prediction. An assessment of the use of intact upper incisors for dose estimation and screening was performed with volunteer subjects who have not been exposed to significant levels of ionizing radiation and patients who have undergone total body irradiation as part of bone marrow transplant procedures. Based on these and previous evaluations of the performance and use of the in vivo tooth dosimetry system, it is concluded that this system could be a very valuable resource to aid in the management of a massive radiological event.
Observations of a pulsating aurora event occurring on 11 February 2008, using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) All‐Sky Imager (ASI) array, indicate a spatially and temporally continuous event with a duration of greater than 15 h and covering a region with a maximum size of greater than 10 h magnetic local time. The optical pulsations are at times locally interrupted or drowned out by auroral substorm activity but are observed in the same location once the discrete aurora recedes. The pulsations following the auroral breakup appear to be brighter and have a larger patch size than before breakup. This suggests that, while the onset of pulsating aurora is not necessarily dependent upon a substorm precursor, the pulsations are affected and possibly enhanced by the substorm process. The long duration of this pulsating aurora event, lasting approximately 8 h without interruption as imaged from Gillam station, is significantly longer than the typical 2–3 h substorm recovery phase, suggesting that pulsating aurora is not strictly a recovery phase phenomenon. This paper is accompanied by a movie of the THEMIS ASI array data, from 0000 to 1715 UT, plotted in mosaic and superimposed onto a map of North America.
In an event where many thousands of people may have been exposed to levels of radiation that are sufficient to cause the acute radiation syndrome, we need technology that can estimate the absorbed dose on an individual basis for triage and meaningful medical decision making. Such dose estimates may be achieved using in vivo electron paramagnetic resonance (EPR) tooth biodosimetry, which measures the number of persistent free radicals that are generated in tooth enamel following irradiation. However, the accuracy of dose estimates may be impacted by individual variations in teeth, especially the amount and distribution of enamel in the inhomogeneous sensitive volume of the resonator used to detect the radicals. In order to study the relationship between interpersonal variations in enamel and EPR-based dose estimates, it is desirable to estimate these parameters nondestructively and without adding radiation to the teeth. Magnetic Resonance Imaging (MRI) is capable of acquiring structural and biochemical information without imparting additional radiation, which may be beneficial for many EPR dosimetry studies. However, the extremely short T2 relaxation time in tooth structures precludes tooth imaging using conventional MRI methods. Therefore, we used zero echo time (ZTE) MRI to image teeth ex vivo to assess enamel volumes and spatial distributions. Using these data in combination with the data on the distribution of the transverse radio frequency magnetic field from electromagnetic simulations, we then can identify possible sources of variations in radiation-induced signals detectable by EPR. Unlike conventional MRI, ZTE applies spatial encoding gradients during the RF excitation pulse, thereby facilitating signal acquisition almost immediately after excitation, minimizing signal loss from short T2 relaxation times. ZTE successfully provided volumetric measures of tooth enamel that may be related to variations that impact EPR dosimetry and facilitate the development of analytical procedures for individual dose estimates.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.