Kinetic signatures of the region surrounding the X-line in asymmetric (magnetopause) reconnection

Kinetic particle-in-cell simulations are used to identify signatures of the electron diffusion region (EDR) and its surroundings during asymmetric magnetic reconnection. A "shoulder" in the sunward pointing normal electric field (EN > 0) at the reconnection magnetic field reversal is a good indicator of the EDR, and is caused by magnetosheath electron meandering orbits in the vicinity of the x-line. Earthward of the X-line, electrons accelerated by EN form strong currents and crescent-shaped distribution functions in the plane perpendicular to B. Just downstream of the X-line, parallel electric fields create field-aligned crescent electron distribution functions. In the immediate upstream magnetosheath, magnetic field strength, plasma density, and perpendicular electron temperatures are lower than the asymptotic state. In the magnetosphere inflow region, magnetosheath ions intrude resulting in an Earthward pointing electric field and parallel heating of magnetospheric particles. Many of the above properties persist with a guide field of at least unity.Comment: Submitted to Geophysical Research Letter

Evolution of suprathermal seed particle and solar energetic particle abundances

We report on a survey of the composition of solar-wind suprathermal tails and solar energetic particles (SEPs) including data from 1998 to 2010, with a focus on 2007 to 2010. The start of solar cycle 24 included several SEP events that were unusually He-poor. We conclude that these He-poor events are more likely related to Q/M-dependent spectral variations than to seed-particle composition changes. We also find that the quiet-time suprathermal Fe/O ratio during the 2008-2009 solar-minimum was dramatically lower (Fe/O ≤ 0.01) than earlier due in part to very low solar activity, but also suggesting contributions from an oxygen-rich source of suprathermal ions of unknown origin

Radiation risks from large solar energetic particle events

Solar energetic particles (SEPs) constitute a radiation hazard to both humans and hardware in space. Over the past few years there have been significant advances in our knowledge of the composition and energy spectra of SEP events, leading to new insights into the conditions that contribute to the largest events. This paper summarizes the energy spectra and frequency of large SEP events, and discusses the interplanetary conditions that affect the intensity of the largest events

How efficient are coronal mass ejections at accelerating solar energetic particles?

The largest solar energetic particle (SEP) events are thought to be due to particle acceleration at a shock driven by a fast coronal mass ejection (CME). We investigate the efficiency of this process by comparing the total energy content of energetic particles with the kinetic energy of the associated CMEs. The energy content of 23 large SEP events from 1998 through 2003 is estimated based on data from ACE, GOES, and SAMPEX, and interpreted using the results of particle transport simulations and inferred longitude distributions. CME data for these events are obtained from SOHO. When compared to the estimated kinetic energy of the associated coronal mass ejections (CMEs), it is found that large SEP events can extract ~10% or more of the CME kinetic energy. The largest SEP events appear to require massive, very energetic CMEs

Two-Dimensional Estimates of Left Ventricular Strains are Significantly Affected by Through-Plane Motion

Background Advanced measures of cardiac mechanics such as left ventricular (LV) strains can be used in conjunction with classical biomarkers to gauge cardiovascular health and improve prediction of patient outcomes. Several imaging techniques, including displacement-encoded magnetic resonance imaging (DENSE), are used to non-invasively assess cardiac mechanics. These data are predominantly acquired in two dimensions (2D) due to simplified post-processing and shorter acquisition times; however, this type of acquisition and subsequent analysis cannot account for through-plane motion caused by longitudinal contraction of the left ventricle. We hypothesized that through-plane motion has a significant effect on 2D strain estimates. Methods Cine DENSE data were acquired in eight healthy volunteers (Age: 27 ± 3 years) with a 3T Siemens Tim Trio scanner. Short-axis slices with 2.8 mm in-plane resolution and an 8 mm slice thickness were acquired to span the entire LV. Displacements were encoded in both through-plane and in-plane directions with an effective temporal resolution of 34 ms. Endocardial and epicardial boundaries were delineated on the magnitude image of all short axis DENSE images. Radial and circumferential strains were computed based upon the deformation of the myocardium relative to the end-diastolic frame. Through-plane displacements were ignored for 2D analysis. For three-dimensional (3D) analysis, a 3D representation of the myocardium derived from the same endocardial and epicardial boundaries was deformed using the measured displacement field. The resulting radial and circumferential strain values were compared directly between the 2D and 3D analyses using a two-tailed paired t-test. Results Two dimensional processing consistently overestimated radial strain and underestimated circumferential strain. Peak circumferential strain was significantly different at the basal and mid-ventricular segments (p = 0.001 and 0.009, respectively). Peak radial strain decreased from the base to the apex in both 2D and 3D analyses; however, 2D significantly overestimated radial strain at the mid-ventricular and apical slices compared to 3D (p = 0.002). Global peak radial and circumferential strains from 3D were 30 ± 5% and -20 ± 2%, respectively, compared to 36 ± 5% and -18 ± 2% for 2D (both p \u3c 0.001). Conclusions Two-dimensional imaging methods for assessing left ventricular mechanics consistently overestimate radial strain and underestimate circumferential strain when compared to three-dimensional imaging. This limitation of two-dimensional imaging is likely due to the through-plane motion of the heart, which is ignored in two-dimensional techniques but easily accounted for when using three-dimensional techniques. Future research needs to determine the clinical and prognostic significance of this difference. Funding This research was funded in part by an NIH Early Independence Award to BKF (DP5 OD012132); contributions made by local businesses and individuals through a partnership between Kentucky Children\u27s Hospital and Children\u27s Miracle network; and the University of Kentucky Cardiovascular Research Center, grant UL1RR033173 from the National Center for Research Resources (NCRR), funded by the Office of the Director, National Institutes of Health (NIH) and supported by the NIH Roadmap for Medical Research. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding sources

Quantification of Right Ventricular Function from Short-Axis Displacement-Encoded Images

Background Right ventricular (RV) function is important in many disease states, but is difficult to quantify from routine MR imaging. Previous work has shown that long-axis deformation/strain is the most critical contributor to global RV function; however, short-axis datasets allow for better coverage of the RV. Thus it would be ideal to be able to quantify RV long-axis function using short-axis slice orientations. We hypothesized that a stack of three-dimensional (3D) displacement encoded (DENSE) images could reliably quantify longitudinal deformation of the RV to overcome the need for acquiring additional long-axis views of the RV. Methods A contiguous stack of cine short-axis DENSE images encompassing the entire RV was acquired with 3D encoding in eight healthy volunteers (Age: 27 ± 3 years) using a 3T Siemens Tim Trio scanner. Endo- and epicardial boundaries were manually drawn on each image to generate a 3D reconstruction of the RV myocardium. The measured displacement field was used to deform the mesh and longitudinal strains were computed at every point throughout the volume. For comparison to the short-axis stack with 3D encoding, a standard four-chamber DENSE image with two-dimensional in-plane displacement encoding was acquired. Similar to the 3D analysis, a mesh was deformed using the measured displacements and was subsequently used to determine longitudinal RV strain values. For comparison with the four-chamber data, only short-axis points lying within the four-chamber imaging slices were used to compute peak longitudinal strain. All strains were compared using a two-tailed paired t-test. Results Right ventricular longitudinal strains derived from short-axis 3D DENSE images (-20 ± 4%) were comparable to values obtained from four-chamber images (-16 ± 2%) (p = 0.14). In addition to obtaining information solely at the four-chamber/short-axis intersection, we computed a global RV longitudinal strain of -17 ± 2% from 3D DENSE data (p = 0.64 relative to four-chamber only). Bland Altman analysis yielded a non-significant bias of 3 ± 11% between four-chamber and short-axis longitudinal strain estimates. Conclusions We have demonstrated that short-axis 3D DENSE imaging allows for accurate characterization of right ventricular longitudinal strain compared to a standard long-axis four-chamber acquisition which is typically used to look at RV function. In addition, 3D DENSE acquired in a short-axis orientation allows for more complete coverage of the RV compared to acquisitions based on long-axis image planes. It is likely that the more complete assessment of RV function provided by 3D DENSE could potentially improve upon the accuracy, reproducibility and prognostic ability of common echocardiographic techniques such as the tricuspid annular plane systolic excursion (TAPSE), but future work will need to investigate this

Observations of Solar Energetic Particles from ^3He-rich Events over a Wide Range of Heliographic Longitude

A prevailing model for the origin of ^3He-rich solar energetic particle (SEP) events attributes particle acceleration to processes associated with the reconnection between closed magnetic field lines in an active region and neighboring open field lines. The open field from the small reconnection volume then provides a path along which accelerated particles escape into a relatively narrow range of angles in the heliosphere. The narrow width (standard deviation 60°. We present the observations of the ^3He-rich event of 2010 February 7, which was detected at all three spacecraft when they spanned 136° in heliographic longitude. Measured fluences of ^3He in this event were found to have a strong dependence on longitude which is well fit by a Gaussian with standard deviation ~48° centered at the longitude that is connected to the source region by a nominal Parker spiral magnetic field. We discuss several mechanisms for distributing flare-accelerated particles over a wide range of heliographic longitudes including interplanetary diffusion perpendicular to the magnetic field, spreading of a compact cluster of open field lines between the active region and the source surface where the field becomes radial and opens out into the heliosphere, and distortion of the interplanetary field by a preceding coronal mass ejection. Statistical studies of additional ^3He-rich events detected at multiple spacecraft will be needed to establish the relative importance of the various mechanisms

Using a Respiratory Navigator Significantly Reduces Variability When Quantifying Left Ventricular Torsion with Cardiovascular Magnetic Resonance

Background: Left ventricular (LV) torsion is an important indicator of cardiac function that is limited by high inter-test variability (50% of the mean value). We hypothesized that this high inter-test variability is partly due to inconsistent breath-hold positions during serial image acquisitions, which could be significantly improved by using a respiratory navigator for cardiovascular magnetic resonance (CMR) based quantification of LV torsion. Methods: We assessed respiratory-related variability in measured LV torsion with two distinct experimental protocols. First, 17 volunteers were recruited for CMR with cine displacement encoding with stimulated echoes (DENSE) in which a respiratory navigator was used to measure and then enforce variability in end-expiratory position between all LV basal and apical acquisitions. From these data, we quantified the inter-test variability of torsion in the absence and presence of enforced end-expiratory position variability, which established an upper bound for the expected torsion variability. For the second experiment (in 20 new, healthy volunteers), 10 pairs of cine DENSE basal and apical images were each acquired from consecutive breath-holds and consecutive navigator-gated scans (with a single acceptance position). Inter-test variability of torsion was compared between the breath-hold and navigator-gated scans to quantify the variability due to natural breath-hold variation. To demonstrate the importance of these variability reductions, we quantified the reduction in sample size required to detect a clinically meaningful change in LV torsion with the use of a respiratory navigator. Results: The mean torsion was 3.4 ± 0.2°/cm. From the first experiment, enforced variability in end-expiratory position translated to considerable variability in measured torsion (0.56 ± 0.34°/cm), whereas inter-test variability with consistent end-expiratory position was 57% lower (0.24 ± 0.16°/cm, p \u3c 0.001). From the second experiment, natural respiratory variability from consecutive breath-holds translated to a variability in torsion of 0.24 ± 0.10°/cm, which was significantly higher than the variability from navigator-gated scans (0.18 ± 0.06°/cm, p = 0.02). By using a respiratory navigator with DENSE, theoretical sample sizes were reduced from 66 to 16 and 26 to 15 as calculated from the two experiments. Conclusions: A substantial portion (22-57%) of the inter-test variability of LV torsion can be reduced by using a respiratory navigator to ensure a consistent breath-hold position between image acquisitions