arXiv subjects

David M. Long

Publications and source records attributed to David M. Long.

86 records · Page 5Linked to original sources

arXiv2017

Moreton and EUV Waves Associated with an X1.0 Flare and CME Ejection

A Moreton wave was detected in active region (AR) 12017 on 29 March 2014 with very high cadence with the Halpha Solar Telescope for Argentina (HASTA) in association with an X1.0 flare (SOL2014-03-29T17:48). Several other phenomena took place in connection with this event, such as low coronal waves and a coronal mass ejection (CME). We analyze the association between the Moreton wave and the EUV signatures observed with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. These include their low-coronal surface-imprint, and the signatures of the full wave and shock dome propagating outward in the corona. We also study their relation to the white-light CME. We perform a kinematic analysis by tracking the wavefronts in several directions. This analysis reveals a high-directional dependence of accelerations and speeds determined from data at various wavelengths. We speculate that a region of open magnetic field lines northward of our defined radiant point sets favorable conditions for the propagation of a coronal magnetohydrodynamic shock in this direction. The hypothesis that the Moreton wavefront is produced by a coronal shock-wave that pushes the chromosphere downward is supported by the high compression ratio in that region. Furthermore, we propose a 3D geometrical model to explain the observed wavefronts as the chromospheric and low-coronal traces of an expanding and outward-traveling bubble intersecting the Sun. The results of the model are in agreement with the coronal shock-wave being generated by a 3D piston that expands at the speed of the associated rising filament. The piston is attributed to the fast ejection of the filament-CME ensemble, also consistent with the good match between the speed profiles of the low-coronal and white-light shock-waves.

astro-ph.SR↗

arXiv2016

Understanding the Physical Nature of Coronal "EIT Waves"

For almost 20 years the physical nature of globally propagating waves in the solar corona (commonly called "EIT waves") has been controversial and subject to debate. Additional theories have been proposed over the years to explain observations that did not fit with the originally proposed fast-mode wave interpretation. However, the incompatibility of observations made using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory with the fast-mode wave interpretation was challenged by differing viewpoints from the twin Solar Terrestrial Relations Observatory spacecraft and higher spatial/temporal resolution data from the Solar Dynamics Observatory. In this article, we reexamine the theories proposed to explain "EIT waves" to identify measurable properties and behaviours that can be compared to current and future observations. Most of us conclude that "EIT waves" are best described as fast-mode large-amplitude waves/shocks that are initially driven by the impulsive expansion of an erupting coronal mass ejection in the low corona.

astro-ph.SR↗

arXiv2016

Unitarisation of EFT Amplitudes for Dark Matter Searches at the LHC

We propose a new approach to the LHC dark matter search analysis within the effective field theory (EFT) framework by utilising the K-matrix unitarisation formalism. This approach provides a reasonable estimate of the dark matter production cross section at high energies, and hence allows reliable bounds to be placed on the cut-off scale of relevant operators without running into the problem of perturbative unitarity violation. We exemplify this procedure for the effective operator D5 in monojet dark matter searches in the collinear approximation. We compare our bounds to those obtained using the truncation method and identify a parameter region where the unitarisation prescription leads to more stringent bounds.

hep-ph↗

arXiv2016

Comparing Experiments to the Fault-Tolerance Threshold

Achieving error rates that meet or exceed the fault-tolerance threshold is a central goal for quantum computing experiments, and measuring these error rates using randomized benchmarking is now routine. However, direct comparison between measured error rates and thresholds is complicated by the fact that benchmarking estimates average error rates while thresholds reflect worst-case behavior when a gate is used as part of a large computation. These two measures of error can differ by orders of magnitude in the regime of interest. Here we facilitate comparison between the experimentally accessible average error rates and the worst-case quantities that arise in current threshold theorems by deriving relations between the two for a variety of physical noise sources. Our results indicate that it is coherent errors that lead to an enormous mismatch between average and worst case, and we quantify how well these errors must be controlled to ensure fair comparison between average error probabilities and fault-tolerance thresholds.

quant-ph↗

arXiv2015

Strong coronal channelling and interplanetary evolution of a solar storm up to Earth and Mars

The severe geomagnetic effects of solar storms or coronal mass ejections (CMEs) are to a large degree determined by their propagation direction with respect to Earth. There is a lack of understanding of the processes that determine their non-radial propagation. Here we present a synthesis of data from seven different space missions of a fast CME, which originated in an active region near the disk centre and, hence, a significant geomagnetic impact was forecasted. However, the CME is demonstrated to be channelled during eruption into a direction + 37+/-10 degree (longitude) away from its source region, leading only to minimal geomagnetic effects. In situ observations near Earth and Mars confirm the channelled CME motion, and are consistent with an ellipse shape of the CME-driven shock provided by the new Ellipse Evolution model, presented here. The results enhance our understanding of CME propagation and shape, which can help to improve space weather forecasts.

astro-ph.SR↗

arXiv2014

The energetics of a global shock wave in the low solar corona

As the most energetic eruptions in the solar system, coronal mass ejections (CMEs) can produce shock waves at both their front and flanks as they erupt from the Sun into the heliosphere. However, the amount of energy produced in these eruptions, and the proportion of their energy required to produce the waves, is not well characterised. Here we use observations of a solar eruption from 2014 February 25 to estimate the energy budget of an erupting CME and the globally-propagating "EIT wave" produced by the rapid expansion of the CME flanks in the low solar corona. The "EIT wave" is shown using a combination of radio spectra and extreme ultraviolet images to be a shock front with a Mach number greater than one. Its initial energy is then calculated using the Sedov-Taylor blast-wave approximation, which provides an approximation for a shock front propagating through a region of variable density. This approach provides an initial energy estimate of $\approx$2.8$\times$10$^{31}$ ergs to produce the "EIT wave", which is approximately 10% the kinetic energy of the associated CME (shown to be $\approx$2.5$\times$10$^{32}$ ergs). These results indicate that the energy of the "EIT wave" may be significant and must be considered when estimating the total energy budget of solar eruptions.

astro-ph.SR↗

arXiv2014

Quasiperiodic acceleration of electrons by a plasmoid-driven shock in the solar atmosphere

Cosmic rays and solar energetic particles may be accelerated to relativistic energies by shock waves in astrophysical plasmas. On the Sun, shocks and particle acceleration are often associated with the eruption of magnetized plasmoids, called coronal mass ejections (CMEs). However, the physical relationship between CMEs and shock particle acceleration is not well understood. Here, we use extreme ultraviolet, radio and white-light imaging of a solar eruptive event on 22 September 2011 to show that a CME-induced shock (Alfvén Mach number 2.4$^{+0.7}_{-0.8}$) was coincident with a coronal wave and an intense metric radio burst generated by intermittent acceleration of electrons to kinetic energies of 2-46 keV (0.1-0.4 c). Our observations show that plasmoid-driven quasi-perpendicular shocks are capable of producing quasi-periodic acceleration of electrons, an effect consistent with a turbulent or rippled plasma shock surface.

astro-ph.SR↗

arXiv2014

CorPITA: An Automated Algorithm for the Identification and Analysis of Coronal "EIT Waves"

The continuous stream of data available from the Atmospheric Imaging Assembly (AIA) telescopes onboard the Solar Dynamics Observatory (SDO) spacecraft has allowed a deeper understanding of the Sun. However, the sheer volume of data has necessitated the development of automated techniques to identify and analyse various phenomena. In this article, we describe the Coronal Pulse Identification and Tracking Algorithm (CorPITA) for the identification and analysis of coronal "EIT waves". CorPITA uses an intensity-profile technique to identify the propagating pulse, tracking it throughout its evolution before returning estimates of its kinematics. The algorithm is applied here to a data-set from February 2011, allowing its capabilities to be examined and critiqued. This algorithm forms part of the SDO Feature Finding Team initiative and will be implemented as part of the Heliophysics Event Knowledgebase (HEK). This is the first fully automated algorithm to identify and track the propagating "EIT wave" rather than any associated phenomena and will allow a deeper understanding of this controversial phenomenon.

astro-ph.SR↗

arXiv2013

Measuring the Magnetic Field Strength of the Quiet Solar Corona Using "EIT Waves"

Variations in the propagation of globally-propagating disturbances (commonly called "EIT waves") through the low solar corona offer a unique opportunity to probe the plasma parameters of the solar atmosphere. Here, high-cadence observations of two "EIT wave" events taken using the Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) are combined with spectroscopic measurements from the Extreme ultraviolet Imaging Spectrometer (EIS) onboard the Hinode spacecraft and used to examine the variability of the quiet coronal magnetic-field strength. The combination of pulse kinematics from SDO/AIA and plasma density from Hinode/EIS is used to show that the magnetic-field strength is in the range ~2-6 G in the quiet corona. The magnetic-field estimates are then used to determine the height of the pulse, allowing a direct comparison with theoretical values obtained from magnetic-field measurements from the Helioseismic and Magnetic Imager (HMI) onboard SDO using PFSS and local-domain extrapolations. While local-scale extrapolations predict heights inconsistent with prior measurements, the agreement between observations and the PFSS model indicates that "EIT waves" are a global phenomenon influenced by global-scale magnetic field.

astro-ph.SR↗

arXiv2012

Kinematic Properties of Globally-Propagating Waves in the Solar Corona

Globally-propagating coronal bright fronts (CBFs) in the solar corona are among the most dramatic manifestations of solar activity, but are not well understood despite strong links with both solar flares and coronal mass ejections. Extreme UltraViolet (EUV) observations from the STEREO and SDO spacecraft are used here to study their kinematics and morphology. The first STEREO observations of a CBF are presented, with the pulse observed in all available EUV passbands (171, 195, 284 and for the first time, 304A). The pulse displayed similar kinematics in all passbands, although the derived pulse velocity and acceleration were found to be strongly influenced by the observing cadence, implying that previous kinematics may have been underestimated. Different techniques for identifying CBFs and deriving their true kinematics were tested, with traditional techniques shown to be prone to undefined user-dependent errors. This was overcome through the development of a statistically rigorous, semi-automated identification algorithm, which was then used to determine the kinematics of four CBF events observed by STEREO. All of the events studied exhibited clear deceleration as well as increases in both spatial and temporal pulse width, indicating that the CBFs are dispersive. A CBF pulse observed by both STEREO and SDO was also studied using the algorithm, with lower initial velocity and weaker deceleration noted in STEREO observations compared to SDO, reaffirming the effects of image cadence on the derived kinematics. The kinematics obtained using SDO were highly passband dependent, suggesting a compressive nature. Significant pulse broadening was also noted in observations from both spacecraft, allowing the dispersion rate of the pulse to be determined. These results indicate that coronal bright fronts are best interpreted as fast-mode magnetoacoustic waves propagating in an inhomogeneous medium.

astro-ph.SR↗

arXiv2011

The Wave Properties of Coronal Bright Fronts Observed Using SDO/AIA

Coronal bright fronts (CBFs) are large scale wavefronts that propagate though the solar corona at hundreds of kilometers per second. While their kinematics have been studied in detail, many questions remain regarding the temporal evolution of their amplitude and pulse width. Here, contemporaneous high cadence, multi-thermal observations of the solar corona from the Solar Dynamic Observatory (SDO) and Solar TErrestrial RElations Observatory (STEREO) spacecraft are used to determine the kinematics and expansion rate of a CBF wavefront observed on 2010 August 14. The CBF was found to have a lower initial velocity with weaker deceleration in STEREO observations compared to SDO (~340 km/s and -72 m/s/s as opposed to ~410 km/s and -279 m/s/s). The CBF kinematics from SDO were found to be highly passband-dependent, with an initial velocity ranging from 379+/-12 km/s to 460+/-28 km/s and acceleration ranging from -128+/-28 m/s/s to -431+/-86 m/s/s in the 335A and 304A passbands respectively. These kinematics were used to estimate a quiet coronal magnetic field strength range of ~1-2 G. Significant pulse broadening was also observed, with expansion rates of ~130 km/s (STEREO) and ~220 km/s (SDO). By treating the CBF as a linear superposition of sinusoidal waves within a Gaussian envelope, the resulting dispersion rate of the pulse was found to be ~8-13 Mm^2 s^-1. These results are indicative of a fast-mode magnetoacoustic wave pulse propagating through an inhomogeneous medium.

astro-ph.SR↗

arXiv2011

Deceleration and Dispersion of Large-scale Coronal Bright Fronts

One of the most dramatic manifestations of solar activity are large-scale coronal bright fronts (CBFs) observed in extreme ultraviolet (EUV) images of the solar atmosphere. To date, the energetics and kinematics of CBFs remain poorly understood, due to the low image cadence and sensitivity of previous EUV imagers and the limited methods used to extract the features. In this paper, the trajectory and morphology of CBFs was determined in order to investigate the varying properties of a sample of CBFs, including their kinematics and pulse shape, dispersion, and dissipation. We have developed a semi-automatic intensity profiling technique to extract the morphology and accurate positions of CBFs in 2.5-10 min cadence images from STEREO/EUVI. The technique was applied to sequences of 171A and 195A images from STEREO/EUVI in order to measure the wave properties of four separate CBF events. Following launch at velocities of ~240-450kms^{-1} each of the four events studied showed significant negative acceleration ranging from ~ -290 to -60ms^{-2}. The CBF spatial and temporal widths were found to increase from ~50 Mm to ~200 Mm and ~100 s to ~1500 s respectively, suggesting that they are dispersive in nature. The variation in position-angle averaged pulse-integrated intensity with propagation shows no clear trend across the four events studied. These results are most consistent with CBFs being dispersive magnetoacoustic waves.

astro-ph.SR↗

arXiv2010

Large-scale Bright Fronts in the Solar Corona: A Review of "EIT waves"

``EIT waves" are large-scale coronal bright fronts (CBFs) that were first observed in 195 Å images obtained using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the \emph{Solar and Heliospheric Observatory (SOHO)}. Commonly called ``EIT waves", CBFs typically appear as diffuse fronts that propagate pseudo-radially across the solar disk at velocities of 100--700 km s$^{-1}$ with front widths of 50-100 Mm. As their speed is greater than the quiet coronal sound speed ($c_s\leq$200 km s$^{-1}$) and comparable to the local Alfvén speed ($v_A\leq$1000 km s$^{-1}$), they were initially interpreted as fast-mode magnetoacoustic waves ($v_{f}=(c_s^2 + v_A^2)^{1/2}$). Their propagation is now known to be modified by regions where the magnetosonic sound speed varies, such as active regions and coronal holes, but there is also evidence for stationary CBFs at coronal hole boundaries. The latter has led to the suggestion that they may be a manifestation of a processes such as Joule heating or magnetic reconnection, rather than a wave-related phenomena. While the general morphological and kinematic properties of CBFs and their association with coronal mass ejections have now been well described, there are many questions regarding their excitation and propagation. In particular, the theoretical interpretation of these enigmatic events as magnetohydrodynamic waves or due to changes in magnetic topology remains the topic of much debate.

astro-ph.SR↗

arXiv2008

The Kinematics of a Globally Propagating Disturbance in the Solar Corona

The kinematics of a globally propagating disturbance (also known as an ``EIT wave") is discussed using Extreme UltraViolet Imager (EUVI) data Solar Terrestrial Relations Observatory (STEREO). We show for the first time that an impulsively generated propagating disturbance has similar kinematics in all four EUVI passbands (304, 171, 195, and 284 A). In the 304 A passband the disturbance shows a velocity peak of 238+/-20 kms-1 within ~28 minutes of its launch, varying in acceleration from 76 ms-2 to -102 ms-2. This passband contains a strong contribution from a Si XI line (303.32 A) with a peak formation temperature of ~1.6 MK. The 304 A emission may therefore be coronal rather than chromospheric in origin. Comparable velocities and accelerations are found in the coronal 195 A passband, while lower values are found in the lower cadence 284 A passband. In the higher cadence 171 A passband the velocity varies significantly, peaking at 475+/-47 kms-1 within ~20 minutes of launch, with a variation in acceleration from 816 ms-2 to -413 ms-2. The high image cadence of the 171 A passband (2.5 minutes compared to 10 minutes for the similar temperature response 195 A passband) is found to have a major effect on the measured velocity and acceleration of the pulse, which increase by factors of ~2 and ~10, respectively. This implies that previously measured values (e.g., using EIT) may have been underestimated. We also note that the disturbance shows strong reflection from a coronal hole in both the 171 and 195 A passbands. The observations are consistent with an impulsively generated fast-mode magnetoacoustic wave.

astro-ph↗