Showing papers by "David Long published in 2013"

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

Abstract: A combination of measurements from the Solar Dynamics Observatory and radiospectroscopy data from the Nancay Radioheliograph now details the mechanism that connects coronal mass ejections from the sun and the acceleration of particles to relativistic speeds. A spatial and temporal correlation between a coronal ‘bright front’ and radio emissions associated with electron acceleration demonstrates the fundamental relationship between the two.

Measuring the Magnetic-Field Strength of the Quiet Solar Corona Using “EIT Waves”

Abstract: 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 global-scale PFSS and local-scale 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.

Improved methods for determining the kinematics of coronal mass ejections and coronal waves

Abstract: Context The study of solar eruptive events and associated phenomena is of great importance in the context of solar and heliophysics Coronal mass ejections (CMEs) and coronal waves are energetic manifestations of the restructuring of the solar magnetic field and mass motion of the plasma Characterising this motion is vital for deriving the dynamics of these events and thus understanding the physics driving their initiation and propagation The development and use of appropriate methods for measuring event kinematics is therefore imperative Aims Traditional approaches to the study of CME and coronal wave kinematics do not return wholly accurate nor robust estimates of the true event kinematics and associated uncertainties We highlight the drawbacks of these approaches, and demonstrate improved methods for accurate and reliable determination of the kinematics Methods The Savitzky-Golay filter is demonstrated as a more appropriate fitting technique for CME and coronal wave studies, and a residual resampling bootstrap technique is demonstrated as a statistically rigorous method for the determination of kinematic error estimates and goodness-of-fit tests Results It is shown that the scatter on distance-time measurements of small sample size can significantly limit the ability to derive accurate and reliable kinematics This may be overcome by (i) increasing measurement precision and sampling cadence; and (ii) applying robust methods for deriving the kinematics and reliably determining their associated uncertainties If a priori knowledge exists and a pre-determined model form for the kinematics is available (or indeed any justifiedfitting-form to be tested against the data), then its precision can be examined using a bootstrapping technique to determine the confidence interval associated with the model/fitting parameters Conclusions Improved methods for determining the kinematics of CMEs and coronal waves are demonstrated to great effect, overcoming many issues highlighted in traditional numerical differencing and error propagation techniques

The English Channel ‘tsunami’ of 27 June 2011 : a probable meteorological source

Abstract: On 27 June 2011 a tsunami struck the Yealm Estuary, near Plymouth, and anomalous tides were experienced from Portugal to the Straits of Dover. These events were caused by a meteotsunami driven by convective cells extending from the Bay of Biscay into the English Channel. This paper explains meteotsunamis, their causes, previous occurrences and, finally, what happened on this day.

Improved methods for determining the kinematics of coronal mass ejections and coronal waves

Abstract: The study of solar eruptive events and associated phenomena is of great importance in the context of solar and heliophysics. Coronal mass ejections (CMEs) and coronal waves are energetic manifestations of the restructuring of the solar magnetic field and mass motion of the plasma. Characterising this motion is vital for deriving the dynamics of these events and thus understanding the physics driving their initiation and propagation. The development and use of appropriate methods for measuring event kinematics is therefore imperative. Traditional approaches to the study of CME and coronal wave kinematics do not return wholly accurate nor robust estimates of the true event kinematics and associated uncertainties. We highlight the drawbacks of these approaches, and demonstrate improved methods for accurate and reliable determination of the kinematics. The Savitzky-Golay filter is demonstrated as a more appropriate fitting technique for CME and coronal wave studies, and a residual resampling bootstrap technique is demonstrated as a statistically rigorous method for the determination of kinematic error estimates and goodness-of-fit tests. It is shown that the scatter on distance-time measurements of small sample size can significantly limit the ability to derive accurate and reliable kinematics. This may be overcome by (i) increasing measurement precision and sampling cadence, and (ii) applying robust methods for deriving the kinematics and reliably determining their associated uncertainties. If a priori knowledge exists and a pre-determined model form for the kinematics is available (or indeed any justified fitting-form to be tested against the data), then its precision can be examined using a bootstrapping technique to determine the confidence interval associated with the model/fitting parameters.

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

Abstract: 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.

An Investigation of the CME of 3 November 2011 and its Associated Widespread Solar Energetic Particle Event

Abstract: Multi-spacecraft observations are used to study the in-situ effects of a large CME erupting from the farside of the Sun on 3 November 2011, with particular emphasis on the associated solar energetic particle (SEP) event. At that time both Solar Terrestrial Relations Observatory (STEREO) spacecraft were located more than 90 degrees from Earth and could observe the CME eruption directly, with the CME visible on-disk from STEREO-B and off the limb from STEREO-A. Signatures of pressure variations in the corona such as deflected streamers were seen, indicating the presence of a coronal shock associated with this CME eruption. The evolution of the CME and an associated EUV wave were studied using EUV and coronagraph images. It was found that the lateral expansion of the CME low in the corona closely tracked the propagation of the EUV wave, with measured velocities of 240+/-19 km/s and 221+/-15 km/s for the CME and wave respectively. Solar energetic particles were observed arriving first at STEREO-A, followed by electrons at the Wind spacecraft at L1, then STEREO-B, and finally protons arriving simultaneously at Wind and STEREO-B. By carrying out velocity-dispersion analysis on the particles arriving at each location, it was found that energetic particles arriving at STEREO-A were released first and the release of particles arriving at STEREO-B was delayed by around 50 minutes. Analysis of the expansion of the CME to a wider longitude indicates that this delay is a result of the time taken for the edge of the CME to reach the footpoints of the magnetic-field lines connected to STEREO-B. The CME expansion is not seen to reach the magnetic footpoint of Wind at the time of solar particle release for the particles detected here, suggesting that these particles may not be associated with this CME.