arXiv subjects

Hamish A. S. Reid

Publications and source records attributed to Hamish A. S. Reid.

At least 19 recordsLinked to original sources

arXiv2026

The Study of Quasi-Periodic Pulsations in Solar and Stellar Flares with SKA

An intensively studied phenomenon which is not described by the standard flare model are quasi-periodic pulsations (QPP) of the flaring emission. As analysis of the QPP phenomenon intrinsically requires a combination of high time and spatial resolutions, especially in the radio band, the unprecedented capabilities of SKA offer us a unique opportunity to reach a breakthrough progress in the observational study of QPP. The SKA-Mid-frequency band falls in a unique window where both coherent emissions from particle acceleration sites and incoherent gyrosynchrotron emissions from non-thermal particles in coronal loops can be studied. With an additional polarisation dimension and the capability to perform wideband spectroscopic imaging, the QPPs in gyrosynchrotron emission ($\ge 1$~GHz) and plasma emission will help understand the local magnetic field modulation due to active phenomena and the response seen in the particle acceleration observable below ~600~MHz. An incomplete list of specific science questions to be addressed with SKA includes (a) the role of QPP in the energy partition in flares, (b) seismology of flaring sites by QPP of different classes, (c ) differences and similarities between QPP in solar and stellar flares, (d) advancing the standard flare model, (e) the physics of repetitive magnetic reconnection: spontaneous vs induced, (f) ML techniques in the detection, classification and analysis of QPP, (g) QPP in weak flares. The latter topic could be especially advanced with SKA which will allow for high-cadence high fidelity radio imaging of weak energy release events.

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arXiv2026

The SKA as a Probe of Heliospheric Turbulence and Radio-wave Propagation Effects

Density turbulence in the heliosphere can impact traversing radio photons originating from anywhere in the universe, leading to distortions of both the spectroscopic and imaging properties of the radio sources. Extra-solar radio sources exhibit scintillation and angular broadening, while monochromatic signals from spacecraft are spectrally broadened. It was recently demonstrated that such impacts, referred to as radio-wave propagation effects, are particularly significant in observations of solar radio bursts excited through the plasma emission mechanism. A comparison of detailed observations with simulations is required in order to quantify the radio-wave propagation effects and disentangle the true radio-source properties from the observed ones, enabling the diagnosis of the heliospheric environment. Consequently, theoretical advancements and our ability to quantify the heliospheric turbulence depend on the quality and quantity of available observations. SKA pathfinders, like LOFAR, have been key to the significant progress recently achieved, but have also highlighted areas where the available observing capabilities are lacking. The SKA's unrivalled sensitivity will be crucial in identifying and distinguishing fine and faint radio structures, where our ability to model them defines whether we can accurately describe the heliospheric turbulence and deduce the fundamental properties of the radio sources. The SKA will be an indispensable tool in observing solar radio emissions from the Sun to beyond 1 au, complemented by ground-based interferometers that can reach frequencies down to the ionospheric cut-off at 10 MHz and space-based radio instruments which cover frequencies down to a few kHz.

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arXiv2026

Coronal Non-Thermal and Doppler Plasma Flows Driven by Photospheric Flux in 28 Active Regions

Magnetohydrodynamic (MHD) waves and/or the braiding of magnetic field lines are largely thought to be responsible for heating the solar corona, both being mechanisms which are driven by the Sun's photospheric magnetic field. Recent modelling work leads us to expect that such heating mechanisms would be seen in the excess broadening (non-thermal velocity) of coronal spectral emission lines and that larger magnitudes of photospheric magnetic flux would generate more heating, but a direct connection between magnetic flux and spectral line broadening has been difficult to establish. We combine measurements of the photospheric magnetic field from SDO/HMI and non-thermal velocity in log T~6.2 coronal plasma from Hinode/EIS for 28 active regions and find a moderate correlation between the two exists in quiescent active regions, consistent with the photospheric field injecting upward Poynting flux into the solar corona and causing coronal heating. We find that no strong correlation with coronal composition makes it difficult to distinguish between MHD wave heating and magnetic field braiding heating using these diagnostics with current instrumentation.

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arXiv2025

Understanding Solar Flares and Energetic Events: Open questions, observational requirements, and instrumental needs over the coming decade

Solar flares are the largest energy-release events in the Solar System, allowing us to study fundamental physical phenomena under extreme conditions. Those include magnetic reconnection, particle acceleration, radiation transport, and various plasma physics processes, all of which occur throughout the heliosphere and rest of the Universe. Flares and eruptive events are also components of geo-effective space weather. Their impacts from a space weather perspective are numerous, such as harm to satellites, disruption to GPS, communications and power systems, and impacts on passenger air travel. A comprehensive understanding of solar flares is therefore not just a compelling science problem, but also important for national security and infrastructure. This white paper (WP) addresses critical open science questions related to solar flares. Key observations and capabilities required to make significant advancements over the coming decade are identified. The UK has a robust and vibrant solar flare community. We are key partners in international collaborations, and also provide instrumentation for existing and upcoming ESA/NASA/JAXA space missions. Continuing this effort over the coming decade is vital to maintain UK leadership in this field, achieve Solar System Advisory Panel roadmap goals, and to work under the UK Space Agency's National Space Strategy Pillars. Several complementary WPs have been submitted that discuss instruments or concepts that would directly address the observational requirements we describe (including: SPARK, solar optimised IFUs, Solar-C/EUVST, and OSIRIS), as well as required numerical modelling efforts and infrastructure.

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arXiv2024

Estimating the total energy content in escaping accelerated solar electron beams

Quantifying the energy content of accelerated electron beams during solar eruptive events is a key outstanding objective that must be constrained to refine particle acceleration models and understand the electron component of space weather. Previous estimations have used in situ measurements near the Earth, and consequently suffer from electron beam propagation effects. In this study, we deduce properties of a rapid sequence of escaping electron beams that were accelerated during a solar flare on 22 May 2013 and produced type III radio bursts, including the first estimate of energy density from remote sensing observations. We use extreme-ultraviolet observations to infer the magnetic structure of the source active region NOAA 11745, and Nançay Radioheliograph imaging spectroscopy to estimate the speed and origin of the escaping electron beams. Using the observationally deduced electron beam properties from the type III bursts and co-temporal hard X-rays, we simulate electron beam properties to estimate the electron number density and energy in the acceleration region. We find an electron density (above $30\ \mathrm{keV}$) in the acceleration region of $10^{2.5}\ \mathrm{cm}^{-3}$ and an energy density of $2\times10^{-5}\ \mathrm{erg\ cm}^{-3}$. Radio observations suggest the particles travelled a very short distance before they began to produce radio emission, implying a radially narrow acceleration region. A short but plausibly wide slab-like acceleration volume of $10^{26}-10^{28}\ \mathrm{cm}^{3}$ atop the flaring loop arcade could contain a total energy of $10^{23}-10^{25}\ \mathrm{erg}$ ($\sim 100$ beams), which is comparable to energy estimates from previous studies.

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arXiv2024

Imaging a large coronal loop using type U solar radio burst interferometry

Solar radio U-bursts are generated by electron beams traveling along closed magnetic loops in the solar corona. Low-frequency ($<$ 100 MHz) U-bursts serve as powerful diagnostic tools for studying large-sized coronal loops that extend into the middle corona. However, the positive frequency drift component (descending leg) of U-bursts has received less attention in previous studies, as the descending radio flux is weak. In this study, we utilized LOFAR interferometric solar imaging data from a U-burst that has a significant descending leg component, observed between 10 to 90 MHz on June 5th, 2020. By analyzing the radio source centroid positions, we determined the beam velocities and physical parameters of a large coronal magnetic loop that reached just about 1.3 $\rm{R_{\odot}}$ in altitude. At this altitude, we found the plasma temperature to be around 1.1 MK, the plasma pressure around 0.20 $\rm{mdyn,cm^{-2}}$, and the minimum magnetic field strength around 0.07 G. The similarity in physical properties determined from the image suggests a symmetric loop. The average electron beam velocity on the ascending leg was found to be 0.21 c, while it was 0.14 c on the descending leg. This apparent deceleration is attributed to a decrease in the range of electron energies that resonate with Langmuir waves, likely due to the positive background plasma density gradient along the downward loop leg.

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arXiv2023

Solar Electron Beam -- Langmuir Wave Interactions and How They Modify Solar Electron Beam Spectra: Solar Orbiter Observations of a Match Made in the Heliosphere

Solar Orbiter's four in-situ instruments have recorded numerous energetic electron events at heliocentric distances between 0.5 and 1au. We analyse energetic electron fluxes, spectra, pitch angle distributions, associated Langmuir waves, and type III solar radio bursts for 3 events to understand what causes modifications in the electron flux and identify the origin and characteristics of features observed in the electron spectrum. We investigate what electron beam properties and solar wind conditions are associated with Langmuir wave growth and spectral breaks in the electron peak flux as a function of energy. We observe velocity dispersion and quasilinear relaxation in the electron flux caused by the resonant wave-particle interactions in the deca-keV range, at the energies at which we observe breaks in the electron spectrum, co-temporal with the local generation of Langmuir waves. We show, via the evolution of the electron flux at the time of the event, that these interactions are responsible for the spectral signatures observed around 10 and 50keV, confirming the results of simulations by Kontar & Reid (2009). These signatures are independent of pitch angle scattering. Our findings highlight the importance of using overlapping FOVs when working with data from different sensors. In this work, we exploit observations from all in-situ instruments to address, for the first time, how the energetic electron flux is modified by the beam-plasma interactions, and results into specific features to appear in the local spectrum. Our results, corroborated with numerical simulations, can be extended to a wider range of heliocentric distances.

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arXiv2022

Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J burst Observations

Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Frequency-ARray (LOFAR) and Parker Solar Probe (PSP) solar radio dynamic spectrum to identify 27 type III bursts and 27 J-bursts during a solar radio noise storm observed on 10 April 2019. We found that their exciter velocities were similar, implying a common acceleration region that injects electrons along open and closed magnetic structures. We describe a novel technique to estimate the density model in coronal loops from J-burst dynamic spectra, finding typical loop apex altitudes around 1.3 solar radius. At this altitude, the average scale heights were 0.36 solar radius, the average temperature was around 1 MK, the average pressure was 0.7 mdyn cm$^{-2}$, and the average minimum magnetic field strength was 0.13 G. We discuss how these parameters compare with much smaller coronal loops.

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arXiv2021

Localised acceleration of energetic particles by a weak shock in the solar corona

Globally-propagating shocks in the solar corona have long been studied to quantify their involvement in the acceleration of energetic particles. However, this work has tended to focus on large events associated with strong solar flares and fast coronal mass ejections (CMEs), where the waves are sufficiently fast to easily accelerate particles to high energies. Here we present observations of particle acceleration associated with a global wave event which occurred on 1 October 2011. Using differential emission measure analysis, the global shock wave was found to be incredibly weak, with an Alfvén Mach number of ~1.008-1.013. Despite this, spatially-resolved type III radio emission was observed by the Nançay RadioHeliograph at distinct locations near the shock front, suggesting localised acceleration of energetic electrons. Further investigation using a magnetic field extrapolation identified a fan structure beneath a magnetic null located above the source active region, with the erupting CME contained within this topological feature. We propose that a reconfiguration of the coronal magnetic field driven by the erupting CME enabled the weak shock to accelerate particles along field lines initially contained within the fan and subsequently opened into the heliosphere, producing the observed type III emission. These results suggest that even weak global shocks in the solar corona can accelerate energetic particles via reconfiguration of the surrounding magnetic field.

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arXiv2021

Fine structure of type III solar radio bursts from Langmuir wave motion in turbulent plasma

The Sun frequently accelerates near-relativistic electron beams that travel out through the solar corona and interplanetary space. Interacting with their plasma environment, these beams produce type III radio bursts, the brightest astrophysical radio sources seen from the Earth. The formation and motion of type III fine frequency structures is a puzzle but is commonly believed to be related to plasma turbulence in the solar corona and solar wind. Combining a theoretical framework with kinetic simulations and high-resolution radio type III observations using the Low Frequency Array, we quantitatively show that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. Our results show how type III fine structure can be used to remotely analyse the intensity and spectrum of compressive density fluctuations, and can infer ambient temperatures in astrophysical plasma, both significantly expanding the current diagnostic potential of solar radio emission.

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arXiv2020

Solar Flare-CME Coupling Throughout Two Acceleration Phases of a Fast CME

Solar flares and coronal mass ejections (CMEs) are closely coupled through magnetic reconnection. CMEs are usually accelerated impulsively within the low solar corona, synchronized with the impulsive flare energy release. We investigate the dynamic evolution of a fast CME and its associated X2.8 flare occurring on 2013 May 13. The CME experiences two distinct phases of enhanced acceleration, an impulsive one with a peak value of ~5 km s$^{-2}$ followed by an extended phase with accelerations up to 0.7 km s$^{-2}$. The two-phase CME dynamics is associated with a two-episode flare energy release. While the first episode is consistent with the "standard" eruption of a magnetic flux rope, the second episode of flare energy release is initiated by the reconnection of a large-scale loop in the aftermath of the eruption and produces stronger nonthermal emission up to $γ$-rays. In addition, this long-duration flare reveals clear signs of ongoing magnetic reconnection during the decay phase, evidenced by extended HXR bursts with energies up to 100--300 keV and intermittent downflows of reconnected loops for >4 hours. The observations reveal that the two-step flare reconnection substantially contributes to the two-phase CME acceleration, and the impulsive CME acceleration precedes the most intense flare energy release. The implications of this non-standard flare/CME observation are discussed.

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arXiv2020

First Observation of a Type II Solar Radio Burst Transitioning Between a Stationary and Drifting State

Standing shocks are believed to be responsible for stationary Type II solar radio bursts, whereas drifting Type II bursts are excited by moving shocks often related to coronal mass ejections (CMEs). Observations of either stationary or drifting Type II bursts are common, but a transition between the two states has not yet been reported. Here, we present a Type II burst which shows a clear, continuous transition from a stationary to a drifting state, the first observation of its kind. Moreover, band splitting is observed in the stationary parts of the burst, as well as intriguing negative and positive frequency-drift fine structures within the stationary emissions. The relation of the radio emissions to an observed jet and a narrow CME was investigated across multiple wavelengths, and the mechanisms leading to the transitioning Type II burst were determined. We find that a jet eruption generates a streamer-puff CME and that the interplay between the CME-driven shock and the streamer is likely to be responsible for the observed radio emissions.

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arXiv2018

The spectral content of SDO/AIA 1600 and 1700 Å filters from flare and plage observations

The strong enhancement of the ultraviolet emission during solar flares is usually taken as an indication of plasma heating in the lower solar atmosphere caused by the deposition of the energy released during these events. Images taken with broadband ultraviolet filters by the {\em Transition Region and Coronal Explorer} (TRACE) and {\em Atmospheric Imaging Assembly} (AIA 1600 and 1700~Å) have revealed the morphology and evolution of flare ribbons in great detail. However, the spectral content of these images is still largely unknown. Without the knowledge of the spectral contribution to these UV filters, the use of these rich imaging datasets is severely limited. Aiming to solve this issue, we estimate the spectral contributions of the AIA UV flare and plage images using high-resolution spectra in the range 1300 to 1900~Å from the Skylab NRL SO82B spectrograph. We find that the flare excess emission in AIA 1600~Å is { dominated by} the \ion{C}{4} 1550~Å doublet (26\%), \ion{Si}{1} continua (20\%), with smaller contributions from many other chromospheric lines such as \ion{C}{1} 1561 and 1656~Å multiplets, \ion{He}{2} 1640~Å, \ion{Si}{2} 1526 and 1533~Å. For the AIA 1700~Å band, \ion{C}{1} 1656~Å multiplet is the main contributor (38\%), followed by \ion{He}{2} 1640 (17\%), and accompanied by a multitude of other, { weaker} chromospheric lines, with minimal contribution from the continuum. Our results can be generalized to state that the AIA UV flare excess emission is of chromospheric origin, while plage emission is dominated by photospheric continuum emission in both channels.

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arXiv2018

Spatial expansion and speeds of type III electron beam sources in the solar corona

A component of space weather, electron beams are routinely accelerated in the solar atmosphere and propagate through interplanetary space. Electron beams interact with Langmuir waves resulting in type III radio bursts. Electron beams expand along the trajectory, and using kinetic simulations, we explore the expansion as the electrons propagate away from the Sun. Specifically, we investigate the front, peak and back of the electron beam in space from derived radio brightness temperatures of fundamental type III emission. The front of the electron beams travelled at speeds from 0.2c--0.7c, significantly faster than the back of the beam that travelled between 0.12c--0.35c. The difference in speed between the front and the back elongates the electron beams in time. The rate of beam elongation has a 0.98 correlation coefficient with the peak velocity; in-line with predictions from type III observations. The inferred speeds of electron beams initially increase close to the acceleration region and then decrease through the solar corona. Larger starting densities and harder initial spectral indices result in longer and faster type III sources. Faster electron beams have higher beam energy densities, produce type IIIs with higher peak brightness temperatures and shorter FWHM durations. Higher background plasma temperatures also increase speeds, particularly at the back of the beam. We show how our predictions of electron beam evolution influences type III bandwidth and drift-rates. Our radial predictions of electron beam speed and expansion can be tested by the upcoming in situ electron beam measurements made by Solar Orbiter and Parker Solar Probe.

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arXiv2018

Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport

Solar type III radio bursts contain a wealth of information about the dynamics of electron beams in the solar corona and the inner heliosphere; currently unobtainable through other means. However, the motion of different regions of an electron beam (front, middle and back) have never been systematically analysed before. We characterise the type III burst frequency-time evolution using the enhanced resolution of LOFAR in the frequency range 30 to 70 MHz and use this to probe electron beam dynamics. Methods. The rise, peak and decay times with a 0.2 MHz spectral resolution were defined for a collection of 31 type III bursts. The frequency evolution is used to ascertain the apparent velocities of the front, middle and back of the type III sources and the trends are interpreted using theoretical and numerical treatments. The type III time profile was better approximated by an asymmetric Gaussian profile, not an exponential as previously used. Rise and decay times increased with decreasing frequency and showed a strong correlation. Durations were smaller than previously observed. Drift rates from the rise times were faster than from the decay times, corresponding to inferred mean electron beam speeds for the front, middle and back of 0.2, 0.17, 0, 15 c, respectively. Faster beam speeds correlate with smaller type III durations. We also find type III frequency bandwidth decreases as frequency decreases. The different speeds naturally explain the elongation of an electron beam in space as it propagates through the heliosphere. The rate of expansion is proportional to the mean speed of the exciter; faster beams expand faster. Beam speeds are attributed to varying ensembles of electron energies at the front, middle and back of the beam.

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arXiv2017

Imaging Spectroscopy of Type U and J Solar Radio Bursts with LOFAR

Radio U-bursts and J-bursts are signatures of electron beams propagating along magnetic loops confined to the corona. The more commonly observed type III radio bursts are signatures of electron beams propagating along magnetic loops that extend into interplanetary space. Given the prevalence of solar magnetic flux to be closed in the corona, it is an outstanding question why type III bursts are more frequently observed than U-bursts or J-bursts. We use LOFAR imaging spectroscopy between 30-80 MHz of low-frequency U-bursts and J-bursts, for the first time, to understand why electron beams travelling along coronal loops produce radio emission less often. The different radio source positions were used to model the spatial structure of the guiding magnetic flux tube and then deduce the energy range of the exciting electron beams without the assumption of a standard density model. The radio sources infer a magnetic loop 1 solar radius in altitude, with the highest frequency sources starting around 0.6 solar radii. Electron velocities were found between 0.13 c and 0.24 c, with the front of the electron beam travelling faster than the back of the electron beam. The velocities correspond to energy ranges within the beam from 0.7-11 keV to 0.7-43 keV. The density along the loop is higher than typical coronal density models and the density gradient is smaller. We found that a more restrictive range of accelerated beam and background plasma parameters can result in U-bursts or J-bursts, causing type III bursts to be more frequently observed. The large instability distances required before Langmuir waves are produced by some electron beams, and the small magnitude of the background density gradients make closed loops less facilitating for radio emission than loops that extend into interplanetary space.

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arXiv2016

Langmuir Wave Electric Fields Induced by Electron Beams in the Heliosphere

Solar electron beams responsible for type III radio emission generate Langmuir waves as they propagate out from the Sun. The Langmuir waves are observed via in-situ electric field measurements. These Langmuir waves are not smoothly distributed but occur in discrete clumps, commonly attributed to the turbulent nature of the solar wind electron density. Exactly how the density turbulence modulates the Langmuir wave electric fields is understood only qualitatively. Using weak turbulence simulations, we investigate how solar wind density turbulence changes the probability distribution functions, mean value and variance of the beam-driven electric field distributions. Simulations show rather complicated forms of the distribution that are dependent upon how the electric fields are sampled. Generally the higher magnitude of density fluctuations reduce the mean and increase the variance of the distribution in a consistent manor to the predictions from resonance broadening by density fluctuations. We also demonstrate how the properties of the electric field distribution should vary radially from the Sun to the Earth and provide a numerical prediction for the in-situ measurements of the upcoming Solar Orbiter and Solar Probe Plus spacecraft.

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arXiv2016

Coronal type III radio bursts and their X-ray flare and interplanetary type III counterparts

Type III bursts and hard X-rays are both produced by flare energetic electron beams. The link between both emissions has been investigated in many previous studies, but no statistical studies have compared both coronal and interplanetary type III bursts with X-ray flares. Using coronal radio events above 100 MHz exclusively from type III bursts, we revisited long-standing questions: Do all coronal type III bursts have X-ray counterparts. What correlation, if any, occurs between radio and X-ray intensities. What X-ray and radio signatures above 100 MHz occur in connection with interplanetary type III bursts below 14 MHz. We analysed data from 2002 to 2011 starting with coronal type III bursts above 100 MHz. We used RHESSI X-ray data greater than 6 keV to make a list of 321 events that have associated type III bursts and X-ray flares, encompassing at least 28 percent of the initial sample of type III events. We examined the timings, intensities, associated GOES class, and any interplanetary radio signature. For our 321 events, the X-ray emission at 6 keV usually lasted longer than type III burst groups at frequencies greater than 100 MHz. A weak correlation was found between the type III radio flux at frequencies below 327 MHz and the X-ray intensity at 25-50 keV, with an absence of events at high X-ray intensity and low type III radio flux. Interplanetary type III bursts less than 14 MHz were observed for 54 percent of the events, increasing when events were observed with 25-50 keV X-rays. A stronger interplanetary association was present when 25-50 keV count rates were above 250 counts per second or 170 MHz fluxes were greater than 1000 SFU, relating to more energetic electrons above 25 keV and events where magnetic flux tubes extend into the high corona. On average type III bursts increase in flux with decreasing frequency, the rate varies from event to event.

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