arXiv subjects

Gert J. J. Botha

Publications and source records attributed to Gert J. J. Botha.

At least 19 recordsLinked to original sources

arXiv2026

Energy redistribution during 2D oscillatory reconnection

Magnetic reconnection is a fundamental energy release mechanism in solar and space plasmas. However, the distribution of released magnetic energy among kinetic and internal (thermal) energy during time-dependent reconnection remains largely unexplored. This work aims to study energy conversion in magnetohydrodynamics via oscillatory reconnection (OR) at a 2D magnetic null point. Using a shock-capturing code, we identify three families of shocks: termination shocks, slow-mode deflection shocks, and Petschek-like shocks (even with uniform resistivity). We measure the time-dependent reconnection rate, finding a peak of $R \approx 0.1$, consistent with a fast reconnection regime. For the first time, we track the energy conversion pathways between magnetic, kinetic and internal energy across the time-dependent reconnecting system. We find energy exchange is strongly region dependent, with shock locations dominating over the diffusion region. In the classical incompressible steady-state reconnection models, the magnetic energy is fully converted into internal and kinetic energy. In contrast, we find that in compressible time-dependent reconnection, the interchange between kinetic and internal energy dominates the system (due to the inclusion of plasma compressibility).

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arXiv2026

Mapping Oscillatory Flows in a Giant Chromospheric Spiral

The solar chromosphere is permeated by complex magnetic fields that guide plasma flows and energy into the corona. This work presents a detailed analysis of a unique, high-resolution observation of a giant chromospheric spiral structure that emerges due to a large magnetic pore, captured by the Swedish 1-m Solar Telescope (SST). A comprehensive data analysis pipeline is developed to automatically detect the edges of 2255 plasma flows (loops) that constitute the spiral, and these are used to extract the kinematics of flows propagating along the magnetic field. The analysis reveals three primary insights into the spiral's physics. First, magnetic curvature is correlated with oscillatory flow dynamics, i.e. regions of high loop curvature exhibit a statistically significant excess of higher-order oscillation modes compared to straighter loops; it is also correlated with higher intensity and longer periods. Second, spatial distribution of oscillation period shows an inverse trend, decreasing from $\sim$3.5 minutes in the pore to $\sim$3 minutes in the outer spiral arms. This is interpreted as a signature of the overlying trans-equatorial quadrupolar coronal loop system compressing the pore's field lines into a near-horizontal orientation, producing a period gradient that challenges the standard expanding canopy model. Finally, the emission signature confirms that oscillating threads represent localised channels of brightness that lie within cooler, absorbing loop material. This study provides the first statistical analysis of oscillatory flows in a large-scale spiral, probing energy flow through the chromosphere through curved magnetic structure.

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arXiv2026

Probing the Solar Corona and the Solar Wind Using Angular Broadening Observations with the SKA

Angular broadening observations of compact radio sources provide a powerful method for probing the solar corona and solar wind. Such observations enable studies of the phase structure function, turbulence amplitude, intermediate-scale density fluctuations, solar-wind heating rates, and dissipation scales. When a compact radio source is observed through coronal or solar-wind plasma, several observable effects can arise: (1) the apparent source size increases because of scattering by turbulent plasma, producing angular broadening; (2) the source flux density decreases because of scattering and absorption; (3) the observed angular broadening can be anisotropic, reflecting anisotropic turbulence in the corona and solar wind; and (4) the position angle of the anisotropy, measured from north through east, can help infer the orientation of the coronal magnetic field. These effects provide insights into the physical processes governing the solar wind and its interaction with electromagnetic waves, and they offer constraints on coronal turbulence and magnetic-field structure. At present, angular broadening studies remain limited and have mostly focused on very bright radio sources such as Tau A. The unprecedented sensitivity and angular resolution of the Square Kilometre Array are expected to greatly expand the number of suitable background sources, opening a new window on the solar corona, solar wind, and heliosphere.

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arXiv2026

Multi-wavelength observations of substructures in solar flare ribbons

Solar flare ribbons are extensive brightenings in the chromosphere during flares, often showing fine scale structuring that reflects the underlying energy release. Using high cadence imaging from the Swedish 1-m Solar Telescope/CRISP during an X1.5-class limb flare on 10 June 2014, we identify and track 232 coherent, thread-like substructures, which we term for the first time ``riblets''. From a statistical analysis, riblets have well defined lifetimes and plane-of-sky speeds (typically 5-15 s and 50-150 km/s respectively), establishing them as distinct ribbon substructures. From analysis of their temporal distributions, their distance-time (X-T) evolution uniquely reveal approximately linear and non-linear (accelerating/decelerating) classes, a discrepancy that may be influenced by projection geometry. From analysis of their spatial distributions, we find no clear correspondence between the properties of adjacent riblets, suggesting that local atmospheric conditions (fine-scale thermodynamic and/or magnetic structuring) govern their kinematics more than spatial variations in electron-beam energy flux. From analysis of their spectral distributions, clusters of riblets do show temporal and spatial coincidence with hard X-ray emission signatures, consistent with episodic electron-beam injection into the chromosphere. Using Fermi/GBM spectroscopy, we derive thick-target parameters suitable for flare simulations, with representative values $δ_{\rm low}\approx 5.93$, $E_{\rm c}\approx 24.7$ keV, and an implied beam energy flux $\mathcal{F}_{\rm beam}\approx 1.5\times 10^{10}$ erg cm$^{-2}$ s$^{-1}$ (based on RHESSI footpoint area). Together, these results identify riblets as the fundamental building block of flare ribbons and provide quantitative constraints for forward tests of riblet formation mechanisms.

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arXiv2026

Fast Magnetoacoustic Wave Behavior within Gravitationally Stratified, Magnetically Inhomogeneous Media

The nature of MHD waves within inhomogeneous media is fundamental to understanding and interpreting wave behavior in the solar atmosphere. We investigate fast magnetoacoustic wave behavior within gravitationally stratified, magnetically inhomogeneous media, by studying a magnetic environment containing a simple 2D X-type magnetic null point. The addition of gravitational stratification fundamentally changes the nature of the system, including breaking the symmetry. There are two main governing effects: the stratified density profile acts in combination with the inhomogeneous magnetic field, creating a large gradient in the Alfven speed and hence a system replete with refraction. The system is investigated using both numerical simulations and a semianalytical WKB solution (via Charpit's method and a fourth-order Runge-Kutta solver) and we find strong agreement between both. The results show a fundamental difference between the stratification-free and stratified cases, including the formation of caustic surfaces and cusps, and we contextualize these results in the theoretical understanding of fast magnetoacoustic waves.

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arXiv2026

First Detailed MeerKAT Imaging Spectroscopy of a Solar Flare

Radio observations provide powerful diagnostics of energy release, particle acceleration, and transport processes in solar flares. However, despite recent progress in radio interferometric imaging spectroscopy, current instruments still face limitations in image fidelity and resolution, restricting detailed spectroscopic studies of flaring regions. Here we present high-fidelity imaging spectroscopy of a M1.3 GOES class flare with MeerKAT, a precursor to the future-generation array SKA-Mid. Radio emissions at the observed frequencies typically originate in the low corona, offering valuable insights into magnetic reconnection and primary energy-release sites. The obtained images achieve an unprecedented dynamic range exceeding 10^3, enabling simultaneous analysis of bright coherent bursts and faint incoherent emission from the active region. Multiple spatially distinct coherent sources are identified, implying contributions from different populations of accelerated electrons. The incoherent emission extends beyond AIA structures, highlighting MeerKAT's ability to detect dilute but hot plasma invisible to Extreme Ultraviolet instruments. Combined with co-temporal Hard X-ray images and magnetic field extrapolations, the radio sources are located within distinct magnetic structures, further revealing their association with different populations of accelerated electrons. These results demonstrate MeerKAT imaging spectroscopy as powerful diagnostics of solar flares and pave the way for future solar flare studies with SKA-Mid.

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arXiv2026

Wave generation via oscillatory reconnection at a three-dimensional magnetic null point

This work conducts a three-dimensional (3D), nonlinear magnetohydrodynamic (MHD) simulation to investigate wave generating, time-dependent reconnection around a magnetic null point. A non-periodic perturbation (in the $xz$-plane) triggers oscillatory reconnection (OR) at the 3D null, resulting in a self-sustained oscillation with a constant period $P$. We investigate the response of the system using three distinct wave proxies (compressible parallel, compressible transverse and incompressible parallel) as well as Spectral Proper Orthogonal Decomposition for decoupling and analyzing the resultant MHD wave behavior. We find that OR generates a slow magnetoacoustic wave of period $P$ that propagates outwards in all directions along the spine and fan plane of the 3D null point. We also find the generation of a propagating Alfvén wave of period $P$, exclusively along the $y$-axis in the fan plane, i.e. in the direction perpendicular to the spine motion. These findings provide new insights into waves generated from a 3D null point and their implications for coronal seismology.

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arXiv2025

Time-dependent Turbulent Electron Acceleration and Transport in Solar Flares

Solar flares are explosive releases of magnetic energy stored in the solar corona, driven by magnetic reconnection. These events accelerate electrons, generating hard X-ray emissions and often display Quasi Periodic Pulsations (QPPs) across the energy spectra. However, the energy transfer process remains poorly constrained, with competing theories proposing different acceleration mechanisms. We investigate electron acceleration and transport in a flaring coronal loop by solving a time-dependent Fokker-Planck equation. Our model incorporates transient turbulent acceleration, simulating the effects of impulsive energy input to emulate the dynamics of time-dependent reconnection processes. We compute the density-weighted electron flux, a diagnostic directly comparable to observed X-ray emissions, across the energy and spatial domains from the corona to the chromosphere. We investigate different time-dependent functional forms of the turbulent acceleration, finding that the functional form of the acceleration source maintains its signature across energy bands (1 to 100 keV) with a response time that is energy dependent (with higher energy bands displaying a longer response time). In addition, we find that (a) for a square pulse the switch on and off response time is different; (b) for a sinusoidal input the periodicity is preserved; and (c) for a damped sinusoidal the decay rate increases with density and higher energy bands lose energy faster. This work presents a novel methodology for analyzing electron acceleration and transport in flares driven by time-dependent sources.

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arXiv2025

The periodicity of three-dimensional oscillatory reconnection

Oscillatory reconnection is a dynamic, magnetic relaxation mechanism in which a perturbed null point reverts back to equilibrium via time-dependent reconnection. In this paper, we investigate the long-term periodic signal generated by a three-dimensional (3D) magnetic null point, when it is perturbed by a non-periodic driver, for a variety of driving amplitudes. We solve the 3D nonlinear magnetohydrodynamic (MHD) equations using a bespoke numerical boundary condition (a sponge region) that damps wave reflections and thus allows the long-term periodic signal at the 3D null point to be investigated. We observe multiple cycles of the 3D oscillatory reconnection mechanism for the first time. We find that the periodicity is both constant and independent of the choice of driving amplitude. Furthermore, the resultant time-dependent current density at the null point normalized by the driving amplitude is invariant. We extract a single period for oscillatory reconnection at a 3D null point, opening the future possibility of using this characteristic period as a diagnostic tool to reveal indirectly the fundamental plasma properties of 3D null points.

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arXiv2025

Solar Vortices as Conduits for Magnetoacoustic Waves: Multi-Layer Coupling and Their Role in Atmospheric Heating

The Sun's atmosphere hosts swirling plasma structures, known as solar vortices, which have long been thought to channel wave energy into higher layers. Until now, no direct observations have confirmed their role in the heating of the atmosphere. Here, we present the first direct evidence that solar vortices act as structured waveguides, carrying magnetoacoustic modes (waves that behave like sound waves but travel through magnetized plasma) that leave clear wave-heating signatures. By mapping vortex regions at multiple heights and analysing the waves they contain, we show that magnetoacoustic waves efficiently transfer energy, offset losses from radiation, and dominate energy transport in the lower chromosphere. These results challenge the long-standing assumption that vortices primarily support twisting disturbances traveling along magnetic field lines (Alfven waves), revealing instead that magnetoacoustic modes play the leading role in the lower atmosphere. This redefines the role of vortices in magnetized plasmas and has broader implications for wave-plasma interactions in regions of strong magnetic fields.

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arXiv2025

The coronal power spectrum from MHD mode conversion above sunspots

Sunspots are intense regions of magnetic flux that are rooted deep below the photosphere. It is well established that sunspots host magnetohydrodynamic waves, with numerous observations showing a connection to the internal acoustic (or p-)modes of the Sun. The p-modes are fast waves below the equipartition layer and are thought to undergo a double mode conversion as they propagate upwards into the atmosphere of sunspots, which can generate Alfvénic modes in the upper atmosphere. We employ 2.5D magnetohydrodynamics (MHD) numerical simulations to investigate the adiabatic wave propagation and examine the resulting power spectra of coronal Alfvénic waves. A broadband wave source is used that has a 1D power spectrum which mimics aspects of the observed p-mode power spectrum. We examine magnetoacoustic wave propagation and mode conversion from the photosphere to the corona. Frequency filtering of the upwardly propagating acoustic waves is a natural consequence of a gravitationally stratified atmosphere, and plays a key role in shaping the power spectra of mode converted waves. We demonstrate that the slow, fast magnetoacoustic waves and Alfvén waves above the equipartition layer have similarly shaped power spectra, which are modified versions of the driver spectrum. Notably, the results reveal that the coronal wave power spectra have a peak at a higher frequency than that of the underlying p-mode driver. This matches observations of coronal Alfvénic waves and further supports the role of mode conversion process as a mechanism for Alfvénic wave generation in the Sun's atmosphere.

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arXiv2024

Self-similar solutions of oscillatory reconnection: parameter study of magnetic field strength and background temperature

Oscillatory reconnection is a specific type of time-dependent reconnection which involves periodic changes in the magnetic topology of a null point. The mechanism has been reported for a variety of magnetic field strengths and configurations, background temperatures and densities. All these studies report an oscillation in the current density at the null point, but also report a variety of periods, amplitudes and overall behaviors. We conduct a parametric study for equilibrium magnetic field strength and initial background temperature, solving 2D resistive MHD equations around a magnetic X-point. We introduce a parameter space for the ratio of internal-to-magnetic energy and find self-similar solutions for simulations where this ratio is below 0.1 (which represents a magnetically-dominated environment or, equivalently, a low-beta plasma). Self-similarity can be seen in oscillations in the current density at the null (including amplitude and period), Ohmic heating and the temperature generated via reconnection jets. The parameter space of energy ratios also allows us to contextualize previous studies of the oscillatory reconnection mechanism and bring those different studies together into a single unified understanding.

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arXiv2024

The Effect of Resistivity on the Periodicity of Oscillatory Reconnection

The oscillatory reconnection mechanism is investigated for a parameter study of eight orders of magnitude of resistivity, with a particular interest in the evolution of the oscillating current density at the null point and its associated periodicity. The resistive, nonlinear MHD simulations are solved in 2.5D for different levels of resistivity. Three methods (wavelet analysis, Fourier transform and ANOVA) are used to investigate the effect of resistivity versus resultant period. It is found that there is an independence between the level of background resistivity and the period of the oscillatory reconnection mechanism. Conversely, it is found that resistivity has a significant effect on the maximum amplitude of the current density and the nature of its decay rate, as well as the magnitude of ohmic heating at the null.

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arXiv2024

Spectroscopic Imaging of the Sun with MeerKAT: Opening a New Frontier in Solar Physics

Solar radio emissions provide several unique diagnostics to estimate different physical parameters of the solar corona, which are otherwise simply inaccessible. However, imaging the highly dynamic solar coronal emissions spanning a large range of angular scales at radio wavelengths is extremely challenging. At GHz frequencies, MeerKAT radio telescope is possibly globally the best-suited instrument at present for providing high-fidelity spectroscopic snapshot solar images. Here, we present the first published spectroscopic images of the Sun made using the observations with MeerKAT in the 880-1670 MHz band. This work demonstrates the high fidelity of spectroscopic snapshot MeerKAT solar images through a comparison with simulated radio images at MeerKAT frequencies. The observed images show extremely good morphological similarities with the simulated images. Our analysis shows that below ~900 MHz MeerKAT images can recover essentially the entire flux density from the large angular scale solar disc. Not surprisingly, at higher frequencies, the missing flux density can be as large as ~50%. However, it can potentially be estimated and corrected for. We believe once solar observation with MeerKAT is commissioned, it will enable a host of novel studies, open the door to a large unexplored phase space with significant discovery potential, and also pave the way for solar science with the upcoming Square Kilometre Array-Mid telescope, for which MeerKAT is a precursor.

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arXiv2023

Oscillatory reconnection as a plasma diagnostic in the solar corona

Oscillatory reconnection is a relaxation process in magnetised plasma, with an inherent periodicity that is exclusively dependent on the properties of the background plasma. This study focuses on the seismological prospects of oscillatory reconnection in the solar corona. We perform three sets of parameter studies (for characteristic coronal values of the background magnetic field, density and temperature) using the PLUTO code to solve the fully compressive, resistive MHD equations for a 2D magnetic X-point. From each parameter study, we derive the period of the oscillatory reconnection. We find that this period is inversely proportional to the characteristic strength of the background magnetic field and the square root of the initial plasma temperature, while following a square root dependency upon the equilibrium plasma density. These results reveal an inverse proportionality between the magnitude of the Alfvén speed and the period, as well as the background sound speed and the period. Furthermore, we note that the addition of anisotropic thermal conduction only leads to a small increase in the mean value for the period. Finally, we establish an empirical formula that gives the value for the period in relation to the background magnetic field, density and temperature. This gives us a quantified relation for oscillatory reconnection, to be used as a plasma diagnostic in the solar corona, opening up the possibility of using oscillatory reconnection for coronal seismology.

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arXiv2022

The independence of oscillatory reconnection periodicity from the initial pulse

Oscillatory reconnection can manifest through the interaction between the ubiquitous MHD waves and omnipresent null points in the solar atmosphere and is characterized by an inherent periodicity. In the current study, we focus on the relationship between the period of oscillatory reconnection and the strength of the wave pulse initially perturbing the null point, in a hot coronal plasma. We use the PLUTO code to solve the fully compressive, resistive MHD equations for a 2D magnetic X-point. Using wave pulses with a wide range of amplitudes, we perform a parameter study to obtain values for the period, considering the presence and absence of anisotropic thermal conduction separately. In both cases, we find that the resulting period is independent of the strength of the initial perturbation. The addition of anisotropic thermal conduction only leads to an increase in the mean value for the period, in agreement with our previous study. We also consider a different type of initial driver and we obtain an oscillation period matching the independent trend previously mentioned. Thus, we report for the first time on the independence between the type and strength of the initializing wave pulse and the resulting period of oscillatory reconnection in a hot coronal plasma. This makes oscillatory reconnection a promising mechanism to be used within the context of coronal seismology.

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arXiv2021

Oscillatory Reconnection of a 2D X-point in a hot coronal plasma

Oscillatory reconnection (a relaxation mechanism with periodic changes in connectivity) has been proposed as a potential physical mechanism underpinning several periodic phenomena in the solar atmosphere including, but not limited to, quasi-periodic pulsations (QPPs). Despite its importance, however, the mechanism has never been studied within a hot, coronal plasma. We investigate oscillatory reconnection in a one million Kelvin plasma by solving the fully-compressive, resistive MHD equations for a 2D magnetic X-point under coronal conditions using the PLUTO code. We report on the resulting oscillatory reconnection including its periodicity and decay rate. We observe a more complicated oscillating profile for the current density compared to that found for a cold plasma, due to mode-conversion at the equipartition layer. We also consider, for the first time, the effect of adding anisotropic thermal conduction to the oscillatory reconnection mechanism, and we find this simplifies the spectrum of the oscillation profile and increases the decay rate. Crucially, the addition of thermal conduction does not prevent the oscillatory reconnection mechanism from manifesting. Finally, we reveal a relationship between the equilibrium magnetic field strength, decay rate, and period of oscillatory reconnection, which opens the tantalising possibility of utilizing oscillatory reconnection as a seismological tool.

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arXiv2017

Observational signatures of a kink-unstable coronal flux rope using Hinode/EIS

The signatures of energy release and energy transport for a kink-unstable coronal flux rope are investigated via forward modelling. Synthetic intensity and Doppler maps are generated from a 3D numerical simulation. The CHIANTI database is used to compute intensities for three Hinode/EIS emission lines that cover the thermal range of the loop. The intensities and Doppler velocities at simulation resolution are spatially degraded to the Hinode/EIS pixel size (1\arcsec), convolved using a Gaussian point-spread function (3\arcsec), and exposed for a characteristic time of 50 seconds. The synthetic images generated for rasters (moving slit) and sit-and-stare (stationary slit) are analysed to find the signatures of the twisted flux and the associated instability. We find that there are several qualities of a kink-unstable coronal flux rope that can be detected observationally using Hinode/EIS, namely the growth of the loop radius, the increase in intensity towards the radial edge of the loop, and the Doppler velocity following an internal twisted magnetic field line. However, EIS cannot resolve the small, transient features present in the simulation, such as sites of small-scale reconnection (e.g. nanoflares)

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