Daniel L. Clarkson

Radio observations of cosmic sources viewed through the turbulent magnetized plasma surrounding the Sun offer unique insights into the properties of plasma turbulence in the solar corona and solar wind, over a wide range of heliocentric distances and latitudes. Using new observations with the MeerKAT radio telescope (a precursor to the mid-frequency Square Kilometre Array; SKA-Mid), we investigate the amplitude of turbulent density fluctuations in the solar corona at about 10 R _⊙ , and we then use this information to evaluate the turbulent energy cascade rate and hence the level of turbulence-associated heating. The remote measurements are compared with MHD simulations of the solar corona and inner heliosphere at high heliospheric latitudes and with Parker Solar Probe equatorial measurements. In equatorial regions, the radio source broadening measurements lead to turbulent energy cascade rates ∼5 × 10 ⁹ erg g ⁻¹ s ⁻¹ at 10 R _⊙ , somewhat consistent with global 3D MHD models and in situ observations; however, higher cascade rates are inferred at high heliospheric latitudes. Our analysis reveals significant variations in the turbulent heating rate ( σ log 10 ϵ ≃ 1.4 ) and source shapes that are more anisotropic than possible in any smooth spherically symmetric turbulence model. Both of these results highlight the influence of the large-scale structure of the solar corona and/or turbulence intermittency on the heating process closer to the Sun. The results provide new insights into the heating of the solar corona and the acceleration of the fast solar wind, especially in regions at high heliospheric latitudes that are not accessible to in situ measurements.

Solar radio noise storms are common phenomena, composed of broadband continuum emission embedded with diverse fine structures, yet their unusually compact apparent sizes remain unexplained. We present frequency-time-resolved imaging spectroscopy of a near-disk-center noise storm observed by the Low Frequency Array between 30 and 40 MHz, together with anisotropic radio-wave scattering simulations. The continuum forms a bright, spatially compact source that drifts across the solar disk over tens of minutes. Across the band, the measured apparent major axis is ∼ 8 . ′ 0 to ∼ 4 . ′ 3 between 31.3 and 38.4 MHz, less than half the size of typical type III burst sources at comparable frequencies. Embedded type I bursts, S-bursts, and spikes exhibit a range of dynamic spectra appearances, yet share nearly identical apparent sizes within uncertainties, suggesting a common size-determining mechanism. Using anisotropic scattering simulations, we show that compact apparent source sizes naturally arise for emission embedded within closed magnetic field structures, where anisotropic turbulence directs radiation away from the observer’s line of sight. Additional modifications arise from enhanced coronal densities, steeper density gradients, reduced turbulence levels, and strong fluctuation anisotropy, but these exert secondary influence. Our results provide a unified explanation for the similar apparent sizes of diverse fine structures in noise storms and demonstrate that the compactness of type I storm sources is governed primarily by the large-scale coronal environment rather than intrinsic differences in emission processes, where the magnetic topology plays a crucial role in determining the observed source size.

Solar radio bursts exhibit complex fine structures that reveal intricate coronal plasma dynamics. Here, we report detection of spike-like repeating burst pairs, characterized by two short-lived (0.1-2 s), narrowband components separated by about 4 s at frequencies 30-50 MHz. Using high-resolution dynamic spectra and spectroscopic imaging, we analyzed 613 burst pairs, measuring their durations, bandwidths, drift rates, flux densities, and spatial characteristics. Imaging links sources to an active region, with earlier components spatially concentrated above the region while delayed components are displaced and exhibit reduced drift rates. Radio-wave propagation simulations support the delayed bursts as turbulent echoes of harmonic emission in anisotropic coronal plasma. The location of the burst sources high in the corona suggests ongoing magnetic reconnection and electron acceleration well above typical flare heights. Our findings offer new insights into coronal turbulence effects while advancing diagnostics of coronal plasma and the elusive nature of solar radio echoes from ground-based transmitters.

Type III solar radio bursts are driven by nonthermal electron beams travelling along heliospheric magnetic fields, with the radio emission frequency drift rate determined by the beam speed and the plasma density profile. Analyzing beam kinematics inferred from the drift rate reveals behavior inconsistent with the emitter moving radially through smooth, monotonically decreasing density. We examine whether these features are driven by disturbances in the guiding magnetic field direction, such as switchbacks, rather than plasma inhomogeneities along the beam path. Using simulations and remote observations of 24 interplanetary type III bursts observed by Parker Solar Probe, we relate measured drift rate variations to local field deflections. In 50% of events, we identify disturbances above a 2 σ noise level that can be attributed to perpendicular deflections of the field between (0.7 and 1.7) R _⊙ , over scales (1.8–6.4) R _⊙ at heliocentric distances (9–30) R _⊙ . The features correspond to either density changes of (10%–30%), or deflections of the field direction by (23°–88°). Further, beam transport simulations show field direction perturbations produce additional observational signatures in type III bursts: delayed emission, intensity breaks, and enhanced emission resembling stria fine structures. In addition, we identified four bursts where the observed variations are more plausibly explained by field deflections, possibly in the form of magnetic switchbacks than by unrealistically large density changes along the field line. The results show that variations in type III burst profiles can arise from magnetic as well as density fluctuations and demonstrate the value of type III bursts as remote probes of inner-heliospheric structure at kilometric wavelengths.

Plasma turbulence cascading from MHD to kinetic scales in the heliospheric plasma is believed to play a key role in coronal heating and fast solar wind acceleration, but the properties of the turbulence remain poorly constrained by observations. Here we compare the ion-scale density fluctuation levels inferred from the properties of solar radio bursts with the magnetic field fluctuation levels obtained through in situ measurements in the inner heliosphere. We find that the observed magnetic and density fluctuation amplitudes are consistent with excitation by kinetic Alfvén waves (KAWs) and/or KAW structures over a broad range of distances from the Sun. We then use the radio diagnostics and the KAW scenario to deduce the radial variation of magnetic fluctuation amplitudes in regions close to the Sun where in situ measurements cannot be obtained. Further, we calculate the energy cascade rate (plasma heating rate) profile over a region that extends from the low corona (∼0.1 R _⊙) into the heliosphere (out to ∼1 au), and compare it to the energy deposition rate required to drive the solar wind. The cascade rate agrees with the available in situ measurements and also provides predictions closer than ∼10 R _⊙ where in situ approaches are not available. The results provide unique diagnostics of the ion-scale plasma turbulence amplitude and energy cascade rate spanning over 3 orders of magnitude in solar distance.

Astrophysical radio sources are embedded in turbulent magnetised environments. In the 1 MHz sky, solar radio bursts are the brightest sources, produced by electrons travelling along magnetic field lines from the Sun through the heliosphere. We demonstrate that the magnetic field not only guides the emitting electrons, but also directs radio waves via anisotropic scattering from density irregularities in the magnetised plasma. Using multi-vantage-point type III solar radio burst observations and anisotropic radio wave propagation simulations, we show that the interplanetary field structure is encoded in the observed radio emission directivity, and that large-scale turbulent channelling of radio waves is present over large distances, even for relatively weak anisotropy in the embedded density fluctuations. Tracing the radio emission at many frequencies (distances), the effects of anisotropic scattering can be disentangled from the electron motion along the interplanetary magnetic field, and the emission source locations are unveiled. Our analysis suggests that magnetic field structures within turbulent media could be reconstructed using radio observations and is found consistent with the Parker field, offering a novel method for remotely diagnosing the large-scale field structure in the heliosphere and other astrophysical plasmas.

Astrophysical radio sources are embedded in turbulent magnetised environments. In the 1 MHz sky, solar radio bursts are the brightest sources, produced by electrons travelling along magnetic field lines from the Sun through the heliosphere. We demonstrate that the magnetic field not only guides the emitting electrons, but also directs radio waves via anisotropic scattering from density irregularities in the magnetised plasma. Using multi-vantage-point type III solar radio burst observations and anisotropic radio wave propagation simulations, we show that the interplanetary field structure is encoded in the observed radio emission directivity, and that large-scale turbulent channelling of radio waves is present over large distances, even for relatively weak anisotropy in the embedded density fluctuations. Tracing the radio emission at many frequencies (distances), the effects of anisotropic scattering can be disentangled from the electron motion along the interplanetary magnetic field, and the emission source locations are unveiled. Our analysis suggests that magnetic field structures within turbulent media could be reconstructed using radio observations and is found consistent with the Parker field, offering a novel method for remotely diagnosing the large-scale field structure in the heliosphere and other astrophysical plasmas.

The fine structures of solar radio bursts reveal complex dynamics in the corona, yet the observed characteristics of these subsecond bursts are additionally complicated by radio wave scattering in the turbulent solar corona. We examine the impact of anisotropic turbulence in radio wave propagation simulations with nonradial magnetic field structures in shaping the morphology, time characteristics, and source positions of fine structures. The apparent sources are found to move along the direction of the magnetic field lines and not along the density gradient, whereas the major axis of the scattered source is perpendicular to the local magnetic field (the scattering anisotropy axis). Using a dipolar magnetic field structure of an active region, we reproduce observed radio fine-structure source motion parallel to the solar limb associated with a coronal loop and provide a natural explanation for puzzling observations of solar radio burst position motions with the Low Frequency Array. Furthermore, the anisotropy aligned with a dipolar magnetic field causes the apparent-source images to bifurcate into two distinct components, with characteristic sizes smaller than in unmagnetized media. The temporal broadening induced by scattering reduces the observed frequency drift rate of fine structures, depending on the contribution of scattering to the time profile. The findings underscore the role of magnetic field geometry and anisotropic scattering for the interpretation of solar radio bursts and highlight that anisotropic scattering produces more than a single source.