arXiv subjects

Iver H. Cairns

Publications and source records attributed to Iver H. Cairns.

At least 19 recordsLinked to original sources

arXiv2026

Plasma frequency waves in Earth's electron foreshock

At Earth's quasi-perpendicular bow shock, electrons can be reflected and accelerated to high velocities, forming beams. These beams excite Langmuir and beam-mode waves, which can then be converted to radio waves. We aim to understand the properties and evolution of Langmuir waves excited in the electron foreshock region using the Magnetospheric Multiscale (MMS) mission. We use fields and particle data from the four MMS spacecraft to investigate the properties of Langmuir/Z-mode waves in Earth's electron foreshock. MMS provides extended high-resolution snapshots of the three-dimensional electric field, enabling detailed analysis of wave properties. Probability distributions of the electric field are used to investigate the evolution of the waves and the role of density fluctuations. Distinct spectral peaks near the electron plasma frequency are often observed, suggestive of simultaneous observations of beam-mode and Langmuir waves, as well as nonlinear electrostatic decay of Langmuir waves or reflection off density gradients. In addition, the electric fields often have large perpendicular components, consistent with Z-mode waves. The statistical results show that the electric fields are largest near the electron foreshock boundary with the solar wind. Both the parallel and perpendicular components of the electric field exhibit close to log-normal probability distribution functions, consistent with predictions from Stochastic Growth Theory. These results suggest that small-scale density perturbations in the ambient plasma, in addition to nonlinear three-wave decay, are crucial to the evolution of Langmuir waves and the generation of radio waves. These results apply to Langmuir waves in the solar wind, such as in Type II and Type III solar radio burst source regions, where the same density fluctuations are expected and large-amplitude Langmuir waves with similar properties are observed.

physics.space-ph↗

arXiv2026

Polarimeter to Unify the Corona and Heliosphere (PUNCH)

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a NASA Small Explorer to determine the cross-scale processes that unify the solar corona and heliosphere. PUNCH has two science objectives: (1) understand how coronal structures become the ambient solar wind, and (2) understand the dynamic evolution of transient structures, such as coronal mass ejections, in the young solar wind. To address these objectives, PUNCH uses a constellation of four small spacecraft in Sun-synchronous low Earth orbit, to collect linearly polarized images of the K corona and young solar wind. The four spacecraft each carry one visible-light imager in a 1+3 configuration: a single Narrow Field Imager solar coronagraph captures images of the outer corona at all position angles, and at solar elongations from 1.5 degrees (6 R$_\odot$) to 8 degrees (32 R$_\odot$); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3 degrees (12 R$_\odot$) to 45 degrees (180 R$_\odot$) from the Sun. PUNCH images include linear-polarization data, to enable inferring the three-dimensional structure of visible features without stereoscopy. The instruments are matched in wavelength passband, support overlapping instantaneous fields of view, and are operated synchronously, to act as a single ``virtual instrument'' with a 90 degree wide field of view, centered on the Sun. PUNCH launched in March of 2025 and began science operations in June of 2025. PUNCH has an open data policy with no proprietary period, and PUNCH Science Team Meetings are open to all.

astro-ph.SR↗

arXiv2024

Frequency Dispersed Ion Acoustic Waves in the Near Sun Solar Wind: Signatures of Impulsive Ion Beams

This work reports a novel plasma wave observation in the near-Sun solar wind: frequency-dispersed ion acoustic waves. Similar waves were previously reported in association with interplanetary shocks or planetary bow shocks, but the waves reported here occur throughout the solar wind sunward of $\sim 60$ solar radii, far from any identified shocks. The waves reported here vary their central frequency by factors of 3 to 10 over tens of milliseconds, with frequencies that chirp up or down in time. Using a semi-automated identification algorithm, thousands of wave instances are recorded during each near-Sun orbit of the Parker Solar Probe spacecraft. Wave statistical properties are determined and used to estimate their plasma frame frequency and the energies of protons most likely to be resonant with these waves. Proton velocity distribution functions are explored for one wave interval, and proton enhancements that may be consistent with proton beams are observed. A conclusion from this analysis is that properties of the observed frequency-dispersed ion acoustic waves are consistent with driving by cold, impulsively accelerated proton beams near the ambient proton thermal speed. Based on the large number of observed waves and their properties, it is likely that the impulsive proton beam acceleration mechanism generating these waves is active throughout the inner heliosphere. This may have implications for acceleration of the solar wind.

astro-ph.SR↗

arXiv2023

The Sun's Alfven Surface: Recent Insights and Prospects for the Polarimeter to Unify the Corona and Heliosphere (PUNCH)

The solar wind is the extension of the Sun's hot and ionized corona, and it exists in a state of continuous expansion into interplanetary space. The radial distance at which the wind's outflow speed exceeds the phase speed of Alfvenic and fast-mode magnetohydrodynamic (MHD) waves is called the Alfven radius. In one-dimensional models, this is a singular point beyond which most fluctuations in the plasma and magnetic field cannot propagate back down to the Sun. In the multi-dimensional solar wind, this point can occur at different distances along an irregularly shaped "Alfven surface." In this article, we review the properties of this surface and discuss its importance in models of solar-wind acceleration, angular-momentum transport, MHD waves and turbulence, and the geometry of magnetically closed coronal loops. We also review the results of simulations and data analysis techniques that aim to determine the location of the Alfven surface. Combined with recent perihelia of Parker Solar Probe, these studies seem to indicate that the Alfven surface spends most of its time at heliocentric distances between about 10 and 20 solar radii. It is becoming apparent that this region of the heliosphere is sufficiently turbulent that there often exist multiple (stochastic and time-dependent) crossings of the Alfven surface along any radial ray. Thus, in many contexts, it is more useful to make use of the concept of a topologically complex "Alfven zone" rather than one closed surface. This article also reviews how the Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission will measure the properties of the Alfven surface and provide key constraints on theories of solar-wind acceleration.

astro-ph.SR↗

arXiv2020

A Flare-Type IV Burst Event from Proxima Centauri and Implications for Space Weather

Studies of solar radio bursts play an important role in understanding the dynamics and acceleration processes behind solar space weather events, and the influence of solar magnetic activity on solar system planets. Similar low-frequency bursts detected from active M-dwarfs are expected to probe their space weather environments and therefore the habitability of their planetary companions. Active M-dwarfs produce frequent, powerful flares which, along with radio emission, reveal conditions within their atmospheres. However, to date, only one candidate solar-like coherent radio burst has been identified from these stars, preventing robust observational constraints on their space weather environment. During simultaneous optical and radio monitoring of the nearby dM5.5e star Proxima Centauri, we detected a bright, long-duration optical flare, accompanied by a series of intense, coherent radio bursts. These detections include the first example of an interferometrically detected coherent stellar radio burst temporally coincident with a flare, strongly indicating a causal relationship between these transient events. The polarization and temporal structure of the trailing long-duration burst enable us to identify it as a type IV burst. This represents the most compelling detection of a solar-like radio burst from another star to date. Solar type IV bursts are strongly associated with space weather events such as coronal mass ejections and solar energetic particle events, suggesting that stellar type IV bursts may be used as a tracer of stellar coronal mass ejections. We discuss the implications of this event for the occurrence of coronal mass ejections from Proxima Cen and other active M-dwarfs.

astro-ph.SR↗

arXiv2019

Comprehensive Characterization of Solar Eruptions With Remote and In-Situ Observations, and Modeling: The Major Solar Events on 4 November 2015

Solar energetic particles (SEPs) are an important product of solar activity. They are connected to solar active regions and flares, coronal mass ejections (CMEs), EUV waves, shocks, Type II and III radio emissions, and X-ray bursts. These phenomena are major probes of the partition of energy in solar eruptions, as well as for the organization, dynamics, and relaxation of coronal and interplanetary magnetic fields. Many of these phenomena cause terrestrial space weather, posing multiple hazards for humans and their technology from space to the ground. Since particular flares, shocks, CMEs, and EUV waves produce SEP events but others do not, since propagation effects from the low corona to 1 AU appear important for some events but not others, and since Type II and III radio emissions and X-ray bursts are sometimes produced by energetic particles leaving these acceleration sites, it is necessary to study the whole system with a multi-frequency and multi-instrument perspective that combines both in-situ and remote observations with detailed modelling of phenomena. This article demonstrates this comprehensive approach, and shows its necessity, by analysing a trio of unusual and striking solar eruptions, radio and X-ray bursts, and SEP events that occurred on 4 November 2015. These events show both strong similarities and differences from standard events and each other, despite having very similar interplanetary conditions and only two are sites and CME genesis regions. They are therefore major targets for further in-depth observational studies, and for testing both existing and new theories and models. Based on the very limited modelling available we identify the aspects that are and are not understood, and we discuss ideas that may lead to improved understanding of the SEP, radio, and space-weather events.

astro-ph.SR↗

arXiv2019

Impedance and voltage power spectra of a monopole antenna in a warm plasma -- derivation and application to CubeSats

The impedance for a monopole antenna is derived and compared with the cases for wire dipole and double-sphere antennas. This derivation produces a new expression for the monopole antenna response function, $F_{m1}(x)$. The monopole, wire dipole, and double-sphere response functions are compared by modeling an antenna in Earth's ionospheric plasma (i.e. electrostatic and collisionless) and predicting the antenna capacitance and voltage power spectra for quasi-thermal noise (QTN) and shot noise. The monopole antenna current distribution is assumed to be a half-triangular current distribution (considering only the positive half of the triangular distribution). The predictions for the shot noise and capacitance presented problems, as the integral over wavenumber-space or $k$-space did not converge for large values of $k$. The derived expression therefore remains a current problem and necessitates future work to determine a more general expression. In this paper we bring the problem of an appropriate analytic monopole antenna response function to the attention of the community and outline a number of tests that can be used to verify any future expression.

physics.space-ph↗

arXiv2019

The Low-Frequency Solar Corona in Circular Polarization

We present spectropolarimetric imaging observations of the solar corona at low frequencies (80 - 240 MHz) using the Murchison Widefield Array (MWA). These images are the first of their kind, and we introduce an algorithm to mitigate an instrumental artefact by which the total intensity signal contaminates the polarimetric images due to calibration errors. We then survey the range of circular polarization (Stokes V) features detected in over 100 observing runs near solar maximum during quiescent periods. First, we detect around 700 compact polarized sources across our dataset with polarization fractions ranging from less than 0.5% to nearly 100%. These sources exhibit a positive correlation between polarization fraction and total intensity, and we interpret them as a continuum of plasma emission noise storm (Type I burst) continua sources associated with active regions. Second, we report a characteristic "bullseye" structure observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. The central component does not match the sign expected from thermal bremsstrahlung emission, and we speculate that propagation effects or an alternative emission mechanism may be responsible. Third, we show that the large-scale polarimetric structure at our lowest frequencies is reasonably well-correlated with the line-of-sight (LOS) magnetic field component inferred from a global potential field source surface (PFSS) model. The boundaries between opposite circular polarization signs are generally aligned with polarity inversion lines in the model at a height roughly corresponding to that of the radio limb. This is not true at our highest frequencies, however, where the LOS magnetic field direction and polarization sign are often not straightforwardly correlated.

astro-ph.SR↗

arXiv2019

Mapping Magnetic Field Lines for an Accelerating Solar Wind

Mapping of magnetic field lines is important for studies of the solar wind and the sources and propagation of energetic particles between the Sun and observers. A recently developed mapping approach is generalised to use a more advanced solar wind model that includes the effects of solar wind acceleration, non-radial intrinsic magnetic fields and flows at the source surface/inner boundary, and conservation of angular momentum. The field lines are mapped by stepping along local magnetic field $\bm{B}$ and via a Runge-Kutta algorithm, leading to essentially identical maps. The new model's maps for Carrington rotation CR 1895 near solar minimum (19 April to 15 May 1995) and a solar rotation between CR 2145 and CR 2146 near solar maximum (14 January to 9 February 2014) are compared with the published maps for a constant solar wind model. The two maps are very similar on a large scale near both solar minimum and solar maximum, meaning that the field line orientations, winding angles, and connectivity generally agree very well. However, close inspection shows that the field lines have notable small-scale structural differences. An interpretation is that inclusion of the acceleration and intrinsic azimuthal velocity has significant effects on the local structure of the magnetic field lines. Interestingly, the field lines are more azimuthal for the accelerating solar wind model for both intervals. In addition, predictions for the pitch angle distributions (PADs) for suprathermal electrons agree at the $90$ -- $95\%$ level with observations for both solar wind models for both intervals.

physics.space-ph↗

arXiv2019

Hit or Miss, Arrival Time, and $B_z$ Orientation Predictions of BATS-R-US CME Simulations at 1 AU

Using a refined setup process, we simulated the propagation of six observed Coronal Mass Ejections (CMEs) with the 2012 Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) code from the Sun to the Earth or STEREO A and compared the outputs with observations. A linear relation between the average CME speed below 6 solar radii and the flux rope current is demonstrated and used to tune the simulations. The simulations correctly predict if and when an observable CME shock reaches one astronomical unit (AU). The arrival time predictions of the CME shocks at 1 AU have an accuracy of 0.9 $\pm$ 1.9 hours. The simulated initial CME speeds and average accelerations are close to the model and data of Gopalswamy et al., 2000. The approach shows promise for predicting the sense of the predominant shock-associated change in the magnetic field component $B_z$. However, the magnetic fields and plasma conditions in the solar wind and CME are not predicted well quantitatively.

physics.space-ph↗

arXiv2019

Unsupervised generation of high dynamic range solar images: A novel algorithm for self-calibration of interferometry data

Solar radio emission, especially at metre-wavelengths, is well known to vary over small spectral ($\lesssim$100\,kHz) and temporal ($<1$\,s) spans. It is comparatively recently, with the advent of a new generation of instruments, that it has become possible to capture data with sufficient resolution (temporal, spectral and angular) that one can begin to characterize the solar morphology simultaneously along the axes of time and frequency. This ability is naturally accompanied by an enormous increase in data volumes and computational burden, a problem which will only become more acute with the next generation of instruments such as the Square Kilometre Array (SKA). The usual approach, which requires manual guidance of the calibration process, is impractical. Here we present the "Automated Imaging Routine for Compact Arrays for the Radio Sun (AIRCARS)", an end-to-end imaging pipeline optimized for solar imaging with arrays with a compact core. We have used AIRCARS so far on data from the Murchison Widefield Array (MWA) Phase-I. The dynamic range of the images is routinely from a few hundred to a few thousand. In the few cases, where we have pushed AIRCARS to its limits, the dynamic range can go as high as $\sim$75000. The images made represent a substantial improvement in the state-of-the-art in terms of imaging fidelity and dynamic range. This has the potential to transform the multi-petabyte MWA solar archive from raw visibilities into science-ready images. AIRCARS can also be tuned to upcoming telescopes like the SKA, making it a very useful tool for the heliophysics community.

astro-ph.IM↗

arXiv2018

Densities Probed by Coronal Type III Radio Burst Imaging

We present coronal density profiles derived from low-frequency (80-240 MHz) imaging of three type III solar radio bursts observed at the limb by the Murchison Widefield Array (MWA). Each event is associated with a white light streamer at larger heights and is plausibly associated with thin extreme ultraviolet rays at lower heights. Assuming harmonic plasma emission, we find average electron densities of 1.8 x10^8 cm^-3 down to 0.20 x10^8 cm^-3 at heights of 1.3 to 1.9 solar radii. These values represent roughly 2.4-5.4x enhancements over canonical background levels and are comparable to the highest streamer densities obtained from data at other wavelengths. Assuming fundamental emission instead would increase the densities by a factor of 4. High densities inferred from type III source heights can be explained by assuming that the exciting electron beams travel along overdense fibers or by radio propagation effects that may cause a source to appear at a larger height than the true emission site. We review the arguments for both scenarios in light of recent results. We compare the extent of the quiescent corona to model predictions to estimate the impact of propagation effects, which we conclude can only partially explain the apparent density enhancements. Finally, we use the time- and frequency-varying source positions to estimate electron beam speeds of between 0.24 and 0.60 c.

astro-ph.SR↗

arXiv2017

Type III Solar Radio Burst Source Region Splitting Due to a Quasi-Separatrix Layer

We present low-frequency (80-240 MHz) radio imaging of type III solar radio bursts observed by the Murchison Widefield Array (MWA) on 2015/09/21. The source region for each burst splits from one dominant component at higher frequencies into two increasingly-separated components at lower frequencies. For channels below ~132 MHz, the two components repetitively diverge at high speeds (0.1-0.4 c) along directions tangent to the limb, with each episode lasting just ~2 s. We argue that both effects result from the strong magnetic field connectivity gradient that the burst-driving electron beams move into. Persistence mapping of extreme ultraviolet (EUV) jets observed by the Solar Dynamics Observatory reveals quasi-separatrix layers (QSLs) associated with coronal null points, including separatrix dome, spine, and curtain structures. Electrons are accelerated at the flare site toward an open QSL, where the beams follow diverging field lines to produce the source splitting, with larger separations at larger heights (lower frequencies). The splitting motion within individual frequency bands is interpreted as a projected time-of-flight effect, whereby electrons traveling along the outer field lines take slightly longer to excite emission at adjacent positions. Given this interpretation, we estimate an average beam speed of 0.2 c. We also qualitatively describe the quiescent corona, noting in particular that a disk-center coronal hole transitions from being dark at higher frequencies to bright at lower frequencies, turning over around 120 MHz. These observations are compared to synthetic images based on the Magnetohydrodynamic Algorithm outside a Sphere (MAS) model, which we use to flux-calibrate the burst data.

astro-ph.SR↗

arXiv2016

On the Directivity of Low-Frequency Type IV Radio Bursts

An intense type IV radio burst was observed by the STEREO Behind (STB) spacecraft located about 144 degres behind Earth. The burst was associated with a large solar eruption that occurred on the backside of the Sun (N05E151) close to the disk center in the STB view. The eruption was also observed by the STEREO Ahead (STA) spacecraft (located at 149 degrees ahead of Earth) as an eruption close to the west limb (N05W60) in that view. The type IV burst was complete in STB observations in that the envelope reached the lowest frequency and then receded to higher frequencies. The burst was partial viewed from STA, revealing only the edge coming down to the lowest frequency. The type IV burst was not observed at all near Earth because the source was 61 degrees behind the east limb. The eruption was associated with a low-frequency type II burst observed in all three views, although it was not very intense. Solar energetic particles were also observed at both STEREOs and at SOHO, suggesting that the shock was much extended, consistent with the very high speed of the CME (about 2048 km/s). These observations suggest that the type IV emission is directed along a narrow cone above the flare site. We confirm this result statistically using the type IV bursts of solar cycle 23.

astro-ph.SR↗

arXiv2016

A new angle for probing field-aligned irregularities with the Murchison Widefield Array

Electron density irregularities in the ionosphere are known to be magnetically anisotropic, preferentially elongated along the lines of force. While many studies of their morphology have been undertaken by topside sounding and whistler measurements, it is only recently that detailed regional-scale reconstructions have become possible, enabled by the advent of widefield radio telescopes. Here we present a new approach for visualising and studying field-aligned irregularities (FAIs), which involves transforming interferometric measurements of TEC gradients onto a magnetic shell tangent plane. This removes the perspective distortion associated with the oblique viewing angle of the irregularities from the ground, facilitating the decomposition of dynamics along and across magnetic field lines. We apply this transformation to the dataset of Loi et al. [2015a], obtained on 15 October 2013 by the Murchison Widefield Array (MWA) radio telescope and displaying prominent FAIs. We study these FAIs in the new reference frame, quantifying field-aligned and field-transverse behaviour, examining time and altitude dependencies, and extending the analysis to FAIs on sub-array scales. We show that the inclination of the plane can be derived solely from the data, and verify that the best-fit value is consistent with the known magnetic inclination. The ability of the model to concentrate the fluctuations along a single spatial direction may find practical application to future calibration strategies for widefield interferometry, by providing a compact representation of FAI-induced distortions.

physics.space-ph↗

arXiv2016

Density duct formation in the wake of a travelling ionospheric disturbance: Murchison Widefield Array observations

Geomagnetically-aligned density structures with a range of sizes exist in the near-Earth plasma environment, including 10-100 km-wide VLF/HF wave-ducting structures. Their small diameters and modest density enhancements make them difficult to observe, and there is limited evidence for any of the several formation mechanisms proposed to date. We present a case study of an event on 26 August 2014 where a travelling ionospheric disturbance (TID) shortly precedes the formation of a complex collection of field-aligned ducts, using data obtained by the Murchison Widefield Array (MWA) radio telescope. Their spatiotemporal proximity leads us to suggest a causal interpretation. Geomagnetic conditions were quiet at the time, and no obvious triggers were noted. Growth of the structures proceeds rapidly, within 0.5 hr of the passage of the TID, attaining their peak prominence 1-2 hr later and persisting for several more hours until observations ended at local dawn. Analyses of the next two days show field-aligned structures to be preferentially detectable under quiet rather than active geomagnetic conditions. We used a raster scanning strategy facilitated by the speed of electronic beamforming to expand the quasi-instantaneous field of view of the MWA by a factor of three. These observations represent the broadest angular coverage of the ionosphere by a radio telescope to date.

physics.space-ph↗

arXiv2015

Solar and Heliospheric Physics with the Square Kilometre Array

The fields of solar radiophysics and solar system radio physics, or radio heliophysics, will benefit immensely from an instrument with the capabilities projected for SKA. Potential applications include interplanetary scintillation (IPS), radio-burst tracking, and solar spectral radio imaging with a superior sensitivity. These will provide breakthrough new insights and results in topics of fundamental importance, such as the physics of impulsive energy releases, magnetohydrodynamic oscillations and turbulence, the dynamics of post-eruptive processes, energetic particle acceleration, the structure of the solar wind and the development and evolution of solar wind transients at distances up to and beyond the orbit of the Earth. The combination of the high spectral, time and spatial resolution and the unprecedented sensitivity of the SKA will radically advance our understanding of basic physical processes operating in solar and heliospheric plasmas and provide a solid foundation for the forecasting of space weather events.

astro-ph.SR↗

arXiv2015

Power spectrum analysis of ionospheric fluctuations with the Murchison Widefield Array

Low-frequency, wide field-of-view (FoV) radio telescopes such as the Murchison Widefield Array (MWA) enable the ionosphere to be sampled at high spatial completeness. We present the results of the first power spectrum analysis of ionospheric fluctuations in MWA data, where we examined the position offsets of radio sources appearing in two datasets. The refractive shifts in the positions of celestial sources are proportional to spatial gradients in the electron column density transverse to the line of sight. These can be used to probe plasma structures and waves in the ionosphere. The regional (10-100 km) scales probed by the MWA, determined by the size of its FoV and the spatial density of radio sources (typically thousands in a single FoV), complement the global (100-1000 km) scales of GPS studies and local (0.01-1 km) scales of radar scattering measurements. Our data exhibit a range of complex structures and waves. Some fluctuations have the characteristics of travelling ionospheric disturbances (TIDs), while others take the form of narrow, slowly-drifting bands aligned along the Earth's magnetic field.

astro-ph.IM↗