Iver Cairns - Profile on Academia.edu (original) (raw)
Papers by Iver Cairns
Geophysical Research Letters, May 25, 2015
Ionization of the Earth's atmosphere by sunlight forms a complex, multilayered plasma environment... more Ionization of the Earth's atmosphere by sunlight forms a complex, multilayered plasma environment within the Earth's magnetosphere, the innermost layers being the ionosphere and plasmasphere. The plasmasphere is believed to be embedded with cylindrical density structures (ducts) aligned along the Earth's magnetic field, but direct evidence for these remains scarce. Here we report the first direct wide-angle observation of an extensive array of field-aligned ducts bridging the upper ionosphere and inner plasmasphere, using a novel ground-based imaging technique. We establish their heights and motions by feature tracking and parallax analysis. The structures are strikingly organized, appearing as regularly spaced, alternating tubes of overdensities and underdensities strongly aligned with the Earth's magnetic field. These findings represent the first direct visual evidence for the existence of such structures.
Publications of the Astronomical Society of Australia, 2001
Beam-driven Langmuir waves in the solar wind are generated just above the electron plasma frequen... more Beam-driven Langmuir waves in the solar wind are generated just above the electron plasma frequency, which fluctuates in the inhomogeneous solar wind plasma. Consequently, propagating Langmuir waves encounter regions in which the wave frequency is less than the local plasma frequency, where they can be reflected, mode converted to transverse electromagnetic waves, and trapped in density wells. The aim here is to investigate Langmuir wave reflection and mode conversion at a linear density gradient for typical solar wind parameters. It is shown that higher mode conversion efficiencies are possible than previously calculated, but that mode conversion occurs in a smaller region of parameter space. In addition, the possibility of detecting mode conversion with in situ spacecraft Langmuir wave observations is discussed.
Journal Of Geophysical Research: Space Physics, Sep 1, 2021
When the interplanetary magnetic field (IMF) is southward, magnetic reconnection occurs at the da... more When the interplanetary magnetic field (IMF) is southward, magnetic reconnection occurs at the dayside low-latitude magnetopause. The reconnection process converts the closed magnetic field lines within the magnetosphere to open magnetic field lines that thread the magnetopause with one footpoint in the high-latitude ionosphere of the Earth, and the other "footpoint" in the solar wind (Cassak & Fuselier, 2016;. The solar wind flow will convect the frozen-in, open field lines over the poles into the tail of the magnetosphere which causes a funnel-shaped "cusp" region in the polar magnetosphere, consisting of open field lines . From the field line geometry presented in Figure for southward IMF, the open magnetic field lines of the cusp on the dayside map to the outer magnetosphere into a region called the low-latitude boundary layer (LLBL). The open magnetic field lines within the LLBL can contain solar wind ions injected across the magnetopause as well as magnetospheric ions that were already present on the field line when reconnection occurred . Typical magnetospheric ion populations within the LLBL consist of low-energy magnetospheric ions (<100 eV) and ring current ions (>10 keV). The ring current ions (e.g., are comprised of energetic ions at 10s of keV drifting westward along the dusk flank from the magnetotail. The low-energy magnetospheric ions have two low-energy contributions : the plasmaspheric plume with typical energies at 10s of eV or less, and the warm plasma cloak (Chappell Abstract On December 08, 2018 the Twin Rocket Investigation of Cusp Electrodynamics 2 (TRICE 2) mission was successfully launched. The mission consisted of two sounding rockets, each carrying a payload capable of measuring electron and ion distributions, electric and magnetic fields, and plasma waves occurring in the northern magnetospheric cusp. This study highlights the ion and wave observations obtained by TRICE 2 in the cusp and observations from the magnetospheric multiscale (MMS) spacecraft at the low-latitude magnetopause two hours prior to the TRICE 2 traversal of the cusp. Within the cusp, typical ion cusp features were observed, that is, energy-latitude dispersion of injected magnetosheath plasma. However, a lower energy population was also measured near the equatorward edge of the cusp on open field lines. Pitch-angle distributions of the low-energy ions suggest that this population was magnetospheric in origin, and not from ionospheric upflows. Data from MMS show that counterstreaming ions were present in the outer magnetosphere and low-latitude boundary layer at similar energies to those observed by TRICE 2 in the cusp. Correlations between the low-energy ions within the cusp and broadband extremely low frequency waves suggest that the low-energy magnetospheric ions that filled the flux tube may have undergone wave-particle interactions. These interactions may cause pitch-angle scattering of low-energy magnetospheric ions closer to the loss cone, thereby allowing them to precipitate into the cusp and be measured by the TRICE 2 sounding rockets.
Annales Geophysicae, Apr 22, 2022
The High-Bandwidth Auroral Rocket (HIBAR) was launched from Poker Flat, Alaska, on 28 January 200... more The High-Bandwidth Auroral Rocket (HIBAR) was launched from Poker Flat, Alaska, on 28 January 2003 at 07:50 UT towards an apogee of 382 km in the nightside aurora. The flight was unique in having three high-frequency (HF) receivers using multiple antennas parallel and perpendicular to the ambient magnetic field, as well as very lowfrequency (VLF) receivers using antennas perpendicular to the magnetic field. These receivers observed five short-lived Langmuir wave bursts lasting from 0.1-0.2 s, consisting of a thin plasma line with frequencies in the range of 2470-2610 kHz that had an associated diffuse feature occurring 5-10 kHz above the plasma line. Both of these waves occurred slightly above the local plasma frequency with amplitudes between 1-100 µV m -1 . The ratio of the parallel to perpendicular components of the plasma line and diffuse feature were used to determine the angle of propagation of these waves with respect to the background magnetic field. These angles were found to be comparable to the theoretical Z-infinity angle that these waves would resonate at. The VLF receiver detected auroral hiss throughout the flight at 5-10 kHz, a frequency matching the difference between the plasma line and the diffuse feature. A dispersion solver, partially informed with measured electron distributions, and associated frequency-and wavevector-matching conditions were employed to determine if the diffuse features could be generated by a nonlinear wave-wave interaction of the plasma line with the lower-frequency auroral hiss waves/lower-hybrid waves. The results show that this interpretation is plausible.
Publications of the Astronomical Society of Australia, 2001
Lower hybrid (LH) drive involves the resonant acceleration of electrons parallel to the magnetic ... more Lower hybrid (LH) drive involves the resonant acceleration of electrons parallel to the magnetic field by lower hybrid waves, often driven by ions with ring or ring-beam distributions. Charge-exchange between hydrogen atoms and protons with relative motions perpendicular to the magnetic field leads to ring distributions of pickup ions, with concomitant perpedicular ion 'heating'. This paper considers the combination of LH drive and charge-exchange in the outflow regions of magnetic reconnection sites in the solar chromosphere and lower corona, showing that the combined mechanism naturally predicts major perpendicular ion heating and parallel electron acceleration, and exploring the mechanism's relevance to specific solar reconnection phenomena, heating of the solar atmosphere, and production of energetic electrons that generate solar radio emission. Although primarily qualitative, analysis shows that the mechanism has numerous attractive aspects, including perpendicular ion heating that increases linearly with ion mass, parallel electron acceleration, predicted ion and electron temperatures that span those of the chromosphere and lower corona, and parallel electron speeds spanning those for type III bursts. Applications to chromospheric explosive events and low-lying active regions, and to heating the chromosphere, appear particularly suitable. Sweeping of plasma frozen-in to chromospheric and coronal magnetic field lines across the neutral atmosphere due to motions of sub-photospheric fields represents an obvious and important generalisation of the mechanism away from reconnection sites. The requirements that the neutrals not be strongly collisionally coupled to the plasma and that sufficient neutrals are available for charge-exchange restricts the LH drive mechanism to above the photosphere but below where the corona is essentially fully ionised. LH drive may thus be important in heating the chromosphere and low corona while other heating mechanisms dominate at higher altitudes. Although attractive thus far, quantitative analyses of LH drive in these contexts are necessary before definitive conclusions are reached.
Journal Of Geophysical Research: Space Physics, Jun 1, 2015
The Langmuir waves associated with an interplanetary type II source region are presented. The typ... more The Langmuir waves associated with an interplanetary type II source region are presented. The type II burst was first observed on 29 November 2013 by STEREO A and B, with the shock crossing STEREO A on 1 December 2013. In the foreshock region upstream of the shock, 11 Langmuir-like waveforms were recorded by STEREO A's Time Domain Sampler on three orthogonal antennas. The observed Langmuir wave electric fields are of large amplitude and aligned with the background magnetic field. Some of the waveforms show evidence of electrostatic decay, and several are consistent with Langmuir eigenmodes of density wells. Harmonic electric fields are observed simultaneously with the Langmuir waveforms and are consistent with fields produced by nonlinear currents. The beam speeds v b exciting the Langmuir waves are estimated from the waveform data, yielding speeds v b ≈ (0.01-0.04)c. These are consistent with previous observations. The beam speeds are slower than those associated with type III solar radio bursts, consistent with the Langmuir wave electric fields being field aligned. The evidence found for electrostatic decay and against strong perpendicular fields, and so low-wave number Langmuir/z-mode waves, suggests that the dominant emission mechanisms for this type II foreshock involve electrostatic decay and nonlinear wave processes, rather than linear-mode conversion. Harmonic radio emission via antenna mechanisms involving Langmuir waves remains possible.
Publications of the Astronomical Society of Australia, 2017
We present techniques developed to calibrate and correct Murchison Widefield Array low-frequency ... more We present techniques developed to calibrate and correct Murchison Widefield Array low-frequency (72-300 MHz) radio observations for polarimetry. The extremely wide field-of-view, excellent instantaneous (u, v)-coverage and sensitivity to degree-scale structure that the Murchison Widefield Array provides enable instrumental calibration, removal of instrumental artefacts, and correction for ionospheric Faraday rotation through imaging techniques. With the demonstrated polarimetric capabilities of the Murchison Widefield Array, we discuss future directions for polarimetric science at low frequencies to answer outstanding questions relating to polarised source counts, source depolarisation, pulsar science, low-mass stars, exoplanets, the nature of the interstellar and intergalactic media, and the solar environment.
Bulletin of the American Physical Society, Nov 12, 2007
Extraordinary-Mode Radiation Produced by Linear-Mode Conversion of Langmuir Waves EUN-HWA KIM, Pr... more Extraordinary-Mode Radiation Produced by Linear-Mode Conversion of Langmuir Waves EUN-HWA KIM, Princeton University, IVER CAIRNS, PETER ROBINSON, University of Sydney -Linear-mode conversion (LMC) of Langmuir waves to radiation near the plasma frequency at density gradients is important for laboratory, space, and astrophysical phenomena. We study LMC in warm magnetized plasmas using numerical electron fluid simulations when the density gradient is parallel to the ambient magnetic field (B 0 ). We demonstrate that LMC can produce extraordinary (x) mode as well as ordinary (o) mode radiation from Langmuir waves, contrary to earlier expectations of o-mode only. Equal amounts of o-and x-mode radiation are produced in the unmagnetized limit. The x-mode efficiency decreases as B 0 increases while the o-mode efficiency oscillates due to an interference phenomenon between incoming and reflected Langmuir or z-modes. Both x-and o-mode should be produced for typical coronal and interplanetary parameters, alleviating the long-standing depolarization problem for type III solar radio bursts.
The Astrophysical Journal, 2019
Solar radio emission, especially at metre-wavelengths, is well known to vary over small spectral ... more Solar radio emission, especially at metre-wavelengths, is well known to vary over small spectral ( 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 ∼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.
Publications of the Astronomical Society of Australia, 2018
We describe the motivation and design details of the ‘Phase II’ upgrade of the Murchison Widefiel... more We describe the motivation and design details of the ‘Phase II’ upgrade of the Murchison Widefield Array radio telescope. The expansion doubles to 256 the number of antenna tiles deployed in the array. The new antenna tiles enhance the capabilities of the Murchison Widefield Array in several key science areas. Seventy-two of the new tiles are deployed in a regular configuration near the existing array core. These new tiles enhance the surface brightness sensitivity of the array and will improve the ability of the Murchison Widefield Array to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ∼3.5. The remaining 56 tiles are deployed on long baselines, doubling the maximum baseline of the array and improving the arrayu, vcoverage. The improved imaging capabilities will provide an order of magnitude improvement in the noise floor of Murchison Widefield Array continuum images. The upgrade retains all of the features that have underpinned the Murchison Widefie...
Radio Science, 2016
Electron density irregularities in the ionosphere are known to be magnetically anisotropic, prefe... more 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 visualizing and studying field‐aligned irregularities (FAIs), which involves transforming interferometric measurements of total electron content 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 data set of Loi et al. (2015a), obtained on 15 October 2013 by the Murchison Widefield Array (MWA) radio telescope and displaying prominent F...
Journal of Geophysical Research: Space Physics, 2016
Geomagnetically aligned density structures with a range of sizes exist in the near-Earth plasma e... more 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 h of the passage of the TID, attaining their peak prominence 1-2 h later and persisting for several more hours until observations ended at local dawn. Analyses of the next 2 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 3. These observations represent the broadest angular coverage of the ionosphere by a radio telescope to date.
Geophysical Research Letters, Dec 15, 1995
Detailed first estimates are presented of angular broadening in the outer heliosphere due to scat... more Detailed first estimates are presented of angular broadening in the outer heliosphere due to scattering of radio waves by density irregularities: The application is to the 2-3 kHz radiation observed by Voyager. Two plausible turbulence models, which account very well for scattering within 1 AU, are extrapolated beyond 10 AU. Both models predict significant angular broadening in the outer heliosphere, accounting semiquantitatively alone for the source sizes inferred from roll modulation data. Predictions are presented for radial variations in the apparent source size if scattering is important. Comparisons with available data argue that scattering is important (and indeed is the dominant contributor to the apparent source size) and that the radiation source is located in the outer heliosphere. Other evidence that scattering is important, such as the fluctuations in apparent source direction and intensity, are also identified. The effects of scattering should be included in future analyses of the 2-3 kHz emissions.
Predicting radio emission from lunar radio anomolies
EAEJA, Apr 1, 2003
Quasilinear Simulation of Harmonic Electromagnetic Emission via Beam-driven Langmuir Waves in the Solar Wind
AGU Fall Meeting Abstracts, Dec 1, 2004
ABSTRACT
Statistics of Waveform and Envelope Fields: Theory, Simulations and Initial Applications to TRICE Data
AGU Fall Meeting Abstracts, Dec 1, 2008
ABSTRACT Plasma waves in space are almost invariably bursty and widely variable in amplitude, mot... more ABSTRACT Plasma waves in space are almost invariably bursty and widely variable in amplitude, motivating statistical approaches such as stochastic growth theory. Recent wave experiments on rockets moving through Earth's auroral regions, as well as the STEREO and Wind spacecraft, have sufficient time resolution to measure the waveform as well as the envelope field. Typically, however, experiments measure the envelope field averaged over long times compared with the wave period. Four sets of new contributions are presented. First, analytic theory is used to predict the distribution of waveform fields for a single mode with known distribution of envelope fields. The distribution P(log Ew) of waveform fields Ew is shown to be proportional to the rectified field Ewa with a ≈ 1.0 for a number of special cases of the distribution P(log Ee) of envelope field Ee. This form arises due to P(log Ew) being proportional to an integral over P(log Ee) that has a square-root singularity in Ee2. Numerical calculations confirm and extend this prediction to wide range of envelope distributions. Second, ensembles of stochastically-driven waves are simulated and the distributions P(log Ew) and P(log Ee) calculated. While small differences exist between the case of a single mode and multiple modes, it is found in general that the results are independent of the product of the wave frequency and decorrelation time. Of importance here is that the distributions P(log Ew) are found to be power-law with index ≈ 1.0 at low Ew, consistent with the analytic prediction. Moreover, the envelope distribution is found to be well fit by the form P(log Ee) ∝ Ee2 exp(- Ee2 / Eth2). This form applies to one- dimensional thermal waves and now, unexpectedly, also to waves driven stochastically near marginal stability. Third, initial calculations show that averaging (boxcar and sliding averages, whether linear or logarithmic) over multiple wave periods leads to both the envelope and waveform distributions being well fitted by lognormal distributions. Fourth, initial comparisons are made with Langmuir-like waves observed in Earth's cusp region by the TRICE rocket. It appears that the foregoing analytic and numerical calculations explain semi-quantitatively the power-law form and index near 1.0 for the waveform distribution of unaveraged fields, the functional form of the envelope distribution of unaveraged fields, and the transition of the waveform and envelope distributions towards lognormal forms with averaging over multiple wave periods. The waves appear consistent with stochastic growth. The theory and simulation results extend stochastic growth theory to measurements on timescales less than or close to the wave period.
Analytic and Numerical Modelling of Plasma Radiation from Earth's Foreshock
AGUFM, Dec 1, 2001
ABSTRACT
Geophysical Research Letters, May 25, 2015
Ionization of the Earth's atmosphere by sunlight forms a complex, multilayered plasma environment... more Ionization of the Earth's atmosphere by sunlight forms a complex, multilayered plasma environment within the Earth's magnetosphere, the innermost layers being the ionosphere and plasmasphere. The plasmasphere is believed to be embedded with cylindrical density structures (ducts) aligned along the Earth's magnetic field, but direct evidence for these remains scarce. Here we report the first direct wide-angle observation of an extensive array of field-aligned ducts bridging the upper ionosphere and inner plasmasphere, using a novel ground-based imaging technique. We establish their heights and motions by feature tracking and parallax analysis. The structures are strikingly organized, appearing as regularly spaced, alternating tubes of overdensities and underdensities strongly aligned with the Earth's magnetic field. These findings represent the first direct visual evidence for the existence of such structures.
Publications of the Astronomical Society of Australia, 2001
Beam-driven Langmuir waves in the solar wind are generated just above the electron plasma frequen... more Beam-driven Langmuir waves in the solar wind are generated just above the electron plasma frequency, which fluctuates in the inhomogeneous solar wind plasma. Consequently, propagating Langmuir waves encounter regions in which the wave frequency is less than the local plasma frequency, where they can be reflected, mode converted to transverse electromagnetic waves, and trapped in density wells. The aim here is to investigate Langmuir wave reflection and mode conversion at a linear density gradient for typical solar wind parameters. It is shown that higher mode conversion efficiencies are possible than previously calculated, but that mode conversion occurs in a smaller region of parameter space. In addition, the possibility of detecting mode conversion with in situ spacecraft Langmuir wave observations is discussed.
Journal Of Geophysical Research: Space Physics, Sep 1, 2021
When the interplanetary magnetic field (IMF) is southward, magnetic reconnection occurs at the da... more When the interplanetary magnetic field (IMF) is southward, magnetic reconnection occurs at the dayside low-latitude magnetopause. The reconnection process converts the closed magnetic field lines within the magnetosphere to open magnetic field lines that thread the magnetopause with one footpoint in the high-latitude ionosphere of the Earth, and the other "footpoint" in the solar wind (Cassak & Fuselier, 2016;. The solar wind flow will convect the frozen-in, open field lines over the poles into the tail of the magnetosphere which causes a funnel-shaped "cusp" region in the polar magnetosphere, consisting of open field lines . From the field line geometry presented in Figure for southward IMF, the open magnetic field lines of the cusp on the dayside map to the outer magnetosphere into a region called the low-latitude boundary layer (LLBL). The open magnetic field lines within the LLBL can contain solar wind ions injected across the magnetopause as well as magnetospheric ions that were already present on the field line when reconnection occurred . Typical magnetospheric ion populations within the LLBL consist of low-energy magnetospheric ions (<100 eV) and ring current ions (>10 keV). The ring current ions (e.g., are comprised of energetic ions at 10s of keV drifting westward along the dusk flank from the magnetotail. The low-energy magnetospheric ions have two low-energy contributions : the plasmaspheric plume with typical energies at 10s of eV or less, and the warm plasma cloak (Chappell Abstract On December 08, 2018 the Twin Rocket Investigation of Cusp Electrodynamics 2 (TRICE 2) mission was successfully launched. The mission consisted of two sounding rockets, each carrying a payload capable of measuring electron and ion distributions, electric and magnetic fields, and plasma waves occurring in the northern magnetospheric cusp. This study highlights the ion and wave observations obtained by TRICE 2 in the cusp and observations from the magnetospheric multiscale (MMS) spacecraft at the low-latitude magnetopause two hours prior to the TRICE 2 traversal of the cusp. Within the cusp, typical ion cusp features were observed, that is, energy-latitude dispersion of injected magnetosheath plasma. However, a lower energy population was also measured near the equatorward edge of the cusp on open field lines. Pitch-angle distributions of the low-energy ions suggest that this population was magnetospheric in origin, and not from ionospheric upflows. Data from MMS show that counterstreaming ions were present in the outer magnetosphere and low-latitude boundary layer at similar energies to those observed by TRICE 2 in the cusp. Correlations between the low-energy ions within the cusp and broadband extremely low frequency waves suggest that the low-energy magnetospheric ions that filled the flux tube may have undergone wave-particle interactions. These interactions may cause pitch-angle scattering of low-energy magnetospheric ions closer to the loss cone, thereby allowing them to precipitate into the cusp and be measured by the TRICE 2 sounding rockets.
Annales Geophysicae, Apr 22, 2022
The High-Bandwidth Auroral Rocket (HIBAR) was launched from Poker Flat, Alaska, on 28 January 200... more The High-Bandwidth Auroral Rocket (HIBAR) was launched from Poker Flat, Alaska, on 28 January 2003 at 07:50 UT towards an apogee of 382 km in the nightside aurora. The flight was unique in having three high-frequency (HF) receivers using multiple antennas parallel and perpendicular to the ambient magnetic field, as well as very lowfrequency (VLF) receivers using antennas perpendicular to the magnetic field. These receivers observed five short-lived Langmuir wave bursts lasting from 0.1-0.2 s, consisting of a thin plasma line with frequencies in the range of 2470-2610 kHz that had an associated diffuse feature occurring 5-10 kHz above the plasma line. Both of these waves occurred slightly above the local plasma frequency with amplitudes between 1-100 µV m -1 . The ratio of the parallel to perpendicular components of the plasma line and diffuse feature were used to determine the angle of propagation of these waves with respect to the background magnetic field. These angles were found to be comparable to the theoretical Z-infinity angle that these waves would resonate at. The VLF receiver detected auroral hiss throughout the flight at 5-10 kHz, a frequency matching the difference between the plasma line and the diffuse feature. A dispersion solver, partially informed with measured electron distributions, and associated frequency-and wavevector-matching conditions were employed to determine if the diffuse features could be generated by a nonlinear wave-wave interaction of the plasma line with the lower-frequency auroral hiss waves/lower-hybrid waves. The results show that this interpretation is plausible.
Publications of the Astronomical Society of Australia, 2001
Lower hybrid (LH) drive involves the resonant acceleration of electrons parallel to the magnetic ... more Lower hybrid (LH) drive involves the resonant acceleration of electrons parallel to the magnetic field by lower hybrid waves, often driven by ions with ring or ring-beam distributions. Charge-exchange between hydrogen atoms and protons with relative motions perpendicular to the magnetic field leads to ring distributions of pickup ions, with concomitant perpedicular ion 'heating'. This paper considers the combination of LH drive and charge-exchange in the outflow regions of magnetic reconnection sites in the solar chromosphere and lower corona, showing that the combined mechanism naturally predicts major perpendicular ion heating and parallel electron acceleration, and exploring the mechanism's relevance to specific solar reconnection phenomena, heating of the solar atmosphere, and production of energetic electrons that generate solar radio emission. Although primarily qualitative, analysis shows that the mechanism has numerous attractive aspects, including perpendicular ion heating that increases linearly with ion mass, parallel electron acceleration, predicted ion and electron temperatures that span those of the chromosphere and lower corona, and parallel electron speeds spanning those for type III bursts. Applications to chromospheric explosive events and low-lying active regions, and to heating the chromosphere, appear particularly suitable. Sweeping of plasma frozen-in to chromospheric and coronal magnetic field lines across the neutral atmosphere due to motions of sub-photospheric fields represents an obvious and important generalisation of the mechanism away from reconnection sites. The requirements that the neutrals not be strongly collisionally coupled to the plasma and that sufficient neutrals are available for charge-exchange restricts the LH drive mechanism to above the photosphere but below where the corona is essentially fully ionised. LH drive may thus be important in heating the chromosphere and low corona while other heating mechanisms dominate at higher altitudes. Although attractive thus far, quantitative analyses of LH drive in these contexts are necessary before definitive conclusions are reached.
Journal Of Geophysical Research: Space Physics, Jun 1, 2015
The Langmuir waves associated with an interplanetary type II source region are presented. The typ... more The Langmuir waves associated with an interplanetary type II source region are presented. The type II burst was first observed on 29 November 2013 by STEREO A and B, with the shock crossing STEREO A on 1 December 2013. In the foreshock region upstream of the shock, 11 Langmuir-like waveforms were recorded by STEREO A's Time Domain Sampler on three orthogonal antennas. The observed Langmuir wave electric fields are of large amplitude and aligned with the background magnetic field. Some of the waveforms show evidence of electrostatic decay, and several are consistent with Langmuir eigenmodes of density wells. Harmonic electric fields are observed simultaneously with the Langmuir waveforms and are consistent with fields produced by nonlinear currents. The beam speeds v b exciting the Langmuir waves are estimated from the waveform data, yielding speeds v b ≈ (0.01-0.04)c. These are consistent with previous observations. The beam speeds are slower than those associated with type III solar radio bursts, consistent with the Langmuir wave electric fields being field aligned. The evidence found for electrostatic decay and against strong perpendicular fields, and so low-wave number Langmuir/z-mode waves, suggests that the dominant emission mechanisms for this type II foreshock involve electrostatic decay and nonlinear wave processes, rather than linear-mode conversion. Harmonic radio emission via antenna mechanisms involving Langmuir waves remains possible.
Publications of the Astronomical Society of Australia, 2017
We present techniques developed to calibrate and correct Murchison Widefield Array low-frequency ... more We present techniques developed to calibrate and correct Murchison Widefield Array low-frequency (72-300 MHz) radio observations for polarimetry. The extremely wide field-of-view, excellent instantaneous (u, v)-coverage and sensitivity to degree-scale structure that the Murchison Widefield Array provides enable instrumental calibration, removal of instrumental artefacts, and correction for ionospheric Faraday rotation through imaging techniques. With the demonstrated polarimetric capabilities of the Murchison Widefield Array, we discuss future directions for polarimetric science at low frequencies to answer outstanding questions relating to polarised source counts, source depolarisation, pulsar science, low-mass stars, exoplanets, the nature of the interstellar and intergalactic media, and the solar environment.
Bulletin of the American Physical Society, Nov 12, 2007
Extraordinary-Mode Radiation Produced by Linear-Mode Conversion of Langmuir Waves EUN-HWA KIM, Pr... more Extraordinary-Mode Radiation Produced by Linear-Mode Conversion of Langmuir Waves EUN-HWA KIM, Princeton University, IVER CAIRNS, PETER ROBINSON, University of Sydney -Linear-mode conversion (LMC) of Langmuir waves to radiation near the plasma frequency at density gradients is important for laboratory, space, and astrophysical phenomena. We study LMC in warm magnetized plasmas using numerical electron fluid simulations when the density gradient is parallel to the ambient magnetic field (B 0 ). We demonstrate that LMC can produce extraordinary (x) mode as well as ordinary (o) mode radiation from Langmuir waves, contrary to earlier expectations of o-mode only. Equal amounts of o-and x-mode radiation are produced in the unmagnetized limit. The x-mode efficiency decreases as B 0 increases while the o-mode efficiency oscillates due to an interference phenomenon between incoming and reflected Langmuir or z-modes. Both x-and o-mode should be produced for typical coronal and interplanetary parameters, alleviating the long-standing depolarization problem for type III solar radio bursts.
The Astrophysical Journal, 2019
Solar radio emission, especially at metre-wavelengths, is well known to vary over small spectral ... more Solar radio emission, especially at metre-wavelengths, is well known to vary over small spectral ( 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 ∼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.
Publications of the Astronomical Society of Australia, 2018
We describe the motivation and design details of the ‘Phase II’ upgrade of the Murchison Widefiel... more We describe the motivation and design details of the ‘Phase II’ upgrade of the Murchison Widefield Array radio telescope. The expansion doubles to 256 the number of antenna tiles deployed in the array. The new antenna tiles enhance the capabilities of the Murchison Widefield Array in several key science areas. Seventy-two of the new tiles are deployed in a regular configuration near the existing array core. These new tiles enhance the surface brightness sensitivity of the array and will improve the ability of the Murchison Widefield Array to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ∼3.5. The remaining 56 tiles are deployed on long baselines, doubling the maximum baseline of the array and improving the arrayu, vcoverage. The improved imaging capabilities will provide an order of magnitude improvement in the noise floor of Murchison Widefield Array continuum images. The upgrade retains all of the features that have underpinned the Murchison Widefie...
Radio Science, 2016
Electron density irregularities in the ionosphere are known to be magnetically anisotropic, prefe... more 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 visualizing and studying field‐aligned irregularities (FAIs), which involves transforming interferometric measurements of total electron content 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 data set of Loi et al. (2015a), obtained on 15 October 2013 by the Murchison Widefield Array (MWA) radio telescope and displaying prominent F...
Journal of Geophysical Research: Space Physics, 2016
Geomagnetically aligned density structures with a range of sizes exist in the near-Earth plasma e... more 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 h of the passage of the TID, attaining their peak prominence 1-2 h later and persisting for several more hours until observations ended at local dawn. Analyses of the next 2 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 3. These observations represent the broadest angular coverage of the ionosphere by a radio telescope to date.
Geophysical Research Letters, Dec 15, 1995
Detailed first estimates are presented of angular broadening in the outer heliosphere due to scat... more Detailed first estimates are presented of angular broadening in the outer heliosphere due to scattering of radio waves by density irregularities: The application is to the 2-3 kHz radiation observed by Voyager. Two plausible turbulence models, which account very well for scattering within 1 AU, are extrapolated beyond 10 AU. Both models predict significant angular broadening in the outer heliosphere, accounting semiquantitatively alone for the source sizes inferred from roll modulation data. Predictions are presented for radial variations in the apparent source size if scattering is important. Comparisons with available data argue that scattering is important (and indeed is the dominant contributor to the apparent source size) and that the radiation source is located in the outer heliosphere. Other evidence that scattering is important, such as the fluctuations in apparent source direction and intensity, are also identified. The effects of scattering should be included in future analyses of the 2-3 kHz emissions.
Predicting radio emission from lunar radio anomolies
EAEJA, Apr 1, 2003
Quasilinear Simulation of Harmonic Electromagnetic Emission via Beam-driven Langmuir Waves in the Solar Wind
AGU Fall Meeting Abstracts, Dec 1, 2004
ABSTRACT
Statistics of Waveform and Envelope Fields: Theory, Simulations and Initial Applications to TRICE Data
AGU Fall Meeting Abstracts, Dec 1, 2008
ABSTRACT Plasma waves in space are almost invariably bursty and widely variable in amplitude, mot... more ABSTRACT Plasma waves in space are almost invariably bursty and widely variable in amplitude, motivating statistical approaches such as stochastic growth theory. Recent wave experiments on rockets moving through Earth's auroral regions, as well as the STEREO and Wind spacecraft, have sufficient time resolution to measure the waveform as well as the envelope field. Typically, however, experiments measure the envelope field averaged over long times compared with the wave period. Four sets of new contributions are presented. First, analytic theory is used to predict the distribution of waveform fields for a single mode with known distribution of envelope fields. The distribution P(log Ew) of waveform fields Ew is shown to be proportional to the rectified field Ewa with a ≈ 1.0 for a number of special cases of the distribution P(log Ee) of envelope field Ee. This form arises due to P(log Ew) being proportional to an integral over P(log Ee) that has a square-root singularity in Ee2. Numerical calculations confirm and extend this prediction to wide range of envelope distributions. Second, ensembles of stochastically-driven waves are simulated and the distributions P(log Ew) and P(log Ee) calculated. While small differences exist between the case of a single mode and multiple modes, it is found in general that the results are independent of the product of the wave frequency and decorrelation time. Of importance here is that the distributions P(log Ew) are found to be power-law with index ≈ 1.0 at low Ew, consistent with the analytic prediction. Moreover, the envelope distribution is found to be well fit by the form P(log Ee) ∝ Ee2 exp(- Ee2 / Eth2). This form applies to one- dimensional thermal waves and now, unexpectedly, also to waves driven stochastically near marginal stability. Third, initial calculations show that averaging (boxcar and sliding averages, whether linear or logarithmic) over multiple wave periods leads to both the envelope and waveform distributions being well fitted by lognormal distributions. Fourth, initial comparisons are made with Langmuir-like waves observed in Earth's cusp region by the TRICE rocket. It appears that the foregoing analytic and numerical calculations explain semi-quantitatively the power-law form and index near 1.0 for the waveform distribution of unaveraged fields, the functional form of the envelope distribution of unaveraged fields, and the transition of the waveform and envelope distributions towards lognormal forms with averaging over multiple wave periods. The waves appear consistent with stochastic growth. The theory and simulation results extend stochastic growth theory to measurements on timescales less than or close to the wave period.
Analytic and Numerical Modelling of Plasma Radiation from Earth's Foreshock
AGUFM, Dec 1, 2001
ABSTRACT