arXiv subjects

Iver H. Cairns

Publications and source records attributed to Iver H. Cairns.

27 records · Page 2Linked to original sources

arXiv2015

Real-time imaging of density ducts between the plasmasphere and ionosphere

Ionization of the Earth's atmosphere by sunlight forms a complex, multi-layered 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.

physics.space-ph↗

arXiv2014

Constraining the amplitude of turbulence in solar corona using observations of angular broadening of radio sources

The angular broadening of compact radio sources observed through a medium having turbulent density irregularities is usually estimated using the phase structure function. We employ an exact formulation for the phase structure function that helps in obtaining an accurate estimate of angular broadening when the baseline lengths are comparable to the inner scale of the turbulent spectrum.

astro-ph.SR↗

arXiv2011

First Spectroscopic Imaging Observations of the Sun at Low Radio Frequencies with the Murchison Widefield Array Prototype

We present the first spectroscopic images of solar radio transients from the prototype for the Murchison Widefield Array (MWA), observed on 2010 March 27. Our observations span the instantaneous frequency band 170.9-201.6 MHz. Though our observing period is characterized as a period of `low' to `medium' activity, one broadband emission feature and numerous short-lived, narrowband, non-thermal emission features are evident. Our data represent a significant advance in low radio frequency solar imaging, enabling us to follow the spatial, spectral, and temporal evolution of events simultaneously and in unprecedented detail. The rich variety of features seen here reaffirms the coronal diagnostic capability of low radio frequency emission and provides an early glimpse of the nature of radio observations that will become available as the next generation of low frequency radio interferometers come on-line over the next few years.

astro-ph.SR↗

arXiv2010

First-order thermal correction to the quadratic response tensor and rate for second harmonic plasma emission

Three-wave interactions in plasmas are described, in the framework of kinetic theory, by the quadratic response tensor (QRT). The cold-plasma QRT is a common approximation for interactions between three fast waves. Here, the first-order thermal correction (FOTC) to the cold-plasma QRT is derived for interactions between three fast waves in a warm unmagnetized collisionless plasma, whose particles have an arbitrary isotropic distribution function. The FOTC to the cold-plasma QRT is shown to depend on the second moment of the distribution function, the phase speeds of the waves, and the interaction geometry. Previous calculations of the rate for second harmonic plasma emission (via Langmuir-wave coalescence) assume the cold-plasma QRT. The FOTC to the cold-plasma QRT is used here to calculate the FOTC to the second harmonic emission rate, and its importance is assessed in various physical situations. The FOTC significantly increases the rate when the ratio of the Langmuir phase speed to the electron thermal speed is less than about 3.

physics.plasm-ph↗

arXiv2006

Nontrapping arrest of Langmuir wave damping near the threshold amplitude

Evolution of a Langmuir wave is studied numerically for finite amplitudes slightly above the threshold which separates damping from nondamping cases. Arrest of linear damping is found to be a second-order effect due to ballistic evolution of perturbations, resonant power transfer between field and particles, and organization of phase space into a positive slope for the average distribution function $f_{av}$ around the resonant wave phase speed $v_ϕ$. Near the threshold trapping in the wave potential does not arrest damping or saturate the subsequent growth phase.

physics.plasm-ph↗

arXiv2004

Properties and Interpretations of Giant Micropulses and Giant Pulses from Pulsars

Giant pulses and giant micropulses from pulsars are distinguished from normal pulsed emission by their large fluxes, rarity, approximately power-law distribution of fluxes and, typically, occurrence in restricted phase windows. Here existing observations of flux distributions are manipulated into a common format and interpreted in terms of theories for wave growth in inhomogeneous media, with the aim of constraining the emission mechanism and source physics for giant pulses and micropulses. Giant micropulses near 2 GHz (PSRs B8033-45 and B1706-44) and 0.4 GHz (PSR B0950+08) have indices $α= 6.5 \pm 0.7$ for the probability distribution $P(E)$ of the electric field $E$, with $P(E) \propto E^{-α}$. Giant pulses (PSRs B0531+24, B1937+214, and B1821-24) have $α$ ranging from $4.6 \pm 0.2$ to $9 \pm 2$, possibly increasing with frequency. These are similar enough to regard giant micropulses and pulses as a single phenomenon with a common physical explanation. The power-law functional form and values of $α$ observed are consistent with predictions for nonlinear wave collapse, but inconsistent with known self-organized critical systems, nonlinear decay processes, and elementary burst theory. While relativistic beaming may be important, its statistics are yet to be predicted theoretically and collapse is currently the favored interpretation. Other possibilities remain, including stochastic growth theory (consistent with normal pulse emission) and, less plausibly, refractive lensing. Unresolved issues remain for all four interpretations and suggestions for further work are given. The differences between normal and giant pulse emission suggest they have distinct source regions and emission processes.

astro-ph↗

arXiv2003

Intrinsic Variability and Field Statistics for the Vela Pulsar: 2. Systematics and Single-Component Fits

Individual pulses from pulsars have intensity-phase profiles that differ widely from pulse to pulse, from the average profile, and from phase to phase within a pulse. Widely accepted explanations do not exist for this variability or for the mechanism producing the radiation. The variability corresponds to the field statistics, particularly the distribution of wave field amplitudes, which are predicted by theories for wave growth in inhomogeneous media. This paper shows that the field statistics of the Vela pulsar (PSR B0833-45) are well-defined and vary as a function of pulse phase, evolving from Gaussian intensity statistics off-pulse to approximately power-law and then lognormal distributions near the pulse peak to approximately power-law and eventually Gaussian statistics off-pulse again. Detailed single-component fits confirm that the variability corresponds to lognormal statistics near the peak of the pulse profile and Gaussian intensity statistics off-pulse. The lognormal field statistics observed are consistent with the prediction of stochastic growth theory (SGT) for a purely linear system close to marginal stability. The simplest interpretations are that the pulsar's variability is a direct manifestation of an SGT state and the emission mechanism is linear (either direct or indirect), with no evidence for nonlinear mechanisms like modulational instability and wave collapse which produce power-law field statistics. Stringent constraints are placed on nonlinear mechanisms: they must produce lognormal statistics when suitably ensemble-averaged. Field statistics are thus a powerful, potentially widely applicable tool for understanding variability and constraining mechanisms and source characteristics of coherent astrophysical and space emissions.

astro-ph↗

arXiv2003

Intrinsic Variability and Field Statistics for the Vela Pulsar: 3. Two-Component Fits and Detailed Assessment of Stochastic Growth Theory

The variability of the Vela pulsar (PSR B0833-45) corresponds to well-defined field statistics that vary with pulsar phase, ranging from Gaussian intensity statistics off-pulse to approximately power-law statistics in a transition region and then lognormal statistics on-pulse, excluding giant micropulses. These data are analyzed here in terms of two superposed wave populations, using a new calculation for the amplitude statistics of two vectorially-combined transverse fields. Detailed analyses show that the approximately power-law and lognormal distributions observed are fitted well at essentially all on-pulse phases by Gaussian-lognormal and double-lognormal combinations, respectively. These good fits, plus the smooth but significant variations in fit parameters across the source, provide strong evidence that the approximately power-law statistics observed in the transition region are not intrinsic. Instead, the data are consistent with normal pulsar emission having lognormal statistics at all phases. This is consistent with generation in an inhomogeneous source obeying stochastic growth theory (SGT) and with the emission mechanism being purely linear (either direct or indirect). A nonlinear mechanism is viable only if it produces lognormal statistics when suitably ensemble-averaged. Variations in the SGT fit parameters with phase imply that the radiation is relatively more variable near the pulse edges than near the center, as found in earlier work. In contrast, Vela's giant micropulses come from a very restricted phase range and have power-law statistics with indices ($6.7 \pm 0.6$) not inconsistent with nonlinear wave collapse. These results imply that normal pulses have a different source and generation mechanism than giant micropulses, as suggested previously on other grounds.

astro-ph↗

arXiv2001

Intrinsic Variability of the Vela Pulsar: Lognormal Statistics and Theoretical Implications

Individual pulses from pulsars have intensity-phase profiles that differ widely from pulse to pulse, from the average profile, and from phase to phase within a pulse. Widely accepted explanations for pulsar radio emission and its time variability do not exist. Here, by analysing data near the peak of the Vela pulsar's average profile, we show that Vela's variability corresponds to lognormal field statistics, consistent with the prediction of stochastic growth theory (SGT) for a purely linear system close to marginal stability. Vela's variability is therefore a direct manifestation of an SGT state and the field statistics constrain the emission mechanism to be linear (either direct or indirect), ruling out nonlinear mechanisms like wave collapse. Field statistics are thus a powerful, potentially widely applicable tool for understanding variability and constraining mechanisms and source characteristics of coherent astrophysical and space emissions.

astro-ph↗