On the propagation and mode conversion of auroral medium frequency bursts

Broughton, M.; LaBelle, J.; Kim, Eun Hwa; Yoon, Peter H.; Cairns, Iver H.

doi:10.1002/2015ja021851

Cited by 3 publications

(2 citation statements)

References 38 publications

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“…In this section, we investigate the effect of secondary electrons on the dynamics of Langmuir turbulence at the F region peak. In order to model the secondary electrons, we decide to employ the Kappa distribution of Broughton et al [] that was determined by fitting the sounding rocket measurements of Newman et al [, ]. The one‐dimensional Kappa distribution function is given by

f_{Kappa} ((), v) = \frac{n_{s e}}{\sqrt{π}} \frac{1}{k^{3 true/ 2} θ_{normals normale}} \frac{normalΓ ((), normalk + 1)}{normalΓ ((), normalk ‐ 1 true/ 2)} {(1 + \frac{v^{2}}{k θ_{s e}^{italic2}})}^{italic‐ k}

where n se is the number density of the secondary population,

θ_{normals normale} = \sqrt{((), \frac{k ‐ 3 / 2}{k}) \frac{italic2 k_{B} T_{normals normale}}{m_{e}}}

is the modified thermal velocity, T se = 18.2 eV is the secondary electron population temperature, k = 1.584 is the velocity spectral index, and

normalΓ ((), t) = true {true\int}_{0}^{\infty} x^{t - 1} e^{prefix- x} d x

.…”

Section: The Effect Of Secondary (Scattered) Electronsmentioning

confidence: 99%

Zakharov simulations of beam‐induced turbulence in the auroral ionosphere

et al. 2016

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Recent detections of strong incoherent scatter radar echoes from the auroral F region, which have been explained as the signature of naturally produced Langmuir turbulence, have motivated us to revisit the topic of beam‐generated Langmuir turbulence via simulation. Results from one‐dimensional Zakharov simulations are used to study the interaction of ionospheric electron beams with the background plasma at the F region peak. A broad range of beam parameters extending by more than 2 orders of magnitude in average energy and electron number density is considered. A range of wave interaction processes, from a single parametric decay, to a cascade of parametric decays, to formation of stationary density cavities in the condensate region, and to direct collapse at the initial stages of turbulence, is observed as we increase the input energy to the system. The effect of suprathermal electrons, produced by collisional interactions of auroral electrons with the neutral atmosphere, on the dynamics of Langmuir turbulence is also investigated. It is seen that the enhanced Landau damping introduced by the suprathermal electrons significantly weakens the turbulence and truncates the cascade of parametric decays.

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f_{Kappa} ((), v) = \frac{n_{s e}}{\sqrt{π}} \frac{1}{k^{3 true/ 2} θ_{normals normale}} \frac{normalΓ ((), normalk + 1)}{normalΓ ((), normalk ‐ 1 true/ 2)} {(1 + \frac{v^{2}}{k θ_{s e}^{italic2}})}^{italic‐ k}

where n se is the number density of the secondary population,

θ_{normals normale} = \sqrt{((), \frac{k ‐ 3 / 2}{k}) \frac{italic2 k_{B} T_{normals normale}}{m_{e}}}

is the modified thermal velocity, T se = 18.2 eV is the secondary electron population temperature, k = 1.584 is the velocity spectral index, and

normalΓ ((), t) = true {true\int}_{0}^{\infty} x^{t - 1} e^{prefix- x} d x

.…”

Section: The Effect Of Secondary (Scattered) Electronsmentioning

confidence: 99%

Zakharov simulations of beam‐induced turbulence in the auroral ionosphere

et al. 2016

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“…This observation has led to speculation about a mechanism whereby Alfvénically accelerated electron beams with energy on the order of a few hundred electron volts excite Langmuir waves, over a range of altitudes, which subsequently convert to L ‐mode radiation, and the combination of electron and (primarily) wave dispersion produces the descending tones (LaBelle, 2011). Some experimental (Broughton et al, 2012) and modeling (Broughton et al, 2016) evidence supports this mechanism. Recently, a correlation has been observed between MFB and Langmuir “caviton” turbulence, concentrated at particular altitudes near 200–300 km in the auroral ionosphere (Akbari et al, 2013).…”

Section: Introductionmentioning

confidence: 94%

Estimating Polar Cap Density and Medium‐Frequency Burst Source Heights Using 2f_ce Roar Radio Emissions

Burnett

LaBelle

2020

JGR Space Physics

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Competing theories exist for the generation mechanism of auroral medium-frequency burst (MFB). In an effort to constrain MFB source heights, this study analyzes 33 events in which MFB and auroral 2f ce roar co-occurred at Sondrestrom, Greenland. Using measurements from an array of receiving antennas, direction-of-arrival calculations indicate that in a given co-occurrence, the elevation angle of MFB typically is higher than that of roar. Ray tracing is used to determine source heights of the MFB signals. Density profiles are obtained from the International Reference Ionosphere (IRI) and shifted in magnitude until each event's roar signals originate at heights where the frequency-matching condition for 2f ce roar generation is satisfied. This shifting method is validated using density measurements from the Sondrestrom incoherent scatter radar (ISR) facility for the two events with available ISR data. After shifting, ray tracing demonstrates that in 25 of the 33 events, burst originates at a height of about 200 km, lower than the typical altitude of peak electron density. However, ISR measurements show that the density profile is enhanced at low altitudes while MFB is observed, peaking in the E region rather than the F region. This finding implies that the MFB sources at 200 km are on the topside of the density peak, in a region of downward pointing density gradient, in qualitative agreement with the mechanism of MFB generation by Langmuir waves in the topside ionosphere. These results also suggest a new method of estimating density in the polar cap using roar signals to calibrate IRI profiles. Plain Language Summary Auroral substorms, which produce the northern lights, also emit radio waves. This study focuses on two types of auroral radio emissions, called roar and burst. Various mechanisms have been proposed to explain burst, and they disagree about its altitude of origin. This study tests these theories by determining burst source heights. An antenna array in Sondrestrom, Greenland, is used to detect roar and burst signals. Their directions of origin are calculated based on the antenna configuration, and the rays are traced backward to their origin. This ray tracing relies on electron density estimates provided by the International Reference Ionosphere (IRI). To make the IRI data more accurate, density values are shifted so that roar rays are traced to their known source altitudes. This method of shifting the IRI profile using roar is a novel way of estimating electron density and is validated using measurements from the incoherent scatter radar system in Sondrestrom. Density measurements also show that as burst occurs, the density peaks at low altitude, below 200 km, whereas burst originates around 200 km in most of the events analyzed. This result provides observational support for theories predicting that burst generation occurs above the altitude of peak electron density. Auroral "roar", also known as cyclotron harmonic emissions, is one of the best-known auroral radio emissions, first observed from ground ...

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Radio emissions of auroral origin observable at ground level: outstanding problems

LaBelle

2023

Front. Astron. Space Sci.

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Auroral radio emissions are of intrinsic interest as part of the Earth’s environment but also provide remote sensing of ionospheric conditions and processes and a laboratory for emission processes applicable to a wide range of space and astrophysical plasmas. At VLF and above, four broad classes of radio emissions occur. All have been observed with ground-based and, in some cases to a lesser degree, with space-based instruments. Related to each type of radio emission, many experimental and theoretical challenges remain, for example: explanations of frequency and time structure, relations to auroral substorms or current systems, and application to remote sensing of the auroral ionosphere. In some cases, basic parameters such as source heights or generation mechanisms are uncertain. Emerging technological advances such as cubesat fleets, ultra-large capacity disk drives, and software defined radio show promise for developing better understanding of auroral radio emissions.

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On the propagation and mode conversion of auroral medium frequency bursts

Cited by 3 publications

References 38 publications

Zakharov simulations of beam‐induced turbulence in the auroral ionosphere

Zakharov simulations of beam‐induced turbulence in the auroral ionosphere

Estimating Polar Cap Density and Medium‐Frequency Burst Source Heights Using 2f_ce Roar Radio Emissions

Radio emissions of auroral origin observable at ground level: outstanding problems

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On the propagation and mode conversion of auroral medium frequency bursts

Cited by 3 publications

References 38 publications

Zakharov simulations of beam‐induced turbulence in the auroral ionosphere

Zakharov simulations of beam‐induced turbulence in the auroral ionosphere

Estimating Polar Cap Density and Medium‐Frequency Burst Source Heights Using 2fce Roar Radio Emissions

Radio emissions of auroral origin observable at ground level: outstanding problems

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Estimating Polar Cap Density and Medium‐Frequency Burst Source Heights Using 2f_ce Roar Radio Emissions