| 1. |
- Graham, Daniel B., et al.
(författare)
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Applying bicoherence analysis to spacecraft observations of Langmuir waves
- 2014
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:5, s. 1367-1374
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Tidskriftsartikel (refereegranskat)abstract
- In type II and type III solar radio bursts and planetary foreshocks, the processes which convert Langmuir waves (LWs) to transverse waves are not well understood. One of the proposed mechanisms for generating transverse waves involves electrostatic (ES) decay followed by coalescence of two LWs. One of the tests used to identify this process is bicoherence analysis. Bicoherence has been applied to spacecraft observations of LWs to yield results consistent with ES decay and coalescence. However, recent work has shown that the harmonic fields produced by LWs are more consistent with sheath rectification and nonlinear currents. It is shown here that sheath rectification and nonlinear currents yield bicoherences similar to those expected for ES decay and coalescence, explaining the bicoherences associated with spacecraft observations of LWs. These results show that bicoherence alone cannot be used to identify ES decay and coalescence and emphasize the importance of sheath rectification. Key Points Sheath rectification and nonlinear currents produce phase-coherent fields Bicoherences are consistent with nonlinear currents and sheath rectification Bicoherence cannot identify electrostatic decay and coalescence
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| 2. |
- Graham, Daniel B., et al.
(författare)
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Constraints on the Formation and Structure of Langmuir Eigenmodes in the Solar Wind
- 2013
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:12, s. 121101-
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Tidskriftsartikel (refereegranskat)abstract
- Localized Langmuir waves are commonly observed in space plasmas and are a potential source of radio waves. Using electric field data from STEREO, it is shown that these localized Langmuir waves are eigenmodes of density wells estimated independently. An analytic model is developed for the eigenmode frequencies. The inferred depths and widths of the density wells typically only allow the zeroth-order Langmuir eigenmode to form, explaining the preponderance of single-peaked waveforms. More complicated waveforms are shown to be consistent with single eigenmode solutions of more complicated density profiles. The inferred depth of the density well increases with Langmuir wave intensity, consistent with the ponderomotive force but not wave packet collapse.
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| 3. |
- Graham, Daniel B., et al.
(författare)
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Dynamical evidence for nonlinear Langmuir wave processes in type III solar radio bursts
- 2014
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Ingår i: J GEOPHYS RES-SPACE. - 2169-9380. ; 119:4, s. 2430-2457
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Tidskriftsartikel (refereegranskat)abstract
- The nonlinear processes and evolution of Langmuir waves in the source regions of type III solar radio bursts are explored in detail. Langmuir waves recorded by the Time Domain Sampler of the STEREO/WAVES instrument can be roughly classified into six groups based on the waveform, power spectra, and field strength perpendicular to the local magnetic field. It is argued that these groups correspond to either different stages of the evolution of Langmuir waves generated by electron beams or differ due to the direction of the magnetic field relative to the solar wind velocity. Approximately half of the observed Langmuir waves have strong perpendicular fields, meaning that understanding how these fields are produced is crucial for understanding type III sources. Most events recorded are either localized waveforms consistent with Langmuir eigenmodes or have two or more spectral peaks consistent with electrostatic (ES) decay of Langmuir/z mode waves. The remaining events appear to correspond to either earlier or later stages of Langmuir wave evolution or are decay events for which the Doppler shift is insufficient to distinguish the beam-driven and product Langmuir waves. This is supported by the fact that most events exceed the threshold for ES decay even though their spectra show no evidence for decay and some of the events are observed when the solar wind flow is approximately perpendicular to the magnetic field, minimizing Doppler shifting. Low-frequency fields produced by intense Langmuir waves are quantitatively consistent with density perturbations produced by the ponderomotive force, ion-acoustic waves produced by ES decay, or sheath rectification. Above the observed nonlinear threshold, quantitative analysis suggests that the observed low-frequency signals are consistent with perturbations produced by ponderomotive effects and ion-acoustic waves produced by ES decay, but effects of sheath rectification may also contribute.
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| 4. |
- Graham, Daniel B., et al.
(författare)
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Electrostatic decay of Langmuir/z-mode waves in type III solar radio bursts
- 2013
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Ingår i: Journal of Geophysical Research-Space Physics. - : American Geophysical Union (AGU). - 2169-9380. ; 118:7, s. 3968-3984
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Tidskriftsartikel (refereegranskat)abstract
- About 40% of the waveforms observed by STEREO during type III solar radio bursts exhibits Langmuir beating and have split spectral peaks, suggestive of decay into product Langmuir and ion-acoustic waves. For lower electron beam speeds v(b)/c less than or similar to 0.1, the spectra of Langmuir events with split spectral peaks are shown to be consistent with electrostatic (ES) decay into Langmuir-like waves with frequencies above the electron plasma frequency. For faster beam speeds vb/c0.1, the spectra are consistent with one or more successive generations of ES decay and an end state of low wave number Langmuir/z-mode waves with strong electric fields perpendicular to the magnetic field. For many of the split spectral-peak events, an intense low-frequency response occurs that is consistent with ion-acoustic waves produced by ES decay, providing further evidence that these events are ES decay. An observed event is shown to be consistent with three successive backscatter decays, but such events are very rare in type III bursts. About 90% of the split spectral-peak events occur when T-i/T-e less than or similar to 0.6. Similarly, over 80% of the split peak events have energy densities above the theoretical nonlinear threshold for ES decay (for reasonable Langmuir damping rates less than or similar to 10(p)(-3)). All events have beam speeds and energy densities below the maxima appropriate for ES decay of beam-driven Langmuir waves.
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| 5. |
- Graham, Daniel B., et al.
(författare)
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Harmonic waves and sheath rectification in type III solar radio bursts
- 2014
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Ingår i: J GEOPHYS RES-SPACE. - 2169-9380. ; 119:2, s. 723-741
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Tidskriftsartikel (refereegranskat)abstract
- In type III solar radio bursts and planetary foreshocks, Langmuir waves are produced by electron beams and converted partially to radio waves by linear and nonlinear processes. Lower amplitude second harmonic electric fields are observed simultaneously during the most intense Langmuir wave events in type III source regions. The electric fields at the harmonic frequencies can arise from various mechanisms, such as radio wave emission by either coalescence or antenna mechanisms, nonlinear currents, harmonics of Langmuir waves, electron trapping in Langmuir wave potentials, and Langmuir wave rectification at the sheath surrounding the spacecraft, or they can result from instrumental harmonics. In this paper the relative powers and electric field vectors of Langmuir waves and the harmonic fields are compared for multiple events. The structure of the harmonic field is shown to be determined by the Langmuir waveform, but the harmonic field direction is typically closely aligned with the solar wind flow. The magnitude, structure, and orientation of the harmonic fields is used to determine which processes are responsible. It is shown that the dominant process generating the observed harmonic fields is Langmuir wave rectification at the sheath surrounding the spacecraft. Key Points Intense Langmuir waves and harmonic fields are observed simultaneously Harmonic fields are primarily produced by sheath rectification Some evidence for nonlinear currents is found
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| 6. |
- Graham, Daniel B., et al.
(författare)
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Langmuir "snakes" and electrostatic decay in the solar wind
- 2013
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 40:10, s. 1934-1939
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Tidskriftsartikel (refereegranskat)abstract
- When Langmuir waves are driven by an electron beam to large amplitudes, they can undergo electrostatic (ES) decay to smaller wave numbers via a series of backscatters. Truncated ES decay, where the number of backscatters is reduced due to damping, is modeled here using the three-dimensional ES Zakharov equations. Langmuir beats develop in snake-like structures parallel to the electron beam direction and are most evident when decay is truncated to a single backscatter. From these results, an analytic form is derived and shown to be consistent with some of the waveforms and spectra observed by STEREO in the source regions of type III solar radio bursts. The agreement between the model and observations provides strong evidence for ES decay and Langmuir snakes parallel to the electron beam and so the ambient magnetic field.
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| 7. |
- Graham, Daniel B., et al.
(författare)
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The Langmuir waves associated with the 1 December 2013 type II burst
- 2015
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 120:6, s. 4126-4141
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Tidskriftsartikel (refereegranskat)abstract
- 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) approximate to(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.
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| 8. |
- Malaspina, David M., et al.
(författare)
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Langmuir wave harmonics due to driven nonlinear currents
- 2013
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 118:11, s. 6880-6888
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Tidskriftsartikel (refereegranskat)abstract
- The conversion of Langmuir waves into electromagnetic radiation near the local plasma frequency (f(pe)) and twice the local plasma frequency (2f(pe)) occurs in diverse heliospheric environments including along the path of type III radio bursts, at interplanetary shocks, and in planetary foreshocks. This radiation has the potential to act as a probe of remote plasma conditions, provided that the conversion mechanism is well understood. One candidate conversion mechanism is the antenna radiation of localized Langmuir waves. Antenna radiation near 2f(pe) requires the presence of nonlinear currents at 2f(pe). In this work, properties of these currents are predicted from theory and compared with observations of Langmuir wave electric fields made using the WAVES instrument on the STEREO spacecraft. It is found that the observed frequency structure, polarization, and wave number ratio are consistent with nonlinear current predictions, once electric fields near 2f(pe)consistent with sheath effects are taken into account.
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