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Found 12 entries in the Bibliography.
Showing entries from 1 through 12
| 2021 |
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Abstract We present a statistical study of density cavities observed in the inner magnetosphere by the Van Allen Probes during four one-month periods: February 2013, July 2013, January 2014 and June 2014. These periods were chosen to allow the survey of all magnetic local times. We find that density cavities are a recurrent feature of the density profiles of in situ measurements in the inner magnetosphere. We further investigate the correlation between the density cavities and the enhancement of fluxes of warm ions with energies of 10-100 eV. The results show that warm ion flux enhancements associated with the density cavities were observed more frequently for H+, then for He+ and the least frequently for O+. The occurrences of the associated flux enhancements were increased when considering only the cavities inside the plasmasphere. Possible mechanisms responsible for the observed warm ion flux enhancements and the role of density cavities on these ion flux enhancements are discussed. Ferradas, C.; Boardsen, S.; Fok, M.-C.; Buzulukova, N.; Reeves, G.; Larsen, B.; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2020JA028326 Magnetosphere: inner; plasmasphere; magnetospheric configuration and dynamics; plasma waves and instabilities; plasma sheet; density cavity; cold ion heating; cold ions; warm Plasma cloak; Van Allen Probes |
| 2020 |
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Fine Harmonic Structure of Equatorial Noise with a Quasiperiodic Modulation Abstract Equatorial noise emissions (fast magnetosonic waves) are electromagnetic waves observed routinely in the equatorial region of the inner magnetosphere. They propagate with wave vectors nearly perpendicular to the ambient magnetic field; that is, they are limited to frequencies below the lower hybrid frequency. The waves are generated by instabilities of ring-like proton distribution functions, which result in their fine harmonic structure with intensity maxima close to harmonics of the proton cyclotron frequency in the source region. Although most equatorial noise emissions are continuous in time, some events exhibit a clear quasiperiodic time modulation of the wave intensity, with typical modulation periods on the order of minutes. We analyze 72 such events (17 observed by the Cluster spacecraft, 55 observed by the Van Allen Probes spacecraft) for which high-resolution data were available. The analysis of the observed harmonic structure allows us to determine source radial distances of the events. It is found that the calculated source radial distances are generally close to the radial distances where the events were observed. The harmonic numbers where the events are generated range between about 12 and 30. Two events for which the spacecraft passed through the generation region were identified and analyzed. No simultaneous ultra-low-frequency magnetic field pulsations and no periodic plasma number density variations were observed. Although the in situ measured proton distribution functions were shown to be responsible for the wave growth, an insufficient resolution of the particle instruments prevented us from detecting a quasiperiodic modulation possibly present in the particle data. Němec, F.; Tomori, A.; Santolik, O.; Boardsen, S.; Hospodarsky, G.; Kurth, W.; Pickett, J.; Kletzing, C.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2020 YEAR: 2020 DOI: 10.1029/2019JA027509 equatorial noise; Fast Magnetosonic Waves; quasiperiodic modulation; Van Allen Probes |
| 2019 |
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Equatorial noise (EN) emissions are observed inside and outside the plasmapause. EN emissions are referred to as magnetosonic mode waves. Using data from Van Allen Probes and Arase, we found conversion from EN emissions to electromagnetic ion cyclotron (EMIC) waves in the plasmasphere and in the topside ionosphere. A low frequency part of EN emissions becomes EMIC waves through branch splitting of EN emissions, and the mode conversion from EN to EMIC waves occurs around the frequency of M/Q=2 (deuteron and/or alpha particles) cyclotron frequency. These processes result in plasmaspheric EMIC waves. We investigated the ion composition ratio by characteristic frequencies of EN emissions and EMIC waves and obtained ion composition ratios. We found that the maximum composition ratio of M/Q=2 ions is ~10\% below 3000 km. The quantitative estimation of the ion composition will contribute to improving the plasma model of the deep plasmasphere and the topside ionosphere Miyoshi, Y.; Matsuda, S.; Kurita, S.; Nomura, K.; Keika, K.; Shoji, M.; Kitamura, N.; Kasahara, Y.; Matsuoka, A.; Shinohara, I.; Shiokawa, K.; Machida, S.; Santolik, O.; Boardsen, S.A.; Horne, R.B.; Wygant, J.F.; Published by: Geophysical Research Letters Published on: 04/2019 YEAR: 2019 DOI: 10.1029/2019GL083024 Arase; EMIC; M/Q=2 ions; Magnetsonic waves; plasmasphere; Van Allen Probes |
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EMIC Wave-Driven Bounce Resonance Scattering of Energetic Electrons in the Inner Magnetosphere While electromagnetic ion cyclotron (EMIC) waves have been long studied as a scattering mechanism for ultrarelativistic (megaelectron volt) electrons via cyclotron-resonant interactions, these waves are also of the right frequency to resonate with the bounce motion of lower-energy (approximately tens to hundreds of kiloelectron volts) electrons. Here we investigate the effectiveness of this bounce resonance interaction to better determine the effects of EMIC waves on subrelativistic electron populations in Earth\textquoterights inner magnetosphere. Using wave and plasma parameters directly measured by the Van Allen Probes, we estimate bounce resonance diffusion coefficients for four different events, illustrative of wave and plasma parameters to be encountered in the inner magnetosphere. The range of electron energies and pitch angles affected is examined to better assess the realistic effects of EMIC-driven bounce resonance on energetic electron populations based on actual, locally observed event-based parameters. Significant local diffusion coefficients (~ > 10-6 s-1) for 50- to 100-keV electrons are achieved for both H+ band wave events as well as He+ band, with diffusion coefficients peaking for near-90\textdegree pitch angles but remaining elevated for intermediate ones as well. Diffusion coefficients for higher-energy 200-keV electrons are typically multiple orders of magnitude lower (ranging from 10-11 to 10-6 s-1) and often peak at lower pitch angles (~20\textendash30\textdegree). These results suggest that both H+ and He+ band EMIC waves can play a role in shaping lower-energy electron dynamics via bounce-resonant interactions, in addition to their role in relativistic electron loss via cyclotron resonance. Blum, L.W.; Artemyev, A.; Agapitov, O.; Mourenas, D.; Boardsen, S.; Schiller, Q.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2019 YEAR: 2019 DOI: 10.1029/2018JA026427 bounce resonance; EMIC wave; energetic electrons; Radiation belts; Van Allen Probes |
| 2018 |
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Recent analysis of an event observed by the Van Allen Probes in the source region outside the plasmapause has shown that fast magnetosonic waves (also referred to as equatorial noise) propagate preferentially in the azimuthal direction, implying that wave amplification should occur during azimuthal propagation. To demonstrate this, we carry out 2-D particle-in-cell simulations of the fast magnetosonic mode at the dipole magnetic equator with the simulation box size, the magnetic field inhomogeneity, and the plasma parameters chosen from the same event recently analyzed. The self-consistently evolving electric and magnetic field fluctuations are characterized by spectral peaks at harmonics of the local proton cyclotron frequency. The azimuthal component of the electric field fluctuations is larger than the radial component, indicating wave propagation mainly along the azimuthal direction. Because the simulation box is within the source region, this also implies wave amplification mainly during azimuthal propagation. The excellent agreement between the wave polarization properties of the present simulations and the recently reported observations is clear evidence that the main wave amplification occurs during azimuthal propagation in the source region. Min, Kyungguk; Boardsen, Scott; Denton, Richard; Liu, Kaijun; Published by: Journal of Geophysical Research: Space Physics Published on: 11/2018 YEAR: 2018 DOI: 10.1029/2018JA026037 2D particle-in-cell simulation; Fast Magnetosonic Waves; Perpendicular propagation; Van Allen Probes |
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Determining the wave vector direction of equatorial fast magnetosonic waves We perform polarization analysis of the equatorial fast magnetosonic waves electric field over a 20 minute interval of Van Allen Probes A Waveform Receiver burst mode data. The wave power peaks at harmonics of the proton cyclotron frequency indicating the spacecraft is near or in the source region. The wave vector is inferred from the direction of the major axis of the electric field polarization ellipsoid and the sign of the phase between the longitudinal electric and compressional magnetic field components. We show that wave vector is preferentially in the azimuthal direction as opposed to the radial direction. From Poynting flux analysis one would infer that the wave vector is primarily in the radial direction. We show that the error in the Poynting flux is large ~ 90\textdegree. These results strongly imply that the wave growth occurs during azimuthal propagation in the source region for this event. Boardsen, Scott; Hospodarsky, George; Min, Kyungguk; Averkamp, Terrance; Bounds, Scott; Kletzing, Craig; Pfaff, Robert; Published by: Geophysical Research Letters Published on: 07/2018 YEAR: 2018 DOI: 10.1029/2018GL078695 equatorial fast magnetosonic; E-field polarization analysis; Poynting Flux analysis; Van Allen Probes; wave vector analysis |
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Electromagnetic wave measurements performed by the two Van Allen Probes spacecraft are used to analyze equatorial noise emissions with a quasiperiodic modulation of the wave intensity. These waves are confined to the vicinity of the geomagnetic equator, and they occur primarily on the dayside. In situ plasma number density measurements are used to evaluate density variations related to the wave occurrence. It is shown that the events are sometimes effectively confined to low density regions, being observed at successive satellite passes over a time duration as long as one hour. The events typically occur outside the plasmasphere, and they are often cease to exist just at the plasmapause. The analysis of the spatial separations of the spacecraft at the times when the events were observed simultaneously by both of them allows us to estimate the event spatial dimensions. It is found that the event spatial extent is typically lower than about 0.25RE in radial distance and within about one hour in magnetic local time. Modulation periods of the events decrease with increasing plasma number density up to about 100cm-3. Principally no dependence is observed at larger densities, possibly indicating a propagation from other locations. emec, F.; ik, O.; Boardsen, S.; Hospodarsky, G.; Kurth, W.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2018 YEAR: 2018 DOI: 10.1029/2018JA025482 equatorial noise; quasiperiodic modulation; RBSP; Van Allen Probes |
| 2017 |
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We analyze a wave event that occurred near noon between 07:03 and 07:08 UT on 23 February 2014 detected by the Van Allen Probes B spacecraft, where waves in the lower hybrid frequency range (LHFR) and electromagnetic ion cyclotron (EMIC) waves are observed to be highly correlated, with Pearson correlation coefficient of ~0.86. We assume that the correlation is the result of LHFR wave generation by the ions\textquoteright polarization drift in the electric field of the EMIC waves. To check this assumption the drift velocities of electrons and H+, He+, and O+ ions in the measured EMIC wave electric field were modeled. Then the LHFR wave linear instantaneous growth rates for plasma with these changing drift velocities and different plasma compositions were calculated. The time distribution of these growth rates, their frequency distribution, and the frequency dependence of the ratio of the LHFR wave power spectral density (PSD) parallel and perpendicular to the ambient magnetic field to the total PSD were found. These characteristics of the growth rates were compared with the corresponding characteristics of the observed LHFR activity. Reasonable agreement between these features and the strong correlation between EMIC and LHFR energy densities support the assumption that the LHFR wave generation can be caused by the ions\textquoteright polarization drift in the electric field of an EMIC wave. Khazanov, G.; Boardsen, S.; Krivorutsky, E.; Engebretson, M.; Sibeck, D.; Chen, S.; Breneman, A.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2017 YEAR: 2017 DOI: 10.1002/2016JA022814 nonlinear phenomena; parametric processes; Van Allen Probes; wave/wave interactions |
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We analyze a wave event that occurred near noon between 07:03 and 07:08 UT on 23 February 2014 detected by the Van Allen Probes B spacecraft, where waves in the lower hybrid frequency range (LHFR) and electromagnetic ion cyclotron (EMIC) waves are observed to be highly correlated, with Pearson correlation coefficient of ~0.86. We assume that the correlation is the result of LHFR wave generation by the ions\textquoteright polarization drift in the electric field of the EMIC waves. To check this assumption the drift velocities of electrons and H+, He+, and O+ ions in the measured EMIC wave electric field were modeled. Then the LHFR wave linear instantaneous growth rates for plasma with these changing drift velocities and different plasma compositions were calculated. The time distribution of these growth rates, their frequency distribution, and the frequency dependence of the ratio of the LHFR wave power spectral density (PSD) parallel and perpendicular to the ambient magnetic field to the total PSD were found. These characteristics of the growth rates were compared with the corresponding characteristics of the observed LHFR activity. Reasonable agreement between these features and the strong correlation between EMIC and LHFR energy densities support the assumption that the LHFR wave generation can be caused by the ions\textquoteright polarization drift in the electric field of an EMIC wave. Khazanov, G.; Boardsen, S.; Krivorutsky, E.; Engebretson, M.; Sibeck, D.; Chen, S.; Breneman, A.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2017 YEAR: 2017 DOI: 10.1002/2016JA022814 nonlinear phenomena; parametric processes; Van Allen Probes; wave/wave interactions |
| 2016 |
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We present a statistical survey of the latitudinal structure of the fast magnetosonic wave mode detected by the Van Allen Probes spanning the time interval of 9/21/2012 to 8/1/2014. We show that statistically the latitudinal occurrence of the wave frequency (f) normalized by the local proton cyclotron frequency (fcP) has a distinct funnel shaped appearance in latitude about the magnetic equator similar to that found in case studies. By comparing the observed E/B ratios with the model E/B ratio, using the observed plasma density and background magnetic field magnitude as input to the model E/B ratio, we show that this mode is consistent with the extraordinary (whistler) mode at wave normal angles (θk) near 90\textdegree. Performing polarization analysis on synthetic waveforms composed from a superposition of extra-ordinary mode plane waves with θk randomly chosen between 87 and 90\textdegree, we show that the uncertainty in the derived wave normal is substantially broadened, with a tail extending down to θk of 60\textdegree, suggesting that another approach is necessary to estimate the true distribution of θk. We find that the histograms of the synthetically derived ellipticities and θk are consistent with the observations of ellipticities and θk derived using polarization analysis. We make estimates of the median equatorial θk by comparing observed and model ray tracing frequency dependent probability occurrence with latitude, and give preliminary frequency dependent estimates of the equatorial θk distribution around noon and 4 RE, with the median of ~4 to 7\textdegree from 90\textdegree at f /fcP = 2 and dropping to ~0.5\textdegree from 90\textdegree at f /fcP = 30. The occurrence of waves in this mode peaks around noon near the equator at all radial distances, and we find that the overall intensity of these waves increases with AE*, similar to findings of other studies. Boardsen, Scott; Hospodarsky, George; Kletzing, Craig; Engebretson, Mark; Pfaff, Robert; Wygant, John; Kurth, William; Averkamp, Terrance; Bounds, Scott; Green, Jim; De Pascuale, Sebastian; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2016 YEAR: 2016 DOI: 10.1002/2015JA021844 EMFISIS; Fast Magnetosonic Waves; latitudinal distribution; statistical study; Van Allen Probes; wave normal angle |
| 2015 |
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Low-harmonic magnetosonic waves observed by the Van Allen Probes Purely compressional electromagnetic waves (fast magnetosonic waves), generated at multiple harmonics of the local proton gyrofrequency, have been observed by various types of satellite instruments (fluxgate and search coil magnetometers and electric field sensors), but most recent studies have used data from search coil sensors, and many have been restricted to high harmonics. We report here on a survey of low-harmonic waves, based on electric and magnetic field data from the EFW double probe and EMFISIS fluxgate magnetometer instruments, respectively, on the Van Allen Probes spacecraft during its first full precession through all local times, from October 1, 2012 through July 13, 2014. These waves were observed both inside and outside the plasmapause (PP), at L shells from 2.4 to ~6 (the spacecraft apogee), and in regions with plasma number densities ranging from 10 to >1000 cm-3. Consistent with earlier studies, wave occurrence was sharply peaked near the magnetic equator. Waves appeared at all local times but were more common from noon to dusk, and often occurred within three hours after substorm injections. Outside the PP occurrence maximized broadly across noon, and inside the PP occurrence maximized in the dusk sector, in an extended plasmasphere. We confirm recent ray-tracing studies showing wave refraction and/or reflection at PP-like boundaries. Comparison with waveform receiver data indicates that in some cases these low-harmonic magnetosonic wave events occurred independently of higher-harmonic waves; this indicates the importance of including this population in future studies of radiation belt dynamics. Posch, J.; Engebretson, M.; Olson, C.; Thaller, S.; Breneman, A.; Wygant, J.; Boardsen, S.; Kletzing, C.; Smith, C.; Reeves, G.; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2015 YEAR: 2015 DOI: 10.1002/2015JA021179 equatorial noise; inner magnetosphere; magnetosonic waves; Van Allen Probes; waves in plasmas |
| 2014 |
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Van Allen Probe Observations of Periodic Rising Frequencies of the Fast Magnetosonic Mode Near simultaneous periodic dispersive features of fast magnetosonic mode emissions are observed by both Van Allen Probes spacecraft while separated in magnetic local time by ~5 hours: Probe A at 15 and Probe B at 9\textendash11 hours. Both spacecraft see similar frequency features, characterized by a periodic repetition at ~180 s. Each repetition is characterized by a rising frequency. Since no modulation is observed in the proton shell distribution, the plasma density, or in the background magnetic field at either spacecraft we conclude that these waves are not generated near the spacecraft but external to both spacecraft locations. Probe A while outside the plasmapause sees the start of each repetition ~40 s before probe B while deep inside the plasmasphere. We can qualitatively reproduce the dispersive features, but not the quantitative details. The cause for this phenomena remains to be identified. Boardsen, S.; Hospodarsky, G.; Kletzing, C.; Pfaff, R.; Kurth, W.; Wygant, J.; MacDonald, E.; Published by: Geophysical Research Letters Published on: 12/2014 YEAR: 2014 DOI: 10.1002/2014GL062020 Fast Magnetosonic Waves; Inner Dayside Magnetosphere; Periodic-Dispersive Features; Van Allen Probes |
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