Signals from very‐low‐frequency transmitters on the ground are known to induce energetic electron precipitation from the Earth's radiation belts. The effectiveness of this mechanism depends on the ...propagation characteristics of those signals in the magnetosphere, and in particular whether the signals are ducted or nonducted along channels of enhanced plasma density, analogous to optical fibers. Here we perform a statistical analysis of in‐situ waveform data collected by the Van Allen Probes satellites that shows that nonducted propagation dominates over ducted propagation in both the occurrence and intensity of the waves. Ray tracing confirms that the latitudinal distribution of wavevectors corresponds to nonducted as opposed to ducted propagation. Our results show the dominant mode of propagation needed to quantify transmitter‐induced precipitation and improve the forecast of electron radiation belt dynamics for the safe operation of satellites.
Plain Language Summary
Very Low Frequency (VLF) signals emitted from ground‐based transmitters can appear in the Earth's magnetosphere, propagating either in ducted mode along magnetic lines or in nonducted mode. The different propagation modes affect how the signals interact with energetic electrons of the radiation belts and precipitate them into the ionosphere. In this study, we present a statistical study using the observations of Alpha transmitter signals by Van Allen Probes satellites and show that nonducted propagation mode dominates over ducted propagation mode in both signal occurrence and wave intensity. Our result is also supported by ray‐tracing simulations. Our study not only resolves the relative contribution of different propagation modes, but also provides critical parameters for quantifying precipitation by VLF transmitter signals and therefore improving forecast of radiation belt electron dynamics.
Key Points
First statistical wave normal analysis on very low frequency (VLF) transmitter signals in the magnetosphere is presented
Dominance of nonducted signals is revealed in the magnetosphere
The power proportions of ducted and nonducted signals are given as a function of L and Kp index
Short‐lived (<1 s) but intense electron precipitation, known as “microbursts,” may contribute significantly to electron losses in the outer radiation belt. Their origin has been suggested to ...correlate with resonant scattering by whistler‐mode chorus waves, but existing models cannot fully explain the properties of microbursts, in particular, the bouncing electron packets in the form of a microburst that have been recently observed. A numerical model is presented that reproduces a series of electron bounce packets in response to individual chorus elements. Results indicate that the actual precipitation only occurs in the leading electron packet whereas subsequent packets form because of the following bounce motions of remaining fluxes. An analysis based on wave propagation and resonance condition yields an approximate time‐energy regime of electron microbursts. Such a model is valuable for interpreting and modeling low Earth‐orbiting satellite observations of electron flux variation in response to the interaction with magnetospheric chorus waves.
Key Points
A numerical model of electron flux variation to chorus elements is presented
Bouncing electron packets are reproduced
Electron microburst duration is estimated by the arrival time‐energy dispersion analysis
Chorus subpackets/subelements are the wave packets occurring at intervals of ∼10–100 msec and are suggested to play a crucial role in the formation of substructures within pulsating aurora. In this ...study, we investigate the evolution of subpackets from the upstream to downstream regions. Using Van Allen Probe A measurements, we have found that the frequency of the upstream subpackets increases smoothly, but that of the downstream subpackets remains almost unchanged. Through a simulation in the real‐size magnetosphere, we have reproduced the subpackets with characteristics similar to those in observations, and revealed that the frequency chirping is influenced by both resonant current of electrons and wave amplitude due to nonlinear physics. Although the resonant currents in the upstream and downstream regions are comparable, the wave amplitude increases significantly during evolution, resulting in lower sweep rate in the downstream region. Our findings provide a fresh insight into the evolution of chorus subpackets.
Plain Language Summary
Subpackets within chorus waves are suggested to play a significant role in producing the substructures within pulsating aurora. How does the frequency change inside subpackets is still an open question. In this study, the subpackets are found with Van Allen Probe A observation to be excited upstream of the magnetic equator, and propagate toward downstream. The frequency of subpackets increases with time in the upstream region, while it keeps almost unchanged in the downstream region. Meanwhile, a particle‐in‐cell simulation has been performed to study the characteristics of subpackets, and the simulation results agree well with those in observations. The frequency variation of subpackets is influenced by both resonant electrons and wave amplitude. Our study provides a clue for better understanding the nonlinear wave‐particle interactions in the evolution of chorus subpackets.
Key Points
The source region of chorus subpackets has been observed by Van Allen Probe A
The chorus subpackets have been investigated via the general curvilinear particle‐in‐cell simulation in the real‐size magnetosphere
Nonlinear physics is a dominant process in the evolution of chorus subpackets
The frequency chirping of chorus waves is commonly observed in the Earth’s inner magnetosphere, but its generation remains an open question. Recently, Liu et al. (2021), ...https://doi.org/10.1029/2021JA029258 reported two unusual rising‐tone (upward chirping) chorus elements. Although the central frequency of constituent subpackets rises, the frequency of a single subpacket is surprisingly downward chirping. With a gcPIC‐δf $\delta f$ simulation in the dipole field, we successfully reproduce this kind of substructure, which contains alternating signs of chirping. Interestingly, both hole and hill structures are formed around the theoretical resonant velocities in the electron phase space, no matter whether the chirping is upward or downward. However, during each chirping interval, only one structure (either a hole or a hill) is associated with wave excitation: the upward chirping is related to the hole, while the hill contributes to the downward chirping. Our study provides a fresh perspective on the theory of frequency chirping in chorus waves.
Plain Language Summary
The frequency chirping is a typical feature of chorus waves in the Earth’s inner magnetosphere, which generally contain either rising‐tone (upward chirping) elements or falling‐tone (downward chirping) elements. Previous theory has suggested that the chirping is due to the nonlinear wave‐particle interaction, where the hole or hill structure is formed in the electron phase space. Recently, Liu et al. (2021), https://doi.org/10.1029/2021JA029258 have observed the upward chirping elements with their subpackets of downward chirping. What electron structure is associated with these elements becomes a puzzle. With a one‐dimensional (1D) general curvilinear particle‐in‐cell (gcPIC) δf simulation in the dipole magnetic field, we successfully reproduce this kind of chorus element, whose frequency contains alternating upward and downward chirping. Interestingly, both the hole and hill structures are formed during a chirping interval, but only one of the two structures is responsible for wave excitation and frequency chirping. The structure of hole‐hill combination provides an important clue into the theory of the frequency chirping in chorus waves.
Key Points
With a gcPIC‐δf $\delta f$ simulation in the dipole field, we reproduce the upward chirping chorus element, whose subpackets are downward chirping
Both hole and hill structures can be formed in the ζ−v‖ $\zeta -{v}_{\Vert }$ phase space, no matter whether the frequency is upward or downward chirping
The time evolution of the hole and hill structures in the phase space leads to the alternating frequency chirping
Russian Alpha radio navigation system (RSDN‐20) emits F1 = 11.9 kHz signals into the magnetosphere which propagate as whistler mode waves. Observed by waveform continuous burst mode from Electric and ...Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on Van Allen Probes, a case is presented and featured with ducted propagation, multiple reflections, and triggered emissions. Both risers and fallers appear in the triggered emissions. We use a ray tracing method to demonstrate ducted propagation, which has a similar wave normal angle near 150° as the observation. The arrival time of reflected signals is estimated using propagation analysis and compared with the observed signal arrival time. To test the nonlinear cyclotron resonance theory, the interaction region scale and the order of chirping rate in triggered emission are estimated. The estimated interaction region scale of MLAT = −3° is smaller than the observed MLAT = −6°. The discrepancy may be caused by the parallel propagation assumption and background field model.
Key Points
Both ducted reflected signals and triggered emissions from transmitters are observed in the magnetosphere
Ducted propagation is confirmed using time delay analysis and ray tracing model
The interaction region for nonlinear growth is determined to be off the equator
A 2‐D GCPIC simulation in a dipole field system has been conducted to explore the excitation of oblique whistler mode chorus waves driven by energetic electrons with temperature anisotropy. The ...rising tone chorus waves are initially generated near the magnetic equator, consisting of a series of subpackets, and become oblique during their propagation. It is found that electron holes in the wave phase space, which are formed due to the nonlinear cyclotron resonance, oscillate in size with time during subpacket formation. The associated inhomogeneity factor varies accordingly, giving rise to various frequency chirping in different phases of subpackets. Distinct nongyrotropic electron distributions are detected in both wave gyrophase and stationary gyrophase. Landau resonance is found to coexist with cyclotron resonance. This study provides multidimensional electron distributions involved in subpacket formation, enabling us to comprehensively understand the nonlinear physics in chorus wave evolution.
Plain Language Summary
Subpackets are a series of wave packets within chorus waves, characterized by wave amplitude modulation. In this study, we investigate the electron distributions in various phase spaces associated with subpacket formation, by performing a two‐dimensional simulation in a dipole field. It is found that the electrons can be trapped in the wave phase space through both cyclotron and Landau resonances. These two resonance interactions can also produce the “bump” and “plateau” shapes in momentum space, as well as the fine density structures in spatial space. Therefore, both cyclotron and Landau resonances play an important role in subpacket formation. Our study provides new inspiration for the nonlinear theory of chorus subpackets.
Key Points
Oblique chorus subpackets are generated in the 2‐D GCPIC simulation model
Electron hole associated with the inhomogeneity factor oscillates with time during subpacket formation
Cyclotron and Landau resonances coexist during subpacket formation
Chorus subpackets are the wave packets with modulated amplitudes in chorus waves, commonly observed in the magnetospheres of Earth and other planets. Nonlinear wave‐particle interactions have been ...suggested to play an important role in subpacket formation, yet the corresponding electron dynamics remain not fully understood. In this study, we have investigated the electron trapping through cyclotron resonance with subpackets, using a self‐consistent general curvilinear plasma simulation code simulation model in dipole fields. The electron trapping period has been quantified separately through electron dynamic analysis and theoretical derivation. Both methods indicate that the electron trapping period is shorter than the subpacket period/duration. We have further established the relation between electron trapping period and subpacket period through statistical analysis using simulation and observational data. Our study demonstrates that the nonlinear electron trapping through cyclotron resonance is the dominant mechanism responsible for subpacket formation.
Plain Language Summary
The spectrum of chorus waves comprises a series of subpackets, characterized by modulated amplitudes within a timescale of ∼10–100 milliseconds. In this study, we have investigated the self‐consistent wave‐particle interactions with subpackets, using two‐dimensional particle‐in‐cell simulations in dipole fields. Cyclotron resonant electrons are trapped in wave phases, and we have measured their trapping period. Since these electrons move in the opposite direction of subpacket propagation, the corresponding trapping period is smaller than the period of subpackets. We have further established the relation between the two periods and validated it through both simulation and observational data. This relation facilitates evaluating electron trapping period from direct measurement of subpackets in observations. Our study sheds important lights on the key role of nonlinear electron trapping through cyclotron resonance in the formation of subpackets.
Key Points
Electron trapping dynamics in the formation of quasi‐parallel chorus subpackets have been investigated
The linkage between electron trapping period and subpacket period is quantified via a geometric relation, where the trapping period is shorter
The proposed relation between electron trapping period and subpacket period is an extension of the classical results of O’Neil (1965)
Signals of powerful ground transmitters at various places have been detected by satellites in near‐Earth space. The study on propagation mode, ducted or nonducted, has attracted much attentions for ...several decades. Based on the statistical results from Van Allen Probes (data from October 2012 to March 2017) and DEMETER satellite (from January 2006 to December 2007), we present the ground transmitter signals distributed clearly in ionosphere and magnetosphere. The observed propagation route in the meridian plane in the magnetosphere for each of various transmitters from the combination of DEMETER and Van Allen Probes data in nighttime is revealed for the first time. We use realistic ray tracing simulation and compare simulation results against Van Allen Probes and DEMETER observation. By comparison we demonstrate that the observed propagation route, with partial deviation from the field lines corresponding to ground stations, provides direct and clear statistical evidence that the nonducted propagation mode plays a main role, although with partial contribution from ducted propagation. The propagation characteristics of VLF transmitter signals in the magnetosphere are critical for quantitatively assessing their contribution to energetic electron loss in radiation belts.
Plain Language Summary
The topic on signal propagation of ground‐based VLF transmitter in ionosphere and magnetosphere has attracted much attention in several decades. But the question about the propagation mode (ducted or nonducted) has not been answered definitely due to lacks of direct support from statistical satellite data. In this paper, the observed propagation route in the meridian plane in the magentosphere for each of various transmitter from the combination of DEMETER and Van Allen Probes data is revealed for the first time. Based on satellite observation and ray tracing simulation, we provide a clear and strong proof that the nonducted mode of transmitter signal plays a main role. The determination of the propagation mode will play a significant role in many fields, including the wave propagation model development and electron population adjustment as radiation belt remediation, which aims at lowering the damage of relativistic particles to the spacecraft and astronaut.
Key Points
Satellite observation of the VLF ground‐based transmitter signals in the ionosphere and magnetosphere is presented
Ray tracing simulation reproduces propagation routine of transmitter signals in the magnetosphere
The observation and simulation both support that the nonducted mode is dominant
Inflammation plays a key role in pathogenesis and rupture of aneurysms. Non-invasively and dynamically monitoring aneurysm inflammation is critical. This study evaluated myeloperoxidase (MPO) as an ...imaging biomarker and therapeutic target for aneurysm inflammation using an elastase-induced rabbit model treated with or without 4-aminobenzoic acid hydrazide (ABAH), an irreversible inhibitor of MPO. Myeloperoxidase-sensitive magnetic resonance imaging (MRI) using Mn-TyrEDTA, a peroxidase activity-dependent contrast agent, revealed weak contrast enhancement in contralateral arteries and decreased contrast enhancement in aneurysm walls with ABAH treatment, indicating MPO activity decreased and inflammation mitigated. This was supported by reduced immune cell infiltration, matrix metalloproteinases (MMP-2 and - 9) activity, ROS production and arterial wall destruction on histology. Finally, the aneurysm expansion rate remained < 50% throughout the study in the ABAH(+) group, but increased gradually in the ABAH(-) group. Our results suggest that inhibition of MPO attenuated inflammation and expansion of experimental aneurysm and MPO-sensitive MRI showed promise as a noninvasive tool for monitoring aneurysm inflammation.