The whistler‐mode chorus emission, a major driver of radiation belt electron energization and precipitation, exhibits significant amplitude modulations on millisecond timescales. These subpacket ...modulations are accompanied by fast changes in the wave normal angle. Understanding the evolution of wave propagation properties inside chorus elements is essential for modeling nonlinear chorus‐electron interactions, but the origin of these rapid changes is unclear. We propose that the variations come from propagation inside thin, field‐aligned cold plasma enhancements (density ducts), which produce differing modulations in parallel and perpendicular wave magnetic field components. We show that a full‐wave simulation on a filamented density background predicts wave vector and amplitude evolution similar to Van Allen Probes spacecraft observations. We further demonstrate that the commonly assumed wide density ducts, in which wave propagation can be studied with ray tracing methods, cannot explain the observed behavior. This indirectly proves the existence of wavelength‐scale field‐aligned density fluctuations.
Plain Language Summary
The evolution of the Earth's outer radiation belt on short timescales is largely determined by interactions of particles with high‐amplitude electromagnetic waves. One type of these electromagnetic emissions, the whistler‐mode chorus, exhibits substantial variations in amplitude and propagation direction on the scale of milliseconds. Such rapid changes influence the interaction between the wave and resonant electrons. It is known that the global propagation properties of chorus can be explained by assuming the presence of increases and decreases in plasma density stretched along magnetic field lines (so‐called density ducts). We assume the existence of wavelength‐scale density ducts and compare two‐dimensional solutions of wave equations with chorus signals detected by the Van Allen Probes spacecraft. We demonstrate that, unlike wide ducts, the small‐scale irregularities can well explain the observed local wave propagation properties. Our simulations thus indirectly prove the existence of small‐scale density fluctuations, which should be accounted for in the analysis of the fine structure of all magnetospheric whistler wave signals.
Key Points
Propagation of lower‐band chorus subpackets near their equatorial source is simulated with finite‐difference time‐domain methods
Narrow, field‐aligned density enhancements (ducts) create different amplitude modulations in parallel and perpendicular wave components
Due to the modulation mismatch, instantaneous wave normal angles exhibit rapid variations, matching the behavior observed by spacecraft
Van Allen Probes observations are used to statistically investigate plasmaspheric hiss wave properties. This analysis shows that the wave normal direction of plasmaspheric hiss is predominantly field ...aligned at larger L shells, with a bimodal distribution, consisting of a near‐field aligned and a highly oblique component, becoming apparent at lower L shells. Investigation of this oblique population reveals that it is most prevalent at L < 3, frequencies with f/fce>0.01 (or f > 700 Hz), low geomagnetic activity levels, and between 1900 and 0900 magnetic local time. This structure is similar to that reported for oblique chorus waves in the equatorial region, perhaps suggesting a causal link between the two wave modes. Ray tracing results from HOTRAY confirm that it is feasible for these oblique chorus waves to be a source of the observed oblique plasmaspheric hiss population. The decrease in oblique plasmaspheric hiss occurrence rates during more elevated geomagnetic activity levels may be attributed to the increase in Landau resonant electrons causing oblique chorus waves to be more substantially damped outside of the plasmasphere. In turn, this restricts the amount of wave power that can access the plasmasphere and evolve into oblique plasmaspheric hiss. These results confirm that, despite the difference in location of this bimodal distribution compared to previous studies, a direct link between oblique equatorial chorus outside of the plasmasphere and oblique hiss at low L shells is plausible. As such, these results are in keeping with the existing theory of chorus as the source of plasmaspheric hiss.
Key Points
Two distinct populations of plasmaspheric hiss are observed, one more field aligned and one more oblique, particularly at low L shells (L < 3)
Oblique hiss is most prevalent during low geomagnetic activity levels and between 1900 and 0900 MLT‐similar to oblique chorus waves
Ray tracing confirms that oblique hiss at low L shells is in keeping with the existing theory of chorus as the source of plasmaspheric hiss
We conduct test particle simulations to study the perturbations in a hot electron velocity distribution caused by a rising chorus element propagating parallel to the ambient magnetic field in the ...Earth's outer radiation belt. The wavefield is constructed from the nonlinear growth theory of chorus emissions of Omura (2021, https://doi.org/10.1186/s40623-021-01380-w), with additional considerations about saturation and propagation of the transverse resonant current being applied to model the subpacket structure. Using Liouville's theorem, we trace electrons back in time to reconstruct the evolution of electron velocity distribution at the magnetic equator. The electromagnetic hole created by nonlinear trapping and transport effects appears as a depression in the velocity distribution, aligned with the resonance velocity curve. We analyze the decrease of particle flux in this depression and estimate the energy resolution, pitch angle resolution, time resolution and geometric factor of particle analyzers needed to observe the perturbation. We conclude that particle detectors on current or recently operating spacecraft are always lacking in at least one of these parameters, which explains the missing direct observations of sharp phase space density depressions during chorus‐electron nonlinear resonant interaction. However, with a dedicated experiment and appropriate measurement strategy, such observations are within the possibilities of the current technology. Similarity of the simulated density perturbation and a step function mathematical model is used to draw an analogy between the backward wave oscillator regime of chorus generation and the nonlinear growth theory.
Plain Language Summary
The plasma environment in the Earth's magnetosphere supports natural growth of various electromagnetic waves, including the whistler‐mode chorus emissions, which consist of nonlinear chirping tones. These emissions can reach large amplitudes and play a major role in energization of radiation belt electrons. Nonlinear theories of chorus generation imply microscopic perturbations in resonant electron populations. A long‐standing problem is that these predictions were never directly confirmed by experimental observations. Here, we analyze perturbations of electron distribution functions numerically, taking into account spacecraft measurements of short subpackets within each chirping element. We reveal distinct perturbations, which are just below the measurability limits of existing spacecraft instruments. We thus explain the current absence of direct measurements of nonlinear effects of chorus on the electron distribution functions. We also suggest measurement strategies for future spacecraft instruments that can increase the number of detected interaction events.
Key Points
We analyze perturbations in a hot electron distribution caused by nonlinear interactions with a model chorus element with fine structure
A stripe structure of phase space density depletions and elevations are observed, associated with individual subpackets
Resolution of spacecraft instruments required to observe the leading most prominent stripe is estimated
Using observations from the Van Allen Probes EMFISIS instrument, coupled with ray tracing simulations, we determine the fraction of chorus wave power with the conditions required to access the ...plasmasphere and evolve into plasmaspheric hiss. It is found that only an extremely small fraction of chorus occurs with the required wave vector orientation, carrying only a small fraction of the total chorus wave power. The exception is on the edge of plasmaspheric plumes, where strong azimuthal density gradients are present. In these cases, up to 94% of chorus wave power exists with the conditions required to access the plasmasphere. As such, we conclude that strong azimuthal density gradients are actually a requirement if a significant fraction of chorus wave power is to enter the plasmasphere and be a source of plasmaspheric hiss. This result suggests it is unlikely that chorus directly contributes a significant fraction of plasmaspheric hiss wave power.
Plain Language Summary
Plasmaspheric hiss waves are typically observed inside a high‐density region of geospace known as the plasmasphere. Chorus waves are typically observed at higher altitudes, beyond the plasmasphere region, where the density is substantially lower. Despite the differences between these two wave types, it has been proposed that chorus waves may propagate in such a way that they enter the plasmasphere, where they become a source of plasmaspheric hiss. However, this mechanism can only occur if chorus waves have a specific set of initial conditions. In this study, we find that chorus waves are rarely observed with these required conditions. Only in a spatially limited region close to the edge of plasmaspheric plume structures, where chorus wave power is typically weaker, do we observe a significant fraction of chorus waves that exist with the conditions required to propagate into the plasmasphere. This result qualitatively indicates that chorus waves may not be a substantial source of plasmaspheric hiss.
Key Points
On a global scale, chorus waves rarely exist with the wave vector orientation required to access the plasmasphere and evolve into hiss
Only in a small region on the duskside, close to plumes, can a substantial fraction of chorus waves propagate into the plasmasphere
This spatial limitation qualitatively indicates that chorus waves may only contribute a small fraction of the plasmaspheric hiss wave power
Quasiperiodic (QP) emissions are whistler‐mode electromagnetic waves observed in the Earth's inner magnetosphere whose intensity has a nearly periodic time modulation with typical modulation periods ...on the order of minutes. Some events exhibit, on top of the main modulation period, an additional fine inner modulation with modulation periods on the order of seconds. We use high‐resolution multi‐component electromagnetic wave data obtained by the Van Allen Probes spacecraft to investigate one such event. Detailed wave analysis demonstrates that the fine inner modulation is due to a wave packet bouncing back and forth between the hemispheres. The presence of a density duct is important for the formation of the event, as demonstrated by the increased ratio of wave power propagating away from the equator (a tentative source region) within the duct. The main QP modulation period corresponds to the plasma number density modulation observed just outside the plasmasphere.
Plain Language Summary
The intensity of electromagnetic waves in the near‐Earth space, the magnetosphere, sometimes has a nearly periodic temporal modulation on the order of minutes. The origin of such waves, so‐called quasiperiodic emissions, is not yet fully understood. On top of the main modulation period, some events exhibit an additional fine inner modulation with periods on the order of seconds. We use wave propagation directions determined from the Van Allen Probes measurements to demonstrate that this shorter modulation corresponds to a wave packet bouncing in between the hemispheres. By examining the ratio of wave power propagating away from and toward the geomagnetic equator (a tentative source region), we further demonstrate that the presence of a region with enhanced density, guiding waves along a given magnetic field line, is important for the formation of the event. Additionally, the main modulation period of the event corresponds to the plasma number density modulation observed just outside the plasmasphere, possibly linked to a plasmapause surface wave. Our results, revealing the presence and origin of the fine inner structure of the waves, provide important observational constraints for models trying to explain the generation of quasiperiodic emissions.
Key Points
Whistler‐mode quasiperiodic event has a fine inner structure that is related to the wave bouncing between hemispheres
Wave generation is related to the presence of a density duct, but the event can also be observed outside the duct
Plasma number density just outside the plasmasphere is modulated with a period that corresponds to the period of the quasiperiodic event
We report the first observations of negative intracloud (IC) dart‐stepped leaders accompanied by regular trains of microsecond‐scale pulses, simultaneously detected by shielded broadband magnetic ...loop antennas and the radio telescope Low Frequency Array (LOFAR). Four investigated pulse trains occurred during complicated IC flashes on 18 June 2021, when heavy thunderstorms hit the Netherlands. The pulses within the trains are unipolar, a few microseconds wide, and with an average inter‐pulse interval of 5–7 μs. The broadband pulses perfectly match energetic, regularly distributed, and relatively isolated bursts of very high frequency sources localized by LOFAR. All trains were generated by negative dart‐stepped leaders propagating at a lower speed than usual dart leaders. They followed channels of previous leaders occurring within the same flash several tens of milliseconds before the reported observations. The physical mechanism remains unclear as to why we observe dart‐stepped leaders, which show mostly regular stepping, emitting energetic microsecond‐scale pulses.
Plain Language Summary
Lightning phenomena inside thunderclouds can be explored using their electromagnetic radiation. To study these processes at small temporal and spatial scales, we combine broadband magnetic loop antennas with the Low Frequency Array (LOFAR) radio telescope. Measurements of broadband antennas acquired during a severe Dutch thunderstorm showed pulse sequences composed of tens microsecond‐scale unipolar pulses, which were surprisingly regularly distributed. Such regular pulse trains have been rarely reported from previous observations. When we thoroughly lined up the timestamps of both simultaneously measuring observational systems, we found that the regular broadband pulses perfectly match with localized isolated bursts of energetic very high frequency radiation detected by LOFAR. The 3D mapping of the radio sources of these bursts allowed us to place the investigated events into the context of the parent intracloud (IC) lightning flash. The results revealed negative IC dart leaders, which propagated along the preconditioned channels originally formed by previous positive or negative IC leaders. Some of these dart leaders then exhibited unusual stepping manifested by the observed regular pulses. We assume that a favorable combination of the conductivity of preexisting lightning channels and the strength of the ambient electric field inside thunderclouds might be needed to trigger this unusual stepping.
Key Points
We observed intracloud negative dart‐stepped leaders producing regular trains of broadband electromagnetic microsecond‐scale pulses
Very High Frequency sources follow channels of previous leaders occurring within the same flash tens of milliseconds before the reported observations
Conductivity of decaying channels and strength of the ambient electric field might act together to trigger this unusual stepping process
Effects of whistler mode hiss waves in March 2013 Ripoll, J.‐F.; Santolík, O.; Reeves, G. D. ...
Journal of geophysical research. Space physics,
July 2017, 2017-07-00, 20170701, Letnik:
122, Številka:
7
Journal Article
Recenzirano
We present simulations of the loss of radiation belt electrons by resonant pitch angle diffusion caused by whistler mode hiss waves for March 2013. Pitch angle diffusion coefficients are computed ...from the wave properties and the ambient plasma data obtained by the Van Allen Probes with a resolution of 8 h and 0.1 L shell. Loss rates follow a complex dynamic structure, imposed by the wave and plasma properties. Hiss effects can be strong, with minimum lifetimes (of ~1 day) moving from energies of ~100 keV at L ~ 5 up to ~2 MeV at L ~ 2 and stop abruptly, similarly to the observed energy‐dependent inner belt edge. Periods when the plasmasphere extends beyond L ~ 5 favor long‐lasting hiss losses from the outer belt. Such loss rates are embedded in a reduced Fokker‐Planck code and validated against Magnetic Electron and Ion Spectrometer observations of the belts at all energy. Results are complemented with a sensitivity study involving different radial diffusion and lifetime models. Validation is carried out globally at all L shells and energies. The good agreement between simulations and observations demonstrates that hiss waves drive the slot formation during quiet times. Combined with transport, they sculpt the energy structure of the outer belt into an “S shape.” Low energy electrons (<0.3 MeV) are less subject to hiss scattering below L = 4. In contrast, 0.3–1.5 MeV electrons evolve in an environment that depopulates them as they migrate from L ~ 5 to L ~ 2.5. Ultrarelativistic electrons are not affected by hiss losses until L ~ 2–3.
Key Points
Computations of daily pitch angle diffusion coefficients and electron lifetimes from properties of hiss waves observed in March 2013
Good agreement found between MagEIS flux observations and 1‐D Fokker‐Planck simulations based on our hiss loss term for quiet times
Combined with transport, hiss waves loss drives the daily energy structure of the radiation belts, with a typical S‐shaped outer belt
Normally, auroral kilometric radiation (AKR) which is emitted in the auroral zones escapes from the Earth. But since a few decades very similar radiations are observed by ground‐based receivers and ...by satellites at altitudes below the AKR generation area. They are called leaked AKR or AKR‐like emissions because it is expected that there are linked to AKR. This paper deals with observations of such AKR‐like emissions observed in the auroral zones (in the North and in the South) by the low‐altitude satellite DEMETER. In total, 2,526 events have been recorded during 6.5 years. These events are not very rare as they occur at least 2% of the time. Although this data set has a severe flaw due to a latitudinal constraint, it was possible to draw interesting properties of these emissions. In fact they are very similar to usual AKR observed at much higher altitudes during auroral activities (the same frequency range, magnetic local time (MLT) sector, and invariant latitude). The main difference concerns a strong asymmetry between the Northern and the Southern hemispheres: (a) the number of AKR‐like emissions in the Northern hemisphere is 32% larger than in the Southern hemisphere but this percentage decreases when the auroral activity increases, and (b) there is an important seasonal effect because the number of events decreases during the winter season both in the North and in the South.
Plain Language Summary
Auroral kilometric radiation (AKR) is the strongest terrestrial radio emission generated between 30 and 800 kHz along the auroral field lines and associated with a discrete auroral arc. This emission is generated by superthermal electrons (several keV) which are injected from the magnetotail when the solar activity increases. Typical location of the source is ∼22 hr of magnetic local time (MLT), ∼70° of invariant latitude, and (2–10)103 km of altitude. AKR propagates from the Earth and should not be able to reach low altitudes because of the ionospheric frequency cutoff. But a few decades ago, AKR‐like emissions have been recorded by low orbiting satellites and even by ground‐based receivers. It was explained by a mode conversion of the wave propagating in inhomogeneous plasma. Up to now, only studies of some AKR‐like events have been reported, and this paper presents the statistical properties of such emissions recorded by the low altitude satellite DEMETER during 6.5 years. AKR‐like emissions display similar characteristics of AKR in terms of frequency, MLT, and invariant latitude. It is shown that more events are observed in the North than in the South, and that there is a seasonal effect (the number of events decreases in winter hemispheres).
Key Points
Numerous auroral kilometric radiation‐like events have been recorded by the low‐altitude satellite DEMETER during moderate and high magnetic activity
There are more events in the Northern than in the Southern hemisphere but this difference decreases when the auroral activity increases
The number of events decreases in winter in each hemisphere
Using Poynting vector measurements of whistler mode chorus emissions detected by the THEMIS spacecraft within the source region, that is, close to the magnetic field minimum, we found both in ...individual events and statistically that chorus elements propagating equatorward had systematically higher frequencies and smaller amplitudes compared with simultaneously observed elements propagating away from the equator. We demonstrate similar features in the results of numerical simulations based on backward wave oscillator equations. It can be qualitatively explained by the nonlinear evolution of the energetic electron distribution function during wave generation. The motion of electrons from the equator is accompanied by a decrease in their velocity component along the magnetic field line due to both the adiabatic mirror force and nonlinear wave‐particle interactions. Thus, the frequency of the chorus elements generated by such electrons and propagating equatorward is higher compared with the elements propagating away from the equator.
Key Points
THEMIS multicomponent wave data on VLF chorus in the source region are analyzed
Opposite Poynting flux directions in shifted frequency bands are observed
Theoretical explanation based on the backward wave oscillator model is proposed
Alpha navigation transmitters are very low frequency (VLF) transmitters operating at mid‐latitudes, which use a specific discrete radiation pattern at three distinct frequencies (11.9, 12.6, and ...14.9 kHz). The transmitters are located in the northern hemisphere, but the radiated signals propagate through the magnetosphere to the conjugate hemisphere, where they are detectable by low‐altitude spacecraft. We present an analysis of such signals detected by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions spacecraft at an altitude of about 660 km. It is found that, due to a Doppler shift, the observed signal frequencies can be at times rather different than the radiated frequencies. This indicates wave propagation at large wave normal angles (close to the resonance cone). Simultaneous observations of the same signal with different Doppler shifts reveal three distinct ways of signal propagation: (i) ducted propagation, (ii) unducted propagation, and (iii) propagation interpreted as only partially ducted. A raytracing analysis is employed to obtain typical wave trajectories corresponding to the individual ways of signal propagation and respective Doppler shifts. A reasonable agreement between the observed and calculated Doppler shifts is obtained. Our results demonstrate the peculiarities of VLF signal propagation throughout the magnetosphere and the possibility of using Doppler shifts to estimate wave normal angles.
Key Points
Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions measurements conjugate to Alpha transmitters show three distinct ways of signal propagation
Two of them are identified as ducted and unducted propagation, the third is likely only partially ducted
A raytracing analysis is employed to explain the observed Doppler shifts
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