An official website of the United States government
Abstract Man‐made very low frequency (VLF) transmitter waves play a critical role in energetic electron scattering and precipitation from the inner radiation belt, a type of which is called wisp precipitation. Wisps exhibit dispersive energy‐versus‐Lspectra due to the evolution of electron cyclotron resonance conditions with near‐monochromatic VLF transmitter waves. Here, we report on such observations of inner belt wisp precipitation events with full pitch angle resolution in the energy range of 50 to ∼500 keV as measured by Electron Loss and Fields Investigation (ELFIN) atL < ∼2 between March 2021 and April 2022. Statistical observations (82 events) reveal occasional (18 events) wisp precipitation events with local bounce‐loss‐cone electron flux enhancements, which provide new information compared with flux enhancements measured in previous studies only in the drift loss cone. Based on magnetic field and plasmaspheric density models, quasilinear theory, and detailed pitch angle distributions of wisps from ELFIN, we have estimated the wisp electron bounce‐averaged pitch angle diffusion coefficients to be of the order of 10−4to 10−2 s−1. These are several orders of magnitude larger than the diffusion rates calculated from models using global statistical averages of VLF transmitter wave power. When using our estimated diffusion coefficients to deduce the associated local transmitter wave amplitudes near the equator, based on quasilinear calculations from a transmitter‐induced electron diffusion model, we find these wave amplitudes to be >1 mV/m. Although probable overestimates, such inferred wave amplitudes exceed the theoretical threshold amplitude for nonlinear interactions, strongly suggesting that it is necessary to include nonlinear effects for an accurate evaluation of energetic electron scattering by transmitter waves.
more »
« less
Periodic Rising and Falling Tone ECH Waves From Van Allen Probes Observations
Abstract Electron cyclotron harmonic (ECH) waves are known to precipitate plasma sheet electrons into the upper atmosphere and generate diffuse aurorae. In this study, we report quasiperiodic rising (3 events) and falling tone (22 events) ECH waves observed by Van Allen Probes and evaluate their properties. These rising and falling tone ECH waves prefer to occur during quiet geomagnetic conditions over the dusk to midnight sector in relatively high‐density (10–80 cm−3) regions. Their repetition periods increase with increasingLshell atL < 6, ranging from ∼60 to 110 s. The wave element duration varies from 10 to 130 s peaking at ∼40 s and the chirping rate peaks at ∼50 (∼−50) Hz/s for rising (falling) tones. Our findings reveal intriguing features of the ECH wave properties, which provide new insights into their generation and potential effects on electron precipitation.
more »
« less
- Award ID(s):
- 1847818
- PAR ID:
- 10374435
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 48
- Issue:
- 7
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Although electron cyclotron harmonic (ECH) waves are the primary contributor to plasma sheet electron scattering loss, experimental verification of their most widely accepted excitation mechanism, loss‐cone instability, has been lacking for decades. Using 10 years of time history of events and macroscale interactions during substorms satellite observations, we investigate ECH wave properties near dipolarization fronts, the predominant source of such waves. To our surprise we find that more than 30% of observed ECH waves have moderately oblique (∼70°) wave normal angles (WNA), much less than the ∼85° expected from classical loss‐cone instability. These moderately oblique WNA ECH waves carry a strong field‐aligned electric field that is used to identify them. They are often observed with cold, dense electrons that exhibit enhanced parallel flux at a few hundred eV energy, which suggests that low‐energy counterstreaming beams (likely of ionospheric origin) might be their free energy source. By solving the linear dispersion relation for parameters representative of such plasma sheet electron distributions, we confirm that ECH waves at WNA ∼ 70° can indeed be driven unstable by such beams. Our work reveals a previously unknown excitation mechanism for ECH waves and exposes the need for quantifying the conditions for and relative importance of beam‐driven waves compared to those excited by the loss‐cone instability in Earth's plasma sheet.more » « less
-
Abstract Electron cyclotron harmonic (ECH) waves, potential drivers for diffuse aurora precipitation, have been extensively investigated for decades. The generation mechanism of ECH waves, however, remains an open question. Theoretical work in 1970s has demonstrated that ECH waves can be excited by loss cone distributions of hot plasma sheet electrons. Recent THEMIS spacecraft observations, however, indicate that the waves can also be excited by low energy electron beams. Utilizing interferometry techniques to analyze the phase difference between electric potentials measured by individual probes on Electric Field Instrument antenna pairs on THEMIS spacecraft, we compute the wavenumber of both beam‐driven ECH waves and loss‐cone‐driven ECH waves. These wavenumber measurements as well as other wave properties obtained from spacecraft measurements prove to be consistent with expectation from linear instability analysis. This provides us with independent verification of the generation mechanism and linear dispersion relation of beam‐driven and loss‐cone‐driven ECH waves. Our statistical results demonstrate that the median value of the wave vectors of beam‐driven ECH waves, characterized by wave normal angles () less than 80°, is 0.011 m−1; and that of loss‐cone‐driven ECH waves, characterized by wave normal angles larger than 85°, is 0.00765 m−1. Direct wavenumber measurements of ECH waves allow us to better understand the interaction between ECH waves and electrons in Earth's magnetosphere.more » « less
-
Abstract Electron cyclotron harmonic waves (ECH) play a key role in scattering and precipitation of plasma sheet electrons. Previous analysis on the resonant interaction between ECH waves and electrons assumed that these waves are generated by a loss cone distribution and propagate nearly perpendicular to the background magnetic field. Recent spacecraft observations, however, have demonstrated that such waves can also be generated by low energy electron beams and propagate at moderately oblique angles . To quantify the effects of this newly observed ECH wave mode on electron dynamics in Earth's magnetosphere, we use quasi‐linear theory to calculate the associated electron pitch angle diffusion coefficient. Utilizing THEMIS spacecraft measurements, we analyze in detail a few representative events of beam‐driven ECH waves in the plasma sheet and the outer radiation belt. Based on the observed wave properties and the hot plasma dispersion relation of these waves, we calculate their bounce‐averaged pitch angle, momentum and mixed diffusion coefficients. We find that these waves most efficiently scatter low‐energy electrons (10–500 eV) toward larger pitch angles, on time scales of to seconds. In contrast, loss‐cone‐driven ECH waves most efficiently scatter higher‐energy electrons (500 eV–5 keV) toward lower pitch‐angles. Importantly, beam‐driven ECH waves can effectively scatter ionospheric electron outflows out of the loss cone near the magnetic equator. As a result, these outflows become trapped in the magnetosphere, forming a near‐field‐aligned anisotropic electron population. Our work highlights the importance of ECH waves, particularly beam‐driven modes, in regulating magnetosphere‐ionosphere particle and energy coupling.more » « less
-
Abstract Electron resonant scattering by whistler‐mode waves is one of the most important mechanisms responsible for electron precipitation to the Earth's atmosphere. The temporal and spatial scales of such precipitation are dictated by properties of their wave source and background plasma characteristics, which control the efficiency of electron resonant scattering. We investigate these scales with measurements from the two low‐altitude Electron Losses and Fields Investigation (ELFIN) CubeSats that move practically along the same orbit, with along‐track separations ranging from seconds to tens of minutes. Conjunctions with the equatorial THEMIS mission are also used to aid our interpretation. We compare the variations in energetic electron precipitation at the sameL‐shells but on successive data collection orbit tracks by the two ELFIN satellites. Variations seen at the smallest inter‐satellite separations, those of less than a few seconds, are likely associated with whistler‐mode chorus elements or with the scale of chorus wave packets (0.1–1 s in time and ∼100 km in space at the equator). Variations between precipitationL‐shell profiles at intermediate inter‐satellite separations, a few seconds to about 1 min, are likely associated with whistler‐mode wave power modulations by ultra‐low frequency waves, that is, with the wave source region (from a few to tens of seconds to a few minutes in time and ∼1,000 km in space at the equator). During these two types of variations, consecutive crossings are associated with precipitationL‐shell profiles very similar to each other. Therefore the spatial and temporal variations at those scales do not change the net electron loss from the outer radiation belt. Variations at the largest range of inter‐satellite separations, several minutes to more than 10 min, are likely associated with mesoscale equatorial plasma structures that are affected by convection (at minutes to tens of minutes temporal variations and ≈[103, 104] km spatial scales). The latter type of variations results in appreciable changes in the precipitationL‐shell profiles and can significantly modify the net electron losses during successive tracks. Thus, such mesoscale variations should be included in simulations of the radiation belt dynamics.more » « less
