Origin of two-band chorus in the radiation belt of Earth

Abstract Naturally occurring chorus emissions are a class of electromagnetic waves found in the space environments of the Earth and other magnetized planets. They play an essential role in accelerating high-energy electrons forming the hazardous radiation belt environment. Chorus typically occurs in two distinct frequency bands separated by a gap. The origin of this two-band structure remains a 50-year old question. Here we report, using NASA’s Van Allen Probe measurements, that banded chorus waves are commonly accompanied by two separate anisotropic electron components. Using numerical simulations, we show that the initially excited single-band chorus waves alter the electron distribution immediately via Landau resonance, and suppress the electron anisotropy at medium energies. This naturally divides the electron anisotropy into a low and a high energy components which excite the upper-band and lower-band chorus waves, respectively. This mechanism may also apply to the generation of chorus waves in other magnetized planetary magnetospheres.

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Generation of two-band chorus waves in the Earth's outer radiation belt

10.5194/egusphere-egu21-380 ◽

2021 ◽

Author(s):

Jinxing Li ◽

Jacob Bortnik ◽

Xin An ◽

Wen Li ◽

Vassilis Angelopoulos ◽

...

Keyword(s):

Electromagnetic Waves ◽

Radiation Belt ◽

High Energy ◽

Outer Radiation Belt ◽

Naturally Occurring ◽

Chorus Waves ◽

High Energy Electrons ◽

Distinct Frequency ◽

Space Environments ◽

Energy Components

<p>Naturally occurring chorus emissions are a class of electromagnetic waves found in the space environments of the Earth and other magnetized planets. They play an essential role in accelerating high-energy electrons forming the hazardous radiation belt environment. Chorus typically occurs in two distinct frequency bands separated by a gap. The origin of this two-band structure remains a 50-year old question. Using measurements from NASA&#8217;s Van Allen Probes we report that banded chorus waves are commonly accompanied by two separate anisotropic electron components. We demonstrate, using numerical simulations, that the initially excited single-band chorus waves alter the electron distribution immediately via Landau resonance, and suppresses the electron anisotropy at medium energies. This naturally divides the electron anisotropy into a low and a high energy components which excite the upper-band and lower-band chorus waves, respectively. This mechanism may also apply to the generation of chorus waves in other magnetized planetary magnetospheres.</p>

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Wave–particle interaction effects in the Van Allen belts

Earth Planets and Space ◽

10.1186/s40623-021-01508-y ◽

2021 ◽

Vol 73 (1) ◽

Author(s):

Daniel N. Baker

Keyword(s):

Electromagnetic Waves ◽

Radiation Belt ◽

Particle Interaction ◽

Outer Zone ◽

High Energy ◽

Particle Interactions ◽

Magnetospheric Substorms ◽

Chorus Waves ◽

Van Allen Probes ◽

Wave Particle Interactions

AbstractDiscovering such structures as the third radiation belt (or “storage ring”) has been a major observational achievement of the NASA Radiation Belt Storm Probes program (renamed the “Van Allen Probes” mission in November 2012). A goal of that program was to understand more thoroughly how high-energy electrons are accelerated deep inside the radiation belts—and ultimately lost—due to various wave–particle interactions. Van Allen Probes studies have demonstrated that electrons ranging up to 10 megaelectron volts (MeV) or more can be produced over broad regions of the outer Van Allen zone on timescales as short as a few minutes. The key to such rapid acceleration is the interaction of “seed” populations of ~ 10–200 keV electrons (and subsequently higher energies) with electromagnetic waves in the lower band (whistler-mode) chorus frequency range. Van Allen Probes data show that “source” electrons (in a typical energy range of one to a few tens of keV energy) produced by magnetospheric substorms play a crucial role in feeding free energy into the chorus waves in the outer zone. These chorus waves then, in turn, rapidly heat and accelerate the tens to hundreds of keV seed electrons injected by substorms to much higher energies. Hence, we often see that geomagnetic activity driven by strong solar storms (coronal mass ejections, or CMEs) commonly leads to ultra-relativistic electron production through the intermediary step of waves produced during intense magnetospheric substorms. More generally, wave–particle interactions are of fundamental importance over a broad range of energies and in virtually all regions of the magnetosphere. We provide a summary of many of the wave modes and particle interactions that have been studied in recent times.

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