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Observations of discrete harmonics emerging from equatorial noise
Michael A Balikhin1, Yuri Y Shprits2, Simon N Walker1
1Department of Automatic Control and Systems Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK.
Nature Communications
|July 15, 2015
Summary
Spacecraft observations reveal structured, periodic plasma waves near Earth's equator. These "equatorial noise" emissions are linked to proton distributions, providing new insights into space plasma physics.
Area of Science:
- Space Plasma Physics
- Wave-particle interactions in space
- Magnetospheric physics
Background:
- Space plasmas exhibit various oscillation modes.
- Noise-like plasma waves near the geomagnetic equator, termed 'equatorial noise', have been observed since the 1970s.
- Previous theories lacked clear observational evidence and realistic modeling.
Purpose of the Study:
- To provide clear observational evidence of the structure of equatorial noise.
- To link observed wave patterns to specific particle distributions.
- To validate theories of equatorial noise generation through modeling.
Main Methods:
- Analysis of data from the Cluster space mission.
- Identification of wave structures and frequencies.
- Observation of proton distributions in velocity space.
- Calculation of wave growth based on ion distributions.
Main Results:
- Cluster mission data revealed highly structured and periodic patterns in equatorial noise.
- Narrow-banded emissions were observed at multiples of the proton gyrofrequency (17th to 30th harmonic).
- Simultaneous observations of ring proton distributions in velocity space were made.
- Calculated wave growth closely matched observed patterns.
Conclusions:
- Equatorial noise is generated by proton distributions.
- The observed waves are coherent and periodic, directly linked to proton gyrofrequency harmonics.
- The findings provide strong observational and modeling support for the generation mechanism of equatorial noise.
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