One Hundred and Fifty kilometer echoes are a type of strong radar echo observed in the valley region of the equatorial ionosphere, whose origin was long standing mystery. Recently, a new upper hybrid (UH) instability theory, driven by high energy photoelectrons, has been proposed to explain most features of 150 km echoes. However, this instability excites high frequency electron modes, whereas radars observe low frequency ion modes. To explain 150 km echoes, the UH instability must ultimately excite ion acoustic modes. This paper describes a set of particle‐in‐cell simulations used to study how photoelectrons interacting with a cold background plasma generate UH waves, and how these drive ion acoustic waves measured by radars. We implement a new electron‐N2collision algorithm to better model the bump on tail features of the photoelectron distribution in the valley region. These simulations show that photoelectrons drive unstable UH waves that strongly enhance ion acoustic waves. We also show a strong frequency dependence on power in the ion line, which explains observational differences between radars, most notably the lack of echoes at ALTAIR. The photoelectron driven UH instability successfully reproduces most features of 150 km echoes associated with naturally enhanced incoherent scattering.
We investigate the mechanism underlying lower hybrid waves associated with high altitude echoes recently detected in the post‐sunset equatorial topside ionosphere and inner plasmasphere by the Jicamarca VHF radar. These waves are visible as prominent sidebands in the echo Doppler spectra. New experimental results and newly processed incoherent scatter radar (ISR) datasets are presented that provide clues as to the conditions in which the echoes and associated waves occur. Numerical simulations are presented which demonstrate the feasibility of an inverse energy cascade coupled with a short wavelength instability, that is, the lower hybrid drift instability, in explaining the waves. An inverse cascade is required for short wavelength lower hybrid waves to extend to the 3 m wavelengths measured by the Jicamarca radar. The simulations were able to reproduce some features of the measurements including the lower hybrid sidebands at 3 m wavelengths, asymmetry in the sidebands, and the damping effect of higher densities and lower altitudes.
more » « less- Award ID(s):
- 2213849
- PAR ID:
- 10397205
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 128
- Issue:
- 2
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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