The Photoelectron‐Driven Upper Hybrid Instability as the Cause of 150‐km Echoes

• Published in: Geophysical research letters 2020-04, Vol.47 (8), p.n/a
• Main Authors: Longley, William J., Oppenheim, Meers M., Pedatella, Nick M., Dimant, Yakov S.
• Format: Article
• Language: English
• Subjects: 150 km echoes; Atmosphere; Coherence; Damping; Echoes; Free electrons; Geomagnetic field; Growth rate; Gyrofrequency; Instability; ionosphere; Landau damping; Magnetic field; Magnetic fields; Photoelectrons; Plasma; Plasma frequencies; Plasma waves; Radar; Radar measurement; Radio; Radio waves; Sunrise; Upper atmosphere; valley region; Wave energy; Wave power
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL087391

150 kilometer echoes are strong, coherent echoes observed by equatorial radars looking close to perpendicular to Earth's magnetic field. Observations over a day show a distinct necklace pattern with echoes descending from 170 km at sunrise to 130 km at noon, before rising again and disappearing overnight. This paper shows that the upper hybrid instability will convert photoelectron energy into plasma wave energy through inverse Landau damping. Using parameters from a WACCM‐X simulation, the upper hybrid wave growth rates over a day show a nearly identical necklace pattern, with bands of positive growth rate following contours of the plasma frequency. Small gaps in altitude with no echoes are explained by thermal electrons Landau damping the instability where the upper hybrid frequency is a multiple of the gyrofrequency. This theory provides a mechanism that likely plays a crucial role in solving a long‐standing mystery on the origin of 150‐km echoes. Key Points Kinetic theory is developed for photoelectron‐driven upper hybrid instability Using WACCM‐X for background parameters, the instability growth rates follow the same morphology as 150‐km echoes Landau damping by thermal electrons creates observed gaps in altitude where the upper hybrid frequency is a multiple of the gyrofrequency Plain Language Summary For decades, large radars have observed strong, unexplained echoes returning from altitudes of 130–170 km in the atmosphere. All radars work by reflecting radio waves off a target and measuring the returned signal. For atmospheric radars, the targets are free electrons within the plasma in the upper atmosphere. Since the free electrons are typically a disordered gas, the radio waves are reflected in random directions. This means some process is needed to create a coherent structure in the plasma for the radio waves to strongly reflect back in the direction of the radar. In this paper, we show that the region between 130–170 km in the upper atmosphere is likely to create and maintain a specific set of plasma waves that act as a coherent structure for radar measurements. We show that predictions of where the plasma waves are generated match well with the observed patterns of these “150‐km echoes.” This is the first research to provide a specific explanation for what causes 150‐km echoes. In understanding this cause, we learned more about the Sun's influence on our upper atmosphere while expanding the capabilities of atmospheric radars.