Review of ionospheric irregularities and ionospheric electrodynamic coupling in the middle latitude region

Abstract

This paper briefly reviews ionospheric irregularities that occur in the E and F regions at mid‐latitudes. Sporadic E (ES) is a common ionospheric irregularity phenomenon that is first noticed in the E layer. ES mainly appears during daytime in summer hemispheres, and is formed primarily from neutral wind shear in the mesosphere and lower thermosphere (MLT) region. Field‐aligned irregularity (FAI) in the E region is also observed by Very High Frequency (VHF) radar in mid‐latitude regions. FAI frequently occurs after sunset in summer hemispheres, and spectrum features of E region FAI echoes suggest that type‐2 irregularity is dominant in the nighttime ionosphere. A close relationship between ES and E region FAI implies that ES may be a possible source of E region FAI in the nighttime ionosphere. Strong neutral wind shear, steep ES plasma density gradient, and a polarized electric field are the significant factors affecting the formation of E region FAI. At mid‐latitudes, joint observational experiments including ionosonde, VHF radar, Global Positioning System (GPS) stations, and all‐sky optical images have revealed strong connections across different scales of ionospheric irregularities in the nighttime F region, such as spread F (SF), medium‐scale traveling ionospheric disturbances (MSTID), and F region FAI. Observations suggest that different scales of ionospheric irregularities are generally attributed to the Perkins instability and subsequently excited gradient drift instability. Nighttime MSTID can further evolve into small‐scale structures through a nonlinear cascade process when a steep plasma density gradient exists at the bottom of the F region. In addition, the effect of ionospheric electrodynamic coupling processes, including ionospheric E‐F coupling and inter‐hemispheric coupling on the generation of ionospheric irregularities, becomes more prominent due to the significant dip angle and equipotentiality of magnetic field lines in the mid‐latitude ionosphere. Polarized electric fields can map to different ionospheric regions and excite plasma instabilities which form ionospheric irregularities. Nevertheless, the mapping efficiency of a polarized electric field depends on the ionospheric background and spatial scale of the field.

Numerical study on matching conditions of Langmuir parametric instability and the formation of Langmuir turbulence in ionospheric heating

“…The SLT theory also showed that ion caviton collapse could be generated, which is related to oscillating two-stream instability. The irregularities can be excited in turbulence process with meter scale, which is smaller than in Natural excitation process with kilometer scale (Liu et al, 2021). The irregularities can effect the wave propagation (Jiang et al, 2020).…”

Study of the Ionospheric Scintillation Radio Propagation Characteristics with Cosmic Observations

“…There are several impact factors that are responsible for the occurrence of ionospheric irregularities [6,7]. Studies reveal that large number of ionospheric irregularities have variable patterns, such as sporadic E (Es), spread F, field-aligned irregularities (FAIs) and plasma bubbles.…”

“…The mid-latitudes ionospheric irregularities are found usually to be associated with atmospheric gravity waves and medium-scale travelling ionospheric disturbances (MSTID) [15,16]. Its major cause is attributed to Perkins instability [7,[17][18][19]. In this theory, a north-south electric field is present in addition to the east-west electric field, leading to the form of rising and falling sheets of ionization.…”

Simulation of Es Layer Modulated by Nonlinear Kelvin–Helmholtz Instability

“…The Es layer modulated by KHI is numerically studied in a linear regime (Bernhardt, 2002), in which relatively weaker KHI can produce large amplitude modulation. The role of KHI mechanism has not been widely recognized yet (Choudhary et al., 2005; Liu et al., 2019, 2020, 2021; Patra et al., 2009). The nonlinear KHI mechanism generating the enhanced horizontal structure of Es layer is not clear.…”