From Field-aligned Irregularities Research Articles

The Local Time and Altitude Dependences of Long‐Lived Nonspecular Meteor Echoes With Weak Magnetic Aspect Sensitivity at Low Latitudes

AbstractLong‐lived nonspecular meteor echoes backscattered from meteor trail nonfield‐aligned irregularities (NFAIs) were observed sporadically at high latitudes. In this study, we employ the observations by the all‐sky radar of Meteor and ionospheric Irregularity Observation System (MIOS) at Ledong (18.4°N, 109°E) in October 2018, January, April, and July 2019, to investigate the occurrences of meteor trail NFAIs at low latitudes. A deep learning method was developed to identify long‐lived meteor echo events with durations of more than 5 s. The long‐lived echo events with triangular shape (a typical feature of nonspecular echo) in radar range‐time intensity maps were manually distinguished from those with nontriangular shape. Based on the cutoff magnetic aspect angle 12° of nonspecular meteor echoes from field‐aligned irregularities (FAIs), the local time and altitude dependences of meteor NFAIs producing long‐lived nonspecular echoes are investigated. A total of 846 meteor events due to the backscatters of NFAIs were observed. These events peak around the morning hours, with the beginning (end) altitudes up to (down to) 120 km (∼80 km). Their mean heights are about 2–6 km higher than specular meteor echoes. The occurrence of NFAI echoes peaks on certain days that is likely associated with meteor showers. The ratio of the observed NFAI echo to specular echo events is, on average, less than 1‰. Possible factors responsible for the NFAIs at low latitudes are discussed based on the current understanding of meteor dust trail.

Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 2. Sounding of electron density, ion effective mass (meff), ion composition (H+, He+, O+), and density irregularities

[1] A companion paper by Sonwalkar et al. (2011) provided new details of whistler mode radio sounding of the altitude range below ∼5000 km by the Radio Plasma Imager (RPI) instrument on the IMAGE satellite. That paper presented frequency-vs- group time delay records of echoes whose raypaths either 1) reversed direction through refraction at altitudes above the ionosphere where the wave frequency was approximately equal to the local lower hybrid resonance frequency flh (magnetospherically reflected or MR echoes), or 2) returned to IMAGE from reflection points along the sharp lower boundary of the ionosphere at ∼90 km (obliquely incident (OI) or normally incident (NI) specularly reflected (SR) echoes). The MR and OI echo paths were shown to form narrow loops, while the NI echo followed the same raypath down and back. Furthermore, the echoes were found to be discrete or broadened in time delay either by multipath propagation or by scattering from field aligned irregularities (FAIs). We begin with a direct interpretive approach, employing a combination of refractive index diagrams, ray tracings, and a plasma density model to predict the detailed frequency-vs-time properties of echoes detected when the sounder is operated over a wide range of whistler mode frequencies (typically 6 kHz to 63 kHz) and the satellite is either above or below the altitude of the maximum flh along the geomagnetic field line B0 in the upper ionosphere. We then consider the inverse problem, estimation of the parameters of the prevailing plasma density model from the observed echo properties. Thanks to variations in the sensitivity of the various echo forms to the altitude profiles of electron density and ion effective mass meff, we use the observed frequency-vs- group time delay (tg − f) details of simultaneously received MR and SR echoes to infer the properties of a diffusive equilibrium model of the plasma, including estimates of the ion composition in the important transition region from the O+-dominated ionosphere to the light ion regime above. Our results on electron density and ion composition measurements are in general agreement with those obtained from in situ measurements on the IMAGE and DMSP-F15 satellites, with bottomside sounding results from nearby Ionosondes, and with values obtained from the IRI-2007 model. We also demonstrate a method of estimating the scale sizes and locations of FAIs located along or near WM echo paths.

Polarization and scattering of a long-duration meteor trail

[1] High-power, large-aperture (HPLA) radars have been used over the past two decades to characterize the plasmas formed both around and behind meteoroids as they enter Earth's atmosphere. These plasmas, referred to as heads and trails, respectively, occur with relative frequency (peak head echo detection rate of ∼1/s) but are extremely diverse and have been difficult to define in a general sense. One particular type of plasma, referred to as the nonspecular trail, occurs when the meteoroid travels quasi-parallel to the radar beam with the radar beam lying quasi-perpendicular to the background magnetic field. Reflection is believed to occur from field-aligned irregularities (FAIs) that form after the trail becomes unstable. While FAI scattering pertains to the majority of nonspecular trails that are short in duration, a subset of these trails, referred to as long-duration trails, still remains open to interpretation. In this paper we present a case study analysis of a long-duration, nonspecular trail and its associated head echo detected with the Advanced Research Project Agency (ARPA) Long-Range Tracking and Identification Radar (ALTAIR), which is an HPLA radar. These data are unique in that they are high resolution (with monopulse angles), dual frequency, and, most importantly, dual polarized, which allows for unprecedented insight into the scattering process from both heads and trails. First, we determine the velocity and mass of the parent meteoroid, which is a particle weighing more than a milligram and is one of the largest meteoroids ever detected by ALTAIR. Second, we determine the peak plasma density and polarization of the head echo and characterize the unique, yet strong returns in the opposite polarization, which may be due to multiple scattering centers within the range gate. Finally, we examine the polarization properties of the trail and discuss the first conclusive evidence of polarization flipping along the trail striations, which we believe corresponds to sharp gradients at the edges of the trail related to turbulent mixing of a dusty plasma that is elongating along the magnetic field. We look into a new idea, namely, the notion that some nonspecular echoes might correspond to a high Schmidt number, dusty plasma, as is found in and above noctilucent clouds. Our results show how polarized return can aid in scattering diagnostics and that single polarization radars must be used with caution for determining head and trail plasma densities given that some of the return can occur in the “unexpected” channel.

Relationship between polar cap patches and field‐aligned irregularities as observed with an all‐sky airglow imager at Resolute Bay and the PolarDARN radar at Rankin Inlet

Simultaneous two‐dimensional observations of airglow enhancement and radar backscatter from field‐aligned irregularities (FAIs) associated with polar cap patches were conducted. The spatial structure of 630 nm airglow from polar cap patches was imaged using an all‐sky airglow imager at Resolute Bay, Canada, while backscatter echoes from decameter‐scale FAIs were observed using the newly constructed HF Polar Dual Auroral Radar Network (PolarDARN) radar at Rankin Inlet, Canada. Both the airglow enhancement and the radar backscatter appeared within a structured region with the spatial extent of about 500–1000 km. The decameter‐scale FAIs were found to extend over the entire region of airglow enhancement associated with polar cap patches, indicating that the polar patch plasma became almost fully structured soon after initiation (within approximately 20–25 min). These findings imply that some rapid structuring process of the entire patch area is involved in addition to the primary gradient‐drift instabilities.

On the sheet-like nature of 150 km (F1) radar echoes

[1] The so-called 150 km (or F1) echoes are shown to arise from field-aligned irregularities (FAI) that are embedded in a sheet-like region. Using a 49.92 MHz radar on Christmas Island, which has an antenna beam directed toward geographic east at a zenith angle of 29.5°, high-resolution Doppler-velocity variations within the radar scattering volume are used to infer that the sheet-like region could have both tilt and curvature. This finding is used to infer that plasma density (N) must be enhanced within the sheet-like region, and that FAI could be generated by an interchange instability, driven by a vertical electric field or a zonal neutral wind that acts on the horizontal component of the gradient in N. How tilted sheet-like structures could form in the F1 region is discussed.

Simultaneous observations of nighttime medium‐scale traveling ionospheric disturbances and E region field‐aligned irregularities at midlatitude

We report on simultaneous observations of very high frequency (VHF) radar backscatter from field‐aligned irregularities (FAI) in the E region and medium‐scale traveling ionospheric disturbances (MSTID) in 630‐nm airglow images. On the night of 6 August 2002, the powerful 46.5‐MHz middle and upper atmosphere (MU) radar with five beam directions at Shigaraki, Japan, observed typical quasiperiodic (QP) echoes from FAI in the E region. During this QP echo event, an all‐sky charge‐coupled device imager at the MU radar site detected MSTID in 630‐nm airglow images. The MSTID also propagated southwestward at 80 m/s with fluctuation amplitudes of 50% from the background intensity and a wavelength of 380 km. Period of the MSTID was almost same as that of the Doppler velocity in the QP echoes. The airglow enhancement (depletion) caused by the MSTID coincided with southeastward (northwestward) velocity in the QP echo. These results indicate that the electric fields associated with the F region MSTID could be closely coupled to those associated with QP echoes in the E region.

Estimation of beam broadening spectrum of radar returns from field-aligned irregularities in ionospheric sporadic E region

Although the backscatter from field-aligned irregularities (FAI) associated with sporadic E (Es) layer are generally believed to be extremely aspect-sensitive, the beam broadening effect induced by the along- and cross-radar-beam drifts of the irregularities on Doppler spectral width of the echoes may be significant and cannot be totally ignored. Prior to the estimate of the beam broadening spectrum of the Es FAI, the locations of the irregularities in the echoing region should be precisely determined. With the knowledge of the interferometer-measured parameters, such as the angular position of the FAI from the radar beam axis and the zonal extent of the irregularities in the echoing region, combined with the observed Doppler velocity, the components of the along- and cross-radar-beam drift velocities can be estimated. Experimental results show that for the present case the magnitudes of the components of the along- and cross-radar-beam drift velocities are, respectively, 22.4 and 11.9 m/s. Taking advantage of these estimated drift velocities, we calculate corresponding beam broadening spectrum in accordance with the expression developed by [Chu, Y.H., Wang, C.Y. Plasma structures of 3-meter type 1 and type 2 irregularities in nighttime mid-latitude sporadic E region. J. Geophys. Res. 108 (A7), 1282, doi:10.1029/2002JA009661, 2003]. A comparison of observed and beam broadening spectral widths shows that the portion of the observed Doppler spectral width which is caused by the beam broadening effect can be as large as more than 26%. Therefore, the beam broadening effect caused by the drift of the field-aligned irregularities in the echoing region can play a decisive role in broadening the Doppler spectral width of the Es FAI and cannot be neglected from the observed Doppler spectrum.

Relationship between propagation direction of gravity waves in OH and OI airglow images and VHF radar echo occurrence during the SEEK-2 campaign

Abstract. We report simultaneous observations of atmospheric gravity waves (AGW) in OI (557.7nm) and OH airglow images and VHF radar backscatter from field-aligned irregularities (FAI) in the E-region during the SEEK-2 (Sporadic-E Experiment over Kyushu 2) campaign period from 29 July to 9 August 2002. An all-sky imager was operated at Nishino-Omote (30.5 N, 130.1 E), Japan. On 14 nights, 17 AGW events were detected in OI and OH airglow images. AGW propagated mostly toward the northeast or southeast. From comparison with the E-region FAI occurrence, which is detected by a nearby VHF radar (31.57MHz), we found that AGW tended to propagate southeastward during FAI events. This result suggests that the interaction between AGW and E-region plasma plays an important role in generating FAI. Furthermore, polarization electric fields generated directly by AGW may contribute to the FAI generation. Keywords. Atmospheric composition and structure (Airglow and aurora), Ionosphere (Ionospheric irregularities, Mid-latitude ionosphere)

Spatial relationship of equatorial plasma bubbles and field‐aligned irregularities observed with an all‐sky airglow imager and the Equatorial Atmosphere Radar

We report, for the first time, simultaneous two‐dimensional observations of 630‐nm airglow depletions and radar backscatter from field‐aligned irregularities (FAI) associated with equatorial plasma bubbles. Spatial distributions of backscatter were obtained by performing east‐west scans with the 47‐MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia. A 630‐nm airglow depletion, caused by a plasma bubble, was simultaneously observed with an all‐sky airglow imager. Both the airglow depletion and backscatter region appeared as band‐like structure elongated in the meridional direction with a zonal width of about 100 km. To compare the spatial structures of backscatter with that of airglow depletion, the backscatter was projected onto a horizontal plane at 250‐km altitude. Backscatter was found to occur within the entire airglow‐depleted region.

Aspect sensitivity of VHF echoes from field aligned irregularities in meteor trails and thin ionization layers

Abstract. The aspect sensitivity of VHF echoes from field aligned irregularities (FAI) within meteor trails and thin ionization layers is studied using numerical models. Although the maximum power is obtained when a radar is pointed perpendicular to the field line (B), substantial power can be obtained off the B direction if the ionization trail/layer is thin. When the FAI length along B is 20 m, the power observed 6° off B is about 10 db below that perpendicular to the B direction. Meteoric FAI echoes can potentially be used to determine the diffusion rate in the mesopause region. Based on the aspect sensitivity analysis, we conclude that the range spread trail echoes far off B observed by powerful VHF radars are likely due to overdense meteors. Our simulation also shows that ionospheric FAI echoes can have an altitude smearing effect of about 4 km if the vertical extension of a FAI-layer is around 100 m, which has often been observed at Arecibo. The altitude smearing effect can account for the fact that the Es-layers observed by the Arecibo incoherent scatter radar are typically much narrower than FAI-layers and the occurrence of double spectral peaks around the Es-layer altitude in FAI echoes.

Magnetic-field-aligned characteristics of plasma bubbles in the nighttime equatorial ionosphere

Measurements of both incoherent-scatter (IS) and backscatter from field-aligned irregularities (FAI) were made in 1978 with ALTAIR, a fully-steerable high-power radar, to investigate the magnetic-field-aligned characteristics of equatorial plasma bubbles. By operating the radar in a latitude-scan IS mode, we were able to map the location and percentage depletion of plasma bubbles as a function of altitude and latitude. By showing that backscatter from FAI is spatially collocated with the upper wall of plasma bubbles, we were able to use the spatial displacement of a field aligned backscatter region to estimate the upward bubble velocity. Besides showing that plasma bubbles are indeed aligned along magnetic field lines, we use this data set to show that a plasma bubble with a percentage depletion of as much as 90% does not have as large an upward velocity as predicted by two-dimensional models. Instead, the inferred bubble velocity is shown to be in better agreement with the bubble velocity predicted by theoretical models using flux-tube-integrated values of electron density and Pedersen conductivity. The need to use flux-tube-integrated values when comparing theory and observation is further stressed by the presence of a non-uniform latitudinal distribution of electron density (i.e. the equatorial anomaly) that was found in the latitude-scan data.