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  1. Abstract

    We present an investigation of the F‐region electron temperature to an intense geomagnetic storm that occurred on 5 August 2011. The investigation is based on the incoherent scatter radar measurements at Arecibo Observatory, Puerto Rico (18.3°N, 66.7°W). The electron temperature exhibits a rapid and intensive enhancement after the commencement of the geomagnetic storm. The electron temperature increases by ∼800 K within an hour, which is seldomly reported at Arecibo. At the same time, a depletion of the electron density is also observed. The daytime perturbations of electron density and temperature are anticorrelated with the correlation coefficient, which is −0.88 and −0.91 on the day and the following day of the geomagnetic storm, respectively. According to the Global Ultraviolet Imager measurements, the ratio of atomic oxygen to molecular nitrogen concentration () decreases dramatically during the storm. Our analysis suggests that the enhancement of the electron temperature is due to the depletion of the electron density, which is likely associated with the decrease of. The reduction ofmaybe caused by a prompt upward plasma motion after the commencement of the geomagnetic storm.

     
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  2. ABSTRACT

    This work presents the result of sporadic meteor radiant density distribution using the Arecibo 430 MHz incoherent scatter radar (ISR) located in Puerto Rico for the first time. Although numerous meteor studies have been carried out using the Arecibo ISR, meteoroid radiant density distribution has remained a mystery as the Arecibo radar cannot measure vector velocity. A numerical orbital simulation algorithm using dynamic programming and stochastic gradient descent is designed to solve the sporadic meteoroid radiant density and the corresponding speed distributions of the meteors observed at Arecibo. The data set for the algorithm comprises over 250 000 meteors from Arecibo observations between 2009 and 2017. Five of the six recognized sporadic meteor sources can be identified from our result. There is no clearly identifiable South Apex source. Instead, there is a broad distribution between +/−30° ecliptic latitude, with the peak density located in the North Apex direction. Our results also indicate that the Arecibo radar is not sensitive to meteors travelling straight into or perpendicular to the antenna beam but is most sensitive to meteors with an arrival angle between 30° and 60°. Our analysis indicates that about 75 per cent of meteoroids observed by the Arecibo radar travel in prograde orbits when the impact probability is considered. Most of the retrograde meteoroids travel in inclined low-eccentricity orbits.

     
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  3. Abstract

    A novel computer vision‐based meteor head echo detection algorithm is developed to study meteor fluxes and their physical properties, including initial range, range coverage, and radial velocity. The proposed Algorithm for Head Echo Automatic Detection (AHEAD) comprises a feature extraction function and a Convolutional Neural Network (CNN). The former is tailored to identify meteor head echoes, and then a CNN is employed to remove false alarms. In the testing of meteor data collected with the Jicamarca 50 MHz incoherent scatter radar, the new algorithm detects over 180 meteors per minute at dawn, which is 2 to 10 times more sensitive than prior manual or algorithmic approaches, with a false alarm rate less than 1 percent. The present work lays the foundation of developing a fully automatic AI‐meteor package that detects, analyzes, and distinguishes among many types of meteor echoes. Furthermore, although initially evaluated for meteor data collected with the Jicamarca VHF incoherent radar, the new algorithm is generic enough that can be applied to other facilities with minor modifications. The CNN removes up to 98 percent of false alarms according to the testing set. We also present and discuss the physical characteristics of meteors detected with AHEAD, including flux rate, initial range, line of sight velocity, Signal‐to‐Noise Ratio, and noise characteristics. Our results indicate that stronger meteor echoes are detected at a slightly lower altitude and lower radial velocity than other meteors.

     
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  4. Abstract

    Based on the meteor winds measured at Mohe (MH; 53.5°N, 122.3°E) and the reanalysis data, we investigate the variations of planetary waves in the mesosphere and lower thermosphere (MLT) region during the 2019/2020 Arctic winter. Four stratospheric polar warmings, including two sudden stratospheric warmings (SSWs), are observed from November 2019 to March 2020. Quasi‐10‐day waves (Q10DWs) are found to be enhanced following three of these warmings in the zonal winds in the MLT region over MH, but unusually weak after the SSW in February 2020. The trigger mechanisms of the enhanced Q10DWs are investigated and the reason for the unusually weak Q10DWs in February is revealed. Upward propagations and in situ generations of Q10DWs are both limited during the February SSW. Our analysis indicates that Q10DWs in the MLT region in February 2020 are largely inhibited by the extremely strong polar vortex and a lack of mesospheric instability.

     
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  5. Abstract

    A quasi‐27‐day wave (Q27DW) caused by the rotational period of solar radiation is commonly observed in the atmospheric dynamics. In the present study, we report an enhancement of a Q27DW during recurrent geomagnetic storms in the autumn of 2018 based on the zonal wind observations in the mesosphere and lower thermosphere (MLT) region over Beijing (BJ, 40.3°N, 116.2°E). According to our analysis, the solar radiation and the seasonal variation are not important in exciting the observed Q27DW. A 27‐day oscillation exists in both solar wind data andKpindex during the recurrent geomagnetic storms. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature and ozone data also reveal a Q27DW signature at 97 km. Using the long‐term observation of BJ meteor radar, two more cases are found during springtime in 2010 and 2018 under the solar quiet condition. Our results indicate that the recurrent geomagnetic storms due to high‐speed solar winds can modulate the temperature and ozone in the MLT region, which is responsible for generating a Q27DW in the MLT zonal winds over BJ. This study suggests that the variation of planetary waves in the MLT neutral winds at mid‐latitude is likely associated with the recurrent geomagnetic storms and high‐speed solar winds.

     
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  6. Abstract

    We present an analysis of planetary‐scale oscillations during sudden stratospheric warming (SSW) events based on data obtained from a meteor radar located at Mohe (MH, 53.5°N, 122.3°E), the Aura satellite and Modern‐Era Retrospective analysis for Research and Applications, Version 2 data (MERRA2). The planetary‐scale oscillations in the mesosphere and lower thermosphere (MLT) region during eight SSW events from 2012 to 2019 have been statistically investigated. Our analysis reveals that the enhancement or the generation of westward propagating quasi 16‐day oscillation with wavenumber 1 (W1) is a common feature during SSWs over MH. A strong enhancement of the quasi 4‐day oscillation during the 2018/2019 SSW is captured by both radar and satellite observations. The amplified quasi 4‐day oscillation has a period of ~4.3 days in both meridional and zonal winds and with a wavenumber of W2 in the zonal component. Using the meteor radar and MERRA2 data, the vertical structure of the quasi 4‐day oscillation from the stratosphere to the lower thermosphere is derived. The upward propagating feature of the quasi 4‐day oscillation in the meridional component indicates that the oscillation is very likely generated in the lower mesosphere. The mesospheric zonal wind reversal after an elevated stratopause event is observed during the SSW, which results in a negative meridional gradient of the quasi‐geostrophic potential vorticity. Our results not only reveal that the amplified quasi 4‐day oscillation in the MLT region is associated with the 2018/2019 SSW but also suggest that the amplification is originally generated around 60 km due to barotropic/baroclinic instability and propagates upward to MLT region.

     
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  7. Abstract

    We present an analysis of helium ion (He+) fraction in an altitude range from about 400 km to around 700 km and its relationship to the ion temperature (Ti) and the vertical ion drift under solar maximum conditions. The data were obtained from the Arecibo incoherent scatter radar during 27 September to 1 October 2014 and 16–20 December 2014. The large He+fraction (>10%) lasts 15 hr per day during the winter solstice, which is 3 times larger than during fall equinox. This difference is caused by the more persistent downward ion drift in the winter. The incremental He+fraction and incrementalTiare well anticorrelated, and the anticorrelation is more prominent during the daytime. These characteristics are associated with whether O+and He+are in diffusive equilibrium. During nighttime, we show that the vertical ion flow is downward causing the He+layer peak altitude to move to an altitude of 500 km from above 650 km. According to our analysis, He+fraction has to be larger than two thirds for diffusive equilibrium to occur above the He+peak height. Therefore, above the He+peak altitude, O+and He+cannot be in diffusive equilibrium with He+being the minor species. The vertical ion flow plays an important role in determining the diurnal variation and seasonal difference of He+distribution and whether He+is in a diffusive equilibrium with O+.

     
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  8. Abstract

    Using data collected from the Arecibo incoherent scatter radar during 5–10 February 2016, we present a study on the quarterdiurnal tide (QDT) from 250 to 360 km. A sudden stratospheric warming (SSW) event occurred on 8 February coincided with our observation. The maximum amplitude of the QDT, at ~37 m/s, is comparable with the diurnal tide and much larger than the semidiurnal tide. The QDT is largely evanescent. Our results manifest that theFregion QDT could be as important as the diurnal and semidiurnal tides. The tidal waves show large variability before and after the commencement of the SSW. Our analysis indicates that the enhancement of the QDT is most likely due to the effect of the SSW. Nonlinear interaction of the diurnal tide with the terdiurnal tide is found to play a significant role in amplifying the QDT during the SSW event.

     
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  9. null (Ed.)