Shvets, Alexander, VNickolaenko, Alexander P.Koloskov, Alexander, VYampolsky, Yuri M....
10页
查看更多>>摘要：We present variations in the source bearings and polarization of the horizontal magnetic field of Q bursts recorded at Antarctic station "Akademik Vernadsky " (65.2458 degrees S, 64.2572 degrees W). We analyzed the parameters of the "polarization ellipse " inscribed in the initial impulse of the Q-burst, which is formed by a direct wave propagating from lightning to the observer along a short path in the Earth-ionosphere cavity. Three days, from March 31 to April 2, 2019, a significant part of the Q burst flux was stably observed in a narrow northerly azimuth sector associated with the direction to lightnings in South America. During this period, regular deviations of the measured azimuths from the northern direction were observed up to 8-10 degrees, to the west when the morning terminator passed and to the east at the evening terminator. Duration of these deviations was about 4 h. The azimuth deviations were accompanied by characteristic oscillations in the "ellipticity " value of the direct wave field, N-shaped during the passage of the evening terminator, and a mirror-symmetric H-shaped oscillation on the morning terminator. We found that, on average, the polarization of the direct wave field is close to linear when the paths from the sources to the observer were on the day side, while at night side its "ellipticity " increases to 0.1. The observed azimuth deviations were qualitatively interpreted using the concept of geometric optics of the reflection of radio waves from the day-night non-uniformity in the Earth-ionosphere cavity. The measurement period was close to the day of the vernal equinox, when the propagation path from the sources and the lines of the solar terminator run almost parallel and radio waves are incident on the day-night interface at "sliding angles", which enhances their "reflection".
Nikitina, L.Fiori, R. A. D.Ghoddousi-Fard, R.Waddington, G. H....
9页
查看更多>>摘要：Ionospheric disturbances, due to photoionization and energetic particle precipitation, occurring both on a regular recurring basis, and heightened due to space weather, impact global positioning, navigation, and timing services provided by Global Navigation Satellite Systems (GNSS). The development of operational services to mitigate the risks to users requires knowledge of how space weather impacts critical systems, and the frequency and magnitude of ionospheric disturbances including extreme events. Analysis of the largest values of total electron content (TEC) is important for the assessment of space weather hazards for systems sensitive to ionospheric effects on radio wave propagation and for the development of risk mitigation strategies. In this study, twenty years of global TEC maps are analyzed to determine the global distribution of the daily maximum TEC. Maximum TEC is shown to reach magnitudes of about 200 TECU in extreme cases, with the largest TEC normally observed at equatorial latitudes. A generalized extreme value distribution has been applied to the daily global maximum TEC and for the high (60 degrees-90 degrees; -90 degrees to -60 degrees), mid (30 degrees-60 degrees; -60 degrees to -30 degrees), and low-latitude (0 degrees-30 degrees; -30 degrees-0 degrees) regions in geographic latitudes to determine the 1-in-44 year (4 solar cycles) and 1-in-100 year maximum TEC with a 95% confidence interval. Additionally, the return period for TEC thresholds defined for safe operation by the International Civil Aviation Organization are discussed. Extreme values provide criteria for the development of operational systems and vulnerability assessment of critical systems dependant on GNSS.
查看更多>>摘要：This paper compares the occurrence of GPS Loss of Lock events from IGS network data to defined values of both the Amplitude scintillation index (S4) from a dedicated scintillation receiver of SCINDA network and the Amplitude scintillation proxy (S4-proxy) from IGS network data measurements in Bahir Dar. LOL may occur at any level of S4 due to other dominating factors; however, in our case studies, the threshold values of both S4 and S4-proxy are greater or equal to 0.7, which is more responsible for the occurrence of LOL. Most of Loss-of-Lock events occurred when both S4 and S4-proxy were greater than or equal to 0.7, implying that LOL occurrence increases with the severity of scintillation and peaks for strong ionospheric scintillation, implying that low S4 and S4-proxy values were associated with infrequent Loss of Lock, whereas high values were usually associated with Loss-of-Lock. According to our findings, there were no Loss-of-Lock events in the absence of strong scintillation operation (S4 and S4-proxy > 0.7). Ionospheric Scintillation never simultaneously occurred on all PRN signals to impact all satellites in the field of view of a receiver simultaneously, however only strong scintillation events S4 > 0.7 and S4-proxy > 0.7 values are causes of Loss-of-Lock. Loss-of-Lock events and strong S4 and S4-proxy values only occurred in the same region of the sky(North-West region) and during the same time, according to our observations. This implies that S4,S4-proxy and Loss-of-Lock are correlated in terms of temporal and azimuth distribution. The absence of geomagnetic storms on the days on which LOL were observed, rules out the likelihood that the occurrence of LOL at equatorial regions is linked to geomagnetic storms. As a result, we discovered that strong ionospheric scintillation is a dominant cause of Loss-of-Lock events in the equatorial region, such as Bahir Dar. Finally, these findings suggest that ionospheric scintillation indices derived from IGS network data, such as S4-proxy values, are equally responsible for determining whether a Loss-of-Lock event is likely to occur and are appropriate for studying ionospheric scintillation in the regions where there are no dedicated scintillation receivers of the SCINDA network.
查看更多>>摘要：We explore a connection between thermospheric density enhancement and increase in thermospheric O 130.4 nm radiance. We observe TIMED/GUVI enhancements in the limb 130.4 nm radiances at-400 and-520 km on the dayside during four intense geomagnetic storms in 2003 and 2004. The enhancements were well correlated with Dst and CHAMP total neutral density at 400 km which represents O density as O is the dominant species at those altitudes. At the 400 and 520 km altitudes, O 130.4 nm emissions are mostly created by two comparable sources: solar resonance scatter and photoelectron impact excitation. The coincident disk 130.4 nm radiances, mostly due to emissions below 200 km (peaked around 130-140 km), were not clearly correlated with the limb radiances. Because the limb 130.4 nm radiances depend on O density, solar EUV and 130.4 nm fluxes, variations in the limb 130.4 nm radiance respond mostly to changes in O density when the solar EUV and 130.4 nm fluxes are stable. This explains the good correlation (correlation coefficients up to 0.98) between the limb 130.4 nm radiance and CHAMP neutral density. Once a quantitative relationship is established between GUVI limb 130.4 nm radiance and neutral density under both quiet and disturbed conditions and at different altitude levels through empirical or radiative transfer modeling, the limb 130.4 nm radiances can be used to retrieve O density profiles in the upper thermosphere.
查看更多>>摘要：The JJI VLF (22.2 kHz) transmitter signal received at two low-latitude stations, one in Port Vila (geog. coord., 17.73S, 168.33E), Vanuatu and other in Suva (18.14S, 178.44E), Fiji, was analyzed for any VLF changes due to 16 Earthquakes (EQs) with magnitudes 5.5 to 7.7, during 2018 (JJI-Vanuatu path, 6.8 Mm) and 2007 to 2018 (JJI-Suva path, 7.5 Mm). The VLF signal amplitude analysis included terminator time (TT), average daytime and nighttime amplitude variation, nighttime fluctuation, and mother Morlet wavelet methods. Out of 16 EQs only eleven EQs have shown subionospheric VLF changes including the decrease in the amplitude for about 2-8 h on the EQ day, unusual shifts in the TT of up to 5-9 min, and the decrease in the average daytime and nighttime signal amplitude of about 1-1.5 dB and 1-5 dB, respectively, on the mainshock day of the EQs. The dA(t) < 0 condition was observed about 4-5 days before the EQ which stabilized after 3-4 days from the EQ day. A decrease in the non-normalized and normalized trend of below -2 sigma (standard deviation) mark was found on the EQ day and an increase in the non-normalized and normalized NF and dispersion of above +20 mark on the day of seismic activity was found. Mother wavelet analysis of EQ associated changes in the signal amplitude showed a strong and enhanced presence of short frequency (~0.05-0.10 mHz) wave-like signatures, a few days prior, on the day of EQ, and after the EQ day as compared to normal days.
Li, AnqiMurtagh, Donal P.Bourassa, Adam E.Degenstein, Douglas A....
8页
查看更多>>摘要：In the mesosphere, the vibrationally excited hydroxyl layer is sensitive to changes in incoming solar flux. An enhanced photodissociation of molecular oxygen will lead to more atomic oxygen production, thus we expect the OH layer emission rate to be positively with the Lyman-alpha flux and the emission height to be negatively correlated. The Optical Spectrograph and InfraRed Imager System (OSIRIS) has recorded the Meinel band centred at 1.53 mu m from 2001 to 2015. In this study, we show how the 11-year solar cycle signature manifests itself in this data set, in terms of OH zenith emission rate and emission height. As expected, the emission height is negatively correlated with the Lyman-alpha flux at all latitudes. The zenith emission rate is positively correlated with the Lyman-alpha & nbsp;flux at most latitudes except near the equator. By the means of a time dependent photochemical model, we show that the changing local time sampling of the Odin satellite was the cause of the observed distortion of the solar cycle signature near the equator.
Zhang, YongliangPaxton, Larry J. J.Schaefer, Robert
4页
查看更多>>摘要：A paper A new method to subtract dayglow for auroral observation of SSUSI in LBH ranges based on the improved AURIC reports a new method to estimate the dayglow intensities in DMSP/SSUSI LBH bands using an improved AURIC model. It is claimed that the new method offers a better alternative than the SSUSI operational algorithm which uses a data based table. The paper showed a few examples and compared them with SSSUI operational results. The comparison indicated that the new method didn't offer any improvement and provided net auroral images with strong residual dayglow. On the other hand, the auroral oval can be easily recognized in the SSUSI data using the operational algorithm, despite some weak residual background which is expected due to count errors in the data. There are likely a few reasons why the method led to poor results: (1) dayglow contribution in SSUSI data covers solar zenith angles (SZA) beyond 90 and the AURIC model is limited to SZA & LE;90, (2) In addition to SZA, SSUSI radiances also depend on look angle (along and cross track pixels). Such a look-angle effect was apparently not reported in the paper. (3) The localized peaks in the plots (radiance versus SZA) were likely due to changes in solar EUV flux, SZA as well as noises caused Southern Atlantic Anomaly, MeV particles at sub-auroral latitude and glint in the Ap dependent data bins. The examples in the paper indicate that the new algorithm is not appropriate to estimate net SSUSI dayglow intensity.
查看更多>>摘要：Tropospheric delay is one of the primary error sources of the Global Navigation Satellite System (GNSS). Usually, a traditional empirical model can be corrected, which uses the trigonometric function of fixed amplitude and frequency for time fitting. However, the traditional models only retain the low-frequency information of the zenith tropospheric delay (ZTD), which limits its application in high time resolution. To solve this problem, a global tropospheric delay grid model, namely AGtrop, is developed based on Anti-Leakage Least-Squares Spectral analysis. The results of the AGtrop model are evaluated by three kinds of sources, including ERA5 data, IGS, and the sounding profile of radiosonde station throughout the year 2019. Compared with the traditional model GPT2w and UNB3m, AGtrop shows better performance, especially in low latitudes and the southern hemisphere. AGtrop can generally achieve a global mean Bias/RMSE of 0.35/2.34 cm in contrast to ERA5 derived ZTD, 0.59/ 2.29 cm in comparison with IGS ZTD, and 0.18/3.71 cm in the match to Radiosonde derived ZTD. Besides, the AGtrop model has an enhancement effect in localized areas. AGtrop model is characterized by simplicity and accuracy, which will be of great significance to real-time GNSS applications.
NSTL
Elsevier