Browsing by Author "Xu, Zhonghua"
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- Characterization of multi-scale ionospheric irregularities using ground-based and space-based GNSS observationsPeng, YuXiang; Scales, Wayne A.; Hartinger, Michael D.; Xu, Zhonghua; Coyle, Shane (2021-07-12)Ionospheric irregularities can adversely affect the performance of Global Navigation Satellite System (GNSS). However, this opens the possibility of using GNSS as an effective ionospheric remote sensing tool. Despite ionospheric monitoring has been undertaken for decades, these irregularities in multiple spatial and temporal scales are still not fully understood. This paper reviews Virginia Tech’s recent studies on multi-scale ionospheric irregularities using ground-based and space-based GNSS observations. First, the relevant background of ionospheric irregularities and their impact on GNSS signals is reviewed. Next, three topics of ground-based observations of ionospheric irregularities for which GNSS and other ground-based techniques are used simultaneously are reviewed. Both passive and active measurements in high-latitude regions are covered. Modelling and observations in mid-latitude regions are considered as well. Emphasis is placed on the increased capability of assessing the multi-scale nature of ionospheric irregularities using other traditional techniques (e.g., radar, magnetometer, high frequency receivers) as well as GNSS observations (e.g., Total-Electron-Content or TEC, scintillation). Besides ground-based observations, recent advances in GNSS space-based ionospheric measurements are briefly reviewed. Finally, a new space-based ionospheric observation technique using GNSS-based spacecraft formation flying and a differential TEC method is demonstrated using the newly developed Virginia Tech Formation Flying Testbed (VTFFTB). Based on multi-constellation multi-band GNSS, the VTFFTB has been developed into a hardware-in-the-loop simulation testbed with external high-fidelity global ionospheric model(s) for 3-satellite formation flying, which can potentially be used for new multi-scale ionospheric measurement mission design.
- Conjugate observations of electromagnetic ion cyclotron waves associated with traveling convection vortex eventsKim, Hyomin; Clauer, C. Robert; Gerrard, Andrew J.; Engebretson, Mark J.; Hartinger, Michael D.; Lessard, Marc R.; Matzka, Juergen; Sibeck, David G.; Singer, Howard J.; Stolle, Claudia; Weimer, Daniel R.; Xu, Zhonghua (2017-07)We report on simultaneous observations of electromagnetic ion cyclotron (EMIC) waves associated with traveling convection vortex (TCV) events caused by transient solar wind dynamic pressure (P-d) impulse events. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft located near the magnetopause observed radial fluctuations of the magnetopause, and the GOES spacecraft measured sudden compressions of the magnetosphere in response to sudden increases in Pd. During the transient events, EMIC waves were observed by interhemispheric conjugate ground-based magnetometer arrays as well as the GOES spacecraft. The spectral structures of the waves appear to be well correlated with the fluctuating motion of the magnetopause, showing compression-associated wave generation. In addition, the wave features are remarkably similar in conjugate hemispheres in terms of bandwidth, quasiperiodic wave power modulation, and polarization. Proton precipitation was also observed by the DMSP spacecraft during the wave events, from which the wave source region is estimated to be 72 degrees-74 degrees in magnetic latitude, consistent with the TCV center. The confluence of space-borne and ground instruments including the interhemispheric, high-latitude, fluxgate/induction coil magnetometer array allows us to constrain the EMIC source region while also confirming the relationship between EMIC waves and the TCV current system.
- First Simultaneous Lidar Observations of Thermosphere-Ionosphere Fe and Na (TIFe and TINa) Layers at McMurdo (77.84 degrees S, 166.67 degrees E), Antarctica With Concurrent Measurements of Aurora Activity, Enhanced Ionization Layers, and Converging Electric FieldChu, Xinzhao; Nishimura, Yukitoshi; Xu, Zhonghua; Yu, Zhibin; Plane, John M. C.; Gardner, Chester S.; Ogawa, Yasunobu (2020-10-28)We report the first simultaneous, common-volume lidar observations of thermosphere-ionosphere Fe (TIFe) and Na (TINa) layers in Antarctica. We also report the observational discovery of nearly one-to-one correspondence between TIFe and aurora activity, enhanced ionization layers, and converging electric fields. Distinctive TIFe layers have a peak density of similar to 384 cm(-3) and the TIFe mixing ratio peaks around 123 km, similar to 5 times the mesospheric layer maximum. All evidence shows that Fe+ ion-neutralization is the major formation mechanism of TIFe layers. The TINa mixing ratio often exhibits a broad peak at TIFe altitudes, providing evidence for in situ production via Na+ neutralization. However, the tenuous TINa layers persist long beyond TIFe disappearance and reveal gravity wave perturbations, suggesting a dynamic background of neutral Na, but not Fe, above 110 km. The striking differences between distinct TIFe and diffuse TINa suggest differential transport between Fe and Na, possibly due to mass separation.
- Geomagnetic Disturbances That Cause GICs: Investigating Their Interhemispheric Conjugacy and Control by IMF OrientationEngebretson, Mark J.; Simms, Laura E.; Pilipenko, Viacheslav A.; Bouayed, Lilia; Moldwin, Mark B.; Weygand, James M.; Hartinger, Michael D.; Xu, Zhonghua; Clauer, C. Robert; Coyle, Shane; Willer, Anna N.; Freeman, Mervyn P.; Gerrard, Andy J. (American Geophysical Union, 2022-10-01)Nearly all studies of impulsive geomagnetic disturbances (GMDs, also known as magnetic perturbation events MPEs) that can produce dangerous geomagnetically induced currents (GICs) have used data from the northern hemisphere. In this study, we investigated GMD occurrences during the first 6 months of 2016 at four magnetically conjugate high latitude station pairs using data from the Greenland West Coast magnetometer chain and from Antarctic stations in the conjugate AAL-PIP magnetometer chain. Events for statistical analysis and four case studies were selected from Greenland/AAL-PIP data by detecting the presence of >6 nT/s derivatives of any component of the magnetic field at any of the station pairs. For case studies, these chains were supplemented by data from the BAS-LPM chain in Antarctica as well as Pangnirtung and South Pole in order to extend longitudinal coverage to the west. Amplitude comparisons between hemispheres showed (a) a seasonal dependence (larger in the winter hemisphere), and (b) a dependence on the sign of the By component of the interplanetary magnetic field (IMF): GMDs were larger in the north (south) when IMF By was >0 (<0). A majority of events occurred nearly simultaneously (to within ±3 min) independent of the sign of By as long as |By| ≤ 2 |Bz|. As has been found in earlier studies, IMF Bz was <0 prior to most events. When IMF data from Geotail, Themis B, and/or Themis C in the near-Earth solar wind were used to supplement the time-shifted OMNI IMF data, the consistency of these IMF orientations was improved.
- Global-GMDs: The global map of geomagnetic disturbancesHu, Hongyi; Xu, Zhonghua (Elsevier, 2024-02)To improve the understanding and monitoring the impacts of geomagnetic disturbances (GMDs) on power grids globally, the presented software, Global-GMDs, uses magnetic field measurements from geomagnetic observatories worldwide and Kriging method to generate global maps of GMDs. It provides better observational information during a solar storm to power grid operations and other crucial infrastructures. It can also help researchers to assess the GMDs prediction model by comparing with Global-GMDs maps and to get better understanding of physics mechanisms.
- The impact and resolution of the GPS week number rollover of April 2019 on autonomous geophysical instrument platformsCoyle, Shane; Clauer, C. Robert; Hartinger, Michael D.; Xu, Zhonghua; Peng, Yuxiang (2021-07-28)Instrument platforms the world over often rely on GPS or similar satellite constellations for accurate timekeeping and synchronization. This reliance can create problems when the timekeeping counter aboard a satellite overflows and begins a new epoch. Due to the rarity of these events (19.6 years for GPS), software designers may be unaware of such circumstance or may choose to ignore it for development complexity considerations. Although it is impossible to predict every fault that may occur in a complicated system, there are a few "best practices" that can allow for graceful fault recovery and restorative action. These guiding principles are especially pertinent for instrument platforms operating in space or in remote locations like Antarctica, where restorative maintenance is both difficult and expensive. In this work, we describe how these principles apply to a communications failure on autonomous adaptive low-power instrument platforms (AAL-PIP) deployed in Antarctica. In particular, we describe how code execution patterns were subtly altered after the GPS week number rollover of April 2019, how this led to Iridium satellite communications and data collection failures, and how communications and data collection were ultimately restored. Finally, we offer some core tenets of instrument platform design as guidance for future development.
- Impact Angle Control of Local Intense dB/dt Variations During Shock-Induced SubstormsOliveira, Denny M.; Weygand, James M.; Zesta, Eftyhia; Ngwira, Chigomezyo M.; Hartinger, Michael D.; Xu, Zhonghua; Giles, Barbara L.; Gershman, Daniel J.; Silveira, Marcos V. D.; Souza, Vítor M. (American Geophysical Union, 2021-12-01)The impact of interplanetary shocks on the magnetosphere can trigger magnetic substorms that intensify auroral electrojet currents. These currents enhance ground magnetic field perturbations (dB/dt), which in turn generate geomagnetically induced currents (GICs) that can be detrimental to power transmission infrastructure. We perform a comparative study of dB/dt variations in response to two similarly strong shocks, but with one being nearly frontal and the other highly inclined. Multi-instrument analyses by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Los Alamos National Laboratory spacecraft show that nightside substorm-time energetic particle injections are more intense and occur faster in the case of the nearly head-on impact. The same trend is observed in dB/dt variations recorded by THEMIS ground magnetometers. THEMIS all-sky imager data show a fast and clear poleward auroral expansion in the first case, which does not clearly occur in the second case. Strong field-aligned currents computed with the spherical elementary current system (SECS) technique occur in both cases, but the current variations resulting from the inclined shock impact are weaker and slower compared to the nearly frontal case. SECS analyses also reveal that geographic areas with dB/dt surpassing the thresholds 1.5 and 5 nT/s, usually linked to high-risk GICs, are larger and occur earlier due to the symmetric compression caused by the nearly head-on impact. These results, with profound space weather implications, suggest that shock impact angles affect the geospace driving conditions and the location and intensity of the subsequent dB/dt variations during substorm activity.
- Mid-Latitude Thermosphere-Ionosphere Na (TINa) Layers Observed With High-Sensitivity Na Doppler Lidar Over Boulder (40.13 degrees N, 105.24 degrees W)Chu, Xinzhao; Chen, Yingfei; Cullens, Chihoko Y.; Yu, Zhibin; Xu, Zhonghua; Zhang, Shun-Rong; Huang, Wentao; Jandreau, Jackson; Immel, Thomas J.; Richmond, Arthur D. (2021-06-16)We report the first lidar observations of regular occurrence of mid-latitude thermosphere-ionosphere Na (TINa) layers over Boulder (40.13 degrees N, 105.24 degrees W), Colorado. Detection of tenuous Na layers (similar to 0.1-1 cm(-3) from 150 to 130 km) was enabled by high-sensitivity Na Doppler lidar. TINa layers occur regularly in various months and years, descending from similar to 125 km after dusk and from similar to 150 km before dawn. The downward-progression phase speeds are similar to 3 m/s above 120 km and similar to 1 m/s below 115 km, consistent with semidiurnal tidal phase speeds. One or more layers sometimes occur across local midnight. Elevated volume mixing ratios above the turning point (similar to 105-110 km) of Na density slope suggest in situ production of the dawn/dusk layers via neutralization of converged Na+ layers. Vertical drift velocity of TINa+ calculated with the Ionospheric Connection Explorer Hough Mode Extension tidal winds shows convergent ion flow phases aligned well with TINa, supporting this formation hypothesis.
- Wavelet Based ULF Pulsation Index for Studying Conjugate ULF Pulsation at High Latitudes and Its Applications to Space WeatherXu, Zhonghua; Kim, H.; Clauer, C. Robert; Weimer, Daniel R.; Deshpande, K. (2016-12-14)A wavelet-based index is described in this study and applied to present geomagnetic Ultra Low Frequency (ULF) pulsations observed in Antarctica and their magnetically conjugate locations in West Greenland. The index is effective for identification of pulsation events in the Pc4-5 frequency range, which is related to the Geomagnetically Induced Currents (GICs) shown by many researchers, and measures important characteristics of ULF pulsations in both the temporal and frequency domains. We discuss how the wavelet indices can be used to monitor geomagnetic pulsations in both hemispheres simultaneously. The wavelet analysis shows valuable information for GIC- related studies, including the spectrum, correlation, and magnitude of the geomagnetic pulsations. The comparison between conjugate locations reveals the similarities and differences of ULF pulsations in both hemispheres. Also, since the Greenland chains are located near the coastal area, while the Antarctic chains are over thousands meters of the ice-sheet on the East Antarctic plateau, inter-hemispheric comparisons of vertical magnetic field perturbations can be used to reveal how sensitive ULF pulsations are to ground conductivity.