Lianhuan Hu

Effects of disturbed electric fields in the low‐latitude and equatorial ionosphere during the 2015 St. Patrick's Day storm

The 2015 St. Patricks day geomagnetic storm with SYM-H value of -233 nT is an extreme space weather event in the current 24th solar cycle. In this work, we investigated the main mechanisms of the profound ionospheric disturbances over equatorial and low latitudes in the Asian-Australian sector and the American sector during this super storm event. The results reveal that the disturbed electric fields, which comprise penetration electric fields (PEFs) and disturbance dynamo electric fields (DDEFs), play a decisive role in the ionospheric storm effects in low latitude and equatorial regions. PEFs occur on March 17 in both the American sector and the Asian-Australian sector. The effects of DDEFs are also remarkable in the two longitudinal sectors. Both the DDEFs and PEFs show the notable local time dependence, which causes the sector differences in the characteristics of the disturbed electric fields. This differences would further lead to the sector differences in the low-latitude ionospheric response during this storm. The negative storm effects caused by the long-duration DDEFs are intense over the Asian-Australian sector, while the repeated elevations of hmF2 and the EIA intensifications caused by the multiple strong PEFs are more distinctive over the American sector. Especially, the storm time F3-layer features are caught on March 17 in the American equatorial region, proving the effects of the multiple strong eastward PEFs.

A statistic study of ionospheric solar flare activity indicator

According to the Chapman ionization theory, an ionospheric solar flare activity indicator (ISFAI) is given by the solar zenith angle and the variation rate of ionospheric vertical total electron content, which is measured from a global network of dual-frequency GPS receivers. The ISFAI is utilized to statistically analyze the ionospheric responses to 1439 M-class and 126 X-class solar flares during solar cycle 23 (1996–2008). The statistical results show that the occurrence of ISFAI peak increases obviously at 3.2 total electron content unit (TECU)/h (1 TECU = 1016 el m−2) and reaches the maximum at 10 TECU/h during M-class flares and 10 TECU/h and 40 TECU/h for X-class flares. ISFAI is closely correlated with the 26–34 nm extreme ultraviolet flux but poorly related to the 0.1–0.8 nm X-ray flux. The central meridian distance (CMD) of flare location is an important reason for depressing relationship between ISFAI and X-ray Flux. Through the CMD effect modification, the ISFAI has a significant dependence on the X-ray flux with a correlation coefficient of 0.76. The ISFAI sensitivity enables to detect the extreme X-class flares, as well as the variations of one order of magnitude or even smaller (such as for C-class flares). Meanwhile, ISFAI is helpful to the calibration of the X-ray flux at 0.1–0.8 nm observed by GOES during some flares. In addition, the statistical results demonstrate that ISFAI can detect 80% of all M-class flares and 92% for all X-class ones during 1996–2008.

Long‐lasting negative ionospheric storm effects in low and middle latitudes during the recovery phase of the 17 March 2013 geomagnetic storm

In this paper, an ionospheric electron density reanalysis algorithm was used to generate global optimized electron density during the March 17-18, 2013 geomagnetic storm by assimilating ~10 low Earth orbit (LEO) satellites based and ~450 ground global navigation satellite system (GNSS) receiver based total electron content (TEC) into a background ionospheric model. The reanalyzed electron density could identify the large-scale ionospheric features quite well during storm time, including the storm enhanced density (SED), the positive ionospheric storm effect during the initial and main phases, and the negative ionospheric storm effect during the recovery phase. The simulations from the thermosphere ionosphere electrodynamics general circulation model (TIEGCM) can reproduce similar large-scale ionospheric disturbances as seen in the reanalysis results. Both the reanalysis and simulations show long-lasting (>17 hours) daytime negative storm effect over the Asia sector as well as hemispheric asymmetry during the recovery phase. Detailed analysis of the Global Ultraviolet Imager (GUVI) derived O/N2 ratio and model simulations indicate that the polar ward meridional wind disturbance, the downward E × B drift disturbance and O/N2 depletion might be responsible for the negative storm effect. The hemispheric asymmetry is mainly caused by the geomagnetic field line configuration, which could cause hemispheric asymmetry in the O/N2 depletion.

Contrasting behavior of the F2 peak and the topside ionosphere in response to the 2 October 2013 geomagnetic storm

In this study, the ionospheric observations from ionosondes, ground-based GPS receivers, Gravity Recovery and Climate Experiment (GRACE) and MetOp-A satellites, and Fabry-Perot interferometer over the Asian-Australian sector have been used to investigate the responses of the F2 peak and the topside ionosphere to the 2 October 2013 geomagnetic storm, particularly during the recovery phase. The comparison between the multiple simultaneous observations revealed a contrasting behavior of the topside ionosphere and the F2 peak in East Asia during the recovery phase. The upward looking total electron content from low-Earth orbit (LEO) satellites did not undergo such depletions as seen in the region near the F2 peak, and they even showed increases. Furthermore, the simulation results of the Thermosphere Ionosphere Electrodynamics General Circulation Model are used to explore the possible mechanisms responsible for the observed features. The model results and observations suggested that the contrasting behavior of the F2 peak and the topside ionosphere is mainly associated with the enhancement of the equatorward winds, albeit the disturbed electric fields could play an important role in producing it.

Seasonal variations of MLT tides revealed by a meteor radar chain based on Hough mode decomposition

Seasonal variations of different tides in the mesosphere and lower thermosphere are investigated from wind observations of a meteor radar chain on the basis of Hough mode decomposition. First, the observed winds are decomposed into different (diurnal, semidiurnal, and terdiurnal) tidal components. Different seasonal patterns are revealed for each component. Pronounced semiannual oscillation (SAO) is presented in the diurnal component, while latitude-dependent seasonal variation is found in the semidiurnal and terdiurnal components. At the low/midlatitude stations, the semiannual/annual oscillation is relatively stronger. Then, Hough mode decomposition is utilized to extract the dominant tidal modes of each decomposed component. It is found that each component is dominated by one of its symmetric tidal modes with strong seasonal dependency. Apparent SAO is observed in the dominant (1, 1) mode; (2, 4) mode is strong in the autumn and winter months (after the September equinox). Based on the extracted results we further map the three-dimensional distribution (latitude × altitude × season) of each tidal component. The mapped results are finally compared with the corresponding values observed by the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Doppler Interferometer (TIDI) and modeled from the global scale wave model (GSWM). Each mapped tidal component agrees well with corresponding TIDI observation in the seasonal variation. Meanwhile, coincidences are found in the seasonal dependency of the diurnal component between the mapped values and the modeled results from GSWM, while difference between them exists in that of the semidiurnal one.

Evidence for lightning‐associated enhancement of the ionospheric sporadic E layer dependent on lightning stroke energy

In this study we analyze the lightning data obtained by the World-Wide Lightning Location Network (WWLLN) and hourly ionospheric data observed by ionosondes located at Sanya and Beijing, to examine the changes in ionospheric electron density in response to the underlying thunderstorms and to investigate the possible connection between lightning discharges and the enhancement of the ionospheric sporadic E(Es) layer. We identify a statistically significant enhancement and a decrease in altitude of the Es layer at Sanya station, in agreement with the results found at Chilton, UK. However, the lightning-associated modification of the Es layer investigated using the same approach is not evident at Beijing station. Furthermore, we compare the responses to weak and strong lightning strokes using WWLLN-determined energies at Sanya in 2012. The lightning-associated enhancement of the Es layer is predominantly attributed to powerful strokes with high stroke energy. A statistically significant intensification of the Es layer with higher-energy strokes at Sanya, along with the statistical dependence of lightning-associated enhancement of the Es layer on stroke energy, leads us to conclude that the magnitude of the enhancement is likely associated with lightning stroke energy.

Development of the Beidou Ionospheric Observation Network in China for Space Weather Monitoring

The Global Navigation Satellite System (GNSS) beacon has been widely used in ionospheric monitoring. The Chinese Beidou Navigation Satellite System (BDS) started to operate in 2012, with three kinds of constellations: the medium Earth orbit (MEO), the inclined geosynchronous satellite orbit (IGSO) and the geosynchronous Earth orbit (GEO). A compact, portable and low power GNSS observation instrument which is named BG2 GNSS ionospheric monitor was developed at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). It is capable of tracking BDS, Global Positing System (GPS) and GLONASS signals with a sampling rate up to 5 Hz. To use the unique BDS GEO signals for ionospheric monitoring, the Beidou Ionospheric Observation Network (BION) utilizing the BG2 GNSS ionospheric monitor was established recently in China with a total of 31 sites. This paper describes the development of the GNSS ionospheric monitor, the construction of the network, data processing, and preliminary scientific research. As demonstrated in the following paper, the BDS GEO total electron content (TEC) data has shown unique value in some specific ionospheric studies due to the fact that it is not influenced by the mixed effect of spatial and temporal ionospheric variability as observed with other GNSS signals. As a part of the BION, the Septentrio PolaRx5s mutli-constellation GNSS ionospheric scintillation monitor will be installed at 14 observation sites at low latitude of China in the near future.

Regional differences of the ionospheric response to the July 2012 geomagnetic storm: STORM-TIME REGIONAL DIFFERENCES

The July 2012 geomagnetic storm is an extreme space weather event in solar cycle 24, which is characterized by a southward interplanetary geomagnetic field lasting for about 30 h below 10 nT. In this work, multiple instrumental observations, including electron density from ionosondes, total electron content (TEC) from Global Positioning System, Jason-2, and Gravity Recovery and Climate Experiment, and the topside ion concentration observed by the Defense Meteorological Satellite Program spacecraft are used to comprehensively present the regional differences of the ionospheric response to this event. In the Asian-Australian sector, an intensive negative storm is detected near longitude ~120°E on 16 July, and in the topside ionosphere the negative phase is mainly existed in the equatorial region. The topside and bottomside TEC contribute equally to the depletion in TEC, and the disturbed electric fields make a reasonable contribution. On 15 July, the positive storm effects are stronger in the Eastside than in the Westside. The topside TEC make a major contribution to the enhancement in TEC for the positive phases, showing the important role of the equatorward neutral winds. For the American sector, the equatorial ionization anomaly intensification is stronger in the Westside than in the Eastside and shows the strongest feature in the longitude ~110°W. The combined effects of the disturbed electric fields, composition disturbances, and neutral winds cause the complex storm time features. Both the topside ion concentrations and TEC reveal the remarkable hemispheric asymmetry, which is mainly resulted from the asymmetry in neutral winds and composition disturbances.

Shear in the zonal drifts of 3 m irregularities inside spread F plumes observed over Sanya

Incoherent scatter radars near magnetic equator regularly measured a vertical shear in zonal drifts of the evening background plasma, with westward drifts below the equatorial F region peak and eastward drifts above. We report here observations of a clear shear structure in the zonal drifts of 3 m irregularities inside spread F (SF) backscatter plumes measured with a 47.5 MHz coherent scatter radar operated at a low-latitude site Sanya (18.4°N, 109.6°E; dip latitude 12.8°N). The radar interferometry analysis on the zonal drifts of the 3 m irregularities yields results consistent with that estimated from the irregularity echo Doppler velocity measurements with multiple beams. It is shown that the SF 3 m irregularities move westward at the lowest altitudes, while at higher altitudes in the same SF plume structure, the 3 m irregularities drift eastward. One interesting point is that the vertical shear of zonal drifts was centered at ~300 km altitude over Sanya, which maps to an apex altitude of ~650 km at magnetic equator and is thus apparently higher than the apex altitudes 250–450 km where the zonal velocity shear has usually been observed. Analysis of the observations suggests that while the possibility of local generation of the shear flow of the irregularities can exist, the possibility of a plasma vortex appearing at relative high altitudes causing the zonal drift shear of F region 3 m irregularities measured over Sanya cannot be totally ruled out.

Observational evidence of high‐altitude meteor trail from radar interferometer

Whether radar meteor echoes occur at high altitudes (above ~130 km) in the Earths atmosphere is a long-standing question within the meteor radar community. Using observations from the Sanya VHF coherent radar interferometer during 11 July to 10 August 2013, we have found a new class of range-spread high-altitude meteor trail echoes (HAMEs), some of which appeared at ~170 km altitude lasting more than 10 s. A statistical analysis on the local time dependence of the identified HAME events shows a maximum around 00–04 LT. The results imply that there could be much more meteor mass input due to meteoroid sputtering at high altitudes in the Earths atmosphere than previously thought.