Browsing by Author "Liu, Libo"
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Item Restricted A case study of ionospheric storm effects during long‐lasting southward IMF Bz‐driven geomagnetic storm(American Geophysical Union, 2014-08-27) Liu, Jing; Liu, Libo; Nakamura, Takuji; Zhao, Biqiang; Ning, Baiqi; Yoshikawa, A.Multiple instrumental observations including GPS total electron content (TEC), foF2 and hmF2 from ionosondes, vertical ion drift measurements from Communication/Navigation Outage Forecasting System, magnetometer data, and far ultraviolet airglow measured by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) are used to investigate the profound ionospheric disturbances at midlatitude and low latitude during the 14–17 July 2012 geomagnetic storm event, which was featured by prolonged southward interplanetary geomagnetic field component for about 30 h below −10 nT. In the East Asian/Australian sector, latitudinal profile of TEC variations in the main phase were characterized by three bands of increments and separated by weak depressions in the equatorial ionospheric anomaly (EIA) crest regions, which were caused by the combined effects of disturbance dynamo electric fields (DDEF) and equatorward neutral winds. In the recovery phase, strong inhibition of EIA occurred and the summer crest of EIA disappeared on 16 July due to the combined effects of intrusion of neutral composition disturbance zone as shown by the TIMED/GUVI O/N2 measurements and long‐lasting daytime westward DDEF inferred from the equatorial electrojet observations. The transit time of DDEF over the dip equator from westward to eastward is around 2200 LT. In the American longitude, the salient ionospheric disturbances in the summer hemisphere were characterized by daytime periodical intrusion of negative phase for three consecutive days in the recovery phase, preceded by storm‐enhanced density plume in the initial phase. In addition, multiple short‐lived prompt penetration electric fields appeared during stable southward interplanetary magnetic field (IMF) Bz in the recovery phase and were responsible for enhanced the EIA and equatorial ionospheric uplift around sunset.Item Restricted Comparative study of the equatorial ionosphere over Jicamarca during recent two solar minima(American Geophysical Union, 2012-01-21) Liu, Libo; Yang, Jun; Le, Huijun; Chen, Yiding; Wan, Weixing; Lee, C. C.It is a critical issue that whether or not the extremely deep solar minimum of solar cycle 23/24 brought serious influences on the Earth's space environment. In this study, we collected and manually scaled the ionograms recorded by a DPS ionosonde at Jicamarca (12.0°S, 283.2°E) to retrieve F layer parameters and electron density (Ne) profiles. A comparative study is performed to evaluate the equatorial ionosphere in solar minima of cycle 22/23 (1996–1997) and 23/24 (2008–2009). The seasonal median values of the critical frequency of F2 layer (foF2) were remarkably reduced in four seasons during the deep solar minimum, compared to those in 1996–1997. It is the first time to find that lower values prevail at most times in 2008–2009 in the F2 layer peak height (hmF2) and Chapman scale height (Hm). The bottomside profile thickness (B0) shows higher values in 2008–2009 than that in 1996–1997 at some daytime intervals, although it also becomes smaller during the rest times. Furthermore, the ionosphere in 2008–2009 is contracted strongly at altitudes above hmF2 and more perceptible in the afternoon hours. The decrease in Ne is strongest in September equinox and weakest in June solstice. The ionospheric responses from solar minimum to minimum are mainly caused by the reduction in solar extreme ultraviolet intensity, and the contribution from dynamical processes competes and is variable. Analysis reveals that semiannual and longer‐scale components are certainly reduced during the deep solar minimum, while shorter scale (e.g., 4 month) components may disrupt the decline picture at some times.Item Restricted Effects of disturbed electric fields in the low‐latitude and equatorial ionosphere during the 2015 St. Patrick's Day storm(American Geophysical Union, 2016-09-02) Kuai, Jiawei; Liu, Libo; Liu, Jing; Sripathi, S.; Zhao, Biqiang; Chen, Yiding; Le, Huijun; Hu, LianhuanThe 2015 St. Patrick's 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 17 March 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 equatorial ionization anomaly intensifications caused by the multiple strong PEFs are more distinctive over the American sector. Especially, the storm time F3 layer features are caught on 17 March in the American equatorial region, proving the effects of the multiple strong eastward PEFs.Item Restricted Equatorial ionospheric electrodynamics during solar flares(American Geophysical Union, 2017-05-09) Zhang, Ruilong; Liu, Libo; Le, Huijun; Chen, YidingPrevious investigations on ionospheric responses to solar flares focused mainly on the photoionization caused by the increased X‐rays and extreme ultraviolet irradiance. However, little attention was paid to the related electrodynamics. In this letter, we explored the equatorial electric field (EEF) and electrojet (EEJ) in the ionosphere at Jicamarca during flares from 1998 to 2008. It is verified that solar flares increase dayside eastward EEJ but decrease dayside eastward EEF, revealing a negative correlation between EEJ and EEF. The decreased EEF weakens the equatorial fountain effect and depresses the low‐latitude electron density. During flares, the enhancement in the Cowling conductivity may modulate ionospheric dynamo and decrease the EEF. Besides, the decreased EEF is closely related to the enhanced ASY‐H index that qualitatively reflects Region 2 field‐aligned current (R2 FAC). We speculated that solar flares may also decrease EEF through enhancing R2 FAC that leads to an overshielding‐like effect.Item Restricted Evidence and effects of the sunrise enhancement of the equatorial vertical plasma drift in the F region ionosphere(American Geophysical Union, 2016-05-13) Zhang, Ruilong; Liu, Libo; Le, Huijun; Chen, YidingRecent studies based on the satellite observations demonstrated that the equatorial vertical plasma drift can also enhance near sunrise in a way similar to the prereversal enhancement. However, it is not clear whether the signature of this sunrise enhancement appears in observations with other sounding techniques. In this work, we explore the Jicamarca (12°S, 283.2°E) incoherent scatter radar measurements to present the evidence of sunrise enhancement in vertical plasma drift on 12 May and 10 June 2004, which are under magnetically quiet and solar minimum conditions. The effects of the sunrise enhancement on the ionosphere are, for the first time, investigated by analyzing the ionograms recorded by the Digisonde Portable Sounder at Jicamarca and conducting the Theoretical Ionospheric Model of the Earth in Institute of Geology and Geophysics, Chinese Academy of Sciences. The observations showed that, during the sunrise enhancement, the F2 layer peak height is lifted remarkably, and the F2 layer peak density and bottomside electron density tend to decrease compared to the days without sunrise enhancements. The simulations indicated that the sunrise enhancement drift can lift the equatorial ionosphere to higher heights and distort the equatorial electron density profiles. What is more, the simulations display an F3 layer in the equatorial F region during the sunrise enhancement, and a new F2 layer develops at lower altitudes under the jointed control of the usual photochemical and dynamical processes.Item Restricted The long‐duration positive storm effects in the equatorial ionosphere over Jicamarca(American Geophysical Union, 2015-01-06) Kuai, Jiawei; Liu, Libo; Liu, Jing; Zhao, Biqiang; Chen, Yiding; Le, Huijun; Wan, WeixingThe long‐duration positive storm (LPS) in the equatorial regions is relatively poorly understood. In this report, we conducted a statistical analysis of the LPS effects in the equatorial ionosphere over Jicamarca (12.0°S, 283.2°E) in 1998–2010. There are 250 geomagnetic storms (minimum Dst < −50 nT) in 1998–2010, but the ionosonde observations at Jicamarca are available only for 204 storms. A total of 46 LPSs are identified in terms of the criterion that the storm time relative deviation of peak density of F2 layer (NmF2) exceeds 25% for more than 6 h. A salient feature is that the occurrence of LPSs tends to decay approximately exponentially on the following days after the main phase of geomagnetic storms. The ratios of the number of equatorial LPSs to that of geomagnetic storms have no obvious dependence on season and solar activity. During the daytime LPSs, the disturbed zonal electric field is mostly westward, as indicated from the geomagnetic field changes in the equatorial American region. For the nighttime LPSs, the significant uplifting of F2 layer caused by an eastward electric field is the most important feature. Therefore, the disturbed electric field should play an essential role in forming the equatorial LPSs.