Occurrence characteristics of branching structures in equatorial plasma bubbles:a statistical study based on all-sky imagers in China

Branching structure(BS)is a very important phenomenon in the evolution of equatorial plasma bubbles(EPBs),the mechanism of which is widely studied from observation and from simulation.However,occurrence characteristics of branching structure of equatorial plasma bubbles(BSEPBs)have not been well addressed.In this work,we used seven-years(2012-2018)of observations from two all-sky imagers to study occurrence of BSEPBs in detail.These data reveal a high incidence of BS in EPB cases;in particular,most EPBs occurring on days with geomagnetic disturbances exhibited BS.Periods when all EPBs exhibited BS increased significantly in the 2014 solar maximum.Occurrence times of BSEPBs varied with local time;most of the BSEPBs began to appear between 21:00 and 22:00 LT.During the solar maximum,some BSEPBs were observed after midnight.The data also reveal that BSEPBs are characterized primarily by two branches or three branches.Multi-branching appeared only in the solar maximum.EPB events with different coexisting branching structures increased from 2012 to 2014 and decreased from 2014 to 2018.These results strongly suggest that BSEPB occurrence is related to solar activity and geomagnetic activity,and thus provide a new perspective for future studies of EPBs as well as enriching our understanding of ionospheric irregularity.

Case study of an Equatorial Plasma Bubble Event investigated by multiple ground-based instruments at low latitudes over China

Observational evidence is insufficient to understand how equatorial plasma bubbles(EPBs)form over low latitudes.The mechanism of plasma-density enhancement(formation of"plasma blobs")at low latitudes is in dispute.In this paper,we use data from multiple ground-based instruments(one all-sky airglow imager,five digisondes,and one Fabry–Perot interferometer)to investigate the evolution of an EPB event that occurred at low latitudes over China on the night of 06 December 2015(06-Dec-2015).We provide observational evidence that an enhanced equatorward wind most likely induced by a substorm could have initiated the Rayleigh–Taylor instability(RTI)that destabilized several EPB depletions in an upwelling region of a large-scale wave-like structure(LSWS)in the bottomside ionosphere.Those EPB depletions were forced to surge poleward,from nearly 10°to 19°magnetic latitude,two hours before midnight.Smaller-scale bifurcations evolved rapidly from tips of airglow depletions by a secondary E×B instability when the aforementioned substorm-induced southwestward wind blew through.During the growth phase of the EPB depletions,a westward polarization electric field inside the LSWS is likely to have compressed plasma downward,inducing the two airglow-type blobs observed in the bottomside ionosphere,by a mechanism of LSWS-blob connection that we propose.We also provide observational evidence of brightness airglow depletions.We find that an enhanced poleward wind associated with a passing-by brightness wave(BW)is likely to have transported plasma to fill the airglow depletions,which finally evolved into brightness airglow structures.This study investigates the physical processes accompanied by the EPB event and those two-airglow blobs observed at low-latitudes over China.

On the solar activity dependence of midnight equatorial plasma bubbles during June solstice periods

The occurrence of midnight Equatorial Plasma Bubbles(EPBs)during the June solstice period of the ascending phase of solar cycle 24,from 2010 to 2014,was studied using data from the 47 MHz Equatorial Atmosphere Radar(EAR)at Kototabang,Indonesia.The analysis shows that the occurrence of midnight hour EPBs was at its maximum during the low solar activity year 2010 and monotonically decreased thereafter with increasing solar activity.Details of the dependence of midnight hour EPB occurrence on solar activity were investigated using SAMI2 model simulation with a realistic input of E×B drift velocity data obtained from the CINDI-IVM onboard the C/NOFS satellite.Results obtained from term-by-term analysis of the flux tube integrated linear growth rate of RT instability indicate that the formation of a high flux tube electron content height gradient(steep vertical gradient)region at higher altitudes,due to the elevated F layer,is the key factor enhancing the growth rate of RT instability during low solar activity June solstices.Other factors are discussed in light of the relatively weak westward zonal electric field in the presence of the equatorward neutral wind and north-to-south transequatorial wind around the midnight hours of low solar activity June solstices.Also discussed are the initial seeding of RT instability by MSTIDs and how the threshold height required for EPB development varies with solar activity.

Interaction between Equatorial Plasma Bubbles and a Medium-Scale Traveling Ionospheric Disturbance,observed by OI 630 nm airglow imaging at Bom Jesus de Lapa,Brazil

OI 630.0 nm airglow observations,from a new observatory at Bom Jesus de Lapa,were used to study the interaction between EPBs(Equatorial Plasma Bubbles)and the MSTID(Medium-Scale Traveling Ionospheric Disturbance)over the Northeast region in Brazil.On the night of September 16 to 17,2020,an EPB was observed propagating eastward,in an apparent fossil stage,until it interacted with a dark band electrified MSTID(e MSTID).After the interaction,four EPBs merged,followed by an abrupt southward development and bifurcations.Analysis of the data suggests that an eastward polarization electric field,induced by the dark band e MSTID,forced the EPB into an upward drift,growing latitudinally along the magnetic field lines and then bifurcating.

Preface to the Special Issue on recent advances in the study of Equatorial Plasma Bubbles and Ionospheric Scintillation

The 2 nd Equatorial Plasma Bubble(EPB)workshop,funded by the Institute of Geology and Geophysics,Chinese Academy of Sciences,and the National Natural Science Foundation of China,took place in Beijing,China during September 13–15,2019.The EPB workshop belongs to a conference series that began in 2016 in Nagoya,Japan at the Institute for Space-Earth Environmental Research,Nagoya University,resulting in a special issue of Progress in Earth and Planetary Science that focused on EPBs.The main goal of the series is to organize in-depth discussion by scientists working on ionospheric irregularities,and solve the scientific challenges in EPB and ionospheric scintillation forecasting.The 2 nd EPB workshop gathered almost 60 scientists from seven countries.A total of 20 invited and contributing papers focusing on ionospheric irregularities and scintillations were presented.Here we briefly comment on 10 papers included in this special issue.

A statistical analysis of conjugate equatorial plasma bubbles based on 630 nm airglow observations

Using the observations of the 630-nm all-sky imagers(ASIs)located in the geomagnetic conjugate points in the American sector from 2014 to 2017,this study statistically analyzed the features of conjugate equatorial plasma bubbles(EPBs),including their occurrence rate,zonal width,location and zonal drift velocity.The results show that the occurrence rate of the EPBs that occur simultaneously at geomagnetic conjugate points is~84%.The zonal widths of the EPBs are mainly~100 km,and the width differences of EPBs between the northern and southern hemispheres are mainly within±30 km.The zonal displacements of the center locations of the northern and southern EPBs are within±50 km.The zonal drift velocities of the northern and southern EPBs are nearly equal.However,it should be noted that the velocity of the EPBs in the northern hemisphere is 10%faster than that in the southern hemisphere.The results suggest that conjugate EPBs are common.Moreover,the non-conjugate EPBs in the northern and southern hemisphere can occur occasionally,which is probably associated with the different ionospheric backgrounds between the two hemispheres.The features of the conjugate EPBs as shown in this study provides support for the nowcasting of EPBs in the conjugate hemispheres.

Multi-instrument study of longitudinal wave structures for plasma bubble seeding in the equatorial ionosphere

Large Scale Wave Structures(LSWS)in the equatorial ionospheric F-region were observed by measuring spatial and temporal variations within detrended total electron content(dTEC)data obtained by ground-based GNSS receivers over the South American continent.By using dTEC-maps,we have been able to produce,for the first-time,two-dimensional representations of LSWS.During the period from September to December,the LSWS frequently occurred starting a few hours prior to Equatorial Plasma Bubble(EPB)development.From 17 events of LSWS observed in 2014 and 2015,wave characteristics were obtained:the observed wavelengths,periods,and the phase speeds are respectively,~900 km,~41 min and~399 m/s;the waves propagated from the northeast to southeast.In some cases the front of the oscillation was meridionally aligned,extending to more than 1600 km,the first time such large extension of the wavefront has been reported.From F-layer bottom height oscillation data,measured by ionosonde,LSWS exhibit two different vertical phase propagation modes,in-phase and downward phase.The former mode indicates the presence of a polarization electric field in the F-layer bottom side;the latter suggests propagation of atmospheric gravity waves.The presence of LSWS near the solar terminator,followed by the development of EPBs,suggests that the upwelling of the F-layer bottom height produces a condition favorable to the development of Rayleigh–Taylor instability.

A measure of ionospheric irregularities:zonal velocity and its implications for L-band scintillation at low-latitudes

We estimate the zonal drift velocity of small-scale ionospheric irregularities at low latitude by leveraging the spaced-receivers technique applied to two GNSS receivers for scintillation monitoring installed along the magnetic parallel passing in Presidente Prudente(Brazil,magnetic latitude 12.8°S).The investigated ionospheric sector is ideal to study small-scale irregularities,being located close to the expected position of the southern crest of the equatorial ionospheric anomaly.The measurement campaign took place between September 2013 and February 2014,i.e.equinox and summer solstice seasons under solar maximum,during which the probability of formation of small-scale irregularities is expected to maximize.We found that the hourly average of the velocity increases up to 135 m/s right after the local sunset at ionospheric altitudes and then smoothly decreases in the next hours.Such measurements are in agreement with independent estimations of the velocity made by the Incoherent Scatter Radar located at the Jicamarca Radio Observatory(magnetic latitude 0.1°N),by the Boa Vista Ionosonde(magnetic latitude 12.0°N),and by applying a recently-developed empirical regional short-term forecasting model.Additionally,we investigated the relationship with the percentage occurrence of amplitude scintillation;we report that it is exponentially dependent on the zonal velocity of the irregularities that cause it.