P. K. Rajesh

CME front and severe space weather

Thanks to the work of a number of scientists who made it known that severe space weather can cause extensive social and economic disruptions in the modern high-technology society. It is therefore important to understand what determines the severity of space weather and whether it can be predicted. We present results obtained from the analysis of coronal mass ejections (CMEs), solar energetic particle (SEP) events, interplanetary magnetic field (IMF), CME-magnetosphere coupling, and geomagnetic storms associated with the major space weather events since 1998 by combining data from the ACE and GOES satellites with geomagnetic parameters and the Carrington event of 1859, the Quebec event of 1989, and an event in 1958. The results seem to indicate that (1) it is the impulsive energy mainly due to the impulsive velocity and orientation of IMF Bz at the leading edge of the CMEs (or CME front) that determine the severity of space weather. (2) CMEs having high impulsive velocity (sudden nonfluctuating increase by over 275u2009kmu2009s−1 over the background) caused severe space weather (SvSW) in the heliosphere (failure of the solar wind ion mode of Solar Wind Electron Proton Alpha Monitor in ACE) probably by suddenly accelerating the high-energy particles in the SEPs ahead directly or through the shocks. (3) The impact of such CMEs which also show the IMF Bz southward from the leading edge caused SvSW at the Earth including extreme geomagnetic storms of mean DstMPu2009<u2009−250u2009nT during main phases, and the known electric power outages happened during some of these SvSW events. (4) The higher the impulsive velocity, the more severe the space weather, like faster weather fronts and tsunami fronts causing more severe damage through impulsive action. (5) The CMEs having IMF Bz northward at the leading edge do not seem to cause SvSW on Earth, although, later when the IMF Bz turns southward, they can lead to super geomagnetic storms of intensity (Dstmin) less than even −400u2009nT.

A new parameter of geomagnetic storms for the severity of space weather

Using the continuous Dst data available since 1957 and H component data for the Carrington space weather event of 1859, the paper shows that the mean value of Dst during the main phase of geomagnetic storms, called mean DstMP, is a unique parameter that can indicate the severity of space weather. All storms having high mean DstMP (≤−250 nT), which corresponds to high amount of energy input in the magnetosphere–ionosphere system in short duration, are found associated with severe space weather events that caused all known electric power outages and telegraph system failures.

Low‐latitude midnight brightness in 630.0 nm limb observations by FORMOSAT‐2/ISUAL

This paper investigates the intense airglow brightness often observed in the 630.0 nm limb images taken using Imager of Sprites and Upper Atmospheric Lightnings (ISUAL), onboard FORMOSAT-2 satellite, where the tangent plane of the measurement falls in the local midnight sector. Most of the images show only single brightness, but in some cases there could be multiple peaks, which sometimes appears to be centered on geographic equator and in some cases falls on either sides of the magnetic equator. In order to understand such intense emission in the near-midnight hours, the observations are simulated using SAMI2 (SAMI2 is Another Model of the Ionosphere) model parameters based on the ISUAL viewing geometry. The simulations reproduced the measured airglow intensity pattern quite remarkably and suggested that the meridional neutral wind and the resulting plasma distribution are closely related with the observed brightness. The intensity and locations of the airglow brightness peaks could potentially be utilized to infer the strength of meridional neutral wind.

Global equatorial plasma bubble growth rates using ionosphere data assimilation

In this study, Rayleigh-Taylor instability growth rates computed using the results of ionosphere data assimilation are used to investigate global plasma bubble occurrence. Thermosphere ionosphere electrodynamics global circulation model forecast results after assimilating total electron content measurements using ground network of global positioning system receivers are used in this work. The calculated growth rates are compared with rate of change of total electron content index (ROTI), estimated from global network of ground based global positioning system receivers, as well as ground based all sky airglow observations carried out over Taiwan. In contrast to the growth rates estimated using the model control run, the results after data assimilation show remarkable agreement with the ROTI. In addition, the all sky imager observations reveal intense bubble occurrence over Taiwan in the nights when the corresponding assimilated growth rates are significant. In the night of the St. Patrick’s day storm on 17 March 2015, no plasma bubbles were recorded in the all sky images over Taiwan, which is supported by the smaller growth rates predicted by the assimilation model. The results further reveal that the significant improvement in the calculated growth rates could be achieved by the accurate forecast of zonal electric field in the data assimilation forecast. The results suggest that realistic estimate or prediction of plasma bubble occurrence could be feasible by taking advantage of the data assimilation approach adopted in this work.

Theoretical study of the ionospheric plasma cave in the equatorial ionization anomaly region

This paper investigates the physical mechanism of an unusual equatorial electron density structure, plasma cave, located underneath the equatorial ionization anomaly by using theoretical simulations. The simulation results provide important new understanding of the dynamics of the equatorial ionosphere. It has been suggested previously that unusual E⇀×B⇀ drifts might be responsible for the observed plasma cave structure, but model simulations in this paper suggest that the more likely cause is latitudinal meridional neutral wind variations. The neutral winds are featured by two divergent wind regions at off-equator latitudes and a convergent wind region around the magnetic equator, resulting in plasma divergences and convergence, respectively, to form the plasma caves structure. The tidal-decomposition analysis further suggests that the cave related meridional neutral winds and the intensity of plasma cave are highly associated with the migrating terdiurnal tidal component of the neutral winds.

Equatorial plasma bubble generation / inhibition during 2015 St. Patrick's Day storm

All sky camera observations carried out over Taiwan showed intense equatorial plasma bubbles (EPB) in 630.0 nm airglow images on consecutive nights of 13-16 March, 2015, but was absent in the following night of 17 March when St. Patricks Day magnetic storm occurred. Rate of total electron content (TEC) index by using Global Positioning System (GPS) network data also confirmed the absence of irregularities on the night 17 March. The results however revealed strong irregularities over Indian sector on the same night. Flux tube integrated Rayleigh-Taylor instability growth rates computed using the prior (forecast) state of Thermosphere-Ionosphere Electrodynamics General Circulation Model output after assimilating the GPS-TEC measurements also agree with the observations, showing smaller values over Taiwan and larger values over India on the night of 17 March. The ionospheric response to the storm over Taiwan that resulted in the apparent inhibition of EPB is investigated in this study by using the data assimilation output. Results indicate that on the night of the magnetic storm, pre-reversal enhancement of zonal electric field over Taiwan was weaker when compared to that over India. Further analysis suggests that the absence of enhancement in the zonal electric field could be due to westward penetration electric field in response to rapid northward turning of interplanetary magnetic field that occurred during the dusk period over Taiwan.

Automatic selection of Dst storms and their seasonal variations in two versions of Dst in 50 years

A computer program is developed to automatically identify the geomagnetic storms in Dst index by applying four selection criteria that minimize non-storm-like fluctuations. The program is used to identify the storms in Kyoto Dst and USGS Dst in 50xa0years (1958–2007). The identified storms (DstMinxa0≤xa0−50xa0nT) are used to investigate their seasonal variations. It is found that the overall seasonal variations of the storm parameters such as occurrence, average intensity (average DstMin) and average strength (average ⟨DstMP⟩) in both versions of Dst exhibit clear semiannual variations with equinoctial maxima and solstice minima; and the maxima and minima in intensity and strength (~±17% each) are less than those in occurrence (~±28%). Wavelet spectra of the storms reveal the existence of distinct semiannual component in four solar cycles (SCs 20–23) and weak longer and shorter-period components in some SCs. The semiannual variation observed also in the mean energy input during the main phase (MP) of the storms estimated from Dst is interpreted in terms of the (1) equinoctial mechanism based on the varying angle between the Earth–Sun line and Earth’s dipole axis and (2) Russell–Mcpherron effect based on the varying angle between the GSM Z-axis and GSE Y-axis; and the yearly range of the dipole tilt angle µ (23.2°) involved in the equinoctial mechanism is found larger than the title angle θ (16.3°) involved in the RM effect.Graphical Abstract.