Bipolar Events Research Articles

Narrow Bipolar Events as indicators of thunderstorm convective strength

Narrow Bipolar Events (NBEs) are impulsive in‐cloud lightning discharges that are commonly detected by both ground‐based and satellite‐based radio‐frequency sensors. In this paper, NBE flash rates are shown to be statistically correlated to cloud‐to‐ground (CG) lightning flash rates as recorded by an array of electric‐field‐change sensors located in Florida. In addition, NBE source heights are found to generally increase with increasing NBE flash rates. The inference is that NBE flash rates and source heights are generally driven by the intensity/strength of the convective updraft in thunderstorms. As a consequence, NBEs represent a meteorologically important type of lightning and provide an excellent target of opportunity for future satellite‐based very high frequency (VHF) global lightning monitors.

Numerical Study on the Earthward Propagating Plasmoids

AbstractThe correlation between plasmoids and geomagnetic substorms has been demonstrated by satellite observations. In addition to the tailward propagating plasmoids with N‐S bipolar signatures, earthward propagating plasmoids are found, which are characterized by plasma sheet S‐N bipolar events and lobe S‐N bipolar signatures. The occurrence rate of the S‐N bipolar events is much lower than that of the N‐S events. The S‐N bipolar events occur primarily during quiet geomagnetic and IMF Bz north period, and they are correlated with small, isolated geomagnetic substorms. A 2.5 dimensional MHD simulation on the earthward propagating plasmoids has been carried out, on base of the specific distribution of the cross‐tail electric fields Ey during geomagnetically quiet times (IMF Bz is northward and | By |≥ Bz). The simulation results of the two cases showed the major features of the flux rope magnetic structures and plasmoids with complex closed magnetic field lines. The similar signatures to tailward propagating plasmoids could be consistent with IMP 8 observations about the earthward propagating plasma sheet S‐N bipolar events. The results displayed a similar configuration to the schema given by Schindler which may explain the low occurrence frequency of the S‐N events, and also showed that the dynamic growth of the magnetic reconnection becomes considerably suppressed for a strong cross‐tail magnetic field By. From this simulation it could be concluded that the magnetic reconnection can be the genetic mechanism of the plasma acceleration and heating in the magnetotail in both geomagnetically active and quiet time, namely, under both southward and northward (with | By |≥ Bz) IMF conditions.

Observations of VHF source powers radiated by lightning

Three‐dimensional lightning mapping observations have been used to estimate the peak source powers radiated by individual VHF events of lightning discharges. The peak powers vary from minimum locatable values of about 1 W typically up to 10–30 kW or more in the 60–66 MHz passband of the receivers. An energetic positive bipolar event radiated in excess of 300 k W peak power. The strongest radiation sources tended to be observed in the upper part of storms, corresponding to the upper positive charge region, where the breakdown is of negative polarity. The results illustrate the bidirectional nature of intracloud discharges, with the largest source powers being along the negative portion of the discharge and an order of magnitude greater than the source powers along the positive portion. Overall, the source powers follow an approximate P−1 distribution for powers above about 100 W. The radiation sources indicate the location of the main charge regions in a storm; sample comparisons with radar data show that the main negative charge coincided with the precipitation core

Initial GEOTAIL survey of magnetic substorm signatures in the magnetotail

Substorm signatures in the magnetotail at radial distances from −30 to −210 Re are studied using magnetic field data acquired with the GEOTAIL spacecraft. The current data set consists of GEOTAIL magnetic field data transmitted in real time to Japan from October 1992 to September 1993, and the corresponding GMS‐4 (140°E geographic, geostationary) energetic particle data and 1‐s Kakioka (L=1.2, 140°E) magnetic field record. We first identify substorms from major increases in the particle flux at the geostationary satellite, then determine accurate onset times from Pi 2 pulsations at the Kakioka ground station, and finally examine the corresponding magnetic field signatures at GEOTAIL. Eighty‐nine substorm events occurred when GEOTAIL was either in the plasma sheet or in the tail lobe, and these events were analyzed with a special emphasis on the temporal development of the north‐south magnetic field component Bz. A clear bipolar signature (northward perturbation followed by southward) is observed in Bz for 53 events. The bipolar events have characteristics consistent with the plasmoid or the traveling compression region reported in earlier studies. In the middle and distant tail (X<−60 Re), most of events seem to indicate the passage of a plasmoid. The present results provide evidence for near‐Earth reconnection and the formation and release of plasmoids in association with substorm onsets. A simple model yields the following plasmoid parameters: near‐Earth reconnection around X=−30 Re, the center of the newly‐created plasmoid around X=−60 Re, and the expansion of the tailward‐moving plasmoid to a dimension of 100 Re in the X direction and almost fully across the distant tail (X=−200 Re).

Multipoint observations of a dayside transient event

Many different processes may take place at the magnetopause. To distinguish the various mechanisms requires simultaneous multipoint observations in the dayside magnetosphere. Such coincident observations were made by a number of spacecraft located in the solar wind, at the magnetopause, at geosynchronous orbit, and at polar orbit on September 23, 1986. Beginning at 1110 UT, the Bz component of the IMF underwent three periodic cycles, each with a duration of about 7 min, and there were 4 solar wind pressure pulses with a period of 10 min, as recorded by the IMP 8 satellite. The ISEE satellite pair crossed the magnetopause near local noon at 1125 UT and recorded two bipolar events in the normal component of the magnetic field at 1115 and 1120 UT. In addition, ISEE 1 observed a bipolar event at 1118 UT that ISEE 2 did not. Periodic oscillations and compressions of the magnetic field were then subsequently seen at geosynchronous orbit in the dawn sector. Greenland magnetometer stations also located in the morning sector recorded transient responses to the solar wind pressure pulses. The East Coast stations (near local noon) apparently observed a different vortex at 1122 UT. Viking auroral images show an activation of a localized feature poleward of the dayside auroral distribution at 1122 UT. An auroral enhancement, seen at 1128 UT in the afternoon sector, moved eastward at a speed of 5 km s−1 and decayed in intensity by 1141 UT. Clear Pc 5 pulsations were seen equatorward of this activation. Differences in both in situ measurements and ionospheric responses to the two bipolar events lead to the conclusion that the 1118 UT event can be interpreted as being associated with reconnection at the same time that the magnetopause was in motion.

Cainozoic oceanic and climatic events: The Indo-Pacific foraminiferal biostratigraphic record

“Datums” in the biostratigraphic record are based on phylogeny, meaning speciation and extinction, and sometimes on coiling changes. We can also employ various nonphylogenetic configurationssuch as disjunct distributions, extratropical excursions and correlations with isotopic spikes. Progress in Cainozoic geohistory depends on rigorous, refined correlations between all environments on the earth. In practice, that means an opportunistic groping “outward” from the most secure and refined realm of our chronology (tropical oceanic succession based on planktonic skeletonized protists) towards the more difficult realms such as larger benthonic foraminifera in neritic sediments, extra-tropical oceanic and neritic successions, continental faunas, floras and palynofloral successions, and, finally and most elusively, continental geomorphic successions. Formal zones are necessary, but their proliferation is not. A Late Eocene example of coincidence in time between planktonic foraminiferal, marine dinoflagellate and terrestrial pollen zones suggests that heterogeneous Oppelzones may be the soundest basis for local and regional stratigraphic systems. The overall Cainozoic climatic deterioration was reversed substantially in the Eocene and in the Miocene by shortlived, far-reaching, bipolar events, signalled by extratropical excursions by tropical-type foraminifera, in some cases correlated solidly with isotopic spikes. Widespread oceanic hiatuses appear to correlate with episodes of global warming and transgression better than with episodes of global cooling, marginal hiatus and of intensified oceanic circulation. Australia's northward “drift” since the Middle Eocene notwithstanding, wholesale diachronism no longer is a useful paradigm; we are better off thinking in terms of virtually isochronous shifts in climate, watermass, and in planktonic and benthonic assemblages.