GEOPHYSICAL RESEARCH LETTERS, VOL. 17, NO. 3, PAGES 259-262, MARCH 1990 SIMULTANEOUS DISTURBANCE OF CONJUGATE IONOSPI-[ERIC REGIONS IN ASSOCIATION WITH IND1VIDU• LIGHTNING FLASHES W. C. Burgess andU.S. Inan STAR Laboratory, Stanford University, Stanford, California Abstract. Characteristic whistler-associated amplitude per- turbations of subionospheric VLF or LF signals ("Trimpi events") observed within one second of eachother at Palmer Station, Antarctica and at Axecibo, Puerto Rico suggest that ionospheric regions in both northern and southern hemispheres are disturbed together in association with individual lightning flashes.During a one hour period on March 21, 1989, the UPM onsets of 44 out of 47 perturbations measured on a 2!.4 kHz signal fromMaryland to Arecibo occurred within 1 s of per- mrbation onsets measured on a 23.4 kHz signal from Hawaii to Palmer Station. Similar activity occurred before and after thisperiod,and on the preceding and following days. The observations are consistent with the disturbance of geomag- netically conjugate ionospheric regions by multiplebounces between hemispheres of bursts of radiation beltelectrons, scat- tered in pitchangleby whistlers in the magnetosphere. Anal- ysis of patterns of perturbations with corresponding whistler uPu and lightning information from thisperiod suggests thatthere wereat leasttwo distinct ionospheric disturbances in each hemisphere. 1. VLF!LF Perturbations Duringthe morning of March 21, 1989, Arecibo,Puerto Rico, and Palmer Station, Antarctica recorded several hun- dred perturbations on signals from five VLF and LF commu- nicationtransmitters in the United States (Table 1). These perturbations were characteristic of Trimpi events[Iraan and Carpenter, 1986], having sudden (0.2 to 2 s) positive or neg- Ave onsets of up to 9 dB in amplitude followed by slow(10 to 100 s) recoveries to prior levels. Between 0900 and 1000 UT these events wereparticularly largeandfrequent on the NSS and48.5 kHz signals at Areciboand on the NPM sig- nalat Palmer Station.The lattersignal pathwasperturbed at least twiceas oftenas any other pathrecorded at either site, averaging 4 perturbations per minute. Partial maps of theten signal paths monitored areshown Ln Figure 1. TABLE 1. Transmitters observed at Arecibo an'd at Palmer. CallSign Transmitter Frequency LatitudeLongitude NSS USN Maryland 21.4 kHz 39øN 76øW NPM USN Hawaii 23.4 kHz 21øN 158øW NAA USN Maine 24.0 kHz 45øN 67øW NLK USN Washington 24.8 kHz 48øN 122øW USAF Nebraska 48.5 kHz 42øN 98øW Copyright 1990 by the American Geophysical Union. P•r number 9fX/[D0123. •-8276/90/90GL-00123503.00 NLK 48.5 NSS NAA NLK 48.5 NSS NAA PA Fig. 1. Great-circle paths of subionospheric signals from the five VLF/LF communication transmitters listed in Table 1 to Arecibo, Puerto Rico (AR) andPalmer Station, Antarctica (PA). Each cloud-to-ground lightning flash recorded by the SUNY-Albany detection network between 0940 and 0950 UT on March 21, 1989 is shown by a + in the upper panel to indicate general thunderstorm activity. Each often additional flashes time-associated withsignal perturbations discussed in the text and occurring between 0900 and 1000 UT ismarked with a ,. The geomagnetic conjugates of all upper panel flashes are similarly marked in thelower panel.Of theten specially marked flashes, the five labeled 'E' were associated with perturbations ofNSS but not 48.5 kHz atArecibo, while the fivelabeled 'W' were associated with perturbations of 48.5 kHz but notNSS at Arecibo. All ten flashes wereassociated with perturbations of NPM at Palmer. The footphnt at!00 km altitude of L = 2 is shown for reference. Forthe purposes of this study, we defix'• an"event" as a characteristic signal arnplitude perturbation with a magnitude not less than 0.2 dB, and treat perturbations •n on differ- ent signals at the same ordifferent sites as a single, simu!~ taneously observed event when the perturbation onsets occur within onesecond of each other. Between 0900 and 1• UT, 129events were observed simultaneously atboth Arecibo and Palmer. Such events accounted for 88%of hhe 147events ob- served at Arecibo butonly 51% of the252events obs.erv:ed 259