Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1043–1047www.elsevier.nl/locate/jastp

The impact of the August 27, 1998, �-ray burst on theSchumann resonances

Colin Pricea ;∗, VadimMushtakbaDepartment of Geophysics and Planetary Sciences, Tel Aviv University, Ramat Aviv 69978, Israel

bDepartment of Civil and Environmental Engineering, MIT, 212 Ulster Heights Road, Woodburne, NY 12788, USA

Received 21 March 2000; accepted 22 January 2001

Abstract

On 27 August 1998, at 10:22UT, an extremely intense gamma ray 6are passed through the solar system. The �-ray burstlasted only 5 min and resulted in the ionization of a large portion of the Earth’s night-side upper atmosphere to levels foundnormally only during the daytime. During this period we were continuously monitoring extremely low frequency (ELF)electromagnetic waves in the range 1–50 Hz, at a new monitoring site established in the Negev Desert, Israel. We haveanalysed our ELF data, including the Schumann resonances, during this period, and no noticeable changes in the ELF signalsare observed as a result of this event. This is very diCerent to observations in the VLF range that showed extreme changes inamplitude and phase resulting from this �-ray burst. Theoretical modeling points to three factors that can explain the diCerencesbetween the VLF and ELF signal responses: (i) diCerent properties of the ELF and VLF signals within the time scale of the�-ray burst; (ii) diCerent paths of these signals during the event; and (iii) the diCerent in6uences of the day=night asymmetryon the propagation of ELF and VLF waves. c© 2001 Elsevier Science Ltd. All rights reserved.

Keywords: ELF; Schumann resonance; �-ray burst

1. Introduction

The existence of natural extremely low frequency (ELF)electromagnetic waves in the atmosphere is primarily dueto global lightning activity (Ogawa et al., 1969; Heck-man et al., 1998). Due to the physical dimensions of theEarth-ionosphere cavity, lightning discharges produce res-onant phenomena within the ELF spectral range, knownas the Schumann resonances (SR) (Schumann, 1952). TheDrst mode of the SR occurs near 8 Hz, being equivalent toa standing wave of wavelength to 40; 000 km (the circum-ference of the Earth).

It has been suggested that the ELF radiation from globallightning activity may be a sensitive indicator of global

∗ Corresponding author. Tel.: +972-3-640-6029; fax: +972-3-640-9282.E-mail addresses: cprice@6ash.tau.ac.il (C. Price), vadimcm@

ieee.org (V. Mushtak).

climate change (Williams, 1992; Fullekrug and Fraser-Smith,1997; Price, 2000). Hence, it is important to know to whatdegree external in6uences on the Earth-ionosphere cavity(solar and comic events) in6uence the SR amplitudes andfrequencies. Sentmann et al. (1996) investigated the eCect oftwo large solar 6ares. They observed no perturbations in theSR intensities monitored at two widely separated stations.

On 27 August 1998, at 10:22UT an extremely intenseX-ray=�-ray 6are passed through the solar system. The 6areoriginated from a magnetar, located 23,000 light years away,known as Soft Gamma Repeater (SGR) 1900+14 (Hurleyet al., 1999). The giant 6are lasted for approximately 5 min,with the intensity varying with a periodicity of 5:16 s, theperiod of rotation of the magnetar. The intense burst of en-ergetic photons produced enhanced ionization at altitudes of30–90 km, resulting in the lower ionosphere of the Earth’snight side being transformed into a daytime ionosphere dur-ing this period. As a result of these large, abrupt changes inthe lower ionosphere, unusually large amplitude and phase

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1044 C. Price, V. Mushtak / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1043–1047

changes were observed in very low frequency (VLF) signalspropagating in the Earth-ionosphere waveguide (Inan et al.,1999; Dowden et al., 1999).

During this period data were being collected at a newSchumann resonance monitoring station recently establishedin the Negev Desert, Israel (Price et al., 1999). The stationis located near the town of Mitzpe Ramon, at the astronom-ical observatory of Tel Aviv University (30◦N; 34◦E). Thesite has very low anthropogenic ELF noise levels, allowingus to record the background SR signals (with amplitudesof pica-Tesla) every day of the year. Since February 1998,we have been continuously recording the horizontal mag-netic components (north–south and east–west) in the spec-tral range of 1–50 Hz. The sampling frequency is 250 Hz.Since January 1999, the vertical electric Deld has also beencollected in this frequency range. All the data are savedas raw time-series, with all spectral analysis done duringpost-processing.

2. ELF observations

The ELF time-series during the period of the �-ray burstis shown in Fig. 1, for a 30-s period at the time of the

Fig. 1. Time series of the horizontal magnetic Delds in the ELFrange during a 30-s period around the �-ray burst (10:22:15UT).

Fig. 2. The ELF spectra during the 15-min period: (a) before and(b) after the event. The bold line is a smoothed curve through thespectra.

�-ray burst. In the upper panel the north–south magneticDeld is presented, while the bottom panel shows the east–west magnetic Deld component (1–50 Hz). The amplitudesof the signals are given here in volts.

Although there is a slight change in the ELF signal pre-cisely at the time of the onset of the �-ray burst (10:22:15UT), such variability in the ELF Delds is noticed also attimes when no �-ray burst occurs. Furthermore, duringthe 5-min period of the burst no systematic change in theELF time series is observed. To further our investigation,we looked at the variability of the spectrum of the aboveELF signals. The spectrum of the ELF time series clearlyshows the well-known Schumann resonances, produced bythe continuous natural lightning activity around the planet(Fig. 2). Since the location of the ELF sources (lightning)vary throughout the diurnal cycle, the spectrum varies ac-cordingly. In Fig. 2a the spectrum of the ELF data duringthe 15-min period prior the �-ray burst is presented, whileFig. 2b shows the spectrum for the 15-min period followingthe �-ray burst.It should be pointed out that the spectra are shown only

for the north–south component of the magnetic Delds, sinceexactly this component receives signals from remote Asian

C. Price, V. Mushtak / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1043–1047 1045

Fig. 3. Variability of the amplitude of the Drst two modes (8 and14 Hz) during a 30-min period around the �-ray burst (shown asvertical line).

Fig. 4. Variability of the frequency of the Drst two modes (8 and14 Hz) during a 30-min period around the �-ray burst (shown asvertical line).

thunderstorm regions at 10:00 UT and, therefore, is sup-posed to be the most sensitive to global changes of the iono-sphere’s properties. The Drst three Schumann modes (near 8,14 and 20 Hz) are clearly visible in the spectra. There is nosigniDcant change in the spectra caused by the �-ray burst.

If we follow the amplitude of the Drst two modes duringthe 30-min period from 1000 to 1030 UT, no change in theamplitude is noticed at the time of the �-ray burst (Fig. 3).Although the variability of the amplitude is approximately50% of the mean on short time scales, no change in ampli-tude is seen at the time of the cosmic event (vertical line inDgure).

Since the ionization of the lower ionosphere in6uences thewaveguide properties, we also checked whether there wasa shift in the peak frequencies of the Schumann resonancesduring this period (Fig. 4). Here too no observable shift is

noticed in the peak frequencies of the Drst two modes. Thenormal variability of the peak frequency is about 0:5 Hz forthe Drst mode, and approximately 1 Hz for the second modebefore, during and after the burst.

3. Theoretical calculations

In order to understand these ‘negative’ observations atthe time of the �-ray burst, a series of theoretical sim-ulations have been computed on the basis of a propaga-tion model taking into account the day=night asymmetry ofthe Earth-ionosphere waveguide. The approach makes useof the two-dimensional telegraph equation (TDTE) methoddeveloped by Kirillov et al. (1997), from a suggestion ofMadden and Thompson (1965) based on analogies betweenprocesses in a two-dimensional transmission line and prop-agation of ELF waves within the waveguide. ELF propa-gation parameters inherent to this approach (two complexcharacteristic altitudes) have been estimated from represen-tative daytime and nighttime aeronomical proDles by meansof a technique similar to that proposed by GreiDnger andGreiDnger (1978). However, the TDTE method is suitablefor considering much more detailed asymmetrical modelsof the waveguide. For the purposes of this paper the modelhas been adapted to provide a fast-running computationalalgorithm for the simplest day=night model of the waveg-uide, distinguishing between daytime and nighttime hemi-spheres, but neglecting angular dependencies of propagationparameters within each hemisphere. Sources of natural ELFDelds have been simulated on the basis of geophysical, me-teorological, and satellite data concerning spatial-temporaldynamics of global lightning activity over major thunder-storm regions (Africa, America, and SE Asia). For each re-gion, the activity has been modeled as a combination of a‘background’ component spatially distributed over the entireterritory of the region and a relatively concentrated thun-derstorm ‘kernel’ migrating westward across the territoryduring the most active period (local afternoon). To bothcomponents, a quasi-Gaussian local time dependence hasbeen ascribed. The sizes of the kernels (two to three mega-meters) has been estimated, in particular, from the resultsof the monitoring of VLF sferics produced by the American(Harth et al., 1982) and African (personal communication,A. Shvets) lightning regions. The applicability of the prop-agation approach and the model of sources have previouslybeen tested by comparison of theoretically computed andexperimentally observed variations of peak frequencies ofSchumann resonances (Mushtak et al., 1999).

Due to the timing of the �-ray burst, the most active light-ning sources in the model were placed in Southeast Asia,implying that the major source of the ELF radiation was,like the observing station, on the day-side of the Earth. Themaximum ELF signals at this time of day in Israel are ob-served in the north–south component of the magnetic Deld.Therefore, the spectrum was calculated for the north–south

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Fig. 5. Simulated spectrum of the north–south magnetic componentobserved in the Negev Desert at 10 UT on 28 August 1998.

component of the magnetic Deld, and compared with theobserved spectra above, also presented for the north–southcomponent only. The theoretical spectra were calculated un-der three diCerent scenarios (Fig. 5). First, under normalconditions with the nighttime ionosphere diCerent to the day-time ionosphere, the “day=night model” spectrum shows theDrst three modes at this time of day, as would have beenobserved from the Mitzpe Ramon site. Similar to the ob-served spectra in Fig. 2, the second mode is not very strong,while often weaker than the third mode at this time of daywith the given source–receiver positions. By changing thenighttime ionosphere into a daytime ionosphere, due to the�-ray burst (Inan et al., 1999), the “day model” curve inFig. 5 shows the expected change in the spectrum. Accord-ing to the model, even within the most active part of the�-ray burst, the amplitude variations observed at the Negevstations should not exceed 25%, while the peak frequenciesshould not vary by more than 1 Hz. Since these values liewithin the natural variability shown in Figs. 3 and 4, it isunderstandable that no noticeable changes in amplitude andfrequency were seen during this event. The “tail” curve inFig. 5 represents the decaying portion of the �-ray burst,or an intermediate situation between the normal day=nightsituation and the maximum of the burst.

If the �-ray burst would have occurred at a diCerent timeof day, when both the most active lightning sources and theobserving station were on the night-side of the Earth, thenwe may have been able to see a change in SR amplitudedue to this event (Fig. 6). At 00:00 UT global lightningactivity is located primarily over the Americas, while thelocal time at the Negev station is 2 a:m. As shown in Fig. 6,the amplitude of the SR spectrumwould have increased quitedramatically by approximately 100%, due to the enhancedionization on the nighttime ionosphere. Although changesin the amplitude would have been seen under the aboveconditions, little changes would have been observed in thefrequencies of the SR modes.

Fig. 6. Simulated spectrum that would have been observed in thenorth–south magnetic component in the Negev Desert if the burstwould have occurred at 00:00 UT.

4. Discussion and conclusions

Although the VLF measurements by Inan et al. (1999)and Dowden et al. (1999) showed signiDcant anomalies asa result of the �-ray burst, no noticeable anomalies weredetected in the ELF signals recorded at the Negev Desertsite. There are three reasons for this diCerence.

1. Properties of signals: ELF (lightning) sources ex-citing SR are natural, random in time, and transmitrelatively broad-band signals arriving at the receiverfrom various directions and distances. Due to the sta-tistical nature of background ELF signals, reasonablystable characteristics of Schumann resonances can beobtained within a time scale of several minutes. Sincethe total duration of the �-ray burst was 5 min, and themost pronounced portion of it (“night turning into day”)lasting about 1 min, the conditions for reliably detectingthis event from ELF observations were not available.On the contrary, the VLF signals observed by Inan et al.(1999) and Dowden et al. (1999) were man-made, rela-tively narrow-band, transmitted from a single geographiclocation. Time required for estimating the amplitude andphase characteristics of the VLF signals takes a fractionof a second, therefore providing enough time not only forthe VLF detection of the event, but also for monitoringits dynamics in detail (the 5:16 s periodicity.)

2. Path of the signals: The VLF anomalies resulting fromthe burst were observed when the VLF waves traveledover the nighttime side of the Earth-ionosphere wave-guide, with both transmitter (Hawaii) and observer(Colorado) under the aCected portion of the ionosphere.The most active ELF (lightning) sources and the receiv-ing station were both in the daytime hemisphere, hencenot experiencing the maximum eCect of the changingproperties of the wave guide during the burst.

3. Day=night asymmetry factor: For VLF waves, thedaytime values of the attenuation rate diCer from the

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nighttime values by up to 6 dB=Mm, while within theSR frequency range (5–30 Hz) the diCerence is −0:05to −0:2 dB=Mm (Galejs, 1972). Even though ELFpropagation has a global character, with the Schumannresonances resulting from the superposition of directand around-the-globe waves, the diCerence due to theburst was not suPcient to produce a noticeable changein the SR parameters. In the hypothetical situation withboth the ELF sources and the observer located in thenighttime hemisphere, the simulated increase of the SRamplitudes (Fig. 6) results mainly from the lower val-ues of the characteristic altitudes over the sources andobserver (Kirillov et al., 1997), and not from the lowerattenuation rates as a result of the “daytime” valuesduring the �-ray burst.

In conclusion, we have shown that there is a reasonableexplanation as to the absence of any noticeable anomaliesin the ELF electromagnetic observations during the periodof the intense �-ray burst that occurred at 10:22 UT on 27August 1998. Although this result may appear to be a neg-ative result to some, it is actually a positive result for thoseresearchers using the Schumann resonance signals to studythe Earth’s climate, and climate change. This study showsthat the SR signals are quite immune to such external forc-ings, and therefore these extraterrestrial events can beignored when looking at the long-term variability of globallightning activity via the Schumann resonances. The re-sults presented here also agree with the previous study bySentmann et al. (1996) where solar 6ares were shown tohave a little in6uence on the Schumann resonances.

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