COMPARISON OF VOLCANO ERUPTIONS IN KAMCHATKA WITH COORDINATES OF ATMOSPHERICS

Transcription

1 COMPARISON OF VOLCANO ERUPTIONS IN KAMCHATKA WITH COORDINATES OF ATMOSPHERICS N.V. Cherneva 1, R.H. Holzworth 2, A.V. Ivanov 1, G.I. Druzhin 1, A.N. Mel'nikov 1 1 Institute of Cosmophysical Research and Radio Wave Propagation (IKIR) FEB RAS, Paratunka, Kamchatskiy krai, , Russia, nina@ikir.ru; 2 University of Washington, Seattle, USA Abstract. The VLF direction finder, developed and constructed in IKIR FEB RAS, carries out continuous registration of atmospherics. This complex meets the requirements of World Wide Lightning Location Network (WWLLN) for receiving stations which allows the Institute to participate in different international projects and investigations on lightning discharges and their relation with other phenomena. One of the directions of such researches is identification of lightning discharges, associated with volcano ash eruptions. The importance of such observations is high as long as determination of ash clouds is important for aviation and may serve as a monitor to identify the beginning of an explosive eruption. On the basis of the present WWLLN: Global Volcanic Lightning Monitor - GVLM ( identification of an ash cloud, caused by an explosive eruption, is carried out. GVLM volcanic data renewed every minute in the case of coincidence with lightning geographic coordinates form an alarm signal. Comparison of direction finding and meteorological data has shown that lightning azimuth distribution, obtained by VLF direction finder of IKIR FEB RAS, quite well coincides with azimuthal distribution calculated on the basis of the worldwide WWLLN station network. Retrospective analysis of VLF direction finder data archive of IKIR FEB RAS over a year and a half period allowed us to determine some cases of lightning locations generated at the moments of Shiveluch, Kizimen and Besymyanniy volcano eruptions. Lightning registration technique Natural electromagnetic field in VLF range is characterized by variety of forms of received radiation. Electromagnetic signals, atmospherics, which are the main source of electromagnetic radiation in the very low frequency range (VLF radiation) from lightning discharges, may propagate at great distances, hundreds and thousands of kilometers (Alpert, 1972). Lightning generation, besides lightning areas, may occur in snow and sand storms as well as in the clouds above erupting volcanoes (Lichter, Osinin, 1978). Directionfinding methods of registration are the most effective ones for spaced sources. Natural electromagnetic field in VLF range is characterized by a variety of forms of received radiation. For spaced sources the most effective are direction-finding methods of registration. Continuous registration of electromagnetic radiation by VLF direction-finder is carried out at the Institute of Cosmophysical Research and Radio Wave Propagation (IKIR) FEB RAS, at Paratunka site. The registration is carried out in the frequency range from 3 to 60 khz. The direction-finder was developed and constructed at IKIR FEB RAS. Signals from thunderstorm sources are received by antenna system of the direction-finder. It consists of two mutually-perpendicular loop antennas and a pin antenna. The pin antenna is used to receive electric component of the electromagnetic field and it consists of a 30-meter mast on the top of which a capacitive attachment is installed. Appearance and allocation of VLF direction finder pin antenna is shown in Fig. 1. Voltage from antenna system output arrive at pre-amplifiers just near the antenna mount, than it is sent to the unit of analogous and digital signal processing via communication cable line. After the multiplication, frequency filtering and digitizing the signals are recorded on a PC. Recording of certain realizations is carried out when a certain threshold voltage of a signal is exceeded. In the result of processing 2 daily files are formed. The first one has signal realization data (current voltage values of electric and magnetic field components), the second one contains the data on certain parameters of realization (data, time, average values of field components, length of realization, azimuthal angles). Furthermore the files are processed by specially developed software which makes it possible to view remote data images. The World Wide Lightning Location Network (WWLLN) includes about seventy receiving stations which over the whole globe allocate lightnings at the distance from several meters up to 10 thousand kilometers from each other. The receiving stations generate radiation arriving via the Earth-ionosphere waveguide with the maximum at the frequency of 10 khz (wave length is 30 km). Five stations, surrounding a thunderstorm, are enough to allocate it. As long as world thunderstorm location depends on the exact time of 71

2 atmospheric arrival to the station and exact station location two GPS signals are necessary. One of them sends a PC the world time (UT): date, month, hour, second; the second one gives the information on station location, its latitude and longitude in degrees and minutes. The former (TSIP) is connected to a PC via COM port. The latter (GPS signal) 1 pulse per second (PPS) is connected to the computer via a sound card (stereo signal and VLF broad band, containing atmospherics). Each of these stations sends the exact (up to several microseconds) time of arrival of thunderstorm discharge impulse (atmospheric) to the central processing PC and in ten minutes these spectrograms from the stations are renewed. Thunderstorm activity registered over the whole world is summarized over the latest minutes Fig.1 Appearance and allocation of VLF direction finder electric pin antenna In the whole WWLLN network is used by the researchers of receiving stations (mainly universities and scientific-research institutes) which join their data to determine world lightning locations (by the time of atmospheric arrival). According to the data received by VLF direction finder, azimuthal distribution of lightning discharges during lightning passing over an observation point is plotted. Visual comparison of direction finding data with WWLLN data allowed us to determine the identity of lightning discharge distribution plots for both data sets. This similarity shows that the results obtained by the direction finder are quite objective and may be used for further joint investigations. Discussion of the results In July, 2009 a strong thunderstorm front passed over Kamchatka peninsula. At Paratunka station the thunderstorm was observed visually and it was accompanied by numerous lightnings and thunder. Fig.2a illustrates the observation results obtained by VLF direction-finder on July 9-13, Azimuth of signal arrival was calculated from the northern direction clockwise. We compared the data on lightning distribution, obtained at Paratunka site, with WWLLN data. Fig. 2b illustrates azimuthal distribution of lightning discharges obtained on the basis of international network data. To calculate atmospheric azimuths from Paratunka we chose the data on lightning discharges within the range of of the northern latitude and of the eastern longitude. In the result we see that the azimuthal lightning distribution obtained from VLF direction-finder coincides quite well with the azimuthal distribution calculated on the basis of WWLLN data. The pattern of the dependence of lightning discharge number on time also coincides. But the number of discharges received by the direction-finder about one order greater than that registered by WWLLN stations. Most likely, it may be explained by the fact that WWLLN stations are located at a big distance from Paratunka site and they do not receive all the discharges occurring near Kamchatka peninsula. 72

3 Fig. 2. Azimuthal distribution of lightning discharges at Paratunka, plotted: a - according to the data of direction-finder observations at Paratunka site; b according to the WWLLN stations. Dots indicate lightning discharges. In the bottom under the azimuthal distribution is the dependence of the received atmospheric number (imp\h) on day time. Observations of electromagnetic field in such a seismically hazardous region as Kamchatka showed that in many cases anomalous field behavior is registered before earthquakes. Application of directionfinding methods gives us the opportunity to detect the source more accurately applying anomalous behavior of radio radiation amplitude (Druzhin, Cherneva, 2005). Scientists of Washington University carried out a scientific research to determine explosive volcanic eruptions in real time ( On October 27, 2010 a signal from WWLLN volcanic system was received for the first time. It registered the beginning of Sheveluch volcano eruption one hour before ash cloud image was received (Hutchins, Holzworth, Rodger and Brundell, 2012). The aim of that study was reliable identification of lightnings associated with ash clouds and caused by active volcano eruptions. Data of ash cloud monitoring of all the volcanoes of the world, updated every minute, compared with the data of lightning coordinates and if they coincide with volcano coordinates an alarm signal is formed. Fig. 3 shows lightning flashes presented both in IKIR and WWLLN data during Shiveluch volcano eruption on October 27, 2010, marked by red according to the color scale of volcano state at the site of Kamchatka Branch of Geophysical Service of RAS and accompanied by about 360 km ash trail in the South-Eastern direction. 73

4 Fig.3. Azimuthal distribution of lightning discharges 27/10/2010: top plot according to IKIR data, bottom plot according to WWLLN data During the analysis of VLF direction finder data archive of IKIR FEB RAS and their comparison with WWLLN data, identification of registered lightning from ash clouds, caused by explosive eruptions of Shiveluch, Kizimen and Besymyanniy volcanoes was carried out. During Kizimen volcano eruption on December 31, 2010, January 6, 2011 and January 12, 2011 accompanied by ash trails of different length 10, 4 and 5 lightning discharges were registered, correspondingly. According to the data of VLF direction-finder azimuthal distribution of lightning discharges at the moment of eruption coincides with azimuthal volcano location in the considered cases. Fig. 4 illustrates two registered cases of lightning from ash clouds caused by explosive eruption of Kizimen (December 31, 2010) and Besymyanniy (April 13, 2011) volcanoes. According to the data of Kamchatka Branch of Geophysical Service of RAS (KF GS RAS) seismic events in volcano structure were observed and 3 earthquakes occurred. At 17:56 a series of surface seismic events occurred which lasted for ~ 20 min and was accompanied by ash outburst and pyroclastic avalanching. According to the data of Alaska Volcano Observatory (AVO) an ash trail was registered at the distance of ~ 43 km to the South-West. At the same time, according to the data of VLF direction finder, ten lightning discharges were registered from 10:01:53 till 18:22 which coordinates coincided with Kizimen volcano location marked by a green line in Fig. 4. During Besymyanniy volcano eruption, marked by a blue line in Fig. 4, 118 weak seismic events in the volcano structure were registered. From 20:20 to 21:00 a seismic event accompanied explosive eruption. At 22:15 there was an ashfall in the region of Krasniy Yar (~45 km to the N-N/W from the volcano. Ash layer was 1 km. From AVO report: according to the data of World Wide Lightning Location Network, numerous flares (lightning) were registered near the volcano. 74

5 Fig.4. Flashes associated with ash eruption of Kizimen (December 31, 2010) and Besymyanniy (April 13, 2011) volcanoes top plot discharge distribution according to IKIR data; bottom WWLLN data. Nonlinear time scale. According to VLF direction finder data azimuthal distribution of lightning discharges coincided with azimuthal location of these volcanoes on the dates of eruptions mentioned above on seismic data. To avoid false signals of volcano explosive eruptions, interpretation of data by statistical methods was carried out. As an example of application of statistical methods two explosive eruptions of Kamchatka volcanoes, Shiveluch ( ) and Besymyanniy ( ), were considered. First of all, data containing information on total number of lightning over the given period were taken from WWLLN site; than a square of Kamchatka peninsula location with definite location and the number of lightning over this territory was selected by SciLab program. As long as we need the data on lightning generated by ashfall of the definite Kamchatka volcanoes (Shiveluch and Besymyanniy), we selected the squares of these volcano locations with 10 km radius. Calculation of azimuth was realized via azimuth calculation on the site ( Shiveluch ' 56036' = ' ' 56038' = ' Bezymyanniy ' 55058' = ' ' 55058' = ' The considered alarm criteria (Kremer, 2004): where n - lightning number (atmospherics), m - eruption number., 75

6 To check the null hypothesis on event occurrence probability equality in two parent populations (having binominal distributions) for the given significance level, observed criteria number was estimated U and according to Laplace function table the critical point Ucr was found by the equality Ф(Ucr) = (1 α)/2. In our case it is 0,4505. If Uobse < Ucr - there is no ground to reject the null hypothesis. If Uobse > Ucr - null hypothesis is rejected. Uobse = 0,4, consequently we do not have ground to reject the null hypothesis. Thus, we may say that the method of detection of explosive eruption beginning by the alarm during registration of atmospherics (lightning), which coordinates or azimuth coincide with volcano location, may be used as an additional method for seismic activity monitoring. Conclusions: 1. Comparison of direction-finding and meteorological data has shown that lightning azimuthal distribution obtained by VLF direction finder of IKIR FEB RAS well agree with azimuthal distribution calculated on the basis of data of WWLLN. 2. On the basis of the present WWLLN network: Global Volcanic Lightning Monitor, reliable identification of ash cloud lightning caused by active volcano eruption is possible. 3. A similar analysis of data archive over a half a year period was carried out applying VLF direction finder and 24 cases were determined allocating lightning generated at the moment of volcano eruption. 4. The system may used to notify the preparation and beginning of volcano eruptions in Kamchatka. References Alpert Y.L. Distribution of electromagnetic waves in the ionosphere. Moscow: Nauka, p. Druzhin G.I., Cherneva N.V. Direction-finding of the sources associated with Kamchatka cyclones, in: Distribution of radio waves: report book of the XXI Russian Scientific Conference. Yoshkar-Ola, V.1. P Kremer N.Sh. Probability theory and mathematical statistics. M.: UNITY, P Lihter Y.I., Osinin V.F. Snowstorm radio noise characteristics, in: Distribution of decametric waves. Moscow: Nauka, P Hutchins, M.L., R. H. Holzworth, C. J. Rodger and J. B. Brundell, Far field power of lightning strokes as measured by the World Wide Lightning Location Network, JTech (J. Atmos. and Ocean. Tech. (AMS), V.29, ,