Analysis of Ionospheric Disturbance Associated With Earth Quakes In Papua New Guinea

25 An analysis of the perturbations in the electron content up to the ionospheric F 2 layer peak and 26 F2 layer peak height (h m F2) variations during earthquake time has been done using ionosonde 27 data observed in the equatorial station Vanimo, Papua New Guinea. Two earth quakes occurred, 28 one of magnitude 7.1 in Sissano in 1998 and the other of magnitude 6.7 in Aitape in 2002 in the 29 western province of Papua New Guinea, have been studied. A decrease in electron content was 30 observed in both the cases a few days prior to the earthquakes. An increase in height of h m F2 31 during night time was also observed during this period. This can be explained in terms of the 32 lithosphere- atmosphere-ionosphere coupling prior to earthquake period.


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Prediction of occurrence of strong earth quakes is a great concern nowadays and a lot of 36 studies using different physical parameters are going on in this area. Seismo-ionospheric study is 37 one among them [Pulinets, 2004;Liu et al., 2011]. Ionosphere is a region of the atmosphere 38 which contains weakly ionized plasma. Changes in the ionosphere are often disturbed by natural 39 phenomena such as volcanic eruptions [Heki, 2006], earth quakes [Liu et al., 2009], solar flares 40 [Krecht and Davis, 1961] and so on. Underground nuclear explosions affect the electron density 41 of the ionosphere [Park et al., 2011]. Total Electron Content (TEC) of the ionosphere is one of 42 the parameters used to study and monitor the ionosphere. TEC is sensitive to strong earth quakes. 43 TEC disturbances increase with earth quake magnitude but decrease with distance from the 44 epicenter [Liu et al., 2006;Zolotov et al, 2012]. 45 In this paper, we have studied the ionospheric disturbances that occurred prior to earth quake in 46 Sissano in 1998 and in Aitape in 2002 in Sandaun province of Papua New Guinea. We have used 47 the critical frequency corresponding to the maximum electron content of F2 layer (f0F2) and the 48 F layer heights (hmF2) of the ionospheric data measured in Vanimo station in Sandaun province 49 of Papua New Guinea. Vanimo station is situated very close to these places. We have also 50 analyzed the geomagnetic activity during this period.  Table 1 60 summarizes the date of occurrence, epicentre, depth and magnitude of the earth quakes 61 considered. The epicentre of the 1998 earth quake observed on 17 th July in Sissano is at a 62 distance of 91 km and that of the 2002 earth quake observed on 10 th January is at a distance of 63 129 km from Vanimo equatorial station.

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F2 region is the most ionized region in the ionosphere. The minimum frequency below 65 which a radio wave is reflected by the ionospheric layer is called the critical frequency. The peak 66 electron density associated with each layer of the ionosphere is associated with a critical 67 frequency. Ionospheric observations can be made using ionosondes (Cooper, et al., 2018). Total  f0F2 is given by (Spalla and Ciraola, 1994). The TEC is related to 75 NmF2 as where  is called the slab thickness which provides an estimation of 76 the width of the vertical electron density profile. (Davies, 1990).

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Ionosonde gives an estimation of vertical TEC up to the peak electron density NmF2. TEC is 78 highly correlated with f0F2 and the correlation coefficient can reach a value of 0.9 (Houminer 79 and Soicher, 1996). The ionospheric region above the NmF2 region cannot be measured by 80 ionosonde. the Aitape earth quake. In short, the decreases in (foF2) 2 observed prior to both the earth quakes 105 seem to be related to the quakes.

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A detailed study is conducted using hourly values of (f0F2) 2 variations during the two 107 periods. The mean value and standard deviation of (f0F2) 2 are calculated in every hour for 31 108 days and are calculated for 24 hours. A graph is plotted with hourly variation of (f0F2) 2 109 along with and for 31 days around the earth quake day. If the (f0F2) 2 variation 110 is outside the range of , it is considered as a fluctuation. Fig. 3a represents the variation 111 of (f0F2) 2 during Sissano earth quake and Fig.3b shows that during Aitape earth quake. During

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. An unusual decrease of (f0F2) 2 is observed on 14 th , 16 th and 18 th of July and their values 114 decrease below 80X10 12 . During Aitape earthquake (Fig.3b) where TECobs. is the daily average of observed value of TEC and TECmean is the mean value of 120 TEC for one month. TEC anomaly is calculated during both the earthquake periods and plotted 121 in Fig. 4. Fig. 4a represents the daily TEC anomaly for 21 days; ten days prior to and ten days 122 after the Sissano earthquake. A negative TEC anomaly of 34% on 4 th , 28% on 16 th and 30% on 123 18 th of July 1998 were observed associated with Sissano earthquake. During Aitape earthquake 124 (Fig. 4b)

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A decrease in daily average (f0F2) 2 is observed prior to the earthquakes (Fig. 2) decrease in electron density is observed three days prior to the Sissano earth quake day (Fig. 3a).

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A decrease in peak electron density is observed on 3 rd , 5 th , 7 th and 9 th of January 2002 prior to the 144 10 th January earth quake in Aitape (Fig. 3b). An increase in electron density is observed on 7 th , 145 11 th , 12 th , 22 nd , 23 rd and 25 th July during Sissano earth quake and on 10 th and 12 th January in 146 Aitape earth quake.

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The height of the peak electron density region reaches a maximum at around 2-3 hours     a) Variation of Dst index (Top) and daily average (f0F2)2 (Bottom) fteen days prior to and fteen days after the Sissano earth quake and b) that after the Aitape earth quake. Vertical dashed lines indicate the earth quake times.

Figure 3
Hourly variation of (f0F2)2 (Blue) along with (Red) and (Green) fteen days prior to and fteen days after (a) the Sissano earth quake and (b) the Aitape earth quake. The vertical dotted line represents the earth quake day.  Variation of height (hmF2) of peak electron density (NmF2) of F2 layer of the ionosphere for days from 13 to 17 of July 1998.

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