Background Geomagnetic Conditions for the Present Study Period
The changes on earth like volcanic and seismic activity could trigger VTEC ionospheric enhancement. The Dst index represents the low latitude disturbance of the annular magnetic field intensity in nT and Kp represents the average geomagnetic intensity on a scale of 0 to 9. The analysis of the global parameters shows Global Parameters of Kp and Dst Variations for the 51 days from the 1st of April 2015 along with the Positive and Negative Anomalies derived from the Interquartile Range method, noted as ∆Dst and ∆Kp identified two major storm conditions during study period (i.e., days 6 and 47) [36] (Fig.2).
The Dst index has dropped drastically to -190 nT and -180 nT these days respectively. After two days of recovery, it appears to be a large storm capable of disrupting both the magnetosphere and the ionosphere. We noticed the positive anomaly in Kp during the two-storm events and the negative anomaly in Dst for the two-storm events.
Lee et al. [36] and Jacobs et al. [37] indicated that Kp<4 and Dst >-50nT could be called geomagnetically quiet periods. In the present study, there is no appreciable solar geomagnetic activity on the ionosphere for 5 days before the events and for a period of 10 to 16 days.
Ionosphere VTEC Variations
We first examined the influence of global geomagnetic effects on daily fluctuations recorded in GPS data. The total vertical electronic content (TECU) is derived from GPS data and the daily mean values of VTEC at these stations are calculated (Figure 3).
The study period can be divided into two categories, geomagnetically disturbed days and quiet days. The Kp values increased on days 6 and 47 (Kp~9) and Dst ~-180nT. These two days are considered geomagnetically disturbed days. At 5 days before both events (i.e. the 25th day; and the 42nd day) and the period from 10th to 16th days, no significant enhancement in the global parameter was detected in raw data and anomalies. These days therefore became quiet days.
The influences of the geomagnetic disturbance on the VTEC of all stations are clearly visible at days 6th and 47th in 10 stations IGS GPS (Fig.3).
Galav and et al. [38] and Kumar and Singh. [39] reported of the amplification of TEC during geomagnetic storms at low and medium latitudes.
Rama Rao and et al. [40] reported of the temporal influence of geomagnetic storm on the navigation system due to GPS‒TEC at different latitudes in the Indian continent. The increased delay during periods of thunderstorm activity reflects the significant increase in TEC in EIA areas. The number of phase shifts in the TEC GPS signal can be seen during the storm weather regime.
The ten GPS locations used in this study can be divided into two categories based on their spatial distribution, namely Equatorial Ionization Anomaly (EIA) Region and Non-Equatorial Ionization Anomaly (Non-EIA) Region. The HYDE, IISC, SGOC, PBRI and CUSV GPS locations are in the region of the Equatorial Ionization Anomaly (EIA).
Due to E×B vertical drift; this region is characterized by an increased electron density at magnetic latitudes of 10°± to 20°± in the F region [43]. AIE can result in a strong latitudinal gradient of the TEC gradient on the equator side and on the polar side, the latter being more intense [42].
Our results are consistent with the EIA, VTEC values ranged from 30 to 90 TECU in the EIA region and from 24 to 45 TECU in the non-EIA region.
On the 6th day, a geomagnetically disturbed day, the increase in VTEC at many stations KIT3, POL2, URUM, LHAZ, LCK4 is observed, about 60% sharp rise compared to the quiet value (Fig.3).
On the 47th day, during a strong magnetic storm, ten IGS stations showed gradual increase in VTEC. The observed VTECs for 10 IGS stations were categorized in ascending order relative to the epicenter in the earthquake preparedness zone [29].
Based on the stations related to the location of the earthquake in Nepal, we have classified the stations into five zones (Fig.1):
‒ LCK-4 (Zone-1);
‒ LHAZ (Zone-2);
‒ HYDE (Zone-3);
‒ URUM, POL-2, PBRI and IISC (Zone-4);
‒ KIT3, CUSV and SGOC (Zone-5).
The two event regimes in the present analyzes are indicated by a dotted line in the raw data. The first dotted line represents magnitude M=7.8 and the second dotted line represents magnitude M=7.3 (Fig.3).
The observed VTEC showed good peaks during geomagnetic storms Kp>5 and Dst<-50 nT and sub geomagnetic storms Kp>4 and Dst around -40 nT. On the 6th and 46th day we noticed a very good increase in the signal of 60% at KIT3, POL-2, URUM, LHAZ and LCK-4 compared to the other stations.
The sub-storm periods have been identified as the 1, 18, 20, 31, 43rd day, where we noticed a 30‒40% enhancement in ionospheric VTEC.
Lee et al. [20] and Huang [22] also reported the enhancement of VTEC during substorm conditions consistent with global solar activity.
We haven’t registered any anomalous magnetic storm conditions on days 15, 22, 26, 28, 36, and 40 at the stations LCK-4 and LHAZ. We observed spikes in the raw data in nearest stations where as we were not noticed such spikes by other remote stations. We saw an increase in observed VTECs of 30‒60% over previous days for nearest IGS GPS stations. Such spikes were not noticed by GPS stations in other areas. The small variation in VTEC (20 to 30%) is generally attributed to the diurnal variability of the ionosphere [41]. We could not distinguish the VTEC variation due to global disturbances based on station locations in the EIA region and the non-EIA region, and they showed a similar trend in both regions during this period.
The nearest stations LCK-4 and LHAZ showed a significant increase in their VTEC value from 28 to 38 from April 22, 2015 their increase is about 45‒50% during quiet days, two days before the event on April 25, 2015, then decreased gradually and returns to normal after the earthquake. Since the earthquake in Nepal occurred at shallow depth, there are three possible explanations for the anomalous variations in TEC, such as:
‒ Acoustic shock waves during topside vibrations [15];
‒ The electric field generated by the voltage variation in the rocks of seismic zones [45];
‒ Release of radon into the lower atmosphere [11].
The release of radon gas from the micro cracks formed in the crust and the surface seems to be the third possible amplification of the pre-earthquake TEC [11]. Therefore, the observed change in VTEC can be correlated with the seismogenic variation just before the earthquake since there is less influence of the global geomagnetic effect on the VTEC.
After the main earthquake of April 25, 2015, the major aftershock occurred on May 12, 2015 (M=7.3, Lat: 27.8090N, Lon: 86.0660E, D: 15 km). This aftershock is also included in our current analysis to study ionospheric variations during these two events 150 km apart. During the main shock, we observed significant variations in VTEC at stations in Zone-1 and Zone-2 three days before the main shock occurred on the 25th day, while we observed sharp peaks on days 36 and 40 before the May 12th aftershock (42nd day). Deviation from this distribution could be related to strong geomagnetic solar activity and impending seismic events. After the event, the significant disturbance was observed in area 1 and area 2 on April 25, 2015, and May 12, 2015 (Fig.3).
Anomalous VTEC from UB, LBs
The deviation of the daily values of the total electronic content near an epicenter a few days before the main shocks has been observed by many researchers [7, 24, 19, 21, 16,55].
For detection of the abnormal signal in the ionospheric parameters, Liu et al. [14] proposed an analysis of protocol based on quartiles, which has been adopted to study the precursor signatures of certain earthquakes [46].
The hourly median (M), lower (first) quartile (LQ) and upper (3rd) quartile (UQ) for the 24 days before and 26 days after the Nepal event were calculated for the same Universal Time (UT) for each station during the study period.
Taking into account the normal distribution with mean (m) and standard deviation (s) for the GPS‒TEC, the expected values of M and LQ and UQ are respectively denoted by m and 1.34 s [47].
The lower bound (LB)=M-1.5(M-LQ) and the upper bound (UB)=M+1.5(UQ-M) are calculated for any TEC anomalies. Here, the probability of the observed TEC in the interval (UB, LB) is about 54%. The UB, LB and TEC variations observed 24 days before and 26 days after the event (Figure 4).
The VTEC as well as the upper and lower bounds for ten IGS stations is shown in Fig. 4. The panels in Fig. 4 are arranged in ascending order of epicentral distance from respective stations. Abnormal VTECs were measured according to the standard protocol of Liu et al. [6]. The blue dotted line shows the upper bound, the red dotted line shows the lower bound, and the green line shows the observed VTEC (Fig.4). The days of the Nepal earthquakes were mentioned with the dotted line at days 25 and 42.
If the observed VTEC crosses UB, it shows the anomalous situation in the ionosphere. The abnormal situation occurs during the disturbance from both external and internal sources. During the days of geomagnetic storms (6 and 47) and substorms (1, 18, 20, 31, 43), we noticed an increase in VTEC on the UB signal. But apart from the external influences of geomagnetic storms and substorms, we also observed a significant increase in VTEC on the UB in LCK4 and LHAZ (i.e. Zone-1 and Zone-2) during the 15th, 22nd, 26th, 36th and 40th days.
As explained in the previous section, there is no external influence (global parameter) on these days and there is no positive or negative anomaly in both Kp and Dst on these days. On obtaining results, we have two sources:
‒ an external source which is a global phenomenon and should be replicated in all areas of EIA and non-EIA;
‒ the second cause of disturbance is earthquake-induced activity, which is local and expected to spread depending on the magnitude [29].
So, in our current analysis, we fixed a good increase in stations in Zone-1 and Zone-2, which are near to the Nepal earthquakes M=7.8 and M=7.3, respectively. We identified as a precursor signal for the 25th event on day 22nd (three days before) and day 15th (10 days before; more distinct) and for the 42nd event of M=7.3 on day 36th (6 days before) which is clearer signal and such variations have not been observed in stations of other zones.
The ionized atmosphere of the Earth consists of several layers, namely mesosphere (60‒85 km), E (85‒155 km), F (155‒550 km), with the main contribution coming from the F-layer. This electron density varies in the equatorial and polar regions for different reasons, with the equatorial regions being affected by geomagnetic activity while the Polar Regions are affected by ionization through the coupling of energetic particles and the magnetosphere. The production of photoelectrons in the ionosphere changed directly with solar irradiance.
Positive slope of the TEC from morning to noon is noted, and with the sudden loss of photoelectrons the negative slope of the TEC is noted from noon to night. The rate of electron production on atomic oxygen and molecular nitrogen could be the causal factor for the positive and negative slopes of the TEC from morning to night. The anomalies could be observed with effects around or after the date of the earthquake [6].
The variations of TEC over the 24-hour cycle can be considered as signals of a Gaussian distribution. The deviation from the Gaussian pattern of the TEC signal has been highlighted as showing the anomalous effect related to earthquakes observed by several researchers [48]. Therefore, the average value of such a signal is far from zero.
The observed TEC increased from the limits of UB, LB in LCK4, LHAZ (Fig.4). The relative amplitude of 54‒60% of the observed VTEC of UB, LB is noted in the study.
Afraimovich and Astafyeva [18] studied a few cases of preseismic precursors using TEC amplification a few days before earthquakes. They found that in some cases the enhancement may reflect global changes in ionization caused by solar and magnetic activity. While in a few cases they were correlated with local seismic activity.
Ouzounov and et al. [49] found that the source of the enhancement of TECs during the Tohoko earthquake was related to the seismogenic origin, as they got a very weak cross-correlation with the global geomagnetic field. Positive and negative anomalies were calculated from the UB, LB analysis above as VTEC←UB and VTEC←‒LB. These positive and negative anomalies have been represented in Figure 5 in ascending order as per the distance. The nearest stations received a stronger signal than the farthest stations.
Atmospheric Anomalies
The diurnal variations of various atmospheric parameters during the earthquake in Nepal are shown in the Figure 6. Atmospheric parameters such as outgoing long wave radiation (OLR), vertical temperature gradient (VTG) were archived from Kalpana satellite space grid data on the epicenter of the earthquake for 51 days. Increase in OLR from 240 to 340 watts/m2, increase in vertical temperature gradient from 4.3 to 23.20 K were observed before Nepal earthquakes. Among all parameters, OLR, AOT and vertical temperature gradients are significantly increased just before the quake with higher coefficients of variation (40‒50%).
Only several researches have reported on the OLR variability in the seismic activity zone during the earthquake preparation period [6, 9, 49]. The rate of change of OLR and radiation is directly proportional to the thermodynamic process over seismically active regions [9].
An abnormal character of the OLR is constructed analogously to the anomalous thermal field [50]. Our results show that the OLR started an upward trend from April 15th and peaked on April 24th and 25th just before the earthquake in Nepal. Increased tectonic activity in the seismic zone increases anomalous latent heat flux and eventually increases radiation rapidly.
Such an increase in radiation has also been observed in most places of recent earthquakes [49, 44], occurred in:
‒ Japan (M=9.3, 2011);
‒ China (M=7.9, 2008);
‒ Italy (M=6.3, 2009);
‒ Samoa (M=7, 2009);
‒ Haiti (M=7, 2010);
‒ Chile (M=8.8, 2010).
The observed increase in the OLR anomaly, VTG, is likely to be correlated with the signal from the earthquake preparatory zone before the earthquake in Nepal.
Spatial Variations of Atmosphere and Ionosphere Parameters
The spatial variation of TEC, OLR and VTG was investigated during the study period near the Nepal earthquake fault radius. The spatial coverage of VTEC over the study period is shown in Figure 7.
VTEC readings are between 25 and 30 TECU on April 24, 2015, just one day before the earthquake struck Nepal in the epicentral region. On the day of the earthquake, the TECU rose to 40‒45. The following day, April 26, 2015, an increase in the level of TECU can also be observed. This is clearly recognizable by the significant changes in VTEC over the seismic zone. The one-to-one relationship in the ionosphere before a large earthquake gives the correct signature.
The spatial coverage of the OLR and the vertical temperature gradient over the study period is shown in Figure 8. The high-resolution datasets were archived from NCEP NCAAR Reanalysis-2 data [51].
The dense gridded variation of the OLR and vertical temperature was observed during the study period. OLR values are in the range of 240‒260 watts/m2 on April 24, 2015, just one day before the Nepal earthquake hit the epicentral region. On the day of the earthquake, the OLR rose to 330-350 watts/m2.
The next day, April 26, 2015, also shows a high OLR value in the range of 280‒290 watts/m2. The possibilities for enhancement in OLR are due to two reasons [52]:
‒ the first possibility is that we can expect the increase by trapping all the outgoing long wave radiation;
‒ the other possibility is that an amplification of OLR by earthquakes and volcanic eruptions.
In our study, the OLR variability is in the range of 110‒130 watts/m2, which may have increased due to the earthquake in Nepal. The increase in OLR near the earthquake site is due to the piezoelectric effect in addition to terrestrial emissivity [9].
Similarly, the vertical temperature variability is shown in Figure 9. We observed that the VTG values vary from 12 to 14 on April 24, 2015, just one day before the earthquake in Nepal in the epicentral region. On the day of the earthquake, the VTG rose in the following units to 22‒24. The next day, April 26, 2015, an increase in the level of VTG in the range of 20‒22 can also be observed. Therefore, thermal anomalies are also observed in the largest faults and areas where major earthquakes occur.