The processes leading to the growth and inhibition of equatorial anomaly before major earthquake (EQ) were viewed in this paper by examining global Total Electron Content (TEC) that contour ed over longitude sectors covering Africa to Pacific, in association with EQ events of Japan (M = 9) that occurred at 135 °E to 145°E and 35°N to 40°N, on March 9 and 11 , 2011 and of Indonesia (M = 8.6) that took place at 2.311° N, 93°E, in April 11, 2012. The paper focuses on the development of abnormal increase in density in the night sector around—10° latitude zone prior to the two major EQ events, though their epicenters are separated widely from the anomaly region. The F-layer density variations from rele vant locations and TEC features obtained from GPS at Appleton anomaly crest station are utilized as supporting inputs. The possible sources leading to the anomalous development in density are discussed in the frame of EQ time con se quences of lithospheric-atmospheric processes between the equator and beyond. The role of electric field generated by pre EQ preparatory activities and dynamical coupling modes through seismic fault line are brought in to the ambit of discussion.
Ionospheric parameters as tools for prediction of an earthquake have been adapted by many workers [1-7]. However, solar geomagnetic influences on ionosphere being significant, the processes of filtering the earthquake (EQ) induced effects from these parameters are difficult exercises. The situation becomes more complex at Appleton anomaly zone [8-11] where ionisation density is transported up by equatorial electric field (E) through EXB drift process. Therefore, at off equatorial stations i.e., round Appleton Anomaly region (±20˚ geomagnetic latitudes), the ionisation density and Total Electron Contents (TEC), have a strong component of migrating electron density. Such movement is also possible from epicentre region during EQ time electric field generated in its preparatory processes [12-14]. Added to these aspects, the anomaly zone undergoes changes with solar geomagnetic environments as well as on coupling dynamics between equatorial and magnetospheric processes [15-18], thereby making EQ time density changes on TEC or ionisation density around the anomaly as multifactor events [19-21]. There are also reports showing apparent modifications in ionisation density and on TEC by pre earthquake influences on lower atmosphere [22- 26].
All these activities make identification of earthquake related dynamics over extended latitudinal and longitudinal sectors very complex especially when anomaly feature is adapted as a tool of EQ precursor as the paper aims. Here, we analyze the growth and inhibition pattern of anomaly zone from global TEC data covering sectors from east Africa to west pacific; the feature so obtained is examined in association with major Japan EQ (JEQ) of March 11, 2011 epicentre at 38.322˚N, 142.369˚E (M = 8.9) and Indonesia EQ (IEQ, M = 8.6) of April 11, 2012 occurred at 2.311˚N, 93˚E. The two events are so selected that EQ time anomaly coupling dynamics could be brought in to focus at widely separated zones around and beyond equatorial anomaly region limited within ±20˚ geomagnetic latitudes. TEC data collected from the GPS receiving setup at Guwahati (26˚10'N, 91˚45'E) along with F-layer peak density (NmF2) data from relevant stations are used as supporting inputs.
A representative profile of TEC obtained from dual frequency GPS observation at Guwahati, an anomaly crest station (26˚10'N, 91˚45'E) is presented in
The very strong JEQ of March 11, 2011, was preceded by another event of March 9 with M = 7.3 occurring at
38.322˚N, 142.369˚E; the global TEC maps prior to, during and after these events obtained from the Australian meteorological department (www.ips/satellite.au) are presented in Figures 2(a)-(d). The TEC contour plots of the figures correspond to 11 UT - 12 UT, i.e. when epicentre zone of 130˚E to 140˚E falls in the night sector, so that abnormality if any could be easily identified from relatively low background night time density.
The IEQ, that occurred at the epicentre 2.3˚N and 93˚E on April 12, 2012 is one of the strongest in recent times with magnitude of M = 8.6. We present in
The magnitude of EEA is determined from the increase in TEC value within the anomaly zone with respect to ones prevailing in the NEA zone at that hour and also the longitudinal spread of the EEA. Adopting these two aspects, the EEA strength is defined in grades (arbitrary unit) from 0.5 to 10, with 40 TEC U as minimum value of the TEC and 80 TECU as the maximum, during the period 10 UT -12 UT. The magnitude so derived is presented from March 1 to 15, 2011 in
The figures show that the magnitude of EEA increased prior to both the JEQs of March 9 and 11 with a sharp decrease on the day of the event. However, this result does not isolate the effects on the EEA of large number of relatively small EQs (
This observation receives support as we do not see any systematic relation between the EEA magnitude (
developed. Thus, the formation of EEA demonstrates a strong association with high latitude EQs predominantly occurring within the 30˚N - 40˚N latitudes over the selected longitudes (
Unlike the March 2011 EQs that occurred mostly at high latitudes, April 2012 events were observed in the −5˚N to +5˚N within our interested longitude zone (90˚E to 140˚E) as displayed in
this hour (10 UT - 11UT) around 20˚E - 30˚E longitude. It is also significant to note that the EEA positions of 95˚ E, 140˚E and 130˚E longitudes coinciding with longitude of the corresponding epicenters which were (a) 2.3˚N, 93˚E, (b) −5.46˚N, 147.12˚E and (c) −1.62˚N, 134.28˚E, respectively.
This analysis along with that for March 2011events thus supports the fact that the development of EEA is coupled to the strong EQ events whether occurred at the equatorial or at high latitude zone.
The explanations to the observed development of the EEA and shift of this anomaly zone towards the longitude of epicentre of the two most devastating recent EQs one at mid latitude and the other at the equator, are difficult to put forward with present understandings on equatorial anomaly, though the underlying physics and dynamics involved with the process are well established. The growth of such anomaly is coupled with conductivity of the ionosphere under an impressed electric field that becomes anisotropic in presence of a magnetic field. The result is generation of three conductivities namely Longitudinal, Pedersen and Hall conductivities. The Hall conductivity amongst the three is significant in this context. Because the Hall current flowing in a direction perpendicular to both the electric and magnetic fields generates a force that moves the plasma upwards through the process E ´ B drift that causes transportation of ionization from the equator to regions off the equator, creating a minimum density over the magnetic equator, while causing maxima at latitudes to 20˚ (geomagnetic) north and south of the equator [8-11]. The effective enhancement of Ionization density or TEC caused by this process at off equatorial stations is generally obtained just after local noon as displayed in
accumulation of ionisation density especially on the night of March 8, (one day prior to EQ of March 9) at Darwin, and a well developed EEA in global TEC map (
The IPS TEC global map shown here is produced with IRI model by taking real-time foF2 data obtained from the region. Therefore, the nighttime increase in NmF2 over Darwin supports the growth of EEA as reflected in the TEC global map. But how such anomaly is coupled to the major EQs occurring at latitude as high as of 35˚N, is a question. It is equally intriguing to note the adjustment of EEA longitude to epicenter longitudes. Considering all the observed features we can also look for the explanation to the hypotheses of EQ time equatorial/low latitude anomaly that suggest pumping of ionization from epicentre to off epicentre location by the EXB drift but possibly in this case the transportation process is significant during anomaly reversal time. It is well known that the day time anomaly caused by the eastward E field reverses after sunset with the development of westward electric filed. Therefore, EXB upward drift decreases during reversal time, goes to negative i.e. downward direction and reverse fountain effects become operational [29,30] and westward electric fields may result to nighttime enhancements in electron concentrations in mid latitude F-region [
The large increase in nighttime TEC near equator prior to the JEQ i.e. the EEA, may be the result of such dumping from epicentre due to EQ induced westward E field, suggesting development of coupling processes between high and low latitude during EQ preparatory processes. The horizontal component of magnetic field at the location of the epicentre though weak, but the situation of zero declination contour lines [32,33] that touches epicentre to the near equatorial high density zone of
Along with this explanation, one has to look for other sources which are active prior to an impending EQ, specialty in the generation of a dynamical coupling from lower to atmosphere. In such an attempt Molchanov and Hayakawa [
One also looks for lower atmospheric perturbations trigged by seismic generated waves as another source in modulating density of ionosphere. The wavelike structures are generated in EQ environment due to modifications in atmospheric parameters like temperature, pressure, humidity. Because, the increase in temperature by the earthquake preparatory processes could develop a differential temperature situation in the steady nocturnal atmosphere thereby sharpening the density gradient or wind shear. In situations when earth’s gravity and the magnitude of stabilizing restoring force introduced by the density gradient are comparable, special structures or waves known as gravity waves are generated. They are quasi longitudinal waves and in presence of strong wind shear these waves break leading to Kelvin Helmholtz instability to generate wave structures. Development of such waves prior to one of the low altitude EQs was also seen by GU Sodar [
It is also important to consider the EQ time radon emission from lithosphere near to the epicentre that may trigger ionization in the atmosphere. The seismic fault zone being in severe stressed condition prior to a major EQ, may release radon gas. In ionosphere, the radon gas not only enhances the electron density, but could modulate the atmospheric electric field, leading to ionospheric perturbation [42,43]. In such situation the component of electric current caused by the Lithospher-AtmosphereIonosphere (LAI) coupling as has been suggested by Freund [
Finally out of all these possible sources we have zeroed down to two main factors in transporting charged particles for development of the EEA zone. One of the factors is the earthquake time downward ExB drift generated by westward electric field that leads to modification in nighttime increase in NmF2/TEC at the equator. Thus, E-H coupling mode in case of the two severe EQ events one in Japan ( lies in fault line, as marked in
nature of which could be seen as an anomaly at near equatorial region in the epicentre longitude zone. This process of accumulation of ionisation density at the 100˚E - 120˚E longitudes is further enhanced by EM focusing effect generated at the EEA location due to the presence of the distortion of earth magnetic field in this sector. The focusing effect in this case can be explained through the simple theory that if the magnetic field converges towards a small region in an otherwise uniform magnetic field over an area, the flux density within the convergence zone would bring the electron current to a focus. The curve in the earth magnetic field (more so in the near horizontal component) at around110˚E and 15˚N, acts like a cone where charged particles if enter magnetic lines, may get trapped and thereby show increase in its density. Before the strong EQ cases as taken up here, the electrons with charge e moving downwards along the magnetic field line by EQ induced electric filed (as discussed above ) with drift velocity v that experience a force of magnitude e (v × B), resulting to a Hall force of Hf = −(v × B). Therefore, with the increase of Hall force, the density increases thereby strengthening the accumulation process of electrons by EQ preparatory processes at the apex of cone zone, in our case at 100˚E longitude and 15˚N latitude. Increase of drift value i.e. of Hall force with magnitude of EQ is the result of spreading of observation of TEC/foF2 far beyond epicentre zone (20, 27, 46, and 47). Here in this paper we consider that this process to be active during the EQ time along with the others and present that the magnetic distortion at special zone may lead to density modulation thereby acting as an identification signature of an EQ event.
As a possible aspect of the future study, we would use the results of this analysis as a basis for constructing quantitative models of the growth and inhibition of equatorial anomaly, to obtain a more reliable and accurate estimations of the EQ in connection with the observable quantities and their relationships.
The authors acknowledge the data support and suggestions received from the Australian meteorological department. The NmF2 data received from Ko-Ichiro Oyama, Institute of Space Science, National Central University, Taiyuan 32001, Taiwan are duly acknowledged.