Ionospheric total electron content variations observed before earthquakes: Possible physical mechanism and modeling
A.A. Namgaladze, O.V. Zolotov, I.E. Zakharenkova, I.I. Shagimuratov, O.V. Martynenko
NNamgaladze A.A. et al. Ionospheric total electron content variations…
УДК
Ionospheric total electron content variations observed beforeearthquakes: Possible physical mechanism and modeling
A.A. Namgaladze , O.V. Zolotov , I.E. Zakharenkova , I.I. Shagimuratov ,O.V. Martynenko Polytechnical Faculty of MSTU, Physics Chair, Murmansk West Department of IZMIRAN, Kaliningrad
Аннотация . На основе GPS наблюдений полного электронного содержания (TEC) ионосферы сиспользованием глобальных и региональных карт TEC, а также отдельных спутниковых измерений , обнаружены возмущения TEC, предшествующие сильным землетрясениям . Для сильныхсреднеширотных землетрясений сейсмо - ионосферные аномалии выглядят как области локальногоувеличения или уменьшения TEC в окрестности эпицентра готовящегося землетрясения . Такиеструктуры формируются в ионосфере за несколько дней до основного сейсмического события . Величинанаблюдаемых модификаций ионосферной плазмы достигает по сравнению с невозмущеннымифоновыми значениями . Область максимального проявления аномалии имеет пространственные размерыболее км по широте и км по долготе . В случае сильных низкоширотных землетрясенийобнаруживаются эффекты , связанные с модификацией экваториальной аномалии : углубление или " заполнение " ионосферного провала электронной концентрации на магнитном экваторе . Предложен физический механизм формирования указанных аномалий . Мы полагаем , что наиболеевероятной причиной наблюдавшихся перед землетрясениями возмущений NmF2 и TEC являетсявертикальный дрейф ионосферной плазмы в F2- области под воздействием зонального электрическогополя сейсмического происхождения . Для проверки этой гипотезы проведен ряд модельных расчётов припомощи глобальной самосогласованной численной модели верхней атмосферы
Земли
UAM (UpperAtmosphere Model).
Предложено пространственное распределение электрического потенциала на границеэпицентральной области , необходимое для существования указанного электрического поля . Параметрыверхней атмосферы , предшествующие готовящемуся сильному землетрясению , моделировались путёмвключения дополнительных источников электрического поля в уравнение для электрическогопотенциала модели UAM, которое решалось численно совместно со всеми остальными уравнениямимодели ( непрерывности , движения и теплового баланса ) для нейтрального и ионизованных газов . Эффективность предложенного механизма исследовалась путём численного моделирования откликаионосферы на воздействие зонального электрического поля , порождённого сейсмогенными источникамидля случаев средних и низких широт . Представлены результаты соответствующих модельных расчётовэлектрического поля и генерированных им эффектов в F2- области ионосферы и плазмосфере Земли . Результаты моделирования хорошо согласуются с данными наблюдений TEC перед сильнымиземлетрясениями как в случае средних , так и низких широт , как по пространственному масштабу , так ипо магнитуде наблюдаемых аномалий . Abstract.
The GPS derived TEC disturbances before earthquakes were discovered in the last years using globaland regional TEC maps, TEC measurements over individual stations as well as measurements along individualGPS satellite passes. For strong mid-latitudinal earthquakes the seismo-ionospheric anomalies look like localTEC enhancements or decreases located in the vicinity of the forthcoming earthquake epicenter. Such structuresare generated in the ionosphere for several days prior to the main shock. The amplitude of plasma modificationreaches the value of 30-90 % relative to the non-disturbed level. The zone of the anomaly maximummanifestation extends larger than 1500 km in latitude and 3500-4000 km in longitude. In case of strong low-latitudinal earthquakes there are effects related with the modification of the equatorial F2-region anomaly:deepening or filling of the ionospheric electron density trough over the magnetic equator.The possible physical mechanism which can cause such anomalies has been proposed. We consider that the mostprobable reason of the NmF2 and TEC disturbances observed before the earthquakes is the vertical drift of theF2-region ionospheric plasma under the influence of the zonal electric field of seismic origin. To check thishypothesis, the model calculations have been carried out with the use of the UAM (Upper Atmosphere Model) –the global numerical model of the Earth’s upper atmosphere. The electric potential distribution at the near-epicenter region boundary required for the electric field maintenance has been proposed. The upper atmospherestate, presumably foregone a strong earthquake, has been modeled by means of switching-on of additional roc. of MSTU, v.12, N 2, 2009 р .308-315 Ключевые слова : ионосферные предвестники землетрясений , эпицентр , сейсмогенное электрическое поле , полное электронноесодержание Key words: ionospheric earthquake precursors, epicenter, seismogenic electric field, total electron content
1. Introduction
Searches of the seismo-ionospheric precursors have been intensively conducted during last 2-3 decadeson the basis of different ground-based and satellite observations, also by means of the special satellite projects ofthe natural hazards monitoring ["COMPASS-1", "COMPASS-2", "Sich-1M", "QuakeSat", "DEMETER"] (seereference list in (
Pulinets, Boyarchuk , 2004)). Nowadays there are vast possibilities to investigate the ionospheremodifications and in particular the ionospheric effects associated with seismic activities by use of the satelliteGlobal Positioning System (GPS) signals measurements (
Liu et al. , 2002;
Plotkin , 2003;
Liu et al. , 2004;
Afraimovich et al. , 2004;
Krankowski et al. , 2006;
Zakharenkova et al. , 2007a). The dense network of GPSreceivers (a few thousands all over the world) fulfills simultaneous coverage in global scale with high temporalresolution. GPS technique provides measurements of the group and phase delays of the signals L =1575 MHzand L =1228 MHz with a 30-sec interval. The ionospheric delay can be transformed into the content of electronsalong the signal path between a GPS satellite and GPS receiver, and then recalculated into its vertical projection.The vertical total electron content (TEC) is very sensitive to changes of the maximal electron concentration(NmF2) in the F2 layer of the ionosphere.The extensive studies of the ionospheric earthquake precursors in the GPS TEC measurements carriedout in the last years have revealed that for strong mid-latitudinal earthquakes the seismo-ionospheric anomaliesvery often (usually) look as local TEC increases and they are situated in the immediate vicinity of the earthquakeepicenter area. The zone of the anomaly maximum manifestation (TEC enhancement more than 35 %) hasa spatial scale of some thousands km in longitude and about 1000 km in latitude ( Zakharenkova et al. , 2006;2007a; 2007b). These sizes are in a good agreement with results obtained from the combined analysis of ground-based and satellite ionosonde measurements. It was found out that the spatial scale of the seismo-modified areaof the ionosphere at the heights of the F-layer maximum during the strong earthquake preparation time has thediameter of 20°-40° in geographical coordinates (
Pulinets , 1998;
Strakhov, Liperovsky , 1999). The size of thearea changes with the earthquake magnitude.
Pulinets et al. (2005) analyzed the GPS TEC measurements for thestrong Mexico earthquake and found out an anomalous ionospheric modification in the spatial (latitude-longitude) distribution of the TEC deviation. The anomalous enhancement of the TEC was registered 3 daysprior to the event and it reached the value of 55 % relative to the background conditions.The negative TEC disturbances (TEC decreases) were also observed, for example for strong Turkeyearthquakes of 1999 (
Ruzhin et al. , 2002) and for several Taiwan and Sumatra earthquakes (
Liu et al. , 2006;
Liu,Chen , 2007).Fig. 1 presents characteristic TEC anomalies observed prior to the mid-latitudinal earthquakes of 25September 2003, Japan and 8 January 2006, Greece (
Zakharenkova et al. , 2007a; 2007b).
105 115 125 135 145 15520304050
02 UT 24.09.2003L it d La t i t ude -10 0 10 20 30 40 501525354555
20 UT 7.01.2006L it d La t i t ude Fig. 1. Differential TEC (%) maps 1 day prior to: 1) the Japanese earthquake of September 25, 2003 (M=8.3),2) the Greece earthquake of January 8, 2006 (M=6.8). The epicenter position is marked by the black dot amgaladze A.A. et al. Ionospheric total electron content variations…
Depueva, Ruzhin , 1995;
Depueva, Rotanova , 2001;
Pulinets, Legen’ka , 2002;
Depueva et al. , 2007).Fig. 2 presents the seismogenic equatorial anomaly observed in the measurements of Alouette-2 satellite 1 dayprior to the Chile earthquake of 12 April 1963 (
Depueva, Ruzhin , 1995). It was found out that one day before theevent the latitudinal dependence of the critical frequency (foF2) on the magnetic inclination (I) looked likea curve with two maxima symmetrically located around the magnetic equator in midnight hours of the local time.The plasma concentration over the epicentral area was reduced more than 10 times related to the normalconditions. Fig. 2. Seismogenic equatorial anomaly (blackcircles) in the ionosphere 1 day prior to theChile earthquake of 12 April 1963 (Alouette-2measurements). Other lines – foF2 variations innon-disturbed time (before and after theearthquake). EQ – the epicenter position(
Depueva, Ruzhin , 1995)
Liu et al. (2001) analyzed the measurements of the Taiwan GPS network and reported significantdecreases in the TEC on the 3rd and 4th days before the Chi-Chi earthquake.
Liu et al. (2004) further examinedthe GPS TEC during all of the 20 M ≥
2. Physical mechanisms of the seismo-ionospheric precursors appearance
Physical interpretation of the seismo-ionospheric precursors appearance has been proposed in numerouspapers (
Sorokin, Chmyrev , 1999;
Pulinets, Boyarchuk , 2004) and it is mainly based on the hypothesis about theseismogenic electric field related with the vertical turbulent transportation of the injected aerosols andradioactive particles (radon isotopes). The increase of the atmospheric radioactivity level during the earthquakepreparation leads to the enlargement of the ionization and electric conductivity of the near-ground atmosphere.The joint action of these processes leads to the intensification of the electric field in the ionosphere up to thevalue of units-tens mV/m (
Chmyrev et al. , 1989).There are strong arguments in favour of the hypothesis about the seismogenic electric field: а ) geomagnetic conjugation of the ionospheric precursors ( Pulinets et al. , 2003), b) effects related with theionospheric F2-region equatorial anomaly controlled by the zonal electric field (
Depueva, Ruzhin , 1995;
Pulinets, Legen’ka , 2002).It was proposed (
Namgaladze et al. , 2007) that the main reason of the appearance of the local anomaliesin the form of the increased (decreased) total electron content of the ionosphere, observed on the base of GPSsignals measurements, is the vertical drift of F2-region ionospheric plasma upward (downward) under theinfluence of the zonal electric field of seismogenic origin directed to the east (west). In the middle latitudes thevertical upward component of the electromagnetic drift, created by the eastward electric field, leads to theincrease of the F2 region electron concentration maximum (NmF2) and TEC due to the plasma transportation tothe regions with lower concentration of the neutral molecules O and N and, consequently, with lower loss rateof dominating ions O + in the ion-molecular reactions ( Brunelli, Namgaladze , 1988).The electric field of the opposite direction (westward) creates the opposite – negative – effect in NmF2and TEC. In the low latitude regions (near the geomagnetic equator) the increase of the eastward electric fieldleads to the deepening of the equatorial anomaly minimum ("trough" over the magnetic equator in the latitudinaldistribution of electron concentration) due to the intensification of the fountain-effect.
3. Model calculations
To check this hypothesis on the zonal electric field as the most probable cause of the observed TECdisturbances before earthquakes, the model calculations were carried out with the use of the UAM – the globalnumerical model of the Earth’s upper atmosphere (
Namgaladze et al. , 1988; 1991; 1998). This first principlemodel describes the Earth’s upper atmosphere behavior by means of solving a system of coupled time-dependent3D continuity, momentum and heat balance equations for the neutral and ionized atmospheric and ionospheric roc. of MSTU, v.12, N 2, 2009 р .308-315 ТЕС increased values which werefound out in (
Pulinets et al. , 2003;
Zakharenkova et al. , 2007b). The first region is a typical middle-latitudeionosphere, the second region is a near-equatorial ionosphere, in which the effects of the electric field are moreessential than at the middle latitudes.In Fig. 3 the numerical grid of geomagnetic coordinates used in the model calculations is shown and itsmesh points in which additional potentials were set, are marked by the circles (dark circles correspond to thepositive potential, light circles correspond to the negative potential).Fig. 3.
The numerical grid of geomagnetic coordinates used in the model calculations and its mesh points inwhich the additional potentials were set noted by the circles (dark circles correspond to the positive potential,light circles correspond to the negative potential)
4. Ionospheric effects created by additional sources of the electric field
The analysis of results of model calculations was carried out on the basis of comparison of the globalmaps of various ionospheric parameters obtained in quiet (without an additional electric field) and disturbed(with additional – presumably seismogenic – sources of an electric field) conditions. For the quiet condition themagneto-quiet day of a June solstice at high solar activity was accepted.The calculated electric potential pattern and horizontal electric field vectors for the sources of 5 and10 kV per mesh point are shown in Fig. 4 for the quiet and disturbed conditions. From this Figure the regions ofeastward electric field above the prospective epicenters of the future earthquakes, appeared when imposing(switching on) additional sources, and the similar magneto-conjugated to them regions in the oppositehemisphere are visible. The symmetry of the potential and electric field concerning the geomagnetic equator iscaused by the ideal electroconductivity of the ionospheric plasma along the geomagnetic field lines and,accordingly, by their electric equipotentiality. The amplitudes of the additional eastward electric fields are ofabout 2-4 mV/m in case of the low-latitude source and 4-10 mV/m in case of the mid-latitude source. They exceedthe background quiet fields (of about 0.2 and 1 mV/m, correspondingly), but they are noticeably smaller than thequiet high-latitude electric fields of magnetospheric origin (15-25 mV/m) obtained in the model calculations. amgaladze A.A. et al. Ionospheric total electron content variations…
Depueva, Ruzhin , 1995;
Ruzhin, Depueva , 1996;
Depueva et al. , 2007) as it is seen from the comparison ofFigs. 2 and 5, 6.The action of the mid-latitudinal source leads to the increase of foF2 and TEC in the near-epicentral andmagnetically conjugated areas of the ionosphere if these regions are lighted by the Sun as our calculations show.Inclusion of the source at night does not cause appreciable effects, they appear after sunrise and are kept aftersunset. The amplitude of enhancements and its spatial sizes are in a good agreement with the correspondingcharacteristics of precursors in the TEC observations. Although the electric field magnitudes in case of the mid-latitudinal source in our calculations are 2-5 times more than in case of low-latitudinal ones, the evoked effectsare weaker than in the near-equatorial regions. roc. of MSTU, v.12, N 2, 2009 р .308-315
5. Summary
As our model calculations show, the very probable reason of the NmF2 and TEC disturbances observedbefore earthquakes is the vertical drift of the F2-region ionospheric plasma under the influence of the zonalelectric field of seismogenic origin. In case of TEC enhancements in the middle latitudes and deepening of theequatorial anomaly trough this field is directed to the east and induces the electromagnetic drift of the plasmaacross the geomagnetic field with velocity directed straight upwards over the magnetic equator and upwards andpole wards in the middle latitudes.The upward plasma drift in the middle latitudes provokes the increase of electron concentration in the F2region of the ionosphere due to decrease of the dominating ions O + loss rate in the ion-molecular reactions; in thelow latitudes it provokes decrease of NmF2 over the magnetic equator due to the fountain-effect intensification.The pattern of the spatial distribution of the seismogenic origin electric field potential has beenproposed. For existence of the eastward electric field in the near-epicentral area it is necessary to dispose thepositive electric charges on the western boundary of this area and the negative charges on the eastern boundary.The efficiency of the proposed mechanism has been investigated by means of the numerical modelcalculations of the ionosphere response to the action of the zonal electric field with amplitude values of the orderof 2-10 mV/m presumably produced by seismogenic sources located in the middle and low latitudes. Results ofthe model calculations have revealed the fine agreement with the TEC and NmF2 anomalies observed beforestrong earthquakes in the middle and low latitudes both in spatial scales and in amplitude characteristics. We donot discuss here the mechanism of generation of the seismogenic electric field and we do not explain, why thepositive charges should accumulate on the western boundary of the near-epicentral region, and negativecharges – on the eastern boundary or vice versa. We show only what kind of electric field can produce the TECand NmF2 disturbances observed before strong earthquakes. amgaladze A.A. et al. Ionospheric total electron content variations… ReferencesAfraimovich E.L., Astafieva E.I., Gokhberg M.B., Lapshin V.M., Permyakova V.E., Steblov G.M.,Shalimov S.L.
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