ANNALS OF GEOPHYSICS, 56, 2, 2013, R0219; doi:10.4401/ag-6241

R0219

Measuring GNSS ionospheric total electron content at Concordia,and application to L-band radiometers

Vincenzo Romano1,*, Giovanni Macelloni2, Luca Spogli1, Marco Brogioni2,Giuditta Marinaro1, Cathryn N. Mitchell3

1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Rome, Italy2 Istituto di Fisica Applicata 'Nello Carrara' (IFAC-CNR), Sesto Fiorentino (Florence), Italy3 University of Bath, Electronic and Electrical Engineering, Bath, United Kingdom

ABSTRACT

In the framework of the project BIS - Bipolar Ionospheric Scintillation

and Total Electron Content Monitoring, the ISACCO-DMC0 andISACCO-DMC1 permanent monitoring stations were installed in 2008.The principal scope of the stations is to measure the ionospheric totalelectron content (TEC) and to monitor the ionospheric scintillations, usinghigh-sampling-frequency global positioning system (GPS) ionosphericscintillation and TEC monitor (GISTM) receivers. The disturbances thatthe ionosphere can induce on the electromagnetic signals emitted by theGlobal Navigation Satellite System constellations are due to the presenceof electron density anomalies in the ionosphere, which are particularly fre-quent at high latitudes, where the upper atmosphere is highly sensitive toperturbations coming from outer space. With the development of presentand future low-frequency space-borne microwave missions (e.g., SoilMoisture and Ocean Salinity [SMOS], Aquarius, and Soil Moisture Ac-tive Passive missions), there is an increasing need to estimate the effects ofthe ionosphere on the propagation of electromagnetic waves that affectssatellite measurements. As an example, how the TEC data collected atConcordia station are useful for the calibration of the European SpaceAgency SMOS data within the framework of an experiment promotedby the European Space Agency (known as DOMEX) will be discussed.The present report shows the ability of the GISTM station to monitorionospheric scintillation and TEC, which indicates that only the use ofcontinuous GPS measurements can provide accurate information on TECvariability, which is necessary for continuous calibration of satellite data.

1. IntroductionThe disturbances that the ionosphere can induce

on the electromagnetic signals emitted by the GlobalNavigation Satellite System (GNSS) constellations aredue to the presence of electron density anomalies inthe ionosphere, which are particularly frequent at highlatitudes where the upper atmosphere is highly sensi-tive to perturbations coming from outer space. Under

disturbed conditions, the ionosphere can show 'patches'of high electron concentrations that can strongly jeop-ardize the satellite signals received at the ground [see,e.g., Yeh and Liu 1982, Wernik et al. 2003]. This is themain reason why scintillation monitoring has a centralrole in the development of forecasting tools within thespace weather activities that are addressed to naviga-tional and positioning systems [see, e.g., Fisher andKunches 2011, Committee on the Societal and Eco-nomic Impacts of Severe Space Weather Events 2008].

The principal scope of the project BIS - BipolarIonospheric Scintillation and Total Electron Content Moni-toring is to measure the ionospheric total electron con-tent (TEC) and to monitor the ionospheric scintillationsusing suitably modified global positioning system (GPS)receivers. In the framework of this project, in January2008 and December 2009, two GPS ionospheric scintil-lation and TEC monitor (GISTM) permanent stationswere installed: ISACCO-DMC0 (latitude, 75.1001°S;longitude 123.333°E; 3280 m a.s.l.) and ISACCO-DMC1 (latitude, 75.106°S; longitude, 123.305°E; 3270m a.s.l.), respectively. Figure 1 shows the locations ofthe ISACCO receivers that were installed in Antarctica inthe framework of the BIS project: two receivers at Con-cordia station, and one receiver at Mario Zucchelli sta-tion. The ISACCO-DMC0 and DMC1 stations enlargedthe GNSS Research and Application for Polar Environ-ment (GRAPE; http://grape.scar.org/) scintillation andTEC receiver network, to cover the lack of experimen-tal observations at the time. Such a network allows un-precedented observation of the polar ionosphere, withextended auroral and polar coverage, making it possibleto map features from mid-latitudes through to polar lat-itudes, and to study the associated ionospheric processes.

Article historyReceived October 22, 2012; accepted March 27, 2013.Subject classification:Total electron content, Antarctica, GNSS, GPS, Faraday rotation, Ionosphere.

Ionospheric parameter determination from high-frequency receivers is useful, not only for continuousmonitoring of the polar ionosphere, but also as a pos-sible application of TEC measurements at the Dome-C site. In particular, in this report, the estimate of theeffects of the ionosphere on the propagation of a low-frequency microwave space-borne radiometer is pre-sented. The choice of the Dome-C area was becausewithin the framework of the European Space AgencySoil Moisture and Ocean Salinity (SMOS) calibrationand validation activities [Kerr et al. 2010], it has beenestablished that the observation of external inde-pendent targets, the characteristics of which are un-related to the instrument on board the SMOS satellite,represents an attractive opportunity for the calibrationprocedure. Apart from stable ocean areas, the EastAntarctic Plateau, and in particular the area near theItalian-French base of Concordia, appears to be par-

ticularly suitable for this purpose [Macelloni et al. 2006].Moreover, to correctly interpret the space-borne data,a precise estimation of the Faraday effect is also re-quired. Indeed, when microwave radiation from theEarth surface propagates through the ionosphere, po-larized field components are rotated by an angle, theFaraday rotation angle, the value of which depends onthe geomagnetic field and ionosphere electronic con-tent along the electromagnetic path. Space-borne ra-diometers typically measure the brightness temperatureat vertical and horizontal polarizations (Tbv, Tbh, re-spectively), and therefore it is important to know howmuch these measured values differ from the actual val-ues of the Tb of the emitting surface, and how thisdifference can vary over time. Such knowledge is par-ticularly important for absolute calibration of the in-struments placed on satellite platforms, as it allowsproperly correcting measured values.

It should be noted that several studies have beendevoted to estimation of the impact of Faraday rota-tion on microwave remote sensing [Le Vine 2000, Yueh2000, Abraham and Le Vine 2004, Freeman 2004, Meiss-ner and Wentz 2006, ], and these have used the Inter-national Geomagnetic Reference Field (http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html) and Interna-tional Reference Ionosphere (http://IRI.gsfc.nasa.gov/)models as sources of geomagnetic field and TEC data,respectively. Whereas these models work relatively wellat a global scale, they are less accurate at the poles, due tothe smaller amounts of long-term and continuous data.In this report, it is demonstrated that the acquisition of

ROMANO ET AL.

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Figure 1. Location of the ISACCO stations in Antarctica, with tworeceivers at Concordia station and one receiver at Mario Zucchellistation.

Figure 2. The ISACCO-DMC0 and DMC1 station block diagram.

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real TEC data improves the estimation of the Faraday ro-tation and thus corrects the measured Tb values.

2. Data and methodsEach experimental station at Dome-C, a block di-

agram of which is illustrated in Figure 2, has a NovAtelOEM4 dual-frequency receiver with special firmware,which comprises the major component of a GPS signalmonitor; this receiver is specifically configured to meas-ure amplitude and phase scintillation from L1 fre-quency GPS signals, and ionospheric TEC from the L1and L2 frequency GPS signals. Software is included inthe GISTM to automatically compute and log the am-plitude scintillation index, S4, and the phase scintillationindex, vU, computed over 1, 3, 10, 30 and 60 s. In addi-tion, the TEC and the TEC rates of change are bothlogged every 15 s. Phase and amplitude data are eitherin the raw form or are detrended (to remove system-atic variations), and these are also logged at 50 Hz. TheGISTM receivers can also compute signal quality pa-rameters, like the carrier-to-noise ratio of both L1 andL2, and the code carrier standard deviation [Van Dieren-dock et al. 1993]. All scintillation and TEC data are storedlocally and are available on the electronic space weatherupper atmosphere (eSWua) database hosted by the Isti-tuto Nazionale di Geofisica e Vulcanologia (INGV) [Ro-mano et al. 2008] (http://www.eswua.ingv.it). Somepictures of the installation of the DMC0 and DMC1 sta-tions are given in Figure 3 and Figure 4, respectively.

In this study, we analyze vertical (v)TEC data ac-quired at the ISACCO-DMC0 station over three years,from January 2008 to December 2010. These data havevery few gaps, and the percentage of total days of dataavailable in each year is above 97.5%, as summarized inTable 1. Vertical TEC (vTEC) values are obtained by pro-jecting the slant (s)TEC directly measured by the GISTMreceiver on the vTEC at the ionospheric piercing point(IPP), accordingly to the following formula:

(1)

where aelev is the elevation angle from the receiver tothe selected satellite, and F(aelev ) is the obliquity factor,

as defined by [Mannucci et al. 1993]:

(2)

where Re is the Earth radius, and HIPP is the height ofthe IPP, which is assumed to be 400 km. As GISTM out-puts a sTEC value every 15 s, we consider the averageof the four values in 1 min to calculate the correspon-ding vTEC. The sTEC was calibrated using the biasvalue furnished by the receiver firmware designer [VanDierendock et al. 1993]. The TEC is a key parameter forcharacterizing ionospheric variability, and its estimationfrom GPS measurements is nowadays an internation-ally acknowledged method for operational monitoringof the ionospheric state. The data analysis shown heremainly relies on the Ground Based Scintillation Clima-tology (GBSC), as described by Spogli et al. [2009, 2010]and Alfonsi et al. [2011]. GBSC produces maps of meanvalues and standard deviations (SD) of different quanti-ties measured by the GISTM receiver, such as thevTEC, and the carrier-to-noise ratio of the L1 frequency

APPLICATION TO TEC MEASUREMENTS AT CONCORDIA

Year %

2008 97.5

2009 100

2010 98.1

Table 1. Percentage of days of available ISACCO-DMC0 data.

Figure 3. The ISACCO-DMC0 station installation (latitude, 75.1001°S; longitude, 123.333°E, 3280 m a.s.l.).

Figure 4. The ISACCO-DMC1 station installation site (latitude,75.106°S; longitude, 123.305°E; 3270 m a.s.l.).

vTEC sTEC/F( )elev= a

( )cos

F

R HR

1

1elev

e IPP

e elev2a

a=

- +c m

(L1C/N). L1C/N is sensitive to possible sources of in-terference from other electromagnetic sources in prox-imity to the field of view of the GISTM antenna. Inparticular, the maps of L1C/N mean and SD can identifyspecific bins in which the signal quality experiences un-expected behavior. The maps of L1C/N allow the char-acterizing of the station by determining possible sourcesof signal distortions that affect the TEC measurements.

The maps are presented here in terms of the geo-graphic coordinates of the IPP, and the bin size waschosen to be 1° latitude × 1° longitude. Maps ofL1C/N are produced for each year, while the vTECmaps are only produced on a tri-monthly basis, to catchthe seasonality of the ionospheric variability. To remove

the contribution of bins with poor statistics, whichcould affect the occurrence estimation, the selected ac-curacy is set at 5% (see Taylor [1997] and Spogli et al.[2009] for application to the GBSC).

3. Results Figure 5 shows the maps of the L1C/N mean (left

column) and SD (right column) in geographic coordi-nates for the three years of data, as 2008 (Figure 5a, b),2009 (Figure 5c, d) and 2010 (Figure 5e, f ). Both theL1C/N mean and SD maps show that the signal qualityis good in each year, with it ranging from 36 dB to 56dB, with mean values and SD generally below 2 dB, ex-cept for a very few bins in the 2010 maps (Figure 5f ). In

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Figure 5. Maps of the L1C/N mean (left column) and SD (right column) in geographic coordinates for the three years of data: 2008 (a, b),2009 (c, d) and 2010 (e, f ).

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general, in 2010, the L1C/N SD was larger than in 2008and 2009, and this is likely to be due to interference froma parabolic antenna installed on the roof of the Con-cordia Station in early 2010. The area most affected bythe degradation of the signal quality is the latitudinalsector between about 60°E and 80°E, and in the latitu-dinal sector ranging from about −85°N to −75°N.

Figure 6 shows the map of the L1C/N SD in geo-graphic coordinates for 2010, superimposed by a blackcurve that represents the IPP of the satellite signalswith elevation angles of 20°, which is the typicalthreshold to remove multipath from observations. Thesignal degradation due to the parabolic antenna is re-moved by applying this filtering.

Figure 7 shows the maps of the vTEC in geo-graphic coordinates of 2008 (top row), 2009 (middlerow) and 2010 (bottom row). Each map refers to a threemonth period, which is representative of the seasonalvariability: January to March (summer), April to June(fall), July to September (winter) and October to De-cember (spring). The vTEC values range from a fewTEC units (TECu) up to about 22 TECu during the aus-tral summer and spring. During the austral night, themean values of vTEC are generally below 10 TECu. Tocatch the seasonality in detail, a month-by-month timeprofile of vTEC is also presented. Thus Figure 8 showsthe monthly variation of the mean vTEC over stationISACCO-DMC0, measured by removing the observa-tions at elevation angles below 20°. This threshold in theelevation allows the removal of the contributions due todegradation of the signal induced by multipath and re-flections, as suggest by Figure 6. The SD of the monthlydistribution of the vTEC is here indicated as the errorbars. Figure 8 shows the variability that is induced by solarradiation and equinoxes, with relative maxima betweenFebruary (2008, 2009) and March (2010), and in October(2008), November (2009) and December (2010). Theselast peaks are due to the combination of the maximumsolar flux during the austral summer and the equinoctialconfiguration that allows the penetration of solar windparticles from the magnetosphere into the ionosphere.This penetration gives rise to enhanced electron densitythat drifts across the polar cap. The time profile of thevTEC suggests also that the vTEC was lower in 2009,with respect to 2008 and 2010, with it being maximum in2010. This reflects the fact that during 2009 the descend-ing phase of the past solar cycle reached its minimum,and then started to increase through 2010.

4. Application to L-band radiometersThe Faraday rotation can be expressed by an in-

tegral of the geomagnetic field and the electron den-sity along the satellite line of sight [Meissner 2006], as

follows:(3)

where:

{F = Faraday rotation angle (clockwise {F >0) (de-grees)

o = frequency (GHz)= geomagnetic field vector (Tesla)

ne = electron density profile as a function of lati-tude, longitude and altitude (m−3)

l = position of the satellite in space (km a.s.l.).

Considering that the electron density has a peak atan altitude of ca. 400 km a.s.l. (corresponding to thepeak of the F2 layer of the ionosphere), and that theTEC measurements give an integrated value of theelectron content of the atmosphere (not a profile), acommon approximation is to assume that all of theelectrons are concentrated in the F2 layer [Davis 1990,Liu et al. 2005]. Thus:

(4)

where d is the Dirac delta function and sTEC (i.e., theslant TEC) is given by:

(5)

The GPS techniques [Sardon et al. 1994] give ameasurement of the sTEC in the ionosphere of up toca. 20,000 km; i.e., the height of the navigation satel-lites. As the altitude of the SMOS satellite orbit is ca.700 km, the sTEC computed by the GPS techniquesalso contains a residual sTEC value above the SMOSsatellite altitude (i.e., from 700 to 20,000 km), whichmust not be taken into account in the calculation of theFaraday rotation. The residual sTEC value depends onthe ionosphere and plasmasphere conditions, and

APPLICATION TO TEC MEASUREMENTS AT CONCORDIA

Figure 6. Map of the L1C/N SD in geographic coordinates for 2010.The black curve represents the IPP of the satellite signals with ele-vation angles of 20°.

Bn sd135 10F e

l

2

18

0

$ ${o

=-

geo#

, , 400kmn h Lat Lon sTEC he $, d -^ ^h h

, ,sTEC n h Lat Lon dlel

0, ^ h#

Bgeo

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Figu

re 7

.Thr

ee-m

onth

ly m

aps o

f th

e vT

EC

in g

eogr

aphi

c co

ordi

nate

s of

2008

(top

row

), 20

09 (m

iddl

e ro

w) a

nd 2

010

(bot

tom

row

).

7

whereas it is in general lower than 10% of the totalsTEC, in some cases it can reach a value of 15% of thetotal [Webb and Essex 1997]. Nevertheless, we mustconsider that the value of the geomagnetic field de-creases very rapidly when the distance from the Earthincreases. For instance, using the International Geo-magnetic Reference Field (IGRF)-11, we determinedthe values of the total intensity F of the geomagneticfield over the Concordia stations, and determined F as52511 nT at 400 km, 39982 nT at 1000 km, and 26769nT at 2000 km, which thus results in an impact on theresidual sTEC values on the Faraday rotation that ismuch lower than 10%. Suggested procedures for cor-recting the residual value using ionospheric models (i.e.IRI, IGS, NeQuick) were reported by Floury [2007]. Be-cause the purpose of the present study is the estima-tion of Faraday rotation using real sTEC data we preferto not use these models and disregard the residualvalue. It should be noted that in this case, the Faradayrotation is overestimated.

By inserting Equations (2) and (3) into Equation(1), the final simplified expression for the Faraday rota-tion angle is as follows:

(6)

where Bo is the geomagnetic field intensity (Tesla) at400 km in altitude, and a is the angle between the lineof sight and the direction of the geomagnetic field(degrees).

It can be demonstrated [Tsang 1991, Yueh 2000] thatthe Tb measured by satellite in horizontal and vertical

polarization is related to the Faraday rotation throughthe following relationships:

(7)

where Q and U are the second and the third Stokes pa-rameters, respectively. For isotropic surfaces, U is zero andreaches a value of tenths of a Kelvin at the L-band forocean surfaces; in this case, it is possible to disregard thisterm. Hence, the effects of the Faraday rotation on themeasured brightness temperature can be expressed by:

(8)

To quantify the impact of real TEC measurements,instead of the use of global ionospheric models, a sys-tematic analysis over the Dome-C area is presentedbelow.

To estimate how the Faraday rotation can affectthe satellite measurements acquired over the Dome-Carea, we have to consider the positions of the SMOSsatellite and Concordia station, and the observationof four lines of sight with respect to the direction ofthe geomagnetic field. In particular, we estimate theFaraday rotation along four different directions, whichare expressed in geographic coordinates using the realTEC data shown in the previous section, and in fourdifferent periods of 2008 ( January, March, June, Sep-tember), to allow for the seasonal variability of theionosphere characteristics. In particular, the average

APPLICATION TO TEC MEASUREMENTS AT CONCORDIA

Figure 8. Mean monthly variation of the vTEC over the whole field of view of the ISACCO-DMC0 station for the three years considered.The error bars show the SD of the distribution.

. ( )cossTEC B1 355 10F 2

4

0$ ${o

a=

2sin cos

sin cos

T T T T Q U

T T T T Q U2

2 2

Va V b V F F

a b F FH H H

2

2

{ {

{ {

D

D

= - = - -

= - = - -

``

jj

sin sinT Q T Tb F V H F2 2, { {D = -^ h

sTEC value is computed using the following steps:– We consider the area of the nominal pixel of the

SMOS satellite (40 km × 40 km) centered over theDome-C station;

– We compute the satellite position along each di-rection considered in the study for a SMOS satellite ob-servation angle of 42°, which is the angle selected tocover the whole swathe and used for the browse prod-uct of the SMOS satellite;

– We calculate the beamwidth of an equivalent an-tenna that would image the pixel from the four differ-ent positions;

– Lastly, since the F2 layer is approximately in themiddle of the line of sight, we used the sTEC meas-urements contained in the four different beams to ob-tain average, maximum, minimum and SD values ofthe sTEC computed over a week.

The sTEC value obtained is then used for the cal-culation of the Faraday rotation according to Equation(6), and finally DTb from Equation (8). In this study, thevalue of the Q = (TV − TH) difference was obtainedfrom the measurements of the DOMEX-2 experimentthat was carried out in Antarctica in 2009-2010 [Macel-loni et al. 2009]. At the incident angle of 42°, which wasconsidered for the computation, the measured value ofthe second Stokes parameter Q was ca. 20 K.

The results obtained for two directions are shownin Figure 9. It is possible to see that the mean bright-ness temperature variation is quite stable, and its valueis ca. 0.1 ±0.04 K for all of the period and directionsconsidered. Usually, the maximum value of DTb slightly

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Figure 9. Computed DTb as a function of time for directions 1 and4 for measurements performed with an observation angle of 42.5°.

Figure 10. DT variations over direction 4 computed using the sTEC data derived from the GPS data (triangles) or the IRI model (red,blue lines).

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exceeds 0.2 K, although it can reach values higher than0.5 K independent of the season and/or the observa-tion direction. It should also be noted that the mini-mum values of DTb were close to the mean values; thismeans that in the considered period, the ionospherewas usually stable and that maximum perturbations arenot common. It is worth noting that the sTEC meas-urements were performed during the quietest periodof the solar activity in decades, and that higher valueswill be reached during the period of the SMOS mission,due to high solar activity periods or events such as coro-nal mass ejections.

To estimate the potential benefit of using in-situGPS measurements instead of the TEC model, the DTbfor direction 4 using TEC data or the model is shown inFigure 10. We can observe that whereas the model cancorrectly predict the average DTb values, it cannot sim-ulate the fluctuations, and in particular the peak events.This can lead to an underestimation of the DTb, whichcan exceed 0.5 K. The computation performed for theother directions showed similar results.

5. ConclusionsIn the framework of the BIS project, two ISACCO

permanent stations have been active since 2008, tomonitor the ionospheric TEC and the ionospheric scin-tillations. Both of these stations have data coveragewith very few data gaps; in particular, for the theISACCO DMC0 station, the percentage of total days ofdata available in each year is above 97.5%. Signal qual-ity was analyzed through the GBSC technique, and itis good for each year, ranging from 36 dB to 56 dB inmean values, with a SD generally below 2 dB, exceptfor a few sky sectors in 2010. In general, the L1C/N SDin 2010 is larger than in 2008 and 2009, and this appearsto be due to interference from a parabolic antenna in-stalled on the roof of the Concordia station in early2010. The vTEC seasonality shows a range from a fewTECu up to about 22 TECu during the austral summerand spring, while during the austral night, vTEC is gen-erally below 10 TECu. The vTEC time profile also re-flects the solar activity, as for 2009; i.e., the minimum ofthe past solar cycle, when it reached its minimum andthen started to increase through 2010.

The vTEC measurements of the DMC0 receiverwere applied to estimate the Faraday rotation, to quan-tify the impact of real measurements instead of the useof global ionospheric models in microwave low-fre-quency satellite measurements. In our case, the appli-cation of the brightness temperature (Tb) acquired bythe SMOS satellite to the measurements shows that theeffects of the Faraday rotation are relatively stable andlead to an influence on the measured Tb of ca. 0.1

±0.04 K for all of the period and directions considered.By comparing the modeled and measured Faraday ro-tation, we observe that while the model can correctlypredict the average DTb values, it cannot simulate thefluctuations, and in particular the peak events. In par-ticular, the analysis performed demonstrates that DTbcan be underestimated by more than 0.5 K. It is difficultto evaluate whether this value is representative of thewhole 11-year solar cycle, and the continuous acquisi-tion of TEC data and ground radiometric data (i.e., theDOMEX-3 experiment) is recommended for the mon-itoring of the absolute performance of space-borne ra-diometers and the inter-calibration of the three L-bandspace-borne radiometric missions (the SMOS, Aquar-ius, and Soil Moisture Active Passive missions) duringhigh solar activity periods.

Acknowledgements. This study was carried out with finan-cial support from the European Space Agency and the Italian Na-tional Research Programme for Antarctica (PNRA). The presentstudy was made possible by the joint French-Italian Concordia pro-gramme, which has established and runs the permanent stations atDome C. Part of this study was carried out in the framework of theGRAPE SCAR Expert Group activities (www.grape.scar.org). Theauthors are grateful to the Italian-French logistic team at Concordiastation for their kind assistance and for their maintenance of the ex-perimental activities.

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*Corresponding author: Vincenzo Romano,Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2,Rome, Italy; email: vincenzo.romano@ingv.it.

© 2013 by the Istituto Nazionale di Geofisica e Vulcanologia. Allrights reserved.

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