Comparing IRI TEC Predictionsto GPS and DigisondeMeasurements at Ebro

M. Mosert (1,4), M. Gende (2), C. Brunini (2), R. Ezquer (3,4,5), D. Altadill (6)

(1) CASLEO, San Juan, Argentina(2) FCAG, Observatorio Astronómico, UNLP, La Plata, Argentina(3) Laboratorio de Ionósfera, Dto. de Física, UNT, Tucumán, Argentina(4) Facultad Regional Tucumán, Universidad Tecnológica Nacional, Argentina(5) CONICET, Argentina(6) Observatori de l´Ebre, Roquetes, España

Abstract

Vertical total electron content obtained with GPS satellites signals (GPSTEC) and total electron content derived from ionograms (ITEC) arecompared with the latest version of the International ReferenceIonosphere, IRI-2000 (IRI TEC). Two years of high solar activity: 2000(Rz=117) and 2001(Rz=111) at the Ebro station (40.4N; 0.3E) areused in the study. The shapes of the three estimations are similar and asit is known, the GPS values are greater than those obtained from thedigisonde. The IRI predictions generally overestimate the ITEC values.An analysis of the variability of the GPS TEC and ITEC is carried out.The diurnal and seasonal variations of the plasmaspheric electron contentare also analyzed.

IntroductionAs it is known , Huang and Reinisch (2001) introduced a new technique forestimating the ionospheric topside profile from the bottom-side electron densityprofile that also allows to derive the total electron content by integration of theentire profile (ITEC).

The increase in availability of TEC data over the last ten years has larguely comefrom a rapid increase in the number of Global Position System (GPS) TEC data overland. Compared with the long record, almost fifty years of ionospheric observationfrom ground-based measurements, the TEC record from GPS is relativelly short.However the increase in the number of GPS sites provides now an important database to study the ionosphere.

In this paper comparisons between IRI-2000 TEC predictions and GPS-TECand ITEC measurements have been done using data from Ebro (40.8°N,0.49°E; Modip= 48.4°). A preliminary study of the day to day variability isalso presented.

Data UsedStation Lat Geog. Long. Geog. Dip ModipEBRO 40.8 0.49 48.4 56.1

Time Periods

Year Month Rz12 UT2000 July (summer) 119.8 00-23

January (winter) 112.9 00-23October (fall) 114.5 00-23April (spring) 120.8 00-23

2001 July (summer) 117.1 00-23January (winter) 108.7 00-23October (fall) 114 00-23April (spring) 107.5 00-23

Monthly Median TEC Values:

ITEC: obtained from digisonde ionograms using the true height inversionprogram NHPC (Reinisch and Huang, 1983; Huang and Reinisch, 1996)

GPS-TEC: obtained using La Plata Ionospheric Model, LPIM (Brunini et al,2001)

IRI-TEC: (h =1000 km) obtained from IRI-2000 version (Bilitza, 2001)

Results

1. Comparing ITEC, GPS-TEC and IRI-TEC

2. Comparisons between ITEC variability andGPS-TEC variability

3. Variations of DTEC= GPSTEC-ITEC

1. Comparing ITEC, GPS-TEC and IRI-TEC

Figures 1 and 2 show the comparison of the monthlymedian values of ITEC, GPSTEC and IRITEC at Ebro forfour months: January, July, April and October, reprentativemonths of winter, summer, spring and fall respectively,during the years 2000 and 2001 (HSA).

0 2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

70

January (winter) 2000

TEC

(TEC

U)

UT

0 2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

70

January (winter) 2001

TEC

(TE

CU

)

UT

0 2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

70

July (summer) 2000

TEC

(TE

CU

)

UT0 2 4 6 8 10 12 14 16 18 20 22

0

10

20

30

40

50

60

70

July (summer) 2001

TEC

(TEC

U)

UT ITEC IRITEC GPSTEC

Ebro

–TE

C - H

SA

2000 2001

Figure 1.

0 2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

70

April (spring) 2000

TEC

(TE

CU

)

UT

0 2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

70

April (spring) 2001

TEC

(TEC

U)

UT

0 2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

70

October (fall) 2000

TEC

(TE

CU

)

UT

0 2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

70

October (fall) 2001

TEC

(TE

CU

)

UT ITEC IRITEC GPSTEC

Ebro

–TE

C -

HSA

2000 2001

Figure 2.

The analysis of the figures 1 and 2 show the comparisonsbetween the IRI VTEC predictions and ITEC and GPSTECmeasurements .

The shape of the of the 3 estimations are quite similar. The GPSvalues , as it is expected, are larger than the ITEC values.The differences DTEC = GPSTEC – ITEC can be considered asa measured of the plasmaspheric contribution. (DTEC will beanalyzed in section 3 of this paper).

The IRI predictions overestimate the ITEC values in all the seasonsduring the 2 years of high solar activity. During winter time the IRImodel overestimates also the GPSTEC values.

2. Comparing ITEC Variability and GPS-TEC VariabilityMosert et al. (2001) presented an analysis of the behavior of ITEC deduced fromionograms recorded at 2 Argentine stations: San Juan and Tucuman duringdifferent seasonal and solar activity conditions and for typical hours of the day.Their results showed that the variability of ITEC (using the variability index V%=(sd/mean)x100) is maximum at hours of minimum TEC; it is greater aroundmidnight that around midday and it decreases with the solar activity. Their resulspresented a good agreement with those found by Ezquer and Adler (1989) andMosert et al. (2004) when they analyzed the Faraday TEC mesurements overTucumán during a period of high solar activity.Ezquer et al. (2002) analyzing the GPS - VTEC over 10 stations of the Americansector during a period of HSA reported that, in general, the variability duringdaylight hours is about 30% of median or less, and that observed for nightimehours is greater than the mentioned porcentage, particularly at last hours of thenight. Moreover, for most of the considered stations the variability observedduring equinox is greater than that observed during solstice.Adeniyi and Radicella (2002) analyzed the ITEC obtained from ionogramsrecorded at an equatorial station. They showed that:

Figures 3, 4, 5 and 6 show the comparisons between ITEC variability and GPSTECvariability , using the variability indexes Cup-Clo, during the different seasons inthe two years of HSA. (The analysis has also been done using the variability index

V%= (SD/ mean)x100

Variability of ITEC

The diurnal and seasonal behavior of the variability is more prominent in some seasonsin the ITEC values than in the GPSTEC values: the variations of the Cup and Cloindexes ar more “smoothed”

The diurnal behavior of the variability in the ITEC values during the two years is notclear. In some months (October 2000 and April 2001) the variability during daytime islower than that observed during nightime.

During the year 2000 the variability of ITEC is slightly greater during winter time(Cup-Clo values range between 0.30-0.50) and fall (0.16-0.35) than during summertime (0.17-0.40) and spring (0.16-0.30).During the year 2001 the seasonal behavior of the variability is different to thatobserved in the year 2000. The lowest variability is observed in October (fall).

Variability of GPS-TEC

The diurnal behavior of the variability of the GPS-TEC values is notclear. However in January 2000 and April and October 2001thevariability shows a tendency to be slightly greater during nighttimethan during daytime.

Regarding to the seasonal behavior in the year 2000 the winter valuesof the index Cup-Clo are larger than those observed in the otherseasons. In the year 2001 the spring values of the index is greaterthan in the other seasons.

The features of the variability of ITEC and GPS-TEC during the years2000 and 2001 are similar if the variability index used isV%= (SD/mean)x 100

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

January 2000 - ITEC

Var

iabi

lity

inde

x

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

July 2000 - ITEC

Var

iabi

lity

inde

x

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5January 2000 - GPS-TEC

Varia

bilit

y in

dex

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Cup Clo

July 2000 - GPS-TEC

Var

iabi

lity

inde

x

UTFigure 3

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5April 2000 - ITEC

Var

iabi

lity

inde

x

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5October 2000 - ITEC

Varia

bilit

y in

dex

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

April 2000 - GPS-TEC

Varia

bilit

y in

dex

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Cup Clo

October 2000 - GPS-TEC

Varia

bilit

y in

dex

UT

Figure 4

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5January 2001 - ITEC

Var

iabi

lity

inde

x

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

July 2001 - ITEC

Var

iabi

lity

inde

x

UT CupITEC CloITEC

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

January 2001 - GPS-TEC

Var

iabi

lity

inde

x

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5July 2001 - GPS-TEC

Varia

bilit

y in

dex

UT CupTEC_GPS CloTEC_GPS

Figure 5

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5April 2001 - ITEC

Var

iabi

lity

inde

x

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5October 2001 - ITEC

Varia

bilit

y in

dex

UT CupITEC CloITEC

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

April 2001 - GPS-TEC

Varia

bilit

y in

dex

UT

0 2 4 6 8 10 12 14 16 18 20 220.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5October 2001 -GPS-TEC

Varia

bilit

y in

dex

UT CupGPS-TEC CloGPS-TEC

Figure 6

3. Variations of DTEC= GPSTEC-ITEC

Figures 7 and 8 show the daily variation of the monthlymedian values of DTEC = GPSTEC-ITEC for therepresentative months of winter (January), summer (July)spring (April) and fall (October).

Figures 9 and 10 show the average seasonal variation of DTECexpressed in percentages (DTEC%) for the two years:2000 (Rz12= 117) and 2001 (Rz12=110).

0 2 4 6 8 10 12 14 16 18 20 22-2

0

2

4

6

8

10

12

14

16

18

20

April 2000 - 2001

DTE

C(T

EC

U)

UT0 2 4 6 8 10 12 14 16 18 20 22

-2

0

2

4

6

8

10

12

14

16

18

20

January 2000 - 2001

DTE

C(T

EC

U)

UT

0 2 4 6 8 10 12 14 16 18 20 22-2

0

2

4

6

8

10

12

14

16

18

20July 2000 - 2001

DTE

C(T

ECU

)

UT

0 2 4 6 8 10 12 14 16 18 20 22-2

0

2

4

6

8

10

12

14

16

18

20

October 2000 - 2001

DTE

C(T

EC

U)

UT DTEC2000 DTEC2001

Figure 7

0 2 4 6 8 10 12 14 16 18 20 22-2

0

2

4

6

8

10

12

14

16

18

20DTEC 2000

DTE

C (T

ECU

)

UT DTECJan DTECApril DTECJul DTECOct

0 2 4 6 8 10 12 14 16 18 20 22-2

0

2

4

6

8

10

12

14

16

18

20DTEC 2001

DTE

C (T

EC

U)

UT

Figure 8

0 2 4 6 8 10 12 14 16 18 20 22

-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70April 2000 - 2001

DTE

C%

UT0 2 4 6 8 10 12 14 16 18 20 22

-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70January 2000 - 2001

DTE

C%

UT

0 2 4 6 8 10 12 14 16 18 20 22

-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70July 2000 - 2001

DTE

C%

UT

0 2 4 6 8 10 12 14 16 18 20 22

-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

October 2000 - 2001

DTE

C%

UT DTEC%2000 DTEC%2001

Figure 9

DTEC %

0 2 4 6 8 10 12 14 16 18 20 22-10-505

10152025303540455055606570

DTEC% - 2000

DTE

C%

UT DTEC Jan DTEC Ap DTEC Jul DTEC Oct

0 2 4 6 8 10 12 14 16 18 20 22-10-505

10152025303540455055606570

DTEC% - 2001

DTE

C%

UT

Figure 10

Diurnal and seasonal variations of DTEC

The curves corresponding to the diurnal variations of DTEC in the two years of HSAshow, in general, minimum values around midday.This behavior is more evident during the solstices (DTEC ranges between 0 and 8TECU) than during the equinox, particularly in April and October 2000.The maximum values are observed around sunrise and sunset. During the periods ofmaximum DTEC the GPS-TEC values exceeds the ITEC values by 6 – 14 TECU inthe solstice and by 10 – 18 TECU in equinox.

A clear seasonal behavior is observed in the DTEC values during the year 2000: Theminimum values are observed during winter time and the maximum values areobserved in April (spring). The values in July (summer) and October (fall) arebetween the winter and spring values. This behavior is observed during the year 2001only during some hours of the day.

Diurnal and seasonal variations of percentage DTEC (DTEC%)

The analysis of the variations of DTEC expressed inpercentages shows:

(1) In general the DTEC % values are lower around middaythan around midnight in the two years studied.

(2) During nightime the DTEC % reaches values of 40-60 % inwinter, 25-50% in summer and 30-55 % in equinox. Thedaytime percentages are generally lower in winter (around5%) than in the other seasons.The maximum values duringdaytime are observed in April (spring) reaching valuesbetween 15 and 25 %. The seasonal variations are moreevident during daytime in the year 2000 and during nightimein the year 2001?

SummaryThis paper studies the diurnal and seasonal behavior of the medianvalues of ITEC and GPSTEC at Ebro during a period of high solaractivity: 2000 (Rz12= 117) and 2001 (Rz12=110). Comparisons betweenboth ITEC and GPSTEC measurements and the IRI-2000 modelpredictions are also done, showing, in general, that the IRIpredictions overestimate the ITEC values and in some cases theGPSTEC values.

The variability study indicates that additional analysis are needed.

The analysis of the DTEC values can be useful to the behavior of theplasmaspheric electron content.

Ionospheric sounding carried out at the same site and simultaneouslywith GPS measurements could contribute to the betterunderstanding of the variations of the total electron content and tothe investigation of the topside ionosphere and plasmasphere.

ReferencesAdeniyi J.O., S.M. Radicella, Preliminary results of ITEC over anEquatorial station, Proceedings Task Force Activity 2001.

Brunini, C., A. Meza and A. Diaz, 2001. Regional Vertical Total ElectronContent using GPS observation, in Proceeding of the 2001 IAG ScientificAssembly (CD edition), 2-7 September 2001, Budapest, Hungary.

Ezquer, R.G., C. Brunini, A. Meza, F. Azpilicueta, M. Mosert, S.M. Radicella,Proceedings Task Force Activity 2001.

Huang X. and Bodo W. Reinisch, Vertical Electron Density Profiles fromDigisonde Ionograms the Average Representative Profiles, Annali diGeofisica, Vol XXXIX, 4, pp 751-756, 1996

Reinisch, Bodo W., and X. Huang, Automatic Calculation of ElectronDensity Profiles from Digital Ionograms, 3, Proceedings of BotomsideIonograms, Radio Science, 18, 477, 1983.

Mosert, M., D. Buresova, R. Ezquer, G. Mansilla, Behaviour of thebottomside electron density profile over Pruhonice, Adv. Space Res., 34,1982-1989, 2004.

Muchas Gracias!

Barreal, San Juan, ArgentinaSunset

The La Plata ionospheric model, described in details by Brunini et al. (2001), is used in this paperto obtain vertical total electron content (VTEC) from single-station GPS observations. Briefly, theVTEC is obtained using the so-called geometry-free linear combination of both L1 and L2 GPS carrierphase observations

where f1 and f2 are the observations, STEC is the line of sight slant total electron content, trand ts are the L1-L2 inter-frequencies electronic delays produced in the hardware of the receiverand the satellite (expressed in linear unities), a=0.105 m/TECU is a constant to convert linear intoTEC units and u is the L1-L2 combined measurement error. “Levelling” the ambiguous carrier phaseobservation to the less precise but unambiguous P-code observations, allow us to reduce the effectof carrier phase ambiguities in the geometry-free linear combination.

The ionosphere is approximated by a single shell of infinitesimal thickness with equivalentVTEC, located at 450 km above the Earth surface. An obliquity factor equals to 1/cosz´, being z´ thezenithal distance of the slant path at the piercing point of the signal on the shell, is used to convertvertical into slant TEC:

The spatial and temporal variability of the VTEC is modeled by an expansion in terms of thegeographic longitude (l) and latitude ( ) of the piercing point and the local time (t)

s1 2 r STEC

1STEC VTECcos z´

0 0VTEC( , , t) A(t) B(t) ( ) C(t) ( )

where l0 and 0 are the geographic coordinates of the GPS station. The time dependentcoefficient A(t), B(t) and C(t) are modeled as trigonometric polynomials.The computation of the VTEC is performed in two consecutive steps:1) Daily solutions are computed to estimate the coefficients of the trigonometric polynomials ofthe VTEC expansion and the inter-frequencies electronic delays for each satellite and thereceiver. All these parameters are jointly estimated by least squares, using the GPS observationsof each station. The sampling rate of the observation was 30 seconds and the elevation cutoffmask was set on 20 degrees. For those stations that have been processed by the InternationalGPS Service (IGS) and for all satellites, the inter-frequencies electronic delay estimates wereconstrained, within a given a-priori standard deviation, to be equal to the values provided by theIGS (Hernández-Pajares, 2003).2) The (previously) estimated inter-frequencies electronic delays are used to correct the GPSobservations and to obtain the VTEC for every observed piercing point, using the followingexpression:

To reduce errors due to the obliquity factor, only observations with a zenithal distance lowerthan 25 degree were considered in this second step.

s1 2 r

cos z´VTEC