Journal of Atmospheric and Solar-Terrestrial Physics 74 (2012) 217–223

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Journal of Atmospheric and Solar-Terrestrial Physics

1364-68

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jastp

Variations in equatorial F2-layer parameters and comparison with IRI-2007during a deep solar minimum

C.C. Lee a,n, B.W. Reinisch b

a General Education Center, Ching-Yun University, Jhongli City, Taoyuan County, Taiwanb Center for Atmospheric Research, University of Massachusetts, Lowell, MA, USA

a r t i c l e i n f o

Article history:

Received 27 January 2011

Received in revised form

11 November 2011

Accepted 12 November 2011Available online 21 November 2011

Keywords:

Ionosphere

Equatorial ionosphere

Ionospheric dynamics

Modeling and forecasting

26/$ - see front matter & 2011 Elsevier Ltd. A

016/j.jastp.2011.11.002

esponding author. Tel.: þ886 3 4581196x653

ail address: cclee@cyu.edu.tw (C.C. Lee).

a b s t r a c t

During the deep solar minimum between 23rd and 24th solar cycles, we study the variations in

F2-layer peak electron density (NmF2), its height (hmF2), and the F2-layer thickness parameter (B0) at

Jicamarca, and compare the observation with IRI-2007 predication. The observation results show that

the hmF2, NmF2, and B0 are greater and smaller during daytime and nighttime, respectively.

The sunrise peaks in hmF2 and B0 appear in all months. However, the post-sunset peaks do not occur

in hmF2 and B0, except hmF2 in the equinox months. Seasonal variations show that the daytime hmF2

and NmF2 are greater in the equinox and summer months, while the daytime B0 are slightly greater in

the equinox and winter months. For the comparison of observation with IRI-2007, the observed hmF2 is

close to the predicted one during 19:00–23:00 LT. The NmF2 of URSI map and observation are close to

each other during 00:00–13:00 LT, while the CCIR map modeled value generally agrees with the

observed ones. Regarding the B0 comparison, the evident discrepancy between observation and

Gulyaeva option is the locations of annual maxima. For the other option, the daily B0 peak is much

broader in the IRI-B0-table option than observation. The CCIR map and Gulyaeva option produce a

slightly better predication of NmF2 and B0, respectively. In contrast to the comparison results in

preceding solar minimum, IRI-2007 provides a worse predication in the deep solar minimum.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The International Reference Ionosphere (IRI) project wasinitiated by the Committee on Space Research (COSPAR) and bythe International Union of Radio Science (URSI) in the late sixties.For almost 5 decades, IRI has been improved and updated by ajoint working group of COSPAR and URSI. Based on the worldwideavailable data observed by ground-based and space-born instru-ments, IRI can provide the ionospheric scientists an internationalstandard of ionospheric parameters. Therefore, IRI is now a widelyused empirical mode of the ionosphere (Bilitza, 1990, 2001;Bilitza and Reinisch, 2007).

In the equatorial ionosphere, many works have applied obser-vations and IRI to study the behaviors of F2-layer in differentlongitudinal sectors. The F2-layer parameters, usually used toinvestigate the F2-layer, are the density (NmF2) and height(hmF2) of F2-layer peak, and F2-layer bottomside parameter(B0). In West Africa sector, many studies have compared theobserved F2-layer parameters with the IRI modeled results atIbadan, Nigeria (7.41N, 3.91E, dip latitude: 6.31S), Ouagadougou,

ll rights reserved.

4; fax: þ886 3 4684024.

Burkina Faso (12.41N, 1.81W, dip latitude: 5.91N), and Korhogo,Cote d’Ivoire (9.31N, 5.41W, dip latitude: 0.71S) (Adeniyi andAdimula, 1995; Adeniyi and Radicella, 1998a, 1998b; Adeniyiet al., 2003; Obrou et al., 2003; Adeniyi et al., 2008; Ehinlafa et al.,2010). In South America, the F2-layer parameters obtained atJicamarca, Peru (121S, 76.91W, dip latitude: 1.01N), as well as S~aoLuı́s (2.31S, 44.21W, dip angle: �0.51) and Palmas (10.21S, 48.21W,dip latitude: 6.61S), Brazil, were compared with the IRI results(Reinisch and Huang, 1996; Abdu et al., 2004; Batista and Abdu,2004; Lee and Reinisch, 2006; Lee et al., 2008; de Jesus et al.,2011). Recently, in Southeast Asia, Wichaipanich et al. (2010)compared the F2-layer parameters between observations and IRIat Chumphon (10.71N, 99.41E, dip 3.22), Thailand.

In the previous works above, the results of observation and IRIhave been reported in different solar epochs, but not in a deepsolar minimum. The recent (23/24) solar minimum, between the23rd and 24th solar cycles, provides an opportunity to examinethe variations in the equatorial F2-layer during extremely lowersolar activity. The F2-layer parameters are observed by theJicamarca digisonde from May 2008 to April 2009. During thisperiod, the monthly values of sunspot number and solar flux(F10.7) were generally less than 4 and 70, respectively. The solarcycle 24 began in December 2008 with the monthly smoothedsunspot number at 1.7. This study further compares the observed

C.C. Lee, B.W. Reinisch / Journal of Atmospheric and Solar-Terrestrial Physics 74 (2012) 217–223218

results with the modeled ones of IRI-2007, and tests the reliabilityof IRI during the deep solar minimum.

2. Data analysis

The observed F2-layer parameters (NmF2, hmF2, and B0) inthis study are obtained from the ionograms of Jicamarca digi-sonde (121S, 76.91W), located near the dip equator. The Jicamarcaionograms are downloaded from the Digital Ionogram DataBase(DIDBase) and the ionograms are manually edited with the SAO-Explorer software package. It is noted that the SAO (StandardArchiving Output) format is a data format to be used fordissemination and archiving of scaled ionogram data (see http://ulcar.uml.edu). NmF2 is estimated from the critical plasmafrequency, foF2, of the F2-layer by NmF2 (el/m3)¼1.24�1010

(foF2/MHz)2. hmF2 and B0 are derived using the true heightinversion algorithm NHPC (Huang and Reinisch, 2001) imbeddedin the SAO-Explorer (Reinisch, 1996; Reinisch and Huang, 1998).In order to investigate the F2-layer behaviors during extremelylow solar activity, the data under geomagnetic quiet-conditionsfrom May 2008 to April 2009 are selected. The geomagnetic quietcondition is defined as SKpr24, where SKp is the sum of theeight 3-hourly Kp indices for a day.

Regarding the modeled values, the hmF2, NmF2, and B0 arepredicted by the IRI-2007 (Bilitza and Reinisch, 2007), the newestversion of IRI. hmF2 is modeled by IRI-2007 through its closecorrelation with the propagation factor M(3000)F2 (Bilitza et al.,1979). For NmF2, the model offers two maps: (1) the InternationalRadio Consultative Committee (CCIR) (CCIR, 1966); (2) the Inter-national Union of Radio Science (URSI) (Rush et al., 1989). The IRI-2007 also provides two options for B0: (1) Gulyaeva’s model(Gulyaeva, 1987); (2) the B0-table (Bilitza et al., 2000). In thisstudy, we compared the observed values with the CCIR map forhmF2, with both the URSI and CCIR maps for NmF2, and with boththe Gulyaeva and B0-table options for B0. It is noted that the IRIuses 12-month running means of the ionospheric IG index forNmF2 (foF2) and of the sunspot number for hmF2 (Bilitza, 1990).Furthermore, the observed Ap and F10.7 indices are inputted inthe IRI modeling to consider the month-to-month variability.

Because the ionosphere can be disturbed even under quietmagnetic conditions (see Gulyaeva and Stanislawska, 2010 andreferences therein), and the CCIR and URSI maps are based onmonthly median data, the monthly median values for each monthcalculated from the observed and modeled data under geomag-netic quiet-conditions are applied in this study.

Fig. 1. Monthly median values of hmF2 under geomagnetic quiet-conditions for

the (a) equinox, (b) summer, and (c) winter months. The equinox months are

September (solid line), October (dashed line), March (circle line), and April

(asterisk line). The summer months are November (bold solid line), December

(plus sign line), January (square line), and February (diamond line). The winter

months are May (bold dash-dot line), June (bold dashed line), July (point line), and

August (cross line).

3. Results and discussion

3.1. Variations in hmF2, NmF2, and B0

In Fig. 1, the monthly median values of hmF2 for three seasonsare presented. The three seasons are the equinox (March, April,September, and October), summer (November–February), andwinter (May–August) months. For the equinox months (Fig. 1a),the hmF2 values have a peak existing at 05:00–06:00 LT, whichare near the time of sunrise. According to Lee et al. (2008), thisincrease in hmF2 altitudes is because the solar production occursat higher altitudes during the sunrise period when the solarzenith angle is greater. Then, hmF2 starts to increase at07:00 LT, and has a maximum value at noontime. During thepost-sunset period, there is a peak at 19:00 LT for all months.After the post-sunset peak, hmF2 begins to decrease duringnighttime. The diurnal variation of hmF2 is mainly caused bythe vertical E�B drift velocity. In the equatorial ionosphere, the

vertical E�B drift velocity is upward and downward duringdaytime and nighttime, respectively (Fejer et al., 1991). And,during the sunset period, the pre-reversal enhancement (PRE) ofthe upward E�B drift velocity (called the PRE upward velocity) isresponsible for the post-sunset peak of hmF2. The PRE upwardvelocity is formed by the development of the F region dynamoelectric field under the post-sunset decay of the E-layer conduc-tivity (Farley et al., 1986).

In the summer months (Fig. 1b), the diurnal variation in hmF2is generally similar to that in the equinox months, exceptNovember. It is noted that there are data gaps during 00:00–05:00 LT and 01:00–04:00 LT for January and December,

Table 1Daytime maximum values of hmF2, NmF2, and B0 for three seasons in the 23/24 and 22/23 solar minimum.

The values in the 22/23 solar minimum are adapted from Lee et al. (2008).

Season 23/24 solar minimum 22/23 solar minimum

hmF2 Equinox 325–345 km 350–355 km

Summer 332–349 km 335–354 km

Winter 319–326 km 350–364 km

NmF2 Equinox 5.0–8.1�1011 el/m3 7.8–9.4�1011 el/m3

Summer 6.1–8.8�1011 el/m3 9.5–10.5�1011 el/m3

Winter 4.4–5.8�1011 el/m3 5.4–5.8�1011 el/m3

B0 Equinox 190–206 km (Mar, Apr, Oct) 242 km (Sep) 198–223 km

Summer 158–187 km (Jan, Nov, Dec) 229 km (Feb) 152–199 km

Winter 190–213 km 235–282 km

Table 2Post-sunset peak values of hmF2 for three seasons in the 23/24 and 22/23 solar

minimum. The values in the 22/23 solar minimum are adapted from Lee et al.

(2008).

Season 23/24 solar minimum 22/23 solar minimum

hmF2 Equinox 330–347 km 331–364 km

Summer – 349–366 km

Winter – –

C.C. Lee, B.W. Reinisch / Journal of Atmospheric and Solar-Terrestrial Physics 74 (2012) 217–223 219

respectively, due to the appearance of spread F. The post-sunsetpeaks in hmF2 do not occur in December–February. This featureindicates that the PRE upward velocity might be too small touplift the F2-layer in these months under extremely low solaractivity. During 10:00–18:00 LT, the difference in hmF2 existsbetween November and other months. This hmF2 behavior inNovember suggests that the vertical velocities are downward andupward during 10:00–14:00 LT and 14:00–1800 LT, respectively.Regarding the winter months, the diurnal variation in hmF2 issimilar to that in other seasons. It is noted that a data gap existsduring 05:00–06:00 LT for July, because the NmF2 is too small tobe detected by the Jicamarca digisonde. In this season, the post-sunset peaks do not appear in hmF2.

Overall, during daytime, the hmF2 values are greater in theequinox and summer months, but smaller in the winter months.In order to compare the results between the 23/24 and preceding(22/23) (Lee et al., 2008) solar minima, Tables 1 and 2 present thedaytime maximum values of three F2-layer parameters and thepost-sunset peak values of hmF2, respectively. It is found that inTable 1, the daytime hmF2 values in all three seasons are smallerin the 23/24 solar minimum. In Table 2, the magnitudes of thepost-sunset hmF2 peaks in the equinox months are also smaller inthe 23/24 solar minimum. And, the post-sunset hmF2 peaksappeared in December–February in the 22/23 solar minimum(Lee et al. 2008), but not in this study. For the winter months, thepost-sunset peaks in hmF2 were not found the 22/23 solarminimum, either (Lee et al., 2008). These results are expectable,since the solar activities are lower in the deep solar minimumthan in the 22/23 solar minimum.

For NmF2, the monthly median values of three seasons areshown in Fig. 2. In Fig. 2a, the NmF2 values in the equinox monthshave a minimum at around 05:00 LT. Then, they start to increase,and have two maxima occurring at around 10:00 and 17:00 LT.After the sunset time, the NmF2 values decrease from 18:00 to05:00 LT. This diurnal behaviors of NmF2 are primarily related tothe solar production and the vertical E�B velocity (e.g. Lee andReinisch, 2006; Lee et al., 2008). During daytime (nighttime), theincrease (decrease) in NmF2 is caused by the larger (smaller)production of solar radiations and the smaller (larger) chemicallosses at higher (lower) altitudes. In addition, at noontime, the

feature called ‘noon bite-out’ is found in this season. Rajaram andRastogi (1977) proposed that the feature of ‘noon bite-out’ ismade mainly by the upward E�B velocity and the neutral windthat blows plasma away from the equator into both hemispheresduring daytime.

In Fig. 2b, it is found that the diurnal variations of NmF2 inDecember, January, and February are similar to those in theequinox months. Therefore, this kind of diurnal variation ismainly caused by the mechanisms, forming the diurnal NmF2variation in the equinox months (e.g. Lee and Reinisch, 2006; Leeet al., 2008). For November, the feature of ‘noon bite-out’ isrelatively obscure. In Fig. 1b, the descending hmF2 during10:00–1400 LT suggests that the plasma might not be lifted tohigher altitudes where the neutral winds can blow plasma awayfrom the equator into both hemispheres. Regarding the wintermonths, the diurnal variations of NmF2 are qualitatively similarto that in the equinox months. The daytime NmF2 values in thisseason are evidently smaller than that in other seasons. Since thedaytime hmF2 values are lower in this season (Fig. 1c), the largerchemical losses would account for the smaller NmF2, in additionto the smaller solar production. Additionally, between the 23/24and 22/23 solar minimums (Lee et al., 2008), it is also expectedthat the NmF2 values are smaller in this solar minimum (seeTable 1).

Fig. 3 displays the monthly median values of B0 for threeseasons. For the equinox (Fig. 3a), the small peak at 06:00 LT isfound in this season. The mechanism for the sunrise B0 peak isthe same as that for the sunrise hmF2 peak. Furthermore, it isfound that in Fig. 3a, the B0 values in the equinox months have asimple diurnal variation. The B0 values start to increase at thepre-sunrise time, and have a maximum at noontime. Then, theybegin to decrease at around 13:00 LT. This diurnal variationwould be related to the behaviors of solar production and verticalE�B velocity, which are primarily responsible for the diurnalNmF2 variations (e.g. Lee and Reinisch, 2006; Lee et al., 2008).In this season, the ‘noon bite-out’ is not found in the B0parameter. This could be because the plasma at low altitudesare unaffected by the movement mechanism, or the solar radia-tion production would be still large enough to replenish theplasma density in the F2 layer bottomside (e.g. Lee and Reinisch,2006; Lee et al., 2008).

In Fig. 3b, the diurnal variations in B0 for December–Februaryare similar to that for the equinox months. However, in Novem-ber, the daytime B0 variation apparently differs from that forother months. It is found that there are two maxima occurring11:00 and 16:00 LT. This daytime behavior of B0 would be causedby the mechanism that forms the daytime hmF2 behavior in thismonth (Fig. 1b). For the winter months (Fig. 3c), the diurnalvariations in B0 are similar to that for the equinox months. For allthree seasons, the post-sunset peaks do not exist in the B0 values.

Fig. 2. Monthly median values of NmF2 under geomagnetic quiet-conditions for

the (a) equinox, (b) summer, and (c) winter months. The presentations of lines are

same as in Fig. 1.

Fig. 3. Monthly median values of B0 under geomagnetic quiet-conditions for the

(a) equinox, (b) summer, and (c) winter months. The presentations of lines are

same as in Fig. 1.

C.C. Lee, B.W. Reinisch / Journal of Atmospheric and Solar-Terrestrial Physics 74 (2012) 217–223220

Overall, the B0 values during daytime are slightly greater in theequinox and winter months than in the summer months. Besides,except February and September (Table 1), the daytime B0 valuesare generally smaller in this solar minimum than the 22/23 solarminimum (Lee et al., 2008). According to Lee (2011), thisanomalous feature in B0 of September 2008 is mainly due tothe difference in the rate of decrease of plasma density betweenat the F2-peak and 200 km.

3.2. Comparison between observation and IRI-2007

Fig. 4 presents the hmF2 values observed by the Jicamarcadigisonde (Fig. 4a) and modeled by IRI-2007 with CCIR option(IRI-CCIR) (Fig. 4b), as well as the difference, DhmF2CCIR, (Fig. 4c)between observation and IRI-CCIR. In Fig. 4c, it is found that the IRI-

CCIR hmF2 predictions are close to the observed values during19:00–23:00 LT for most months. However, during 07:00–18:00 LT,the negative values indicate that the IRI-CCIR model overestimatesthe hmF2 values at all months. The results are similar to thosein the previous studies during low solar activity. Adeniyi andAdimula (1995), Adeniyi et al. (2003), Ehinlafa et al. (2010), andWichaipanich et al. (2010) found that the IRI modeled values weresignificantly higher than the observed hmF2 during daytime. On theother hand, Obrou et al. (2003) Batista and Abdu (2004), and Leeet al. (2008) reported that the observed hmF2 is generally close tothe IRI-CCIR hmF2 model prediction during daytime under low solaractivity. Furthermore, between 00:00 and 06:00 LT, IRI-CCIR under-estimates the hmF2 values, especially in the winter months. Theseunderestimations are mainly because the IRI-CCIR does not modelthe sunrise peaks in hmF2. The post-sunset hmF2 peaks in the

C.C. Lee, B.W. Reinisch / Journal of Atmospheric and Solar-Terrestrial Physics 74 (2012) 217–223 221

equinox months are not predicted by the IRI-CCIR model, either.In the previous studies under low solar activity, the sunrise and post-sunset hmF2 peaks were not produced by the IRI-CCIR model(Adeniyi and Adimula, 1995; Adeniyi et al., 2003; Obrou et al.,2003; Batista and Abdu, 2004; Lee et al., 2008; Ehinlafa et al., 2010;Wichaipanich et al., 2010).

Fig. 4. Diurnal variations in hmF2 of (a) the Jicamarca digisonde and (b) IRI-CCIR, as

difference of hmF2 between observation and IRI-CCIR.

Fig. 5. Diurnal variations in NmF2 of (a) the Jicamarca digisonde observations, (b) I

DNmF2CCIR (e) are the differences of NmF2 between observation and IRI-URSI and IRI-

For NmF2, the observed values (Fig. 5a) and IRI-2007 values ofURSI (IRI-URSI) (Fig. 5b) and CCIR (IRI-CCIR) (Fig. 5d) maps areshown in Fig. 5. The NmF2 differences, DNmF2URSI and DNmF2CCIR,between observation and IRI-2007 are also shown in Fig. 5c and e,respectively. For the IRI-URSI map (Fig. 6c), the model results agreewith the observation ones generally during 00:00–13:00 LT for most

well as (c) the DhmF2CCIR during May 2008–April 2009, where DhmF2CCIR is the

RI-URSI, and (d) IRI-CCIR during May 2008–April 2009. The DNmF2URSI (c) and

CCIR model predictions, respectively.

Fig. 6. Diurnal variations in B0 of (a) the Jicamarca digisonde, (b) IRI-Gulyaeva, and (d) IRI-B0-table during May 2008–April 2009. The DB0Gulyaeva (c) and DB0B0-table

(e) are the differences of B0 between observations and IRI-Gulyaeva and IRI-B0-table model predictions, respectively.

C.C. Lee, B.W. Reinisch / Journal of Atmospheric and Solar-Terrestrial Physics 74 (2012) 217–223222

months. The DNmF2URSI values are significantly positive at 09:00–11:00 LT of March and at 01:00–02:00 LT of April. The significantnegative differences are found during 08:00–14:00 LT of Februaryand September–December, and during 14:00–23:00 LT for mostmonths. Regarding the IRI-CCIR map, the modeled values aregenerally close to the observed ones. The positive DNmF2CCIR valuesare significant at 14:00–16:00 LT for April. The negative differencesare found generally during at 08:00–20:00 LT for February, 16:00–23:00 LT for March, and 09:00–10:00 LTfor October–December.Some of these results are similar to that in the earlier studies underlow solar activity (Adeniyi and Adimula, 1995; Batista and Abdu,2004; Lee et al., 2008; Wichaipanich et al., 2010). Overall, both mapshave a better prediction in the winter months. This result isconsistent with those in Adeniyi et al. (2003) and Lee et al. (2008),but is different from that in Adeniyi and Adimula (1995). Moreover,between the two maps, the observed values agree slightly betterwith the IRI-CCIR values than with the IRI-URSI ones. Adeniyi et al.(2003), Lee et al. (2008), and Wichaipanich et al. (2010) alsoreported that the IRI-CCIR predication is slightly better during solarminimum.

In Fig. 6, the B0 values obtained from the Jicamarca observa-tion (Fig. 6a) and predicted by the Gulyaeva (IRI-Gulyaeva)(Fig. 6b) and B0-table (IRI-B0-table) (Fig. 6d) options of IRI-2007, as well as the B0 differences (DB0Gulyaeva and DB0B0-table)(Fig. 6c and e) are displayed. For the IRI-Gulyaeva options, theobserved values are close to the modeled ones in January–Apriland October. For May–September, because the apparent differ-ence between observation and IRI-Gulyaeva is the locations ofannual maxima during daytime, the positive values are significant

during 09:00–16:00 LT. For the IRI-B0-table option, the amplitudeof the daily B0 variation is close to the observed values for mostmonths, except for February, June, and September. The significantdifference between observation and IRI-B0-table is that the dailyB0 peak is much broader for IRI-B0-table. Moreover, the sunrisepeaks are not predicted by both the IRI-Gulyaeva and IRI-B0-tableoptions for all months. These results are similar to the previousstudies under low solar activity (Adeniyi and Radicella, 1998a;Batista and Abdu, 2004; Lee et al., 2008). Furthermore, theIRI-Gulyaeva generally has a better predication for the B0 para-meter. This differs from the results of Lee et al. (2008), in whichthe IRI-B0-table provided a slightly better simulation.

Following the comparison, we contrast the comparison resultsat Jicamarca in this study and in Lee et al. (2008). For hmF2, thedifferences between observation and IRI-CCIR of these twostudies are close to each other. However, for NmF2 and B0, IRIprovides a better prediction in Lee et al. (2008), except IRI-Gulyaeva. This result indicates that the IRI might be not able topredict the equatorial F2-layer correctly during a deep solarminimum. It would not be surprising since the representativedataset, coming from a comparable solar minimum, is notincluded in IRI. Nevertheless, the results in this study can helpto improve the IRI model in this new condition.

4. Conclusion and summary

The ionograms observed by the Jicamarca digisonde betweenMay 2008 and April 2009 are used to investigate the equatorial

C.C. Lee, B.W. Reinisch / Journal of Atmospheric and Solar-Terrestrial Physics 74 (2012) 217–223 223

F2-layer during a deep solar minimum. The monthly median valuesof NmF2, hmF2, and B0 of the equatorial F2-layer under geomag-netic quiet-conditions are analyzed. Further, these observed resultsare compared with the IRI-2007 predictions.

For observations, hmF2 has a peak near the sunrise time for all 12months. Then, the higher hmF2 values during the daytime are relatedto the upward vertical E�B velocity, while the lower hmF2 valuesduring nighttime are associated to the downward velocity. During thesunset period, a post-sunset peak in hmF2, produced by the PREupward velocity, appears only in the equinox months. For the seasonalvariation in hmF2, the daytime values are greater in the equinox andsummer months, but smaller in the winter months. Regarding thediurnal variation in NmF2, the greater values in daytime are formedmainly by the higher hmF2, as well as the associated greaterproduction and smaller loss rates, while the smaller NmF2 in night-time are primarily due to the lower hmF2, smaller production rate,and greater loss rates. Furthermore, the seasonal variation in NmF2also shows that the daytime values are greater in the equinox andsummer months, but smaller in the winter months. For the B0parameter, a peak exists in the sunrise period for all months. Thecauses for the diurnal variation in B0 are same as that in NmF2. Duringthe sunset period, the post-sunset peak in B0 is not found in anymonths. For the seasonal variation, the B0 values during daytime areslightly greater in the equinox and winter months than in the summermonths. Overall, the daytime values of hmF2, NmF2, and B0 aresmaller for this deep solar minimum than the 22/23 solar minimum.

The comparison of hmF2 between observation and IRI-CCIR, theobserved hmF2 generally agrees with the IRI-CCIR hmF2 predictionduring 19:00–23:00 LT for most months. The significant negativedifferences are found during 07:00–18:00 LT at all months. Moreover,the sunrise peaks for all months and the post-sunset peaks for theequinox months are not modeled by IRI-CCIR. For the NmF2comparison, the modeled values of IRI-URSI map are close to thatof observation generally during 00:00–13:00 LT for most months. Thedifferences are significantly positive during 09:00–11:00 LT forMarch, but significantly negative during 14:00–23:00 LT for mostmonths. For the other map, the IRI-CCIR map modeled value generallyagrees with the observed value. Overall, IRI-CCIR produces a slightlybetter prediction than IRI-URSI at Jicamarca. Regarding the B0comparison, the apparent difference between observation and IRI-Gulyaeva is the locations of annual maxima. For the IRI-B0-tableoption, the significant differences come from the broader daily B0peak in the model results. IRI-2007 does not model the sunrise peaksin B0 parameter for either B0 option. In general, the prediction of IRI-Gulyaeva option is slightly better than the other one. Besides, atJicamarca, the IRI predications are generally better in the 22/23 solarminimum than this deep solar minimum.

Since 1920s, the 23/24 solar minimum is the deepest one. Theobservations by ground-based and space-born instruments underextremely low solar activities were not included in IRI. Therefore, it isnot surprising that IRI cannot predict the ionosphere well during thisdeep solar minimum. In order to improve the reliability of IRI, thecomparisons of worldwide observed data with IRI-2007 results during2008–2009 should be made in future to know what kind ofmodification is required.

Acknowledgments

CCL was supported by the grant of National Science Council NSC99-2111-M-231-002-MY3. BWR was supported by the AF grant#FA8718-06-C-0072. The authors would like to thank Dr. J. Chau ofthe Jicamarca Radio Observatory, Peru, for the high quality ionogramdata and UML for access to DIDBase (http://ulcar.uml.edu/DIDBase/).The authors also thank the National Geophysical Data Center (NGDC)(http://www.ngdc.noaa.gov/) for providing data of Ap, sunspot

number, and F10.7 solar flux, and NASA’s National Space ScienceData Center (NSSDC) for providing the IRI-2007 model.

References

Abdu, M.A., Batista, I.S., Reinisch, B.W., Carrasco, A.J., 2004. Equatorial F-layerheights, evening prereversal electric field, and night E-layer density in theAmerican sector: IRI validation with observation. Advances in Space Research34, 1953–1965.

Adeniyi, J.O., Adimula, I.A., 1995. Comparing the F2 layer model of IRI withobservations at Ibadan. Advances in Space Research 15, 141–144.

Adeniyi, J.O., Radicella, S.M., 1998a. Diurnal variation of ionospheric profileparameters B0 and B1 for an equatorial station at low solar activity. Journalof Atmospheric and Solar-Terrestrial Physics 60, 381–385.

Adeniyi, J.O., Radicella, S.M., 1998b. Variation of bottomside profile parameters B0and B1 at high solar activity for an equatorial station. Journal of Atmosphericand Solar-Terrestrial Physics 60, 1123–1127.

Adeniyi, J.O., Bilitza, D., Radicella, S.M., Willoughby, A.A., 2003. Equatorial F2-peakparameters in the IRI model. Advances in Space Research 31, 507–512.

Adeniyi, J.O., Radicella, S.M., Adimula, I.A., Willoughby, A.A., Oladipo, O.A., Olawepo,O., 2008. Validation of B0 and B1 in the IRI-2001 model at low solar activity forIlorin an equatorial station. Advances in Space Research 42, 691–694.

Batista, I.S., Abdu, M.A., 2004. Ionospheric variability at Brazilian low andequatorial latitudes: comparison between observation and IRI model.Advances in Space Research 34, 1894–1900.

Bilitza, D., 1990. International Reference Ionosphere 1990, NSSDC 90-22, Green-belt, Maryland, USA.

Bilitza, D., 2001. International reference ionosphere 2000. Radio Science 36, 261–275.Bilitza, D., Reinisch, B.W., 2007. International Reference ionosphere 2007:

Improvements and new results. Advances in Space Research 42, 599–609.Bilitza, D., Sheikh, N.M., Eyfrig, R., 1979. A global model for the height of the F2-

peak using M(3000)F2 values from the CCIR mumerical map. Telecommunica-tion Journal 46, 549–553.

Bilitza, D., Radicella, S.M., Reinisch, B.W., Adeniyi, J.O., MosertGonzalez, M.E.,Zhang, S.R., Obrou, O., 2000. New B0 and B1 models for IRI. Advances in SpaceResearch 25, 89–95.

CCIR, 1966. Comite Consultatif International des Radiocommunications, Report340-1, 340-6, International Telecommunication Union, Gen�eve, Switzerland.

de Jesus, R., Sahai, Y., Guarnieri, F.L., Fagundes, P.R., de Abreu, A.J., Pillat, V.G., Lima,W.L.C., 2011. F-region ionospheric parameters observed in the equatorial andlow latitude regions during medium solar activity in the Brazilian sector andcomparison with the IRI-2007 model results. Advances in Space Research 47,718–728.

Ehinlafa, O.E., Falaiye, O.A., Adeniyi, J.O., 2010. Comparison of observed hmF2 andIRI 2007 model with M(3000)F2 estimation of hmF2 at low solar activity for anequatorial station. Advances in Space Research 46, 89–93.

Farley, D.T., Bonelli, E., Fejer, B.G., 1986. The pre-reversal enhancement of thezonal electric field in the equatorial ionosphere. Journal of GeophysicalResearch 91, 13723–73728.

Fejer, B.G., de Paula, E.R., Gonzales, S.A., Woodman, R.F., 1991. Average vertical and zonalplasma drift over Jicamarca. Journal of Geophysical Research 96, 13901–13906.

Gulyaeva, T.L., 1987. Progress in ionospheric informatics based on electron densityprofile analysis of ionogram. Advances in Space Research 7, 39–48.

Gulyaeva, T.L., Stanislawska, I., 2010. Magnetosphere-associated storms and theautonomous storms in the ionosphere-plasmasphere environment. Journal ofAtmospheric and Solar-Terrestrial Physics 72, 90–96.

Huang, X., Reinisch, B.W., 2001. Vertical electron content from ionograms in realtime. Radio Science 36 (2), 335–342.

Lee, C.C., 2011. Equatorial B0 anomaly in September under extremely low solar activity.Journal of Geophysical Research 116, A05325. doi:10.1029/2010JA016394.

Lee, C.C., Reinisch, B.W., 2006. Quiet-condition hmF2, NmF2, and B0 variations atJicamarca and comparison with IRI-2001 during solar maximum. Journal ofAtmospheric and Solar-Terrestrial Physics 68, 2138–2146.

Lee, C.C., Reinisch, B.W., Su, S.Y., Chen, W.S., 2008. Ouiet-time variations of F2-layer parameters at Jicamarca and comparison with IRI-2001 during solarminimum. Journal of Atmospheric and Solar-Terrestrial Physics 70, 184–192.

Obrou, O.K., Bilitza, D., Adeniyi, J.O., Radicella, S.M., 2003. Equatorial F2 layer peakheight and correlation with vertical ion drift and M(3000)F2. Advances inSpace Research 31, 513–520.

Rajaram, G., Rastogi, R.G., 1977. Equatorial electron densities-seasonal and solarcycle changes. Journal of Atmospheric and Terrestrial Physics 39, 1175–1182.

Reinisch, B.W., 1996. Modern ionosondes. In: Kohl, H., Ruester, R., Schlegel, K.(Eds.), Modern Radio Science. , European Geophysical Society, Katlenburg-Lindau, Germany, pp. 440–458.

Reinisch, B.W., Huang, X., 1996. Low latitude digisonde measurements andcomparison with IRI. Advances in Space Research 18, 5–12.

Reinisch, B.W., Huang, X., 1998. Finding better B0 and B1 parameters for the IRI F2profile function. Advances in Space Research 22, 741–747.

Rush, C., Fox, M., Bilitza, D., Davies, K., McNamara, L., Stewart, F., PoKempner, M.,1989. Ionospheric mapping-an update of fF2 coefficients. TelecommunicationJournal 56, 179–182.

Wichaipanich, W., Supnithi, P., Ishii, M., Maruyama, T., 2010. Ionospheric variationat Thailand equatorial latitude station: comparison between observations andIRI-2001 model prediction. Advances in Space Research 45, 284–293.