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Advances in Space Research 54 (2014) 2151–2158

Variations in Electron Content Ratio and Semi-thickness Ratioduring LSA and MSA periods and some Cyclone Genesis Periods

using COSMIC satellite observations

Gopal Mondal a,⇑, Manojit Gupta b, Goutam Kumar Sen a

a School of Oceanographic Studies, Jadavpur University, Kolkata 700032, Indiab Department of Mathematics, Jadavpur University, Kolkata 700032, India

Received 26 February 2014; received in revised form 12 July 2014; accepted 17 July 2014Available online 29 July 2014

Abstract

In this study for the first time, COSMIC satellite data have been used to deduce values of ionospheric Electron Content Ratio (ECR)and Semi-thickness Ratio (Rtb) for Low Solar Activity (LSA) (2008) and Moderate Solar Activity (MSA) (2012) periods over the Indianlow-latitude (15–30�N) region with 80–95�E longitude. These two ratios provide sensitive information about bottom and topside iono-sphere for different geophysical conditions. Extraction of suspected patterns and discrepancies unfold that the deviations between ECRand Rtb values during LSA period are comparatively higher than that of MSA period when the diurnal variability in these two param-eters is flatter along with the diurnal-dips during pre-noon hours. The correlative relationship of ECR exhibits low association withNmF2 and anti-correlation with HmF2, whereas its correlation with Rtb is extremely high. During Cyclone Genesis Period (CGP) strongdips in ECR and Rtb values with respect to pre and post CGP occurred which helps to take decisive conclusion about the ionosphericvariations to be dominant through getting relatively higher Ne concentration in the bottom side part of the ionosphere.� 2014 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: ECR; Rtb; LSA; MSA; CGP

1. Introduction

The Ionosphere is considered as a shell-layered ofcharged particles surrounding the Earth above the heightof peak electron concentration in F2 layer (HmF2) whichseparates the ionosphere in bottom-side and topside parts.The knowledge of the vertical distribution of electron (Ne)concentration in the ionosphere is very important for theestimation of ionospheric effect on the Earth-space radiolink, remote sensing at L-band and evaluation of naviga-tion system, etc. (Sethi and Dabas, 2006). All these effectsare proportional to the Integrated Electron Content

http://dx.doi.org/10.1016/j.asr.2014.07.017

0273-1177/� 2014 COSPAR. Published by Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +91 9062882361.E-mail address: gopalmondal18@gmail.com (G. Mondal).

(IEC) which in turn intimately depends on Ne distributionalong the ray-path, connecting satellite to receivers. How-ever, IEC delivers one of the most important quantitativecharacteristic (Kenpankho et al., 2011) about the verticalframework of ambient ionosphere. DasGupta et al.(2007) have reported that large spatial gradient in TotalElectron Content (TEC; 1TECU = 1016 el/m2) variationsexist in the equatorial ionosphere. However, the Ne distri-bution sustained diurnal, latitudinal, longitudinal, seasonaland solar activity variations (e.g., Chen et al., 2009;Sripathi, 2012; Su et al., 1998). Gulyaeva (2007) haveshown that the Semi-thickness Ratio (Rtb) varies more reg-ularly than absolute value of the Semi-thickness (B0)parameter. It has been reported that the ionosphere alsoshows anomalous perturbation due to geomagnetic storm

2152 G. Mondal et al. / Advances in Space Research 54 (2014) 2151–2158

(Dabas et al., 1980; Gulyaeva, 2007), large magnitudeearthquake (Hsiao et al., 2009; Singh et al., 2010) and thun-derstorm activity (Rai et al., 2006), etc. Some earlier studies(e.g., DasGupta et al., 2007; Sethi et al., 2004; Sethi andDabas, 2006; Sripathi, 2012) have unfolded so many inter-esting characteristic of Indian-equatorial ionosphere.Though, the traditional ground-based measurements (e.g.,Sethi and Mahajan, 2002; Sun et al., 2012) are sufficientto determine the bottom-side ionospheric behaviour, themodern space-based measurements (Aragon-Angel et al.,2009) are essential to attest the topside variability in precisemanner (Reinisch and Huang, 1998). Due to the coinci-dence of Tropical Cyclone (TC) with geomagnetic stormthe authors in Afraimovich et al. (2008) could not manageto detect the disturbance due to the generation of typhoonSAOMAI. Lin (2012a, 2012b), Polyakova and Perevalova(2013) and Tian et al. (2010) also have analyzed the TECvariations in the low-latitude ionosphere during typhoons.Their analysis showed that the TEC in ionosphere near theTC genesis area has a large increase which had an impor-tant impact on the ionosphere. However, the variationsin IEC and B0 parameters characterised in bottom sideand topside ionosphere during some CGPs have not beenstudied so far. Incorporating the knowledge of recent stud-ies (Gulyaeva, 2007; Sethi et al., 2004; Sethi and Dabas,2006) and using modern space-based measurements(Aragon-Angel et al., 2009; Chuo et al., 2011; Liu et al.,2008, 2010, 2011) reasonable estimation of the inherentvariations in IEC and B0 parameters characterised in bot-tom side and topside ionosphere could be performed.

In the present paper an attempt has been made to studythe variations in IEC and B0 with the help of COSMICradio occultation (RO) measurements over the Indianlow-latitude (15–30�N) region with 80–95�E longitude dur-ing Low Solar Activity (LSA) period, Moderate SolarActivity (MSA) period and some CGPs. Section 2 describesdata and method of analysis. Result and Discussion arementioned in Section 3, and summary is given in Section 4.

2. Data and method of analysis

To examine the change in ionospheric parameters due tothe solar flux and geomagnetic storm activities, the solar10.7 cm flux F10.7 data and

PKp index data for year

2008 and 2012 are obtained from the World Data Centrewebsite (http://www.ukssdc.ac.uk) of UKSSDC, UK andfrom the websites (http://wdc.kugi.kyoto-u.ac.jp) ofKyoto, Japan, respectively. For the solar flux variabilitythe years, 2008 (F10.7 < 80) and 2012 (80 6 F 10:7 6 150)are considered as the LSA and MSA period respectively.Also, for the days free from geomagnetic storm, thoseRO-profiles have analyzed which are associated toP

Kp 6 30: The Constellation Observing System for Mete-orology, Ionosphere and Climate (COSMIC)/FormosaSatellite 3 [FORMOSAT-3 (F3/C)] observations providesnear about 2500 Vertical Electron Density (VED) profilesthroughout the globe per day (e.g., Ely et al., 2012;

Kakinami et al., 2012). For the present study, suitableVED profiles (namely “ionPrf” data in the website, a level2vertical profile data) for the years 2008 and 2012, obtainedfrom COSMIC website (http://www.cosmic.ucar.edu), aresorted out such that the geographical position (Lat, Lon)of Maximum Electron Density (NmF2) lies within the box-car region with the box size 15� (15–30�N, 80–95�E) lati-tude by longitude for noon time (10.00–14.00 LT) andmidnight time (20.00–02.00 LT). Thereafter, the Bottom-side Integrated Electron Content (BIEC) and Topside Inte-grated Electron Content (TIEC) are calculated by the fol-lowing formulae:

ðBIEC; TIECÞ ¼Z ðHmF 2;TaltÞ

ðBalt;HmF 2ÞNeðzÞdz ð1Þ

where, Balt, is the Mean Sea Level Altitude (MSL_Alt) ofthe bottom point of the profiles; HmF2, the MSL_Alt ofNmF2 in F layer; and Talt, the MSL_Alt of the top pointof the profiles. For COSMIC satellite, the Talt generallyconfined in between 750–800 km altitude.

Now we define Electron Content Ratio (ECR) asfollows:

ECR ¼ TIEC=BIEC ð2Þ

Meanwhile, the Bottom-side Semi-thickness (B0bot) andTopside Semi-thickness (B0top) parameters are calculatedfor the half-peak density (Ne = 0.5 * NmF2) height belowand above the F2 peak (HmF2) using the formulae(Gulyaeva, 2007):

B0bot ¼ HmF 2� h0:5bot ð3ÞB0top ¼ h0:5top � HmF 2 ð4Þ

Hence, the Semi-thickness Ratio (Rtb) is calculatedusing B0bot and B0top with the formula (Gulyaeva, 2007):

Rtb ¼ B0top=B0bot ð5Þ

The Rtb provides sensitive information about the bot-tom side and topside ionosphere for different geophysicalconditions. In this study the diurnal, seasonal and solaractivity variations of ECR and Rtb for LSA and MSA per-iod are investigated using the COSMIC satellite VED pro-files. Also, the comparative variations of these two ratiosduring (15 days period) three deleterious cyclones Nargis(27th April–3rd May 2008), Rashmi (25–27th October2008) and Nilam (28th October–1st November 2012) gen-erated over Bay of Bengal are analysed. All the VED pro-files (80, 56 and 23 profiles are studied for Nargis, Rashmiand Nilam) in the 15� box size region including the respec-tive cyclone track have been considered to analyse.

3. Result and discussion

3.1. Solar flux andP

Kp index variations

To study the effects of solar activity and geomagneticactivity, actual data of

PKp index and F10.7 are plotted

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in Fig. 1. Some earlier studies have admitted that solaractivity (Lee and Reinisch, 2006, 2007) and geomagneticactivity (Dabas et al., 1980; Park et al., 2010; Sun et al.,2012) have obvious association with ionospheric variabil-ity. The left panel (Fig. 1a), however, indicates the solaractivity variations for the LSA (2008) and MSA (2012)periods. Analyzing Fig. 1a it is observed that the solaractivity for the MSA period is 2–2.5 times higher than thatof LSA period with periodicity varying from 21 days to33 days (But, maximum numbers of phases are of 26 and29 days). Also, during beginning of the year, 2012, twolong-duration low-phases are seen. In this year, consider-able fluctuations of solar activity occurred during Juneand July months, whereas for the LSA year (2008) theSun is unprecedently clam except 2–3 cases (one at begin-ning of January and other at beginning of April). In 2008the solar EUV flux level lower by nearly 15%(Lastovicka, 2013; Lean et al., 2011) than that of previoussolar minimum (a periodical phenomenon generally associ-ated with the period of 11 year), happened in 1996. Mini-mum solar EUV flux can help to infer about the naturalvariability of ionospheric electrodynamics (Sripathi,2012). In Fig. 1b, the geomagnetic

PKp values for the

years, 2008 and 2012, are depicted using black lines forthe year, 2008 and red lines for 2012. On 29th February,9th March, 26th March and 23rd April of 2008 the

PKp

values exceed the limit 30 which is considered as the cut-off between low and high geomagnetic activity, whereasfor the year 2012, 19 days associated to enhanced geomag-netic activity. Solar activity (Lee and Reinisch, 2006, 2007;Liu et al., 2011) and geomagnetic activity (Park et al., 2010;Sun et al., 2012) presumably influence the ionospheric var-iability through several rigorous mechanisms.

3.2. Diurnal variations of ECR and Rtb

In Fig. 2(a–f) the variations in seasonal mean TIEC,BIEC (using Eq. (1)), B0bot (using Eq. (3)), B0top (using

Fig. 1. (a) The day-to-day solar flux at F10.7 cm for the year 2008 and 2012;Low Solar Activity (LSA) period (2008) and red coloured line for Moderateduring 2012) and 732 (366 for both 2008 and 2012) valid data points are incorpcolour in this figure legend, the reader is referred to the web version of this a

Eq. (4)), NmF2 and HmF2 values are depicted for differentLTs of the days during LSA and MSA periods, whereas theFig. 3(a–f) shows the diurnal variability in ECR (black line)and Rtb (red line) values, calculated using Eqs. (2) and (5)respectively, for different seasons during LSA and MSAperiods. 2521 (1409 profile for 2008 and 1112 profile for2012) VED profiles have been considered to analyze thediurnal variability over this region. Interestingly all fourparameters (TIEC, BIEC, B0bot and B0top) during MSAperiod achieve noticeably higher (�1.9–5.4, 1.6–6.7, 1.0–1.7 and 0.7–2.1 times for TIEC, BIEC, B0bot and B0top,respectively) values in comparison to LSA period. StudyingFig. 3 we see that ECR exhibits more or less similar varia-tions pattern as that of Rtb having diurnal-peaks duringpost-midnight hours, whereas diurnal-dips are seen to bein between 09.00–11.00 LT hours except in winter seasons.During winter season the diurnal-dips occurred mostlyduring pre-midnight hours. In general, the seasonal meanvalues of ECR (�1.3–3.00 for LSA and 1.4–2.7 for MSA)are prominently higher than that of Rtb values (�0.9–2.0for LSA and 1.00–2.1 for MSA) for a particular LocalTime (LT) with minimum near the noontime. The heightdifferences between topside (with respect to LEO satelliteheight (750 km)) and bottom side ionosphere, taking start-ing point of D layer at 70 km altitude, are nearly 180–350 km in LSA period and 71–317 km in MSA period.The electron content of this extra height in topside iono-sphere certainly helps to achieve excessive TIEC whichindirectly influences ECR to account for higher values incomparison to Rtb. When solar activity is in concern, dur-ing MSA period both the parameters (ECR and Rtb) dis-play less diurnal variability in comparison to LSA periodmaintaining low deviations in hour-to-hour values, espe-cially during noon-time (deviations at noon time in ECRlies in 0.05–0.34 and that of Rtb lies in 0.15–0.5 withrespect to MSA period considering all the seasons). Itshould be mentioned here that in equatorial ionospherethe low rate of chemical losses in comparison to high rate

and (b)P

Kp index for 2008 and 2012; black coloured line represents theSolar Activity (MSA) period (2012). Here 726 (361 during 2008 and 365orated in (a) and (b), respectively. (For interpretation of the references to

rticle.)

Fig. 2. The diurnal variations of seasonal mean (a) TIEC; (b) BIEC; (c) B0top; (d) B0bot; (e) NmF2; and (f) HmF2 for LSA (2008) and MSA (2012)period obtained using COSMIC satellite VED profile data in the latitude range 15–30�N and longitude range 80–95�E during: Winter (black line), Spring(red line), Summer (green line) and Autumn (blue line) seasons. 2521 (1409 during 2008 and 1112 during 2012) profiles have been used to investigate theseasonal variations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. The diurnal variation of ECR (black line) and Rtb (red line) for LSA (2008, cross symbols) and MSA (2012, vertical bar symbols) during (a)Winter; (b) Spring; (c) Summer; and (d) Autumn seasons. Here 2521 profiles have been used for 2008 (1409) and 2012 (1112). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

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of ion production due to photo ionisation at higher altitudeis responsible for increase in B0 parameter during post-sun-rise period (Lee and Reinisch, 2006; Sripathi, 2012). There-after, at noontime though, the so-called “noon bite-out”mechanism sensitively controlled the variation of iono-spheric peak parameters (NmF2, HmF2), B0 remainsun-affected from this type of natural wind generated mech-anism (Lee and Reinisch, 2006; Sripathi, 2012). Moreover,the vertical E � B drift velocity variation sensitively influ-ences the ionospheric electrodynamics over this tropicalregion (Liu et al., 2010; Sun et al., 2012) due to horizontalgeomagnetic field lines. Without any distraction, our studyyields that during noontime both B0bot and BIEC param-eters play dominant role in Rtb and ECR variations respec-tively (depicted in Fig. 3) are causing to get diurnal-dips.Evidently, the height dependence of sunrise effect on Neconcentration is responsible for relatively higher photoionisation in the lower ionosphere at noontime in compar-ison to upper layer. One thing may be noted here that thedynamic variability in B0 (for example see Fig. 2c and d) issharper than that of IEC (for example see Fig. 2a and b)because B0 is dependent on height of particular Ne concen-tration which varies rapidly for day-to-day, hour-to-hour(Lee and Reinisch, 2006; Sethi and Mahajan, 2002),whereas IEC is sum up of Ne concentration representingthe ionospheric Ne shape profile (Sethi and Dabas, 2006).Hence, the inherent evolution in Rtb and ECR variationsare constrained by the modifications in B0 and IEC param-eters characterised in bottom side and topside parts of theionosphere.

3.3. Correlation analysis

Fig. 4 shows the typical scatter plot of mean ECR valuesversus mean Rtb, NmF2 and HmF2 values during LSAand MSA conditions. The left panels are for noontimeobservation, whereas the right panels are for midnight timeobservation. The red coloured symbols represent the valuesof the parameters for MSA period, whereas the black col-oured symbols for LSA period. The correlation coefficients(R) of ECR with Rtb, NmF2 and HmF2 values are alsodisplayed in the upper right-corner of each panel. The anal-ysis of coefficients shows that the correlations betweenECR and Rtb (e.g., see Fig. 4a) at noontime (10.00–14.00LT) are higher (�0.83, 0.89) at LSA and MSA period,whereas for midnight time (22.00–02.00 LT) these coeffi-cients are comparatively small (�0.56, 0.80), shown inFig. 4d. The high correlation between ECR and Rtb valuesdemonstrate that the physical processes controlling Rtbvariations might also be responsible for ECR variations.Our study reveals that ECR have almost the same levelof correlative relationship with NmF2 (see Fig. 4b and e)as that of Rtb, reported in Gulyaeva (2007), related withcritical frequencies (foF2) which in turns is a proxy ofNmF2. Meanwhile, the correlative relationship of ECRwith HmF2 are in general anti-correlated (e.g., for example�0.23 and �0.30 at noontime and �0.46 and �0.24 at

midnight time) which advocate to decide that increase inHmF2 is associated to decrease in ECR except at pre-mid-night during MSA period (see Fig. 4f) when more diffusivenature of seasonal mean HmF2 is encountered. Hence, itshould be noted that the ionosphere is separated in bottomside and topside (Nsumei et al., 2010) part with respect toHmF2, but, the ratios (ECR, Rtb) of the parameters (IEC,B0) characterised from these two parts (bottom-side andtopside) maintain an excellent correlation which help usto conclude that the height of NmF2 (i.e., HmF2) is themost essential parameter to know the instantaneous frame-work of the Ionosphere at any spatial locations and Iono-sphere is a strongly coupled system with respect to itsbottom and topside parts.

3.4. ECR and Rtb variations during cyclone genesis time

The most surprising results are demonstrated in Fig. 5for the daily variations in (weighted-mean values of hourlydata) ECR and Rtb values during 15 days period includingthe CGP of three catastrophic cyclones, namely, Rashmi(25–27th October 2008; Fig. 5a), Nargis (27th April–3rdMay 2008; Fig. 5a) and Nilam (28th October–1st Novem-ber 2012; Fig. 5b) generated over the Bay of Bengal. Here,near about 159 (80, 56 and 23 profiles for Nargis, Rashmiand Nilam respectively) VED profiles have been analyzedto detect the variability. The data gaps (in Fig. 5b) aredue to the non-availability of COSMIC-RO data over thecyclone genesis region for the corresponding days and dif-ferent symbols denote the days of peak intensity of respec-tive cyclones, obtained from IMD website (http://www.imd.gov.in/). The significant lower phases in the evo-lution of these two ratios (ECR, Rtb) in between genera-tion and precipitation of these cyclones are clearindication of dominant variations in bottom-side part ofthe parameters (IEC and B0) through getting relativelyhigher values, as no geomagnetic storm activity and largegradient in solar flux variations have occurred during therespective periods. This interesting feature is more promi-nent, especially in higher solar activity period, in the vari-ations of Rtb (0.8(R), 0.56(NI) and 1.17(N) at the daysof peak intensity of respective cyclones) than that ofECR (1.3(R), 0.7(NI) and 1.37(N) at the days of peakintensity of respective cyclones), as the dynamic variabilityin B0bot (see Fig. 2d) is sharper than that of BIEC (seeFig. 2b), because the instantaneous variations in B0botare dependent on height of particular Ne concentrationwhich varies rapidly for day-to-day and hour-to-hour(Lee and Reinisch, 2006; Sethi and Mahajan, 2002),whereas BIEC is the sum up of vertical Ne concentrationbellow HmF2 representing the bottom side Ne shape pro-file (Sethi and Dabas, 2006). Some studies (Tian et al.,2010; Lin, 2012a, 2012b) have addressed that the iono-spheric TEC increased by noticeable amount before land-ing of Typhoons and its highest variation amplitude wasdetected at the most intense state of the cyclone when windspeed and area (of typhoon), both are maximum. But, the

Fig. 4. The scatter plot of ECR vs. Rtb (a, d), NmF2 (b, e) and HmF2 (c, f) for noontime (left panels) and midnight time (right panels) during LSA (2008)and MSA (2012) period. The correlation coefficients (R) for LSA (black symbols) and MSA (red symbols) are placed at the upper-right corner of eachpanel. Nearly 519 (249 for 2008 and 270 for 2012) data points have been used here. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

Fig. 5. The day-to-day weighted averaged values of ECR and Rtb are plotted for the period of 15 days including the life cycle of three cyclones: (a) Nargis(N) and Rashmi (R) during LSA period; and (b) Nilam (NI) during MSA period. The most intense state of the life cycle of these three cyclones is displayedby different symbols: triangle (Nargis), delta (Rashmi) and square (Nilam). Here 159 (80, 56 and 23 profiles for Nargis, Rashmi and Nilam) data pointshave been used.

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analysis we carried out has revealed that the B0bot andBIEC values noticeably increased in comparison to T0topand TIEC near the intense state of the CGPs which indi-rectly helps to diminish both Rtb and ECR to a decisivelevel.

4. Summary

In the present study, for the first time, the diurnal andseasonal variations of ECR are analysed, and comparedwith the diurnal and seasonal variations of Rtb over theboxcar region having box size of 15� (15–30�N, 80–95�E)longitude by latitude using COSMIC radio-occultation(RO) profiles. The correlation analysis of ECR with Rtband ionospheric peak parameters (NmF2, HmF2) are alsomade. In summary, the major features of the present studyare outlined as follows:

(1) In general, ECR sustained higher values than that ofRtb values throughout the day (see Fig. 3a–d) havingexcellent correlations (see Fig. 4a and b) betweenthem at local noon-time and midnight time underany solar activity conditions.

(2) ECR exhibits low correlation with NmF2 and anti-correlation with HmF2 at local noontime and mid-night time of LSA and MSA period over this region(see Fig. 4c–f).

(3) During CGPs the dominant variations in the lowerionosphere due to higher Ne concentration conduceto diminish both ECR and Rtb to a decisive level(see Fig. 5a–b).

(4) Our study also admits that bottom side and topsideionosphere are strongly coupled each other whichwas reported in Gulyaeva (2007).

Acknowledgements

The research work presented here is carried out throughfinancial support from CSIR (Council of Scientific andIndustrial Research), Ministry of Earth Science, India.COSMIC satellite data were downloaded from websitehttp://www.cosmic.ucar.edu. Cyclone related informationhas been taken from IMD website http://www.imd.gov.in/. The author would like to thanks WDC Geomag-netic Data Centre, Kyoto University, Japan for providinggeomagnetic indices and UKSSDC, UK for providingF10.7 cm solar flux data. The author would also like toacknowledge the reviewers for their earnest comments.

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