Solar cycle effect on temporal and spatial variation of ...

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ScienceDirect Advances in Space Research 54 (2014) 290–305 www.elsevier.com/locate/asr

Solar cycle effect on temporal and spatial variation of topside ion density measured by SROSS C2 and ROCSAT 1 over the Indian longitude sector Pradip Kumar Bhuyan ⇑, Saradi Bora Centre for Atmospheric Studies, Dibrugarh University, Dibrugarh 786004, India Available online 23 July 2013

Abstract We investigated the diurnal, seasonal and latitudinal variations of ion density Ni over the Indian low and equatorial topside ionosphere within 17.5°S to 17.5°N magnetic latitudes by combining the data from SROSS C2 and ROCSAT 1 for the 9 year period from 1995 to 2003 during solar cycle 23. The diurnal maximum density is found in the local noon or in the afternoon hours and the minimum occurs in the pre sunrise hours. The density is higher during the equinoxes as compared to that in the June and December solstice. The local time spread of the daytime maximum ion density increases with increase in solar activity. A north south asymmetry with higher ion density over northern hemisphere in the June solstice and over southern hemisphere in December solstice has been observed in moderate and high solar activity years. The crest to crest distance increases with increase in solar flux. Ion density bears a nonlinear relationship with F10.7 cm solar flux and EUV flux in general. The density increases linearly with solar flux up to 150 sfu (1 sfu = 1022Wm2Hz1) and EUV flux up to 50 units (109 photons cm2 s1). But beyond this the density saturates. Inverse saturation and linear relationship have been observed in some season or latitude also. Inter-comparison of the three solar activity indices F10.7 cm flux, EUV flux and F10.7P (= (F10.7 + F10.7A)/2, where F10.7A is the 81 day running average value of F10.7) shows that the ion density correlates better with F10.7P and F10.7 cm fluxes. The annual average daytime total ion density from 1995 to 2003 follows a hysteresis loop as the solar cycle reverses. The ion density at 500 km over the Indian longitude sector as obtained by the international reference ionosphere is in general lower than the measured densities during moderate and high solar activity years. In low solar activity years the model densities are equal or higher than measured densities. The IRI EIA peaks are symmetric (±10°) in equinox while densities are higher at 10°N in June solstice and at 10°S in the December solstice. The model density follows F10.7 linearly up to about F10.7 > 150 sfu and then saturates. Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Equatorial ionosphere; Equatorial ionization anomaly; IRI; Solar flux; Hysteresis

1. Introduction The ionosphere is coupled to the solar wind and interplanetary magnetic field via the magnetosphere. As the magnitude of the solar radiation varies with various time scales, the study of solar activity modulation of the ionosphere helps us to understand the ionospheric structure and evolution. The ionospheric plasma density exhibits long-term e.g. solar cycle and short periodic variability due to geomagnetic storms, solar flares etc. In the past, the solar activity effects of several ionospheric parameters ⇑ Corresponding author. Tel.: +91 3732370224; fax: +91 3732370323.

E-mail address: [email protected] (P.K. Bhuyan).

such as electron density, total electron content, plasma temperature, peak electron density NmF2 and peak height hmF2 of the layer have been investigated extensively. But due to insufficient spatial and temporal coverage of data, a complete global pattern of the solar cycle influence on the ionosphere is not available till date. Several researchers have reported a nonlinear relationship between plasma density and F10.7 cm solar flux when the radiation flux exceeds some threshold values and after that a saturation effect is noticed. (Bhuyan et al., 1983; Balan et al., 1993; Park et al., 2008; Chen et al., 2009; Liu et al., 2007b). Balan et al. (1993, 1994) and Park et al. (2008) found that the solar EUV flux increases linearly with F10.7 cm flux up to F10.7  150–160 sfu

0273-1177/$36.00 Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2013.07.017

P.K. Bhuyan, S. Bora / Advances in Space Research 54 (2014) 290–305

(1 sfu = 1022W m2 Hz1) beyond which the slope of the solar EUV flux becomes less steep. This nonlinearity between F10.7 and the solar EUV flux clearly affects the ionospheric responses to solar activity. On the other hand, in some cases saturation has been observed when NmF2 or TEC are plotted against F10.7 but not with solar EUV flux (Liu et al., 2004, 2006; Rishbeth, 1993; Sethi et al., 2002; Zhang and Holt, 2007). Liu et al. (2003) and Lei et al. (2005) found that saturation occurs for both foF2 and solar EUV and most profound saturation effect occurs at the equatorial anomaly crest regions. Chen et al. (2009) pointed three kinds of patterns in the variations of plasma density as linear, saturation and amplification. Liu et al. (2009) analyzed the JPL GIM TEC data over one solar cycle to investigate the overall climatology of the ionosphere. They found strong solar activity sensitivity in low latitudes and that the saturation effect in the mean TEC versus F10.7 cm flux is more pronounced at low latitudes while an amplification (curvature opposite to saturation) effect is seen in the mean TEC versus solar EUV at higher latitude. It has also been reported that similar to the hysteresis effect for magnetic materials, foF2 also exhibits a hysteresis effect during a solar cycle i.e. foF2 has different values at the same solar level during the ascending and descending phase of a solar cycle (Huang, 1963; Kane, 1992, 2005; Mikhailov and Mikhailov, 1995; Ortiz de Adler and Manzano, 1995; Ortiz de Adler and Elias, 2008; Chakraborty and Hajra, 2008 etc.). The international reference ionosphere (IRI) is an empirical model developed under the aegis of a joint COSPAR and URSI working group and provides analytical representation of monthly average or median value of electron density, electron content, electron temperature, ion temperature and ion composition as a function of height, location, local time and sunspot number for magnetically quiet conditions. Since inception (Rawer et al., 1978) the IRI has steadily improved with newer data and better modeling techniques leading to release of different versions that tries to narrow the gap between predictions and observations which have been found to be substantial particularly at low latitudes. Bilitza et al. (2011) have described the latest version of the model and reviewed the efforts being undertaken towards future improvements. Long term study of the temporal and spatial variation of topside total ion density over the Indian low and equatorial latitudes has not been attempted so far to our knowledge. The objective of the present work is to study the effect of solar cycle on the diurnal, seasonal and latitudinal variation of total ion density by combining the data measured by the SROSS C2 and ROCSAT 1 satellites over the Indian equatorial and low latitudes during solar cycle 23 from 1995 to 2003. The primary focus is on the effect of solar activity on the growth and decay of the equatorial ionization anomaly at 500 km altitude and variation of total ion density vis-a-vis three solar activity indices F10.7, F10.7P and EUV. The long-term data are further utilized to examine the predictability of the IRI over this latitude

291

longitude sector for period of low to very high solar activity. 2. Data P Ion density ( of O+, O2+, He+ and H+ densities) data from SROSS C2 (1995–2000) and total ion density data from ROCSAT 1 (1999–2003) are used for the present study which covers the ascending and descending phase of the solar cycle 23. The combined data from the two satellites spanning 9 years provides a unique opportunity to study the effect of solar activity on temporal and spatial variation of ion density over the Indian longitude sector. SROSS C2 RPA data are available from 1994 to 2001 within the latitude belt ±34° and longitude belt 40–100°E at an average altitude 500 km. The longitudinal extent of the ROCSAT 1 data is restricted to ±30° of 70°E to match the spatial coverage of SROSS C2. Details of the Retarding Potential Analyzer (RPA) and Ionospheric Plasma and Electrodynamics Instrument (IPEI) sensors on board SROSS C2 and ROCSAT 1 and the data retrieval procedures are given by Garg et al. (2003) and Yeh et al. (1999). The average altitude of SROSS C2 was 500 km, while that of ROCSAT-1 was 600 km. Therefore, to study the solar cycle effect, the ROCSAT – 1 data at 600 km is normalized to 500 km using the following equation and the IRI height profile for every hour

Fig. 1. Illustration of the spatial distribution of total ion density at 1200 LT in equinox as observed by SROSS C2 for solar minimum (1995) and ROCSAT 1 for solar maximum (2000). The solid continuous lines are the average densities at 5° latitude intervals.

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Fig. 2a. 2D representation in local time and year of observation of total ion density at 500 km altitude during 1995–2003 over MLAT 10°N (top), equator (middle) and 10°S (bottom).

ðNiROCSAT1 Þð500 kmÞ ¼ ðNi ROCSAT1 Þð600 kmÞ þ DðNiIRI500  NiIRI600 Þ

ð1Þ

In the IRI, the topside electron density profile is normalized to the F2 peak density NmF2 and height hmF2. The absolute value of electron density at a fixed height is, therefore, determined by the models for NmF2 and hmF2 and by the model for the parameters that determine the topside profile shape (Bilitza et al., 2012). Further, while the IRI foF2/NmF2 model uses an ionosphere effective solar index IG12, sunspot number Rz12 is used to describe the solar activity effect on hmF2. Bilitza et al. (2012) have found from a comparison of these two indices over the last six solar cycles that the Rz12 does not sufficiently describe the solar activity variation of the F region with most pronounced differences during solar minima and maxima. Therefore, over or underestimation of the peak height and inadequate representation of the effect of solar activity might result in inaccurate estimation of density by the IRI to some extent and consequently on the estimation of ion

density using Eq. (1). In the present case measurements at 500 km and 600 km altitudes were available simultaneously for the two high sunspot years 1999 and 2000. The simultaneous data have been utilized to remove the bias, if any, in the normalized data. Examples of the data distribution within the selected latitude longitude zone for two years representing low and high sunspot years are shown in Fig. 1 for 1200 LT. The measured densities are grouped into bins of ±2.5° at 5° latitude intervals. The EUV flux data are the 0.1–50 nm EUV flux as monitored by solar EUV monitor (SEM) spectrometer on board the Solar Heliosphere observatory (SOHO). The F10.7 cm flux data are taken from the SPIDER website and the adjusted values of F10.7 are adopted for analysis. The annual mean F10.7 cm flux varied from the minimum 72 in 1996 to the maximum 183 in 2001. The year is divided into three seasons: June solstice (May, June, July, August), December solstice (November, December, January, February) and equinoxes (March, April, September, October).

P.K. Bhuyan, S. Bora / Advances in Space Research 54 (2014) 290–305

3. Results and discussion 3.1. Diurnal and seasonal variation The diurnal and seasonal variations of electron density in the topside ionosphere covering different levels of solar activity over the Indian equatorial and low latitudes have been reported earlier (Bhuyan and Chamua, 2002; Bhuyan et al. 2003; Bhuyan and Borgohain, 2006). Bhuyan and Chamua (2002) studied the electron density measured by the Hinotori satellite at 600 km altitude within ±25° over the Indian longitude sector during the high solar activity years 1981–1982 and observed that the electron density maximizes between 1200 and 1600 LT and minimum density is reached around 0400 LT. The equatorial ionization anomaly (EIA) was found to be absent at 600 km during this period of moderate and high solar activity. Bhuyan et al. (2003) reported the diurnal and seasonal variation of electron density measured over the Indian zone by the SROSS C2 during the low solar activity period 1995– 1996 and found that asymmetric ionization anomaly is

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present in the equinox and December solstice while the EIA is not so well developed in the June solstice. Su et al. (1995) have reported that the electron density at 600 km measured by the Hinotori has its highest value within a broad range of latitudes around the magnetic equator and the density is higher in the summer hemisphere. Bhuyan and Borgohain (2006) have reported the diurnal and seasonal variations of O+, H+, He+ and O2+ ions measured at the altitude of 500 km during solar minimum. They found that O+ density is minimum (108 m3) before local sunrise (0400 LT) and reaches its peak (1011 m3) near local noon or in the afternoon hours (1200–1600 LT) depending on season and latitude of observation. The minimum and maximum densities vary with season and latitude. The O+ ion concentrations reached its diurnal maximum earlier in June solstice than that in December solstice. The growth and decay of ion concentration during the daylight hours was found to be sharper in June solstice compared to that in December solstice and equinox. Borgohain and Bhuyan (2010) found that O+ dominates the altitudes of 500–600 km in the Indian equa-

Fig. 2b. 2D representation in local time and year of observation of IRI – 2012 simulated ion density at 500 km altitude during 1995–2003 over MLAT 10°N (top), equator (middle) and 10°S (bottom).

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Peak Ion Density (× 1011 m -3)

June solstice

December solstice

Equinoxes

1995

1995

1995

1996

1996

1996

1997

1997

1997

Magnetic Latitude (o) Fig. 3a. Latitudinal variation of average ion density at 1200 LT for 1995–1997 along 70° ± 30°E longitudes. The solid lines indicate the observed data and the dotted lines are for the IRI – 2012 simulated data.

torial and low latitude sector. Day and nighttime O+ was found to have a good positive correlation with F10.7 cm solar flux. The diurnal and seasonal variations of total ion density along the 70° ± 30° longitude sector over three magnetic

Peak Ion Density (× 1011m -3)

June solstice

18

latitudes (10°N, 10°S and equator) are studied from 1995 to 2003. In Fig. 2a the diurnal variation of density in the three seasons are shown. The density is higher during the equinoxes than that during June and December solstice. It is seen from this figure that the time of occurrence of

December solstice

Equinoxes

1998

1998

1998

1999

1999

1999

2000

2000

2000

12 6 0

Magnetic Latitude (°) Fig. 3b. Same as in Fig. 3a for 1998, 1999 and 2000.

P.K. Bhuyan, S. Bora / Advances in Space Research 54 (2014) 290–305

June solstice

December solstice

Equinoxes

2001

2001

2001

2002

2002

2002

2003

2003

2003

Peak Ion Density (× 1011m -3)

295

Magnetic Latitude (o) Fig. 3c. Same as in Fig. 3afor 2001, 2002 and 2003.

diurnal maximum and minimum density varies with season. The diurnal maximum density occurs at local noon or in the afternoon hours and the minimum is found in the pre sunrise hours. Over 10°N MLAT, during June solstice the diurnal maximum density is found in 2000 as 2.09  1012 m3 at around 1300 LT. The diurnal minimum is 0.06  1011 m3 at around 0400 LT in 1997. During December solstice an evening enhancement of density is seen in 2000 and diurnal maximum is found as 2.47  1012 m3 at around 2000 LT. The diurnal minimum is found as 0.04  1011 m3 in 1997 at around 0500 LT.

During the equinoxes the diurnal maximum density is found in 2001 as 2.49  1012 m3 at around 1300 LT and minimum is 0.08  1011 m3 in 1997 at around 0400 LT. Over the magnetic equator, during June solstice the diurnal maximum is found in 2000 as 1.5  1012 m3 at around 1800 LT and minimum is 0.03  1011 m3 at 0300 LT in 1997. In 1995 the diurnal maximum is found very early at 0900 LT and in 2001 it is delayed and occurs at around 1600 LT. During December solstice, the maximum occurs in 2001 as 1.92  1012 m3 at around 1100 LT and the minimum is 0.07  1011 m3 at 0500 LT in 1995. During equinoxes the diurnal maximum is

Table 1 Latitudinal position of the anomaly crests and crest to crest distance in the three seasons obtained for measured and IRI data from 1999 to 2003. Season

Year

Mean F10.7

Crest position (°N)

IRI crest position (°N)

Crest position (°S)

IRI crest position (°S)

Crest to crest distance (°)

IRI crest to crest distance (°)

June solstice

1999 2000 2001 2002 2003 1999 2000 2001 2002 2003 1999 2000 2001 2002 2003

168 190 159 177 130 157 167 187 185 129 135 186 204 179 131

7 7 7 9 Equator 5 Not found 5 9 Equator 5 8 10 9 7

– – – – – 7 5 5 7 7 7.5 7.5 7.5 10 10

10 10 8 10 10 11 10 10 10 10 10 15 10 15 10

– – – – – 5 5 5 5 10 7.5 7.5 7.5 10 10

17 17 15 19 10 16 – 15 19 10 15 21 20 24 15

– – – – – 12 10 10 12 17 15 15 15 20 20

December solstice

Equinoxes

296

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Fig. 4. Variation of crest to crest distance with seasonal mean F10.7 cm solar flux from 1999 to 2003: observed (left panel) and IRI (right panel). The solid lines are the linear regression fitted through the data points.

2.36  1012 m3 at 1400 LT in 2000 and the minimum is 0.05  1011 m3 found at 0400 LT in 1997. Over 10°S MLAT in the June solstice the diurnal maximum density is found in 2000 as 1.80  1012 m3 at 1200 LT. and the minimum density is 0.01  1011 m3 at 2300 LT in 1997. There is a decrease of density in 2001 and again it increases in 2002. During December solstice, the diurnal maximum density is found as 1.85  1012 m3 at 1700 LT in 2000 and the minimum is 0.05  1011 m3 found at 0400 LT in 1995. In equinoxes, the diurnal maximum is 2.22  1012 m3 at 1200 LT found in 2002 and the minimum density is found in 1997 as 0.02  1011 m3 at 0100 LT. From Fig. 2a it is seen that the ion density reaches its diurnal maximum around local noon and remains nearly at the same level till afternoon hours. The local time spread of the daytime maximum ion density increases with increase in solar activity. During high solar activity equinoxes broad daytime maxima is found between 1000 LT to 1800/2000 LT. The diurnal minimum is found at 0400– 0500 LT. An evening enhancement of ion density is also seen during December solstice over the anomaly crest latitudes ±10°. During June solstice evening enhancement is seen in 1995–1996 over the equator and 10°N. The observed broadening of occurrence of diurnal maximum

during high solar activity period might be due to increase in F peak height and subsequent lowering of recombination rate leading to sustained daytime peak ionization for an extended period. The figure also shows a north south asymmetry with higher ion density over northern hemisphere in June solstice and over southern hemisphere in December solstice. The hemispherical asymmetry observed at 500 km altitude is similar (i.e. higher EIA crest density in the summer hemisphere) to the asymmetry observed and reported earlier for F peak density. It is well known that the topside ionosphere is controlled by the transport and dynamic processes involving the neutral wind, E  B drift and diffusions. The thermospheric winds push the plasma up and down and transport from one hemisphere to the other influences the latitudinal structure of the plasma density. Liu et al. (2007a) using the HWM-93 model showed that the during solstice months the direction of wind is different for both the hemispheres. Since the equator ward wind push the plasma to higher altitude and hence lower recombination resulting higher density in the off equatorial region. A difference in neutral wind among the hemispheres will cause hemispheric asymmetry in the occurrence of anomaly peak density. Lin et al. (2007) studied the time evolution of the F peak ionization over 90°E and found that the hemispheric

P.K. Bhuyan, S. Bora / Advances in Space Research 54 (2014) 290–305

25

June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

297

20 15

Total ion density (×1011m-3)

10 5 0 25 20 15 10 5 0 25 20 15 10 5 0 0

50

100

150

200

250 0

50

100

150

200

250 0

50

100

150

200

250

Solar F10.7 flux Fig. 5. Variation of monthly mean daytime (1000–1400 LT) ion density from 1995 to 2003 with solar F10.7 cm flux over MLAT 10°N (top), equator (middle), 10°S (bottom). The solid and dotted lines represent the best fit 2nd order polynomial regression through the observed and IRI simulated data points respectively.

asymmetry is a function of time and the density is higher in the forenoon hours and lower in the afternoon hours in the southern hemisphere and vice versa in July–August, 2006. The local time variation of ion density obtained from the IRI and shown in Fig. 2b from 1995 to 2003 plotted as in Fig. 2a indicates that the model densities are lower than the measured densities by about 50% during the high solar activity daytime peak hours. The difference between the measured and model density reduces in years of low solar activity. The IRI exhibits hemispherical asymmetry in the solstices. The densities are higher at 10°N in June solstice and at 10°S in the December solstice. In the equinoxes the daytime densities are nearly same at ±10°. 3.2. Growth and decay of the equatorial ionization anomaly Due to the formation of equatorial ionization anomaly, a trough of ion density over the magnetic equator and two crests on both sides of the equator are found. The EIA crest strength and position have been found to exhibit seasonal bias by many observers (e.g. Rush and Richmond, 1973; Lin et al., 2007; Chen et al., 2008). The strength of the EIA peak and the crest to crest distance also varies with latitude and solar activity (Liu et al., 2007a,b; Chakraborty and Hajra, 2008; Bhuyan and Bhuyan, 2009). To study the latitudinal variation the total ion density is plotted from

15°N to 15°S MLAT at 5° latitude intervals for 1200 LT in Figs. 3a–3c from 1995 to 2003 separately for the three seasons. The anomaly in general is found to be well developed a few hours around local noon. In 1995, the EIA is not well developed in the southern hemisphere. The ion density during June solstice is higher in the northern hemisphere and peak is found over 10°N (Ni 1.78  1011 m3). During equinoxes, the hemispheric asymmetry is more prominent and the density peaks over 5°N (Ni 6.1  1011 m3). During December solstice peak density is found over 5°N (Ni 4.07  1011 m3). The plasma density is low during this low solar activity year 1995 and full grown EIA on both hemispheres have not been observed at the altitude of 500 km. In 1996, during the June solstice EIA peaks are found over 10°S (Ni 2.33  1011 m3) and 5°N (Ni 2.49  1011 m3). The trough is found over the magnetic equator (Ni 2.16  1011 m3). In the December solstice the EIA is not developed and peak density is found over 5°N (Ni 2.12  1011 m3). In equinoxes, toward the southern hemisphere, the peak density is found over 10°S (Ni 2.87  1011 m3) and towards the northern hemisphere the peak density is found over 10°N (Ni 2.97  1011 m3). A depression of ion density is found over 5°N (Ni 1.88  1011 m3).

P.K. Bhuyan, S. Bora / Advances in Space Research 54 (2014) 290–305

Total ion density (×1011m -3)

298

June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

Solar F10.7 flux Fig. 6. Variation of monthly mean nighttime (2200–0200 LT) ion density from 1995 to 2003 with solar F10.7 cm flux over MLAT 10°N (top), equator (middle), 10°S (bottom). The solid and dotted lines respectively represent the best fit 2nd order polynomial regression through the observed and IRI simulated data points.

In 1997, the EIA is absent in the December solstice with peak density over the equator (Ni 2.98  1011 m3). During the June solstice density peaks over 5°N (Ni 2.11  1011 m3) and the equatorial trough shifts to 10°S. During the equinoxes, two peaks of density form over 5°N (Ni 5.39  1011 m3) and 10°S (Ni 11 3 5.58  10 m ) with the depression of ion density near the equator (Ni 4.75  1011 m3). In 1998, during the June solstice the EIA southern peak is higher than the northern peak. During equinoxes, two crests are found over ±10°. In the southern hemisphere peak Ni 9.01  1011 m3 and in the northern hemisphere peak Ni 1.15  1012 m3. The trough in this season is found over the equator (Ni 7.25  1011 m3). Due to unavailability of data the formation of EIA during winter is not clear. It is seen from the above analysis that at 500 km altitude over the Indian longitude sector the EIA has a distinct seasonal variability and not equally developed in both the hemispheres in the solstices during low and moderate solar activity periods. In equinoxes, on the other hand, the anomaly is found to be better developed for the same levels of solar activity. At the peak of the solar cycle 23 in 1999, during June solstice in the southern hemisphere the peak density is found over 10°S (Ni 7.40  1011 m3) and in the northern

hemisphere it is found over 7°N (Ni 7.86  1011 m3). In December solstice, the EIA crest in the southern hemisphere (at 10°S) is higher than the EIA crest in northern hemisphere (at 5°N). In the equinoxes, the anomaly crests form over 10°S (Ni 9.71  1011 m3) and 5°N (Ni 9.92  1011 m3). The trough density over the equator is found to be 8.34  1011 m3. In 2000, during June solstice the EIA at 5°N (Ni 9.77  1011 m3) is higher than that at 10°S (Ni 9.08  1011 m3). During December solstice, southern peak formed over 10°S (Ni 1.21  1011 m3) and the northern peak is not well developed. During the Equinoxes, the ionization density peaks over 15°S (Ni 1.44  1012 m3) and 10°N (Ni 1.53  1012 m3). In 2001, during the December solstice the peak density is found over 10°S (Ni 1.28  1012 m3) and 10°N (Ni 1.35  1012 m3), the northern crest density being marginally higher than the southern crest density. In the equinox of the same year the crest density over 10°S (Ni 1.34  1012 m3) is found to be much lower than the crest density over 10°N (Ni 1.73  1012 m3). The trough of EIA is found over the equator (Ni 1.26  1012 m3). In 2002, the EIA peaks during June solstice are seen over 10°S (Ni 1.14  1012 m3) and 10°N (Ni1.29  1012 m3) having minimum density over 5°S (Ni 1.00  1012 m3). During December solstice the peaks

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25

June solstice

December solstice

June solstice

December solstice

June solstice

December solstice

299

Equinoxes

20 15

Total ion density (×1011m -3)

10 5 0 25

Equinoxes

20 15 10 5 0 25

Equinoxes

20 15 10 5 0 0

10

20

30

40

50

60

70

80

0

10

20

30

40

50

60

70

80 0

10

20

30

40

50

60

70

80

EUV0.1-50 nm(109 photon cm-2 s-1) Fig. 7. Monthly mean daytime (1000–1400 LT) ion density from 1995 to 2003 plotted against corresponding solar EUV flux over MLAT 10°N (top), equator (middle), 10°S (bottom). The solid and dotted lines represent the best fit 2nd order polynomial regression through the observed and IRI simulated data points respectively.

are formed over 10°S (Ni1.26  1012 m3) and over 5°N (Ni 1.28  1012 m3). The trough is found over 5°S (Ni 1.19  1012 m3). In the equinoxes the peak density in the southern hemisphere (15°S, Ni 1.68  1012 m3) is higher than the peak density in northern hemisphere (10°N, Ni 1.61  1012 m3). The equatorial trough density is 1.30  1012 m3. In the moderate activity year 2003, EIA is not distinguishable in both the solstices. Instead a broad peak is found between 10°S and 5°N. During equinoxes, the EIA northern crest Ni 1.08  1011 m3 over 5°N is higher than the southern crest over 10°S (Ni 1.00  1012 m3). The trough is found over 5°S (Ni 9.37  1011 m3). Figs. 3a–3c also show the corresponding noontime densities obtained from the IRI. The IRI EIA peaks are symmetric (±10°) in equinox whereas in the June solstice though the northern crest forms around 10°N in all years the southern crest is found at 10°S only in the solar minimum years 1995–1997. In the December solstice, the northern crest is located at 10°N. The southern crest forms at 5°S in 1999–2002 and beyond 10°S during 1995–1997. In June solstice, the IRI generally overestimates the measured density. In December solstice too the IRI density is higher at about all latitudes from 1995 to 2000 but becomes lower in 2001–2003. In equinox, however, the IRI underestimates the densities at all latitudes and in all years except in 1996.

The position of the EIA crests and crest to crest distances from 1995 to 2003 are presented in Table 1 from which the annual and seasonal variations become clear. It is seen that from 1999 to 2003 when the anomaly was present in all seasons, the crest to crest distances varied with season and also with solar activity. This point is further elucidated by plotting the observed and IRI predicted crest to crest distances against the mean F10.7 cm solar flux for each season of the corresponding year during 1999–2003 when the EIA peaks have been unambiguously observed (Fig. 4). From the figure it is seen that the observed crest to crest distance increases with increase in solar flux i.e. the anomaly moves away from the equator during high solar activity. But contrary to observation the IRI shows negative correlation and hence reversal of the anomaly crests towards the equator with increase in solar activity. Su et al. (1995) reported that the crest to trough ratio and the latitudinal position of the crests vary with solar and geomagnetic activity. Balan et al. (1995) showed that the field aligned plasma flow caused by neutral wind is responsible for asymmetric EIA formation about the geomagnetic equator. It was also shown that the strength of the anomaly decreases with altitude and disappears at about 800 km (Balan et al., 1997). The variation of the EIA crest position over the Indian longitude sector as

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Total ion density (×1011m -3)

300

June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

EUV0.1-50 nm(109 photon cm-2 s-1) Fig. 8. Monthly mean nighttime (2200–0200 LT) ion density from 1995 to 2003 plotted against corresponding solar EUV flux over MLAT 10°N (top), equator (middle), 10°S (bottom). The solid and dotted lines represent the best fit 2nd order polynomial regression through the observed and IRI simulated data points respectively.

seen from the present analysis is in agreement with the results of Liu et al. (2007a). They have studied the solar activity dependence of EIA structure for low, moderate and high activity levels differently for noon and post sunset local time sectors using CHAMP satellite data at 400 km altitude. The results showed that the EIA crests tend to move pole ward with increasing solar activity. This trend is found to be stronger in the post sunset sector than in the noon sector. Also they calculated the crest to trough ratio at different solar activity levels and found higher ratio in high solar activity period than that in the low activity period. Bhuyan and Bhuyan (2009) using electron density data from SROSS C2 satellite found that the EIA crest position in the northern hemisphere is closest to the geomagnetic equator in 1996 and furthest from the equator in 1998 and 1999 during all the seasons. Also positive correlation has been reported between latitudinal position of northern EIA crest and F10.7 and EEJ. The observed variation of crest to crest distance with solar cycle may be due to combined effect of solar activity and EEJ. The changes in the EEJ will result in changes in the E  B drift. Increase in E  B drift velocity would lift ionization to higher altitudes and consequently the plasma would move further in latitude along the field lines and vice versa.

3.3. Ion density and solar flux In Fig. 5 the monthly mean daytime (1000–1400 LT) ion density is plotted against F10.7 for the three seasons over 10°N, equator and 10°S magnetic latitudes. The solid lines represent the 2nd order polynomial fit across the data points. From the figure it is seen that the dependence of ion density on solar F10.7 flux varies with season and latitude. During June solstice, the trend line is linear up to F10.7 150 sfu, but beyond this, the density saturates over all latitudes. During December solstice, over 10°N the density varies linearly with F10.7 but over 10°S and the equator after F10.7 160 sfu an inverse saturation i.e. faster increase in ion density as compared to that in solar flux producing a concave curvature is seen. During equinoxes over all latitudes the saturation of density starts when F10.7 reaches 150 sfu. The monthly average daytime ion density obtained from the IRI and plotted alongwith the measured density is lower during moderate to high solar activity period (F10.7 > 100 sfu) while in low solar activity period (F10.7 < 100 sfu), the model densities are higher than the measured densities. The model density follows F10.7 linearly up to about F10.7 > 150 sfu and then saturates. The correlation obtained for the measured and model densities are nearly equal in all cases except in over the equator and

Total ion density (×1011m -3)

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June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

301

F10.7P Fig. 9. Variation of mean monthly daytime (1000–1400 LT) ion density with F10.7P over 10°N (top), equator (middle), 10°S (bottom) MLAT during 1995– 2003. The solid and dotted lines are the 2nd order polynomial fit that represent the best R2 values for observed and IRI predicted density.

10°N. In Fig. 6 the variation of nighttime (2200–0200 LT) ion density with solar F10.7 flux is shown. From the figure it is seen that during June solstice, the trend lines indicate saturation over the three latitudes but the curvature is not prominent. During December solstices the density saturates over 10°N when F10.7 150 sfu. Over the equator the trend is linear whereas the inverse saturation effect is seen over 10°S. This indicates a faster response of the F region to change in solar flux for high activity level which is contrary to the observed saturation or slow response of the F region seen in most cases. In equinoxes, saturation beyond 180 sfu is seen over the equator and 10°S while the reverse is true for 10°N. Nighttime density predicted by the IRI shows better correlation with F10.7 and matches well with measured data as seen from the closeness of the trend lines. Nighttime density increases linearly initially with F10.7 upto about 150/160 sfu and then tends to saturate. In the June solstice, however, the model densities are generally higher than measured densities. In Fig. 7 the daytime ion density is plotted against solar EUV flux for the three latitudes and season as in Fig. 5. During June solstice the density saturates when EUV flux 45–50 units (109 photons cm2 s1) over all the three latitudes. During December solstice over the equator and 10°S density saturates beyond EUV 50 units though the trend is not sharp over the equator. The inverse saturation effect is seen over 10°N. During the equinoxes over 10°N

the progression of density with EUV flux is nearly linear while over the equator the density saturates beyond EUV 45 units and over 10°S it saturates when EUV 40 units. Nighttime ion density is similarly plotted against with EUV flux in Fig. 8. It is seen from the figure that nighttime ion density bears a nonlinear relationship with EUV flux showing saturation over the equator in June solstice and over 10°N in December solstice and inverse saturation in other seasons and latitudes. The nighttime density obtained from the IRI plotted against EUV solar flux in Fig. 8 shows large scatter in all cases and the R2 values between model density and EUV are much lower compared to the R2 values obtained for measured densities. Model densities exhibit nonlinear relationship with EUV in the solstices while in equinox the trend lines are linear. In Fig. 9 and Fig. 10 the variation of daytime and nighttime ion density with F10.7P (=(F10.7 + F10.7A)/2, where F10.7A is the 81 day running average value of F10.7) is shown. During June solstice daytime ion density saturates when F10.7P 150. During December solstice inverse nonlinearity is noticed over the equator and 10°S while over 10°N ion density indicates saturation with F10.7P. During the equinoxes saturation of density is observed over all the latitudes for F10.7P 170. The variation of model density with the derived solar flux index F10.7P appears to be nearly linear and correlation is higher than that obtained for the measured densities. Similarly both measured and

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Total ion density (×1011m -3)

302

June solstice

December solstice

June solstice

December solstice

Equinoxes

June solstice

December solstice

Equinoxes

F10.7P Fig. 10. Variation of monthly mean nighttime (2200–0200 LT) ion density with corresponding F10.7P over 10°N (top), equator (middle), 10°S (bottom) MLAT during 1995–2003. The solid and dotted lines are the 2nd order polynomial fit that represent the best R2 values for observed and IRI predicted density.

Table 2 Coefficients of correlation, R obtained for total ion density versus F10.7, EUV and F10.7P. Latitude

10°N

Equator

10°S

Season

June solstice December solstice Equinoxes June solstice December solstice Equinoxes June solstice December solstice Equinoxes

F10.7

EUV

F10.7P

Niobs

NiIRI

Niobs

NiIRI

Niobs

NiIRI

0.84 0.84 0.90 0.93 0.92 0.88 0.93 0.95 0.92

0.88 0.94 0.89 0.92 0.96 0.94 0.89 0.96 0.93

0.77 0.72 0.84 0.89 0.70 0.77 0.90 0.73 0.82

0.87 0.85 0.88 0.90 0.86 0.90 0.89 0.86 0.90

0.85 0.82 0.90 0.93 0.92 0.89 0.93 0.94 0.94

0.90 0.96 0.91 0.95 0.97 0.96 0.91 0.97 0.94

predicted nighttime density shows saturation as well as the inverse effect. The correlation coefficients between daytime average total ion density and solar F10.7, EUV and F10.7P calculated from the regression analysis as given in the figures are shown in Table 2 for inter comparison. It is seen from the Table that ion density correlates better with F10.7 and F10.7P is better than that with EUV flux. However, there is no significant difference between the correlation obtained for F10.7 and F10.7P. The response of F region ionization to EUV flux on short time scales may be slower compared to

its response to F10.7 leading to better correlation of density with F10.7 than with EUV flux. Though the ionospheric plasma is produced as a result of photo ionization by the solar radiations earlier studies have shown that its density varies with solar activity in a rather complicated way. Bhuyan et al. (1983) reported saturation effect of TEC and foF2 over Delhi, a northern low latitude station during solar cycle 21. The non linear variation of TEC with solar flux was reported by Balan et al. (1993) for southern mid latitude. Balan et al. (1994, 1996) observed that NmF2 and TEC vary nonlinearly with

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flux becomes less steep. Liu et al. (2009) analyzed the JPL GPS TEC data for one solar cycle to investigate the overall climatology of the ionosphere. The mean TEC data are averaged over low, middle and high latitude bands in the southern and northern hemisphere separately and both hemisphere together. They found that the saturation effect in the mean TEC versus F10.7 is more pronounced at low latitudes while mean TEC increases more rapidly for higher solar EUV fluxes (inverse effect). 3.4. Annual variation and hysteresis effect Fig. 11. Year to year variation of total ion density from 1995 to 2003 against annual mean solar F10.7 cm flux over the magnetic equator. The colored dots indicate the mean F10.7 cm solar flux for respective years.

Fig. 12. Year to year variation of total ion density from 1996 to 2003 with annual mean solar EUV flux over the magnetic equator. The colored dots indicate the mean EUV flux for respective years.

F10.7 cm solar flux and linearly with solar EUV flux. They proposed that this may be due to the nonlinearity between EUV flux and F10.7. Saturation pattern is reported in case of NmF2 and F10.7 by Liu et al. (2006) and attributed it to the ionospheric dynamics and neutral compositions. Liu et al. (2011) have reviewed the solar activity effects on the ionosphere and reported that three patterns of ionospheric parameters have been observed when different set of proxies are used to represent the effect of solar radiation flux on the ionosphere. Liu et al. (2009) have shown that saturation occurs in mean TEC value when F10.7 approaches values greater than 200 sfu more evident at low latitudes; while an amplification effect (reverse saturation) is detected in mean TEC versus solar EUV at higher latitudes. Liu et al. (2007a) studied total ion density measurements for ten years from DMSP spacecraft to examine the dependence of plasma densities in the topside ionosphere at mid and low latitudes on solar activity. They found that there is a strong linear dependence of ion density at 848 km altitude with F10.7 when F10.7 lies in the range 100–250 units but a nonlinear dependence on EUV measured by SEM/SOHO. Park et al. (2008) found that the solar EUV flux increases linearly with F10.7 cm flux up to 150–160 sfu beyond which the slope of the solar EUV

To examine the annual variation of ion density with solar activity for the nine year period the annual average daytime density over the equator is plotted against corresponding annual mean F10.7 and EUV flux respectively in Figs. 11 and 12. When the data points are joined chronologically a hysteresis effect i.e. the value of ion density being different at the same level of solar activity during the ascending and descending leg of the solar cycle 23 has been observed in both the figures. The hysteresis effect is one important phenomenon of long term variation of ionospheric parameters which has not been understood fully. Huang (1963) mentioned that there may be larger solar radiation present in the falling part of the solar cycle than in the rising part. Titheridge (1974) found a lag of six months in the changes in ion composition. It is known that the geomagnetic activity in the descending phase of the solar cycle is generally stronger than that during the ascending phase. Kane (1992) noted that the hysteresis effect is found to be small at low and high latitudes but substantial at middle latitudes and is attributed to possible geomagnetic storm effects. Mikhailov and Mikhailov (1995) postulated that the observed hysteresis effect is due to the difference in the geomagnetic effect between the two solar activity epochs. Kane (2005) attributed the hysteresis effect to the delayed response of solar EUV flux to the F10.7 variation. Ortiz de Adler and Elias (2008) studied the foF2 hysteresis effect during three solar cycles 20, 21 and 22. They found a latitudinal variation of the hysteresis magnitude with a trough at the magnetic equator and crest at about ±30o geomagnetic latitudes and suggested that the hysteresis magnitude may be controlled by the geomagnetic activity. Chakraborty and Hajra (2008) using the long term (1978–1990) TEC data for Calcutta, a station below the northern crest of the EIA found both positive and negative hysteresis effect. They explained the effect in terms of cumulative effect of geomagnetic disturbance as the number of disturbed days is greater in the descending phase than that in the ascending phase. During solar cycle 23 the disturbed days (Dst < 50 nT) for the period 1995– 2003 have been identified. For the ascending half (1995– 2000) the number of disturbed days is 8, 2, 9, 20, 14 and 29 respectively. For the descending half the number of disturbed days is 33, 35 and 36 respectively from 2001 to 2003. The number of disturbed days per year during the descending half of the solar cycle is more than that during the

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