Available online at www.sciencedirect.com
Advances in Space Research 43 (2009) 1676–1682
www.elsevier.com/locate/asr
The equatorial ionization anomaly at the topside F region
of the ionosphere along 75°E
P.K. Bhuyan *, K. Bhuyan
Department of Physics, Dibrugarh University, Dibrugarh 786004, India
Received 31 October 2007; received in revised form 15 September 2008; accepted 15 September 2008
Abstract
Electron density measured by the Indian satellite SROSS C2 at the altitude of 500 km in the 75°E longitude sector for the ascending
half of the solar cycle 22 from 1995 to 1999 are used to study the position and density of the equatorial ionization anomaly (EIA). Results
show that the latitudinal position and peak electron density of the EIA crest and crest to trough ratios of the anomaly during the 10:00–
14:00 LT period vary with season and from one year to another. Both EIA crest position and density are found to be asymmetric about
the magnetic equator and the asymmetry depends on season as well as the year of observation, i.e., solar activity. The latitudinal position
of the crest of the EIA and the crest density bears good positive correlation with F10.7 and the strength of the equatorial electrojet (EEJ).
Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Ionosphere; Topside ionosphere; Equatorial ionization anomaly (EIA); Equatorial electrojet (EEJ); SROSS C2
1. Introduction
The equatorial ionization anomaly (EIA) is characterized by a depression in ionization densities (or trough) at
the geomagnetic equator and two peaks (crests) on either
side of the equator at about 15° magnetic latitudes (Appleton, 1946). Mitra (1946) suggested that the trough exists
because plasma produced by photo ionization at great
heights over the magnetic equator diffuses downwards
and outwards to the north and south leaving depletion at
the equator. Martyn (1947) offered the explanation that
the mutually perpendicular east–west electric field and
north–south geomagnetic field give rise to an upward electrodynamic (E B) drift of plasma during the daytime. As
the plasma is lifted to greater heights, it diffuses downward
along geomagnetic field lines towards higher latitudes
under the influence of gravity and pressure gradients and
produces the anomaly. The ratio of the electron density
at the crest to the electron density at the trough is a measure of the intensity of the anomaly. The crest to trough
*
Corresponding author.
E-mail address: [email protected] (P.K. Bhuyan).
ratio is highest near the height of the F2 region peak and
decreases both downward and upward. The crest-to-trough
ratio and the latitudinal position of the crests vary with
solar and geomagnetic activity (Su et al., 1995). The EIA
is also asymmetric about the geomagnetic equator caused
by field aligned plasma flow due to neutral winds (Balan
et al., 1995). Another factor that affects the F region ionization at low latitudes is the strength of the electric field
that drives the equatorial electrojet (Dabas et al., 1984).
The EEJ is driven by the Pedersen east–west electric field
in the lower E region (Fejer, 1981). The F region east–west
drifts are driven by the F region vertical electric field, which
is coupled along the magnetic field lines to the E region.
Electron density in the equatorial F region is affected by
the variations in electrojet electric field and due to the E
and F region coupling. Bhuyan et al. (2003) have reported
a positive correlation between electron density in the topside F region over India and the strength of the EEJ.
The anomaly crests in both hemispheres occur at lower
altitudes and become weaker with height. At about
800 km, the equatorial anomaly disappears (Balan et al.,
1997). From a study of in situ electron density measured
by the Hinotori satellite during 1981–1982, a period of
0273-1177/$36.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.asr.2008.09.027
P.K. Bhuyan, K. Bhuyan / Advances in Space Research 43 (2009) 1676–1682
national Reference Ionosphere (IRI) 2001 model is compared with the measured data.
2. Results
In Fig. 1, the measured electron density in equinox for
the 10:00–14:00 LT period is plotted against latitude separately for the years 1995–1999. The solid line depicts the
1.00E+13
1995
1.00E+12
1.00E+11
1.00E+10
1.00E+13-20
-10
0
10
20
30
40
1996
-10
0
10
20
30
40
1997
-10
0
10
20
30
40
1998
-10
0
10
20
30
40
1999
1.00E+12
1.00E+11
1.00E+10
1.00E+13-20
Electron density (m-3)
moderate to high solar activity, Su et al. (1995) found that
at 600 km altitude the electron density has its highest value
within a broad range of latitude around the geomagnetic
equator. At higher latitudes, the electron density decreases
with increasing latitude in both hemispheres and is higher
in the summer hemisphere than in the winter hemisphere.
Bhuyan et al. (2003) reported that the electron density at
500 km altitude in the 75°E longitude sector measured
by the SROSS C2 satellite during low solar activity showed
asymmetric equatorial ionization anomaly. In the equinoxes, electron density maximizes at 10°N and 5°S geomagnetic latitudes and the peak is higher in the Northern
Hemisphere than in the Southern Hemisphere. In the
December solstice, the ionization peak in the south is
higher than that in the north, whereas it was observed that
the EIA is not well developed at this altitude in the June
solstice. There has been a shift of the EIA crest from
10°N in the equinoxes to 5°N in the December solstice.
Neutral winds blowing across the magnetic equator facilitate or retard the diffusion along magnetic field lines
depending upon wind direction and the magnetic field
geometry (Anderson and Roble, 1981). Consequently, the
anomaly is quite often asymmetric about the magnetic
equator in its magnitude (Balan et al., 1995) and location
of the trough (Huang, 1986). Bilitza et al. (1996) have
reported local time, latitude, longitude and seasonal variation of the EIA by using ionospheric total electron content
(IEC) data derived from the Topex/Poseidon altimeter
measurements. Satellite borne ionospheric measurements
have been carried out in the past on satellite missions like
the AE, DE, ISIS, Aeros, etc. Yet the data from topside
F region measurements over Indian equatorial and low latitudes are sparse. The Japanese Hinotori satellite in a near
circular orbit of 600 km provided an ideal database for
study of temporal and spatial variations of electron density
in the topside ionosphere (Watanabe et al., 1995; Su et al.,
1995). But the data are limited to a period of medium and
high solar activity. The Indian satellite SROSS C2
launched in May 1994 was in a 630 by 430 km orbit till
2000. The average height of the satellite was 500 km. It
carried Retarding Potential Analyzer (RPA) sensors for
measuring electron and ion parameters. ROCSAT carried
out plasma density measurements over equatorial and
low latitudes from the high to medium solar activity years
2002–2004 (Yeh et al., 2001; Su et al., 2002; Le et al., 2003;
Burke et al., 2004; Kil et al., 2004; Lin et al., 2005; Kil and
Paxton, 2006).
In this paper, we investigate the seasonal and annual
variations of the structure of the equatorial ionization
anomaly and their association with the equatorial electrojet
and solar activity in the Indian zone during the ascending
half of the solar cycle 22 for the period 1995–1999. The
monthly mean F10.7 during the period varied from 69 (in
the equinoxes of 1996) to 164 (in June solstice of 1999).
The seasonal and annual variations of the electron density
at the altitude of 500 km for the time space configuration
similar to that of the SROSS C2 and predicted by the Inter-
1677
1.00E+12
1.00E+11
1.00E+10
1.00E+13-20
1.00E+12
1.00E+11
1.00E+10
1.00E+13-20
1.00E+12
1.00E+11
1.00E+10
-20
-10
0
10
20
30
40
Geomagnetic Latitude (°N)
Fig. 1. Latitudinal distribution of electron density measured by the
SROSS C2 at 500 km along 75°E at the equinoxes. The solid and dashed
lines are the measured averages and the IRI 2001 predicted densities,
respectively, at 5° latitude intervals.
P.K. Bhuyan, K. Bhuyan / Advances in Space Research 43 (2009) 1676–1682
density averaged at 5° latitude intervals and the dashed line
shows the density predicted by the International Reference
Ionosphere 2001 model at similar intervals within ±30°.
Measured electron density at the altitude of 500 km exhibits a trough at the magnetic equator and peak at a location
away from the equator every year. The data coverage is
predominantly confined to the north of the geomagnetic
equator. Therefore, the position and the strength of the
EIA’s northern arm are easily distinguishable from the figure, whereas the southern part of the EIA could not be
clearly identified. The location of the northern crest of
the anomaly is closest to the geomagnetic equator
(4°N) in 1996 and furthest from the equator (14°N) in
1998 and 1999. In the years 1995 and 1997, the northern
crest of the EIA forms at 9°N and 10°N, respectively.
The positions of the EIA crests have been found to be independent of the time of the day, i.e., for forenoon and afternoon hours. The ionization density also varies from one
year to another in this season. Density at the crest is lowest
in 1996 (2.1 1011 m 3) and highest in 1999 (1.05 1012 m 3). The crest-to-trough ratios are 1.78, 1.68,
1.96, 1.59 and 1.18, respectively, from 1995 to 1999.
There is larger spread in the densities during low solar
activity period of 1995–1996 than during the remaining
years partly because the data during the low activity
years are more than during the moderate or high activity
years from 1997 to 1999. The SROSS C2 satellite did not
carry on board memory for the RPA payload and the
data were received when the satellite passes were over
the tracking station at Bangalore and due to operational
reasons and data retrieval procedure seasonal or annual
data sets are not uniform. The electron density predicted
by the IRI shows a broad trough and two crests at 15°S
and 20°N geomagnetic latitudes. The position of the EIA
crests does not vary with solar activity. It may also be
noted that the electron density predicted by the IRI is
closer to the measured density within ±10° of the geomagnetic equator but at higher latitudes IRI overestimates electron density.
The latitudinal distribution of measured and predicted
electron density (10:00–14:00 LT) at the altitude of
500 km is shown in Fig. 2 for the December solstice. The
figure indicates that the crest of the northern equatorial
ionization anomaly as measured by the SROSS C2 in this
season is closest to the equator in 1996 (4°N) and shifts
northward a little in the following years to 5°N in 1997
and 6.5°N in 1998. In 1995 and 1999, the EIA crest is further away and forms around 8°N. The peak ionization at
the anomaly crest is equal to 3.2 1011 m 3, 1.7 1011 m 3, 2.2 1011 m 3, 5.2 1011 m 3 and 11 1011
m 3 from 1995 to 1999, respectively. The crest-to-trough
ratio is 1.02, 1.2, 1.26, 1.39 and 1.1, respectively.
The density as well as the strength of the anomaly is less
compared to those in the equinoxes. The latitudinal and
year to year variations of the IRI predicted density are similar to those in the equinoxes. IRI overestimates the topside
density in the Indian equatorial and low latitudes. How-
1.00E+13
1995
1.00E+12
1.00E+11
1.00E+10
1.00E+13
1996
1.00E+12
1.00E+11
1.00E+10
1.00E+13
-3
Electron densit y (m )
1678
1997
1.00E+12
1.00E+11
1.00E+10
1.00E+13
1998
1.00E+12
1.00E+11
1.00E+10
1.00E+13
1999
1.00E+12
1.00E+11
1.00E+10
-20
-10
0
10
20
30
Geomagnetic Latitude (°N)
Fig. 2. Latitudinal distribution of electron density measured by the
SROSS C2 at 500 km along 75°E at the December solstice. The solid and
dashed lines are the measured averages and the IRI 2001 predicted
densities, respectively, at 5° latitude intervals.
ever, the difference between measurement and prediction
is less around the geomagnetic equator.
In the June solstice (Fig. 3), the EIA is not well developed in the low activity years of 1995 to 1997. The crestto-trough ratio of the northern EIA is around 1 in these
years. This ratio increases to 1.73 in the moderate activity
year 1998 and to 1.83 in the high solar activity year 1999.
The crest of the northern EIA forms at the latitude of
P.K. Bhuyan, K. Bhuyan / Advances in Space Research 43 (2009) 1676–1682
1.00E+13
1995
1.00E+12
1.00E+11
1.00E+10
1.00E+13-20
-10
0
10
20
30
1996
-10
0
10
20
30
1997
-10
0
10
20
30
1998
-10
0
10
20
30
1999
1.00E+12
-3
Electron density (m )
1.00E+11
1.00E+10
1.00E+13-20
1.00E+12
1.00E+11
1.00E+10
1.00E+13-20
1.00E+12
1.00E+11
1.00E+10
1.00E+13-20
1.00E+12
1679
the density at the trough (i.e., at about the equator), the
ratio of crest density to trough density in each season of
the years of observation from 1995 to 1999 are given in
Table 1. It is seen from the table that in the equinoxes,
the anomaly is well developed when solar activity is low,
i.e., in the years 1995 to 1997. The crest density is nearly
double the trough density in 1997. As solar activity rises
to moderate and higher levels, the crest to trough ratio
declines. Minimum crest to trough ratio is observed in
the high activity year 1999. In the summer solstice, the
trend reverses, i.e., the crest to trough ratio is low in low
solar activity and high in the years of moderate and high
solar activity. In the December solstice, the crest to trough
ratio does not exhibit any particular trend. The basic features of IRI predicted density in the June solstice is more
or less similar to that in the winter solstice or in the equinoxes. However, in the low activity years IRI overestimates
the density significantly at all latitudes. Fig. 4 illustrates the
relationship between the latitudinal position of the crest of
the EIA north of the geomagnetic equator in each season
with the corresponding F10.7 and the strength of the equatorial electrojet. The EEJ has been calculated using the procedure followed in Bhuyan et al. (2003). It is seen that the
position of the crest bears good positive correlation with
solar activity (see Fig. 4). The latitudinal position of the
EIA crest is also positively correlated with EEJ as expected;
however, the correlation is low as compared to that with
solar flux. In the December solstice, correlation between
the position of the crest and EEJ is insignificant. In
Fig. 5, the electron density at the crest of the northern
EIA averaged over the seasons is plotted against corresponding F10.7 and the EEJ as in Fig. 4. The electron density at the crests of the northern EIA is positively correlated
with F10.7 and EEJ (see Fig. 5). However, the correlation
between density and F10.7 is very good (0.97) compared
to that between density and EEJ (0.61).
3. Discussion
1.00E+11
1.00E+10
-20
-10
0
10
20
30
Geomagnetic Latitude (°N)
Fig. 3. Latitudinal distribution of electron density measured by the
SROSS C2 at 500 km along 75°E at the June solstice. The solid and
dashed lines are the measured averages and the IRI 2001 predicted
densities, respectively, at 5° latitude intervals.
7°N in 1996 and gradually moves away from the magnetic equator to higher latitudes with increase in solar
activity. EIA’s northern crest is furthest from the equator
at 14°N in the high activity year 1999. A significant rise
in ionization density of the crest is observed in the moderate and high activity years.
The latitudinal position of the EIA crest north of the
geomagnetic equator, the corresponding electron density,
Ionization in the F region is directly proportional to the
solar ionizing radiations. Change in the solar flux at XUV
is one of the factors responsible for the observed seasonal
and inter-annual variability in the ionization density.
Another factor that directly affects the F region ionization
at equatorial and low latitude ionosphere is the strength of
the electric field that drives the EEJ. The EEJ is driven by
the Pedersen east west electric field in the lower E region.
Changes in electric field strength results in changes in
E B drift and subsequently changes in the amount of
plasma lifted up that diffuses downward to low latitudes
along the magnetic field lines. Bhuyan et al. (2003) have
reported a positive correlation between day and nighttime
electron density at 500 km over 10°N magnetic latitude
with EEJ during the low activity period of 1995–1996 in
all seasons. Rama Rao et al. (1983) had found that during
a strong electrojet day, the crest of the anomaly as seen
through total ionospheric electron content tends to form
1680
P.K. Bhuyan, K. Bhuyan / Advances in Space Research 43 (2009) 1676–1682
Table 1
Year to year variation of latitudinal positions of crest and crest to trough ratio for all year corresponding to different seasons.
Season
Year
Latitudinal position of
crest (geomag., °N)
Crest density
(m 3)
Trough density
(m 3)
Crest to trough
ratio
Equinox
1995
1996
1997
1998
1999
9.4
3.9
9.9
13.8
14.2
5.60e+11
3.57e+11
6.61e+11
9.67e+11
1.25e+11
3.15e+11
2.12e+11
3.38e+11
6.67e+11
1.05e+12
1.78
1.68
1.96
1.59
1.18
December solstice
1995
1996
1997
1998
1999
8.1
4.1
5.0
6.5
8.0
3.24e+11
1.73e+11
2.32e+11
5.18e+11
1.13e+12
2.23e+11
1.44e+11
1.84e+11
3.74e+11
1.03e+11
1.45
1.20
1.26
1.39
1.10
June solstice
1995
1996
1997
1998
1999
9.0
6.8
8.0
12.4
14.2
1.63e+11
2.10e+11
2.09e+11
8.60e+11
1.94e+12
1.60e+11
1.86e+11
1.99e+11
4.98e+11
1.05e+12
1.02
1.13
1.05
1.73
1.83
10
16
Latitudinal position of EIA crest ( °N)
r = 0.561
Equinox
r = 0.539
8
12
6
8
4
4
2
0
16
14
Equinox
12
r = 0.806 December Slostice
10
r = 0.228
December Solstice
12
10
8
8
6
6
4
4
2
2
16
14
16
r = 0.907 June Solstice
14
r =0.867
June Solstice
12
12
10
10
8
8
6
6
4
4
2
50 60 70 80 90 100 110 120 130 140 150
F10.7
50
60
70
80
90
100
110
delta H , nT
Fig. 4. Plot of the latitudinal position of the equatorial ionization anomaly (EIA) crest against F10.7 and equatorial electrojet EEJ.
northward of the normal latitudinal position. Bhuyan
(1993) has found that the EIA peaks at different latitudes
in different seasons. The EEJ and Sq. current systems are
coupled interactive systems. The mean daytime Sq. focus
is asymmetric about the dip equator and the total intensities of the Sq. current system vary with season (Matsushita
and Campbell, 1967). It may be suggested that the EEJ
resulting out of the asymmetric current system produce
P.K. Bhuyan, K. Bhuyan / Advances in Space Research 43 (2009) 1676–1682
2.5E+12
2.5E+12
-3
Electron densit y ( m )
r = 0.982
Equinox
2.0E+12
2.0E+12
1.5E+12
1.5E+12
1.0E+12
1.0E+12
5.0E+11
5.0E+11
1.0E+09
1.0E+09
2.5E+12
r =0.606
Equinox
r = 0.642
December solstice
r =0.605
June Solstice
2.5E+12
r = 0.974
December solstice
2.0E+12
2.0E+12
1.5E+12
1.5E+12
1.0E+12
1.0E+12
5.0E+11
5.0E+11
1.0E+09
1.0E+09
2.5E+12
2.5E+12
r = 0.973
June solstice
2.0E+12
2.0E+12
1.5E+12
1.5E+12
1.0E+12
1.0E+12
5.0E+11
5.0E+11
1.0E+09
50
1681
60 70
80
90 100 110 120 130 140 150
F10.7
1.0E+09
50
60
70
80
90
100
delta H, nT
Fig. 5. Plot of the equatorial ionization anomaly (EIA) crest density against F10.7 and equatorial electrojet EEJ.
seasonal variations in the magnitude of the vertical drift
velocities resulting in lifting up of ionization to varying
heights above the equator. Consequently, the anomaly
forms at different latitudes. However, better correlation
of both EIA crest density and position with solar ionizing
flux than that with EEJ needs to be investigated further
with extended data and theoretical models.
Acknowledgements
This work was done with partial support from the Indian CAWSES program. The authors express their sincere
thanks to S.C. Garg, PI, P. Subrahmanyam and all others
involved in the SROSS C2 RPA project. The authors wish
to thank Mr. P. Mahanta for his help in preparation of the
manuscript.
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