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Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–1673

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Seasonal characterization of the equatorial electrojet heightrise over Brazil as observed by the RESCO 50MHz

back-scatter radar

C.M. Denardini�, M.A. Abdu, E.R. de Paula, J.H.A. Sobral, C.M. Wrasse

Instituto Nacional de Pesquisas Espaciais, P.O. 515, S. J. Campos, SP, Brazil

Available online 24 August 2005

Abstract

In this paper, we present the seasonal characteristics of the post-noon rise of the equatorial electrojet 3-m

irregularities scattering region observed over Sao Luıs, Brazil (2:3�S, 44:2�W, dip:�0:5�). The study is based on a 1 year

data set collected by the 50MHz coherent back-scatter radar (also known by the acronym RESCO), that started

operation in 1998. Using a method to estimate the moments from each individual back-scattered power profiles that

constitute the standard range–time–intensity (RTI) maps we were able to determine the following representative

parameters of the electrojet: the total power back-scattered by the electrojet irregularities (EJP), the thickness of the

electrojet back-scattered power profile (EJT), the height of the center of the back-scatter region, that is, the power

profile, (EJC), and the noise level corresponding to each power profile (EJN). The parameterization was applied to all

selected daily RTI maps from 2002. The analysis was carried out by grouping the data according to the radar beam

angle (tilted 30� westward or eastward), the magnetic disturbance indices Kp and the season, which enables us to

quantify the differences in the parameters on these bases. The results are presented and discussed here focusing on the

post-noon ascent of the EJC. We will also present results on the east–west asymmetry in the radar back-scattered echo

power confirming our previous results, and on the appearance of a scattering region after sunset during magnetically

quiet conditions around the southern summer solstice.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: E-region; Back-scatter radar; Equatorial electrojet; Plasma irregularities height rise

1. Introduction

The equatorial electrojet (EEJ) is an eastward current

sheet that flows in the ionospheric plasma at the E layer

heights between about 90 and 120 km centered at the

magnetic equator and extends in a zone of about 600 km

in the north–south direction (Forbes, 1981; Reddy,

1989). The EEJ current can drive plasma instabilities

e front matter r 2005 Elsevier Ltd. All rights reserve

stp.2005.04.008

ing author. Tel.: +5512 3945 7156;

5 6990.

ess: [email protected] (C.M. Denardini).

leading to the formation of field-aligned plasma

irregularities. Two types of plasma instabilities are

known to operate in the equatorial E-region. The

Farley–Buneman instability, also known as two-stream

instability (Farley, 1963; Buneman, 1963), manifests

itself as type-1 radar echo Doppler spectrum, and is

excited when the electron drift velocity with respect to

the ions, exceeds the threshold value of ion-acoustic

speed. The gradient drift instability (GDI), which

produces type-2 Doppler spectrum of radar echoes,

operates in regions where the ambient E-field and the

density gradient must be in the same direction (Simon,

d.

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ARTICLE IN PRESSC.M. Denardini et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–16731666

1963; Hoh, 1963). During daytime when the polarization

E-field is upwards, the GDI mechanism may operate in

height regions where the density gradient is upward and

during nighttime vice versa. This mechanism is similar to

the gravity instability mechanism (Rosenbluth and

Longmire, 1957).

Equatorial plasma irregularities have been extensively

investigated using VHF coherent and incoherent scatter

radars during the last few decades. In the American

sector pioneering investigations have been carried out

over Jicamarca–Peru (Cohen and Bowles, 1967; Balsley,

1969; Cohen, 1973; Fejer et al., 1975; Farley, 1985).

Extensive investigations of electrojet irregularities have

been conducted from Trivandrum, India since the 1970s

(Prakash et al., 1971; Reddy and Devasia, 1981;

Somayajulu et al., 1994), and investigations in Brazil

were started recently over Sao Luıs (Abdu et al., 2002,

2003; Denardini et al., 2004; de Paula and Hysell, 2004).

Additionally, many important basic aspects of the

electrojet irregularity processes have been discussed in

the literature, like the dependence of the irregulari-

ties phase velocity on the polarization electric field

(St.-Maurice et al., 1986) and the spectral asymmetries

in the EEJ (Kudeki et al., 1985).

The magnetic equatorial region has a large long-

itudinal extension (�40�) in Brazil. It also possesses

certain peculiarities in the geomagnetic field configura-

tion that are distinctly different from other longitude

sectors. A notable peculiarity is the large magnetic

declination angle (being �21�W) at the RESCO radar

site, which is significantly different from that of

Jicamarca (being �4�E).

In this paper, we intent to focus the discussion on the

seasonal behavior of EJC. We will also show the

appearance of a scattering region after sunset during

magnetically quiet conditions around the summer

solstice and a consistent presence of the east–west power

asymmetry in the back-scatter radar signal.

2. Radar sounding description and data analysis

technique

RESCO (Radar de ESpalhamento COerente) is a VHF

coherent back-scatter radar system located at Sao Luıs -

SLZ (2:3�S, 44:2�W, dip:�� 0:5�). It consists of a

modular system of 8 transmitters each one having

capability of up to 15 kW peak power and operates at

50MHz, sensitive to field aligned 3-m irregularities.

During the reported observations it was operated with 4

transmitters connected to the antenna array and a

transmitting peak power of about 40 kW. The coaxial-

collinear antenna array consists of 768 dipoles arranged

in 16 antennas aligned in magnetic north–south direc-

tion. The antenna beam width is 3� and 7� wide in the

geomagnetic N–S and E–W planes, respectively, and can

be oriented to three fixed directions: vertical, westward

and eastward (tilted 30� with respect the zenith), or can

be operated in a beam switching mode, commutating

between any two directions. The beam switching is

usually done at a rate of 1-min for each direction when

running EEJ experiments. At each beam position the

radar transmits a pulse train at a rate of 512Hz, at the

beginning of every 6 s thus providing 10 power spectra

per minute. After each transmitted pulse the correspond-

ing echo is received, processed, sampled and stored. For

the present study of the diurnal EEJ the transmitted

pulse width was set at 20ms, the inter-pulse period was

set to 1ms to avoid ambiguity in echo detection, the time

delay before enabling echoes acquisition was set to

620ms giving an initial height at around 80 km, the width

of sampling window was set at 20ms (3 km in range �

2.6 km in height, since we have used oblique transmis-

sion only) and the number of sequential samples per

echo was set to 16, which gave a height range for the

present investigation from 80 to 120 km of altitude, with

2.6 km vertical sample resolution. The vertical height

resolution calculated from the pulse width and the beam

width considerations in the E–W plane is 7.6 km. (For

details on the radar configuration and operation, see,

also Abdu et al., 2002)

The basic data analysis consists of building one

spectrogram per sampled height; each spectrogram

being a contour map of spectral power plotted in a

format of Doppler frequency versus local time. Time

variation of the total received power from a given height

is obtained by integrating in frequency each spectrum of

the corresponding spectrogram. A daily range–time–in-

tensity (RTI) map is then produced from the time

variations of the total received power according to the

height they were obtained (see Fig. 1). Next step in the

data analysis is to parameterize each vertical profile of

back-scattered power from the EEJ. We used an

algorithm to determine the representative Gaussian

parameters through estimation of the moment similar

to that used by Reddy et al. (1987). The estimated

parameters are: the total power back-scattered by

the EEJ irregularities (EJP), the thickness of the EEJ

back-scattered power profile (EJT), as seen by radar, the

center height of the back-scattered power region in the

EEJ (EJC) and the noise level of the power profile

(EJN). All these parameters are referred to the scattering

volume containing 3-m irregularities and are measured

in terms of the time delay from the leading edge of the

transmitted pulse. Regarding the EJC estimation, one

should remember that the theoretical vertical height

resolution estimated is 7.6 km, while the sampled height

interval is �2:6 km (corresponding to a transmitted

pulse width of 20ms). This means that 16 adjacent

sampling pulses produce, from the received echo, 16

adjacent height samples at intervals of 2.6 km within the

total sampled height interval extending from 80 to

ARTICLE IN PRESS

Fig. 1. Mean RTI maps for (a) summer solstice, (b) winter solstice and (c) equinoxes obtained for (left) the quiet period (Kpp3þ) and

(right) the disturbed period (Kp43þ), using the RESCO radar beam tilted (upper panels) 30� west and (lower panels) 30� east in beam

switching mode. The color scale gives the signal power in dBm.

C.M. Denardini et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–1673 1667

120 km. This height range covers the normally expected

EEJ associated 3-m irregularity echo region, known to

be centered around 105 km (see, for example, Fejer et

al., 1975; Prakash and Muralikrishna, 1981). We may

note further that the estimated theoretical height

resolution of 7.6 km can result in somewhat smeared

out structures, observed with the 2.6 km height sam-

pling, which does not affect the present study of the EJC

characteristics, and especially the relative changes in the

EJC height with LT (to be discussed later). Thus even

though a precise estimation of the EJC height may have

a limitation imposed by the theoretical height resolution,

the data on the change in EJC height with local time is

accurate enough for the purpose of the analysis

presented in this paper. Furthermore, we have calculated

the EJC for each vertical profile individually and then

averaged them for all days of observation considered in

this study. We have determined that the error in

ARTICLE IN PRESS

Table 1

Data set classification according to the magnetic activity and

Southern hemisphere seasons

Season Month Days

Quiet Period

Summer January 21, 22, 23, 28, 29, 30

November 14, 15, 16, 18, 19, 26, 28, 29

December 02, 03, 05, 06, 09, 10, 11, 12

Winter May 24, 28, 29

June 20, 21, 24, 25, 26, 27

July 24, 26

Spring September 12, 13, 17, 20

November 08, 09

Autumn February 14, 15, 16, 18, 20, 21, 22, 23, 24, 25, 26

March 19, 21, 22, 26, 27, 28

April 23, 24, 25, 26, 29, 30

May 02

Disturbed Period

Summer December 01, 04, 07, 08

November 13, 17, 20, 22, 23, 24, 25, 27, 30

Winter May 23, 27

July 23, 25

Spring August 14, 26

September 10, 11

November 05, 06, 07, 10, 11, 12

Autumn February 17, 19, 27, 28

March 20, 25

April 22

C.M. Denardini et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–16731668

estimating EJC is typically �0:9 km, though it is variable

according to the conditions of signal-to-noise ratio.

The parameterization was applied individually over

all days selected for this analysis. The resulting para-

meters were grouped according to the beam direction,

magnetic activity and season. Table 1 presents the

classification of the selected days. The Kp index

(Rostoker, 1972) was used for magnetic activity

classification. For values of Kp43þ the data was

classified as disturbed. The season refers to the southern

hemisphere, and the parameters grouped into a given

season were sample averaged to obtain seasonally

averaged diurnal variation of the parameters. We have

obtained such diurnal patterns for EJP, EJT, EJC and

EJN, in the form of their respective R–T maps for the

two subgroups based on Kp values, and the results are

presented and discussed in the following section with

specific focus on the EJC height variation.

3. Results

Fig. 1 shows the seasonally averaged RTI maps with a

time resolution of 2min. An outstanding feature we see

in these maps is an appreciable difference in the echo

power received from the opposite beams. A comparison

between the upper and the lower panels reveals that the

echo power is always higher in the westward beam than

in the eastward beam at all local times and seasons. This

east–west asymmetry in power is consistent with the first

observation of such asymmetry reported for Jicamarca

by Balsley (1970). We have calculated the west-to-east

power ratio to be of the order of four around the peak

heights of the EEJ, but precise values will be given in a

separate analysis of the EJP and EJT in an ensuing

work. It should be also remembered that the radar is

located south of dip equator at the dip latitude �0:5�,and that the antenna array is magnetically north–south

aligned, so that we expect equivalent EEJ irregularity

regions illuminated by the opposite beams. Thus, the

asymmetry in echo power detected by our radar refers

obviously to a location slightly southward of the EEJ

central latitude, that is, the dip equator, whereas the

asymmetry reported over Jicamarca radar corresponded

to 1� north of the dip equator.

Another interesting phenomenon can be observed in

the RTI maps for the summer period. An enhanced

scattering region is formed between about 1800 and

1930LT. The local time of occurrence of this region

matches exactly with the twilight at the E-region heights

during summer. The occurrence of an echo region near

(and after) 1800LT in December that is associated with

enhanced phase velocity was described in a recent

publication by Abdu et al. (2002). The present results

seem to bring out clearly the RTI characteristics of this

phenomenon. This aspect will not be discussed further in

this paper, however. The main characteristic of the EEJ

echoes that we intend to discuss in this paper is the

evening rise of the scattering region.

An examination of the RTI maps of Fig. 1 reveals the

regular presence of rising EEJ scattering region during

the post-noon hours (from �1400 to �1700 LT)

irrespective of the different magnetic and seasonal

conditions of their observation. Such a behavior has

been observed also in the results presented by Fejer et al.

(1975) over Jicamarca although these authors did not

focus their attention on this point. Height shift in the

EEJ echo region associated with the morning and

evening prereversal electric field enhancement has been

reported over Thumba, India by Muralikrishna and

Prakash (1978) which they attributed either to a shift in

the vertical profile of the Hall polarization field or in the

electron density gradients. They have shown a height

shift in the echo peak position from 760ms ð�114 kmÞ at1610LT to 810ms ð�121:5 kmÞ at 1830LT.Our results show such height shift to start at 1400LT

(near 100 km) and rising by about 7–8 km until 1700LT.

In the Brazilian sector it was first reported over Sao Luıs

based on the RESCO data by Abdu et al. (2002), and

more recently based on the 30MHz back-scatter radar

data by de Paula and Hysell (2004). Since the rise/ascent

of the scattering region is observed during all seasons

and irrespective of the magnetic condition, we may

consider its presence to be a characteristic of the daily

ARTICLE IN PRESSC.M. Denardini et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–1673 1669

behavior of the EEJ on the Brazilian sector. However,

the degree of the ascent of the scattering region seems to

have a seasonal dependence. It looks to be more

pronounced in summer, with the EJC rising from

around 100 up to 108 km of altitude. During equinoxes

the rising is also very clear, but the height range covered

seems to be quite smaller (from 101 to 107 km only)

compared to that of summer. During winter the rising

feature is not clearly defined in the RTI maps.

Unfortunately, the data set collected during winter does

not have a good statistic for both quiet and disturbed

periods. Hence, the less appreciable rise during winter

seen in these maps should not be considered conclusive.

To confirm the rising feature in the RTI maps and to

try to understand the probable driving mechanism, we

have performed an analysis on the seasonal dependence

of the local time variation of the EJC. Fig. 2 presents the

diurnal variation of the mean EJC obtained for

disturbed (‘‘a’’ and ‘‘b’’) and quiet (‘‘c’’ and ‘‘d’’)

magnetic conditions, separated according to the radar

9899

100101102103104105106107108109110111

08 09 10 11 12 13 14 15 16 17 18 19

979899

100101102103104105106107108109110111

98 % Conf.: ± 2.78 km98 % Conf.: ± 4.36 km98 % Conf.: ± 1.49 km


WEST

ALT

ITU

DE

(km

)

DISTURBED SUMMER

DISTURBED WINTER

DISTURBED EQUINOX

LOCAL TIME (h)

ALT

ITU

DE

(km

)

QUIET SUMMER QUIET WINTER QUIET EQUINOX

(a)

(c)

Fig. 2. Diurnal variation of the mean EJC, obtained from disturbe

according the radar beam and season. The daily mean limits of confid

were calculated for 98% of statistical confidence.

beam orientation and season. According to these plots,

the EJC ascent starts at about 1400LT, and rises by

5–7 km in a period of about 4 h giving an exponential

rising shape to the EJC graph. The mean EJC values

obtained for the eastward beam present a higher level of

scattering due to the higher uncertainty when estimating

each individual EJC from the vertical power profiles.

The lower power received from the eastward beam

corresponds to reduced signal-to-noise ratio, which

increases the variability of the individual EJC around

their most probable value, resulting in higher values for

the same limit of confidence (98% in all cases). The

mean EJC values obtained for the disturbed condition

appears more spread out than those for the quiet

condition. This is not surprising since the disturbed

conditions are known to cause larger day-to-day

variability in the EEJ characteristics including the

EJC. During disturbed time the energy deposition in

the high latitude thermosphere changes the global

thermospheric circulation and, as a consequence,

20 09 10 11 12 13 14 15 16 17 18 19 20



EAST

LOCAL TIME (h)

(d)

(b)

d (‘‘a’’ and ‘‘b’’) and quiet (‘‘a’’ and ‘‘b’’) periods, separated

ence are presented at the right bottom corner in each panel and

ARTICLE IN PRESS

Table 2

Parameter of the exponential growing (h ¼ h0 þ xeDt=t) fitted to

the different EJC curves, restrict to the period between 1400

and 1700LT, where x ¼ 1 km.

Season Parameter Westward Eastward

Quiet Period

Summer solstice h0 99.4 km 99.5 km

t 1.58 h 1.17 h

Correlation 0.99437 0.94487

Winter solstice h0 99.4 km 100.3 km

t 5.67 h 2.81 h


Equinoxes h0 100.9 km 100.1 km

t 1.80 h 3.19 h


Disturbed Period

Summer solstice h0 100.1 km 99.9 km

t 1.50 h 1.78 h


Winter solstice h0 101.6 km 101.6 km

t 3.84 h 3.13 h


Equinoxes h0 99.9 km 101.0 km

t 1.82 h 1.40 h


C.M. Denardini et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–16731670

disturbance dynamo electric fields extend to equatorial

latitudes (Blanc and Richmond, 1980), where the EEJ

current system and irregularity development are drasti-

cally modified. Transient events which occur under

disturbed condition, such as the prompt penetration of

magnetospheric electric fields to the equatorial region

also serve as a source of large variability in the EEJ

processes. See, for example, Reddy (1989) and Abdu

et al. (2003).

In order to quantify the rising feature of the EJC we

used an exponential fitting of the form h ¼ h0 þ xeDt=t

(with the scale unit x being equal to 1 km) for the EJC

curves, restricted to the period between 1400 and

1700LT. The initial height (h0) is defined as the height

1 km below the lowest EJC found in the time interval. By

means of curve fitting we determined the time constants

(t) and initial heights (h0) for all cases presented above,

and the results are listed in the Table 2, where the linear

correlation index can also be found.

4. Discussion

The basic condition for the development of the EEJ is

the partial or total inhibition of the vertical Hall current

driven by the primary east–west (dynamo) electric field

Ep. This inhibition produces a strong vertical Hall

polarization electric field that in turn enhances the

horizontal current. This vertical electric field Ez is given

by (Fejer and Kelley, 1980):

Ez � ðni=OiÞ½Ep=ð1þ cÞ�, (1)

where c ¼ nine=OiOe, being ni and ne the ion and

electron collision frequencies and Oi and Oe the ion

and electron gyrofrequencies. Based on plasma fluid

equations Fejer et al. (1975) have derived a general

linear dispersion relation for the plasma wave angular

frequency given by

or ffi kðV e þ cV iÞ=ð1þ cÞ, (2)

where V e and V i are, respectively, the horizontal

electron and ion drift velocities, and k is the westward

component of the wave vector. Knowing that horizontal

velocity V e ¼ Ez=B0 (where B0 is the geomagnetic field

intensity), and neglecting the ion drift velocity, which is

reasonable at E-region heights, the wave phase velocity

from Eqs. (1) and (2) is given by

Vf ¼ oi=kffi ðni=OiÞ½ðEp=B0Þð1þ cÞ2�. (3)

As stated by Fejer et al. (1975) the maximum drift

velocity occurs at a height where when c ¼ 1 (i.e., where

nine ¼ OiOe) while the phase velocity maximizes where

c ¼ 1=3, which occurs near 105 km. We examined the

possible change in the height of c ¼ 1=3 that can be

caused by possible variation in the collision frequencies

between 1400 and 1700LT. The electron-neutral and ion-

neutral collision frequencies are, respectively, given by

ni ¼ ð2:6� 10�9ÞnA�1=2n and (4a)

ne ¼ ð5:4� 10�9ÞnT1=2, (4b)

where n denotes neutral atmosphere density, An denotes

mean molecular mass and T e is electron temperature

which is equal to the neutral temperature. A change in the

neutral temperature can cause a corresponding change in

the atmospheric scale height and therefore in the neutral

density leading to variations in both ne and ni. We used

the MSIS-E-90 model to calculate the increase in height,

at which c � 1=3 occurred, from 1400 to 1700LT as

result of the change in collision frequencies. The change

in height was found to be of the order of 2 km, which is

much smaller than the observed height increase. There-

fore, changes in the collision rates itself does not seem to

explain the observed post-noon ascent of the EJC.

Given that the E region behaves close to an a-Chapman layer, we then calculated the rising of the

reduced height of the peak-ion production (zm) observed

in the afternoon period. According to the Chapman’s

theory, the electron density (ne) varies as ½cos w�1=2, andthe ion production rate (q) varies as ½cos w], w being the

solar zenith angle, and the zm given by

zm ¼ ln½cosðwÞ�. (5)

ARTICLE IN PRESS

14:00 14:30 15:00 15:30 16:00 16:30 17:00

14:00 14:30 15:00 15:30 16:00 16:30 17:00

14:00 14:30 15:00 15:30 16:00 16:30 17:00

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

(EJC

- E

JCo)

/ S

cale

Hei

ght

LOCAL TIME (h)

Quiet Winter21-JUN-2002Westward BeamEJCo = 100 kmExos. Temp. = 1037 K

(EJC

- E

JCo)

/ S

cale

Hei

ght Quiet Equinox

20-MAR-2002Westward BeamEJCo = 100.5 kmExos. Temp. = 1037 K

RESCO Back-Scatter 50 MHz Coherent Radar - SZL - Brasil(E

JC -

EJC

o) /

Sca

le H

eigh

t Quiet Summer22-DEC-2002Westward BeamEJCo = 99.5 kmExos. Temp. = 1037 K

α-Chapman with qm -> (COSχ)



α-Chapman with qm -> (COSχ)2/5



Fig. 3. Peak-production height (zm) of an a-Chapman layer, calculated for the case when electron density varies as ½cosðwÞ�2:k for three

situations: summer, winter and equinox. The mean EJC are superimposed on the corresponding graphs.

C.M. Denardini et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–1673 1671

ARTICLE IN PRESSC.M. Denardini et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–16731672

Since the electron density varies as ½cos w�1=2, the E

region critical frequency foE, as measured, for example,

by a digisonde, should vary with solar zenith angle

as ½cos w�1=4. Thus, we used the representation

foE / ½cos w�k, which corresponds to ne / ½cos w�2:k,and q / ½cos w�4:k and calculated the variation of zm as

a function of local time between 1400 and 1700LT for

two k values, k ¼ 0:1 and k ¼ 0:25. The results are

shown in the Fig. 3 together with the mean EJC values

for three situations: summer, winter and equinox. The

solar zenith angle was calculated using the appropriate

astrometric consideration to the latitude of the RESCO

radar site. According to these calculations an explana-

tion for the EJC rising through the vertical displacement

of the peak ionization height in the E region seems to be

acceptable. We note that most of the EJC values lie

within the two curves corresponding to the two k values,

0.1 and 0.25. This range of k values is in agreement with

the values required to describe the foE variation

observed by digisonde (see, for example Hargreaves,

1992). In summer the EJC variation shows very good

fitting for k values that lie in the middle between 0.1 and

0.25, while the results for equinox shows reasonable

fitting for k values closer to 0.25, and the winter values

are more scattered for reasons commented earlier.

The growth rate of the gradient drift plasma

instability is dependent on the electron density gradient

length LN, as per equation (Fejer and Kelley, 1980):

G ¼1

1þ corV i

OiLNk

� �� 2ane, (6)

where a is the recombination coefficient and the other

terms have been defined before. The smaller the gradient

length the higher is the growth rate. Thus, the ascent of

the EJC would suggest a corresponding ascent of the

region of the smallest LN with increasing solar zenith

angle, during the post-noon period, in conformity with

the behavior of an a-Chapman layer. The values of the

time constant t obtained in the fitting are an indication

that our explanation based on the rising of the electron

density gradient region of an a-Chapman layer may

indeed be an acceptable one based on the following

consideration. The time constants obtained for the

summer period are lower than those for equinox. Table

2 shows that the initial heights of EJC before it start

rising are quite the same for both seasons and the

shorter time constant for summer results in larger EJC

at 1700LT in this season than in equinoxes (Fig. 2). The

behavior of EJC in relation to the peak height (zm)

variations in Fig. 3 also shows that, if the E region

behaves like an a-Chapman layer, the time constant t forthe EJC ascent corresponds, according to its cos wdependence, to a smaller value of k in equinox than in

summer. Regarding the winter period, as mentioned

before, the data set collected does not represent a good

statistic but it still shows the rising feature of the EJC

that fits in with the explanation presented here.

Comparing the time constants t from the quiet and

disturbed periods we see that those from quiet period are

usually larger than those from disturbed period. This

shows up in correspondingly different EJC attained at

1700LT. If we look at the initial heights (quiet vs.

disturbed) on the Table 2 we see no much difference

among them. However, the top EJC height at 1700LT in

the graphs of Fig. 2 are higher during disturbed periods,

especially during summer. Thus, it appears that the EJC

rise is faster during disturbed periods. Unfortunately, we

do not have an explanation now for the higher evening

heights attained by the EJC during disturbed period.

Finally, we would like to emphasize that all fitting

presented a high correlation index, except for the winter.

Better values were obtained for the quiet period, which

is not surprising due to inherent characteristics of the

disturbed time.

5. Conclusions

Our analysis on height of the EEJ center (EJC) has

shown its mean diurnal behavior as being characterized

by an ascent during the post-noon period extending into

the evening. The ascending region presents an exponen-

tial shape with EJC rising by 5–7 km in a period of 4 h. A

tentative explanation for this rising feature was

suggested in terms of the solar zenith angle dependent

variation of the peak-production height (zm) of an a-Chapman layer. We have found the k values ranging

between 0.1 and 0.25 in the relation zm ¼ ½cosðwÞ�2:k, inagreement with the values obtained from digisonde data

base. The mentioned rising of the whole E-region,

following the theory of Chapman, also explains the

seasonal difference of the EJC ascent.

We have also observed two other interesting phenom-

ena: one of them concerns the strong East–West

asymmetry in the received echo power and the other

one concerns the confirmation of the appearance of a

scatter region after the sunset, between 1800 and

1930LT, (observed previously by Abdu et al., 2002),

mainly around summer solstice. They were not discussed

in this paper but the analysis work is continuing.

Acknowledgements

This work was supported by FAPESP (Fundac- ao de

Amparo a Pesquisa de Estado de Sao Paulo) through

thematic project Grant Nr. 99/00437. Support received

through CNPq (Conselho Nacional de Pesquisa e

Desenvolvimento) Grants Nr. 520185/95-1 and 522919/

96-0 is also acknowledged. C. M. D. also wishes to

thank FAPESP for the financial support to his doctoral

ARTICLE IN PRESSC.M. Denardini et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1665–1673 1673

degree Program Nr. 98/16156-8, and for his post-

doctoral program through the Grant Nr. 03/01146-7.

M. A. A. wishes to acknowledge the support from

CNPq through the Process Nr. 520185/95-1. E. R. de P.

was partially supported by CNPq under Grant Nr.

502223/91-0. C. M. W. was supported by CNPq under

Grant Nr. 150039/04-2.

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