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The low-latitude ionospheric tomography network (LITN)—initial results
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The low-latitude ionospheric tomography network (LITN)-initial results C. R. Huang. National (Receiwd
H. C. Yeh and W. H. Tsai
C. H. Liu.
Central
University,
Chung-Li,
Taiwan,
R.O.C.
3 October 1995; accepted 9 September
1996)
Abstract-The
Low-latitude Ionospheric Tomography Network (LITN). a chain of six stations located along the 12l’E meridian receiving signals from the Transit NNSS satellites, carries out tomographic investigations of the ionosphere in this equatorial anomaly region. The technical aspects of the network are introduced. Because of the steep latitudinal gradients of the ionosphere in this region. a special procedure has been developed to facilitate the reconstruction process. Model simulations are carried out to help formulate the best reconstruction algorithm. Initial results from the first set of data from the full network are presented. and the diurnal behavior of the anomaly discussed. Comparisons of the reconstructed electron density profiles with those derived from ionograms and the reconstructed vertical TECs with those observed appear to indicate that the LITN can be used to provide a two-dimensional image of the ionosphere in the equatorial region. c 1997 Elsevier Science Ltd
1. INTRODUCTION
In the past decade or so, Computerized Ionospheric Tomography (CIT) has attracted the interests of investigators in a number of related research areas including ionospheric physics, imaging technology as well as remote sensing. The goal of CIT is to apply the tomographic technique to image the electron density distribution in the ionosphere using Total Electron Content (TEC) data collected at a number of receiving stations. As is well known, TEC is the line integral of the electron density along a given path which usually coincides with the propagation path of the radio beacon signal from a satellite to a receiving station (Yeh and Swenson, 1961: Yeh and Liu, 1972). Ever since the advent of artificial satellites, TEC has been used to investigate a variety of ionospheric phenomena such as the equatorial bubble (e.g. Yeh et al., 1979). ionosphere trough (e.g. Rodger rt al., 1992); equatorial anomaly (e.g. Huang et a/., 1989). and the traveling ionosphere disturbance (e.g. Georges and Hooke, 1970; Yeh and Liu. 1972). The fact that relatively inexpensive equipment is used in measuring TEC made the technique widely accessible to researchers all over the world and the Satellite Beacon community has become a very active group in ionosphere research (Klobuchar and Whitney, 1966; Leitinger et al., 1975). One major drawback of the technique is that it can only provide the path integrated electron density distribution. Variations of the
ionosphere along the path usually are averaged out. Therefore, the idea of applying the tomographic technique was naturally developed when multi-station TEC data from orbiting satellites became available. Austen et al. (1986) introduced the ionospheric tomographic imaging system at the Beacon Satellite Symposium in Oulu, Finland. Since then considerable efforts from many research groups have been directed toward the development of robust reconstruction algorithms (Austen et al., 1988; Na and Lee, 1991; Raymund et al.. 1990, 1994; Fremouw et al., 1992; Liu and Raymund, 1994). Several campaigns have been organized in Europe and America to acquire actual data for reconstruction (Kersley et al.. 1993; Klobuchar et a/., 1992). The results have been encouraging. Since these measurements were mostly limited to the high- and middlelatitude regions, plans to build a low-latitude tomographic system in the Asian sector were initiated in the early 1990s. The idea is to set up a chain of receiving stations along the 121 “E longitude to study the equatorial anomaly in that sector. Using TEC data derived from the NNSS Transit satellite at a single station in Taiwan, Huang et (11. (1989) showed that the behavior of the ionosphere in the anomaly region is very complicated with large day-to-day variability in the magnitude, position and extent of the anomaly. These variabilities certainly are related to the electrodynamic effects of the whole equatorial ionosphere including the effects of the neutral atmosphere (Anderson. 1973; 1553
1554
C. R. Huang et
Kelley, 1994). The two-dimensional image of the ionosphere obtained from a tomographic system in this region will add crucial information about the structure of the anomaly which in turn will help us understand the electrodynamics of the equatorial ionosphere. Initial results from a chain of four stations were reported by Yeh et al. (1994) and Huang et al. (1996). Since June 1994 the six-station chain has been in place and is now in routine operation. In this paper. the Low-latitude Ionospheric Tomography Network (LITN) will be described and some initial results using the full network will be presented. In Section 2, the technical details of the LITN will be discussed. Because of the steep latitudinal variation of the ionosphere in the anomaly region, special care must be taken in setting up the reconstruction algorithm. These points are discussed in Section 3. using results from simulation as guidance. In Section 4, actual observational data from the NNSS are presented and vertical TECs will be discussed. Tomographic reconstructions are carried out in Section 5 and the results are compared with those derived from ionograms. Some conclusions are given in Section 6.
2. LOW-LATITUDE
IONOSPHERIC
TOMOGRAPHY
NETWORK (LITN)
The LITN consists of a chain of six Naval Navigation Satellite System (NNSS) receivers with data acquisition systems located along the 121”E longitude. As shown in Fig. 1, the stations are at Shanghai (31.0’N, 12l.O”E), Wenzhou (28.O”N, 121.O”E), Chung-Li(25.0°N, 121.0cE), Kaohsiung(22S’N, (16.4”N, 121.O”E) 121 .O”E), Baguio and Manila( 14.6”N. 121 .O”E). The network is operated jointly by research groups from the National Central University (NCU) and National Sun Yet-Sen University in Taiwan; Wuhan University in mainland China and the University of Illinois at Urbana-Champaign in the U.S.A. In addition, the Manila Observatory assists NCU to maintain the two stations in the Philippines. At each station. the system consists of a JMR receiver which records the 400MHz and 150 MHz signals from the Transit satellite. A simple phase detector is used to derive the phase difference between the two signals. This is known as the differential Doppler in the literature (Leitinger, 1994) and is proportional to the relative TEC. A PC-LPM-16 A/D card serves as the interface between the JMR and a 386SX 16MHz personal computer with 1 Mbyte RAM and 40 Mbyte internal hard disk. The output of the computer goes to a 1 Gbyte external magnetic optical disk drive which completes the whole data
al.
taking system. Six identical systems are placed at the six locations, respectively. The data acquisition software is written in TURBO C+ + under the DOS environment. The differential Doppler phase and the ephemeris information are recorded on optical disks, stored for off-line processing. The ephemeris information gives the geometry for each path and the differential Doppler measures the relative slant TEC along that path. A whole month’s worth of data can be stored on a single optical disk.
3. CIT RECONSTRUCTION
PROCEDURES
Referring to Fig. 1, for any given path p at any station, the measured phase difference Y between the signals at the two frequencies is related to the slant TEC C,yfor that path by (Leitinger et al., 1975) ‘I’+$,
= DC,y,C, = j N,ds P
(1)
where N, is the electron density, a,, the unknown initial phase for a given receiver, and D is a proportional constant. Only when @‘. is found, can one obtain the absolute TEC from the measured data Y. Therefore, the first step in the reconstruction procedure is the determination of BO. To determine @“, Leitinger et al. (1975) proposed a two-station procedure. First, the vertical TEC C,, is defined by C,. = ; Ned/i. 0
c4
This is an integration along a vertical path from the ground up to the satellite height h,. To convert the slant TEC, C,, to the vertical TEC, C,, one introduces a mean ionospheric height, h,,,,, usually 50 km or so above the height of maximum electron density. Referring to Fig. 2, the path p intersects this mean height at point P, making an angle Z with the vertical at this point. The projection of P on the ground, P’, is called the subionospheric point corresponding to the path p. The vertical TEC, C,., for P’is given by C,cosZ. For a given receiving station, using data from the moving satellite, one obtains the vertical TEC as a function of latitude on the ground. It has been shown by simulation that with a reasonable choice of the mean height. the vertical TEC obtained in this way as a function of latitude can be very close to the true vertical TEC distribution (Leitinger et al., 1975). Thus, one should expect the vertical TEC determined from the slant TEC at all stations to be very close to each other. This provides us with a way to determine the unknown phase using data from more than one station. Following Leitinger et al. (1975) from equation (1)
Low-latitude
ionosphere
tomography
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(a) TEC integration
paths
Path of NNSS Satellite
._.. . .
Kaohsiung 22.5”N
I
Chungli 25”N
Earth’s surface
(b) D
50’
c
45’
D
-
2
_
40’
=
35'
300
25’
35’
5 5
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Mainland
China 20”
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-
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- 10”
30’
g .$
25’
0 z
20’
I
150
- 5”
10”
- 0”
5”
- -5”
00 . 1100
2 $ Q
-10” 115’
120”
125”
Longitude Fig.
3 .o 5 k
z 8 al Q
P) 3 .=
130”
135”
140”
(East)
I. (a) Tomography geometry showing satellite to ground station ray paths as an NNSS satellite traverses the image region over the receiving stations.(b) Locations of the six stations of the LITN.
C. R. Huang
1556
hm
R
P’
Fig. 2. Geometry for converting slant TEC to vertical TEC for an assumed mean ionospheric height, h,,,,. Note that spherical geometry is used.
for two stations point k
1 and 2, we have for subionospheric
Y,,++@O, = DC,,/cosZ,,
k = I,2 ,..., 3
Y Ik + Q,z = DC,.,/cosZ,,
h(x) = 280-t lOO[l +exp-(x-
= 0.
(4)
The two unknowns $,, and OoZcan be determined by using the least squares method. requiring the minimization of the error E
E=
following the procedure described previously. For any latitude, the true vertical TEC was first calculated from the model. An iterative procedure is designed to find the right mean height for this latitude such that the vertical TEC at this latitude derived from the corresponding slant TEC is equal to the true vertical TEC. This procedure is repeated for all relevant latitudes and in Fig. 4 the resulting mean ionospheric height as a function of latitude is shown. We note that indeed it is not constant. Due to the propagation geometry, the variations were very fast around the chosen station Baguio and they are not shown in the figure. What this means is that in the equatorial anomaly region where there are substantial latitudinal variations in electron density distribution, in order to make the vertical TEC derived from the slant TEC to be the same as the true vertical TEC, one needs to use a mean height distribution in the conversion process somewhat similar to the one shown in Fig. 4. For our simulation, the function for the mean height was chosen as
(3)
where C,.,is the vertical TEC at the subionospheric point k. Combining these two equations, we obtain cosZ,~(Y,~+~01)-COSZ?~(Y~~+~“~)
et al.
2 [cosZ,,(Y,~+~,,)--cosz,~(~~~+~~~)]~
L=l
(5) where II is the number of subionospheric points chosen. The technique can be extended to cases where there are more than two stations. Leitinger (1994) has carried out extensive simulations for the multi-station technique and found that for mid-latitude the choice of 400 km for the mean ionospheric height works reasonably well. However, when we tried to use 400 km as the mean ionospheric height for the equatorial anomaly region, the results were not satisfactory. This obviously is due to the fact that latitudinal variation of the ionosphere can be rather large in this region such that the mean height can no longer be taken as constant. To examine the situation more closely, we carried out a simulation using the IRI-90 model for the equatorial anomaly region. Figure 3(a) shows the contour plots for the model. For a given station, say Baguio, the slant TEC, C,. was calculated from the model for an assumed satellite pass. This set of slant TEC data was then used to derive a set of vertical TEC data
17.5’)-‘km
(6)
where x is the latitude in degrees. Figure 3(b) shows the slant TEC for the six stations calculated from the model in Fig. 3(a). To test the procedure, an initial value was subtracted from each set such that the minimum TEC value for each station was set to zero as shown in Fig. 3(c). These were the relative slant TECs for each station. Following the modified multi-station minimization procedure described previously in which the mean ionospheric height was taken as that given in equation (6) the initial constants were determined. Table 1 compares the results with the actual values as well as those determined by assuming constant mean heights and other methods. We note that the constants obtained by using the modified multi-station procedure are very close to the actual values while the results from the other methods are not as satisfactory. The last two rows in the table present the results derived from another method. Slant TEC data from the satellite GOES were available at Lunpin near Chung-Li. Assuming a mean ionospheric height of 400 km, the subionospheric point for this path was at (22.6’ N, 131 .7cE) which is very close to Kaohsiung in latitude but about 10” off to the east in longitude. The vertical TEC was calculated at this point using 400 km as the mean ionospheric height (see Fig. 4). This was taken as the vertical TEC at Kaohsiung. This value was then used at each station to determine the initial constants. The results shown in the last two rows of the Table indicate that the constants are reasonable
1557
(a)
‘:::
IRI contour at Sep 14, 1994
09:OO UT (el/cm3)
:
20
30
Geographic (b) 2’0e+18
Absolute
slant TEC obtained
40
50
Lat. (ON)
from Sep 14, 1994 RI-90
Model (121 E)
8
I
I
I
20
30
40
Geo-latitude (C) Relative
slant TEC
50
(ON)
from (b) minus actual constants
2.0e+l8
solid line : ma N”
1.5e+l8
E L L l.Oe+lB F E m z
5.0e+17
o.oe+oo 0
10(-l
.2)
20(8.8) Geographic
30(18.8) (geomagnetic)
40(28.8)
50
latitude
Fig. 3. IRI-90 model for simulation. Parameters are: 14 September, Latitude 0”-5ON-‘. Longitude 121”E, sunspot number = 30, UT = 0500, (a) The contour, (b) Absolute slant TEC of six receiving stations deduced from the model. (c) The corresponding relative slant TEC.
1558
C. R. Huang Mean Height
1::
et al.
denoting the electron density in the j-th pixel by x,. Then for the i-th path, equation (7) can be approximated by
F-----J
F g
(8)
or in matrix notation
300.0
al
=
i = 1,2,...,m
C, = f A&, ,=1
400.0
C=AX 200.0 I 100.0 0.0
where A is a matrix whose elements denote the length of the path-pixel intersections for each path. Note that C and X are column vectors for absolute slant TEC and electron density, respectively. A is an M x n matrix where m is the number of TEC values from all paths at all receiving sites. The elements in A depend on the geometry of the paths and can be computed once the experimental configuration is fixed. The task of CIT is to invert equation (9) to obtain the electron density vector X. Recently, Raymund (1994) reviewed the CIT reconstruction algorithms proposed by various investigators. Special features of each algorithm were examined. Because of the special geometry of the ionospheric tomography problem, there are intrinsic limitations in the reconstruction, independent of the algorithm applied (Raymund et al., 1994). One of the most commonly used algorithms is the Algebraic Reconstruction Technique, or ART, first introduced in CIT by Austen et al. (1988). This is an iterative procedure for solving equation (9). The iteration starts by assuming an initial guess J?‘. From equation (9) C’ is computed. Comparing C? with the observed data C, a difference vector is obtained. This difference is used to construct the next solution in the iteration. For the (k+ I)-th iteration, we have
1
1
0
lO(-1.2) 20(&E) 30(18.8)40(28.8) Geographic (Geomagnetic)Lat.
Fig. 4. Mean ionospheric
height deduced lation
50
from model simu-
for the three southern stations but bad for the three northern stations. Therefore, from the results of simulation, we have shown that the modified multi-station method is quite suitable for determining the initial constants in the equatorial anomaly region. The technique has been tested on several other IRI-90 models and the results were all satisfactory. For cases of lower electron density such as during evening hours, the best choice of the range for mean heights was 320 km to 430 km. With the initial constants determined, the absolute slant TEC can be obtained from the data. We start by considering the slant TEC along any path p between the satellite and a receiver C= lN,ds P where the subscript s is omitted for convenience. For tomographic applications, the TEC along some path p is approximated by a finite sum of segments of the integral (7). This is carried out by dividing the twodimensional ionosphere into a set of n pixels and
Table 1. Summary
of estimated
(9)
P”
= x”+f(C-C”)
(10)
whereJ’(CCk) indicates the scheme to distribute the difference in the measured TEC and the reconstructed TEC from the k-th iteration. A modified version of
unknown
constants
using variety
of methods.
Station (Location)
Manila (14.6’N)
Baguio (16.4”N)
Kaoshiung (22.5’N)
Chung-Li (25”N)
Wenzhou (28”N)
Shanghei (3l”N)
Actual constants Modified multistation (error percentage) Leitinger (400 km fixed) Leitinger (350 km fixed) GOES TEC (400 km) GOES TEC (350 km)
4.417 4.478 (1.38%) 5.498 5.083 4.382 4.916
4.830 4.823 (0.14%) 5.730 5.319 4.709 5.038
4.930 4.916 (0.28%) 5.518 5.071 4.835 4.818
4.328 4.348 (0.46%) 4.840 4.383 3.929 3.843
3.491 3.524 (0.9%) 3.952 3.449 2.726 2.616
2.153 2.740 (0.47%) 3.183 2.512 1.692 1.728
*The ionospheric
mean height being adjusted
using a sigmoid
function
as in equation
(6)
Low-latitude ionosphere tomography ART is the so-called multiplicative ART (MART) in which the correction in each iteration is obtained by making a multiplicative modification to X rather than an additive correction (Raymund et al., 1990). In this paper, MART will be used. To test the reconstruction procedure, a simulation was carried out. We used the IRI-90 model shown in Fig. 3(a) as the example. The relative TECs shown in Fig. 3(c) were used in various methods to determine the initial constants. These were added back to obtain the absolute slant TECs. The MART algorithm was then applied to the data to reconstruct the ionosphere. The initial guess was another IRI-90 model with different parameters. Figure 5 shows the resulting reconstruction together with the original model. We note that the resulting contours in Fig. 5(b) and Fig. 5(d) match those for the original model shown in Fig. 3(a) rather well. In Fig. 5(c) a reconstruction using absolute TECs obtained from GPS information is also shown; the result is not as good. Vertical TEC profiles are calculated from each reconstructed ionosphere. In Fig. 5(e), note that the result using the modified multistation method fits the original data as well as in the case when the actual constants were used, while the result using the GPS method is not satisfactory.
4. OBSERVATIONAL
DATA FROM LITN
On 14 September 1994 all six stations were in operation. Six good passes of the satellites were recorded in all stations giving us data covering the whole day. Relative slant TECs were derived from the differential Doppler measured at the six stations. Figure 6 shows the relative TEC for the six stations for the six time periods covering the whole 24 hours of the day. However, as we can see, the length of the data varies from station to station due to the nature of the passes and local reception conditions. Applying the modified multi-station technique to these data, the initial constants were first determined for each case. At the same time, the method also estimated the vertical TEC for each station. These are shown in Fig. 7. First we note that the vertical TECs derived from data from the six stations coincide with each other implying that the multi-station method has performed reasonably well in determining the unknown constants. Also, the resulting vertical TEC shown in the figure could be taken as the actual variation of vertical TEC along the latitudes at different hours of day. At 0850 LT. a hint of the anomaly started to appear just north of the equator. The anomaly started to grow and moved to the North. It persisted in its general shape as late as 20:20 LT. The vertical TEC around local midnight
1559
showed a rather flat ionosphere with low electron density. The peak anomaly crest was found to occur at about 7”N geomagnetic latitude. This falls within the range of 6”to 13”N geomagnetic latitude for the locations of the peak in this region as reported by Huang et al. (1989, 1991).
5. RECONSTRUCTED
IONOSPHERE
FOR 14 SEPTEMBER
1994
With the unknown constants determined, the relative TECs obtained during the observational campaign on 14 September 1994 at the six stations were converted into absolute TECs. These were then used for tomographic reconstruction. It turned out that the quality of the data at different stations was not uniform. Therefore, not all the data could be used for reconstruction. For the pass at 0050 UT, for example, data from Baguio and Shanghai could not be used. For the 0500 UT pass, data from Manila was missing. Baguio and Chung-Li could not provide good data for sufficient duration for the 16:45 UT pass. For the last pass at 23: 15 UT, only two stations recorded data of sufficient quality and duration. Therefore, reconstruction was carried out in cases where at least four stations provided good data. The algorithm used for reconstruction for this study was MART. For the initial guess the IRI-90 model was chosen. Figure 8 shows the contours of the reconstructed ionosphere for the five passes. As in most reconstruction cases using MART (Austen et al., 1988; Raymund et al., 1990), the slant TECs obtained from the reconstructed ionosphere from the stations were very close to the observed slant TECs. On the other hand, it is of interest to see how well the vertical TECs derived from the reconstruction compared with the ‘observed’ vertical TECs described in the previous section. Figure 9 shows the five vertical TECs computed from the reconstructed contours. Compared with those shown in Fig. 7, we note that the general features coincide quite well including the latitudinal variation. the magnitude and the position of the maximum of the anomaly. However, for the 05:OO UT pass, the vertical TEC maximum seems to be shifted slightly towards the North. This seems to be due to the fact that data from Manila were not available for this reconstruction. That this is the case was confirmed by performing the reconstruction using simulated data. When data from Manila were taken out in the reconstruction, a similar shift of the contours to the North was found in the reconstructed image. The phenomenon was not found in cases when data from other stations such as Chung-Li were missing. This
560
C. R. Huang
,ooo
(a) I’
et al.
(b) REC.for
Initial guess (M-90) .‘.‘..’
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I 40
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Latitude
Fig. 5. (a) Initial guess: IRI-90 model for 27 March with parameters: Latitude 14.51-31”N, Longitude 121”E, sunspot number = 30. UT = 0900, (b) Reconstruction using actual absolute slant TEC, (c) Reconstruction using initial constants obtained from GPS information, (d) Reconstruction using initial constants obtained by the modified multi-station method, (e) Vertical TEC derived from contours in (a), (b), (c), (d).
Low-latitude ionosphere tomography
1561
(a) Relative slant TEC from LITN at 08:50 LT (00:50 UT) OSCAR 25
(d) 20120LT (12:20 UT) OSCAR 25 ‘.0e+18 8 8.0e+l7
6.0e+l7
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0
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Geographic(Geomagnetic)
50
60
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0
lO(-1.2)20(8.8)30(18.8)40(28.8)
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(b) 13:OOLT (05:OOUT) OSCAR 23
(e) 00:45LT (16:45 UT) OSCAR 32 l.Oe+lB
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50
60
-10
0
lO(m1.2)20(8.8) 30(18.8)40(28.8)
50
Fig. 6. Relative slant TEC for the six stations for the six passes on 14 September 1994. derived from the differential Doppler phase data.
may be due to the fact that the maximum of the anomaly is very close to Manila. More examples, both from simulated and real data, should be examined. Another test for the reconstruction is to compare the electron density profiles calculated from the recon-
strutted ionosphere with those obtained from ionograms. Ionograms from Manila were obtained. Unfortunately most of the ionograms on 14 September 1994 were of poor quality. Only those for 09: 15 UT and 12:20 UT could be used to deduce the
60
C. R. Huang
et al.
(a) Estimated vertical TEC at Sep. 14.1994 0850 LT (00~50 UT)
(d) 20:20 LT (12:20 UT)
L
4.08+17
3
2
3.08+17
u'
2.0e+17
i l.Oe+17
1.oe+15 1 0
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40(28.8)
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I
0
I
I
I
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I
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ZO(8.8)
30(18.8)
40(28.8)
50
Lat.
(e) OOz45LT(16~45 UT)
(b) 1303 LT (05:OOUT)
:_;f
1.oe+15
1
0
I
I
I
I
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
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i
0
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
50
(f)O7:15LT(23:15UT)
(c) 17:15 LT (09:15 UT)
I
I
I
I
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
I
I
I
I
4.0e+17 3.0e+17
2.0e+17
l.Oe+17
l.Oe+15 * 0
50
l.Oe+15
0
I
I
I
I
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
50
Fig. 7. Vertical TEC converted from slant TEC shown in Fig. 6 using the modified multi-station method.
true height profiles using the software POLAN (Titheridge, 1985). Figure 10 shows the comparison of the profiles. We note that except for the discrepancy in the maximum value of the electron density at 09: 15 UT, the results compared reasonably well. Figure 11 shows the electron density profiles at each
station for different times of day obtained from the reconstructed ionosphere. These profiles provide information about the diurnal variations of the ionosphere at different latitudes of the Asian equatorial anomaly region. To summarize the results from the reconstruction: the tomographically reconstructed
Low-latitude
tomography
ionosphere
1000
1000
600
600
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-2 600 E .E"
g E P
I"
ii
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600
400
200 16
18
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20
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(ON)
20
30
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(ON)
l&45
(“N)
800
600 E .P I"
400
200
05x00 UT Geo-latitude
(ON)
UT Geo-latitude
1000
600
-2 g 800 E .9J I"
400
200 15
20
30
25
09:15 UT Geo-latitude
(ON)
Initial guess
Fig. 8. Contours of the reconstructed images of the ionosphere at different times of day on 14 September 1994. (f) is the initial guess for reconstruction with parameters: IRI-90, 14 September. Latitude 14.5-31”N, Longitude 121 “E. sunspot number = 26, UT = 1600.
564
C. R. Huang (a)00:50 I
Fz ‘E > P F z ‘F f
et al.
UT
I
(d)l2:20 UT I
I
I
4.0e+17
-
-
4.0e+17
-
3.0e+17
-
-
3.0e+17
-
2.0e+17
-
-
2.0e+17
-
l.Oe+17
-
-
l.Oe+17
-
1.Oe+l5
0
I I I I lO(-1.2) 20(8.8) 30(18.8) 40(28.8) Geographic (Geomagnetic) Lat.
1 .Oe+l5 50
0
I
-
-
4.0e+17
-
3.Oe+l7
-
-
3.0e+17
-
2.0e+17
-
-
2.0e+l7
-
1 .Oe+l7
-
-
l.Oe+17
-
l.Oe+15
0
I
0
I
I
50
(e) 18:45 UT I
I
4.0e+17
1 .Oe+l5
I
I I I I 10(-l .2) 20(8.8) 30(18.8) 40(28.8) Geographic (Geomagnetic) Lat.
(b)OS:OOUT I
I
I
I
10(-l .2) 20(8.8) 30(18.8) 40(28.8) Geographic (Geomagnetic) Lat.
50
I
I
I
1
I
I
I
I
10(-l .2) 20(8.8) 30(18.8) 40(28.8) Geographic (Geomagnetic) Lat.
50
(c)09:15 UT
4.0e+l7
’
’
-i
3.0e+l7 2.0e+l7 1 .Oe+17 l.Oe+l5
t ~ 0
i
10(-l .2)
20(8.8)
30(18.8)
40(28.8)
50
Geographic (Geomagnetic) Lat.
Fig. 9. Reconstructed
vertical
TEC calculated
image shows the development and the decay of the anomaly on 14 September 1994. A latitudinal density gradient started to develop as early as 08:50LT. It grew into an anomaly peak around noon time. The peak was located between 1Yto 20”N geographic latittide at approximately 325 km height. The anomaly
from the contours
in Fig. 8.
persisted as late as 20:20 LT. The ionosphere became rather flat latitudinally around local midnight. 6. CONCLUSIONS
The Low-latitude Ionospheric Tomography Network (LITN) is introduced in this paper. This is a
Low-latitude
ionosphere
tomography
(a) Compare 17:15 LT reconstruction with MA true height
1565 (b) Compare 20:20 LT reconstruction with MA true height
I
O.Oe+OO Fig.
solid : Tomography
solid : Tomography
circle : lonogram
circle : lonogram
iSe+12 50e+ll 1.Oe+l2 Electron density (el/mh3)
10. Comparisons
of reconstructed
2.0e+12
electron
density
I 2.0e+ll
I 4.0e+ll
for Manila
with those
0.0 O.Oe+OO profiles
I 6.0e+ll
-
&Oe+l
derived from
ionograms.
network of six stations along the 121 “E meridian using signals received from NNSS Transit satellites for imaging the two-dimensional electron density distributions of the ionosphere in the equatorial anomaly region of the Asian Sector. In order to overcome the difficulty involved in obtaining the absolute TEC from the observed data, a modified multi-station method has been proposed. This method appears to yield better estimates of the unknown initial constants for the equatorial anomaly region. Initial results from a campaign where for the first time data from six stations became available have been presented in this paper. These results indicate that the LITN can be used to obtain a tomographic image of the ionosphere in the equatorial anomaly region. The diurnal behavior of the ionospheric structure in the reconstructed image is consistent with the predicted behavior as well as earlier observations of the equatorial anomaly in this region (Huang et al., 1989; Huang and Cheng, 1991). Electron density profiles obtained from the reconstructed ionosphere compared reasonably well with true-height profiles derived from the available ionograms at Manila. Also. the shape, location of the maximum and the magnitude of the vertical TEC profiles calculated from the reconstructed ionosphere coincide with those for the vertical TEC derived directly from the observed data. This, to certain degree, is a self-consistency check of the reconstruction procedure. Therefore it appears that the tomographic reconstruction using data from the LITN may be regarded as a credible means for monitoring the ionosphere in this region. A systematic study of the two-dimensional behavior of the Asian
sector equatorial anomaly will be carried out in the future to understand its diurnal, seasonal variations, its response to storms and related physical mechanisms. There are also technical aspects related to the LITN that need our attention. Although the reason is not quite understood yet, it seems that data from the southernmost station, Manila, in the network are especially crucial to the reconstruction. On the other hand, we may have not found the most robust reconstruction algorithms yet. Also, operations at each receiving station need to be improved to yield data with better quality such that we always have the full coverage of the six stations of the network. One of the challenges in a further development of CIT is to improve the accuracy of the height distributions of the electron density in the reconstruction. Attempts are being made to make use of a priori information, such as that obtained from ionosondes, in the reconstruction in order to compensate for the deficiency caused by the lack of horizontal ray paths in the usual CIT configurations (Liu and Raymund, 1994). Acknowledgements-The research reported in this paper is supported by grants NSC-82-SP-008-04, NSC-83-NSPO-BRDD-008-04, NSC-84-2612-M-008-002 and NSC-85-2612M-008-001 from the National Science Council. ROC. The work by University of Illinois personnel is supported by the U. S. National Science Foundation through grant ATM9121660. The data from Wenzhou and Shanghai were provided by Professors J. S. Xu and S. Y. Ma from Wuhan University. the data from Manila and Baguio by Dr. F. V. Badillo from Manila Observatory, and the data from Kaoshiung by National Sun Yat-Sen University. Discussions with K. C. Yeh, Steve Franke and Andrei V. Izotov are acknowledged with pleasure.
1
1566
C. R. Huang
et al.
(a)Manila site
(b)Saguio site
1 600.0
600.0
600.0
I 1 O.Oe+OO5.0e+ll
0.0
I
I
l.Oe+l2 1.5e+12 Electrondensity(eVnF3)
2.0e+12
0.0 1 O.Oe+OO
I
I
I
50e+ll
1.08+12
1.5e+12
(c)Kaoshiuna site
I
2.0e+12
(d)Chungfi site
600.0
800.0
600.0
Circle
: 0850
LT
Square : 1300 LT
600.0
Triangle : 1715 LT
400.0
Star
: 2020 LT : 24:45LT
l.Oe+l2
l&+12
Plus
200.0 0.0
1
I
I
l.Oe+12
1.5e+l2
I
O.Oe+OO5.0e+ll
2.0e+12
0.0 O.Oe+OO
5.0e+ll
(e) Wenzhou site
Fig.
(f)Shanghai site
600.0
600.0
600.0
600.0
0.0 O.Oe+OO
11.Reconstructed
5.0e+ll
l.Oe+12
l&+12
2.0e-I
electron density profiles for the six stations 1994.
0.01
O.Oe+OO5.0e+ll
for different
1.08+12
1.5e+l2
times of day on 14 September
REFERENCES Anderson D. Austen J. R., Franke
S. J., Liu C. H. and Yeh K. C.
Austen J. R., Franke S. J. and Liu C. H. Fremouw E. J., Secan J. A. and Howe B. M. Georges T. M. and Hooke W. H. Huang Y. N., Cheng K. and Chen S. W. Huang Y. N. and Cheng K.
2.0e+12
1973 1986 1988 1992 1970 1989 1991
Planet. spare Sci. 21,421442. Proc. Beacon Satellite Symp.. Finland, 25-35. Radio Sri. 23, 299. Radio Sci. 21, 721-732. J.geophys. Rex 75, 6295-6308. J.geophys. Res. 94, 13515-13522. J.geophys. Res. 96, 13953-13965.
University
of Oulu,
2.0e+l2
Low-latitude Huang C. R.. Liu C. H., Yeh H. C., Tsai W. H., Wang C. J.. Yeh K. C., Lin K. H. and Tsai H. L. Kelley M. C. Kersley L., Heaton J. A. T., Pryse S. E. and Raymund T. D. Klobuchar J. A. and Whitney H. E. Klobuchar J. A.. Kuenzler H. J.. Pakula W. A. Fougere P. F.. Sheehan R. E., Doherty P. H., Buonsanto M. J., Foster J. C. and Langley R. B. Leitinger R., Schmidt G. and Tauriainen A. Leitinger R. Liu C. H. and Raymund T. D.
Na H. and Lee H. Raymund T. D.. Austen J. R., Franke S. J., Liu C. H., Klobuchar J. A. and Stalker J. Raymund T. D. Raymund T. D., Franke S. J. and Yeh K. C. Rodger A. S., Moffett R. J. and Quegan S. Titheridge J. E.
Yeh K. C. and Swenson G. W Yeh K. C. and Liu C. H. Yeh K. C., Soicher H. and Liu C. H. Yeh H. C.. Franke S. J.. Yeh K. C.. Liu C. H.. Raymund T. D., Chen H. H., Irotov A. V., Liu J. Y.. Wu J., Lin K. H. and Chen S. W.
ionosphere
tomography
1567
1996
Adv. space Res. 18, 237-240.
1994 1993
The Earth’s Ionosphere. Academic Ann. Geophys 11, 1064-1074.
1966 1992
Radio Sci. 1, 1149-l 154. Proc. Beacon Satellite Symp., 313-318.
1975 1994 1994
J.Geophys. 41, 201-213. Int. J. imaging syst. Tech 5, 8696. Proceeding of COSPAR Colloquium on Low-latitude Ionospheric Physics held in Taipei, Taiwan, 9-12 November, ed. F. S. Kuo, 285-293. ht. J. imaging syst. Tech 3, 354-365. Radio Sci. 25, 77 1.
1991 1990 1994 1994 1992 1985
1961 1972 1979 1994
Press.
MIT, Cambridge,
MA.
Int. J. imaging syst. Tech. 5, 75-85. J. atmos. terr. Phys. 56, 7377757. J. atmos. terr. Phvs. 54, l-30. Ionogram analysis with generahsed program POLAN. Rept. UAG-93, World Data Cent. A for Sol.-Terr. Phys.. Washington, D. C. J.geophys. Res. 66, 1061-1067. Rev. geophys. space Phys. 10,631-709. J. geophvs. Res. 84, 6589-6594. Proceeding of COSPAR Colloquium on Low-latitude Ionospheric Physics held in Taipei, Taiwan, 9-12 November, ed. F. S. Kuo, 295-303.
All nghts
PII: SOOZl-9169(96)00156-0
reserved. Printed m Great Bntam 136&6X26/97 $17 OO+O.OO
The low-latitude ionospheric tomography network (LITN)-initial results C. R. Huang. National (Receiwd
H. C. Yeh and W. H. Tsai
C. H. Liu.
Central
University,
Chung-Li,
Taiwan,
R.O.C.
3 October 1995; accepted 9 September
1996)
Abstract-The
Low-latitude Ionospheric Tomography Network (LITN). a chain of six stations located along the 12l’E meridian receiving signals from the Transit NNSS satellites, carries out tomographic investigations of the ionosphere in this equatorial anomaly region. The technical aspects of the network are introduced. Because of the steep latitudinal gradients of the ionosphere in this region. a special procedure has been developed to facilitate the reconstruction process. Model simulations are carried out to help formulate the best reconstruction algorithm. Initial results from the first set of data from the full network are presented. and the diurnal behavior of the anomaly discussed. Comparisons of the reconstructed electron density profiles with those derived from ionograms and the reconstructed vertical TECs with those observed appear to indicate that the LITN can be used to provide a two-dimensional image of the ionosphere in the equatorial region. c 1997 Elsevier Science Ltd
1. INTRODUCTION
In the past decade or so, Computerized Ionospheric Tomography (CIT) has attracted the interests of investigators in a number of related research areas including ionospheric physics, imaging technology as well as remote sensing. The goal of CIT is to apply the tomographic technique to image the electron density distribution in the ionosphere using Total Electron Content (TEC) data collected at a number of receiving stations. As is well known, TEC is the line integral of the electron density along a given path which usually coincides with the propagation path of the radio beacon signal from a satellite to a receiving station (Yeh and Swenson, 1961: Yeh and Liu, 1972). Ever since the advent of artificial satellites, TEC has been used to investigate a variety of ionospheric phenomena such as the equatorial bubble (e.g. Yeh et al., 1979). ionosphere trough (e.g. Rodger rt al., 1992); equatorial anomaly (e.g. Huang et a/., 1989). and the traveling ionosphere disturbance (e.g. Georges and Hooke, 1970; Yeh and Liu. 1972). The fact that relatively inexpensive equipment is used in measuring TEC made the technique widely accessible to researchers all over the world and the Satellite Beacon community has become a very active group in ionosphere research (Klobuchar and Whitney, 1966; Leitinger et al., 1975). One major drawback of the technique is that it can only provide the path integrated electron density distribution. Variations of the
ionosphere along the path usually are averaged out. Therefore, the idea of applying the tomographic technique was naturally developed when multi-station TEC data from orbiting satellites became available. Austen et al. (1986) introduced the ionospheric tomographic imaging system at the Beacon Satellite Symposium in Oulu, Finland. Since then considerable efforts from many research groups have been directed toward the development of robust reconstruction algorithms (Austen et al., 1988; Na and Lee, 1991; Raymund et al.. 1990, 1994; Fremouw et al., 1992; Liu and Raymund, 1994). Several campaigns have been organized in Europe and America to acquire actual data for reconstruction (Kersley et al.. 1993; Klobuchar et a/., 1992). The results have been encouraging. Since these measurements were mostly limited to the high- and middlelatitude regions, plans to build a low-latitude tomographic system in the Asian sector were initiated in the early 1990s. The idea is to set up a chain of receiving stations along the 121 “E longitude to study the equatorial anomaly in that sector. Using TEC data derived from the NNSS Transit satellite at a single station in Taiwan, Huang et (11. (1989) showed that the behavior of the ionosphere in the anomaly region is very complicated with large day-to-day variability in the magnitude, position and extent of the anomaly. These variabilities certainly are related to the electrodynamic effects of the whole equatorial ionosphere including the effects of the neutral atmosphere (Anderson. 1973; 1553
1554
C. R. Huang et
Kelley, 1994). The two-dimensional image of the ionosphere obtained from a tomographic system in this region will add crucial information about the structure of the anomaly which in turn will help us understand the electrodynamics of the equatorial ionosphere. Initial results from a chain of four stations were reported by Yeh et al. (1994) and Huang et al. (1996). Since June 1994 the six-station chain has been in place and is now in routine operation. In this paper. the Low-latitude Ionospheric Tomography Network (LITN) will be described and some initial results using the full network will be presented. In Section 2, the technical details of the LITN will be discussed. Because of the steep latitudinal variation of the ionosphere in the anomaly region, special care must be taken in setting up the reconstruction algorithm. These points are discussed in Section 3. using results from simulation as guidance. In Section 4, actual observational data from the NNSS are presented and vertical TECs will be discussed. Tomographic reconstructions are carried out in Section 5 and the results are compared with those derived from ionograms. Some conclusions are given in Section 6.
2. LOW-LATITUDE
IONOSPHERIC
TOMOGRAPHY
NETWORK (LITN)
The LITN consists of a chain of six Naval Navigation Satellite System (NNSS) receivers with data acquisition systems located along the 121”E longitude. As shown in Fig. 1, the stations are at Shanghai (31.0’N, 12l.O”E), Wenzhou (28.O”N, 121.O”E), Chung-Li(25.0°N, 121.0cE), Kaohsiung(22S’N, (16.4”N, 121.O”E) 121 .O”E), Baguio and Manila( 14.6”N. 121 .O”E). The network is operated jointly by research groups from the National Central University (NCU) and National Sun Yet-Sen University in Taiwan; Wuhan University in mainland China and the University of Illinois at Urbana-Champaign in the U.S.A. In addition, the Manila Observatory assists NCU to maintain the two stations in the Philippines. At each station. the system consists of a JMR receiver which records the 400MHz and 150 MHz signals from the Transit satellite. A simple phase detector is used to derive the phase difference between the two signals. This is known as the differential Doppler in the literature (Leitinger, 1994) and is proportional to the relative TEC. A PC-LPM-16 A/D card serves as the interface between the JMR and a 386SX 16MHz personal computer with 1 Mbyte RAM and 40 Mbyte internal hard disk. The output of the computer goes to a 1 Gbyte external magnetic optical disk drive which completes the whole data
al.
taking system. Six identical systems are placed at the six locations, respectively. The data acquisition software is written in TURBO C+ + under the DOS environment. The differential Doppler phase and the ephemeris information are recorded on optical disks, stored for off-line processing. The ephemeris information gives the geometry for each path and the differential Doppler measures the relative slant TEC along that path. A whole month’s worth of data can be stored on a single optical disk.
3. CIT RECONSTRUCTION
PROCEDURES
Referring to Fig. 1, for any given path p at any station, the measured phase difference Y between the signals at the two frequencies is related to the slant TEC C,yfor that path by (Leitinger et al., 1975) ‘I’+$,
= DC,y,C, = j N,ds P
(1)
where N, is the electron density, a,, the unknown initial phase for a given receiver, and D is a proportional constant. Only when @‘. is found, can one obtain the absolute TEC from the measured data Y. Therefore, the first step in the reconstruction procedure is the determination of BO. To determine @“, Leitinger et al. (1975) proposed a two-station procedure. First, the vertical TEC C,, is defined by C,. = ; Ned/i. 0
c4
This is an integration along a vertical path from the ground up to the satellite height h,. To convert the slant TEC, C,, to the vertical TEC, C,, one introduces a mean ionospheric height, h,,,,, usually 50 km or so above the height of maximum electron density. Referring to Fig. 2, the path p intersects this mean height at point P, making an angle Z with the vertical at this point. The projection of P on the ground, P’, is called the subionospheric point corresponding to the path p. The vertical TEC, C,., for P’is given by C,cosZ. For a given receiving station, using data from the moving satellite, one obtains the vertical TEC as a function of latitude on the ground. It has been shown by simulation that with a reasonable choice of the mean height. the vertical TEC obtained in this way as a function of latitude can be very close to the true vertical TEC distribution (Leitinger et al., 1975). Thus, one should expect the vertical TEC determined from the slant TEC at all stations to be very close to each other. This provides us with a way to determine the unknown phase using data from more than one station. Following Leitinger et al. (1975) from equation (1)
Low-latitude
ionosphere
tomography
1555
(a) TEC integration
paths
Path of NNSS Satellite
._.. . .
Kaohsiung 22.5”N
I
Chungli 25”N
Earth’s surface
(b) D
50’
c
45’
D
-
2
_
40’
=
35'
300
25’
35’
5 5
/I--
Mainland
China 20”
2
_
>
-
- 15”
vi
- 10”
30’
g .$
25’
0 z
20’
I
150
- 5”
10”
- 0”
5”
- -5”
00 . 1100
2 $ Q
-10” 115’
120”
125”
Longitude Fig.
3 .o 5 k
z 8 al Q
P) 3 .=
130”
135”
140”
(East)
I. (a) Tomography geometry showing satellite to ground station ray paths as an NNSS satellite traverses the image region over the receiving stations.(b) Locations of the six stations of the LITN.
C. R. Huang
1556
hm
R
P’
Fig. 2. Geometry for converting slant TEC to vertical TEC for an assumed mean ionospheric height, h,,,,. Note that spherical geometry is used.
for two stations point k
1 and 2, we have for subionospheric
Y,,++@O, = DC,,/cosZ,,
k = I,2 ,..., 3
Y Ik + Q,z = DC,.,/cosZ,,
h(x) = 280-t lOO[l +exp-(x-
= 0.
(4)
The two unknowns $,, and OoZcan be determined by using the least squares method. requiring the minimization of the error E
E=
following the procedure described previously. For any latitude, the true vertical TEC was first calculated from the model. An iterative procedure is designed to find the right mean height for this latitude such that the vertical TEC at this latitude derived from the corresponding slant TEC is equal to the true vertical TEC. This procedure is repeated for all relevant latitudes and in Fig. 4 the resulting mean ionospheric height as a function of latitude is shown. We note that indeed it is not constant. Due to the propagation geometry, the variations were very fast around the chosen station Baguio and they are not shown in the figure. What this means is that in the equatorial anomaly region where there are substantial latitudinal variations in electron density distribution, in order to make the vertical TEC derived from the slant TEC to be the same as the true vertical TEC, one needs to use a mean height distribution in the conversion process somewhat similar to the one shown in Fig. 4. For our simulation, the function for the mean height was chosen as
(3)
where C,.,is the vertical TEC at the subionospheric point k. Combining these two equations, we obtain cosZ,~(Y,~+~01)-COSZ?~(Y~~+~“~)
et al.
2 [cosZ,,(Y,~+~,,)--cosz,~(~~~+~~~)]~
L=l
(5) where II is the number of subionospheric points chosen. The technique can be extended to cases where there are more than two stations. Leitinger (1994) has carried out extensive simulations for the multi-station technique and found that for mid-latitude the choice of 400 km for the mean ionospheric height works reasonably well. However, when we tried to use 400 km as the mean ionospheric height for the equatorial anomaly region, the results were not satisfactory. This obviously is due to the fact that latitudinal variation of the ionosphere can be rather large in this region such that the mean height can no longer be taken as constant. To examine the situation more closely, we carried out a simulation using the IRI-90 model for the equatorial anomaly region. Figure 3(a) shows the contour plots for the model. For a given station, say Baguio, the slant TEC, C,. was calculated from the model for an assumed satellite pass. This set of slant TEC data was then used to derive a set of vertical TEC data
17.5’)-‘km
(6)
where x is the latitude in degrees. Figure 3(b) shows the slant TEC for the six stations calculated from the model in Fig. 3(a). To test the procedure, an initial value was subtracted from each set such that the minimum TEC value for each station was set to zero as shown in Fig. 3(c). These were the relative slant TECs for each station. Following the modified multi-station minimization procedure described previously in which the mean ionospheric height was taken as that given in equation (6) the initial constants were determined. Table 1 compares the results with the actual values as well as those determined by assuming constant mean heights and other methods. We note that the constants obtained by using the modified multi-station procedure are very close to the actual values while the results from the other methods are not as satisfactory. The last two rows in the table present the results derived from another method. Slant TEC data from the satellite GOES were available at Lunpin near Chung-Li. Assuming a mean ionospheric height of 400 km, the subionospheric point for this path was at (22.6’ N, 131 .7cE) which is very close to Kaohsiung in latitude but about 10” off to the east in longitude. The vertical TEC was calculated at this point using 400 km as the mean ionospheric height (see Fig. 4). This was taken as the vertical TEC at Kaohsiung. This value was then used at each station to determine the initial constants. The results shown in the last two rows of the Table indicate that the constants are reasonable
1557
(a)
‘:::
IRI contour at Sep 14, 1994
09:OO UT (el/cm3)
:
20
30
Geographic (b) 2’0e+18
Absolute
slant TEC obtained
40
50
Lat. (ON)
from Sep 14, 1994 RI-90
Model (121 E)
8
I
I
I
20
30
40
Geo-latitude (C) Relative
slant TEC
50
(ON)
from (b) minus actual constants
2.0e+l8
solid line : ma N”
1.5e+l8
E L L l.Oe+lB F E m z
5.0e+17
o.oe+oo 0
10(-l
.2)
20(8.8) Geographic
30(18.8) (geomagnetic)
40(28.8)
50
latitude
Fig. 3. IRI-90 model for simulation. Parameters are: 14 September, Latitude 0”-5ON-‘. Longitude 121”E, sunspot number = 30, UT = 0500, (a) The contour, (b) Absolute slant TEC of six receiving stations deduced from the model. (c) The corresponding relative slant TEC.
1558
C. R. Huang Mean Height
1::
et al.
denoting the electron density in the j-th pixel by x,. Then for the i-th path, equation (7) can be approximated by
F-----J
F g
(8)
or in matrix notation
300.0
al
=
i = 1,2,...,m
C, = f A&, ,=1
400.0
C=AX 200.0 I 100.0 0.0
where A is a matrix whose elements denote the length of the path-pixel intersections for each path. Note that C and X are column vectors for absolute slant TEC and electron density, respectively. A is an M x n matrix where m is the number of TEC values from all paths at all receiving sites. The elements in A depend on the geometry of the paths and can be computed once the experimental configuration is fixed. The task of CIT is to invert equation (9) to obtain the electron density vector X. Recently, Raymund (1994) reviewed the CIT reconstruction algorithms proposed by various investigators. Special features of each algorithm were examined. Because of the special geometry of the ionospheric tomography problem, there are intrinsic limitations in the reconstruction, independent of the algorithm applied (Raymund et al., 1994). One of the most commonly used algorithms is the Algebraic Reconstruction Technique, or ART, first introduced in CIT by Austen et al. (1988). This is an iterative procedure for solving equation (9). The iteration starts by assuming an initial guess J?‘. From equation (9) C’ is computed. Comparing C? with the observed data C, a difference vector is obtained. This difference is used to construct the next solution in the iteration. For the (k+ I)-th iteration, we have
1
1
0
lO(-1.2) 20(&E) 30(18.8)40(28.8) Geographic (Geomagnetic)Lat.
Fig. 4. Mean ionospheric
height deduced lation
50
from model simu-
for the three southern stations but bad for the three northern stations. Therefore, from the results of simulation, we have shown that the modified multi-station method is quite suitable for determining the initial constants in the equatorial anomaly region. The technique has been tested on several other IRI-90 models and the results were all satisfactory. For cases of lower electron density such as during evening hours, the best choice of the range for mean heights was 320 km to 430 km. With the initial constants determined, the absolute slant TEC can be obtained from the data. We start by considering the slant TEC along any path p between the satellite and a receiver C= lN,ds P where the subscript s is omitted for convenience. For tomographic applications, the TEC along some path p is approximated by a finite sum of segments of the integral (7). This is carried out by dividing the twodimensional ionosphere into a set of n pixels and
Table 1. Summary
of estimated
(9)
P”
= x”+f(C-C”)
(10)
whereJ’(CCk) indicates the scheme to distribute the difference in the measured TEC and the reconstructed TEC from the k-th iteration. A modified version of
unknown
constants
using variety
of methods.
Station (Location)
Manila (14.6’N)
Baguio (16.4”N)
Kaoshiung (22.5’N)
Chung-Li (25”N)
Wenzhou (28”N)
Shanghei (3l”N)
Actual constants Modified multistation (error percentage) Leitinger (400 km fixed) Leitinger (350 km fixed) GOES TEC (400 km) GOES TEC (350 km)
4.417 4.478 (1.38%) 5.498 5.083 4.382 4.916
4.830 4.823 (0.14%) 5.730 5.319 4.709 5.038
4.930 4.916 (0.28%) 5.518 5.071 4.835 4.818
4.328 4.348 (0.46%) 4.840 4.383 3.929 3.843
3.491 3.524 (0.9%) 3.952 3.449 2.726 2.616
2.153 2.740 (0.47%) 3.183 2.512 1.692 1.728
*The ionospheric
mean height being adjusted
using a sigmoid
function
as in equation
(6)
Low-latitude ionosphere tomography ART is the so-called multiplicative ART (MART) in which the correction in each iteration is obtained by making a multiplicative modification to X rather than an additive correction (Raymund et al., 1990). In this paper, MART will be used. To test the reconstruction procedure, a simulation was carried out. We used the IRI-90 model shown in Fig. 3(a) as the example. The relative TECs shown in Fig. 3(c) were used in various methods to determine the initial constants. These were added back to obtain the absolute slant TECs. The MART algorithm was then applied to the data to reconstruct the ionosphere. The initial guess was another IRI-90 model with different parameters. Figure 5 shows the resulting reconstruction together with the original model. We note that the resulting contours in Fig. 5(b) and Fig. 5(d) match those for the original model shown in Fig. 3(a) rather well. In Fig. 5(c) a reconstruction using absolute TECs obtained from GPS information is also shown; the result is not as good. Vertical TEC profiles are calculated from each reconstructed ionosphere. In Fig. 5(e), note that the result using the modified multistation method fits the original data as well as in the case when the actual constants were used, while the result using the GPS method is not satisfactory.
4. OBSERVATIONAL
DATA FROM LITN
On 14 September 1994 all six stations were in operation. Six good passes of the satellites were recorded in all stations giving us data covering the whole day. Relative slant TECs were derived from the differential Doppler measured at the six stations. Figure 6 shows the relative TEC for the six stations for the six time periods covering the whole 24 hours of the day. However, as we can see, the length of the data varies from station to station due to the nature of the passes and local reception conditions. Applying the modified multi-station technique to these data, the initial constants were first determined for each case. At the same time, the method also estimated the vertical TEC for each station. These are shown in Fig. 7. First we note that the vertical TECs derived from data from the six stations coincide with each other implying that the multi-station method has performed reasonably well in determining the unknown constants. Also, the resulting vertical TEC shown in the figure could be taken as the actual variation of vertical TEC along the latitudes at different hours of day. At 0850 LT. a hint of the anomaly started to appear just north of the equator. The anomaly started to grow and moved to the North. It persisted in its general shape as late as 20:20 LT. The vertical TEC around local midnight
1559
showed a rather flat ionosphere with low electron density. The peak anomaly crest was found to occur at about 7”N geomagnetic latitude. This falls within the range of 6”to 13”N geomagnetic latitude for the locations of the peak in this region as reported by Huang et al. (1989, 1991).
5. RECONSTRUCTED
IONOSPHERE
FOR 14 SEPTEMBER
1994
With the unknown constants determined, the relative TECs obtained during the observational campaign on 14 September 1994 at the six stations were converted into absolute TECs. These were then used for tomographic reconstruction. It turned out that the quality of the data at different stations was not uniform. Therefore, not all the data could be used for reconstruction. For the pass at 0050 UT, for example, data from Baguio and Shanghai could not be used. For the 0500 UT pass, data from Manila was missing. Baguio and Chung-Li could not provide good data for sufficient duration for the 16:45 UT pass. For the last pass at 23: 15 UT, only two stations recorded data of sufficient quality and duration. Therefore, reconstruction was carried out in cases where at least four stations provided good data. The algorithm used for reconstruction for this study was MART. For the initial guess the IRI-90 model was chosen. Figure 8 shows the contours of the reconstructed ionosphere for the five passes. As in most reconstruction cases using MART (Austen et al., 1988; Raymund et al., 1990), the slant TECs obtained from the reconstructed ionosphere from the stations were very close to the observed slant TECs. On the other hand, it is of interest to see how well the vertical TECs derived from the reconstruction compared with the ‘observed’ vertical TECs described in the previous section. Figure 9 shows the five vertical TECs computed from the reconstructed contours. Compared with those shown in Fig. 7, we note that the general features coincide quite well including the latitudinal variation. the magnitude and the position of the maximum of the anomaly. However, for the 05:OO UT pass, the vertical TEC maximum seems to be shifted slightly towards the North. This seems to be due to the fact that data from Manila were not available for this reconstruction. That this is the case was confirmed by performing the reconstruction using simulated data. When data from Manila were taken out in the reconstruction, a similar shift of the contours to the North was found in the reconstructed image. The phenomenon was not found in cases when data from other stations such as Chung-Li were missing. This
560
C. R. Huang
,ooo
(a) I’
et al.
(b) REC.for
Initial guess (M-90) .‘.‘..’
I”.
1000 , ,
‘I
actual constant
,
,
800
800 z 5 E ”
600
-$ E c”
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400
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Rec. for New method
1000
800 E-
6,0e+,7
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Vertical
T:C
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600
I
400
comparison
,
,
5.0e+17
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4.0e+17
Diamond
: initial guess
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Triangle
: actual
case
Circle
: model
Star
: new
constants
method
3.0e+17
I-
2.0e+i
7
1 .Oe+17 l IO
I
I 20(8.8)
Geographic
I 40
30(18.8)
(geomagnetic)
Latitude
Fig. 5. (a) Initial guess: IRI-90 model for 27 March with parameters: Latitude 14.51-31”N, Longitude 121”E, sunspot number = 30. UT = 0900, (b) Reconstruction using actual absolute slant TEC, (c) Reconstruction using initial constants obtained from GPS information, (d) Reconstruction using initial constants obtained by the modified multi-station method, (e) Vertical TEC derived from contours in (a), (b), (c), (d).
Low-latitude ionosphere tomography
1561
(a) Relative slant TEC from LITN at 08:50 LT (00:50 UT) OSCAR 25
(d) 20120LT (12:20 UT) OSCAR 25 ‘.0e+18 8 8.0e+l7
6.0e+l7
2.Oe+17
-10
0
lO(-1.2)20(8.8) 30(18.8)40(28.8)
Geographic(Geomagnetic)
50
60
O.Oe+OQ
-10
0
lO(-1.2)20(8.8)30(18.8)40(28.8)
50
60
Lat.
(b) 13:OOLT (05:OOUT) OSCAR 23
(e) 00:45LT (16:45 UT) OSCAR 32 l.Oe+lB
I
I
I
I
I
I
8.0t?+l7 -
2.0e+17
o.oe+cKl -10
0
lO(-1.2)20(8.8) 30(18.8)40(28.8)
50
60
6.0e+17
-
4.0e+17
-
2.0e+17
-
O.Oe+OO
-10
I
I
I
I
I
lO(-1.2)ZO(8.8) 30(18.8)40(28.8)
(007:15LT(23:15UT)
(c) 17:15 LT (09:15 UT) NOVA 11 l.Oe+lB
0
I
1
l.Oe+lB
8.0e+l7
8.0e+l7
I
I
I
50
1
60
OSCAR31 I
I
I
-
6.01?+17 -
4.oe+17
2.0e+17
O.Oe+OO
4.0e+17
-
1
1 t -10
0
lO(-1.2)20(8.8) 30(18.8)40(28.8)
50
60
-10
0
lO(m1.2)20(8.8) 30(18.8)40(28.8)
50
Fig. 6. Relative slant TEC for the six stations for the six passes on 14 September 1994. derived from the differential Doppler phase data.
may be due to the fact that the maximum of the anomaly is very close to Manila. More examples, both from simulated and real data, should be examined. Another test for the reconstruction is to compare the electron density profiles calculated from the recon-
strutted ionosphere with those obtained from ionograms. Ionograms from Manila were obtained. Unfortunately most of the ionograms on 14 September 1994 were of poor quality. Only those for 09: 15 UT and 12:20 UT could be used to deduce the
60
C. R. Huang
et al.
(a) Estimated vertical TEC at Sep. 14.1994 0850 LT (00~50 UT)
(d) 20:20 LT (12:20 UT)
L
4.08+17
3
2
3.08+17
u'
2.0e+17
i l.Oe+17
1.oe+15 1 0
I
I
I
I
iO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
Geographic(Geomagnetic)
50
l.Oe+l5
I
0
I
I
I
I
I
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
50
Lat.
(e) OOz45LT(16~45 UT)
(b) 1303 LT (05:OOUT)
:_;f
1.oe+15
1
0
I
I
I
I
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
I 50
i
0
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
50
(f)O7:15LT(23:15UT)
(c) 17:15 LT (09:15 UT)
I
I
I
I
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
I
I
I
I
4.0e+17 3.0e+17
2.0e+17
l.Oe+17
l.Oe+15 * 0
50
l.Oe+15
0
I
I
I
I
lO(-1.2)
ZO(8.8)
30(18.8)
40(28.8)
50
Fig. 7. Vertical TEC converted from slant TEC shown in Fig. 6 using the modified multi-station method.
true height profiles using the software POLAN (Titheridge, 1985). Figure 10 shows the comparison of the profiles. We note that except for the discrepancy in the maximum value of the electron density at 09: 15 UT, the results compared reasonably well. Figure 11 shows the electron density profiles at each
station for different times of day obtained from the reconstructed ionosphere. These profiles provide information about the diurnal variations of the ionosphere at different latitudes of the Asian equatorial anomaly region. To summarize the results from the reconstruction: the tomographically reconstructed
Low-latitude
tomography
ionosphere
1000
1000
600
600
1563
-2 600 E .E"
g E P
I"
ii
400
600
400
200 16
18
0050
20
24
22
UT Geo-latitude
26
26
15
(ON)
20
30
25
12:20 UT Geo-latitude
(ON)
l&45
(“N)
800
600 E .P I"
400
200
05x00 UT Geo-latitude
(ON)
UT Geo-latitude
1000
600
-2 g 800 E .9J I"
400
200 15
20
30
25
09:15 UT Geo-latitude
(ON)
Initial guess
Fig. 8. Contours of the reconstructed images of the ionosphere at different times of day on 14 September 1994. (f) is the initial guess for reconstruction with parameters: IRI-90, 14 September. Latitude 14.5-31”N, Longitude 121 “E. sunspot number = 26, UT = 1600.
564
C. R. Huang (a)00:50 I
Fz ‘E > P F z ‘F f
et al.
UT
I
(d)l2:20 UT I
I
I
4.0e+17
-
-
4.0e+17
-
3.0e+17
-
-
3.0e+17
-
2.0e+17
-
-
2.0e+17
-
l.Oe+17
-
-
l.Oe+17
-
1.Oe+l5
0
I I I I lO(-1.2) 20(8.8) 30(18.8) 40(28.8) Geographic (Geomagnetic) Lat.
1 .Oe+l5 50
0
I
-
-
4.0e+17
-
3.Oe+l7
-
-
3.0e+17
-
2.0e+17
-
-
2.0e+l7
-
1 .Oe+l7
-
-
l.Oe+17
-
l.Oe+15
0
I
0
I
I
50
(e) 18:45 UT I
I
4.0e+17
1 .Oe+l5
I
I I I I 10(-l .2) 20(8.8) 30(18.8) 40(28.8) Geographic (Geomagnetic) Lat.
(b)OS:OOUT I
I
I
I
10(-l .2) 20(8.8) 30(18.8) 40(28.8) Geographic (Geomagnetic) Lat.
50
I
I
I
1
I
I
I
I
10(-l .2) 20(8.8) 30(18.8) 40(28.8) Geographic (Geomagnetic) Lat.
50
(c)09:15 UT
4.0e+l7
’
’
-i
3.0e+l7 2.0e+l7 1 .Oe+17 l.Oe+l5
t ~ 0
i
10(-l .2)
20(8.8)
30(18.8)
40(28.8)
50
Geographic (Geomagnetic) Lat.
Fig. 9. Reconstructed
vertical
TEC calculated
image shows the development and the decay of the anomaly on 14 September 1994. A latitudinal density gradient started to develop as early as 08:50LT. It grew into an anomaly peak around noon time. The peak was located between 1Yto 20”N geographic latittide at approximately 325 km height. The anomaly
from the contours
in Fig. 8.
persisted as late as 20:20 LT. The ionosphere became rather flat latitudinally around local midnight. 6. CONCLUSIONS
The Low-latitude Ionospheric Tomography Network (LITN) is introduced in this paper. This is a
Low-latitude
ionosphere
tomography
(a) Compare 17:15 LT reconstruction with MA true height
1565 (b) Compare 20:20 LT reconstruction with MA true height
I
O.Oe+OO Fig.
solid : Tomography
solid : Tomography
circle : lonogram
circle : lonogram
iSe+12 50e+ll 1.Oe+l2 Electron density (el/mh3)
10. Comparisons
of reconstructed
2.0e+12
electron
density
I 2.0e+ll
I 4.0e+ll
for Manila
with those
0.0 O.Oe+OO profiles
I 6.0e+ll
-
&Oe+l
derived from
ionograms.
network of six stations along the 121 “E meridian using signals received from NNSS Transit satellites for imaging the two-dimensional electron density distributions of the ionosphere in the equatorial anomaly region of the Asian Sector. In order to overcome the difficulty involved in obtaining the absolute TEC from the observed data, a modified multi-station method has been proposed. This method appears to yield better estimates of the unknown initial constants for the equatorial anomaly region. Initial results from a campaign where for the first time data from six stations became available have been presented in this paper. These results indicate that the LITN can be used to obtain a tomographic image of the ionosphere in the equatorial anomaly region. The diurnal behavior of the ionospheric structure in the reconstructed image is consistent with the predicted behavior as well as earlier observations of the equatorial anomaly in this region (Huang et al., 1989; Huang and Cheng, 1991). Electron density profiles obtained from the reconstructed ionosphere compared reasonably well with true-height profiles derived from the available ionograms at Manila. Also. the shape, location of the maximum and the magnitude of the vertical TEC profiles calculated from the reconstructed ionosphere coincide with those for the vertical TEC derived directly from the observed data. This, to certain degree, is a self-consistency check of the reconstruction procedure. Therefore it appears that the tomographic reconstruction using data from the LITN may be regarded as a credible means for monitoring the ionosphere in this region. A systematic study of the two-dimensional behavior of the Asian
sector equatorial anomaly will be carried out in the future to understand its diurnal, seasonal variations, its response to storms and related physical mechanisms. There are also technical aspects related to the LITN that need our attention. Although the reason is not quite understood yet, it seems that data from the southernmost station, Manila, in the network are especially crucial to the reconstruction. On the other hand, we may have not found the most robust reconstruction algorithms yet. Also, operations at each receiving station need to be improved to yield data with better quality such that we always have the full coverage of the six stations of the network. One of the challenges in a further development of CIT is to improve the accuracy of the height distributions of the electron density in the reconstruction. Attempts are being made to make use of a priori information, such as that obtained from ionosondes, in the reconstruction in order to compensate for the deficiency caused by the lack of horizontal ray paths in the usual CIT configurations (Liu and Raymund, 1994). Acknowledgements-The research reported in this paper is supported by grants NSC-82-SP-008-04, NSC-83-NSPO-BRDD-008-04, NSC-84-2612-M-008-002 and NSC-85-2612M-008-001 from the National Science Council. ROC. The work by University of Illinois personnel is supported by the U. S. National Science Foundation through grant ATM9121660. The data from Wenzhou and Shanghai were provided by Professors J. S. Xu and S. Y. Ma from Wuhan University. the data from Manila and Baguio by Dr. F. V. Badillo from Manila Observatory, and the data from Kaoshiung by National Sun Yat-Sen University. Discussions with K. C. Yeh, Steve Franke and Andrei V. Izotov are acknowledged with pleasure.
1
1566
C. R. Huang
et al.
(a)Manila site
(b)Saguio site
1 600.0
600.0
600.0
I 1 O.Oe+OO5.0e+ll
0.0
I
I
l.Oe+l2 1.5e+12 Electrondensity(eVnF3)
2.0e+12
0.0 1 O.Oe+OO
I
I
I
50e+ll
1.08+12
1.5e+12
(c)Kaoshiuna site
I
2.0e+12
(d)Chungfi site
600.0
800.0
600.0
Circle
: 0850
LT
Square : 1300 LT
600.0
Triangle : 1715 LT
400.0
Star
: 2020 LT : 24:45LT
l.Oe+l2
l&+12
Plus
200.0 0.0
1
I
I
l.Oe+12
1.5e+l2
I
O.Oe+OO5.0e+ll
2.0e+12
0.0 O.Oe+OO
5.0e+ll
(e) Wenzhou site
Fig.
(f)Shanghai site
600.0
600.0
600.0
600.0
0.0 O.Oe+OO
11.Reconstructed
5.0e+ll
l.Oe+12
l&+12
2.0e-I
electron density profiles for the six stations 1994.
0.01
O.Oe+OO5.0e+ll
for different
1.08+12
1.5e+l2
times of day on 14 September
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2.0e+l2
Low-latitude Huang C. R.. Liu C. H., Yeh H. C., Tsai W. H., Wang C. J.. Yeh K. C., Lin K. H. and Tsai H. L. Kelley M. C. Kersley L., Heaton J. A. T., Pryse S. E. and Raymund T. D. Klobuchar J. A. and Whitney H. E. Klobuchar J. A.. Kuenzler H. J.. Pakula W. A. Fougere P. F.. Sheehan R. E., Doherty P. H., Buonsanto M. J., Foster J. C. and Langley R. B. Leitinger R., Schmidt G. and Tauriainen A. Leitinger R. Liu C. H. and Raymund T. D.
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Int. J. imaging syst. Tech. 5, 75-85. J. atmos. terr. Phys. 56, 7377757. J. atmos. terr. Phvs. 54, l-30. Ionogram analysis with generahsed program POLAN. Rept. UAG-93, World Data Cent. A for Sol.-Terr. Phys.. Washington, D. C. J.geophys. Res. 66, 1061-1067. Rev. geophys. space Phys. 10,631-709. J. geophvs. Res. 84, 6589-6594. Proceeding of COSPAR Colloquium on Low-latitude Ionospheric Physics held in Taipei, Taiwan, 9-12 November, ed. F. S. Kuo, 295-303.