D-region electron density measurements by MF radar in the middle and high latitudes

Adv. Space Res. Vol. 25, No. I, pp. 25-32, 2000 0 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273 I 177/00 $20.00 + 0.00

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D-REGION ELECTRON DENSITY MEASUREMENTS IN THE MIDDLE AND HIGH LATITUDES

BY MF RADAR

K. Igarashi’, Y. Murayama’, M. Nagayama2, S. Kawana2

1Communication Research Laboratory, 4-2-l Nukuikita-machi, Koganei-shi, Tokyo 184-879.5Japan ’ Wakkanai Radio Observatory, Communication Research Laboratory, 2-3-20 Midori Wakkanai-shi, 0970004 Japan

ABSTRACT The second MF radar was deployed at Wakkanai (45.36O N, 141.81” E) in September, 1996, in addition to the MF radar at Yamagawa (3 1.20’ N, 130.62’ E). The Wakkanai MF radar has a capability of electron density measurements in D-region. The differential absorption technique is applied in order to measure the electron density profile in the D-region regularly. Observations of both of wind and electron density are conducted every 2 minutes alternately at Wakkanai. Monthly variations of electron density profile in the D-region are presented and discussed by comparing with the W-95 model. Preliminary electron density profiles in the D-region obtained during the test operation of MF radar at Poker Flat (65.1” N, 147.5” W) are also compared with the IRI-95 model. Monthly mean variations of electron density profiles are discussed for Wakkanai and Poker Flat. Seasonal variations of electron density profile in the D-region are compared between northern and southern hemispheres using the IR195 model. 01999

ELECTRON

DENSITY MEASUREMENTS

COSPAR.

Published

by Elsevier Science Ltd.

WITH MF RADAR

The electron density profiles in the D-region can be measured with differential absorption experiment (D.A.E.) using the partial reflection of middle-high frequency radio waves from the lower ionosphere (e.g. Belrose, 1970). In the Wakkanai MF radar this differential absorption experiment was applied for measuring the electron density profiles in the D-region. The observations of wind and electron density profile are conducted every 2 minutes alternately. The technique of DAE is based on the following relation. Ax/Ao=RxRo

l

exp[ - 2 s h (kx - ko)dh ]

(1)

where kx, ko are absorption coefftcients for the extraordinaly (X) and ordinaly (0) mode waves at height h, Rx/R0 is the ration of X- to O-mode reflection coefficients at height h, and Ax/A0 is the amplitude ratio of X- to O-mode signal returns at height h, respectively. The real time processing DAE software developed in Adelaide University was applied to the MF radars at Wakkanai and Poker Flat (Holdsworth, 1998). In this DAE technique the collision frequency profile Y (h), magnetic field strength B(h), and its

26

K. lgarashi @Icrl

declination profile 8 n (h) are used to calculate the reflection coefficient ratio and absorption coefficient. Choice of collision frequency profile and assessment of the DAE method are discussed in the other articles (e.g. Gregory and Manson, 1969) The main characteristics of Wakkanai MF radar are shown in Table 1. Table 1. Characteristics of Wakkanai MF radar Location Peak Envelope Power Operating Frequency Half Power Pulse Width Transmitting/Receiving antenna Sampling Interval Observation modes Operated period COMPARISON OF ELECTRON MF RADAR MEASUREMENT Electron Density Measurements

45.4” N, 141.7” E 50 kW RMS 1.9585 MHz 48 Y set four cross dipole array at 15 m above ground, polarization 2 km FCA, SCA, DAE,DPE since Sep. 19, 1996 DENSITY

PROFILES

BETWEEN

for circular

THE IR195 MODEL AND THE

at Wakkanai

The Wakkanai MF radar has been continued to observe the winds and the electron density profile in the altitude 60 - 100 km by averaging for 2 minutes alternately every 4 minutes since September, 1996. Figure 1 shows the comparison results between the XI-95 model and monthly averaged values of the measured electron density profiles at 12 h LT in March, June, September, and December of 1997. Above about 85 km the electron density measurement is difficult due to a small amplitude ratio of the extraordinary mode signal and the ordinary mode signal, Measured electron density in the altitude 70 km - 80 km is lower than the IRI-95 model. The measured electron density profile in December, 1997 is good agreement with the IRI-95 model. Below 70 km the measured electron density is larger than the IRI-95 model. The DAE method assumes profiles of magnetic field model IGRF and atmospheric pressure model from CIRA86. These assumptions and interpolations may cause the difference between measured electron density profiles and real electron density profiles. In this paper we are comparing the DAE results obtained by using these models with the IRI-95 model. Monthly variations of D-region electron density profile at Wakkanai are shown in Figure 2. The measurement of electron density above 80 km was difficult due to rapidly decrease of the amplitude ratio Ax/Ao. Reliability of measured electron densities and causes of errors were discussed in great detail in the many literatures (e.g., Belrose, 1970). In the altitude around 75 km the long period variation of electron density appears from January to May. Two-layer structure may be indicated around 65 km during summer period in Figure 2. Two-layer structure in D-region electron densities has been suggested by Thrane et. al. (1968). But below 65 km differential absorption is negligible. So the DAE measurement below 65 km may not be reliable. Comuarison of Electron Densitv Profiles in Northern and Southern Hemisphere The monthly variations of electron density profile by the IRI-95 model are shown at noon for Wakkanai, Japan and Adelaide, Australia in Figure 3 (a) and Figure 3 (b). The response of lower ionosphere is different between summer and winter. Seasonal changes of noon electron densities at 65 - 90km have been reported about differential absorption experiments with a partial reflection radar during 1963-1967 at Christchurch (44’ S, 173” E) by Gregory and Manson (1969). These results showed a single

27

D-Region Middle and High Latitudes

.c 0

r

b

4

N

t.,,,“,‘l~.““~“l~““.“‘l”“~““l z z2 0

0

0

m

r.

co

i=

0 0

0

0

Ez

m

r.

F2 -

(u7)1~313H

(~4)1~313H

Fig. 1 (a) Comparison of the electron density profiles using the IRI-95 model (dashed lines) and monthly mean values of the D-region electron density (solid lines) measured by the Wakkanai MF radar in March and June of 1997.

‘b ‘b

.

b

Fig. 1 (b) Comparison of the electron density profile using the W-95 model (dashed lines) and monthly mean values of the D-region electron densities (solid lines) measured by the Wakkanai MF radar in September and December of 1997.

28

K. lgarashl et al.

Wakkanai

MF radar:DAE

Electron

density

12h

LT, 1997

80

65

60 JAN.

I FEB.

I MAR.

I APR.

MAY

I

I

I

JUN.

JUL.

I

AUG.

SEP.

I

I

I

OCT.

NOV.

1 DEC.

MONTH

Fig. 2

Contour plots of monthly averaged electron density profiles at noon in 1997 at Wakkanai

IRl95

MODEL(12h

loo-

LT), I

WAKKANAI, I,.“X,U

-

1997 I

I

I

_#Q4

-4

.0%104 5.0x10

-_,

90 :

3

70

604 JAN.

FEB.

MAR.

APR.

MAY

JUN.

JUL.

AUG.

SEP.

OCT.

NOV.

DEC.

MONTH

Fig. 3 (a) Contour plots of monthly variations of electron density profiles from the IRI-95 at Wakkanai (45.36” N, 141.81” E) in 1997.

29

D-Region Middle and High Latitudes

IRl95

MODEL(12h

LT),

ADELAIDE,

1997

80

JAN.

FEB.

MAR.

APR.

MAY

JUN.

JUL.

AUG.

SEP.

OCT.

NOV.

DEC.

MONTH

Fig. 3 (b) Contour plots of monthly variations of electron density profiles from the IRI-95 at Adelaide (35’ S, 138” E) in 1997. dominant maximum, at mid-winter above 80 km and an constant mean electron density below 80 km by applying a solar zenith angle’s factor of the form I/cos”~ x This feature is a good agreement with the IRI model. Recent measurements by MF radar at Adelaide was reported by Holdsworth et al. (1998). This result presented a single dominant maximum at mid-winter above 68 km clearly at Adelaide in the southern hemisphere. on D-v

Me-

at Poker

In order to make a test experiment for investigating interferences at Poker Flat the MF radar was operated for only day time from November 17 , 1997 to July 15, 1998 at a peak power level of 10 kW, a frequency of 2.43 MHz and a pulse width of 27 p s. Two cross dipole antennas were used for transmission and reception. A electron density profile measurement in the D-region was conducted for testing the DAE (Differential Absorption Experiment) Figure 4 (a) and Figure 4 (b) show the comparison results between the IRI-95 model and the measured electron density profiles at 12 h LT from December 1997 to March 1998. The KG95 profiles in all cases give lower electron density values than those measured by MF radar below 86 km. The electron density of IRI-95 model indicates extremely low electron density rather than the measured electron density below especially 70 km. Monthly variations of D-region electron density profile at Poker Flat are shown in Figure 5. The electron densities observed at Poker Flat at about 75 km are larger than those observed at Wakkanai. Around 75 km the changes in electron density are between -350 - 600 electrons/cm3. The electron density enhancements seem to be indicated at 68 - 75 km in July, 1998. At about 65 km the changes in electron density are between -275 - 350 electrons/cm3. The values of electron density between 62 - 65 km are 275 - 450 electrons/ cm3. These electron densities are larger than those obtained by the IRI-95 model. The electron densities measured with the 2.75 MHz partial reflection radar at Tromso showed 300 - 400

K. lgarashi et al.

h b

‘? -0

0 0

m

0

cc

0

r.

k2

g

:

Eis

0 I--

::

(W)lHXlt-I

(W)LH~I~H

Fig. 4 (a) Comparison of the electron density profile using the RI-95 model (dashed lines) and the Dregion electron densities (solid lines) measured by MF radar at Poker Flat in December, 1997 and January,

‘Q

b

Fig. 4 (b) Comparison of the electron density profile using the IRI-95 model (dashed lines) and the Dregion electron densities (solid lines) measured by MF radar at Poker Flat in February, 1998 and March, 1998.

31

D-Region Middle and High Latmdes

Poker

60t

DEC

Flat

MF radar:DAE

I

JAN.

I

FEB.

Electron

density

I

I

APR.

MAR.

I

MAY

12h

LT, 1998

1

I

JUN.

JUL

MONTH Fig. 5 Contour plots of monthly averaged electron density profiles at local noon from December, July, 1998 at Poker Flat.

1997 to

electrons/ cm3 in the altitude 62-65km for the PCA recovery interval (Rastogi et al., 1982). The electron density profiles observed at Tromsla showed persistent features below 68 km in the recovery phase of a PCA event. The observation values at Poker Flat seem to be similar values with these results at Tromso. In the aurora1 zone it was suggested that a weak background flux of ionizing electrons may be present even during quiet conditions. A contribution of weak background flux to electron density enhancements should be considered for improvements of the IRI model in the high latitude (Thrane et al., 1968). SUMMARY

We have presented recent observation results of D-region electron density profiles with the differential absorption experiments (DAE) of MF radars at Wakkanai and Poker Flat. During the low solar activity periods in 1997 the Wakkanai MF radar showed a good agreement of electron density profile with the IRI-95 model in December, 1997. On the other hand the measured electron densities below 80 km was lower than the IRI-95 model for March, June, and September in 1997. At Poker Flat the measured electron densities in the altitude 62-84 km are larger than those from the IRI-95 model. The DAE profile assumes profiles of magentic field and pressure model. So this is one of the difficulties with direct comparison with the IRI model. It may explain higher electron density profiles than the N-95 at Poker Flat that a weak background flux of ionizing electrons may be present even during quiet conditions. Bilitza (1998) suggested that the main problem in D-region modeling remains the scarcity of reliable rocket measurements and the relatively poor long term data base of ground based radar measurements. In spite of several limitations of DAE, continuous observations of electron densities in the lower ionosphere with MF radar are very useful to understand the variabilities of lower ionosphere and to contribute to an improvement of the IRI.

32

K. lgarashi et al.

ACKNOWLEDGMENTS The MF radar at Poker Flat has been constructed as one of the collaboration projects based on a science and technology cooperation agreement between the Geophysical Institute, Universty of Alaska, USA and Communications Research Laboratory, Ministry of Posts and Telecommunications, Japan. Don Rice of Geophysical Research Institute is appreciated for conducting the test experiments of MF radar at Poker Flat. D. A. Holdsworth of ATRAD is appreciated for evaluating the electron density profile of Wakkanai and Poker Flat. The Science and Technology Agency of Japan is thanked for a research grant to this evaluation. REFERENCES Belrose, J. S., Radio wave probing of the ionosphere by the partial reflection of radio waves (from heights below 100 km), J. Atmos. Terr. Phys., 32, 567-596 (1970). Bilitza, D., The E- and D-region in IRI, Adv. Space Res., 21, 6, 871-874 (1998). Gregory, J. B., and A. H. Manson, Seasonal variations of electron densities below 100 km at mid-latitude - 1 I , J. Atmos. Terr. Phys., 3 1, pp. 683-701 (1969). Holdsworth, D.A. private communication (1998) Rastogi, P.K., A.Brekke, O.Holt and T. Hansen, Variability of D-region electron densities at Tromso, ./. Atmos. Terr. Phys., 44, pp. 3 13-323 (1982). Thrane, E.V., A.Haug, B. Bjelland, M. Anastassiades and E. Tsagakis, Measurements of D-region electron densities during the International Quiet Sun Years, J. Atmos. Terr. Phys., 30, pp. 135-I 50 (1968).