2 Observations

2.1 The MU radar array

The MU radar is primarily used for Doppler shift measurements of the echo signals which are incoherently scattered by the atmospheric particles at various heights. Since the detailed description of the MU radar has been given elsewhere (Fukao et al. 1985a, 1985b), here we will only briefly describe the system which is an active-phased array consisting of 475 crossed 3-element Yagis arranged within a circle of 103-m diameter. It has 475 transmitter-receiver modules, each connected to crossed Yagis by two equal-length coaxial cables. By phasing the two orthogonal Yagis by means of a phase switch, the right-hand or left-hand polarization are obtained. Each transmitter-receiver module has a preamplifier and a phase shifter. There is only one beam and, by electronically controlling the phasing parameters, its direction can be changed from one position to the next, as rapidly as 0.4 ms. The Yagi elements are not physically tilted as the array is phased to various zenith angles. The beam direction can be tilted as far as $30\hbox{$^\circ$ }$ from the zenith. A basic triangular grid is used to arrange the array elements. The element spacing is $0.7 \lambda$ and no significant grating lobes appear for $Z \leq 30 ^{\circ}$, where Z is the zenith angle. The first side lobe level is theoretically -18 dB with respect to the main beam. The maximum effective area of the array is 8300 m²and the half-power beam width is $3{{\rlap.}{^{\circ}}}6$ at the zenith. As Z increases, the effective area decreases in proportion to ${\rm cos}\,Z$ because of foreshortening and the beam within a tipped direction also broadens as $3{{\rlap.}{^{\circ}}}6\, {\rm sec}\,Z$. Since the beam direction of the MU radar can be tilted up to $30 ^{\circ}$ from the zenith, we can observe the sky in the declination range from $+4{{\rlap.}{^{\circ}}}85$to $+64{{\rlap.}{^{\circ}}}85$, and the reduction in effective area is less than 15%. The array polarization is almost purely circular in every direction at least for $Z \leq 30 ^{\circ}$.

Since the 475 preamplifiers are simultaneously working during observation, it is practically impossible to make calibrations. It is therefore necessary to calibrate the MU radar observations using calibrated independent data. The system noise is about 2300 K. Table 1 shows the main instrumental parameters of the MU radar array.

 

 

Table 1: Parameters of the MU radar

Operating frequency	46.5 MHz
Array	475 crossed 3-element Yagis
Half-power beam width	$3{{\rlap.}{^{\circ}}}6 (\alpha )\times 3{{\rlap.}{^{\circ}}}6\, {\rm sec}\,Z\,(\delta)$
Polarization	circular (RH or LH) or linear
Effective area	8300 m2 at the zenith
Beam direction	$0\hbox{$^\circ$ }\leq Z^{\rm a} \leq 30\hbox{$^\circ$ }$; $0\hbox{$^\circ$ }\leq A^{\rm b} \leq 360\hbox{$^\circ$ }$
Receiver bandwidth	1.65 MHz
Array coordinates	$34{{\rlap.}{^{\circ}}}85$ N, $136{{\rlap.}{^{\circ}}}10$ E

$^{\rm a}$

Zenith angle.

$^{\rm b}$

Azimuth.

Figure 1: Set of maps presented in equatorial coordinates (Epoch 1950). Each panel covers 6 hours in right ascension, and $61\hbox {$^\circ $ }$ $(+4\hbox {$^\circ $ }\leq \delta \leq +65\hbox {$^\circ $ })$ in declination. Contours are labelled in Kelvin (see text). Arrows on contour lines point towards decreasing temperatures

Figure 1: continued

Figure 1: continued

Figure 1: continued

Figure 2: Set of maps presented in galactic coordinates. The contour levels are the same as those in Fig. 1 (see text). Arrows on contour lines point towards decreasing temperatures

Figure 2: continued

Figure 2: continued

Figure 3: Map presented in galactic coordinates in equal-area projection

Figure 4: Colour coded Aitoff projection map of the 45-MHz survey in galactic coordinates. The code is shown in the figure. The contours begin at 10000 K and continue in steps of 5000 K

2.2 Survey observations

Our observations were made in the periods 1985-1989 and 1997-1999, each consisting of 2-4 day runs. Since some of the observations experienced interference and apparent gain changes, we used only selected data for this survey. The results presented in this paper are based on the data taken during February 24-26, 1988, and February 9-11, 1998. Data sets taken on other occasions were used as a complement in the analysis.

In the observations, the MU radar was used as a meridian transit instrument by taking advantage of the quick steerability of the beam direction. For each one-day observation a total of 16 or 18 declinations were observed. The beam direction was changed every 1 ms and the selected declinations were cyclically scanned. The declinations observed in 1988 (two different sets of 16 beam directions) and 1998 (one set of 18 beam directions) runs are shown in Table 2. We have not used all the declinations of the 1998 data, but only those which have not been observed in 1988.

   

Table 2: Observed declination strips in 1988 and 1998

$\delta$ (1988)	$\delta$ (1998)	$\delta$ (1988)	$\delta$(1998)
( $\hbox{$^\circ$ }$)	( $\hbox{$^\circ$ }$)	( $\hbox{$^\circ$ }$)	( $\hbox{$^\circ$ }$)
4.85	4.85	34.85	34.85
--	6.85	36.85	--
8.85	--	--	38.85
--	10.85	40.85	40.85
12.85	12.85	--	42.85
--	14.85	44.85	--
16.85	--	--	46.85
--	18.85	48.85	--
20.85	--	--	50.85
21.85	--	52.85	--
--	22.85	--	54.85
24.85	--	56.85	--
--	26.85	58.85	58.85
28.85	--	60.85	--
--	30.85	--	62.85
32.85	--	64.85	--