1 Introduction

Since the beginning of radio astronomy several surveys of the galactic background have been made at various frequencies, but most of them covered only the limited region near the galactic plane (see e.g., Salter & Brown 1988).

It is presently known that the background continuum radiation is a mixture of thermal free-free emission from ionized hydrogen and non-thermal synchrotron emission resulting from the interaction of cosmic ray electrons with the galactic magnetic field. At frequencies higher than about 1 GHz, the diffuse thermal component becomes important in the galactic plane; for example, at 1420 MHz it may reach 30% to 50% (Reich 1986). The non-thermal component is important at low frequencies and increases with decreasing frequency until the free-free absorption due to ionized hydrogen sets in at around a few tens of MHz. Continuum surveys in this range (say 20 MHz $\leq \nu \leq$ 100 MHz) are therefore most important to investigate the structure of the magnetic field in our Galaxy, the spatial and energy distributions of cosmic ray electrons as well as those of the synchrotron radio emission. However, only a few surveys, discussed in Sect. 5, have been made in this frequency range because of inherent difficulties. For example, to achieve a good angular resolution at low frequencies it is necessary to build a large array; terrestrial interference (man-made and natural) generally becomes serious with decreasing frequency; the ionospheric opacity increases as the frequency decreases, and changes in opacity result in spurious intensity variations of the incoming background radiation. Furthermore, solar radio bursts fatally disturb the observation of the background radiation during high solar activity periods.

In this paper we present the results of a northern survey at 45 MHz. The observations were made with the MU (Middle and Upper atmosphere) radar located at Shigaraki, Japan. The data processing was performed at the Maipu Radio Observatory, Chile, and the final maps were obtained at the Max-Planck-Institut für Radioastronomie, Germany, making use of the NOD2 program package (Haslam 1974).