Introduction

There exist a number of scientific questions to be addressed by solar observations in the 2 to 20 MHz frequency range. Energy releases, low in the solar atmosphere, produce coronal shock waves that generate coronal type II radio bursts. Ground-based observations have shown that these bursts usually die away in the 15 to 30 MHz range. Similar bursts are observed from spacecraft at frequencies below 2 MHz as interplanetary (IP) type II events and IP type II bursts are known to be generated by the IP shock waves associated with coronal mass ejections (CMEs). CMEs can have profound geophysical effects and it is of great importance to understand the physics of their origin and propagation. One key question is whether or not the IP shock waves are extensions of the coronal shock waves (Cane 1997). The most direct way to answer this question would be to observe a coronal type II burst that continued through the 20 to 2 MHz range to become an IP type II event, or to show that this does not occur. However, for reasons discussed below, this frequency range is virtually unobserved. Other questions to be addressed through observations in the 2 to 20 MHz range include the connection between shock associated (SA) events (Cane et al 1981) observed in the IP medium and the herringbone structures that are observed in coronal type II bursts above 20 MHz. There appears to be a time correlation between these phenomena, but no direct connection has been made because of the frequency gap in the observations. SA events are of particular interest because they are well correlated with major proton events in the Earth's environment (Cane and Stone 1984), (Kahler et al 1986).

Radio observations from the surface of the Earth at frequencies below 20 MHz are very difficult in the daytime because of high levels of terrestrial interference. Above 20 MHz the interfering signals are normally far enough apart in frequency that solar bursts can be observed between them but below this frequency interfering signals virtually fill the entire spectrum. This is because interfering signals in this frequency range propagate world-wide. Wide-band, impulsive emissions from lightening strokes also propagate over long distances at frequencies below 20 MHz and are another troublesome source of interference. Because of these problems, very few ground-based observations are attempted.

Observations with space-borne systems are also uncommon in the 2 to 20 MHz frequency range. Below the ionospheric critical frequency for vertical propagation () the ionosphere will normally shield spacecraft from Earth-generated interference. Daytime values of are 3 MHz or so, and above this frequency interference can break through the ionosphere to greatly disturb observations made from spacecraft located near the Earth. The problem is diminished by locating the spacecraft at a large distance from Earth, such as at the inner Lagrangian point, L1, but telemetry restrictions then limit the amount of spectral information that can be conveniently returned to Earth. For example, the WAVES experiment (Bougeret et al 1995) on the WIND spacecraft, that is located at L1, observes up to 14 MHz with limited spectral and temporal resolution.

Tasmania is known to have unusually low daytime ionospheric densities, which mitigates the problem of over-the-horizon interference from distant sources and allows solar bursts to propagate to the Earth's surface at quite low frequencies. On Bruny Island, an island off the southeast coast of Tasmania, locally generated interference is uncommon. Employing this good location from which to explore the possibilities for decametric solar observations, the Bruny Island Radio Spectrometer (BIRS) has been built. BIRS is an adaptive spectrometer designed to make use of modern digital techniques for the mitigation of terrestrial interference. The system is designed primarily for solar observations but it is also usable for Jupiter observing and for monitoring any other transient decametric radio sources. At the present time, it is adjusted for swept-frequency observations in the 3 to 37 MHz band. The upper portion of this band overlaps that covered by the Culgoora Radiospectrograph (Presage et al 1994), which is at nearly the same longitude, making data comparison convenient. It has also been found useful to make detailed comparisons between BIRS and WAVES observations of the same bursts to delineate structure requiring high temporal and spectral resolution and to compare the flux density calibrations of the two instruments.

© Copyright Astronomical Society of Australia 1997