Measurements using Optic and RF Waves

Chapter 12Long Baseline Decameter Interferometrybetween Nançay and LOFAR¹

 

 

 

12.1. Introduction

LOFAR (LOw Frequency ARray) is a very large array of low frequency (LF) antennas, currently under construction in the Netherlands (www.lofar.org). LOFRA is an interferometer in which each antenna is actually a phased array of elementary antennas (crossed dipoles), called “stations”. In its original version, LOFAR was made of 77 stations, 32 of which gathered in a compact configuration (“virtual core”), the other 45 providing interferometric baselines up to 100 km from the “virtual core” (the current LOFAR version in construction - LOFAR Phase 1 will consist of about 40, i.e. 20 + 20, stations). The frequency ranges covered by LOFAR are 30–80 MHz and 110–240 MHz, on both sides of the FM band. At these frequencies, 100 km baselines provide an angular resolution of 20″ at 30 MHz and 7″ at 200 MHz (proportional to the wavelength λ).

Many scientific programs could take advantage of a 10 times better resolution, typically of order of an arcsecond. These scientific objectives are described in detail in Vogt (2006). Here we will restrict ourselves to a single example: the fast imaging of LF radiosources in Jupiter's magnetosphere, whose scientific implications, described in (Zarka, 2004), include:

– improved mapping of the surface planetary magnetic field, via imaging of instantaneous cyclotron sources of the highest frequency;

– improved mapping of the Jovian plasma environment (especially ...