IPS From LOFAR: A Complement to Thomson Scattering Studies

Information about the large scale physical properties of the inner heliosphere plasma can only be obtained by employing remote sensing techniques. The two most useful measurement techniques for this are Thomson scattering, used by Solar Mass Ejection Imager (SMEI), and Interplanetary Scintillation (IPS). Both these techniques are sensitive to the distribution of properties of the solar wind plasma along the entire line-of-sight through the medium. The two measurement techniques are sensitive to different properties of the same physical plasma. IPS is sensitive to the fluctuations in the refractive index of the medium (~ δ n_e²) and their spectral index, the perpendicular component of velocity of the solar wind, the anisotropy in electron density fluctuations caused by the magnetic field and their inner scale. Thomson scattering, on the other hand, is sensitive only to the distribution of the electron density along the line of sight. The data from both these techniques are suitable for tomographic reconstructions, yielding three dimensional visualisations of the inner heliosphere. Heliospheric tomography will benefit significantly from the denser sky coverage and the improved signal-to-noise IPS measurements promised by the upcoming instruments and the simultaneous use of Thomson scattering data. The unquestionable synergy between the information obtained from these two techniques should be exploited to arrive at significantly better constrained tomographic reconstructions. We are now assessing the potential for space weather applications, including IPS studies, of the Low Frequency Array (LOFAR), an aperture synthesis radio interferometer covering the 10-240 MHz range. The unique design of this instrument allows the possibility of high sensitivity observations of up to 4000 IPS sources a day. This unprecedented ability will increase the sampling of the inner heliosphere by ∼2 orders of magnitude compared to the present IPS instruments and improve the signal-to-noise of individual measurements. This paper will describe some aspects of the LOFAR design and outline its potential IPS measurement capabilities.