Astronomy and Astrophysics – Astronomy
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh61a06k&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SH61A-06
Astronomy and Astrophysics
Astronomy
6954 Radio Astronomy, 7513 Coronal Mass Ejections, 7534 Radio Emissions
Scientific paper
Solar type II radio emissions arise from shocks in the solar corona and interplanetary medium. The movement of these shocks away from the sun triggers radio emissions at successively lower frequencies, which equal the electron plasma frequency (or its harmonic) in the coronal and interplanetary medium. Many of these shocks are driven by CMEs, thus the estimation from radio measurements of the shock speed is an important quantity in models that predict the arrival of CMEs at Earth. The estimation of shock speed from radio measurements is usually done by equating the observed radio emission frequency to models of the coronal and interplanetary electron density, such as the well-known Saito or Newkirk models. In this speed estimation process, it is usually implicitly assumed that the emission takes place on the expanding shock along the normal to the solar surface so that the source locations at successively lower frequencies lie along a radial from the sun. In reality, there is considerable evidence for radio emission to take place on the flanks of shocks, thus the radio source regions can lie along lines that are distinctly non-radial. The error in speed determination due to the radial assumption is investigated for various coronal density models. These errors can be quite large near the sun where radio measurements are generally made by ground-based observatories. However, far from the sun, in the frequency range where radio measurements are usually made by space-borne radio telescopes, the radial assumption is not so important.
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