On the feasibility of imaging coronal mass ejections at radio wavelengths

Abstract

Abstract. Coronal mass ejections (CMEs) can have a profound impact on the interplanetary medium and the near-Earth environment. We discuss the feasibility of detecting coronal mass ejections at radio wavelengths with a ground-based instrument. In particular, we explore the possibility that a radio telescope employing Fourier synthesis imaging techniques can detect thermal bremsstrahlung emission from CMEs. Using a simulated database from such a telescope, we explore three detection schemes: direct detection, an approximate differential detection scheme, and an "exact" differential detection scheme. We conclude that thermal bremsstrahlung emission from CMEs can be detected by such a telescope provided differential techniques are employed. While the approximate differential detection scheme may be sufficient for CMEs viewed near the solar limb, detection of CMEs against the solar disk may require the more sensitive exact differential scheme. The detection and imaging of nonthermal radio emissions from CMEs is also discussed. IntroductionA coronal mass ejection (CME) is an eruption of coronal magnetic field and plasma from the Sun into interplanetary space. CMEs can have a profound influence on the interplanetary medium and the near-Earth environment. The relationship between CMEs, flares, prominence eruptions, coronal and interplanetary shocks, and various kinds of radio bursts is complex and is an extremely active area of research. In white light (as observed by a space-based coronagraph, for example), CMEs typically show a three-part structure composed of a relatively fast, bright, leading arc of emission from material at coronal temperatures, followed by a dark cavity, and slower moving, dense, cool (prominence) material. In soft X rays (SXRs), eruptive morphologies have been identified with overlying CMEs, but no direct counterpart to the white light CME has been conclusively identified [Hudson et al., 1996].In this paper we investigate the detectability of the radio counterpart to white light CMEs via the thermal bremsstrahlung (flee-free) radiation they emit. In particular, we consider detection schemes best suited to an imaging instrument operating at radio wavelengths. While Rodriguez [1996] has considered the idea of detecting CMEs using radar techniques in the 10-80 MHz range, such techniques probe the upper corona and rely upon the detection of the Doppler-shifted signal reflected from the CME. In contrast, we consider direct detection of the radiation emitted by the CME and by associated phenomena at significantly higher radio frequencies.Observations of CMEs at radio wavelengths offer several advantages over those in other wavelength regimes. Because there is no occulting disk, CMEs can, in principle, be imaged against the disk of the Sun in a manner similar to SXR detec- free-free emission, the temperature and emission measure can be derived and compared with complementary soft X ray and white light observations. If the emission is due to gyrosynchrotron emission from nonthermal electrons entrained ...

Science with the Murchison Widefield Array

“…Metre and decimetre frequencies are regarded as the most suitable in which to try to image the synchrotron, gyrosynchrotron, and/or thermal bremsstrahlung emission from the CME plasma (Bastian & Gary 1997). Such measurements offer very useful and unique diagnostic capabilities, including the ability to constrain the CME magnetic field remotely.…”

Estimation of the Three‐Dimensional Space Speed of a Coronal Mass Ejection Using Metric Radio Data

“…One can observe activity at any longitude similar to that at EUV and X-ray wavelengths ( Ramesh 2000). In addition, the frontal structure of a CME has a large optical depth at meter wavelengths and can be readily observed ( Bastian & Gary 1997;Gopalswamy 1999;Ramesh et al 2003). Making use of the above-mentioned advantages, we present an independent method for estimating the three-dimensional space speed of a CME close to its onset using metric radio data.…”

Remote Sensing of the Heliosphere with the Murchison Widefield Array

“…The advantages of radio observations over those from optical and soft X-ray have been appreciated for some time now (Bastian and Gary, 1997) and the metric and decimetric band is widely regarded as the most promising part of the spectrum for CME studies. With only a handful of successful attempts reported in the literature (Sheridan et al, 1978;Bastian et al, 2001;White et al, 2006;Maia et al, 2007;Magdalenić et al, 2008), direct radio detection of CME emission has remained largely beyond the reach of existing instrumentation.…”