There is great interest in developing a means of detecting and predicting the arrival times of Earthward-directed Coronal Mass Ejections (CMEs), the main cause of increasingly costly geomagnetic storms. Until now the best way of studying CMEs has been from space, which is expensive and often unreliable. Furthermore, as Figure 2.6 illustrates, space-based coronagraphs are not well suited for studying Earthward-bound CMEs. A revolutionary ground-based technique of detecting and tracking CMEs with a long-wavelength solar radar is now being considered, and LOFAR would be an ideal imaging receiver for such experiments. LOFAR's pioneering of the technique of Solar radar in conjunction with a suitably equipped transmitting facility could open up an entirely new and exciting field of solar research. There are related LOFAR radar applications in both ionospheric and magnetospheric physics. We describe here only the solar radar application
Figure 2.6 A LASCO coronagraph image of a transversely directed CME, obtained from the SOHO spacecraft located at the Earth-Sun L1 point. Coronagraph images are most effective for determining transverse motions of CMEs. Figure courtesy of NRL, Code 7600.
The El Campo radar (see Figure 2.7), built by the Lincoln Laboratory, detected 38 MHz radar echoes from the Sun for a period of 9 years in the 1960's. Huge, rapidly-moving targets were occasionally observed but this was before the space-borne coronagraph discovery of coronal mass ejections (CMEs), and the physical nature of these “targets” was a mystery. It is now thought that CMEs were being observed.
The reliable detection and monitoring of CMEs is of great practical importance. CMEs that impact the Earth's magnetosphere can result in hundreds of millions of dollars in damage to spacecraft, communications, and electrical power systems. A low-frequency transmitter coupled with a high-angular-resolution receiving array would form an extremely cost-effective system to detect and track CMEs. Rodriguez (1996) has summarized the potential of 10-80 MHz radar systems for detecting CMEs; Figure 2.8 illustrates the basic principle. The Doppler shift introduced by different parts of an outward-moving CME will result in a characteristic frequency- and time-dependent signature in the reflected signal. The rich information inherent in this measurement could open an entirely new window on CME studies, yielding their angular distribution, ranges, and line-of-sight velocities.
Figure 2.7 Two examples of solar radar spectra observed by James in 1964.The data from 3 April are typical of 70 percent of El Campo spectra.The data from 29 April shows two scattering regions;the signal from the upper (more distant)region is likely caused by radar waves refracted to and from regions lying near the edge of the solar disk.At times James also observed echoes from regions lying up to 5 solar radii from the center of the sun (one solar radius equals 700 Mm).The availability today of extensive,high quality supporting observations,in combination with future radar imaging observations with the LOFAR,would allow identification of the source regions and scattering mechanisms of solar radar echoes. After James (1968).
Figure 2.8 Constraining CME velocities from solar radar imaging and Doppler.
Figure 2.8 shows that combining the radial velocity obtained from the Doppler shift with the transverse velocity obtained from imaging is required to determine the CME total velocity vector. This would allow for accurate predictions of CME Earth-arrival times. Transmitting facilities now exist (e.g. Arecibo Observatory and over-the-horizon radars that are no longer required for military purposes), but adequate receiving facilities are needed for such a project.
Currently, space-based coronagraphs are used to detect and track CMEs. However, space-borne white-light coronagraphs detect the Thompson-scattered light from the CME material and therefore are less sensitive to structures propagating at large angles out of the plane of the sky, such as Earth-directed CMEs. An array of coronagraphs in the Earth's orbit could provide the stereoscopic view to determine this information, but at significant cost and a limited, somewhat unpredictable lifetime. A cheaper and more reliable low-frequency radar system (incorporating LOFAR as its prototype imaging receiver) offers a great cost advantage over such space-based CME monitoring schemes.
Aside from the macroscopic physics of direct interest to the space weather program, there is great potential in unraveling the microscopic physics of the solar radar scattering mechanism. An understanding of this mechanism is key to solving the puzzles of the spectral shape, the large Doppler spread, the Doppler shift, and the variation in solar radar cross section. Various mechanisms have been suggested, including turbulence in the local medium, fluctuations in the altitude of the plasma resonance level due to electron density fluctuations in the solar wind, ion acoustic waves, and coherent lower hybrid waves. The possibility of coherent coronal plasma waves is especially intriguing as it is a topic of current interest, and a search for such waves was included in the observing program for the Solar and Heliospheric Observatory (SOHO) spacecraft.
Once the microphysics is understood, solar radar could become a probe of the solar plasma on a par with the modern coherent and incoherent radars used to probe the earth's ionosphere. In particular, the astonishingly accurate plasma physical theory of incoherent scatter radar has allowed a detailed and productive study of our nearest space plasma in a way undreamed of before its discovery. A similar scientific potential may be hidden in the coronal scattering mechanism.
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