From cjl@haystack.mit.edu Mon Mar 24 19:40:00 2003 Date: Mon, 10 Mar 2003 11:02:42 -0500 (EST) From: Colin Lonsdale To: Steve Rawlings Cc: SKA ISAC , blitz@gmc.berkeley.edu, ccarilli@cv3.cv.nrao.edu, Carole Jackson , cordes@astrosun.astro.cornell.edu, dbacker@astron.berkeley.edu, dj@sgra.jpl.nasa.gov, dole@haystack.mit.edu, dwilner@cfa.harvard.edu, edvige@arcetri.astro.it, ems@physics.usyd.edu.au, fbriggs@astro.rug.nl, fowen@cv3.cv.nrao.edu, Mike Garrett , gfl@aaoepp.aao.gov.au, hfalcke@mpifr-bonn.mpg.de, john@astro.umn.edu, Michael Kramer , lazio@nrl.navy.mil, lferetti@ira.bo.cnr.it, p181bck@mpifr-bonn.mpg.de, russ@ras.ucalgary.ca, Sean.Dougherty@hia.nrc.ca, vdhulst@astro.rug.nl, Richard Schilizzi , schilizzi@skatelescope.org Subject: Re: last stand of the matrix On Sun, 9 Mar 2003, Steve Rawlings wrote: > > PS: no input on CMEs or solar-system bodies, so I've winged this. > I'd be grateful of checks on these areas too! > Steve, Here is a brief summary of what I know on CME studies, and more generally, probes of the IPM with both passive and active techniques relevant to SKA. Radar studies of CMEs come into their own at very low frequencies (below, say, 20 MHz). The radar cross section drops quickly as the CME moves out due to expansion and rarefaction, and a precipitous drop in the plasma frequency, so that direct radar detection/imaging of CMEs is relevant only for CME's within a few solar radii of the sun. For this reason, I believe, the SKA is much better suited for passive studies, for which there are multiple promising techniques. (A possible caveat on the plasma frequency issue relates to the "snowplow" effect mentioned below under direct imaging, but I'm still not enthused by CME radar for SKA). Passive techniques include, but are probably not limited to: - Direct imaging of CME-related synchrotron radiation - Interplanetary scintillation measurements - Scatter broadening measurements - Faraday rotation measurements Direct imaging of synchrotron radiation has been achieved at 164 MHz by the Nancay group, for one exceptionally radio-loud CME. The emissivity can be expected to drop rapidly with distance from the sun, and with increasing frequency, but the sensitivity of SKA surely would enable more detections. It is unlikely that long baselines would be of use. Resolutions of an arcminute or so would be ideal. These things move quickly, so good instantaneous imaging would be important. Obviously, frequencies below 1 GHz, extending as low as possible, are the most important. As the disturbance evolves, there is a "snowplow" effect, and it's possible that the leading edge bowshock could produce detectable emission much further out. Again, low frequencies and low resolutions are most relevant, one assumes. It seems to me that all designs can do this well, with those that operate with higher sensitivity at lower frequencies preferred. Instantaneous uv coverage at moderate angular resolution is potentially an issue for large-D designs. IPS measurements are potentially very powerful, and are currently in use to attempt 3-D reconstructions of the solar wind, under a variety of simplifying assumptions. Their efficacy depends on the number of sources that can be measured in a given span of time, their distribution on the sky, and the availability of measurements from multiple locations on the ground simultaneously. IPS signals are fluctuations on timescales measured in milliseconds, so we are talking about phased-array voltage sum time series measurements. The benefit of multiple (at least 4, preferably more) measurement locations on the earth, separated by a few hundred km, is that one can determine the velocity vector of the IPS "shadow" as it passes across the earth's surface by cross-correlating the IPS fluctuation signal from each location. The range of solar radii for which IPS is both possible and accurate is strongly frequency dependent. Therefore, ideally one would like to observe a wide range of frequencies, say 200 MHz to 2 GHz, to get a wide range of radii. A SKA design capable of observing in many directions at once has an advantage, as does one that is capable of very rapid repointing. Covering the sky very quickly is more important than getting huge SNR in this instance - the SKA has lots of sensitivity. the European tile design is ideal, while designs that lend themselves to flexible subarraying come in second. LOFAR will do superbly, especially compared to current facilities, but the upper frequency limit restricts how close to the sun we can go. Higher frequencies probed by SKA would be a good complement. Scatter broadening is a strong function of wavelength (lambda squared), and is a good probe of regions close to the sun at SKA wavelengths. Long baselines will help here. SKA sensitivity will give us lots of sources to measure. This is an imaging technique, and effective imaging FOV (including multibeaming) is an important figure of merit. The imaging must be instantaneous - one wants a time-series of scattering size measurements for each source in the FOV, and one wants to cover the entire circumsolar region in as short a time as possible. Again, I think sensitivity will not be the limitation, and instead pointing agility (both electronic and physical) is key. The tile design has a clear advantage. Both for this and IPS, the Ozcyl design is attractive because of the partial electronic pointing. Flexible subarraying is valuable. The Chinese design lacks long baselines which will reduce the range of solar radii that can be investigated at a given frequency. Faraday rotation measurements hold the potential of measuring the magnetic field in the solar wind plasma. This is a worthy goal because the geoeffectiveness of solar wind disturbances strongly depends on the field configuration. This seems like a very attainable goal for SKA, with requirements similar to those for IPS and scatter broadening, except that long baselines are probably not required - we will be looking for strongly polarized background sources, not self-absorbed VLBI-scale cores. In summary, for these types of passive study, I rate the Euro tile design as ideal, followed by the Ozcyl. Most other small-D designs are a toss-up, and all will probably do very well. There are questions about the pointing agility and subarraying potential of both the large-D designs, and the Chinese design has a small strike against it due to the lack of long baselines for scatter broadening measurements.