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====== Jupiter DAM (decametric emission) interferometry ====== Starting with LOFAR cycle 0, Philippe Zarka, Olaf Wucknitz and others tried to observe decametric Jupiter bursts interferometrically with LOFAR. Prior to that we investigated possible observing modes using commissioning time, mostly unsuccessful because of technical problems with the correlator. ==== Goal ==== All our knowledge about the DAM emission is from the analysis of dynamic spectra plus very limited information from early VLBI observations. The latter only provided upper limits on the size of emission regions, but no spatial information. The main goal of our LOFAR observations is to localise the sources and measure their motion as function of time and frequency. Clock offsets and drifts and the ionosphere modify the observed phases, and these effects have to be calibrated before we can measure positions. The general approach in interferometry is to use known reference sources to calibrate the ionosphere and clocks, but this does not work in our case because of the very low sensitivity of observations below 30 MHz and the low density of sufficiently strong reference sources. To overcome this problem, we decided to concentrate on times with expected emission from both hemispheres, so that we can calibrate the emission regions against each other. In other words we can then only measure relative offsets and motion between the hemispheres, but that should be possible without external reference sources. This approach is still far from trivial, because the emission from the northern hemisphere is mostly right-handed circularly polarised, while the southern emission is left handed. The ionosphere does not only produce a frequency-dependent delay, which is the same for both parts, but also a polarisation-dependent delay, which is also known as Faraday rotation. For purely circular emission, we still had no way to determine the amount of Faraday rotation (differentially between stations) so that a systematic ionospheric shift between the hemispheres could not be calibrated. However, the polarisation also has a small linear component, so that both hemispheres have at least some level of right and left handed polarisation, which allows us to calibrate not only the overall delay, but also the differential Faraday rotation. If this really works, we can measure positions of emission regions relative to each other. ==== Cycle 0 observations ==== The successful {{:projects:jupiter:wucknitz-zarka-jupiterdam-final.pdf|cycle 0 proposal}} provided us with data for a couple of Dutch and international stations, all recorded centrally at CEP in the so-called fly's eye mode, in which full complex voltage data are recorded. Data were not further divided into channels per subband, because we want to keep the full time resolution and can channelise in a more flexible way later. With the early version of the correlator used, this means that delay compensation for all stations was only applied by shifting the data stream by integer samples. Whenever we channelise further, we have to correct for the remaining phase slope over each subband. Data are correlated offline from the full data streams. Channelisation is either done with a simple FFT or with a polyphase filter with arbitrary weighting function. Here it is important that the original integer-sample delay compensation is exactly consistent with the later phase correction. Because my delay model may be slightly different from the one in the correlator, jumps by integer samples may be slightly offset in time. Because of this potential problem, I keep a list of times when these jumps happen, so that short ranges around them can be excluded from the analysis. ==== Results ==== * [[projects:jupiter:first|First successful interferometry: L202279 2014-01-24 20:00-21:25 UTC]]