Strong Scintillation Coincident with Solar Eclipse Onset

The regular KAIRA experiment includes a beam pointing at the strong radio source Cassiopeia-A to monitor ionospheric scintillation.  Over the period of the solar eclipse on 20th March 2015 no obvious variation in this scintillation pattern has been noted so far, although the scintillation was very active because of the CME which hit a couple of days previously.  However, scintillation of Cygnus-A is different: In this case the line of sight to the radio source passed through the eclipse shadow. 

Although the experiment didn't include a dedicated beam on Cygnus-A (another frustration - the intention was to change Cas-A to Cyg-A, but this was forgotten in the lead-up to observation), it is obvious in a few of the regular riometry beams and, most importantly, it's an obvious source in the all-sky imaging.  The image shows the scintillation in amplitude (top plot) and phase (lower plot - see earlier 'blog posts for information on how this is calculated) found by analysing the intensity and location of Cygnus-A in the all-sky images taken every second.  The start of the visible eclipse from KAIRA was just after 09:00 UT, but the shadow in the F-region will have been larger and started earlier.  This corresponds neatly with the period of very strong amplitude scintillation, suggesting that this is associated with the onset of the eclipse.

Simultaneous phase and intensity scintillation from Cas A

The scintillation due to the ionosphere of Cassiopeia A seen on Christmas Day 2013 from KAIRA, in which the source was observed to shift position and shape, was followed exactly 48 hours later by a period of calm when Cassiopeia A could be seen in the same part of the sky but with virtually no scintillation.  This meant that the quiet day could be used to ascertain the "real" position of Cassiopeia A at the same local sidereal time and the movement of the source relative to this due to the scintillation measured.   The technique described in Monday's 'blog post was used to calculate phase  directly from the imaging, both in terms of the location of the source relative to the image centre and relative to the "real" position measured on the quiet day.

In the plots above, the top two panels show dynamic spectra of the intensities measured on both days using beam-formed observations.  The third plot shows the phase for the location of Cassiopeia A in the all-sky images, relative to the baseline between LBA antenna numbers 45 and 34.  The scintillation is easily seen in the rapid variation of phase seen on the 25th December 2013, with the phase on the 27th being almost constant (the slight downwards trend is due only to the daily movement of the source across the sky).  The lower plot is phase calculated from the relative positions of Cassiopeia A on each day and illustrates the movement of the source around its "real" location.

Phase calculation in all-sky imaging

For an interferometer such as KAIRA, the raw data product for imaging purposes is a set of "visibilities" which are essentially the cross-correlations of the voltages sampled by each antenna-pair in the array.  These encode the amplitudes and phases of the interference fringes of any source in the field of view.  An image is obtained via a Fourier-transform-type relation between visibilities and intensities (source brightness) known as the van Cittert - Zernike relation.

For convenience, the antenna co-ordinates are converted into a set of baselines between each pair in a uvw co-ordinate system perpendicular to the source direction where u is towards the east, v is towards the north and w is towards the source.  In the case of KAIRA all-sky images, the source direction is zenith.  The image transform from the visibilities then creates a "flattened" image (orthographic projection) of the visible sky with co-ordinates l,m corresponding directly to u,v (the w direction is neglected in this case).   The phase for a given point on the image and antenna baseline is simply the dot-product of the l,m co-ordinates of the point on the image (basically the normalised number of pixels from the centre in each direction) and the u,v co-ordinates of the antenna baseline (expressed in wavelengths in this case).

The left-hand image above plots the phases for each antenna pair for the location of Cassiopeia A in an image from 16:00 UTC on 2013-12-27 (the image is Hermitian and so only one half is shown).  These have not been arranged in terms of the array layout on the ground, but do show the alternate positive (yellow-red) and negative (blues) phases of the interference pattern.  The maximum phase occurs for the baseline between antennas 43 and 34, indicated by the end of the white line.  A plot of phase with baseline length for antenna pairs along this baseline is given on the right and shows the phase increasing with baseline length. 

The plot above indicates the location and direction of this baseline on the array and demonstrates that this represents the closest baseline alignment to the direction of Cassiopeia A as seen in the image.