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.

CME Impact as seen from KAIRA

All day yesterday (17th March), the space environment around Earth was being blasted by a CME which launched from the Sun a few days previously.  The solar wind speed measured just upstream from the Earth reached 500-600 km/s and the interplanetary magnetic field stayed southwards with values of -15-20 nT.  All this resulted in spectacular displays of aurora visible across the whole of Fenno-Scandinavia and at least as far south as Wales in the UK.  Here at Kilpisjärvi it was, naturally, cloudy and snowing all night...  However, KAIRA was observing as usual and the beamlet statistics indicate a very nice set of data!

The plot is of data from all beams in the current standard background task experiment (including a number of riometry beams covering the sky, a beam on the strong radio source Cas A and a beam on the Sun or a pulsar depending on whether or not the Sun is up).  The beam on Cas A is the most obvious as it is the one with all the scintillation!  However, of great interest to a number of SGO researchers are the darker vertical stripes, consistent over all the beamlets (which means all the beams in whichever direction they're pointing).  These are likely to correspond to strong riometric absorption events.  Some of them can be seen to be "wobbling" a bit in time, indicating some movement of absorption patches across the sky.  This will be a very interesting dataset indeed!

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.

Scintillation time series from all-sky images

The standard KAIRA experiment, which is always left running if the station is not being used for campaign experiments, includes a beam which continually observes the strong natural radio source Cassiopeia A at several frequencies spread over the KAIRA low-band.  Simultaneously, all-sky images are taken at a single observing frequency with a one second cadence, such as the example below, in which Cassiopeia A is easily identified (it's the strongest source, the other easily identifiable source is Cygnus A).

On Chirstmas Day 2013 a period of very strong ionospheric scintillation (the "twinkling" of radio sources due to density variations in the ionosphere) was observed, for which a movie of all-sky images was created in which the dominant sources can be seen flickering and sometimes changing position and shape (see the KAIRA blog post).  I am now in the process of analysing these data to investigate the phase-shifting of the radio signal from Cassiopeia A caused by the density variations moving through the ionosphere.  As a first stage of this, I've been looking at the intensity of Cassiopeia A as measured from the images and comparing it to the intensity measured by the beam-formed data looking directly at this source.  A first comparison is below.

As can be seen, the two time series' match almost exactly.  Whilst this is hardly surprising, it is a relief to know that the code used to identify Cassiopeia A in the imagery is working as expected.

Broadband Meter-Wavelength Observations of Ionospheric Scintillation

This week, the first KAIRA paper detailing the ionospheric scintillation observations we've been performing, and the "scintillation arc" phenomenon we've noted in the KAIRA blog (http://kaira.sgo.fi/2014/07/ionospheric-scintillation-arcs.html) was accepted for publication in the Journal of Geophysical Research.  The image shown is of a poster presented at the recent European Space Weather Week which summarises this work, along with a few more recent results taken with LOFAR.  The abstract of the paper follows and the full work can already be found online (http://onlinelibrary.wiley.com/enhanced/doi/10.1002/2014JA020406/).

Intensity scintillations of cosmic radio sources are used to study astrophysical plasmas like the ionosphere, the solar wind, and the interstellar medium. Normally these observations are relatively narrow band. With Low Frequency Array (LOFAR) technology at the Kilpisjarvi Atmospheric Imaging Receiver Array (KAIRA) station in northern Finland we have observed scintillations over a 3 octave bandwidth. “Parabolic arcs”, which were discovered in interstellar scintillations of pulsars, can provide precise estimates of the distance and velocity of the scattering plasma. Here we report the first observations of such arcs in the ionosphere and the first broad-band observations of arcs anywhere, raising hopes that study of the phenomenon may similarly improve the analysis of ionospheric scintillations. These observations were made of the strong natural radio source Cygnus-A and covered the entire 30-250 MHz band of KAIRA. Well-defined parabolic arcs were seen early in the observations, before transit, and disappeared after transit although scintillations continued to be obvious during the entire observation. We show that this can be attributed to the structure of Cygnus-A. Initial results from modeling these scintillation arcs are consistent with simultaneous ionospheric soundings taken with other instruments, and indicate that scattering is most likely to be associated more with the topside ionosphere than the F-region peak altitude. Further modeling and possible extension to interferometric observations, using international LOFAR stations, are discussed.