ALMA Memos 332 and 361 reported test comparisons between phase fluctuations predicted by line-of-sight PWV measurements from 183-GHz water-vapour radiometers at Chajnantor and the phase signal measured by the NRAO site-test interferometer observing an 11.2-GHz. geostationary satellite beacon. Though data from the two methods showed good correlation for long periods, an unexpected strong high-frequency fluctuation was found to occur in the interferometer phase, sometimes persisting for several hours. No counterpart for this phenomenon was found in the radiometer data, but a similar effect was seen on the ESO site-test interferometer located on the same baseline but observing a different 11.2-GHz satellite. If the effect proved to be directly proportional to frequency it would lead to complete decorrelation of the astronomical signal at wavelengths of order one millimetre with no prospect of correction using the radiometer measurements. This memo reports investigations of the phenomenon to establish its likely physical origin and hence how it would scale to higher frequencies. We show that the phase variations are accompanied by amplitude variations, indicating some kind of scintillation effect. Moreover the inferred velocity of the underlying disturbances (or waves) is so high as to rule out their being any kind of small-scale tropospheric irregularities not visible to the radiometers; rather, it is consistent with typical propagation speeds of ionospheric disturbances. The occurrence and time of onset in the interferometer data appear to exhibit some agreement with Range-Time-Intensity (RTI) plots from the JULIA back-scatter radar experiments from the Jicamarca Radio Observatory in Peru. We present some statistics showing the incidence of the effect in the interferometer data over a 5-year period, revealing seasonal and diurnal patterns which suggest that the predominant cause is the post-sunset ionospheric disturbances well-known to occur at low magnetic latitudes and collectively termed Equatorial Spread F.
View a pdf version of ALMA Memo #459.
Last modified: 2003-04-22