LOFAR Interplanetary Scintillation g-level Calculations Using Multiple Simultaneous Observations

Abstract:

Interplanetary scintillation (IPS) is the variation in intensity of a signal originating from a distant, compact, radio source, related to variations in the density of interplanetary plasma flowing out through interplanetary space from the Sun. The strength of the scintillation, as measured using a "scintillation index", m, is a quantity related to the solar wind density and so a useful measure for space weather purposes. However, the values of this m-index are also dependent on the radio source used to observe IPS, instrumental effects (such as sensitivity/ collecting area, bandwidth, and observing frequency). In order to determine changes in the solar wind density, and to compare those using separate instruments, we calculate a "g-level", which is a normalisation of the m-index for radio source differences and distance from the Sun, thus allowing density structure to be inferred from a tomographic inversion of the results of many different observations taken as the Sun rotates. However, instrumental differences remain, leading to potential difficulties when attempting to amalgamate values calculated by different telescopes, and limiting their use in heliospheric models. The LOw Frequency ARray (LOFAR) observes with a wide bandwidth in the frequency range 10-250MHz and consists of a dense core of 24 stations within a ~4km diameter and 14 stations spread further afield across the northeast area of The Netherlands, and a further 14 stations spread internationally (six across Germany, three in Poland, and one each in France, Ireland, Latvia, Sweden, and the UK). Further international expansion is under way. In this work, we use LOFAR space-weather campaign observations of IPS from October 2016 in order to understand how m-index and g-level are affected by observing with different LOFAR stations, with repeat observations of radio sources over multiple days. This data set, in all, includes observations of around 20 radio sources per day and 13 stations/components of LOFAR often simultaneously observing the same radio source. Although each LOFAR station was built using the same hardware design, the instrumental noise from each station is not the same, making LOFAR ideal for a study of this nature. This is the first time in which LOFAR m-indices and g-levels have been made available and explored.