The Earth’s magnetic field shields us from the constant stream of charged particles emitted from the sun known as solar wind. Occasionally, large eruptions of solar material, called Coronal Mass Ejections (CMEs), launch dense and highly-magnetised material at high speeds through the solar wind. This can also strongly affect many of the technologies upon which we increasingly rely, including GPS navigation, satellite communications, and electric power grids.
The CMEs set off a whole chain of processes in the Earth’s magnetosphere and ionosphere which can result in geomagnetic “storming” and spectacular displays of the aurora, but can also affect our technologies. “Space weather” studies use observations, data analyses, and modelling, to understand and ultimately predict the complex state of and interactions between the Sun, solar wind, magnetosphere, and ionosphere, and the what is their impact on our technological systems.
Astronomers at ASTRON use the unique capabilities of LOFAR to study fundamental plasma processes in the solar corona. These processes are the ultimate drivers of space weather, from the expansion of the corona through interplanetary space as the solar wind, to eruptive events such as solar flares and CMEs, which cause particle acceleration, powerful electromagnetic emission, and the ejection of vast quantities of solar material into interplanetary space.
We can obtain vital information about the initial launch speed of CMEs by measuring the rate of change of solar bursts with frequency. When combined with imaging of the bursts across the frequency range, we can predict the trajectory for CMEs.
Interplanetary scintillation – the scintillation of signals from compact radio sources due to small-scale density structure in the solar wind – has been used for studies of the solar wind for over half a century.
Daily observations of the interplanerary scintillation (ISP) with LOFAR are used to image the 3-D structure of both speed and density of solar wind throughout the inner heliosphere. Such measurements represent the only ground-based method by which the 3-D solar wind can be probed and efforts are now underway to incorporate them into space weather forecasting models.
The wide geographical spread of LOFAR makes it possible to have a highly-detailed spatial view of the scintillation pattern “flowing” over the different LOFAR stations. Any sudden or short-lived variations in velocity can be seen easily in this kind of visualisation, as can any differences in flow direction. The spatial extent of individual scintles, related to the scale size of density structures in the solar wind, can also be viewed in an approximate sense from these images. This technique potentially allows any fine-scale changes in velocity or density structures to be investigated in a way that cannot be achieved with other instruments.
Interplanetary Magnetic Field
The sun is a big magnet whose magnetic field is carried through interplanetary space by the solar wind. The strength and direction of this interplanetary magnetic field (IMF) as it encounters the Earth’s magnetosphere has a large impact on space weather and its effect on the Earth. We currently obtain information about the IMF in the vicinity of the Earth via
- satellite measurements from spacecrafts at the L1 Lagrangian point
- measurements of the magnetic field at the solar surface and magnetohydrodynamic (MHD) models that make predictions on how this field is spreading in space.
Remote-sensing observations of these parameters, essential to provide the global view necessary for accurate modelling, is a challenging proposition.
The polarization of radio signals is affected by the presence of magnetic field (an effect known as Faraday rotation). Daily observations of the Faraday rotation (FR) of an incoming polarised radio signal with LOFAR offers a unique tool to obtain 3-D measurement of the global interplanetary magnetic field.
To accurately reconstruct IMF, we need very accurate measurements of FR, which we acquire from LOFAR’s high bandwidth observations of pulsars. LOFAR is currently the only European facility with such capabilities. A major challenge in our work is to decouple the effects of the IMF on the radio signals from those due to propagation through the interstellar medium and the Earth’s ionosphere.
Signals traveling through the Earth’s ionosphere experience scintillations and other distortions because of turbulence. Strong scintillation can disrupt satellite signals, such as those from GPS satellites and other modern communication technologies. Currently, most studies use satellite data (usually single frequency) to investigate ionospheric scintillation. With LOFAR we can expand such measurements across a much wider bandwidth to gain extra information essential to be able to
- fully understand and model the scintillation and the conditions giving rise to it
- inform efforts to predict its occurrence and severity.
LOFAR observations of radio wave scintillation over a wide range of wavelengths also contributes to a wide range of scientific endeavors (link to paper or some other source) to
- identify plasma structures, and their spatial and temporal evolution
- have precise characterisation of ionospehric scattering that can improve calibration of radio astronomy facilities
- model Faraday rotation.