Exploring the low-frequency Universe with the MWA.

Space Weather: Solar, Heliospheric and Ionospheric Science

This science working group targets multiple aspects of solar bursts as they travel from the surface of the Sun to the Earth, with applications such as improving early warnings of solar storms to protect satellites, power grids, communications networks and other infrastructure.

Probing the near-Earth environment

The MWA holds great promise for innovative contributions to solar, heliospheric and ionospheric (SHI) science and to space weather applications.

The overarching goal of the MWA for SHI science is to conduct observations from the Sun, through the heliosphere, to the near-Earth environment, thus providing measurements of the Sun and Earth as a coupled system. Using various radio techniques, the MWA seeks to observe and locate radio bursts on the Sun that lead to Coronal Mass Ejections (CME), determine the density, velocity and magnetic fields of the CMEs as well as the background heliosphere, and measure fluctuations in the Earth’s ionosphere during quiet and geomagnetically disturbed conditions.

Interplanetary Scintillation

Interplanetary Scintillation (IPS) is essentially the radio analogue of optical twinkling of stars due to the Earth’s atmosphere. The plane wave-front from a distant radio source picks up phase corrugations as it traverses the density fluctuations in the solar wind as illustrated in the schematic below. These phase corrugations develop into an interference pattern by the time they reach an Earth-based observer. The motion of the solar wind sweeps this interference pattern past the observing telescope giving rise to intensity fluctuations which are referred to as interplanetary scintillations. In the weak scattering regime, the power spectrum of the intensity fluctuations can be modeled in terms of the velocity, the strength of scattering, and a few other physical properties of the solar wind such as density, through which the radiation has traveled.

The MWA’s powerful multi-beaming ability will allow it to observe 16 sources simultaneously in its wide field-of-view. This will increase the density of sampling of the heliosphere by more than an order of magnitude, addressing one of the most constraining bottleneck of existing observations and will allow it to better deal with the time evolution of the solar wind over the 27 day period. Furthermore, the higher sensitivity of the MWA will allow it access to a larger number of sources in the sky.

Close collaboration between the MWA and other groups that gather and analyze IPS measurements from various observatories an analysis centers is anticipated. This includes STELab, UCSD, Ooty, EISCAT, and MEXART. A common format suitable for processing data from all these observatories, including the MWA, is planned. It is recognized that the availability of data from the southern hemisphere at the longitude of the MWA is expected to greatly complement and augment the measurements from the other observatories.

Faraday Rotation

A primary objective of the Faraday Rotation (FR) measurement is to diagnose magnetized plasmas by determining variations in the Rotation Measure (RM) along lines of sight (LOS) from the MWA through the plasmas to distant sources. The RM is proportional to the integral along the LOS of the electron number density and the projection of the vector magnetic field along the LOS.

The MWA heliospheric FR measurements are aimed at
(a) improving our understanding of the evolution of the solar magnetic field from the corona into interplanetary space,
(b) characterizing the magnetic field strength and orientation within the flux ropes of Coronal Mass Ejections (CMEs) before they arrive at Earth, and
(c) characterize coronal turbulence by measuring the power spectrum of RM fluctuations and by monitoring source depolarization.

Solar Burst Imaging

By its design, the MWA will provide excellent imaging capabilities that can be applied to form images of thermal and non-thermal solar emission with the high time and frequency resolution needed to perform diagnostics on plasma motion, shock formation, and particle acceleration. The MWA will thus be used to take snapshots of the Sun at a standard cadence for a long term archive of coronal morphology, and will be triggered at high resolution to follow transient events such as CMEs. Particular transient phenomena of interest will be Type II and Type III radio bursts caused by accelerated electrons associated with shock waves and magnetic reconnection.

Emphasis on the MWA observations will be placed on Type II bursts which have been associated with fast CMEs and shocks. Their imaging and precise location would serve to monitor the evolution of CMEs using the IPS and FR techniques which were noted above, thus providing a compelling a near-complete tracking of these important space weather phenomena.

Ionospheric Structure

The Earth’s ionosphere and plasmasphere introduce challenges to the calibration of the MWA due to its operation at low frequencies (<300 MHz). As a result of the required careful calibration of the array to compensate for refractive errors of the received radio signals due to the plasma, the MWA will be capable of determining ionospheric variations on short temporal (~10 sec) and spatial (~ 1 km) scales. This by-product which yields 'relative' ionospheric variations over the array can then be used to study ionospheric structure.

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