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 techniquesthat will be outlined, 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. The MWA goals and approaches for SHI science are described by Salah et al. (2005) and by Bowman et al (2013).
Briefly, the Interplanetary Scintillation (IPS) technique will be used to measure solar wind density and velocity, and the Faraday rotation technique will be used to estimate heliospheric magnetic fields that are crucial to the determination of the CME geo-effectiveness, i.e. whether a particular CME will couple with the Earth's magnetic field when it impinges on the magnetosphere thereby resulting in space weather effects. Knowledge of the magnetic field evolution at the earliest time in the CME trajectory is important for space weather prediction. Another technique will rely on high-fidelity imaging of solar radio bursts in order to couple their occurrence, particularly Type II bursts, to the development of CMEs. The final technique which will contribute to studies of ionospheric structure will be based on the required calibration of the MWA since observations by the low frequency array must be precisely corrected for the effects of the Earth's ionosphere in order for the MWA to operate successfully as a radio imaging array. The results of that correction, both on a relative and absolute level, will therefore be a useful by-product for ionospheric science at a southern hemisphere site. Each of these techniques and applications is summarized briefly in the following sections.
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 as illustrated in Figure 1. 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 geometry for IPS observations is illustrated in Figure 2. IPS observables are line-of-sight (LOS) integrals and hence they are sensitive to the properties of the solar wind all along the LOS. The propagation time from the Sun to a point on the LOS can vary by up to a few days, and the LOS projected back on the Sun spans around 100 degrees in length. An IPS observation is thus sensitive to the solar wind arising over an extended window in time and from an extended region on the solar surface.
In a Sun-centered frame, the Earth rotates around the Sun every 27 days. The rotation of the Sun and the outward motion of the solar wind allow an Earth-based telescope to obtain many different perspective views of the long-lived stable features in the solar wind. A large number of lines-of-sight to observed IPS sources, filling the entire inner heliosphere, can therefore be obtained and used for tomographic reconstruction of the distribution of the solar wind in the inner heliosphere as has been successfully done at various radio observatories. An example from the UCSD analysis of data from the STELab IPS array is shown in Figure 3 for the storm of July 13, 2000, illustrating the density structures emanating from the Sun towards orbiting Earth (Jackson and Hick, 2005).
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 (FR)
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 characterize coronal turbulence by measuring the power spectrum of RM fluctuations and by monitoring source depolarization.
Figure 1 illustrates a representation of the quiescent solar wind. The plasma density is encoded in the colors, while the magnetic field direction is shown by the arrows. The line-of-sight (in red) to a distant radio source passes through this medium, and experiences Faraday rotation. The amount of rotation is large at the MWA frequencies, and the measurements can yield constraints on the magnetoionic properties of the plasma. By observing many independent lines-of-sight, a 3-D representation can be built up.
To observe Faraday rotation, the MWA must find and use a sufficient surface density of polarized background sources. A recent survey at Westerbork of polarization at 340-370 MHz found 13 extra-galactic sources in an area of less than 35 square degrees, exhibiting typical polarized intensities of ~20 mJy and readily measurable RM. (Haverkorn et al., 2003). In the 200-300 MHz range of the MWA demonstrator, with a field of view in the 200-400 square degree range, we can expect over 100 sources in 5-minute integration with a single pointing. A survey of polarized sources will be conducted with the MWA as a first step in the process.
Figure 2 illustrates the region (yellow) where Faraday rotation observations with the MWA are expected to yield useful constraints on the heliospheric plasma properties. The MWA observing frequency range from 80 to 300 MHz is shown on the abscissa, and the ordinate shows Rotation Measure (left) and the heliospheric distance at which the quiescent heliosphere would generate that amount of rotation (right). The different lines correspond to various constraints. The sloping lower lines are set by instrumental sensitivity, and the sloping upper lines by scattering and loss of phase coherence across the frequency channels. The horizontal red lines indicate levels of ionospheric Faraday rotation, which must be calibrated. For absolute calibration to remove the ionospheric effects, the MWA will utilize GPS observations which have been shown to be possible to a level of a few percent.
A simulation of the Faraday rotation from the flux rope that would have been observed by the MWA at 150 MHz during event of 28 October 2003 is shown in Figure 3 (J. Kasper), roughly six hours after the eruption of the CME. A substantial Faraday rotation signal is expected to be observable with the MWA whose measurement accuracy is estimated at 4 degrees for a 20 mJy source with an integration time of 5 minutes. Once the Faraday rotation is measured, an inversion or model fitting is then required to determine the magnetic field in the CME, utilizing knowledge of the electron density structure from other observations (e.g. STEREO) or from IPS as a first order estimate. Several groups have developed methods for such calculations (e.g. Liu et al, 2006). The observational and analysis techniques will be demonstrated with the MWA as a high priority as soon as the full array is deployed, and the MWA is then expected to contribute to space weather predictions in the long term once the operational capabilities are established and verified.
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.
It is expected that the 32-tile system which will be the first phase of MWA construction, will be capable of important measurements of Type III bursts with fine frequency (~10 kHz) and time (~50 msec) resolution. The figure (D. Oberoi) illustrates observations of such bursts made with an interferometer centered at 100 MHz consisting of only three tiles during prototype tests at Mileura in Western Australia in 2005. Burst emission with fine frequency and time resolution is observed in some of the 1-second snapshot images.
The MWA time resolution will be decreased when the full 512 tile system is deployed due to the large data volume that must be handled. A cadence of a few seconds is then expected but will be sufficient to characterize the relatively slower drift rate of Type II bursts with frequency (~-0.2 MHz/s) and lasting many minutes within the observing frequency range of the MWA. The need to localize the bursts with as fine an angular resolution as possible (~2 arcmin) has driven the deployment of 16 tiles outside the core region extending the array baselines to 3 km.
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.
In addition to the observations of relative ionospheric structure from the MWA calibration system, there is a need for 'absolute' determination of ionospheric and plasmaspheric electron content to correct for Faraday rotation of radio sources as part of the heliospheric study outlined above. The ionosphere and plasmasphere introduce a rotation of the same magnitude as expected from the heliosphere, and therefore must be accurately compensated. For this absolute measurement of ionospheric-plasmaspheric electron content and Faraday rotation, it is planned to utilize GPS observations, aided by empirical models where necessary.
Three dual-frequency GPS receivers (GSV4004B with Novatel GPS702 antennas), provided by AFRL/AFOSR, are planned for deployment at the MWA site. In December 2006, the GPS systems were tested at Haystack Observatory and were operated simultaneously with the Millstone Hill Incoherent Scatter Radar (ISR) to validate their performance. The figure (J. Salah) shows the variation during the period 13-15 December 2006 of total electron content (TEC) in TECunits (1 TECU=1016 electrons/m2) with time, shown in hours from start of experiment at 12 UT on 13 December. The GPS measurements were obtained with one of the MWA GPS receivers using signals from various GPS satellites viewed from Haystack at high elevation. The red curve is the result of integrating high-resolution (3 km) profiles of electron density from the ISR over the altitude range 100-1000 km.
Good agreement is found between the GPS and ISR, with evidence of plasmaspheric contribution to TEC (1000-22000 km) of about 3 TECU during the daytime on 13 December (~18 UT). A major magnetic storm occurred on 14 December resulting in an enhancement of TEC by a factor of 2, depletion of the plasmasphere, and occurrence of large oscillations (2-3 TECU) during the night after the storm (48-56 UT). In the recovery phase from the storm on the following day (15 December), the TEC remained low with only 5 TECU recorded by both the ISR and GPS.
Presently, the three GPS receivers have been deployed at the MWA site in Boolardy, Western Australia, and TEC observations commenced in August 2008. Two of the receivers are deployed at a baseline distance of ~35 km with the third to be installed at an equivalent distance perpendicular to the first baseline forming a triangle around the MWA. The MWA GPS receivers will also be combined with other receivers in Western Australia which will form an important product for the overall study of ionospheric structure in the southern hemisphere. The MWA location in Western Australia represents a mirror conjugate point to the American sector in the northern hemisphere and is at roughly the same geomagnetic latitude as the Caribbean area which has been found to be a source of large plasma enhancements during solar storms resulting in the propagation of plumes of ionization that cause serious space weather effects on navigation signals. Whether similar behavior is found in the southern hemisphere will be determined from the planned observations.
Haverkorn, M., Katgert P., & de Bruyn, A.G., "Characteristics of the structure in the Galactic polarized radio background at 350 MHz", Astron. Astrophys., 403, 1031, 2003.
Jackson, B. V. and P. P. Hick, "Three-Dimensional Tomography of Interplanetary Disturbances", in: D.E. Gary and C.U. Keller (eds.), "Solar and Space Weather Radiophysics, Current Status and Future Developments", Chapter 17, Astrophysics and Space Science Library 314, 355-386, 2005.
Liu, Y., W. B. Manchester, J. C. Kasper, J. D. Richardson and J. W. Belcher, "Determining the magnetic field orientation of coronal mass ejections from Faraday rotation", Astrophys. J., 665:1439-1447, 2007.