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Solar-Heliospheric-Ionospheric Science

The MWA, which represents the next generation radio array instruments, holds great promise for innovative contributions to Solar, Heliospheric and Ionospheric (SHI) science and to space weather applications. The key features of MWA which make it a well adapted for these research areas are:

Most of what we know about the Sun and heliosphere comes from either observations of the solar disk over a large and impressive range of frequencies (from Gamma rays to very low frequency radio waves) using a variety of instruments including imaging devices like coronographs to spectrometers. Once the solar wind moves out of the field of view of the cornographs it remains essentially unobservable till it reaches satellite-based observatories near Earth which directly sample the plasma and provide a wealth of data about solar wind parameters like velocity, density, magnetic field, and composition at that location.  Most of our satellite- based observatories tend to be in Earth orbit around 1 AU, and until the recent launch of STEREO, are unable to sample the vast intervening region from close to solar surface to 1 AU.  This region is crucial to improving our understanding of the solar wind and supporting space weather predictions particularly if crucial information about heliospheric magnetic fields is obtained. The MWA seeks to help fill this large gap in our observations through the application of remote sensing radio techniques.  

Two of the techniques that will be used for the MWA observations include Interplanetary Scintillations (IPS) and Faraday Rotation (FR).  The goal of these techniques is to allow measurements of the various key solar wind parameters, such as density, velocity and magnetic fields in interplanetary space.  Magnetic field observations are crucial to the determination of the geo-effectiveness of coronal mass ejections (CME), 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 therefore important for space weather prediction. A third technique is based on imaging solar radio bursts in order to couple their occurrence, particularly Type II bursts, to the development of CMEs. Finally, since the MWA operates at low frequencies, precise correction for the effects of the ionosphere through the use of innovative calibration techniques is critically important for its successful operation as a radio imaging array.  The results of that correction, both on a relative and absolute level, will therefore be a useful byproduct for ionospheric science at a southern hemisphere site.  Each of these techniques and applications are summarized briefly below.  

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, though which the radiation has traveled.


The geometry for IPS observations is illustrated in the figure below. 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.

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 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 under discussion. 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

The 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.

The figure below 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.

The diagram below 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.


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 coronal mass ejections (CME). 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 below illustrates observations of such bursts made with an interferometer consisting of only three tiles at Mileura during prototyping tests.  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 512T 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.

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.

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 below 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.

When operated at the MWA site, the GPS receivers combined with other receivers in Australia will also form a useful product for the 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.






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