Low Frequency Solar Imaging Using the Murchison Widefield

Array and CASAMeagan Crowley

MIT Haystack Observatory

Mentors: Colin Lonsdale and Leo Benkevitch

REU Symposium

August 11, 2016

Outline● The Murchison Widefield Array

● What We’re Looking for

○ CMEs

○ Faraday Rotation

● Why We’re Looking

○ Predicting Geomagnetic Storms

● Imaging

○ Fourier Transforms

○ CASA

● Results: 11/4/2015

● Conclusions

● Looking Towards the Future

The Murchison Widefield Array● Shire of Murchison, Australia (pop. 114)

● 80-300 MHz (3.75 -1 m wavelength)

● Suited for LF solar imaging

● Stats:

○ 128 tiles of 4x4 antennas

○ 8128 baselines

○ Up to 3 km baseline

○ 0.5 sec temporal resolution

○ 40 kHz spectral resolution

○ Bow tie dipole antenna

Top: Close up of one tile

Source:https://www.skatelescope.org

Bottom: Central Region of MWA (~1.5 km)

Source:http://www.mwatelescope.org

What We’re Looking For: Coronal Mass Ejections

● CMEs

○ High density plasma clouds ejected from the Sun

○ Associated with plasma disturbances

○ Shocks lead to Type II Radio Bursts

○ Also associated with other bursts e.g. Type III

● Type II Solar Bursts

○ Slow frequency drift

● Type III Solar Bursts

○ Fast frequency drift

○ Caused by reconnection events

● This work focuses on specific CME from Nov. 4th 2015

○ Strong Type II emission present

Type II & III Bursts, Frequency v. Time. (White 2007)

What We’re looking for: Faraday Rotation● Radio waves passing through magneto-ionic medium change polarization

● Direction, magnitude of magnetic field

● Better knowledge of magnetic field improves prediction of space weather (1-2 days before impact)

Two illustrations of faraday rotation (Sources: left: http://astronomyonline.org, right: http://www.jb.man.ac.uk)

Why we’re looking: Predicting Geomagnetic Storms● Space weather can have huge effects on Earth

○ Power grids, satellites, communication, GPS, gas pipeline corrosion, interference with space travel

○ Carrington Event

● Two ways of enhancing prediction capabilities

○ Faraday Rotation measurements of galactic background

■ Not addressed in the current work

○ High fidelity solar imaging

■ Looking for faint CME synchrotron emission

■ Requires multi-frequency analysis

Figure showing the spectral variability of LF solar emission as

detected by one MWA baseline. Courtesy of Colin Lonsdale

Imaging: Fourier Transforms● Data is in the form of visibilities

○ Interference pattern

■ Phase, amplitude, spatial scale

○ U-V plane

■ Distribution of baselines

● Fourier Transforms

○ Spatial frequency ----> radio brightness distribution

FT Relationship for Wide-field Radio Astronomy:

1. Example u-v plot of measurement set made using

plotms in CASA

2. Fourier transformed dirty image of same plot

using CLEAN in CASA

Source: Taylor, G. B. et al (1999)

Imaging: CASA● Common Astronomy Software Applications Program developed by National Radio Astronomy

Observatory (NRAO)

● Process:

○ Calibrating Data

■ Instrumental gains

■ Phase and amplitude

○ CLEAN algorithm (Jan Högbom 1974)

■ Deconvolution

● Fills in for unmeasured visibilities

● Reduces sidelobes

● Measures of success:

○ Dynamic Range

○ Fidelity

Calibrated and Cleaned Solar Image, 144 MHz

Made using CASA

Results: Type II Burst on November 4th 2015

Difference images of Sun with

Type II Burst at 144 MHz.

Courtesy of Kamen Kozarev

Composite images of burst with overlay of solar

disk at 144, 178, and 216 MHz.

Courtesy of Patrick McCauley

Integral Radio emission of

burst at 144 and 216 MHz with

overlay of solar disk.

Courtesy of Kamen Kozarev

Conclusions● CASA difficulties

○ Opaque software

○ Imaging:

■ Sidelobes

■ Sampling errors

■ Calibration and cleaning errors

● CenA possibly not best calibrator

■ Maintaining fidelity

● Able to achieve highest dynamic range yet (~2,000)

● Close to 32,000 images made from 1632 GB of data

Looking towards the Future● Need much higher dynamic range (~10,000-100,000)

● Van Vleck Corrections

● High solar activity events

● Spectroscopic imaging

● >2 Petabytes amount of data waiting to be imaged

● Much more work to be done

ThanksColin Lonsdale and Leo Benkevitch, my mentors

Divya Oberoi, John Morgan, Kamen Kozarev, Patrick McCauley and Iver Cairns

Lynn Matthews and Vincent Fish for CASA help

REU program and fellow REUs

Haystack Staff

The MWA and Team

The NSF

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ApJ The Astrophysical Journal 728.2 (2011): n. pag. IOPScience. Web. 8 Aug. 2016.

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University of New Mexico. Web. 8 Aug. 2016.

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