LIMB BRIGHTENING ON THE SUN AT 21 CM USING A 3-ELEMENT
INTERFEROMETER
Jonathon Oiler
University of Wisconsin
Mentor: Alan E.E. Rogers
MIT Haystack Observatory
ABSTRACT: During the summer, three small radio telescopes (SRTs) were used for observing the Sun during the Sun’s Quiet Phase almost daily over a two month period. We used the three SRTs
as a 3-element interferometer to observe the entire Sun. The baselines used were 7.77 meters, 20.12 meters, and 26.67 meters. Data from the Sun were taken and software was used to calculate the
visibility amplitudes and closure phase. A modeling program was used to replicate the Sun and a comparison of closure phases between the SRT data and the modeling data was performed with a
weighted least-squares fit. We found evidence for limb brightening around the equatorial region of the Sun and determined that the modeling software fit the data better with limb brightening (the
outer 20 percent of the solar disk’s diameter) used in conjunction with a translatable sunspot of varying brightness. The equatorial limb brightening was found to contribute 30 ± 10 percent of the
total solar radio flux.
Three Small Radio Telescopes
Observation and Data Collection
Monthly Average Number of Sunspots
• 2.3 meter diameter
• Used as 3-element interferometer
• Baselines of 7.77 m, 20.12 m, and 26.67 m
• Data sampling process controlled by GPS
• VLBI mode allowed for continuous tracking of the Sun
• Observations performed during the Quiet Sun
• Relatively few sunspots
• Important to minimize enhanced brightening effects
• Viewed at 21 cm
• Observations taken almost daily from June ‘06 to August ‘06
• Approximately 8 hour coverage each day
• Daily images of Sun catalogued and visually compared to data
• Limb brightening caused by increased optical thickness
• Optically thicker where path length is longer (limbs)
SRT 3-element interferometer in small triangle observing the Sun
Modeling the Sun
SOHO satellite image (July 7, 2007)
• Custom software used to calculate closure phase and amplitudes
• Sun modeled as uniform brightness until outer 20% radius reached
• Brightening ramped up until end of disk is reached
• Roaming sunspot added to make data a better fit
• Sunspot can having varying brightness
• Data over all days shows rotation of Sun and matches satellite images
Nobeyama image (July 7, 2007)
Results and Discussion
• Absence of limb brightening is not a good fit
• Circularly symmetric brightening is not a good fit (Smerd, 1950)
• Polar limb brightening is not a good fit
• Equatorial limb brightening is a good fit (Christiansen and Warburton, 1955)
• Results from electron density increase near the equator due to magnetic fields
• Magnetic field lines are open at poles and closed at equator
• Ratio of limb brightening to total brightening: 0.3 ± 0.1
Uniform brightening until outside 20% of disk, where brightening is ramped up
Plots of baseline phases and closure phase
Modeling image of Sun over all days of analysis
Polar brightening fit gets worse as more
brightening is added.
A possible explanation for equatorial brightening: Open magnetic field lines at the pole allow electrons to escape while closed
field lines at the equator trap the electrons causing them to concentrate