Monica G. Bobra
Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305
On May 9, 2016, Mercury made a 7.5 hour transit across the Sun. Mercury transits across the Sun approximately thirteen times a century; the last time Mercury made such a transit was ten years ago, on 8 November 2006. This time, however, both the NASA Solar Dynamics Observatory’s Helioseismic and Magnetic Imager (HMI) instrument and the ESA Solar and Heliospheric Observatory’s Michelson Doppler Imager (MDI) instrument captured the transit. Here, we show a composite image of Mercury’s transit taken by each instrument.
Figure 1 | An HMI composite continuum intensity image with the 9 May 2016 Mercury transit sampled every 20 minutes is shown above. These continuum intensity data were specially created from HMI level-1 filtergram data such that the cadence of the images capture Mercury’s ingress and egress. For the purpose of a composite image, using these data is a purely aesthetic choice.
The real difficulty in making a composite image1 is to capture the annulus of scattered light around Mercury, which has an intensity comparable to other regions of the Sun. For HMI, this annulus is about a couple of pixels in width. However, the MDI telescope’s front window has physically degraded over time. This and other factors contribute to more scattered light within the instrument. As such, the annulus of scattered light around Mercury in the MDI images is much larger than that in the HMI images.
Figure 2 | A MDI composite continuum intensity image with the 9 May 2016 Mercury transit sampled at an irregular cadence is shown above. We don’t have data during the time of the ingress, but we do see the egress.
Because MDI’s vantage point, at L1, is different from HMI’s, in inclined geosynchronous orbit, the transit path was different for each instrument. The transit times were different as well — the transit ended some two hours earlier from MDI’s perspective, with egress at 16:55 UT, while HMI observed egress at 18:44 UT. The next transit of Mercury will occur on 11 November 2019.
The images that appear here are courtesy of NASA/SDO and the HMI science team.