Remote Observations of the Lunar Sodium Exosphere

Abstract:

We have designed, built and installed a small robotic coronagraph at the Winer Observatory in Sonoita, Arizona, in order to observe the sodium exosphere out to one-half degree around the Moon. We have observed every available clear night from our home base at Goddard Space Flight Center for several lunations in 2015, and thus have a long baseline of sodium exospheric calibrated images. We employ an Andover temperature-controlled 1.5 Å wide narrow-band filter centered on the sodium D2 line, and a similar 1.5 Å filter centered blueward of the D2 line by 5 Å. This filter would yield a sodium signal at least 24% of the scattered lunar light at first quarter. Exposures of 10 minutes are required to image the sodium corona at good signal to noise. Following each exposure pair, taking a 0.1 sec exposure with the open filter collects on- and off-band images of the lunar surface. An example of our resulting image of the sodium corona is shown in Figure 1, with the image of the moon's disk (taken subsequently to the occulted coronal image) superimposed on the occulting disk, thus showing the position and phase of the moon under the disk. We will compare our lunar model derived from these observations with the data from the UV spectrograph onboard the LADEE spacecraft.

Figure 1. An image of the lunar sodium corona obtained on March 26, 2015 is shown with the lunar surface superimposed on the coronagraphic image. Using various sized occulting disks, depending on lunar phase, we observe the corona very close to the lunar surface with no scattered light.