First High Time Resolution FPI Observations of the Daytime Thermosphere During the Eclipse Over Svalbard on 20th March 2015
Abstract:Daylight observations of the upper atmosphere have long been a goal of the ground-based optical community. Fabry-Perot Interferometer observations of the airglow emission of atomic oxygen at 630.0 nm are used as a measure of thermospheric winds and temperatures at an altitude of around 240 km. However, airglow is only about 10 times the intensity of starlight. Adding extra etalons (up to a triple etalon FPI) to filter out sunlight has been attempted by a few groups, including ours, over the decades. However, the alignment of multiple etalons is extremely tricky, and long exposures (several minutes) are required, which reduces the capacity to observe the dynamic behaviour of the upper thermosphere.
In this paper we show FPI observations made during the solar eclipse on the 20th March 2015. A total eclipse occurred over Svalbard for 2 minutes 27 seconds from 10:10 – 10:13 UT. This is within the time window when Svalbard passes under the magnetic cusp. There are 24 hours of darkness at Svalbard during the period November to January, which allows continuous FPI observations, including cusp measurements. However, the standard nighttime observing period is 18:55-10:16 UT for March equinox. During this tiny window of time we measured the vertical winds at very high time resolution using a 15 second exposure with our narrow angle FPI; and were able to make a single exposure for 104 seconds with our Scanning Doppler Imager (SCANDI). The SCANDI provided an all-sky observation, divided into 61 sectors, of horizontal winds and temperatures as a context for the high time resolution vertical winds. The observations are compared with FPI-SCANDI December cusp measurements and the UCL Coupled Middle Atmosphere Thermosphere (CMAT2) global circulation model simulations for December 2014 and March 2015. This is an opportunity to test the model daylight winds with direct observations.