High-latitude observations on SOFIA

We present the results of a study that offers an observing scenario that ca n significantly enhance the scientific return from the Stratospheric Observ atory for Infrared Astronomy (SOFIA). By examining atmospheric data from a satellite mission, we found that at typical SOFIA flight altitudes from 37, 000 to 45,000 feet, it can be advantageous to fly into high-latitude region s-for example, north of +40 degrees. This is because of the fact that the w ater vapor overburden (WVO) and the frequency of cloud occurrence at these latitudes are less than if the flights are centered above Moffett Field, Ca lifornia, where the SOFIA base will be located. It has also been shown that for certain science projects, the amount of time on target can be consider ably extended with high-latitude flights. Compared to flights on the Kuiper Airborne Observatory (KAO), SOFIA's predecessor, which typically took 6.5- 8 hours, an increase of up to 50% of target time is possible without exceed ing the often required very low line-of-sight (LOS) WVO of approximate to 1 0 mum. When flying into northern latitudes where the tropopause, and hence the major water vapor distribution, is lower than over the tropics, it beco mes possible to observe at correspondingly lower altitudes, for example, 37 ,000 or 39,000 feet. This permits longer but fewer flights, which include l egs at lower altitudes, and makes observatory operations easier. Therefore, we have studied flights from Moffett Field, California, to Canada, as well as week-long flight series out of Anchorage, Alaska. In the winter months, the air over Canada and Alaska is very dry at SOFIA altitudes. Those fligh ts demonstrate that the LOS WVO is almost always below 10 mum. Additionally , the study shows that the LOS WVO for a fixed set of Galactic plane object s is about the same for north-shifted SOFIA flights-elevation range 20 degr ees -60 degrees -as for the Moffett-centered KAO flights-elevation range 35 degrees -75 degrees.