Ionospheric observations from formation flying spacecraft

Abstract

The equatorial ionosphere exhibits remarkable variability on a variety of spatial and temporal scales, and therefore observations of this region remain critical. Remote sensing from the ground offers an opportunity to get high resolution observations, but only at a few locations. Remote sensing from space offers the possibility of characterizing the region on a global-scale but can miss some of the smaller-scale features. In situ observations from space capture the smallest observable scales, but only at the location of the spacecraft. Here we utilize a time period in which ICON and FORMOSAT-7/COSMIC-2 (F7/C2) FM4 were flying in formation - maintaining approximately the same relative position of less than a few hundred km of separation for approximately 2 weeks. Using in situ observations, small-scale feature can be seen by both spacecraft, which allows for any spatial and temporal ambiguity to be resolved and spatial gradients to be calculated. Results for observations equatorial plasma bubbles yield results similar to those in other studies, with the depth of depletion reducing with apex height. A small-scale uplift of the ionosphere and corresponding ion density enhancement that appear after the pre-reversal enhancement are seen. This small-scale feature spans only 8 min of local time. While the observations available cannot definitely determine the origin of this feature, it could be associated with a polarization field that precedes the formation of an equatorial plasma bubble. Quasi-periodic oscillations in the ion density and drifts are seen on more than one day, with spatial scales along-track of ∼ 500–1000 km. Using data from both spacecraft, it is shown that these are consistent with travelling ionospheric disturbances, in one case with a horizontal wavelength of 570 km and period of 24 min. The demonstration of such a variety of features that can be examined in this way during just a 2-week period of serendipitous conjunctions between these two spacecraft highlights the potential for future missions that are planned to maintain such a configuration over a more prolonged period of time.