Global Profiles of ULF Wave Power in the Quiet-time Magnetosphere

Abstract

Since their discovery in the mid-nineteenth century, ultralow frequency (ULF) waves, often associated with magnetic storms, have been the subject of numerous investigations whose results have provided a significant contribution to our understanding of space weather. The recent launches of multi-spacecraft missions such as Cluster and THEMIS have produced new data that can be applied to the study of ULF waves in Earth’s magnetosphere. The large amount of data from these missions offers an unprecedented opportunity for a large-scale investigation of magnetospheric ULF wave activity. During the most recent minimum in the sunspot cycle (2006–08), the average annual number of geomagnetic storms was the fewest for nearly the past 40 years. This period was marked by extended intervals during which the magnetosphere was in a quiet, steady state. Such unusual conditions, along with the abundance of spacecraft data, have provided a rare opportunity to study wave power in the quiet magnetosphere—a topic that has been investigated far less than wave power in the active magnetosphere. This is the focus of our current work. Our primary objective in this investigation is to develop statistical “maps” of ULF wave power throughout the quiet magnetosphere using available magnetic field data from the five THEMIS satellites. These global maps serve not only as useful references for work by other researchers, but also contribute to our understanding of the connection between the solar wind and space weather. Here, we present our computed global power maps followed by a discussion of the dependence of ULF wave power on the solar wind during quiet magnetospheric conditions.