New Science from Archived Data: Hawkeye Observations of High-Latitude Reconnection

Volume 11, Number 4, December 1995
By Ramona Kessel

Although magnetic reconnection in the high-latitude region of the Earth's magnetosphere was predicted for northward interplanetary magnetic field (IMF) conditions many years ago and its picture was developed with the help of simulations, it was not identified in spacecraft observations until recently. (This is in contrast to years of observations and studies of low-latitude magnetic merging associated with southward IMF.) Nearly 35 years ago Jim Dungey predicted that during periods when the interplanetary magnetic field (IMF) enveloping the Earth's magnetosphere had a northward component, merging of magnetic field lines and associated energy release should take place poleward of the cusps at the high-latitude magnetopause. (See Figure 1 below.) At the high-latitude magnetopause the Earth's locally southward directed magnetic field "reconnects" to the IMF's locally northward directed magnetic field. At the reconnection site there is a magnetically neutral point or region (shown in Figure 1 below as a gap), and the magnetic configuration is unstable. Plasma is accelerated by an induced electric field tangential to the magnetopause, and the energy associated with the magnetic configuration is converted into particle energy. The arrows in Figure 1 indicate the flow directions.

Figure 1

Only a few satellites had orbits such that they could observe the high-latitude magnetopause: the High Energy Orbiting Satellite (HEOS) 2, Prognoz-7, and Hawkeye; and each is about 20 years old. Only Hawkeye data were preserved at full time resolution and are available via a publicly accessible archive. Because of the easy access provided by NSSDC's near-line archive and the state of the art tools that have been developed in the past few years, Hawkeye data look like those from a "new" satellite. This has enabled new science discoveries such as high-latitude reconnection reported at recent science meetings by this author and her colleagues.

The Hawkeye satellite (Explorer 52) collected data from June 3, 1974, until April 28, 1978, supplying nearly four years of continuous coverage. Hawkeye flew in a polar orbit with an inclination to the Earth's equator of nearly 90 deg, an apogee of 20-21 Earth Radii, and an orbital period of 51.3 hours. (See Figure 1 above.) Three instruments were flown on Hawkeye by the University of Iowa to measure magnetic fields, plasmas, and low frequency electrostatic and electromagnetic waves. For further details and Hawkeye data plots, see NSSDC's Hawkeye page at URL

Hawkeye observations have provided evidence of reconnection at the relatively unexplored high-latitude magnetopause. Recent simulations have provided corroborating evidence for the reconnection studies from Hawkeye at least qualitatively, in terms of flow directions expected for high-latitude reconnection during northward IMF. Because the Hawkeye data were preserved, an important discovery was possible, and the way is paved for in-depth statistical studies with the entire four-year data set. These data represent one of this country's national treasures and are not easily replaceable. Even though its data are nearly two decades old, Hawkeye clearly remains a key data source for high-latitude magnetosphere observations. Old data and new technologies are combining to facilitate the progress of science!

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Author:Miranda Beall
Curators: Erin Gardner and Miranda Beall
Responsible Official: Dr. Joseph H. King, Code 633
Last Revised: 21 Nov 1996 [EDG]