Observations of Solar-Wind-Magnetosphere Coupling at the Earth's Magnetopause [and Discussion]

C. J. Farrugia, S. Schwartz, M.J. Rycroft, D. A. Bryant
1989 Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences  
Significant observations have been made with the Active Magnetospheric Particle Tracer Explorers data-set on the signatures of reconnection, thought to be the major process responsible for the coupling of the solar wind to the terrestrial magnetosphere. We review results reached by some of these studies. Recent theoretical ideas on reconnection at the terrestrial magnetopause, both time-independent and timedependent, are also briefly discussed. Two data examples from the International Sun-Earth
more » ... rnational Sun-Earth Explorer mission are revisited and interpreted in the light of these newer developments. C.J. FA R R U G IA O bservational signatures General When studying magnetic field data related to magnetopause crossings, it is usual to present the magnetic field vectors in boundary normal coordinates introduced by Russell & Elphic in 1978 (figure 1). One determines first the vector N , the normal to the notional m agnetopause (there are several methods). L is defined such that the iVX-plane shown in figure 1 contains the geocentric solar magnetospheric (gsm) z-axis and L points northw ards along the m agnetopause, while M completes the right-handed triad and points westwards. Let us imagine an outbound crossing of the dayside m agnetopause when the m agnetosheath and magnetosphere fields have antiparallel BL components are shown here, Bh and BN. As we move away from the E arth, we encounter bipolar variations in the normal field component, shown here as a positive followed by a negative excursion. Such signatures are called flux transfer events (ftes). By definition, ftes are magnetic signatures and they were first discovered by Russell & Elphic (1978) . The BN variation is seen accom panied by changes in the BL component, ftes are observed up to ca. 1 away from the magnetopause. This latter is the current layer separating the sheath from the magnetosphere and we can identify it in figure 2 as the region in which BL reverses sign from a consistently positive to a consistently negative orientation. As the m agnetopause is crossed, jetting plasma is seen; this is w hat we shall hereafter call the high-speed flow region (hsfr). O n the sheath side of the magnetopause, rapid, short-duration excursions of the Bh com ponent are seen. They are associated with clear BN activity which is, however, not manifestly FTE-like. These we call ' BLspikes'. Once inside the sheath, further ftes are seen, of the same polarity as those in the magnetosphere. This hypothetical m agnetopause crossing therefore has three signatures, hsfrs, ftes and Bl spikes, which we also see in real spacecraft data. We shall now com m ent on each of these in turn. 2.2. High-speed flow regions
doi:10.1098/rsta.1989.0024 fatcat:grcfea65q5a5hhze34otuehieq