8.3 Magnetic clouds and coronal mass ejections

An important departure from the steady or quasi-static picture presented above occurs as coronal mass ejections (CMEs) drag closed magnetic loops with them into the solar wind. In situ observations show occasional features called magnetic clouds (MCs, Burlaga et al., 1981 ) which appear to be the interplanetary manifestation of CMEs. Magnetic clouds are regions $-~ 3 -6× 107 km$ in diameter (at $1 AU$ ) in which the density and temperature are significantly lower, and magnetic field strength is significantly greater, than in the ambient solar wind. These properties suggest that the MC is a distinct magnetically dominated structure ( $b < 0.1$ , Bothmer and Schwenn, 1998 ), which has expanded on its journey from the Sun. The arrival at $1 AU$ of a MC can often be associated with a previous prominence disappearance or CME at the solar surface (Bothmer and Schwenn, 1994 ). In a few cases, the trailing portion of a MC has been observed to contain a density enhancement whose properties could be associated with filament material.

Magnetic clouds are often marked by the presence of high-energy electrons flowing in both directions along the magnetic field. Such instances of bidirectional heat flux indicate that the magnetic field lines remain anchored at both ends (Gosling et al., 1987 ; Larson et al., 1997 ). The MC is therefore believed to be a closed magnetic flux rope entrained in the solar wind. Since the wind is supersonic (and super-Alfvénic) at $1 AU$ , magnetic tension will not be able to retract the flux and it will ultimately be dragged ever outward. The continual occurrence of CMEs will thereby tend to increase the Sun’s open magnetic flux (Gosling, 1975 ). Observational evidence indicates, however, that the net open flux does not monotonically increase, but rather varies up and down by a small amount in approximate phase with the photosphere’s magnetic cycle (King, 1979 ; Wang et al., 2000 ). This indicates either that CMEs do not actually open any flux, or that the opening is opposed by closing down previously open field lines, perhaps at different places and times.

The topology of solar wind field lines is inferred from the direction in which the high-energy (halo) electron population is conducting heat. Uni-directional heat flux indicates connection at one end (open) and bi-directional heat flux indicates connection at both ends (closed). The natural signature of a field line open at both ends (a U-loop) would be an absence of heat flux electrons, known as a heat flux drop-out. McComas et al. (1989 ) found 25 instances of heat flux dropout in ISEE-3 plasma data covering the last 4.5 months of 1978. These events were often found near sector boundaries, making it even more likely that they signified the creation of a U-shaped magnetic loop through reconnection. Lin and Kahler (1992 ) re-investigated these events using data from electrons of still higher energy ( $2-8.5 keV$ ) which they took to be more reliable indicators of connectivity. This data revealed that at least 8 of the heat flux dropouts identified by McComas et al. (1989 ) were in fact connected to the solar surface, and only 2 remained unambiguously U-shaped. They attributed the discrepancy to the presence of enhanced scattering which made the field lines appear open, at least to electrons of low enough energies.

Based on the observed variation in radial field strength at $1 AU$ (King, 1979 ) it is believed that the total open flux does vary over the solar cycle. This requires the introduction of new open field lines during the rising phase, and their subsequent destruction during the declining phase. There is clear evidence that CMEs carry closed field lines into interplanetary space, thereby increasing the open flux. It is not yet clear, however, where, when or how open field lines are closed down in the declining phase.