Propagation of sudden disturbances through a nonhomogeneous solar wind

In the earliest studies of the propagation of solar flare disturbances through the solar wind the problem was treated as being spherically symmetric. However, typically interplanetary conditions at 1 AU are dominated by a series of high-velocity streams, each preceded by a density peak. Earlier calculations have indicated that velocity and density inhomogeneities have important effects on transit times and strengths of propagating disturbances. Other workers have numerically investigated the effect of streams on the propagation of shocks confined to remain strong throughout the propagation to 1 AU. However, the constraint that the interplanetary shock remain strong throughout its propagation is unrealistic, since the shocks observed in space at 1 AU often have low or intermediate Mach numbers. We here report on a calculation in which a sudden spherically symmetric disturbance is introduced at 30 Rs and allowed to propagate through a solar wind containing a velocity stream. No restriction is made on the evolution of the strength of the disturbance as it propagates. It was found that at 1 AU the disturbance remained strong in the low-velocity part of the stream and that it propagated from the sun relatively slowly. In the high-velocity region the disturbance reached 1 AU more rapidly but was weaker. Near the density peak the form of the disturbance seen at 1 AU was highly distorted. In other words, it is shown that the calculated transit time, strength, and form of the disturbance at 1 AU depend markedly on the position within the stream at which it is observed.