Co-rotating interaction regions and their associated shocks are not the only transient phenomena in the solar wind. Data from the LASCO coronagraph on SOHO makes it clear that many transient releases of matter occur from the Sun, some but not all in association with solar flares. Historically the primary evidence for shocks in the corona and solar wind came from type II solar radio bursts (whose drift speeds appeared small enough to be associated with a shock moving at the Alfven or fast mode speeds) and from the ``Sudden Storm Commencement (SSC)'' component of geomagnetic activity. Until recently it was thought that some shocks observed in the solar wind were associated with blast waves initiated by flares and others were driven ahead of plasma clouds ejected from the Sun (``coronal mass ejections or CMEs''). Now, however, the current belief is that all interplanetary shocks are associated with CMEs.
The basic situation is illustrated in Figure 11.14 [Cravens, 1997]: a dense, fast, magnetized loop or cloud of plasma is ejected from the Sun, moving like a piston into the pre-existing solar wind and creating a compression region bounded by a forward shock.
Figure 11.14: Schematic of a coronal mass ejection in the form of a magnetic cloud [Cravens, 1997] with a shock.
The CME/magnetic cloud often has a force-free configuration and may remain magnetically connected to the Sun even beyond 1 AU; its plasma composition and characteristics are often very different from the pre-existing solar wind. Figure 11.15 shows the plasma and magnetic field data for one CME event observed near 1 AU. Note the forward shock, the compression of the plasma density and magnetic field, and the slow rotation of the magnetic field vector inside the magnetic cloud itself (this is a characteristic of a force-free field configuration).
Figure 11.15: Plasma parameters of a CME and associated shock observed near 1 AU [Gosling, 1996].
Arguing by analogy with the two shocks associated with the pressure pulse in CIRs, one might expect that CMEs would also drive a reverse shock, at least beyond 1 AU. Figure 11.16 shows that this is the case [Gosling, 1996]. Similar to CIR shocks, travelling interplanetary shocks also accelerate particles and generate enhanced plasma waves and radio emissions.
Figure 11.16: Example of a CME and associated forward/reverse shock pair observed by Ulysses near 5 AU [Gosling, 1996].
Travelling interplanetary shocks and CME's affect Earth's ``space weather'' environment in a number of ways. First, the change in magnetic field across the shock causes a time-varying EMF which can overload transformers etc. on communication cables and power grids. Second, the large plasma pressure of the CME and compression region can significantly move the bow shock and magnetosphere Earthward, leading to major currents that can couple to the ionosphere, the ring current, and to terrestrial cables. Third, the shock can inject large numbers of energetic particles into Earth's inner magnetosphere, worsening the radiation environment for spacecraft and increasing the ring current. These effects will be discussed more in Chapter 15.