Magnetic field and dynamic pressure ULF fluctuations in coronal-mass-ejection-driven sheath regions

Abstract

Compressed sheath regions form ahead of interplanetary coronal mass ejections (ICMEs) that are sufficiently faster than the preceding solar wind. The turbulent sheath regions are important drivers of magnetospheric activity, but due to their complex internal structure, relatively little is known on the distribution of the magnetic field and plasma variations in them. In this paper we investigate ultra low frequency (ULF) fluctuations in the interplanetary magnetic field (IMF) and in dynamic pressure (<I>P</I>_dyn) using a superposed epoch analysis of 41 sheath regions observed during solar cycle 23. We find strongest fluctuation power near the shock and in the vicinity of the ICME leading edge. The IMF and <I>P</I>_dyn ULF power have different profiles within the sheath; the former is enhanced in the leading part of the sheath, while the latter is increased in the trailing part of the sheath. We also find that the ICME properties affect the level and distribution of the ULF power in sheath regions. For example, sheath regions associated with strong or fast ICMEs, or those that are crossed at intermediate distances from the center, have strongest ULF power and large variation in the power throughout the sheath region. The weaker or slower ICMEs, or those that are crossed centrally, have in general considerably weaker ULF power with relatively smooth profiles. The strong and abrupt decrease of the IMF ULF power at the ICME leading edge could be used to distinguish the ICME from the preceding sheath plasma

Depleting effects of ICME-driven sheath regions on the outer electron radiation belt

“…We note that the sheath ULF powers and dynamic pressure (leading to the compression of the magnetosphere) are highly correlated: seven events in the most compressed magnetosphere class are also among the events with the largest ULF ( B Z,GSM and P dyn ) power, and seven in the least compressed magnetosphere class are among those with lowest ULF power. This was to be expected since Kilpua et al [] concluded that both ULF levels are high for fast ICMEs. In contrast, the sign of is equally distributed in the least/most compressed and low/high ULF power classes (i.e., 5 events with and 5 with in each set of 10 events).…”

Coronal mass ejections and their sheath regions in interplanetary space

“…Different fluctuation levels are also visible in Fig. 19 that shows superposed epoch analysis of Ultra Low Frequency (ULF) fluctuations in the IMF north-south component over the 3-10-min range for 41 sheath + ICME-events during Solar Cycle 23 (Kilpua et al 2013a). The transition from the sheath to ICME is featured by a clear and abrupt drop in the fluctuation power.…”

“…Liu et al (2006a) performed a superposed epoch analysis to show that the plasma in particular in the sheaths of shock driving MCs is generally mirror unstable and there hence should be mirror mode waves. The superposed epoch analysis by Kilpua et al (2013a) showed that the power of the magnetic field and dynamic pressure ULF waves (see also Fig. 19 for the B Z ULF power) in ICME sheaths peaks close to the shock and close to the sheath-ICME boundary.…”

“…This is expected as sheaths The red and blue lines are upper and lower quartiles, respectively. The length of all sheaths is scaled to 10 h (see Kilpua et al 2013a) largely form from the "nominal" solar wind plasma that is expected to have shorter confinement times in the corona and/or lower source temperatures than plasma that constitutes CME flux ropes (excluding the possible filament material). The radial extensions of the sheaths are typically smaller than those of the ICMEs.…”

Dynamics of large‐scale solar wind streams obtained by the double superposed epoch analysis

“…For example, in the work of Simms et al [2010] the measured points only near two reference times are processed by the SEA method, and points between them are not processed and ignored. In other works (see, e.g., papers by , Kilpua et al [2013Kilpua et al [ , 2015, Hietala et al [2014]) the durations of all intervals are made equal by artificial change of distance between points. We use the "double" (with two reference times) SEA method , that is, we rescale the duration of the interval of all SW types in such a manner that, respectively, beginning and end for all intervals of selected SW type coincide.…”