Kinetic magnetic holes in the high-speed streams during solar cycle 23

Highlights

• Statistics of MH properties in 15 high-speed Alfven streams have been collected.

• MH occurrence rate increases at solar cycle minimum and early growth phase.

• MH events were found with full shape and size retention while transferring for 0.7 ​Mm.

Abstract

One of the important elements of solar wind turbulence is the so-called “magnetic holes” (MHs), sharp temporary drops in the intensity of the interplanetary magnetic field. Using solar wind data from the ACE and Wind spacecraft, distribution and evolution of kinetic-type MHs are investigated. In particular, in the first part of this study, we examine fifteen high-speed Alfvénic streams with a high helicity value, which are mainly CIR streams. The period from 1997 to 2011 is covered, with one stream per year selected. A total of 207 kinetic MHs with duration of 3–129 ​s were detected in 15 high-speed streams with a total duration of 1004 ​h. The occurrence rate of holes is about 5 events per day (0.2 per hour), which far exceeds the average frequency of MH observation in the slow wind. The median value of the MH depth is 1.52 ​nT, the maximum is 22.5 ​nT. The occurrence rate of holes monotonically increases with decreasing their duration. During the solar cycle, the probability of MH observation increases at minimum and during the early growth phase (1997–1998 and 2008–2010), reaching 0.32 events per hour. In the second part, the dynamics of MHs during their convective transfer by the solar wind was tracked. We used 1-s ACE and Wind magnetometer data during the movement of the Wind SC from the magnetosphere to the libration point L1 against the background of the passage of five high-speed streams. We studied the evolution of MHs observed consistently first on the ACE SC, and then, as a result of their transfer by the wind stream, to the Wind SC. Fifteen MHs were selected whose transfer time between satellites deviated from the calculated one by no more than 15%. Of these, 9 ​MHs are of the linear type, 5 ​MHs are of rotational type, and one more was observed against the background of the chaotic behavior of the magnetic field components. The evolution of the MHs during their convective transfer was diverse: from the full shape retention to its significant change both towards the deepening of the hole and the steepening of its front, and towards its significant spreading. The possible modulation effect of MHs on wave phenomena in the magnetosphere is discussed.

Introduction

The behavior of the magnetosphere-ionosphere system during disturbances is largely dependent on the level and nature of turbulence in the solar wind flow disturbing the magnetosphere. Turbulence per se belongs to the category of fundamental physical processes; it is a phenomenon of complex nonlinear flow or motion of a liquid or plasma (Matthaeus et al., 2002). Turbulence connects macro and microprocesses: it interacts, on the one hand, with large-scale flows and structures; on the other hand, it is involved in microscopic or kinetic processes. In the solar wind, turbulence is involved in such key processes as the origin of the solar magnetic field (turbulent dynamo), wave-driven heating of the corona, acceleration of the solar wind, coronal mass ejections, a set of nonlinear MHD effects, turbulent reconnection, wave processes. Turbulence and waves are two sides of the same phenomenon, it is difficult to draw a boundary between them: an increase in the wave amplitude with the transition to a nonlinear mode generates turbulence, on the other hand, turbulence generates waves.

One of the elements of turbulence in the solar wind is a sharp decrease in the magnetic field, the so-called magnetic holes (MHs) associated with current sheets. Turner et al. (1977), were the first to pay attention to MHs, analyzing the measurements of the interplanetary magnetic field (IMF) by the IMP-6 spacecraft (SC). The drop in magnetic pressure in the MH is compensated by a corresponding increase in the plasma gas pressure; total pressure is maintained (Winterhalter et al., 1994), therefore MHs can be very stable when observed at large distances. Most likely, MHs are involved in local reconnection processes in the solar wind. Zurbuchen et al. (2001) identified a separate class of kinetic MHs for the description of which the equations of magneto hydrodynamics are inapplicable due to the small size of these objects, comparable to the gyroradius of solar wind protons. Thus, MHs are present in turbulence of two scales: both macroscopic and kinetic. MHs also differ in the behavior of the magnetic field vector inside the hole: linear MHs are called holes in which the field vector retains its position. Holes with rotation of the field will be called here magnetic rotational holes. In addition to the solar wind, MHs were observed and studied in a magnetosheath (Huang et al., 2017), the plasma layer of the geomagnetic tail (Zhang et al., 2017), and in radiation belts (Vovchenko et al., 2018). Their origin is not unambiguous. Many models of MH formation mechanisms have been published. Most often, mirror instability was called as a source of MHs (Tsurutani et al., 1982; Winterhalter et al., 1994; Zhang et al., 2009). However, quite often MHs are observed without fulfilling the conditions of the mirror instability criterion (Tsurutani et al., 2009), therefore, many other mechanisms have been proposed, for example, nonlinear evolution (Buti et al., 2001), phase steepening of large-scale Alfvén waves (Tsurutani et al., 2002), wave-wave interaction (Vasquez, 2007; Tsubouchi, 2009), soliton type solutions (Baumgärtel, 1999).

Despite the abundance of publications on MHs, very few satellite studies have been conducted to track the dynamics of MHs during their movement in space. Only two publications are available on observations of MH transport by the solar wind between two SC (Tsurutani et al., 2005; Potapov, 2018). There are also no studies on variations of the MH occurrence rate in the solar cycle. Here we are trying to fill these gaps. Section 2 first describes the source magnetic field measurements from ACE and Wind SC and processing methods, then provides statistical data on single-satellite MH observations at libration point L1 during the passage of 15 high-speed solar wind CIR streams from 1997 to 2011. Section 3 considers statistics of magnetic holes in high-speed Alfvénic streams. Section 4 describes the results of correlated observations of MH sequentially by the two SC. Discussion and concluding remarks are provided in Section 5.