Inside the Black Box: Magnetic Reconnection and the Magnetospheric Multiscale Mission

Abstract

The motivation for the recently launched Magnetospheric Multiscale mission is learning about the process of magnetic reconnection, especially the physics of what is called the diffusion region. The diffusion region is often treated as a black box but is the home of very important physics, which is of great significance to understanding space weather. This article is a brief review of what is known-and not known-about the diffusion region in magnetic reconnection, written for the broad space weather community and its stakeholders (with an appendix for readers interested in more technical matters). The focus is on the physics of magnetic reconnection and the diffusion region, why it has been challenging to study, how MMS will contribute, and how the community will benefit from its measurements. Setting the StageNASA's Magnetospheric Multiscale (MMS) mission was successfully launched on 12 March 2015. The mission was conceived to help understand the somewhat mysterious process called magnetic reconnection. While it is widely appreciated that magnetic reconnection plays a crucial role in space weather (it is discussed in more than 70 Space Weather Journal articles), the many stakeholders of space weather may not be familiar with the process, why understanding it is useful for science and for space weather applications, and how MMS will contribute. The goal of this brief review is to offer clarity on these questions to the space weather community, its industrial stakeholders, and to policy makers. What Is Magnetic Reconnection?Magnetic reconnection, often simply referred to as reconnection, is a process that takes place in gases of sufficiently high temperature that electrons can remain apart from their nuclei. Such gases are called plasmas, and they naturally occur in every star in the universe and most of the regions between stars and between planets. They also can be produced on Earth; fluorescent light bulbs, TV and computer screens, and neon lights all contain plasmas. Plasmas are also important to the pursuit of energy production through fusion, where gases are made to be hot enough that their nuclei stick together when they collide.Many plasmas, including those in stars and in interstellar and interplanetary space, are accompanied by a magnetic field; magnetic fields in plasmas are important because they interact strongly with the charged particles in plasmas, whereas they hardly have any effect on neutral gases such as the air we breathe. In the simplest description of a plasma (called magnetohydrodynamics, which is simply a description of a plasma as if it was a fluid like water except that it interacts with magnetic fields), one can show that magnetic field lines retain their identity. This means that one can follow magnetic field lines as they effectively move through space with their surrounding plasma.While appealing due to its simplicity, the model is grossly oversimplified. It turns out that magnetic field lines, when they point in opposite directions in a small region of space, can effectively...

3D turbulent reconnection: Theory, tests, and astrophysical implications

“…In order to account for these difficulties with magnetic flux-freezing, one might find it tempting to appeal to additional effects beside collisional resistivity. For instance, one might consider additional terms, such as Hall effect and electron pressure anisotropy, in the generalized Ohm's law (electron momentum equation) (Burch et al, 2016;Cassak, 2016). This does not answer the question, unfortunately, how magnetic reconnection events can be fast in fluids with high rates of collisions.…”

“…On the contrary, in 3D turbulent reconnection the loops can shrink all the way with no constraints arising from the artificial dimensional constraints. In fact, later research (see Dahlin, Drake, and Swisdak, 2014, 2015, 2016 has shown that when the problem of tearing is considered in the space of correct dimensions, i.e. in 3D, the acceleration efficiency is increasing.…”

Magnetic Reconnection in the Space Sciences: Past, Present, and Future

“…In the process, we aim to convey the rich and interesting history from whence magnetic reconnection came, the excitement of current magnetic reconnection research, and the challenges and rewards of advances in magnetic reconnection research. However, this paper is not intended as a comprehensive review of magnetic reconnection research and applications; many excellent reviews are available in other articles (e.g., Paschmann et al, 1979;Antiochos et al, 1999;Goodman & Uzdensky, 2008;Cassak, 2016;Drake et al, 2009;Gonzalez & Parker, 2013;Moore et al, 2015), and therefore, many references to important past and current work on the subject will be omitted.…”

Space physics and policy for contemporary society

“…In this Commentary, we discuss what space physics is, its societal relevance and its impact on U.S. policy. A number of pieces have been written about the related topic of space weather [ Baker , ; Baker and Lanzerotti , ] and its societal and policy impacts [ Fry , ; Lanzerotti , ; Schrijver , ; Jonas and McCarron , ; Cassak , ; Gaunt , ; Bonadonna et al , ]; the emphasis for this Commentary is the broader field of space physics.…”