UNDERSTANDING SPACE WEATHER Part III: The Sun’s Domain KEITH STRONG, NICHOLEEN VIALL, JOAN SCHMELZ, AND JULIA SABA The solar wind is a continuous, varying outflow of high-temperature plasma, traveling at hundreds of kilometers per second and stretching to over 100 au from the Sun. his paper is the third in a series of five papers Our intention is to provide a basic context so that about understanding the phenomena that cause they can produce more informative and interesting T space weather and their impacts. These papers program content with respect to space weather. are a primer for meteorological and climatological Strong et al. (2012, hereafter Part I) described the students who may be interested in the outside forces production of solar energy and its tortuous journey that directly or indirectly affect Earth’s neutral at- from the solar core to the surface. That paper dealt mosphere. The series is also designed to provide with long-term variations of the solar output. Strong background information for the many media weather et al. (2017, hereafter Part II) addressed issues related forecasters who often refer to solar-related phenom- to short-term solar variability (primarily flares and ena such as flares, coronal mass ejections (CMEs), CMEs). This paper (Part III) deals with the variations coronal holes, and geomagnetic storms. Often those in the outflow of particles from the Sun throughout the presentations are misleading or definitively wrong. solar system to its outer boundaries: the Sun’s domain (see Fig. 1). The forthcoming Part IV (“The Sun–Earth Connection”) will deal with how the variable solar AFFILIATIONS: STRONG,* VIALL, AND SABA*—NASA Goddard radiation, magnetic field, and particle outputs inter- Space Flight Center, Greenbelt, Maryland; SCHMELZ—Universities act with Earth’s magnetic field and atmosphere. The Space Research Association, Columbia, Maryland, and University concluding Part V (“Impacts on Life and Society”) of Memphis, Memphis, Tennessee will discuss the tangible effects of space weather on * Emeritus our increasingly technology-dependent civilization. CORRESPONDING AUTHOR: Dr. Keith T. Strong, The quiescent state of the Sun produces the con- [email protected] tinual stream of particles and magnetic fields called The abstract for this article can be found in this issue, following the the solar wind. The Sun undergoes frequent violent table of contents. explosions resulting in solar flares and CMEs. Both DOI:10.1175/BAMS-D-16-0204.1 are generally understood to be the result of a huge In final form 23 January 2017 magnetic reconnection event on the Sun. A solar flare ©2017 American Meteorological Society For information regarding reuse of this content and general copyright is the rapid increase in photons (radiation from γ rays information, consult the AMS Copyright Policy. to radio waves) and propagates isotropically to arrive at Earth about 8 min later (i.e., at the speed of light) to AMERICAN METEOROLOGICAL SOCIETY DECEMBER 2017 | 2593 Unauthenticated | Downloaded 10/05/21 02:39 AM UTC the solar surface while the other end is connected to the heliospheric fields), the solar wind particles can stream out into interplan- etary space. The Sun exports nearly 2 Mt s−1 of solar wind particles, mostly protons and electrons with some α particles (He nuclei) and trace amounts of heavier elements. The solar wind is accel- erated to speeds of 200– 800 km s−1 with a mean of ~450 km s−1 at Earth. These flows exceed both the speed of sound and Alfvén veloc- ity (the speed at which a low-frequency oscillation FIG. 1. The domain of the Sun (the heliosphere) stretches from the Sun itself of the ions and magnetic (center of image) to the edge of interstellar space. The ambient conditions in field propagates through the the inner solar system are determined by the nature of the solar wind. The outer edge of the solar system comprises zones characterized by the solar medium). Ultimately, the wind’s interaction with the interstellar medium as the solar system plows energy that heats the solar through it. These interactions give rise to many complex physical processes corona and accelerates the that can affect our space weather. [Figure from Gilbert et al. (2009).] solar wind comes from the magnetic energy built up impact Earth’s thermosphere and ionosphere. A CME by the tangling photospheric motions. However, the consists of the magnetic field and plasma expelled from physical mechanisms by which the Sun accelerates the Sun into interplanetary space at several hundred the plasma to these high speeds are, as yet, unknown. kilometers per second and so can arrive at Earth days The electron density of the solar wind starts later. CMEs often erupt at speeds significantly different out at over 108 cm−3 near the Sun and falls with from that of the ambient solar wind. Both the initial distance from the Sun according to the inverse magnetic reconnection and the resulting CME shock square law, ranging from 2 to 50 cm−3 at 1 au (where can accelerate particles to extremely high energies, 1 au ≡ 149 597 870 700 m). The solar wind is charge creating solar energetic particles (SEPs) [see the reviews neutral, so that the proton and electron number densi- by Schwenn (2006) and Pulkkinen (2007)]. There is ties are essentially the same, with the protons carrying a higher occurrence of these solar explosions during most of the momentum. The total mass of the solar solar maximum, although solar flares and CMEs can wind impinging on the entire Earth is about 7 kg s−1, occur during solar minimum too. carrying a kinetic energy on the order of 7 × 1011 J (i.e., about 0.0004% of the radiant energy Earth receives CORONAL HOLES AND THE SOLAR WIND. from the Sun in a second). However, most of the solar EUV and X-ray images of the solar corona show large wind is diverted around Earth by the magnetosphere areas of low emissions, called coronal holes. The solar except during solar storms, so the actual mass and magnetic fields in coronal holes are relatively weak, energy flux directly injected into geospace from the predominantly radial, and connected to interplanetary solar wind is considerably less. magnetic fields. As the primary emission process in the The solar wind is basically a collisionless plasma lower corona is collisional excitation, which increases whose flow becomes faster than collisions can equili- rapidly with temperature and density, coronal holes, brate the ionization states, so they become “frozen being less dense and cooler than their surroundings, in” and do not evolve subsequently with propagation. emit less radiation and hence appear darker. Consequently, the concept of temperature is different Where the coronal fields are open (i.e., one in the solar wind from that in a collisional gas like end of the field line connects directly back to Earth’s atmosphere. The different components of the 2594 | DECEMBER 2017 Unauthenticated | Downloaded 10/05/21 02:39 AM UTC solar wind are coupled via wave–particle interactions Coronal holes exist almost continuously through- as opposed to classical Coulomb collisions. The solar out the ~11-yr solar activity cycle. Solar wind wind velocity distribution often has a Maxwellian form; from coronal holes flows outward at speeds up to the protons have a thermal velocity of about 40 km s−1, 800 km s−1, thus being termed the high-speed solar which corresponds to a temperature of about 0.1 MK. wind (McComas et al. 1998). When coronal holes ap- The solar wind permeates the entire solar system, pear at lower latitudes, especially when they cross the a huge volume of space (of order 1031 km3). It is cur- solar equator, these high-speed streams of particles rently sampled in situ by a small fleet of interplanetary sweep past Earth once every solar rotation (~27 days). spacecraft at just a few points, mostly in orbits near the Higher-latitude coronal holes can also become geoef- ecliptic plane or around a few of the planets. Many of fective as a result of the tilt of the solar magnetic the solar wind phenomena vary on a wide range of tem- dipole and the inclination of the solar spin axis with poral and spatial scales. Therefore, these intermittent respect to the ecliptic. These recurring patterns can measurements leave key properties of the solar wind last for many months, making their impacts some- (e.g., temperature, composition, velocity, density, ion- what predictable (see Fig. 2). ization state, and magnetic field) vastly undersampled. In the corona the magnetic pressure exceeds the Remote sensing instruments give a global view plasma pressure, so the magnetic field constrains and of the solar wind and CMEs as they flow out from guides the high-temperature plasma. However, as the Sun. For example, the simultaneous white-light the solar corona transitions into the solar wind, the images from the two Solar Terrestrial Relations Ob- ratio of the plasma pressure to the magnetic pressure servatory (STEREO) spacecraft (Howard et al. 2008) changes, and the plasma pressure takes control. In allow for a 3D reconstruction of CMEs, from which the accelerated solar wind, that ratio is on average the electron density and speed can be estimated. The 1, but can range from ~0.1 to 10. In the region near images can also be used to measure relative density the magnetic equator, the magnetic field pressure changes and the velocity of the ambient solar wind, declines and the ratio can become very large. particularly where the solar wind is first accelerated Once accelerated, the solar wind plasma flows (e.g., Sheeley 1999; Viall et al. 2010). out radially, so the magnetic field, constrained by FIG.