Academic journal article Proceedings of the American Philosophical Society

Everyday Solar Storms . . . and Their Everyday Impacts 1

Academic journal article Proceedings of the American Philosophical Society

Everyday Solar Storms . . . and Their Everyday Impacts 1

Article excerpt

Our Home in Space

"Look again at that dot. That's here. That's home. That's us . . . a mote of dust suspended in a sunbeam. The Earth is a very small stage in a vast cosmic arena." - Carl Sagan (1934-1996)

The view of Earth from a distance of 7.2 billion km (4 billion miles), shown in Figure 1,2 indeed underscores Earth's tiny cosmic presence- and hints at its susceptibility to the Sun's vagaries. In our orbit of radius 149.6 million km around the Sun, we depend entirely on, and are potential victims of, its energetic output.

Solar electromagnetic radiation makes Earth habitable. As Figure 2 shows, solar energy, in the form of photons, travels directly to Earth from the Sun's surface and atmosphere, reaching us in just 8 minutes. It is the balance of this incoming radiative energy (primarily at visible wavelengths) with outgoing thermal energy (at near-infrared wavelengths), which the solar-warmed Earth radiates back to space, that establishes the temperature of Earth's surface and atmosphere. The Sun's visible radiation varies only minimally, by a tenth of a percent or so over decades (Lean 2010).

Electromagnetic radiation is not the only form of energy that the Sun sends to Earth. Figure 3 depicts how the Sun and Earth are also intimately connected in a quite different way-by a solar wind of magnetic fields and plasma that blows by Earth. Magnetic fields from the Sun's surface extend through its outer atmosphere-the corona-and pervade the space environment between the Sun and the Earth-the heliosphere. They deform Earth's intrinsic magnetic field forming a magnetospheric bubble that, to a large extent, protects Earth's underlying atmosphere and surface-and our societal infrastructure-from energetic particles in the solar wind plasma and magnetosphere.

Unruly Sun: Driver of Space Weather

The Sun's magnetic fields are in constant motion. Convective, diffusive, and meridional flows continually jostle the field line footprints, which are anchored beneath the Sun's visible surface. When magnetic fields interact, they can abruptly reconfigure, producing eruptions that perturb the outflow of both electromagnetic and plasma energy. Frequently-but not always-enhanced electromagnetic radiation is evident at this time from the region of the Sun where the fields distort and reconnect. The flares are observed primarily as enhanced radiation at extreme ultraviolet and X-ray wavelengths. An ejection of mass from the outer solar atmosphere often results. Coronal mass ejections (CMEs) spew outward from the Sun into the surrounding heliosphere as a cloud of plasma and magnetic fields producing shocks that propagate through the solar wind (Vita-Finzi 2015; current issue). Figure 4 depicts the morphology of an eruptive solar event whose origin on the Sun is such that the Earth, in its transit around the Sun, is in the path of the CME.

Eruptive events perturb the heliosphere initially in the vicinity of the location of the event on the Sun and increasingly over a wider volume of the solar system as the Sun's rotation generates a spiraling of the outward-spreading magnetic fields and plasma. The original location of the CME on the Sun and its subsequent transit through the heliosphere determine the extent of its interaction with objects in the solar system, including the Earth. Because of its miniscule cosmic footprint (Figure 1), Earth mainly avoids significant disruption of its local space environment by the Sun's eruptions. The magnetosphere (with an up-wind diameter of ~128,000 km) encompasses only about 0.013% of the circumference of Earth's orbit around the Sun (940 million km).

Figure 5 shows, on the left, an image of an eruptive solar event on 15 March 2015, observed by dual instruments on the twin spacecraftof NASA's Solar Stereo Observatory (STERO). On the right are simulations using the WSA-ENLIL model that the National Oceanic and Atmospheric Administration's (NOAA's) Space Weather Prediction Center (http://www. …

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