ERIK MARTIN WILLÈN: Space Weather

Monday, September 19, 2016

Space Weather

To the casual observer, the Sun may appear unimpressive from 93 million miles (150 million km - 1 AU) away but upon closer examination – in the extreme ultraviolet region of the spectrum, it becomes evident that it’s characterized by unpredictable and explosive surface activity. The Sun creates highly variable and complex conditions in the space, as well. We call these conditions ‘space weather’. Space weather is an emerging multidisciplinary field within space sciences that studies how solar activity influences Earth’s space environment.

Our Sun continuously bathes Earth in solar energy, in the forms of: electromagnetic radiation (visible light, microwaves, radio waves, infrared, ultraviolet, X-ray, gamma rays) and corpuscular radiation (streams of subatomic particles such as protons, electrons, and neutrons). The Sun is a magnetic variable star, and like most stars, it’s composed of superheated plasma; a collection of negatively charged electrons and positively charged ions. Its magnetic fields are produced by electric currents that are generated by the movement of the charged particles. The electrically conductive solar plasma acts like a viscous fluid, so the plasma near the poles rotates slower than the plasma at the equator. This differential rotation results in a twisting and stretching of the magnetic field lines, leading to the formation of sunspots, solar flares and CMEs.

The Sun’s overall magnetic field is quite weak compared to sunspots, which are localized regions of intense magnetism (magnetic loops that poke out of the photosphere), and they can be 1000 times stronger than the Sun’s average field. Above sunspot regions, the Sun’s magnetic field lines twist and turn like rubber bands, and when the field lines interact, the confined coronal plasma is accelerated to several million miles per hour in a powerful magnetic eruption. The cloud of extremely hot and electrically charged plasma expelled from the active region is called a coronal mass ejection, or CME for short. CMEs aimed at Earth are called halo events or halo CMEs because of the way they look in coronagraph images; the coronagraph instrument will detect it as a gradually expanding ring around the Sun. As the CME moves away from the Sun, it pushes an interplanetary shock wave before it, amplifying the solar wind speed, and magnetic field strength, as well. The Sun’s magnetic field isn’t confined to the star, the interplanetary magnetic field (IMF) is carried into interplanetary space by the solar wind and CMEs.

Depending on how the IMF is aligned in relationship to our geomagnetic field, there can be various results when the CME arrives. Some particles get deflected around Earth – thanks to the invisible magnetic “bubble”, called the magnetosphere (it’s actually non-spherical), but a small amount of ionized particles can still get into our near-Earth environment (geospace), mostly via the magnetotail. The magnetosphere is formed when the flow of the solar wind impacts the Earth’s magnetic (dipole) field. The overall shape of Earth’s magnetosphere is influenced by the speed, density and temperature of the solar wind: the dayside is continuously compressed by the solar wind, and the nightside is stretched out into a tear drop shaped magnetotail. Our magnetosphere is an extremely dynamic region and it’s filled with a variety of current systems.

When a powerful CME hits Earth, electrons in the magnetosphere cascade into the ionosphere at the polar regions, creating the so-called Birkeland or field-aligned current that flows along the main geomagnetic field. If the CME’s polarity matches that of Earth’s magnetic field (Northward IMF), our magnetosphere may deflect some of the highly charged particles. The problems occur when the CME’s polarity is the opposite of Earth’s (Southward IMF) because it can cause a geomagnetic storms and brief magnetospheric substorms that disrupt Earth’s own magnetic environment.

Changes in the ionosphere trigger bright aurorae that are, in fact, the visual manifestation of the interaction between solar energetic particles and the high-altitude atmosphere. Solar energetic particles are high-energy charged particles, they can induce voltages and currents in power grids and cause large-scale power and radio blackouts, temporary operational anomalies, damage to spacecraft electronics. During geomagnetic storms, the energy transferred into the ionosphere by the Birkeland current heats up (Joule heating) the atmosphere, which consequently rises and increases drag on low-altitude satellites.

Fortunately, there is a fleet of observing spacecraft monitoring the Sun’s activity across a wide range of electromagnetic wavelengths. Their continuous observations and measurements of solar and geospace variability gives us the ability to prepare and respond to potentially harmful space weather events.

Source: http://spaceplasma.tumblr.com/post/139233168464/space-weather-to-the-casual-observer-the-sun/embed

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