8. Energizing Space
- Interplanetary space contains hot, charged pieces of the Sun, known as the solar wind, and a smaller number of more energetic charged particles, the cosmic rays, emitted during the explosion of dying stars, called supernovae.
- When the Sun is most active, there are fewer cosmic rays arriving at Earth and more solar particles impact Earth, and when the Sun is in the inactive part of its 11-year cycle the solar wind is weaker and the number of cosmic rays hitting Earth is greater.
- Invisible magnetic fields emerge out of the south magnetic pole of the Earth, loop through space, and enter the north magnetic pole.
- The Earthís dipolar magnetic field deflects charged particles, and diverts the solar windís electrons and protons around the Earth.
- The Earthís dipolar magnetic field hollows out a cavity, called the magnetosphere, within the Sunís relentless winds.
- Our planetís magnetism has a symmetric shape near the Earth, but the solar wind produces a non-spherical, asymmetric magnetic shape further out, with a bow shock on the dayside and a long magnetotail on the night side.
- The Earthís magnetic barrier is imperfect, and the tempestuous solar wind can buffet and even penetrate the magnetosphere.
- The Sunís winds bring the solar and terrestrial magnetic fields together on the night side of the Earth, where magnetic fields that point in the opposite direction can merge together. Electrons and protons can enter the magnetosphere at this point of magnetic reconnection, and these particles can become accelerated within the magnetosphere.
- Doughnut-shaped belts of energetic electrons and protons girdle the Earthís equator. These Van Allen belts are fed by the Sunís winds and by cosmic rays coming from interstellar space.
- When coronal mass ejections are directed toward the Earth, they can distort its magnetic field, feeding low-energy charged particles into the outer Van Allen belt.
- Cosmic rays can supply protons to the inner Van Allen belt by first creating neutrons when colliding with our atmosphere. The neutrons decay into protons after entering the magnetic cage of the inner Van Allen belt.
- The northern and southern lights, the auroras, are always present near the north and south poles of the Earth.
- The auroras are caused by high-speed electrons, which energize oxygen and nitrogen molecules in the atmosphere and cause them to fluoresce.
- When viewed from space, the auroras form an oval centered on each magnetic pole, where magnetic fields guide energetic electrons down into the Earthís upper atmosphere.
- Variations in the Earthís magnetic field are called geomagnetic storms. They make compass needles quiver.
- Geomagnetic storms come in two varieties: intense, sporadic, non-recurrent storms and moderate, 27-day recurrent storms.
- Intense, non-recurrent geomagnetic storms are caused when coronal mass ejections encounter the Earthís magnetosphere with the right magnetic alignment.
- Studies from SOHO demonstrate that Earth-directed coronal mass ejections generally precede geomagnetic storms, but about three quarters of them do not result in even moderate geomagnetic activity.
- Intense, non-recurrent geomagnetic storms occur more frequently at the maximum in the Sunís 11-year magnetic activity cycle.
- Intense, non-recurrent geomagnetic storms are accompanied by extremely bright auroras.
- Low-level, recurrent geomagnetic storms, with a 27-day repetition period, are produced by co-rotating interaction regions in the solar wind. When the fast-speed and slow-speed solar winds meet, they produce one of these co-rotating interaction regions.
- The 27-day recurrent geomagnetic storms produce relatively faint auroras, noticed mainly during the minimum in the 11-year activity cycle.
- Space weather refers to condition on the Sun and in the solar wind, magnetosphere, ionosphere and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can affect human life and health.
- The Sun powers space weather, producing coronal mass ejections and/or solar flares that cause gusts and squalls in the tempestuous solar wind. They appear more frequently near the maximum of the 11-year solar activity cycle.
- Energetic protons accelerated by solar flares or coronal mass ejections can cripple spacecraft and seriously endanger unprotected astronauts that venture into outer space. Sun storms can also disrupt global radio communications and disable satellites used for navigation, military reconnaissance or surveillance, and communication, from cell phones to pagers, with considerable economic, safety and security consequences.
- Impulsive solar flares eject protons and electrons into interplanetary space with energies that are thousands and even millions of times greater than those usually present in the solar wind.
- Solar flares interfere with ground-based radio communications that use the ionosphere. Flares also endanger communications satellites.
- Solar energetic particle events, of high-energy electrons, protons or heavier ions, arrive at the Earth from the Sun following solar flares or coronal mass ejections.
- The solar proton events are the most energetic and therefore the most dangerous solar energetic particles. They can severely affect the health of unprotected astronauts traveling outside the Earthís protective magnetosphere, and they are capable of penetrating spacecraft to damage or disrupt sensitive technical systems. The strongest events produce radiation doses that might be lethal to astronauts fixing a spacecraft in outer space or taking a walk on the Moon or Mars.
- Energetic charged particles generated during solar flares will only threaten our planet or any other place in space if they occur at just the right place on the Sun, at one end of a spiral magnetic field line that connects the flaring region to that location.
- Solar energetic particles can be accelerated to very high energies in solar flares or by interplanetary shocks driven by fast coronal mass ejections, abbreviated CMEs. The flare-associated particles are produced at the Sun and follow the interplanetary magnetic fields, while the CME-driven shocks can cross magnetic fields lines and accelerate particles all the way from the Sun to the Earth.
- It takes 8 minutes for the radiation of a solar flare to travel from the Sun to the Earth, and by the time you see it the flare radiation has arrived. Very energetic solar-flare particles travel at nearly the velocity of light, and can similarly arrive with no warning. The direct in situ detection of high-speed electrons at a spacecraft can nevertheless give about an hourís advance warning of the arrival of more dangerous, energetic protons at the same location.
- Coronal mass ejections are not deflected by interplanetary magnetic fields, and they take days to travel from the Sun to the Earth.
- When encountering the Earth, coronal mass ejections can compress the magnetosphere below the orbits of geosynchronous satellites that hover above one place on Earth, exposing the satellites to the full force of the solar wind.
- Coronal mass ejections that strike the Earth can generate power surges on transmission lines that could cause electrical power blackouts of entire cities.
- Changes in the ionosphere resulting from solar flares or coronal mass ejections can attenuate or disrupt high-frequency radio wave communications that utilize reflection from the ionosphere to carry signals to distances beyond the local horizon. Even during moderately intense flares, long-distance radio communications can be temporarily silenced over the Earth's entire sunlit hemisphere.
- Space weather interference with radio communication can be avoided by using short-wavelength, ultra-high-frequency signals that pass right through the ionosphere to satellites that can relay the transmissions to other locations. Signals in this frequency range are nevertheless also vulnerable to the aurora currents in the ionosphere, and can be degraded or completely lost during times of high geomagnetic activity.
- With adequate warning, we can defend ourselves from violent Sun-driven space weather. National centers and defense agencies therefore continuously monitor the Sun from ground and space to forecast threatening solar activity.
- Helioseismology is used to detect magnetically complex and strong active regions on the hidden backside of the Sun, providing more than a week of warning before they rotate into view to threaten the Earth.
- Potential solar outbursts can be forecast by monitoring the twist and shear of active-region magnetic fields using vector magnetograms on the ground or in space, by the detection of twisted soft X-ray sigmoid shapes, or by observing emerging or swirling flows beneath the active regions.
- When initiated, threatening coronal mass ejections can be accompanied by solar flares, halos coronagraph images, coronal X-ray or extreme-ultraviolet dimming and global extreme-ultraviolet waves.
- The STEREO mission uses two spacecraft to track coronal mass ejections in space.
Copyright 2010, Professor Kenneth R. Lang, Tufts University