4. Sun-Earth Connection
Geomagnetic storms and Terrestrial Auroras
Large, sporadic geomagnetic storms
We have known about significant variations in the Earth’s magnetic field for almost three centuries. They are detected by irregular movements in the direction that compass needles point, with typical fluctuations lasting seconds to days. These variations are caused by invisible geomagnetic storms that rage in the magnetic fields far above our atmosphere.
Solar wind disturbances driven by fast coronal mass ejections are now thought to produce the most intense geomagnetic storms, at least during the maximum in the Sun’s activity cycle. Slow coronal mass ejections do not produce such events because they lack the strong magnetic fields and high speeds required to stimulate intense magnetic activity on Earth. The Earth intercepts about 70 coronal mass ejections per year when solar activity is at its peak, and less than 10 will have the punch needed to produce large, geomagnetic storms.
The solar wind generally moves slower than coronal mass ejections, so the ejection plows through the solar wind on its way into interplanetary space, driving a huge shock wave far ahead of it (Fig. 8.6). When directed at the Earth, these shocks ram into the terrestrial magnetic field and trigger the initial phase, or sudden commencement, of a large geomagnetic storm a few days after the mass ejection leaves the Sun.
Strong interplanetary magnetic fields are also generated by fast coronal mass ejections (Fig. 8.6). It is their intense magnetism and high speed that account for the main phase of a powerful magnetic storm, provided that the magnetic alignment is right. The Earth’s field is generally directed northward in the outer day side magnetosphere, so a fast coronal mass ejection is more likely to merge and connect with the terrestrial field if it points in the opposite southward direction. The rate of magnetic reconnection, and hence the rate at which energy is transferred from the solar wind to the magnetosphere, increases with the strength and speed of the interplanetary magnetic field. The energy gained drives currents that make the intense magnetic storm.
Moderate, 27-day recurrent geomagnetic activity
Unlike the great sporadic storms, moderate geomagnetic activity does not exhibit a well defined connection with sunspots or any other indicator of solar magnetic activity. Indeed, these weaker events, sometimes referred to as substorms, can occur when there are no visible sunspots. A solar connection is nevertheless indicated by their 27-day repetition period, corresponding to the rotation period of the Sun at low solar latitudes when viewed from the moving Earth. Centuries.
The recurrent activity is linked to long-lived, high-speed streams in the solar wind that emanate from coronal holes. When the Sun is near a lull in its 11-year activity cycle, the fast wind streams rushing out of coronal holes can extend to the plane of the solar equator. When this fast wind overtakes the slow-speed, equatorial one, the two wind components interact, like two rivers merging to form a larger one. This produces shock waves and intense magnetic fields that rotate with the Sun (Fig. 8.7). Such Co-rotating Interaction Regions, or CIRs for short, can periodically sweep past the Earth, producing moderate geomagnetic activity every 27 days. Near solar maximum, at the peak of the 11-year activity cycle, coronal mass ejections dominate the interplanetary medium, producing the most intense geomagnetic storms, and the low-level activity is less noticeable.
Forceful coronal mass ejections can generate exceptionally intense auroras when solar activity is at its peak. When their magnetic field are swept around and next to the Earth’s magnetotail, the interaction can create an opening in the Earth’s magnetic barrier, allowing solar wind particles and energy to pour into the plasma sheet at the center of the magnetotail. The energy gained during this process not only produces intense geomagnetic storms; it also accelerates the infiltrating solar wind particles and local particles already in the magnetosphere. At such times, the accelerated electrons hurtle along magnetic conduits connected to the upper atmosphere, or ionosphere, in both polar regions, generating spectacular auroras. Today spacecraft look down on the auroras from high above, showing them in their entirety (Fig. 8.8). They form an oval centered at each magnetic pole, resembling a fiery halo.
The northern or southern lights, named the aurora borealis and aurora australis in Latin, are one of the most magnificent and earliest-known examples of solar-terrestrial interaction. They illuminate the cold, dark Arctic and Antarctic skies with curtains of green and red light that dance and shimmer across the night sky far above the highest clouds (Fig. 8.9), pulsating and flickering for hours to days.
The reason that auroras are usually located near the polar regions is that the Earth’s magnetic fields guide energetic electrons there. The auroral lights form when high-speed electrons rain down along the Earth’s magnetic field lines into the upper atmosphere in the polar regions, like electricity making the gas in a neon light shine. The cascade of electrons collides with oxygen and nitrogen in our atmosphere, boosting them to higher energies and causing them to glow. It is something like the beam of electrons that strikes the screen of your color television set, making it glow in different colors depending on the type of chemicals (phosphors) that coat the screen.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University