It follows one communicates published in english on the day (15/06) in the website of the National Institute for Space Research (INPE) noting that researchers at INPE and NASA figure out the causes of the 'Solar Minimum'.
INPE and NASA’s Researchers Reveal
What Caused the Latest “Solar Minimum
The solar minimum occurs approximately every 11 years and is defined by the number of black spots on the sun. More sunspots generally mean more activity and eruptions on the sun and vice versa. The number of sunspots can change from cycle to cycle, and 2008 saw the longest and weakest solar minimum since 1960, when scientists have been monitoring the sun with space-based instruments.
National Institute for Space Research (INPE) along with NASA's Jet Propulsion Laboratory scientists have studied about magnetic effects on Earth due to the sun. Such effects are usually harmless, with the only obvious sign of their presence being the appearance of auroras near the poles. However, in extreme cases, they can cause power grid failures on Earth or induce dangerous currents in long pipelines, so it is valuable to know how the geomagnetic effects vary with the sun.
Signed by Bruce Tsurutani of NASA, and Walter D. Gonzalez and Ezequiel Echer of INPE, the paper appeared in “Annales Geophysicae”reports that these effects on Earth did in fact reach a minimum but some eight months later. Check out the paper.
According to this paper factors in the speed of the solar wind, and the strength and direction of the magnetic fields embedded within it, helped produce this anomalous low. “Historically, the solar minimum is defined by sunspot number. But the geomagnetic effects on Earth reached their minimum quite some time later, in 2009. So we decided to look at what caused the geomagnetic minimum," say space weather scientists.
Geomagnetic effects basically amount to any magnetic changes on Earth due to the sun, and they're measured by magnetometer readings on the surface of the Earth. Knowing what situations cause and suppress intense geomagnetic activity on Earth is a step toward better predicting when such events might happen.
Three things help determine how much energy from the sun is transferred to Earth's magnetosphere from the solar wind: the speed of the solar wind, the strength of the magnetic field outside Earth's bounds (known as the interplanetary magnetic field) and which direction it is pointing, since a large southward component is necessary to connect successfully to Earth's magnetosphere and transfer energy. The team examined each component in turn.
The researchers noted that in 2008 and 2009, the interplanetary magnetic field was the lowest it had been in the history of the space age. This was an obvious contribution to the geomagnetic minimum. But since the geomagnetic effects didn't drop in 2008, it could not be the only factor.
To examine the speed of the solar wind, they turned to NASA's Advanced Composition Explorer (ACE), which is in interplanetary space outside the Earth's magnetosphere, approximately 1 million miles toward the sun. The ACE data showed that the speed of the solar wind stayed high during the sunspot minimum. Only later did it begin a steady decline, correlating to the timing of the decline in geomagnetic effects.
The next step was to understand what caused this decrease. The team found a culprit in something called coronal holes. Coronal holes are darker, colder areas within the sun's outer atmosphere. Fast solar wind shoots out the center of coronal holes at speeds up to 500 miles per second, but wind flowing out of the sides slows down as it expands into space.
Usually, at solar minimum, the coronal holes are at the sun's poles. Therefore, Earth receives wind from only the edges of these holes, and it's not as fast as wind from sun’s central region. But in 2007 and 2008, the coronal holes were not confined to the poles as normal.
Those coronal holes lingered at low latitudes to the end of 2008. Consequently, the center of the holes stayed firmly pointed towards Earth, sending fast solar wind in Earth's direction. Only as they finally appeared closer to the poles in 2009 did the speed of the solar wind at Earth begin to slow down so the geomagnetic effects and sightings of the aurora along with it.
Coronal holes seem to be responsible for minimizing the southward direction of the interplanetary magnetic field as well. The solar wind's magnetic fields oscillate on the journey from the sun to Earth. These fluctuations are known as “Alfvén waves”. The wind coming out of the centers of the coronal holes has large fluctuations, meaning that the southward magnetic component – like that in all the directions – is fairly large. The wind that comes from the edges, however, has smaller fluctuations, and comparably smaller southward components. So, once again, coronal holes at lower latitudes would have a better chance of connecting with Earth's magnetosphere and causing geomagnetic effects, while mid-latitude holes would be less effective.
Working together, these three factors – low interplanetary magnetic field strength, combined with slower solar wind speed and smaller magnetic fluctuations due to coronal hole placement – create the perfect environment for a geomagnetic minimum.
Between 1645 and 1715, there was a long decrease in observation of sunspots and auroras. This period is known as "Maunder Minimum". Researchers believe that at that time might happened a confluence of effects similar to 2009.
The solar minimum occurs when fewer sunspots like these appear.
Image credit: NASA/Goddard Space Flight Center
Source: WebSite of the National Institute for Space Research (INPE)