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Naval Research: The Sun - Inflows

Astronomers spend a lot of time studying what flows away from the Sun, such as supercharged particles, hot gases, light, heat, and other types of energy. They want to know what is coming toward the Earth so that we can learn how to protect our astronauts, satellites, and communications. Now scientists believe that by studying material that flows into the Sun, they can better understand what comes out of the Sun.

The inflowing material was not discovered until 1997, when researchers studying images from the Large-Angle Spectrometric Coronagraph (LASCO) onboard the SOHO satellite noticed gas moving toward the Sun at speeds of 31 to 62 miles per second (50 to 100 km/s). The gas was somehow moving against the powerful solar wind, which, at this distance from the Sun carries material outward at 75 miles per second (120 km/s).

Dr. Neil Sheeley and Dr. Yi-Ming Wang at the Naval Research Laboratory, in Washington, D.C., had seen material head toward the Sun before, but these inflows were different. Appearing at about 1.7 million miles (2.7 million km) above the Sun's surface, the inflows would gain speed at first, then slow as they neared the Sun, only to stop altogether about two solar radii [435,000 miles (700,000 km)] above the surface. If gravity were causing the inflows to fall, they would have sped up and traveled much faster-and they would not have stopped falling.

To get a better look at the faint inflows, which are difficult to notice in images showing bright features, the researchers created special videos that show only what is moving or changing in brightness. The videos are made by subtracting one image from another taken a short time later. Anything that does not change in the time between images, does not appear in the final image. If you were to use this method on two sequential images of a stationary rock, your final image would be blank! If a spider were crawling across that rock, all you'd see is the spider.

Figuring it out

Drs. Sheeley and Wang then compared their videos with maps of the Sun's magnetic fields made from images taken by the Extreme Ultraviolet Imager Telescope (EIT) and data obtained at the National Solar Observatory. They found that the inflows occurred at the boundaries between regions of strong negative and positive magnetic fields. As material is sent out from the Sun, it carries with it a magnetic field. Large loops of magnetic field lines form, towering high above the Sun's surface. Scientists now believe that these lines sometimes cross, then snap to form new connections which collapse toward the Sun, carrying with them clouds of charged particles.

The inflow regions seem to have staying power, and researchers have been able to follow them over the course of many months. Some of the inflows first appear on the left side of the Sun, then rotate out of view as the Sun turns. The inflows reappear two weeks later on the right side of the Sun, then pass out of view and reappear on the Sun's left side after two more weeks. The number of inflows has increased with the level of sunspot activity that has occurred since SOHO first began observing the Sun in 1996.

A mixed up Sun

The Earth has a simple magnetic field, called a dipole, with one north (positive) and one south (negative) magnetic pole, like a bar magnet. Magnetic field lines run from a dipole's north pole to its south pole. The Sun, on the other hand, has a very complex magnetic field. During times of little sunspot activity, the Sun's magnetic field resembles the Earth's simple dipole. However, sunspot activity increases and decreases on an 11-year cycle. During times of great sunspot activity, the Sun's magnetic field lines get twisted and pulled in ways that cause regions of north and south polarity to appear far from the geographical poles of the star. These fields give rise to several forms of solar activity, including sunspots, prominences, coronal mass ejections-and inflows. By studying the inflows, scientists hope to better understand the Sun's magnetic field and how it affects life on Earth.