Space Scientists Detect Disturbances on Sun

5/20/1996 - 65-96r
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An international group of space scientists announced that they have made the first detections of global short-term disturbances in the solar corona. These disturbances result in global mass ejections from the Sun, which in turn have varied, often massive, effects on Earth's magnetosphere. Their findings were at the Spring Meeting of the American Geophysical Union in Baltimore, MD on May 20. In contrast, previous observations have revealed large outbursts called coronal mass ejections (CMEs) which are propelled outward in a more single direction.

The group of American, German, French, and British scientists, under the direction of Dr. Guenter Brueckner of the Naval Research Laboratory (NRL), says that these detections of short-term (hours to days) global coronal disturbances may lead to a credible explanation of the previously known CMEs, and the cause of the solar wind. The findings will also help scientists formulate a model of the interaction of these events with the Earth's magnetosphere and may lead to the ability to forecast magnetospheric activity.

The group used observations from NRL's Large Angle Spectro Coronagraph (LASCO) instrument on board the ESA-NASA Solar and Heliospheric Observatory (SOHO) satellite.

The new short-term global coronal disturbances in the plane of the solar equator, are all around the Sun not just in a single direction. Many of these global coronal disturbances have been detected since the LASCO-SOHO observations began in January 1996. "So they must be a very common and probably very important process on the Sun," Dr. Brueckner said.

LASCO uses three coronagraphs to observe the outer solar atmosphere from the solar limb to a distance of 22 million km. Coronagraphs are special telescopes that can image the faint corona in the presence of the glaring light of the visible solar disk. These observations can only be carried out from space because of the scattering of sunlight in the Earth's atmosphere.

CMEs are clouds of hot (1-2 million degrees) gasses ejected from the Sun at extremely high speeds (from several hundreds to 2000 km per second). After acceleration at the Sun, they travel through interplanetary space and reach Earth in 2.5 to 5 days. When they reach the Earth, CMEs cause disturbances in the magnetosphere, which trigger auroras, make magnetic navigation at high latitudes difficult, and sometimes cause current spikes in high-voltage power lines, resulting in power outages and occasionally in destruction of power equipment. They also can damage or destroy Earth-orbiting satellites.

LASCO began making observations on December 29, 1995. On January 15, less than 3 weeks later, and to the surprise of everybody involved in the project, the #3 coronagraph observed a large CME. Then, only 4 days after the #2 coronagraph was activated, LASCO's #3 and #2 coronagraphs observed a second large event.

This second event, because it was seen in the field of view of two coronagraphs, could be followed from 1.1 million km above the solar surface out to 15 million km. Figure 1 is a frame of the event during an eleven-hour observation. Bright clouds are seen traveling outward in the equatorial plane with speeds ranging from 90 km per second to 540 km per second over both the east and the west limb of the Sun. The acceleration takes place over a distance of 15 million km. The sectors above the solar limb in which the bright clouds are seen seem to extend over an angle
of 120 degrees in the equatorial plane. Although the instruments cannot see material moving toward or away from the Earth-Sun line, it is safe to assume that the CME extended all the way around the Sun. A faint event can be seen in this picture above the south pole of the Sun.

A thin magnetic current sheet forms around the Sun during minimums of the solar cycle. The acceleration of the global coronal disturbances seem to occur in this sheet. The upper and lower boundary layers of this sheet have opposite magnetic polarity. Consequently, the two boundary layers attract and form a "lid," thus trapping hot coronal material in the corona's outer layers. The amount of material stored varies greatly with time. The upper panel of Figure 2 shows the current sheet during a quiet period; the lower panel shows the same area two days prior to the February 3 CME. The obvious increase in brightness prior to the CME indicates that more material is stored before a global coronal disturbance than during quiet coronal periods. It is assumed that the current sheet will blow open when the pressure inside exceeds a certain value. An instability will then accelerate and release the stored hot coronal material as a global coronal disturbance.

The configuration of the solar magnetic field during the minimum of the solar cycle, which the Sun is approaching now, leads to the formation of this current sheet. Weak polar fields are bending over toward the equator to form the current sheet in the upper corona, which can be seen in Figure 3 by following the polar boundaries of the hot corona. The corona over the poles at this time of the solar cycle is much cooler and therefore does not show up in this picture.

Many more global coronal disturbances have been observed by LASCO since February 3. They seem to be a regular feature of the corona during the minimum of solar activity. This explains why the Sun was found to be so active during the so-called "quiet" phase of its eleven year cycle.

Future LASCO research will address the connection between global coronal disturbances and the Earth's magnetic field. Observations of these new global coronal disturbances at the Earth carried out from other satellites must be correlated with those of LASCO before a credible picture of the Sun-Earth connection can emerge.

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