Recent SOHO Highlights|
Solar Data Analysis Center at NASA Goddard
In a rare celestial spectacle near Earth's own star, two comets
were seen plunging into the Sun's atmosphere in close
succession on June 1 and 2. The demise of the comets was followed
the same day by a dramatic ejection of hot gas and
magnetic energy known as a coronal mass ejection. The observations
were made by the Large-Angle Spectrometric
Coronagraph (LASCO) on the Solar and Heliospheric Observatory
|>From April 27 through May 6, ISTP spacecraft and observatories got a sample of what is to come during the maximum of the solar cycle. In the span of 10 days, the Sun produced seven coronal mass ejections, two X-class flares (the most energetic type), and at least two energetic particle events. At Earth, several major magnetic storms upset several spacecraft, brought auroras to lower-than-normal latitudes, and forced power companies to reconfigure the grid in New England.|
On April 29, a "halo" CME left the Sun and its shock arrived at the
spacecraft within 53 hours. The shock arrived faster than any
detected so far by SOHO. A magnetic storm followed on May 2.
rumors and anecdotal reports, the failure of the Equator-S
not necessarily a result of CME or magnetic storm.
ISTP observers have noted that sunspots are becoming more complex and moving with a clockwise rotation--telltale properties of proton flare sunspots.
On May 1-2, ISTP observed two halo CMEs, as well as an X-class
High-energy protons from the flare arrived at SOHO within 30
a major magnetic storm developed on May 4.
During the storm, the disturbance storm-time index (Dst) reached -218 on the scale of 0 to -220. It is the largest storm of the current solar cycle.
An ISTP ground station (CANOPUS) measured electric currents in the ionosphere well above 2000 nanotesla, about 3-4 times the norm for solar minimum. The January 1997 event that knocked out Telstar 401 had currents of 1800 nT.
In response to the magnetic storm, power companies in New England reduced their power sharing capacity with Canadian utilities. Auroras were reported as far south as Boston and Chicago.
||May 6 also was an extremely active day: ISTP observatories detected a slow CME, a moderate-speed halo CME, and a fast CME, though none led to magnetic storms. However, an X-class flare burst from the Sun, raining high-energy protons toward Earth. The protons upset the Polar spacecraft, forcing controllers to shut it down for several hours. All of the instruments and electronics have since been restored.|
Researchers from the International Solar-Terrestrial Physics (ISTP) program are currently tracking a coronal mass ejection (CME) that left the Sun late on January 2 and began arriving at Earth around 10 a.m. Eastern Time on January 6. CMEs are eruptions of electrically charged gas from the Sun that can trigger magnetic storms around Earth. Such eruptions--which are becoming more frequent as the Sun builds up toward the maximum of its 11-year cycle--occasionally disturb spacecraft, navigation and communications systems, and electric power grids.
The Wind, Polar, and Geotail spacecraft, as well as a network of smaller satellites and ground-based observatories are now monitoring the interplanetary storm as it crosses paths with Earth. Scientists are observing changes in the strength of Earth's magnetic field and radiation belts, while gathering images of Earth's auroras.
Forecasters at the Space Environment Center of the National Oceanic and Atmospheric Administration predicted the CME would begin arriving during the latter half of January 6 and would continue through January 7. The disturbance to Earth's magnetic field and space environment is not expected to be particularly strong; however, observers at high latitudes (Canada, Scandinavia, etc.) are likely to see aurora tonight and tomorrow.
On January 2, scientists operating the Solar and Heliospheric Observatory (SOHO) spacecraft detected a "halo" type coronal mass ejection erupting from the Sun at approximately 500 km/s (more than 1 million miles per hour). The SOHO team alerted the rest of ISTP to the possibility of an Earthbound storm. In research presented at the December meeting of the American Geophysical Union, ISTP researchers announced that "halo" CMEs almost always result in magnetic activity at Earth. Halo CMEs are so named because they appear as expanding halos around the Sun when seen from Earth.
ISTP is a joint, comprehensive effort to observe and understand our star, the Sun, and its effects on Earth's environment in space. The primary participating institutions include NASA, the European Space Agency (ESA), the Japanese Institute of Space and Astronautical Sciences (ISAS), the Russian Space Research Institute (IKI).
To view the same data and images as ISTP scientists, visit the
Event web page at http://pwg.gsfc.nasa.gov/istp/events/1998jan3.
For more information about ISTP and the physics of the Sun and Earth, go to http://pwg.gsfc.nasa.gov/istp/outreach.
For the official U.S. space weather forecast, visit http://www.sec.noaa.gov/today.html.
An image to accompany this story is available at http://pwg.gsfc.nasa.gov/istp/events/1998jan3/images.jpg.
Current images of Earth's aurora as seen from space are available
Using spacecraft and supercomputers, scientists from the International Solar Terrestrial Physics (ISTP) program have developed a new theory for the explosive, high velocity coronal mass ejections (CMEs) that will erupt from the Sun with increasing frequency during the maximum of the new solar cycle. CMEs are eruptions of electrically charged gas from the Sun that can trigger magnetic storms around Earth. Such storms occasionally disturb spacecraft, navigation and communications systems, and electric power grids.
Recent experimental and theoretical observations from ISTP indicate that the interaction of magnetic fields high above the Sun's surface, not low in the corona, as was previously thought, allows tremendous energy to build up and to release CMEs at speeds approaching 2000 kilometers per second. Scientists also have determined that halo CMEs almost always lead to geomagnetic storms, an observation of great consequence for the prediction of space weather. Finally, ISTP researchers have been able to create realistic visual simulations of the effects that CMEs can have on Earth's magnetic field.
Coronal mass ejections are the largest structures that erupt from the Sun and they one of the principal ways that the Sun ejects material and magnetic energy into the solar system. Most CMEs travel at 400 km per second, but fast CMEs can reach speeds double or triple that. The first fast CME of the new solar cycle was observed on November 6, 1997. Slow or fast, the eruptions produce magnetic clouds that can cause geomagnetic storms and increase the intensity of the aurora (Northern and Southern lights). Fast CMEs also can accelerate protons in interplanetary space to the point where they can harm spacecraft or astronauts in space.
Inspired by images of CMEs collected by the Large Angle Spectrometric Coronograph on the SOHO spacecraft, Dr. Spiro Antiochos of the U.S. Naval Research Laboratory in Washington, DC, has proposed a new answer to the long-standing question of how the Sun can build up the energy to produce the violent explosions of fast CMEs. Using supercomputers from NASA and the Department of Defense, Antiochos has created a model that simulates the complex, interwoven magnetic structures of the Sun. After observing how magnetic fields abut and interact, Antiochos theorizes that the Sun's magnetic fields tend to restrain each other and force the buildup of tremendous energy.
When a stressed magnetic field low in the Sun's atmosphere pushes higher into the corona, it gets held back by the surrounding and overlying fields. As the stressed field builds up more potential energy, it pushes harder against these magnetic ropes and moves higher into the corona. Eventually, through a process known as "magnetic reconnection"-- where opposing magnetic lines of force merge and cancel--the field is released from its bonds and escapes the Sun as high speed.
Antiochos compares the process to that of filling a helium balloon. "If you fill it without anchoring it, the balloon will slowly drift upward," he said. "But if you hold the balloon down as you fill it, you can generate a lot of upward force, which makes the balloon take off at higher speed once you release it. Fast CMEs are released in the same way." Antiochos added that such high-speed bursts will be more frequent as the Sun approaches its maximum period of activity in 2000-2001.
In other research, ISTP teams headed by David Webb of Boston College and the Research Laboratory at Hanscom Air Force Base and by Dr. Guenter Brueckner of the Naval Research Laboratory used ISTP's SOHO and Wind spacecraft to study nine "halo" type CMEs that occurred from December 1996 to May 1997. They found that such events almost always result in magnetic activity at Earth. Halo CMEs are so named because they appear as expanding halos around the Sun when seen from the perspective of Earth.
According to Dr. Nancy Crooker, a space physicist at Boston University, detection of halo events "will vastly improve" methods of predicting geomagnetic storms. "If you see a halo CME, you are pretty sure that you are going to see a storm at Earth," Crooker said. "In the past, scientists could only look at solar flares to know if a storm was headed toward Earth. And while some flares have accompanying CMEs, some don't. But now we have instruments sensitive enough to see the CMEs themselves."
ISTP does not predict space weather. Forecasting is the domain of the Space Environment Center of the National Oceanic and Atmospheric Administration.
Crooker and Webb also have identified the signature of halo CMEs on the Sun's surface. These remnants of CMEs show up in X rays and ultraviolet light as the sudden brightening of magnetic arches, with dimming areas on either side. These dimming areas seem to mark the roots of CMEs already launched into space.
Downwind from those eruptions, the invisible magnetic shell (or magnetosphere) that shields Earth from the Sun's particles and radiation is regularly shaped and shorn by CMEs. To better understand this interaction, Dr. Charles Goodrich, of the University of Maryland, has developed a series of animations that depict how the magnetosphere responds to the shock of a CME.
Using a Cray C-90 and other powerful computers, Goodrich and colleagues have for the first time depicted the evolution of the magnetosphere as it is bombarded by a real CME. The simulation reconstructs 42 hours of time centered around the arrival of a CME on January 10, 1997. The simulation reveals the shape and orientation of Earth's magnetic field, as well as the density of plasma in Earth's space environment.
In fact, the real January CME produced magnetic storms and spectacular auroral displays, and poured as much as 1400 Gigawatts of electricity into the atmosphere (almost double the power generating capacity of the United States). The magnetic cloud from the CME smacked the magnetosphere with a burst of plasma 30 times denser than the normal solar wind. The shock compressed the leading edge of the magnetosphere inside geosynchronous orbit, where many satellites are positioned.
"We have created a global picture of what is going on in the magnetosphere," said Goodrich. "Since we only have a few spacecraft, and they can only make point measurements, this is the only way to look at the whole system. Such simulations will provide researchers with insights about the conditions that lead to--and perhaps trigger-- geomagnetic storms", Goodrich added.
In the past year, the primary spacecraft of ISTP--SOHO, Wind, Polar, and Geotail--and the program's network of cooperating satellites, ground sensors, and theory centers have monitored approaching interplanetary storms for the first time, from their genesis to their impact on Earth. ISTP was designed as a joint, comprehensive effort to observe and understand our star, the Sun, and its effects on Earth's environment in space. Participating institutions include NASA, the European Space Agency (ESA), the Japanese Institute of Space and Astronautical Sciences (ISAS), the Russian Space Research Institute (IKI), as well as the Max Planck Institute, the U.S. National Oceanic and Atmospheric Administration, the Los Alamos National Laboratory, the U.S. Air Force, the Canadian Space Agency, the British Antarctic Survey, the U.S. National Science Foundation, and the Johns Hopkins Applied Physics Laboratory.
On September 24, a coronal mass ejection lifted off the Sun with the classic signature of an event that should cause geomagnetic storms. Scientists detected a Moreton wave (see figure), several flares, and Type II radio bursts associated with the shock fronts of a CME. NOAA issued a press release and alerted the physics community about a likely strong impact on Earth. Three days later, the CME missed SOHO, Wind, Polar, and Earth, reminding scientists that spectacular solar events don't necessarily mean effects at Earth.
On September 28, SOHO spied another CME (see figure) leaving the Sun. Like the effective January 1997 event, this "halo" CME did not have the markings of a storm-inducing event--no Moreton waves, no flares, no Type II radio bursts, no X-ray signatures. NOAA predicted near-normal space weather, but ISTP alerted scientists to a possible impact. On October 1, Polar and ISTP ground stations detected bright aurora, a geomagnetic storm, and substorms heralding the arrival of the CME at Earth.
The LASCO large angle coronagraph (C3) on the Solar and
Observatory (SOHO) has discovered 14 new comets since the mission
in December 1995. None of the 14 were observed by any ground based
observers because they are too small and were too close to the Sun.
These observations represent the low end of the mass/brightness
distribution for comets (There is possible relevance of these
observations to those of Lou Frank, who reports thousands of small
objects hitting the Earth each day -- see June ISTP news). 12 of
comets were clearly members of the Kreutz family of sun grazers (as
shown in the image,
taken on 24 December 1996). This is known based on
their orbits. The remaining 2 had unusual orbits: one was a
"corona-grazer" (~10R); the second had a perihelion of 0.13 AU.
For reporters and editors seeking more information about any ISTP news item, contact Mike Carlowicz, science writer for ISTP, at firstname.lastname@example.org