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The Sun
Views of the 'Bastille Day' Sun
July 2000

SOHO image of the 'Bastille Day' Sun provided by NASA's Goddard Space Flight Center

Activity on the Sun peaks in its eleven-year cycle when our star is most tempestuous and the Earth is buffeted with powerful solar gusts.

As the Sun's stormy season arrived at its most recent zenith in the second half of the year 2000, Earth-bound solar scientists had an excellent view through the largest coordinated fleet of spacecraft and ground observatories ever assembled to observe angry outbursts of solar radiation and predict the impact of turbulent space weather.

Scientists said the images received and data read in 2000 were beyond the wildest expectations of astronomers just one generation before.

On July 13, 2000, NASA and NOAA tracked one intense solar storm they nicknamed the "Bastille Day event." Scientists on Earth used data received from the orbiting Solar and Heliospheric Observatory (SOHO) and NOAA's Geostationary Operational Environmental Satellites (GOES) to anticipate the bright solar flare and its ensuing energetic proton shower. SOHO is a joint project of the European Space Agency and NASA.

The flare coincided with a coronal mass ejection from the Sun which sent billions of tons of plasma into space traveling at 4 million miles per hour. That is twice the normal speed. A coronal mass ejection is an eruption of the electrified gas known as plasma from the Sun.

ACE Blinded. NOAA forecasters usually rely on data from the Advanced Composition Explorer (ACE) spacecraft to provide one hour notice before the start of a geomagnetic storm on Earth. Unfortunately, the July 2000 solar shower blinded ACE's most important detectors. Without that reliable data, forecasters had to wait until Earth's magnetic field became distorted before they knew for sure that the disturbance had arrived from the Sun.

A G5 geomagnetic storm raged for nearly nine hours after the solar shower's impact. G5 is the most intense level.

The effects of the Bastille Day storm were widespread. Cameras and star-tracking navigation devices on several satellites were flooded with solar particles. Measurements from particle detectors and other instruments on several NOAA and NASA spacecraft were either degraded or temporarily shut down. The Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA) was sent tumbling in orbit.

On the ground, aurora light shows were seen as far south as El Paso, Texas. Power companies suffered geomagnetically induced currents that tripped capacitors and damaged at least one transformer. Global positioning system (GPS) accuracy was degraded for several hours.



Images from Space. A number of International spacecraft provided extensive data and extraordinary images showing the development and character of the July 2000 Bastille Day event:


Solar and Heliospheric Observatory (SOHO)

The Sun Before and During Solar Maximum
Click this link to see a pair of false color SOHO Extreme Ultraviolet Telescope photos comparing the quiet Sun's atmosphere (left) with the stormy Sun (right).
Credit: NASA and ESA


Before 'Bastille Day'
During 'Bastille Day'
After 'Bastille Day'
The "Bastille Day" storm began with a powerful x-class solar flare captured in this sequence of images by SOHO's Extreme Ultraviolet Telescope. The left image was taken just before the flare. The bright area in the middle image shows the rapid and intense variation in brightness associated with the flare. (The horizontal streak is merely an effect in the telescope caused by the intense brightness of the flare.) In the right image, the flare released streams of high-energy protons which peppered the ultraviolet telescope's sensors just six minutes later.
Credit: NASA and ESA



Before 'Bastille Day'
During 'Bastille Day'
After 'Bastille Day'
This sequence of images of the Sun by SOHO's LASCO camera also was taken just before, during and right after the massive "Bastille Day" eruption from the Sun. The solid circle in the middle of each picture is the camera's occulting disk, which blocks out intense light so that the Sun's tenuous corona is visible. So-called "halo events" are coronal mass ejections moving almost directly toward Earth. As they loom larger and larger, they appear to envelope the Sun, forming a halo around our star. The "during" image shows a spectacular coronal mass ejection eruption of electrified gas billowing away from the surface of the Sun after the onset of the solar flare. The "after" image shows LASCO's cameras peppered with high-energy protons associated with the earlier solar flare.
Credit: NASA and ESA





Transition Region and Coronal Explorer (TRACE)

Before the flare
The Flare
The Slinky Flare
These photos were snapped by NASA's Transition Region and Coronal Explorer (TRACE) spacecraft in the morning of July 14, 2000, at 9:41, 10:23, and 12:31 Universal Time. In these closeups of the "Bastille Day" flare, the areas covered are about 186,000 miles across, which is large enough to span 23 Earths. The images are false color and show radiation emitted by gas at about 2.7 million degrees Fahrenheit. The third image shows a slinky formation of coronal loops -- immense arches of hot electrically-charged gas that erupted from the Sun's surface and followed invisible lines of magnetic force in the solar atmosphere.

TRACE saw a magnetic reconnection event powering the flare. Magnetic reconnection occurs when oppositely directed lines of magnetic force become compressed and distorted in close proximity to each other. Much like a rubber band snaps suddenly if twisted too much, the distorted magnetic field lines break and reconnect to oppositely directed lines, releasing tremendous energy. At the beginning, overlying, tenuous, very hot coronal loops -- invisible in the before picture -- hold down the slightly cooler, denser, stretched-out loops in the magnetically active region. As the stress increases, the overlying magnetic field reconnects to the stressed loops themselves, releasing the tension like a lid blowing off a pressure cooker. The original cool loops fly outward, and their energy is released as heat. The heat raises the temperature causing more material to boil out of the surface of the Sun to fill the newly formed, relaxed loops.
Credit: NASA and Lockheed Martin Solar and Astrophysics Laboratory





Polar Plasma Laboratory (POLAR)

Earth during the 'Bastille Day' event
The POLAR satellite was launched in 1996 to obtain data from above Earth's poles. High above the poles, the particles of the solar wind and the energy of the wind can find their way into the magnetosphere. At lesser altitudes, energy is transferred from electric fields and electromagnetic waves to electrons that then plunge into the atmosphere to create the aurora. At intermediate altitudes nearer the equator, the satellite passes through the Earth's trapped radiation, the Van Allen belts. Out of the polar ionosphere flows plasma into the magnetosphere. Particles and energy flow from the geomagnetic tail into the atmosphere.

Some 31 hours after the "Bastille Day" flare and coronal mass ejection from the Sun, the Earth was in the midst of an intense geomagnetic storm. The plasma's magnetic field interacted with Earth's, allowing solar wind energy to enter our planet's magnetosphere and generate spectacular auroral displays. False color photos by the POLAR satellite at the height of the storm show the intensity of the storm. Black-red areas in the image are the most intense auroral activity. The large area over the United States extends as far south as Florida as a result of the exceptionally strong geomagnetic storm. Auroras usually are seen only at high latitudes.
Credit: NASA and the University of Iowa




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