Interplanetary Shock Wave Passes Earth

A wave of plasma and magnetic fields from a Feb. 17 coronal mass ejection passed by Earth on Feb. 20, 2000. Geomagnetic activity is up, but this event may not cause significant aurora.

February 21, 2000 -- An interplanetary wave of ionized gas and magnetic fields passed by Earth on February 20, 2000 at 2100 UT. The shock front was caused by a full halo coronal mass ejection (CME) that erupted from the Sun on February 17, 2000. Geomagnetic activity has increased as a result of the interplanetary wave, but it appears there will be no significant aurora over the lower 48 U.S. states. The NOAA Space Environment Center forecasts a 30% chance of minor geomagnetic storm activity at middle latitudes today, decreasing to only 15% tomorrow.

Above: These data from NASA's Advanced Composition Explorer (ACE) spacecraft show the speed of the solar wind measured 1.5 million km from Earth at the L1 Sun-Earth libration point. The discontinuity, where the wind velocity jumps from 330 km/s to nearly 450 km/s, marks the passage of an interplanetary shock wave caused by a solar coronal mass ejection on February 17. Events detected by ACE usually reach Earth about an hour later.

When fast-moving material from a CME flows away from the Sun, it piles up against slower-moving gas that had been ejected earlier. This produces a sharp, dense shock front like the one ACE detected yesterday. When the shock wave passed ACE around 2100 UT, the solar wind density soared from 3 to over 10 protons per cubic centimeter.

Right: Click on this image to see what happens when a coronal mass ejection strikes our planet's magnetosphere.

About an hour later the shock wave reached Earth's magnetosphere, a region of space controlled by our planet's magnetic field. The magnetosphere forms a shield that helps protect Earth from solar wind storms. Yesterday's shock front carried with it a magnetic field with a strong southerly component. South-pointing magnetic fields are sometimes able to partially cancel the Earth's magnetic field at the point of impact and create a chink in our magnetic armor. Plasma (ionized gas) can then enter the magnetosphere through this weak point. Aurora often follow such an injection of solar plasma. This time, however, the magnetic field of the disturbance flipped northward after only 4 hours. Mid-latitude aurora were not reported last night, and forecasters have downgraded the chances for aurora during the next 48 hours.

While this space weather event might not put on much of an auroral show, we could be in store for a dazzling display late this week or next. A large coronal hole is just approaching the Sun's central meridian. Coronal holes are regions of low magnetic field strength where high speed solar wind particles can escape into interplanetary space. When the energetic wind stream reaches Earth, it can buffet the magnetosphere and trigger aurora. Stay tuned to SpaceWeather.com for this developing story.

For more news and information about space weather, please see SpaceWeather.com. Technical information about current space weather conditions may be found at the NOAA Space Environment Center.

Web Links

Here Comes the Sun - February 18, 2000 Space Science News. A full halo coronal mass ejection appears headed directly for Earth.

Small Sunspot, Big Flare - February 6, 2000 Space Science News. The brightest solar flare of the current solar cycle erupts from a diminutive sunspot group.

Advanced Composition Explorer - home page.

Solar Flares - Learn more about solar flares and coronal mass ejections at this excellent web site from NASA/Goddard.

SpaceWeather.com -follow the latest events on the Sun

Coronal Mass Ejections -from the Marshall Space Flight Center

SOHO home page -real-time images, screen savers, and more