Friday, September 21, 2007
A rare, timely conjunction of ground-based instrumentation and a dozen satellites has helped scientists better understand how electrons in space can turn into ‘killers’.
‘Killer’ electrons are highly energetic, negatively charged particles found in near-Earth space. They can critically, and even permanently, damage satellites in orbit, including telecommunication satellites, and pose a hazard to astronauts.
Several theories have been formulated in the past to explain the origin of killer electrons, and many uncoordinated observations have already been performed. Recently, scientists got a boost in their understanding of this hazardous phenomenon. This was possible thanks to a unique set of data, collected simultaneously, by a global armada of ground and space observatories during the recovery phase of a large geomagnetic storm.
In the aftermath of the storm, the CARISMA (Canadian Array for Realtime Investigations of Magnetic Activity) magnetometer chain observed a type of Ultra Low Frequency (ULF) electromagnetic wave, well-known for creating killer electrons. CARISMA observed the so-called ‘Pc5 waves’ continuously, for many hours, during the recovery phase of a large geomagnetic storm on 25 November 2001. In the meantime, they were also picked up by more than half a dozen scientific satellites located inside Earth’s magnetic environment, or magnetosphere, including NASA’s Polar mission.
Meanwhile, ESA’s four Cluster satellites were located at the boundary of Earth’s magnetosphere, called the magnetopause. They observed undulations, or disturbances of the magnetopause, at the same frequency as that of Pc5 waves observed from inside the magnetosphere.
Taking into account data from all satellites, Earth-based radars and magnetometers, Rae's team were able to reveal the mechanism behind the scenario.
During this event, the velocity of solar wind - a continuous stream of solar particles impacting and shaping Earth’s magnetosphere – was measured at approximately 750 km/s, nearly twice its average speed. The impact of this fast flow of solar particles on Earth’s magnetosphere induced the undulations observed by Cluster.
In turn, these undulations drove compressional waves, which propagated inward from the magnetopause towards Earth. Close to the location of the Polar satellite, these compressional waves coupled with Earth’s magnetic field lines, making the field lines resonate at the frequency of Pc5 waves, which are able to create killer electrons.
Earth’s magnetosphere is a very large, complex and variable system. This makes the understanding of ULF waves, together with the mechanisms for the energy transfer from space to ground, a very difficult matter.
These new results on ULF waves and killer electrons once again highlight the need for simultaneous observations from space and ground. Only with constant monitoring with ground-based instruments data obtained in space can we put into a global context.
Mars Odyssey spacecraft has discovered entrances to seven possible caves on the slopes of a Martian volcano. The find is fueling interest in potential underground habitats and sparking searches for caverns elsewhere on the Red Planet.
Very dark, nearly circular features ranging in diameter from about 100 to 250 meters (328 to 820 feet) puzzled researchers who found them in images taken by NASA's Mars Odyssey and Mars Global Surveyor orbiters. Using Mars Odyssey's infrared camera to check the daytime and nighttime temperatures of the circles, scientists concluded that they could be windows into underground spaces.
Evidence that the holes may be openings to cavernous spaces comes from the temperature differences detected from infrared images taken in the afternoon and in the pre-dawn morning. From day to night, temperatures of the holes change only about one-third as much as the change in temperature of surrounding ground surface.
They are cooler than the surrounding surface in the day and warmer at night. Their thermal behavior is not as steady as large caves on Earth that often maintain a fairly constant temperature, but it is consistent with these being deep holes in the ground.
Whether these are just deep vertical shafts or openings into spacious caverns, they are entries to the subsurface of Mars. Somewhere on Mars, caves might provide a protected niche for past or current life, or shelter for humans in the future.
The holes "Seven Sisters," are at some of the highest altitudes on the planet, on a volcano named Arsia Mons near Mars' tallest mountain.
These are at such extreme altitude, they are poor candidates either for use as human habitation or for having microbial life. Even if life has ever existed on Mars, it may not have migrated to this height.
The observations have prompted researchers using Mars Odyssey and NASA's newer Mars Reconnaissance Orbiter to examine the Seven Sisters. The goal is to find other openings to underground spaces at lower elevations that are more accessible to future missions to Mars.
The key to finding these was looking for temperature anomalies at night -- warm spots. That instrument produced both visible-light and infrared images researchers used for examining the possible caves.