ESA Finds High-Energy Particles Charged In Polar Cusps
June 8, 2012

ESA Finds High-Energy Particles Charged In Polar Cusps

Data gathered by several European Space Agency (ESA) spacecraft suggests that high-energy particles which fill cavities within the Earth's magnetic field are accelerated locally within the cusps themselves as they cross through areas characterized by different electric potential, the organization announced earlier this week.

In a Tuesday announcement, the ESA said that the information, which was gathered during the Cluster mission in February 2003, shows that the particles are accelerated while drifting along potential gradients that were the result of magnetic reconnection events.

The discovery will play a vital role in the ongoing debate regarding how and where these particles are accelerated, according to the agency.

The cusps of the magnetosphere, which occur above both the North and South Poles in areas where the magnetic field is extremely weak, are the only areas where the magnetosphere does not shield the Earth from the stream of electrically charged particles released by the Sun and known as the solar wind.

"Through the cusps, particles from the solar wind can access the upper layer of Earth's atmosphere -- the ionosphere — and thus can have an impact on activities such as telecommunication networks," the ESA explained. "As the solar wind's direct gateway to Earth's atmosphere, the cusps are a key element in our planet's magnetic environment and have been the target of investigation for decades."

Measurements have demonstrated that the cusps are often densely populated with highly energetic particles, including 40-plus keV electrons and 28-plus keV protons and oxygen ions, they noted. Those observations have led to what the ESA refers to as "an intense debate" regarding specifically how and where they are accelerated to those high energy levels. Theories include the magnetosphere, the bow shock, and the cusps themselves, though prior to the Cluster mission observations, no solid conclusions could be drawn.

"Our analysis of the Cluster data and comparison with simulations provide the first robust evidence in favor of a local acceleration mechanism: particles are being energized within the cusps," study leader Katariina Nykyri from the US-based Embry-Riddle Aeronautical University (ERAU), said in a statement.

She added that the research, which has been published in a special edition of the Journal of Atmospheric and Solar-Terrestrial Physics, involved sifting through the first five year's worth of Cluster data in order to discover "an event during which the spacecraft moved from the magnetosphere into the cusps in a way that best enabled us to study how the cusp's properties vary in space and time."

That event came on Valentine's Day in 2003, when the four Cluster vehicles entered the northern cusp. The scientists noted that there were large separations of nearly 5,000km between each spacecraft, and that the magnetic field shifted from northward to southward, in turn changing the site of magnetic reconnection events from one side of the cusp to the opposite side.

"As they crossed the cusp“¦ the four Cluster spacecraft encountered a cavity whose structure was varying in space and time due to magnetic reconnection. Hence, they repeatedly entered and exited regions of weaker and stronger magnetic field," the ESA said. "During these crossings, the Cluster spacecraft probed the distribution of electrons, protons and oxygen ions. The data revealed that these particles have substantially higher energy within the cavity, where the magnetic field is low, whereas their energy declines farther away from it. Furthermore, the data indicated that most electrons were moving almost perpendicularly to the magnetic field's direction."

"The fact that we observe large amounts of high-energy particles in the cavity and hardly any in the magnetosheath — where the magnetic field is stronger — already suggests that the particles are being energized locally," added Nykyry. "Nevertheless, our smoking-gun argument is the direction of motion of the electrons in the cavity," because if the acceleration had taken place at either the bow shock of the magnetosphere, the electrons would have primarily moved parallel to the magnetic field within the diamagnetic cavity, not perpendicular to it.