12 aprile 2010

Aurore boreali ed elettroni killer, il sole si risveglia

Oggi Repubblica riportava la notizia della "scoperta" delle particelle che scatenano le aurore boreali. Ho ricostruito la fonte: era un'Ansa relativa al meeting della Royal Astronomical Society, dove è stato presentato uno studio di Colin Forsyth sui dati raccolti con i satelliti della missione Cluster, in particolare l'esperimento Plasma Electron And Currents Experiment (PEACE). Secondo Forsyth le aurore boreali e australi vengono "accese" dall'interazione tra gli strati più alti della nostra atmosfera e gli elettroni delle fasce esterne che vengono tenute insieme dal campo magnetico planetario.

Cluster takes first look at acceleration processes driving aurora

Scientists from University College London (UCL) have made the first direct observations of charged particles that lead to some of the brightest aurora using the Cluster spacecraft. Dr Colin Forsyth will present the results at the RAS National Astronomy Meeting (NAM2010) in Glasgow on Monday 12th April.
The aurora, or northern and southern lights, are caused by highly energetic charged particles, normally held in space by Earth’s magnetic field, colliding with Earth’s upper atmosphere. As these high-energy particles collide with molecules in the atmosphere they lose energy, causing the atmospheric molecules to glow and heating the atmosphere. The result of is spectacular displays of shimmering curtains of red, green and blue light normally seen above the polar regions, but occasionally seen as far south as northern England.
Despite their frequent occurrence, there are still many questions regarding the physical processes behind the aurora. The particles that excite the aurora are accelerated up to high energies in a region extending to around 50 000 km (31 000 miles) above the atmosphere. By understanding the accelerating processes in this region, scientists hope to further understand the aurora.
Launched in 2000, the joint European Space Agency (ESA) and NASA Cluster mission consists of four identical spacecraft flying in a close formation around the Earth. Each spacecraft carries a suite of instruments to study the charged particles and electromagnetic fields in the space environment around the Earth known as the magnetosphere. The multi-point perspective of the Cluster spacecraft allows scientists build up a 3D picture of the magnetosphere.
Dr. Colin Forsyth has been leading an international team hoping to directly measure the acceleration of charged particles above the aurora. At NAM2010, Dr. Forsyth will present data from the Plasma Electron And Currents Experiment (PEACE), built by UCL’s Mullard Space Science Laboratory, showing this acceleration in action.
“The Cluster spacecraft have been manoeuvred such that one of them was at a higher altitude than the others when they passed over the auroral regions” said Dr. Forsyth. “We were then able to simultaneously measure the particle energies at different heights and thus their acceleration. These exciting new results will give us new insight into the accelerating processes and the transfer of energy from the magnetosphere into the atmosphere”.
These new observations are the first step in understanding the processes behind the aurora and its impact on the atmosphere. Dr. Forsyth and his team aim to link these and similar observations to observations of large-scale processes in the magnetosphere and detected on the ground in the auroral regions. This could be a key factor in understanding how energy from the magnetosphere affects Earth’s atmosphere.

In realtà Forsyth aveva presentato i suoi studi anche in occasione della conferenza PEACE nel marzo scorso e sul sito della missione Cluster si trova anche, per quel periodo, un interessante comunicato relativo a uno studio molto simile volto a identificare i meccanismi di accelerazione dei cosiddetti "elettroni killer": elettroni altamente energetici prodotti nella cintura esterna, la fascia di Van Allen (tenuta insieme dal campo magnetico del nostro pianeta). Questi elettroni sono detti killer perché riescono a perforare le schermature dei satelliti e a provocare minuscole scariche, veri e propri fulmini in scala ridotta, che possono danneggiare l'elettronica di bordo. I dati raccolti da Cluster e analoghe sonde orbitali hanno permettono di identificare il fenomeno delle "scosse" interplanetarie provocate dalle eiezioni di massa coronale come quella che si è verificata domenica 11 aprile. La massa percuote l'involucro rappresentato dalle linee del campo magnetico terrestre generando onde magnetiche a frequenze VLF e ULF. I ritmici colpi di frusta, specie quelli a frequenze ULF, finiscono per indurre negli elettroni della Fascia di Van Allen una forte e rapida - su scale di soli 15 minuti - accelerazione.
Conoscere questi dettagli potrà aiutarci in futuro a prendere le contromisure necessarie per proteggere sonde e astronauti in orbita, ma chissà che non ci serva anche per affinare la nostra capacità di prevedere le condizioni radiopropagative.

Shocking recipe for 'killer electrons'
11 Mar 2010

Interplanetary shocks can create "killer electrons" in the near-Earth space environment within 15 minutes of the shock reaching the Earth's protective magnetic bubble. The underlying mechanism for this process has now been revealed as a result of a rare configuration of satellites, including Cluster, SOHO and Double Star.
For decades we have known that our near-Earth space environment is intimately linked to the Sun's activity. However, models of this relationship are still not accurate enough to predict - in detail - the impact on Earth of violent explosions (known as coronal mass ejections) on the Sun. In particular, it is not yet possible to determine where and to which extent a specific region of near-Earth space might be harmful for a spacecraft or perturb sat-nav signals.
This situation is rapidly improving. Thanks to an armada of scientific spacecraft, we live in a period of unprecedented opportunity for remote and in situ observations of the Sun and the near-Earth space environment. A recent study, led by Qiugang Zong from Peking University (China) and University of Massachusetts Lowell (USA), has investigated the relationship between interplanetary shocks, triggered by coronal mass ejections (CME), and so-called "killer-electrons", and uncovered the underlying mechanism.
"Killer electrons" are highly energetic particles trapped in the Earth's outer radiation belt. Their name derives from the fact that, due to their energy, they can penetrate the thick shielding of satellites and cause microscopic lightning strikes which damage and sometimes destroy vital onboard electronic components.
Theories show that several physical processes can accelerate electrons to these harmful energies; the predominant processes are interaction with waves either in the Very Low Frequency (3 to 30 kHz) domain or in the Ultra Low Frequency (between 0.001 to 1 Hz) domain. Up until recently it has been unclear which process is predominantly at work in the Earth's radiation belts after the impact of an interplanetary shock.
On 7 November 2004, a strong interplanetary shock impacted upon the magnetosphere, the Earth's magnetic bubble. The speed and the orientation of the wave front induced by this shock were determined using measurements obtained by instruments on the Cluster and Double Star satellites, along with other satellites widely spread across the magnetosphere. At geostationary altitude, the magnetosphere extends over roughly 84,000 km. Thus, having nine scientific satellites (four Cluster spacecraft, two Double Star spacecraft, NOAA GOES-10 and GOES-12, and the NASA Polar spacecraft) distributed over this large area of space during the impact of an interplanetary shock makes it a rare event to study.
"While the constant flow of solar wind particles propagates at an average speed of 500 km/s, the wave front propagation speed was more than 1200 km/s at geostationary orbit (36,000 km altitude) compared to 660 km/s in the plasmasphere", says Qiugang Zong lead author of the paper describing this result.
For this event, the amount of energetic electrons in the outer radiation belt started to increase almost immediately after the shock arrival. This substantial rise of "killer electrons" is found to be caused by a two-step process: The initial acceleration is due to the strong shock-related magnetic field compression. Immediately after the impact of the interplanetary shock, its passage across the magnetosphere triggered the Earth's magnetic lines to wobble at Ultra Low Frequencies (ULF). In turn, these ULF waves were found to effectively accelerate seed electrons, provided by the first step, to become "killer electrons".
"Both VLF and ULF waves accelerate electrons in the Earth’s radiation belts, but with different time scales. The ULF waves are much faster to do that than the VLF, due to their much larger amplitudes. They can explain the short time interval between shock impact and electrons being accelerated up to harmful energies", says Zong. "Data from the four Cluster satellites allowed the identification of ULF waves able to accelerate electrons", says Malcolm Dunlop, Rutherford Appleton Laboratory, Didcot (UK), and co-author of this study. "The Cluster constellation was also key to estimate the time needed for seed electrons to become ‘killer electrons’, after only 15 minutes!" added Zong.
"These new findings can help us to improve the models predicting the radiation environment in which satellites and astronauts operate. With solar activity now ramping up, we expect more of these shocks to impact our magnetosphere over the months and years to come", says Philippe Escoubet, Cluster project scientist at the European Space Agency. "Fortunately", he added, "even after almost 10 years in operation, the Cluster satellites are in excellent condition and can continue to quantify these effects".

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