Tempeste solari: tutta la meteorologia in una app

Ora che l'attività solare è tornata a fare notizia con i brillamenti e le eiezioni di massa coronale che caratterizzano la rampa in salita del ciclo 24, è decisamente il tempo di attrezzarsi al meglio per usufruire di tutte le informazioni che Internet riserva all'appassionato di "meteorologia spaziale". Parecchi siti istituzionali e hobbystici contengono una marea di dati generati dalle osservazioni al suolo e soprattutto da quelle orbitali. I lettori di questo blog cononsceranno le risorse messe per esempio a disposizione dalla NASA, dalla NOAA americana, dal belga Solar Influences Data Analysis Center, dall'australiano IPS Ionospheric Prediction Service, dai radioamatori di SolarHam, il progetto congiunto ESA/NASA, Helioviewer e anche dagli italiani del CNR con il sito di previsione ionosferica GIFINT. Ma in epoca di smartphone queste stesse informazioni cominciano a essere accessibili anche sottoforma di app.

Io per esempio conoscevo Sun Viewer (oggi "NASA Space Weather Media Viewer") e 3D Sun, che visualizza in tempo reale i dati provenienti dalle sonde in orbita eliocentrica STEREO. Francesco Clemente, che ringrazio, mi ha segnalato la recente uscita NASA SWx, una potente applicazione iPhone che permette di accedere a una vasta quantità di grafici osservativi e predittivi riferiti a sole, eliosfera, magnetosfera, ionosfera e superficie planetaria. Il programma viene descritto in dettaglio in questa recensione apparsa sul sito dell'Istituto di astrofisica di Trieste, l'INAF. L'applicazione è disponibile per iPhone e per Android. Girellando per l'iTunes Store ho trovato anche una app, SWx Monitor, del coreano Space Environment Lab, più condensata nelle sue informazioni ma altrettanto utile. Il difetto di tutte queste app, che hanno il grosso vantaggio di essere tutte gratuite, è quello di dare praticamente per scontato tutta la teoria che c'è dietro la meteorologia spaziale, la fisica delle macchie, del magnetismo e del vento solari, l'interazione tra il vento solare e la magnetosfera terrestre, le dinamiche della ionosfera. Una teoria che da sola non potremmo condensare nemmeno in tre corsi di laurea, ma merita di essere divulgata. Fa eccezione 3D Sun, che visualizza anche qualche notizia e qualche spiegazione. Per fortuna su Web si trovano parecchie risorse, come lo Space Weather Primer preparato dalla NOAA, lo Space Weather Center della NASA, l'eccellente sezione Education dell'IPS australiano. Restano i collegamenti tra la meteorologia spaziale e la propagazione delle onde radio, materia studiata dai radioamatori e dai DXer. Ma il discorso sarebbe molto lungo e per nulla conclusivo.

Le forme del vento (solare) e altri Camps (In)visibles

Da un autorevole blog scientifico Renato Bruni segnala un post relativo a uno strano lavoro di videoart basato sull'elaborazione audiovisuale dei dati raccolti dalla rete di magnetometri canadese CARISMA della Università dell'Alberta. L'opera è stata realizzata da una coppia di artisti, Ruth Jarman e Joe Gerhardt meglio conosciuti come Semiconductorfilms. Il loro lavoro si chiama "20 Hz" ed è stato ottenuto rielaborando i file che CARISMA archivia da anni sul sito del Canadian Space Science Data Portal per mostrare, letteralmente, gli effetti del vento solare durante una tempesta magnetica, un insieme di suoni e forme geometriche che ricordano un caleidoscopio e ci appaiono misteriose e inquietanti.

Un esoterico laboratorio di ricerca che effettua le sue osservazioni nello spettro delle radiofrequenze ULF diventa così uno dei protagonisti di Camps Invisibles, una esposizione che si concluderà il prossimo 4 marzo e attualmente in corso presso la galleria di arte moderna Arts Santamonica di Barcellona (se vi sentite più a vostro agio con l'inglese piuttosto che con il catalano, ecco il link a Invisible Fields). La mostra vuole esplorare in chiave artistica le conseguenze dei nostri studi sulle radioonde nella comprensione del mondo e dell'universo.

Missione THEMIS, ecco come funziona lo "spaziomoto"

I satelliti della missione THEMIS aiutano gli scienziati a spiegare i meccanismi di un nuovo tipo di evento "magneto-sismico" all'interfaccia tra plasma solare e campo magnetico terrestre. L'analogia con i terremoti induce i geofisici a parlare di spaziomoto, con onde di shock che si propagano dalla coda della magnetosfera rimbalzano tra linee di campo, come una palla o la crosta terrestre in caso di terremoti. Il fenomeno secondo gli scienziati potrebbe scatenare aurore boreali e tempeste di ionizzazione che a loro volta andrebbero a interferire con le comunicazioni e sistemi GPS.

Spacequakes Rumble Near Earth

Rumbles without sound

Auroras rain down

Magnetic fields shake

Beware the spacequake

July 27, 2010: Researchers using NASA's fleet of five THEMIS spacecraft have discovered a form of space weather that packs the punch of an earthquake and plays a key role in sparking bright Northern Lights. They call it "the spacequake."

A spacequake in action. Click to launch a computer-simulated movie created by Walt Feimer of Goddard's Scientific Visualization Lab.

A spacequake is a temblor in Earth's magnetic field. It is felt most strongly in Earth orbit, but is not exclusive to space. The effects can reach all the way down to the surface of Earth itself.

"Magnetic reverberations have been detected at ground stations all around the globe, much like seismic detectors measure a large earthquake," says THEMIS principal investigator Vassilis Angelopoulos of UCLA.

It's an apt analogy because "the total energy in a spacequake can rival that of a magnitude 5 or 6 earthquake," according to Evgeny Panov of the Space Research Institute in Austria. Panov is first author of a paper reporting the results in the April 2010 issue of Geophysical Research Letters (GRL).

In 2007, THEMIS discovered the precursors of spacequakes. The action begins in Earth's magnetic tail, which is stretched out like a windsock by the million mph solar wind. Sometimes the tail can become so stretched and tension-filled, it snaps back like an over-torqued rubber band. Solar wind plasma trapped in the tail hurtles toward Earth. On more than one occasion, the five THEMIS spacecraft were in the line of fire when these "plasma jets" swept by. Clearly, the jets were going to hit Earth. But what would happen then? The fleet moved closer to the planet to find out.

"Now we know," says THEMIS project scientist David Sibeck of the Goddard Space Flight Center. "Plasma jets trigger spacequakes."

According to THEMIS, the jets crash into the geomagnetic field some 30,000 km above Earth's equator. The impact sets off a rebounding process, in which the incoming plasma actually bounces up and down on the reverberating magnetic field. Researchers call it "repetitive flow rebuffing." It's akin to a tennis ball bouncing up and down on a carpeted floor. The first bounce is a big one, followed by bounces of decreasing amplitude as energy is dissipated in the carpet.

"We've long suspected that something like this was happening," says Sibeck. "By observing the process in situ, however, THEMIS has discovered something new and surprising."

The surprise is plasma vortices, huge whirls of magnetized gas as wide as Earth itself, spinning on the verge of the quaking magnetic field. "When plasma jets hit the inner magnetosphere, vortices with opposite sense of rotation appear and reappear on either side of the plasma jet," explains Rumi Nakamura of the Space Research Institute in Austria, a co-author of the study. "We believe the vortices can generate substantial electrical currents in the near-Earth environment."

Acting together, vortices and spacequakes could have a noticeable effect on Earth. The tails of vortices may funnel particles into Earth's atmosphere, sparking auroras and making waves of ionization that disturb radio communications and GPS. By tugging on surface magnetic fields, spacequakes generate currents in the very ground we walk on. Ground current surges can have profound consequences, in extreme cases bringing down power grids over a wide area.

After THEMIS discovered the jets and quakes, Joachim Birn of the Los Alamos National Lab in New Mexico conducted a computer simulation of the rebounding process. Lo and behold, vortices appeared in good accord with THEMIS measurements. Moreover, the simulations suggest that the rebounding process can be seen from Earth's surface in the form of ripples and whirls in auroral displays. Ground stations report just such a phenomenon.

"It's a complicated process, but it all fits together," says Sibeck.

The work isn't finished. "We still have a lot to learn," he adds. "How big can spacequakes become? How many vortices can swirl around Earth at once--and how do they interact with one another?"

Stay tuned for answers from THEMIS.

Vortices swirl

plasma a'twirl

Richter predicts

a magnitude six

ESA, la missione Cluster insegue gli ioni in fuga

Ignoravo totalmente il fenomeno descritto in questo articolo dedicato ai primi risultati della missione delle sonde Cluster. L'ESA spiega che in corrispondenza dei poli l'alta atmosfera terrestre "perde" un piccolo flusso di ioni di ossigeno e atri gas, che seguendo le linee di flusso magnetico vengono prima intrappolati nella magnetosfera e in seguito rientrano in parte nelle quote superiori dell'atmosfera. In base a un meccanismo ancora non del tutto spiegato questi ioni ricevono una forte accelerazione e al loro rientro, fortemente energizzati, rischiano di provocare danni ai satelliti e alle missioni spaziali. Dalla missione Cluster ci si aspetta appunto un chiarimento, grazie alla distribuzione spaziale delle sonde che consente di analizzare gli eventi in una prospettiva diversa. Chissà che i flussi di particelle rientranti non contribuiscano in qualche modo ad alimentare i meccanismi delle aurore polari e ad altri fattori che influiscono sulla propagazione dei segnali radio.

ESA Science & Technology
28-Aug-2008

Cluster examines Earth-escaping ions

27 Aug 2008
Oxygen ions are constantly escaping from the Earth's upper atmosphere through the magnetic polar caps. This has long been observed but the underlying mechanism has eluded scientists - until now. A recent study reports the first experimental assessment of a physical mechanism that makes a significant contribution to the acceleration of oxygen ions towards the centre of the terrestrial magnetic tail, along magnetic field lines. This assessment was made possible as a result of several years of data collected by the European Space Agency's Cluster mission.

Figure 1 Artist’s impression of the magnetosphere of an exoplanet (NASA/ESA)

Figure 2 Illustration of ions flowing out from the polar cap towards the magnetotail (NASA/ESA)

Prior to the space age, scientists believed that the magnetic environment of Earth was mainly filled with particles of solar origin forming an immense comet-like cloud of electrified gas surrounding our planet (see Figure 1).
Later, observations with scientific satellites surprisingly revealed that oxygen, helium and hydrogen ions from the Earth's upper atmosphere leaked into space from regions near the Earth's poles, (see Figure 2).
This is no cause for concern. This leakage is tiny compared to the volume of the Earth's atmosphere, but it has changed our vision of the composition of the Earth's environment. The magnetosphere is now known to be partially filled by a fountain of energized gas blowing from each of Earth's poles (see, for example, Huddleston et al., 2005).
Satellites in the 1980s and 1990s revealed an intriguing fact about these escaping particles: the higher in altitude they are observed, the more energy or velocity they have. Some of these accelerated particles are lost in space but the rest of them accumulate in the center of the magnetic tail, or plasmasheet, and eventually return back to Earth in a kind of boomerang effect. Moreover, this input in terms of density increases significantly during periods of geomagnetic disturbance as reported by Lennartsson and Shelley (1986) who found that the average density of oxygen ions can increase by almost an order of magnitude with respect to quiet times in the plasma sheet at a distance of between 10 to 23 Earth radii (1 Earth radius or RE = 6378 km).
In other words, low-energy particles from the upper atmosphere escape, are accelerated on the way, provide a significant input of matter to the plasma sheet from where some of them return back to Earth with high-energy, thus becoming a hazard for satellites and astronauts. But how can they be accelerated and escape the Earth's gravity field?
Several possible acceleration mechanisms have been proposed by space scientists but determining which one is dominant, if any, is still the subject of hot debate. In spite of decades of measurements in space, this topic remains an open question due to the fact that it is experimentally challenging to assess quantitatively the role of any acceleration process. For instance, the distances involved are considerable (tens of thousands of kilometres). Moreover, this phenomenon has to be understood in terms of spatial and temporal evolution. Thus, a quantitative assessment requires several years of data collection and, ideally, data from a constellation of satellites since assumptions have to be made when interpreting data gathered by a single scientific satellite.
A recent study reports the first statistical investigation of outflowing oxygen ion beams collected over a three year period (2001 to 2003) by the four Cluster satellites flying in constellation at between 5 RE and 12 RE over the polar regions. For the first time, the role of a specific acceleration process for such outflowing oxygen ions, called centrifugal acceleration (see Cladis, 1986), is quantitatively assessed in the polar cap at high altitude without any assumptions related to observations by a single satellite.

Figure 3 Distribution of the observed parallel velocities (blue bars) and the parallel velocity resulting from the cumulative centrifugal acceleration observed at all altitudes below each observation point (red bars).(Credit: Nilsson et al., 2008)

"Four spacecraft was exactly what we needed to fully estimate all terms of a textbook equation describing the centrifugal acceleration mechanism, a well known physical process to accelerate outflowing oxygen ions along magnetic field lines towards the plasma sheet", comments Dr. Hans Nilsson, lead author of this study.
Centrifugal acceleration can be regarded as a sling-shot effect that occurs when ions are moving across magnetic field lines, whose shape changes in the presence of an electric field. When ions move over a polar cap of Earth, the changing shape of the magnetic field lines bends the drift path induced by the electric field normal to the magnetic field. The centrifugal acceleration associated with this bending shoots the ions away from the Earth along the magnetic field lines. This effect provides more energy to heavy ions like oxygen compared to the lighter helium ions and protons. It is the changing shape of the field-lines that can be resolved by the use of the four Cluster spacecraft.
"And it turns out that the role of the centrifugal acceleration mechanism is significant and may explain a large fraction of the parallel velocities observed at high altitude above the polar caps", reports Nilsson in his paper published in the 4 February 2008 issue of Annales Geophysicae.
"This scientific discovery is an excellent example of what can be done with the four satellites of the Cluster mission", says Philippe Escoubet, Cluster project scientist at the European Space Agency.

Reference publication

Nilsson, H., et al., "An assessment of the role of the centrifugal acceleration mechanism in high altitude polar cap oxygen ion outflow", Ann. Geophys., 26, 145-157, 2008.

Related publications

Huddleston, M. M., et al., "An examination of the process and magnitude of ionospheric plasma supply to the magnetosphere", J. Geophys. Res., 110, A12202, 2005, DOI:10.1029/2004JA010401.
Cladis, J. B., "Parallel acceleration and transport of ions from polar ionosphere to plasma sheet", Geophys. Res. Lett., 13, 893-896, 1986.
Lennartsson, W., & Shelley, E. G., "Survey of 0.1- to 16- keV/e plasma sheet ion composition", J. Geophys. Res., 91, 3061, 1986.

Le aurore, battito cardiaco della magnetosfera

Un nuovo importante studio, effettuato da Applied Physics Lab della Johns Hopkins University e da Aeronautica Militare USA su una mole di dati raccolti da satelliti NASA e da altre agenzie, conferma che fenomeni come l'aurora boreale e altre turbolenze magnetiche non sono, come si credeva, correlati ai forti campi elettrici del vento solare e quindi ai brillamenti nell'atmosfera del sole. La correlazione più certa sembra essere quella, magnetica, ai confini tra i campi magnetici solare e terrestre. E' ciò che i ricercatori chiamano "battito cardiaco della magnetosfera": il tasso di "connessione" tra le linee di campo magnetico del sole e della terra (Radiopassioni aveva trattato questo stesso argomento qualche tempo fa parlando di uno studio molto simile).

Il testo del comunicato stampa rilasciato per l'occasione è interessante, ma ancora più dettagliato è il materiale diffuso per la presentazione della ricerca davanti alla American Geophysical Union a San Francisco, lo scorso 11 dicembre. Gli autori dello studio sottolineano di aver ricavato una singola formula che sulla base delle osservazioni magnetiche a circa 40 mila chilometri di quota (all'interfaccia tra i due campi magnetici) fornisce accurate previsioni sull'intensità delle aurore e altri dieci parametri significativi.

New findings indicate that the aurora and other near-Earth space weather are driven by the rate at which the Earth’s and Sun’s magnetic fields connect, or merge, and not by the solar wind’s electric field as was previously assumed. The merging occurs at a spot between the Earth and Sun, roughly 40,000 miles above the planet’s surface, and appears fundamental to the circulation of particles and magnetic fields throughout near-Earth space.
Researchers at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and the Air Force Research Laboratory at Hanscom Air Force Base (Massachusetts) will announce the results of their study at the Fall American Geophysical Union Meeting in San Francisco on Dec. 11. The researchers, led by Patrick Newell of APL, have developed a formula that describes the merging rate of the magnetic field lines and predicts 10 different types of near-Earth space weather activity, such as the aurora and magnetic disturbances.
“Having this formula is a big step forward for understanding how the Sun and Earth interact,” says Newell. And that understanding could help predict the space weather that affects communications, navigation, and the health of humans in space.
The space between the Earth and Sun is not empty, but filled with energetic particles, most of which are generated in the solar atmosphere. Temperatures of a few million degrees accelerate a stream of these particles, called the solar wind, to roughly one million miles per hour. Space weather scientists had long assumed that near-Earth space weather phenomena could best be predicted by the behavior of the solar wind electric field. However, Newell and his colleagues were the first to put this theory to a rigorous test with many data sets from a number of years.

Tags: radioascolto, radiofonia, radio, dxing.