La missione Cluster "ascolta" la stazione radio aurorale

All'acuto sguardo di Piero Bianucci sulla Stampa non sono sfuggite due notizie dell'ESA sulle missioni Cluster e SOHO. Partiamo dal secondo caso, simpatico ma un po' marginale per i nostri interessi. La missione SOHO, un osservatorio orbitale dedicato al geomagnetismo e all'attività solare, ha stabilito un record con ben 1.500 nuove comete registrate. Il fatto di inquadrare spesso il nostro astro lo pone in una posizione avvantaggiata rispetto agli osservatori terrestri, che tuttavia sono i veri fautori di questa caccia al pezzo di ghiaccio con la coda: sono loro che esaminando le immagini trasmesse da SOHO ogni 15 minuti si accorgono di quei puntini in movimento.
Le notizie che arrivano da Cluster sono invece molto più pertinenti ai nostri interessi propagativi, anche se a ben considerare le ultime scoperte torneranno utili a chi i segnali radio li vuole ascoltare dai pianeti lontani. La missione costitituita da quattro satelliti disposti in un particolare array pensato per conferire una cornice referenziale alle osservazioni radio, ha appurato il funzionamento della Auroral Kilometric Radiation, o radiazione aurorale nelle bande chilometriche, una particolare emissione radio originata dal nostro pianeta nel cono ionosferico sopra i poli, la regione in cui si producono le aurore. E' un emissione formata da una miriade di fonti che "sparano" in una zona dello spettro compresa tra i 50 e i 500 kHz. Cluster ha consentito di stabilire un nuovo modello per la modalità e la polarizzazione del segnale, che invece di essere banalmente a forma di cono svuotato come proposto una trentina d'anni fa, risponderebbero a una teoria più recente più simile a quanto riscontrato nelle magnetosfere di altri pianeti e cioè a un segnale orientato su un piano tangente all'area di ionizzazione da cui parte l'emissione. Questo può aiutare molto i radioastronomi impegnati nella ricerca sulle magnetosfere non terrestri e su eventuali segnali di vita intelligente. Il sito dell'ESA riporta alcuni file ottenuti convertendo in frequenze audio il variabile spettro del segnale AKR misurato con lo strumento WBD appositamente costruito dall'Università dello Iowa, un ricevitore di campi elettrici e magnetici a frequenza comprese tra 25 Hz e 577 kHz (in pratica un ricevitore spaziale di radiofari e frequenze naturali). Davvero eccezionale.

Beamed radio emission from Earth

27 Jun 2008

A recent study reveals how the most powerful emission of terrestrial origin, the Auroral Kilometric Radiation (AKR), is beamed into space. This result was obtained using data collected by the four satellites of the Cluster mission. These new data were found to be inconsistent with two leading AKR beaming theories developed 30 years ago, but support a more recent one. The result has important implications for radio studies of planetary magnetospheres, including radio searches for exo-planets.

Seen from deep space, the Earth is a powerful planetary radio source, comparable to Jupiter, with maximum output power in the 50-500 kHz range. At such frequencies, the dominant emission is Auroral Kilometric Radiation (AKR), a natural electromagnetic wave generated in the auroral zones. (An auroral zone is an annular ring around each magnetic pole where auroras are most likely to occur, see Figure 1.)
AKRs are intimately linked with auroras, or more precisely with the energetic electron beams that cause the auroras. (As a reminder: it is the precipitation of such beams and their interactions with oxygen and nitrogen atoms which illuminate the sky at an altitude of about 100 km.)
Satellite observations since the 1970's have greatly enhanced our knowledge of AKR. In particular, these observations have enabled us to pin down the physical process generating this powerful emission. But even 40 years after its discovery, a fundamental problem remains: in which direction is AKR beamed with respect to the Earth's magnetic field?

Open question: How is AKR beamed in relation to Earth's magnetic field?

The question is actually more complicated than it sounds. As revealed by high-time resolution data, AKR is in fact composed of many elementary radiation sources (follow the link on the right-hand menu to listen to the sound of AKR). So far, scientists have modelled the angular beaming characteristics of AKR in a statistical sense, mainly using data collected by a single satellite. As a result, a few beaming theories have been proposed but the only way to discriminate between them is to directly measure the angular beaming pattern of individual AKR bursts. This is what has been recently achieved thanks to data collected by the ESA/NASA Cluster mission.
The Cluster constellation consists of four satellites, each equipped with the same type of sensitive radio wave receiver - the WideBanD (WBD), built by the University of Iowa, USA, and financed by NASA. By flying in formation above AKR sources, this space fleet provides scientists with a unique opportunity to determine the angular power pattern of individual AKR bursts.
First of all, the WBD system records waves with sufficient time and frequency resolution to isolate emission from individual elementary AKR sources.
In addition, since the Cluster constellation forms a 2-dimensional array in the sky, it can simultaneously sample the burst from different viewpoints.
Finally, by measuring the differential delays between all pairs of spacecraft (6 combinations in total), the locations of individual AKR bursts can also be determined (Mutel et al., 2003). This provides a spatial filter, which isolates radiation from a single region and allows the array to sample individual burst power patterns.

Cluster data compatible with recent model; rules out older ones

In a recent study, Dr. Robert Mutel (University of Iowa) and colleagues report the analysis of more than 12 000 individual AKR bursts collected by the Cluster mission (see Figure 3). "Thanks to Cluster, we could demonstrate that individual AKR bursts do not radiate in the manner described by two models that were proposed some 30 years ago. However, these new data do back up the numerical model developed 10 years ago by Louarn and Le Quéau", says Mutel, lead author of this discovery which is reported in Geophysical Research Letters on 09 April 2008.
"AKR is similar to radio emissions that emanate from other magnetospheres at Jupiter, Saturn, Uranus and Neptune but also certain solar and stellar radio bursts. The radio emission is a result of a plasma instability that frequently develops in astrophysical magnetospheres which can efficiently convert free energy present in energetic particles into radiating electromagnetic waves. Its full comprehension is therefore of crucial importance", wrote Philippe Louarn (CNRS, Toulouse, France) in a recent review on the subject.
The new result clarifies how the radiation is beamed, which is in a narrow plane tangent to the magnetic field at the source, rather than a hollow cone as previously suggested.
"This has important implications for the study of AKR from other planets, stellar magnetospheres, and even the search for exo-planets by radio telescopes", notes Melvyn Goldstein, NASA Cluster project scientist.
"This result is a major scientific breakthrough obtained by the Cluster mission", says Philippe Escoubet, Cluster project scientist at the European Space Agency.

Related publications

Mutel, R. L., Christopher, I. W., & Pickett, J. S., "Cluster multi-spacecraft determination of AKR angular beaming", Geophys. Res. Lett., 35, L07104, 2008, doi: 10.1029/2008GL033377.

Louarn, P., "Generation of auroral kilometric radiation in bounded source regions", Lect. Notes Phys, 687, 55-86, 2006, doi: 10.1007/3-540-33203-0_3

Louarn, P., & Le Quéau, D., "Generation of the auroral kilometric radiation in plasma cavities-II. The cyclotron maser instability in small size sources", Planet. Space Sci., 44, 211– 224, 1996, doi: 10.1016/0032-0633(95)00122-0

Mutel, R. L., Gurnett, D. A., Christopher, I. W., Pickett, J. S., & Schlax, M., "Locations of auroral kilometric radiation bursts inferred from multispacecraft wideband Cluster VLBI observations: 1. Description of technique and initial results", J. Geophys. Res., 108(A11), 1398, 2003, doi: 10.1029/2003JA010011.