16 aprile 2009

Un sole a bassa energia potrebbe essere normale

Ho scoperto grazie a una delle tante mailing list che frequento un'altra interessante fonte di informazione sull'attività solare e sulle teorie legate ai suoi meccanismi più attivi, dai brillamenti (flares) alle eiezioni di massa coronale. Si tratta del wiki curato dal team di ricercatori di RHESSI la missione satellitare lanciata nel 2002 per studiare spettroscopicamente i fenomeni solari ad alta energia.
Leif Svalgaard e Hugh Hudson si chiedono se non sia il caso, per chi studia questi fenomeni, di cominciare a preoccuparsi visto che il prolungato minimo solare ha drasticamente ridotto il numero di brillamenti da un sole caratterizzato da lunghi periodi senza macchie. Non solo, anche il flusso alla lunghezza d'onda di 10,7 centimetri appare in costante calo, segno di un abbassamento di quella che potremmo definire attività corrente del sole. La conclusione, dicono i ricercatori, è che no, non è ancora il caso di spaventarsi, perché abbiamo ampia evidenza storica di un sole ancora meno attivo di così. Semmai sono stati gli ultimi cicli, quelli immediatamente precedenti il numero 24 a essere stati particolarmente agitati. La transizione tra l'ultimi ciclo e l'attuale appena iniziato ricorderebbe condizioni riscontrate poco più di un secolo fa, nel passaggio dal ciclo 13 al 14. Naturalmente i dati raccolti allora erano diversi e non prevedevano misurazioni del flusso a 10,7 centimetri.
L'indirizzo del wiki è questo (per leggere gli articoli cliccare su "nuggets"). La pagina del RHESSI dell'Università di California a Berkeley si aggiunge a quella del Goddard Space Center della NASA dove si trovano molti dati raccolti dalla sonda.

Cycle 24 - don't panic yet!
From RHESSI Wiki

Published: 13 April 2009
Leif Svalgaard and Hugh Hudson

We're in an extended period of minimal solar activity (see Nugget 91 for a previous look at this issue). Without flares, RHESSI is missing its most important observational work, and there has not been even a C-class flare yet this year. Will sunspots and flares ever return? How unusual is this behavior? In this Nugget we conclude that it is too soon to panic, but that certainly we're seeing an interesting diminished level of activity -a level most of us have not seen before.
The 10-cm radio flux from the Sun, with its daily index F10.7, is one of the basic standard tools for gauging the level of solar activity. This index has been generated in an unbroken string since 1947. It is derived from careful radio flux measurements made in Canada and pioneered by the early radio astronomer A.E. Covington.
The figure on the left might give cause for alarm if one is interested in observations of solar flares. It shows (red line) the variation of F10.7 monthly means for the two years prior to the present time. The black line shows the mean of the four previous cycles, registered by summed epoch analysis on simple Gaussian fits to their preceding maxima as references. The four time series from prior maxima from are averaged, using the Gaussian peak time as a reference, month by month; the range bars show the standard deviations of these means for each month. This procedure does not allow for the possibility of different cycle durations, something that seems fairly obvious from the sunspot record, but it is hard to be quantitative about this. Our approach here is to use the most direct approach to analysis of the most objective of the indices, and Figure 1 is the result. If one interprets the range bars as true error bars, and did not know (or believe) that cycles could have different lengths, this figure would provide compelling evidence that Cycle 24 is special, and that it might be time for fans of solar flares to "panic." Our discussion below, however, shows that this would be premature.
A closer look (Figure 2 below) shows the daily values of three indices: F10.7, the total solar irradiance TSI, and the classical sunspot number. There is a clear apperance of an up-turn in the least "noisy" of these indices, F10.7, although there are other small variations that we do not understand well. But probably F10.7 is giving us an early warning about the sudden increase of Cycle 24 spots. These should appear within the next few weeks or months, though, so please check the daily updates of the indices on our Web page.
In Figure 3 we do a comparison of the contributions of old- and new-cycle activity to the minimum periods between Cycles 21/22, 22/23, and 23/24 (the present one). These are counts of 'region days' per month (normalized to 30 days), defined as the number of days when an active region (one with a NOAA number) was visible within 70 degrees of central meridian, and then summed for every region. Different cycles (as determined from the magnetic polarity of the spots) are coded with a different color. Yearly smoothed counts are shown as the 'smoother' curves. The detailed bottom three panels show blown-up views of the transitions between cycles. Note that the 23/24 transition is indeed diffferent, and that Cycle 24 has just barely begun (see also the weak increase in F10.7 visible in Figure 2).
Although this transition may look unusual to us, for the Sun it may just be business as usual. The current transition looks very much like the one between cycles 13 and 14, 107 years ago. Not only were the sunspot numbers (or 'region counts') very similar, but the heliospheric magnetic field back then behaved very similarly to what we observe today, as seen in Figure 4.
In summary it is probably too soon to panic. In the modern era (Figure 1) there is no precedent for such a protracted activity minimum, but there are historical records from a century ago of a similar pattern. We do expect activity to pick up fairly suddenly soon. In the meanwhile this is a good opportunity to use the excellent new data available from many satellites and ground-based observatories without interference from new flux emergence. We can hope to learn a great deal about how low-level activity works in the network and in the polar caps.

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