Wednesday, December 4, 2013

Could Comet ISON's Demise Lead to Solar Physics Insights?

Space Thanksgiving
ISON  (left) taken on Nov. 25 by NASA's Stereo-A spacecraft as it approaches the Sun.

In the aftermath of the disruption of Comet ISON, the question now becomes whether we can salvage scientific information, especially on the Sun's corona.   Undoubtedly, any break up detected and 'unpacked' over its sling shot bypass will also divulge information on the comet composition itself - but my interest is in the value in terms of solar revelations.

The fact is that the anatomy of coronal solar loops is a key subject of interest, including to solar flare forecasting.   To fix ideas some of the physical-structural details in a typical solar loop are shown below:

We can observe the dynamics of such loops using a variety of space-based instruments, but they are still not fully understood. For example, the central issue of which magnetic field lines loop back on the Sun (as depicted) and can contain the million plus degree plasma, and which are forced open in the heliosphere (the region above the solar surface through which the solar wind extends) is still not resolved. While we can analyze the magnetic topology and field line behavior to an extent we can't explicitly tie them to particular solar active regions. One major reason is because they occur in a domain (the lower corona) from which light is hardly emitted and spacecraft can't venture (they'd be incinerated).

As shown in Fig. 2,  the typical solar loop is anchored in a more dense medium in which convection occurs. To see convection in action, get a transparent (glass) saucepan and fill it halfway with water, then heat it to boiling. Observe laterally through the glass when boiling point is attained. Elements (bubbles)  of water are heated near the flame, then rise on expansion to the top, are cooled, then descend back to the source of heat. This occurs in analogous fashion in the Sun except we are talking about elements of solar plasma.  The overturning plasma and small-scale field bundles (entrained within it) supply non-radiative or 'convective' energy to the solar atmosphere. The dissipation of that energy then heats the outer atmosphere (corona) to 1-3 million degrees Kelvin or some 300 times hotter than the temperature of the solar surface (photosphere) itself.  This outer atmosphere then radiates in the high frequency band of the electro-magnetic spectrum, namely in x-rays and EUV (extreme ultra-violet).

Fortunately, we aren't completely hamstrung because nature has provided us with natural probes: sun-grazing comets.  For example, in July, 2011, the comet designated C /2011 N3 or 'N3' for short, transited the solar corona and its tail lit up in the extreme ultraviolet. N3 passed to within 110,000 km of the solar surface, losing up to 100 tons per second of its mass as it did so. A half year later, comet C/2011 W3 ('Lovejoy') passed to within 135,000 km of the surface.  Most noteworthy, the comet's tail was observed moving in response to forces exerted by both the Sun's magnetic field and its atmosphere.

What is the critical key to a useful sun-grazing comet? Primarily it's the mass. While the deep corona is way too harsh an environment for spacecraft, the large initial masses  of sun-grazer comets often enable them to survive long enough to deduce something about the coronal environment.  In this context, we look at the so-called Lorentz force: F = q(E + v X B)  which acts on ionized cometary material (i.e. with gases that have lost one or more electrons).  Then, the resulting ion motions reveal the local orientation of the coronal magnetic field (which shapes the loops).  This is possible even as the comet's ions decelerate and settle into the coronal plasma.

What physical factors can we use to predict the comet tail's evolution, say as a useful indicator of coronal field lines? One is the ratio of the energy density of the coronal magnetic field,( B2 / 2uo ) to the kinetic energy density of the plasma in the comet's tail (i.e.  0.5 r v2).   For example, in the case of Comet N3, the comet's inertia dominated, so as the cometary plasma decelerated during collisions with the corona, the latter's magnetic field became strongly deformed.  By contrast, in Lovejoy's case, the solar magnetic field - reflected in its energy density - appeared to hold its own. Hence, each comet delivered key information on the behavior of the corona in relation to ambient solar factors.

In addition, information was gleaned from the varying deflections of Lovejoy's tail- showing a highly inhomogeneous medium. Subsequent state of the art computer modeling of the solar corona confirmed the interpretation and revealed a striking consistency between observed tail and the orientation of the magnetic field in the solar corona. In other words, Lovejoy really came through as an excellent solar probe -  and provided critical knowledge for projecting future behavior of solar loops.

But let's not get ahead of ourselves. The ultimate goal is to create or construct a global model of the corona. But for this we need a full, 3D spherical map of the Sun. At the present time (or at least until we can get a solar orbiting space craft) only the field in front of the Sun can be reliably measured and then only to latitudes between 70 degrees north and south heliographic. The Sun's rate of rotation is roughly 27 days meaning that for up to two weeks our observational access is blocked until the given active region comes back into view.

Given, this only about one fourth of the solar surface can be accurately mapped from observations of its magnetic field. In effect, no matter how many 'ideal' comet probes we can make use of, we can't get a genuine handle on the corona until we get a full sphere map of it. Maybe when the "war" in Afghanistan is finally ended, and we bring those troops back home, we can use some of the money saved to launch a solar orbiter.

Despite Lovejoy's short (2 -3 day) survival after passing through the solar corona, it did behave enough as a probe to permit a computer model to confirm certain properties of the corona.  What about Comet ISON?  Unfortunately, in terms of learning more about the corona itself, ISON is of limited value. ISON's perihelion distance of 1.2 million km put it much farther out in the corona than Lovejoy's (135,000 km) passage.  Since plasma density drops rapidly with altitude in the corona  ISON's solar interaction will be more dominated by waves and turbulence. 

In this case, it might be possible for ISON's demise to provide us with some useful information about ion acoustic and Alfven wave behavior (see e.g.

 at the level of the high corona. To check this, we could make use of modeled solar parameters at that altitude, say the coronal pressure, P = 2 n T (where n is the number density and T the temperature), as well as the thermal conductivity: k  = k o  T 5/2   and  thence obtain the coronal heat flux density           q = -k grad T and work these into appropriate wave equations, to check for consistency with computer models.

To what extent this can be done from ISON data remains to be seen, but I am confident we can at least extract something of value to further our understanding of the Sun's corona - at least at the level that ISON transited.

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