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

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 / 2u_o ) 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. http://brane-space.blogspot.com/2011/04/getting-handle-on-alfven-waves-1.html

 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.

SO What Happened to Comet ISON? ANS. It's Still Too Close to the Sun

 In my Nov 1st blog post I noted that Comet ISON (an acronym for the International Scientific Optical Network in Russia, which discovered it in September 2012) was not yet visible. Earlier, I invoked temporary forecasts to say it ought to be brilliant by the dawn of Nov. 28, today. Well that didn't happen either. Why? The reason is that the comet is actually grazing the Sun's corona today - at a distance of about 730,000 miles (1.2 million kilometers). 


Nor is there any guarantee it will survive this close passage because at such proximity it will be subject to the Sun's heat and gravitational forces, the combination of which could see it disintegrate.  This is the fate forecast by Carey M. Lisse, a senior research scientist at the Johns Hopkins Applied Physics Laboratory during a press conference on Tuesday.

 Readers may recall in my October 19 blog post I cited an ASTRONOMY magazine article ('Comet Ison Blazes Into Glory', November, p. 54):

"Optimists tout numbers that have  many amateur astronomers giddy with excitement. If they're right the comet could rival the full Moon when it passes closest to the Sun at perihelion November 28 and remain as bright as the planet Jupiter for several days around then."

In retrospect and as I will show below, "bright as the full Moon" at perihelion-  was not likely to be a reality after later observations. However, the comet did undergo some brightening as noted at the space.com website: 


"From Nov. 13 to 21, Comet ISON brightened by 3.5 magnitudes on the scale used to determine the brightness of objects in space. That's a 25-fold increase in brightness to the observer! Between Nov. 19 and 21 alone, ISON more than doubled in brightness"


For those who want to observe the comet, the new advice from many websites is to be watching just before sunrise or just after sunset for the 12-14 days after its solar perihelion passage today. I likely won't bother to dig out my binoculars until at least Tuesday or Wednesday. I suspect earlier than that will not likely allow a decent sighting since ISON is still too close to the Sun.

Why the scientific interest in ISON? Mainly because it's an Oort Cloud comet, formed a few million years ago, probably by the gravitational nudge of a passing star. That 'nudge' sent it hurtling toward the inner solar system for the first time and quite possibly the last.   Scientists have never before been able to watch an Oort Cloud comet dive past the Sun.

In all probability - from the data we now have- the early ASTRONOMY estimates of a 'full Moon' brightness at perihelion was overblown.  As one report noted yesterday, scientists at first thought ISON was several miles in diameter and would become the “Comet of the Century,” "rivaling the brightness of a full moon."   But as the comet passed Mars, the NASA orbiter took pictures showing that it was three-quarters of a mile wide at most, smaller than most comets.   In this case, the comet wouldn't reach the brightness forecast, i.e. in ASTRONOMY.


Michelle Thaller, a NASA astrophysicist appearing this morning on MSNBC, gave an exuberant depiction of the comet from an "optimist's perspective". So she is hopeful it will survive it's close passage.  She compared it to Comet Lovejoy, which also made a close solar pass, and noted that it survived its passage. She said ISON is "quite a bit larger than Lovejoy" was - thereby implying a much better chance at survival. However, with a puckish look on her face he wryly noted "that if it breaks up it's in some ways better for NASA because we'll get to see what's inside".  She added we will have clues to what happened in our early solar system from 4 1/2 billion years ago.  Heady stuff indeed, but I suspect most ordinary folk will be hoping that break up doesn't happen!


As to the observing basics, and assuming it survives, Thaller noted it will be visible just before dawn and very near the Sun. As December progresses, she noted, it will be displaced farther and farther away from the Sun- until by Dec. 17 it will be in the East-southeast sky and also be visible at night. She said that "by the time you get to late December it should be roughly where the Big Dipper is in the sky. So assuming it survives you might be in for a very nice show in the sky".


One other remark of Thaller's is somewhat suspect, when she claimed a comet like ISON could be a "very nice probe of the Sun" adding that when such a comet gets very close it "interacts with the Sun's magnetic field, with the solar wind...so in some ways we're doing an experiment".

However, as a recent article in Physics Today ('Comets as Solar Probes', October, p. 27) has pointed out, ISON's perihelion distance of 1.2 million km puts it much farther out in the Sun's 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 than by collisions and Lorentz forces, i.e. F = q(E + v X B). It is the latter which is truly useful in assessing key parameters for solar MHD or magneto-hydrodynamics.  However, as the the article notes, ISON will allow us to probe a region in the "nascent solar wind". Not as terrific as the denser corona, but not to be sneezed at.

We will wait to see what happens after today!