Thursday, April 2, 2009

Where will the Sun's magnetic field hit bottom?

At the moment the Sun's magnetic field, "the interplanetary magnetic field," is bottoming out with a long solar minimum, a low point in solar activity between the slow dying of Cycle 23 and the unexpectedly slow start to Cycle 24.

The Sun's relative quiet presents an opportunity to further determine any natural floor, what the "absolute" bottom background might be, to the interplanetary magnetic field, without the background noise of its eleven-year swing between often dramatic and chaotic peak activity.

This may not seem as immediately important to the earthbound as the health of Earth's magnetic field (though Earth's magnetic field's shielding energy is, in some measure, determined by our orbital vector perpendicular through the Sun's particle streams and magnetic field lines) but for machines and humans traveling outside Earth's magnetic field an improved understanding if the interplanetary magnetic field is essential.

Between its peak strength at solar max and its weakest at solar minimum , the in-fall of sometimes very heavy and energetic cosmic rays varies by half. The stronger the interplanetary magnetic field the greater the protection against interstellar cosmic rays, and it is literally a toss up if travel beyond Low Earth Orbit is "safer" for humans and their equipment at solar max or solar minimum.

At solar max the interplanetary magnetic field is strongest, providing a statistically important shield against Galactic Cosmic Rays, but it also increases likelihood travelers will encounter a dangerous "Solar Particle Event."

Solar Flares and attendant Coronal Mass Ejections can, of course, occur at anytime,though they are more likely at solar max. Even during the present protracted solar minimum, flares have been observed, along with coronal holes allowing the hot breath of solar wind, consisting mostly of protons, to gust away from the Sun's photosphere.

It is within today's design and materials technologies and spacecraft design to greatly shield life and equipment from most solar wind. Though rarer, a heavy cosmic ray consisting of a stripped nucleon of primordal metal and traveling near the speed of light might be shattered by its encounter with an aluminum-titanium hull of sufficient thickness but a resulting shower of secondary particles actually would increase the likelihood that an astronaut would receive a wider cone of ionizing radiation.

Generating a magnetic field has been suggested as a way to change the direction of infalling cosmic rays. But the effective size and strength of such a field's strength would need to be more than hundreds of kilometers in radius, to name just one known issue with this solution.

The fact remains that reducing the probability of radiation exposure induced death to below 4 percent, over an individual astronaut's lifetime, for a trip to Mars using presently available speeds is still beyond our capability. Though this will change, reducing the likelihood of being dosed by a wide range of cosmic rays by 50 percent, possible within the interplanetary magnetic field at solar max, is not a small factor in mitigating risk in long periods traveling in deep space.

Living on the Moon immediately provides a shield from half the infall of interstellar radiation, and under 15 meters of lunar regolith doses come down to levels on Earth at sea level.

It may be possible, where it would be unthinkable on Earth, to build interplanetary transport vessels shielded by lunar concrete and propelled on trips to Mars and elsewhere using rail guns and unimpeded by atmospheric drag.

Of a more basic concern to engineers and policy makers, it may prove very unwise to travel to Mars without first building an infrastructure on Earth's Moon. This is especially likely if the Sun's present solar minimum persists.

It may be that the downslope from solar max may be driven by CMEs, carrying away kinks of the Sun's internally twisted magnetic field, up and away from the Sun and temporarily reducing the interplanetary magnetic field by as much as ten percent. A CME may be the way the Sun balances out the tensions in its magnetic field that wind up as portions of the Sun rotate at different speeds.

Because the Sun's magnetic field swaps out polarity between its hemispheres from one cycle to the next, the interplanetary magnetic field changes polarity.

On first glance it may seem finding an absolute bottom, a basic and unchanging level to the interplanetary magnetic field, would be impossible. Scientists at the Russian Academy of Sciences, however, have published an examination of the interplanetary magnetic field between 1976 and 2000. They claim there is no point when the interplanetary magnetic field reaches "zero," and this is backed by observations of the Sun over this past year's lengthy solar minimum.

In 2008-2009, the sun has produced brief outcroppings of sunspots at polarities and latitutdes that clearly mark them as a beginning to the next solar maximum, with its next expected peak in activity now expected in 2013. Over the past summer however, months after Cycle 24 officially started, sunspots from Cycle 23 briefly appeared.

In The floor in the interplanetary magnetic field (Yermolaev, et. al. 2009) predicts a floor to the interplanetary magnetic field at 4.65 ± 6.0 nT.

Today's measurement of the interplanetary magnetic field on was 4.1 nT, and their report also "agrees well" with observations over the past thirty years.

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