* Astronomy

Title: Turbulence in the Solar Corona
Authors: Steven R. Cranmer (Harvard-Smithsonian CfA)

The solar corona has been revealed in the past decade to be a highly dynamic nonequilibrium plasma environment. Both the loop-filled coronal base and the extended acceleration region of the solar wind appear to be strongly turbulent, but direct observational evidence for a cascade of fluctuation energy from large to small scales is lacking. In this paper I will review the observations of wavelike motions in the corona over a wide range of scales, as well as the macroscopic effects of wave-particle interactions such as preferential ion heating. I will also present a summary of recent theoretical modelling efforts that seem to explain the time-steady properties of the corona (and the fast and slow solar wind) in terms of an anisotropic MHD cascade driven by the partial reflection of low-frequency Alfven waves propagating along the superradially expanding solar magnetic field. Complete theoretical models are difficult to construct, though, because many of the proposed physical processes act on a multiplicity of spatial scales (from centimetres to solar radii) with feedback effects not yet well understood. This paper is thus a progress report on various attempts to couple these disparate scales.

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Helium may act as a "throttle" for the solar wind, setting its minimum speed, according to new results with NASA's Wind spacecraft.
The solar wind is a diffuse stream of electrically conducting gas (plasma) constantly blowing from the sun.

"This result gives us another clue about how the solar wind is accelerated, which may help us better understand space weather" - Dr. Justin Kasper of the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology, Cambridge, Mass., lead author of a paper on this research that appeared in the Astrophysical Journal May 1.

When turbulent solar wind hits Earth's magnetic field, it can cause magnetic storms that overload power lines and radiation storms that disrupt spacecraft.
The new research could also lead to a deeper understanding of plasma physics, which is of interest because stars are made of plasma and plasma is used in advanced devices like plasma TVs and experimental fusion reactors.

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-- Edited by Blobrana at 01:24, 2007-05-18

Satellites and space observatories beware! Microscopic protons are shooting out of the Sun at speeds thousands of times faster than bullets. Although these protons cannot be spotted with the naked eye, they can cause some serious damage.

For example, if enough protons hit a human body, the person could suffer from enough damaged cells that it would make them sick or even kill them. This type of damage has a name: "radiation sickness." Fortunately for living creatures, Earth's atmosphere stops most of the speeding protons coming in from space. The protons just hit air molecules and not human cells.
However, unlike humans, satellites and space observatories, like NASA's Spitzer Space Telescope, aren't so lucky. There is nothing to shelter them from the solar protons zipping around in space. Thus, astronomers on Earth sometimes take extra precautions to protect their space telescopes during solar storms, when large amounts of protons are shot out of the Sun.
What are Protons?
Every atom in the universe has at least one proton, and atoms are the microscopic building blocks of all of the matter we can see, including stars, planets, rocks, and people. Since the Sun is more than a hundred times bigger than the Earth, we know that there are a LOT of atoms with their protons just in the Sun. Astronomers estimate that there are approximately 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 protons living in the Sun. That's a one with 57 zeros following it!

That's more than the total number of grains of sand in the world. In fact, if you had one grain of sand for each proton in the Sun, you could cover the entire surface of the Earth millions of miles deep with sand. You could even pile sand way out past the Moon, which is a quarter of a million miles away.
The Sun is extremely hot, hotter than anything on Earth except the inside of a nuclear explosion. When atoms are heated up that much they come apart and the protons that are usually trapped inside of them are free to fly around at incredible speeds. Although heat may have released protons from the confines of an atom, most are still trapped by the Sun's gravity. However, some of them are fast enough to escape even the Sun's gravity, and race out to the planets and beyond. The fastest protons that leave the Sun fly through space at hundreds or thousands of miles per second.

In every nook and cranny of space between the planets of our solar system, there are solar protons are racing around moment by moment. This is called the "solar wind," because it's like a wind of protons (and electrons) blowing out from the Sun in all directions. Any object in space, be it a spacecraft, an astronaut, or a planet, is getting hit all the time with these. Most of the time there aren't enough of the dangerously fast solar protons leaving the Sun to hurt anything or anybody in space.
But sometimes the Sun has a big explosion on its surface. These eruptions are called "solar storms" when they throw fast protons or electrons out into space. They can produce a lot of dangerous, fast protons that can have speeds as much as 100,000 times faster than bullets. Because of these solar storms, scientists use the term "space weather."
Sometimes during a solar storm, a whole lot of electrons race out from the Sun at high speeds. If lots of solar electrons reach the Earth all at once, it can mess up our electric power lines and even cause power blackouts. In fact, a solar storm event in 1989 almost caused the US one of its worst power blackouts ever, and it did make 6 million people in Quebec lose power for several hours.

Solar storms can sometimes make big parts of interplanetary space fill up with fast protons called a solar "proton event." Solar proton events are the most dangerous space weather event. They are dangerous to both astronauts and spacecraft. Whenever there is a big proton event, the astronauts in the International Space Station hurry to a specially protected part of the station for safety. They wait there until the proton event dies down. A space suit doesn't give enough protection during a big proton event.
Since protons are too small to be seen we can't see them coming, but we can detect them when they get here. There are special proton detectors on several ongoing space missions like SOHO and GOES. These detectors alert us when a big blast of fast protons arrives at Earth, but not before.

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The Chandra X-ray Observatory survey of nearby sun-like stars suggests there is nearly three times more neon in the sun and local universe than previously believed. If true, this would solve a critical problem with understanding how the sun works.

"We use the sun to test how well we understand stars and, to some extent, the rest of the universe. But in order to understand the sun, we need to know exactly what it is made of" - Jeremy Drake of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Mass.

It is not well known how much neon the sun contains. This is critical information for creating theoretical models of the sun. Neon atoms, along with carbon, oxygen and nitrogen, play an important role in how quickly energy flows from nuclear reactions in the sun's core to its edge, where it then radiates into space.

The rate of this energy flow determines the location and size of a crucial stellar region called the convection zone. The zone extends from near the sun's surface inward approximately 125,000 miles. The zone is where the gas undergoes a rolling, convective motion much like the unstable air in a thunderstorm.

"This turbulent gas has an extremely important job, because nearly all of the energy emitted at the surface of the sun is transported there by convection" - Jeremy Drake .



These Chandra results reassured astronomers the detailed physical theory behind the solar model is secure. Scientists use the model of the sun as a basis for understanding the structure and evolution of other stars, as well as many other areas of astrophysics.

Neon, along with atoms of carbon, nitrogen and oxygen, plays an important role in regulating the rate at which energy flows from nuclear reactions in the Sun's core to its surface. The character of the energy flow changes dramatically about 125,000 miles from the surface on the Sun, where the stately diffusion of heat suddenly converts to a convective motion much like the unstable air in a thunderstorm.

The location of this turbulent region, called the convection zone, has been deduced to fairly high precision from the study of oscillations of the surface of the Sun (a technique called helioseismology in analogy of the use of oscillations of the Earth to study its interior). The location of the convection zone can also be deduced to equal precision from theoretical calculations based on among other things, the abundance of neon.

This is where astrophysicists get heartburn. The two determinations disagree. Several scientists have proposed that the paradox could be resolved if the solar abundance of neon is in fact about three times larger than the currently accepted value. This value is based on indirect estimates, since gas at the relatively cool 6,000 degree Celsius surface temperature of the Sun gives off no characteristic radiation at optical wavelengths.
However, a gas heated to millions of degrees produces a distinct neon signal in X-rays. The upper atmospheres, or coronas, of stars like the Sun have temperatures of millions of degrees, so the solar corona would seem to be a good place to settle the argument (not with Chandra - the bright solar radiation would irreparably damage the telescope).
Unfortunately, the solar X-rays come from numerous localized loops of hot gas that vary from location to location and time to time, complicating the interpretation of the data on neon. Jeremy Drake of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, MA and his colleague Paola Testa of the Massachusetts Institute of Technology in Cambridge, came up with an ingenious approach to the problem. They used Chandra to measure the neon abundance in 21 Sun-like stars within a distance of 400 light years.



The relative amount of neon in these stars was, on average, almost three times more neon than is measured for the Sun, just the amount needed to bring the solar oscillation observations and the theoretical model into agreement.
So, for the moment, astrophysicists can feel that their model of the Sun may be okay after all, and they can continue to boldly extrapolate this understanding to the rest of the Universe.

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