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TOPIC: Solar wind


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RE: Biggest X-class flare

Solar Fireworks Signal New Space Weather Mystery

The most intense burst of solar radiation in five decades accompanied a large solar flare on January 20. It shook space weather theory and highlighted the need for new forecasting techniques, according to several presentations at the American Geophysical Union (AGU) meeting this week in New Orleans.

The solar flare, which occurred at 7:00 UT, tripped radiation monitors all over the planet and scrambled detectors on spacecraft. The shower of energetic protons came minutes after the first sign of the flare. This flare was an extreme example of the type of radiation storm that arrives too quickly to warn interplanetary astronauts.

"This flare produced the largest solar radiation signal on the ground in nearly 50 years. But we were really surprised when we saw how fast the particles reached their peak intensity and arrived at Earth." - Dr. Richard Mewaldt of the California Institute of Technology, Pasadena, Calif. He is a co-investigator on NASA's Advanced Composition Explorer (ACE) spacecraft.
Normally it takes two or more hours for a dangerous proton shower to reach maximum intensity at Earth after a solar flare. The particles from the January 20 flare peaked about 15 minutes after the first sign.

"That's important because it's too fast to respond with much warning to astronauts or spacecraft that might be outside Earth's protective magnetosphere. In addition to monitoring the sun, we need to develop the ability to predict flares in advance if we are going to send humans to explore our solar system." - Dr. Richard Mewaldt.

The event shakes the theory about the origin of proton storms at Earth.
"Since about 1990, we've believed proton storms at Earth are caused by shock waves in the inner solar system as coronal mass ejections plough through interplanetary space. But the protons from this event may have come from the sun itself, which is very confusing." - Professor Robert Lin of the University of California at Berkeley. He is principal investigator for the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI).

The origin of the protons is imprinted in their energy spectrum, as measured by ACE and other spacecraft, which matches the energy spectrum of gamma-rays thrown off by the flare, as measured by RHESSI.
"This is surprising because in the past we believed the protons making gamma-rays at the flare were produced locally and the ones at the Earth were produced instead by shock acceleration in interplanetary space. The similarity of the spectra suggests they are the same." - Professor Robert Lin.

Solar flares and coronal mass ejections (CMEs), associated giant clouds of plasma in space, are the largest explosions in the solar system. They are caused by the build-up and sudden release of magnetic stress in the solar atmosphere above the giant magnetic poles we see as sunspots. The Transitional Region and Coronal Explorer (TRACE) and the Solar and Heliospheric Observatory (SOHO) spacecraft are devoted to observing the sun and identifying the root causes of flares and CMEs, with an eye toward forecasting them.

"We do not know how to predict the flow of energy into and through these large flares. Instruments like TRACE give us new clues with each event we observe." - Dr. Richard Nightingale of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alta, Calif.

TRACE has identified a possible source of the magnetic stress that causes solar flares. The sunspots that give off the very largest (X-class) flares appear to rotate in the days around the flare.
"This rotation stretches and twists the magnetic field lines over the sunspots. We have seen it before virtually every X-flare that TRACE has observed since it was launched and more than half of all flares in that time." - Dr. Richard Nightingale.

However, rotating sunspots are not the whole story. The unique flare came at the end of a string of five other very large flares from the same sunspot group, and no one knows why this one produced more sudden high energy particles than the first four.

"It means we really don't understand how the sun works. We need to continue to operate and exploit our fleet of solar-observing spacecraft to identify how it works." - Professor Robert Lin.

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Posts: 131433
RE: Solar wind

A Chinese-German team of scientists have identified the magnetic structures in the solar corona where the fast solar wind originates. Using images and Doppler maps from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer and magnetograms delivered by the Michelson Doppler Imager (MDI) on the space-based Solar and Heliospheric Observatory (SOHO) of ESA and NASA, they observed solar wind flowing from funnel-shaped magnetic fields which are anchored in the lanes of the magnetic network near the surface of the Sun. These observations are presented in the April 22 issue of Science magazine. The research leads to a better understanding of the magnetic nature of the sources of the solar wind, a stream of tenuous and hot plasma (electrically conductive gas) that affects the Earth's space environment.

The solar wind consists of protons, alpha particles (two-fold ionized helium), heavy ions and electrons flowing from the surface of the Sun with speeds ranging from 300 to 800 km/s. The heavy ions in the coronal source regions emit radiation at certain ultraviolet wavelengths. When they flow towards Earth, as they do when tracing the nascent solar wind, the wavelengths of the ultraviolet emission become shorter, a phenomenon called the Doppler Effect, which is well known in its acoustic variant, for example, from the change in tone of the horn of a police car while approaching to or receding from the listener. In the solar case, plasma motion towards us, which means away from the solar surface, is detected as blue shift in the ultraviolet spectrum, and thus can be used to identify the beginning of the solar wind outflow.

A SUMER ultraviolet spectrum is similar to what is seen when a prism separates white light into a rainbow of distinct colours. The ultraviolet radiation is however invisible to the human eye and cannot penetrate the Earth's atmosphere. By analyzing ultraviolet emission obtained by SUMER on the space observatory SOHO from space, solar physicists can learn a great deal about the Sun and infer the gas temperature, chemical composition, and motion in the various atmospheric layers.


This picture was constructed from measurements which were made on September 21, 1996 on SOHO (see the April 22 issue of SCIENCE magazine) with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer (SUMER) providing Doppler spectroscopy of the coronal plasma, with the Michelson Doppler Imager (MDI) delivering magnetogramms of the solar photosphere, and The Extreme ultraviolet Imaging Telescope (EIT) giving the context image of the Sun in the left corner. The SUMER spectrometer analyzes ultraviolet light which is emitted by the hot gas in the Sun's atmosphere, and is ideally suited for studying atmospheric motions. Careful data analysis, involving subtle wavelength calibration and coronal magnetic-field extrapolation was required before the slow outward motions could be identified at various heights above the solar surface, and their links with the magnetic field guiding the flow could be established. The figure illustrates location and geometry of three-dimensional magnetic field structures in the solar atmosphere. The magenta coloured curves illustrate open field lines, and the dark grey solid arches show closed ones. In the lower plane, the magnetic field vertical component obtained at the photosphere by MDI is shown. In the upper plane, inserted at 20,600 km, we compare the Ne VIII Doppler shift with the model field. The shaded area indicates where the outflow speed of highly charged neon ions is larger than 7 km/s. Note the funnel constriction by pushing and crowding of neighbouring loops. The scale of the figure is significantly stretched in the vertical direction. The smaller figure in the lower right corner shows a single magnetic funnel, with the same scale in both vertical and horizontal directions.

"The fine magnetic structure of the source region of solar wind has remained elusive" said first author Prof. Chuanyi Tu, from the Department of Geophysics of the Peking University in Beijing, China. "For many years, solar and space physicists have observed fast solar wind streams coming from coronal regions with open magnetic field lines and low light intensity, the so called coronal holes. However, only by combining complex observations from SOHO in a novel way have we been able to infer the properties of the sources inside coronal holes. The fast solar wind seems to originate in coronal funnels with a speed of about 10 km/s at a height of 20,000 kilometres above the photosphere".
"The fast solar wind starts to flow out from the top of funnels in coronal holes with a flow speed of about 10 km/s", states Prof. Tu. "This outflow is seen as large patches in Doppler blue shift (hatched areas in the above figure) of a spectral line emitted by Ne+7 ions at a temperature of 600,000 Kelvin, which can be used as a good tracer for the hot plasma flow. Through a comparison with the magnetic field, as extrapolated from the photosphere by means of the MDI magnetic data, we found that the blue-shift pattern of this line correlates best with the open field structures at 20,000 km."

The SUMER spectrometer scrutinized the sources of the solar wind by observing ultraviolet radiation coming from a large area of the northern polar region of the Sun.
"The clear identification of the detailed magnetic structure of the source, now being revealed as coronal funnels, and the determination of the release height and initial speed of the solar wind are important steps in solving the problems of mass supply and basic acceleration. We can now focus our attention on studying further plasma conditions and physical processes that occur in the expanding coronal funnels and in their narrow necks anchored in the magnetic network"- Prof. Eckart Marsch, co-author of the Science paper.

Solving the nature and origin of the solar wind is one of the main goals for which SOHO was designed. It has long been known to the astronomical community that the fast solar wind comes from coronal holes. What is new here is the discovery that these flows start in coronal funnels, which have their source located at the edges of the magnetic network. Just below the surface of the Sun there are large convection cells. Each cell has magnetic fields associated with it, which are concentrated in the network lanes by magneto-convection, where the funnel necks are anchored. The plasma, while still being confined in small loops, is brought by convection to the funnels and then released there, like a bucket of water is emptied into an open water channel.

"Previously it was believed that the fast solar wind originates on any given open field line in the ionization layer of the hydrogen atom slightly above the photosphere", says Prof. Marsch, "However, the low Doppler shift of an emission line from carbon ions shows that bulk outflow has not yet occurred at a height of 5,000 km. The solar wind plasma is now considered to be supplied by plasma stemming from the many small magnetic loops, with only a few thousand kilometres in height, crowding the funnel. Through magnetic reconnection plasma is fed from all sides to the funnel, where it may be accelerated and finally form the solar wind."

The SUMER instrument was built under the leadership of Dr. Klaus Wilhelm, who is also a co-author of the paper, at the Max Planck Institute for Solar System Research (formerly Max Planck Institute for Aeronomy) in Lindau, Germany, with key contributions from the Institut d'Astrophysique Spatiale in Orsay, France, the NASA Goddard Space Flight Centre in Greenbelt, Maryland, the University of California in Berkeley, and with financial support from German, French, USA and Swiss national agencies. SOHO has been operating for almost ten years at a special vantage point in space 1.5 million kilometres from the Earth, on the sunward side of the Earth. SOHO is a project of international collaboration between the European Space Agency and NASA. It was launched on an Atlas II-AS rocket from NASA's Kennedy Space Centre, Florida, in December 1995 and is operated from the Goddard Space Flight Centre.


-- Edited by Blobrana at 01:05, 2006-07-27



Posts: 131433

A layer deep in the solar atmosphere can be used to estimate the speed of the solar wind, a stream of electrified gas that constantly blows from the Sun. Estimating the speed of the solar wind will improve space weather forecasts, which will aid human exploration of the planets.
The solar wind flows from the Sun's hot, thin, outer atmosphere, the "corona". The researchers were surprised to discover that the structure of the Sun's cooler, dense lower atmosphere, called the chromosphere, could be used to estimate the speed of the solar wind.
This was unexpected because the solar wind is a phenomenon of the corona, and the chromosphere is so deep -- it's the layer just above the Sun's visible surface.

"It's like discovering that the source of the river Nile is another 500 miles inland." -Dr. Scott McIntosh of the Southwest Research Institute, Boulder, Colo., lead author of a paper on this research published May 10 in the Astrophysical Journal.
The new work promises to increase the accuracy of space radiation forecasts. The Sun occasionally launches billion-ton blasts of electrified gas, called coronal mass ejections (CMEs), into space at millions of kilometres per hour. If a fast CME is ploughing through slow solar wind, a shock builds up in front of the CME that accelerates the electrically charged solar wind particles. These fast particles can disrupt satellites and are hazardous to unprotected astronauts.
"Just as knowing more details about the atmosphere helps to predict the intensity of a hurricane, knowing the speed of the solar wind helps to determine the intensity of space radiation storms from CMEs." - Dr. Robert Leamon (co-author), of L-3 Government Services at NASA's Goddard Space Flight Centre, Greenbelt, Md.
Like wind on Earth, the solar wind is gusty, ranging in speed from about approximately 350 km/second to 700 km/second.

Since the solar wind is made up of electrically-charged particles, it responds to magnetic fields that permeate the solar atmosphere. Solar wind particles flow along invisible lines of magnetic force like cars on a highway. When the magnetic field lines bend straight out into space, as they do in "coronal hole" regions, the solar wind acts like cars on a drag strip, racing along at high speed. When the magnetic field lines bend sharply back to the solar surface, like the pattern of iron filings around a bar magnet, the solar wind acts like cars in city traffic and emerges relatively slowly. Scientists have known this for over thirty years and used it to give a crude estimate for the speed of the solar wind -- either fast or slow.
In the new work, the team has tied the speed of the solar wind as it blows past Earth to variations deeper in the solar atmosphere than had previously been detected (or even expected). By measuring the time taken for a sound wave to travel between two heights in the chromosphere, they were able to determine that the chromosphere is effectively "stretched thin" below coronal holes with their open magnetic fields, but compressed below magnetically closed regions.
The team used the observation to derive a continuous range of solar wind speeds from the structure of the chromosphere. The wider the chromospheric layer is, the more it is being allowed to expand by open magnetic fields and the faster the solar wind will blow. This new method is more precise than the old "fast or slow" estimate.
NASA's Transition Region and Coronal Explorer (TRACE) spacecraft was used to measure the speed of sound waves in the chromosphere, and NASA's Advanced Composition Explorer (ACE) spacecraft was used to take measurements of the solar wind speed as it blew by the Earth. Comparing the data from the two spacecraft gave the connection.
"Prior to this discovery, we could only determine solar wind speed from spacecraft that were roughly in line between the Earth and the Sun, like ACE, WIND, and the Solar and Heliospheric Observatory. This spacecraft fleet was placed along the Earth-Sun line because we need to know about the space weather coming our way. However, compared to the size of our solar system, this is a very narrow range; it's like looking through a soda straw. With this discovery, we can use TRACE to build up images that can predict the solar wind speed throughout half the solar system." - Dr. Joe Gurman, solar researcher at NASA Goddard.


-- Edited by Blobrana at 01:43, 2005-05-21

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