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Sun Storms

Solar Flares: UV Image
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The Sun's surface is anything but quiescent. Violent eruptions of gas mark the onset of solar flares that last on average only 20 minutes. Scientists still do not fully understand what exactly causes solar flares, much less how to predict them. But exhaustive studies have implicated powerful magnetic fields emerging from active regions associated with sunspots.

Sunspot Polarities
Permeating the Sun's gaseous layers, intense magnetic fields trap the gas, preventing it from escaping the surface. The image to the left illustrates the magnetic polarity of sunspots, and shows where the looping magnetic fields exit the sun. The red coloring signifies north poles; south poles are indicated in blue. Sunspots reveal where the powerful magnetic fields project into lower layers of hot gas (the photosphere).

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The stronger the magnetic fields in a given region, the more concentrated the gas trapped beneath the surface. Measuring the concentrations of the solar gases, therefore, provides a means to estimate the strength and shape of the magnetic fields that constrain them.

Mapping the Sun's Gas Layers

The Sun is enveloped in distinct layers of hot gas. The outer layers consists of the photosphere, chromosphere, transition layer, and the corona.

Innermost of these layers lies the photosphere. At temperatures of about 6000 degrees Celsius (about 11,000 degrees Fahrenheit), the photosphere shines more-or-less uniformly in the visible portion of the spectrum.

Sunspots: Optical View

A few sunspots show up as dark regions in the picture on the left. Here, strong magnetic fields prevent the convective flow of heat upwards from deeper inside the Sun. As a result, the gas in these regions cools off, emits less visible radiation, and therefore appears darker. Some sunspots are as large as Earth!

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UV Image of the Sun
This ultraviolet image more clearly shows the next layer outwards, the transition layer. Regions where strong magnetic fields concentrate the greatest amounts of gas are white, indicating strong emissions. Between the photosphere and chromosphere and the transition layer, the temperature increases from 6000 to over half-a-million degrees Celsius (11,000 to over 1 million degrees Fahrenheit). At these higher temperatures, ultraviolet light is emitted.

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The outermost corona exhibits the highest temperature, between 0.5 and 5 million degrees Celsius (1 to 9 million degrees F), and emits radiation across the entire spectrum, from visible light to x-rays.

X-Ray Image of the Sun

The dark regions indicate where the Sun's magnetic fields extend into outer space, confining the hot gases. The bright areas show where the coronal gases are trapped by the more powerful magnetic fields associated with active regions.

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At times of high solar activity, the number of sunspots increases and their associated magnetic fields grow in strength; thousands-fold stronger compared with the fields prevailing at quieter times. Enormous columns of gas called prominences hang above the sunspot regions. Some of these erupt further, shooting material outwards at about 1000 kilometers (600 miles) per second. The most violent are the solar flares which are thought to result from the collapse of strong magnetic fields emerging from active regions associated with sunspots.

From X-rays, ultraviolet and visible light, to radiowaves and high energy particles, solar flares emit all types of radiation. The X-ray, ultraviolet, visible and radio emissions, travelling at the speed of light, reach Earth a little over 8 minutes after a flare erupts. Ionizing the Earth's atmosphere, this radiation can seriously disrupt long distance radio communications. High energy particles reach Earth about a day later, and become trapped by its magnetic field.

By combining observations in the x-ray, ultraviolet, visible and radio portions of the spectrum, astronomers aim to map the magnetic fields and trace their role in the production of solar flares. When it comes to studying the Sun's radio emissions, several researchers are turning to the BIMA array.

Probing the Sun's Magnetic Fields

Observation of Solar Flares in Millimeter Wavelengths

Though the causes of solar flares are not precisely known, scientists know that magnetic fields play a crucial role. Produced in active regions of the Sun's surface, high energy charged particles, specifically electrons, spiral outwards along the powerful, arching magnetic fields, releasing radio-freqency radiation at millimeter and other wavelengths.

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Employing the BIMA array, researchers can image the sun's active regions at high spatial resolution. The resulting images reveal the shape of the flare by tracing the radio emissions of the outbound electrons. Such studies will help scientists better understand the forces causing eruption of solar flares.

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NCSA. Last modified 11/11/95