Solar Activity The Solar Wind

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1 Solar Activity The Solar Wind The solar wind is a flow of particles away from the Sun. They pass Earth at speeds from 400 to 500 km/s. This wind sometimes gusts up to 1000 km/s. Leaves Sun at highest speeds from coronal holes. Stretches magnetic field lines away from the Sun. When particles arrive at Earth, they are funneled toward the north and south poles by Earth s magnetic field. When they hit the atmosphere, they cause it to glow. This produces the Aurora Borealis and the Aurora Australis. Earth s Magnetic Field 1

2 Coronal Loops Magnetic Heating the of the Solar Corona Coronal loop = arch-shaped region where plasma is trapped by the Sun s magnetic field. There may be other mechanisms that transfer heat to the corona, such as microflares with temperatures up to 5,000,000 K. All of them involve the interaction of solar matter with magnetic fields. Prominences Dense, relatively cool clouds of gas that look dark, and are called filaments, when viewed with the photosphere as background. Look either like loops or irregular clouds when seen on the limb. Extend tens of thousands or (in rare cases) even 200,000 km into the corona. Last as long as 2 or 3 months. Most of them erupt and eject material into the solar wind. 2

3 Large Eruptive Prominence (HeII Filtergram at 30.4 nm) 24 July 1999 Image from Coronal Mass Ejection Sometimes, clouds containing billions of tons of matter are ejected from the corona at speeds up to 1000 km/s. This phenomenon is called a coronal mass ejection. Coronal mass ejections are usually, but not always, associated with prominences or flares. When the Sun is least active (solar minimum), there may be only one cme per week. When it is most active, there may be 2 or 3 per day. 3

4 Solar Flares Magnetic field in an active region suddenly changes its structure. Large numbers of rapidly-moving ions and electrons are released. Much more energetic eruption than a prominence. These energetic particles collide with coronal gas and raise its temperature up to 40 million K. Contributes up to 1% to the Sun s brightness. Most of this is X and UV radiation that reaches Earth in about 8 minutes. Electrons and ions reach Earth in a few hours or days. Effect of a Solar Flare on Earth Ionization of Earth s upper atmosphere is increased. Enhancement of ionosphere disrupts long-range radio communications. Power surges may occur, causing damage to transformers and resulting in blackouts. Eventually, ejected coronal matter also reaches Earth. Unless they are shielded for protection, astronauts are endangered by the increased radiation. The Aurora Borealis and Aurora Australis are brighter than usual and are seen farther from the poles. Bombardment of Earth s upper atmosphere causes it to expand and engulf some satellites. Friction with the satellite surface reduces its energy, causing the radius of its orbit to increase 4

5 Sunspots Dark spots seen in the photosphere 2500 km to 50,000 km in diameter Large ones can last for months. Occur in groups. Associated with magnetic fields Spots within a group are paired, one being a north magnetic pole and the Penumbra other a south magnetic pole. Approx. Size of Earth Consist of a dark inner part (umbra) and a surrounding lighter part (penumbra). Are found in active regions along with flares and prominences. T photosphere ~ 5800 K, T spot ~ 4300 K. Though dark in contrast to the surrounding photosphere, they are actually several times brighter than the full moon. Umbra Sunspot Penumbral Filaments (Swedish 1-meter Solar Telescope at the Roque de los Muchachos Observatory on La Palma, Canary Islands) 5

6 The Sunspot Number Cycle The number of Sunspots reaches a maximum approximately every 11 years. This cycle is not yet understood well enough to make accurate predictions. Solar Magnetogram Dark areas are north poles and bright areas are south poles. Magnetic field strength up to 10,000 times as large as at Earth s surface. Leading (farthest east) spot in each group in the northern hemisphere is a south pole. Leading (farthest east) spot in each group in the southern hemisphere is a north pole. The Sunspot polarities switch at the beginning of a new cycle. In the next cycle, the leading Sunspot in the northern hemisphere will be a north pole. 6

7 The Maunder Minimum E. Walter Maunder noticed that there was a period from 1645 to 1715 when very few Sunspots were observed. Recent research shows that this corresponded to a period of low solar activity. It may be one of the causes of the little ice age, a period from about 1600 to 1850 when the weather was unusually cold in Europe and North America. The Maunder Butterfly Diagram This is a plot of Sunspot latitude as a function of date. For each date, a dot is placed on the diagram at the latitude of each Sunspot observed. The first Sunspots to appear at the beginning of a cycle are about 35º north and south of the Sun s equator. Subsequent spots are formed closer to the equator. 7