Storms on the Sun

Storms on the Sun

Solar “activity” is a term which describes the irregular, sometimes explosive motions of gas and radiation which occur on the photosphere and in the Sun’s atmosphere. This activity is powered by the Sun’s energy output or luminosity, its differential rotation, and the twisting of magnetic fields.

Expressions of Solar Activity:

Sunspots: localized, blotchy, “dark” regions visible on the photosphere

·  ~ 1500 K cooler than the average photospheric temperature, hence they appear dark in contrast to the hotter, brighter surroundings.

·  range in size from a few hundred to a few 10s of thousands of kilometers. The largest sunspots exceed the size of Earth.

·  strongly magnetic (compared to surrounding regions). Magnetic fields hold gas in place, allowing it to cool more than the surrounding gas.

·  occur in pairs, with opposing polarity (one spot is a north magnetic pole, the other is a south pole).

·  Alter their appearance on day-to-day timescales. Large spots may last for 1 or 2 solar rotations (up to 60 days).

Prominences: huge flame-like structures extending from the photosphere into

the lower corona

·  Plasma within the prominences is both rising from and falling to the photosphere. Individual prominences can last hours to days.

·  Their shape is defined by magnetic fields; gas is constrained to move along magnetic field lines.

·  Are usually located near sunspot pairs

Flares: brief, unpredictable, extremely energetic eruptions of atomic particles

and high-energy radiation (UV, X-rays) from the photosphere

·  eject streams of ionized gas into the corona

·  can eject material directly into the solar wind

·  Gas temperatures in the cores of flares may reach 100 million K

·  Powered by the release of magnetic energy, which can accelerate protons and electrons to near light-speed. As these atomic particles crash into other atoms, they radiate their kinetic energy away in the form of high energy radiation.

Coronal mass ejections: massive magnetized blobs of gas ejected from the

corona, which may, if pointed in the correct direction, affect the Earth

Most of these manifestations of solar activity occur within active regions on the photosphere, in which the Sun’s magnetic field is twisted and compressed, storing up a tremendous amount of energy.

The Solar Wind:

·  A spherical stream of atomic particles (protons, electrons, helium nuclei), which escape from the Sun constantly. The solar system is embedded within this wind.

·  Produced by the high temperatures in the corona. The gas at the top of the corona is hot enough to escape the Sun’s gravity.

·  During periods of high solar activity, the wind is stronger, carrying more mass away from the Sun, with more energy.

·  Most of the solar wind escapes through coronal holes, regions in the corona where the magnetic field lines extend into interplanetary space. Gas flows most freely along these lines.

·  The solar wind can affect the Earth by dumping atomic particles into the Earth’s magnetosphere, which cause aurora and disruptions in radio communications.

The Solar Cycle:

All solar activity waxes and wanes in an average 22-year cycle.

·  The number of sunspots rises and falls during each 11-year cycle. Then, the polarity of the spots reverses for the next 11 years, as the number of spots rises and declines again.

·  Sunspot locations migrate to lower latitudes during each cycle, after starting out at high latitude

·  Flares, mass ejections, and the shape of the corona all vary in tune with this cycle. Activity peaks at the time of sunspot maximum

·  The time between sunspot maxima is not precisely 11 years, but varies from 7 to 15 years.

·  The solar cycle appears to be caused by the stretching and twisting of magnetic fields due to the Sun’s rotation and convection at the base of the photosphere. The magnetic field strength rises, peaks, then energy is explosively released from the magnetic fields as the field lines relax to their original, smooth north/south orientation. The magnetic field reverses itself and the cycle begins again.

·  Four centuries of observing the Sun have provided a baseline for exploring the solar cycle.

·  For unknown reasons, solar activity apparently can “turn off” for decades, as happened from 1645 to 1715. During this time, termed the Maunder Minimum, sunspots were virtually absent, and overall solar activity was low.

·  The subtle effects of solar activity on Earth’s climate are being explored by astronomers, climatologists and atmospheric scientists today,