Solar Flares and Lunar Tides

The Sun’s Differential Rotation Since the Sun is a gaseous body rather than solid, different latitudes can rotate at different speeds. Its equator rotates once every 25 days, while regions near the poles rotate every 30 days.

The Sun’s Magnetic Field Imagine the Sun as a bar magnet, with magnetic field lines cutting through it. Because the equator is rotating faster than the poles, the magnetic field lines near the equator are pulled around the Sun faster, resulting in loops and kinks in the magnetic field.

Creation of Sunspots The magnetic field loops and kinks poke through the Sun’s surface. They absorb energy from areas on the surface that they pass through, making those spots cooler, and therefore darker.

Prominences and Flares Eventually, just like a rubber band, the loops and kinks in the magnetic field will break and release their energy in a huge explosion. Solar Prominence Solar Flare

The Sun in X-rays Because the Sun’s temperature is about 6000°, it emits mostly at optical wavelengths. However, solar flares are extremely hot, so they produce lots of X-rays.

The Solar Cycle After the magnetic field lines break, they straighten out and the cycle begins again. One cycle is completed every 11 years, which is seen in the varying number of sunspots over time.

The Solar Cycle Solar Maximum March 29, 2001 Solar Minimum Oct. 11, 2004 After the magnetic field lines break, they straighten out and the cycle begins again. One cycle is completed every 11 years, which is seen in the varying number of sunspots over time.

The Maunder Minimum Sunspots are easy to see –you don’t need a telescope (just project the sun through a pinhole). So good data on the Sun exists all the way to the time of Galileo. In the 1600’s, the earth went through a mini- ice age: Europe and Asia were abnormally cold. This coincided with an unusually low number of sunspots.

Effects on Earth When the Sun has a lot of sunspots, solar flares, and prominences  The earth is warmed by all the additional energy  The earth is bombarded with cosmic rays (i.e., high energy hydrogen and helium nuclei that are ejected from the Sun. In other words, a stronger solar wind.) The earth’s magnetic field and atmosphere protects us from these particles; those that get through are funneled into the atmosphere at the poles.

Aurorae When the solar wind hits the earth’s atmosphere, the particles excite electrons bound to atoms of oxygen and nitrogen. When the electrons fall back down, they produce emission lines.

Sun-grazing Comets

Tides on Earth A tidal force is the difference in gravity from one side of a body to the other that is exerted by a 2nd object. The Moon exerts a tidal force on the Earth that causes the oceans facing the Moon to bulge out toward it, and the oceans on the opposite side of the Earth to bulge out away from the Moon. These varying ocean levels are called the tides.

Tides on Earth The Sun also causes tides, but they are smaller than the Lunar tides. When the Moon and Sun are aligned (during New and Full Moons), they pull the oceans in the same direction. This is when the high tide is highest, and low tide is lowest. During the quarter moons, the Sun partially cancels the Lunar tides, so the tides are less extreme.

Tides on Earth

Tidal Friction Because of friction between the Earth’s crust and the oceans, the rotation of Earth pulls the ocean’s tidal bulge out of alignment with the Moon. This has 2 effects: 1) The gravitational pull of the Moon on the tidal bulge slows Earth’s rotation (by a few milliseconds per century). This will continue until Earth keeps the same face towards the Moon. 2) The gravitation pull of the tidal bulge on the Moon slings the the Moon ahead in its orbit, causing its orbit to expand (by a few cm per year).

Tidal Friction

Tides on the Moon Just as the Moon exerts tidal forces on Earth, Earth also exerts tidal forces on the Moon. Since Earth is much more massive than the Moon, its tidal forces are much stronger. So Earth’s tidal forces have already slowed the Moon’s rotation to the point that it always keeps the same face towards Earth. As a result, there’s a far side of the Moon that we never see from Earth. We didn’t know what it looked like until satellites orbited the Moon and photographed the far side.

The Moon’s Rotation