Today: Upper Atmosphere/Ionosphere

Today: Upper Atmosphere/Ionosphere • Review atmospheric Layers – Follow the energy! – What heats the Stratosphere? • Mesosphere – Most turbulent layer – why? • Ionosphere – What is a Plasma? The Solar Spectrum: The amount of energy the Sun produces at a given wavelength is determined by its temperature. This is a general property of any Black Body such as a star or the heating element on the stove. • The Sun is about 5270 K and produces most light in the visible. • The atmosphere is transparent in the visible and so over half of the solar energy reaches the ground. • Some gasses absorb certain wavelengths of sunlight, and thus some energy is absorbed directly into the atmosphere. Depositing Energy Energy from the Sun may be transmitted directly to the ground, absorbed in the atmosphere, or reflected from clouds or the Sun back into space. Some re-radiated heat from the ground is absorbed by the atmosphere, further heating it. The Troposhphere The troposphere is the region of the atmosphere we live in. The primary source of energy in the troposphere is heat (infrared light ) radiated from the ground. This means that it is warmest at the bottom and coolest at the top. Temperature drops about 11.5 ° F for each km of altitude. When pressure and temperature (with temp. faster) drop with altitude it triggers convection . Convection makes the troposphere unstable, but in a good way. Dominant Region The Troposphere contains 80% of the mass in the atmosphere and 99% of the water in the atmosphere. Water Vapor, CO 2, methane, nitrous oxide, ozone, chloroflorocarbons are all greenhouse gases Convection (both horizontal and vertical) produces weather in the atmosphere. Storms occur in the troposphere. The top of the troposphere occurs where the temperature change switches from decreasing with altitude to increasing. The top of the troposphere is called the tropopause . It’s altitude depends on latitude and season. It’s about -60° F at the tropopause. The Stratosphere: The stratosphere is the atmospheric layer directly above the tropopause. It has much of the remaining mass of the atmosphere and is nearly as important as the troposphere for living things. The primary source of energy in the stratosphere is internal . Incoming ultraviolet light is absorbed by Ozone (O3). O3 + UV -> O2 + O Much of the O3 that is destroyed is at the top of the stratosphere. Temperature rises with altitude. The stratosphere is stable . The Role of Ozone: Ozone does more than heat the stratosphere. Without ozone absorption, UV radiation would kill most things living on land! Human activity is destroying stratospheric ozone! Chlorofluorocarbons (from spray cans) are transported to the stratosphere in tropospheric convection. They contain chlorine. ⇒ Cl + O3 ClO + O2 ⇒ ClO + O Cl + O2 The above is called a catalytic reaction. The O3 is destroyed, but not Cl! Because the stratosphere is stable, the Cl is trapped. This is a big problem…. The Mesosphere: Above the Stratopause is another, thin atmospheric layer called the Mesosphere . Energy input to the Mesosphere comes from the formation of Ozone, which occurs near the stratopause (the bottom). ⇒ O2 + UV O + O ⇒ O + O2 O3 • Like the troposphere, temps in the mesosphere drop rapidly with altitude. • The rapid drop in temperature makes this the most turbulent part of the atmosphere. • The mesopshere is where meteors -100 32 and spacecraft begin to break up. Temperature (F) The Thermosphere & Ionosphere Above the Mesopause the atmosphere begins to heat rapidly as the atmosphere absorbs solar UV and X-Ray radiation. The thermosphere is the hottest region of the atmosphere, but also far less dense than the lower layers. In the thermosphere, the mixing of atmospheric gasses begins to break down. Lighter gasses rise to the top. Some solar photons are so energetic that they break atoms apart, creating charge layers (the ionosphere - several layers – see next lecture!). Temperatures in the thermosphere are very dependent on sunlight. It cools dramatically at night and heats -100 1500 (750) Temperature (F) up when the Sun is active. The Exosphere: Unlike the other layers of the atmosphere there is no thermopause . What happens instead is that it simply extends into space getting thinner, hotter, and lighter . Eventually the gasses become so hot and light on average that they are moving fast enough to escape the Earth altogether. We call this region the Exosphere . The exosphere is so thin that it is a better vacuum than we can make in a laboratory on Earth. It’s really space, but “Earth-Space”. Ions and Aurora Once ions are formed in the atmosphere it becomes part of a large circuit (like a resistor) that connects via the magnetic field to the magnetosphere. Ions and electrons in the magnetosphere flow into the atmosphere where they collide with thermospheric gasses and give off light. This is called AURORA Substorm starts, then expands Dayside UV and EUV Aurora borealis during substorm Neutral and charged particles in the ionosphere neutral charged Ion Populations: • The D region is located in the Mesosphere from 60-90 km. • Its ion population is + + dominated by NO and O2 • The E region is located in the Thermosphere from 90-150 km. • Its ion population is also + + dominated by NO and O2 • The F region is located in the Thermosphere from 150-800 km. • Its ion population is dominated by O+ So, what is a plasma? • A plasma is an ionized gas (‘ionized’ means that electrons have been removed from some or all of the atoms, leaving positive ions, and free electrons) • A plasma is often referred to as the “4 th state of matter” • A plasma is a good conductor and can reflect, or slow down radio waves • 99.9% of the universe is said to be a plasma Image Credit link Charged and Neutral Structure Extreme Variability! F E.

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