An Introduction to the Upper Atmosphere Thomas Immel Space Sciences Laboratory UC Berkeley

Earth's Atmosphere Ionosphereand Aurora The Different Levels of the Atmosphere are: Troposphere: This is the lowest atmospheric layer and is about seven miles (11 km) thick. Most clouds and weather are found in the troposphere. The troposphere is thinner at the poles (averaging about 8km thick) and thicker at the equator (averaging about 16km thick). The temperature decreases with altitude. Stratosphere: The stratosphere is found from about 7 to 30 miles (11-48 kilometers) above the Earth’s surface. In this region of the atmosphere is the ozone layer, which absorbs most of the harmful ultraviolet radiation from the Sun. The temperature increases slightly with altitude in the stratosphere. The highest temperature in this region is about 32 degrees Fahrenheit or 0 degrees Celsius. Mesosphere: The mesosphere is above the stratosphere. Here the atmosphere is very rarefied, that is, thin, and the temperature is decreasing with altitude, about –130 Fahrenheit (-90 Celsius) at the top. Thermosphere: The thermosphere starts at about 55 kilometers. The temperature is quite hot; here temperature is not measured using a thermometer, but by looking at the motion and speed of the rarefied gases in this region, which are very energetic but would not affect a thermometer. Temperatures in this region may be as high as thousands of degrees. Exosphere: The exosphere is the region beyond the thermosphere. Ionosphere: The ionosphere overlaps the other atmospheric layers, from above the Earth. The air is ionized by the Sun’s ultraviolet light. These ionized layers affect the transmittance and reflectance of radio waves. Different ioniosphere layers are the D, E (Heaviside-Kennelly), and F (Appleton) regions.

What is important to know? Most importantly, you can’t understand the upper atmosphere without some understanding of what lies below. So we will look at the lower atmosphere. “Understanding” means a knowledge of conditions such as temperature, how it changes and why.

Atmospheric Conditions These three parameters and how they vary with altitude are some of the defining characteristics of an atmosphere Temperature Pressure Composition What controls these conditions? Physics Chemistry Solar Energy Input is an example of a process that involves both of these.

Solar Energy drives the whole atmosphere

Solar Energy drives the whole atmosphere It heats the Earths surface, driving winds and weather in the lower atmosphere

Solar Energy drives the whole atmosphere It heats the Earths surface, driving winds and weather in the lower atmosphere It deposits energy directly into the atmosphere which is converted to heat and chemical energy O2 N2 O2+ O+ N+ O N2+ N O2 N2

Solar Energy drives the whole atmosphere The 11-year solar cycle in sunspots is reflected in the temperature and density in the upper atmosphere, as we will see.

Solar Energy drives the whole thing The 11-year solar cycle in sunspots is reflected in the temperature and density in the upper atmosphere, as we will see. Solar wind energy is converted into aurora, which also adds heat to the upper atmosphere.

A simple atmosphere Imagine a planet with a solid surface and no inputs from a sun. What do the pressure and temperature look like as a function of altitude? Let’s build a model to see what turns out.

Build a model atmosphere Add a small layer of gas to Planet X. The pressure at the top of the layer is the same as space (zero). The pressure at the bottom depends on the weight of the gas you added. Planet X

Build a model atmosphere Pressure = 0 Planet X Height=1 Pressure = 1 At the top of the atmosphere, the pressure is zero, and at the bottom, lets define that as pressure=1

An atmosphere is made of gas Gasses obey a simple physical law. Pressure is proportional to density and temperature. Or, as pressure increases, so will the density and/or temperature

Build a model atmosphere So let’s follow the rules. With the addition of the second layer on top of the first, the lower layer will compress and get hotter (indicated with color) Height=1 Height=0.75

Build a model atmosphere Pressure The pressure at the bottom is still twice the value in the middle. But the increase happens over a shorter distance. Height=1 1 Height=0.75 2

Build a model atmosphere Pressure 1 Planet X 2 3 Keep going! Temperatures go up at the bottom as we add layers to the top. Pressure levels continue to get closer together. 4 5 6

Build a model atmosphere Pressure Altitude Temperature In a simple atmosphere, the pressures increase faster at the bottom, while the temperatures drop roughly linearly throughout. So, lets look at Earth’s atmosphere and see if our model works.

Earth’s lower and middle atmosphere The pressure drops very rapidly with increasing altitude, from about 1000 units at the surface to 1 at 45 km. And it just keeps dropping. The temperature drops regularly from the surface up to the “tropopause”, and then the atmosphere starts to heat up again!

Earth’s lower and middle atmosphere Reversals in the temperature trend defines the boundaries of the three regions of the atmosphere shown here. The huge departure from the temperature curve we expected from our simple model is due to stratospheric ozone. We forgot to include that. :)

Earth’s lower and middle atmosphere The upper atmosphere is usually considered to begin at the mesopause. There is a hint there, that the temperatures are again increasing with altitude.

Earth’s lower and middle atmosphere 400 300 Altitude (kilometers) 200 Sure enough, the temperatures well exceed value reached below. That’s why the upper-atmosphere is called the thermosphere 100 400 500 600 700 800 900 1000 Temperature (Kº)

The upper atmosphere Like the stratosphere, the temperatures in the thermosphere get hotter with altitude because of absorption of solar ultraviolet radiation. The difference is that ozone absorbs light that is just out of the visible range. The thermospheric gas absorbs extreme ultraviolet and X-rays.

The upper atmosphere Sunspot cycle - Solar Visible Variation The thermospheric gas absorbs extreme ultraviolet and X-rays. The sun is much more variable at these wavelengths over its 11-year sunspot cycle than it is in visible/near-UV wavelengths.

The upper atmosphere Sunspot cycle - Solar Extreme UV Variation The thermospheric gas absorbs extreme ultraviolet and X-rays. The sun is much more variable at these wavelengths over its 11-year sunspot cycle than it is in visible/near-UV wavelengths.

Solar Maximum Temp Solar Minimum Temp 400 300 Altitude (kilometers) Solar Maximum Temp 200 Solar Minimum Temp 100 The variation in Extreme ultraviolet radiation between solar minimum and solar maximum is so large that the temperature in the thermosphere vary by 100s of degrees 400 500 600 700 800 900 1000 Temperature (Kº)

Solar Maximum Temp Solar Minimum Temp 400 300 Altitude (kilometers) Solar Maximum Temp 200 Solar Minimum Temp 100 The variation in Extreme ultraviolet radiation between solar minimum and solar maximum is so large that the temperature in the thermosphere vary by 100s of degrees 400 500 600 700 800 900 1000 Temperature (Kº)

Solar Maximum Density Solar Minimum Density 400 With the huge increase in temperatures, you find that the pressure does not decrease so rapidly with height, especially at solar maximum. 300 Altitude (kilometers) 200 Solar Minimum Density 100 1e-5 1e-4 0.001 0.01 0.1 1 10 100 1000 Pressure (hPa)

Who cares about the density of the thermosphere? March 23, 2001 Mir Reentry

These guys care Feb ‘06 ISS altitude 344 km - Time for a boost!

Finally, Composition Gas Percent of Atmosphere

Finally, Composition Gas Percent of Atmosphere Nitrogen, N2

Finally, Composition Gas Percent of Atmosphere Nitrogen, N2 78%

Finally, Composition Gas Percent of Atmosphere Nitrogen, N2 78% Oxygen, O2

Finally, Composition Gas Percent of Atmosphere Nitrogen, N2 78% Oxygen, O2 21%

Finally, Composition Gas Percent of Atmosphere Nitrogen, N2 78% Oxygen, O2 21% Carbon Dioxide,CO2 0.036% +0.0014% per decade

Finally, Composition O O2 N2 500 400 Altitude (kilometers) 300 200 100 Lower Density Higher In the thermosphere, the gas is thin enough that the separate species can separate out by mass. The proportions change dramatically with altitude. Atomic Oxygen dominates above 150-200 km.

Finally, Ion Composition 500 Nighttime Daytime 400 O+ Altitude (kilometers) 300 200 NO+ O+ 100 N2+ Lower Density Higher The thermosphere is the source of all the plasma in the ionosphere. These region coexist, and interact strongly.

Studying the Upper Atmosphere Ultraviolet imagers in orbit can be used to observe composition changes in the thermosphere induced by auroral storms. Noon Noon Noon

Studying the Upper Atmosphere Ultraviolet imagers in orbit can be used to quantify the energy input to high latitudes that cause thermospheric and ionospheric storms

Studying the Upper Atmosphere The same ultraviolet imagers can be used to study the ionosphere. It is actually very structured and dynamic.

Studying the Upper Atmosphere Solar Flare effects on the upper atmosphere can also be measured with orbiting ultraviolet imagers.

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