Jan 27, 2020
Solar Cycle 25 is intensifying--and Earth's upper atmosphere is responding.

"The Thermosphere Climate Index (TCI) is going up rapidly right now," reports Linda Hunt of Science Systems and Applications, Inc. "It has nearly tripled in the past year."

TCI is a number published daily by NASA, which tells us how hot Earth's upper atmosphere is. The thermosphere*, our atmosphere's highest layer, literally touches space and is a sort of "first responder" to solar activity. Hunt created this plot showing how TCI has unfolded during the last 7 solar cycles. Solar Cycle 25 (shown in blue) is just getting started:


"So far Solar Cycle 25 is well ahead of the pace of Solar Cycle 24," notes Hunt. If this trend continues, the thermosphere could soon hit a 20-year high in temperature.

Before we go any farther, a word of caution: This does not mean Earth is about to heat up. The thermosphere is hundreds of kilometers above our heads. Here on the planet's surface we do not feel its heat; summer days are no warmer when TCI is "hot." As Dr. Marty Mlynczak of NASA notes, "energy driving the climate system near Earth's surface is hundreds of thousands of times greater than in the thermosphere." As far as we know, cyclical warming and cooling of the thermosphere by the solar cycle does not affect climate.

Nevertheless, the thermosphere is important. When it heats up, as it is doing now, it also puffs up. Think of a marshmallow held over a campfire. The thermosphere can expand upward so much it actually touches Earth-orbiting satellites. Almost 40 Starlink satellites fell out of the sky earlier this year as a result of aerodynamic drag up there.


TCI might also have some predictive value. Hunt's plot shows that the index is on an upward trajectory that most closely mimics Solar Cycle 20, which peaked back in the 1970s. Solar Cycle 20 was an above average solar cycle with plenty of solar activity. Coincidentally, a new prediction for Solar Cycle 25 based on the arrival of the Termination Event suggests the same thing: 25 could be the new 20. If this turns out to be true, Solar Cycle 25 would be stronger than its immediate predecessor, weak Solar Cycle 24.

You can follow the progress of TCI as Solar Cycle 25 unfolds. It is published every day right here on


* Thermosphere is a layer of Earth's atmosphere that is directly above the mesosphere and below the exosphere. It extends from about 90 km (56 miles) to between 500 and 1,000 km (311 to 621 miles) above our planet.

The thermosphere is directly above the mesosphere and below the exosphere. Earth's ionosphere, composed of several regions overlaps with and shares the same space as the thermosphere. UCAR/Randy Russell

Temperatures climb sharply in the lower thermosphere (below 200 to 300 km altitude), then level off and hold fairly steady with increasing altitude above that height. Solar activity strongly influences temperature in the thermosphere. The thermosphere is typically about 200° C (360° F) hotter in the daytime than at night, and roughly 500° C (900° F) hotter when the Sun is very active than at other times. Temperatures in the upper thermosphere can range from about 500° C (932° F) to 2,000° C (3,632° F) or higher.

The boundary between the thermosphere and the exosphere above it is called the thermopause. At the bottom of the thermosphere is the mesopause, the boundary between the thermosphere and the mesosphere below.

Although the thermosphere is considered part of Earth's atmosphere, the air density is so low in this layer that most of the thermosphere is what we normally think of as outer space. In fact, the most common definition says that space begins at an altitude of 100 km (62 miles), slightly above the mesopause at the bottom of the thermosphere. The space shuttle and the International Space Station both orbit Earth within the thermosphere!

Below the thermosphere, gases made of different types of atoms and molecules are thoroughly mixed together by turbulence in the atmosphere. Air in the lower atmosphere is mainly composed of the familiar blend of about 80% nitrogen molecules (N2) and about 20% oxygen molecules (O2). In the thermosphere and above, gas particles collide so infrequently that the gases become somewhat separated based on the types of chemical elements they contain. Energetic ultraviolet and X-ray photons from the Sun also break apart molecules in the thermosphere. In the upper thermosphere, atomic oxygen (O), atomic nitrogen (N), and helium (He) are the main components of air.

Much of the X-ray and UV radiation from the Sun is absorbed in the thermosphere. When the Sun is very active and emits more high-energy radiation, the thermosphere gets hotter and expands or "puffs up". Because of this, the height of the top of the thermosphere (the thermopause) varies. The thermopause is found at an altitude between 500 km and 1,000 km or higher. Many satellites orbit within the thermosphere and changes in the density of (the very, very thin) air at orbital altitudes, brought on by heating and expansion of the thermosphere, generates a drag force on satellites. Engineers must take this varying drag into account when calculating orbits, and satellites occasionally need to be boosted higher to offset the effects of the drag force.

High-energy solar photons also tear electrons away from gas particles in the thermosphere, creating electrically-charged ions of atoms and molecules. Earth's ionosphere, composed of several regions of such ionized particles in the atmosphere, overlaps with and shares the same space with the electrically neutral thermosphere.

Like the oceans, Earth's atmosphere has waves and tides within it. These waves and tides help move energy around within the atmosphere, including the thermosphere. Winds and the overall circulation in the thermosphere are largely driven by these tides and waves. Moving ions, dragged along by collisions with the electrically neutral gases, produce powerful electrical currents in some parts of the thermosphere.

Finally, the aurora (the Southern and Northern Lights) primarily occur in the thermosphere. Charged particles (electrons, protons, and other ions) from space collide with atoms and molecules in the thermosphere at high latitudes, exciting them into higher energy states. Those atoms and molecules shed this excess energy by emitting photons of light, which we see as colorful auroral displays.


Now we understand the mechanism which caused so many Starlink satellites to slow and burn up in the upper atmosphere - the heat expansion of the thermosphere which caused the drag on the low Earth satellites.
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