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Earth’s Upper Atmosphere Cooling Dramatically, Cosmic Rays Continue To Increase As Deep Solar Minimum Approaches

Meteorologist Paul Dorian, Perspecta, Inc.

New data from NASA’s SABER instrument on board NASA’s TIMED satellite confirms that our atmosphere is losing heat energy near the edge of space as we approach solar minimum. In fact, if current trends continue, it could soon set a Space Age record for cold.

Daily observations of the number of sunspots since 1 January 1977 according to Solar Influences Data Analysis Center (SIDC). The thin blue line indicates the daily sunspot number, while the dark blue line indicates the running annual average. The recent low sunspot activity is clearly reflected in the recent low values for the total solar irradiance. Compare also with the geomagnetic Ap-index. Data source: WDC-SILSO, Royal Observatory of Belgium, Brussels. Last day shown: 31 October 2018. Last diagram update: 1 November 2018.


The sun is blank again today and has been without sunspots about 60% of the time this year as the current historically weak solar cycle heads towards the next solar minimum. Solar cycle 24 is currently on pace to be the weakest sunspot cycle with the fewest sunspots since cycle 14 peaked in February 1906. Solar cycle 24 continues a recent trend of weakening solar cycles which began with solar cycle 21 that peaked around 1980. The last time the sun was this blank in a given year on a percentage basis was 2009 during the last solar minimum when 71% of the time was spotless. That last solar minimum actually reached a nadir in 2008 when an astounding 73% of the year featured a spotless sun – the most spotless days in a given year since 1913. All indications are that the fast-approaching next solar minimum may be even quieter than the last one which was the deepest in nearly a century.

One of the natural consequences of a solar minimum is for the upper part of the Earth’s atmosphere to cool down. Another natural impact of decreasing solar activity is the weakening of the ambient solar wind and its magnetic field which, in turn, allows more cosmic rays to penetrate the solar system. The intensification of cosmic rays can have important consequences on such things as the safety of airline passengers and astronauts in space, Earth’s cloud cover and climate, and possibly even on lightning.


Possible record-breaking cooling in the thermosphere

One of the most important ways the solar cycle affects our planet is by cooling off the thermosphere during solar minimum. The thermosphere is the part of the earth’s atmosphere that begins at about 50 miles (80 kilometers) above the earth’s surface, extends to outer space, and is characterized by steadily increasing temperature with height. New data from NASA’s SABER instrument on board NASA’s TIMED satellite confirms the notion that our atmosphere is losing heat energy near the edge of space as we approach solar minimum. In fact, if current trends continue, it could soon set a Space Age record for cold according to NASA. There is good news and bad news in all of this. The good news is that when the thermosphere cools, it actually shrinks, thereby reducing drag on satellites in low Earth orbit which can increase the life of a satellite. The bad news is that this cooling aloft tends to delay the natural decay of space junk resulting in a more cluttered environment around Earth.

Thermosphere Climate Index (TCI)

The SABER instrument has been in orbit for only 17 years and it monitors infrared radiation from carbon dioxide (CO2) and nitric oxide (NO), two substances that play a vital role in the energy output of our thermosphere, the very top level of our atmosphere. By measuring the infrared glow of these molecules, SABER can assess the thermal state of gas at the very top of the atmosphere in the thermosphere. NASA has actually created an index called the Thermosphere Climate Index (TCI) to measure how much NO is dumped from the thermosphere into outer space. During the solar maximum phase, TCI is very high and it is low at the time of solar minimum.

According to NASA, the TCI is currently very low indeed as SABER is measuring 33 billion watts of infrared power from NO which is ten times smaller than what is seen during more active phases of a solar cycle. In fact, TCI numbers are very close to setting a record and that could very well happen sometime during the next several months (source). Although SABER has been in orbit for only 17 years, NASA has calculated the TCI going all the way back to the 1940s. According to NASA, “SABER taught us to do this by revealing how TCI depends on other variables such as geomagnetic activity and the sun’s UV output–things that have been measured for decades”.

Above: An historical record of the Thermosphere Climate Index. Mlynczak and colleagues recently published a paper on the TCI showing that the state of the thermosphere can be discussed using a set of five plain language terms: Cold, Cool, Neutral, Warm, and Hot. Source:

Cosmic Rays

Galactic cosmic rays are high-energy particles originating from outside the solar system that can impact the Earth’s atmosphere. Our first line of defense from cosmic rays comes from the sun as its magnetic field and the solar wind combine to create a ‘shield’ that fends off cosmic rays attempting to enter the solar system. The shielding action of the sun is strongest during Solar Maximum and weakest during Solar Minimum with the weakening magnetic field and solar wind. The intensity of cosmic rays varies globally by about 15% over a solar cycle because of changes in the strength of the solar wind, which carries a weak magnetic field into the heliosphere, partially shielding Earth from low-energy galactic charged particles

Evidence of an increase in stratospheric radiation

One way to monitor cosmic ray penetration into the Earth’s upper atmosphere is to measure stratospheric radiation over an extended period of time. “” has led an effort since 2015 to monitor radiation levels in the stratosphere over the US with frequent high-altitude helium balloon flights. These balloons contain sensors which detect X-rays, gamma-rays and neutrons in the stratosphere produced by the crash of primary cosmic rays into Earth’s atmosphere. The sensors sample energies between 10 keV and 20 MeV, spanning the range of medical X-ray machines, airport security devices, and “killer electrons” in Earth’s radiation belts.

Above: Dose rates at the Regener-Pfotzer Maximum, a layer of peak radiation about 20 km above Earth’s surface. Source

The initial monitoring effort focused on the state of California, but has since expanded to a dozen other states including the following: NV, OR, WA, ID, WY, KS, NE, MO, IL, ME, NH, VT. Every one of these states has seen an upward trend in radiation according to ranging from +20% in central California to +33% in Maine. The latest points (circled in red) were gathered during a ballooning campaign in August-October 2018.

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