Auringosta purkautuu jatkuva plasmavirta, jota kutsutaan aurinkotuuleksi

Solar wind driven particle precipitation affects winter climate in polar regions

Changes in space climate driven by long-term changes in solar activity have a significant impact on Earth’s atmosphere and climate. Understanding the complex system requires cooperation between space physics and climate science.

The PROSPECT project studies the long-term forecasting of solar wind, geomagnetic activity and the energetic particles of near space and their effects on the climate.

“In a few years we hope to be able to connect changes in space climate with long-term weather predictions. This could improve forecasting average winter weather, for example”, says Docent Timo Asikainen at the Space Climate research group at the University of Oulu.

Particles destroy ozone in the upper atmosphere

The Sun emits a constant flow of plasma known as the solar wind. As it flows from regions of open magnetic field in the solar corona, the solar wind forms fast flows that cause charged particles of near-Earth space to precipitate down into the upper atmosphere. There they create the colourful auroral lights.

The aurorae are a fascinating phenomenon, but Timo Asikainen is more interested in the impact of particle precipitation on the Earth's atmosphere and climate.

Since the 1970s it has been known that in addition to the aurorae, particle precipitation from space into the upper atmosphere causes formation of molecules which can destroy ozone.

The ozone layer absorbs UV radiation from the Sun, forming the stratosphere at an altitude of about 10-50 km, where the temperature rises with altitude. The temperature of the stratosphere depends on the amount of ozone, and when this is altered as a result of particle precipitation, the air pressure and the strength of winds also change. The dynamics of the entire stratosphere change.

“In the past 15 years we have learned that particle precipitation from space changes not only changes the conditions in the stratosphere. The stratospheric changes also affect the lower atmosphere, thus affecting winds and weather on the surface of the Earth”, Timo Asikainen says.

An observational image of energetic electrons raining into the atmosphere at a height of about 100 km and triggering a chain of chemical reactions (molecular balloons) which, further down in the stratosphere, produce ozone destruction at an altitude of about 30-50 km. Ozone devastation colds the polar stratosphere, thereby enhancing the polar orb of the stratosphere circulating around the polar region (a strong wind blowing from the west, represented by curving arrows which also move downwards). Further down on the surface of the earth, this enhanced polar vortex also affects winds and directs them from the Atlantic to northern Europe (red arrows down).
The picture also shows one of POES, Polar Orbiting Environmental Satellite satellites, which measure particles entering the atmosphere on the polar track.


Particle precipitation partly explains the NAO variations

The US National Oceanic and Atmospheric Administration (NOAA) has been sending satellites into polar orbit around Earth since the late 1970s. Among other things, the satellites measure energetic particles which strike the atmosphere in the Auroral oval.

This 40-year dataset is interesting for both space and climate scientists, and the material has also opened new possibilities for the study of the climatic impact of space climate.

“We are interested in the winter climate because the particle precipitation can destroy ozone when the Sun does not shine - that is, in winter. In the summer sunshine quickly destroys the molecules that destroy ozone”, Asikainen says.

In winter, the polar vortex, a strong westerly wind, is formed in the stratosphere, which also affects winds on the surface of the Earth. When large amounts of particles precipitate down into the upper atmosphere, they lead to ozone loss, altering the temperature of the stratosphere enough to strengthen the polar vortex. This is reflected on the Earth’s surface as a positive NAO index. The NAO (North Atlantic Oscillation) phenomenon characterizes the location and strength of large air pressure centres in the Atlantic and is the dominant factor affecting the winter weather in large regions of the Northern Hemisphere.

In the positive phase NAO produces windy, rainy, and mild winters in Northern Europe, like the ongoing winter in Finland. A weak polar vortex produces negative NAO and brings us a severe winter.

“When the satellite measurements of particle precipitation were compared with temperatures measured around the world in the same period of time, we got a statistically significant correlation for large areas in the Northern Hemisphere”, Asikainen says.

Intense particle precipitation is associated to mild weather in northern parts of Eurasia, and cold weather in North America. NAO causes a similar pattern: when the NAO index is positive, strong westerly winds keep the winter warm and rainy in Northern Europe. When the NAO index is negative, winds from the Arctic Ocean lead to severe winter weather.

On the right, a picture of the Sun taken at the wavelength of visible light, i.e. like a regular camera at very short shutter speed, visible sunspot groups. The time series in the image illustrate a few long series of data used in space air research.
On green: approximately 40 years of direct satellite measurements, a combination of energetic electrons coming into the Earth's atmosphere.
In red: from geomagnetic measurements reconstructed estimate of the speed of the solar wind in the last hundred years.
With purple: the longest unified time series for geomagnetic activity (the so-called AA index), starting from 1868 and continuing to the present day.
In blue: 400 year series of sunspots. This set of data is the longest indicator of solar activity based on direct measurements.


Indirect information from the phases of sunspot cycles

Forty years is a short period of time in climate studies, and consequently, Timo Asikainen is also interested in much longer time periods. Historical records of geomagnetic disturbances and sunspots give indirect information on the particle precipitation. Geomagnetic disturbances have been measured from the mid-19th century and sunspots from the 17th century.

“The fast solar wind streams emanating from coronal holes maximize in the declining phase of the sunspot cycle, 3 - 4 years after the sunspot maximum. This is when geomagnetic disturbances usually peak too. When we know the times of the sunspot maxima and minima, we also know when particle precipitation has maximized”, says Asikainen.
When these indirect measures of particle precipitation were compared with temperatures on the Earth's surface over a long period, the particles explained about a fifth of the variation in the NAO index.

However, the situation is complicated by the fact that the internal state of the atmosphere seems to affect the strength of the particle precipitation mechanism. If these internal factors could be understood in greater detail, the effects of the particles could be predicted better.

“If the stratospheric wind at the equator blows from the west, the effect of the particle precipitation on the polar vortex is not visible.  But if we only take into account the winters when the wind in the stratosphere over the equator blows from the east, and compare the particle precipitation with the NAO index, the particles explained nearly half of the variation in the NAO index in the past 40 years”, Asikainen says.

Most of the variability in average winter weather is related to internal variation in the climate system. For example, particle precipitation does not explain the mildness of the current winter, as it has been rather low, while the NAO index has been strongly positive. Therefore, the role that the particle precipitation plays in the big picture still requires much study.

“The next sunspot cycle should start soon and in 5-6 years the cycle will start to decline. Then we can expect fast solar wind flows and increased particle precipitation. It will probably bring us more mild winters.”

Measurements are made using satellites, which are replaced every few years, and the results of the measurements have not been directly comparable. Timo Asikainen has spent several years calibrating satellite measurement results and has transformed the data into a single series. Photo: Juha Sarkkinen

Text: Satu Räsänen

Main image: The images of the wavelength combined with the image of the Sun showing magnetically active areas (bright) and large coronal gaps (dark ones), from which fast solar wind flows (yellow lines).


Last updated: 18.3.2020