Space weather forecasting still difficult due to insufficient observations
Coronal mass ejections (CME), huge clouds of plasma and magnetic fields, are the cause of nearly all major geomagnetic storms. In recent years, computer simulations developed for forecasting CMEs have improved the capacity to predict these events, but the accuracy of the forecasts is still modest. Forecasting CMEs is not easy due to the long distance between the Sun and the Earth, the sparse observation network, and the difficulty of assessing the structure of CMEs, explains Academy Research Fellow Emilia Kilpua.
Our society is becoming increasingly dependent on technology that can be damaged by space weather. When a CME enters the Earth's magnetosphere, it causes various severe changes in near-Earth space, for example, in auroral currents and the radiation belts surrounding the Earth. The changes provide us with magnificent aurora displays, but they are also the cause of harmful disruptions in GPS tracking systems and radio traffic, as well as satellite and power failures. For this reason, space weather forecasting is important.
Preferably the users of space weather services require a 24-hour notice. This means that the forecasts are based on indirect solar observations. Determining the direction and speed of earthbound CMEs is very difficult. The plasma clouds can only be observed with any reliability for the first tenth of their journey towards the Earth once they have expelled away from the Sun. After a CME has been reliably observed for the final time, its direction and speed can change significantly.
”As a result, a CME predicted to hit Earth with direct impact may only graze the Earth's magnetosphere, causing very little solar activity. Correspondingly, the trajectory of the limb CME may change so that it hits the Earth and causes a major geomagnetic storm. If the orientation of the CME's magnetic field is northward, it will not cause a storm, even if this powerful high-speed CME does hit Earth with direct impact. Similarly, interaction with surrounding solar wind and other CMEs may significantly alter the structure of the CME and its ability to drive geomagnetic storms," says Kilpua.
In 2006, NASA launched STEREO probes that have enabled the imaging of plasma clouds from various angles, improving the ability to assess the direction and speed of CMEs. However, the distance of STEREO probes changes constantly, and therefore they cannot be used in continuous space weather forecasting.
The University of Helsinki's Kumpula Space Centre performs a wide range of basic research that benefits space weather forecasting. It produces services for society, such as solar activity-level reporting, and in the future, it will provide forecasts based on solar observations. Kilpua's research team uses STEREO measurements to study how CMEs rotate and change direction after being ejected from the Sun.
“We have a particular interest in understanding how solar super storms are born,” Kilpua says. In July 2012, a powerful plasma cloud missed the Earth, but collided with a STEREO-A probe. The eruption was one of the strongest ever observed, and it is believed to have been even as strong as the one that caused the Carrington event in 1859, during which aurora displays were seen in Hawaii and the telegraph network was inoperable for a long time. Super storms are often caused by a cloud of plasma and magnetic field interacting with the Earth's magnetic field.
More information: Academy Research Fellow Emilia Kilpua, emilia.kilpua(at)helsinki.fi
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