A layer deep in the solar atmosphere can be used to estimate the speed of the solar wind, a stream of electrified gas that constantly blows from the Sun. Estimating the speed of the solar wind will improve space weather forecasts, enhancing our ability to protect communications, navigation, and other satellites from the effects of solar storms. We will also be able to warn human explorers on their way to the planets of the severity of those storms.

The sun's atmosphere is threaded with magnetic fields (yellow lines). Areas with closed magnetic fields give rise to slow, dense solar wind (short, dashed, red arrows), while areas with open magnetic fields -- so-called "coronal holes" -- yield fast, less dense solar wind streams (longer, solid, red arrows). In addition to the permanent coronal holes at the Sun's poles, coronal holes can sometimes occur closer to the Sun's equator, as shown here just right of center. Credit: ESA/NASA

The solar wind flows from the Sun's hot, thin, outer atmosphere, the "corona". The researchers were surprised to discover that the structure of the Sun's cooler, dense lower atmosphere, called the chromosphere, could be used to estimate the speed of the solar wind.

This was unexpected because the solar wind originates in the corona, and the chromosphere is much deeper: it lies just above the Sun's visible surface. "It's like discovering that the source of the river Nile is another 500 miles inland," said Dr. Scott McIntosh of the Southwest Research Institute, Boulder, Colo., lead author of a paper on this research published in the Astrophysical Journal.

The new work promises to increase the accuracy of space radiation forecasts. The Sun occasionally launches billion-ton blasts of electrified gas, called coronal mass ejections (CMEs), into space at millions of miles (kilometers) per hour. If a fast CME is plowing through slow solar wind, a shock builds up in front of the CME that accelerates the electrically charged solar wind particles. These fast particles can disrupt satellites and are hazardous to unprotected astronauts.

"Just as knowing more details about the atmosphere helps to predict the intensity of a hurricane, knowing the speed of the solar wind helps to determine the intensity of space radiation storms from CMEs," said co-author Dr. Robert Leamon of L-3 Government Services at NASA's Goddard Space Flight Center, Greenbelt, Md.

Like wind on Earth, the solar wind is gusty, ranging in speed from about 750,000 miles per hour (approximately 350 km/second) to 1.5 million miles per hour (700 km/second).

Since the solar wind is made up of electrically-charged particles, it responds to magnetic fields that permeate the solar atmosphere. Solar wind particles flow along invisible lines of magnetic force like cars on a highway. When the magnetic field lines bend straight out into space, as they do in "coronal hole" regions, the solar wind acts like cars on a drag strip, racing along at high speed. When the magnetic field lines bend sharply back to the solar surface, like the pattern of iron filings around a bar magnet, the solar wind acts like cars in city traffic and emerges relatively slowly. Scientists have known this for over thirty years and used it to give a crude estimate for the speed of the solar wind -- either fast or slow.

In the new work, the team has tied the speed of the solar wind as it blows past Earth to variations deeper in the solar atmosphere than had previously been detected (or even expected). By measuring the time taken for a sound wave to travel between two heights in the chromosphere, they were able to determine that the chromosphere is effectively "stretched thin" below coronal holes with their open magnetic fields, but compressed below magnetically closed regions.

The team used the observation to derive a continuous range of solar wind speeds from the structure of the chromosphere. The wider the chromospheric layer is, the more it is being allowed to expand by open magnetic fields and the faster the solar wind will blow. This new method is more precise than the old "fast or slow" estimate.

NASA's Transition Region and Coronal Explorer (TRACE) spacecraft was used to measure the speed of sound waves in the chromosphere, and NASA's Advanced Composition Explorer (ACE) spacecraft was used to take measurements of the solar wind speed as it blew by the Earth. Comparing the data from the two spacecraft gave the connection.

"Prior to this discovery, we could only determine solar wind speed from spacecraft that were roughly in line between the Earth and the Sun, like ACE, WIND, and the Solar and Heliospheric Observatory. This spacecraft fleet was placed along the Earth-Sun line because we need to know about the space weather coming our way. However, compared to the size of our solar system, this is a very narrow range; it's like looking through a soda straw. With this discovery, we can use TRACE to build up images that can predict the solar wind speed throughout half the solar system," said Dr. Joe Gurman, a solar researcher at NASA Goddard.