Space Weather

Living With a Star Guide to Space Weather

Just as cities can be buffeted by winds, flood, and rain, so too the entire Earth is bathed in a million-mile-per-hour wind of charged particles from the Sun that affects many spacecraft and terrestrial systems such as communication and electrical power networks. Understanding the solar wind, its magnetic field, and the energetic particles that flow through it from the Sun and our galaxy will also be crucial for human exploration in space. The state, dynamics, and effects on the Earth and human concerns of the particles and fields outside the Earth's immediate environment are what is now termed "space weather." This page provides a number of links to images, movies, and graphs that will give you a view of the current weather in space; many also provide data from earlier observations. Two good overview sites are SpaceWeather.com and NOAA/SEC Today's Space Weather. If you want to go directly to links to answer specific space weather questions, click here, or read on for a quick overview.

The Sun and Solar Wind

Space weather begins below the surface of the Sun where the turbulent overturning of the outer layer of the Sun (the convection zone) continually drives magnetic fields through the solar surface (photosphere). These magnetic fields provide the dominant energy above the photosphere, especially in the very hot (millions of Kelvins) corona that starts after a transition region within a very small distance of the photosphere. The release of magnetic energy, in not fully understood ways, leads to the rapid energization of charged particles through the flaring of active regions and the release of massive loops of field and plasma known as Coronal Mass Ejections (CMEs).

The solar energy releases and their precursor regions are visible in photographs of the Sun and its surrounding atmosphere. The flare particles and associated X-rays move rapidly toward the Earth where they are measured by orbiting spacecraft. The radiation comes straight to us and the particles move along the spiraling interplanetary magnetic field lines, arriving somewhat later. The CMEs, which may or may not have associated flares, take one to four days to reach the Earth, accelerating particles along the way, and causing the major "magnetic storms" when they reach the Earth. Spacecraft orbiting outside of the Earth's magnetic influence measure the interplanetary fields before they reach the Earth, giving an hour or so warning of the impending effects.

In addition to these sporadic releases of energy, the Sun's atmosphere is continually flowing outwards to form the solar wind, of which CMEs are only a part. The solar wind consists of both charged particles and associated magnetic fields. Areas in which the magnetic field open into the interplanetary medium are dark in X-ray and many EUV images of the Sun are called coronal holes. Solar wind plasma from coronal holes flows more quickly than plasma from other areas on the Sun, and the presence of a hole near the sun's equator facing Earth can also lead to disturbances in Earth's magnetic field (the "magnetosphere").

The Earth's Magnetic Field and Upper Atmosphere

The Earth's magnetosphere and ionosphere respond directly and indirectly to solar radiation, and solar wind plasma, energetic particles, and magnetic fields. On the sunlit side of the Earth, the enhanced radiation associated with solar flares enhances ionospheric ionization and can disrupt radio communication. If the Sun falls within the field of view of a ground radar, solar radio waves can disrupt the radar's operation. The Earth's magnetic field channels energetic particles into the polar caps, where they can also enhance ionization and disrupt communication. Southward solar wind magnetic fields enable widespread entry of solar wind mass, energy, and momentum into the region of space dominated by the Earth's magnetic field. The energy entering the dayside magnetosphere is generally transferred to the nightside and stored within the stretched magnetic field lines of the Earth's magnetotail.

This transfer of energy results in the antisunward motion of ionospheric plasma over the Earth's polar caps. This flow can be observed directly by radars, or inferred from ground magnetometer observations of ionospheric currents. During quasi-periodic (~3 hours) substorms, field lines snap back into the familiar dipolar (bar magnet) configuration, injecting copious fluxes of newly energized ions and electrons into the Van Allen radiation belts of the inner magnetosphere. Some of the ions and electrons precipitate (stream along) magnetic field lines into the Earth's atmosphere, where they create beautiful (and as yet poorly understood) optical and ultraviolet displays of aurorae in the high latitude nightside sky, as well as repeatable and significant variations in the magnetic field strength and direction observed on the ground at high latitudes. During sequences of substorms, known as geomagnetic storms, radiation levels within the magnetosphere can rise by many orders of magnitude. The radiation is dangerous for astronauts and spacecraft. The injected ions and electrons drift in opposite directions around the Earth, resulting in a ring current that depresses and disturbs magnetic field strengths and directions over the entire surface of the Earth. During large storms, the Earth's field may be so weakened that aurora can penetrate down to latitudes over the continental United States.

Links:

LWS Space Weather Data FAQ
Space Weather Primer, an overview of the affects of space weather from the NOAA/Space Envioronment Center.
A guide to using online data for space weather forecasting from the Royal Observatory of Belgium.

Draft web site 6/18/04, tak