What is NASA's THEMIS Mission?
THEMIS is an acronym for Time History of Events and Macroscale Interactions during Substorms.
THEMIS is NASA's first five identical-satellite mission, launched as a constellation on Feb. 17, 2007, to resolve the tantalizing mystery of what causes the spectacular sudden brightening of the Northern Lights or the auroral borealis - the fiery skies over Earth's North Pole. These lights are the visible manifestations of sudden large energy releases (called substorms) in near-Earth space, out to half the way to the moon. THEMIS will answer the 30-year-old question: Where and when do substorms start?
What are substorms?
Substorms are fundamental modes of explosive energy released in Earth's environment. They are often embedded within large space storms, but can also occur in isolation. It is believed that some of the most intense space storms – the ones producing the most penetrating radiation – are accompanied by substorms. Understanding substorms is a prerequisite to understanding space weather and protecting commercial satellites and humans in space from the adverse effects of particle radiation.
Are substorms always a factor in space weather?
Most large storms are punctuated by substorms. In some cases repetitive substorms, called sawtooth events, generate intense particle acceleration in Earth's radiation belts. In others, isolated substorms are related with visible auroral streamers rushing towards lower latitudes. Scientists believe substorms may act like storm catalysts, replenishing the radiation belt particles with fresh populations from large distances. The exact relationship of the substorms to the parent storm's severity is still unclear. But being an important and visible part of large storms, substorms need to be understood and modeled in order to make progress in understanding and predicting space weather phenomena.
How often do substorms occur?
A substorm is a relatively common, and typically benign phenomenon, recurring on average once per four hours, and with frequency that is 50 percent larger during spring than during winter or summer, due to the preferential orientation of Earth's bar-like magnet relative to the magnetic field that emanates from the sun. While substorms take place during both low and high solar activity, and so they are easy to find and study, they are also embedded within large storms - the ones producing intense radiation damaging satellites and threatening humans in space.
The sun's magnetic field is arranged near the ecliptic plane in a ballerina-like skirt that extends throughout the heliosphere. When Earth goes above or below this skirt, due to crossing one of its folds, it encounters high-speed streams of solar particles that cause recurrent large storms. Those recurrent storms take place typically once per 27 days, the solar rotation period. These are most pronounced in the declining phase of the solar cycle, i.e., as we approach solar minimum.
During solar maximum, the sun's magnetic field near the base of the solar corona, becomes less organized, as sunspots create multiple bar-like magnets near the solar surface. At those times the sun occasionally emits high-speed blobs of strongly magnetized, high-speed plasma called coronal mass ejections. If those ejections encounter Earth they may cause severe storms. Damaging radiation produced by interplanetary shocks ahead of the ejections can also cause severe space weather effects.
Both types of storms, the ones recurring during the declining phase of the solar cycle and the random ones at solar max, are accompanied by substorms that may enhance space weather effects. Approximately once per year, a very strong event will be marked by the creation of a new population in the radiation belts, auroras as far south as California and severe effects on the telecommunications and global positioning system (GPS) satellites. The National Oceanic and Atmospheric Administration wants to predict those strong events, both in terms of intensity and when they will occur. Toward that goal, NASA strives to understand what makes them so severe. THEMIS is a stepping stone for reaching that understanding.
What are the two competing theories that THEMIS is investigating?
Magnetospheric substorms occur in the Earth's magnetic "tail", which extends behind Earth, along its shadow, and extends beyond the moon, deep into interplanetary space. There are two main theories proposed to explain the trigger (onset) of magnetospheric substorms in the magnetotail: The Current Disruption theory, and the Magnetic Reconnection theory. The two competing theories can be distinguished by accurate timing. Timing determines the trigger mechanism and how it sets the entire magnetosphere into motion. The time history of these events and their macro-scale interactions, during substorms is the primary goal of the THEMIS mission.
The first theory suggests that Current Disruption, which occurs around 50,000 miles (80,000km) in altitude above the equator and is due to electromagnetic turbulence that disrupts the flow of intense space currents, is the substorm trigger mechanism.
The Current Disruption theory states that the cascade of events that constitutes a substorm starts close to Earth, the region where Earth's bar magnet influence starts waning and the solar wind distortion of the magnetosphere starts taking over. That region is a conduit of intense space currents that are required to flow in the equatorial plane from dawn to dusk. When the currents are weak this is possible, but when the currents get very intense (as is the case under conditions of intense solar wind energy input), that region develops electromagnetic turbulence. The laminar current path, supporting normal space current flow is disrupted. The current finds an easier path, a short-circuit, directly through the ionosphere at low altitude. The dissipation of that current is what causes the aurora to start to brighten, and the turbulent heating sets off a local implosion in space, which triggers the substorm. This is the current disruption hypothesis of the substorm onset. This phenomenon happens 1/6 of the way to the moon (roughly 50,000mi or 80,000km above the equator). Two nearby satellites are needed to measure the wave propagation and determine the trigger point of origin and onset time.
The second theory suggests that Magnetic Reconnection, which occurs even further away, at approximately 100,000 miles (160,000km) in altitude above the equator due to spontaneous conversion of magnetic energy into heat and particle acceleration, is responsible for triggering the avalanche of substorm energy.
The Magnetic Reconnection theory of substorms states that phenomena start further away, where the Earth's magnetic field is stretched into a long magnetotail, which resembles a wind-sock. In that environment, Earth's field lines connected to its two poles are stretched far away, and compressed together like stretched rubber bands. At some point they snap, and re-connect into stretched U-shaped loops, that are now free to contract. The slingshot-like contraction accelerates particles towards Earth and powers the aurora. Two satellites are needed to bracket the reconnection site and determine the precise trigger location and time of onset.