The NASA Radiation Belt Storm Probes (RBSP) mission addresses how populations of high energy charged particles are created, vary, and evolve in space environments, and specifically within Earth's magnetically trapped radiation belts. RBSP, with a nominal launch date of August 2012, comprises two spacecraft making in situ measurements for at least 2 years in nearly the same highly elliptical, low inclination orbits (1.1 × 5.8 RE, 10• ). The orbits are slightly different so that 1 spacecraft laps the other spacecraft about every 2.5 months, allowing separation of spatial from temporal effects over spatial scales ranging from ∼0.1 to 5 RE. The uniquely comprehensive suite of instruments, identical on the two spacecraft, measures all of the particle (electrons, ions, ion composition), fields (E and B), and wave distributions (dE and dB) that are needed to resolve the most critical science questions. Here we summarize the high level science objectives for the RBSP mission, provide historical background on studies of Earth and planetary radiation belts, present examples of the most compelling scientific mysteries of the radiation belts, present the mission design of the RBSP mission that targets these mysteries and objectives, present the observation and measurement requirements for the mission, and introduce the instrumentation that will deliver these measurements. This paper references and is followed by a number of companion papers that describe the details of the RBSP mission, spacecraft, and instruments.

The NASA Radiation Belt Storm Probes (RBSP) mission addresses how populations of high energy charged particles are created, vary, and evolve in space environments, and specifically within Earth's magnetically trapped radiation belts. RBSP, with a nominal launch date of August 2012, comprises two spacecraft making in situ measurements for at least 2 years in nearly the same highly elliptical, low inclination orbits (1.1 × 5.8 RE, 10 • ). The orbits are slightly different so that 1 spacecraft laps the other spacecraft about every 2.5 months, allowing separation of spatial from temporal effects over spatial scales ranging from ∼0.1 to 5 RE. The uniquely comprehensive suite of instruments, identical on the two spacecraft, measures all of the particle (electrons, ions, ion composition), fields (E and B), and wave distributions (dE and dB) that are needed to resolve the most critical science questions. Here we summarize the high level science objectives for the RBSP mission, provide historical background on studies of Earth and planetary radiation belts, present examples of the most compelling scientific mysteries of the radiation belts, present the mission design of the RBSP mission that targets these mysteries and objectives, present the observation and measurement requirements for the mission, and introduce the instrumentation that will deliver these measurements. This paper references and is followed by a number of companion papers that describe the details of the RBSP mission, spacecraft, and instruments.

Magnetospheric substorms explosively release solar wind energy previously stored in Earth's magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at approximately 10 R(E) (R(E): Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at approximately 20 to 30 R(E). We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 R(E), at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

We have assembled a data set of 1821 magnetopause crossings. Separate fits to subsets of this data set determine the magnetopause location as a function of solar wind dynamic pressure and interplanetary magnetic field orientation. Solar wind dynamic pressure variations produce self‐similar magnetopause motion on time scales of one hour or longer. We verify the pressure balance relationship between the solar wind dynamic pressure and the location of the subsolar magnetopause. We quantify the relationship between the IMF Bz, region l Birkeland current strength, the position of the subsolar magnetopause, and the shape of the dayside magnetosphere. Cross sections of the dayside magnetopause in planes perpendicular to the Earth‐Sun line are oblate.

Brief, impulsive, large-amplitude (•ip/p -1) solar wind dynamic pressure pulses, recurring on time scales of 5 to 15 min, are common just upstream of the Earth's bow shock. When each pulse strikes the magnetopause, it launches a fast-mode compressional wave in the magnetosphere that can propagate antisunward faster than the magnetosheath flow. Consequently, the magnetopause bulges outward ahead of each contraction associated with a pressure pulse. These ridges generally propagate antisunward, although sunward motion is common on the early post-noon magnetopause. The greatest amplitude (-1 to 2 RE) magnetopause motion occurs on the prenoon magnetopause, at high-latitudes, and during periods of southward interplanetary magnetic field. The signatures of the pressure-pulse-driven magnetopause motion include a bipolar magnetic field signature normal to the nominal magnetopause, a rotation of the magnetic fidd away from both magnetosheath and magnetospheric orientations, a mixture of magnetosheath and magnetospheric plasmas, and high-speed magnetosheath plasma flows. The magnetopause boundary motion, in turn, drives transient compressions and shears in the dayside magnetospheric magnetic field. These compressions and shears map to the dayside auroral ionosphere, where the ground signatures produced by a single, brief, solar wind dynamic pressure pulse are an antisunward moving (sunward at early post-noon local times) double-convection vortex, associated with northsouth magnetic field perturbations, increased ELF/VLF wave activity, precipitating particles, and cosmic noise absorption. The ionospheric and magnetospheric signatures driven by solar wind pressure pulses greatly resemble those previously associated with flux transfer events. SOLAR WIND AND MAGNETOSHEATH DYNAMIC PRESSURE VARIATIONSIn this section, we consider the characteristics of previously reported solar wind and magnetosheath dynamic pressure variations, emphasizing the variations associated with solar wind shocks, holes, and tangential discontinuities. We discuss recent evidence that the bow shock itself may modulate the solar wind dynamic pressure applied to the magnetosphere. Solar WindAt least three solar wind features are associated with significant dynamic pressure variations: shocks, holes, and tangential discontinuities. The properties of corotating and traveling solar wind shocks are relatively well known. They bring increases (and occasionally decreases) in the solar wind density, velocity, and dynamic pressure to the Earth every several hours to days [Burlaga, 1969]. Although the dynamic pressure can increase by a factor of as much as 20 across solar wind shocks, factors of 3 are more common [Siscoe et al., 1968b]. Corotating shocks are aligned with the spiral IMF [Siscoe, 1972].By contrast, the dynamic pressure changes associated with tangential discontinuities [Burlaga, 1968[Burlaga, , 1969 and holes [Turner et al., 1977] are poorly known, partly because high time resolution plasma parameters were not previously available. Such small-scale fe...