Abstract. We investigate the timing of popular substorm onset signatures to understand their temporal relationship. Proxies for substorm onsets include auroral breakups, high-latitude magnetic bays, low-latitude Pi2 bursts, dispersionless injections at geostationary orbits, and auroral kilometric radiation. We use the auroral breakup, identified with Polar UVI images, as a common reference time frame to calibrate the others. Results, illustrated by two well-defined auroral substorms, unambiguously indicate that none of the four frequently used substorm onset proxies can provide a consistent timing of substorm onset. This inconsistency in substorm onset timing is attributed as a consequence of •emporal and spatial limitations on each observational technique. A delay between the proxy identifiers and the auroral breakup is found to be typical. It is therefore strongly suggested from this study result that a common reference time frame for substorm onset is necessary, and we propose it should be auroral breakups. We argue that there is a need for an intercalibration of magnetospheric substorm phenomenology by using a unified definition of the substorm onset.

[1] A three-dimensional (3-D) magnetic field configuration in force balance with a realistic plasma pressure distribution can provide more accurate evaluation of the role of magnetic field on plasma sheet dynamics and M-I coupling. We used Geotail and Time History of Events and Macroscale Interactions During Substorms (THEMIS) data to establish an empirical model for nightside equatorial isotropic plasma pressure to r = 30 R E for Kp = 0-5 and for solar wind dynamic pressure (P SW ) = 1.5 and 3 nPa. The model pressure is used in the companion paper for modeling a 3-D force-balanced pressure and magnetic field equilibrium. Larger convection during higher Kp drives the plasma sheet further earthward, resulting in larger increase of pressure and pressure gradient at smaller radial distance. On the other hand, magnetosphere compression by increasing P SW enhances pressure and pressure gradient mainly in the tail plasma sheet. While both pressure and radial gradients are enhanced with increasing Kp or P SW , there is no significant azimuthal pressure variation statistically under all Kp and P SW conditions. The empirical pressures well reproduce these statistical profiles with very high correlation coefficients. Additionally, comparisons with pressure gradients computed using two simultaneous measurements from two THEMIS spacecraft show reasonable agreement. Furthermore, our model provides more accurate pressure gradients than previous empirical models. The model magnetic field distributions obtained in the companion paper from requiring force balance with these empirical pressure profiles are also found to be consistent with the magnetic field observations, indicating that our equilibria well represent realistic 3-D pressure and magnetic field configurations.

The year 1992 marks the quincentenary jubilee of the famous voyage of Christopher Columbus to the New World, a trip which initiated sustained contact between Europe and the American continent. Courageous explorations often lead to advancement of mankind, be they in uncharted territory or science. As much as Columbus was unaware of what lay in store for his voyage, we were just as poorly informed about what lay beyond our home planet when we began space exploration about three decades ago. There is much similarity among the pioneering spirits characteristic of both endeavors. It is thus fitting to celebrate this quincentenary occasion by declaring 1992 International Space Year (ISY). In conjunction with the COSPAR Meeting and the International Convention of the World Space Congress to be held in Washington, D.C., from August to September 1992, a 4‐day symposium on the initial results from the Solar‐Terrestrial Energy Program (STEP) Facilities and Theory Campaigns will be held at Johns Hopkins University, Applied Physics Laboratory, Laurel, Md., August 24–27. 1992.

Solar terrestrial (ST) sciences started centuries ago and branched into different disciplines. Starting with naked eye to highly sophisticated novel experimental techniques, observations have revealed the secrets of the Sun, heliosphere, magnetosphere, plasmasphere, and ionosphere-atmosphere components of the ST system. Theories and theoretical models have been developed for the different components independently and together. World-wide efforts under different umbrella are being persuaded to understand the challenges of the ST system. The onset problem and role of O + ions in sub-storm physics are two issues that are hotly debated. The onset problem is whether sub-storm is triggered by magnetic reconnection in the tail region at 15-20 Re or by a current disruption at ~12 Re. The issue on O + role is whether O + ions affect the dynamics of sub-storms under magnetic storm and non-storm conditions differently. This special issue of Geoscience Letters contains a collection of 15 papers on the history and development of solar terrestrial sciences including auroral sub-storms. Over half of the papers are based on the presentations in a session on the same topic organized at the AOGS (Asia Oceania geosciences Society) General Assembly held in Singapore during 02-07 August 2015. The rest of the papers from outside the assembly also falls within the theme of the special issue. The papers are organized in the order of history and development of ST coupling, sub-storms, and outer heliosphere.