An algorithm is presented for predicting the ground‐based Dst index solely from a knowledge of the velocity and density of the solar wind and the north‐south solar magnetospheric component of the interplanetary magnetic field. The three key elements of this model are an adjustment for solar wind dynamic pressure, an injection rate linearly proportional to the dawn‐to‐dusk component of the interplanetary electric field which is zero for electric fields below 0.5 mV m−1, and an exponential decay rate of the ring current with an e folding time of 7.7 hours. The algorithm is used to predict the Dst signature of seven geomagnetic storm intervals in 1967 and 1968. In addition to being quite successful, considering the simplicity of the model, the algorithm pinpoints the causes of various types of storm behavior. A main phase is initiated whenever the dawn‐to‐dusk solar magnetospheric component of the interplanetary electric field becomes large and positive. It is preceded by an initial phase of increased Dst if the solar wind dynamic pressure increases suddenly prior to the main phase. The recovery phase is initiated when the injection rate governed by the interplanetary electric field drops below the ring current decay rate associated with the ring current strength built up during the main phase. Variable recovery rates are generally due to additional injection during the recovery phase. This one algorithm accounts for magnetospheric behavior at quiet and at disturbed times and seems capable of predicting the behavior of Dst during even the largest of storms.

The semiannual variation in geomagnetic activity is well established in geomagnetic data Its explanation has remained elusive, however. We propose, simply, that it is caused by a semiannual variation in the effective southward component of the interplanetary field. The southward field arises because the interplanetary field is ordered in the solar equatorial coordinate system, whereas the interaction with the magnetosphere is controlled by a-magnetospheric system. Several simple models utilizing this effective modulation of the southward component of the interplanetary field are examined. One of these closely predicts the observed phase and amplitude of the semiannual variation. This model assumes that northward interplanetary fields are noninteracting and that the interaction with southward fields is ordered in solar magnetospheric coordinates. The prediction of the diurnal variation of the strength of the interaction at the magnetopause by this model, does not, however, match the diurnal variation of geomagnetic activity as derived from ground-based data. However, predictions of the dependence of geomagnetic activity on the polarity of the interplanetary magnetic field and of a 22-year cycle in geomagnetic activity are confirmed by studies of ground-based data. It appears that the mechanism controlling the semiannual variation of geomagnetic activity has been identified but that a quantitative model must await further refinements in our knowledge of the solar wind-magnetosphere coupling.

The FIELDS instrumentation suite on the Magnetospheric Multiscale (MMS) mission provides comprehensive measurements of the full vector magnetic and electric fields in the reconnection regions investigated by MMS, including the dayside magnetopause and the night-side magnetotail acceleration regions out to 25 Re. Six sensors on each of the four MMS spacecraft provide overlapping measurements of these fields with sensitive crosscalibrations both before and after launch. The FIELDS magnetic sensors consist of redundant flux-gate magnetometers (AFG and DFG) over the frequency range from DC to 64 Hz, a search coil magnetometer (SCM) providing AC measurements over the full whistler mode spectrum expected to be seen on MMS, and an Electron Drift Instrument (EDI) that calibrates offsets for the magnetometers. The FIELDS three-axis electric field measurements are provided by two sets of biased double-probe sensors (SDP and ADP) operating in a highly symmetric spacecraft environment to reduce significantly electrostatic errors. These sensors are complemented with the EDI electric measurements that are free from all local spacecraft perturbations. Cross-calibrated vector electric field measurements are thus produced from DC to 100 kHz, well beyond the upper hybrid resonance whose frequency provides an accurate determination of the local electron density. Due to its very large geometric factor, EDI also provides very high time resolution (∼ 1 ms) ambient electron flux measurements at a few selected energies near 1 keV. This paper provides an overview of the FIELDS suite, its science objectives and measurement requirements, and its performance as verified in calibration and cross-calibration procedures that result in anticipated errors less than 0.1 nT in B and 0.5 mV/m in E. Summaries of data products that result from FIELDS are also described, as well as algorithms for cross-calibration. Details of the design and performance characteristics of AFG/DFG, SCM, ADP, SDP, and EDI are provided in five companion papers.

Abstract.A stream interaction region (SIR) forms when a fast solar stream overtakes a slow stream, leading to structure that evolves as an SIR moves away from the Sun. Based on Wind (1995 -2004) and ACE (1998ACE ( -2004 in situ observations, we have conducted a comprehensive survey of SIRs at one AU, including a separate assessment of the longer-lasting corotating interaction regions (CIRs) that recur on more than one solar rotation. In all there are 196 CIRs, accounting for about 54% of the 365 SIRs. The largest proportion of CIRs to SIRs (64%) appears in 1999, and the smallest proportion (49%) is in 2002. Over the ten years, the annual number of SIR events varies little, from 32 up to 45. On average, the occurrence rate of shocks at SIRs at one AU is about 24%. Seventy percent of the SIRs with shocks have only forward shocks, more than twice the percentage of SIRs with only reverse shocks. This preponderance of forward shocks is consistent with the deflections of forward and reverse shocks relative to the ecliptic plane. In order to help address the effect of SIRs and CIRs on geomagnetic activity, we determine the solar-cycle variation of the event duration, scale size, the change in velocity from slow stream to fast stream, and the solar-cycle variation of the maximum magnetic field, peak total perpendicular pressure, and other properties. These statistics also provide a baseline for future studies at other heliocentric distances and for validating heliospheric models.