The interplanetary magnetic field (IMF) By‐dependent distribution of field‐aligned currents in the cleft region is studied, using the magnetic field and plasma data from 47 passes of Dynamics Explorer (DE) 2. These orbits were chosen on the conditions that cusp/cleft particles are detected and that at the same time the IMF By and Bz components satisfy the criteria |By|≥5 nT and |Bz|≤5 nT during the satellite's crossing of the relevant field‐aligned current region. When By is positive (negative) in addition to satisfying these conditions, there is a strong eastward (westward) magnetic perturbation caused by a pair of field‐aligned current sheets, consisting of an equatorward sheet with downward (upward) current and a poleward sheet having upward (downward) current. These By‐dependent field‐aligned currents in the equatorward and the poleward sheets are referred to as the low‐latitude cleft current (LCC) and the high‐latitude cleft current (HCC), respectively. The cusp/cleft electron precipitation region and the LCC region overlap with each other to a varying degree irrespective of the sign of By. For positive (negative) By, LCC has the same direction as the morning (afternoon) region 1 current or the afternoon (morning) region 2 current. Thus an interpretation has been given in the past that the LCC region is an extension of the region 1 or region 2 current system. However, in this paper we present an alternative view that the LCC region is not an extension of the region 1 or region 2 current system and that a pair of LCC and HCC constitutes the cleft field‐aligned current regime. The proposed pair of cleft field‐aligned currents is explained with a qualitative model in which this pair of currents is generated on the open field lines that have just been reconnected on the dayside magnetopause. The model assumes a quasi‐steady reconnection operating within certain longitudinal width extending to both sides of the stagnation point on the dayside magnetopause. The reconnected flux tubes move under the influences of the field tension and the magnetosheath flow. When the magnetosheath By is positive, the northern hemisphere field lines reconnected on the eastward side of the stagnation point are pulled toward higher latitudes, and the field lines reconnected on the westward side of the stagnation point are pulled along the dawnside magnetopause flank. The electric fields associated with these motions are present immediately inside the magnetopause (rotational discontinuity). This is the source region of LCC and HCC. The electric fields are transmitted along the field lines to the ionosphere, creating a poleward electric field and a pair of field‐aligned currents when By is positive; the pair of field‐aligned currents consists of a downward current at lower latitudes (LCC) and an upward current at higher latitudes (HCC). In the By negative case, the model explains the reversal of the field‐aligned current direction in the LCC and HCC regions.

[1] We studied dynamics of O + ions during the superstorm that occurred on 29-31 October 2003, using energetic (9-210 keV/e) ion flux data obtained by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite and neutral atom data in the energy range of 10 eV to a few keV acquired by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Since the low-energy neutral atoms are created from the outflowing ionospheric ions by the charge exchange process, we could examine variations of ionospheric ion outflow with the IMAGE/LENA data. In the near-Earth plasma sheet of X GSM $ À6 R E to À8.5 R E , we found that the H + energy density showed no distinctive differences between the superstorm and quiet intervals (1-10 keV cm À3 ), while the O + energy density increased from 0.05-3 keV cm À3 during the quiet intervals to $100 keV cm À3 during the superstorm. The O + /H + energy density ratio reached 10-20 near the storm maximum, which is the largest ratio in the near-Earth plasma sheet ever observed by Geotail, indicating more than 90% of O + in the total energy density. We argued that such extreme increase of the O + /H + energy density ratio during the October 2003 superstorm was due to mass-dependent acceleration of ions by storm-time substorms as well as an additional supply of O + ions from the ionosphere to the plasma sheet. We compared the ion composition between the ring current and the near-Earth plasma sheet reported by previous studies and found that they are rather similar. On the basis of the similarity, we estimated that the ring current had

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In order to enhance the reproduction of the recovery phase D st index of a geomagnetic storm which has been shown by previous studies to be poorly reproduced when compared with the initial and main phases, an artificial neural network with one hidden layer and error back-propagation learning has been developed. Three hourly D st values before the minimum D st in the main phase in addition to solar wind data of IMF southward-component B s , the total strength B t and the square root of the dynamic pressure, √ nV 2 , for the minimum D st , i.e., information on the main phase was used to train the network. Twenty carefully selected storms from 1972-1982 were used for the training, and the performance of the trained network was then tested with three storms of different D st strengths outside the training data set. Extremely good agreement between the measured D st and the modeled D st has been obtained for the recovery phase. The correlation coefficient between the predicted and observed D st is more than 0.95. The average relative variance is 0.1 or less, which means that more than 90% of the observed D st variance is predictable in our model. Our neural network model suggests that the minimum D st of a storm is significant in the storm recovery process.

[1] Poleward-moving auroral forms (PMAF), the ionospheric signatures of flux transfer events (FTEs), are intermittent phenomena observed in the cusp during negative interplanetary magnetic field intervals. Previous meridian scanning photometer-based observations showed that the distribution of the separation time between successive PMAFs is maximum at $3.5 min. In this paper we present initial results from a new high-sensitivity all-sky imager, which was set up at Longyearbyen, Svalbard, Norway in October 2011. The 630.0-nm all-sky images taken with a time resolution of 4 s reveal that one of the PMAFs that occurred with such typical separation times on 29 December 2011 comprises two consecutive auroral bursts-the first occurring in the PMAF immediately after it was seen and the second, $2 min later, following almost the same route. This observation provides evidence that one PMAF could reflect double FTEs-there is not always a one-to-one correspondence between FTEs and PMAFs. Reconnection on the dayside magnetopause would be modulated with a period of $2 min during the course of transient reconnection.