The commonly used formula H = DCF + DR, where DCF and DR are the effects of the magnetopause and ring current, respectively, neglects contribution of the cross‐tail current to the Dst variation. The formula allows us to explain satisfactorily the observed relation of the Dst variation to the ring current intensity but faces difficulties in explaining other experimental facts. First, the equatorward shift of the auroral oval cannot be caused by the sole enhancement of the ring current. Second, the observed relation of the Dst growth rate to the southward IMF component [Burton et al., 1975] does not have any quantitative explanation up to now. We suggest using a different formula, H = (2μ0psw)1/2 + DR‐Fout/2S. The formula is obtained from the conditions of magnetic flux conservation and pressure balance. The flux Fout is directed mainly to the nightside of the magnetosphere. Hence the term Fout/2S describes the effect of the crosstail current and a part of the magnetopause currents. During quiet periods, each term in the right‐hand side of our formula is of the order of tens of nanoteslas. During storm time, each term can rise to hundreds of nanoteslas. The flux Fout grows after the interplanetary magnetic field (IMF) becomes southward owing to the flux transport from the dayside to the magnetotail. The growth rate is described by the formula dFout/dt = U − Fout/τF + ηF, where U is the electric potential difference between the dawnside and duskside of the magnetosphere and τF and ηF are constant. The voltage U depends linearly on the IMF southward component. Combining the latter formula with the expression for H yields a relationship between the Dst growth rate and the IMF southward component close to the observed one. Since the auroral oval is mapped predominantly to the plasma sheet of the magnetotail, the growth of Fout during a storm allows us to explain the equatorward shift of the auroral oval. Another prediction from our theory is the erosion of the stable trapping region in which the equatorial cross section S is related to the flux Fout by the equation S1/2[S(2μ0psw)1/2 + Fout] = 3π3/2(ME + MRC), where ME and MRC are the magnetic moments of the Earth and ring current, respectively. Growth of Fout leads to the decrease of S and to the earthward movement of the dayside magnetopause. During storms this effect can be stronger than that of the region 1 Birkeland current, also moving the magnetopause earthward.

[1] On 13 December 2004, morningside Sun-aligned auroral arcs were observed at about 8 MLT over Svalbard (around 75 MLAT) during about 1.5 hours after the IMF B z component had turned to strongly northward (+10 nT). The arcs appeared periodically (6.7 min) and moved poleward at a velocity of about 700 m/s. The arcs occurred during substorm recovery and appeared to develop from the area of enhanced luminosity seen in the western (nightside) horizon toward east. The electric fields and currents associated with the arcs have been calculated from the EISCAT Svalbard radar observations. The arcs were associated with periodic spatial structures of 270 km in latitudinal width. Each of the 270-km wide structure consists of four specific FAC regions: the upward FAC region 75-km wide containing the optical arc, the return downward FAC region 100-km wide poleward of the arc; and a secondary weaker arc equatorward of the main arc with a pair of FACs similar to the main arc, but narrower in width. The interchange instability with the ionospheric feedback is suggested as a suitable generation mechanism for such kind arcs.

Abstract. Global ultraviolet auroral images from the IM-AGE satellite were used to investigate the dynamics of the dayside auroral oval responding to a sudden impulse (SI) in the solar wind pressure. At the same time, the TV all-sky camera and the EISCAT radar on Svalbard (in the pre-noon sector) allowed for detailed investigation of the auroral forms and the ionospheric plasma flow. After the SI, new discrete auroral forms appeared in the poleward part of the auroral oval so that the middle of the dayside oval moved poleward from about 70 • to about 73 • of the AACGM latitude. This poleward shift first occurred in the 15 MLT sector, then similar shifts were observed in the MLT sectors located more westerly, and eventually the shift was seen in the 6 MLT sector. Thus, the auroral disturbance "propagated" westward (from 15 MLT to 6 MLT) at an apparent speed of the order of 7 km/s. This motion of the middle of the auroral oval was caused by the redistribution of the luminosity within the oval and was not associated with the corresponding motion of the poleward boundary of the oval. The SI was followed by an increase in the northward plasma convection velocity. Individual auroral forms showed poleward progressions with velocities close to the velocity of the northward plasma convection. The observations indicate firstly a pressure disturbance propagation through the magnetosphere at a velocity of the order of 200 km/s which is essentially slower than the velocity of the fast Alfvén (magnetosonic) wave, and secondly a potential (curl-free) electric field generation behind the front of the propagating disturbance, causing the motion of the auroras. We suggest a physical explanation for the slow propagation of the disturbance through the magnetosphere and a model for the electric field generation. Predictions of the model are supported by the global convection maps produced by the SuperDARN HF radars. Finally, the interchange instability and the eigenmode toroidal Alfvén oscillations areCorrespondence to: A. Kozlovsky (alexander.kozlovsky@oulu.fi) discussed as possible generation mechanisms for the dayside auroral forms launched by the SI.

Abstract.On 7 December 2000, TV ASC camera in Barentsburg (Svalbard) observed pre-noon (at 09:00-10:00 MLT) rayed auroral arcs, which occurred at the poleward edge of the auroral oval after an IMF transition from B y -dominated (B y = +8.8, B z = +4.3) to strongly northward dominated (B y = +2.7, B z = +8.6). The arcs appeared from the area of enhanced luminosity seen in the western (nightside) horizon, and developed to the east, progressing at a velocity of about 1.5 km/s. Simultaneously, the arcs were drifting poleward at a velocity of 300-500 m/s, whose value was equal to the F-region ionospheric plasma drift velocity observed by the Incoherent Scatter Radar (ESR). The arc appearance and motion corresponded well to the poleward expansion of the auroral oval following the IMF shift, which was observed by the UVI on board the Polar satellite. The observed auroras were associated with closed LLBL indicated by the particle precipitation data from DMSP satellites showing also several-keV electrons of PS origin. The observations allow us to suggest that the arcs arise due to the interchange instability that starts to develop at the boundary between the magnetospheric plasma and the magnetosheath flux tubes entering the closed magnetosphere due to the reconnection beyond the cusp after the IMF changes. The interchange instability can be suggested as a possible mechanism for the formation of the LLBL.

Abstract.We have examined several cases of magnetosphere compression by solar wind pressure pulses using a set of instruments located in the noon sector of auroral zone. We have found that the increase in riometric absorption (sudden commencement absorption, SCA) occurred simultaneously with the beginning of negative or positive magnetic variations and broadband enhancement of magnetic activity in the frequency range above 0.1 Hz. Since magnetic variations were observed before the step-like increase of magnetic field at equatorial station (main impulse, MI), the negative declinations resembled the so-called preliminary impulse, PI. In this paper a mechanism for the generation of PI is introduced whereby PI's generation is linked to SCA -associated precipitation and the local enhancement of ionospheric conductivity leading to the reconstruction of the ionospheric current system prior to MI. Calculation showed that PI polarity depends on orientation of the background electric field and location of the observation point relative to ionospheric irregularity. For one case of direct measurements of electric field in the place where the ionospheric irregularity was present, the sign of calculated disturbance corresponded to the observed one. High-resolution measurements on IRIS facility and meridional chain of the induction magnetometers are utilized for the accurate timing of the impact of solar wind irregularity on the magnetopause.