This paper documents a series of brief, strong (Δp/p = 1), dynamic pressure oscillations that occurred in the region upstream of the Earth's bow shock during a period of radial interplanetary magnetic field (IMF). The analyzed set of oscillations, which may be either intrinsic solar wind or bow shock‐related phenomena, recur approximately every 8–10 min, and their magnetic field signatures occur nearly simultaneously over great distances transverse to the Earth‐Sun line. The pressure oscillations appear to drive tailward‐moving magnetopause surface wavelets. In turn, the surface wavelets can be identified as hydromagnetic waves with strong compressional components in the outer magnetosphere and as quasi‐periodic variations in electron precipitation and high‐latitude ground pulsations. We use observations by spacecraft in the outer dayside magnetosphere to predict geosynchronous and subsolar magnetic field strengths, the location of the subsolar magnetopause, the solar wind dynamic pressure, and variations in the energetic magnetospheric ion flux.

Abstract. An anomalous geomagnetic sudden commencement (SC) occurred on March 24, 1991. It is characterized by an exceptionally large and sharp impulse observed in its initial part along the noon meridian in middle and low latitudes. The analysis of the SC was made by using high time resolution digital data from the 210 ø Meridian Magnetometer Chain in the west Pacific, Sub-Auroral Magnetometer Network (SAMNET) in the United Kingdom and southern Scandinavia, the EISCAT Magnetometer Cross in northern Scandinavia and Svalbard, and Canopus in Canada together with other ground and satellite (GOES 6, GOES 7, CRRES, and GMS) data. The results of the analysis suggest that the pulse observed at lower-latitude ground stations was caused by the propagation of a strong magnetospheric compression of short duration (less than 1 min) which has never been observed before this event. The HF Doppler observation in Kyoto near local noon seems to be consistent with existence of the bipolar electric field associated with the propagating compressional magnetic pulse. The SAMNET stations and CRRES in the early morning also detected positive pulses which delays 30-50 s from the pulses in noon sector. Although the delay in the peak time of the pulse observed on the ground is consistent with ionospheric hydromagnetic wave propagation from the dayside to the nightside with finite speed, the initial onset time of the pulse on the ground was almost simultaneous everywhere suggesting the existence of an "almost instantaneous" propagation mode below the ionosphere. [1993] successfully simulated this event by particle acceleration due to a sharp compressional electromagnetic pulse launched from the magnetopause at 1500 LT in the very initial stage of the SC. The CRRES actually observed an 130 nT monopolar magnetic pulse and an associated bipolar electric pulse with a peak-topeak amplitude of 80 mV/m. The duration of the pulse is about 2 min. Such a large and sharp pulse has never been observed before in this region of the magnetosphere. The SC itself might be produced by a coronal mass ejection (CME) driven interplanetary shock which was presumably related with a 3B optical flare at 2246 UT on March 22, 1991.Since the analyses of this event have mostly been limited to particle data so far, it is necessary to study properties of the electromagnetic pulse observed by CRRES in more detail by analyzing other magnetic data. Although the CRRES observations and the simulation by Li et al. [1993] greatly contributed to understand this peculiar event, we have to recognize that the CRRES observations were made at one point in the magnetosphere. It is difficult therefore to discuss in detail about excitation and propagation of the observed pulse. Analyses of SCs made so far [see Araki, 1994] show that a simple compression of the magnetosphere causes a complex global distribution of amplitude and waveform of SC. The main positive H component increase of SCs is often preceded by a positive or negative impulse of short duration. This preceding impulse ...

“Upstream waves,” generated in the solar wind upstream of a quasi‐parallel bow shook, are believed to be a major source of the Pc 3‐4 pulsation activity observed in the dayside magnetosphere. In an attempt to better understand the means by which “upstream wave” energy is transmitted from the solar wind into the magnetosphere, we compared simultaneous data from ISEE 1 and 2 in the upstream solar wind, AMPTE IRM in the subsolar magnetosheath, and AMPTE CCE in the dayside magnetosphere. Our observations indicate that dayside magnetospheric Pc 3‐4 pulsation activity and low IMF cone angles are associated with increased turbulence in the subsolar magnetosheath magnetic field (with large amplitude fluctuations both parallel and transverse to the average field direction), and with increased and highly variable levels of energetic magnetosheath particles. Fourier analysis of the magnetic field fluctuations shows broadband increases in wave power from 0.01 Hz to at least 0.5 Hz, but with peak power at Pc 3‐4 frequencies; there is no evidence in our data set of narrow‐band magnetic field variations in the magnetosheath at these times. Purely compressional waves, which are at times observed in the subsolar magnetosheath, have a somewhat narrower frequency distribution, but are associated with neither upstream wave activity nor magnetospheric pulsations.

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Abstract. A geomagnetic negative sudden impulse (SI-) observed on May 13, 1995, was examined using magnetic field data from ETS VI and GOES 7 in the magnetosphere and ground geomagnetic observation networks. The SI-was caused by a sudden decrease in the solar wind dynamic pressure at the front boundary of a magnetic cloud embedded in a density enhancement region. The amplitude and the fM1 time •t K•kiok• geomagnetic observatory (m•gnetic l•titude 26.9 ø, m•gnetic longitude 208.3 ø) were 26 nT and 10 rain, respectively. Although SI-has been considered to be the mirror image of geomagnetic positive sudden impulse (SI +) or sudden commencement (SC), we found the polarization distribution of the SIconsistent with that of SC. We suggest that the contribution from the longitudinal movement of a twin-vortex ionospheric current system is dominant to produce the polarization of SC and SI-. We also discuss the relationship between the angle at which discontinuities in the solar wind impinge upon the magnetosphere and the geomagnetic response. IntroductionThe geomagnetic sudden commencement ( [1961] was not demonstrated in their study because of the lack of high time resolution geomagnetic data. This is one of reasons why we have made a comprehensive study of an SI-in this paper.