[1] Global auroral images are used to investigate how specific types of solar wind change relate to the resulting type of auroral-region disturbance, with the goal of determining fundamental response types. For not strongly southward IMF conditions (B z^À 5 nT), we find that IMF changes that are expected to reduce the convection electric field after^30 min of negative IMF B z cause typical substorms, where expansion phase auroral activity initiates within the expected location of the Harang electric field reversal and expands in $10 min to cover $5 hours of MLT. For not strongly southward IMF conditions, solar wind dynamic pressure (P dyn ) enhancements compress the entire magnetosphere, leading to a global auroral enhancement with no evidence for substorm bulge-region aurora or current wedge formation. Following prolonged strongly southward IMF (B z ] À8 nT), an IMF change leading to convection electric field reduction gives a substorm disturbance that is not much different from substorms for less strongly southward IMF conditions, other than the expansion phase auroral bulge region seems to expand somewhat more in azimuth. However, under steady, strongly southward IMF conditions, a P dyn enhancement is found to cause both compressive auroral brightening away from the bulge region and a Harangregion substorm auroral brightening. These two auroral enhancements merge together, leading to a very broad auroral enhancement covering $10-15 hours of MLT. Both current wedge formation and compressive energization in the inner plasma sheet apparently occur for these events. We also find that interplay of effects from a simultaneous IMF and P dyn change can prevent the occurrence of a substorm, leading to what we refer to as null events. Finally, we apply the plasma sheet continuity equation to the IMF and pressure driven substorm responses and the null events. This application suggests that solar wind changes cause substorm onset only if the changes lead to a reduction in the strength of convection within the inner plasma sheet.

[1] Enhancements in relativistic electron fluxes in the outer radiation belt often occur following magnetic storms and have been suggested to result from resonant interactions with enhanced whistler-mode chorus emissions observed on the dawnside. Using observations during a period of persistent high-speed, corotating, solar wind streams, we investigate the aspects of solar wind-magnetosphere coupling that lead to these enhanced chorus emissions. We find that relativistic electron energization occurs in association with large-amplitude Alfvén waves within the high-speed streams. These waves last for multiday periods and cause multiday intervals having intermittent periods of significantly enhanced convection. The enhanced convection periods are followed by repetitive substorm onsets caused by the Alfvén wave related repetitive reductions in convection. We use these substorm onsets, identified using geosynchronous particles and midlatitude H components, as indicators of preceding periods of enhanced convection and of reductions in convection. We use ground-based chorus observations from the Halley station VLF/ ELF Logger Experiment (VELOX) instrument to indicate magnetospheric chorus intensities. These data give evidence that the periods of enhanced convection that precede substorm expansions lead to the enhanced dawnside chorus wave. We also see that the enhanced solar wind densities n sw ahead of high-speed streams are associated with significant energetic electron loss at geosynchronous orbit and that the subsequent flux increases appear to not begin until n sw drops below $5 cm À3 even if the solar wind speed increases earlier. The sequence of loss during the leading interval of high n sw , followed by energization during high-speed streams, occurs whether or not the high n sw interval leads to a magnetic storm.

[1] Geosynchronous energetic particle fluxes are used to examine the differences and similarities between the particle disturbances due to an enhancement in solar wind dynamic pressure P dyn and those caused by substorms. Disturbances are also distinguished by IMF conditions. First, for not strongly southward IMF conditions (weakly southward or northward IMF), we find that the magnetospheric compression by a P dyn enhancement usually causes particle fluxes to increase simultaneously at all energy channels. The increase is global around the Earth, but it usually occurs first on the dayside and then propagates to the nightside within a few minutes. We also find that a magnetospheric compression sometimes leads to a flux decrease or no flux change for at least one energy channel at some MLTs, which we attribute to the shape of radial profiles at constant adiabatic invariants. However, we find no evidence for substorm-like injections in our P dyn enhancement events when the IMF is not strongly southward. Following prolonged strongly southward IMF, substorms caused by IMF changes that lead to convection electric field reduction and are not associated with a P dyn change generate flux disturbances that are quite similar to typical substorm flux disturbances for less strongly southward IMF conditions. However, the dispersionless injection front is found over a much wider azimuthal region, sometimes extending to the late afternoonside for protons. We find that under prolonged steady, strongly southward IMF conditions, a P dyn enhancement leads to a two-mode type disturbance. The disturbance due to magnetospheric compression can be clearly identified and is seen primarily on the dayside, and a substorm-like injection associated with current wedge formation is seen on the nightside. The dayside compression effect is seen in both species, but is often more easily identified in the proton fluxes than in the electron fluxes. The substorm-like injection feature is also seen in both species but is usually more evident in the electron fluxes. In the events studied here, the dayside compression disturbance precedes the substorm-like injection on the nightside by a few minutes.

[1] Satellite observations often show that relativistic electron fluxes that decrease during a geomagnetic storm main phase do not recover their prestorm level even when the storm has substantially recovered. A possible explanation for such sustained flux dropout is that the electrons that move to larger shells (L shells) aided by the disturbance storm time (Dst) effect associated with the main phase geomagnetic field depression may be suffering drift loss to the magnetopause, resulting in irreversible (nonadiabatic) flux decreases during a geomagnetic storm. In this study, we have numerically evaluated the drift loss effect by combining it with the Dst effect and including off-equatorially mirroring electrons for three different storm conditions obtained by averaging 95 geomagnetic storms that occurred from 1997 to 2002. Using the Tsyganenko T02 model and our own simplified method, we estimated the storm time flux changes based on the guiding center orbit dynamics. Assuming that there is no competing source mechanism taking place at the same time, our calculations of the electron fluxes at equatorial midnight suggest that the drift loss when combined with the Dst effect can be responsible for flux dropouts, which can be seen even inside the geosynchronous orbit during storm periods. Specifically, by evaluating omnidirectional flux values at three specific times that correspond to the storm onset time, the time of minimum Dst value, and the end of the Dst recovery, we have obtained the following numerical results. First, for the strong storm with −150 nT < Dst min ≤ −100 nT, the combined drift loss and Dst effect can cause a complete dropout of the electron flux for r ≥ ∼5 R E at the end of the storm recovery. A nearly full recovery of the particle flux by the adiabatic Dst effect is seen only for r < ∼5 R E . For the moderate storm with −100 nT < Dst min ≤ −50 nT, the overall flux decrease level at the end of the storm recovery is less significant compared to that of the strong storm. However, the combined loss effect can still penetrate into r ∼ 5 R E , leading to some partial dropout of the flux. For the severe storm with Dst min ≤ −150 nT, the flux dropout is far more significant than for the other two storms, indicating that the combined drift loss and Dst effect can even reduce the flux level at an inner region of r ∼ 4 R E . But in this case, the solar wind dynamic pressure is so high that the dayside magnetopause can cross the geosynchronous orbit. Consequently, the flux dropouts seen in actual observations can be primarily attributed to a fast and direct loss to the magnetopause at times when the magnetopause crosses the geosynchronous orbit. It is possible that our numerical results may quantitatively change to some extent with different magnetospheric models and assumptions and may also change depending on the validity of the fully adiabatic invariants assumption.Citation: Kim, K. C., D.-Y. Lee, H.-J. Kim, E. S. Lee, and C. R. Choi (2010), Numerical estimates of drift loss and Dst effect for outer rad...

[1] This paper reports on several substorms observed under northward interplanetary magnetic field (IMF) conditions, the intensity of which was at least as significant as that of typical substorms under moderately southward IMF conditions. Such northward IMF periods were identified during the recovery phase of three strong storms. In the case of each storm, two or more substorms occurred successively, being separated by $1.8-5 h, while the IMF condition continued persistently northward. The substorms are clearly evidenced by auroral and other complementary observations. For the most intense substorms, the auroral breakup occurred at the magnetic latitude of $58°, and for the others it was between 60°and 65°. The polar cap size prior to each onset was substantial despite the northward IMF conditions. The auroral expansion following each onset lasted from a few up to several magnetic local time hours and exhibited a clear poleward expansion feature. For most of the events studied, geosynchronous magnetic dipolarizations preceded by field stretching and/or energetic particle injections occurred. The occurrence of such (intense) substorms implies that a certain (large) amount of energy remains in the tail even under northward IMF conditions. The occurrence of two or more successive substorms further implies that even after the release of a certain amount of energy triggered by the substorm, the tail can still have a substantial amount of energy left, which can be released by a subsequent substorm(s). We conjecture that an intense substorm during a northward IMF period can be expected when such a period belongs to the recovery phase of an intense storm mainly because of large energy loading done by preceding southward IMF B z during the storm's main (and some early recovery) phase. In addition we argue that substorm energy can also be supplied by other mechanisms of the solar wind-magnetosphere coupling under northward IMF conditions such as dayside reconnection in the presence of a substantial IMF B y component.

[1] Substorms sometimes occur repetitively with a period of $1-4 hours. In this paper we examine repetitive substorms, identified using particle injections and positive H bays on the nightside, that we find to occur during corotating high-speed streams associated with coronal holes. The high-speed streams often last for several days and are accompanied by large amplitude Alfvén waves of the interplanetary magnetic field (IMF). We find that repetitive substorms occur every $1-4 hours, regardless of the solar cycle phase, whenever the Earth's magnetosphere is impinged by these high-speed streams. We further find that a significant number of these substorms are associated with repetitive northward turnings of the Alfvénic IMF, each northward turning preceded by weakly-to-moderately southward IMF, i.e., B z $ À3.6 nT for $29 min on the average. We present eight example intervals where most of the repetitive substorms were associated with a northward turning. Statistically, for 63.5% of 312 substorms we are able to identify a reasonable association with a northward turning. While limitations of the Weimer-mapped IMF used here and the spatial structure of the Alfvénic IMF prevent us from estimating a precise figure for the percentage of IMF triggered substorms, our results indicate that many of the repetitive substorms are likely due to repetitive triggering by the Alfvénic IMF.

[1] We have investigated the characteristics of solar wind and magnetospheric conditions associated with the occurrence of geosynchronous relativistic electron events. Most of the geosynchronous relativistic events for April 1999 to December 2002 are found to occur during prolonged (a number of days) quiet intervals following the appearance of highspeed solar wind streams. In a typical relativistic event, the electron fluxes begin to increase by orders of magnitude when the solar wind density drops after reaching a sharp peak at the leading edge of a high-speed stream. The increased fluxes stay at a high level until the quiet solar wind conditions cease. In addition, enhanced ULF wave activity and substorm injections of 10s to 100s keV electrons are observed at the time of the large flux increases in the events. We found that geosynchronous relativistic events can be observed only when both the solar wind and magnetospheric wave/ substorm injection conditions are favorable regardless of whether or not a magnetic storm takes place. These observations suggest the following scenario for the occurrence of a geosynchronous relativistic electron event: (1) Quiet solar wind conditions (i.e., no strong solar wind pressure and large southward turnings of IMF B z ) can lead to stable and more dipole-like magnetospheric configurations in which the geosynchronous orbit is located well inside the trapping boundary of the energetic electrons. (2) If a large population of MeV electrons is generated (by some acceleration process(es) involving enhanced ULF wave and substorm injections) in the inner magnetosphere, it can be trapped and effectively accumulated to a high intensity. (3) The high electron flux can persist for a number of days in the geosynchronous region as long as the solar wind conditions remain quiescent. The occurrence of a geosynchronous relativistic electron event requires not only the proper acceleration process and sufficient seed electrons but also no significant loss process that dominates over any acceleration/source.

Physics of electromagnetic ion cyclotron (EMIC) waves is complicated by inclusion of heavy ions. In particular, He+ ions in the magnetosphere have long been considered to play important roles. Motivated by recent observations, we examine the effect of the inclusion of hot anisotropic He+ ions in addition to the usual hot anisotropic protons. We solve the kinetic dispersion relation for this examination and find the following results. First, inclusion of hot anisotropic He+ ions leads to the growth of EMIC waves at frequencies below the He+ gyrofrequency (He band) and a reduction of the EMIC wave growth rates (or damping of the waves) at frequencies between the proton and He+ gyrofrequencies (H band). Second, this effect is more dramatic for higher temperatures of He+ that would play a role in damping EMIC waves for both frequency bands and especially for cases without a He+ temperature anisotropy. Lastly, the effect is more prominent for cold plasma dominant conditions such as the region inside the plasmasphere or plume than for hot proton dominant conditions such as the region outside the plasmasphere. We propose that this last effect can at least partially explain the satellite observations indicating the preferred (though not exclusive) occurrence of He band waves inside the plasmasphere for the times when hot anisotropic He+ ions are supplied from the plasma sheet and ring current.