We investigate two abnormal CME-Storm pairs that occurred on 2014 September 10 - 12 and 2015 March 15 - 17, respectively. The first one was a moderate geomagnetic storm (Dstmin  -75 nT) driven by the X1.6 high speed flare-associated CME (1267 km s−1) in AR 12158 (N14E02) near solar disk center. The other was a very intense geomagnetic storm (Dstmin  -223 nT) caused by a CME with moderate speed (719 km s−1) and associated with a filament eruption accompanied by a weak flare (C9.1) in AR 12297 (S17W38). Both CMEs have large direction parameters facing the Earth and southward magnetic field orientation in their solar source region. In this study, we inspect the structure of Interplanetary Flux Ropes (IFRs) at the Earth estimated by using the torus fitting technique assuming self-similar expansion. As results, we find that the moderate storm on 2014 September 12 was caused by small-scale southward magnetic fields in the sheath region ahead of the IFR. The Earth traversed the portion of the IFR where only the northward fields are observed. Meanwhile, in case of the 2015 March 17 storm, our IFR analysis revealed that the Earth passed the very portion where only the southward magnetic fields are observed throughout the passage. The resultant southward magnetic field with long- duration is the main cause of the intense storm. We suggest that 3D magnetic field geometry of an IFR at the IFR-Earth encounter is important and the strength of a geomagnetic storm is strongly affected by the relative location of the Earth with respect to the IFR structure.

Because of the small number of spacecraft available in the Earth’s magnetosphere at any given time, it is not possible to obtain direct measurements of the fundamental quantities, such as the magnetic field and plasma density, with a spatial coverage necessary for studying, global magnetospheric phenomena. In such cases, empirical as well as physics-based models are proven to be extremely valuable. This requires not only having high fidelity and high accuracy models, but also knowing the weakness and strength of such models. In this study, we assess the accuracy of the widely used Tsyganenko magnetic field models, T96, T01, and T04, by comparing the calculated magnetic field with the ones measured in-situ by the GOES satellites during geomagnetically disturbed times. We first set the baseline accuracy of the models from a data-model comparison during the intervals of geomagnetically quiet times. During quiet times, we find that all three models exhibit a systematic error of about 10% in the magnetic field magnitude, while the error in the field vector direction is on average less than 1%. We then assess the model accuracy by a data-model comparison during twelve geomagnetic storm events. We find that the errors in both the magnitude and the direction are well maintained at the quiet-time level throughout the storm phase, except during the main phase of the storms in which the largest error can reach 15% on average, and exceed well over 70% in the worst case. Interestingly, the largest error occurs not at the Dst minimum but 2–3 hours before the minimum. Finally, the T96 model has consistently underperformed compared to the other models, likely due to the lack of computation for the effects of ring current. However, the T96 and T01 models are accurate enough for most of the time except for highly disturbed periods.

The dynamics of the outer zone radiation belt has received a lot of attention mainly due to the correlation between the occurrence of enhancing relativistic electron flux and spacecraft operation anomalies or even failures (e.g., Baker et al. 1994). Relativistic electron events are often observed during great storms associated with ultra low frequency (ULF) waves. For example, a large buildup of relativistic electrons was observed during the great storm of March 24, 1991 (e.g., Li et al. 1993; Hudson et al. 1995; Mann et al. 2013). However, the dominant processes which accelerate magnetospheric radiation belt electrons to MeV energies are not well understood. In this paper, we present observations of Pc5 ULF waves in the recovery phase of the Bastille day storm of July 16, 2000 and electron and proton flux simultaneously oscillating with the same frequencies as the waves. The mechanism for the observed electron and proton flux modulations is examined using groundbased and satellite observations. During this storm time, multiple packets of discrete frequency Pc5 ULF waves appeared associated with energetic particle flux oscillations. We model the drift paths of electrons and protons to determine if the particles drift through the ULF wave to understand why some particle fluxes are modulated by the ULF waves and others are not. We also analyze the flux oscillations of electrons and protons as a function of energy to determine if the particle modulations are caused by a ULF wave drift resonance or advection of a particle density gradient. We suggest that the energetic electron and proton modulations by Pc5 ULF waves provide further evidence in support of the important role that ULF waves play in outer radiation belt dyanamics during storm times.

Thermospheric wind observations from high to mid latitudes are compared with the newly developed Multiscale Atmosphere Geospace Environment (MAGE) model for the Nov 3–4 geomagnetic storm. The observation and simulation comparison shows a very good agreement and is better at high latitudes in general. We were able to identify a thermospheric poleward wind reduction possibly linked to a northward turning of the Interplanetary Magnetic Field (IMF) at ~22 UT on Nov 3 and an enhancement of the poleward wind to a southward turning near 10 UT on Nov 4 at high latitudes. An IMF southward turning may have led to an enhancement of equatorward winds at Boulder, Colorado near midnight. Simultaneous occurrence of aurora may be associated with an IMF By turning negative. The MAGE model wind simulations are consistent with observations in these cases. The results show the model can be a very useful tool to further study the magnetosphere and ionosphere coupling on short time scales.

Challenging Minisatellite Payload (CHAMP) satellite magnetic data are used to investigate the latitudinal variation of the storm-time meso-scale field-aligned currents by defining a new metric called the FAC range. Three major geomagnetic storm events are considered. Alongside SymH, the possible contributions from solar wind dynamic pressure and interplanetary magnetic field (IMF) BZ are also investigated. The results show that the new metric predicts the latitudinal variation of FACs better than previous studies. As expected, the equatorward expansion and poleward retreat are observed during the storm main phase and recovery phase respectively. The equatorward shift is prominent on the northern duskside, at ~58° coinciding with the minimum SymH and dayside at ~59° compared to dawnside and nightside respectively. The latitudinal shift of FAC range is better correlated to IMF BZ in northern hemisphere dusk-dawn magnetic local time (MLT) sectors than in southern hemisphere. The FAC range latitudinal shifts responds better to dynamic pressure in the duskside northern hemisphere and dawnside southern hemisphere than in southern hemisphere dusk sector and northern hemisphere dawn sector respectively. FAC range exhibits a good correlation with dynamic pressure in the dayside (nightside) southern (northern) hemispheres depicting possible electrodynamic similarity at day-night MLT sectors in the opposite hemispheres.

Storm sudden commencements (SSCs) occur due to a rapid compression of the Earth's magnetic field. This is generally believed to be caused by interplanetary (IP) shocks, but with exceptions. In this paper we explore possible causes of SSCs other than IP shocks through a statistical study of geomagnetic storms using SYM-H data provided by the World Data Center for Geomagnetism – Kyoto and by applying a superposed epoch analysis to simultaneous solar wind parameters obtained with the Advanced Composition Explorer (ACE) satellite. We select a total of 274 geomagnetic storms with minimum SYM-H of less than –30nT during 1998-2008 and regard them as SSCs if SYM-H increases by more than 10 nT over 10 minutes. Under this criterion, we found 103 geomagnetic storms with both SSC and IP shocks and 28 storms with SSC not associated with IP shocks. Storms in the former group share the property that the strength of the interplanetary magnetic field (IMF), proton density and proton velocity increase together with SYM-H, implying the action of IP shocks. During the storms in the latter group, only the proton density rises with SYM-H. We find that the density increase is associated with either high speed streams (HSSs) or interplanetary coronal mass ejections (ICMEs), and suggest that HSSs and ICMEs may be alternative contributors to SSCs.