N. Buzulukova

Spontaneous formation of dipolarization fronts and reconnection onset in the magnetotail

We present full-particle simulations of 2-D magnetotail current sheet equilibria with open boundaries and zero driving. The simulations show that spontaneous formation of dipolarization fronts and subsequent formation of magnetic islands are possible in equilibria with an accumulation of magnetic flux at the tailward end of a sufficiently thin current sheet. These results confirm recent findings in the linear stability of the ion tearing mode, including the predicted dependence of the tail current sheet stability on the amount of accumulated magnetic flux expressed in terms of the specific destabilization parameter. The initial phase of reconnection onset associated with the front formation represents a process of slippage of magnetic field lines with frozen-in electrons relative to the ion plasma species. This non-MHD process characterized by different motions of ion and electron species generates a substantial charge separation electric field normal to the front.

The Comprehensive Inner Magnetosphere-Ionosphere Model

Simulation studies of the Earths radiation belts and ring current are very useful in understanding the acceleration, transport, and loss of energetic particles. Recently, the Comprehensive Ring Current Model (CRCM) and the Radiation Belt Environment (RBE) model were merged to form a Comprehensive Inner Magnetosphere-Ionosphere (CIMI) model. CIMI solves for many essential quantities in the inner magnetosphere, including ion and electron distributions in the ring current and radiation belts, plasmaspheric density, Region 2 currents, convection potential, and precipitation in the ionosphere. It incorporates whistler mode chorus and hiss wave diffusion of energetic electrons in energy, pitch angle, and cross terms. CIMI thus represents a comprehensive model that considers the effects of the ring current and plasmasphere on the radiation belts. We have performed a CIMI simulation for the storm on 5–9 April 2010 and then compared our results with data from the Two Wide-angle Imaging Neutral-atom Spectrometers and Akebono satellites. We identify the dominant energization and loss processes for the ring current and radiation belts. We find that the interactions with the whistler mode chorus waves are the main cause of the flux increase of MeV electrons during the recovery phase of this particular storm. When a self-consistent electric field from the CRCM is used, the enhancement of MeV electrons is higher than when an empirical convection model is applied. We also demonstrate how CIMI can be a powerful tool for analyzing and interpreting data from the new Van Allen Probes mission.

Magnetic reconnection, buoyancy, and flapping motions in magnetotail explosions

A key process in the interaction of magnetospheres with the solar wind is the explosive release of energy stored in the magnetotail. Based on observational evidence, magnetic reconnection is widely believed to be responsible. However, the very possibility of spontaneous reconnection in collisionless magnetotail plasmas has been questioned in kinetic theory for more than three decades. In addition, in situ observations by multispacecraft missions (e.g., THEMIS) reveal the development of buoyancy and flapping motions coexisting with reconnection. Never before have kinetic simulations reproduced all three primary modes in realistic 2-D configurations with a finite normal magnetic field. Moreover, 3-D simulations with closed boundaries suggest that the tail activity is dominated by buoyancy-driven instabilities, whereas reconnection is a secondary effect strongly localized in the dawn-dusk direction. In this paper, we use massively parallel 3-D fully kinetic simulations with open boundaries to show that sufficiently far from the planet explosive processes in the tail are dominated by reconnection motions. These motions occur in the form of spontaneously generated dipolarization fronts accompanied by changes in magnetic topology which extend in the dawn-dusk direction over the size of the simulation box, suggesting that reconnection onset causes a macroscale reconfiguration of the real magnetotail. In our simulations, buoyancy and flapping motions significantly disturb the primary dipolarization front but neither destroy it nor change the near 2-D picture of the front evolution critically. Consistent with recent multiprobe observations, dipolarization fronts are also found to be the main regions of energy conversion in the magnetotail.

TWINS stereoscopic imaging of multiple peaks in the ring current

Global, ion equatorial flux distributions and energy spectra are presented from stereoscopic Two Wide-Angle Imaging Neutral-Atom Spectrometers (TWINS) 1 and TWINS 2 energetic neutral atom (ENA) images for two time periods, 29 May 2010, 1330–1430 UT and 26 May 2011, 1645–1715 UT. The first is just after the main phase of a weak (minimum SYM/H ≈ −70 to −80 nT) corotating interaction region-driven geomagnetic storm. The second is during a relatively quiet period. The global ion distributions show multiple spatial peaks that are coincident with peaks in the AE index. The energy spectra have a primary maximum in the 15–20 keV range. Below the energy maximum, the flux is Maxwellian. Above the main maximum, the flux is either significantly below that of a Maxwellian or has a second component with a maximum in the 40–50 keV range. For the 29 May 2010, 1330–1430 UT time period, the flux from the TWINS stereoscopic images is compared to the results from TWINS 1 and TWINS 2 alone illustrating the advantage of stereoscopic viewing. The flux deconvolved from the TWINS images also shows spatial and temporal correlations with Time History of Events and Macroscale Interactions during Substorms (THEMIS) in situ measurements. Magnetic field dipolarizations observed by GOES support the existence of a peak in the ion flux in the midnight/dawn sector. In summary, increased spatial resolution from TWINS stereoscopic ENA images is demonstrated. Multiple peaks in the ion flux of trapped particles in the ring current are observed. THEMIS electrostatic analyzer in situ ion flux measurements and GOES geosynchronous magnetic field measurements are consistent with the spatial and temporal structure obtained.

First results using TWINS‐derived ion temperature boundary conditions in CRCM

We have integrated dynamic, spatiotemporally resolved ion temperature boundary conditions into the Comprehensive Ring Current Model (CRCM), which are based on 2-D equatorial maps derived from the Two Wide-Angle Imaging Neutral-Atom Spectrometers (TWINS) energetic neutral atom (ENA) data. The high-speed stream-driven event on 22 July 2009 is simulated and compared against an identical simulation using a statistically derived boundary condition model. ENA-derived temperatures allow users to include event-specific observations associated with a dynamic plasma sheet. This method also provides temperatures in the important region between geosynchronous orbit and the plasma sheet, a region which existing empirical models exclude. We find that the spatial and energy distributions of ring current flux and pressure have sensitive dependence on boundary conditions during this event. The coupling of boundary conditions to the time history of the convection field strength also plays an important role by throttling the influence of the boundary plasma on the inner magnetosphere. Simulated moments and spectra from our simulations are compared with remotely imaged ion temperatures from TWINS and also in situ energy spectra and temperature moments from Time History of Events and Macroscale Interactions during Substorms-D. Storm time dusk-dawn asymmetries consistent with observational data, such as Zhang et al. (2006), are reproduced well when CRCM is provided with the event-specific boundary model. A hot localized structure observed by TWINS at geosynchronous midnight during a strong northward interplanetary magnetic field interval is also reproduced with this boundary model, whereas the empirical boundary model fails to yield this feature.

Impact of Solar Wind on the Earth Magnetosphere: Recent Progress in the Modeling of Ring Current and Radiation Belts

When solar wind interacts with the Earth’s magnetosphere it causes disturbances in the near-Earth plasma environment. Large disturbances result in geomagnetic storms, and affect not only Earth magnetosphere but also space-borne and ground-based technological systems. The systematic studies of cause-effect relations between solar wind variations and resulting disturbances in near-Earth plasma environment as well as the construction of relevant numerical models are important subjects of Space Weather, which is currently a very active topic of research.

Dynamics Geomagnetic Storm on 7–10 September 2015 as Observed by TWINS and Simulated by CIMI

For the first time, direct comparisons of the equatorial ion partial pressure and pitch angle anisotropy observed by TWINS and simulated by CIMI are presented. The TWINS ENA images are from a 4-day period, 7–10 September 2015. The simulations use both the empirical Weimer 2K and the self-consistent RCM electric potentials. There are two moderate storms in succession during this period. In most cases, we find that the general features of the ring current in the inner magnetosphere obtained from the observations and the simulations are similar. Nevertheless, we do also see consistent contrasts between the simulations and observations. The simulated partial pressure peaks are often inside the observed peaks and more toward dusk than the measured values. There are also cases in which the measured equatorial ion partial pressure shows multiple peaks that are not seen in the simulations. This occurs during a period of intense AE index. The CIMI simulations consistently show regions of parallel anisotropy spanning the night side between approximately 6 and 8RE, whereas the parallel anisotropy is seen in the observations only during the main phase of the first storm. The evidence from the unique global view provided by the TWINS observations strongly suggests that there are features in the ring current partial pressure distributions that can be best explained by enhanced electric shielding and/or spatially localized, short-duration injections.