Global, collisional model of high‐energy photoelectrons

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95539/1/grl8976.pd

The effects of dynamic ionospheric outflow on the ring current

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94583/1/jgra20739.pd

A bounce‐averaged kinetic model of the ring current ion population

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94668/1/grl7966.pd

Modeling ring current ion and electron dynamics and plasma instabilities during a high‐speed stream driven storm

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94593/1/jgra21840.pd

The two‐way relationship between ionospheric outflow and the ring current

It is now well established that the ionosphere, because it acts as a significant source of plasma, plays a critical role in ring current dynamics. However, because the ring current deposits energy into the ionosphere, the inverse may also be true: the ring current can play a critical role in the dynamics of ionospheric outflow. This study uses a set of coupled, first‐principles‐based numerical models to test the dependence of ionospheric outflow on ring current‐driven region 2 field‐aligned currents (FACs). A moderate magnetospheric storm event is modeled with the Space Weather Modeling Framework using a global MHD code (Block Adaptive Tree Solar wind Roe‐type Upwind Scheme, BATS‐R‐US), a polar wind model (Polar Wind Outflow Model), and a bounce‐averaged kinetic ring current model (ring current atmosphere interaction model with self‐consistent magnetic field, RAM‐SCB). Initially, each code is two‐way coupled to all others except for RAM‐SCB, which receives inputs from the other models but is not allowed to feed back pressure into the MHD model. The simulation is repeated with pressure coupling activated, which drives strong pressure gradients and region 2 FACs in BATS‐R‐US. It is found that the region 2 FACs increase heavy ion outflow by up to 6 times over the noncoupled results. The additional outflow further energizes the ring current, establishing an ionosphere‐magnetosphere mass feedback loop. This study further demonstrates that ionospheric outflow is not merely a plasma source for the magnetosphere but an integral part in the nonlinear ionosphere‐magnetosphere‐ring current system.Key PointsRegion 2 field‐aligned currents drive additional ionospheric O+ outflowThis additional outflow feeds the ring current, creating a feedback systemIonospheric outflow is a tightly coupled piece of the M‐I systemPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112284/1/jgra51836.pd

Energy Content of the Stormtime Ring Current

Given the important role the ring current plays in magnetospheric energetics, it is essential to understand its strength and evolution in disturbed times. There are currently three main methods for deducing the strength of the ring current: measuring ground magnetic perturbations, measuring high-altitude magnetic perturbations, or directly measuring ring current particles. The use of ground magnetometers is the most convenient, and many use the ground magnetometer-derived Dst index as a proxy for the ring current. Recent work suggests, however, that a substantial portion of Dst may not be caused only by the ring current but also by local induction effects or other magnetospheric currents, so simply using the Dst index may yield inaccurate results. This study uses direct particle measurements to calculate the strength of the ring current and compares this to the measured Dst values. We investigate several magnetic storm intervals, using the Polar Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) to measure ring current ions. We then use the Dessler-Parker-Sckopke relation to compare this to the measured Dst. This analysis is used both to understand the general behavior of the ring current compared to Dst as well as to compare the usefulness of the Dst proxy for different types of storms. Ring current ions are shown in this analysis to contribute, on average, half of the Dst depression, with a large variation among individual events

An improved empirical model of electron and ion fluxes at geosynchronous orbit based on upstream solar wind conditions

A new empirical model of the electron fluxes and ion fluxes at geosynchronous orbit (GEO) is introduced, based on observations by Los Alamos National Laboratory (LANL) satellites. The model provides flux predictions in the energy range ~1 eV to ~40 keV, as a function of local time, energy, and the strength of the solar wind electric field (the negative product of the solar wind speed and the z component of the magnetic field). Given appropriate upstream solar wind measurements, the model provides a forecast of the fluxes at GEO with a ~1 h lead time. Model predictions are tested against in‐sample observations from LANL satellites and also against out‐of‐sample observations from the Compact Environmental Anomaly Sensor II detector on the AMC‐12 satellite. The model does not reproduce all structure seen in the observations. However, for the intervals studied here (quiet and storm times) the normalized root‐mean‐square deviation < ~0.3. It is intended that the model will improve forecasting of the spacecraft environment at GEO and also provide improved boundary/input conditions for physical models of the magnetosphere.Key PointsNew model of electron and ion fluxes at GEO (driven by ‐vBz) provides a ~1 h forecast of fluxes in the energy range ~1 eV to ~40 keVThe main benefit from the new model is the ability to predict the fluxes at GEO in advanceForecasts are a good match to observations during quiet times and storm timesPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134149/1/swe20339_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134149/2/swe20339.pd

Tomography of a Cryo-immobilized Yeast Cell Using Ptychographic Coherent X-Ray Diffractive Imaging

The structural investigation of noncrystalline, soft biological matter using x-rays is of rapidly increasing interest. Large-scale x-ray sources, such as synchrotrons and x-ray free electron lasers, are becoming ever brighter and make the study of such weakly scattering materials more feasible. Variants of coherent diffractive imaging (CDI) are particularly attractive, as the absence of an objective lens between sample and detector ensures that no x-ray photons scattered by a sample are lost in a limited-efficiency imaging system. Furthermore, the reconstructed complex image contains quantitative density information, most directly accessible through its phase, which is proportional to the projected electron density of the sample. If applied in three dimensions, CDI can thus recover the sample's electron density distribution. As the extension to three dimensions is accompanied by a considerable dose applied to the sample, cryogenic cooling is necessary to optimize the structural preservation of a unique sample in the beam. This, however, imposes considerable technical challenges on the experimental realization. Here, we show a route toward the solution of these challenges using ptychographic CDI (PCDI), a scanning variant of coherent imaging. We present an experimental demonstration of the combination of three-dimensional structure determination through PCDI with a cryogenically cooled biological sample—a budding yeast cell (Saccharomyces cerevisiae)—using hard (7.9 keV) synchrotron x-rays. This proof-of-principle demonstration in particular illustrates the potential of PCDI for highly sensitive, quantitative three-dimensional density determination of cryogenically cooled, hydrated, and unstained biological matter and paves the way to future studies of unique, nonreproducible biological cells at higher resolution

Excitation of EMIC waves detected by the Van Allen Probes on 28 April 2013

Abstract We report the wave observations, associated plasma measurements, and linear theory testing of electromagnetic ion cyclotron (EMIC) wave events observed by the Van Allen Probes on 28 April 2013. The wave events are detected in their generation regions as three individual events in two consecutive orbits of Van Allen Probe-A, while the other spacecraft, B, does not detect any significant EMIC wave activity during this period. Three overlapping H+ populations are observed around the plasmapause when the waves are excited. The difference between the observational EMIC wave growth parameter (Eh) and the theoretical EMIC instability parameter (Sh) is significantly raised, on average, to 0.10 ± 0.01, 0.15 ± 0.02, and 0.07 ± 0.02 during the three wave events, respectively. On Van Allen Probe-B, this difference never exceeds 0. Compared to linear theory (Eh\u3eSh), the waves are only excited for elevated thresholds