Yihong Wu

Understanding the spatial variability and correlation lengths throughout Stream Interaction Regions (SIRs) has importance both for fundamentally understanding the structuring of the solar wind and SIRs and for space weather operations. SIR observations from Advanced Composition Explorer and Wind are utilized to characterize the spatial variability of these structures and to compute correlation lengths of the solar wind speed and magnetic field magnitude for different regions of the SIR. In general, the probability of agreement between time‐shifted magnetic field magnitudes and solar wind speeds decreases with increasing spacecraft separation, with this degradation being more pronounced in the magnetic field magnitude. While the slow compressed region of the SIR has the steepest decline in agreement for the magnetic field magnitude, it is the shallowest decline for the percent agreement of solar wind speed. Despite these trends, the correlation length is longest in the slow compressed region for both magnetic field magnitude and solar wind speed, with the magnetic field magnitude generally having a longer correlation length than speed. These findings are consistent with current sheet layering within the compressed regions of the SIR and impacts of large‐amplitude Alfvénic fluctuations within the compressed and pristine fast solar wind. These results also further demonstrate the limitations of treating structures in the solar wind as laminar slabs, rather than being highly structured and variable, which may have impacts on uncertainties in space weather forecasting.

Understanding the spatial variability and correlation lengths throughout Stream Interaction Regions (SIRs) has importance both for fundamentally understanding structuring of the solar wind and SIRs and for space weather operations. SIR observations from ACE and Wind are utilized to characterize the spatial variability of these structures and to compute correlation lengths of the solar wind speed and magnetic field magnitude for different regions of the SIR. In general, the probability of agreement between time-shifted magnetic field magnitudes and solar wind speeds decrease for increasing spacecraft separation, with this degradation being more pronounced in the magnetic field magnitude. While the slow compressed region of the SIR has the steepest decline in agreement for the magnetic field magnitude, it is the shallowest decline for the percent agreement of solar wind speed. Despite these trends, the correlation length is longest in the slow compressed region for both magnetic field magnitude and solar wind speed, with the magnetic field magnitude having generally a longer correlation length than speed. These findings are consistent with current sheet layering within the compressed regions of the SIR and impacts of large-amplitude Alfvénic fluctuations within the compressed and pristine fast solar wind. These results also further demonstrate the limitations of treating structures in the solar wind as laminar slabs, rather than being highly structured and variable, which may have impacts on uncertainties in space weather forecasting.

The origin of hard X-rays and γ-rays emitted from the solar atmosphere during occulted solar flares is still debated. The hard X-ray emissions could come from flaring loop tops rising above the limb or coronal mass ejection shock waves, two by-products of energetic solar storms. For the shock scenario to work, accelerated particles must be released on magnetic field lines rooted on the visible disk and precipitate. We present a new Monte Carlo code that computes particle acceleration at shocks propagating along large coronal magnetic loops. A first implementation of the model is carried out for the 2014 September 1 event, and the modeled electron spectra are compared with those inferred from Fermi Gamma-ray Burst Monitor (GBM) measurements. When particle diffusion processes are invoked, our model can reproduce the hard electron spectra measured by GBM nearly 10 minutes after the estimated on-disk hard X-rays appear to have ceased from the flare site.

Interstellar pickup ions (PUIs) play a significant part in mediating the solar wind (SW) interaction with the interstellar medium. In this paper, we examine the details of spatial variation of the PUI velocity distribution function (VDF) in the SW by solving the PUI transport equation. We assume the PUI distribution is isotropic resulting from strong pitch-angle scattering by wave–particle interaction. A three-dimensional model combining the MHD treatment of the background SW and neutrals with a kinetic treatment of PUIs throughout the heliosphere and the surrounding local interstellar medium has been developed. The model generates PUI power-law tails via second-order Fermi process. We analyze how PUIs transform across the heliospheric termination shock and obtain the PUI phase space distribution in the inner heliosheath including continuing velocity diffusion. Our simulated PUI spectra are compared with observations made by New Horizons, Ulysses, Voyager 1, 2, and Cassini, and a satisfactory agreement is demonstrated. Some specific features in the observations, for example, a cutoff of PUI VDF at v = V _SW and a f ∝ v ⁻⁵ tail in the reference frame of the SW, are well represented by the model.