Solar Wind ∼0.15–1.5 keV Electrons around Corotating Interaction Regions at 1 au

Here we present a statistical study of the ∼0.15–1.5 keV suprathermal electrons observed in uncompressed/compressed slow and fast solar wind around 59 corotating interaction regions (CIRs) with good measurements by Wind 3DP from 1995 through 1997. For each of these CIRs, we fit the strahl and halo energy spectra at ∼0.15–1.5 keV to a Kappa function with a Kappa index κ and kinetic temperature T eff. We find that the ∼0.15–1.5 keV strahl electrons behave similarly in both slow and fast wind: the strahl number density n s positively correlates with the solar wind electron temperature T e and interplanetary magnetic field magnitude ∣B∣, while the strahl pitch angle width Θ s decreases with the solar wind speed V sw. These suggest that the strahl electrons are generated by a similar/same process at the Sun in both slow and fast wind that produces these correlations, and the scattering efficiency of strahl in the interplanetary medium (IPM) decreases with V sw. The ∼0.15–1.5 keV halo electrons also behave similarly in both slow and fast wind: the halo parameter positively correlates with the corresponding strahl parameter, and the halo number density n h positively correlates only with T e . These indicate that the halo formation process in the IPM retains most of the strahl properties, but it erases the relationship between n s and ∣B∣. In addition, κ in compressed wind distributes similarly to that in uncompressed wind, for both the strahl and halo. It shows that CIRs at 1 au are not a significant/effective acceleration source for the strahl and halo.

Solar Rotation Multiples in Space-Weather Effects

The solar rotation period is the most prominent mid-term periodicity in the temporal behaviour of solar, heliospheric, and geomagnetic parameters. It is also a cause of the repeatedly appearing geomagnetic storms originating from the corotating interaction regions (CIRs). Since geomagnetic CIR-driven storms have a natural periodic character, and geomagnetic storms impact energy infrastructure via geomagnetically induced currents, it is of interest whether this periodic character is also noticeable in the temporal behaviour of electrical-grid failures (EGFs), which, at least to some extent, might be of solar origin.

Interplanetary Shock-driven Magnetopause Compressions in Gorgon Global-MHD Simulations

<p>Fast-forward interplanetary interplanetary shocks, as occur at the forefront of interplanetary coronal mass ejections and at corotating interaction regions, can rapidly compress the magnetopause inside the drift paths of electrons and protons, and expose geosynchonous satellites directly to the solar wind.  Here, we use Gorgon Global-MHD simulations to study the response of the magnetopause to different fast-forward interplanetary shocks, with strengths extending from the median shocks observed during solar minimum up to that representing an extreme space weather event. The subsequent magnetopause response can be characterised by three distinct phases; an initial acceleration as inertial forces are overcome, a rapid compression well-represented by a power law, and large-scale damped oscillatory motion of the order of an Earth radius, prior to reaching pressure-balance equilibrium. The subsolar magnetopause is found to oscillate with notable frequencies in the range of 2–13 mHz over several periods of diminishing amplitudes.  These results provide an explanation for similar large-scale magnetopause oscillations observed previously during the extreme events of August 1972 and March 1991 and highlight why static magnetopause models break down during periods of strong solar wind driving.</p>

An extensive spectroscopic time series of three Wolf–Rayet stars – II. A search for wind asymmetries in the dust-forming WC7 binary WR137

ABSTRACT We present the results of a 4-month, spectroscopic campaign of the Wolf–Rayet dust-making binary, WR137. We detect only small-amplitude random variability in the C iii λ5696 emission line and its integrated quantities (radial velocity, equivalent width, skewness, and kurtosis) that can be explained by stochastic clumps in the wind of the WC star. We find no evidence of large-scale periodic variations often associated with Corotating Interaction Regions that could have explained the observed intrinsic continuum polarization of this star. Our moderately high-resolution and high signal-to-noise average Keck spectrum shows narrow double-peak emission profiles in the H α, H β, H γ, He ii λ6678, and He ii λ5876 lines. These peaks have a stable blue-to-red intensity ratio with a mean of 0.997 and a root mean square of 0.004 commensurate with the noise level; no variability is found during the entire observing period. We suggest that these profiles arise in a decretion disc around the O9 companion, which is thus an O9e star. The characteristics of the profiles are compatible with those of other Be/Oe stars. The presence of this disc can explain the constant component of the continuum polarization of this system, for which the angle is perpendicular to the plane of the orbit, implying that the rotation axis of the O9e star is aligned with that of the orbit. It remains to be explained why the disc is so stable within the strong ultraviolet radiation field of the O star. We present a binary evolutionary scenario that is compatible with the current stellar and system parameters.

Radio variability from corotating interaction regions threading Wolf–Rayet winds

ABSTRACT The structured winds of single massive stars can be classified into two broad groups: stochastic structure and organized structure. While the former is typically identified with clumping, the latter is typically associated with rotational modulations, particularly the paradigm of corotating interaction regions (CIRs). While CIRs have been explored extensively in the ultraviolet band, and moderately in the X-ray and optical, here we evaluate radio variability from CIR structures assuming free–free opacity in a dense wind. Our goal is to conduct a broad parameter study to assess the observational feasibility, and to this end, we adopt a phenomenological model for a CIR that threads an otherwise spherical wind. We find that under reasonable assumptions, it is possible to obtain radio variability at the 10 per cent level. The detailed structure of the folded light curve depends not only on the curvature of the CIR, the density contrast of the CIR relative to the wind, and viewing inclination, but also on wavelength. Comparing light curves at different wavelengths, we find that the amplitude can change, that there can be phase shifts in the waveform, and the entire waveform itself can change. These characterstics could be exploited to detect the presence of CIRs in dense, hot winds.

Comparative statistical study of characteristics of plasma in planar and non-planar ICME sheaths during solar cycles 23 and 24

ABSTRACT Planar magnetic structures (PMS) are often observed in sheath regions driven by interplanetary coronal mass ejections (ICMEs) and in corotating interaction regions (CIRs). Here, we study plasma properties statistically within planar and non-planar ICME sheath regions using in situ data from the Advanced Composition Explore (ACE) spacecraft. The study includes 420 ICME-driven sheaths from 1998–2017. We found that 146 ($\sim 35{{\ \rm per\ cent}}$) ICME-driven sheaths are planar, whereas 274 ($\sim 65{{\ \rm per\ cent}}$) are non-planar. This study found that the average plasma temperature, density, speed, plasma beta, thermal pressure and magnetic pressure are higher in planar sheaths than in non-planar sheaths. This implies that high compression plays an essential role in the formation of PMS in sheath regions. Interestingly, our analysis reveals explicitly that the strength of the southward/northward magnetic field component is almost double in planar sheath regions compared with non-planar sheath regions. This suggests that planar sheaths are more geoeffective than non-planar sheaths.