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Register a new userTime dependent solar modulation of cosmic rays from solar minimum to solar maximum
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We study the time-dependent modulation effect and derive the local interstellar spectra (LIS) for the cosmic ray (CR) proton, helium, boron and carbon. A two-dimensional modulation model including the variation of the interplanetary environment with time is adopted to describe modulation process. The propagation equation of CRs in the heliosphere is numerically solved by the package Solarprop. We derive the LIS by fitting the latest results of several experiments, including Voyager 1, PAMELA, BESS-POLARII and ACE, during low solar activity periods. We further study the modulation in the polarity reversal periods with the PAMELA proton data. We find that the rigidity dependence of the diffusion coefficient is critical to explain the modulation effect during reversal periods. Our results also indicate a power law relation between the diffusion coefficient and the magnitude of the heliospheric magnetic field (HMF) at the Earth.
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The solar wind speed plays a key role in the transport of CME out of the Sun and ultimately determines the arrival time of CME-driven shocks in the heliosphere. Here, we develop an empirical model of the solar wind parameters at the inner boundary (18 solar radii, Rs) used in our global, 3D MHD model (G3DMHD) or other equivalent ones. The model takes solar magnetic field maps at 2.5 Rs (which is based on the Potential Field Source Surface, PFSS model) and interpolates the solar wind plasma and field out to 18 Rs using the algorithm of Wang and Sheeley [1990a]. A formula V_{18Rs} = V1 + V2 fs^{alpha} is used to calculate the solar wind speed at 18 Rs, where V1 is in a range of 150-350 km/s, V2 is in the range of 250-500 km/s, and fs is an expansion factor, which was derived from the Wang and Sheeley (WS) algorithm at 2.5 Rs. To estimate the solar wind density and temperature at 18 Rs, we assume an incompressible solar wind and a constant total pressure. The three free parameters are obtained by adjusting simulation results to match in-situ observations (Wind) for more than 54 combination of V1, V2 and {alpha} during a quiet solar wind interval, CR2082. We found V18Rs = (150 +/- 50) + (500 +/- 100) fs^-0.4 km/s performs reasonably well in predicting solar wind parameters at 1 AU not just for CR 2082 but other quiet solar period. Comparing results from the present study with those from WSA [Arge et al. 2000; 2004] we conclude that i) Results of using V_{18Rs} with the full rotation data (FR) as input to drive G3DMHD model is better than the results of WSA using FR, or daily updated. ii) When using a modified daily updated 4-day-advanced solar wind speed predictions WSA performs slightly better than our G3DMHD. iii) When using V_{18Rs} as input, G3DMHD model performs much better than the WSA formula. We argue the necessity of the extra angular width ({theta}b) parameter used in WSA.
We investigate the solar modulation effect with the long time cosmic ray proton and helium spectrum measured by AMS-02 on the time scale of a Bartels rotation (27 days) between May 2011 and May 2017. The time-span covers the negative heliospheric magnetic field polarity cycle, the polarity reversal period and the positive polarity cycle. The unprecedented accuracy of AMS-02 observation data provide a good opportunity to improve the understanding of the time dependent solar modulation effect. In this work, a two-dimensional solar modulation model is used to compute the propagation of cosmic rays in the heliosphere. Some important ingredients of the model which reflect the global heliospherical environment are taken from the observations. The propagation equation is numerically solved with the pubic Solarprop code. We find that the drift effect is suppressed during the high solar activity period but nearly recovered in the first half of 2017. The time-dependent rigidity dependence of the mean free path is critical to reproduce the observations between August 2012 and October 2015.
Both coronal holes and active regions are source regions of the solar wind. The distribution of these coronal structures across both space and time is well known, but it is unclear how much each source contributes to the solar wind. In this study we use photospheric magnetic field maps observed over the past four solar cycles to estimate what fraction of magnetic open solar flux is rooted in active regions, a proxy for the fraction of all solar wind originating in active regions. We find that the fractional contribution of active regions to the solar wind varies between 30% to 80% at any one time during solar maximum and is negligible at solar minimum, showing a strong correlation with sunspot number. While active regions are typically confined to latitudes $pm$30$^{circ}$ in the corona, the solar wind they produce can reach latitudes up to $pm$60$^{circ}$. Their fractional contribution to the solar wind also correlates with coronal mass ejection rate, and is highly variable, changing by $pm$20% on monthly timescales within individual solar maxima. We speculate that these variations could be driven by coronal mass ejections causing reconfigurations of the coronal magnetic field on sub-monthly timescales.
The Wide-field Imager for Solar PRobe (WISPR) obtained the first high-resolution images of coronal rays at heights below 15 R$_odot$ when the Parker Solar Probe (PSP) was located inside 0.25 au during the first encounter. We exploit these remarkable images to reveal the structure of coronal rays at scales that are not easily discernible in images taken from near 1 au. To analyze and interpret WISPR observations, which evolve rapidly both radially and longitudinally, we construct a latitude versus time map using the full WISPR dataset from the first encounter. From the exploitation of this map and also from sequential WISPR images, we show the presence of multiple substructures inside streamers and pseudostreamers. WISPR unveils the fine-scale structure of the densest part of streamer rays that we identify as the solar origin of the heliospheric plasma sheet typically measured in situ in the solar wind. We exploit 3D magnetohydrodynamic models, and we construct synthetic white-light images to study the origin of the coronal structures observed by WISPR. Overall, including the effect of the spacecraft relative motion toward the individual coronal structures, we can interpret several observed features by WISPR. Moreover, we relate some coronal rays to folds in the heliospheric current sheet that are unresolved from 1 au. Other rays appear to form as a result of the inherently inhomogeneous distribution of open magnetic flux tubes.
We report on results of imaging and spectral studies of X-ray emission from Jupiter observed by Suzaku. In 2006 Suzaku had found diffuse X-ray emission in $1unicode{x2013}5$ keV associated with Jovian inner radiation belts. It has been suggested that the emission is caused by the inverse-Compton scattering by ultra-relativistic electrons ($ sim 50 $ MeV) in Jupiters magnetosphere. To confirm the existence of this emission and to understand its relation to the solar activity, we conducted an additional Suzaku observation in 2014 around the maximum of the 24th solar cycle. As a result, we successfully found again the diffuse emission around Jupiter in $1unicode{x2013}5$ keV and also point-like emission in $0.4unicode{x2013}1$ keV. The luminosity of the point-like emission which was probably composed of solar X-ray scattering, charge exchange, or auroral bremsstrahlung emission increased by a factor of $ sim 5$ with respect to 2006, most likely due to an increase of the solar activity. The diffuse emission spectrum in the $1unicode{x2013}5$ keV band was well-fitted with a flat power-law function ($ Gamma = 1.4 pm 0.1 $) as in the past observation, which supported the inverse-Compton scattering hypothesis. However, its spatial distribution changed from $ sim 12 times 4 $ Jovian radius (Rj) to $ sim 20 times 7 $ Rj. The luminosity of the diffuse emission increased by a smaller factor of $ sim 3 $. This indicates that the diffuse emission is not simply responding to the solar activity, which is also known to cause little effect on the distribution of high-energy electrons around Jupiter. Further sensitive study of the spatial and spectral distributions of the diffuse hard X-ray emission is important to understand how high-energy particles are accelerated in Jupiters magnetosphere.
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