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Register a new userOn the Dependency between the Peak Velocity of High-speed Solar Wind Streams near Earth and the Area of Their Solar Source Coronal Holes
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The relationship between the peak velocities of high-speed solar wind streams near Earth and the areas of their solar source regions, i.e., coronal holes, has been known since the 1970s, but it is still physically not well understood. We perform 3D magnetohydrodynamic (MHD) simulations using the European Heliospheric Forecasting Information Asset (EUHFORIA) code to show that this empirical relationship forms during the propagation phase of high-speed streams from the Sun to Earth. For this purpose, we neglect the acceleration phase of high-speed streams, and project the areas of coronal holes to a sphere at 0.1 au. We then vary only the areas and latitudes of the coronal holes. The velocity, temperature, and density in the cross section of the corresponding highspeed streams at 0.1 au are set to constant, homogeneous values. Finally, we propagate the associated high-speed streams through the inner heliosphere using the EUHFORIA code. The simulated high-speed stream peak velocities at Earth reveal a linear dependence on the area of their source coronal holes. The slopes of the relationship decrease with increasing latitudes of the coronal holes, and the peak velocities saturate at a value of about 730 km/s, similar to the observations. These findings imply that the empirical relationship between the coronal hole areas and high-speed stream peak velocities does not describe the acceleration phase of high-speed streams, but is a result of the high-speed stream propagation from the Sun to Earth.
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Coronal holes (CHs) are regions of open magnetic flux which are the source of high speed solar wind (HSSW) streams. To date, it is not clear which aspects of CHs are of most influence on the properties of the solar wind as it expands through the Heliosphere. Here, we study the relationship between CH properties extracted from AIA (Atmospheric Imaging Assembly) images using CHIMERA (Coronal Hole Identification via Multi-thermal Emission Recognition Algorithm) and HSSW measurements from ACE (Advanced Composition Explorer) at L1. For CH longitudinal widths $Deltatheta_{CH}<$67$^{circ}$, the peak SW velocity ($v_{max}$) is found to scale as $v_{max}~approx~330.8~+~5.7~Deltatheta_{CH}$~km~s$^{-1}$. For larger longitudinal widths ($Deltatheta_{CH}>$67$^{circ}$), $v_{max}$ is found to tend to a constant value ($sim$710~km~s$^{-1}$). Furthermore, we find that the duration of HSSW streams ($Delta t$) are directly related to the longitudinal width of CHs ($Delta t_{SW}$~$approx$~0.09$Deltatheta_{CH}$) and that their longitudinal expansion factor is $f_{SW}~approx 1.2~pm 0.1$. We also derive an expression for the coronal hole flux-tube expansion factor, $f_{FT}$, which varies as $f_{SW} gtrsim f_{FT} gtrsim 0.8$. These results enable us to estimate the peak speeds and durations of HSSW streams at L1 using the properties of CHs identified in the solar corona.
We comparatively studied the long-term variation (1992-2017) in polar brightening observed with the Nobeyama Radioheliograph, the polar solar wind velocity with interplanetary scintillation observations at the Institute for Space-Earth Environmental Research, and the coronal hole distribution computed by potential field calculations of the solar corona using synoptic magnetogram data obtained at Kitt Peak National Solar Observatory. First, by comparing the solar wind velocity (V) and the brightness temperature (T_b) in the polar region, we found good correlation coefficients (CCs) between V and T_b in the polar regions, CC = 0.91 (0.83) for the northern (southern) polar region, and we obtained the V-T_b relationship as V =12.6 (T_b-10,667)^{1/2}+432. We also confirmed that the CC of V-T_b is higher than those of V-B and V-B/f, where B and f are the polar magnetic field strength and magnetic flux expansion rate, respectively. These results indicate that T_b is a more direct parameter than B or B/f for expressing solar wind velocity. Next, we analyzed the long-term variation of the polar brightening and its relation to the area of the polar coronal hole (A). As a result, we found that the polar brightening matches the probability distribution of the predicted coronal hole and that the CC between T_b and A is remarkably high, CC = 0.97. This result indicates that the polar brightening is strongly coupled to the size of the polar coronal hole. Therefore, the reasonable correlation of V-T_b is explained by V-A. In addition, by considering the anti-correlation between A and f found in a previous study, we suggest that the V-T_b relationship is another expression of the Wang-Sheeley relationship (V-1/f) in the polar regions.
The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 Rs, allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSPs fourth solar encounter, likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with spectral index close to -5/3 rather than -3/2), a lower Alfvenicity, and a 1/f break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ~4{deg} from the HCS, suggesting ~8{deg} as the full-width of the streamer belt wind at these distances. While the majority of the Alfvenic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind.
We have studied the relationship between the solar-wind speed $[V]$ and the coronal magnetic-field properties (a flux expansion factor [$f$] and photospheric magnetic-field strength [$B_{mathrm{S}}$]) at all latitudes using data of interplanetary scintillation and solar magnetic field obtained for 24 years from 1986 to 2009. Using a cross-correlation analyses, we verified that $V$ is inversely proportional to $f$ and found that $V$ tends to increase with $B_{mathrm{S}}$ if $f$ is the same. As a consequence, we find that $V$ has extremely good linear correlation with $B_{mathrm{S}}/f$. However, this linear relation of $V$ and $B_{mathrm{S}}/f$ cannot be used for predicting the solar-wind velocity without information on the solar-wind mass flux. We discuss why the inverse relation between $V$ and $f$ has been successfully used for solar-wind velocity prediction, even though it does not explicitly include the mass flux and magnetic-field strength, which are important physical parameters for solar-wind acceleration.
We demonstrate how the parameters of a Gibson-Low flux-rope-based coronal mass ejection (CME) can be constrained using remote observations. Our Multi Scale Fluid-Kinetic Simulation Suite (MS-FLUKSS) has been used to simulate the propagation of a CME in a data driven solar corona background computed using the photospheric magnetogram data. We constrain the CME model parameters using the observations of such key CME properties as its speed, orientation, and poloidal flux. The speed and orientation are estimated using multi-viewpoint white-light coronagraph images. The reconnected magnetic flux in the area covered by the post eruption arcade is used to estimate the poloidal flux in the CME flux rope. We simulate the partial halo CME on 7 March 2011 to demonstrate the efficiency of our approach. This CME erupted with the speed of 812 km/s and its poloidal flux, as estimated from source active region data, was 4.9e21 Mx. Using our approach, we were able to simulate this CME with the speed 840 km/s and the poloidal flux of 5.1e21 Mx, in remarkable agreement with the observations.
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