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Register a new userReconstruction of Helio-latitudinal Structure of the Solar Wind Proton Speed and Density
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Justyna M. Sok\\'o{\\l}
Publication date
2015
fields
Physics
and research's language is
English
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The modeling of the heliosphere requires continuous three-dimensional solar wind data. The in-situ out-of-ecliptic measurements are very rare, so that other methods of solar wind detection are needed. We use the remote-sensing data of the solar wind speed from observations of interplanetary scintillation (IPS) to reconstruct spatial and temporal structures of the solar wind proton speed from 1985 to 2013. We developed a method of filling the data gaps in the IPS observations to obtain continuous and homogeneous solar wind speed records. We also present a method to retrieve the solar wind density from the solar wind speed, utilizing the invariance of the solar wind dynamic pressure and energy flux with latitude. To construct the synoptic maps of the solar wind speed we use the decomposition into spherical harmonics of each of the Carrington rotation map. To fill the gaps in time we apply the singular spectrum analysis to the time series of the coefficients of spherical harmonics. We obtained helio-latitudinal profiles of the solar wind proton speed and density over almost three recent solar cycles. The accuracy in the reconstruction is, due to computational limitations, about 20%. The proposed methods allow us to improve the spatial and temporal resolution of the model of the solar wind parameters presented in our previous paper (Soko{l} et al. 2013) and give a better insight into the time variations of the solar wind structure. Additionally, the solar wind density is reconstructed more accurately and it fits better to the in-situ measurements from Ulysses.
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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.
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.
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.
Solar wind measurements in the heliosphere are predominantly comprised of protons, alphas, and minor elements in a highly ionized state. The majority of low charge states, such as He$^{+}$, measured in situ are often attributed to pick up ions of non-solar origin. However, through inspection of the velocity distribution functions of near Earth measurements, we find a small but significant population of He$^+$ ions in the normal solar wind whose properties indicate that it originated from the Sun and has evolved as part of the normal solar wind. Current ionization models, largely governed by electron impact and radiative ionization and recombination processes, underestimate this population by several orders of magnitude. Therefore, to reconcile the singly ionized He observed, we investigate recombination of solar He$^{2+}$ through charge exchange with neutrals from circumsolar dust as a possible formation mechanism of solar He$^{+}$. We present an empirical profile of neutrals necessary for charge exchange to become an effective vehicle to recombine He$^{2+}$ to He$^{+}$ such that it meets observational He$^{+}$ values. We find the formation of He$^{+}$ is not only sensitive to the density of neutrals but also to the inner boundary of the neutral distribution encountered along the solar wind path. However, further observational constraints are necessary to confirm that the interaction between solar $alpha$ particles and dust neutrals is the primary source of the He$^{+}$ observations.
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.
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