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In situ local shock speed and transit shock speed  [PDF]
S. Watari,T. Detman
Annales Geophysicae (ANGEO) , 2003,
Abstract: A useful index for estimating the transit speeds was derived by analyzing interplanetary shock observations. This index is the ratio of the in situ local shock speed and the transit speed; it is 0.6–0.9 for most observed shocks. The local shock speed and the transit speed calculated for the results of the magnetohydrodynamic simulation show good agreement with the observations. The relation expressed by the index is well explained by a simplified propagation model assuming a blast wave. For several shocks the ratio is approximately 1.2, implying that these shocks accelerated during propagation in slow-speed solar wind. This ratio is similar to that for the background solar wind acceleration. Keywords. Interplanetary physics (Flare and stream dynamics; Interplanetary shocks; Solar wind plasma)
Heliolatitude and time variations of solar wind structure from in situ measurements and interplanetary scintillation observations  [PDF]
J. M. Sokol,M. Bzowski,M. Tokumaru,K. Fujiki,D. J. McComas
Physics , 2011, DOI: 10.1007/s11207-012-9993-9
Abstract: The 3D structure of solar wind and its evolution in time is needed for heliospheric modeling and interpretation of energetic neutral atoms observations. We present a model to retrieve the solar wind structure in heliolatitude and time using all available and complementary data sources. We determine the heliolatitude structure of solar wind speed on a yearly time grid over the past 1.5 solar cycles based on remote-sensing observations of interplanetary scintillations, in situ out-of-ecliptic measurements from Ulysses, and in situ in-ecliptic measurements from the OMNI-2 database. Since the in situ information on the solar wind density structure out of ecliptic is not available apart from the Ulysses data, we derive correlation formulae between solar wind speed and density and use the information on the solar wind speed from interplanetary scintillation observations to retrieve the 3D structure of solar wind density. With the variations of solar wind density and speed in time and heliolatitude available we calculate variations in solar wind flux, dynamic pressure and charge exchange rate in the approximation of stationary H atoms.
Reconstruction of Helio-latitudinal Structure of the Solar Wind Proton Speed and Density  [PDF]
Justyna M. Sokó?,Pawe? Swaczyna,Maciej Bzowski,Munetoshi Tokumaru
Physics , 2015, DOI: 10.1007/s11207-015-0800-2
Abstract: 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 (Sok\'o{\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.
A Numerical Investigation of the Recurrent High-speed Jets as a Possibility of Solar Wind Origin  [PDF]
Liping Yang,Jiansen He,Hardi Peter,Chuanyi Tu,Lei Zhang,Eckart Marsch,Linghua Wang,Xueshang Feng
Physics , 2015,
Abstract: In the solar atmosphere, jets are prevalent and they are significant for the mass and energy transport. Here we conduct numerical simulations to investigate the mass and energy contributions of the recently observed high-speed jets to the solar wind. With a one-dimensional hydrodynamic solar wind model, the time-dependent pulses are imposed at the bottom to simulate the jets. The simulation results show that without other energy source, the injected plasmas are accelerated effectively to be a transonic wind with a substantial mass flux. The rapid acceleration occurs close to the Sun, and the resulting asymptotic speed, number density at 0.3 AU, as well as mass flux normalized to 1 AU are compatible with in situ observations. As a result of the high speed, the imposed pulses generate a train of shocks traveling upward. By tracing the motions of the injected plasma, it is found that these shocks heat and accelerate the injected plasmas successively step by step to push them upward and eventually allow them to escape. The parametric studies show that increasing the speed of the imposed pulses or their temperature gives a considerably faster, and hotter solar wind, while increasing their number density or decreasing their recurring period only bring a denser solar wind. These studies provide a possibility that the ubiquitous high-speed jets are a substantial mass and energy contributions to the solar wind.
Speed evolution of fast CME/shocks with SOHO/LASCO, WIND/WAVES, IPS and in-situ WIND data: analysis of kilometric type-II emissions
A. Gonzalez-Esparza ,E. Aguilar-Rodriguez
Annales Geophysicae (ANGEO) , 2009,
Abstract: Fast CME/shocks propagating in the interplanetary medium can generate kilometric Type II (km-TII) radio emissions at the local plasma frequency and/or its harmonic, so these radio emissions provide a means of remotely tracking CME/shocks. We apply a new analysis technique, using the frequency drift of km-TII spectrum obtained by the Thermal Noise Receiver (TNR) of the WIND/WAVES experiment, to infer, at some adequate intervals, the propagation speed of six CME/shocks. We combine these results with previously reported speeds from coronagraph white light and interplanetary scintillation observations, and in-situ measurements, to study the temporal speed evolution of the six events. The speed values obtained by the km-TII analysis are in a reasonable agreement with the speed measurements obtained by other techniques at different heliocentric distance ranges. The combination of all the speed measurements show a gradual deceleration of the CME/shocks as they propagate to 1 AU. This new technique can be useful in studying the evolution of fast CME/shocks when adequate intervals of km-TII emissions are available.
Comparative Study of MHD Modeling of the Background Solar Wind  [PDF]
C. Gressl,A. M. Veronig,M. Temmer,D. Odstrcil,J. A. Linker,Z. Mikic,P. Riley
Physics , 2013, DOI: 10.1007/s11207-013-0421-6
Abstract: Knowledge about the background solar wind plays a crucial role in the framework of space weather forecasting. In-situ measurements of the background solar wind are only available for a few points in the heliosphere where spacecraft are located, therefore we have to rely on heliospheric models to derive the distribution of solar wind parameters in interplanetary space. We test the performance of different solar wind models, namely Magnetohydrodynamic Algorithm outside a Sphere/ENLIL (MAS/ENLIL), Wang-Sheeley-Arge/ENLIL (WSA/ENLIL), and MAS/MAS, by comparing model results with in-situ measurements from spacecraft located at 1 AU distance to the Sun (ACE, Wind). To exclude the influence of interplanetary coronal mass ejections (ICMEs), we chose the year 2007 as a time period with low solar activity for our comparison. We found that the general structure of the background solar wind is well reproduced by all models. The best model results were obtained for the parameter solar wind speed. However, the predicted arrival times of high-speed solar wind streams have typical uncertainties of the order of about one day. Comparison of model runs with synoptic magnetic maps from different observatories revealed that the choice of the synoptic map significantly affects the model performance.
Solar winds along curved magnetic field lines  [PDF]
Bo Li,Li-Dong Xia,Yao Chen
Physics , 2011, DOI: 10.1051/0004-6361/201116668
Abstract: Both remote-sensing measurements using the interplanetary scintillation (IPS) technique and in situ measurements by the Ulysses spacecraft show a bimodal structure for the solar wind at solar minimum conditions. At present what makes the fast wind fast and the slow wind slow still remains to be answered. While a robust empirical correlation exists between the coronal expansion rate $f_c$ of the flow tubes and the speeds $v$ measured in situ, further data analysis suggests that $v$ depends on more than just $f_c$. We examine whether the non-radial shape of field lines, which naturally accompanies any non-radial expansion, could be an additional geometrical factor. We solved the transport equations incorporating the heating due to turbulent Alfv\'en waves for an electron-proton solar wind along curved field lines given by an analytical magnetic field model, representative of a solar minimum corona. The field line shape is found to influence substantially the solar wind parameters, reducing the asymptotic speed by up to $\sim 130$ km s$^{-1}$, or by $\sim 28%$ in relative terms, compared with the case neglecting the field line curvature. This effect was interpreted in the general framework of energy addition in the solar wind: Relative to the straight case, the field line curvature enhances the effective energy deposition to the subsonic flow, resulting in a higher proton flux and a lower terminal proton speed. Our computations suggest that the field line curvature could be a geometrical factor which, in addition to the tube expansion, substantially influences the solar wind speed. Furthermore, at solar minima although the field line curvature unlikely affects the polar fast solar wind, it does help make the wind at low latitudes slow, thereby helping better reproduce the Ulysses measurements.
Real-time solar wind prediction based on SDO/AIA coronal hole data  [PDF]
T. Rotter,A. M. Veronig,M. Temmer,B. Vrsnak
Physics , 2015, DOI: 10.1007/s11207-015-0680-5
Abstract: We present an empirical model based on the visible area covered by coronal holes close to the central meridian in order to predict the solar wind speed at 1 AU with a lead time up to four days in advance with a 1hr time resolution. Linear prediction functions are used to relate coronal hole areas to solar wind speed. The function parameters are automatically adapted by using the information from the previous 3 Carrington Rotations. Thus the algorithm automatically reacts on the changes of the solar wind speed during different phases of the solar cycle. The adaptive algorithm has been applied to and tested on SDO/AIA-193A observations and ACE measurements during the years 2011-2013, covering 41 Carrington Rotations. The solar wind speed arrival time is delayed and needs on average 4.02 +/- 0.5 days to reach Earth. The algorithm produces good predictions for the 156 solar wind high speed streams peak amplitudes with correlation coefficients of cc~0.60. For 80% of the peaks, the predicted arrival matches within a time window of 0.5 days of the ACE in situ measurements. The same algorithm, using linear predictions, was also applied to predict the magnetic field strength from coronal hole areas but did not give reliable predictions (cc~0.2).
A turbulence-driven model for heating and acceleration of the fast wind in coronal holes  [PDF]
A. Verdini,M. Velli,W. H. Matthaeus,S. Oughton,P. Dmitruk
Physics , 2009, DOI: 10.1088/2041-8205/708/2/L116
Abstract: A model is presented for generation of fast solar wind in coronal holes, relying on heating that is dominated by turbulent dissipation of MHD fluctuations transported upwards in the solar atmosphere. Scale-separated transport equations include large-scale fields, transverse Alfvenic fluctuations, and a small compressive dissipation due to parallel shears near the transition region. The model accounts for proton temperature, density, wind speed, and fluctuation amplitude as observed in remote sensing and in situ satellite data.
Stellar Winds on the Main-Sequence I: Wind Model  [PDF]
C. P. Johnstone,M. Güdel,T. Lüftinger,G. Toth,I. Brott
Physics , 2015, DOI: 10.1051/0004-6361/201425300
Abstract: Aims: We develop a method for estimating the properties of stellar winds for low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a range of distances from the star. Methods: We use 1D thermal pressure driven hydrodynamic wind models run using the Versatile Advection Code. Using in situ measurements of the solar wind, we produce models for the slow and fast components of the solar wind. We consider two radically different methods for scaling the base temperature of the wind to other stars: in Model A, we assume that wind temperatures are fundamentally linked to coronal temperatures, and in Model B, we assume that the sound speed at the base of the wind is a fixed fraction of the escape velocity. In Paper II of this series, we use observationally constrained rotational evolution models to derive wind mass loss rates. Results: Our model for the solar wind provides an excellent description of the real solar wind far from the solar surface, but is unrealistic within the solar corona. We run a grid of 1200 wind models to derive relations for the wind properties as a function of stellar mass, radius, and wind temperature. Using these results, we explore how wind properties depend on stellar mass and rotation. Conclusions: Based on our two assumptions about the scaling of the wind temperature, we argue that there is still significant uncertainty in how these properties should be determined. Resolution of this uncertainty will probably require both the application of solar wind physics to other stars and detailed observational constraints on the properties of stellar winds. In the final section of this paper, we give step by step instructions for how to apply our results to calculate the stellar wind conditions far from the stellar surface.
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