ترغب بنشر مسار تعليمي؟ اضغط هنا

On the production of He$^+$ of solar origin in the solar wind

70   0   0.0 ( 0 )
 نشر من قبل Yeimy Rivera
 تاريخ النشر 2020
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

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.



قيم البحث

اقرأ أيضاً

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 f ound 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.
The Solar Wind Electrons Alphas and Protons experiment on the Parker Solar Probe (PSP) mission measures the three-dimensional electron velocity distribution function. We derive the parameters of the core, halo, and strahl populations utilizing a comb ination of fitting to model distributions and numerical integration for $sim 100,000$ electron distributions measured near the Sun on the first two PSP orbits, which reached heliocentric distances as small as $sim 0.17$ AU. As expected, the electron core density and temperature increase with decreasing heliocentric distance, while the ratio of electron thermal pressure to magnetic pressure ($beta_e$) decreases. These quantities have radial scaling consistent with previous observations farther from the Sun, with superposed variations associated with different solar wind streams. The density in the strahl also increases; however, the density of the halo plateaus and even decreases at perihelion, leading to a large strahl/halo ratio near the Sun. As at greater heliocentric distances, the core has a sunward drift relative to the proton frame, which balances the current carried by the strahl, satisfying the zero-current condition necessary to maintain quasi-neutrality. Many characteristics of the electron distributions near perihelion have trends with solar wind flow speed, $beta_e$, and/or collisional age. Near the Sun, some trends not clearly seen at 1 AU become apparent, including anti-correlations between wind speed and both electron temperature and heat flux. These trends help us understand the mechanisms that shape the solar wind electron distributions at an early stage of their evolution.
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$20% on monthly timescales within individual solar maxima. We speculate that these variations could be driven by coronal mass ejections causing reconfigurations of the coronal magnetic field on sub-monthly timescales.
Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted owing to photospheric motions. It is suggested that the twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through observing satellite. We show that the total pressure of several observed twisted tubes resembles the theoretically expected profile. The twist of isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. We have also found some evidence that the flux tube walls can be associated with local heating of the plasma and elevated proton and electron temperatures. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. Stability analysis of twisted tubes shows that the critical twist angle of the tube with a homogeneous twist is 70$^0$, but the angle can further decrease owing to the motion of the tube with regards to the solar wind stream. The tubes with a stronger twist are unstable to the kink instability, therefore they probably can not reach 1 AU.
It is widely reported that the power spectra of magnetic field and velocity fluctuations in the solar wind have power law scalings with inertial-range spectral indices of -5/3 and -3/2 respectively. Studies of solar wind turbulence have repeatedly de monstrated the impact of discontinuities and coherent structures on the measured spectral index. Whether or not such discontinuities are self-generated by the turbulence or simply observations of advected structures from the inner heliosphere has been a matter of considerable debate. This work presents a statistical study of magnetic field and velocity spectral indices over 10 years of solar-wind observations; we find that anomalously steep magnetic spectra occur in magnetically dominated intervals with negative residual energy. However, this increase in negative residual energy has no noticeable impact on the spectral index of the velocity fluctuations, suggesting that these intervals with negative residual energy correspond to intermittent magnetic structures. We show statistically that the difference between magnetic and velocity spectral indices is a monotonic function of residual energy, consistent with previous work which suggests that intermittency in fluctuations causes spectral steepening. Additionally, a statistical analysis of cross helicity demonstrates that when the turbulence is balanced (low cross-helicity), the magnetic and velocity spectral indices are not equal, which suggests that our observations of negative residual energy and intermittent structures are related to non-linear turbulent interactions rather than the presence of advected pre-existing flux-tube structures.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا