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Register a new userModulation of Galactic Cosmic Rays in the Inner Heliosphere over Solar Cycles
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The 11-year and 22-year modulation of galactic cosmic rays (GCRs) in the inner heliosphere are studied using a numerical model developed by Qin and Shen in 2017. Based on the numerical solutions of Parkers transport equations, the model incorporates a modified Parker heliospheric magnetic field, a locally static time delayed heliosphere, and a time-dependent diffusion coefficients model in which an analytical expression of the variation of magnetic turbulence magnitude throughout the inner heliosphere is applied. Furthermore, during solar maximum, the solar magnetic polarity is determined randomly with the possibility of $A>0$ decided by the percentage of the north solar polar magnetic field being outward and the south solar polar magnetic field being inward. The computed results are compared with several GCR observations, e.g., IMP 8, SOHO/EPHIN, Ulysses, Voyager 1 & 2, at various energies and show good agreement. It is shown that our model has successfully reproduced the 11-year and 22-year modulation cycles.
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Since the beginning of the space exploration era, solar activity was observed at its lowest level during 2006 to 2009. During this period, the PAMELA space experiment observed spectra for galactic cosmic rays, specifically for protons, electrons and positrons over a wide energy range, during what is called an A < 0 solar magnetic polarity cycle. Drift theory predicts a difference in the behaviour for these oppositely charge particles during A < 0 cycles. An opportunity was thus created to study the predicted charge-sign-dependent modulation, also now for very quiet heliospheric conditions. A comprehensive three-dimensional, drift modulation model has been used to study the solar modulation for cosmic rays in detail with extensive comparison to the observed PAMELA spectra for the mentioned period. First, this was done for protons and secondly for electrons, as already published, to test and to authenticate the modelling approach and then to come to a better understanding and appreciation of the underlying physics, such as diffusion and drift theory. The results were also used to make predictions of how cosmic rays are differently modulated down to low energies (1 MeV) for the two magnetic polarity cycles of the Sun, and what role drifts play in this process. All computed solutions are based on new very local interstellar spectra, now also done for positrons. This report is focussed on detailed aspects of the solar modulation of positrons during the extraordinary quiet solar modulation period from 2006 to 2009. For the first time, a meaningful modulation factor in the heliosphere is computed for positrons, from 50 GeV down to 1 MeV, as well as the electron to positron ratios as a function of time and rigidity for the mentioned period.
The slow solar wind is typically characterized as having low Alfvenicity. However, Parker Solar Probe (PSP) observed predominately Alfvenic slow solar wind during several of its initial encounters. From its first encounter observations, about 55.3% of the slow solar wind inside 0.25 au is highly Alfvenic ($|sigma_C| > 0.7$) at current solar minimum, which is much higher than the fraction of quiet-Sun-associated highly Alfvenic slow wind observed at solar maximum at 1 au. Intervals of slow solar wind with different Alfvenicities seem to show similar plasma characteristics and temperature anisotropy distributions. Some low Alfvenicity slow wind intervals even show high temperature anisotropies, because the slow wind may experience perpendicular heating as fast wind does when close to the Sun. This signature is confirmed by Wind spacecraft measurements as we track PSP observations to 1 au. Further, with nearly 15 years of Wind measurements, we find that the distributions of plasma characteristics, temperature anisotropy and helium abundance ratio ($N_alpha/N_p$) are similar in slow winds with different Alfvenicities, but the distributions are different from those in the fast solar wind. Highly Alfvenic slow solar wind contains both helium-rich ($N_alpha/N_psim0.045$) and helium-poor ($N_alpha/N_psim0.015$) populations, implying it may originate from multiple source regions. These results suggest that highly Alfvenic slow solar wind shares similar temperature anisotropy and helium abundance properties with regular slow solar winds, and they thus should have multiple origins.
The scaling of the turbulent spectra provides a key measurement that allows to discriminate between different theoretical predictions of turbulence. In the solar wind, this has driven a large number of studies dedicated to this issue using in-situ data from various orbiting spacecraft. While a semblance of consensus exists regarding the scaling in the MHD and dispersive ranges, the precise scaling in the transition range and the actual physical mechanisms that control it remain open questions. Using the high-resolution data in the inner heliosphere from Parker Solar Probe (PSP) mission, we find that the sub-ion scales (i.e., at the frequency f ~ [2, 9] Hz) follow a power-law spectrum f^a with a spectral index a varying between -3 and -5.7. Our results also show that there is a trend toward and anti-correlation between the spectral slopes and the power amplitudes at the MHD scales, in agreement with previous studies: the higher the power amplitude the steeper the spectrum at sub-ion scales. A similar trend toward an anti-correlation between steep spectra and increasing normalized cross helicity is found, in agreement with previous theoretical predictions about the imbalanced solar wind. We discuss the ubiquitous nature of the ion transition range in solar wind turbulence in the inner heliosphere.
The anisotropy of solar wind turbulence is a critical issue in understanding the physics of energy transfer between scales and energy conversion between fields and particles in the heliosphere. Using the measurement of emph{Parker Solar Probe} (emph{PSP}), we present an observation of the anisotropy at kinetic scales in the slow, Alfvenic, solar wind in the inner heliosphere. textbf{The magnetic compressibility behaves as expected for kinetic Alfvenic turbulence below the ion scale.} A steepened transition range is found between the inertial and kinetic ranges in all directions with respect to the local background magnetic field direction. The anisotropy of $k_perp gg k_parallel$ is found evident in both transition and kinetic ranges, with the power anisotropy $P_perp/P_parallel > 10$ in the kinetic range leading over that in the transition range and being stronger than that at 1 au. The spectral index varies from $alpha_{tparallel}=-5.7pm 1.0$ to $alpha_{tperp}=-3.7pm 0.3$ in the transition range and $alpha_{kparallel}=-3.12pm 0.22$ to $alpha_{kperp}=-2.57pm 0.09$ in the kinetic range. The corresponding wavevector anisotropy has the scaling of $k_parallel sim k_perp^{0.71pm 0.17}$ in the transition range, and changes to $k_parallel sim k_perp^{0.38pm 0.09}$ in the kinetic range, consistent with the kinetic Alfvenic turbulence at sub-ion scales.
The first computation of the compressible energy transfer rate from $sim$ 0.2 AU up to $sim$ 1.7 AU is obtained using PSP, THEMIS and MAVEN observations. The compressible energy cascade rate $varepsilon_C$ is computed for hundred of events at different heliocentric distances, for time intervals when the spacecraft were in the pristine solar wind. The observational results show moderate increases of $varepsilon_C$ with respect to the incompressible cascade rate $varepsilon_I$. Depending on the level of compressibility in the plasma, which reach up to 25 $%$ in the PSP perihelion, the different terms in the compressible exact relation are shown to have different impact in the total cascade rate $varepsilon_C$. Finally, the observational results are connected with the local ion temperature and the solar wind heating problem.
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