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Register a new userSolar modulation of cosmic ray positrons in a very quiet heliosphere
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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.
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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.
Low energy cosmic rays are modulated by the solar activity when they propagation in the heliosphere, leading to ambiguities in understanding their acceleration at sources and propagation in the Milky Way. By means of the precise measurements of the $e^-$, $e^+$, $e^-+e^+$, and $e^+/(e^-+e^+)$ spectra by AMS-02 near the Earth, as well as the very low energy measurements of the $e^-+e^+$ fluxes by Voyager-1 far away from the Sun, we derive the local interstellar spectra (LIS) of $e^-$ and $e^+$ components individually. Our method is based on a non-parametric description of the LIS of $e^-$ and $e^+$ and a force-field solar modulation model. We then obtain the evolution of the solar modulation parameters based on the derived LIS and the monthly fluxes of cosmic ray $e^-$ and $e^+$ measured by AMS-02. {bf To better fit the monthly data, additional renormalization factors for $e^-$ and $e^+$ have been multiplied to the modulated fluxes.} We find that the inferred solar modulation parameters of positrons are in good agreement with that of cosmic ray nuclei, and the time evolutions of the solar modulation parameters of electrons and positrons differ after the reversal of the heliosphere magnetic field polarity, which shows clearly the charge-sign dependent modulation effect.
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.
We explore the tail of various waiting time datasets of processes that follow a nonstationary Poisson distribution with a sinusoidal driver. Analytically, we find that the distribution of large waiting times of such processes can be described using a power law slope of -2.5. We show that this result applies more broadly to any nonstationary Poisson process driven periodically. Examples of such processes include solar flares, coronal mass ejections, geomagnetic storms, and substorms. We also discuss how the power law specifically relates to the behavior of driver near its minima.
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