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تسجيل مستخدم جديدA Pilot Study of Interplanetary Scintillation with FAST
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Observations of Interplanetary Scintillation (IPS) are an efficient remote-sensing method to study the solar wind and inner heliosphere. From 2016 to 2018, some distinctive observations of IPS sources like 3C 286 and 3C 279 were accomplished with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the largest single-dish telescope in the world. Due to the 270-1620 MHz wide frequency coverage of the Ultra-Wideband (UWB) receiver, one can use both single-frequency and dual-frequency analyses to determine the projected velocity of the solar wind. Moreover, based on the extraordinary sensitivity owing to the large collecting surface area of FAST, we can observe weak IPS signals. With the advantages of both the wider frequency coverage and high sensitivity, also with our radio frequency interference (RFI) mitigation strategy and an optimized model-fitting method developed, in this paper, we analyze the fitting confidence intervals of the solar wind velocity, and present some preliminary results achieved using FAST, which points to the current FAST system being highly capable of carrying out observations of IPS
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Observation of interplanetary scintillation (IPS) beyond Earth-orbit can be challenging due to the necessity to use low radio frequencies at which scintillation due to the ionosphere could confuse the interplanetary contribution. A recent paper by Ka
plan {it et al} (2015) presenting observations using the Murchison Widefield Array (MWA) reports evidence of night-side IPS on two radio sources within their field of view. However, the low time cadence of 2,s used might be expected to average out the IPS signal, resulting in the reasonable assumption that the scintillation is more likely to be ionospheric in origin. To verify or otherwise this assumption, this letter uses observations of IPS taken at a high time cadence using the Low Frequency Array (LOFAR). Averaging these to the same as the MWA observations, we demonstrate that the MWA result is consistent with IPS, although some contribution from the ionosphere cannot be ruled out. These LOFAR observations represent the first of night-side IPS using LOFAR, with solar wind speeds consistent with a slow solar wind stream in one observation and a CME expecting to be observed in another.
We study periods of elevated energetic particle intensities observed by STEREO-A when the partial pressure exerted by energetic ($geq$83 keV) protons ($P_{EP}$) is larger than the pressure exerted by the interplanetary magnetic field ($P_{B}$). In th
e majority of cases, these periods are associated with the passage of interplanetary shocks. Periods when $P_{EP}$ exceeds $P_{B}$ by more than one order of magnitude are observed in the upstream region of fast interplanetary shocks where depressed magnetic field regions coincide with increases of the energetic particle intensities. When solar wind parameters are available, $P_{EP}$ also exceeds the pressure exerted by the solar wind thermal population ($P_{TH}$). Prolonged periods ($>$12 h) with both $P_{EP}$$>$$P_{B}$ and $P_{EP}$$>$$P_{TH}$ may also occur when energetic particles accelerated by an approaching shock encounter a region well-upstream of the shock characterized by low magnetic field magnitude and tenuous solar wind density. Quasi-exponential increases of the sum $P_{SUM}$=$P_{B}$+$P_{TH}$+$P_{EP}$ are observed in the immediate upstream region of the shocks regardless of individual changes in $P_{EP}$, $P_{B}$ and $P_{TH}$, indicating a coupling between $P_{EP}$ and the pressure of the background medium characterized by $P_{B}$ and $P_{TH}$. The quasi-exponential increase of $P_{SUM}$ implies a convected exponential radial gradient $partial{P_{SUM}}/partial{r}$$>$0 that results in an outward force applied to the plasma upstream of the shock. This force can be maintained by the mobile energetic particles streaming upstream of the shocks that, in the most intense events, drive electric currents able to generate diamagnetic cavities and depressed solar wind density regions.
A statistical analysis of 15,210 electron velocity distribution function (VDF) fits, observed within $pm$2 hours of 52 interplanetary (IP) shocks by the $Wind$ spacecraft near 1 AU, is presented. This is the second in a three-part series on electron
VDFs near IP shocks. The electron velocity moment statistics for the dense, low energy core, tenuous, hot halo, and field-aligned beam/strahl are a statistically significant list of values illustrated with both histograms and tabular lists for reference and baselines in future work. The beam/strahl fit results in the upstream are currently the closest thing to a proper parameterization of the beam/strahl electron velocity moments in the ambient solar wind. This work will also serve as a 1 AU baseline and reference for missions like $Parker Solar Probe$ and $Solar Orbiter$. The median density, temperature, beta, and temperature anisotropy values for the core(halo)[beam/strahl] components, with subscripts $ec$($eh$)[$eb$], of all fit results respectively are $n{scriptstyle_{ec(h)[b]}}$ $sim$ 11.3(0.36)[0.17] $cm^{-3}$, $T{scriptstyle_{ec(h)[b], tot}}$ $sim$ 14.6(48.4)[40.2] $eV$, $beta{scriptstyle_{ec(h)[b], tot}}$ $sim$ 0.93(0.11)[0.05], and $mathcal{A}{scriptstyle_{ec(h)[b]}}$ $sim$ 0.98(1.03)[0.93]. The nuanced details of the fitting method and data product description were published in Paper I and the detailed analysis of the results will be shown in Paper III.
Analysis of model fit results of 15,210 electron velocity distribution functions (VDFs), observed within $pm$2 hours of 52 interplanetary (IP) shocks by the Wind spacecraft near 1 AU, is presented as the third and final part on electron VDFs near IP
shocks. The core electrons and protons dominate in the magnitude and change in the partial-to-total thermal pressure ratio, with the core electrons often gaining as much or more than the protons. Only a moderate positive correlation is observed between the electron temperature and the kinetic energy change across the shock, while weaker, if any, correlations were found with any other macroscopic shock parameter. No VDF parameter correlated with the shock normal angle. The electron VDF evolves from a narrowly peaked core with flaring suprathermal tails in the upstream to either a slightly hotter core with steeper tails or much hotter flattop core with even steeper tails downstream of the weaker and strongest shocks, respectively. Both quasi-static and fluctuating fields are examined as possible mechanisms modifying the VDF but neither is sufficient alone. For instance, flattop VDFs can be generated by nonlinear ion acoustic wave stochastic acceleration (i.e., inelastic collisions) while other work suggested they result from the combination of quasi-static and fluctuating fields. This three-part study shows that not only are these systems not thermodynamic in nature, even kinetic models may require modification to include things like inelastic collision operators to properly model electron VDF evolution across shocks or in the solar wind.
Analysis of 15314 electron velocity distribution functions (VDFs) within $pm$2 hours of 52 interplanetary (IP) shocks observed by the emph{Wind} spacecraft near 1 AU are introduced. The electron VDFs are fit to the sum of three model functions for th
e cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The best results were found by modeling the core as either a bi-kappa or a symmetric (or asymmetric) bi-self-similar velocity distribution function, while both the halo and beam/strahl components were best fit to bi-kappa velocity distribution function. This is the first statistical study to show that the core electron distribution is better fit to a self-similar velocity distribution function than a bi-Maxwellian under all conditions. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The range of values defined by the lower and upper quartiles for the kappa exponents are $kappa{scriptstyle_{ec}}$ $sim$ 5.40--10.2 for the core, $kappa{scriptstyle_{eh}}$ $sim$ 3.58--5.34 for the halo, and $kappa{scriptstyle_{eb}}$ $sim$ 3.40--5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents are $s{scriptstyle_{ec}}$ $sim$ 2.00--2.04, and asymmetric bi-self-similar core exponents are $p{scriptstyle_{ec}}$ $sim$ 2.20--4.00 for the parallel exponent, and $q{scriptstyle_{ec}}$ $sim$ 2.00--2.46 for the perpendicular exponent. The nuanced details of the fit procedure and description of resulting data product are also presented. The statistics and detailed analysis of the results are presented in Paper II and Paper III of this three-part study.
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