HomeDate: 2019-01-02
The electron density power spectrum with scales of 50m to 15au in the local interstellar medium is obtained from in situ observations of Voyager 1. By combining the in situ data with the earlier ground remote observations, we now have the turbulent spectrum extending from 50m to 100 light-years, over 16 orders of magnitude.
The sun emits high-speed flows of protons and electrons, i.e., the solar wind, leading to the formation of heliosphere. As shown in Figure 1b, the heliopause separates the solar wind from the interstellar medium. The interstellar space refers to the huge gaps between stars, and the matter that fills in such space is the interstellar medium. The interstellar medium consists of mostly molecular, atomic and ionized hydrogen particles. Unlike neutral fluids in hydrodynamics (HD), the motion of charged particles is highly influenced by the magnetic field that resides in the interstellar space and the heliosphere. The study of the fluid dynamics of such magnetized and ionized matter is called magnetohydrodynamics (MHD). In 1941, Kolmogorov proposed the first hydrodynamic turbulence model and gave the well-known power law index, -11/3, of energy spectrum. MHD turbulence models have been proposed by several authors.
In 1976, Lee and Jokipii first suggested that the interstellar turbulence at the length scale λ of 10^8m to 10^18m (100 light-years) also has a Kolmogorov-like spectrum based on observations of radio wave scintillations and interstellar clouds. The scintillations of pulsar radio wave by interstellar turbulent electrons provide the turbulence power at the scale λ=10^8m, and the observations of interstellar clouds provide the turbulence power at λ=10^18m. The slope of the line connecting the powers in logarithmic scale at λ=10^8m and at λ=10^18m is about -11/3 (Figure 2a). Many later ground observations confirmed this speculation. Armstrong et al. (1995) constructed the composite spectrum extending from 10^6.4m to 10^18m, a.k.a. the Big Power Law in the sky, based on several observational results (Figure 2b). So far the observational techniques for interstellar turbulence were all based on ground observations.
In 1977, two satellites named Voyager 1 & 2 were launched. The early objective was to study the solar wind, Jupiter and Saturn as well as other planets inside the heliosphere. Owing to the surprisingly good condition in the power supply and the scientific equipment of Voyagers, the mission was extended to the exploration of the interstellar medium. Voyager 1 entered the local interstellar medium in 2012 and traveled more than 23 astronomical units beyond the heliopause to date (Figure 1b). Recently, NASA announced that Voyager 2 crossed the outer edge of the heliosphere and entered the interstellar medium on 2018 November 5.
Thanks to the Voyager science team, Academician Lou-Chuang Lee and Dr. Kun-Han Lee at Institute of Earth Sciences use the data from in situ measurements to determine the spectrum of the turbulent density fluctuations from 50m to 2.25x10^12m (15 au) in the local interstellar medium. For the length scale of 10^6m-2.25x10^12m, lying within the inertial range, our results show great consistency with earlier remote observations (Figure 2c). They also obtain the turbulence spectrum at finer scales of ion and electron kinetic range from 50m to 10^6m (purple and part of green dots in Figure 2c), and the data show an interesting result with an enhanced spectral power.
By combining our in situ measured spectrum and the earlier remote observed data, they obtain the composite spectrum, the Grand Power Law in the Milky Way, extending from 50m to 10^18m (100 light-years), over 16 orders of magnitude in length scale (Figure 2d). The results are published in Nature Astronomy by K. H. Lee and L. C. Lee on 2018 December 31 (DOI: 10.1038/s41550-018-0650-6).
Contact: Ms. Hung, +886-2-2783-9910#1135, tin2868@earth.sinica.edu.tw
