Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Applicability of Taylor's Hypothesis during Parker Solar Probe perihelia   We investigate the validity of Taylor's Hypothesis (TH) in the analysis of
Alfv\'enic fluctuations of velocity and magnetic fields in solar wind streams
measured by Parker Solar Probe (PSP)~during the first four encounters. We use
PSP velocity and magnetic field measurements from 24 h intervals selected from
each of the first four encounters. The applicability of TH is investigated by
measuring the parameter $\epsilon=\delta u_0/\sqrt{2}V_\perp$, which quantifies
the ratio between the typical speed of large-scale fluctuations, $\delta u_0$,
and the local perpendicular PSP speed in the solar wind frame, $V_\perp$. TH is
expected to be applicable for $\epsilon\lesssim0.5$ when PSP is moving nearly
perpendicular to the local magnetic field in the plasma frame, irrespective of
the Alfv\'en Mach number $M_{\rm A}=V_{\rm SW}/V_{\rm A}$, where $V_{\rm SW}$
and $V_{\rm A}$ are the local solar wind and Alfv\'en speed, respectively. For
the four selected solar wind intervals we find that between 10% to 60% of the
time the parameter $\epsilon$ is below 0.2 when the sampling angle (between the
spacecraft velocity in the plasma frame and the local magnetic field) is
greater than $30^\circ$. For angles above $30^\circ$, the sampling direction is
sufficiently oblique to allow one to reconstruct the reduced energy spectrum
$E(k_\perp)$ of magnetic fluctuations from its measured frequency spectra. The
spectral indices determined from power-law fits of the measured frequency
spectrum accurately represent the spectral indices associated with the
underlying spatial spectrum of turbulent fluctuations in the plasma frame.
Aside from a frequency broadening due to large-scale sweeping that requires
careful consideration, the spatial spectrum can be recovered to obtain the
distribution of fluctuation's energy among scales in the plasma frame.

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The categories of this paper are: math-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: q-fin
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The categories of this paper are: patt-sol
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The categories of this paper are: supr-con
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The categories of this paper are: chao-dyn
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The categories of this paper are: adap-org
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The categories of this paper are: nlin
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The categories of this paper are: nucl-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: funct-an
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-ex
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The categories of this paper are: cs
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The categories of this paper are: gr-qc