Evolution of solar wind discontinuities in the inner heliosphere
PSP and Earth conjunctions and alignments
Motivation
Studying the radial evolution of solar wind discontinuities from synergistic observations of PSP and Earth-orbiting missions (ARTEMIS, Wind) during aligned intervals.
How does the discontinuities change with the radial distance from the Sun?
How is solar wind discontinuities formed? What is the physical mechanisms?
Generated at or near the sun?
Locally generated in the interplanetary space by turbulence?
Methods
In-situ synergistic observations (Alignment)
Similar type of solar wind or Same solar solar wind
How to define alignment?
Trajectory & Orbits (theory): ballistic approximation of Parker spiral
Plasma properties (observation):
Invariants: plasma composition, ionization states, mass flux \(n u r^2\)
Correlation: magnetic field / plasma velocity
Simulation: Trace plasma parcels evolution
High-speed solar wind radial evolution
Perrone et al. (2019)
The radial dependence of the proton number density, \(n_p\) and magnetic field magnitude, \(B\), is given by
\[ n_p = (2.4 ± 0.1)(R/R_0)^{−(1.96±0.07)} cm^{−3} \\ B = (5.7 ± 0.2)(R/R_0)^{−(1.59±0.06)} nT \]
The faster decrease of the magnetic than kinetic pressure is reflected in the radial proton plasma beta variation
\[ β_p = P_k/P_B = (0.55 ± 0.04)(R/R_0)^{(0.4±0.1)} \]
Examples of discontinuities
In general discontinuities observed by PSP are with
shorter duration (1~5 secs)
compared with 5-10 secs
large current density (100-1000 nA/m^2)
compared with 1-10 nA/m^2
Results
Comparison of Discontinuity Properties
K1: Normalized discontinuities properties does not vary much with radial distance
Comparison between Alfven speed and plasma speed change
For rotational discontinuities, the plasma velocity jump across them: the plasma flow velocity \(v_l\) (the solar wind velocity projected onto \(l\)) changes due to changes in \(B_l\), \(Δv_l=±Δv_A\) with \(v_A= B_l / \sqrt{4 n m}\).
K2: \(Δv_l\),l demonstrates a strong correlation with \(Δv_{A}\), albeit being consistently smaller.
Estimated Anisotropy
The equation for \(Δv_A\) includes a factor depending on the anisotropy of the plasma.
Pressure anisotropy \(Λ = \mu (P_∥ - P_⊥) / B^2\).
Comparison between Estimated and Observed Anisotropy
Conclusion
- The properties of the discontinuities is related to the local plasma parameters
- Thickness => ion inertial length
- Current density => Alfven velocity (current density)
- Normalized thickness and current density of discontinuities remain constant with radial distance
- Anisotropy of the plasma is expected to be larger near the sun to explain the observed speed change ratio \(Δv_i/Δv_A\).
References
Velli et al. (2020)
Artemyev et al. (2018) examined the evolution of transient currents in the solar wind using a data set compiled from observations of the same solar wind flow at Earth’s and Mars’s orbits.
> we show that it consists of several processes: discontinuity thinning (decrease in thickness normalized by the ion inertial length), intensification of currents normalized to the proton thermal current (i.e., the product of proton charge, density, and thermal velocity), and increase in the compressional component of magnetic field variations across discontinuities.
Anistropy
Shen et al. (2024)
pressure anisotropy is defined as \(Λ = \mu (P_∥ - P_⊥) / B^2\).