Magnetic Topology of Actively Evolving and Passively Convecting Structures in the Turbulent Solar Wind

Multipoint in situ observations of the solar wind are used to identify the magnetic topology and current density of turbulent structures. We find that at least 35% of all structures are both actively evolving and carrying the strongest currents, actively dissipating, and heating the plasma. These structures are comprised of $\sim 1/5$ 3D plasmoids, $\sim 3/5$ flux ropes, and $\sim 1/5$ 3D $X$ points consistent with magnetic reconnection. Actively evolving and passively advecting structures are both close to log-normally distributed. This provides direct evidence for the significant role of strong turbulence, evolving via magnetic shearing and reconnection, in mediating dissipation and solar wind heating.

Figure 1

Solar wind magnetic field and bulk flow parameters observed by Cluster on 21-Feb-2004: (a) ion number density $n_i$ (blue) and solar wind ion bulk speed ${\mathrm{v}}_i$ (red); (b) magnetic field vector components in GSE coordinates ($B_x$-red, $B_y$-green, $B_z$-blue) and magnitude (black). (c),(d) Normalized invariants $nQ$ and $nR$ of the magnetic field gradient tensor.

Figure 2

(a) Probability density of counts $P(\zeta )$, combined for intervals 1–5. Blue vertical lines indicate the limits of passive (active) magnetic field configurations. (b),(c) Scatter plots of ($nR$, $nQ$) pairs combined for intervals 1–5: (b) a FFMFC, $|\zeta |>0.94$ and (c) magnetic field departing from FFMFC, $|\zeta |<0.94$. The points are colored according to the current density magnitude: red—$|j|>{\sigma }_j$, blue (plotted over the red)—$|j|<{\sigma }_j$, where ${\sigma }_j$ is a standard deviation of $|j|$. Solid green line shows the curve of $D=0$. Black cross in panel (b) indicates the error estimated from the magnetometer uncertainty of 0.1 nT.

Figure 3

Conditional joint probability density for combined intervals 1–5: (a) $p(nR,nQ||\zeta |>0.94)$; (b) $p(nR,nQ||\zeta |<0.94)$. Solid magenta line indicates $D=0$ curve.

Figure 4

Combined probability density $P(j^{*}||\zeta |>0.94)$ (red squares) and $P(j^{*}||\zeta |<0.94)$ (blue squares), where the current is standardized $j*=(|j|-⟨|j|⟩)/{\sigma }_{|}j|$. Black lines plot a normal distribution for the logarithm of $P(j^{*})$ with zero mean and unit standard deviation.