Solar Wind: Fast Streams Inside Slow Ones express
The Sun's corona looks like a shaggy dandelion: bright petal-like loops hold hot gas near the surface, while long, tenuous jets escape into space between them. These jets, the heliospheric current sheet, stretch across the entire solar system. But how exactly plasma transitions from closed loops into open flow has remained a mystery.
A probe flying at 11.7 solar radii from the center pierced one of these jets, a streamer, for the first time. Instruments recorded an electric field of 400 millivolts per meter. That's like a discharge appearing between the ends of a battery in your pocket, but on a cosmic scale. Such a field breaks the simple rule that "plasma and magnetic field move together," so it's explained by an extra term in Ohm's law — the Hall effect.
Fifteen such crossings showed that plasma inside streamers races at 326 km/s, while outside it's only 266 km/s. Previously, streamers were thought to be the source of the slow solar wind. Now it's clear: there are fast streams inside them. This changes our understanding of how the Sun feeds the entire heliosphere.
Original abstract
as written by the authors arXiv:2603.15930
A major goal of solar physics is understanding the transition of the medium from the closed-loop magnetic configuration of the corona to the open structure of the heliospheric current sheet. The evolution of solar wind streamers, an essential component of this transition, has been observed in-situ for the first time by measuring a pass through a streamer stalk at 11.7 solar radii. A plasma rest frame DC electric field, reaching an amplitude of 400 mV/m, was observed as the magnetic field changed from +1100 nT to -1100 nT. This electric field violates the frozen in condition so it must be understood via the non-MHD Hall term of the Generalized Ohm's Law. Two plasma regimes containing the large electric field were observed. In the first regime, the magnetic field was large, the ion beta was small (~0.1), the ratio of the ion skin depth to ion gyroradius was large (>10), and the current of ~0.3 mA/m2 was carried by electrons. In the second regime, the magnetic field was small, the ion beta was large (~10), the ion skin depth was equal to the ion gyroradius, and the ~10 mA/m2 current was carried by the ions. Also, measurements during 15 such crossings showed that the plasma flow speed inside the current sheets exceeded that outside six times and the 326 km/sec average speed inside the current sheets exceeded the 266 km/sec outside. Such findings challenge the traditional consensus that streamers are the source of the "slow" solar wind.
The electric field in a streamer is comparable to that inside a running microwave oven, but stretched over thousands of kilometers.