Magnetic twist: a source and property of space weather

TL;DR

This paper investigates magnetic helicity in the heliosphere, linking solar dynamo processes to space weather phenomena through simulations and observational data, revealing scale-dependent helicity sign reversals.

Contribution

It provides new evidence of magnetic helicity density in the heliosphere and explains its scale-dependent sign reversal using numerical models and solar wind data.

Findings

Solar dynamo produces negative small-scale and positive large-scale helicity.

Heliospheric magnetic helicity exhibits opposite sign patterns compared to the solar dynamo.

Magnetic helicity sign reversal is explained by the magnetic helicity flux and its gradient.

Abstract

We present evidence for finite magnetic helicity density in the heliosphere and numerical models thereof, and relate it to the magnetic field properties of the dynamo in the solar convection zone. We use simulations and solar wind data to compute magnetic helicity either directly from the simulations, or indirectly using time series of the skew-symmetric components of the magnetic correlation tensor. We find that the solar dynamo produces negative magnetic helicity at small scales and positive at large scales. However, in the heliosphere these properties are reversed and the magnetic helicity is now positive at small scales and negative at large scales. We explain this by the fact that a negative diffusive magnetic helicity flux corresponds to a positive gradient of magnetic helicity, which leads to a change of sign from negative to positive values at some radius in the northern hemisphere.