Evolution of the solar wind speed with heliocentric distance and solar cycle. Surprises from Ulysses and unexpectedness from observations of the solar corona

TL;DR

This paper analyzes Ulysses data to reveal how solar wind speed varies with distance, latitude, and solar cycle, showing complex behaviors and unexpected variations explained by coronal hole flows and a new MHD model.

Contribution

It presents a detailed analysis of solar wind speed evolution with heliocentric distance and solar cycle, highlighting deviations from classical models and introducing a new MHD model to explain observed variations.

Findings

Solar wind speed depends on latitude and solar activity cycle.

Deviations from Parker's profile are significant and vary with conditions.

A new MHD model explains the impact of coronal hole flows on V(r).

Abstract

An extensive analysis of Ulysses observations of the solar wind speed V from 1990 to 2008 is undertaken. It is shown that the evolution of V with heliocentric distance r depends substantially on both the heliolatitude and the solar activity cycle. Deviations from the predicted Parker's profile of V(r) are so considerable that cannot be explained by a scarcity of measurements or other technical effects. In particular, the expected smooth growth of the solar wind speed with r is typical only for the solar activity maximum and for low heliolatitudes (lower than +/-40deg), while at high latitudes, there are two V(r) branches: growing and falling. In the solar activity maximum, V increases toward the solar pole in the North hemisphere only; however, in the South hemisphere, it decreases with heliolatitude. In the minimum of solar activity, the profile of V(r) at low heliolatitudes has a local minimum between 2 and 5 AU. This result is confirmed by the corresponding data from other spacecraft (Voyager 1 and Pioneer 10). Unexpected spatial variations in V at low heliolatitudes can be explained by the impact of coronal hole flows on the V(r) profile since the flows incline to the ecliptic plane. To reproduce the impact of spatial variations of V in the polar corona on the behavior of V at low heliolatitudes, a stationary one-fluid ideal MHD-model is developed with account of recent results on imagery of the solar wind speed in the corona up to 5.5 solar radii obtained on the basis of combined observations from SOHO/UVCS, LASCO, and Mauna Loa.