Category: Seminar

Speaker: Marco Velli

Affiliation: EPSS, UCLA

Date: Friday, April 17, 2026

Time: 3:30PM

Zoom/Meeting Link: https://ucla.zoom.us/j/97828298609?pwd=kbJEOQ2YHlxZifxQW1uT7SQkiBmchT.1

Abstract

It has been established since the Helios epoch and confirmed by Ulysses and SOHO that the sources of fast solar wind streams at solar minimum are the polar coronal holes, while slower solar wind streams have contributions from different sources. The larger than expected filling factor of slow solar wind has been attributed to flows coming from coronal hole boundaries, i.e., regions with large expansion factors, or from regions where the mapping of the magnetic field from the photosphere into the heliosphere is complex, as identified for example by the squashing factor, and known as the S-Web.

The observations by Parker Solar Probe that much of the solar wind, independently of speed, is dominated by Alfvénic fluctuations, and the frequent observation of slow Alfvénic solar wind, previously observed relatively rarely in Helios and Wind data, provide evidence for a picture of solar wind origins that incorporates both the expansion factor and S-web paradigms: both coronal holes with large expansion and S-Web regions act as slow solar wind sources, with the difference that highly expanding coronal holes provide Alfv.nic slow streams, while the S-web wind is unlikely to exhibit strong Alfvénic correlations.

As far as the fast solar wind is concerned, we focus on a new aspect associated with the interaction of spherically polarized Alfvén waves and the background wind. It arises from the continuous presence of spherically polarized Alfvénic fluctuations, in the form of switchbacks and patches of switchbacks, that lead the solar wind to be formed of multitudes of one-sided jets. We call the average effect of such jets the Gosling boost, as Jack Gosling was the first to recognize such one-sided jets over the baseline unperturbed solar wind expansion. Here we show how the Gosling boost provides direct empirical evidence for the acceleration of the wind by Alfvénic fluctuations and discuss the more general question

of the origin and acceleration of Alfvénic solar wind streams.

Speaker: Tien Vo

Affiliation: LASP, University of Colorado, Boulder

Date: Friday, April 10, 2026

Time: 3:30PM

Zoom/Meeting Link: https://ucla.zoom.us/j/97828298609?pwd=kbJEOQ2YHlxZifxQW1uT7SQkiBmchT.1

Abstract

Turbulence is an important mechanism for energy conversion in weakly collisional plasmas. In fluid turbulence, energy injected into the system at large spatial scales is transferred to smaller scales until it is dissipated as heat through collisions. In the interim between energy injection and dissipation, kinetic effects can also mediate the dissipation of energy below characteristic fluid (ion) scales. There is a longstanding question concerning plasmas with low collisional rates: How does this energy conversion process end without collisions? In addition, this classical understanding of turbulence is, in nature, based on a velocity-averaged theory of Vlasov-Boltzmann systems. The energy in consideration is that of the plasma bulk motion; the transfer process is one through space. Since a Vlasov-Boltzmann description concerns both space and velocity, we can also ask the question: Is there a conjugate spatial-averaged theory of turbulence, one that describes a turbulent energy transfer instead through velocity?

In this talk, we explore these two questions regarding turbulence in space and in velocity with MMS observations in two regions in the Earth’s magnetosphere. In the magnetotail, explosive large-scale magnetic reconnection generates strong turbulence with low density and background field. The electromagnetic field spectra are well resolved below electron scales, suitable for turbulence studies of kinetic effects. We show evidence of a sub-electron kinetic range where the energy transfer process appears to complete. In the magnetosphere, the distribution function is well resolved with MMS instruments, suitable for turbulence studies of fine structures in velocity space near fluid scales. We show the first ever statistical observation of velocity-space cascade using data from the MMS unbiased magnetosheath campaign