Sudden and Semi-Permanent Flow Speed Decreases as a

Sudden and Semi-Permanent Flow Speed Decreases as a Source of Roughly Radial Magnetic Fields in the Heliosphere* J. T. Gosling LASP, University of Colorado *Based on Gosling and Skoug, JGR, 107 (A 10), 1327, 2002 and simulations produced by P. Riley.

Intervals of Roughly Radial Heliospheric Magnetic Fields First studied by Neugebauer and Goldstein [1997] and Neugebauer et al. [1997]. Tend to occur when solar wind speed is declining. Field tends to be “quiet”. Often occur in association with ICMEs. Radial extent (duration) tends to increase with distance from Sun. Observed at all heliographic latitudes near solar maximum. All portions of an “event” appear to have left the Sun at ~ same time. Events are transient; they do not recur on subsequent solar rotations.

More Intervals of Roughly Radial Magnetic Field Intervals of roughly radial magnetic field also frequently occur without any obvious ICME associations.

Archimedean Spirals in Solar Equatorial Plane Resulting from a Steep, Semi-permanent Longitudinal Gradient in Flow Speed in Clockwise Sense as Observed Above North Solar Pole Sketch ignores effects of pressure gradients that: 1. Develop as the structure propagates into heliosphere. 2. Modify the flow velocity. 3. Cause the field lines to be slightly different spirals. Footpoint motions and waves and turbulence also ignored. Next slide illustrates field configuration resulting from a sudden decrease in flow speed occurring at a solar site such as illustrated here, which might correspond to the trailing (eastern) boundary of a coronal hole.

Snapshots at Two Later Times of Idealized Magnetic Field Structure Produced by an Earlier Sudden, Localized Decrease in Solar Wind Speed at the Sun at “ 12 o’clock” Simple kinematic model Sudden decrease in flow speed produces a radial field segment connecting pre and post-decrease spirals. Radial segment propagates radially away from Sun and grows in size with increasing heliocentric distance. Sharp speed decrease evolves into a gradual speed decline. Dynamic effects associated with pressure gradients produced by sudden drop in flow speed alters the field structure that actually results from the decrease (reason for doing simulations).

Summary of Kinematic Model Predicts that roughly radial field intervals are result of sudden speed decreases in solar wind outflow. Simple kinematic model reproduces basic features of observed roughly radial field events, including: 1. Their roughly radial fields. 2. Their occurrence during intervals of gradually declining speed. 3. Their occurrence on both closed and open field lines. 4. Their increased radial extents at large heliocentric distances. 5. Their strong tendency not to recur on subsequent solar rotations. 6. The fact that all portions of a given event appear to depart from the Sun at approximately the same time.

Final Comments on Kinematic Model We can invert discussion and argue: Observations of intervals of roughly radial HMF are strong evidence that flow speed does often decrease abruptly near the Sun. These sudden speed decreases must be a relatively common occurrence at boundaries between coronal regions emitting fast and slow flows both within the normal wind and within ICMEs. The flow speed decreases may arise from interchange reconnection. Sudden flow speed decreases produce decreases in the azimuthal field but do not initially affect any meridional field component. Thus sudden speed decreases may be more common than would be inferred from intervals of large Br/B alone.

A 3 -Dimensional MHD Simulation of a Large Drop in Speed (Riley) Latitude Color-Coded for Br/B 560 km/s 300 km/s Speed Perturbation at Sun (560 to 300 km/s in a 10˚x 10˚ area) Longitude

Snapshots of Radial Velocity and Total (scaled) pressure

Snapshots of Azimuthal Velocity and Total (scaled) Pressure Azimuthal flows induced by new pressure gradients resulting from the speed decrease are primarily responsible for tilting the field in the disturbance toward the spiral direction.

Simulation Results Rarefaction produced by sudden speed drop produces dynamic effects that alter the field structure in the heliosphere predicted by the kinematic model: Sharp corners in B are smoothed out. Field perturbation tilts more toward the spiral direction. Magnitude of Br/B decreases with distance, but Br/B still relatively large at 4. 7 AU and the region of large Br/B grows with increasing distance, as in the kinematic model. Simulations support idea that the transient, roughly radial field events are result of sudden large decreases in flow speed.