Measurements of the Orientation of the Heliospheric Magnetic Field Neil Murphy Jet Propulsion Laboratory

What direction do we expect the heliospheric magnetic field to point? If the magnetic field is radial when the field becomes frozen into the radially flowing solar wind, the field remains oriented along flow streamlines. In a frame of reference rotating with the sun, these streamlines form Archimedean spirals, as do the magnetic field lines - transforming into an inertial coordinate system leaves the magnetic field carrying this spiral orientation - the Parker spiral: Of course, the heliospheric magnetic field almost never points along the Parker spiral. However, in many cases, the long-term average magnetic field direction is closely aligned with the expected spiral, although there are notable exceptions

Why do we care? There are several motivations for studying the average orientation of the heliospheric magnetic field (HMF) over various time scales: –It controls the diffusion of cosmic rays throughout the heliosphere –It determines the magnetic connectivity between the earth (or other places we care about) and sources of energetic particles –It may tell us something about physical processes at and close to the sun –It can tell us something about how the solar wind evolves as it flows outward from the sun –It might tell us something about how the HMF interacts with the termination shock and heliosheath

Some past observations, and the questions they raise Ness & Wilcox, 1967, Schatten et al., 1968 –Persistent variations from Parker Spiral, in both quiet and disturbed periods (effects of gradients) Smith & Bieber, 1991, 1992 –≈1.5° underwinding of IMF from OMNI data at 1 AU Bieber, 1988, Smith & Bieber, 1992, 1993, Sabbah, 1995, 1996 –Up to 2.5° north-south asymmetry in the IMF from OMNI data at 1 AU, similar signature in PVO data Smith & Phillips, 1997 –Underwinding due to transients (CME’s, shocks), asymmetry still present Winterhalter et al., 1987, 1989, 1990 –Persistent departures from Parker model in field azimuth and magnitude - Pioneer data AU, normalized by 1 AU data Murphy et al.,1996 –Variance in azimuthal field angle strongly correlated with variance in elevation angle Forsyth et al., 1995 –Persistent departure from Parker spiral over sun’s southern pole (> 60° latitude) Jones et al., 1998 –Radial HMF intervals seen by Ulysses Murphy et al, 2002, Schwadron, 2002 –Field in CRR’s significantly more radial than Parker spiral Gosling & Skoug, 2002 –Radial fields in transient events

In this coordinate system, the Z axis is the same as N in the RTN system, but the X-Y (R-T) plane is rotated so that the X- axis is aligned with the expected inward Parker spiral (other authors have used the outward spiral direction). The spiral direction is calculated using two-hour average solar wind speeds.

From Smith and Phillips 1997 This paper demonstrated that the small differences between the long- term average over-winding reported previously was due to transient events in the solar wind, not some continuous process Transients did not account for the persistent small difference between the winding angle between inward and outward sectors

From Forsyth et al., 1995 A large scale over-winding of the Heliospheric field at high latitudes

From Winterhalter et al., 1989 Blah Showed a consistent deficit in |B| compared to data taken at 1 AU Also showed deficit in B , i.e a more radial field than expected

One year distributions from Ulysses for 1991 and 1992 For 1991, the field distribution is Parker-like and symmetrical For 1992, the field distribution shows significantly more strucure, although the average field is still aligned along the spiral. During thiss period, Ulysses began to encounter the periodic co- rotating streams typical of the solar wind during solar minimum

Sixty day distributions from Ulysses for 1991 and 1992 A two month period in 1991 the distribution is asymmetric, and beginning to diverge from the PS Towards the end of 1992, a two month field distribution deviates substantially from the expected PS and is highly structured. Variations in  are correlated with variations in . At this point Ulysses is more than 30° below the heliographic equator

A single co-rotating rarefaction region (CRR) Note: there is a significant difference between the mean and most probable field directions

Four examples of CRRs seen by Ulysses

Distributions for these four CRRs B - t=26dA - t=0 C - t=97dD - t=106d

Pioneer 10 compression regions from 1972 to 1975

Pioneer 10 rarefaction regions from 1972 to 1975

Conclusions In many circumstances the HMF does not, on average, follow the expected Parker spiral –Small deviations in long term averages seen in data at 1 AU have been attributed to the effects of CMEs and associated shocks –An unexplained large departure (≈25°) was seen by Ulysses over the sun’s southern pole –During solar minimum, variations in field azimuth angle are correlated with variations in elevation angle –CRRs show consistent large (10s of degrees) under-winding of the HMF which appears to be a steady state effect. –These non-Parker fields are seen consistently at larger distances from the sun, but there is no clear evolution (too little data?) - many periods show significant structure, with apparent over-winding and under-winding –The trailing edges of ICMEs also have significant periods of more radial fields

When the first spacecraft moved beyond the confines of the magnetosphere, it was discovered the interplanetary magnetic field was structured into sectors, within which the average magnetic field orientation was consistent with Parker’s prediction for steady state supersonic solar wind expanding with an embedded magnetic field. Since these earliest observations the degree to which the field is oriented as expected from Parker’s picture has been studied extensively. Both long-term small (≈1 degree) deviations and short-term large (10s of degree) deviations have been reported, as well as both steady state and transient departures from Parker’s model. These departures have been attributed to both processes occurring throughout interplanetary space and in the regions close to the sun where the field becomes frozen into the supersonic flow. In this paper we will examine the observational evidence for departures from Parker’s model and the circumstances under which they occur.