111. Increasing occurrence of inverted magnetic fields from 0.3 to 1 au

July 24, 2020, from uksp_nug_ed

Author: Allan Macneil, Mathew Owens, Mike Lockwood, Matthew Lang, Sarah Bentley (University of Reading) and Robert Wicks (University of Northumbria)

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Introduction

Local inversions in the heliospheric magnetic field (HMF) are observed at a range of solar distances and latitudes by in situ solar wind spacecraft. Figure 1 shows a schematic example of inverted and uninverted magnetic field.

Figure 1. Schematic of two magnetic field lines which follow a Parker spiral configuration (a and c) and one which is locally inverted (b) as viewed from heliographic north.

Recent observations of numerous rapid, Alfvénic inversions (known as ‘switchbacks’) by the new Parker Solar Probe (PSP) mission at distances down to 0.16 au have led to renewed interest in this phenomenon [2, 3]. Finding the origins of these inversions, and specifically whether they are formed at the Sun or through in-transit processes, is of particular interest. Knowledge of some solar origin for these inversions could help to reveal processes occurring in the corona which contribute to the production of the solar wind, such as jets and interchange reconnection [4, 5]. In this nugget, we describe results which put constraints on the origins of HMF inversions by quantifying the change in inversion occurrence as a function of distance r, as measured by the Helios 1 spacecraft.

Helios Observations of Inverted Field

Helios 1 observed the low-latitude solar wind and HMF over distances of 0.3 to 1 au, over several orbits from 1974 to 1981. For this data set, we classify 40 s cadence magnetic field samples as corresponding to either inverted or uninverted HMF. This is done by combining the magnetic field polarity (defined relative to the nominal Parker spiral direction) with the beam direction of the suprathermal electron strahl (which traces an anti-sunward path along the field). Once all valid data has been classified, we bin the samples into bins of r for analysis.

Evolution of Relative Inverted Field Occurrence

For our binned data, we calculate the fractions of all valid samples which correspond to inverted and uninverted HMF. These results are plotted against r in Figure 2.

Figure 2. Plot of the fractional occurrence, in 14 radial distance bins, of uninverted and inverted HMF samples. Error bars are calculated based on the number of samples in each bin. A line of best fit, gradient m, is shown for each HMF type. Shaded regions indicate bounds calculated using upper and lower fitting errors in the gradient and intercept.

The occurrence of inverted HMF increases between 0.3 and 1 au, at the expense of uninverted HMF. The relative number of inverted HMF samples increases by a factor of around 4. This result implies that inverted HMF over this distance range is primarily created through some driving process in the heliosphere. If most inversions formed at the Sun, then we would expect inverted HMF occurrence to instead drop-off with r, as the inversions gradually decay.

Direction of Magnetic Field Deflection

Field and plasma properties associated with inversions can provide evidence as to what mechanisms are driving the creation of inverted HMF. We calculate the azimuthal ‘deflection angle’, ΔϕP, of each sampled magnetic field vector away from the nominal Parker spiral direction. The strahl beam direction is used again to remove magnetic sector dependence, such that |ΔϕP| is 0° when the field is unperturbed, and |ΔϕP| > 90° indicates that the field has been deflected to the point of inversion.

Figure 3. Normalised histograms of ΔϕP in six equally sized radial distance bins. The central bin distance is labelled on each panel. Blue (orange) sectors of each panel indicate uninverted (inverted) HMF. Grey lines show the part of the histogram in the inverted sectors of each histogram, re-normalised to show detail.

Figure 3 shows that the distribution of ΔϕP gradually broadens with r. This supports the above interpretation that the increase in inverted HMF is driven by the gradual deflection of the field away from the nominal Parker spiral direction, as more samples exceed |ΔϕP| = 90°.

Generation of Inversions

In Figure 4 we show some simple schematics of possible processes, adapted from suggestions in [6], which could generate inverted magnetic fields in the heliosphere.

Figure 4. Schematics of processes which may create HMF inversions in the solar wind. The drawn field lines lie in the ecliptic plane, and would follow the Parker spiral if unperturbed. Green arrows in each case indicate the direction of rotation which the field undergoes to become inverted. Panel a.: inversion of a flux tube which threads a stream shear. Panels b. and c.: inversion of flux tube which is draped over ejecta. Panels d. and e.: inversion of field by waves and turbulence.

Panels a to c show that the action of convecting plasma in the solar wind can drive inversions into the field. The angle between the background Parker spiral field and the radial propagation of these elements means that inversions can only be generated through a deflection in the positive ΔϕP direction. Meanwhile, inversions created by waves and turbulence can result from deflection of the field in either direction. Comparison of the wings of the distributions in Figure 3 at the positive and negative extremes reveals that there is no strong bias towards inversion through either clockwise or anti-clockwise deflection. Thus, of the presented processes, only waves and turbulence are consistent with our observations. We note that the schematics here do not account for more complex possible effects, such as the interaction between stream shears and turbulence [7] or the expansion of inverted structures [8].

Conclusions

We have shown that the occurrence of inverted HMF gradually increases over the distance range 0.3 to 1 au. This indicates that most of these inversions are being actively driven into the HMF, instead of being a remnant of some process at the Sun. Analysis of the azimuthal deflection angle of inverted HMF suggests that waves and turbulence may be the dominant process in creating these inversions. While these results demonstrate that in situ driving of inversions takes place, they do not rule out that inversions may also be generated by processes at the Sun. This is particularly true for the frequent near-Sun switchbacks observed by PSP. These results raise an interesting question: as the switchbacks which dominate the PSP encounters become common on approaching the Sun, at what distance does the occurrence of inverted HMF start to (presumably) increase?

References

  • [1] Macneil, A. R., et al., MNRAS 494 3 (2020)
  • [2] Bale, S. D., et al., Nature 576 7786 (2019)
  • [3] Kasper, J. C., et al., Nature 576 7786 (2019)
  • [4] Horbury, T. S., Matteini, L., and Stansby, D., MNRAS 478 2 (2018)
  • [5] Crooker, N. U., et al. JGR: Space Phys 109 A3 (2004)
  • [6] Lockwood, M., Owens, M. J., and Macneil, A. R., Sol Phys 294 6 (2019)
  • [7] Landi, S., Hellinger, P., and Velli, M., GRL 33 14 (2006)
  • [8] Jokipii,i J. R., Kota, J., GRL 16 L1 (1989)


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