3.3.1. Direction of Breg
The local galactic magnetic field near the Sun at galactic radius rg = 8 kpc points toward l 80° ± 15°, i.e., clockwise as seen from the North Galactic Pole, and with the regular component strength 2 µgauss. There is a somewhat larger random component, as derived from radio RM observations of QSOs and galaxies (first reported in Fig. 1 of Morris & Berge, 1964; confirmed later in Fig. 3 of Gardner et al., 1969).
In the Scutum arm, the first detection of an excess rotation measure of 75 radians/m2 was found, and the magnetic field was found to be directed clockwise as viewed from the north pole of our Galaxy (Fig. 1 in Vallée et al., 1988b).
In the Perseus arm, the first detection of an excess rotation measure of 30 radians/m2 was found (Fig. 2 in Vallée, 1983d), with the magnetic field directed clockwise as viewed from the north pole of our Galaxy.
Averaging over three spiral arms in our Galaxy (Sagittarius, Perseus, Scutum) plus the Orion spur, one finds a mean arm rotation measure of 85 radians/m2 (e.g., Table 1 in Vallée 1984b), corresponding to a uniform component of 3 µGauss for the magnetic field.
Going radially inward toward the Galactic center, one encounters a first magnetic field reversal near rg = 7 kpc - the magnetic field direction at rg < 7 kpc goes counterclockwise as seen from the North Galactic Pole, as obtained by radio RM observations of QSO and galaxies (first reported in Fig. 3 in Simard-Normandin & Kronberg, 1979), and confirmed one year later by radio RM observations of pulsars in the Milky Way (reported in Fig. 5 in Thomson and Nelson, 1980). More pulsar data re-confirmed this first magnetic field reversal (Rand & Kulkarni, 1990). RM analysis of pulsar data have generally confirmed the RM analysis of QSO and galaxies data (e.g., Rand, 1994).
Continuing radially inward toward the Galactic center, a second magnetic field reversal is observed near the Scutum arm at rg = 5.5 kpc - the magnetic field direction at rg < 5.5 kpc goes clockwise as seen from the North Galactic Pole, as obtained by radio RM observations of QSO and galaxies (first reported in Fig. 1 of Vallée et al., 1988b), and confirmed 6 years later by radio RM observations of pulsars in the Milky Way (reported in Fig. 5 of Rand & Lyne, 1994).
Going radially outward toward the galactic anticenter, no magnetic field reversal is encountered in the Perseus arm and "Perseus + I" arm at rg > 10 kpc - the magnetic field there follows the clockwise direction as seen from the North Galactic Pole, with a strength of ~ 2 µGauss near the Perseus arm obtained by radio RM observations of QSO and galaxies (first reported in Fig. 2 of Vallée, 1983d; see also Vallée 1983c), and confirmed by radio RM observations of pulsars in the Milky Way (e.g., Fig. 3 in Lyne & Smith 1989). The lack of any radial reversal of the magnetic field in the outer Galaxy has been suggested by Simard-Normandin & Kronberg (1980), from early RM data.
Clegg et al. (1992) obtained the Rotation Measures of 56 sources in a selected part of the sky, mostly bounded by 45° < l < 93° and -5° < b < + 5° . The small RM increase in their Fig. 9 for the pulsar data near a distance of 4 kpc from the sun is localized to a small angular distance in longitude (between 70° and 90°), to a small radial distance range (between 4 and 5 kpc from the sun), and to a small angular area in latitude (between -10° and +10°) since it is not seen in the RM of quasars well above and well below the galactic plane. This resembles the effects of a local interstellar magnetic superbubble/shell there (not the effect of a large scale spiral arm magnetic field - i.e., Feature D in their Fig. 12 could be a superbubble). Thus the reality of a more distant large-scale magnetic field reversal is in doubt. Fig. 6d in Rand & Lyne (1994) corroborates Fig. 9 in Clegg et al. (1992), in showing a localized, not a large-scale, feature D. Clegg et al. (1992) also showed a large negative RM for extragalactic sources near l = 70° and l = 88° (their Fig. 9), which indicate a large scale magnetic field going away from the sun at > 20 kpc (not plotted in their Fig. 12 beyond Feature D, at > 20 kpc).
The combined effects of weaker, more distant superbubbles will average out, and only the effects from the most nearby superbubbles will stand out in the RM sky distribution.