Rotation Measures of Extragalactic Sources Behind the Southern
Galactic Plane: New Insights into the Large-Scale Magnetic Field
of the Inner Milky Way
J. C. Brown,1 M. Haverkorn,2,3 B. M. Gaensler,4,5,6 A. R. Taylor,1 N. S. Bizunok,5
N. M. McClure-Griffiths,7 J. M. Dickey,8 and A. J. Green4
ABSTRACT
We present new Faraday rotation measures (RMs) for 148 extragalactic radio sources behind
the southern Galactic plane (253◦ ≤ ℓ ≤ 356◦, |b| ≤ 1.5◦), and use these data in combination
with published data to probe the large-scale structure of the Milky Way’s magnetic field. We
show that the magnitudes of these RMs oscillate with longitude in a manner that correlates with
the locations of the Galactic spiral arms. The observed pattern in RMs requires the presence of
at least one large-scale magnetic reversal in the fourth Galactic quadrant, located between the
Sagittarius-Carina and Scutum-Crux spiral arms. To quantitatively compare our measurements
to other recent studies, we consider all available extragalactic and pulsar RMs in the region we
have surveyed, and jointly fit these data to simple models in which the large-scale field follows
the spiral arms. In the best-fitting model, the magnetic field in the fourth Galactic quadrant
is directed clockwise in the Sagittarius-Carina spiral arm (as viewed from the North Galactic
pole), but is oriented counter-clockwise in the Scutum-Crux arm. This contrasts with recent
analyses of pulsar RMs alone, in which the fourth-quadrant field was presumed to be directed
counter-clockwise in the Sagittarius-Carina arm. Also in contrast to recent pulsar RM studies,
our joint modeling of pulsar and extragalactic RMs demonstrates that large numbers of large-scale
magnetic field reversals are not required to account for observations.
Subject headings: Galaxy: structure — ISM: magnetic fields — polarization
1Department of Physics and Astronomy, University
of Calgary, T2N 1N4, Canada; jocat@ras.ucalgary.ca;
russ@ras.ucalgary.ca
2Jansky Fellow, National Radio Astronomy Observatory
3Astronomy Department, UC-Berkeley, 601 Campbell
Hall, Berkeley, CA 94720; marijke@astro.berkeley.edu
4School of Physics A29, The University of Syd-
ney, NSW 2006, Australia; bgaensler@usyd.edu.au;
agreen@physics.usyd.edu.au
5Harvard-Smithsonian Center for Astrophysics,
60 Garden Street, Cambridge, MA 01238;
nbizunok@cfa.harvard.edu
6Alfred P. Sloan Research Fellow, Australian Research
Council Federation Fellow
7Australia Telescope National Facility, CSIRO, PO
Box 76, Epping, NSW 1710, Australia; Naomi.McClure-
Griffiths@csiro.au
8Physics Department, University of Tasma-
nia, Private Bag 21, Hobart TAS 7001, Australia;
1. Introduction
The Galactic magnetic field is now recognized
as a fundamental component of the interstellar
medium, and plays a critical role in the formation
and evolution of structures in the Milky Way. An
important prediction in models of the large-scale
magnetic field, in both the Milky Way and in other
galaxies, is the existence of magnetic field reversals
(regions of magnetic shear across which the field
changes direction by roughly 180◦). Determining
the number and location of these magnetic rever-
sals is essential to understanding Galactic evolu-
tion (Shukurov 2005). While the majority of the
recent studies suggest several large-scale reversals
in the Galaxy along its radius, it is interesting to
John.Dickey@utas.edu.au
http://arxiv.org/abs/0704.0458v1
note that most other galaxies exhibit either one re-
versal or none (Beck 2007). Is our Galaxy unique
this way, or is it simply a difference in observing
methods?
The large-scale Galactic magnetic field is con-
centrated in the disk, and is most often studied via
observations of rotation measure (RM), the mea-
surable consequence of Faraday rotation. For a
source that emits linearly polarised radiation at
an angle φ◦, the received radiation will have a po-
larisation angle at a wavelength, λ [m] , given by:
φ = φ◦ + λ
2 0.812
neB · dl = φ◦ + λ
2 RM (1)
where ne [cm
−3] is the electron density andB [µG]
is the magnetic field along the propagation path
dl [pc]. The RM integral is over the path from the
source to the observer.
For more than four decades, the RMs of both
pulsars and extragalactic radio sources (EGS)
have been used to probe the Galactic magnetic
field. This has led to a series of clear conclu-
sions. First, in the local spiral arm, the field
is unquestionably directed clockwise (CW), as
viewed from the North Galactic pole (Manchester
1972, 1974; Heiles 1996a). The field in the first
quadrant (Q1; 0◦ ≤ ℓ ≤ 90◦; see Figure 1) of
the Sagittarius-Carina spiral arm is reversed rel-
ative to the local arm (Thomson & Nelson 1980;
Simard-Normandin & Kronberg 1980; Lyne & Smith
1989), implying a large-scale field reversal be-
tween the local arm and the Q1 component of the
Sagittarius-Carina arm. Some evidence suggests
that this reversed field extends into the fourth-
quadrant (Q4; 270◦ ≤ ℓ ≤ 360◦) component of the
Sagittarius-Carina arm (eg. Rand & Lyne 1994;
Han et al. 1999; Frick et al. 2001).
At larger distances, the existence of other large-
scale reversals in the Galaxy remains unclear.
Ideally, reconstruction of the Galactic magnetic
field should utilize information from both pulsars
and extragalactic sources. However, until recently
there have been very few EGS RMs available at
low-latitude that might be used to study the field
in the disk. Studies that have utilized pulsar RMs
alone are constrained by the comparatively sparse
sampling of pulsars on the sky, which can make it
difficult to map the field in complicated regions.
Three recent pulsar RM studies are as follows.
Using pulsar RMs, Weisberg et al. (2004) inves-
tigated the existence and location of reversals in
Q1, concluding that a reversal occurs between each
arm so that the magnetic fields in adjacent arms
are oppositely directed. While the evidence pre-
sented by Weisberg et al. (2004) for a reversal
between the local arm and the Sagittarius-Carina
arm in Q1 is indisputable, their evidence for addi-
tional reversals is based on limited data, and they
acknowledge that the evidence for a reversal into
the Q4-component of the Sagittarius-Carina arm
is not well-defined.
Vallée (2005) investigated azimuthal field con-
figurations in the Galaxy using pulsar RMs aver-
aged in concentric rings. He concluded that best-
fit for this model was an overall clockwise mag-
netic field with a 2 kpc wide counter-clockwise
(CCW) ring, located between 4 and 6 kpc from
the Galactic center (note that a non-standard
Solar-circle radius of 7.2 kpc was assumed in this
study). In this model, the Galactic field has two
large-scale reversals, and the Q1 component of
the Sagittarius-Carina arm is CCW, while the Q4
component is CW.
Using 223 new pulsar RM observations primar-
ily in the fourth quadrant, in conjunction with pre-
vious pulsar RMs, Han et al. (2006) concluded
there is a reversal at every arm-interarm boundary,
so that the fields in the arms are directed CCW,
and the interarm regions are directed CW. A po-
tential inconsistency with this model is that the
majority of the pulsars distributed along the Q4
component of the Sagittarius-Carina spiral arm
have what Han et al. (2006) describe as ‘unex-
pectedly positive’ RMs, which they suggest is the
influence of HII regions along the lines-of-sight to
the affected pulsars (see also Mitra et al. 2003).
Recent surveys of the Galactic plane at high
resolution and at multiple wavelengths have as-
sisted greatly in the study of the Galactic mag-
netic field by addressing the previous paucity of
low-latitude EGS RM data (eg. Brown & Taylor
2001). One such survey is the Southern Galac-
tic Plane Survey (SGPS; McClure-Griffiths et al.
2005; Haverkorn et al. 2006b). The SGPS images
low latitudes in the third and fourth quadrants of
the Galaxy, complementing the Canadian Galactic
Plane Survey in the northern hemisphere (CGPS;
Taylor et al. 2003). Rotation measures calculated
from these data were used by Haverkorn et al.
(2006a) to explore the structure of the small-scale
field. Here, we present the tabulation of these
RMs and use them to examine the structure of
the large-scale field.
2. Rotation Measure Calculations
The initial set of observations for the SGPS
(ie. Phase I) spans an area of 253◦ ≤ ℓ ≤
358◦ and |b| ≤ 1.5◦. The observations were
done with the Australia Telescope Compact Ar-
ray (ATCA) in New South Wales, Australia. For
details about the SGPS observations and polari-
metric data reduction, see Gaensler et al. (2001)
and Haverkorn et al. (2006b).
RMs were calculated using twelve separate 8
MHz bands centered on frequencies between 1336
MHz and 1432 MHz. The proximity of the 8
MHz bands allows for unambiguous RM calcula-
tions using the algorithm designed by Brown et al.
(2003b) for the CGPS, with appropriate modifica-
tions for the ATCA. Specifically, an RM calcula-
tion was considered reliable if the source was suf-
ficiently polarized (>0.3%), had sufficient signal
to noise (>2) across at least 3 pixels, was Fara-
day thin (Vallée 1980), and had a consistent value
across the source.
Of the 215 polarised source candidates identi-
fied, 148 sources had RMs that successfully passed
the screening tests discussed above. These new
data are given in Table 1. Three of our 148 sources
had a RM determined by Gaensler et al. (2001),
as part of the SGPS test region (325.5◦ ≤ ℓ ≤
332.5◦,−0.5 ≤ b ≤ 3.5◦). The previously deter-
mined values of these sources are within the er-
rors of the new values quoted in Table 1. These
sources are indicated by footnote marker d. Two
additional test region RM sources fall within the
latitude range of the SGPS but they lie within the
noise perimeter of our data (see Gaensler et al.
2001) and were not observable. There is one other
previously observed RM from an EGS, reported
by Broten et al. (1988), that falls within our field
at ℓ = 307.1◦, b = 1.2◦. This source is resolved
in the SGPS and exhibits a large gradient across
the source. Consequently, it failed the screening,
though its calculated RM (+183± 23 rad m−2) is
in agreement with the previously determined value
(+185± 1 rad m−2).
3. Observations of the Galactic Magnetic
Field
Figure 2 shows the RMs of EGS and pulsars in
the SGPS region. Most of the pulsars (99 of 120)
are from Han et al. (2006). The remaining are
from Taylor et al. (2000) and Han et al. (1999).
The most striking feature of Figure 2 is the change
in sign of RM from predominantly positive at low
longitudes to predominantly negative at high lon-
gitudes for both the pulsars and EGS at ℓ ∼ 304◦,
though it is more prominent for the EGS. A change
in sign of the overall trend in RM can only come
from a change of direction in the dominant line-
of-sight component of the magnetic field. The fact
that the RM sign change is abrupt indicates that
this directional change is the result of a physi-
cal field reversal, rather than a viewing angle ef-
fect such as that observed towards ℓ ∼ 180◦ (eg.
Brown & Taylor 2001). Furthermore, the abrupt-
ness indicates a thin current sheet and an associ-
ated large gradient in the magnetic field (Heiles
1996b).
By averaging the EGS RMs in ℓ across the sky
to reduce small-scale variations (for example, due
to the small-scale field or intrinsic effects from the
EGS themselves), we can obtain more information
about the large-scale field. The top panel in Fig-
ure 3 shows the RM data from the SGPS, both
raw and binned plotted as a function of Galac-
tic longitude. The middle panel shows these data
smoothed. In all three presentations of the EGS
data, an oscillating pattern of RM with longitude
is visible. The transition from positive to nega-
tive RMs remains at ℓ ∼ 304◦, indicated by the
solid vertical line. In the bottom panel of Figure
3, we plot the individual pulsar RMs as a function
of Galactic longitude. These data were not aver-
aged in ℓ because of the variation and uncertainty
in the pulsar distances. In spite of this, there are
features seen here similar to the oscillatory pattern
observed in the EGS data.
The dashed and dotted lines in the middle panel
of Figure 3 are the approximate longitudes of |RM|
maxima and minima respectively for the SGPS
data. In Figure 1, we show these lines as lines-of-
sight overlaid on a top-down view of the Galaxy
where the grey scale is the Galactic electron den-
sity model of Cordes & Lazio (2002), hereafter
CL02. Interestingly, the blue dashed lines (|RM|
maxima) tend to have the longest continuous frac-
tion of their length along a spiral arm or through
the central annulus put in the CL02 to correspond
to the molecular ring. This is consistent with the
expectation that EGS RMs should be dominated
by the spiral arms as a result of the higher electron
densities in these regions (Han et al. 2006). The
strong positive RMs in the longitude range around
ℓ ∼ 292◦, seen in both the EGS and pulsars, sug-
gest the magnetic field in the Q4-component of the
Sagittarius-Carina arm is directed CW. The same
conclusion was reached by Caswell et al. (2004)
using data from a distant supernova remnant.
This CW field presents a simple explanation for
the positive pulsar RMs identified by Han et al.
(2006) as part of a ‘Carina anomaly’.
Similarly, the strong negative RMs in the lon-
gitude range around ℓ ∼ 312◦ suggest the mag-
netic field in the Q4-component of the Scutum–
Crux arm is directed CCW. Therefore, a magnetic
field reversal must reside between the Sagittarius–
Carina arm and the Scutum–Crux arm in Q4. Fur-
thermore, the strong evidence for a field reversal
between the local and Sagittarius–Carina arms in
Q1 suggests the reversal must slice through the
Sagittarius–Carina arm if this reversal is contin-
uous between Q1 and Q4, such as in the model
proposed by Vallée (2005). The subsequently im-
plied lack of alignment of the magnetic field with
the Sagittarius-Carina arm is consistent with ear-
lier observations of at least a 5◦ offset in pitch an-
gle between the orientation of the local field and
that of the local spiral arm (Beck 2007).
4. Modeling the Large-Scale Magnetic
Field
As a separate approach to qualitative obser-
vational analysis of the data, we can fit global
magnetic field models to the RM data (with as-
sumptions regarding the electron distribution in
the Galaxy), to explore the structure of the uni-
form field and the location or existence of mag-
netic field reversals. Here we use the CL02 electron
density model and the technique of Brown (2002)
which uses linear inversion theory (Menke 1984)
to obtain a least-squares fit to the data. With this
method, the boundaries of magnetic field regions
are fixed a priori but the strength and direction of
the field within these regions are not. The model is
fit to the observed RMs to derive the strength and
direction within each region. This contrasts previ-
ous analyses which also employed separate models
for the electron density and magnetic field, but
where the direction of the field was an input to
the model (e.g. Indrani & Despande 1998).
We present here a simple model where we con-
sider nine magnetic field regions, eight of which
are either arm or interarm regions, and the ninth
corresponds to the molecular ring. This model
does not include the differing pitch angle between
the magnetic and spiral arms discussed in section
3, but is instead designed to be directly compa-
rable with the results of Weisberg et al. (2004)
and Han et al. (2006). The region boundaries are
delineated by the green spirals in Figure 4. The
constraints for the model field within each arm or
interarm region are: 1) a log-spiral with a pitch
angle of 11.5◦ as shown; 2) a field strength that
is inversely proportional to Galactic radius (eg.
Heiles 1996b; Beck 2001; Brown et al. 2003a);
3) a zero vertical component; 4) a coherent di-
rection within each region. We set the magnetic
field within the molecular ring to have the same
constraints as in the arms, except that the field is
assumed to be purely azimuthal. At Galactic radii
less than 3 kpc or greater than 20 kpc, or at more
than 1 kpc from the mid-plane, the field is set to
zero.
We constrain the model using RM data from
individual sources only within the SGPS region
(120 pulsars, 148 SGPS EGS, and 1 EGS from
Broten et. al 1988; see sections 2 and 3).1 For
modeling purposes, we assume that the inter-
galactic contribution to the EGS RMs is neg-
ligible (Simard-Normandin & Kronberg 1980;
Gaensler et al. 2005), so that the EGS may be
considered to reside at Galactocentric radii of 20
kpc. Most of the published pulsar distances used
are based on the CL02 model, and are therefore
consistent with this model. As a consequence of
the limited sky coverage of the RM data used here,
we confine our analysis of the result to within the
SGPS longitudes.
The best-fit output for this model is a CW field
everywhere except for a CCW field in the Scutum-
Crux arm and in the molecular ring, as shown in
1Global models using data from all quadrants is beyond the
scope of this paper, but will be presented in a future paper.
Figure 4. Figure 5 shows a plot of both the mea-
sured and modeled RMs for the individual SGPS
EGS data (top panel), these data averaged and
smoothed as in the middle panel of Figure 3 (mid-
dle panel), and the measured and modeled RMs
for individual pulsars (bottom panel).
This model is able to closely reproduce the RM
structure seen in the smoothed SGPS EGS data
(recall that the fit is to the individual EGS and
pulsar data) . However, there are two (relatively
small) discrepancies between the model and the
observed data towards the outer Galaxy. The first
is in the vicinity around ℓ ∼ 275 deg, where the
measured data are more negative than the mod-
eled data as also seen in the top panel Figure 5. In
Figure 2, there is a contained region of small, neg-
ative RM between 270◦ < l < 283◦ at b > 0◦, re-
sembling a magnetic bubble like that discussed by
Clegg et al. (1992) and Brown & Taylor (2001).
RMs at these longitudes nominally should be dom-
inated by the field in the local arm. Thus, if the
negative RMs seen here were to be attributed to
the large-scale field, this would imply that the
field is directed counter-clockwise in the local arm.
This is contrary to the many studies that show
the field is clockwise in the local arm, as discussed
in section 1. Interestingly, this region was previ-
ously identified in the Parkes 2.4-GHz survey as
containing a polarized feature of unknown origin
(Duncan et al. 1997). Since the lines-of-sight in
the outer Galaxy are considerably smaller through
the interstellar medium, compared to lines-of-sight
through the inner Galaxy, it is likely that this lo-
calized feature is dominating the RM for these
EGS. As a result, averaging the negative RMs
through this localized feature with the otherwise
positive RMs of the local arm creates the effect of
RM ∼ 0 at l ∼ 275◦.
The other discrepancy is around ℓ ∼ 265◦
where the measured RMs are more positive than
the modeled RMs. Lines-of-sight at this longi-
tude are again through the outer Galaxy where
localized features including the Gum nebula
(Chanot & Sivan 1983) could dominate the RM.
As seen in the top panel of Figure 3 there are two
EGS sources at ℓ = 263◦ that have RMs roughly
double that of neighboring sources which biases
the average RM near this longitude. Interestingly,
there is also a localized peak in the pulsar data
near this longitude, as seen in the bottom panel
of Figure 3. The model tries to fit both the nega-
tive region around ℓ ∼ 275◦ and the more positive
region around ℓ ∼ 265◦, with the result being a
compromise between the two.
Although the individual pulsar data are noisier
than the averaged EGS data, the trends of the
pulsar data are also reproduced by this model.
In particular, the model supports our observa-
tional conclusion that the Q4-component of the
Sagittarius-Carina arm is directed CW, while the
Q4-component of the Scutum-Crux arm is directed
Finally, we note that the direction of the Norma
arm field is not well constrained by this model or
by the data. Regardless of whether the field in the
Norma arm is oriented CW or CCW, the results
from this model contrast the previous suggestions
of reversals with every arm (Weisberg et al. 2004)
or at every arm-interarm boundary (Han et al.
2006).
5. Summary
We present the rotation measures for 148 ex-
tragalactic sources found in the southern Galactic
plane survey. The oscillations of rotation mea-
sure with longitude revealed by these sources,
and as also seen in pulsar RM data, highlight
the dominating effect of the spiral arms on rota-
tion measure. Both empirically and with a di-
rect fit to measurements, the new data show con-
clusively that the field is directed clockwise in
the fourth-quadrant component of the Sagittarius-
Carina arm, and that a field reversal exists be-
tween the Sagittarius-Carina arm and the Scutum-
Crux arms in the fourth quadrant.
A definitive measurement of the number of
large-scale magnetic reversals in the Galaxy can
only emerge from an analysis that includes pulsar
and EGS RM data at all Galactic longitudes, and
which considers a wide range of distinct field con-
figurations. In addition, the technique presented
here is constrained to geometries imposed by the
CL02 model. With these caveats, the results from
our study of southern RMs indicate that far fewer
magnetic reversals are needed to explain the data
than other recent studies have suggested.
We thank the anonymous referee for the insight-
ful comments that have improved this manuscript.
The Australia Telescope is funded by the Com-
monwealth of Australia for operation as a National
Facility managed by CSIRO. This work was facili-
tated in part by an associateship grant to JCB by
the Alberta Ingenuity Fund. MH acknowledges
support from the National Radio Astronomy Ob-
servatory, which is operated by Associated Uni-
versities, Inc., under cooperative agreement with
the NSF. BMG and MH acknowledge the support
of the NSF through grant AST-0307358.
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This 2-column preprint was prepared with the AAS LATEX
macros v5.2.
Table 1
Rotation Measures of the SGPS
ℓ a b a α δ I b m c RM
(◦) (◦) (J2000) (J2000) (mJy) (%) (rad m−2)
253.30 0.89 08 19 46.2 -34 42 05 179 5.8 −8 ± 12
253.52 0.83 08 20 08.4 -34 55 20 21 6.4 −50 ± 26
253.68 -0.60 08 14 47.2 -35 51 08 72 3.5 −349 ± 27
254.16 -0.34 08 17 10.3 -36 06 01 64 5.5 −338 ± 19
254.60 -0.87 08 16 12.7 -36 46 05 62 4.3 −15 ± 24
254.81 0.93 08 24 08.9 -35 55 21 87 13.7 +84 ± 13
254.95 0.63 08 23 20.7 -36 11 56 41 4.9 +8 ± 32
255.16 0.24 08 22 21.9 -36 36 03 92 5.3 −35 ± 16
255.27 0.16 08 22 20.2 -36 43 53 84 3.4 +26 ± 23
255.36 -0.26 08 20 52.1 -37 02 48 136 9.0 +97 ± 10
255.36 0.50 08 23 59.5 -36 37 00 94 8.0 −115 ± 13
256.14 0.25 08 25 11.9 -37 24 00 92 5.3 +44 ± 17
256.64 -0.22 08 24 43.7 -38 04 40 104 4.7 +172 ± 15
257.47 0.54 08 30 20.4 -38 18 27 46 9.5 +23 ± 20
257.71 -0.66 08 26 02.2 -39 12 13 117 4.0 +144 ± 20
257.92 0.65 08 32 07.7 -38 36 33 383 2.3 +76 ± 14
258.52 1.02 08 35 30.1 -38 52 23 118 2.7 +196 ± 26
258.77 0.08 08 32 23.4 -39 37 38 131 3.4 +221 ± 16
259.05 -0.72 08 29 49.9 -40 19 35 123 5.7 +175 ± 12
259.05 -0.75 08 29 45.1 -40 20 52 103 10.0 +150 ± 12
259.77 1.22 08 40 15.2 -39 44 39 33 5.8 +250 ± 29
260.41 -0.43 08 35 21.8 -41 14 50 101 3.0 +221 ± 18
260.52 -0.55 08 35 11.4 -41 24 44 25 5.8 +247 ± 31
260.69 -0.23 08 37 05.4 -41 21 08 224 3.2 +204 ± 12
263.20 1.07 08 50 56.2 -42 30 54 165 5.6 +739 ± 14
263.22 1.08 08 51 02.6 -42 31 32 164 4.4 +826 ± 19
263.50 0.17 08 48 12.7 -43 19 14 139 0.9 +260 ± 28
264.24 0.88 08 53 48.3 -43 25 59 112 11.0 +406 ± 9
265.69 0.85 08 58 54.5 -44 33 41 106 1.8 +70 ± 28
266.14 1.08 09 01 33.6 -44 44 49 66 3.2 +211 ± 29
266.27 0.66 09 00 15.2 -45 07 17 95 3.4 +396 ± 18
267.03 0.04 09 00 27.1 -46 06 15 58 3.8 +298 ± 21
267.17 0.47 09 02 50.7 -45 55 36 598 0.3 +323 ± 28
268.62 0.58 09 08 56.5 -46 55 53 231 2.1 +256 ± 15
269.05 0.17 09 08 54.5 -47 31 15 118 3.6 +456 ± 16
269.55 0.45 09 12 09.7 -47 41 35 156 1.9 +137 ± 18
270.56 -0.85 09 10 32.9 -49 19 14 58 5.5 −152 ± 23
270.91 0.93 09 19 52.0 -48 20 10 57 6.2 −149 ± 22
271.22 -0.35 09 15 37.0 -49 27 04 66 2.5 +215 ± 29
271.30 -0.06 09 17 12.3 -49 18 17 83 5.0 +136 ± 16
Table 1—Continued
ℓ a b a α δ I b m c RM
(◦) (◦) (J2000) (J2000) (mJy) (%) (rad m−2)
271.52 -1.01 09 13 54.3 -50 07 34 120 1.7 +75 ± 28
271.70 -0.38 09 17 30.7 -49 49 16 116 3.5 −93 ± 18
272.36 0.62 09 24 45.8 -49 34 44 21 5.6 −296 ± 28
273.46 0.68 09 29 58.1 -50 17 41 18 13.6 −106 ± 19
273.57 1.28 09 33 00.8 -49 55 47 229 1.9 +20 ± 25
274.77 0.25 09 34 14.3 -51 30 18 51 3.9 −76 ± 22
275.02 0.82 09 37 51.7 -51 15 07 26 8.4 −101 ± 25
275.48 -0.68 09 33 32.0 -52 40 18 282 2.8 +248 ± 13
275.56 1.02 09 41 17.5 -51 27 26 281 6.3 +16 ± 7
275.56 -0.20 09 36 03.4 -52 22 03 46 2.9 +112 ± 30
275.83 0.16 09 38 55.3 -52 16 41 170 9.3 −107 ± 7
275.86 0.94 09 42 25.0 -51 42 48 29 11.0 0 ± 21
276.46 0.89 09 45 09.7 -52 08 29 153 2.5 −17 ± 20
277.44 0.86 09 49 58.5 -52 47 04 10 12.1 −71 ± 28
277.78 -0.73 09 44 52.1 -54 13 45 127 2.0 −36 ± 23
277.78 -0.81 09 44 32.3 -54 17 32 690 7.3 −77 ± 3
278.04 0.75 09 52 37.5 -53 15 08 635 7.8 −104 ± 3
278.37 0.15 09 51 47.9 -53 55 46 18 13.7 −4 ± 24
278.43 0.53 09 53 42.8 -53 40 12 42 4.4 +20 ± 27
278.47 -0.30 09 50 22.2 -54 20 21 22 6.6 −116 ± 24
279.04 -0.88 09 50 55.5 -55 09 04 242 2.4 +239 ± 17
279.09 0.89 09 58 46.2 -53 47 21 47 5.0 −120 ± 25
279.15 -0.63 09 52 36.5 -55 01 08 36 11.2 +341 ± 24
279.15 -0.65 09 52 32.6 -55 02 37 44 6.2 +329 ± 20
279.33 0.80 09 59 41.3 -54 00 11 35 7.8 −157 ± 18
279.80 1.21 10 03 53.7 -53 57 24 25 9.7 −145 ± 25
280.53 0.81 10 06 18.9 -54 42 57 37 5.4 −10 ± 23
280.62 -0.14 10 02 53.8 -55 32 09 134 3.7 −112 ± 15
282.07 -0.78 10 08 36.2 -56 54 09 723 0.9 +862 ± 16
282.46 0.24 10 15 09.3 -56 17 28 109 2.0 +256 ± 23
284.30 0.81 10 28 39.3 -56 48 09 80 2.4 −547 ± 25
285.15 0.96 10 34 33.2 -57 06 17 268 0.9 +168 ± 31
285.60 0.62 10 36 12.7 -57 37 38 44 3.3 +368 ± 35
286.04 -1.05 10 32 43.2 -59 17 54 2826 0.4 +809 ± 14
286.89 0.59 10 44 37.5 -58 16 43 58 3.0 +324 ± 28
288.27 -0.70 10 49 34.1 -60 03 21 255 0.6 +491 ± 25
290.81 0.74 11 12 45.4 -59 47 27 1858 0.4 +419 ± 23
292.90 -0.02 11 26 31.6 -61 13 37 52 6.6 +349 ± 28
293.39 0.73 11 32 19.4 -60 40 22 67 5.0 +121 ± 20
293.73 0.63 11 34 43.2 -60 52 06 30 3.4 +116 ± 23
Table 1—Continued
ℓ a b a α δ I b m c RM
(◦) (◦) (J2000) (J2000) (mJy) (%) (rad m−2)
294.29 -0.90 11 35 31.6 -62 29 13 32 4.2 +449 ± 26
294.38 -0.75 11 36 37.4 -62 22 07 265 0.7 +470 ± 24
295.17 0.01 11 44 55.1 -61 51 26 57 3.5 +363 ± 22
295.23 -1.05 11 43 02.3 -62 53 56 114 2.1 −207 ± 22
295.29 -1.23 11 43 05.7 -63 04 55 226 1.6 −43 ± 29
296.18 -0.59 11 52 05.2 -62 40 59 118 3.7 +752 ± 14
296.90 0.14 11 59 31.2 -62 07 15 452 1.6 +1113 ± 11
297.67 0.77 12 06 52.0 -61 38 54 283 2.6 +570 ± 11
299.42 -0.23 12 20 29.8 -62 53 37 55 4.0 +535 ± 30
299.51 -1.10 12 20 24.8 -63 46 10 192 2.9 +315 ± 21
300.25 -0.01 12 27 57.6 -62 45 42 109 4.1 +123 ± 14
300.47 -0.99 12 29 06.5 -63 45 08 112 4.9 +412 ± 18
300.65 -0.41 12 31 12.4 -63 11 40 809 0.9 +358 ± 19
301.14 -0.09 12 35 39.6 -62 54 30 375 1.3 +350 ± 18
301.70 0.25 12 40 45.4 -62 36 01 21 5.2 +296 ± 33
302.60 -1.17 12 48 26.2 -64 02 26 162 1.7 +159 ± 25
303.30 0.51 12 54 36.3 -62 21 25 149 3.5 −370 ± 14
304.53 1.00 13 05 00.1 -61 49 31 32 6.1 +40 ± 28
305.62 -1.16 13 15 52.5 -63 54 09 44 10.5 −61 ± 28
306.87 0.02 13 25 47.5 -62 35 15 215 1.0 −197 ± 27
306.92 -0.70 13 27 00.7 -63 17 46 275 2.0 +52 ± 16
307.20 -0.84 13 29 39.3 -63 23 47 41 8.1 +382 ± 18
308.64 -0.62 13 41 56.1 -62 55 38 40 2.8 −133 ± 27
308.73 0.07 13 41 33.5 -62 14 06 92 1.4 −661 ± 29
308.93 0.40 13 42 41.5 -61 52 38 152 2.5 −752 ± 17
309.06 0.84 13 43 00.9 -61 24 52 670 2.3 −504 ± 10
310.20 -1.04 13 56 09.7 -62 59 30 46 8.5 −584 ± 19
312.37 -0.04 14 11 37.1 -61 25 54 129 2.3 −438 ± 28
313.96 -0.76 14 26 14.9 -61 34 58 88 2.3 −480 ± 22
313.99 0.94 14 21 36.9 -59 59 01 507 1.8 −828 ± 17
314.02 1.01 14 21 40.3 -59 54 20 757 1.7 −579 ± 20
314.50 0.30 14 27 14.8 -60 24 07 86 5.3 −738 ± 19
314.82 0.89 14 27 57.3 -59 43 58 92 3.7 −507 ± 25
316.64 1.15 14 40 15.0 -58 47 30 64 4.6 −525 ± 27
317.54 -0.57 14 52 27.0 -59 58 11 152 2.1 +395 ± 27
318.53 0.30 14 56 18.1 -58 44 29 326 1.0 +53 ± 29
319.34 1.08 14 58 58.6 -57 40 31 1101 0.7 +241 ± 21
319.39 0.74 15 00 28.7 -57 57 04 85 1.4 +279 ± 30
320.83 0.88 15 09 19.7 -57 07 19 454 0.8 −8 ± 20
321.48 1.02 15 12 54.7 -56 40 20 933 2.0 −243 ± 8
Table 1—Continued
ℓ a b a α δ I b m c RM
(◦) (◦) (J2000) (J2000) (mJy) (%) (rad m−2)
321.58 -0.76 15 20 29.3 -58 07 41 1831 1.1 −138 ± 9
322.05 -0.95 15 24 17.5 -58 01 49 78 5.4 −397 ± 17
323.15 -0.52 15 29 19.5 -57 03 49 47 2.9 +83 ± 25
324.77 0.61 15 34 10.5 -55 12 30 161 3.9 −66 ± 14
325.81 1.08 15 38 05.1 -54 13 08 608 0.6 −15 ± 31 d
325.83 -0.30 15 43 57.7 -55 18 26 247 1.8 +356 ± 18
326.69 -1.16 15 52 28.2 -55 27 04 95 9.3 −142 ± 22
327.31 0.88 15 47 01.4 -53 28 29 256 1.2 −189 ± 26 d
328.36 -0.41 15 58 00.3 -53 48 30 493 0.3 −721 ± 35
329.48 0.22 16 00 57.1 -52 36 04 599 0.7 −100 ± 25
330.12 -1.08 16 09 49.5 -53 08 33 218 3.0 −931 ± 25
332.14 1.03 16 10 10.8 -50 13 21 151 2.8 −754 ± 22 d
333.72 -0.27 16 22 55.5 -50 03 31 108 2.5 +204 ± 29
335.32 0.60 16 26 00.4 -48 18 36 352 2.9 −138 ± 11
337.06 0.85 16 32 01.7 -46 52 39 100 2.5 −739 ± 32
339.65 -0.24 16 46 44.6 -45 40 00 222 2.2 −398 ± 19
342.16 -0.74 16 57 52.7 -44 02 49 264 2.2 +127 ± 15
342.62 -0.45 16 58 15.4 -43 30 19 50 3.9 −913 ± 30
343.29 0.60 16 56 01.8 -42 19 58 102 4.7 −1035 ± 16
345.22 0.68 17 02 02.9 -40 45 50 39 12.6 +183 ± 21
347.40 -1.04 17 16 09.8 -40 02 24 72 4.0 −524 ± 36
349.65 -0.36 17 19 58.3 -37 48 15 133 3.3 +110 ± 20
350.52 -0.73 17 23 58.9 -37 18 18 127 2.2 +277 ± 32
351.31 -0.53 17 25 22.9 -36 32 16 200 3.3 −247 ± 15
351.82 0.17 17 23 56.8 -35 43 34 118 6.1 +134 ± 16
352.13 1.15 17 20 51.7 -34 54 49 116 6.5 +76 ± 30
355.43 -0.81 17 37 32.3 -33 14 40 76 4.2 +601 ± 21
356.57 0.87 17 33 45.9 -31 22 37 116 1.6 +985 ± 30
aThe identified location is the peak of the gaussian fit to the source in
polarised intensity. All sources were either unresolved or partially resolved.
b I is the peak-pixel Stokes I value of the interferometric data.
c m is the fractional polarisation (linear polarised intensity divided by
Stokes I) averaged over the FWHM pixels used for RM calculation.
dA RM for this source was calculated by Gaensler et al. (2001) using
a simpler approach than the more rigorous method used here. These new
values should replace the previously determined values.
Norma
Scutum-Crux
Sagittarius-
Carina
Perseus
local
molecular
l=253o
l=358o
Fig. 1.— View of the Galaxy from above. The grey scale is the CL02 electron density model, with labels
indicating the spiral arms, and the asterisk indicating the location of the Sun. Quadrants 1, 2, 3, and 4 are
labeled with Q1, Q2, Q3 and Q4, respectively. The bounding longitudes of the SGPS (Phase I) are indicated
by black lines and are labeled. The circles represent the smoothed-averaged SGPS RMs as shown in the
middle panel of Figure 3. Filled (open) circles indicate positive (negative) RM, with the size of the circles
linearly proportional to |RM|, truncated between 59 and 592 rad m−2 (the |RM|max from the middle panel of
Figure 3) so that sources with |RM| < 59 rad m−2 are set to 59 rad m−2. The blue dashed (dotted) lines are
also from the middle panel of Figure 3, and indicate the approximate longitudes of |RM| maxima (minima)
in the SGPS RM data. The solid blue line is the longitude where the RMs transition from primarily positive
to primarily negative (ℓ ∼ 304◦).
      
350 330 310 304 290 270 250
PULSARS
Galactic Longitude
Fig. 2.— Rotation measures for sources in the SGPS. Top panel: extragalactic RM sources; Bottom panel:
pulsar RM sources. Grey filled symbols indicate positive rotation measure, black open symbols indicate
negative rotation measures; sizes of symbols are linearly proportional to the magnitude of RM truncated
between 100-600 rad m−2, so that sources with |RM| < 100 rad m−2 are set to 100 rad m−2, and those with
|RM| > 600 rad m−2 are set to 600 rad m−2. In the top panel, the square at ℓ = 307.1◦, b= 1.2◦ represents
the EGS RM previously observed by Broten et al. (1988), while the squares at ℓ ∼ 330◦ represent the EGS
RMs of the SGPS test region. The circles in the top panel represent the SGPS EGS as given in Table 1.
The vertical line at ℓ = 304◦ is the approximate longitude where the EGS RMs change sign. The dashed
boxes indicate boundaries of the SGPS region.
             
-1200
   SGPS EGS
(raw, binned)
             
-1200
SGPS EGS
(smoothed)
360 350 340 330 320 310  300 290 280 270 260 250
-1200
PULSARS
  (raw) 
Galactic Longitude
Fig. 3.— RM versus Galactic longitude for RM sources in the SGPS region. Top panel: circles represent the
individual SGPS EGS sources (errors are smaller than the symbol size), while open red diamonds represent
data averaged into into independent longitude bins containing 9 sources (the end bin at 255◦ contains 13
sources). Where symbol size permits, the error bars are the standard deviation in longitude and RM for each
bin. Middle panel: purple diamonds represent boxcar-averaged SGPS EGS data over 9 degrees in longitude
with a stepsize of 3 degrees. In contrast to the binned data in the top panel, the error bars are the standard
error of the mean. The solid line marks the approximate longitude of transition from predominantly positive
RMs to negative RMs (l ∼ 304◦) Dotted lines (dashed lines) mark approximate longitudes of minimum
(maximum) |RM| in SGPS data. Bottom panel: squares represent the individual pulsars with known RM in
the SGPS region (errors are smaller than the symbol sizes).
Fig. 4.— A simple model of the magnetic field in the SGPS region, constrained using individual RMs of
149 EGS and 120 pulsars. Colored regions indicate the total strength for the model fit of magnetic field
corresponding to the regions identified by the green lines as discussed in the text. Red (blue) shading indicates
a clockwise (counter-clockwise) field direction as viewed from the Galactic north pole. The direction of the
field within the arms is also indicated by arrows. The open-head arrow on the Norma arm indicates the field
direction in this arm is not well-constrained in this model configuration with the data used. The bounding
longitudes of the SGPS (Phase I) are indicated by black lines and are labeled. The grey scale is the CL02
electron density model with labels indicating the spiral arms.
            
-1200
SGPS EGS
(raw)
Model
            
-1200
SGPS EGS
(smoothed)
Model
360 350 340 330 320 310 300 290 280 270 260 250
-1200
Pulsars (raw)
Model
Galactic Longitude
Fig. 5.— Comparison of modeled RMs (green symbols) and observed RMs. The modeled RMs are calculated
using the CL02 electron density model and the magnetic field model shown in Figure 4. The format of this
figure follows that of Figure 3, where the individual SGPS EGS are shown in the top panel, smoothed-
averaged SGPS EGS data are shown in the middle panel, and the individual pulsars are presented in the
bottom panel.
	Introduction
	Rotation Measure Calculations
	Observations of the Galactic Magnetic Field
	Modeling the Large-Scale Magnetic Field
	Summary

  We present new Faraday rotation measures (RMs) for 148 extragalactic radio
sources behind the southern Galactic plane (253o < l < 356o, |b| < 1.5o), and
use these data in combination with published data to probe the large-scale
structure of the Milky Way's magnetic field. We show that the magnitudes of
these RMs oscillate with longitude in a manner that correlates with the
locations of the Galactic spiral arms. The observed pattern in RMs requries the
presence of at least one large-scale magnetic reversal in the fourth Galactic
quadrant, located between the Sagittarius- Carina and Scutum-Crux spiral arms.
To quantitatively compare our measurements to other recent studies, we consider
all available extragalactic and pulsar RMs in the region we have surveyed, and
jointly fit these data to simple models in which the large-scale field follows
the spiral arms. In the best-fitting model, the magnetic field in the fourth
Galactic quadrant is directed clockwise in the Sagittarius-Carina spiral arm
(as viewed from the North Galactic pole), but is oriented counter- clockwise in
the Scutum-Crux arm. This contrasts with recent analyses of pulsar RMs alone,
in which the fourth-quadrant field was presumed to be directed
counter-clockwise in the Sagittarius- Carina arm. Also in contrast to recent
pulsar RM studies, our joint modeling of pulsar and extragalactic RMs
demonstrates that large numbers of large-scale magnetic field reversals are not
required to account for observations.

Introduction
The Galactic magnetic field is now recognized
as a fundamental component of the interstellar
medium, and plays a critical role in the formation
and evolution of structures in the Milky Way. An
important prediction in models of the large-scale
magnetic field, in both the Milky Way and in other
galaxies, is the existence of magnetic field reversals
(regions of magnetic shear across which the field
changes direction by roughly 180◦). Determining
the number and location of these magnetic rever-
sals is essential to understanding Galactic evolu-
tion (Shukurov 2005). While the majority of the
recent studies suggest several large-scale reversals
in the Galaxy along its radius, it is interesting to
John.Dickey@utas.edu.au
http://arxiv.org/abs/0704.0458v1
note that most other galaxies exhibit either one re-
versal or none (Beck 2007). Is our Galaxy unique
this way, or is it simply a difference in observing
methods?
The large-scale Galactic magnetic field is con-
centrated in the disk, and is most often studied via
observations of rotation measure (RM), the mea-
surable consequence of Faraday rotation. For a
source that emits linearly polarised radiation at
an angle φ◦, the received radiation will have a po-
larisation angle at a wavelength, λ [m] , given by:
φ = φ◦ + λ
2 0.812
neB · dl = φ◦ + λ
2 RM (1)
where ne [cm
−3] is the electron density andB [µG]
is the magnetic field along the propagation path
dl [pc]. The RM integral is over the path from the
source to the observer.
For more than four decades, the RMs of both
pulsars and extragalactic radio sources (EGS)
have been used to probe the Galactic magnetic
field. This has led to a series of clear conclu-
sions. First, in the local spiral arm, the field
is unquestionably directed clockwise (CW), as
viewed from the North Galactic pole (Manchester
1972, 1974; Heiles 1996a). The field in the first
quadrant (Q1; 0◦ ≤ ℓ ≤ 90◦; see Figure 1) of
the Sagittarius-Carina spiral arm is reversed rel-
ative to the local arm (Thomson & Nelson 1980;
Simard-Normandin & Kronberg 1980; Lyne & Smith
1989), implying a large-scale field reversal be-
tween the local arm and the Q1 component of the
Sagittarius-Carina arm. Some evidence suggests
that this reversed field extends into the fourth-
quadrant (Q4; 270◦ ≤ ℓ ≤ 360◦) component of the
Sagittarius-Carina arm (eg. Rand & Lyne 1994;
Han et al. 1999; Frick et al. 2001).
At larger distances, the existence of other large-
scale reversals in the Galaxy remains unclear.
Ideally, reconstruction of the Galactic magnetic
field should utilize information from both pulsars
and extragalactic sources. However, until recently
there have been very few EGS RMs available at
low-latitude that might be used to study the field
in the disk. Studies that have utilized pulsar RMs
alone are constrained by the comparatively sparse
sampling of pulsars on the sky, which can make it
difficult to map the field in complicated regions.
Three recent pulsar RM studies are as follows.
Using pulsar RMs, Weisberg et al. (2004) inves-
tigated the existence and location of reversals in
Q1, concluding that a reversal occurs between each
arm so that the magnetic fields in adjacent arms
are oppositely directed. While the evidence pre-
sented by Weisberg et al. (2004) for a reversal
between the local arm and the Sagittarius-Carina
arm in Q1 is indisputable, their evidence for addi-
tional reversals is based on limited data, and they
acknowledge that the evidence for a reversal into
the Q4-component of the Sagittarius-Carina arm
is not well-defined.
Vallée (2005) investigated azimuthal field con-
figurations in the Galaxy using pulsar RMs aver-
aged in concentric rings. He concluded that best-
fit for this model was an overall clockwise mag-
netic field with a 2 kpc wide counter-clockwise
(CCW) ring, located between 4 and 6 kpc from
the Galactic center (note that a non-standard
Solar-circle radius of 7.2 kpc was assumed in this
study). In this model, the Galactic field has two
large-scale reversals, and the Q1 component of
the Sagittarius-Carina arm is CCW, while the Q4
component is CW.
Using 223 new pulsar RM observations primar-
ily in the fourth quadrant, in conjunction with pre-
vious pulsar RMs, Han et al. (2006) concluded
there is a reversal at every arm-interarm boundary,
so that the fields in the arms are directed CCW,
and the interarm regions are directed CW. A po-
tential inconsistency with this model is that the
majority of the pulsars distributed along the Q4
component of the Sagittarius-Carina spiral arm
have what Han et al. (2006) describe as ‘unex-
pectedly positive’ RMs, which they suggest is the
influence of HII regions along the lines-of-sight to
the affected pulsars (see also Mitra et al. 2003).
Recent surveys of the Galactic plane at high
resolution and at multiple wavelengths have as-
sisted greatly in the study of the Galactic mag-
netic field by addressing the previous paucity of
low-latitude EGS RM data (eg. Brown & Taylor
2001). One such survey is the Southern Galac-
tic Plane Survey (SGPS; McClure-Griffiths et al.
2005; Haverkorn et al. 2006b). The SGPS images
low latitudes in the third and fourth quadrants of
the Galaxy, complementing the Canadian Galactic
Plane Survey in the northern hemisphere (CGPS;
Taylor et al. 2003). Rotation measures calculated
from these data were used by Haverkorn et al.
(2006a) to explore the structure of the small-scale
field. Here, we present the tabulation of these
RMs and use them to examine the structure of
the large-scale field.
2. Rotation Measure Calculations
The initial set of observations for the SGPS
(ie. Phase I) spans an area of 253◦ ≤ ℓ ≤
358◦ and |b| ≤ 1.5◦. The observations were
done with the Australia Telescope Compact Ar-
ray (ATCA) in New South Wales, Australia. For
details about the SGPS observations and polari-
metric data reduction, see Gaensler et al. (2001)
and Haverkorn et al. (2006b).
RMs were calculated using twelve separate 8
MHz bands centered on frequencies between 1336
MHz and 1432 MHz. The proximity of the 8
MHz bands allows for unambiguous RM calcula-
tions using the algorithm designed by Brown et al.
(2003b) for the CGPS, with appropriate modifica-
tions for the ATCA. Specifically, an RM calcula-
tion was considered reliable if the source was suf-
ficiently polarized (>0.3%), had sufficient signal
to noise (>2) across at least 3 pixels, was Fara-
day thin (Vallée 1980), and had a consistent value
across the source.
Of the 215 polarised source candidates identi-
fied, 148 sources had RMs that successfully passed
the screening tests discussed above. These new
data are given in Table 1. Three of our 148 sources
had a RM determined by Gaensler et al. (2001),
as part of the SGPS test region (325.5◦ ≤ ℓ ≤
332.5◦,−0.5 ≤ b ≤ 3.5◦). The previously deter-
mined values of these sources are within the er-
rors of the new values quoted in Table 1. These
sources are indicated by footnote marker d. Two
additional test region RM sources fall within the
latitude range of the SGPS but they lie within the
noise perimeter of our data (see Gaensler et al.
2001) and were not observable. There is one other
previously observed RM from an EGS, reported
by Broten et al. (1988), that falls within our field
at ℓ = 307.1◦, b = 1.2◦. This source is resolved
in the SGPS and exhibits a large gradient across
the source. Consequently, it failed the screening,
though its calculated RM (+183± 23 rad m−2) is
in agreement with the previously determined value
(+185± 1 rad m−2).
3. Observations of the Galactic Magnetic
Field
Figure 2 shows the RMs of EGS and pulsars in
the SGPS region. Most of the pulsars (99 of 120)
are from Han et al. (2006). The remaining are
from Taylor et al. (2000) and Han et al. (1999).
The most striking feature of Figure 2 is the change
in sign of RM from predominantly positive at low
longitudes to predominantly negative at high lon-
gitudes for both the pulsars and EGS at ℓ ∼ 304◦,
though it is more prominent for the EGS. A change
in sign of the overall trend in RM can only come
from a change of direction in the dominant line-
of-sight component of the magnetic field. The fact
that the RM sign change is abrupt indicates that
this directional change is the result of a physi-
cal field reversal, rather than a viewing angle ef-
fect such as that observed towards ℓ ∼ 180◦ (eg.
Brown & Taylor 2001). Furthermore, the abrupt-
ness indicates a thin current sheet and an associ-
ated large gradient in the magnetic field (Heiles
1996b).
By averaging the EGS RMs in ℓ across the sky
to reduce small-scale variations (for example, due
to the small-scale field or intrinsic effects from the
EGS themselves), we can obtain more information
about the large-scale field. The top panel in Fig-
ure 3 shows the RM data from the SGPS, both
raw and binned plotted as a function of Galac-
tic longitude. The middle panel shows these data
smoothed. In all three presentations of the EGS
data, an oscillating pattern of RM with longitude
is visible. The transition from positive to nega-
tive RMs remains at ℓ ∼ 304◦, indicated by the
solid vertical line. In the bottom panel of Figure
3, we plot the individual pulsar RMs as a function
of Galactic longitude. These data were not aver-
aged in ℓ because of the variation and uncertainty
in the pulsar distances. In spite of this, there are
features seen here similar to the oscillatory pattern
observed in the EGS data.
The dashed and dotted lines in the middle panel
of Figure 3 are the approximate longitudes of |RM|
maxima and minima respectively for the SGPS
data. In Figure 1, we show these lines as lines-of-
sight overlaid on a top-down view of the Galaxy
where the grey scale is the Galactic electron den-
sity model of Cordes & Lazio (2002), hereafter
CL02. Interestingly, the blue dashed lines (|RM|
maxima) tend to have the longest continuous frac-
tion of their length along a spiral arm or through
the central annulus put in the CL02 to correspond
to the molecular ring. This is consistent with the
expectation that EGS RMs should be dominated
by the spiral arms as a result of the higher electron
densities in these regions (Han et al. 2006). The
strong positive RMs in the longitude range around
ℓ ∼ 292◦, seen in both the EGS and pulsars, sug-
gest the magnetic field in the Q4-component of the
Sagittarius-Carina arm is directed CW. The same
conclusion was reached by Caswell et al. (2004)
using data from a distant supernova remnant.
This CW field presents a simple explanation for
the positive pulsar RMs identified by Han et al.
(2006) as part of a ‘Carina anomaly’.
Similarly, the strong negative RMs in the lon-
gitude range around ℓ ∼ 312◦ suggest the mag-
netic field in the Q4-component of the Scutum–
Crux arm is directed CCW. Therefore, a magnetic
field reversal must reside between the Sagittarius–
Carina arm and the Scutum–Crux arm in Q4. Fur-
thermore, the strong evidence for a field reversal
between the local and Sagittarius–Carina arms in
Q1 suggests the reversal must slice through the
Sagittarius–Carina arm if this reversal is contin-
uous between Q1 and Q4, such as in the model
proposed by Vallée (2005). The subsequently im-
plied lack of alignment of the magnetic field with
the Sagittarius-Carina arm is consistent with ear-
lier observations of at least a 5◦ offset in pitch an-
gle between the orientation of the local field and
that of the local spiral arm (Beck 2007).
4. Modeling the Large-Scale Magnetic
Field
As a separate approach to qualitative obser-
vational analysis of the data, we can fit global
magnetic field models to the RM data (with as-
sumptions regarding the electron distribution in
the Galaxy), to explore the structure of the uni-
form field and the location or existence of mag-
netic field reversals. Here we use the CL02 electron
density model and the technique of Brown (2002)
which uses linear inversion theory (Menke 1984)
to obtain a least-squares fit to the data. With this
method, the boundaries of magnetic field regions
are fixed a priori but the strength and direction of
the field within these regions are not. The model is
fit to the observed RMs to derive the strength and
direction within each region. This contrasts previ-
ous analyses which also employed separate models
for the electron density and magnetic field, but
where the direction of the field was an input to
the model (e.g. Indrani & Despande 1998).
We present here a simple model where we con-
sider nine magnetic field regions, eight of which
are either arm or interarm regions, and the ninth
corresponds to the molecular ring. This model
does not include the differing pitch angle between
the magnetic and spiral arms discussed in section
3, but is instead designed to be directly compa-
rable with the results of Weisberg et al. (2004)
and Han et al. (2006). The region boundaries are
delineated by the green spirals in Figure 4. The
constraints for the model field within each arm or
interarm region are: 1) a log-spiral with a pitch
angle of 11.5◦ as shown; 2) a field strength that
is inversely proportional to Galactic radius (eg.
Heiles 1996b; Beck 2001; Brown et al. 2003a);
3) a zero vertical component; 4) a coherent di-
rection within each region. We set the magnetic
field within the molecular ring to have the same
constraints as in the arms, except that the field is
assumed to be purely azimuthal. At Galactic radii
less than 3 kpc or greater than 20 kpc, or at more
than 1 kpc from the mid-plane, the field is set to
zero.
We constrain the model using RM data from
individual sources only within the SGPS region
(120 pulsars, 148 SGPS EGS, and 1 EGS from
Broten et. al 1988; see sections 2 and 3).1 For
modeling purposes, we assume that the inter-
galactic contribution to the EGS RMs is neg-
ligible (Simard-Normandin & Kronberg 1980;
Gaensler et al. 2005), so that the EGS may be
considered to reside at Galactocentric radii of 20
kpc. Most of the published pulsar distances used
are based on the CL02 model, and are therefore
consistent with this model. As a consequence of
the limited sky coverage of the RM data used here,
we confine our analysis of the result to within the
SGPS longitudes.
The best-fit output for this model is a CW field
everywhere except for a CCW field in the Scutum-
Crux arm and in the molecular ring, as shown in
1Global models using data from all quadrants is beyond the
scope of this paper, but will be presented in a future paper.
Figure 4. Figure 5 shows a plot of both the mea-
sured and modeled RMs for the individual SGPS
EGS data (top panel), these data averaged and
smoothed as in the middle panel of Figure 3 (mid-
dle panel), and the measured and modeled RMs
for individual pulsars (bottom panel).
This model is able to closely reproduce the RM
structure seen in the smoothed SGPS EGS data
(recall that the fit is to the individual EGS and
pulsar data) . However, there are two (relatively
small) discrepancies between the model and the
observed data towards the outer Galaxy. The first
is in the vicinity around ℓ ∼ 275 deg, where the
measured data are more negative than the mod-
eled data as also seen in the top panel Figure 5. In
Figure 2, there is a contained region of small, neg-
ative RM between 270◦ < l < 283◦ at b > 0◦, re-
sembling a magnetic bubble like that discussed by
Clegg et al. (1992) and Brown & Taylor (2001).
RMs at these longitudes nominally should be dom-
inated by the field in the local arm. Thus, if the
negative RMs seen here were to be attributed to
the large-scale field, this would imply that the
field is directed counter-clockwise in the local arm.
This is contrary to the many studies that show
the field is clockwise in the local arm, as discussed
in section 1. Interestingly, this region was previ-
ously identified in the Parkes 2.4-GHz survey as
containing a polarized feature of unknown origin
(Duncan et al. 1997). Since the lines-of-sight in
the outer Galaxy are considerably smaller through
the interstellar medium, compared to lines-of-sight
through the inner Galaxy, it is likely that this lo-
calized feature is dominating the RM for these
EGS. As a result, averaging the negative RMs
through this localized feature with the otherwise
positive RMs of the local arm creates the effect of
RM ∼ 0 at l ∼ 275◦.
The other discrepancy is around ℓ ∼ 265◦
where the measured RMs are more positive than
the modeled RMs. Lines-of-sight at this longi-
tude are again through the outer Galaxy where
localized features including the Gum nebula
(Chanot & Sivan 1983) could dominate the RM.
As seen in the top panel of Figure 3 there are two
EGS sources at ℓ = 263◦ that have RMs roughly
double that of neighboring sources which biases
the average RM near this longitude. Interestingly,
there is also a localized peak in the pulsar data
near this longitude, as seen in the bottom panel
of Figure 3. The model tries to fit both the nega-
tive region around ℓ ∼ 275◦ and the more positive
region around ℓ ∼ 265◦, with the result being a
compromise between the two.
Although the individual pulsar data are noisier
than the averaged EGS data, the trends of the
pulsar data are also reproduced by this model.
In particular, the model supports our observa-
tional conclusion that the Q4-component of the
Sagittarius-Carina arm is directed CW, while the
Q4-component of the Scutum-Crux arm is directed
Finally, we note that the direction of the Norma
arm field is not well constrained by this model or
by the data. Regardless of whether the field in the
Norma arm is oriented CW or CCW, the results
from this model contrast the previous suggestions
of reversals with every arm (Weisberg et al. 2004)
or at every arm-interarm boundary (Han et al.
2006).
5. Summary
We present the rotation measures for 148 ex-
tragalactic sources found in the southern Galactic
plane survey. The oscillations of rotation mea-
sure with longitude revealed by these sources,
and as also seen in pulsar RM data, highlight
the dominating effect of the spiral arms on rota-
tion measure. Both empirically and with a di-
rect fit to measurements, the new data show con-
clusively that the field is directed clockwise in
the fourth-quadrant component of the Sagittarius-
Carina arm, and that a field reversal exists be-
tween the Sagittarius-Carina arm and the Scutum-
Crux arms in the fourth quadrant.
A definitive measurement of the number of
large-scale magnetic reversals in the Galaxy can
only emerge from an analysis that includes pulsar
and EGS RM data at all Galactic longitudes, and
which considers a wide range of distinct field con-
figurations. In addition, the technique presented
here is constrained to geometries imposed by the
CL02 model. With these caveats, the results from
our study of southern RMs indicate that far fewer
magnetic reversals are needed to explain the data
than other recent studies have suggested.
We thank the anonymous referee for the insight-
ful comments that have improved this manuscript.
The Australia Telescope is funded by the Com-
monwealth of Australia for operation as a National
Facility managed by CSIRO. This work was facili-
tated in part by an associateship grant to JCB by
the Alberta Ingenuity Fund. MH acknowledges
support from the National Radio Astronomy Ob-
servatory, which is operated by Associated Uni-
versities, Inc., under cooperative agreement with
the NSF. BMG and MH acknowledge the support
of the NSF through grant AST-0307358.
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This 2-column preprint was prepared with the AAS LATEX
macros v5.2.
Table 1
Rotation Measures of the SGPS
ℓ a b a α δ I b m c RM
(◦) (◦) (J2000) (J2000) (mJy) (%) (rad m−2)
253.30 0.89 08 19 46.2 -34 42 05 179 5.8 −8 ± 12
253.52 0.83 08 20 08.4 -34 55 20 21 6.4 −50 ± 26
253.68 -0.60 08 14 47.2 -35 51 08 72 3.5 −349 ± 27
254.16 -0.34 08 17 10.3 -36 06 01 64 5.5 −338 ± 19
254.60 -0.87 08 16 12.7 -36 46 05 62 4.3 −15 ± 24
254.81 0.93 08 24 08.9 -35 55 21 87 13.7 +84 ± 13
254.95 0.63 08 23 20.7 -36 11 56 41 4.9 +8 ± 32
255.16 0.24 08 22 21.9 -36 36 03 92 5.3 −35 ± 16
255.27 0.16 08 22 20.2 -36 43 53 84 3.4 +26 ± 23
255.36 -0.26 08 20 52.1 -37 02 48 136 9.0 +97 ± 10
255.36 0.50 08 23 59.5 -36 37 00 94 8.0 −115 ± 13
256.14 0.25 08 25 11.9 -37 24 00 92 5.3 +44 ± 17
256.64 -0.22 08 24 43.7 -38 04 40 104 4.7 +172 ± 15
257.47 0.54 08 30 20.4 -38 18 27 46 9.5 +23 ± 20
257.71 -0.66 08 26 02.2 -39 12 13 117 4.0 +144 ± 20
257.92 0.65 08 32 07.7 -38 36 33 383 2.3 +76 ± 14
258.52 1.02 08 35 30.1 -38 52 23 118 2.7 +196 ± 26
258.77 0.08 08 32 23.4 -39 37 38 131 3.4 +221 ± 16
259.05 -0.72 08 29 49.9 -40 19 35 123 5.7 +175 ± 12
259.05 -0.75 08 29 45.1 -40 20 52 103 10.0 +150 ± 12
259.77 1.22 08 40 15.2 -39 44 39 33 5.8 +250 ± 29
260.41 -0.43 08 35 21.8 -41 14 50 101 3.0 +221 ± 18
260.52 -0.55 08 35 11.4 -41 24 44 25 5.8 +247 ± 31
260.69 -0.23 08 37 05.4 -41 21 08 224 3.2 +204 ± 12
263.20 1.07 08 50 56.2 -42 30 54 165 5.6 +739 ± 14
263.22 1.08 08 51 02.6 -42 31 32 164 4.4 +826 ± 19
263.50 0.17 08 48 12.7 -43 19 14 139 0.9 +260 ± 28
264.24 0.88 08 53 48.3 -43 25 59 112 11.0 +406 ± 9
265.69 0.85 08 58 54.5 -44 33 41 106 1.8 +70 ± 28
266.14 1.08 09 01 33.6 -44 44 49 66 3.2 +211 ± 29
266.27 0.66 09 00 15.2 -45 07 17 95 3.4 +396 ± 18
267.03 0.04 09 00 27.1 -46 06 15 58 3.8 +298 ± 21
267.17 0.47 09 02 50.7 -45 55 36 598 0.3 +323 ± 28
268.62 0.58 09 08 56.5 -46 55 53 231 2.1 +256 ± 15
269.05 0.17 09 08 54.5 -47 31 15 118 3.6 +456 ± 16
269.55 0.45 09 12 09.7 -47 41 35 156 1.9 +137 ± 18
270.56 -0.85 09 10 32.9 -49 19 14 58 5.5 −152 ± 23
270.91 0.93 09 19 52.0 -48 20 10 57 6.2 −149 ± 22
271.22 -0.35 09 15 37.0 -49 27 04 66 2.5 +215 ± 29
271.30 -0.06 09 17 12.3 -49 18 17 83 5.0 +136 ± 16
Table 1—Continued
ℓ a b a α δ I b m c RM
(◦) (◦) (J2000) (J2000) (mJy) (%) (rad m−2)
271.52 -1.01 09 13 54.3 -50 07 34 120 1.7 +75 ± 28
271.70 -0.38 09 17 30.7 -49 49 16 116 3.5 −93 ± 18
272.36 0.62 09 24 45.8 -49 34 44 21 5.6 −296 ± 28
273.46 0.68 09 29 58.1 -50 17 41 18 13.6 −106 ± 19
273.57 1.28 09 33 00.8 -49 55 47 229 1.9 +20 ± 25
274.77 0.25 09 34 14.3 -51 30 18 51 3.9 −76 ± 22
275.02 0.82 09 37 51.7 -51 15 07 26 8.4 −101 ± 25
275.48 -0.68 09 33 32.0 -52 40 18 282 2.8 +248 ± 13
275.56 1.02 09 41 17.5 -51 27 26 281 6.3 +16 ± 7
275.56 -0.20 09 36 03.4 -52 22 03 46 2.9 +112 ± 30
275.83 0.16 09 38 55.3 -52 16 41 170 9.3 −107 ± 7
275.86 0.94 09 42 25.0 -51 42 48 29 11.0 0 ± 21
276.46 0.89 09 45 09.7 -52 08 29 153 2.5 −17 ± 20
277.44 0.86 09 49 58.5 -52 47 04 10 12.1 −71 ± 28
277.78 -0.73 09 44 52.1 -54 13 45 127 2.0 −36 ± 23
277.78 -0.81 09 44 32.3 -54 17 32 690 7.3 −77 ± 3
278.04 0.75 09 52 37.5 -53 15 08 635 7.8 −104 ± 3
278.37 0.15 09 51 47.9 -53 55 46 18 13.7 −4 ± 24
278.43 0.53 09 53 42.8 -53 40 12 42 4.4 +20 ± 27
278.47 -0.30 09 50 22.2 -54 20 21 22 6.6 −116 ± 24
279.04 -0.88 09 50 55.5 -55 09 04 242 2.4 +239 ± 17
279.09 0.89 09 58 46.2 -53 47 21 47 5.0 −120 ± 25
279.15 -0.63 09 52 36.5 -55 01 08 36 11.2 +341 ± 24
279.15 -0.65 09 52 32.6 -55 02 37 44 6.2 +329 ± 20
279.33 0.80 09 59 41.3 -54 00 11 35 7.8 −157 ± 18
279.80 1.21 10 03 53.7 -53 57 24 25 9.7 −145 ± 25
280.53 0.81 10 06 18.9 -54 42 57 37 5.4 −10 ± 23
280.62 -0.14 10 02 53.8 -55 32 09 134 3.7 −112 ± 15
282.07 -0.78 10 08 36.2 -56 54 09 723 0.9 +862 ± 16
282.46 0.24 10 15 09.3 -56 17 28 109 2.0 +256 ± 23
284.30 0.81 10 28 39.3 -56 48 09 80 2.4 −547 ± 25
285.15 0.96 10 34 33.2 -57 06 17 268 0.9 +168 ± 31
285.60 0.62 10 36 12.7 -57 37 38 44 3.3 +368 ± 35
286.04 -1.05 10 32 43.2 -59 17 54 2826 0.4 +809 ± 14
286.89 0.59 10 44 37.5 -58 16 43 58 3.0 +324 ± 28
288.27 -0.70 10 49 34.1 -60 03 21 255 0.6 +491 ± 25
290.81 0.74 11 12 45.4 -59 47 27 1858 0.4 +419 ± 23
292.90 -0.02 11 26 31.6 -61 13 37 52 6.6 +349 ± 28
293.39 0.73 11 32 19.4 -60 40 22 67 5.0 +121 ± 20
293.73 0.63 11 34 43.2 -60 52 06 30 3.4 +116 ± 23
Table 1—Continued
ℓ a b a α δ I b m c RM
(◦) (◦) (J2000) (J2000) (mJy) (%) (rad m−2)
294.29 -0.90 11 35 31.6 -62 29 13 32 4.2 +449 ± 26
294.38 -0.75 11 36 37.4 -62 22 07 265 0.7 +470 ± 24
295.17 0.01 11 44 55.1 -61 51 26 57 3.5 +363 ± 22
295.23 -1.05 11 43 02.3 -62 53 56 114 2.1 −207 ± 22
295.29 -1.23 11 43 05.7 -63 04 55 226 1.6 −43 ± 29
296.18 -0.59 11 52 05.2 -62 40 59 118 3.7 +752 ± 14
296.90 0.14 11 59 31.2 -62 07 15 452 1.6 +1113 ± 11
297.67 0.77 12 06 52.0 -61 38 54 283 2.6 +570 ± 11
299.42 -0.23 12 20 29.8 -62 53 37 55 4.0 +535 ± 30
299.51 -1.10 12 20 24.8 -63 46 10 192 2.9 +315 ± 21
300.25 -0.01 12 27 57.6 -62 45 42 109 4.1 +123 ± 14
300.47 -0.99 12 29 06.5 -63 45 08 112 4.9 +412 ± 18
300.65 -0.41 12 31 12.4 -63 11 40 809 0.9 +358 ± 19
301.14 -0.09 12 35 39.6 -62 54 30 375 1.3 +350 ± 18
301.70 0.25 12 40 45.4 -62 36 01 21 5.2 +296 ± 33
302.60 -1.17 12 48 26.2 -64 02 26 162 1.7 +159 ± 25
303.30 0.51 12 54 36.3 -62 21 25 149 3.5 −370 ± 14
304.53 1.00 13 05 00.1 -61 49 31 32 6.1 +40 ± 28
305.62 -1.16 13 15 52.5 -63 54 09 44 10.5 −61 ± 28
306.87 0.02 13 25 47.5 -62 35 15 215 1.0 −197 ± 27
306.92 -0.70 13 27 00.7 -63 17 46 275 2.0 +52 ± 16
307.20 -0.84 13 29 39.3 -63 23 47 41 8.1 +382 ± 18
308.64 -0.62 13 41 56.1 -62 55 38 40 2.8 −133 ± 27
308.73 0.07 13 41 33.5 -62 14 06 92 1.4 −661 ± 29
308.93 0.40 13 42 41.5 -61 52 38 152 2.5 −752 ± 17
309.06 0.84 13 43 00.9 -61 24 52 670 2.3 −504 ± 10
310.20 -1.04 13 56 09.7 -62 59 30 46 8.5 −584 ± 19
312.37 -0.04 14 11 37.1 -61 25 54 129 2.3 −438 ± 28
313.96 -0.76 14 26 14.9 -61 34 58 88 2.3 −480 ± 22
313.99 0.94 14 21 36.9 -59 59 01 507 1.8 −828 ± 17
314.02 1.01 14 21 40.3 -59 54 20 757 1.7 −579 ± 20
314.50 0.30 14 27 14.8 -60 24 07 86 5.3 −738 ± 19
314.82 0.89 14 27 57.3 -59 43 58 92 3.7 −507 ± 25
316.64 1.15 14 40 15.0 -58 47 30 64 4.6 −525 ± 27
317.54 -0.57 14 52 27.0 -59 58 11 152 2.1 +395 ± 27
318.53 0.30 14 56 18.1 -58 44 29 326 1.0 +53 ± 29
319.34 1.08 14 58 58.6 -57 40 31 1101 0.7 +241 ± 21
319.39 0.74 15 00 28.7 -57 57 04 85 1.4 +279 ± 30
320.83 0.88 15 09 19.7 -57 07 19 454 0.8 −8 ± 20
321.48 1.02 15 12 54.7 -56 40 20 933 2.0 −243 ± 8
Table 1—Continued
ℓ a b a α δ I b m c RM
(◦) (◦) (J2000) (J2000) (mJy) (%) (rad m−2)
321.58 -0.76 15 20 29.3 -58 07 41 1831 1.1 −138 ± 9
322.05 -0.95 15 24 17.5 -58 01 49 78 5.4 −397 ± 17
323.15 -0.52 15 29 19.5 -57 03 49 47 2.9 +83 ± 25
324.77 0.61 15 34 10.5 -55 12 30 161 3.9 −66 ± 14
325.81 1.08 15 38 05.1 -54 13 08 608 0.6 −15 ± 31 d
325.83 -0.30 15 43 57.7 -55 18 26 247 1.8 +356 ± 18
326.69 -1.16 15 52 28.2 -55 27 04 95 9.3 −142 ± 22
327.31 0.88 15 47 01.4 -53 28 29 256 1.2 −189 ± 26 d
328.36 -0.41 15 58 00.3 -53 48 30 493 0.3 −721 ± 35
329.48 0.22 16 00 57.1 -52 36 04 599 0.7 −100 ± 25
330.12 -1.08 16 09 49.5 -53 08 33 218 3.0 −931 ± 25
332.14 1.03 16 10 10.8 -50 13 21 151 2.8 −754 ± 22 d
333.72 -0.27 16 22 55.5 -50 03 31 108 2.5 +204 ± 29
335.32 0.60 16 26 00.4 -48 18 36 352 2.9 −138 ± 11
337.06 0.85 16 32 01.7 -46 52 39 100 2.5 −739 ± 32
339.65 -0.24 16 46 44.6 -45 40 00 222 2.2 −398 ± 19
342.16 -0.74 16 57 52.7 -44 02 49 264 2.2 +127 ± 15
342.62 -0.45 16 58 15.4 -43 30 19 50 3.9 −913 ± 30
343.29 0.60 16 56 01.8 -42 19 58 102 4.7 −1035 ± 16
345.22 0.68 17 02 02.9 -40 45 50 39 12.6 +183 ± 21
347.40 -1.04 17 16 09.8 -40 02 24 72 4.0 −524 ± 36
349.65 -0.36 17 19 58.3 -37 48 15 133 3.3 +110 ± 20
350.52 -0.73 17 23 58.9 -37 18 18 127 2.2 +277 ± 32
351.31 -0.53 17 25 22.9 -36 32 16 200 3.3 −247 ± 15
351.82 0.17 17 23 56.8 -35 43 34 118 6.1 +134 ± 16
352.13 1.15 17 20 51.7 -34 54 49 116 6.5 +76 ± 30
355.43 -0.81 17 37 32.3 -33 14 40 76 4.2 +601 ± 21
356.57 0.87 17 33 45.9 -31 22 37 116 1.6 +985 ± 30
aThe identified location is the peak of the gaussian fit to the source in
polarised intensity. All sources were either unresolved or partially resolved.
b I is the peak-pixel Stokes I value of the interferometric data.
c m is the fractional polarisation (linear polarised intensity divided by
Stokes I) averaged over the FWHM pixels used for RM calculation.
dA RM for this source was calculated by Gaensler et al. (2001) using
a simpler approach than the more rigorous method used here. These new
values should replace the previously determined values.
Norma
Scutum-Crux
Sagittarius-
Carina
Perseus
local
molecular
l=253o
l=358o
Fig. 1.— View of the Galaxy from above. The grey scale is the CL02 electron density model, with labels
indicating the spiral arms, and the asterisk indicating the location of the Sun. Quadrants 1, 2, 3, and 4 are
labeled with Q1, Q2, Q3 and Q4, respectively. The bounding longitudes of the SGPS (Phase I) are indicated
by black lines and are labeled. The circles represent the smoothed-averaged SGPS RMs as shown in the
middle panel of Figure 3. Filled (open) circles indicate positive (negative) RM, with the size of the circles
linearly proportional to |RM|, truncated between 59 and 592 rad m−2 (the |RM|max from the middle panel of
Figure 3) so that sources with |RM| < 59 rad m−2 are set to 59 rad m−2. The blue dashed (dotted) lines are
also from the middle panel of Figure 3, and indicate the approximate longitudes of |RM| maxima (minima)
in the SGPS RM data. The solid blue line is the longitude where the RMs transition from primarily positive
to primarily negative (ℓ ∼ 304◦).
      
350 330 310 304 290 270 250
PULSARS
Galactic Longitude
Fig. 2.— Rotation measures for sources in the SGPS. Top panel: extragalactic RM sources; Bottom panel:
pulsar RM sources. Grey filled symbols indicate positive rotation measure, black open symbols indicate
negative rotation measures; sizes of symbols are linearly proportional to the magnitude of RM truncated
between 100-600 rad m−2, so that sources with |RM| < 100 rad m−2 are set to 100 rad m−2, and those with
|RM| > 600 rad m−2 are set to 600 rad m−2. In the top panel, the square at ℓ = 307.1◦, b= 1.2◦ represents
the EGS RM previously observed by Broten et al. (1988), while the squares at ℓ ∼ 330◦ represent the EGS
RMs of the SGPS test region. The circles in the top panel represent the SGPS EGS as given in Table 1.
The vertical line at ℓ = 304◦ is the approximate longitude where the EGS RMs change sign. The dashed
boxes indicate boundaries of the SGPS region.
             
-1200
   SGPS EGS
(raw, binned)
             
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SGPS EGS
(smoothed)
360 350 340 330 320 310  300 290 280 270 260 250
-1200
PULSARS
  (raw) 
Galactic Longitude
Fig. 3.— RM versus Galactic longitude for RM sources in the SGPS region. Top panel: circles represent the
individual SGPS EGS sources (errors are smaller than the symbol size), while open red diamonds represent
data averaged into into independent longitude bins containing 9 sources (the end bin at 255◦ contains 13
sources). Where symbol size permits, the error bars are the standard deviation in longitude and RM for each
bin. Middle panel: purple diamonds represent boxcar-averaged SGPS EGS data over 9 degrees in longitude
with a stepsize of 3 degrees. In contrast to the binned data in the top panel, the error bars are the standard
error of the mean. The solid line marks the approximate longitude of transition from predominantly positive
RMs to negative RMs (l ∼ 304◦) Dotted lines (dashed lines) mark approximate longitudes of minimum
(maximum) |RM| in SGPS data. Bottom panel: squares represent the individual pulsars with known RM in
the SGPS region (errors are smaller than the symbol sizes).
Fig. 4.— A simple model of the magnetic field in the SGPS region, constrained using individual RMs of
149 EGS and 120 pulsars. Colored regions indicate the total strength for the model fit of magnetic field
corresponding to the regions identified by the green lines as discussed in the text. Red (blue) shading indicates
a clockwise (counter-clockwise) field direction as viewed from the Galactic north pole. The direction of the
field within the arms is also indicated by arrows. The open-head arrow on the Norma arm indicates the field
direction in this arm is not well-constrained in this model configuration with the data used. The bounding
longitudes of the SGPS (Phase I) are indicated by black lines and are labeled. The grey scale is the CL02
electron density model with labels indicating the spiral arms.
            
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SGPS EGS
(raw)
Model
            
-1200
SGPS EGS
(smoothed)
Model
360 350 340 330 320 310 300 290 280 270 260 250
-1200
Pulsars (raw)
Model
Galactic Longitude
Fig. 5.— Comparison of modeled RMs (green symbols) and observed RMs. The modeled RMs are calculated
using the CL02 electron density model and the magnetic field model shown in Figure 4. The format of this
figure follows that of Figure 3, where the individual SGPS EGS are shown in the top panel, smoothed-
averaged SGPS EGS data are shown in the middle panel, and the individual pulsars are presented in the
bottom panel.
	Introduction
	Rotation Measure Calculations
	Observations of the Galactic Magnetic Field
	Modeling the Large-Scale Magnetic Field
	Summary
