2. Radio continuum observations

The NPS is seen as the most prominent feature of the high latitude distribution of radio continuum emission. This radio continuum spur (RCNPS) has been mapped by large area surveys at many frequencies between 2.2 and 1420 MHz and shown by its nonthermal spectrum to be radiating by the synchrotron process. This con­clusion is well confirmed by the extremely high precentages of linear polarization found for the radiation at the higher frequencies. As observations of the total intensity and linear polar zation of the continuum emission provide us with infor­mation on essentially different physical characteristics of the source we will deal with them separately.

1. Total intensity

Although the bright arc of the RCNPS was such an intense feature on early conti­nuum surveys, where, and if, any extension of the arc was to be found was not at all clear. A number of authors (Large et al. 1962; Davies 1964; Guidice 1971) connected the feature with the fainter emission arc near (300°, 65°)¹, now usually considered to be part of Loop IV. However, using a stereographic projection, Large et al. (1966) noted that the RCNPS followed a small circle in the sky to excellent approximation. They demonstrated that a weak continuum ridge, lying along / zz 268° and of some 25° length, fitted well with the predicted path of this small circle, despite some divergence from the circle below b = 25°. Although any emission linking the two ridges is below the sensitivities of existing surveys and the proposed connection is purely geometrical, their association has been widely accepted. The hypothetical circular object in the sky, of which these ridges form the observable radio segments, has become known as Loop I. It should be stressed that the exact geometry, and even reality, of such a feature is not a complete certainty. In particular, evidence for the continuation of Loop I below the Galactic plane is at best fragmentary. On the other hand, observations described below of the Л 21cm neutral hydrogen line, and of soft x-rays, strongly support the NPS being part of a larger coherent structure. Berkhuijsen et al. (1971) fitted a small circle to the radio ridge peaks and derived a centre for the Loop I feature of (329°.0 ± 1°.5, 17°.5 it 3°.0) and a diameter of 116° ± 4°. Over the 155° arc of the circle where data was measurable, the r.m.s. deviation of the observed ridge from the fitted circle was 0°.9.

The ridge through (300°, 65°), originally considered to be the extension of the RCNPS, was shown by Large et al. (1966) to form part of a semicircular feature in its own right. Berkhuijsen et al. (1971) named this Loop IV. The large, semicircular features, Loop II (The Cetus Arc) and Loop III, are also discussed by Berkhuijsen et al. These authors fitted a small circle to Loop IV but from a recent 1420 MHz survey of part of this loop, Reich & Steffen (1981) suggested that the continuum ridge deviated^ significantly from the Berkhuijsen et al. fit. The present author has refitted the circular geometry of Loop IV using the recent 408 MHz maps of Haslam et al. (1982). He finds a centre of (315^e.75 ±3°.0, 48°.25 ± 1°.0) with a diameter of 39°.75 ± 2°.0 and an r.m.s. deviation of only 0°.6. This solution is within the quoted errors of the earlier work.

Apart from Loop IV, Large et al. (1966) catalogued a number of other continuum ridges within Loop I. These were confirmed and added to by a number of authors (Merkelijn & Davis 1967; Holden 1969; Berkhuijsen 1971; Haslam et al. 1981). Large et al. found that, with the notable exception of Loop IV, most of the ridges were roughly concentric with Loop I. Berkhuijsen (1971) came to a similar general conclusion but noted that a few ridges in her extensive collection of these features deviated markedly from the rule. She also pointed out that a number of ridges apparently associated with Loop I, including some side branches of the main RCNPS ridge, were situated outside the small circle defining the loop. This obser­vation, together with the linear polarization studies of the same author and the Hi observations (see sections 2.2 and 3), indicates that the zone of influence of Loop I is even more extensive than suggested by the RCNPS. Holden (1969) had already concluded that emission outside the main ridge of the RCNPS at lower latitudes appeared to be an extension of the object. Observing at 178 MHz with the rela­tively high resolution of 20 arcmin, Holden detected abrupt steps in the edge gradients of both the RCNPS and the internal and external ridges. Across a typical step, the brightness temperature increased abruptly by some 100 K. Despite their small angular width, being often unresolved to the 178 MHz beam, the steps had typical lengths of 6°. For a distance to such a step of lOOpc, the linear projected size across the step would be ^ 0.4pc, whereas its projected length would be « 12pc.

The main ridge of the RCNPS possesses a number of distinctive features. Davies (1964) showed that its outer edge has a considerably steeper gradient than the inner edge and further suggested that the brightness temperatures within the loop are higher than those outside. He concluded that both these results are consistent with the physical object giving rise to Loop I having a shell structure. On this assumption he computed that such a shell would have a thickness of about 7-11% of the radius. Guidice (1971) derived a typical radial profile across the RCNPS (figure 3) which agreed with both the suggestions of Davies. He fitted a shell with a thickness of 13.5% of its radius to the profile, noting that this thickness was probably overestimated by 15-25% owing to beam broading. Haslam et al. (1964) discovered an intriguing feature of the RCNPS from their 240 MHz observations at lower latitudes. The main ridge of the Spur appeared to display a narrow neck atll°^fe<16° with the width of the ridge increasing above these latitudes. Although Holden (1969) concluded that the NPS did not extend below b « II^е, Berkhuijsen et al. (1971) traced the main ridge to below b zz 8° and could follow the bright inner ridge passing through (22°, 14°) to b zz 4°. Recently, Sofue & Reich (1979) published 1420 MHz maps of the RCNPS at low latitudes having the highest resolution (H.P.B.W. = 10 arcmin) yet available for the object. These authors believe they can follow the RCNPS from (33°, 20°) right down to (22°, 3°). For the narrow neck they find a typical width of 0°.8, with no evidence for a particular narrowing of the ridge in the neck, although the ridge does split into several subridges for b > 11°. They do, however, find the ridge to be narrower at the lowest latitudes and the narrowest width of the RCNPS seems to increase quasilinearly with latitude over the entire range 3° < b < 20°. The narrow, low latitude extension between (24°, 6°) and (22°, 3°) seems to be a genuine extension

L oop I (i the North Polar Spur)

I 1 I I I i

6CP 80° 40° 30° 20° 10°

ANGLE FROM CENTRE OF SPUR

Figure 3. An experimentally determined, radial, radio continuum profile of NPS through the position RA = 14^h, dec = 20°. Note that RA = 15^b10^m, dec = —17°, (345°, 34°) was taken as the Loop I centre in the preparation of the profile. (Guidice 1971).

of the main ridge, with a width of about 0°.3. No further extension into the plane or at southern latitudes is found.

Since the days of the earliest surveys there has been little doubt that the RCNPS radiates predominatly by the sychrotron mechanism. This is demonstrated both by its steep radio spectrum above 100 MHz and the high percentage polarization found for the emission above 400 MHz. Neverthless, the exact form of the radio continuum spectrum is still not well defined, with the possibility existing of a flatten­ing, or even turnover, in the lpw frequency spectrum. In addition, it has sometimes been suggested that the spectral properties of the object may be partly shaped by the presence of ionized material either within the object or in the foreground.

At decimetre wavelengths, the most detailed study of the NPS spectrum has been made by Berkhuijsen (1971) who used the technique of T—T plots to derive spectral indices (S « v“*) between 240 and 820 MHz. The values of a she obtained had typical absolute errors of 0.25, although the random internal errors were much smaller. For the latitude range 17° < b < 43°, Berkhuijsen found 0.55 < a < 0.72, with the lowest values at the lowest latitudes. She noted that a similar trend was present in

the spectral index maps made between 85 and 150 MHz by Landecker (1969). From her T—T plots Berkhuijsen concluded that (i) for constant brightness temperatures at 240 MHz, the 820 MHz temperatures inside the main ridge of the NPS were higher than those outside; (ii) the slope of the T~T plots was the same inside and outside the NPS; (iii) the temperature difference between the T — T segments inside and outside the NPS was zz 0.4К at 820 MHz; and (iv) the temperature increase set in over the inner gradient of the NPS near 1=20°, having its maximum value on top of the inner ridges. To explain these effects she proposed the presence of a smooth, low spectral index component between 20° < l ^ 30° over the entire latitude range considered. She believed that this component consisted of ionized hydrogen with an average emission measure of about 70cnr~^epc, and a maximum emission measure of 170 cm~⁶ pc at b = 27° where a low value of a was found.

At low frequencies the situation is rather less clear, with the early studies often producing mutually contradictory results. Davies (1964) obtained a = 0.55 ± 0.08 between 38 and 178 MHz. Bridle (1967) suggested that the NPS had a lower spect­ral index between 17.5 and 81.5 MHz than the surrounding regions, and computed a = 0.1 ± 0.2. Baldwin (1967) reported that the Penticton 10 MHz observations of Purton showed absorption patches along the NPS ridge and Berkhuijsen (1971) identified these with her proposed Hu component described above. More recent measurements seem to clarify the situation somewhat, and very low values of spectral index at decametric wavelengths no\v seem less likely. First, Guidice (1971) concluded from four frequency observations that the spectral index between 20 and 40 MHz is a = 0.5 ± 0.15. Caswell (1976) produced an updated Penticton 10 MHz map of the northern sky which included the section of the NPS earlier than right ascension 15^h 30^m. No absorption patches are apparent and the author concludes that from 10 MHz to 38 or 178 MHz the spectrum of the RCNPS is at least as steep as, perhaps steeper than, the surrounding Galactic emission. He points out that Bridle obtained his value of a after subtraction of a steeper than average underlying background. Finally, Novaco & Brown (1978) claim that the RCNPS can be seen in emission on their low resolution observations made between 2.2 and

2. MHz. They find that at 1.3 MHz the feature is no longer visible and this they attribute to absorption in the intervening interstellar medium.

Spectral information concerning Loop IV is scarce, though Reich & Steffen (1981) note that Loop IV has a nonthermal spectrum.

2. Linear polarization

Linear polarization of the radio emission from the RCNPS can be clearly seen from observations covering the frequency range 408-1415 MHz. Typical of these studies are the extensive series of observations made during the 1960s using the 25m Dwin- geloo telescope at a number of frequencies and recently produced as an atlas of maps by Brouw & Spoelstra (1976). The 1415 MHz map from this compendium is shown in figure 4 and demonstrates the high polarization over the NPS.

A noteworthy early study of polarization data was made by Bingham (1967) who combined his own 1407 MHz data, covering virtually the whole RCNPS, with already existing lower frequency Dutch results. Subsequently, the full Dutch obser­vations were interpreted in a series of papers (Berkhuijsen 1971; Spoelstra 1971, 1972).

B

c

о

?70

Figure 4. Distribution of linear polarization at 1411 MHz. The length of the polarization vectors are proportional to the intensity of the polarized radiation, whereas the direction of the vectors gives the polarization angle (i.e. direction of the electric field component). The circles indicate the, presence of point sources which may have influenced the results. (Brouw & Spoelstra 1976).

ingham presented a map of rotation measure (RM) over the NPS for b ^ 40°. He found that the sign of RM was mostly positive with its magnitude increasing towards lower latitudes. Towards the north Galactic pole, the RMs were very small and alternated in sign suggesting that the magnetic field traversed by the radiation was essentially perpendicular to the line of sight (i.e. parallel to the Galactic plane). He took the existence of extensive polarization on the 408 MHz Dutch maps to mean that most of the rotation is occurring between the NPS and the sun, rather than within the source which would give large front-back depolarization at this frequency. Spoelstra presented RM maps separately for the low and high latitude sections of the NPS in his two publications. At b > 50° his derived values of RM are rather higher than those of Bingham, with an average value of ^ 4 rad m^-2. Below b = 50° the rotation measures are generally ^ 14 rad m^-2, the

higher values being found where the projected magnetic field direction becomes perpendicular to the NPS (see below). Spoelstra concluded that for generally accepted values of the interstellar magnetic field of l-5juG and an electron density of 0.06 cm"³, the RMs are consistent with a distance of 50-100pc. This strongly suggests that the NPS is a relatively local object.

Using the values of RM to derive intrinsic polarization vector directions, Bingham found that above b s 40° the magnetic field in the emission regions is essentially parallel to the NPS for at least 15° on either side of the ridge peak. For high latitudes Spoelstra arrived at a similar conculsion, but below b zz 45° he revealed a very different situation. Within an area 30° < / < 35°, 26° < b < 40° the intrinsic polarization position angles imply a magnetic field that is essentially perpendicular to the NPS. This is also so for positions on the internal ridges running through (24°, 27°) and (24°, 35°). Perhaps significantly, this area of the RCNPS is where the radio continuum structure appears to show a superposed semicircular feature on, and inside, the main ridge (Haslam, Salter & Sieber, personal communication). This circular structure (figure 5) can be followed over

Figure 5. The semicircular radio continuum feature superposed on the main ridge of the NPS. The contours are from the 408 MHz all-sky survey of Haslam et al. (1982) and are labelled in K. Arrows on contour lines point clockwise around minima in the brightness distribution and anticlock­wise around maxima. (Haslam, Salter & Sieber, personal communication).

200° of a circle centred on (25°.8 ± 0°.6, 33°.3 ± 0°.6) and of diameter 13°.5 ± 0.8. Its ridge peak deviates from this best-fit small circle by some 0°.35. Spoelstra showed that above b=45° the polarization vectors swing round from parallel to perpendicular to the NPS ridge. This effect sets in at lower latitudes just outside the spur ridge.

To estimate the percentage polarization of the RCNPS emission Bingham plotted

his polarization temperatures against the total power temperatures Tb at each

point. At high latitudes, where the RM’s are small, he found a strong correlation between the two quantities. The slope of the best-fit straight line to the plot suggest­ed a uniform degree of polarization on the RCNPS of some 50-60% when observed with a resolution of 2°. Such a high value implies that the magnetic field within the object is well aligned in depth. Berkhuijsen also investigated the percen­tage polarization of the RCNPS radiation using plots of T* against Tb at a given latitude. From these plots she arrived at the following conclusions :

( i ) Below b ^ 30° no relationship existed between T_B and Tb.

2. For b > 30° straight lines fitted the plots well, giving a polarization percen­tage that increased with increasing latitude.

3. For 50° < b < 70° the polarization percentage at 1411 MHz was about 74%, roughly the maximum possible value for synchrotron emission. She believed that the somewhat lower values found by Bingham were caused by depolarization due to the larger beamwidth of the Cambridge observations.

4. The polarization percentage at 820 MHz was invariably lower than at 1411 MHz. This she ascribed to a combination of the depolarizing effect of the greater Faraday rotation, and the larger telescope beamwidth, at the lower frequency.

( v) She believed the polarization percentages to be somewhat lower on the RCNPS ridge peak which she noted would be consistent with Faraday depolarization in the H н layer she had proposed to be present there (see section 2.1).

(vi) From the Tb values at the T_B - 0 intercept of the best-fit straight lines, she

concluded that an essentially uripolarized background of rather uniform intensity underlies the RCNPS emission at all latitudes considered.

Using a similar technique, Spoelstra confirmed that the polarization percentages were close to the theoretical maximum for 65° < b < 75°, demonstrating again the extreme regularity of the field. Below b = 50° he still found regions with polari­zation percentages greater than, or equal to, 75% of the theoretical maximum. He noted that the highest percentages there are found at b = 35° and b = 50°. He estimated that in these regions of the NPS the ratio of the strength of the ordered magnetic field component perpendicular to the line of sight to that of the random field component is about 2.8. However, he agreed with Berkhuijsen that near the b = 20° lower latitude limit of his survey, there is no linear polarization markedly greater than that of the general Galactic background. Interestingly, at no point did Spoelstra find any positional shift between the maximum of ^fhe total power and polarized intensities.

In the latitude range 40° < b < 55°, Bekhuijsen found that the high polarized intensities at 1411 MHz continued far outside the RCNPS to about / = 60°. From the position angles of the polarization vectors she suggested that this effect was related to Loop I. This, together with the existence of total power ridges and steps outside the RCNPS (see section 2.1), led her to conclude that the influence of Loop I can be seen considerably beyond the main ridge of the RCNPS. Spoelstra pointed out that the radiation from the ridges inside Loop I is highly polarized. Where ridges meet he found lower polarization percentages, presumably due to the vector­summing of components of different position angle within his beam. Spoelstra could not detect any direct continuation of the NPS beyond / ss 320° in the pola­rized intensity distribution. Finally, in no part of the NPS could he find any • correlation between either the polarized intensity or RM distributions and that of the surface density of neutral hydrogen (see section 3).

Spoelstra (1973) noted that at 1415 MHz no polarization was evident for the continuum emission from the section of Loop IV above b ^ 50°. This could be taken to imply that Loop IV is farther from the sun than the NPS and hence suffers greater interstellar depolarization.

2. Evidence from small diameter sources .

Small diameter radio sources have been used in a number of attempts to cast light on features of the NPS. The first suggested correlation came when Edge et al. (1959) reported an excess of sources of flux density > 8 Jy at 159 MHz in the region of the NPS. Foster (1961) derived a similar result for sources above 6 Jy at 178 MHz. Holden (1969), however, examined the distribution of source down to 2Jy at 178 MHz and found no significant excess. The same conclusion was reached by Sofue & Reich (1979) for the distribution of sources on the NPS down to 0.25 Jy at 1420 MHz. It seems that the existence of a statistically significant excess of small diameter sources connected with the NPS is not tenable on the present evidence.

An interesting possibility was raised by Shapirovskaya (1978) who noticed that the fifteen then-known, low frequency, variable radio sources all lie in the directions of cotitinuum loops, spurs or ridges. She believed this to be statistically significant and derived a "scale size of 10¹³ cm for irregularities in the loops which she postulated to be causing the variability through scintillation. However, Fanti et al. (1982) have since pointed out that her conclusions on the sky distribution of low frequency variables are no longer supported by the distribution of the present increased number of known, or suspected, variables.

A somewhat different effect has been suggested by Rickard & Cronyn (1979). These authors reexamined the Cambridge 81.5 MHz interplanetary scintillation results, dividing the sky into boxes of about 15° x 15° and comparing the number of nonscintillators in each box with that expected. Away from the Galactic plane only three boxes possessed an excess of nonscintillators at the > 2c level. These three boxes were contiguous, stretching from b = 10° to 65° at / ^ 60°. The probability that the only three boxes should be contiguous by chance is put at j^O.05% The authors note that the line of boxes is parallel to the NPS and suggest that the lack of scintillators could be due to scattering in an outer turbulence shell of the NPS. As the longitude of such a feature would be some 90° away from the centre of Loop I, the sun should be located close to, if not actually inside, the turbulence shell itself.

The distribution of rotation measures (RM) of polarized extragalactic radio sources is believed to show the influence of Loop I. Gardner et al. (1969) and Vallee & Kronberg (1975) invoked the NPS to explain the anomolous zone of RM north of the Galactic plane. This effect is highlighted by Simard-Normandin & Kronberg (1980) who point out that whereas the RMs at negative latitudes are mostly negative for 0° < / < 180° and positive for 180° < / < 360°, at positive latitudes they are positive for 0° < l < 90°, with the sign alternating in successive quadrants. To explain this they propose that Loop I is perturbing the local magnetic field at positive latitudes in the first and fourth quadrants. Inoue & Tabara (1982) also find the same anomolous zone of positive RM near / = 40° for 30° < b < 60° and similarly conclude that it is due to the NPS.