The Heliosphere and the Solar Cycle
The long accepted view of the shape of the heliosphere (HS) is that it is a comet-like object with a long tail opposite to the direction in which the solar system moves through the local interstellar medium (LISM). However, in the paper by Parker (The Stellar Wind Regions, 1961) we find from figure 1 (in the original paper) that the comet-like shape occurs under certain conditions namely: ‘The streamlines of the subsonic, nearly incompressible, hydrodynamic flow of a stellar wind beyond the shock transition (r=R) in the presence of a subsonic interstellar wind
carrying no significant magnetic field.’ Indeed, Parker described the special condition under which a comet-like shaped HS would occur:
‘Steady subsonic interstellar wind without interstellar magnetic field’. (1) (my emphasis).
Parker, however, did model a HS constrained by a: ‘Large scale interstellar field in the absence of significant interstellar gas pressure and interstellar wind’, only this HS is not comet-like.
So, the accepted model of the HS is one based on the assumption that there is no significant LISMF. But we now know that there must exist a significant magnetic field, coupled with a slower relative inflow motion we can ask- would a comet-like HS form at all?
In this paper: ‘Imaging the Interaction of the Heliosphere with the Interstellar Medium from Saturn with Cassini’ by S. M. Krimigis, et al. 2009, the authors suggest, following a review of data from the Cassini spacecraft and based on the morphology of the ‘Cassini Belt’, that the HS is indeed shaped as Parker described, if it were influenced by a: ‘
Large scale interstellar field in the absence of significant interstellar gas pressure and interstellar wind’ and not the conventional comet-like shape, the authors admit that: ‘It is very different from the contemporary paradigm.’ (2)
To my mind a picture is emerging that is at odds with the mainstream view of the HS.
Standard interpretation of the alignment of the Heliosphere
Looking at an all-sky map we find that the ‘nose’ of the comet-like HS lies at a point between the constellations Ophiuchus and Scorpius, in the general direction of the galactic centre (0 degrees, galactic longitude). On the same map we find
Voyager 1also lies within the constellation Ophiuchus whilst
Voyager 2 can be found in the constellation Telescopium. Viewed from Earth both Voyager spacecraft are headed ‘upwind’ in the ‘nose’ of the HS (the HS ‘tail’ can be found ‘downwind’ in the constellation of Taurus, 180 degrees galactic longitude).
Viewed from Earth
Voyager 2 is remarkably close to the structure known as the IBEX Ribbon. Is it possible that one or both of the Voyager spacecraft have measured conditions in the outer HS influenced by the IBEX Ribbon? Furthermore, if the ‘comet-like’ HS model is wrong, as observations of the Cassini Belt suggest, where would that leave the Voyager spacecraft and what are they measuring?
From the alternative model i.e. a ‘diamagnetic bubble’ suggested by Krimigis, et al. and for the purpose of this discussion I will use the term ‘Heliotube’ (HT) to describe the nature of the environment from which the Sun draws energy to sustain the solar discharge.
- Galactic Coordinates looking toward 270 degrees longitude. For illustrative purposes only, not to scale.
- Galactic Coordinates looking toward 0 degrees longitude. For illustrative purposes only, not to scale.
From the illustrations above it can be seen that the ‘X’ of the pinched HT approximately matches the latitudinal extent of both the Heliospheric Current Sheet (HCS) and Cassini Belt.
Voyager 1 and Voyager 2
Voyager 1 encountered the Termination Shock (TS) at a distance of 94 AU in December 2004, however, between July 2007 and April 2010 the radial component of the solar wind speed observed by
Voyager 1 in the heliosheath decreased linearly from 60 km/s to 0 km/s at a rate (−18.8 ± 1.5) km/s/yr as
Voyager 1 moved from 103.9 AU to 113.2 AU. The radial velocity remained near 0 km/s from April 2010 to February 2011 as
Voyager 1 moved from 113.2 AU to 118.0 AU, this region was labelled the ‘stagnation region’ by mission scientists.
Although the radial velocity had fallen to zero (in some cases the flow actually took on negative values)
Voyager 1 found that the azimuthal component had grown- taking on values comparable to solar wind radial velocities (26 km/s at the distance of
Voyager 1).Puzzlingly, the magnetic field strength recorded by
Voyager 1 did not increase as mission scientists had expected.
Voyager 2 encountered the TS at a distance of 84 AU in August 2007 and is now (2016) at a distance of 112 AU. Since encountering the TS
Voyager 2 has not experienced a deceleration region or stagnation region of the solar wind, the flow has remained remarkably constant averaging 150 km/s. Voyager mission scientists admit that the ‘observations of steady speeds are not predicted by models and are not understood’.
Why such a marked difference if both spacecraft are immersed in an HS shaped by a ‘subsonic interstellar wind’?
Slow Solar Wind and the IBEX Ribbon
The origin of the IBEX Ribbon is considered to lie outside of the HS at approximately 140 AU, from mainstream papers we find that the ‘…
ribbon is possibly consistent with an external source of particles (Heerikhuisen et al. 2010; Chalov et al. 2010), that lies beyond the heliopause, and “sits” in a “background” of ENA (energetic neutral atoms) emissions, referred to as globally distributed flux (GDF), that is created by the CE (charge exchange) interactions from the inner heliosheath (Schwadron et al. 2009; McComas et al. 2009; Funsten et al. 2009; Fuselier et al. 2009).’, and ‘The ribbon flux is notably superimposed on a slowly varying ENA flux that is referred to as the globally distributed flux (GDF) and is likely a separate emission population.’ (3)
Observations by
IBEX show that the ribbon appears narrowest at energies of 0.7 and 1.1 keV, which are energies associated with characteristic Slow Solar Wind (SSW) speeds of 370 and 460 km/s, respectively. What is the SSW doing at 140 AU?
Of course, the SSW as we understand it, within the HS, is probably not really present at 140 AU but I have suggested that the SSW is slow as it represents the interaction of an incoming electron current with an outgoing electron deficient current- the origin of this interaction now appears to extend at least to the IBEX Ribbon itself.
At higher energies the IBEX Ribbon, which is approximately aligned with the ecliptic equator, broadens and eventually merges with the GDF. At higher energies the GDF is approximately aligned with the galactic equator and eventually forms the Cassini Belt.
The Cassini Belt and IBEX Ribbon
From its vantage point orbiting the planet Saturn the
Cassini spacecraft discovered a belt of ENA emissions in the outer HS. The emissions were found to be organised on galactic coordinates, ‘
Contrary to theoretical expectations (Axford 1973), the ENA emissions are moderately well organized in Galactic coordinates, following roughly the Galactic equator with a 30 degree tilt in latitude, rather than in ecliptic coordinates. In principle, the heliosheath is dominated/ formed by the influence of the shocked solar wind from inside the heliopause. On the other hand, the apparent symmetry in Galactic coordinates point toward an external source of influence in the formation of the ENAs inside the heliosheath, possibly lying beyond the heliopause.’
At certain energies emissions from the IBEX Ribbon and Cassin Belt are similar: ‘
As a result, the qualitative similarities between the IBEX intensities in the highest TOF channel (E = 4.29 keV) and INCA (E > 5 keV) are striking, almost identical in both the formed structure (position in the sky, width, etc.) and ENA intensity range. Whether this is viewed as the evolution of one structure from the ribbon shape into the belt shape as a function of energy or as two separate structures that overlap in energy (and could in fact originate at different radial distances along the LOS) remains a matter for further inquiry.’ (3)
The energy levels of ENAs associated with the Cassini Belt are typical of the Fast Solar Wind (FSW). We now see both types of solar wind reflected in ENA emissions from the outer HS and perhaps beyond. The IBEX Ribbon reflects the SSW and is approximately centred on the ecliptic equator, the Cassini Belt reflects the FSW and is approximately centred on the galactic equator.
Given the currently accepted distances of the IBEX Ribbon and Cassini Belt it is difficult to see how they could be one structure, rather the energies concerned indicate ENAs associated with the Fast Solar Wind (FSW) form a torus in the heliosheath organised on galactic coordinates. That is not to say that the two structures are unrelated their relationship is clearly a dynamical one,
‘…such as the bimodal nature of the solar wind, i.e., the co-existence of a fast and slow solar wind, that is reflected in a hardening of the IBEX spectra at >1 keV toward the ecliptic poles. However, the GDF spectra can generally be described by a single power-law function (with the exception of the high northern and southern latitudes; e.g., Dayeh et al. 2011), a fact that provides further evidence that the GDF and the ribbon are distinct features that originate from different source plasma populations (heliosheath and outside the heliopause, respectively).’ This evidence points ‘…toward an external source of influence in the formation of the ENAs inside the heliosheath (i.e. Cassini Belt), possibly lying beyond the heliopause (i.e. IBEX Ribbon).’ (3), moreover, both structures evolve over the solar cycle.
Solar Cycle 23 and 24
Both the
IBEX and
Cassini spacecraft observed the outer HS during the descending phase of solar cycle 23 (SC23) through to Solar Minimum, while the new solar cycle (SC24) began in 2010.
Cassini conducted observations of this region in 2003, shortly after Solar Maximum. I have previously described the changes that occurred in the Cassini Belt over this period. By the time of the first all sky map from
IBEX SC23 was approaching Solar Minimum. What changes in the outer HS did
IBEX record?
IBEX recorded changes as the solar cycle progressed from minimum to maximum conditions; at Solar Minimum (2009) we find concentrated high emissions from the IBEX Ribbon, as if current was arriving from a concentrated HT ‘pinch’. As the solar cycle progressed to Solar Maximum (2013) emissions from the IBEX Ribbon steadily fell but not in all areas of the Ribbon;
‘…From 2009 to 2012, ENA fluxes are declining at all energies in both the northern and southern portions of the Ribbon. However, in 2013, while the northern Ribbon ENAs continue to show declining fluxes, especially for the highest two energies, the southern Ribbon ENA fluxes appear to have flattened out or even slightly recovered. Thus, the evolution of the Ribbon fluxes in the two hemispheres has become quite different in 2013. We note that this difference cannot be simply explained by the difference in the survival probability corrections, which diverge between the north and south in 2012–2013 as these corrections are only a few percent different, especially at the higher energies were the Ribbon flux evolution becomes so different.
‘Perhaps this divergence can be explained by significant differences in the distance to the Ribbon source regions in the north and south, especially at higher latitudes/energies. The southern portion of the Ribbon is almost certainly closer to the Sun than the northern portion for several reasons: (1) the Ribbon does not extend to as high latitudes in the south as in the north, (2) the southern portions of the Ribbon are largely on the upwind, and thus compressed, side of the heliosphere, while the northern portions wrap back around the north pole on its downwind side, and (3) the heliosphere is compressed in the south compared to the north, owing to the inclination of the strong external field surrounding the heliosphere (McComas et al. 2009c; Schwadron et al. 2009; Opher et al. 2009).’ (4)
At higher energy levels we also see a reduction in ENA emissions, at Solar Maximum the FSW has all but disappeared in the inner HS and this reduction is reflected in emissions associated with the FSW detected by
IBEX.
The GDF and Cassini Belt change over the solar cycle
‘…our current understanding is that the GDF evolves with increasing ENA energy to form the identified belt at high energies. The belt possibly corresponds to a “reservoir” of particles that exist within the heliosheath, moving in a great circle along the nose to tail direction, passing through the ecliptic poles, constantly replenished by new particles from the solar wind.’ (3) In galactic coordinates the ‘reservoir’ of particles would form a torus- the Cassini Belt- at the inner edge of the outer HS ‘moving in a great circle’ in direct response to changes in the strength of emissions in the IBEX Ribbon.
Over SC23 and SC24 IBEX Ribbon intensities fell asymmetrically, researchers attribute this to the compression of the HS- but what if we are actually witnessing changing intensities of a Birkeland Current? In a pinched HT we would expect to see features associated with twisting currents, did IBEX reveal such features and could they have been misinterpreted by researchers based on preconceived ideas?
Does the Heliotail Exist?
A comet-like HS was expected to have ‘nose’ and ‘tail’ features but observations of the Cassini Belt show no asymmetry between the ‘nose’ and the ‘tail’ regions. This led to the suggestion of a ‘tailless’ HS by Krimigis et al. (2009), which explained the Cassini INCA ENA measurements using a modified ‘diamagnetic bubble’ concept that, as we have already seen, was proposed by Parker (1961).
Despite the evidence provided by
Cassini, researchers continue to discuss the structure of the HS as if it is actually comet-like. All-sky maps produced by IBEX continue to be labelled as the ‘heliotail’ by researchers simply because it is viewed as being ‘downwind’ of the heliospheric ‘nose’; but observations of this region reveal emissions that are generally no different to other regions of the HS and as with the IBEX Ribbon the ‘heliotail’ structure is ordered on ecliptic coordinates.
‘
Unlike the IBEX Ribbon, which may well come from ENA emissions beyond the heliopause (e.g., from a “secondary ENA source; McComas et al. 2009b; Heerikhuisen et al. 2010; Chalov et al. 2010; Schwadron & McComas 2013), emissions from the heliotail are almost certainly coming from the region beyond the termination shock, but still inside the heliopause, just as they do for the rest of the globally distributed flux (McComas et al. 2009b; Schwadron et al. 2011).’ (5)
Researchers have also identified two ‘lobes’ in the ‘heliotail’. Furthermore,
IBEX observations demonstrate ‘
the twisting or tilting of the two lobed slow solar wind plasma sheet toward the direction of the external magnetic field. This tilting shows that even within a relatively short heliotail observed over several hundreds of AU with ENAs, the effect of the magnetic tension force, T, of the external magnetic field is strong enough to start squeezing the tail and rotating it toward alignment with the external field configuration.’ (5)
The two ‘lobes’ are present in data returned by
Cassini: ‘
Also, indicated in Figure 10(b) (by red contour lines and semi-transparent blue fill) are the offset heliotail “lobes” identified in the 4.3 keV IBEX skymaps by McComas et al. (2013). In terms of the brighter features, the belt roughly overlaps the ribbon in the “nose” hemisphere, but the belt is clearly present in the “tail” hemisphere where the ribbon is absent. For the dimmest features, both of the IBEX “lobes” fall rather well into the INCA “basins,” although the basins extend considerably further in latitude (up to 60 degrees north or south) versus the lobes (not much beyond 30 degrees north and south).’ (3)
So, the
IBEX ‘lobes’, regions of reduced ENA emissions match the reduced ENA emission areas found by
Cassini- INCA ‘basins’. When the HS is viewed in galactic coordinates do the
IBEX ‘lobes’ form part of a ‘heliotail’? I suggest that they do not, rather:
‘…The minima of the ENA emissions (ENA basins) in ecliptic coordinates reside in the center of the image (e.g., see Figure 1) and reappear beyond the belt at 120 degrees to about −170 degrees in longitude at the respective edges of the image. These ENA basins roughly coincide with the Galactic north and south poles (Figure 2), with a ∼30 degree tilt with respect to the Galactic equator.’ (3) (my emphasis)
As we find with the HS ‘nose’ we find a twisting and ‘draping’ of the magnetic field due to a pinch in the ‘tail’ region of the HT. The slight differences between the ‘nose’ and ‘tail’ are due to an asymmetry in the pinch i.e. the angle of the IBEX Ribbon. The ‘lobes’ or ‘basins’ are simply regions away from either the IBEX Ribbon and Cassini Belt, in galactic coordinates, we are looking ‘up’ and ‘down’ the ‘X’ of the pinched HT into a region of depleted ENA emissions as this alignment is asymmetric it gives the impression of a ‘tail’.
Summary
We have seen that the comet-like model of the HS suggested by Parker is based on assumed conditions no longer supported by data gathered during the space-age; multiple lines of evidence now suggest that the interstellar magnetic field is relatively strong.
In-situ measurements by the Voyager spacecraft do not sit well with the comet-like HS model. The azimuthal solar wind recorded by
Voyager 1 suggests that
Voyager 1 exited the HS at the ‘waist’ of the pinched hourglass shaped HT.
Voyager 2 measuring a steady solar wind speed from the general direction of the IBEX Ribbon appears to be recording the ‘pinch’ current. Observations from
Voyager 2 may continue to surprise mission scientists for as long as the spacecraft continues to function.
When IBEX Ribbon emissions are high we find Solar Minimum conditions in the inner HS.
When IBEX Ribbon emissions are low or fragmented we find Solar Maximum conditions in the inner HS.
Global properties of the entire HS change over the solar cycle, these changes are reflected in emissions in both the IBEX Ribbon and Cassini Belt.
The IBEX Ribbon straddles the ‘waist’ of the pinched hourglass shaped HT. At the centre of the pinch we find a torus of trapped particles aligned to galactic coordinates- the Cassini Belt.
Observations of the IBEX Ribbon over the 22 year Hale cycle will be required to build a complete picture of the Sun’s interaction with its environment.
Hourglass shaped planetary nebulae tend to exhibit a preferred alignment- based on galactic coordinates the HT would display a similar alignment.
The Stellar Cycle of M2-9
It is known that bipolar planetary nebulae, the stellar equivalent of the HT, show a ‘preference for a particular alignment’ many ‘have their long axes aligned along the plane of our galaxy’ (
https://www.eso.org/public/unitedkingdom/news/eso1338/), we would expect the Sun’s environment to display a similar alignment.Observations of one such planetary nebula, M2-9, reveal changes over an 18 year period that may be analogous to the solar cycle, comparable to the HT if it were in glow mode.
(See:
http://apod.nasa.gov/apod/image/0706/m2 ... orradi.gif)
References:
1.Parker. E. N. 1961. The Stellar Wind Regions.
American Astronomical Society February 1961
2.Krimigis. S. M. et al. 2009. Imaging the Interaction of the Heliosphere with the Interstellar Medium from Saturn with Cassini.
Science 326:971, November 2009.
3.Dialynas. K. et al. 2013. A Three-Coordinate System (Ecliptic, Galactic, ISMF) Spectral Analysis Of Heliospheric ENA Emissions Using Cassini/INCA Measurements.
The Astrophysical Journal 778:40, November 2013.
4.McComas. D. J. et al. 2014. IBEX: The First Five Years (2009-2013).
The Astrophysical Journal Supplement Series 213:20, July 2014.
5.McComas. D. J. et al. 2013. The Heliotail Revealed By The Interstellar Boundary Explorer.
The Astrophysical Journal 771:77, July 2013.