International Association of Geomagnetism and (IAGA) 2nd Symposium: Solar – Interaction © 2010 Cairo University Press December 4th–8th, 2009, Cairo, Egypt – L. Damé & A. Hady (eds) DOI: 10.1012/S120027852006

The Dynamic

Marius Potgieter, Stefan Ferreira and Du Toit Strauss

Unit for (USP), North-West University, 2520 Potchefstroom, South Africa. [email protected]

Abstract. The global features of the heliosphere influence what happens inside its boundaries on a variety of time-scales so that the relation between the dynamics of the heliosphere, its shape and geometry, and solar activity has become increasing important. Galactic cosmic rays serve as excellent probes for this purpose, conveying vital information on global heliospheric changes in the way that they respond. By observing neutral and charged particles, including cosmic rays, over a wide range of energies on various spacecraft and at , a better understanding is gained about heliospheric phenomena including space and space . Significant progress is made in this field, stimulated by the recent observations in the outer heliosphere by the two Voyager spacecraft and in the inner heliosphere by Ulysses and several other space missions. Progress and issues in this research field are briefly discussed. Keywords: Heliosphere, solar activity, cosmic rays.

1 The Dynamic Heliosphere

The interstellar space between the and nearby is filled with plasmas, magnetic fields, neutral and charged particles. The Sun moves through the local (ISM) with a velocity of ~26 km.s-1 so that an interface with the ISM is formed. The prevents this medium from flowing into the large volume dominated by the Sun, called the heliosphere. Because it is moving through the ISM, the heliosphere is asymmetrical with respect to the Sun, with the tail region much more extended than the nose region, the direction in which it is moving. It extends over at least 500 AU in its equatorial regions and at least 250 AU in the plane. The main constituents of this interface with the local ISM, shown in Figure 1, are the termination (TS), the heliopause (HP) and perhaps also a (BS), with the region between the TS and the HP defined as the inner heliosheath and the outer heliosheath located between the HP and the BS. The latter is expected to be rather weak. For a review of these heliospheric features and work, see [1]. The HP separates the solar wind and local ISM so that it may be considered the outer boundary of the heliosphere. A prediction of MHD-HD (magneto- hydrodynamic - hydrodynamic) modeling is that the solar wind creates a TS where it drops from supersonic (400800 km.s–1) to subsonic speeds [e.g. 2, 3]. It is predicted that the heliospheric structure is dynamically asymmetric, yielding a ratio of ~1:2 for the upwind-to-downwind TS distance. However, in the nose direction the TS 33 Marius Potgieter, Stefan Ferreira and Du Toit Strauss

Fig. 1 Illustration of the heliospheric geometry, structure and boundaries using contour plots of the proton number density in the X-Y and Y-Z planes, obtained with a 3-dimensional HD (hydrodynamic) model with the associated number density profiles in the heliospheric upwind, downwind, crosswind and polar directions in the lower panels [3]. movement is relatively restricted. The heliosphere is elongated in its polar directions because of the latitudinal variation of the solar wind momentum flux. This asymmetry may become more pronounced during conditions. Recently, Richardson et al. [4] reported that the radial flow speeds in the heliosheath vary between 80 and 200 km.s-1 with an average speed of 138 km.s-1, as measured by Voyager 2. These flows suggest that the heliosphere is wider than it is high. The TS position oscillates significantly with solar activity. In Table 1 recent estimates of the TS and HP positions, in AU, are summarized. In this respect, Snyman [24] modeled the distance to the TS as it varies over an 11 year cycle in comparison with the positions of Voyager 1 and Voyager 2. This is shown in Fig. 2. It follows that the TS could come inwards, as close as 78 AU, and as far out as 96 AU from the Sun over a . The Voyager 1 spacecraft encountered the TS in December 2004 at a distance of ~94 AU from the Sun, at a polar angle of ~60º. Its twin spacecraft Voyager 2 crossed the TS in August 2007 at 83.7 AU in the southern hemisphere, at ~120º and ~10 AU closer to the Sun than found by Voyager 1, in accord with the 34 The Dynamic Heliosphere

Fig. 2 The computed radial heliocentric position (in AU) of the TS at 34.1° heliolatitude (solid black line) and at 270° (solid blue line) for the period 1975 to 2010. Also shown are the radial positions of Voyager 1 and 2 at the time. The vertical dashed line indicates the time when Voyager 1 crossed the TS [24]. predictions shown in Figure 2. This additional asymmetry could indicate an asymmetric pressure from an interstellar , from transient-induced shock motion, or from the solar wind dynamic pressure [4, 5]. Recently, Opher et al. [23] reported on the strength and orientation of the magnetic field in the interstellar medium near the heliosphere and found that the field strength in the local interstellar medium is 3.7–5.5 G (previous estimates were 1.8– 2.5 G). This field is also tilted 20–30° from the interstellar medium flow direction and is at an angle of about 30° from the Galactic plane. This indicates that the heliosphere is in one hemisphere to a larger extent ‘magnetically squeezed’ than in the other half, causing this additional asymmetry in the north-south direction. Another exciting and very recent observation is that of energetic neutral atoms (ENAs) produced by charge exchange between fast ions and slow neutral atoms. A major part of the ENA flux comes from regions where both the flux of parent ions and the neutral atom density are high so that at the energies that are measured by the IBEX mission, the main contribution is expected from the (inner) heliosheath. This appears not to be the case [22], yet another scientific surprise from beyond the TS. Stone et al. [5] reported also that the intensity of 4-5 MeV protons accelerated by the TS near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in particle populations at the two locations. Companion papers in Nature (July 2008) reported on the plasma, magnetic field, plasma-wave and lower energy particle observations at the TS and beyond. Voyager 1 is approaching the HP at about 3 AU per year so the anticipation is that the spacecraft may cross the HP within the next few years. It will then move into interstellar space for the first time.

Table 1. Heliocentric distances to the solar wind termination shock (TS) and the heliopause (HP), estimated from observations and models, for three main heliospheric directions; the ‘nose’ is the direction in which the heliosphere is moving through the local interstellar medium.

Nose Poles Tail TS: (86 ± 9) AU TS: (130 ± 12) AU TS: (200 ± 15) AU HP: (135 ± 10) AU HP: (200 ± 30) AU HP: > 500 AU

35 Marius Potgieter, Stefan Ferreira and Du Toit Strauss

2 Cosmic Rays in the Heliosphere

Cosmic rays are charged particles with energies ranging from ~1 MeV to as high as thousands of PeV. Charged particles present in the heliosphere are classified in four main populations: (1) Galactic CRs which originated far outside the heliosphere, probably accelerated during supernova explosions. When arriving at Earth, these particles are composed of ~98% nuclei (mostly protons), fully stripped of all their electrons, and ~2% electrons, fewer positrons and anti-protons. (2) Solar energetic particles which originate mainly from solar flares, coronal mass ejections and shocks in the interplanetary medium. They occur sporadically, and may have energies up to hundreds of MeV, observed in the inner heliosphere usually only for several hours mainly during activity. These events are directly linked to what is called . (3) The anomalous component which originally were interstellar neutral atoms that got singly ionized in the heliosphere. They are transported by the solar wind, as so-called pick-up ions, to the outer heliosphere where they get accelerated up to a ~100 MeV through various processes. (4) The Jovian electrons which dominate the observed low energy electron spectrum, up to 30 MeV, within the first 10 AU from the Sun.

Fig. 3 A comparison of computed and observed spectra, by the Voyager 1, of anomalous Oxygen in the heliosheath. Modelling results are shown at a polar angle of 55°, at the radial distances as indicated, approximating the Voyager trajectory. Data set is for long term averages over four 5-AU-wide radial ranges and illustrates how the anomalous Oxygen intensities evolve beyond the TS which is at 94 AU in this case [25].

The principal acceleration mechanism for the ACRs has been considered to be diffusive shock acceleration. However, at the location of the TS observed by the two Voyager spacecraft, there was no direct evidence of this process occurring for typical ACR energies [5]. This suggests that the source is elsewhere on the TS or somewhere 36 The Dynamic Heliosphere inside the heliosheath. Higher energy ACRs thus seem disappointingly unaffected by the TS. On the other hand, low-energy ions (E < 3 MeV/nucleon) are clearly accelerated. These unmistakably accelerated particles (low-energy ACRs) have since become known as termination shock particles [5]. For a review of these research challenges, see [6]. Alternative mechanisms have been suggested by several authors so that this topic is quite controversial. In Figure 3, a comparison is shown of computed and observed spectra, by Voyager 1, of anomalous Oxygen in the heliosheath. The modelling results are shown at a polar angle of 55°, at the radial distances as indicated, approximating the Voyager trajectory. The data illustrate how the anomalous Oxygen intensities evolve beyond the TS which is at 94 AU in this case. The modelling approach includes classical diffusive shock acceleration, adiabatic heating and momentum diffusion (stochastic acceleration) of these particles beyond the TS [7, 25]. The power-law profiles exhibited at the lower energies is a characteristic of diffusive shock acceleration for the lower energy ACRs. Life on Earth is protected against CRs by three space 'frontiers', the first one arguably the least appreciated: (1) The dynamic heliosphere with the solar wind and the accompanying turbulent heliospheric magnetic field (HMF). (2) The Earth's magnetic field, with its dynamic character, e.g., large decreases have been occurring over southern Africa over the past decades. This means that significant changes in the geomagnetic cut-off rigidity for CRs occur, sufficiently large over the past 400 years so that the change in CR flux impacting the Earth may approximate the relative change in flux over a solar cycle [8]. The withstands the space weather that the Sun produces, and also reverses its magnetic polarity on a very long- term (the last one was ~780 000 years ago, so that the next reversal is considered long overdue). (3) Finally, the Earth’s atmosphere, with all its complex physics and chemistry, where the CR intensity decreases exponentially with increasing atmospheric pressure. The most important variability time scale related to solar activity is the 11-year cycle. This quasi-periodicity is convincingly reflected in records of since the early 1600's, and in other solar activity indices, also in the galactic CR intensity observed at ground and sea level especially since the 1950's when neutron monitors (NMs) were widely deployed as CR detectors during the International Geophysical Year (IGY1957). The period 2006-2008 was celebrated as the 50th anniversary of the IGY and was called the International Heliophysical Year (www.ihy2007.org). This campaign is now followed by the International Space Weather Initiative (http://iswi.cu.edu.eg and http://www.iswi-secretariat.org). The CR behaviour as a footprint of what they had experienced before being observed, is shown in Figure 4. Another important and interesting phenomenon shown in this figure is the 22-year cycle, caused by the gradient, curvature and current sheet drifts of these CR particles which are directly related to the global gradients and curvatures of the HMF and the fact that it reverses polarity every 11 years. Because drift patterns are reversed for oppositely charged particles, electrons (or protons) and positrons (or anti-protons) behave differently inside the heliosphere during a given solar magnetic polarity cycle. They therefore probe different regions of the heliosphere during the same cycle. Simultaneous measurements of galactic CR electrons and positrons (protons and anti- protons) are thus a crucial test for our understanding of charge-sign modulation in the heliosphere, in particular how the drifts that they experience are affected by 37 Marius Potgieter, Stefan Ferreira and Du Toit Strauss

Fig. 4 Results from the Hermanus Monitor in South Africa, illustrating the 11- year and 22-year cycles and the large step-like decreases and recoveries. Peaks are formed during the A < 0 solar magnetic field polarity in contrast to A > 0 cycles when the modulation profile is flattish. Intensity is corrected for atmospheric pressure to get rid of seasonal and daily variations. Note that CRs have reached the highest level over five solar cycles in 2009. turbulence as a function of decreasing energy, position in the heliosphere and over a complete solar activity cycle [9,10]. Observations of these cosmic anti-particles are presently made by the PAMELA mission [26]. In this context a very useful proxy for solar activity is the so-called ‘tilt angle’ of the wavy heliospheric current sheet (HCS), which turned out to be one of the most successful modulation parameters after it was realized that gradient and curvature drifts of CR particles should play an important role. The “tilt angles” of the HCS, shown in Figure 5, are widely used for CR data interpretation and in numerical modeling. For a detailed discussion, see also [11,12,13,19]. The modulation effects of this wavy HCS and global drifts, the subsequent 22-year cycle and corresponding charge-sign dependence have been studied in detail [13,19]. It was found that periods of maximum CR modulation are more complex; they may last only three years (e.g., 1969–1971), or up to six years (e.g., 1979–1984), or may temporarily be dominated by a massive CR decrease as in 1991. Underlying patterns are obscured by an apparent randomness which makes modeling of long-term modulation difficult. A major issue in modulation modeling is the spatial, energy (or rigidity) and time-dependence of the modulation process as determined by three types of diffusion coefficients [7,9]. As yet, no ab initio modulation theory exists, one in which the diffusion coefficients are determined on the basis of our understanding of heliospheric turbulence and diffusion. For example, the slope of the turbulence spectrum determines the energy dependence of the scattering mean free path with respect to the background HMF, obviously of vital

38 The Dynamic Heliosphere

Fig. 5 Tilt angles of the wavy heliospheric current sheet; maximum values indicate extreme solar activity when the polarity of the solar magnetic field changes; minimum values indicate when solar modulation is at its minimum such as in 2008-2009. (Courtesy of Wilcox ; see http://wso.stanford.edu. for the two approaches in calculating these values). importance to CR propagation studies. The time dependence of the transport of energetic particles results from the time dependence of the solar wind and HMF turbulence. Using basic and phenomenological approaches, progress is being made in this important field of heliospheric physics. For overviews of the basic theory, and appropriate transport equation for the modulation of CRs in the heliosphere, see [9,10,15,19]. There are several additional short periodicities evident in NM and other CR data, e.g., the 25-27-day variation caused by the rotating Sun, and the daily variation caused by the Earth's rotation. These variations seldom have magnitudes of more than 1% with respect to the previous quite times. The well-studied corotating effect is caused mainly by interaction regions (CIRs) created when a faster solar wind overtakes a previously released slow solar wind. They usually merge as they propagate outwards to form various types of interaction regions, the largest ones are known as global merged interaction regions (GMIRs) [14]. They are related to what happened to the HMF at an earlier stage and are linked to coronal mass ejections (CMEs) that are always prominent with increased solar activity but dissipate during solar minimum. These GMIRs propagate with the solar wind speed into the heliosheath so that an isolated GMIR may cause a decrease similar in amplitude than the 11-year cycle (as shown for 1991 in Figure 4) but it usually lasts only several months. A series of GMIRs, however, may contribute significantly to 11-year modulation during periods of increased solar activity in the form of relatively large discrete steps, increasing the overall amplitude of the 11-year cycle [13,15]. The recent solar minimum activity period was extra-ordinary long, lasting two years longer than the previous ones (Figure 4). The HMF (measured and estimated) at Earth was at its lowest value since a 100 years ago [21]. Consequently, CRs had 39 Marius Potgieter, Stefan Ferreira and Du Toit Strauss reached the highest intensity level over five solar cycles in 2009. The next solar activity cycle is predicted to have significantly lower sunspots than the previous ones. Could this be the first indication that we are about to observe a solar cycle much longer than the 11-year and 22-year cycles?

3 The Heliosphere, Cosmic Rays and Space Climate

Marsh and Svensmark [18] found a correlation between the flux of CRs and the global average of low cloud cover on Earth, which on its part affects the climatic temperature. The galactic CR flux is not expected to be constant along the trajectory of the in the galaxy. Interstellar conditions, also locally, should differ significantly over time-scales of millions of years, such as when the Sun moves in and out of the galactic spiral arms [16]. Shaviv [20] speculated that the major ice ages on Earth might even be triggered by encounters of the heliosphere with its galactic environment, given that the CR-clouds relation holds. It is widely accepted that the concentration of 10Be nuclei in polar ice exhibits temporal variations in response to changes in the flux of the primary CRs over hundreds of years ([17] and references therein). As reviewed by Scherer et al. [19], computations indicate that the galactic CR flux reaching the heliosphere is indeed not constant over very long time scales. These kind of variations in galactic CR intensity are significant enough so that it should have a measurable effect on space climate in the heliosphere and a clear imprint on the terrestrial archive. The interpretation of the cosmogenic archives is importance for our understanding of variations of galactic CR spectra and of the and as such of great interest to astrophysics. The correlation of cosmogenic with climate archives gives valuable information regarding the question to what extent the Earth’s climate is driven by extraterrestrial and extraheliospheric forces. Besides solar activity as the internal driver of heliospheric dynamics, and the obvious space weather connection, the structure of the heliosphere is thus also determined by external factors caused by a changing interstellar environment.

4 Discussion and Conclusions

Heliospace physics forms a part of the universal physical processes that can be used to gain better understand of the features and characteristics of space, from geospace to heliospace, into interstellar space, and finally into galactic space. Heliospheric physics, in addition to geospace physics and solar-terrestrial physics, has become most relevant and is actively studied. Recent crossings of the TS by the two Voyager spacecraft have been a major accomplishment that has renewed the interest in CR modulation, the physics of the heliosheath, and what is to be expected beyond the HP when these spacecraft move into the local interstellar medium. Cosmic ray variability contributes to the understanding of the importance of the complex field of space weather and climate. Only recently has the dynamics of the heliosphere been studied and appreciated, in particular its role in CR variability and 40 The Dynamic Heliosphere ultimately its role in space climate. Observations of galactic and anomalous CRs in the outer and inner heliosphere, together with the solar wind and magnetic field, have also caused new scientific controversies. The acceleration of the anomalous CRs at the TS was thought to be caused mainly by diffusive shock acceleration but new information and modeling show that neglected mechanisms such as stochastic acceleration and solar wind adiabatic heating may be equally important. Several challenges need to be addressed, examples are: What is the spatial-time-dependence of the global strength and structure of the TS? How are energetic particles accelerated beyond the TS? What are the global properties of the plasmatic flow beyond the TS and what is happening to it in the heliotail? How does the interstellar flow interact with the heliosphere beyond the HP? Understanding both the macro and micro- physics of the dynamic heliosphere will give the theoretical and modeling tools to study broader issues in astrophysics. The study of the heliosheath, the heliopause and the heliospheric interface with the local interstellar medium and how galactic and anomalous CRs respond to the global dynamics thereof will be one of the prominent heliospace research topics for the coming years. It is essential to fully understand the dynamics of the heliosphere, the time variation of local ISM, and beyond, in order to appreciate the variations of the order of thousands of years, and much longer, that seem to exist in the flux of galactic CRs. The effects of the dynamics of the heliosphere on CR modulation and subsequently on climate has been studied quantitatively only recently, an aspect that will become increasingly important. Variations in galactic CR intensity are significant so that it should have a measurable effect on space climate in the heliosphere and should have a clear imprint on historic climatic archives.

Acknowledgments. MSP thanks the organizers for the invitation to participate in the December 2009 workshop in Cairo. Financial support of the South African National Research Foundation (NRF) and the SA Centre for High Performance Computing (CHPC) is valued. The authors are grateful for many informative discussions on the heliosphere with Bill Webber, Klaus Scherer, Horst Fichtner, Bernd Heber and Ingo Büsching.

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