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The Dynamic Heliosphere

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The Dynamic Heliosphere International Association of Geomagnetism and Aeronomy (IAGA) 2nd Symposium: Solar Wind – Space Environment Interaction © 2010 Cairo University Press December 4th–8th, 2009, Cairo, Egypt – L. Damé & A. Hady (eds) DOI: 10.1012/S120027852006 The Dynamic Heliosphere Marius Potgieter, Stefan Ferreira and Du Toit Strauss Unit for Space Physics (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 Earth, a better understanding is gained about heliospheric phenomena including space weather and space climate. 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 Sun and nearby stars is filled with plasmas, magnetic fields, neutral and charged particles. The Sun moves through the local interstellar medium (ISM) with a velocity of ~26 km.s-1 so that an interface with the ISM is formed. The solar wind 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 polar 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 bow shock (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 solar minimum 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 solar cycle. 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 magnetic field, 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 solar maximum activity. These events are directly linked to what is called space weather. (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].
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