3 Particle Probes of the HMF

While in situ magnetometer measurements can make extremely accurate observations of the local magnetic field strength and orientation, information about the global heliospheric magnetic field topology, particularly HMF connectivity to the solar surface, can be probed through energetic particle observations.

3.1 Suprathermal electrons

Suprathermal electrons (STEs) have energies well above the thermal plasma (e.g., > 70 eV), allowing them to stream along the HMF and carry the heat flux away from the Sun (Feldman et al., 1975Jump To The Next Citation Point; Rosenbauer et al., 1977Jump To The Next Citation Point). As the STEs move out into the heliosphere, a strong STE field-aligned beam, or “strahl,” is formed by conservation of magnetic moment. This strahl serves as an effective tracer of heliospheric magnetic field topology. As STEs move much faster along magnetic fields than the bulk plasma flow, they also act as near-instantaneous indicators of magnetic connection to the Sun.

View Image

Figure 10: A sketch of heliospheric magnetic topology inferred from suprathermal electron observations. Left panel: A view of the ecliptic plane, with magnetic field lines shown as black arrows and the anti-sunward suprathermal electron flux shown as red arrows. Right panel: The STE pitch-angle distribution seen by a spacecraft in near-Earth orbit. At (a), the field is part of an inward-polarity sector, so the STE strahl is anti-parallel to the field. Similarly, at (c), the outward sector results in a parallel strahl. At (b), the magnetic field is connected to the Sun at both ends, resulting in both parallel and anti-parallel strahls, or counterstreaming. At (d), there is no solar connection, so no strahl is seen.

Figure 10View Image shows a sketch of the relation between heliospheric magnetic topology and suprathermal electrons. The left panel shows a view of the ecliptic plane, with magnetic field lines shown as black arrows and the anti-sunward STEs shown as red arrows. The right panels shows the expected STE pitch-angle distribution seen by a spacecraft in near-Earth orbit. “Open” heliospheric magnetic flux has a single connection to the Sun, as shown at (a) and (c), and will therefore result in a single strahl (Feldman et al., 1975; Rosenbauer et al., 1977). A strahl parallel or anti-parallel to the HMF reveals the polarity of the magnetic foot point connected to the Sun, regardless of any “folding” or twisting of the field between the Sun and point of observation (Crooker et al., 2004bJump To The Next Citation Point). Thus at (a), the field is part of an inward-polarity sector, so the STE strahl is anti-parallel to the field. Similarly, at (c), the outward sector results in a parallel strahl. At (b), the HMF forms a closed loop, with the magnetic field connected to the Sun at both ends. Thus, while the magnetic field threads the source surface to form open solar flux, it is closed in the heliosphere. This results in both parallel and anti-parallel strahls (Gosling et al., 1987Jump To The Next Citation Point), commonly referred to as bi-directional electrons (BDEs) or counterstreaming electrons (CSEs). This signature may only be present for newly added heliospheric loops, as the apex of a loop will continue to move anti-sunward meaning the loop length will increase such that the CSE signature is lost by pitch-angle scattering (Hammond et al., 1996; Maksimovic et al., 2005; Owens et al., 2008b). This CSE signature is closely correlated with interplanetary coronal mass ejections (Gosling et al., 1987Jump To The Next Citation Point, see also Section 4.2). It should be noted, however, that CSEs can also result from open heliospheric flux when STEs are reflected at discontinuities, particularly strong shocks (Gosling et al., 1993a), and through pitch-angle focussing and mirroring on open field lines (Gosling et al., 2001; Steinberg et al., 2005). Thus, care must be taken when interpreting STE data in terms of magnetic connectivity.

Finally, at (d), the HMF has no connection to the photosphere, forming a disconnected loop in the heliosphere. No strahl is expected on such a flux system, and periods of “heat flux dropouts” (HFDs; McComas et al., 1989) or, more accurately, “electron dropouts” (EDs; Owens and Crooker, 2007Jump To The Next Citation Point) are expected. However, they are observed to be extremely rare in solar wind observations (Pagel et al., 2005Jump To The Next Citation Point, 2007). Note that this does not necessarily mean that the disconnection of heliospheric flux is uncommon, just that the signature of disconnection is only fleetingly observable at 1 AU (see Owens and Crooker, 2007Jump To The Next Citation Point, for more detail).

3.2 High-energy particles

While suprathermal electrons are ubiquitous in the solar wind, there are also intermittent bursts of much higher energy particles, both electrons and ions, which result from particle acceleration at solar flare sites and at shock fronts driven by solar eruptions and stream interaction regions (SIRs). As the flare-associated impulsive solar energetic particles (SEPs) have distinct launch times, the dispersion in arrival times of particles of different energies can provide information about the length of the field line connecting the observer and the source (e.g., Larson et al., 1997; Chollet and Giacalone, 2011; Kahler et al., 2011). When the particle acceleration site can be reliably determined (e.g., using extreme ultra-violet or soft X-ray observations of a flare), the spatial connection between the observer and the Sun can also be inferred. Energetic particles accelerated at SIR-driven shock fronts (see Section 2.5) have been particularly useful for understanding changing connectivity of the HMF to the photosphere (Fisk, 1996Jump To The Next Citation Point, see also Section 5.4).

Energetic particles from non-solar sources can also reveal information about the large-scale heliospheric magnetic field. Energetic electrons released by the Jovian magnetosphere (Teegarden et al., 1974; Chenette et al., 1974) provide a point source in the heliosphere which can be used to infer magnetic connectivity to Jupiter and, hence, the large-scale structure of the HMF (Chenette, 1980; Moses, 1987; Owens et al., 2010). Galactic cosmic rays (GCRs) (e.g., Usoskin, 2013Jump To The Next Citation Point, and references therein), which originate outside the solar system, are near isotropic. Thus, changes in GCR flux can reveal information about the large-scale HMF, particularly the total open solar flux (OSF) and heliospheric current sheet orientation (e.g., Ferreira and Potgieter, 2003; Alanko-Huotari et al., 2007, see also Section 5.5.3). Cosmic-ray intensity in the heliosphere rises and falls in anticorrelation with the OSF and, hence, shows a strong 11-year solar cycle variation. Cosmic-ray intensity, however, also shows a 22-year cycle (Webber and Lockwood, 1988; Smith, 1990), with alternate cycles displaying “peak-” and “dome-like” variations. This is primarily due to differing cosmic ray drift patterns in alternate global solar magnetic polarities (Jokipii et al., 1977), though there is some evidence that the OSF and latitudinal extent of the heliospheric current sheet are enhanced during odd-numbered solar cycles relative to even ones, which may lead to direct modulation of cosmic rays by differing heliospheric structure (Cliver and Ling, 2001; Thomas et al., 2013).

While less directly relevant to this review, we note there are a host of other non-solar energetic particles present in the heliosphere which are of great interest to a range of scientific areas. Pickup ions are formed when neutral particles become ionized and entrained in the solar wind and, hence, reveal information about both the neutral interstellar medium and the inner heliospheric dust distribution (see Gloeckler et al., 2001, for an excellent review of the subject). As pickup ions can contribute as much as 10% of the abundance of solar wind ions in the outer heliosphere, they can affect solar wind dynamics at large heliocentric distances. Energetic neutral atoms (ENAs), on the other hand, are high-energy charged particles which charge exchange with the solar wind to become neutral. As they are demagnetised, they travel large distances undisturbed, enabling remote sensing of magnetospheres or distant heliospheric structure, such as the heliopause (Gruntman, 1997). The Interstellar Boundary Explorer (IBEX) mission (McComas et al., 2009) is currently mapping the structure of the heliopause through ENA imaging, as discussed in Section 2.6.

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