Heliopause

Three nested boundaries: shock, sheath, pause

The heliosphere — the bubble of plasma carved out of the local interstellar medium by the Sun's wind — has three nested boundaries. From the Sun outward:

The termination shock. The solar wind leaves the Sun supersonic, with Mach number M = v / c_s of order 5–10. As it expands radially the magnetic and ram pressure fall as r⁻². At some distance the wind ram pressure equals the local interstellar pressure (magnetic + thermal + ram). The wind cannot remain supersonic past that point and shocks down to subsonic speeds. The termination shock — at about 85–95 AU upstream — is a fast-mode MHD shock at which the wind speed drops by roughly a factor of two, density and temperature jump up, and magnetic field strength increases. Voyager 1 crossed it on 16 December 2004 at 94 AU; Voyager 2 on 30 August 2007 at 84 AU. The 10-AU difference between the two crossings is real and reflects the asymmetry of the heliosphere driven by the local interstellar magnetic-field direction.

The heliosheath. Between the termination shock and the heliopause is the heliosheath, where the post-shock subsonic solar wind sloshes around as it deflects to flow tailward around the heliopause. It is hot (10⁵–10⁶ K), magnetized, and turbulent. Its thickness in the upstream direction is about 30 AU; downstream it is much thicker. Pickup ions — interstellar atoms that drifted into the heliosphere, ionized, and were swept up by the solar wind — provide most of the pressure in the heliosheath. The heliosheath is also where energetic neutral atoms (ENAs) are produced by charge-exchange between heliosheath protons and inflowing interstellar hydrogen.

The heliopause. The outer boundary of the heliosphere. Here the heliosheath plasma meets the local interstellar plasma, with sharply different magnetic-field direction and chemical composition. There is a tangential discontinuity (no plasma flow across) but mass is exchanged via charge-exchange and possibly magnetic reconnection. The heliopause is where the solar magnetic field gives way to the interstellar magnetic field. Voyager 1 saw the magnetic-field magnitude jump by ~ 50% and direction shift by ~ 25° in a single day on 25 August 2012, simultaneous with a tripling of the cosmic-ray flux.

The boundary regions in numbers

Region	Distance from Sun	Plasma origin	Speed	Density (H+)	Magnetic field
Inner heliosphere	0.05–10 AU	Solar wind	400–800 km/s (supersonic)	5 cm⁻³ at 1 AU	5 nT at 1 AU (Parker spiral)
Outer heliosphere	10–90 AU	Solar wind	Decelerating slowly	~ 10⁻³ cm⁻³	~ 0.05 nT
Termination shock	85–95 AU	Wind shocks down	Drops to subsonic (~ 100 km/s)	Jumps ~ 2×	Jumps ~ 2×
Heliosheath	~ 95–120 AU	Shocked wind + pickups	~ 100 km/s, deflecting	~ 0.002 cm⁻³	~ 0.1 nT, turbulent
Heliopause	110–170 AU	Tangential discontinuity	Discontinuity	0.002 → 0.08 cm⁻³	~ 0.5 nT (LISM)
Local interstellar medium	> heliopause	Galactic plasma	~ 23 km/s relative	0.05–0.1 cm⁻³	~ 0.5 nT

The plasma in the local interstellar medium at the heliopause is ~ 30 times denser than the heliosheath plasma despite both being technically vacuum by laboratory standards. The interstellar magnetic field is also stronger than the locally compressed solar field beyond the heliosheath. These contrasts are what make the boundary detectable in situ.

What Voyager actually saw

Voyager 1's heliopause crossing was not anticipated as a single sharp event. The spacecraft entered a "magnetic highway" region in May 2012 — a transition zone in which low-energy heliospheric particles disappeared, intermediate-energy galactic cosmic rays became unusually abundant, but the magnetic-field direction had not yet changed. This was already a region structurally different from the heliosheath. Then on 25 August 2012, in a span of less than 24 hours, the cosmic-ray flux made its final jump, the magnetic-field magnitude rose, and the field direction rotated. Plasma instrument readings were also consistent with a transition into a denser, colder medium — but Voyager 1's plasma instrument had failed in 1980, so direct density measurements were inferred only later from plasma-wave instrument observations of locally generated Langmuir waves.

The Voyager 1 "exit" was instead clinched by the plasma-wave detection. Coronal mass ejections from the Sun had launched in March 2012 and reached the heliopause region in October–November 2013 and April–May 2014. When they did, they drove brief, narrow Langmuir-wave activity at frequencies of 2.4 kHz — corresponding via the plasma-frequency relation f_p = 8980 √n_e to electron density n_e ≈ 0.08 cm⁻³, very different from heliosheath values of ~ 0.002 cm⁻³ and consistent with the local interstellar medium. The combination of magnetic-field and plasma-density data confirmed the August 2012 crossing.

Voyager 2, with its plasma instrument still working, made an even cleaner crossing in November 2018. The plasma density jumped from heliosheath levels to ~ 0.05 cm⁻³, the temperature dropped from ~ 30,000 K to ~ 7,500 K, and the cosmic-ray flux jumped by a factor of three — all in less than a day. Voyager 2's data confirmed and refined what Voyager 1's incomplete instrument suite had implied.

Worked example: termination-shock distance from pressure balance

Where would we expect the termination shock to be? At the shock the solar-wind ram pressure equals the local interstellar total pressure. Take typical wind values at 1 AU and scale outward as r⁻²:

P_ram_wind(r) = ρ_w(r) × v_w² = ρ_w(1 AU) × (1 AU / r)² × v_w²

With wind density at 1 AU = 5 protons/cm³ × m_p = 8.35 × 10⁻²¹ kg/m³ and wind speed 450 km/s = 4.5 × 10⁵ m/s:

P_ram_wind(1 AU) = 8.35 × 10⁻²¹ × (4.5 × 10⁵)²
                = 1.69 × 10⁻⁹ Pa
                ≈ 1.7 nPa

The local interstellar medium has total pressure (thermal + magnetic + ram + cosmic-ray) of roughly

P_LISM ≈ n_LISM k_B T + B_LISM² / (2μ₀) + ½ ρ_LISM v_rel²
       (each term is roughly 0.1–1 pPa, total ~ a few pPa)
       ≈ 4 × 10⁻¹³ Pa

Setting P_ram_wind(r_TS) = P_LISM:

r_TS² = (1 AU)² × P_ram_wind(1 AU) / P_LISM
      = 1 × 1.69 × 10⁻⁹ / 4 × 10⁻¹³
      = 4225 AU²
 r_TS ≈ 65 AU

The simple analytic estimate gives ~ 65 AU, somewhat smaller than the observed 85–95 AU. The discrepancy is because the simple estimate ignores pickup-ion pressure inside the heliosphere and the deflection of the wind around the heliosheath. A more careful MHD calculation reproduces the observed distance, but the order-of-magnitude argument captures the right physics: the termination shock sits where solar ram pressure has fallen to interstellar total pressure.

The heliopause sits ~ 25 AU farther out, in the post-shock subsonic region where pressure balance is between heliosheath thermal-plus-magnetic pressure and the interstellar total. The exact distance depends sensitively on the local interstellar magnetic-field direction — the "ribbon" feature seen by IBEX shows that the field is tilted by ~ 60° to the wind flow direction, distorting the heliopause asymmetrically.

Variants and extensions

• Bow shock or bow wave? Earlier models predicted a bow shock outside the heliopause where the relative motion between Sun and ISM is supersonic. IBEX measurements of the LISM speed (23 km/s) and Alfvén speed (~ 30 km/s) at the heliopause showed the relative motion is sub-Alfvénic, so no shock forms — only a "bow wave". This was a 2012 revision of textbook diagrams.
• Heliotail geometry. The downstream end of the heliosphere is poorly mapped because no spacecraft has flown that direction. IBEX ENA maps suggest a "two-lobed" or "croissant-shaped" tail rather than a simple comet-tail extending hundreds of AU. The Cassini INCA imager added complementary maps from inside Saturn's orbit.
• Solar-cycle variability. The heliopause distance varies with the solar cycle. At solar maximum the increased wind ram pressure pushes the boundary outward by 5–10 AU; at minimum it relaxes inward. Voyagers 1 and 2 crossed during periods of moderate solar activity; future missions might catch the boundary breathing.
• Astropauses around other stars. Massive stars with strong winds carve "astropauses" much larger than our heliopause. NASA's Hubble has imaged the bow shock of the runaway star ζ Ophiuchi (an O9.5 V star) compressed by its supersonic motion through the interstellar medium. Astropauses are observable in young stellar populations and around AGB stars.
• Local Bubble context. The heliosphere sits inside the Local Bubble — a low-density, hot cavity in the interstellar medium ~ 300 light-years across, evacuated by supernovae 10–20 Myr ago. The local interstellar cloud the Sun is currently inside (the "Local Interstellar Cloud" or G-cloud) is denser than the surrounding bubble. The Sun is expected to leave the LIC within ~ 50,000 years and enter a different cloud, with consequences for the heliopause distance.

Where the heliopause shows up

• Voyager 1 and 2 (1977–). The only direct in-situ measurements. Both spacecraft are still transmitting data from outside the heliopause, on declining radioisotope-thermoelectric power that will run out around 2025–2028. They will continue moving outward into the local interstellar medium for thousands of years thereafter, dead but coasting.
• Interstellar Boundary Explorer (IBEX, 2008–). Earth-orbiting NASA mission mapping the heliopause via energetic neutral atom emission. Discovered the IBEX ribbon, mapped its temporal evolution over a solar cycle, and continues to monitor large-scale heliospheric structure. IBEX is the primary global tool for studying the heliopause.
• Cassini INCA (2004–2017). The Ion and Neutral Camera on Cassini, originally designed for Saturn's magnetosphere, also produced ENA maps of the heliopause from Saturn's distance. INCA's higher-energy maps complement IBEX's lower-energy maps.
• Cosmic-ray modulation studies. Galactic cosmic rays must penetrate the heliopause and propagate inward through the heliosphere to reach Earth. Their fluxes anti-correlate with solar activity (the 11-year cosmic-ray cycle). The Voyager pre- and post-crossing flux ratio of 3× is the cleanest measurement of the heliosphere's cosmic-ray modulation efficiency.
• Interstellar Mapping and Acceleration Probe (IMAP, scheduled 2025). Successor to IBEX, with much higher angular resolution and energy range. Will map the heliopause structure during the upcoming solar cycle and resolve the IBEX ribbon morphology in detail.

The IBEX ribbon: an unexpected discovery

When IBEX produced its first all-sky ENA map in 2009 the team expected to see a smoothly varying emission pattern reflecting the global heliopause structure. Instead the map showed a narrow, bright band of enhanced ENA emission about 20° wide that traced a great circle across the sky, peaking at energies of 1–4 keV. The "ribbon" sat at locations where the local interstellar magnetic field was perpendicular to the line of sight — strongly suggesting a magnetic-geometry origin.

The leading explanation invokes secondary ENAs. Solar-wind protons charge-exchange with interstellar neutrals beyond the heliopause, becoming neutral hydrogen atoms moving outward. Some of these in turn charge-exchange again with interstellar protons in the local cloud, leaving behind a population of "pickup" protons that are then deflected by the local interstellar magnetic field. The pickup protons charge-exchange a third time and the resulting ENAs travel back toward Earth. The geometry concentrates ENA emission along sightlines where the line-of-sight component of the interstellar magnetic field is zero — exactly where the ribbon is observed. The ribbon is therefore an indirect imprint of the local interstellar magnetic field on the heliopause.

The model is not yet uniquely confirmed. IMAP, with higher resolution and broader energy coverage, is the next test.

Common pitfalls

• Calling the heliopause "the edge of the solar system". In plasma terms it is, but the gravitational reach of the Sun extends to the Oort cloud at ~ 50,000 AU, far beyond the heliopause. Comets are still gravitationally bound to the Sun out there. The heliopause is the edge of solar plasma influence, not gravitational influence.
• Confusing the termination shock and the heliopause. The termination shock is where the wind goes subsonic; the heliopause is where the heliosheath ends and the interstellar medium begins. Voyager 1 crossed the shock in 2004 and the heliopause in 2012 — they are distinct boundaries 25–30 AU apart.
• Assuming spherical symmetry. The heliosphere is comet-shaped (or possibly croissant-shaped) due to the Sun's motion through the interstellar medium and the LISM magnetic-field tilt. Upstream the heliopause is at ~ 120 AU; downstream it may be at hundreds of AU.
• Treating Voyager's instruments as fully working. Voyager 1's plasma instrument failed in 1980; only Voyager 2 has direct plasma data at the heliopause. Voyager 1's crossing was inferred from cosmic-ray and magnetometer data plus later plasma-wave detections of CME-driven Langmuir waves.
• Forgetting that the heliopause moves. The boundary breathes with the solar cycle, expanding outward at maximum and inward at minimum by 5–10 AU. Voyager 2 crossed at almost the same distance as Voyager 1 mostly by chance — the boundary distance varies from 110 AU to 170 AU depending on direction and time.