Disconnecting Solar Magnetic Flux

Disconnection of open magnetic flux by reconnection is required to balance the injection of open flux by CMEs and other eruptive events. Making use of recent advances in heliospheric background subtraction, we have imaged many abrupt disconnection events. These events produce dense plasma clouds whose distinctie shape can now be traced from the corona across the inner solar system via heliospheric imaging. The morphology of each initial event is characteristic of magnetic reconnection across a current sheet, and the newly-disconnected flux takes the form of a “U”-shaped loop that moves outward, accreting coronal and solar wind material. We analyzed one such event on 2008 December 18 as it formed and accelerated at 20 m/s^2 to 320 km/s, expanding self-similarly until it exited our field of view 1.2 AU from the Sun. From acceleration and photometric mass estimates we derive the coronal magnetic field strength to be 8uT, 6 Rs above the photosphere, and the entrained flux to be 1.6x10^11 Wb (1.6x10^19 Mx). We model the feature’s propagation by balancing inferred magnetic tension force against accretion drag. This model is consistent with the feature’s behavior and accepted solar wind parameters. By counting events over a 36 day window, we estimate a global event rate of 1/day and a global solar minimum unsigned flux disconnection rate of 6x10^13 Wb/y (6x10^21 Mx/y) by this mechanism. That rate corresponds to ~0.2 nT/y change in the radial heliospheric field at 1 AU, indicating that the mechanism is important to the heliospheric flux balance.

💡 Research Summary

The paper tackles a long‑standing problem in heliophysics: how the Sun’s open magnetic flux, which is continually added by coronal mass ejections (CMEs) and other eruptive events, is removed so that the interplanetary magnetic field (IMF) does not grow without bound. The authors argue that magnetic reconnection in the corona can disconnect previously open flux, producing a characteristic “U‑shaped” plasma loop that drifts outward, sweeps up coronal and solar‑wind material, and eventually disappears into interplanetary space.

To demonstrate this process, the authors exploit recent advances in heliospheric background subtraction applied to data from the SECCHI suite on board STEREO‑A. By removing the F‑corona, stray light, and the quasi‑static K‑corona, they isolate the faint Thomson‑scattered signal from free electrons. In a 36‑day interval (December 2008), they identify twelve distinct “U” or “V” shaped features that propagate from the low corona out to beyond 1 AU. One event on 2008‑12‑18 is examined in detail because it is especially clear and can be traced from its origin at 04:00 UT on 12 December through the entire heliospheric field of view.

The event begins with a pinch‑off of the streamer belt at a heliographic longitude of roughly –10° ± 5°, forming a bright, narrow “U” loop. The loop accelerates at 20 m s⁻², reaching a speed of about 320 km s⁻¹, and expands self‑similarly as it travels outward. By converting the observed brightness to electron column density using the Thomson scattering formula, the authors estimate the plasma mass and its increase due to accretion of ambient solar‑wind material.

Assuming that the acceleration is supplied by magnetic tension, they equate the tension force Fₜ = B²A/(2μ₀) to the product of the measured mass and acceleration. Solving for the magnetic field yields B ≈ 8 µT (≈80 mG) at a height of 6 R☉. Using the inferred cross‑sectional area of the loop, the magnetic flux that becomes disconnected is Φ ≈ 1.6 × 10¹¹ Wb (1.6 × 10¹⁹ Mx).

A simple dynamical model is then constructed that balances this tension force against a drag term representing the momentum loss to the solar wind as the loop sweeps up plasma (accretion drag). The model reproduces the observed velocity profile when reasonable solar‑wind parameters (density ≈ 5 cm⁻³, speed ≈ 350 km s⁻¹) are adopted, providing an internal consistency check on the derived magnetic field and flux values.

Counting the twelve events in the 36‑day sample and extrapolating to a full year gives an average global rate of roughly one disconnection event per day. Multiplying by the per‑event flux yields a total open‑flux removal rate of about 6 × 10¹³ Wb yr⁻¹ (6 × 10²¹ Mx yr⁻¹). This corresponds to a change in the radial heliospheric field at 1 AU of roughly –0.2 nT per year, a magnitude comparable to the observed secular decline of the IMF during the recent solar minimum.

The authors conclude that reconnection‑driven flux disconnection is a quantitatively significant process for maintaining the Sun‑Earth magnetic flux balance. Their work demonstrates that modern heliospheric imaging, combined with sophisticated background subtraction, can directly capture the full life cycle of these elusive structures. Limitations remain: the events are observed primarily off the ecliptic, the mass and magnetic‑field estimates rely on simplified geometry (e.g., cylindrical “U” loops), and the sample size is modest. Future work involving multi‑viewpoint observations, higher‑resolution coronagraphs, and three‑dimensional MHD simulations will be essential to refine the global flux‑budget and to understand how reconnection sites are distributed over the solar surface.