On the nature of the in-ecliptic interplanetary magnetic fields two-humped distribution at 1AU

It was found out that the distribution’s shape of the in-ecliptic (as well as radial) component of the interplanetary magnetic field (IMF) significantly changes with the heliocentric distance, which poorly corresponds to classical models of the solar wind and the interplanetary magnetic field (IMF) expansion. For example, distributions of the radial photospheric and the source surface’s magnetic field in the ecliptic plane are Gaussian-like, the distribution of the radial IMF component at the Earth orbit demonstrates two-humped shape, and it becomes again Gaussian-like at 3-4 AU. These differences lead to lack of correspondence between simulations of the IMF behaviour at 1 AU and observations. Our results indicate that picture of the IMF expansion into space is more complicated than usually considered, and the sector structure is not the only source of the two-humped shape of the in-ecliptic or radial IMF component. We have analysed data from different spacecraft at the distances from 0.29 AU to 4 AU and found that the shape of the radial IMF component distribution strongly depends on a heliocentric distance and a heliolatitude. The “two-humped IMF” effect is most brightly expressed at low heliolatitudes at 0.7-2 AU, but it fully disappears at 3-4 AU. There is also dependence of the IMF distributions’ view on a solar cycle due to active processes, such as solar flares and CMEs. We suppose that the in-ecliptic solar wind field at 1 AU is influenced by solar active regions in a high degree, and actually the distribution is the three-humped: two humps correspond to the IMF from the middle and high heliolatitudes and the third one is the theoretically expected distribution from the solar field nearby the heliomagnetic equator. Vanishing of the IMF zero-component with the distance from the Sun partially could be a result of a magnetic reconnection at the current sheets in the solar wind.

💡 Research Summary

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The paper investigates the puzzling “two‑humped” distribution of the interplanetary magnetic field (IMF) radial component (Br) observed near the ecliptic plane, focusing on how this distribution evolves with heliocentric distance, heliographic latitude, and solar‑cycle phase. Using a comprehensive set of spacecraft measurements—OMNI2 (near‑Earth), Helios 2 (0.29 AU), Pioneer Venus Orbiter (0.7 AU), Voyager 1 (2–4 AU), and Ulysses (high‑ and low‑latitude observations from 1 to 4 AU)—the authors construct histograms of Br (and the horizontal components Bx, By) spanning more than three solar cycles (1977‑2009).

Key observational findings are:

1. Distance dependence – At 0.29 AU the Br histogram shows three distinct peaks, suggesting contributions from high‑, mid‑, and low‑latitude solar wind streams. At 0.7 AU the classic two‑humped shape (two symmetric peaks around ±1 nT with a pronounced deficit near zero) is most prominent. By 2 AU the two peaks are still discernible, but by 3–4 AU the distribution becomes essentially Gaussian, indicating that the “zero‑field” deficit has vanished.

2. Latitude dependence – Ulysses data reveal that high‑latitude solar wind (|latitude| > 40°) retains a two‑humped distribution even out to 3–4 AU, although the peak amplitudes are smaller (±0.8 nT) than those at low latitudes (±1.5 nT). This suggests that the high‑latitude IMF contributes a distinct, weaker component that mixes with the low‑latitude flow near the ecliptic.

3. Solar‑cycle modulation – During solar maximum (e.g., 1979‑81, 1989‑91, 1999‑01) the histograms broaden and the peaks diminish, reflecting the increased occurrence of CMEs and corotating interaction regions (CIRs) that inject strong, irregular magnetic fields. In contrast, solar minima display sharper, more symmetric two‑humped structures.

4. Vertical component behavior – The Bz component remains Gaussian and centered on zero throughout, contradicting any hypothesis that the missing near‑zero values in Bx/By are simply transferred to Bz by a tilt of the heliospheric current sheet (HCS).

The authors argue that the traditional explanation—sector‑boundary crossings (SBC) of a thin HCS—cannot account for the observed deficit of near‑zero Br values at 1 AU. Instead, they propose two complementary mechanisms:

• Latitudinal mixing – Solar wind streams originating from mid‑ and high‑latitude coronal holes are deflected toward the ecliptic, superimposing their magnetic signatures on the low‑latitude flow. This creates a “three‑humped” underlying distribution (high‑latitude, mid‑latitude, and equatorial contributions). The observed two‑humped shape at 1 AU is then a projection of this more complex structure.

• Magnetic reconnection at current sheets – As the solar wind expands, reconnection events at the HCS and other large‑scale current sheets can annihilate magnetic neutral lines, effectively removing Br≈0 occurrences. This process would naturally diminish the central dip of the histogram with increasing distance, leading to the Gaussian shape seen beyond ~3 AU. The authors link this to the concept of an IMF “floor” (a minimum average field strength of ~4.6 nT at 1 AU and ~3.2 nT at high latitudes) reported in earlier studies.

By combining statistical analysis with physical reasoning, the paper concludes that IMF expansion is far more intricate than the simple radial, Parker‑spiral picture. The sector structure is only one of several contributors to the two‑humped distribution; latitudinal mixing and distance‑dependent reconnection play essential roles. The work highlights the need for three‑dimensional, time‑dependent MHD modeling that incorporates high‑latitude wind streams and dynamic reconnection processes to reproduce the observed evolution of IMF statistics. Future investigations, possibly leveraging data from Parker Solar Probe and Solar Orbiter, should aim to resolve the relative importance of these mechanisms and to quantify how they shape the heliospheric magnetic field throughout the solar cycle.