Flux Erosion of Magnetic Clouds by Reconnection With the Sun's Open Flux

Abstract

Magnetic clouds (MCs) are flux rope magnetic structures forming a subset of solar coronal mass ejections, which have significant space weather impacts. The geoeffectiveness of MCs depends on their properties, which evolve during their interplanetary passage. Based on an analysis of observations spanning two solar cycles, we establish that MCs interacting with the ambient solar wind magnetic field (i.e., heliospheric open flux) lose a substantial amount of their initial magnetic flux via magnetic reconnection, which, in some cases, reduce their geoeffectiveness. We find a linear correlation between the eroded flux of MCs and solar open flux which is consistent with the scenario that MC erosion is mediated via the local heliospheric magnetic field draping around an MC during its interplanetary propagation. The solar open flux is governed by the sunspot cycle. This work therefore uncovers a hitherto unknown pathway for solar cycle modulation of the properties of MCs.Plain Language Summary Vast eruptions of magnetized plasma from the Sun known as coronal mass ejections (CMEs) are the main contributors to severe space weather conditions. The magnetic properties of CMEs-which determine their geoeffectiveness-evolve during their interplanetary journey. Therefore, it is important to understand the physical processes that govern their evolution. Based on in situ satellite observations, and modeling here, we provide evidence that the magnetic flux of CME associated magnetic clouds erode over their interplanetary journey due to reconnection with the ambient heliospheric open flux. We also demonstrate that this flux erosion impacts the geoeffectiveness of magnetic clouds in some cases. This mechanism establishes a novel connection between the solar cycle variation of the heliospheric open flux and space weather in the vicinity of solar system planets.

Uncovering erosion effects on magnetic flux rope twist

“…During interplanetary propagation, FRs can interact and reconnect with the ambient interplanetary magnetic field. Reconnection of FR field lines can lead to substantial erosion of FR flux in the azimuthal plane (i.e., the plane formed by the FR axis and spacecraft trajectory in the FR frame), and therefore to the imbalance of the azimuthal flux (Ruffenach et al 2012(Ruffenach et al , 2015Pal et al 2020). In contrast, if no erosion occurs during interplanetary propagation and flux is conserved in the FR azimuthal plane, the MC azimuthal flux measured at 1 AU in situ ideally should be equal to the initial FR flux.…”

“…It is evident from Dasso et al (2006) that erosion may remove flux and helicity from FRs and result in asymmetry in the FR azimuthal flux. Pal et al (2020) further showed that FR erosion is modulated by the solar cycle and may affect the geoeffectiveness of FRs.…”

“…Regardless of their intrinsic configuration, the magnetic flux and helicity of FRs may be affected by interaction with the ambient solar wind magnetic field as they propagate outward in interplanetary space. This interaction may occur via magnetic reconnection when FR fields are oppositely directed to the local heliospheric magnetic field, which can drape around the FR during its interplanetary propagation (Gosling & McComas 1987;Ruffenach et al 2015;Pal et al 2020;McComas et al 1988). Several studies suggest that reconnection may occur either at the front or back of FRs (Ruffenach et al 2015;Pal et al 2020).…”

“…This interaction may occur via magnetic reconnection when FR fields are oppositely directed to the local heliospheric magnetic field, which can drape around the FR during its interplanetary propagation (Gosling & McComas 1987;Ruffenach et al 2015;Pal et al 2020;McComas et al 1988). Several studies suggest that reconnection may occur either at the front or back of FRs (Ruffenach et al 2015;Pal et al 2020). Reconnection can decrease the flux of the FR by peeling off the FR outer layers.…”

Evidence From Galactic Cosmic Rays That the Sun Has Likely Entered a Secular Minimum in Solar Activity

“…A poloidal dipolar field, on the other hand, is created by the shift of energy from the toroidal field to the poloidal field due to the dispersion and decay of the BMRs via surface flux transport processes during the declining phase of the solar cycle (Babcock, 1961;Bhowmik & Nandy, 2018;Leighton, 1969). The largescale solar magnetic field that is tied to the solar activity cycles as described above governs the coronal conditions and plays a role in balancing the heliospheric open flux and the resulting HMF (see e.g., Pal et al, 2020;Schwadron et al, 2010).Many attempts have been made to understand the cause of the longer-term solar variations. These variations dictate the amplitude of the solar cycles (…”