Measurement of the Open Magnetic Flux in the Inner Heliosphere down to 0.13AU

Abstract

(Abridged) Aim: We attempt to determine robust estimates of the heliospheric magnetic flux (${\Phi }_H$) using Parker Solar Probe (PSP) data, analyze how susceptible this is to overestimation compared to the true open flux (${\Phi }_{open}$), assess its dependence on time and space, and compare it to simple estimates from Potential Field Source Surface (PFSS) models. Methods: We compare different methods of computation using data from PSP, STEREO A and Wind. The effects of fluctuations and large scale structure on the estimate are probed by using measured radial trends to produce synthetic data. Best estimates are computed as a function of time and space, and compared to estimates from PFSS models. Results: Radially-varying fluctuations of the HMF vector and variation of the Parker spiral angle cause the standard metrics of the mean and mode to evolve with radius independent of the central value about which the vector fluctuates. This is best mitigated by projecting the vector into the background Parker spiral direction. Nevertheless, we find a small enhancement in flux close to 1AU. The fraction of locally inverted field lines grows with radial distance from the Sun which remains a possible physical reason for this excess, but is negligible at PSP`s perihelia. Similarly, the impact of fluctuations in general is much reduced at PSP`s perihelia. The overall best estimate is ~2.5 nT AU2 . No strong dependence on latitude or longitude is apparent. The PFSS models predict lower values from 1.2 to 1.8 nT AU2. Conclusions: The heliospheric flux is robustly estimated relative to a mean Parker spiral direction at PSP`s perihelia where the decay of fluctuations and weakening importance of local flux inversions means ${\Phi }_H$ ~ ${\Phi }_{open}$. Despite this, the estimate remains too high to be explained by PFSS models, supporting the idea that coronal models underestimate the open magnetic flux.

17 pages, 10 figures main text; 4 pages, 4 figures appendix; submitted to Astronomy and Astrophysics, September 11 2020, Revised November 1 2020, Accepted November 4 2020

References in corpus (11)

• Array Programming with NumPy
• The Solar Orbiter mission -- Science overview
• Switchbacks in the near-Sun magnetic field: long memory and impact on the turbulence cascade
• The Open Flux Problem
• Magnetic connectivity of the ecliptic plane within 0.5 AU : PFSS modeling of the first PSP encounter
• Estimating Total Open Heliospheric Magnetic Flux
• Coronal Magnetic Field Topology From Total Solar Eclipse Observations
• The Near-Sun Streamer Belt Solar Wind: Turbulence and Solar Wind Acceleration
• Global solar magnetic field organization in the outer corona: influence on the solar wind speed and mass flux over the cycle
• The Evolution of Inverted Magnetic Fields Through the Inner Heliosphere
• Using Parker Solar Probe observations during the first four perihelia to constrain global magnetohydrodynamic models

Cited by in corpus (16)

• Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum
• A solar source of Alfvénic magnetic field switchbacks: {\em in situ} remnants of magnetic funnels on supergranulation scales
• Evolution of large-amplitude Alfvén waves and generation of switchbacks in the expanding solar wind
• Coronal Hole Detection and Open Magnetic Flux
• The Near-Sun Streamer Belt Solar Wind: Turbulence and Solar Wind Acceleration
• Prediction and Verification of Parker Solar Probe Solar Wind Sources at 13.3 R${}_{\odot }$
• Using Parker Solar Probe observations during the first four perihelia to constrain global magnetohydrodynamic models
• Magnetograph Saturation and the Open Flux Problem
• The preferential orientation of magnetic switchbacks, implications for solar magnetic flux transport
• Global Coronal Equilibria with Solar Wind Outflow
• Density And Velocity Fluctuations of Alpha Particles in Magnetic Switchbacks
• The Kinetic Expansion of Solar-Wind Electrons: Transport Theory and Predictions for the very Inner Heliosphere
• A New Component from the Quiet Sun: Synchrotron Radiation from Galactic Cosmic-Ray Electrons
• Which Component of Solar Magnetic Field Drives the Evolution of Interplanetary Magnetic Field over Solar Cycle?
• Transport of Cosmic ray electrons from 1 AU to the Sun
• What Determines the Brightness of the Magnetically Open Solar Corona?: Insights from Three-dimensional Radiative Magnetohydrodynamic Simulations and Observations