arXiv subjects

John W. Bonnell

Publications and source records attributed to John W. Bonnell.

21 records · Page 2Linked to original sources

arXiv2019

Magnetic connectivity of the ecliptic plane within 0.5 AU : PFSS modeling of the first PSP encounter

We compare magnetic field measurements taken by the FIELDS instrument on Parker Solar Probe (PSP) during its first solar encounter to predictions obtained by Potential Field Source Surface (PFSS) modeling. Ballistic propagation is used to connect the spacecraft to the source surface. Despite the simplicity of the model, our results show striking agreement with PSPs first observations of the heliospheric magnetic field from 0.5 AU (107.5 Rs) down to 0.16 AU (35.7 Rs). Further, we show the robustness of the agreement is improved both by allowing the photospheric input to the model to vary in time, and by advecting the field from PSP down to the PFSS model domain using in situ PSP/SWEAP measurements of the solar wind speed instead of assuming it to be constant with longitude and latitude. We also explore the source surface height parameter (RSS) to the PFSS model finding that an extraordinarily low source surface height (1.3-1.5Rs) predicts observed small scale polarity inversions which are otherwise washed out with regular modeling parameters. Finally, we extract field line traces from these models. By overlaying these on EUV images we observe magnetic connectivity to various equatorial and mid-latitude coronal holes indicating plausible magnetic footpoints and offering context for future discussions of sources of the solar wind measured by PSP.

astro-ph.SR↗

arXiv2018

Impact of Residual Energy on Solar Wind Turbulent Spectra

It is widely reported that the power spectra of magnetic field and velocity fluctuations in the solar wind have power law scalings with inertial-range spectral indices of -5/3 and -3/2 respectively. Studies of solar wind turbulence have repeatedly demonstrated the impact of discontinuities and coherent structures on the measured spectral index. Whether or not such discontinuities are self-generated by the turbulence or simply observations of advected structures from the inner heliosphere has been a matter of considerable debate. This work presents a statistical study of magnetic field and velocity spectral indices over 10 years of solar-wind observations; we find that anomalously steep magnetic spectra occur in magnetically dominated intervals with negative residual energy. However, this increase in negative residual energy has no noticeable impact on the spectral index of the velocity fluctuations, suggesting that these intervals with negative residual energy correspond to intermittent magnetic structures. We show statistically that the difference between magnetic and velocity spectral indices is a monotonic function of residual energy, consistent with previous work which suggests that intermittency in fluctuations causes spectral steepening. Additionally, a statistical analysis of cross helicity demonstrates that when the turbulence is balanced (low cross-helicity), the magnetic and velocity spectral indices are not equal, which suggests that our observations of negative residual energy and intermittent structures are related to non-linear turbulent interactions rather than the presence of advected pre-existing flux-tube structures.

astro-ph.SR↗

arXiv2018

Electrostatic steepening of whistler waves

We present surprising observations by the NASA Van Allen Probes spacecraft of whistler waves with substantial electric field power at harmonics of the whistler wave fundamental frequency. The wave power at harmonics is due to nonlinearly steepened whistler electrostatic field that becomes possible in the two-temperature electron plasma due to whistler wave coupling to the electron-acoustic mode. The simulation and analytical estimates show that the steepening takes a few tens of milliseconds. The hydrodynamic energy cascade to higher frequencies facilitates efficient energy transfer from cyclotron resonant electrons, driving the whistler waves, to lower energy electrons.

physics.space-ph↗