The possibility that charged particles are accelerated statistically in a “sea” of small-scale interacting magnetic flux
ropes in the supersonic solar wind is gaining credence. In this Letter, we extend the Zank et al. statistical transport
theory for a nearly isotopic particle distribution by including an escape term corresponding to particle loss from a
finite acceleration region. Steady-state 1D solutions for both the accelerated particle velocity distribution function
and differential intensity are derived. We show Ulysses observations of an energetic particle flux enhancement
event downstream of a shock near 5 au that is inconsistent with the predictions of classical diffusive shock
acceleration (DSA) but may be explained by local acceleration associated with magnetic islands. An automated
Grad-Shafranov reconstruction approach is employed to identify small-scale magnetic flux ropes behind the shock.
For the first time, the observed energetic particle “time-intensity” profile and spectra are quantitatively compared
with theoretical predictions. The results show that stochastic acceleration by interacting magnetic islands accounts
successfully for the observed (i) peaking of particle intensities behind the shock instead of at the shock front
as standard DSA predicts; (ii) increase in the particle flux amplification factor with increasing particle energy;
(ii) increase in distance between the particle intensity peak and the shock front with increasing energy; and
(iv) hardening of particle power-law spectra with increasing distance downstream of the shock.
more »
« less
An Unusual Energetic Particle Flux Enhancement Associated with Solar Wind Magnetic Island Dynamics
The possibility that charged particles are accelerated statistically in a “sea” of small-scale interacting magnetic flux
ropes in the supersonic solar wind is gaining credence. In this Letter, we extend the Zank et al. statistical transport
theory for a nearly isotopic particle distribution by including an escape term corresponding to particle loss from a
finite acceleration region. Steady-state 1D solutions for both the accelerated particle velocity distribution function
and differential intensity are derived. We show Ulysses observations of an energetic particle flux enhancement
event downstream of a shock near 5 au that is inconsistent with the predictions of classical diffusive shock
acceleration (DSA) but may be explained by local acceleration associated with magnetic islands. An automated
Grad-Shafranov reconstruction approach is employed to identify small-scale magnetic flux ropes behind the shock.
For the first time, the observed energetic particle “time-intensity” profile and spectra are quantitatively compared
with theoretical predictions. The results show that stochastic acceleration by interacting magnetic islands accounts
successfully for the observed (i) peaking of particle intensities behind the shock instead of at the shock front
as standard DSA predicts; (ii) increase in the particle flux amplification factor with increasing particle energy;
(ii) increase in distance between the particle intensity peak and the shock front with increasing energy; and
(iv) hardening of particle power-law spectra with increasing distance downstream of the shock.
more »
« less
- Award ID(s):
- 1707247
- NSF-PAR ID:
- 10096123
- Date Published:
- Journal Name:
- The Astrophysical journal. Letters to the editor
- Volume:
- 864
- Issue:
- 2
- ISSN:
- 2471-4259
- Page Range / eLocation ID:
- L34
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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