An in-situ interplanetary "U-burst": Observation and results

TL;DR

This paper reports the observation of a rare in-situ electron event linked to a solar type III radio burst, analyzes the electron distribution and magnetic field configuration, and proposes a model explaining the observed U-burst profile.

Contribution

It presents the first in-situ observation of an interplanetary U-burst and introduces a flux-rope model that reproduces the radio emission profile and explains secondary electron features.

Findings

Electrons arrived mainly from the anti-solar direction, indicating Sunward motion.

A flux-rope model successfully reproduces the U-burst profile.

Evidence of secondary electron energy distribution was observed.

Abstract

We report and examine the observation of an unusual and rare in-situ electron observation associated with a solar type III radio burst on 24 December 1996. This radio event was accompanied by high energy electrons, measured by the Solid State Telescope (SST) on-board WIND spacecraft. The type III radio emission started at $\approx$ 13:10~UT and was associated to a C2.1 GOES-class flare whose maximum was at 13:11 UT and hosted by the active region NOAA 8007/8004, located on the west limb at N05$^\circ$ W74$^\circ$/N06$^\circ$ W85$^\circ$. During this event, the observation of an electron energy distribution likely to be associated with the radio emission was registered. The electrons arrive at the spacecraft predominantly from the anti-solar direction, suggesting that their general motion is Sunward along a closed magnetic field line. Leblanc et al. (1999) propose a model in which energetic electrons are injected into a coronal flux tube at one of its footpoints, releasing standard type-III emission. As the magnetic field then directs them back toward the magnetic-conjugate footpoint of the first, the electrons release subsequent emission whose radio profile is a quasi-time reversal of the standard. We have constructed a cylindrical flux-rope facsimile of this scenario that reproduces the U-burst profiles. We also report observational features indicating a secondary electron energy distribution and propose a scenario that explains this feature.