Introduction

It is well established that the hot, magnetic loops associated with solar flares appear to expand outward after the most explosive phase of many flares. This apparent expansion is believed to be a manifestation of the formation of new magnetic loops in the corona through magnetic reconnection.

Theoretical expectations

In the simplest picture, oppositely directed magnetic field lines that are roughly vertical relative to the solar surface pinch together, where they reconnect and form new field lines that snap both upward and downward, away from the reconnection region (see illustration below). The new, upward-moving field lines form a large coronal loop that may become a coronal mass ejection (CME). The new downward-moving field lines form a relatively compact coronal loop or arcade of loops. This compact loop continues to build up, somewhat like adding more and more layers to an onion, as long as the magnetic reconnection continues above it.

Magnetic reconnection also extracts part of the energy from the original magnetic field. This heats the plasma contained in the reconnected magnetic field lines and accelerates many of the charged particles in the plasma to high energies. Thus, the newly formed magnetic loops are freshly heated, while the older loops in the layers below are cooling. This continued build up of new, hot magnetic layers is seen as the apparent outward expansion of the flare loop.

RHESSI Results

RHESSI observations have added a new twist to this picture. Observations of hot, flare loops in X-rays have shown that in most cases the loop contracts downward during the early, most explosive part of the flare before the apparent outward expansion is observed. An example of this is shown in the figure below. This downward contraction was not predicted by flare models. The higher energy and, therefore, higher temperature X-ray source is observed to be at a higher altitude than the source seen at lower energies. It also contracts more rapidly than the looptop source seen at lower energies.

Discussion

What is the significance of this downward contraction? One possibility is that we are seeing the new magnetic field as it snaps downward from the reconnection region. The observed downward motion is much slower than the rate at which the magnetic field is ejected from the reconnection region. However, the downward motion will slow as the new magnetic field settles onto the existing layers. This downward motion could only be observed if the rate of buildup of new magnetic field is slower than the speed of this downward settling. Perhaps this is the case early in the flare. A specific model that accounts for heating in the contracting magnetic field lines has recently been proposed.

A possible problem with this explanation is that there is evidence that the loop footpoints also contract inward early in the flare. (For an intriguing example, see the second figure in the recent RHESSI Nugget by Paolo Grigis.) Such an inward motion of the footpoints is not expected in this explanation.

An alternative explanation is tied to the onset and development of magnetic reconnection in the flare. During explosive reconnection energy is rapidly extracted from the magnetic field in the corona above the hot flare loops. This could result in a sort of implosion that compresses the flare loops downward. An associated elongation and thinning of the reconnection region could explain both the downward contraction of the looptop and the inward motion of the footpoints.

Whatever the answer, it is likely to be fundamental to our understanding of flares.

Biographical note: Gordon Holman is an astrophysicist and RHESSI Co-Investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland