David M. Rust

The origin and development of the May 1997 magnetic cloud

A complete halo coronal mass ejection (CME) was observed by the SOHO Large-Angle and Spectrometric Coronagraph (LASCO) coronagraphs on May 12, 1997. It was associated with activity near Sun center, implying that it was aimed earthward. Three days later on May 15 an interplanetary shock and magnetic cloud/flux rope transient was detected at the Wind spacecraft 190 RE upstream of Earth. The long enduring southward magnetic fields associated with these structures triggered a geomagnetic storm. The CME was associated with a small coronal arcade that formed over a filament eruption with expanding double ribbons in Hα emission. The flare was accompanied by a circular EUV wave, and the arcade was flanked by adjacent dimming regions. We surmise that these latter regions marked the feet of a flux rope that expanded earthward into the solar wind and was observed as the magnetic cloud at Wind. To test this hypothesis we determined key parameters of the solar structures on May 12 and compared them with the modeled flux rope parameters at Wind on May 15. The measurements are consistent with the flux rope originating in a large coronal structure linked to the erupting filament, with the opposite-polarity feet of the rope terminating in the depleted regions. However, bidirectional electron streaming was not observed within the cloud itself, suggesting that there is not always a good correspondence between such flows and ejecta.

Expansion of an X-ray coronal arch into the outer corona

An asymmetric, expanding arch, photographed in the inner corona with an X-ray telescope on 13 August, 1973, is identified as the source of the mass ejected in a white light transient in the outer corona. The morphology, angular position, estimated mass and apparent rate of upward acceleration of the lower coronal arch are similar to those of the arch seen passing through the outer corona. The mass of material removed from the lower corona is estimated at 2 × 1015 g, and the upward movement is consistent with a constant acceleration of 12.5 m s−2 between 1.3 and 5 R⊙.

Coronal X-ray enhancements associated with Hα filament disappearances

A survey of soft X-ray images from Skylab has revealed a class of large-scale transient X-ray enhancements in the lower corona which are typically associated with the disappearance of Hα filaments away from active regions. Contemporary with the Hα filament disappearance, X-ray emitting structures appeared at or near the filament location with shape and size resembling the filament. Eventually these structures faded, but the filament cavity was no longer obvious. Typically the peak of the X-ray event lagged the end of the filament disappearance by tens of minutes. The durations of the coronal X-ray enhancements were considerably longer than the associated Hα filament disappearances. Major flare effects, such as chromospheric brightenings, typically were not associated with these X-ray events.One event analyzed quantitatively had a peak temperature between 1.8 and 2.7 × 106 K, achieved a peak density of ≈109 cm−3 and resulted in an enhancement in the plasma pressure over the conditions of the preexisting coronal cavity of at least a factor of 7. The mass of the coronal X-ray emitting material was about 10% that of the preexisting filament and the thermal energy of the coronal event was on the order of 1029 erg, about 10% of the mechanical energy of the Hα filament eruption. The event appeared to cool by radiative losses and not by thermal conduction. It is likely that the coronal enhancements are caused by heating of an excess of previously cooler material, either from the filament itself, or by compression of coronal material by a changing magnetic field.

Coronal disturbances and their terrestrial effects

Coronal disturbances lead to geomagnetic storms, proton showers, auroras and a wide variety of other phenomena at Earth. Yet, attempts to link interplanetary and terrestrial phenomena to specific varieties of coronal disturbances have achieved only limited success. Here, several recent approaches to prediction of interplanetary consequences of coronal disturbances are reviewed. The relationships of shocks and energetic particles to coronal transients, of proton events to γ-ray bursts, of proton events to microwave bursts, of geomagnetic storms to filament eruptions and of solar wind speed increases to the flare site magnetic field direction are explored. A new phenomenon, transient coronal holes, is discussed. These voids in the corona appear astride the long decay enhancements (LDEs) of 2–50 Å X-ray emission that follow Hα filament eruptions. The transient holes are similar to long-lived coronal holes, which are the sources of high speed solar wind streams. There is some evidence that transient coronal holes are associated with transient solar wind speed increases.

Soft X-ray observations of large-scale coronal active region brightenings

One-hundred fifty-six large-scale enhancements of X-ray emission from solar active regions were studied on full-disk filterheliograms to determine characteristic morphology and expansion rates for heated coronal plasma. The X-ray photographs were compared with Hα observations of flares, sudden filament disappearances, sprays and loop prominence systems (LPS). Eighty-one percent of the X-ray events were correlated with Hα filament activity, but only forty-four percent were correlated with reported Hα flares. The X-ray enhancements took the form of loops or arcades of loops ranging in length from 60 000 km to 520 000 km and averaging 15 000 km in width. Lifetimes ranged from ≥3 hr to >24 hr. Event frequency was ∼1.4 per day. X-ray loop arcades evolved from sharp-edged clouds in cavities vacated by rising Hα filaments. Expansion velocities of the loops were ∼50 km s-1 immediately after excitation and 1–10 km s-1 several hours later. These long-lived loop arcades are identified with LPS, and it is suggested that the loops outlined magnetic fields which were reconnecting after filament eruptions. Another class of X-ray enhanced loops stretched outside active regions and accompanied sprays or lateral filament ejections. Hα brightenings occurred where these loops intersected the chromosphere. Inferred excitation velocities along the loops ranged between ∼300 and 1200 km s-1. It is suggested that these loops outlined closed magnetic fields guiding slow mode shocks from flares and filament eruptions.

Physical parameters in long-decay coronal enhancements

X-ray images have been studied quantitatively to determine electron temperature and density as functions of time in two long-decay X-ray enhancements (LDEs). This is the first study of the X-ray emission from LDEs to include all corrections for scattering and vignetting. Derived electron density is about twice that found by Vorpahl et al. (1977) and by Smith et al. (1977) in the same events. Our results are combined with those for two other LDEs to find their general characteristics. The LDEs all had the form of arcades of very bright loops which were 1–3 × 106 K hotter at the apices than along the legs. This temperature structure was maintained for at least 8 hr in each case. From this it is inferred that continual heating was taking place at the loop apices. Each LDE was preceded by a filament eruption and a white-light transient. Each was associated with a loop prominence system (LPS) composed of cool (Te < 105 K) loops nested 2–8 × 103 km below the hot LDE loops. And, although the energy release rates in the four events varied greatly even 4 hr after onset, they all had similar growth rates (loop height vs time ≅ 1 km s−1). Event lifetimes were very long, from 24 to 72 hr. After a survey of published models, it is concluded that only a magnetic reconnection model (e.g., Kopp and Pneuman, 1976) is consistent with these observations of the LDE-LPS phenomenon.

An active role for magnetic fields in solar flares

Magnetic fields in the low corona are the only plausible source of energy for solar flares. Other energy sources appear inadequate or uncorrelated with flares. Low coronal magnetic fields cannot be measured accurately, so most attention has been directed toward measurements of the photospheric magnetic fields from which coronal developments may be inferred. Observations of these magnetic fields are reviewed. It is concluded that, except possibly for the largest flares, changes in the photospheric magnetic fields in flaring centers are confined to evolutionary changes associated with emergence of new magnetic flux. Flare observations with the 10830 Å line of helium, in particular, are discussed. It is concluded that the brightest flare knots appear near points of emergent magnetic flux. Pre-flare activation and eruptions of Hα filaments are discussed. It is concluded that the rapid motions in filaments indicate unambiguously that the magnetic fields in the low corona are severely disrupted prior to most flares. The coronal signature of Hα filament eruptions is illustrated with soft X-ray photographs from the S-054 experiment of the NASA Skylab mission. An attempt is made, by studying X-ray flare morphology, to determine whether flares grow by reconnections between adjacent or intertwined magnetic elements or by triggering, in which each flaring loop drives adjacent loops to unstable states. It is concluded that successive loop brightenings are most easily interpreted as the result of magnetic field reconnections, although better time resolution is required to settle the question. A model of magnetic field reconnections for flares associated with filament activation and emerging magnetic flux is presented.

Flare loops heated by thermal conduction

AbstractA flare observed with the Hard X-Ray Imaging Spectrometer (HXIS) was studied during its rise to maximum temperature and X-ray emission rate. Two proximate flare loops, of lengths 2.8 × 109 cm and 1.1 × 1010 cm, rose to temperatures of 21.5 × 106 K and 30 × 106 K, respectively, in 30 s. Assuming equal heat flux F into each loop from a thermal source at the point where they met, we derive a simple relationship between temperature T and loop length

The work of the diode array: He 10 830 observations of spicules and subflares

L:T = (\tfrac{7}{2}(FL/K))^{2 7}