Mass flow and the validity of ionization equilibrium on the Sun

Ionization equilibrium is a useful assumption which allows temperatures and other plasma properties to be deduced from spectral observations. Inherent to this assumption is the premise that the ion stage densities are determined solely by atomic processes which are local functions of the plasma temperature and electron density. However, if the time scale of plasma flow through a temperature gradient is less than the characteristic time scale for an important atomic process, deviations from the ionization stage densities expected for equilibrium will occur which could introduce serious errors into subsequent analyses. In the past few years, significant flow velocities in the upper solar atmosphere have been inferred from observations of emission lines originaing in the transition region (about 10⁴–10⁶ K) and corona. In this paper, three models of the solar atmosphere (quiet Sun, coronal hole, and a network model) are examined to determine if the emission expected from these model atmospheres could be produced from equilibrium ion populations when steady flows of several kilometers per second are assumed. If the flows are quasi-periodic instead of steady, spatial and temporal averaging inherent in the observations may allow for the construction of satisfactory models based on the assumption of ionization equilibrium. Representative emission lines are analysed for the following ions: C iii, iv, O iv, v, vi, Ne vii, viii, Mg ix, x, Si xii, and Fe ix–xiv. Two principle conclusions are drawn. First, only the iron ions are generally in equilibrium for steady flows of 20 km s^–1. For carbon and oxygen, ionization equilibrium is not a valid assumption for steady flows as small as 1 km s^–1. Second, the three models representing different solar conditions behave in a qualitatively similar manner, implying that these results are not particularly model dependent over the range of temperature gradients and electron densities thus far inferred for the Sun. In view of the flow velocities which have been reported for the Sun, our results strongly suggest caution in using the assumption of ionization equilibrium for interpreting spectral lines produced in the transition region.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Proposed major format change to geomagnetic activity reports and forecasts produced by the SESC,Boulder, Colorado,U.S.A.

The Space Environment Services Center (SESC) provides real-time information on the state of the solar-terrestrial environment. In particular, the geomagnetic field is closely monitored. Summaries of geomagnetic activity are issued daily, as are forecasts for each of the next three days. Customers are alerted when activity rises above selected thresholds of theK- andA-indices. This present geomagnetic reporting and forecasting format is no longer the most appropriate for the expanding number of agencies and industries which the SESC serves. These customers need more detailed information about the geographic distribution of geomagnetic activity stated in plainer language. The proposed new format would quantitatively describe geomagnetic activity in terms of 6 qualitative categories: quiet, unsettled, active, minor storm, major storm, and severe storm. Summaries and forecasts would be made for 2 specific latitude zones: mid-latitudes (20–50 deg geomagnetic) and high-latitudes (50–80 deg geomagnetic). The format adapts well to longer-range (27-day) forecasts.     

A large arcade along the inversion line,observed on May 19, 1992 by Yohkoh,and enhancement of interplanetary energetic particles

A large arcade associated with a long-duration soft X-ray emission was observed on May 19, 1992 by the Yohkoh soft X-ray telescope. This large arcade was formed along the inversion line and a filament eruption was observed as part of this event. Also associated with this event were solar energetic particles and an interplanetary shock observed near Earth. This event supports the idea that coronal mass ejections are large-scale eruptions along an inversion line, or a heliospheric current sheet. However, this event implies that present models on eruptions are not sufficient.     

Geomagnetic quiet day selection

Based on published literature and the response to a questionnaire sent to geomagnetic field, ionospheric and magnetospheric researchers, several methods of choosing periods of quiet conditions based on geomagnetic records, as well as other observed parameters, have been identified. Caveats with respect to using geomagnetic indices to select quiet periods include the following:

A large proton event associated with solar filament activity

We report observations made from several interplanetary spacecraft, of the large low-energy particle event of 23–27 April, 1979 associated with solar filament activity. We discuss the intensity, spectral and directional evolution of the event as observed in the energy range 35–1600 keV on ISEE-3, located ～ 0.99 AU from the Sun upstream of the Earth. We demonstrate that the shock disturbance propagating through the interplanetary medium and observed at ISEE-3 on 24/25 April strongly controls the particle event. From a comparison of the ISEE-3 observations with those on other spacecraft, in particular on Helios-2, located at 0.41 AU heliocentric distance near the Sun-Earth line, we identify the solar filament erupting on late 22 April near central meridian as the trigger for the propagating shock disturbance. This disturbance which comprises a forward shock and a reverse shock at the orbit of ISEE-3 is found to be the main source of the energetic proton population observed.     

Three solar filament disappearances associated with interplanetary low-energy particle events

Three low-energy particle events (35–1600 keV) associated with interplanetary shocks, detected at 1 AU by ISEE-3, have been identified as originating in solar disappearing filaments instead of large flares. This increases to fourteen the number of events of this kind presently known. The observational characteristics of these non-flare generated events are similar to the ones of the other eleven events already known (i.e., absence of type II or IV bursts, weak X-ray emission, H brightening in the surroundings of the filament disappearance, frequent presence of a double-ribbon event, slow propagation of the generated interplanetary shock, lack of shock deceleration).