What is a Hyder flare?
Flares are intense brightenings that occur in the solar chromosphere. Flares are generally observed from Earth using narrow band filters, typically with a bandwidth of less than 0.1 nm, and often centred on the Hydrogen-alpha wavelength of 656.3 nm. (Flares also have counterparts, that is, sudden outbursts, in the radio and X-ray spectrum).
Most flares occur around active regions associated with sunspot groups. However, occasionally a flare (sudden brightening) is observed well away from an active region or sunspot group. These flares are invariably associated with the sudden disappearance of a large (thick, long, 'bushy') dark solar filament, and are termed Hyder flares.
Why are Hyder flares so named?
Max Waldmeier wrote a paper in 1938 which described the phenomenon of suddenly disappearing filaments (disparition brusque), and mentioned that these can be associated with flare-like brightenings, but it was left to Charles Hyder to postulate the first comprehensive mechanism for the such flares.
Following on work from his doctoral thesis with the University of Colorado in Boulder (1964), Hyder published two papers in the second volume of the journal Solar Physics (1967) in which the mechanism by which Hyder flares might occur was discussed in detail. Hyder was then on the staff of the (US) Air Force Cambridge Research Laboratories at the Sacramento Peak Observatory in New Mexico.
It was these papers in Solar Physics by which Hyder's name became associated with the flares in question, even though he was by no means the first to observe them.
What are the characteristics of Hyder flares?
As previously mentioned, the name Hyder flare is given to a flare that occurs away from an active region or sunspot group and that is associated with the sudden disappearance of a dark filament. The appearance of these flares can range from a string of bright knots on one or both sides of the filament (or rather, the position previously occupied by the filament, sometimes called the filament channel), to a single or double ribbon flare. The ribbons are parallel to the filament channel. If only one ribbon is present, it will lie to one side of the channel, whereas if two parallel ribbons occur, one ribbon will lie on one side of the filament channel, and the other ribbon will lie on the opposite side.
One interesting characteristic of Hyder flares is that they usually develop or rise to maximum brightness much more slowly than do the more common flares associated with active regions. The larger Hyder flares may take 30 to 60 minutes to rise to a peak intensity, and then they may last for several hours. Although they may attain a large area, they usually have a relatively low intensity. Thus, classifications for a large Hyder flare may read 2F (F=faint), 2N (N=normal) or possibly even 3F. This contrasts to an active region flare in which 3F is very rare. An active region flare that attains sufficient area to put it into the importance class 3, will invariably have either an N or more usually a B (B=brilliant) brightness classification.
X-ray flares and radio (microwave) bursts associated with the optical Hyder flare, are also generally long lived phenomenon and are classified as the gradual rise and fall type of event (in contrast to the impulsive and complex events associated with large active region flares).
Generally Hyder flares are not associated with energetic particle emission or geomagnetic storms (implying that they may not be associated with a coronal mass ejection). However, this is not always the case, as a large halo CME observed by the LASCO solar coronagraph on board the SOHO spacecraft was most definitely associated with a Hyder flare (2N/M1) observed on 12 September 2000. This same complex also appeared to have produced energetic protons at geosynchronous orbit with energies in excess of 100 MeV, and in substantial numbers at energies of 10 MeV. It is believed that the sudden storm commencement observed at 0450UT 15 September, and the subsequent minor geomagnetic storm was produced by this particular CME.
What produces Hyder flares?
Hyder's explanation of the flare type now named after him depended on the observational evidence that (1) often the flare was a parallel ribbon flare with one ribbon each side of the filament channel, and (2) that geomagnetic storms were not associated with these flares. This led to the speculation that the filamentary material was not ejected far into the corona, but in fact fell back to the chromosphere producing the flare.
Stable or quiescent filaments are believed to lie in and along a magnetic trough. It is thought that the sudden disappearance of such a filament is due to a reconfiguration of the field. In essence, the magnetic trough becomes a magnetic ridge (the bottom of the trough elevating in a period of tens of minutes to become the peak of the ridge). In this process, the filamentary material (cooler gas) is thought to be accelerated into the corona. Hyder's explanation is that, in the case of the Hyder flare, some or even most of the filament material, instead of suffering acceleration and ejection, falls down the sides of the magnetic ridge and interacts with the lower chromospheric material producing the flare. If the infall process is symmetrical, then the double parallel ribbon flare will result, if asymmetrical, then only one ribbon results. If the infall is sporadic, or the material insufficient, then only bright knots of flare are produced. Hyder did calculations to show that the kinetic energy of the infalling material should be sufficient to provide the required flare energy release observed.
The Hyder mechanism has come into question. Some people (notably Zirin) have questioned whether infall occurs, stating that the magnetic reconfiguration must always produce ejection. The respective roles of flares and CMEs in solar active processes has also been hotly debated, and this has implications for the exact mechanism of Hyder flares. We certainly have enough observational evidence to show that Hyder flares can be associated with both CMEs and energetic particle production. For the moment, the question of Hyder flare production mechanism appears unresolved, and will probably be sidelined until the more significant (and undoubtedly related) issue of CME - flare production mechanism is sorted out.
The bottom line is that at this stage in solar physics we do not really know what produces a flare nor what produces a CME. There are competing theories, but all tend to have deficiencies with respect to matching the observational evidence. We certainly believe that they all depend on the reconfiguration of magnetic fields as their primary energy source, but in the final analysis, we really only believe this because we can conceive of no other solar energy source of sufficient magnitude.