Rieger and coworkers discovered in 1984 a strong period of ~154 days in hard solar flares, at least since the
solar cycle 19.[2] The period has since been confirmed in most heliophysics data and the
interplanetary magnetic field, and is commonly known as the Rieger period.[3]
Rieger-type periodicities
Besides numerous confirmations of PR, its
resonanceharmonics were reported as well, including 5⁄6PR, 2⁄3PR, 1⁄2PR, 1⁄3PR, and 1⁄5PR, i.e., ~128, ~102, ~78, ~51, and ~31 days, called Rieger-type periodicities.[4] Types of data periodic with Rieger cycles include
solar flares,
photosphericmagnetic flux, group
sunspot numbers, and
proton speed. Various longer (1–2 years) modulations also were reported in almost all heliophysics data types. Besides the above mentioned, data types that exhibit long-periodic dynamics include solar flare index,
solar radio flux, and others, except for the coronal index and 10.7 cm
solar flux.[5]
So far, these periodicities have been reported in different ranges, depending on data, location, epoch, and methodology, as 155–160 days, 160–165 days, 175–188 days, and 180–190 days.[6] Most of those studies indicate a leading periodicity ranging from 152 to 158 days, which appears to be dominant particularly in the time phase from ~1979–1983, corresponding to the
solar maximum activity.[7]
Origin of Rieger resonance
Various proposals exist as to the origin of the underlying resonant process behind PR in the dynamics of Sun-ejected particles and its modulations and harmonics, including possible influences of planetary constellations on the Sun.[8][9] One such report found that a damped periodically forced nonlinear oscillator, which exhibits both periodic and chaotic behavior, can simulate the process described by Rieger periodicities.[10] The entire Rieger resonance was detected in the interplanetary magnetic field as well, including
Earth's vicinity.[11]
Other work
High-energy solar flares
In 1989, Rieger provided strong evidence that flares with emissions >10 MeV are visible only near the
solar limb.[12] Gamma-ray-emitting flares are observed from sites located predominantly near the limb of the Sun; this effect was observed for flares detected at energies >0.3 MeV, but it is at energies >10 MeV that the effect is particularly pronounced.[13] Since in both of these cases the bulk of the emission is
bremsstrahlung from primary electrons, these results imply that the radiating electrons are
anisotropic. Thus, the anisotropy could result from the mirroring of the charged particles in the convergent chromospheric magnetic fields.
The emissions are strongly anisotropic, with more emission in the directions tangential to the
photosphere than in directions away from the
Sun.[14] In order to account for the anisotropy of the
gamma-ray emission from high energy solar flares, invoked are electron transport in the
coronal region and magnetic mirroring of converging magnetic flux tubes beneath the
solar transition region. As the gaseous models of the Sun cannot support the existence of a real surface, another mechanism must act as a surface.
Artificial comet
Rieger was involved in the MPE early research initiatives, including the first
artificial comet, created by a cloud of
bariumions, and which was released by the German IRM (Ion Release Module) satellite in 1985.[15]
^Rieger, E.; Share, G. H.; Forrest, D. J.; Kanbach, G.; Reppin, C.; Chupp, E. L. (1984). "A 154-day periodicity in the occurrence of hard solar flares?". Nature. 312 (5995). Springer Science and Business Media LLC: 623–625.
Bibcode:
1984Natur.312..623R.
doi:
10.1038/312623a0.
ISSN0028-0836.
S2CID4348672.
^Bai, Taeil; Cliver, E. W. (1990). "A 154 day periodicity in the occurrence rate of proton flares". The Astrophysical Journal. 363. American Astronomical Society: 299.
Bibcode:
1990ApJ...363..299B.
doi:
10.1086/169342.
ISSN0004-637X.
^Ramaty, R., Simnett, G. M. (1991) Accelerated particles in solar flares. In: Sonett, C. P., Giampapa, M. S., Matthews, M. S. (Eds.)
The Sun in Time, The University of Arizona Press, Tucson, AZ, pp. 232–259.
^Miller, James A.; Ramaty, Reuven (1989). "Relativistic electron transport and bremsstrahlung production in solar flares". The Astrophysical Journal. 344. American Astronomical Society: 973.
Bibcode:
1989ApJ...344..973M.
doi:
10.1086/167865.
ISSN0004-637X.