Erich Rieger (1 October 1935 – 14 August 2021) was a German astrophysicist who spent his research career at the

Max Planck Institute for Extraterrestrial Physics The Max Planck Institute for Extraterrestrial Physics is part of the Max Planck Society, located in Garching, near Munich, Germany. In 1991 the Max Planck Institute for Physics and Astrophysics split up into the Max Planck Institute for Extraterr ...

(MPE) near

Munich Munich is the capital and most populous city of Bavaria, Germany. As of 30 November 2024, its population was 1,604,384, making it the third-largest city in Germany after Berlin and Hamburg. Munich is the largest city in Germany that is no ...

. He is notable for his 1984 discovery of the

period Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

of ~154 days in

solar flares A solar flare is a relatively intense, localized emission of electromagnetic radiation in the Stellar atmosphere, Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar partic ...

. Since the discovery, the period has been confirmed in most heliophysics data in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

, including the

interplanetary magnetic field The interplanetary magnetic field (IMF), also commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar System ...

, and has become known as the ''Rieger period'' (P_R). Rieger died on 14 August 2021, at the age of 85.

Rieger periodicities

Rieger period

Rieger and coworkers discovered in 1984 a strong period of ~154 days in hard solar flares, at least since the

solar cycle 19 Solar cycle 19 was the nineteenth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 10.5 years, beginning in April 1954 and ending in October 1964. The International Geophysical Year occurred ...

. The period has since been confirmed in most heliophysics data and the

, and is commonly known as the ''Rieger period''.

Rieger-type periodicities

Besides numerous confirmations of P_R, its

resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

harmonics In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st harm ...

were reported as well, including P_R, P_R, P_R, P_R, and P_R, i.e., ~128, ~102, ~78, ~51, and ~31 days, called ''Rieger-type periodicities''. Types of data periodic with Rieger cycles include

solar flare A solar flare is a relatively intense, localized emission of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and ot ...

photospheric The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esca ...

magnetic flux In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the we ...

, group sunspot numbers, and

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

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. 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. 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 Solar maximum is the regular period of greatest solar activity during the Sun's 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. On average, the solar cycle take ...

activity.

Origin of Rieger resonance

Various proposals exist as to the origin of the underlying resonant process behind P_R in the dynamics of Sun-ejected particles and its modulations and harmonics, including possible influences of planetary constellations on the Sun. 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. The entire Rieger resonance was detected in the interplanetary magnetic field as well, including

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

's vicinity.

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 Limb darkening is an optical effect seen in stars (including the Sun) and planets, where the central part of the disk appears brighter than the edge, or ''limb''. Its understanding offered early solar astronomers an opportunity to construct model ...

. 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. Since in both of these cases the bulk of the emission is

bremsstrahlung In particle physics, bremsstrahlung (; ; ) is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic ...

from primary electrons, these results imply that the radiating electrons are

anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...

. 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 The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

than in directions away from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

. In order to account for the anisotropy of the

gamma-ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists ...

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 The solar transition region is a region of the Sun's atmosphere between the upper chromosphere and corona. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere: * Below, gr ...

. 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

barium Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s, and which was released by the German IRM (Ion Release Module) satellite in 1985.Rieger, E. (2013) Explosive rocket men. In
Reflections on 50 years of extraterrestrial research, 1963–2013
Monograph on the occasion of the golden jubilee of the Max Planck Institute for Extraterrestrial Physics, pp. 18–21.

References

External links

Max Planck Gesellschaft

Max Planck Institute for Astronomy

NASA

European Space Agency
{{DEFAULTSORT:Rieger, Erich German physicists Technical University of Munich alumni 1935 births Scientists from Würzburg Max Planck Institutes researchers 20th-century German physicists 21st-century German scientists German astrophysicists 2021 deaths