Erich Otto Ernst Rieger (born 1935 in

Würzburg Würzburg (; Main-Franconian: ) is a city in the region of Franconia in the north of the German state of Bavaria. Würzburg is the administrative seat of the ''Regierungsbezirk'' Lower Franconia. It spans the banks of the Main River. Würzburg ...

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...

) is a German astrophysicist who spent his research career at the

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

(MPE) near

Munich Munich ( ; german: München ; bar, Minga ) is the capital and most populous city of the German state of Bavaria. With a population of 1,558,395 inhabitants as of 31 July 2020, it is the third-largest city in Germany, after Berlin and ...

. He is notable for his 1984 discovery of the

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

of ~154 days in solar flares. 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 "Solar S ...

, including the

interplanetary magnetic field The interplanetary magnetic field (IMF), now more 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 Sy ...

, and has become known as the ''Rieger period'' (P_R).

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 may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. " solar panels") ** Solar calendar, a calendar whose dates indica ...

. 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 describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...

harmonics A harmonic is a wave with a frequency that is a positive integer multiple of the '' fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', ...

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 an intense localized eruption 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 other solar phe ...

photospheric The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...

magnetic flux, group sunspot numbers, and

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

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 Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...

. 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 tak ...

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 harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

'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. 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 ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...

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

anisotropic Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...

. 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. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...

than in directions away from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. In order to account for the anisotropy of the

gamma-ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically sh ...

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, gra ...

. 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

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 conve ...

s, and which was released by the German IRM (

Ion Release Module An ion () is an atom or molecule with a net electric charge, 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 po ...

) 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 Living people 1935 births Scientists from Würzburg Max Planck Institutes researchers 20th-century German scientists 21st-century German scientists German astrophysicists