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Owing to the ever-present solar wind, our vast solar system is full of plasmas. The turbulent solar wind, together with sporadic solar eruptions, introduces various space plasma processes and phenomena in the solar atmosphere all the way to Earth’s ionosphere and atmosphere and outward to interact with the interstellar media to form the heliopause and termination shock. Remarkable progress has been made in space plasma physics in the last 65 years, mainly due to sophisticated in situ measurements of plasmas, plasma waves, neutral particles, energetic particles, and dust via space-borne satellite instrumentation. Additionally, high-technology ground-based instrumentation has led to new and greater knowledge of solar and auroral features. As a result, a new branch of space physics, i.e., space weather, has emerged since many of the space physics processes have a direct or indirect influence on humankind. After briefly reviewing the major space physics discoveries before rockets and satellites (Section I), we aim to review all our updated understanding on coronal holes, solar flares, and coronal mass ejections, which are central to space weather events at Earth (Section II), solar wind (Section III), storms and substorms (Section IV), magnetotail and substorms, emphasizing the role of the magnetotail in substorm dynamics (Section V), radiation belts/energetic magnetospheric particles (Section VI), structures and space weather dynamics in the ionosphere (Section VII), plasma waves, instabilities, and wave-particle interactions (Section VIII), long-period geomagnetic pulsations (Section IX), auroras (Section X), geomagnetically induced currents (GICs, Section XI), planetary magnetospheres and solar/stellar wind interactions with comets, moons and asteroids (Section XII), interplanetary discontinuities, shocks and waves (Section XIII), interplanetary dust (Section XIV), space dusty plasmas (Section XV), and solar energetic particles and shocks, including the heliospheric termination shock (Section XVI). This article is aimed to provide a panoramic view of space physics and space weather.

Graphical abstract:

Updated summaries of the August 1972 and March 1989 space weather events have been constructed. The features of these two events are compared to the Carrington 1859 event and a few other major space weather events. It is concluded that solar active regions release energy in a variety of forms (X‐rays, EUV photons, visible light, coronal mass ejection (CME) plasmas and fields) and they in turn can produce other energetic effects (solar energetic particles (SEPs), magnetic storms) in a variety of ways. It is clear that there is no strong one‐to‐one relationship between these various energy sinks. The energy is often distributed differently from one space weather event to the next. Concerning SEPs accelerated at interplanetary CME (ICME) shocks, it is concluded that the Fermi mechanism associated with quasi‐parallel shocks is relatively weak and that the gradient drift mechanism (electric fields) at quasi‐perpendicular shocks will produce harder spectra and higher fluxes. If the 4 August 1972 intrinsic magnetic cloud condition (southward interplanetary magnetic field instead of northward) and the interplanetary Sun to 1 au conditions were different, a 4 August 1972 magnetic storm and magnetospheric dawn‐to‐dusk electric fields substantially larger than the Carrington event would have occurred. Under these special interplanetary conditions, a Miyake et al. (2012), https://doi.org/10.1038/nature11123‐like extreme SEP event may have been formed. The long duration complex 1989 storm was probably greater than the Carrington storm in the sense that the total ring current particle energy was larger. All the main information for the August 1972 and March 1989 space weather events have been gathered together for the readership. Various features of these two events have been compared and contrasted to the Carrington event and several other recent major events. It is thought that major energetic MeV to GeV particle acceleration occurs at interplanetary shocks instead of acceleration at the solar flare site. Two popular mechanisms for shock particle acceleration are: (a) Fermi acceleration between waves upstream and downstream of a shock, and (b) particle gradient drift in the direction of an electric field along the shock surface. The second mechanism is concluded to form a harder particle spectrum with higher fluxes. If the intrinsic conditions of the 4 August 1972 coronal mass ejection were different (southward interplanetary magnetic fields) and that propagated to the Earth without major evolution or distortion, a magnetic storm equal to or more intense than the Carrington event would have occurred. We have also constructed a scenario involving altered interplanetary space for the August 1972 event which could lead to a high Mach number quasiperpendicular shock and possibly a solar particle event comparable to the Miyake et al. (2012, https://doi.org/10.1038/nature11123) tree ring event. The August 1972 and March 1989 space weather events are summarized and compared to the Carrington plus a few other recent eventsWith different solar and interplanetary conditions, the August 1972 ICME could have caused a storm greater than the Carrington stormThe complex March 1989 storm was a “stealth magnetic storm”, probably greater ring‐current energy‐wise than the Carrington storm The August 1972 and March 1989 space weather events are summarized and compared to the Carrington plus a few other recent events With different solar and interplanetary conditions, the August 1972 ICME could have caused a storm greater than the Carrington storm The complex March 1989 storm was a “stealth magnetic storm”, probably greater ring‐current energy‐wise than the Carrington storm

Low frequency (LF) ~22 Hz to 200 Hz plasmaspheric hiss was studied using a year of Polar plasma wave data occurring during solar cycle minimum. The waves are found to be most intense in the noon and early dusk sectors. When only the most intense LF (ILF) hiss was examined, they are found to be substorm dependent and most prominent in the noon sector. The noon sector ILF waves were also determined to be independent of solar wind ram pressure. The ILF hiss intensity is independent of magnetic latitude. ILF hiss is found to be highly coherent in nature. ILF hiss propagates at all angles relative to the ambient magnetic field. Circular, elliptical, and linear/highly elliptically polarized hiss have been detected, with elliptical polarization the dominant characteristic. A case of linear polarized ILF hiss that occurred deep in the plasmasphere during geomagnetic quiet was noted. The waveforms and polarizations of ILF hiss are similar to those of intense high frequency hiss. We propose the hypothesis that ~10–100 keV substorm injected electrons gradient drift to dayside minimum B pockets close to the magnetopause to generate LF chorus. The closeness of this chorus to low altitude entry points into the plasmasphere will minimize wave damping and allow intense noon‐sector ILF hiss. The coherency of ILF hiss leads the authors to predict energetic electron precipitation into the mid latitude ionosphere and the electron slot formation during substorms. Several means of testing the above hypotheses are discussed. Intense Low Frequency (~22 to 200 Hz) is found to be coherent at all local times. The waves are most intense in the noon sectorIntense noon sector hiss is substorm dependent and solar wind pressure independent. A hypothesis of min. B pocket chorus origin is givenLF hiss can be circularly, elliptically and linearly polarized. The majority of waves are elliptically polarized

While Yi, Reid, Xue, Younger, Spargo, et al. (2017, http://doi:10.1002/2017JA024446) described the observational results showing solar wind high‐speed stream (HSS) impacts on the mesosphere over Antarctica, the specific physical mechanism behind them was not discussed. We discussed here how magnetospheric wave‐particle interactions and energetic ~20‐ to 50‐keV electron precipitation into the auroral zone atmosphere (diffuse auroras) during HSS intervals can cause the observed effects in the mesosphere. During HSS substorm events, fresh ~10‐ to 100‐keV electrons are injected and scattered by waves creating the diffuse auroraThe ~20‐ to 50‐keV portion of the precipitating electrons deposit most of their energy at heights of ~95 km down to ~85 kmThe energy deposition creates NOx and3O3loss. This leads to atmospheric cooling and modulation of zonal wind speeds and neutral densities

Intense ~300‐Hz to 1.0‐kHz plasmaspheric hiss was studied using Polar plasma wave data. It is found that the waves are coherent in all local time sectors with the wave coherency occurring in approximately three‐ to five‐wave cycle packets. The plasmaspheric hiss in the dawn and local noon time sector are found to be substorm (AE*) and storm (SYM‐H*) dependent. The local noon sector is also solar wind pressure dependent. It is suggested that coherent chorus monochromatic subelements enter the plasmasphere (as previously suggested by ray tracing models) to explain these plasmaspheric hiss features. The presence of intense, coherent plasmaspheric hiss in the local dusk and local midnight time sectors is surprising and more difficult to explain. For the dusk sector waves, either local in situ plasmaspheric wave generation or propagation from the dayside plasmasphere is possible. There is little evidence to support substorm generation of the midnight sector plasmaspheric hiss found in this study. One possible explanation is propagation from the local noon sector. The combination of high wave intensity and coherency at all local times strengthens the suggestion that the electron slot is formed during substorm intervals instead of during geomagnetic quiet (by incoherent waves). Plasmaspheric hiss is found to propagate at all angles relative to the ambient magnetic field, θkB. Circular, elliptical, and linear polarized plasmaspheric hiss have been detected. No obvious, strong relationship between the wave polarization and θkBwas found. This information of hiss properties should be useful in modeling wave‐particle interactions within the plasmasphere. Plasmaspheric hiss is found to be coherent (at all local times). The coherency occurs in packets of ~3 to 5 cycles. For the dawn and noon local time sectors, a scenario of substorm and solar wind pressure generation of outer zone chorus with further propagation into the plasmasphere is supported by the data analysis results. The predominant wave polarization of hiss is found to be elliptical, with some minor presence of circular and linear polarizations. This is in general agreement with theoretical expectations.The presence of intense, coherent plasmaspheric hiss strongly supports the new hypothesis that the electron slot is formed during substorms rather than geomagnetic quiet periods. The loss of relativistic E ~ 1MeV electrons for the inner magnetosphere (L > 6) may be due to wave‐particle interactions with coherent plasmaspheric hiss. Intense plasmaspheric hiss is coherent at all local timesApproximately 3 to 5 cycle coherent plasmaspheric hiss are outer zone chorus subelements that have propagated into the plasmasphereMost of plasmaspheric hiss is elliptically polarized with some circular and linear polarizations; these features are consistent with theory

Solar wind turbulence within high‐speed streams is reviewed from the point of view of embedded single nonlinear Alfvén wave cycles, discontinuities, magnetic decreases (MDs), and shocks. For comparison and guidance, cometary plasma turbulence is also briefly reviewed. It is demonstrated that cometary nonlinear magnetosonic waves phase‐steepen, with a right‐hand circular polarized foreshortened front and an elongated, compressive trailing edge. The former part is a form of “wave breaking” and the latter that of “period doubling.” Interplanetary nonlinear Alfvén waves, which are arc polarized, have a ~180° foreshortened front and with an elongated trailing edge. Alfvén waves have polarizations different from those of cometary magnetosonic waves, indicating that helicity is a durable feature of plasma turbulence. Interplanetary Alfvén waves are noted to be spherical waves, suggesting the possibility of additional local generation. They kinetically dissipate, forming MDs, indicating that the solar wind is partially “compressive” and static. The ~2 MeV protons can nonresonantly interact with MDs leading to rapid cross‐field (~5.5% Bohm) diffusion. The possibility of local (~1 AU) generation of Alfvén waves may make it difficult to forecast High‐Intensity, Long‐Duration AE Activity and relativistic magnetospheric electrons with great accuracy. The future Solar Orbiter and Solar Probe Plus missions should be able to not only test these ideas but to also extend our knowledge of plasma turbulence evolution. Interplanetary Alfvénic turbulence is studied from an observational microstructural viewpoint. We use cometary turbulence as a guide and for comparison to interplanetary turbulence. It is shown that single wave cycles reveal much of the ongoing physics. Alfvén waves phase‐steepen forming a high‐frequency end, leaving a low‐frequency end. This is a form of “wave breaking” and “period doubling” occurring at the same time. If Alfvén waves occur at all scale sizes, this can explain the Kolmogrov‐type spectra found in all studies. The interplanetary medium is also highly “compressive.” This is caused by the magnetic decreases detected at the ends of the Alfvén waves. It is thought that this is a kinetic process associated with the dissipation of the Alfvén waves. The interplanetary Alfvén waves are often arc‐polarized spherical waves implying a local source of generation. Some theoretical mechanisms for local generation are cited. Finally, it is shown that the MDs can cause rapid cross‐field diffusion of energetic solar flare particles, perhaps explaining their broad distributions in solar longitude. Single cycle Alfvén waves impinging on the magnetosphere cause strong and continuous auroral activity. Interplanetary turbulence in high‐speed streams is reviewed from a microstructual observational viewpoint for the first timeSingle wave characteristics can give information on the development of turbulenceSingle wave “period doubling” and “wave breaking” have been found

An alternative scenario to the Ngwira et al. (2014, https://doi.org/10.1002/2013JA019661) high sheath densities is proposed for modeling the Carrington magnetic storm. Typical slow solar wind densities (~5 cm−3) and lower interplanetary magnetic cloud magnetic field intensities (~90 nT) can be used to explain the observed initial and main phase storm features. A second point is that the fast storm recovery may be explained by ring current losses due to electromagnetic ion cyclotron wave scattering. The 1859 Carrington magnetic storm is the largest storm in recorded history. It is used as a model of an "extreme storm" by the U.S. Homeland Security for effects of such an event on the U.S. population. Computer modelers have tried to duplicate the magnetic ground signatures of the storm that were published in Tsurutani et al. (2003, https://doi.org/10.1029/2002JA009504). Some have used extremely high solar wind densities, values which have never been detected in the space age. Here we explain why assumptions of such high densities are unnecessary. The densities used in Ngwira et al. (2014) are unnecessarily too high. Such densities are not neededIt is shown that typical solar wind densities can be used to explain the Carrington storm featuresHigh solar wind densities are not needed to explain the Carrington storm short recovery phase. An alternate mechanism is given

The Colaba, India ∼−1,600 nT magnetic spike caused by an interplanetary sheath magnetic field inducing a “dayside R1‐field aligned current wedge” during the Carrington magnetic storm proposed by Ohtani (2022), https://doi.org/10.1029/2022JA030596seems highly improbable. Normal interplanetary magnetic field intensities of ∼5 nT have previously been shown to be sufficient to explain the ∼+120 nT sudden impulse (SI+) observed at Colaba during the storm (Tsurutani et al., 2018, https://doi.org/10.1002/2017JA024779). Magnetohydrodynamic theory (Kennel et al., 1985, https://doi.org/10.1029/GM034p0001) predicts a maximum of 4× magnetic field compression by a fast shock, giving an interplanetary sheath field of ∼20 nT, a value too low to support the Ohtani (2022), https://doi.org/10.1029/2022JA030596hypothesis. The Ohtani (2022), https://doi.org/10.1029/2022JA030596(and G. Siscoe et al., 2006, https://doi.org/10.1016/j.asr.2005.02.102) claim of a further 10× amplification of the interplanetary sheath fields has not been verified in near‐Earth interplanetary sheaths. The original (Tsurutani et al., 2003, https://doi.org/10.1029/2002JA009504) hypothesis that an interplanetary coronal mass ejection magnetic cloud (MC) having southward magnetic fields of ∼90 nT caused the Carrington magnetic storm main phase of peak SYM‐H/Dst = −1,760 nT seems more likely. The short time between the SI+and the storm main phase onset implies a foreshortened interplanetary sheath. The extremely rapid recovery of the magnetic storm was hypothesized by Tsurutani et al. (2018), https://doi.org/10.1002/2017JA024779as being due to nonlinear ring current losses. We point out that the Hydro‐Quebec 1989 storm was caused by multiple shock‐sheaths and MCs (Lakhina & Tsurutani, 2016, https://doi.org/10.1186/s40562‐016‐0037‐4) unlike the interplanetary causes of the Carrington storm. The Hydro‐Quebec event was a “stealth” magnetic storm. Tsurutani et al. (2018), https://doi.org/10.1002/2017JA024779have previously shown that an ordinary interplanetary plasma density of ∼5 cm−3and magnetic field of ∼5 nT are consistent with the observed Colaba SI+of ∼+120 nT. A maximum possible 4× shock compression would give a sheath field strength of ∼20 nT, a value too low to have caused the Carrington magnetic storm and the Colaba magnetic decrease of 1,600 nT as suggested by Ohtani (2022), https://doi.org/10.1029/2022JA030596. The cause of the Carrington storm was most probably a Bz∼ −90 nT component inside an interplanetary magnetic cloud (Lakhina et al., 2012, https://doi.org/10.1029/2011GM001073; Tsurutani et al., 2003, https://doi.org/10.1029/2002JA009504). The G. Siscoe et al. (2006), https://doi.org/10.1016/j.asr.2005.02.102hypothesis of further sheath magnetic field amplification by a factor of up to ∼10× has not been observed (no significant amplification has been noted at all). It is concluded that the SYM‐H/Dst value of the Carrington storm was −1,760 nT, the original value determined in Tsurutani et al. (2003), https://doi.org/10.1029/2002JA009504. We disagree with Ohtani (2022), https://doi.org/10.1029/2022JA030596that an interplanetary sheath magnetic field could have caused the Carrington storm spike at ColabaWe argue that the sheath fields would be too low to have created the Carrington magnetic stormThere is no evidence that interplanetary sheath magnetic fields can be amplified by up to a factor of 10× We disagree with Ohtani (2022), https://doi.org/10.1029/2022JA030596that an interplanetary sheath magnetic field could have caused the Carrington storm spike at Colaba We argue that the sheath fields would be too low to have created the Carrington magnetic storm There is no evidence that interplanetary sheath magnetic fields can be amplified by up to a factor of 10×

A study of dayside plasmaspheric hiss at frequencies from ~22 Hz to ~1.0 kHz was carried out by using 1 year of Polar data. It is shown that intense, dayside plasmaspheric hiss is correlated with solar wind pressure with P> 2.5 nPa. The dayside effect is most prominent in the ~300 to ~650 Hz range. Intense dayside waves are also present during SYM‐H< −5 nT. The latter is centered at local noon, with the greatest intensities in the L = 2 to 3 region. Assuming drift of ~25 keV electrons from midnight to the wave magnetic local time, plasmaspheric hiss is shown to be highly correlated with precursor AE* and SYM‐H* indices, indicating that the hiss is associated with substorms and small injection events. Our hypothesis is that both sets of waves originate as outer zone (L = 6 to 10) chorus and then propagate into the plasmasphere. Fourteen high‐intensity dayside plasmaspheric hiss events were analyzed to identify the wave k, polarization, and the degree of coherency. The waves are found to be obliquely propagating, elliptically polarized and quasi‐coherent (~0.5 to 0.8 correlation coefficient). It is hypothesized that the dayside plasmaspheric hiss is quasi‐coherent because the chorus has been recently generated in the outer magnetosphere and have propagated directly into the plasmasphere. It is possible that the quasi‐coherency of the dayside hiss at L = 2 to 3 may be an alternate explanation for the generation of the energetic particle slot region. There are two sources of dayside plasmaspheric hiss. One source is solar pressure, and the other is substorms/small injection eventsDayside plasmaspheric hiss is typically elliptically polarized and quasi‐coherent. Models should include these features in future codesThe dayside wedge plasmaspheric hiss has a maximum intensity at L = 2 to 3, indicating that these waves could be causing the electron slot

Intense, midnight‐to‐dawn sector, near‐equatorial, chorus rising tones which cross frequencies of ∼0.5fce$\sim 0.5{f}_{\text{ce}}$have been analyzed to determine their structures and possible substructures. Upper band (f≥0.5fce$f\ge 0.5{f}_{\text{ce}}$) chorus and “gap” (f∼0.5fce$f\sim 0.5{f}_{\text{ce}}$) chorus are examined in detail for the first time. It is found that upper band chorus and gap chorus are composed of the same structure as lower band (f≤0.5fce$f\le 0.5{f}_{\text{ce}}$) chorus: they are composed of short‐duration subelements, which are monochromatic with σ≤1%$\sigma \le 1\%$. These findings have strong implications for the chorus element generation mechanism. Following Kennel and Petschek (1966, https://doi.org/10.1029/JZ071i001p00001) the overall chorus riser is most likely generated by anisotropic (T⊥/T∥${T}_{\perp }/{T}_{{\Vert} }$> 1) ∼10–100 keV substorm‐injected electrons. Assuming cyclotron resonance, the upper band chorus is generated by the low energy portion of the electron spectrum. The often‐present gap at ∼0.5fce$\sim 0.5{f}_{\text{ce}}$is related to Landau/cyclotron damping. This however is not the end of the story. There is another type of two‐frequency chorus (called Type 2) for which the lower band is not well connected to the upper band. A Type 2 chorus reported previously by Fu et al. (2014, https://doi.org/10.1002/2014JA020364) has also been studied in detail. Both the lower band and upper band are composed of subelements which are monochromatic. Such a similar fine structure for the different type of chorus may imply a similar generation mechanism, for which the difference between them is just the energy range of resonant energetic electrons. One mechanism discussed here, generation by phase bunched electrons, will be tested in the near future. Understanding chorus structure and microstructure is essential toward understanding the wave generation mechanisms and wave‐particle interaction consequences. In this paper we show that upper band (f≥0.5fce$f\ge 0.5{f}_{\text{ce}}$) chorus and gap chorus (f∼0.5fce$f\sim 0.5{f}_{\text{ce}}$) are composed of substructures (subelements) which are monochromatic with σ≤1%$\sigma \le 1\%$. These are the same features of lower band (f≤0.5fce$f\le 0.5{f}_{\text{ce}}$) chorus. The Kennel‐Petschek theory therefore needs to be enlarged such that phase‐bunching of ∼10–100 keV substorm injected anisotropic electrons occur, which then “lase” to yield the monochromatic wave subelements. Coherent and monochromatic chorus can explain the rapid burstiness of ionospheric microburst X‐ray structures. There is another type of upper band chorus, called Type 2 upper band chorus, where the lower band chorus elements are not clearly connected to the upper band chorus. A Type 2 chorus reported previously by Fu et al. (2014, https://doi.org/10.1002/2014JA020364) has been examined in this paper. The apparently unrelated upper band has been found to be composed of subelements which are monochromatic in nature. Thus the different type of chorus may be excited by a similar generation mechanism, for which the difference between them is only the energy interval of the resonant energetic electrons. Type 1 upper band chorus and gap chorus are found to be composed of short duration subelements which are monochromatic (σ≤1%$\sigma \le 1\%$)Since all parts of chorus risers have similar constructions, it is most probable that they are generated by anisotropic narrow energy‐range phase‐bunched electronsA Type 2 chorus (no connection between the lower band and upper band) was studied. The upper band was composed of monochromatic subelements Type 1 upper band chorus and gap chorus are found to be composed of short duration subelements which are monochromatic (σ≤1%$\sigma \le 1\%$) Since all parts of chorus risers have similar constructions, it is most probable that they are generated by anisotropic narrow energy‐range phase‐bunched electrons A Type 2 chorus (no connection between the lower band and upper band) was studied. The upper band was composed of monochromatic subelements

Electromagnetic Ion Cyclotron (EMIC) rising tone emissions are important in understanding the nonlinear wave evolution and interaction with the energetic particles. We present observations of rising tone emissions of EMIC waves by THEMIS A, D, and E spacecraft in the outer magnetosphere. These emissions with subpacket structures in the proton band were observed during the time interval 14:20 UT to 14:30 UT on September 9, 2010. The observed waves are left‐handed polarized with wave normal angles less than 30°. THEMIS A was closest to the equator and was in a higher L‐shell than THEMIS E and D. The smallest radial separation is ∼2,000 km between THEMIS E and D spacecraft. This configuration of THEMIS allows us to investigate the subpackets of rising tone EMIC waves observed simultaneously at 14:23 UT by three spacecraft. Hilbert Huang Transformation (HHT) is applied to show the variations of the instantaneous frequency and the observed wave amplitude. The direction of energy flow is determined from the analysis of the Poynting flux. There is a rapid nonlinear growth of the EMIC subpackets within one wavelength. Subpackets are dynamic in nature as their structure changes within one wave period, which is further supported by the nonlinear wave growth theory. Optimum and threshold amplitudes for the EMIC wave growth are calculated beside the nonlinear transition time (TN). Observed ion energies and pitch angle spectra of the ion fluxes are consistent with the energy associated with the Landau and cyclotron resonance conditions. Electromagnetic Ion Cyclotron (EMIC) waves are observed below the proton gyrofrequency and play an important role in the magnetospheric dynamics through the ion heating and precipitation of relativistic electrons. They often appear as a series of repetitive structures (known as subpackets) with increasing frequencies, known as rising tone emissions in their entirety. These rising tone emissions are believed to be generated near the geomagnetic equator by the anisotropic distribution of energetic ions (T⊥T∥>1). These emissions are self‐sustaining after the primary linear growth of the triggering wave at lower frequencies. Although previous simulations and theory showed that the source regions of these rising tone emissions move along the magnetic field line, direct observational evidence was missing. Our article provides a case study where these emissions are observed simultaneously by three THEMIS probes in the outer magnetosphere. Adopting a multipoint observation technique, we show there are scattered source regions with an extent greater than the EMIC wavelength, and the subpacket structure changes nonlinearly within one wave period. This analysis provides crucial information about the dynamics of the fine structures of rising emissions and gives an idea about the 3D extent of subpackets. Simultaneous observation of an Electromagnetic Ion Cyclotron (EMIC) wave event by THEMIS A, D, and E separated by a few 1,000 kmSource regions of EMIC waves are scattered around the equator close to the magnetopauseThe subpackets drastically change their amplitudes and frequencies within one wavelength of EMIC waves Simultaneous observation of an Electromagnetic Ion Cyclotron (EMIC) wave event by THEMIS A, D, and E separated by a few 1,000 km Source regions of EMIC waves are scattered around the equator close to the magnetopause The subpackets drastically change their amplitudes and frequencies within one wavelength of EMIC waves

Three lower‐band (f< 0.5 fce) chorus riser elements detected in the dayside generation region were studied in detail using the Van Allen Probe data. Two subelements/wave packets within each riser were examined for their wave “frequency” constancy within seven consecutive wave cycles. The seven wave cycles contained the maximum amplitudes of the subelements/packets. Maximum variance B1 zero crossings were used for the identification of wave cycle start and stop times. It is found that the frequency is constant to within ~3% (one standard deviation), with no evidence of upward frequency sweeping over the seven cycles. Continuous wavelet power spectra for the duration of the seven cycles confirm this conclusion. The implication is that a chorus riser element is composed of coherent approximately “monochromatic” steps instead of a gradual sweep in frequency over the whole element. There was no upward frequency stepping where the wave amplitude was the largest, contrary to the sideband theory prediction. It is shown that a chorus riser involves instability of cyclotron resonant energetic electrons from ~6 to ~40 keV at L= 5.8, that is, essentially the whole substorm electron energy spectrum. The above findings may have important consequences for possible wave generation mechanisms. Some new ideas for mechanisms are suggested in conclusion. Chorus rising tone elements/packets are found to be monochromaticThe largest amplitude waves within a subelement have constant frequencyThe ends of the packets have the greatest frequency deviations; increases and decreases are detected at both ends