Electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency are frequently observed in the solar wind, yet their generation mechanism is still an open question. Based on the Wind data from 2005−2015, this paper carries out a statistical study on the plasma characteristics associated with the occurrence of ECWs. The probability density distributions (PDDs) of proton temperature anisotropy (T _⊥/T _∥) and proton parallel beta (β _∥) are investigated, where ⊥ and ∥ refer to perpendicular and parallel to the background magnetic field, respectively. The PDDs depend on solar wind types as well as wave polarizations, and those for ECWs with left-handed (LH) polarization exhibit considerable differences from the PDDs for ambient solar winds. The distributions of occurrence rates of LH ECWs in (β _∥, T _⊥/T _∥) space show a tendency that the occurrence rates increase with proton temperature anisotropies. The β _∥ with the maximum of occurrence rates is near 0.1 when T _⊥/T _∥ > 1, while it is around 1 when T _⊥/T _∥ < 1. The presence of alpha−proton differential flow with large kinetic energy corresponds to a much higher occurrence rate as well as the domination of LH polarization of ECWs. Based on these observations and existing theories, we propose that the proton cyclotron and parallel firehose instabilities with effects of alpha−proton differential flow are likely responsible for the local generation of LH ECWs in the solar wind. The generation mechanism of right-handed (RH) ECWs seems to be complicated and more discussions are needed in future research.

Small interplanetary magnetic flux ropes (SIMFRs) are commonly observed by spacecraft at 1 AU, and their origin still remains disputed. We investigated the counterstreaming suprathermal electron (CSE) signatures of 106 SIMFRs measured by Wind during 1995–2005. We found that 79 (75%) of the 106 flux ropes contain CSEs, and the percentages of counterstreaming vary from 8% to 98%, with a mean value of 51%. CSEs are often observed in magnetic clouds (MCs), and this indicates these MCs are still attached to the Sun at both ends. CSEs are also related to heliospheric current sheets (HCSs) and the Earth's bow shock. We divided the SIMFRs into two categories: The first category is far from HCSs, and the second category is in the vicinity of HCSs. The first category has 57 SIMFRs, and only 7 of 57 ropes have no CSEs. This ratio is similar to that of MCs. The second category has 49 SIMFRs; however, 20 of the 49 events have no CSEs. This ratio is larger than that of MCs. These two categories have different origins. One category originates from the solar corona, and most ropes are still connected to the Sun at both ends. The other category is formed near HCSs in the interplanetary space.

Electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency are common wave activities in the solar wind and have attracted much attention in recent years. This paper investigates 82,809 ECWs based on magnetic field data from the Solar Terrestrial Relations Observatory‐A mission between 2007 and 2013. Results show that ECWs may last for just a few seconds or incessantly for several tens of minutes. The time fraction of ECW storms among all solar wind is about 0.9%; the storms are obtained with the duration threshold of 10 min, amplitude criterion of 0.032 nT, and time separation limit of 3 min for combination of intermittent ECWs. Most of ECWs have their amplitudes less than 1 nT, while some ECWs have large amplitudes comparable to the ambient magnetic field. The distributions of the durations and amplitudes of these ECWs are characterized by power law spectra, respectively, with spectrum indexes around 4. Statistically, there seems to be a tendency that ECWs with a longer duration will have a larger amplitude. Observed ECW properties are time dependent, and the median frequency of left‐hand ECWs can be lower than that of right‐hand ECWs in some months in the spacecraft frame. The percentage of left‐hand ECWs varies in a large range with respect to months; it is much low (26%) in a month, though it frequently exceeds 50% in other months. Characteristics of ECWs with concurrent polarizations are also researched. The present study should be of importance for a more complete picture of ECWs in the solar wind.

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Type III solar radio bursts (SRBs) produced by fast electron beams (FEBs) traveling along solar magnetic fields are the best known and the most important kind of SRBs because of their clearest association with FEBs as well as most frequent observations during solar activities. However, the physics of their emitting mechanism has been a controversial issue. Based on the electron cyclotron maser (ECM) instability driven directly by a magnetized FEB, whose physics is fairly well known from the Earth's auroral kilometric radiation, this paper proposes a self‐consistent mechanism for type III SRBs, in which the Alfvén wave (AW) produced by the current instability of the beam‐return current system associated with the FEB, called the self‐generated AW, plays an important and crucial role. Taking into account the return‐current effect of the FEB, the growth rate and the saturation intensity of the self‐generated AW are estimated. Then the effects of the self‐generated AW on the ECM emission via the ECM instability driven by the magnetized FEB are further investigated. The results show that the self‐generated AW can significantly influence and change the physical properties of the ECM emission. In particular, this novel ECM emission mechanism can effectively overcome the main difficulties of the conventional ECM emission mechanism in application to type III SRBs and may potentially provide a self‐consistent physics scenario for type III SRBs.