Photoplethysmography (PPG) technology has been used to develop small, wearable, pulse rate sensors. These devices, consisting of infrared light-emitting diodes (LEDs) and photodetectors, offer a simple, reliable, low-cost means of monitoring the pulse rate noninvasively. Recent advances in optical technology have facilitated the use of high-intensity green LEDs for PPG, increasing the adoption of this measurement technique. In this review, we briefly present the history of PPG and recent developments in wearable pulse rate sensors with green LEDs. The application of wearable pulse rate monitors is discussed.

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Hinode discovered a beautiful giant jet with both cool and hot components at the solar limb on 2007 February 9. Simultaneous observations by the Hinode SOT, XRT, and TRACE 195Å satellites revealed that hot (∼ 5 × 10 6 K) and cool (∼ 10 4 K) jets were located side by side and that the hot jet preceded the associated cool jet (∼ 1 − 2 min.). A current-sheet-like structure was seen in optical (Ca II H), EUV (195Å), and soft X-ray emissions, suggesting that magnetic reconnection is occurring in the transition region or upper chromosphere. Alfvén waves were also observed with Hinode SOT. These propagated along the jet at velocities of ∼ 200 km s −1 with amplitudes (transverse velocity) of ∼5-15 km s −1 and a period of ∼ 200s. We performed two-dimensional MHD simulation of the jets on the basis of the emerging flux -reconnection model, by extending Yokoyama and Shibata's model. We extended the model with a more realistic initial condition (∼ 10 6 K corona) and compared our model with multi-wavelength observations. The improvement of the coronal temperature and density in the simulation model allowed for the first time the reproduction of the structure and evolution of both the cool and hot jets quantitatively, supporting the magnetic reconnection model. The generation and the propagation of Alfvén waves are also reproduced self-consistently in the simulation model.

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