tokens: region that is magnetically connected to the bow shock and is filled with ions or electrons backstreaming from the bow shock (Eastwood et al. 2005 ). The backstreaming particles can excite many instabilities, generating various waves (Johlander et al. 2018 ; Shan et al. 2018 ; Shi et al. 2020 ) and transient structures (Schwartz et al. 2018 ; Bai et al. 2020 ; Turner et al. 2020 ). In recent years, a new type of magnetic structure, electron-scale magnetic holes, has been revealed to exist upstream of the terrestrial bow shock (Yao et al. 2019a ; Wang et al. 2020b , 2020c ) as well as in the magnetosheath (Huang et al. 2017a ; Yao et al. 2017 ) and current sheet (Ge et al. 2011 ; Sun et al. 2012 ). The sizes of these holes are within 2 ρ i (ion gyroradii) (Sun et al. 2012 ; Wang et al. 2020d ), and their cross sections have complicated shapes (Liu et al. 2020b )—some are circular (Liu et al. 2019 ) or like a two-dimensional sheet (Wang et al. 2020d ). Bipolar electron velocities are often observed in the cross section, suggesting that there are electron vortices inside the structures (Huang et al. 2017a ; Wang et al. 2020c ). In contrast, electron-scale magnetic holes have little effect on the ion population (Balikhin et al. 2012 ; Sundberg et al. 2015 ). These structures can trap electrons, and have rich wave activity (Huang et al. 2017b , 2019 ; Yao et al. 2019b ). Several mechanisms have been provided to explain their generation, such as electron mirror instabilities (Yao et al. 2019a ), tearing-mode instabilities (Balikhin et al. 2012 ), solitary waves (Ji et al. 2014 ; Li et al. 2016 ), and electron vortex magnetic holes (Haynes et al. 2015 ). Since they can survive up to 100 electron gyroperiods (Haynes et al. 2015 ) and they can propagate with the ambient flow (Wang et al. 2020d ), it is difficult to identify whether an electron-scale magnetic hole in the terrestrial foreshock is locally generated or originates from the upstream undisturbed solar wind. Thus, the generation mechanism of electron-scale magnetic holes upstream of the bow shock is still not fully understood. Magnetic peaks are another kind of magnetic structure, characterized by an enhancement of the magnetic field strength (Balikhin et al. 2010 ). They widely exist in the planetary magnetosheath (Horbury et al. 2004 ; Balikhin et al. 2010 ) and in the heliosheath (Burlaga et al. 2006 ). In the heliosheath, their sizes are of the order of 20 ρ i (Burlaga et al. 2006 ). They mainly appear in the mirror-unstable region, and they are suggested to be generated by ion mirror instabilities (Soucek et al. 2008 ). Using observations of the four Cluster spacecraft, Horbury et al. ( 2004 ) studied the propagation of magnetic peaks based on timing analysis (Harvey 1998 ). The authors found that their propagation velocities are scattered around zero within the timing errors in the plasma frame, and suggested that they are stationary in the plasma frame. The authors also found that their
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