Hongchun Zhang

Evidence that crater flux transfer events are initial stages of typical flux transfer events

[1] Bipolar magnetic perturbations along the normal to the local magnetopause associated with field magnitude enhancements are signatures of typical flux transfer events (T-FTEs) and are interpreted as evidence of encounters with magnetic flux ropes with strong core fields. If the field magnitude dips at the center of the signature, we identify the event as a crater FTE (C-FTE). In the multiple-spacecraft data of the Time History of Events and Macroscale Interactions During Substorms (THEMIS) between 1 May and 31 October 2007, we have identified 622 FTEs of which only 23 manifested C-FTE signatures. We analyze a C-FTE (30 July 2007) that evolved into a T-FTE and compare its properties with those of a T-FTE (May 20, 2007). For all 23 C-FTEs and 35 confirmed T-FTEs, we compare solar wind conditions and internal plasma and field properties. The similarity of solar wind properties for events in the two classes suggests that differences in their structures are not related to the solar wind conditions. Systematic differences in internal peak fields (B C-FTE < B Magnetosphere < B T-FTE ) and averaged number densities (N T-FTE < 0.5 x N Magnetosheath < N C-FTE ) between the two groups are consistent with the evolution of C-FTEs into T-FTEs. We propose that parallel flows inside C-FTEs deplete the internal ion densities and reduce the thermal pressures as the central field magnitude increases to maintain pressure balance.

The origins of the anomalous warming in the California coastal ocean and San Francisco Bay during 2014–2016

During 2014 exceptionally warm water temperatures developed across a wide area off the California coast and within San Francisco Bay (SFB) and persisted into 2016. Observations and numerical model output are used to document this warming and determine its origins. The coastal warming was mostly confined to the upper 100 meters of the ocean and was manifested strongly in the two leading modes of upper ocean (0-100 m) temperature variability in the extra-tropical eastern Pacific. Observations suggest that the coastal warming in 2014 propagated into nearshore regions from the west while later indicating a warming influence that propagated from south to north into the region associated with the 2015-16 El Nino event. An analysis of the upper ocean (0-100 m) heat budget in a Regional Ocean Modeling System simulation confirmed this scenario. The results from a set of sensitivity runs with the model in which the lateral boundary conditions varied supported the conclusions drawn from the heat budget analysis. Concerning the warming in the SFB, an examination of the observations and the heat budget in an unstructured-grid numerical model simulation suggested that the warming during the second half of 2014 and early 2016 originated in the adjacent California coastal ocean and propagated through the Golden Gate into the Bay. The finding that the coastal and Bay warming are due to the relatively slow propagation of signals from remote sources raises the possibility that such warming events may be predictable many months or even several seasons in advance.