Properties of Daily Helium Fluxes

We present the precision measurement of 2824 daily helium fluxes in cosmic rays from May 20, 2011 to October 29, 2019 in the rigidity interval from 1.71 to 100 GV based on 7.6×10^{8} helium nuclei collected with the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station. The helium flux and the helium to proton flux ratio exhibit variations on multiple timescales. In nearly all the time intervals from 2014 to 2018, we observed recurrent helium flux variations with a period of 27 days. Shorter periods of 9 days and 13.5 days are observed in 2016. The strength of all three periodicities changes with time and rigidity. In the entire time period, we found that below ∼7  GV the helium flux exhibits larger time variations than the proton flux, and above ∼7  GV the helium to proton flux ratio is time independent. Remarkably, below 2.4 GV a hysteresis between the helium to proton flux ratio and the helium flux was observed at greater than the 7σ level. This shows that at low rigidity the modulation of the helium to proton flux ratio is different before and after the solar maximum in 2014.

The March 2012 Heat Wave in Northeast America as a Possible Effect of Strong Solar Activity and Unusual Space Plasma Interactions

In the past two decades, the world has experienced an unprecedented number of extreme weather events, some causing major human suffering and economic damage. The March 2012 heat wave is one of the most known and broadly discussed events in the Northeast United States (NE-USA). The present study examines in depth the possible influence of solar activity on the historic March 2012 heat wave based on a comparison of solar/space and meteorological data. Our research suggests that the historic March 2012 heat wave (M2012HW) and the March 1910 heat wave (M1910HW), which occurred a century earlier in NE-USA, were related to Sun-generated special space plasma structures triggering large magnetic storms. Furthermore, the largest (Dst = −222 nT) magnetic storm during solar cycle 24 in March 2015 (only three years later than the March 2012 events) occurred in relation to another heat wave (M2015HW) in NE-USA. Both these heat waves, M2012HW and M2015HW, resemble each other in many ways: they were characterized by extremely huge temperature increases ΔΤΜ = 30° and 32° (with maximum temperatures ΤΜ = 28° and 23°, respectively) during a positive North Atlantic Oscillation index, the high temperatures coincided with large-scale warm air streaming from southern latitudes, they were accompanied by superstorms caused by unexpected geoeffective interplanetary coronal mass ejections (ICMEs), and the ICME-related solar energetic particle (SEP) events were characterized by a proton spectrum extending to very high (>0.5 GeV) energies. We infer that (i) all three heat waves examined (M2012HW, M2015HW, M1910HW) were related with strong magnetic storms triggered by effective solar wind plasma structures, and (b) the heat wave in March 2012 and the related solar activity was not an accidental coincidence; that is, the M2012HW was most probably affected by solar activity. Future case and statistical studies are needed to further check the hypothesis put forward here, which might improve atmospheric models in helping people’s safety, health and life.

Energization of the Ring Current by Substorms

The substorm process releases large amounts of energy into the magnetospheric system, although where the energy is transferred to and how it is partitioned remains an open question. In this study, we address whether the substorm process contributes a significant amount of energy to the ring current. The ring current is a highly variable region, and understanding the energization processes provides valuable insight into how substorm-ring current coupling may contribute to the generation of storm conditions and provide a source of energy for wave driving. In order to quantify the energy input into the ring current during the substorm process, we analyze Radiation Belt Storm Probes Ion Composition Experiment and Helium Oxygen Proton Electron ion flux measurements for H+, O+, and He+. The energy content of the ring current is estimated and binned spatially for L and magnetic local time. The results are combined with an independently derived substorm event list to perform a statistical analysis of variations in the ring current energy content with substorm phase. We show that the ring current energy is significantly higher in the expansion phase compared to the growth phase, with the energy enhancement persisting into the substorm recovery phase. The characteristics of the energy enhancement suggest the injection of energized ions from the tail plasma sheet following substorm onset. The local time variations indicate a loss of energetic H+ ions in the afternoon sector, likely due to wave-particle interactions. Overall, we find that the average energy input into the ring current is ∼9% of the previously reported energy released during substorms.

Relativistic Electrons Produced by Foreshock Disturbances Observed Upstream of Earth's Bow Shock

Charged particles can be reflected and accelerated by strong (i.e., high Mach number) astrophysical collisionless shock waves, streaming away to form a foreshock region in communication with the shock. Foreshocks are primarily populated by suprathermal ions that can generate foreshock disturbances-large-scale (i.e., tens to thousands of thermal ion Larmor radii), transient (∼5-10  per day) structures. They have recently been found to accelerate ions to energies of several keV. Although electrons in Saturn's high Mach number (M>40) bow shock can be accelerated to relativistic energies (nearly 1000 keV), it has hitherto been thought impossible to accelerate electrons beyond a few tens of keV at Earth's low Mach number (1≤M<20) bow shock. Here we report observations of electrons energized by foreshock disturbances to energies up to at least ∼300  keV. Although such energetic electrons have been previously observed, their presence has been attributed to escaping magnetospheric particles or solar events. These relativistic electrons are not associated with any solar or magnetospheric activity. Further, due to their relatively small Larmor radii (compared to magnetic gradient scale lengths) and large thermal speeds (compared to shock speeds), no known shock acceleration mechanism can energize thermal electrons up to relativistic energies. The discovery of relativistic electrons associated with foreshock structures commonly generated in astrophysical shocks could provide a new paradigm for electron injections and acceleration in collisionless plasmas.